JP2002156669A - Apparatus for converting wavelength - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an apparatus for converting wavelengths which can realize stable wavelength conversion without depending on the polarization state of a signal light. SOLUTION: The wavelength conversion apparatus includes a multiplexer 10 which multiplexes pump light and signal light, a wavelength conversion element 1 which generates output light polarized in the x direction by converting the wavelength of a linearly polarized component in the x direction among the pump light and the signal light, an optical circulator 11 which outputs the light of the multiplexer 10 to the wavelength conversion element 1, and outputs light from the wavelength conversion element 1 to a port 11c, a reflecting mirror 20, and a Faraday rotator 22 which makes the difference of polarization directions between a light beam which goes from the wavelength conversion element 1 to the reflecting mirror 20 and a light beam which is reflected by the reflecting mirror 20 and returns to the wavelength conversion element 1, into 90 degrees. The intensity IL of output light is made constant without depending on the incident polarizing angle ϕ of signal light λs.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength converter that can convert the wavelength of input light into light having another wavelength.

[0002]

2. Description of the Related Art In the field of communication using optical fibers, large-capacity, high-speed data transmission is required. In particular, wavelength multiplexing (W
DM) and optical time-division multiplexing (OTDM) are promising because they can significantly increase the transmission capacity of optical fibers. A wavelength conversion technique for converting into a carrier wavelength becomes important.

For example, in an existing optical communication network,
The mainstream is single-wavelength optical transmission using a 1.3 μm band as a carrier wavelength, which has a small optical fiber loss, and is generally laid to replace a telephone communication network in a city. on the other hand,
BACKGROUND ART In a trunk optical communication network connecting cities, wavelength multiplex optical transmission using a 1.5 μm band suitable for wavelength multiplex transmission as a carrier wavelength is mainly used.

When both optical communication networks are connected, since the carrier wavelengths are different from each other, an optical signal flowing through one network is first converted into an electric signal and then converted into an optical signal using a carrier wavelength suitable for the other network. There is a need to. Then, the performance of optical communication is limited by the capability of electric signal processing.

[0005] Therefore, if the carrier wavelength of one network can be directly converted to the carrier wavelength of the other network, electric signal processing is not required, and high performance of optical communication can be effectively maintained. Therefore, an optical mixing technique for converting the carrier wavelength is indispensable.

In such wavelength conversion, the second harmonic generation (SHG) and the sum frequency generation (SF) due to the nonlinear optical effect are performed.
In order to utilize G), difference frequency generation (DFG), parametric conversion, and the like, a material having a high nonlinear optical effect is desired.

As a related prior art, Japanese Patent Laid-Open No. 10-2
No. 13826, JP-A-2000-10130, literature (IE
IC Trans. Electron. VolE83-C, No. 6 p869 (2000)).

[0008]

The nonlinear optical effect largely depends on the polarization state of the input light and the orientation of the nonlinear optical material. For example, when light linearly polarized in a predetermined direction passes through an optical fiber, it is affected by dispersion of the optical fiber, etc.
The polarization state at the exit of the optical fiber cannot generally be specified.

In addition, since nonlinear optical materials generally have polarization dependence, if the polarization state of input light changes for each carrier wavelength, the wavelength conversion efficiency is not constant, and the intensity of the wavelength-converted output light becomes unstable. Become.

An object of the present invention is to provide a wavelength conversion device that can realize stable wavelength conversion without depending on the polarization state of signal light.

[0011]

SUMMARY OF THE INVENTION The present invention includes a wavelength conversion element for performing wavelength conversion on a linearly polarized light component in a first direction, and a reflection element for reflecting light passing through the wavelength conversion element and returning the reflected light to the wavelength conversion element. And a polarization rotating means for providing a polarization direction difference of 90 degrees between light traveling from the wavelength conversion element to the reflection element and light reflected by the reflection element and returning to the wavelength conversion element. .

According to the present invention, the wavelength conversion element has a polarization dependency capable of wavelength conversion with respect to the linear polarization component in the first direction.
When the pump light and the signal light are input, the first direction component of the pump light and the signal light is wavelength-converted by the wavelength conversion element, and the second direction component orthogonal to the first direction is not wavelength-converted.

Next, when the light is reflected by the polarization rotating means, the first direction component is orthogonal to the first direction of the wavelength conversion element and thus is not wavelength-converted, while the second direction component is parallel to the first direction of the wavelength conversion element. Wavelength conversion. As a result, the first direction component is wavelength-converted on the outward path, and the second direction component is wavelength-converted on the return path. The combined intensity of the wavelength-converted output light becomes constant and depends on the polarization state of the signal light. Without this, stable wavelength conversion can be realized.

Further, the present invention is characterized in that a wavelength selective reflection element is provided between the wavelength conversion element and the polarization rotating means and selectively reflects the pump light.

According to the present invention, when the pump light and the signal light are inputted by providing the wavelength selective reflection element for selectively reflecting the pump light between the wavelength conversion element and the polarization rotating means, the pump light is provided. The first direction component of the signal light is wavelength-converted by the wavelength conversion element, the second direction component orthogonal to the first direction is not wavelength-converted, and then only the pump light is returned to the wavelength conversion element by the wavelength selective reflection element. hand,
The signal light passes through as it is.

Next, when the light is reflected by the polarization rotating means, 45
The first direction component and the second direction component are rotated by 45 degrees around the optical axis by the polarization rotation element, and pass through the wavelength selective reflection element as it is. Then, the first direction component of the signal light is not wavelength-converted because it is orthogonal to the first direction of the wavelength conversion element, while the second direction component of the signal light is parallel to the first direction of the wavelength conversion element, so that the wavelength conversion is performed. At this time, the first of the pump light
Directional components contribute to wavelength conversion. Therefore, the first direction component of the pump light can contribute to the wavelength conversion on the outward path and the return path, so that the wavelength conversion efficiency is further improved. Preferably, when linearly polarized light in the first direction is used as the pump light, the wavelength conversion efficiency is further improved. Can be improved.

The present invention also provides a polarization splitting element made of a birefringent material which splits light to be wavelength-converted into two linearly polarized light components orthogonal to each other by a beam walk-off and travels each of the two linearly polarized light components to two different optical axes. Wavelength conversion, comprising: a wavelength conversion element for converting the wavelength according to each linear polarization component traveling along two optical axes; and a polarization multiplexing element for multiplexing two lights passing through the wavelength conversion element. Device.

According to the present invention, the light is separated into two linearly polarized light components by beam walk-off, then the wavelength is converted for each of the polarized light components, and then combined again, so that the combined intensity of the wavelength-converted output light is constant. Thus, stable wavelength conversion can be realized without depending on the polarization state of the input light.

Further, according to the present invention, the linearly polarized light in the first direction
The linear polarization in the second direction orthogonal to the first direction is applied to the optical axis in the second direction.
A polarization separation element that separates the light into an optical axis, a first 90-degree polarization rotation element that rotates the polarization direction of light traveling along the second optical axis from the polarization separation element by 90 degrees, and a first light from the polarization separation element. A wavelength conversion element that performs wavelength conversion on a linear polarization component in a first direction with respect to light traveling along the axis and light traveling along the second optical axis from the first 90-degree polarization rotation element; A second 90-degree polarization rotation element for rotating the polarization direction of light traveling along the first optical axis by 90 degrees, and light and wavelength conversion traveling from the second 90-degree polarization rotation element along the first optical axis A polarization multiplexing element for multiplexing light traveling along a second optical axis from the element.

According to the present invention, the wavelength conversion element has a polarization dependence capable of converting the wavelength of the linearly polarized light component in the first direction, and the pump light and the signal light are inputted to the polarization separation element arranged in the preceding stage. Then, the polarization splitting element sets linearly polarized light in the first direction with respect to the pump light and the signal light to the first optical axis,
The linearly polarized light in the second direction orthogonal to the first direction is separated into the second optical axis, and the pump light and the signal light traveling along the first optical axis are wavelength-converted by the wavelength conversion element. On the other hand, when the pump light and the signal light traveling along the second optical axis pass through the first 90-degree polarization rotation element, the polarization directions become parallel to the first direction, and the wavelengths are converted by the wavelength conversion element.

The pump light and the signal light traveling from the wavelength conversion element along the first optical axis pass through the second 90-degree polarization rotation element, so that the polarization direction becomes parallel to the second direction, and the polarization light becomes a polarization multiplexing element. On the other hand, pump light and signal light traveling along the second optical axis from the wavelength conversion element enter the polarization multiplexing element as they are, and light along both optical axes is multiplexed. as a result,
The first directional component is wavelength-converted on the first optical axis, the second directional component is wavelength-converted on the second optical axis, and the combined intensity of the wavelength-converted output light becomes constant, and the polarization state of the signal light is changed. Stable wavelength conversion can be realized without depending on it.

Further, according to the present invention, the linearly polarized light in the first direction
The linear polarization in the second direction orthogonal to the first direction is applied to the optical axis in the second direction.
A polarization separation element that separates the light into an optical axis, a first wavelength conversion element that performs wavelength conversion on a linear polarization component in a first direction with respect to light traveling along the first optical axis from the polarization separation element, and a polarization separation element. A second wavelength conversion element that performs wavelength conversion on a linearly polarized light component in the second direction with respect to light traveling along the second optical axis, light that has passed through the first wavelength conversion element, and light that has passed through the second wavelength conversion element And a polarization multiplexing element for multiplexing the wavelengths.

According to the present invention, the first wavelength conversion element is the first wavelength conversion element.
The second wavelength conversion element has a polarization dependency capable of wavelength conversion with respect to the linear polarization component in the second direction, and is disposed in front of the second polarization conversion element. When pump light and signal light are input to the polarization splitting element, the polarization splitting element sets linear polarization in the first direction to the first optical axis and linear polarization in the second direction orthogonal to the first direction to the pump light and the signal light. The pump light and the signal light that are separated into two optical axes and travel along the first optical axis are wavelength-converted by the first wavelength conversion element, and the pump light and the signal light that travel along the second optical axis are the second light. The wavelength is converted by the wavelength conversion element.

From the first wavelength conversion element, along the first optical axis,
The pump light and the signal light, which respectively travel along the second optical axis from the second wavelength conversion element, are multiplexed by the polarization multiplexing element. As a result, the first direction component is wavelength-converted by the first wavelength conversion element, the second direction component is wavelength-converted by the second wavelength conversion element, the combined intensity of the wavelength-converted output light becomes constant, and the signal light Independent of polarization state,
Stable wavelength conversion can be realized.

Also, the present invention provides a method for converting linearly polarized light in a first direction into a first direction.
The linear polarization in the second direction orthogonal to the first direction is applied to the optical axis in the second direction.
A polarization separation element that separates the light into an optical axis, a 90-degree polarization rotation element that rotates the polarization direction of light that has passed from the polarization separation element along the second optical axis by 90 degrees,
A wavelength conversion element that performs wavelength conversion on a linear polarization component in a first direction with respect to light traveling along an optical axis and light traveling from a 90-degree polarization rotation element along a second optical axis; Light traveling along the optical axis and the second optical axis is reflected, light along the first optical axis passes through the wavelength conversion element, and light along the second optical axis is converted into the wavelength conversion element and the 90-degree polarization rotation. A wavelength conversion device comprising: a reflection element that passes in the order of elements and is multiplexed in a polarization separation element.

According to the present invention, the wavelength conversion element has a polarization dependency capable of converting the wavelength of the linearly polarized light component in the first direction, and the pump light and the signal light are input to the polarization separation element disposed at the preceding stage. Then, the polarization splitting element sets linearly polarized light in the first direction with respect to the pump light and the signal light to the first optical axis,
The linearly polarized light in the second direction orthogonal to the first direction is separated into the second optical axis, and the pump light and the signal light traveling along the first optical axis are wavelength-converted by the wavelength conversion element. On the other hand, when the pump light and the signal light traveling along the second optical axis pass through the 90-degree polarization rotation element, the polarization directions become parallel to the first direction, and the wavelengths are converted by the wavelength conversion element.

When the pump light, the signal light and the output light traveling from the wavelength conversion element along the first optical axis and the second optical axis, respectively, are reflected by the reflection element, they pass through the same wavelength conversion element again and are wavelength-converted. You. When the pump light, the signal light, and the output light traveling along the second optical axis pass through the 90-degree polarization rotation element, the polarization direction becomes parallel to the second direction and enters the polarization separation element. Pump light, signal light and output light traveling along both optical axes are multiplexed by the polarization splitting element. As a result, the first direction component is wavelength-converted on the first optical axis, the second direction component is wavelength-converted on the second optical axis, the combined intensity of the wavelength-converted output light becomes constant, and the signal light Stable wavelength conversion can be realized without depending on the polarization state.

Also, the present invention provides a method for converting linearly polarized light in a first direction into a first polarized light.
The linear polarization in the second direction orthogonal to the first direction is applied to the optical axis in the second direction.
A polarization separation element that separates the light into an optical axis, a first wavelength conversion element that performs wavelength conversion on a linear polarization component in a first direction with respect to light traveling along the first optical axis from the polarization separation element, and a polarization separation element. A second wavelength conversion element that converts the wavelength of the linearly polarized light component in the second direction with respect to the light traveling along the second optical axis, and a light traveling along the first optical axis from the first wavelength conversion element and a second wavelength conversion element. A reflecting element for reflecting light traveling along the second optical axis from the wavelength conversion element, and combining the light passing through the first wavelength conversion element and the light passing through the second wavelength conversion element in a polarization splitting element; A wavelength converter characterized by comprising:

According to the invention, the first wavelength conversion element is the first wavelength conversion element.
The second wavelength conversion element has a polarization dependency capable of wavelength conversion with respect to the linear polarization component in the second direction, and is disposed in front of the second polarization conversion element. When pump light and signal light are input to the polarization splitting element, the polarization splitting element sets linear polarization in the first direction to the first optical axis and linear polarization in the second direction orthogonal to the first direction to the pump light and the signal light. The pump light and the signal light that are separated into two optical axes and travel along the first optical axis are wavelength-converted by the first wavelength conversion element, and the pump light and the signal light that travel along the second optical axis are the second light. The wavelength is converted by the wavelength conversion element.

When the pump light and the signal light traveling from the wavelength conversion element along the first optical axis and the second optical axis, respectively, are reflected by the reflection element, they pass through the same wavelength conversion element again and are wavelength-converted. The light is multiplexed by the polarization multiplexing element. As a result, the first direction component is wavelength-converted by the first wavelength conversion element, the second direction component is wavelength-converted by the second wavelength conversion element, the combined intensity of the wavelength-converted output light becomes constant, and the signal light Stable wavelength conversion can be realized without depending on the polarization state.

Further, according to the present invention, the linearly polarized light in the first direction
The linear polarization in the second direction orthogonal to the first direction is applied to the optical axis in the second direction.
A polarization separation element that separates the light into an optical axis, and a wavelength conversion element that performs wavelength conversion on a linear polarization component in a first direction with respect to light traveling along the first optical axis from the polarization separation element.
A polarization multiplexing element for multiplexing light traveling along the first optical axis from the wavelength conversion element and light traveling along the second optical axis from the polarization separation element; and reflecting light passing through the polarization multiplexing element. And a polarization rotating means such that the difference in polarization direction between light traveling from the wavelength conversion element to the reflection element and light reflected by the reflection element and returning to the wavelength conversion element is 90 degrees. A wavelength converter characterized by the above.

According to the present invention, the wavelength conversion element has a polarization dependence capable of converting the wavelength of the linearly polarized light component in the first direction, and the pump light and the signal light are input to the polarization separation element disposed in the preceding stage. Then, the polarization splitting element sets linearly polarized light in the first direction with respect to the pump light and the signal light to the first optical axis,
The linearly polarized light in the second direction orthogonal to the first direction is separated into the second optical axis, and the pump light and the signal light traveling along the first optical axis are wavelength-converted by the wavelength conversion element. The pump light and signal light traveling along the axis are not wavelength-converted. Pump light and signal light traveling from the wavelength conversion element along the first optical axis and the second optical axis, respectively, are multiplexed by the polarization multiplexing element.

Next, when the light is reflected by the polarization rotating means, the polarization direction of the light is rotated by 90 degrees. Further, the polarization multiplexing element separates the pump light and the signal light traveling on the first optical axis on the outward path into the second optical axis, and separates the pump light and the signal light traveling on the second optical axis on the outward path into the first optical axis. Then, the wavelength of the light along the first optical axis is converted by the wavelength conversion element, but the wavelength of the light along the second optical axis is not converted. Next, the pump light and the signal light traveling from the wavelength conversion element along the first optical axis and the second optical axis, respectively, are multiplexed by the polarization separation element. As a result, the first direction component is wavelength-converted on the outward path, and the second direction component is wavelength-converted on the return path. The combined intensity of the wavelength-converted output light becomes constant and depends on the polarization state of the signal light. Without this, stable wavelength conversion can be realized.

Further, the present invention provides a method for converting linearly polarized light in the first direction into the first direction.
The linear polarization in the second direction orthogonal to the first direction is applied to the optical axis in the second direction.
A polarization separation element that separates the light into an optical axis, a first 90-degree polarization rotation element that rotates the polarization direction of light traveling along the second optical axis from the polarization separation element by 90 degrees, and a first light from the polarization separation element. A wavelength conversion element that performs wavelength conversion on a linear polarization component in a first direction with respect to light traveling along the axis and light traveling along the second optical axis from the first 90-degree polarization rotation element; A second 90-degree polarization rotation element for rotating the polarization direction of light traveling along the first optical axis by 90 degrees, and light and wavelength conversion traveling from the second 90-degree polarization rotation element along the first optical axis A polarization multiplexing element that multiplexes light traveling along the second optical axis from the element, and a reflection element that reflects light passing through the polarization multiplexing element and returns the light to the wavelength conversion element. Wavelength conversion by reflecting the light going to The difference in the polarization direction of the light returning to the child is a wavelength conversion device characterized by comprising a polarization rotation means such that 90 degrees.

According to the present invention, the wavelength conversion element has a polarization dependency capable of converting the wavelength of the linearly polarized light component in the first direction, and the pump light and the signal light are input to the polarization separation element disposed at the preceding stage. Then, the polarization splitting element sets linearly polarized light in the first direction with respect to the pump light and the signal light to the first optical axis,
The linearly polarized light in the second direction orthogonal to the first direction is separated into the second optical axis, and the pump light and the signal light traveling along the first optical axis are wavelength-converted by the wavelength conversion element. On the other hand, when the pump light and the signal light traveling along the second optical axis pass through the first 90-degree polarization rotation element, the polarization directions become parallel to the first direction, and the wavelengths are converted by the wavelength conversion element.

The pump light and the signal light traveling from the wavelength conversion element along the first optical axis pass through the second 90-degree polarization rotation element, so that the polarization direction becomes parallel to the second direction, and the polarization light becomes a polarization multiplexing element. On the other hand, pump light and signal light traveling along the second optical axis from the wavelength conversion element enter the polarization multiplexing element as they are, and light along both optical axes is multiplexed.

Next, when the light is reflected by the polarization rotating means, the polarization direction of the light is rotated by 90 degrees. Further, the polarization multiplexing element separates the pump light and the signal light traveling on the first optical axis on the outward path into the second optical axis, and separates the pump light and the signal light traveling on the second optical axis on the outward path into the first optical axis. Then, the pump light and the signal light traveling along the first optical axis are wavelength-converted by the wavelength conversion element and directly enter the polarization splitting element. On the other hand, when the pump light and the signal light traveling along the second optical axis pass through the second 90-degree polarization rotation element, the polarization direction becomes parallel to the first direction, and the wavelength is converted by the wavelength conversion element. 1
After passing through the 90-degree polarization rotation element, the polarization direction becomes parallel to the second direction and enters the polarization separation element. The pump light and the signal light traveling along both optical axes are multiplexed by the polarization separation element. As a result, the first direction component and the second direction component are wavelength-converted in the forward path and the return path, and the combined intensity of the wavelength-converted output light is constant, and does not depend on the polarization state of the signal light and is stable. Wavelength conversion can be realized.

Further, according to the present invention, the linearly polarized light in the first direction is
The linear polarization in the second direction orthogonal to the first direction is applied to the optical axis in the second direction.
A polarization separation element that separates the light into an optical axis, a first wavelength conversion element that performs wavelength conversion on a linear polarization component in a first direction with respect to light traveling along the first optical axis from the polarization separation element, and a polarization separation element. A second wavelength conversion element that converts the wavelength of the linearly polarized light component in the second direction with respect to the light traveling along the second optical axis, and a light traveling along the first optical axis from the first wavelength conversion element and a second wavelength conversion element. A polarization multiplexing element that multiplexes light traveling along the second optical axis from the wavelength conversion element, and a reflection element that reflects light that has passed through the polarization multiplexing element and returns the light to the wavelength conversion element. A wavelength conversion device comprising: a polarization rotating unit that causes a difference in polarization direction between light traveling toward the reflection element and light reflected by the reflection element and returning to the wavelength conversion element to be 90 degrees.

According to the present invention, the first wavelength conversion element is the first wavelength conversion element.
The second wavelength conversion element has a polarization dependency capable of wavelength conversion with respect to the linear polarization component in the second direction, and is disposed in front of the second polarization conversion element. When pump light and signal light are input to the polarization splitting element, the polarization splitting element sets linear polarization in the first direction to the first optical axis and linear polarization in the second direction orthogonal to the first direction to the pump light and the signal light. The pump light and the signal light that are separated into two optical axes and travel along the first optical axis are wavelength-converted by the first wavelength conversion element, and the pump light and the signal light that travel along the second optical axis are the second light. The wavelength is converted by the wavelength conversion element.

From the first wavelength conversion element along the first optical axis,
The pump light and the signal light, which respectively travel along the second optical axis from the second wavelength conversion element, are multiplexed by the polarization multiplexing element.

Next, when the light is reflected by the polarization rotating means, the polarization direction of the light is rotated by 90 degrees. Further, the polarization multiplexing element separates the pump light and the signal light traveling on the first optical axis on the outward path into the second optical axis, and separates the pump light and the signal light traveling on the second optical axis on the outward path into the first optical axis. And the light along the first optical axis is the first
The wavelength is converted by the wavelength conversion element, and the light along the second optical axis is converted by the second wavelength conversion element.

From the first wavelength conversion element along the first optical axis,
The pump light and the signal light traveling along the second optical axis from the second wavelength conversion element and the second wavelength conversion element are multiplexed by the polarization separation element. As a result, the first direction component and the second direction component are wavelength-converted in the forward path and the return path, and the combined intensity of the wavelength-converted output light is constant, and does not depend on the polarization state of the signal light and is stable. Wavelength conversion can be realized.

Also, in the present invention, the polarization rotating means rotates the polarization direction of the light by 45 degrees, and the 45 degree rotation element for reflecting the light passing through the 45 degree polarization rotation element and returning the light to the 45 degree rotation element. It is comprised of an inversion element, and the 45-degree rotation element is provided between the wavelength conversion element and the reflection element.

According to the present invention, the polarization direction of the light incident on the polarization rotation means can be rotated by 90 degrees and returned to the wavelength conversion element.

Further, the present invention is characterized in that the polarization rotating means comprises a λ / 4 plate provided between the reflection element and the wavelength conversion element and the reflection element.

According to the present invention, the polarization direction of the light incident on the polarization rotating means can be rotated by 90 degrees and returned to the wavelength conversion element.

[0047]

FIG. 1 is a block diagram showing a first embodiment of the present invention. The wavelength conversion device includes a multiplexer 10, an optical circulator 11, the wavelength conversion element 1, a Faraday rotator 22, a reflection mirror 20, and the like.

The multiplexer 10 has an input port 10a to which the pump light (wavelength λp) is input and an input port 10b to which the signal light (wavelength λs) is input, and multiplexes the pump light and the signal light. And output along the same optical axis.

The optical circulator 11 outputs the light input to the port 11a to the port 11b, and outputs the light input to the port 11b to the port 11c.

The wavelength conversion elements 1 and 2 are formed of a non-linear optical material such as LiNbO ₃ (abbreviation LN), LiTaO ₃ (abbreviation LT), KNbO ₃ (abbreviation KN), KTiOPO ₄ (abbreviation KTP), and here, polarization. QPM (Quasi Phase Matchin) in which the directions are alternately reversed with a period of coherence length
g: quasi-phase matching) element. The polarization direction of the wavelength conversion element 1 is the x direction (parallel to the paper and perpendicular to the optical axis).
It is arranged so that it may become parallel.

The Faraday rotator 22 rotates the polarization direction of the light by 45 degrees around the optical axis in a predetermined direction. The reflecting mirror 20 reflects the incident light on the same optical axis.

When the conversion constant d33 of the nonlinear optical material is used in the wavelength conversion, when the polarization direction of the wavelength conversion element, the polarization direction of the pump light, and the polarization direction of the signal light coincide with each other, the wavelength-converted output light (wavelength λL) occur in the same polarization direction.

For example, when performing cascade type difference frequency generation and performing wavelength conversion in 1.5 μm band optical fiber communication,
The wavelength λs is the C band (1.53 to 1.56 μm), the wavelength λL is the L band (1.56 to 1.61 μm), and the wavelength λp
Is set to 1.56 μm, which is the center between the C band and the L band, and the wavelength λp of the pump light, the wavelength λs of the signal light, and the wavelength λL of the output light satisfy the relationship of Expression (1). 1 / λL = 2 / λp−1 / λs (1)

Next, the operation will be described. For example, linearly polarized light having a ratio of the x-direction component to the y-direction component of 1: 1 is used as the pump light. It is assumed that the polarization direction of the signal light fluctuates with time and is uncertain.

The pump light and the signal light are multiplexed by the multiplexer 10 and pass through the wavelength conversion element 1 from the port 11a of the optical circulator 11 via the port 11b.
Then, the x-direction components of the pump light and the signal light are wavelength-converted into output light polarized in the x-direction by the wavelength conversion element 1 polarized in the x-direction. The y-direction components of the pump light and the signal light are not wavelength-converted because they are orthogonal to the polarization direction of the wavelength conversion element 1.

Next, when the pump light, the signal light, and the output light pass through the Faraday rotator 22, the polarization direction of each light is rotated by 45 degrees. The polarization direction is further rotated by 45 degrees, and after being rotated by 90 degrees as compared with before the incidence of the Faraday rotator 22, the light is re-input to the wavelength conversion element 1. At this time, since the x-direction components of the pump light and the signal light are rotating in the y-direction, the wavelength is not converted by the wavelength conversion element 1, while the y-direction components of the pump light and the signal light are rotating in the x-direction. Therefore, the wavelength is converted by the wavelength conversion element 1. As a result, the x-direction component before input is wavelength-converted on the outward path, and the y-direction component before input is wavelength-converted on the return path.

The wavelength-converted output light is extracted from the port 11b of the optical circulator 11 via the port 11c.

The incident polarization angle φ of the signal light, the electric field intensity Ep of the pump light, the electric field intensity Es of the signal light, the clockwise conversion efficiency η
Using r, the left-handed conversion efficiency ηl, and the optical length L of the wavelength conversion element 1, the output light intensity IL is expressed by Expression (2). IL = (ηr · L · Ep / √2 · Es · cosφ) ² + (ηl·L · Ep / √2 · Es · sinφ) ² … (2)

Here, since the light passes through the same place on the outward path and the return path of the wavelength conversion element 1, ηr = ηl, and the intensity IL of the output light is expressed by the following equation (3). IL = (ηr · L · Ep · Es) ² /2… (3)

Therefore, the intensity IL of the output light becomes constant without depending on the incident polarization angle φ of the signal light, and stable wavelength conversion can be realized even if the polarization state of the signal light changes.

FIG. 2 is a configuration diagram showing a second embodiment of the present invention. The wavelength conversion device includes a multiplexer 10, an optical circulator 11, a wavelength conversion element 1, a wavelength selective reflection mirror 21, a Faraday rotator 22, and a reflection mirror 20.
Etc.

The multiplexer 10 has an input port 10a to which the pump light (wavelength λp) is input and an input port 10b to which the signal light (wavelength λs) is input, and multiplexes the pump light and the signal light. And output along the same optical axis.

The optical circulator 11 outputs the light input to the port 11a to the port 11b, and outputs the light input to the port 11b to the port 11c.

The wavelength conversion element 1 is composed of LN, LT, KN, K
Here, an example using a QPM element formed of a non-linear optical material such as TP and in which the polarization direction is alternately inverted with the period of the coherence length is shown. The wavelength conversion element 1 is arranged so that the polarization direction is parallel to the x direction.

The wavelength selective reflection mirror 21 is composed of a dichroic mirror or the like, and has a characteristic of reflecting pump light and passing signal light and output light. At this time, it is preferable that the pump light be set to a wavelength corresponding to the second harmonic of the fundamental wavelength, because better characteristics are obtained as the reflection wavelength and the transmission wavelength are farther apart.

The Faraday rotator 22 rotates the polarization direction of the light by 45 degrees around the optical axis in a predetermined direction. The reflecting mirror 20 reflects the incident light on the same optical axis.

When the conversion constant d33 of the nonlinear optical material is used in the wavelength conversion, when the polarization direction of the wavelength conversion element, the polarization direction of the pump light, and the polarization direction of the signal light match each other, the wavelength-converted output light (wavelength λL) occur in the same polarization direction.

For example, by performing a simple difference frequency generation,
When performing wavelength conversion in 5 μm band optical fiber communication, the wavelength λ
s is set to C band, wavelength λL is set to L band, wavelength λp is set to 0.78 μm, which is half of the central wavelength of 1.56 μm between C band and L band, wavelength λp of pump light, wavelength λs of signal light, and output The wavelength λL of the light satisfies the relationship of Expression (1A). 1 / λL = 1 / λp−1 / λs (1A)

Next, the operation will be described. For example, linearly polarized light in the x direction is used as the pump light. It is assumed that the polarization direction of the signal light fluctuates with time and is uncertain.

The pump light and the signal light are multiplexed by the multiplexer 10 and pass through the wavelength conversion element 1 from the port 11a of the optical circulator 11 via the port 11b.
Then, the x-direction components of the pump light and the signal light are wavelength-converted into output light polarized in the x-direction by the wavelength conversion element 1 polarized in the x-direction. The y-direction components of the pump light and the signal light are not wavelength-converted because they are orthogonal to the polarization direction of the wavelength conversion element 1.

Next, the wavelength selective reflection mirror 21 reflects only the pump light from the wavelength conversion element 1 and reenters it into the wavelength conversion element 1, and transmits the signal light and the output light. As a result, the pump light linearly polarized in the x direction can contribute to wavelength conversion on the outward path and the return path.

Next, when the signal light and the output light pass through the Faraday rotator 22, the polarization direction of each light is rotated by 45 degrees. When the light is reflected by the reflection mirror 20, it passes through the Faraday rotator 22 again and the polarization direction of each light is changed. It is further rotated 45 degrees, and 90 degrees compared with before the Faraday rotator 22 is incident.
After the rotation, the light passes through the wavelength selective reflection mirror 21 and is input again to the wavelength conversion element 1. At this time, since the x direction component of the signal light is rotating in the y direction, the wavelength conversion element 1 does not perform wavelength conversion. On the other hand, the y direction component of the signal light is rotating in the x direction. Is wavelength-converted. As a result, the x-direction component before input is wavelength-converted on the outward path, and the y-direction component before input is wavelength-converted on the return path.

The wavelength-converted output light is extracted from the port 11b of the optical circulator 11 via the port 11c.

In the configuration shown in FIG. 2, it is also possible to generate a cascade type difference frequency. The wavelength λs is in the C band, the wavelength λL is in the L band, and the wavelength λp is 1.56 μm which is the center of the C and L bands. Is set and the wavelength λ of the pump light
p, the wavelength λs of the signal light, and the wavelength λL of the output light are given by Equation (1).
Is established. 1 / λL = 2 / λp−1 / λs (1)

In this case, since the wavelengths of the pump light, the signal light and the output light are close to each other, the wavelength selective reflection mirror 21 is an optical element having a steeper wavelength selection characteristic, for example, a fiber type such as a fiber Bragg grating (FBG). It is preferable to use a filter or a narrow band optical coating filter.

Here, the incident polarization angle φ of the signal light, the electric field intensity Ep of the pump light, the electric field intensity Es of the signal light, the clockwise conversion efficiency ηr, the clockwise conversion efficiency η1, and the optical length L of the wavelength conversion element 1 are used. Since the pump light contributes in a reciprocating manner, the intensity IL of the output light is expressed by Expression (4). IL = (ηr · L · Ep · Es · cosφ) ² + (ηl·L · Ep · Es · sinφ) ² … (4)

Here, since the light passes through the same place in the forward path and the return path of the wavelength conversion element 1, ηr = ηl, and the intensity IL of the output light is represented by the following equation (5). IL = (ηr · L · Ep · Es) ² … (5)

Therefore, the intensity IL of the output light becomes constant without depending on the incident polarization angle φ of the signal light, and stable wavelength conversion can be realized even if the polarization state of the signal light changes. In addition, since the pump light contributes in a round trip, the output light intensity IL
Is twice as large as that of FIG. 1, and the wavelength conversion efficiency can be further improved.

FIG. 3 is a configuration diagram showing a third embodiment of the present invention. The wavelength conversion device includes a multiplexer 10, a polarization separation element 3, a wavelength conversion element 1, a polarization multiplexing element 4, and 90-degree polarization rotation elements 5, 6, and the like.

The multiplexer 10 has an input port 10a to which the pump light (wavelength λp) is input, and an input port 10b to which the signal light (wavelength λs) is input, and multiplexes the pump light and the signal light. And output along the same optical axis.

The wavelength conversion element 1 is composed of LN, LT, KN, K
Here, an example using a QPM element formed of a non-linear optical material such as TP and in which the polarization direction is alternately inverted with the period of the coherence length is shown. The wavelength conversion element 1 is arranged so that the polarization direction is parallel to the x direction.

The polarization separation element 3 and the polarization multiplexing element 4
Crystal obtained by cutting a birefringent optical crystal obliquely with respect to the crystal axis (for example, YV cut at 45 degrees from the c-axis)
O ₄ ) etc., and by the beam walk-off effect,
In the incident light, the linear polarization in the x direction is regarded as extraordinary light e and the optical axis Q
1, the linearly polarized light in the y direction is separated into the optical axis Q2 as ordinary light o, or x incident along the optical axis Q1.
It has the function of combining linearly polarized light in the direction and linearly polarized light in the y direction incident along the optical axis Q2.

The 90-degree polarization rotating elements 5 and 6 are constituted by half-wave plates or the like, and have a function of rotating the polarization direction of incident light by 90 degrees.

When the conversion constant d33 of the nonlinear optical material is used in the wavelength conversion, when the polarization direction of the wavelength conversion element, the polarization direction of the pump light, and the polarization direction of the signal light match each other, the wavelength-converted output light (wavelength λL) occur in the same polarization direction.

For example, when performing cascade type difference frequency generation and performing wavelength conversion in 1.5 μm band optical fiber communication,
The wavelength λs is set to the C band, the wavelength λL is set to the L band, the wavelength λp is set to 1.56 μm which is the center of the C band and the L band, and the wavelength λp of the pump light, the wavelength λs of the signal light, and the wavelength λL of the output light are: Equation (1) holds. 1 / λL = 2 / λp−1 / λs (1)

Next, the operation will be described. For example, linearly polarized light in which the ratio between the x-direction component and the y-direction component is 1: 1 at a position where the light enters the polarization separation element 3 as pump light, that is, linearly polarized light rotated by 45 degrees with respect to the main axis of the polarization separation element 3 Use It is assumed that the polarization direction of the signal light fluctuates with time and is uncertain.

When the pump light and the signal light are multiplexed by the multiplexer 10 and are incident on the polarization splitter 3, linearly polarized light in the x direction of the pump light and the signal light is separated along the optical axis Q1, and X by the wavelength conversion element 1 polarized to
The wavelength is converted into output light polarized in the direction, then converted into linearly polarized light in the y direction by the 90-degree polarization rotation element 6, and enters the polarization multiplexing element 4. On the other hand, the linearly polarized light in the y direction of the pump light and the signal light is separated along the optical axis Q2 in the polarization separating element 3,
Is converted into linearly polarized light in the
The wavelength is converted to output light polarized in the direction,
to go into.

The polarization multiplexing element 4 multiplexes pump light, signal light and output light incident along the optical axes Q1 and Q2,
Output along the same optical axis.

As described above, since the x-direction component is wavelength-converted on the optical axis Q1 and the y-direction component is wavelength-converted on the optical axis Q2, the combined intensity of the wavelength-converted output light becomes constant, and the polarization of the signal light becomes constant. Stable wavelength conversion can be realized regardless of the state.

FIG. 4 is a configuration diagram showing a fourth embodiment of the present invention. The wavelength converter includes a multiplexer 10, a polarization splitter 3, wavelength converters 1 and 2, a polarization multiplexer 4, an optical delay element 7, and the like.

The multiplexer 10 has an input port 10a to which the pump light (wavelength λp) is input, and an input port 10b to which the signal light (wavelength λs) is input, and multiplexes the pump light and the signal light. And output along the same optical axis.

The wavelength conversion elements 1 and 2 are LN, LT, K
QP formed of a non-linear optical material such as N or KTP, in which the polarization direction is alternately inverted with the period of the coherence length
An example using an M element will be described. The wavelength conversion element 1 is arranged so that the polarization direction is parallel to the x direction. Wavelength conversion element 2
Are arranged such that the polarization direction is parallel to the y direction.

The polarization separation element 3 and the polarization multiplexing element 4
Crystal obtained by cutting a birefringent optical crystal obliquely with respect to the crystal axis (for example, YVO cut at 45 degrees from the c-axis)
₄ ) etc., and the linear polarization in the x direction of the incident light is regarded as extraordinary light e due to the beam walk-off effect.
Then, the linearly polarized light in the y direction is separated into the optical axis Q2 as ordinary light o, or the linearly polarized light in the x direction incident along the optical axis Q1 and the linearly polarized light in the y direction incident along the optical axis Q2. Has the function of multiplexing.

The optical delay element 7 is made of a transparent material or the like having a predetermined optical length, and matches the optical lengths of the optical axes Q1 and Q2 from the plane of incidence of the polarization separation element 3 to the plane of emission of the polarization multiplexing element 4. In addition, it has a function of eliminating a phase difference of output light generated along the optical axes Q1 and Q2 and a pulse delay time difference in the case of a pulse. If such a phase difference is practically negligible, the optical delay element 7 can be omitted.

When the conversion constant d33 of the nonlinear optical material is used in the wavelength conversion, when the polarization direction of the wavelength conversion element, the polarization direction of the pump light, and the polarization direction of the signal light coincide with each other, the wavelength-converted output light (wavelength λL) occur in the same polarization direction.

For example, when performing cascaded difference frequency generation and performing wavelength conversion in 1.5 μm band optical fiber communication,
The wavelength λs is set to the C band, the wavelength λL is set to the L band, the wavelength λp is set to 1.56 μm which is the center of the C band and the L band, and the wavelength λp of the pump light, the wavelength λs of the signal light, and the wavelength λL of the output light are: Equation (1) holds. 1 / λL = 2 / λp−1 / λs (1)

Next, the operation will be described. For example, linearly polarized light in which the ratio between the x-direction component and the y-direction component is 1: 1 at a position where the light enters the polarization separation element 3 as pump light, that is, linearly polarized light rotated by 45 degrees with respect to the main axis of the polarization separation element 3 Use It is assumed that the polarization direction of the signal light fluctuates with time and is uncertain.

When the pump light and the signal light are multiplexed by the multiplexer 10 and are incident on the polarization splitting element 3, linearly polarized light in the x direction of the pump light and the signal light is separated along the optical axis Q1, and X by the wavelength conversion element 1 polarized to
The wavelength is converted to output light polarized in the direction,
to go into. On the other hand, in the polarization separation element 3, the linearly polarized light in the y direction of the pump light and the signal light is separated along the optical axis Q2, and the wavelength is converted into output light polarized in the y direction by the wavelength conversion element 1 polarized in the y direction. Then, the light enters the polarization multiplexing element 4 after passing through the optical delay element 7.

The polarization multiplexing element 4 multiplexes the pump light, the signal light and the output light incident along the optical axes Q1 and Q2,
Output along the same optical axis.

Thus, the x-direction component is wavelength-converted on the optical axis Q1, and the y-direction component is wavelength-converted on the optical axis Q2. Thus, the combined intensity of the wavelength-converted output light is constant, and the polarization of the signal light is Stable wavelength conversion can be realized regardless of the state.

FIG. 5 is a configuration diagram showing a fifth embodiment of the present invention. The wavelength conversion device includes a multiplexer 10, an optical circulator 11, a polarization separation element 3, a wavelength conversion element 1,
It comprises a reflection mirror 20, a 90-degree polarization rotation element 5, an optical delay element 7, and the like.

The multiplexer 10 has an input port 10a to which the pump light (wavelength λp) is input, and an input port 10b to which the signal light (wavelength λs) is input, and multiplexes the pump light and the signal light. And output along the same optical axis.

The optical circulator 11 outputs the light input to the port 11a to the port 11b, and outputs the light input to the port 11b to the port 11c.

The wavelength conversion element 1 is composed of LN, LT, KN, K
Here, an example using a QPM element formed of a non-linear optical material such as TP and in which the polarization direction is alternately inverted with the period of the coherence length is shown. The wavelength conversion element 1 is arranged so that the polarization direction is parallel to the x direction.

The polarized light separating element 3 is a crystal obtained by cutting a birefringent optical crystal obliquely with respect to the crystal axis (for example, 4 × from the c axis).
It is composed of YVO ₄ ) cut in the direction of 5 degrees, etc., and is made to emit linearly polarized light in the x direction as an extraordinary light e on the optical axis Q1 and linearly polarized light in the y direction as ordinary light o due to the beam walk-off effect. The x-direction linearly polarized light and the optical axis Q2 separated from each other along the axis Q2 or incident along the optical axis Q1.
And a function of multiplexing the linearly polarized light in the y-direction incident along.

The 90-degree polarization rotating element 5 is constituted by a half-wave plate or the like, and has a function of rotating the polarization direction of incident light by 90 degrees.

The optical delay element 7 is made of a transparent material or the like having a predetermined optical length, and matches the optical lengths of the optical axes Q1 and Q2 from the plane of incidence of the polarization separation element 3 to the plane of emission of the polarization multiplexing element 4. In addition, it has a function of eliminating a phase difference of output light generated along the optical axes Q1 and Q2 and a pulse delay time difference in the case of a pulse. If such a phase difference is practically negligible, the optical delay element 7 can be omitted.

When the conversion constant d33 of the nonlinear optical material is used in the wavelength conversion, when the polarization direction of the wavelength conversion element, the polarization direction of the pump light, and the polarization direction of the signal light match each other, the wavelength-converted output light (wavelength λL) occur in the same polarization direction.

For example, when performing cascade type difference frequency generation and performing wavelength conversion in 1.5 μm band optical fiber communication,
The wavelength λs is set to the C band, the wavelength λL is set to the L band, the wavelength λp is set to 1.56 μm which is the center of the C band and the L band, and the wavelength λp of the pump light, the wavelength λs of the signal light, and the wavelength λL of the output light are: Equation (1) holds. 1 / λL = 2 / λp−1 / λs (1)

Next, the operation will be described. For example, linearly polarized light in which the ratio between the x-direction component and the y-direction component is 1: 1 at a position where the light enters the polarization separation element 3 as pump light, that is, linearly polarized light rotated by 45 degrees with respect to the main axis of the polarization separation element 3 Use It is assumed that the polarization direction of the signal light fluctuates with time and is uncertain.

When the pump light and the signal light are multiplexed by the multiplexer 10 and enter the polarization splitter 3 from the port 11a of the optical circulator 11 via the port 11b, the pump light and the signal light in the x direction The linearly polarized light is separated along the optical axis Q1, and is wavelength-converted into output light polarized in the x direction by the wavelength conversion element 1 polarized in the x direction. On the other hand, in the polarization separation element 3, the linearly polarized light in the y direction of the pump light and the signal light is separated along the optical axis Q2, and then converted into linearly polarized light in the x direction by the 90-degree polarization rotation element 5, and the optical delay element After passing through 7, the wavelength conversion element 1 converts the wavelength into output light polarized in the x direction.

The pump light, signal light and output light traveling from the wavelength conversion element 1 along the optical axes Q1 and Q2, respectively,
When reflected by the reflection mirror 20, the same wavelength conversion element 1
And the wavelength is converted. Pump light, signal light, and output light that travel along the optical axis Q1 enter the polarization splitting element 3 as they are. The pump light, the signal light, and the output light traveling along the optical axis Q2 pass through the optical delay element 7 and then enter the polarization separation element 3 with the polarization direction parallel to the y direction by the 90-degree polarization rotation element 5. .

The pump light, signal light and output light traveling along the optical axes Q 1 and Q 2 are multiplexed by the polarization splitting element 3. The output light is supplied to the port 11 of the optical circulator 11.
b is taken out through the port 11c.

Thus, the x-direction component is wavelength-converted on the optical axis Q1, and the y-direction component is wavelength-converted on the optical axis Q2. Thus, the combined intensity of the wavelength-converted output light becomes constant, and the polarization of the signal light is changed. Stable wavelength conversion can be realized regardless of the state.

FIG. 6 is a configuration diagram showing a sixth embodiment of the present invention. The wavelength conversion device includes a multiplexer 10, an optical circulator 11, a polarization separation element 3, a wavelength conversion element 1,
It comprises a reflection mirror 20, an optical delay element 7, and the like.

The multiplexer 10 has an input port 10a to which the pump light (wavelength λp) is input and an input port 10b to which the signal light (wavelength λs) is input, and multiplexes the pump light and the signal light. And output along the same optical axis.

The optical circulator 11 outputs the light input to the port 11a to the port 11b, and outputs the light input to the port 11b to the port 11c.

The wavelength conversion elements 1 and 2 are LN, LT, K
QP formed of a non-linear optical material such as N or KTP, in which the polarization direction is alternately inverted with the period of the coherence length
An example using an M element will be described. The wavelength conversion element 1 is arranged so that the polarization direction is parallel to the x direction. Wavelength conversion element 2
Are arranged such that the polarization direction is parallel to the y direction (perpendicular to the plane of the paper and perpendicular to the optical axis).

The polarization splitting element 3 is a crystal obtained by cutting a birefringent optical crystal obliquely with respect to the crystal axis (for example, 4 ×
It is composed of YVO ₄ ) cut in the direction of 5 degrees, etc., and is made to emit linearly polarized light in the x direction as an extraordinary light e on the optical axis Q1 and linearly polarized light in the y direction as ordinary light o due to the beam walk-off effect. The x-direction linearly polarized light and the optical axis Q2 separated from each other along the axis Q2 or incident along the optical axis Q1.
And a function of multiplexing the linearly polarized light in the y-direction incident along.

The optical delay element 7 is made of a transparent material or the like having a predetermined optical length, and matches the optical lengths of the optical axes Q1 and Q2 from the plane of incidence of the polarization separation element 3 to the plane of emission of the polarization multiplexing element 4. In addition, it has a function of eliminating a phase difference of output light generated along the optical axes Q1 and Q2 and a pulse delay time difference in the case of a pulse. If such a phase difference is practically negligible, the optical delay element 7 can be omitted.

When the conversion constant d33 of the nonlinear optical material is used in the wavelength conversion, when the polarization direction of the wavelength conversion element, the polarization direction of the pump light, and the polarization direction of the signal light coincide with each other, the wavelength-converted output light (wavelength λL) occur in the same polarization direction.

For example, when performing cascaded difference frequency generation and performing wavelength conversion in 1.5 μm band optical fiber communication,
The wavelength λs is set to the C band, the wavelength λL is set to the L band, the wavelength λp is set to 1.56 μm which is the center of the C band and the L band, and the wavelength λp of the pump light, the wavelength λs of the signal light, and the wavelength λL of the output light are: Equation (1) holds. 1 / λL = 2 / λp−1 / λs (1)

Next, the operation will be described. For example, linearly polarized light in which the ratio between the x-direction component and the y-direction component is 1: 1 at a position where the light enters the polarization separation element 3 as pump light, that is, linearly polarized light rotated by 45 degrees with respect to the main axis of the polarization separation element 3 Use It is assumed that the polarization direction of the signal light fluctuates with time and is uncertain.

When the pump light and the signal light are multiplexed by the multiplexer 10 and enter the polarization splitter 3 from the port 11a of the optical circulator 11 via the port 11b, the x-direction of the pump light and the signal light The linearly polarized light is separated along the optical axis Q1, and is wavelength-converted into output light polarized in the x direction by the wavelength conversion element 1 polarized in the x direction. On the other hand, the linearly polarized light in the y direction of the pump light and the signal light in the polarization splitting element 3 is separated along the optical axis Q2, passes through the optical delay element 7, and is then polarized by the wavelength converting element 2 polarized in the y direction. The wavelength is converted to output light polarized in

Pump light, signal light and output light traveling from the wavelength conversion element 1 along the optical axes Q1 and Q2 respectively are:
When the light is reflected by the reflection mirror 20, the light passes through the same wavelength conversion elements 1 and 2 again and is wavelength-converted. Pump light, signal light, and output light that travel along the optical axis Q1 enter the polarization splitting element 3 as they are. The pump light, the signal light, and the output light that travel along the optical axis Q2 pass through the optical delay element 7 and then enter the polarization separation element 3.

The pump light, signal light and output light traveling along the optical axes Q 1 and Q 2 are multiplexed by the polarization splitting element 3. The output light is supplied to the port 11 of the optical circulator 11.
b is taken out through the port 11c.

Thus, the x-direction component is wavelength-converted on the optical axis Q1, and the y-direction component is wavelength-converted on the optical axis Q2. Thus, the combined intensity of the wavelength-converted output light becomes constant, and the polarization of the signal light is changed. Stable wavelength conversion can be realized regardless of the state.

FIG. 7 is a configuration diagram showing a seventh embodiment of the present invention. The wavelength conversion device includes a multiplexer 10, an optical circulator 11, a polarization separation element 3, a wavelength conversion element 1,
It is composed of the polarization multiplexing element 4, the Faraday rotator 22, the reflection mirror 20, and the like.

The multiplexer 10 has an input port 10a to which the pump light (wavelength λp) is input, and an input port 10b to which the signal light (wavelength λs) is input, and multiplexes the pump light and the signal light. And output along the same optical axis.

The optical circulator 11 outputs the light input to the port 11a to the port 11b, and outputs the light input to the port 11b to the port 11c.

The wavelength conversion element 1 is composed of LN, LT, KN, K
Here, an example using a QPM element formed of a non-linear optical material such as TP and in which the polarization direction is alternately inverted with the period of the coherence length is shown. The wavelength conversion element 1 is arranged so that the polarization direction is parallel to the x direction.

The polarization separation element 3 and the polarization multiplexing element 4
Crystal obtained by cutting a birefringent optical crystal obliquely with respect to the crystal axis (for example, YVO cut at 45 degrees from the c-axis)
₄ ) etc., and the linear polarization in the x direction of the incident light is regarded as extraordinary light e due to the beam walk-off effect.
Then, the linearly polarized light in the y direction is separated into the optical axis Q2 as ordinary light o, or the linearly polarized light in the x direction incident along the optical axis Q1 and the linearly polarized light in the y direction incident along the optical axis Q2. Has the function of multiplexing.

[0133] The Faraday rotator 22 rotates the polarization direction of the light by 45 degrees around the optical axis in a predetermined direction. The reflecting mirror 20 reflects the incident light on the same optical axis.

When the conversion constant d33 of the nonlinear optical material is used in the wavelength conversion, when the polarization direction of the wavelength conversion element, the polarization direction of the pump light, and the polarization direction of the signal light coincide with each other, the wavelength-converted output light (wavelength λL) occur in the same polarization direction.

For example, when performing cascade type difference frequency generation and performing wavelength conversion in 1.5 μm band optical fiber communication,
The wavelength λs is set to the C band, the wavelength λL is set to the L band, the wavelength λp is set to 1.56 μm which is the center of the C band and the L band, and the wavelength λp of the pump light, the wavelength λs of the signal light, and the wavelength λL of the output light are: Equation (1) holds. 1 / λL = 2 / λp−1 / λs (1)

Next, the operation will be described. For example, linearly polarized light in which the ratio between the x-direction component and the y-direction component is 1: 1 at a position where the light enters the polarization separation element 3 as pump light, that is, linearly polarized light rotated by 45 degrees with respect to the main axis of the polarization separation element 3 Use It is assumed that the polarization direction of the signal light fluctuates with time and is uncertain.

When the pump light and the signal light are multiplexed by the multiplexer 10 and enter the polarization beam splitter 3 from the port 11a of the optical circulator 11 via the port 11b, the x-direction of the pump light and the signal light The linearly polarized light is separated along the optical axis Q1, and is wavelength-converted into output light polarized in the x direction by the wavelength conversion element 1 polarized in the x direction.
It enters the polarization multiplexing element 4. On the other hand, the linearly polarized light in the y direction of the pump light and
The light enters the polarization multiplexing element 4 without being wavelength-converted by the wavelength conversion element 1.

The polarization multiplexing element 4 multiplexes the pump light, the signal light and the output light incident along the optical axes Q1 and Q2,
Output along the same optical axis.

Next, when the pump light, the signal light, and the output light pass through the Faraday rotator 22, the polarization direction of each light is rotated by 45 degrees. The polarization direction is further rotated by 45 degrees, and is rotated by 90 degrees compared to before the Faraday rotator 22 is incident. When the light enters the polarization multiplexing device 4 again, the linearly polarized light in the x direction is separated along the optical axis Q1, and
The wavelength is converted into output light polarized in the x direction by the wavelength conversion element 1 polarized in the direction, and enters the polarization separation element 3. The linearly polarized light in the y direction is separated along the optical axis Q2, and enters the polarization separation element 3 without wavelength conversion by the wavelength conversion element 1.

The polarization splitting element 3 multiplexes the pump light, signal light and output light incident along the optical axes Q1 and Q2,
Output along the same optical axis.

The wavelength-converted output light is extracted from the port 11b of the optical circulator 11 via the port 11c.

As described above, the x-direction component before the input is the optical axis Q1 →
The wavelength-converted output light travels in the order of the optical axis Q2 and is wavelength-converted on the outward path, and the y-direction component before input travels in the order of the optical axis Q2 → the optical axis Q1 and is wavelength-converted on the return path. The combined intensity becomes constant, and stable wavelength conversion can be realized without depending on the polarization state of the signal light. Further, since the optical lengths through which the x-direction component and the y-direction component pass before input match, the influence of phase dispersion (PMD) can be prevented.

FIG. 8 is a configuration diagram showing an eighth embodiment of the present invention. The wavelength conversion device includes a multiplexer 10, an optical circulator 11, a polarization separation element 3, a wavelength conversion element 1,
It is composed of a polarization multiplexing element 4, 90-degree polarization rotation elements 5, 6, a Faraday rotator 22, a reflection mirror 20, and the like.

The multiplexer 10 has an input port 10a to which the pump light (wavelength λp) is input and an input port 10b to which the signal light (wavelength λs) is input, and multiplexes the pump light and the signal light. And output along the same optical axis.

The optical circulator 11 outputs the light input to the port 11a to the port 11b, and outputs the light input to the port 11b to the port 11c.

The wavelength conversion element 1 is composed of LN, LT, KN, K
Here, an example using a QPM element formed of a non-linear optical material such as TP and in which the polarization direction is alternately inverted with the period of the coherence length is shown. The wavelength conversion element 1 is arranged so that the polarization direction is parallel to the x direction.

The polarization separation element 3 and the polarization multiplexing element 4
Crystal obtained by cutting a birefringent optical crystal obliquely with respect to the crystal axis (for example, YVO cut at 45 degrees from the c-axis)
₄ ) etc., and the linear polarization in the x direction of the incident light is regarded as extraordinary light e due to the beam walk-off effect.
Then, the linearly polarized light in the y direction is separated into the optical axis Q2 as ordinary light o, or the linearly polarized light in the x direction incident along the optical axis Q1 and the linearly polarized light in the y direction incident along the optical axis Q2. Has the function of multiplexing.

The 90-degree polarization rotating elements 5 and 6 are constituted by half-wave plates or the like, and have a function of rotating the polarization direction of incident light by 90 degrees.

The Faraday rotator 22 rotates the polarization direction of light by 45 degrees around the optical axis in a predetermined direction. The reflecting mirror 20 reflects the incident light on the same optical axis.

When the conversion constant d33 of the nonlinear optical material is used in the wavelength conversion, when the polarization direction of the wavelength conversion element, the polarization direction of the pump light, and the polarization direction of the signal light coincide with each other, the wavelength-converted output light (wavelength λL) occur in the same polarization direction.

For example, when wavelength conversion is performed by 1.5 μm band optical fiber communication by performing cascade type difference frequency generation,
The wavelength λs is set to the C band, the wavelength λL is set to the L band, the wavelength λp is set to 1.56 μm which is the center of the C band and the L band, and the wavelength λp of the pump light, the wavelength λs of the signal light, and the wavelength λL of the output light are: Equation (1) holds. 1 / λL = 2 / λp−1 / λs (1)

Next, the operation will be described. For example, linearly polarized light in which the ratio between the x-direction component and the y-direction component is 1: 1 at a position where the light enters the polarization separation element 3 as pump light, that is, linearly polarized light rotated by 45 degrees with respect to the main axis of the polarization separation element 3 Use It is assumed that the polarization direction of the signal light fluctuates with time and is uncertain.

When the pump light and the signal light are multiplexed by the multiplexer 10 and enter the polarization beam splitter 3 from the port 11a of the optical circulator 11 via the port 11b, the pump light and the signal light in the x direction The linearly polarized light is separated along the optical axis Q1, and is wavelength-converted into output light polarized in the x direction by the wavelength conversion element 1 polarized in the x direction.
Next, the light is converted into linearly polarized light in the y direction by the 90-degree polarization rotation element 6 and enters the polarization multiplexing element 4. On the other hand, the linearly polarized light in the y direction of the pump light and the signal light is separated along the optical axis Q2 in the polarization separating element 3 and then converted into linearly polarized light in the x direction by the 90-degree polarization rotating element 5, and the wavelength conversion element The wavelength is converted into output light polarized in the x direction by 1;
It enters the polarization multiplexing element 4.

The polarization multiplexing element 4 multiplexes the pump light, signal light and output light incident along the optical axes Q1 and Q2,
Output along the same optical axis.

Next, when the pump light, the signal light, and the output light pass through the Faraday rotator 22, the polarization direction of each light is rotated by 45 degrees. The polarization direction is further rotated by 45 degrees, and is rotated by 90 degrees compared to before the Faraday rotator 22 is incident. When the light again enters the polarization multiplexing element 4, the linearly polarized light in the x direction is separated along the optical axis Q2, and x
The wavelength is converted into output light polarized in the x direction by the wavelength conversion element 1 polarized in the direction, converted into linearly polarized light in the y direction by the 90-degree polarization rotation element 5, and enters the polarization separation element 3. The linearly polarized light in the y direction is separated along the optical axis Q1, and 9
The light is converted into linearly polarized light in the x-direction by the 0-degree polarization rotation element 5, wavelength-converted into output light polarized in the x-direction by the wavelength conversion element 1, and enters the polarization separation element 3.

The polarization separating element 3 multiplexes the pump light, the signal light and the output light incident along the optical axes Q1 and Q2,
Output along the same optical axis.

The wavelength-converted output light is extracted from the port 11b of the optical circulator 11 via the port 11c.

Thus, the x-direction component before the input is changed from the optical axis Q1 →
The wavelength-converted output light travels in the order of the optical axis Q2 and is wavelength-converted on the outward path, and the y-direction component before input travels in the order of the optical axis Q2 → the optical axis Q1 and is wavelength-converted on the return path. The combined intensity becomes constant, and stable wavelength conversion can be realized without depending on the polarization state of the signal light. In addition, since the optical lengths through which the x-direction component and the y-direction component pass before input match, the influence of phase dispersion (PMD) can be prevented.

FIG. 9 is a block diagram showing a ninth embodiment of the present invention. The wavelength conversion device includes a multiplexer 10, an optical circulator 11, a polarization separation element 3, wavelength conversion elements 1 and 2,
, A polarization multiplexing element 4, a Faraday rotator 22,
It is composed of a reflection mirror 20 and the like.

The multiplexer 10 has an input port 10a to which the pump light (wavelength λp) is input, and an input port 10b to which the signal light (wavelength λs) is input, and multiplexes the pump light and the signal light. And output along the same optical axis.

The optical circulator 11 outputs the light input to the port 11a to the port 11b, and outputs the light input to the port 11b to the port 11c.

The wavelength conversion elements 1 and 2 are LN, LT, K
QP formed of a non-linear optical material such as N or KTP, in which the polarization direction is alternately inverted with the period of the coherence length
An example using an M element will be described. The wavelength conversion element 1 is arranged so that the polarization direction is parallel to the x direction. Wavelength conversion element 2
Are arranged such that the polarization direction is parallel to the y direction.

The polarization separation element 3 and the polarization multiplexing element 4
Crystal obtained by cutting a birefringent optical crystal obliquely with respect to the crystal axis (for example, YVO cut at 45 degrees from the c-axis)
₄ ) etc., and the linear polarization in the x direction of the incident light is regarded as extraordinary light e due to the beam walk-off effect.
Then, the linearly polarized light in the y direction is separated into the optical axis Q2 as ordinary light o, or the linearly polarized light in the x direction incident along the optical axis Q1 and the linearly polarized light in the y direction incident along the optical axis Q2. Has the function of multiplexing.

[0164] The Faraday rotator 22 rotates the polarization direction of the light by 45 degrees around the optical axis in a predetermined direction. The reflecting mirror 20 reflects the incident light on the same optical axis.

When the conversion constant d33 of the nonlinear optical material is used in the wavelength conversion, when the polarization direction of the wavelength conversion element, the polarization direction of the pump light, and the polarization direction of the signal light coincide with each other, the wavelength-converted output light (wavelength λL) occur in the same polarization direction.

For example, when wavelength conversion is performed by 1.5 μm band optical fiber communication by performing cascade type difference frequency generation,
The wavelength λs is set to the C band, the wavelength λL is set to the L band, the wavelength λp is set to 1.56 μm which is the center of the C band and the L band, and the wavelength λp of the pump light, the wavelength λs of the signal light, and the wavelength λL of the output light are: Equation (1) holds. 1 / λL = 2 / λp−1 / λs (1)

Next, the operation will be described. For example, linearly polarized light in which the ratio between the x-direction component and the y-direction component is 1: 1 at a position where the light enters the polarization separation element 3 as pump light, that is, linearly polarized light rotated by 45 degrees with respect to the main axis of the polarization separation element 3 Use It is assumed that the polarization direction of the signal light fluctuates with time and is uncertain.

When the pump light and the signal light are multiplexed by the multiplexer 10 and enter the polarization splitter 3 from the port 11a of the optical circulator 11 via the port 11b, the pump light and the signal light in the x direction The linearly polarized light is separated along the optical axis Q1, and is wavelength-converted into output light polarized in the x direction by the wavelength conversion element 1 polarized in the x direction.
It enters the polarization multiplexing element 4. On the other hand, the linearly polarized light in the y direction of the pump light and
Wavelength conversion element 2 separated along 2 and polarized in the y-direction
The wavelength is converted into output light polarized in the y-direction by the controller, and enters the polarization multiplexing element 4.

The polarization multiplexing element 4 multiplexes the pump light, signal light and output light incident along the optical axes Q1 and Q2,
Output along the same optical axis.

Next, when the pump light, the signal light, and the output light pass through the Faraday rotator 22, the polarization direction of each light is rotated by 45 degrees. The polarization direction is further rotated by 45 degrees, and is rotated by 90 degrees compared to before the Faraday rotator 22 is incident. When the light enters the polarization multiplexing device 4 again, the linearly polarized light in the x direction is separated along the optical axis Q1, and
The wavelength is converted into output light polarized in the x direction by the wavelength conversion element 1 polarized in the direction, and enters the polarization separation element 3. The linearly polarized light in the y direction is separated along the optical axis Q2, and is wavelength-converted by the wavelength conversion element 2 into output light polarized in the y direction.
The light enters the polarization separation element 3.

The polarization splitting element 3 multiplexes the pump light, signal light and output light incident along the optical axes Q1 and Q2,
Output along the same optical axis.

The wavelength-converted output light is extracted from the port 11b of the optical circulator 11 via the port 11c.

As described above, the x-direction component before the input is the optical axis Q1 →
The light travels in the order of the optical axis Q2 and is wavelength-converted in the forward path and the return path. The y-direction component before input travels in the order of the optical axis Q2 → the optical axis Q1 and is wavelength-converted in the forward path and the return path. The combined intensity of the output light becomes constant, and stable wavelength conversion can be realized without depending on the polarization state of the signal light. Also,
Since the optical lengths through which the x-direction component and the y-direction component pass before input also match, the phase dispersion (PMD: Polarization Mode
Dispersion) can be prevented. Corresponding components in the embodiments of FIGS. 1 to 9 are denoted by the same reference numerals.

Note that the same effect can be obtained by replacing the Faraday rotator 22 with a λ / 4 plate in FIGS. 1, 2, 7, 8, and 9.

[0175]

As described above, according to the present invention, the wavelength-converted output is obtained by performing the wavelength conversion on the linearly-polarized light component in the first direction and the linearly-polarized light component in the second direction orthogonal to the first direction. Since the combined intensity of light is constant, stable wavelength conversion can be realized without depending on the polarization state of the signal light.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a first embodiment of the present invention.

FIG. 2 is a configuration diagram showing a second embodiment of the present invention.

FIG. 3 is a configuration diagram showing a third embodiment of the present invention.

FIG. 4 is a configuration diagram showing a fourth embodiment of the present invention.

FIG. 5 is a configuration diagram showing a fifth embodiment of the present invention.

FIG. 6 is a configuration diagram showing a sixth embodiment of the present invention.

FIG. 7 is a configuration diagram showing a seventh embodiment of the present invention.

FIG. 8 is a configuration diagram showing an eighth embodiment of the present invention.

FIG. 9 is a configuration diagram showing a ninth embodiment of the present invention.

[Explanation of symbols]

 1, 2 wavelength conversion element 3 polarization separation element 4 polarization multiplexing element 5, 6 90-degree polarization rotation element 7 optical delay element 10 multiplexer 11 optical circulator 20 reflection mirror 21 wavelength selective reflection mirror 22 Faraday rotator

Claims (12)

[Claims] 1. A wavelength conversion element for performing wavelength conversion on a linear polarization component in a first direction, and a reflection element for reflecting light passing through the wavelength conversion element and returning the reflected light to the wavelength conversion element, wherein the wavelength conversion element is used as a reflection element. A wavelength conversion device, comprising: a polarization rotating unit that causes a difference in polarization direction between a traveling light and light reflected by a reflection element and returned to the wavelength conversion element to be 90 degrees. 2. The wavelength conversion device according to claim 1, further comprising a wavelength selection reflection element provided between the wavelength conversion element and the polarization rotating means for selectively reflecting the pump light. 3. A polarization splitting element made of a birefringent material that splits the light to be wavelength-converted into two linearly polarized light components orthogonal to each other by beam walk-off and advances each of the light to two different optical axes. A wavelength conversion device comprising: a wavelength conversion element that converts a wavelength according to each linearly polarized light component that travels; and a polarization multiplexing element that multiplexes two lights that have passed through the wavelength conversion element. 4. A method according to claim 1, wherein the first direction is a linearly polarized light in a first direction, and the first direction is a first optical axis.
A polarization separation element for separating linearly polarized light in a second direction orthogonal to the direction into a second optical axis, respectively, and a first 90 for rotating the polarization direction of light traveling along the second optical axis from the polarization separation element by 90 degrees Linear polarization component in the first direction with respect to light traveling along the first optical axis from the polarization splitting element and light traveling along the second optical axis from the first 90-degree polarization rotating element. A wavelength conversion element that performs wavelength conversion, a second 90-degree polarization rotation element that rotates the polarization direction of light traveling along the first optical axis from the wavelength conversion element by 90 degrees, and a second 90-degree polarization rotation element. A wavelength conversion device, comprising: a polarization multiplexing element that multiplexes light traveling along a first optical axis and light traveling along a second optical axis from a wavelength conversion element. 5. A method according to claim 1, wherein the first direction is linearly polarized light in a first optical axis.
A polarization separation element that separates linearly polarized light in a second direction orthogonal to the direction into a second optical axis, and wavelength conversion of linearly polarized light component in the first direction with respect to light traveling from the polarization separation element along the first optical axis. A first wavelength conversion element that performs wavelength conversion on linearly polarized light components in the second direction with respect to light traveling along the second optical axis from the polarization separation element; A wavelength multiplexing device comprising: a polarization multiplexing device that multiplexes the light that has passed through and the light that has passed through the second wavelength conversion device. 6. A method according to claim 1, wherein the first direction is a linearly polarized light in a first direction, and the first direction is a first optical axis.
A polarization separation element that separates linearly polarized light in a second direction orthogonal to the direction into a second optical axis, and a 90-degree polarization rotation that rotates the polarization direction of light passing from the polarization separation element along the second optical axis by 90 degrees An element; light traveling along the first optical axis from the polarization splitting element;
A wavelength conversion element for performing wavelength conversion on a linearly polarized light component in a first direction with respect to light traveling along the second optical axis from the 0-degree polarization rotation element; and a wavelength conversion element extending along the first and second optical axes from the wavelength conversion element. The light traveling along the first optical axis passes through the wavelength conversion element, the light traveling along the second optical axis passes through the wavelength conversion element and the 90-degree polarization rotation element in this order. A wavelength conversion device, comprising: a reflection element for multiplexing the elements. 7. A method according to claim 1, wherein the first direction is a linearly polarized light in a first direction, and the first direction is a first optical axis.
A polarization separation element that separates linearly polarized light in a second direction orthogonal to the direction into a second optical axis, and wavelength conversion of linearly polarized light component in the first direction with respect to light traveling from the polarization separation element along the first optical axis. A first wavelength conversion element that performs wavelength conversion, a second wavelength conversion element that performs wavelength conversion on linearly polarized light components in the second direction with respect to light traveling along the second optical axis from the polarization separation element, and a first wavelength conversion element. The light that travels along the first optical axis and the light that travels along the second optical axis from the second wavelength conversion element is reflected, and the light that has passed through the first wavelength conversion element and the light that has passed through the second wavelength conversion element A wavelength conversion device, comprising: a reflection element for multiplexing in a polarization separation element. 8. The method according to claim 1, wherein the first direction is a linearly polarized light in a first direction, and the first direction is a first optical axis.
A polarization separation element that separates linearly polarized light in a second direction orthogonal to the direction into a second optical axis, and wavelength conversion of linearly polarized light component in the first direction with respect to light traveling from the polarization separation element along the first optical axis. , A polarization multiplexing element that multiplexes light traveling along the first optical axis from the wavelength conversion element and light traveling along the second optical axis from the polarization splitting element, and a polarization multiplexing element. A reflection element that reflects the light that has passed through and returns the wavelength conversion element to the wavelength conversion element, and the difference in polarization direction between the light traveling from the wavelength conversion element to the reflection element and the light reflected by the reflection element and returning to the wavelength conversion element is 90 degrees. A wavelength conversion device comprising such polarization rotation means. 9. The method according to claim 1, wherein the first direction is linearly polarized light in a first direction and the first direction is a first optical axis.
A polarization separation element for separating linearly polarized light in a second direction orthogonal to the direction into a second optical axis, respectively, and a first 90 for rotating the polarization direction of light traveling along the second optical axis from the polarization separation element by 90 degrees Linear polarization component in the first direction with respect to light traveling along the first optical axis from the polarization splitting element and light traveling along the second optical axis from the first 90-degree polarization rotating element. A wavelength conversion element that performs wavelength conversion, a second 90-degree polarization rotation element that rotates the polarization direction of light traveling along the first optical axis from the wavelength conversion element by 90 degrees, and a second 90-degree polarization rotation element. A polarization multiplexing element for multiplexing the light traveling along the first optical axis and the light traveling along the second optical axis from the wavelength conversion element; and a wavelength conversion element for reflecting light passing through the polarization multiplexing element. Includes a reflection element that returns to the Wavelength converter difference in the polarization direction of the light reflected back by the light and the reflective element towards the child in the wavelength conversion element is characterized in that it comprises a polarization rotation means such that 90 degrees. 10. A polarization separation element for separating linearly polarized light in a first direction into a first optical axis and linearly polarized light in a second direction orthogonal to the first direction into a second optical axis, respectively. A first wavelength conversion element for performing wavelength conversion on a linearly polarized light component in a first direction with respect to light traveling along the optical axis; and a straight line in a second direction with respect to light traveling along the second optical axis from the polarization separation element. A second wavelength conversion element that performs wavelength conversion on the polarized light component; and a light that travels along the first optical axis from the first wavelength conversion element and a light that travels along the second optical axis from the second wavelength conversion element are combined. And a reflection element that reflects light passing through the polarization multiplexing element and returns the light to the wavelength conversion element. The light traveling from the wavelength conversion element to the reflection element and reflected by the reflection element return to the wavelength conversion element. Polarization such that the difference in the polarization direction of light is 90 degrees. Wavelength converter, characterized in that it comprises a rotating means. 11. The apparatus according to claim 11, wherein the polarization rotating means sets the polarization direction of the light to four.
A 45-degree rotation element for rotating the light by a 5-degree angle; and the inverting element for reflecting light passing through the 45-degree polarization rotation element and returning the reflected light to the 45-degree rotation element. The wavelength converter according to any one of claims 1, 2, 8, 9, and 10, which is provided between the elements. 12. The apparatus according to claim 1, wherein said polarization rotation means comprises a λ / 4 plate provided between said reflection element and said wavelength conversion element, and said reflection element.
The wavelength converter according to any one of claims 9 and 10.