JPH05165075A - Stabilized wave-length converting device - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a parametric wavelength converter with stabilized output wavelength. A part of the output light of the parametric wavelength conversion element 102 is made incident on the light absorbing gas cell 106, the intensity of the output light is detected by a photodetector 107, and the detection output is wavelength-controlled via a feedback circuit 108. By returning to the mechanism 109 and controlling the temperature of the element 102 and the applied electric field, the phase matching condition of the parametric wavelength conversion is adjusted, and the element 102 is adjusted.
The wavelength of the output light is tuned to a specific absorption line of the gas in the gas cell 106.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength converter used in the field of optical transmission and the like, and more particularly to a stabilized wavelength converter having a wide wavelength tunable range and a stabilized output wavelength.

[0002]

2. Description of the Related Art As a wavelength conversion device used in the field of optical transmission, a bulk type or resonator type parametric wavelength conversion device has been studied. From the Maxwell equation, the electromagnetic wave propagation in the nonlinear medium used for parametric wavelength conversion is

[0003]

[Equation 1]

It is represented by Here, E is the electric field, μ ₀ is the magnetic permeability in vacuum, σ is the conductivity, ε is the dielectric constant of the medium, and P _NL is the non-linear polarization generated in the medium.

The electromagnetic field is set to Z at angular frequencies ω ₁ , ω ₂ and ω ₃ .
Consider the following three plane waves traveling in the direction.

[0006]

[Equation 2]

[0007]

[Equation 3]

[0008]

[Equation 4]

Here, k is the wave number vector of light, E (z) is the complex amplitude of the electric field, and * is the complex conjugate.

The instantaneous value of the total electric field strength is

[0011]

[Equation 5]

It is represented by

The non-linear polarization P _NL is given by d being a quadratic non-linear optical coefficient.

[0014]

## EQU6 ## P _NL = dE ² (6)

From equations (5) and (6), P _NL in equation (1) is

[0016]

[Outer 1]

Will include terms such as

These components have a new angular frequency (ω ₁ + ω
₂ ), (ω ₃ −ω ₂ ), so ω ₁ , ω ₂ , ω
It is generally not synchronized with the _three components and cannot be the source of their excitation.

However, only when ω ₃ = ω ₁ + ω ₂ occurs from the third term of the equation (1).

[0020]

[Outside 2]

The term oscillates at an angular frequency of ω ₁ + ω ₂ = ω ₃ and can be an excitation source of ω ₃ .

[0022] This means that the omega ₁ and omega ₂ waves from omega ₃ waves, or omega ₃ energy exchange to omega ₁ and omega ₂ wave from wave occurs.

[0023] Thus, in the parametric wavelength conversion consists omega ₃ of the pump _{light, ω 3 = ω 1 + ω} 2 satisfy the omega ₁ of the signal light, and omega to generate _two of the idler light.

Assuming that ω ₃ = ω ₁ + ω ₂ holds, the following equation (7) is finally obtained by rewriting equation (1).

[0025]

[Equation 7]

Equation (7) is derived from the contribution of the second term on the right side to E ₁ ,
It shows that E ₂ can be increased, and the degree of increase is

[0027]

[Outside 3]

That is, the maximum value is shown when k ₃ = k ₁ + k ₂ .

Therefore, k ₃ = k ₁ + k ₂ is required for efficient generation of ω ₁ light and ω ₂ light.

Therefore, the angular frequencies of the pump light, the signal light, and the idler light in parametric wavelength conversion depend on the energy conservation conditions.

[0031]

[Equation 8] ω ₃ = ω ₂ + ω ₁ (8) From the phase matching condition

[0032]

[Expression 9] k ₃ = k ₂ + k ₁ (9) must be satisfied.

Here, ω is the angular frequency of light, k is the wave number vector of light, and subscripts 1, 2 and 3 correspond to signal light, idler light, and pump light, respectively.

In equation (9), n is the refractive index in the medium,

[0035]

[Expression 10] ω ₃ n ₃ = ω ₂ n ₂ + ω ₁ n ₁ (10) can be rewritten.

In crystals, the refractive index depends on frequency, crystal orientation, temperature, and electric field. Therefore, the crystal orientation, temperature,
When the electric field is changed, n ₁ , n ₂ and n ₃ change. Here, changing the crystal orientation is equivalent to changing the incident angle to the crystal. Therefore, when ω ₃ is constant, ω ₁ and ω ₂ satisfying the expressions (8) and (10) are changed by changing the crystal orientation, the temperature, and the electric field.

Therefore, in the parametric wavelength conversion, the output wavelength can be selected by controlling the crystal orientation, temperature and electric field.

As described above, in these devices, the phase matching condition for parametric wavelength conversion controls the angle of light incident on the wavelength conversion element, the temperature of the wavelength conversion element, the electric field applied to the wavelength conversion element, and the like. Although the output wavelength can be adjusted by adjusting the output wavelength, the output wavelength has conventionally been generally stabilized only by stabilizing each external condition.

FIG. 6 shows an example of a parametric wavelength conversion device which stabilizes the output wavelength by stabilizing the temperature of the element. Here, 601 is a pump light source,
Reference numeral 602 denotes a second-order nonlinear optical crystal having a parametric effect as a parametric wavelength conversion element, 603 a heater for controlling the temperature of the nonlinear optical crystal 602, 604.
Is a temperature sensor attached to the nonlinear optical crystal 602, 60
A temperature control mechanism 5 controls the heater 603 according to the detection output from the temperature sensor 604. 606 is pump light emitted from the light source 601 and incident on the nonlinear optical crystal 602, and 607 is output light from the nonlinear optical crystal 602.

First, the pump light 606 is incident on the nonlinear optical crystal 602 from the light source 601. Here, pump light 60
6 is wavelength-converted by the parametric effect according to the phase matching condition of the wavelength conversion element, and output light 607 is obtained. In consideration of the pump light wavelength and the target output light wavelength, the phase matching condition of the wavelength conversion element is determined in advance, and the incident light angle to the wavelength conversion element, the temperature of the wavelength conversion element, etc. are set. The temperature control mechanism 605 detects a deviation from the set value of the temperature of the wavelength conversion element by the temperature sensor 604,
The output wavelength is stabilized by adjusting the current flowing through the heater 603 to stabilize the temperature of the nonlinear optical crystal 602.

[0041]

In the conventional parametric wavelength conversion device as described above, the phase matching condition is applied to the angle of the incident light to the wavelength conversion element, the temperature of the wavelength conversion element, and the wavelength conversion element. The output wavelength can be selected by adjusting it by controlling the electric field, etc., but the output wavelength has a large dependence on the incident light angle, temperature, and electric field, so the external conditions were individually stabilized. Therefore, it is difficult to obtain a stable output wavelength.

For example, when lithium niobate is used as the material having second-order nonlinearity, the output wavelength is 0.
It has a temperature dependence of 06 μm-0.1 μm / ° C. Therefore, the heat sensor 604 and the heater 603 allow the element 6
Even if the temperature of 02 is controlled to ± 0.01 ° C, a wavelength variation of 0.6 nm-1 nm will occur.

As described above, according to the conventional method of independently stabilizing the external conditions, the output wavelength varies due to the variation of other unstabilized external conditions such as the variation of the pump light wavelength and the deviation of the incident light angle. I couldn't hold back.

The present invention has been made in view of the above points, and an object of the present invention is to provide a parametric wavelength conversion device having a stabilized output wavelength.

[0045]

In order to achieve such an object, the invention described in claim 1 uses a material having a second-order nonlinear optical effect, and a parametric wavelength conversion element that receives pump light, A light absorbing gas cell that allows at least a part of the output light from the wavelength conversion element to pass therethrough, a light detector that detects light emitted from the gas cell, and an output signal from the light detector that processes the output signal. A circuit that outputs a signal indicating the deviation of the wavelength of the output light from the wavelength conversion element from the peak of a specific absorption line of gas in the gas cell, and a signal that indicates the deviation from the circuit, and the wavelength conversion is performed. And a wavelength control mechanism for adjusting the phase matching condition of the element.

According to a second aspect of the present invention, the wavelength control mechanism sets the phase matching condition in the parametric wavelength conversion to the angle of incident light with respect to the wavelength conversion element, the temperature of the wavelength conversion element, and the wavelength conversion element. It is characterized in that the wavelength of the output light from the wavelength conversion element is selected by adjusting at least one of the applied electric fields.

[0047]

In the parametric wavelength converter according to the present invention, the phase matching condition is adjusted by controlling the incident light angle to the wavelength conversion element, the temperature of the element, the electric field applied to the element, and the output wavelength is selected. can do. Therefore, at least a part of the output light from the wavelength conversion element is passed through a gas cell having a specific absorption line, and a signal indicating the deviation of the wavelength of the output light from the absorption line is fed back to the wavelength control mechanism. It is possible to stabilize the wavelength of output light to a desired absorption line wavelength.

[0048]

Embodiments of the present invention will now be described in detail with reference to the drawings.

FIG. 1 is a diagram schematically showing the construction of an embodiment of the wavelength stabilizing wavelength converter according to the present invention. Here, 101 is a pulse drive LD light source, and 102 is a light source 101.
A second-order nonlinear optical crystal having a parametric effect of receiving the output light from the pump as a pump light 111, 103 a heater for controlling the temperature of the nonlinear optical crystal 102, 104 an electrode for applying an electric field to the nonlinear optical crystal 102, 1
Reference numeral 05 denotes a beam splitter that splits the output light 112 of the nonlinear optical crystal 102 and outputs a monitor light 113 and a main output light 115. Reference numeral 106 denotes a light absorbing gas cell that receives one output light from the beam splitter 105, that is, the monitor light 113. , 107 is the emitted light 1 after passing through the gas cell 106.
14 is a photodetector, 108 is a feedback circuit that receives the output of the photodetector 107, 109 is the output of the feedback circuit 108 that drives the heater 103, and applies an electric field to the electrode 104 to adjust the phase matching condition. This is a wavelength control mechanism for

In this apparatus, first, the pump light 111
Is incident on the nonlinear optical crystal 102. This pump light 11
1 is wavelength-converted by the parametric effect according to the phase matching condition of the wavelength conversion element 102, and the output light 112 is obtained. The output light 112 is split into a main output light 115 and a monitor light 113 by the beam splitter 105. The monitor light 113 passes through the gas cell 106 having a sharp absorption line at a specific wavelength. Next, the intensity of the emitted light 114 from the light absorbing gas cell 106 is supplied to the feedback circuit 108 as an electric signal from the photodetector 107. The feedback circuit 108 detects a deviation of the emission light wavelength from the absorption line peak based on the change of the electric signal, and supplies a signal for eliminating the deviation to the wavelength control mechanism 109. The wavelength control mechanism 109 controls the temperature of the parametric wavelength conversion element 102 and the element 102.
The wavelength of the output light 115 from the element 102 is made to match the wavelength of a specific absorption line by the gas cell 106 by changing one or both of the electric fields applied to the. That is, when the output wavelength changes and deviates from the peak of this absorption line, the amount of light received by the photodetector 107 increases, and the feedback circuit 108 senses the deviation. As a result, a wavelength control signal for canceling the shift is sent to the wavelength control mechanism 109, and the wavelength control mechanism 109 causes the wavelength conversion element 102 to operate.
Or the electric field applied to the wavelength conversion element 102 is changed.

FIG. 2 shows changes in the output wavelength due to changes in the temperature of the element 102.

FIG. 3 shows changes in the output wavelength due to the electric field applied to the element 102.

Therefore, it becomes possible to control the wavelength by adjusting the phase matching condition of the wavelength conversion element 102 by controlling the temperature and the electric field corresponding to the output wavelength change.
Initialization of the pump light wavelength and the phase matching condition for determining the output light wavelength is performed by aiming at the absorption line peak.

For example, in the configuration of FIG. 1, the pump light source 101 is pulse-driven 0.83 μm LD, the wavelength conversion element 102 is a core diameter 3 μm grown in a direction of 48 ° from the crystal axis, length 70 cm, loss 0.1 dB. / Cm of lithium niobate single crystal fiber, the light absorption gas cell 106 has a length of 2 cm, and C ₂ H ₂ gas is 760T.
It was possible to stabilize the output wavelength of the wavelength conversion element 102 by using the cell sealed with orr.

FIG. 4 shows the absorption line of this gas cell 106. Of the absorption lines by this gas cell 106, 1.53
The output wavelength was stabilized using the absorption line of 1588 μm. For the pump light output of 1 W, the wavelength conversion element 102
As the output of 100 mW of 1.5 μm band light was obtained.

FIG. 5 shows the measurement results of the time variation of the oscillation wavelength before and after the stabilizing operation according to the present invention, which was obtained in this example. In FIG. 5, T0 is a period before stabilization and T1 is a period after stabilization. As can be seen from FIG. 5, when wavelength stabilization is performed only by temperature control as in the conventional case, the wavelength fluctuation range is around 6 GHz, but after stabilization by the stabilization operation using the absorption line of the gas cell, It was confirmed that the fluctuation range could be reduced to 10 MHz or less, and it was shown that the present invention improved the stability by two digits or more.

A similar stabilizing operation is performed by using 1.52-1.55.
Obviously, it can also be carried out with other C ₂ H ₂ absorption lines in the μm wavelength range. Also, other molecular absorption lines,
Alternatively, it is obvious that it can be applied to other wavelength ranges by using atomic absorption lines. For example, in order to stabilize the wavelength in the 1.3 μm band, atomic absorption lines of Ar and Kr, NH ₃ ,
In order to stabilize the wavelength in the 1.5 μm band by using molecular absorption lines of CH ₄ , C ₂ H ₂ , and C ₂ H ₄ , in addition to the gases mentioned in this example, Ar, Ne, Kr atomic absorption line,
It is also possible to use the molecular absorption lines of HCN, NH ₃ , D ₂ O and HDO.

Further, although the single crystal fiber type element is used as the wavelength conversion element 102 in the present embodiment, it is obvious that the present invention can be implemented by using the substrate waveguide type element or the bulk crystal type element. In this example, the phase matching condition for parametric wavelength conversion was adjusted by the temperature and the electric field to stabilize the output wavelength.However, in the case of using the bulk crystal type element, it was further determined by the incident light angle to the element. Can also stabilize the output wavelength.

Further, although the semiconductor laser is used as the pump light source in this embodiment, it is obvious that the present invention can be carried out by using a laser of higher output.

[0060]

As described above, according to the present invention, the fluctuation of the wavelength of the output light output from the parametric wavelength conversion element is directly monitored by the gas cell, and the output showing the fluctuation is passed through the feedback circuit, By feeding back to the wavelength control mechanism and adjusting the phase matching condition, the output wavelength can be stabilized. Unlike the conventional method that independently stabilizes each external condition without monitoring the output wavelength, the present invention shows superiority in the stability of the output wavelength and has a higher wavelength stability than the conventional one. You can get sex.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of a stabilized wavelength conversion device according to the present invention using a traveling waveform single crystal fiber as a parametric wavelength conversion element.

FIG. 2 is a characteristic diagram showing dependence of an output light wavelength output from a parametric wavelength conversion element on a temperature change.

FIG. 3 is a characteristic diagram showing dependence of an output light wavelength output from a parametric wavelength conversion element on an applied electric field.

FIG. 4 is a characteristic diagram showing absorption lines of gas cells used in Examples of the present invention.

FIG. 5 is a characteristic diagram showing a wavelength fluctuation range before and after wavelength stabilization according to the present invention.

FIG. 6 is a configuration diagram showing an example of a conventional parametric wavelength conversion device by a stabilization method.

[Explanation of symbols]

 101 Semiconductor Laser 102 Parametric Wavelength Conversion Element 103 Temperature Control Heater 104 Electric Field Control Electrode 105 Beam Splitter 106 Light Absorption Gas Cell 107 Photodetector 108 Feedback Circuit 109 Wavelength Control Mechanism 111 Pump Light 112 Output Light from Wavelength Conversion Element 113 Monitor Light 114 Emitted light from gas cell 115 Main output light 601 Pump light light source 602 Parametric wavelength conversion element 603 Temperature control heater 604 Temperature sensor 605 Temperature control mechanism 606 Pump light 607 Output light

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kenichi Kuboji 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (2)

[Claims] 1. A parametric wavelength conversion element that receives pump light and uses a material having a second-order nonlinear optical effect; and a light absorption gas cell that allows at least a part of output light from the wavelength conversion element to enter and pass through. A photodetector for detecting the emitted light from the gas cell, the wavelength of the output light from the wavelength conversion element by processing the output signal from the photodetector, from the peak of a specific absorption line of the gas in the gas cell A stabilized wavelength provided with a circuit for outputting a signal indicating a shift, and a wavelength control mechanism for receiving a signal indicating the shift from the circuit and adjusting a phase matching condition of the wavelength conversion element. Converter. 2. The wavelength control mechanism sets a phase matching condition in parametric wavelength conversion among an angle of incident light with respect to the wavelength conversion element, a temperature of the wavelength conversion element, and an electric field applied to the wavelength conversion element. The wavelength of the output light from the wavelength conversion element is selected by controlling at least one of them to select the wavelength of the output light.
The stabilized wavelength conversion device described.