JP2004119721A - Wavelength locker element - Google Patents

Abstract

An object of the present invention is to provide a small-sized, low-cost wavelength locker element having no polarization dependency.
A three-beam Wollaston prism for splitting monitor light output from a semiconductor laser into three beams, a photodiode for detecting the intensity of the first monitor light split, and a second monitor light for splitting the second monitor light. An etalon filter 6 for filtering with predetermined wavelength selection characteristics, a photodiode 11 for detecting the intensity of the second monitor light output from the etalon filter 6, and a photodiode 12 for detecting the intensity of the third monitor light branched. The output of the photodiode 10 and the output of the photodiode 12 are added to form a reference signal, and the output of the photodiode 11 is used as a wavelength change signal.
[Selection] Fig. 2

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a wavelength locker element that is a component of a wavelength locker module that controls the wavelength of a semiconductor laser or the like used as a light source for optical wavelength division multiplexing communication, and in particular, uses a three-beam Wollaston prism as an optical signal branch element. And a wavelength locker element having no polarization dependence.
[0002]
[Prior art]
With the rapid spread of the Internet, large volumes of data such as images have been frequently transferred. Therefore, there is an urgent need to increase the capacity of the transmission path, and communication by the high-density wavelength division multiplexing (hereinafter, referred to as DWDM) system for multiplexing and transmitting optical signals of many wavelengths on one optical fiber cable has been put to practical use. , And its density has been increased. In order to multiplex optical signals of many wavelengths on one optical fiber, it is necessary to keep the optical signals at a constant wavelength, and the semiconductor laser as the light source is required to operate stably with high accuracy. . Since semiconductor lasers have a property that the wavelength of light changes due to factors such as temperature, the semiconductor laser generally has a function of detecting and controlling the fluctuation of the wavelength of an optical signal and keeping the wavelength of the optical signal constant. A wavelength locker module is used.
[0003]
FIG. 5 is a functional diagram showing an example in which a conventional wavelength locker element is incorporated in a laser oscillator. FIG. 1 shows only the functions necessary for explaining the wavelength locker element. The laser oscillator includes a semiconductor laser 1, a driving unit 2 for driving the semiconductor laser, and a wavelength locker for detecting a change in the wavelength of an optical signal. The wavelength locker device 3 includes a device 3 and a control unit 4 that controls the driving unit 2 of the semiconductor laser based on the detection result. The wavelength locker device 3 includes a branch device 5 that branches the monitor light output from the semiconductor laser 1 and a branch. An etalon filter 6 for filtering the first monitor light with a predetermined wavelength selection characteristic, a photodiode 7 for detecting the intensity of the wavelength-selected first monitor light, and detecting the intensity of the branched second monitor light A photodiode 8 is provided.
[0004]
Next, the function of monitoring the fluctuation of the wavelength of the semiconductor laser 1 will be described. The wavelength of the monitor light output from the semiconductor laser 1 is selected in the etalon filter 6 received via the branch element 5.
FIG. 6 shows an example of the wavelength selection characteristics of the etalon filter. As shown in the figure, the wavelength selection characteristic of the etalon filter is such that the wavelength is assigned to the horizontal axis, the output intensity of light is assigned to the vertical axis, and the output intensity when an optical signal of wavelength λ is input is P0. When −Δλ fluctuates, the output intensity decreases to P−. On the other hand, when the input wavelength fluctuates by + Δλ, the output intensity increases to P +. Therefore, if the intensity of the optical signal output from the etalon filter is detected by using the photodiode, the change in the wavelength of the optical signal can be recognized.
Therefore, the output of the etalon filter 6 is detected by the photodiode 7 and an electric signal corresponding to the fluctuation of the wavelength of the semiconductor laser 1 is obtained, and the electric signal is transmitted to the drive unit 2 of the semiconductor laser 1 via the control unit 4. Provides feedback and wavelength control.
[0005]
On the other hand, the optical signal detected by the photodiode 7 changes not only when the wavelength of the semiconductor laser 1 changes but also when the intensity of the optical signal changes. Therefore, in order to detect a change in the intensity of the optical signal of the semiconductor laser 1, the second monitor light obtained by splitting the monitor light of the semiconductor laser 1 by the splitter 5 is directly detected by the photodiode 8. Therefore, the control unit 4 compares the output of the photodiode 7 with the output of the photodiode 8 to determine whether the change in the detection output by the photodiode 7 is due to the wavelength variation or the intensity variation. Identify. Based on the detection results of the photodiodes 7 and 8, the control unit 4 controls the driving unit 2 of the semiconductor laser 1 and performs a predetermined control to stabilize the optical signal.
[0006]
FIG. 7 is a functional diagram showing an example of a conventional wavelength locker element. FIG. 7A shows a branching device 5a which reflects a part of an optical signal and branches it, such as a half mirror, a beam splitter, or a TAP filter, and the like. The first monitor light is input to a photodiode 7 via an etalon filter 6 to be converted into an electric signal, and the second monitor light is input to a photodiode 8 to be branched. Convert to electrical signals. On the other hand, FIG. 7B shows a means for transmitting and branching an optical signal, such as a prism, as a branching element 5b. The branching element 5b receives monitor light output from a semiconductor laser and branches. The first monitor light is input to a photodiode 7 via an etalon filter 6 and converted into an electric signal, and the second monitor light is input to a photodiode 8 and converted into an electric signal.
[0007]
[Problems to be solved by the invention]
However, in a conventional wavelength locker element, an input optical signal is polarized because a built-in half mirror, a beam splitter, a TAP filter, or a branching element such as a two-beam Wollaston prism has polarization dependence. Accordingly, when the optical signal is branched, the branching ratio is affected by the polarization, and the level of the branched optical signal changes despite the fact that the wavelength and the light amount of the light source do not change.
[0008]
FIGS. 8A and 8B are functional diagrams showing the state of branching of a conventional optical signal. FIG. 8A shows a case where an optical signal input to a branching element is not polarized, and FIG. 1 shows a case where an optical signal to be transmitted is polarized. The splitting element shown in this drawing has a function of splitting the optical signal 100 in two directions at a ratio of 50:50. When the input optical signal is not polarized as shown in FIG. The split optical signals are at the same level. On the other hand, when the input optical signal is polarized as shown in (b), the level of the branched optical signal differs depending on the degree of polarization. Therefore, when a biased optical signal is input to the branching element, an error occurs in the branching ratio, and a detection error occurs when detecting the output intensity and wavelength fluctuation of the semiconductor laser.
[0009]
FIG. 9 is a diagram illustrating a detection error due to polarization dependence of a conventional branch element. (1) in the figure indicates the output intensity of the optical signal detected through the etalon filter, and has a predetermined wavelength selection characteristic. In the figure, (2) and (3) indicate the output intensities obtained by directly detecting the optical signal, and (2) indicates the output intensities at the time of splitting without error due to the polarization in the splitter. And (3) is the output intensity at which the optical signal split by the influence of the polarization in the splitting element is reduced. Therefore, assuming that the point ● where (1) and (2) intersect is the wavelength of the standard optical signal, a wavelength error has occurred by Δλ due to the influence of polarization.
[0010]
Next, when using means for branching by reflecting a part of an optical signal, such as a conventional half mirror, etc., in order to arrange the photodiode, the half mirror, etc. so as not to interfere with each other in the optical path, Since a certain amount of space is required and the optical axis becomes complicated, it is difficult to reduce the size of the wavelength locker element, and the optical path adjustment for aligning the center of the photodiode light receiving surface with the optical axis becomes complicated. The problem had arisen.
[0011]
FIG. 10 is an explanatory diagram when an optical path is adjusted in a conventional wavelength locker element using a half mirror. In the figure, the monitor light output from the semiconductor laser is input to a half mirror 5a and branched, and the first monitor light is input to a photodiode 7 via an etalon filter 6 and converted into an electric signal. The second monitor light reflected at 5a is input to the photodiode 8 and converted into an electric signal. The optical path adjustment in this example needs to be performed independently for each of the photodiodes 7 and 8, and the assembly process of the wavelength locker element becomes complicated.
The present invention has been made to solve the above-described problem, and has as its object to provide a small-sized, low-cost wavelength locker element that has no polarization dependence.
[0012]
[Means for Solving the Problems]
In order to achieve the above object, a wavelength locker element according to the present invention has the following configuration.
The wavelength locker element according to claim 1, wherein the output light of the semiconductor laser is detected by a monitor means and a wavelength fluctuation of the semiconductor laser is detected. A branching element that branches into three beams of monitor light and third monitor light, a first photodetector that detects the intensity of the first monitor light that is an ordinary ray, and an ordinary ray and an extraordinary ray are added. An etalon filter that filters the second monitor light with a predetermined wavelength selection characteristic, a second photodetector that detects the intensity of output light from the etalon filter, and an intensity of the third monitor light that is an extraordinary ray A third photodetector for detecting the first photodetector and the detection output of the first photodetector and the detection output of the third photodetector, and outputs the sum as a reference signal. The detection output of Configured to output as.
[0013]
A wavelength locker element according to claim 2, wherein the output light of the semiconductor laser is detected by a monitor means and a wavelength fluctuation of the semiconductor laser is detected. A branching element that branches into three beams of monitor light and third monitor light, a multiplexer that optically multiplexes the first monitor light, which is an ordinary ray, and the third monitor light, which is an extraordinary ray, A first photodetector for detecting the output intensity of the wave filter, an etalon filter for filtering the second monitor light obtained by adding the ordinary ray and the extraordinary ray with a predetermined wavelength selection characteristic, and an output from the etalon filter. A second photodetector that detects the intensity of light, outputs a detection output of the first photodetector as a reference signal, and outputs a detection output of the second photodetector as a wavelength change signal. Constitute.
[0014]
According to a third aspect of the present invention, in the wavelength locker element, the branch element is a three-beam Wollaston prism.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail with reference to the illustrated embodiments.
The present embodiment is characterized in that a three-beam Wollaston prism is used as a branching element. As disclosed in Japanese Patent Publication No. 19522/1992, a three-beam Wollaston prism is formed by joining at least two crystals so that their optical axes are non-perpendicular to each other. The first light beam is an ordinary light beam, the second light beam is a sum of the ordinary light beam and the extraordinary light beam, and the third light beam is an extraordinary light beam. The branched second light beam is a light beam having no polarization dependency, and the sum of the first light beam and the third light beam also has the property of not having polarization dependency. .
[0016]
FIG. 1 shows how laser light is split into three beams using a three-beam Wollaston prism. As shown in the figure, the split light (1) is an ordinary ray, the split light (2) is an ordinary ray + an extraordinary ray, the split light (3) is an extraordinary ray, and the split light (1) and the split light (3) are Are added and used as a light beam having no polarization dependence. Therefore, the light beam of the split light (1) + the split light (3) is used as a reference signal, and the light beam of the split light (2) is used as a wavelength change signal. On the other hand, when the laser beam is branched by the three-beam Wollaston prism, the branching ratio has a relationship of branching light (1): branching light (2): branching light (3) = 1: 2: 1 as shown in FIG. When set, the reference signal and the wavelength change signal have a 2: 2 relationship.
[0017]
FIG. 2 is a configuration diagram showing a first embodiment of the wavelength locker element according to the present invention. FIG. 1 shows a three-beam Wollaston prism 9 that splits monitor light output from a semiconductor laser into three beams, a photodiode 10 that detects the intensity of the first monitor light that is split, and a second monitor light that splits the second monitor light. An etalon filter 6 for filtering with predetermined wavelength selection characteristics, a photodiode 11 for detecting the intensity of the second monitor light output from the etalon filter 6, and a photodiode 12 for detecting the intensity of the third monitor light branched. The output of the photodiode 10 and the output of the photodiode 12 are added to form a reference signal, and the output of the photodiode 11 is used as a wavelength change signal.
[0018]
As described above, since the reference signal and the wavelength change signal constructed according to the present embodiment do not have polarization dependence, no branching error occurs even if the monitor light input to the three-beam Wollaston prism 9 is polarized. .
This embodiment is a wavelength locker element using a prism, and the three-beam Wollaston prism 9, the etalon filter 6, and the photodiodes 10, 11, and 12 are arranged on the optical axis. The size can be reduced, and the optical path can be easily adjusted when assembling the wavelength locker element.
[0019]
FIG. 3 is a diagram showing an example of a method of adjusting the optical path of the wavelength locker element according to the present invention. As shown in the figure, the three photodiodes are fixed to the three photodiodes 13 and the optical path can be easily adjusted by adjusting the three photodiodes 13 in the front-rear direction.
Further, the three-beam Wollaston prism does not require an optical film for branching and is an inexpensive optical component, and thus contributes to the cost reduction of the wavelength locker element. In a three-beam Wollaston prism, two prisms having different crystal optical axis directions are bonded to each other, rather than performing an optical function by an optical thin film of an optical element. Things.
[0020]
FIG. 4 is a configuration diagram showing a second embodiment of the wavelength locker element according to the present invention. In the figure, when adding the first monitor light and the third monitor light branched by the three-beam Wollaston prism, they are added in the state of an optical signal using a light multiplexer. A three-beam Wollaston prism 9 for branching the monitor light output from the semiconductor laser into three beams, a multiplexer 14 for multiplexing the branched first monitor light and the third monitor light, and a multiplexer 14 A photodiode 15 for detecting the intensity of the optical signal multiplexed by the above, an etalon filter 6 for filtering the branched second monitor light with a predetermined wavelength selection characteristic, and a second monitor light output from the etalon filter 6. The output of the photodiode 15 is used as a reference signal, and the output of the photodiode 11 is used as a wavelength change signal.
[0021]
As an example of a method of multiplexing the first monitor light and the third monitor light, an optical waveguide element is used, and the first monitor light and the third monitor light are made incident on the waveguide of the optical waveguide element to form a waveguide. There is known a method of creating a photosynthetic wave by synthesizing the light within the same.
As described above, since the reference signal and the wavelength change signal constructed according to the present embodiment do not have polarization dependence, no branching error occurs even if the monitor light input to the three-beam Wollaston prism 9 is polarized. .
[0022]
Also, in the present embodiment, similarly to the first embodiment, the wavelength locker element can be reduced in size, and two photodiodes can be used. Further, the first monitor light and the third monitor light are used. Are optically combined using an optical waveguide element, and the size can be further reduced. Even when assembling the wavelength locker element, the optical path can be easily adjusted as in the first embodiment, and the three-beam Wollaston prism does not require an optical film for branching and is an inexpensive optical component. This contributes to the cost reduction of the wavelength locker element.
[0023]
【The invention's effect】
As described above, the invention according to claims 1 to 3 uses a low-cost three-beam Wollaston prism to constitute a wavelength locker element having no polarization dependency, and makes it easy to adjust the optical path during assembly. In addition, a wavelength locker element that is reduced in size can be realized, and exhibits a great effect in using the wavelength locker element.
[Brief description of the drawings]
FIG. 1 shows how laser light is split into three beams using a three-beam Wollaston prism.
FIG. 2 is a configuration diagram showing a first embodiment of a wavelength locker element according to the present invention.
FIG. 3 is a diagram showing an example of a method of adjusting an optical path of a wavelength locker element according to the present invention.
FIG. 4 is a configuration diagram showing a second embodiment of the wavelength locker element according to the present invention.
FIG. 5 is a functional diagram showing an example in which a conventional wavelength locker element is incorporated in a laser oscillator.
FIG. 6 is an example of a wavelength selection characteristic of an etalon filter.
FIG. 7 is a functional diagram showing an example of a conventional wavelength locker element.
FIG. 8 is a functional diagram showing a state of branching of a conventional optical signal.
FIG. 9 is a diagram illustrating a detection error due to polarization dependence of a conventional branch element.
FIG. 10 is an explanatory view when adjusting an optical path in a conventional wavelength locker element using a half mirror.
[Explanation of symbols]
1 .... Semiconductor laser, 2 .... Driver,
3 ・ ・ wavelength locker element 、 4 ・ ・ control unit 、
5, 5a, 5b ··· branch element, 6 ··· etalon filter,
7. Photodiode, 8. Photodiode,
9. 3 beam Wollaston prism,
10. Photodiode, 11 Photodiode,
12 photodiodes, 13 triple photodiodes,
14..combiner, 15 ... photodiode

Claims (3)

In a wavelength locker element that detects output light of a semiconductor laser by monitoring means and detects wavelength fluctuation of the semiconductor laser,
A branch element that branches the output light of the semiconductor laser into three beams of a first monitor light, a second monitor light, and a third monitor light;
A first photodetector that detects the intensity of the first monitor light that is an ordinary ray,
An etalon filter that filters the second monitor light to which the ordinary ray and the extraordinary ray have been added with predetermined wavelength selection characteristics,
A second photodetector for detecting the intensity of the output light from the etalon filter;
A third photodetector that detects the intensity of the third monitor light that is an extraordinary ray, and adds the detection output of the first photodetector and the detection output of the third photodetector to determine a reference. A wavelength locker element that outputs a signal and outputs a detection output of a second photodetector as a wavelength change signal. In a wavelength locker element that detects output light of a semiconductor laser by monitoring means and detects wavelength fluctuation of the semiconductor laser,
A branch element that branches the output light of the semiconductor laser into three beams of a first monitor light, a second monitor light, and a third monitor light;
A multiplexer that optically multiplexes the first monitor light that is an ordinary ray and the third monitor light that is an extraordinary ray,
A first photodetector for detecting the output intensity of the multiplexer;
An etalon filter that filters the second monitor light to which the ordinary ray and the extraordinary ray have been added with predetermined wavelength selection characteristics,
A second photodetector that detects the intensity of the output light from the etalon filter, outputs a detection output of the first photodetector as a reference signal, and outputs a detection output of the second photodetector to a wavelength. A wavelength locker element that outputs a change signal. 3. The wavelength locker element according to claim 1, wherein the branch element is a three-beam Wollaston prism.

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