JP2584688Y2 - Frequency stabilized laser light source - Google Patents

Description

[Detailed description of the invention]

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a frequency-stabilized laser light source used for coherent optical communication and the like, and more particularly to a frequency-stabilized laser light source with reduced influence of interference by an optical fiber.

[0002]

2. Description of the Related Art FIG. 7 shows an example of a conventional frequency stabilized laser light source. In FIG. 7, 1 is a semiconductor laser, 2 is a beam splitter, 3 is an absorption cell in which C ₂ H ₂ or the like is sealed, 4 is a photodetector, 5 is the oscillation frequency of the semiconductor laser 1 controlled by an output signal of the photodetector 4. Control circuit.

An optical output of a semiconductor laser 1 is incident on a beam splitter 2 and is split into two. One of the optical outputs of the beam splitter 2 is output as stabilized output light, and the other is incident on the photodetector 4 via the absorption cell 3. The output of the photodetector 4 is input to a control circuit 5, and the control circuit 5 outputs a control signal to control the optical output frequency of the semiconductor laser 1 to the absorption signal frequency of the absorption cell 3.

Here, in the configuration shown in FIG. 7, the semiconductor laser 1, the beam splitter 2, the absorption cell 3, and the photodetector 4 must constitute an integrated optical unit. For this reason, there is a conventional example as shown in FIG. In FIG. 8, reference numeral 1a denotes a semiconductor laser module to which an optical fiber can be connected, 3 denotes an absorption cell, 6 and 8 denote optical fibers, 7 denotes an optical coupler, and 9 and 10 denote output beams of the optical fibers 6 and 8 as parallel beams. , 11 is a beam splitter, 1
2 and 13 are photodetectors, 14 is a subtracter for subtracting the output of the photodetector 12 from the output of the photodetector 13, 15 is a lock-in amplifier, 16 is an oscillator, 17 is a current control circuit, and 18 is an adder. is there. Reference numeral 50 denotes an absorption cell unit including the absorption cell 3, the collimating lens 10, the beam splitter 11, the photodetector 12, the photodetector 13, and the subtractor 14.
Reference numeral 1 denotes control circuit means including a lock-in amplifier 15, an oscillator 16, a current control circuit 17, and an adder 18. In this configuration, by using an optical fiber, the absorption cell unit 50 can be separated as an independent optical unit.

The optical output of the semiconductor laser module 1a is made incident on an optical coupler 7 by an optical fiber 6 and split into two. One of the optical outputs of the optical coupler 7 is an optical fiber 6
The light is output as stabilized output light via a collimating lens 9, and the other light is incident on a beam splitter 11 via an optical fiber 8 and a collimating lens 10. The beam splitter 11 further splits the incident light into two, one of which is incident on the photodetector 12 and the other is incident on the photodetector 13 via the absorption cell 3. The outputs of the photodetector 12 and the photodetector 13 are subtracted by a subtractor 14 to obtain a lock-in amplifier 15.
Is input to In the lock-in amplifier 15, the output of the subtractor 14 is synchronously detected by the output of the oscillator 16, and the current control circuit 1
7 is input. The current control circuit 17 outputs a control signal to control the optical output frequency of the semiconductor laser module 1a to the absorption signal frequency of the absorption cell 3. Further, this control signal is added as the frequency control signal of the semiconductor laser module 1a after the output of the oscillator 16 is added by the adder 18.

Here, the light intensity before and after the absorption cell 3 is detected by the photodetector 12 and the photodetector 13, and the gains of the photodetector 12 and the photodetector 13 are adjusted so that both outputs become equal. This compensates for the amplitude modulation component caused by the modulation of the semiconductor laser and the fluctuation of the light intensity caused by the interference in the optical fibers 6 and 8 and the like.

[0007]

However, in the conventional frequency stabilized laser light source shown in FIG. 8, when the polarization changes in the optical fibers 6 and 8, the branching ratio in the beam splitter 11 fluctuates and the frequency drift occurs. Will occur. To remove this frequency drift, the beam splitter 11
It is necessary to adjust the gain of the photodetector 12 and the photodetector 13 every time the branching ratio changes at the time. Therefore, an object of the present invention is to realize a frequency stabilized laser light source capable of compensating for a frequency drift even if the polarization changes in the optical fiber and the branching ratio in the beam splitter fluctuates.

[0008]

According to a first aspect of the present invention, there is provided an absorption cell having an absorption line at a specific frequency, wherein the absorption line controls the frequency of a semiconductor laser light source. In a frequency-stabilized laser light source for stabilizing a frequency, a semiconductor laser, first branch means for branching the optical output of the semiconductor laser into two, and polarized light for inputting the optical output of the first branch means Means, a second branch means for branching the light output of the polarizing means into first and second light outputs, an absorption cell for receiving the first light output of the second branch means, and the absorption cell A first photodetector for receiving an optical output, a second photodetector for receiving a second optical output of the second branching means, and a second light from the output of the first photodetector A subtractor for subtracting the output of the detector, and the half based on the output of the subtractor. It is characterized in that a control circuit means for controlling the frequency of the body laser. In a second aspect of the present invention, a frequency-stabilized laser light source that includes an absorption cell having an absorption line at a specific frequency, and controls the frequency of the semiconductor laser light source on the absorption line to stabilize the frequency.
A semiconductor laser, a first branching unit that branches the optical output of the semiconductor laser into two, a polarization branching unit that branches the optical output of the first branching unit into two, and a first branch of the polarization branching unit. A second light splitting means for splitting the light output of the second splitting means into two, a first absorption cell receiving the first light output of the second splitting means, and a second light receiving means for receiving the light output of the first absorption cell. 1 light detector, a second light detector that receives the second light output of the second branching means, and a third branch that branches the second light output of the polarization branching means into two. Means, a second absorption cell for receiving the first light output of the third branch means, and the second absorption cell.
A third photodetector receiving the light output of the absorption cell, a fourth photodetector receiving the second light output of the third branching means, and a first photodetector of the first and third photodetectors. A subtractor for subtracting the outputs of the second and fourth photodetectors from the output; and control circuit means for controlling the frequency of the semiconductor laser based on the output of the subtractor. is there.

[0009]

The polarization beam splitter provided between the collimator lens and the beam splitter converts the output light of the collimator lens including the elliptically polarized light in the optical fiber into only the linearly polarized light in a certain direction and enters the beam splitter. As a result, the branching ratio in the beam splitter does not change. Further, a polarizing beam splitter provided between the collimating lens and the beam splitter converts the output light of the collimating lens including the elliptically polarized light or the like in the optical fiber into two output lights having linear polarization in a certain direction, and outputs both output lights. Is incident on the beam splitter. As a result, the branching ratio in the beam splitter does not change, and the S / N is improved.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings.
FIG. 1 is a block diagram showing a configuration of a first embodiment of a frequency stabilized laser light source according to the present invention. 8 are given the same reference numerals. In FIG. 1, reference numeral 19 denotes a polarizing beam splitter as a polarizing means, which is provided between the collimator lens 10 and the beam splitter 11. Also,
50a is an absorption cell 3, a collimating lens 10, a beam splitter 11, a photodetector 12, a photodetector 13, and a subtractor 1.
4 and a polarizing beam splitter 19.

The operation of the embodiment shown in FIG. 1 will be described with reference to FIGS. A possible cause of the fluctuation of the branching ratio in the beam splitter 11 is that the mode in the optical fiber becomes elliptically polarized light due to temperature or the like. In FIG. 1, a polarization beam splitter 19 is provided in front of the beam splitter 11 so that linearly polarized light in a certain direction is incident on the beam splitter 11.

In FIG. 2, the light output "A" of the collimating lens 10 is normally incident as linearly polarized light in an optical fiber mode such as "B" or "C" in the drawing. These light outputs are polarized by the polarization beam splitter 19 into linearly polarized light in a certain direction such as "d" in the figure, and are split into "e" and "f" by the beam splitter 11, and are respectively absorbed by the absorption cell 3 and the photodetector. 12 is incident.

However, when the mode in the optical fiber becomes elliptically polarized due to temperature or the like, the polarization beam splitter 19
The input / output changes temporally, for example, in the order shown in FIGS. 3 (A), (B), (C) and (D). In FIG. 3, “a”, “c”, “e”, and “g” indicate the input of the polarization beam splitter 19, and “b”, “d”, “f”, and “h” indicate the output of the polarization beam splitter 19. ing. At this time, even if the input of the polarization beam splitter 19, that is, the output of the polarization beam splitter 19 is a linear polarization in a certain direction even if the polarization of the optical fiber changes from a straight line to an ellipse or a circle, the light is split by the beam splitter 11. The ratio does not change.

On the other hand, the polarization of the optical fiber is
Alternatively, the light intensity changes to a circle, and the light intensity becomes "d" in FIG.
Alternatively, the light intensity changes like “f”, but the change rates of the light intensity detected before and after the absorption cell 3 are the same, so that these light intensities are detected by the light detector 12 and the light detector 13, and both are detected. The difference is compensated for by taking the difference between.

FIG. 4 shows an output example of the lock-in amplifier 15 which is a component of the control circuit 51 according to the embodiment shown in FIG. FIG. 4A is a frequency characteristic curve diagram of the conventional example of FIG. 8, and FIG. 4B is a frequency characteristic curve diagram of the present embodiment. In FIG. 4 (A), the frequency fluctuates within a few minutes from “a” to “b” or “b” to “b”, causing a frequency drift. In FIG. Does not fluctuate over time. The stabilized output frequency is about 95% due to the output fluctuation "c" from "a" to "b" in FIG.
A frequency drift of about MHz is caused. On the other hand, the frequency drift of the stabilized output frequency due to the output fluctuation of "d" in FIG. 4B is about 0.9 MHz, which is about 1/100 as compared with the conventional example.

In the embodiment shown in FIG. 1, a polarizing beam splitter 19 is used to input linearly polarized light in a fixed direction to the beam splitter 11, but instead of the polarizing beam splitter 19, a Glan-Thompson prism, It is also possible to use a Glan-Taylor prism or the like.

FIG. 5 is a block diagram showing a second embodiment of the frequency stabilized laser light source according to the present invention.
1 are given the same reference numerals. In FIG. 5, 11a is a beam splitter, 12a and 13a are photodetectors, 20 is a λ / 2 plate, and 21 is a mirror. Further, the polarization beam splitter 19 is used as a polarization splitting means, and 50b is an absorption cell 3, a collimating lens 1
0, beam splitter 11a, photodetector 12a, photodetector 13a, subtractor 14, polarization beam splitter 19, λ
2 is an absorption cell section composed of a plate 20 and a mirror 21.

In FIG. 2, the collimating lens 10
When the light output “A” of FIG. 2 is incident on the polarization beam splitter 19, the light output in the direction of “G” which is the reflected light is “D” in FIG. And output as linearly polarized light in the vertical direction.

Therefore, in FIG.
The zero light output is split by the polarizing beam splitter 19 into transmitted light “a” and reflected light “b” output light.
The output light of “a” in FIG. 5 has a polarization horizontal to the drawing, and the output light of “b” in FIG. 5 has a polarization perpendicular to the drawing. Here, the output light of “b” in FIG. 5 passes through the λ / 2 plate 20 so as to be polarized horizontally to the drawing, ie, “c” in FIG.
Output light. In FIG. 5, the output lights "a" and "c" are each split by the beam splitter 11a, one of the output lights is incident on the photodetector 12a, and the other of the output lights is transmitted through the absorption cell 3. Via photodetector 13
a. Further, the outputs of the photodetectors 12a and 13a are subtracted by the subtractor 14 and output to the control circuit means 51.

As a result, compared with the first embodiment shown in FIG. 1, since all the light incident on the absorption cell section 50b is used for control, the S / N is improved. Also, no matter what kind of polarized light enters the absorption cell unit 50b, the light intensity incident on the photodetectors 12a and 13a is the same, so that an absorption cell unit having less polarization dependence can be configured.

FIG. 6 is a structural block diagram showing a third embodiment of the frequency stabilized laser light source according to the present invention.
1 are given the same reference numerals. In FIG. 6, 3a and 3b are absorption cells, 11b and 11c are beam splitters, 12b, 12c, 13b and 13c photodetectors, and 21 is a mirror. The polarization beam splitter 19 is used as a polarization splitting means.
c denotes absorption cells 3a and 3b, collimating lens 10, beam splitters 11b and 11c, photodetectors 12b,
2 c, 13 b and 13 c, a subtractor 14, a polarizing beam splitter 19 and a mirror 21.

The absorption cell section 50c having the structure shown in FIG.
As in the case of the second embodiment shown in FIG.

The absorption cell used in the first, second and third embodiments can be constructed in various wavelength bands by changing the sealing material. Although the λ / 2 plate is provided on the reflected light side of the polarizing beam splitter, it may be provided on the transmitted light side, that is, on the portion “A” in FIG.

[0024]

[Effect of the invention] As is clear from the above explanation,
The present invention has the following effects. A polarization beam splitter is installed between the collimator lens and the beam splitter, and only linearly polarized light in a certain direction is incident on the beam splitter, so that frequency drift can be compensated even if the polarization changes in the optical fiber. A laser light source can be realized. In addition, by using the reflected light of the polarization beam splitter for control, it is possible to realize a frequency-stabilized laser light source having a good S / N ratio and less polarization dependence.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a first embodiment of a frequency stabilized laser light source according to the present invention.

FIG. 2 is a perspective view illustrating a relationship between a polarization beam splitter and polarization in the beam splitter.

FIG. 3 is a characteristic curve diagram showing an input / output example of a polarizing beam splitter.

4 is a characteristic curve diagram showing an output example of a lock-in amplifier in the embodiment of FIG. 1 and the conventional example of FIG. 8;

FIG. 5 is a block diagram showing a second embodiment of the frequency stabilized laser light source according to the present invention.

FIG. 6 is a configuration block diagram showing a third embodiment of the frequency stabilized laser light source according to the present invention.

FIG. 7 is a configuration block diagram showing an example of a conventional frequency stabilized laser light source that does not use an optical fiber.

FIG. 8 is a configuration block diagram showing an example of a frequency stabilized laser light source using a conventional optical fiber.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Semiconductor laser 1a Semiconductor laser module 2,11,11a, 11b, 11c Beam splitter 3,3a, 3b Absorption cell 4,12,12a, 12b, 12c, 13,13a, 1
3b, 13c Photodetector 5 Control circuit 6, 8 Optical fiber 7 Optical coupler 9, 10 Collimating lens 14 Subtractor 15 Lock-in amplifier 16 Oscillator 17 Current control circuit 18 Adder 19 Polarizing beam splitter 20 λ / 2 plate 21 Mirror 50 , 50a, 50b, 50c Absorption cell section 51 Control circuit means

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-3-244175 (JP, A) JP-A-3-129891 (JP, A) JP-A-3-225981 (JP, A) 89575 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H01S 3/133

Claims (1)

(57) [Scope of request for utility model registration] 1. A frequency stabilized laser light source comprising an absorption cell having an absorption line at a specific frequency, wherein the absorption line controls the frequency of a semiconductor laser light source to stabilize the frequency. First splitting means for splitting the optical output of the first splitter into two, polarization splitting means for splitting the optical output of the first splitter into two, and splitting the first optical output of the second splitter into two. The second to branch
A first absorption cell that receives the first light output of the second branching means, a first photodetector that receives the first light output of the absorption cell, and the second A second photodetector that receives the second optical output of the splitting unit; and a third photodetector that splits the second optical output of the polarization splitting unit into two.
A second absorption cell that receives the first light output of the third branching means, a third photodetector that receives the second absorption cell light output, and the third A fourth light detector that receives the second light output of the branching unit, a subtractor that subtracts the outputs of the second and fourth light detectors from the outputs of the first and third light detectors, Control frequency means for controlling the frequency of the semiconductor laser based on the output of the subtracter.