JP2736105B2 - Semiconductor laser device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to a semiconductor laser device that can be used for coherent optical communication.

(Prior Art) In a semiconductor laser device generally used for coherent optical communication, the laser power is detected by detecting the optical power and oscillation wavelength (oscillation frequency) of the semiconductor laser and controlling the oscillation output of the semiconductor laser based on the detected output. Light is stabilized. FIG. 4 shows the configuration of a conventional apparatus.

In FIG. 4, reference numeral 11 denotes a semiconductor laser, and a laser beam emitted from the front of the semiconductor laser 11 is an optical lens 12,1.
The laser light condensed on the end of the transmission optical fiber 14 by 3 and emitted from the rear is converted into parallel light by the optical lens 15 and then split by the beam splitter 16 into two systems. One of the laser beams is condensed on the first photodetector 18 by the optical lens 17, and the optical power of the laser beam can be monitored based on the detection output.
The other laser beam is made incident on a Fabry-Perot resonator comprising reflecting mirrors 19 and 20 which are spaced apart by a length L in the optical axis direction and arranged in parallel and opposed to each other. The resonator laser light resonates at a specific frequency to stabilize the oscillation wavelength of the semiconductor laser 11, and the laser light transmitted therethrough is condensed by the optical lens 21 on the second photodetector 22, The wavelength of the laser beam can be monitored by the detection output.

By the way, in the above Fabry-Perot resonator, the transmitted light is
Output with the free spectral interval determined by C / (2nL) as the cycle. C represents the speed of light, n represents the refractive index in the Fabry-Perot resonator, and L represents the distance between the reflecting mirrors. FIG. 5 shows frequency versus transmitted light intensity characteristics.

However, the conventional semiconductor laser device as described above has the following problems.

Now, assuming that a feedback system is constructed so that point A has a resonance frequency in FIG.
If the laser does not oscillate at a frequency in the range a when the power is turned on, the Fabry-Perot resonator cannot pull the frequency of the laser beam to the point A. That is, in a normal Fabry-Perot resonator, the set frequency is not at the center of the pull-in frequency band, so that it is difficult to perform stable wavelength control of laser light. Also,
In order to arbitrarily determine the set frequency, it is necessary to finely adjust the distance L of the Fabry-Perot resonator with submicron accuracy or to finely adjust the incident angle on the Fabry-Perot resonator. The former can be realized using a piezo element, but has a problem in stability after fine adjustment. The latter has a problem that the finesse also changes because the angle of incidence is changed. That is, it was difficult to adjust the set frequency in the conventional semiconductor laser device.

(Problems to be Solved by the Invention) As described above, in the conventional semiconductor laser device, since the set frequency of the laser beam is not at the center of the frequency band in which the Fabry-Perot resonator can be pulled, it is necessary to stably control the wavelength. And it is not easy to fine-tune the set frequency.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problem, and has as its object to provide a semiconductor laser device that can easily perform fine adjustment of a set frequency of laser light.

[Structure of the Invention] (Means for Solving the Problems) In order to achieve the above object, a semiconductor laser device according to the present invention includes a semiconductor laser capable of controlling an oscillation wavelength, and a laser beam generated by the semiconductor laser. A branching unit that branches into two or more systems, an optical power detection unit that detects optical power from one of the laser beams branched by the branching unit, and the other laser beam that is branched by the branching unit according to the wavelength of the laser beam. First polarization converting means for converting elliptical polarization into elliptically polarized light, second polarization converting means for converting laser light transmitted through this means into linearly polarized light, and polarization change of laser light transmitted through this means. A polarizing beam splitter that converts the optical power into a change in optical power according to the angle, receives the laser beam transmitted through the polarizing beam splitter, detects the optical power of the laser beam, and detects the And a wavelength detecting section for reading the length change, characterized by comprising a control means for controlling the oscillation of the semiconductor laser on the basis of the detection results of the optical power detector and the wavelength detector.

(Operation) According to the semiconductor laser device having the above configuration, the oscillation output of the semiconductor laser is branched into two systems, one of which is guided to the first optical power detection means to detect the optical power, and the other is the first polarized light. The light is polarized into elliptically polarized light by the conversion means, converted into linearly polarized light by the second polarization conversion means, and the wavelength change is converted into the optical power change by the polarization beam splitter. Since the oscillation of the semiconductor laser is controlled based on the detection results of the optical power and the wavelength, the laser beam can be easily stabilized, and the oscillation wavelength of the semiconductor laser can be changed by changing the polarization angle of the polarization beam splitter. It can be fine-tuned easily.

(Embodiment) An embodiment of the present invention will be described below with reference to FIGS. However, in FIG. 1, the same parts as those in FIG. 4 are denoted by the same reference numerals, and description thereof is omitted here.

FIG. 1 shows the configuration of the semiconductor laser 11. The oscillation wavelength of the semiconductor laser 11 used here can be controlled by an external control voltage. The laser light emitted from the front of the semiconductor laser 11 enters the optical fiber 14 as in FIG. 4, and the laser light emitted from the rear is split into two systems by the beam splitter 16 as in FIG. One is incident on a first photodetector 18 for monitoring the optical power. The other laser light includes a first birefringent substance 23 and a second birefringent substance 24.
Then, after passing through a / 4 wavelength plate 25 and a polarizing beam splitter 26, the light is condensed on a second photodetector 22 for wavelength monitoring by an optical lens 21.

The detection signals of the first and second photodetectors 18 and 22 are both sent to the wavelength control circuit 27. The wavelength control circuit 27 detects the optical power and wavelength of the laser light from the two input detection signals, obtains a wavelength error from the detected values, generates a control voltage corresponding to the error, and supplies the control voltage to the semiconductor laser 11. Thus, the oscillation wavelength of the semiconductor laser 11 is controlled.

The first and second birefringent substances 23 and 24 are installed such that the C axis forms an angle of 45 ° with the polarization direction, and the quarter-wave plate 25 has the C axis parallel or perpendicular to the polarization direction. It is installed so that it becomes. Further, the polarization angle of the polarization beam splitter 24 is determined according to the set frequency of the semiconductor laser 11. For example,
If the first birefringent material is calcite and the second birefringent material 24 is rutile, they are installed so that their optical axes are orthogonal to each other, and the length in the light traveling direction is set to approximately 6: 1. .

The operation principle of the above configuration will be described below with reference to FIGS.

First, the birefringent substances 23 and 24 are parallel and perpendicular to the C axis.
x, considering taking y coordinates, if the electric field of the laser beam e _x, and e _y, field just before entrance of the first birefringent material _{23, e xi = Asinωt ... (1} ) e yi = Asinωt ... (2) can be expressed. The second immediately after transmission of the birefringent material 24 electric field e _xo, e _yo is Can be expressed as Here, n _ho and n _he are the refractive index of ordinary light and extraordinary light of calcite, respectively, n _ro and n _re are the refractive index of ordinary light and extraordinary light of rutile, respectively, and l ₁ and l ₂ are the first calcite, respectively. And the length of the second birefringent substance 24 made of rutile in the optical axis direction.

Now, the phase terms shown in equations (3) and (4) Is set to δ, and the temperature characteristic of δ is calculated, Becomes The dash “′” in the equation (5) means a value of ΔT = 0, and α ₁ and α ₂ are thermal expansion coefficients of calcite and rutile.

In the equation (5), the condition that the term of ΔT becomes 0, that is, think of. In equation (6) By substituting the physical constant of, we can rearrange as 0.89l ₁ = 5.2l ₂ … (7). That is, if l ₁ : l ₂ = 5.8: 1 ≒ 6: 1, the expression (6) is satisfied and the phase difference δ is constant regardless of the temperature.

Next (3) and (4) obtaining the expression was erased _{^{ωt e xo 2 + e xo 2}} -2e xo e yo cosδ = A 2 sin 2 δ from equation (8). This is an elliptic equation, which means that the light becomes elliptically polarized light after passing through the birefringent material. Further, when coordinate transformation is performed on the X and Y axes rotated 45 ° from the x and y axes, (8)
The equation can be modified as (1−cos δ) e _X ² + (1 + cos δ) e _Y ² = A ² sin ² δ (9) This is an elliptic equation with the X and Y axes as major or minor axes.

The elliptically polarized light formed by the first and second birefringent substances 23 and 24 is further polarized by a quarter-wave plate 25. That is, the phase terms in equations (3) and (4) are
_1, rewritten as _{φ 2, e xo = Asin (} ωt + φ 1) ... (10) e yo = Asin (ωt + φ 2) ... a (11). The electric field components of the coordinate axes obtained by rotating this by 45 ° are e _X , e
_{Expressed as Y} , Becomes When this light passes through the quarter-wave plate 25, the phase term of e _X increases by π / 2. Here, _assuming that the electric field components are e _Xh and e _Yh , the laser light after passing through the quarter-wave plate From the Y-axis from equations (14) and (15) It becomes linearly polarized light inclined by the angle of.

To summarize the above, the laser light that has passed through the birefringent substances 23 and 24 has a long axis and a short axis X and a short axis as shown by an ellipse a in FIG.
It becomes elliptically polarized light determined by the Y axis. Further, when the light passes through the quarter-wave plate 25, as shown in FIG. It becomes linearly polarized light inclined by the angle of. At this time, under the condition satisfying the expression (6), the polarization direction is stable without depending on the temperature, Given by

The laser light that has passed through the quarter-wave plate 25 is condensed on the second photodetector 22 by the optical lens 21 via the polarization beam splitter 26, and the polarization beam splitter 26 detects a change in the wavelength of the laser light. Since the light is converted into a change in intensity, the second photodetector 22 can detect the plane of polarization of the laser light as a change in light intensity. FIG. 3 shows the light intensity characteristics after passing through the polarizing beam splitter 26.

Now, as can be seen from equation (16), when the wavelength (frequency) changes, the plane of polarization rotates and the light intensity changes in a sinusoidal manner. Therefore, by rotating the polarization beam splitter 26, the dashed line in FIG. 3 can be translated as indicated by the solid line. Thus, for example, when attempting to fixed in the figure point A wavelength, indicating an intensity of the A ₁ point on a wavy line,
If caused to translate its properties by rotating the polarization beam splitter 26 can be adjusted to A ₂ points on the solid line. That is, fine adjustment can be easily performed, and after the fine adjustment, the fixed wavelength A is substantially at the center of the pull-in wavelength range a, so that stable feedback control can be performed. In the above embodiment, the wavelength control circuit 27 monitors the optical power and wavelength of the laser light from the detection signals of the first and second photodetectors 18 and 22, and the oscillation output of the semiconductor laser 11 based on the monitored values. Has feedback control.

As numerical examples, l ₁ ′ = 6 [mm], l ₂ ′ = 1 [mm], λ
= 1.5 [μm], it can be seen from equation (16) that the period of the sine wave in FIG. 3 is 1.7 [nm]. in this way,
It can be said that the semiconductor laser device having the above configuration has an extremely sensitive wavelength detecting function.

Therefore, in the above configuration, since the fixed wavelength is at the center of the pull-in wavelength range, stable wavelength control is possible, and fine adjustment of the fixed wavelength can be performed very easily. Further, it has excellent characteristics that it is stable against a temperature change and has high wavelength detection sensitivity, so that it is suitable for coherent optical communication.

Note that the present invention is not limited to the above-described embodiment. For example, a polarization plate may be adjusted by inserting a wave plate between birefringent materials having good temperature stability. Further, instead of rotating and adjusting the polarization beam splitter, a similar effect can be obtained by inserting a half-wave plate and adjusting the rotation.

[Effects of the Invention] As described above, according to the present invention, it is possible to provide a semiconductor laser device capable of easily performing fine adjustment of the set frequency of laser light.

[Brief description of the drawings]

FIG. 1 is a view showing a configuration of an embodiment of a semiconductor laser device according to the present invention, FIG. 2 is a view for explaining an elliptically polarized wave conversion means of the embodiment, and FIG. FIG. 4 is a diagram for explaining the fine adjustment means, FIG. 4 is a diagram showing the configuration of a conventional semiconductor laser device, and FIG. 5 is a diagram for explaining the operation of the conventional semiconductor laser device. 11… Semiconductor laser, 12,13,15,18,21 …… Optical lens,
14 ... optical fiber, 16 ... beam splitter, 17,22 ...
… Photodetectors, 19, 20… Reflector, 23, 24… Birefringent substances,
25 ... quarter wave plate, 26 ... polarization beam splitter, 27 ...
... Control circuit.

Claims (4)

(57) [Claims] 1. A semiconductor laser capable of controlling an oscillation wavelength, a branching means for branching laser light generated by the semiconductor laser into two or more systems, and an optical power from one of the laser lights branched by the branching means. An optical power detection unit for detecting, a first polarization conversion unit for converting the other laser beam branched by the branching unit into elliptically polarized light whose ellipticity changes according to the wavelength of the laser beam, and a laser beam transmitted through this unit. Second polarization conversion means for converting the light into linearly polarized light, a polarization beam splitter for converting a wavelength change of the laser light transmitted through the means into an optical power change in accordance with a polarization angle, and a laser beam transmitted through the polarization beam splitter. Receives light and detects the optical power of the laser light,
A wavelength detector that reads a change in the wavelength of the laser light from the detection result, and a control unit that controls oscillation of the semiconductor laser based on each detection result of the optical power detector and the wavelength detector. Semiconductor laser device. 2. The first polarization conversion means has two types of birefringent substances arranged on the optical axis of the laser beam so that the difference in the optical path length of each birefringent substance is constant regardless of the temperature. The semiconductor laser device according to claim 1, wherein: 3. The two types of birefringent materials are one of calcite and the other of rutile, each of which is installed so that their optical axes are orthogonal to each other, and specifies the length of the light traveling direction to be approximately 6: 1. The semiconductor laser device according to claim 2, wherein: 4. The apparatus according to claim 1, wherein said second polarization conversion means is a quarter-wave plate, and the oscillation wavelength of said semiconductor laser is adjusted by changing a polarization angle of said polarization beam splitter. The semiconductor laser device according to item (1).