JPH07302948A - Wavelength stabilizing device - Google Patents

Abstract

PURPOSE:To improve a semiconductor laser in a S/N ratio in a wavelength stabilization control by a method wherein a first quarter-wave plate is arranged on an optical path between a polarized beam splitter and a wavelength discriminator, and a second quarter-wave plate is located on the optical path of a second light flux and fixed to a case. CONSTITUTION:A polarized beam splitter 28 is provided in front of an optical source unit 12 confronting light ray emitted from the unit 12, and a quarter-wave plates 32 is provided on an optical path between the polarized beam splitter 28 and an etalon 34 which serves as a wavelength discriminator. A quarter-wave plate 30 is disposed on the optical path of light reflected from the polarized beam splitter 28. The quarter-wave plate 30 is fixed to a case 10 and made to serve a window through which a measuring light 54 is projected. By this setup, a returning light harmful to the wavelength stabilization of a semiconductor laser can be remarkably lessened, so that the semiconductor laser is capable of emitting laser rays very stable in wavelength.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength stabilizing device used for a light source such as a laser length measuring device.

[0002]

2. Description of the Related Art Conventionally, a laser length measuring device has been known which measures a moving displacement of an object with the oscillation wavelength of a semiconductor laser as a length measuring reference. The oscillation wavelength of the semiconductor laser depends on the injection current and the temperature. Therefore, in order to stabilize the wavelength that is the length measurement reference, the injection current and the temperature may be controlled.

An example of a conventional wavelength stabilizing device is shown in FIG. A temperature sensor 20 and a Peltier element 22 are attached to the semiconductor laser 14. Further, a temperature control unit 26 that drives the current of the Peltier element 22 so that the output of the temperature sensor 20 reaches a predetermined set temperature is provided.

The collimator lens 16, the optical isolator 18, the half mirror 100, the etalon 34, and the photodiode 3 are located in front of the light emitting direction of the semiconductor laser 14.
6 are arranged in order. In addition, the photodiode 36
A current controller 38 for controlling the injection current of the semiconductor laser 14 is provided so as to maintain the output of the semiconductor laser 14 at the set value.

The light emitted from the semiconductor laser 14 is
The collimator lens 16 collimates the light, passes through the optical isolator 18, and travels toward the half mirror 100.
The light 52 that has passed through the optical isolator 18 is partially reflected by the half mirror 100 and the rest is reflected by the half mirror 100.
Through. The light 54 reflected by the half mirror 100 is used as measurement light for a device such as a length measuring device. The light 56 transmitted through the half mirror 100 enters the etalon 34, and the transmitted light of the etalon 34 enters the photodiode 36. The output of the photodiode 36 is input to the current controller 38, and the current controller 38 controls the injection current of the semiconductor laser 14 so that the output of the photodiode 36 is maintained at the set value.

[0006]

In this structure, the return light of the measurement light 54 is reflected by the half mirror 100,
Alternatively, the return light from the etalon 34 is the half mirror 10.
It may pass through 0 and go to the semiconductor laser 14. Since the optical isolator 18 is provided between the half mirror 100 and the semiconductor laser 14, most of the light traveling to the semiconductor laser 14 is blocked by the optical isolator 18, but some reaches the semiconductor laser 14.

Even with such a weak return light, the oscillation state of the semiconductor laser 14 becomes unstable, which is not preferable for controlling wavelength stabilization. Reference: Frequency control of semiconductor laser and its application (Applied Physics Vol. 58, No. 10, 198
9) ”, it is pointed out that the returning light must be reduced to about 60 decibels. A typical optical isolator has a return light rate of about 30 dB, so
In many cases, the return light rate of 60 decibels is secured by stacking the two layers, but it becomes expensive.

Further, in controlling the wavelength stabilization, the amount of light incident on the photodetector 36 has an appropriate amount range in terms of S / N ratio. In that case, the half mirror 100 needs to be designed and manufactured so that the branching ratio of the transmitted and reflected light amounts becomes a predetermined ratio so that the light amount incident on the photodetector 36 becomes a desired light amount.

However, since the transmittance of the etalon 34 greatly varies depending on the processing accuracy of the etalon 34, the amount of light actually incident on the photodetector 36 varies. S / N
There may be a problem in comparison. An object of the present invention is to realize an S / N ratio in the wavelength stabilization control of a semiconductor laser and a reduction of light returning to the semiconductor laser with an inexpensive device.

[0010]

A wavelength stabilizing device of the present invention is a light source unit having a semiconductor laser and a collimator lens built-in, a temperature control means for controlling the temperature of the semiconductor laser, and a first light emitted from the light source unit. Based on the output of the photodetector and the polarization beam splitter that splits the light flux of the second light flux, the wavelength discriminator that discriminates the wavelength of the first light flux, the photodetector that receives the light that has passed through Current controlling means for controlling the injection current of the semiconductor laser, the first quarter-wave plate arranged on the optical path between the polarization beam splitter and the wavelength discriminator, and the optical path for the second luminous flux. And a second quarter-wave plate fixed to the housing.
The light emitted from the second quarter-wave plate is used for other devices.

[0011]

The light emitted from the semiconductor laser is collimated by the collimator lens and emitted from the light source unit. A part of the light emitted from the light source unit passes through the polarization beam splitter, and the rest is reflected by the polarization beam splitter to be divided into a first light flux and a second light flux. The first light flux enters the wavelength discriminator through the first quarter-wave plate, and the transmitted light thereof enters the photodetector. The second light flux passes through the second quarter-wave plate and is emitted to the outside of the housing to be used by another device. At this time, the temperature control means controls so as to keep the temperature of the semiconductor laser constant, and the current control means controls the injection current of the semiconductor laser so as to keep the output of the photodetector at a constant value.

The return light of the second light beam emitted from the second quarter-wave plate passes through the second quarter-wave plate again and goes to the polarization beam splitter. This light is the second quarter
Since the light has passed through the wave plate twice, the light immediately after being split by the polarization beam splitter has a polarization plane orthogonal to that of the light. Therefore, in the polarization beam splitter, the light is deflected so as not to go to the light source unit. Further, since the return light from the wavelength discriminator also passes through the first quarter-wave plate twice, it is deflected by the polarization beam splitter so as not to go to the light source unit.

[0013]

Embodiments of the present invention will now be described with reference to the drawings. The light source unit 12 includes a semiconductor laser 14, a collimator lens 16, and an optical isolator 18. A temperature sensor, such as a thermistor 20, for detecting the temperature near the semiconductor laser is provided near the semiconductor laser 14. A Peltier element 22 for adjusting the temperature near the semiconductor laser is attached to the light source unit 12. Radiation fins 24 are provided on the Peltier element 22. Further, a temperature control unit 26 that drives the Peltier element 22 so as to maintain the output of the thermistor 20 at the set temperature is provided.

In front of the light emitting direction of the light source unit 12, a polarization beam splitter 28, a quarter wavelength plate 32,
A wavelength discriminator such as an etalon 34 and a photodetector such as a photodiode 36 are sequentially arranged in a line. A current control unit 38 that supplies an injection current to the semiconductor laser 14 according to the output of the photodiode 36 is provided.
A quarter wave plate 30 is arranged on the optical path of the light reflected by the polarization beam splitter 28. The quarter-wave plate 30 is attached to the housing 10, and the measurement light 54
Has become a window to eject.

The light source unit 12 is held in the housing 10 so as to be rotatable about the optical axis within a predetermined angle range. The holding structure will be described with reference to FIGS. 2 and 3. The light source unit is, as shown in FIG.
And a holding member 40 for holding the collimator lens 16 and the optical isolator 18. The holding member 40 has a columnar shape including a flange portion 42. It is inserted into an opening 50 provided in the housing wall 48. As shown in FIG.
The flange portion 42 is formed with a long hole 44 that is curved along a circumference having a predetermined radius. A screw 46 is screwed into the housing wall 48 through the elongated hole 44. The holding member 40 is
It can be rotated with the screw 46 loosened, and is fixed by tightening the screw 46.

In FIG. 1, the light emitted from the semiconductor laser 14 is converted into parallel light by the collimator lens 16, passes through the optical isolator 18, and passes through the light source unit 12.
Is ejected from. The emitted light 52 of the light source unit 12 enters the polarization beam splitter 28, part of which is transmitted through the polarization beam splitter 28, and the rest is reflected by the polarization beam splitter 28. Since the light emitted from the light source unit 12 is linearly polarized light, the ratio of transmitted light to reflected light is determined by the direction of the optical axis of the polarization beam splitter 28 and the emitted light 52.
It is determined by the relative relationship with the direction of the polarization plane of, and is changed by rotating the light source unit 12 around the optical axis. The light reflected by the polarization beam splitter 28 is a quarter wavelength plate 3
After passing through 0, the measurement light 54 is emitted to the outside of the housing 10. The light transmitted through the polarization beam splitter 28 is ¼
Only the light of the specific wavelength discriminated therein enters the etalon 34 through the wave plate 32 and enters the photodiode 36. The current control unit 38 controls the injection current of the semiconductor laser 14 so that the output of the input photodiode 36 is maintained at the set value.

The return light, which is the measurement light 54 reflected by another device and enters the inside of the housing 10 again, passes through the quarter-wave plate 30 twice, and therefore, is returned immediately after being reflected by the polarization beam splitter 28. Since the plane of polarization of the light is orthogonal to that of the light, it is transmitted through the polarization beam splitter 28 this time and hardly goes to the light source unit 12 as indicated by reference numeral 58. Further, the reflected light of the light 56 incident on the etalon 34 passes through the quarter-wave plate 32 twice, so that it is reflected by the polarization beam splitter 28 this time, and is almost not directed to the light source unit 12 as indicated by reference numeral 58. Absent. In addition,
Quarter wave plates 30 and 32 and polarization beam splitter 28
The light that could not be cut by the head goes to the light source unit 12, which is blocked by the optical isolator 18 and does not reach the semiconductor laser 14.

By the way, in general, a glass plate is used for a window in a wavelength stabilizing device. Therefore, when passing through the window glass, the amount of light decreases and the wavefront is disturbed. In particular, it is necessary to avoid the disturbance of the wavefront as much as possible, considering that the wavelength stabilizer is often used for interferometric measurement. In the present embodiment, as shown in FIG. 1, the quarter-wave plate 30 is attached to the housing 10 and is a window itself. The reduction of the light quantity and the disturbance of the wavefront when it is performed are suppressed.

Next, adjustment of the amount of light transmitted through the polarization beam splitter 28 will be described. The etalon 34 is shown in FIG.
As shown in, it has a periodic transmission characteristic with respect to the wavelength. In the current controller 38, the injection current to the semiconductor laser 14 is slightly modulated, the variation of the transmitted light amount of the etalon 34 is detected by the photodiode 36, and it is controlled so that it always becomes the mode peak (λ 0). It is preferable that the amount of light incident on the photodiode 36 is large from the top of the S / N, but if a large amount of light is incident beyond its dynamic range, the output will be saturated, and the above control cannot be performed. . Therefore, the polarization beam splitter 28
It is necessary to suppress the amount of light that passes through within a predetermined range.

The light emitted from the semiconductor laser 14 is linearly polarized light having a polarization plane in a specific direction with respect to the semiconductor laser, and the polarization beam splitter transmits light having a specific polarization component. Further, the light source unit 12 is held so as to be rotatable about the optical axis within the predetermined angle range as described above. Therefore, when the light source unit 12 is rotated, the amount of transmitted light changes. That is, by rotating the light source unit 12, the light amount of the transmitted light of the polarization beam splitter 28 is adjusted.

As described above, in this embodiment, in addition to the optical isolator 18, the polarization beam splitters 28 and 1/4 are provided.
The wave plates 30 and 32 doubly remove the returning light. Therefore, the return light, which is harmful to the wavelength stabilization of the semiconductor laser, is significantly reduced, and a wavelength stabilization device that emits light with a very stable wavelength can be obtained. Further, the conditions required for the optical isolator can be relaxed as compared with the conventional example in which the returning light is shielded only by the optical isolator. Therefore, the cost of the optical isolator used is low, and the cost of the device can be reduced.

Further, by rotating the light source unit 12, the amount of light incident on the photodiode 36 can be set to a desired amount, so that wavelength control can be accurately performed. Therefore, it is possible to obtain a wavelength stabilizing device that emits light having a very stable wavelength.

[0023]

According to the present invention, the return light from other devices and the return light from the wavelength discriminator are deflected by the polarization beam splitter so as not to go to the light source unit, so that light of a very stable wavelength is emitted. A wavelength stabilization device that emits is obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an embodiment of a wavelength stabilizing device of the present invention.

FIG. 2 is a view showing a holding structure of a light source unit.

3 is a view showing a cross section taken along line III-III in FIG.

FIG. 4 is a diagram showing transmittance characteristics of an etalon.

FIG. 5 is a diagram showing a configuration of a conventional wavelength stabilizing device.

[Explanation of symbols]

12 ... Light source unit, 14 ... Semiconductor laser, 16 ... Collimating lens, 20 ... Thermistor, 22 ... Peltier element, 26 ... Temperature control part, 28 ... Polarization beam splitter, 30 and 32 ... 1/4 wavelength plate, 34 ... Etalon , 36
... photodiode, 38 ... current control section.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hirohisa Fujimoto 2-43-2 Hatagaya, Shibuya-ku, Tokyo Inside Olympus Optical Co., Ltd.

Claims (2)

[Claims] 1. A light source unit containing a semiconductor laser and a collimator lens, temperature control means for controlling the temperature of the semiconductor laser, and a polarization beam splitter for splitting the light emitted from the light source unit into a first light flux and a second light flux. A wavelength discriminator that discriminates the wavelength of the first light flux, a photodetector that receives the light that has passed through the wavelength discriminator, and a current control means that controls the injection current of the semiconductor laser based on the output of the photodetector. , A first quarter-wave plate disposed on the optical path between the polarization beam splitter and the wavelength discriminator, and a second 1 fixed on the housing, which is located on the optical path of the second light flux.
A wavelength stabilizing device including a / 4 wavelength plate, and the light emitted from the second ¼ wavelength plate is used for another device. 2. The wavelength stabilizing device according to claim 1, further comprising means for holding the light source unit rotatably around the optical axis of the emitted light of the semiconductor laser.