JP4713769B2 - High-frequency superposition operation inspection system for semiconductor laser - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-frequency superposition operation inspection apparatus for a semiconductor laser, and in particular, inspects for the presence or absence of a high-frequency superposition operation of a semiconductor laser whose emission spectrum is made into a multimode by superimposing a high frequency on an injection current to a single mode semiconductor laser. The present invention relates to a high-frequency superposition operation inspection apparatus for a semiconductor laser, which is suitable for use at the time.
[0002]
[Prior art]
As described in Japanese Patent Laid-Open No. 2000-88528, an apparatus for measuring the size and displacement of a measurement object using a laser beam is used to measure the length of a shadow caused by the measurement object arranged within the parallel scanning range of the laser beam. As described in Japanese Patent Publication No. 4-49048, the position of the reflected light of the laser beam projected on the measurement target surface is measured. Using a triangulation optical displacement meter that measures changes due to displacement of the measurement object surface using a position detection element such as a PSD, or an autofocus mechanism as described in JP-A-7-43148 Then, there is an optical displacement meter of a focusing type that measures the displacement of the measurement target surface from the change of the focusing position.
[0003]
In such a dimension measuring apparatus using laser light, a multimode semiconductor laser having a diffused oscillation spectrum has been used in the past. However, single mode semiconductor lasers with concentrated oscillation spectra have become mainstream, and multimode semiconductor lasers have become difficult to obtain.
[0004]
However, in an optical system using a single mode semiconductor laser, the following phenomenon may be a problem.
[0005]
(1) A sudden change in the optical path due to a sudden change in the center of gravity of the light wavelength accompanying a mode hopping in which the oscillation mode changes due to a change in ambient temperature and the emission spectrum changes.
[0006]
(2) Unnecessary light interference.
[0007]
(3) Return light induced noise.
[0008]
As an effective technique for solving these problems, the applicant of Japanese Patent Application No. 2000-393223 has made the emission spectrum multi-mode by superimposing a high frequency of, for example, several hundred MHz on the injection current to the semiconductor laser (high frequency superposition). Proposed).
[0009]
However, there is a possibility that the semiconductor laser does not perform the high-frequency superimposition operation due to malfunction of the electronic circuit, line abnormality, etc., and is not in the multimode. Therefore, it is necessary to inspect the semiconductor laser for the high-frequency superimposition operation. .
[0010]
The following method can be considered as a technique for that purpose.
[0011]
(1) A method of capturing multi-mode emission spectrum using an optical spectrum analyzer.
[0012]
(2) A method of observing the interference intensity of two light beams and catching a decrease in coherence.
[0013]
(3) A method of capturing a reduction in return light induced noise using a spectrum analyzer.
[0014]
[Problems to be solved by the invention]
However, all of these methods require a large space for the entire inspection system. Further, except for the optical fiber input type, not only a long time and skill are required for adjusting the optical axis of the optical element, but also the environmental resistance is poor. Furthermore, the methods (1) and (2) have a problem that an expensive measuring instrument such as an optical spectrum analyzer or an interferometer is required.
[0015]
The present invention has been made to solve the above-described conventional problems, and it is an object of the present invention to simplify the operation and adjustment and shorten the inspection time.
[0016]
[Means for Solving the Problems]
The present invention relates to a high-frequency superposition operation inspection apparatus for a semiconductor laser, an optical element capable of dynamically changing the polarization state of the semiconductor laser, a birefringent optical fiber into which light emitted from the optical element is incident, A polarizer having a transmission axis rotated by 45 ° with respect to the main axis of the birefringent optical fiber, which is incident on the outgoing light from the birefringent optical fiber, and between orthogonal polarization modes generated in the birefringent optical fiber. The optical path difference is set to be longer than the coherence length of the semiconductor laser light during the high frequency superimposing operation and shorter than the coherence length of the semiconductor laser light when the high frequency superimposing operation is not performed. This problem is solved by examining the presence or absence of a high-frequency superposition operation from the change in the light output of the polarizer when the polarization state of light is changed.
[0017]
Similarly, a high-frequency superimposing operation inspection apparatus for a semiconductor laser, an optical element capable of dynamically changing the polarization state of the semiconductor laser, a fiber depolarizer to which light emitted from the optical element is incident, and the depolarizer The optical path difference between the orthogonal polarization modes generated by the depolarizer is longer than the coherence length of the semiconductor laser light during the high frequency superimposing operation, and the high frequency superimposing operation is not performed. Inspects whether there is a high-frequency superposition operation based on the change in the light output of the polarizer when the optical element changes the polarization state of the semiconductor laser light by setting it to be shorter than the coherence length of the semiconductor laser light at the time By doing so, the same problem is solved.
[0018]
Similarly, a high-frequency superimposing operation inspection apparatus for a semiconductor laser, a fiber-type depolarizer on which a semiconductor laser beam is incident, and a variable type for converting output light in a state where the high-frequency of the depolarizer is not superimposed into a linearly polarized state A phase retarder and a rotatable polarizer on which light emitted from the phase retarder is incident, and an optical path difference between orthogonal polarization modes generated in the depolarizer is a high-frequency superposition operation of the semiconductor laser light longer than the coherence length, and, set to be shorter than the coherence length of the semiconductor laser light when not in the high frequency superimposing operation, the polarization at the time of changing the polarization state of the semiconductor laser beam by the pre-Symbol polarizer The above problem is similarly solved by inspecting the presence or absence of the high-frequency superposition operation from the change in the light output of the child.
[0019]
The optical element can be a variable retarder or a polarization rotator.
[0020]
In addition, a coupling lens is provided on the entrance side of the optical element, the birefringent optical fiber, or the depolarizer to increase the incident efficiency of the semiconductor laser light.
[0021]
Further, a microstage for adjusting the incident position of the semiconductor laser light is provided to facilitate the optical axis adjustment.
[0022]
In addition, a light receiver and a light receiving amplifier are provided on the exit side of the polarizer so that a special measuring device such as an optical power meter is not required, and a light output level can be observed using a general-purpose voltage measuring device. It is.
[0023]
Further, an analog / digital conversion unit and a display unit are provided on the output side of the light receiving amplifier to form an independent stand-alone inspection apparatus.
[0024]
Light propagating through the birefringent optical fiber is divided into orthogonal polarization modes. The optical path difference that occurs between the orthogonal polarization modes increases in proportion to the length of the birefringent optical fiber. Therefore, a birefringent optical fiber having a certain length causes an optical path difference larger than the coherence length between orthogonal polarization modes for light having a coherence length smaller than a certain value. Therefore, for light whose coherence length is smaller than a certain value, the light output (light quantity) is almost the same regardless of the polarization state at the time of incidence, while the coherence length is larger than a certain value. For light, consider an optical system in which the light output varies greatly depending on the polarization state at the time of incidence.
[0025]
Here, the light emitted from the semiconductor laser has the properties that (1) the degree of polarization is high, and (2) the coherence length is shortened by high-frequency superposition.
[0026]
Therefore, the optical path difference between orthogonal polarization modes that occurs in birefringent optical fibers (including those used in fiber-type depolarizers) is longer than the coherence length of the semiconductor laser light during high-frequency superposition operation, and high-frequency superposition operation If an optical system that is shorter than the coherence length of the semiconductor laser when not being used is used, the high-frequency superimposing operation of the semiconductor laser can be inspected.
[0027]
The present invention is based on such knowledge.
[0028]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0029]
In the first embodiment of the inspection apparatus 10 according to the present invention, as shown in FIG. 1, an optical element 12 capable of dynamically changing the polarization state of a semiconductor laser beam and light emitted from the optical element 12 are incident. A birefringent optical fiber 14; and a polarizer 16 into which light emitted from the birefringent optical fiber 14 is incident and whose transmission axis is rotated by 45 ° with respect to the main axis of the birefringent optical fiber 14. The optical path difference between the orthogonal polarization modes generated in the refractive optical fiber 14 is longer than the coherence length of the semiconductor laser light during the high frequency superimposing operation and shorter than the coherence length of the semiconductor laser light when the high frequency superimposing operation is not performed. Is set to
[0030]
In the figure, 18 is an optical power meter for measuring the amount of light output from the polarizer 16.
[0031]
As the optical element 12, for example, a variable mode retarder and a polarization rotator that are provided with a rotation mechanism or the like so that the thickness and presence of a wave plate or the like can be dynamically switched, as well as a single mode optical fiber (twist, pinch, When stress is applied by bending or the like, the polarization state of the internal light can be changed).
[0032]
The light output of the polarizer 16 does not depend on the polarization state of the incident wave before entering the birefringent optical fiber 14, and the light amount is always ½ of the incident light if the optical path loss is ignored.
[0033]
In this embodiment, when the optical element 12 is rotated or stress is applied to dynamically change the polarization state of the semiconductor laser light, if the semiconductor laser performs a high frequency superposition operation, the polarizer 16 Although the optical output hardly changes, the optical output of the polarizer 16 changes greatly if the high frequency superposition operation is not performed. Therefore, the measurement of the amount of light such as the optical power meter 18 is provided on the exit side of the polarizer 16, and the presence or absence of the high frequency superposition operation is determined by determining whether the rate of change is qualitatively small or large. Can be easily inspected. At this time, it is not always necessary to quantitatively obtain highly accurate data.
[0034]
Next, a second embodiment of the present invention will be described in detail.
[0035]
In the present embodiment, as shown in FIG. 2, the optical element 12 that can dynamically change the polarization state of the semiconductor laser light as in the first embodiment, and the fiber type in which the emitted light from the optical element 12 is incident. A depolarizer (depolarizer) 24 and a polarizer 16 to which light emitted from the depolarizer 24 is incident, and an optical path difference between orthogonal polarization modes generated in the depolarizer 24 is a semiconductor laser beam during high frequency superposition operation. It is set to be longer than the coherence length of the semiconductor laser light and shorter than the coherence length of the semiconductor laser light when the high frequency superimposing operation is not performed.
[0036]
As shown in detail in FIG. 3, the depolarizer 22 is configured by connecting the main axes of two birefringent optical fibers 14A and 14B having a length of 1: 2, for example, by tilting each other by 45 °. In the first embodiment, it can be considered that the polarizer 16 is disposed in place of the birefringent optical fiber 14B on the exit side of the second embodiment.
[0037]
The relationship between the main axis of the depolarizer 24 and the transmission axis of the polarizer 16 in the second embodiment is arbitrary.
[0038]
In the present embodiment, when the polarization state of the semiconductor laser light is dynamically changed by the optical element 12, the optical output of the polarizer 16 hardly changes if the semiconductor laser is performing a high-frequency superposition operation. If not operating, the light output of the polarizer 16 will change significantly. Therefore, by providing a light quantity measurement system such as the optical power meter 18 on the exit side of the polarizer 16, the presence or absence of the high-frequency superimposing operation can be easily inspected.
[0039]
Next, a third embodiment of the present invention will be described in detail.
[0040]
In the present embodiment, as shown in FIG. 4, a fiber type depolarizer 24 into which a semiconductor laser beam is incident, a variable type retarder 26 for making the output light of the depolarizer 24 into a linearly polarized state, and the retarder 26, a rotatable polarizer 28 on which light emitted from the laser beam 26 is incident, the optical path difference between the orthogonal polarization modes generated in the depolarizer 24 is longer than the coherence length of the semiconductor laser light during high frequency superposition operation, and It is set to be shorter than the coherence length of the semiconductor laser light when the high frequency superimposing operation is not performed.
[0041]
In a state where the high frequency is not superimposed, the light emitted from the semiconductor laser has a high degree of polarization, and the light output may be circularly polarized by passing through the depolarizer 24. In this circularly polarized state, the light output hardly changes even when the polarizer 28 is rotated. That is, although the high-frequency superimposing operation is not performed, the light output does not change, and thus the inspection determination according to the present invention may not be possible. In order to prevent this, in the present embodiment, by adjusting the variable retarder 24, the light output in the state where the high frequency is not superimposed is surely kept in the linearly polarized state. The variable retarder 24 can be adjusted when inspecting light sources having different wavelengths using a wave plate or the like.
[0042]
As a result, when the transmission axis of the polarizer 28 is rotated, the output of the polarizer 28 hardly changes if the semiconductor laser is performing a high frequency superimposing operation. However, if the semiconductor laser is not performing the high frequency superimposing operation, the polarizer 28 is not changed. The light output varies greatly. Therefore, by providing a light amount measurement system such as the optical power meter 16 on the exit side of the polarizer 28, the presence or absence of the high-frequency superimposing operation can be easily inspected.
[0043]
Here, the light input to the polarizers 16 and 28 is obtained by using the depolarizer 24 of the second and third embodiments including the two birefringent optical fibers 14A and 14B shown in FIG. As shown in FIG. 3, the first embodiment including only the first birefringent optical fiber 14A shown in FIG. 3 is divided into four light waves having optical path differences equal to or greater than the coherence length. As shown in FIG. 2, the light wave is divided into two light waves having optical path differences equal to or greater than the coherence length.
[0044]
That is, the light wave before entering the first birefringent optical fiber 14A indicated by A in FIG. 3 is indicated by the x axis and the y axis when one light wave (the middle triangle) enters the birefringent optical fiber 14A. It shows that it is divided into orthogonal polarization modes (triangles facing each direction). Since the optical path difference between the orthogonal polarization modes propagating in the left birefringent optical fiber 14A increases in the central light wave indicated by B in FIG. 3 and exceeds the coherence length lc of this light, the optical fiber 14A It is shown that the light emitted from the light does not interfere with each other and is divided into two light waves whose polarization directions are orthogonal. The right light wave indicated by C in FIG. 3 is converted into a new x-ray in the right birefringent optical fiber 14B inclined by 45 ° with respect to the main axis of the left birefringent optical fiber 14A. This shows that the light emitted from the fiber 14B is divided into four light waves that do not interfere with each other because the optical path difference is larger than the coherence length. ing. At this time, the amount of light is uniformly distributed in all polarization directions.
[0045]
Therefore, the first embodiment can narrow the range of the optical path difference in the entire light wave, and has the following advantages.
[0046]
That is, the emission spectrum when the high-frequency superposition method is applied to the single mode semiconductor laser to be converted into the multimode may have a large degree of spectral modulation. The condition of the optical path difference necessary for obtaining a state of low coherence with respect to such light is not limited to being not less than the coherence length. As shown in FIG. 5, the second coherence peak, the third coherence peak,..., When the nth coherence peak is large, even if the optical path difference is equal to or greater than the coherence length, the optical path difference corresponding to the position of each coherence peak If it is, it shows high coherence. There are two ways to avoid this effect of visibility.
[0047]
(1) An optical path difference is designed in an optical path difference region where the visibility coherence peak is sufficiently small (a sufficiently large optical path difference is set).
[0048]
(2) An optical path difference is designed in an optical path difference area avoiding each coherence peak of visibility (for example, an optical path difference is designed in an optical path difference area having low coherence between the first coherence peak and the second coherence peak. To do).
[0049]
Here, as described above, in the first embodiment, the range of the optical path difference in the entire light wave can be narrower than in the second and third embodiments, and thus the design of the above (2) is easy. . This is an advantage of the first embodiment.
[0050]
In each of the above embodiments, the output of the polarizer 16 or 28 is measured by a light quantity measurement system separate from the inspection apparatus 10 such as the optical power meter 18, but the modification shown in FIG. As described above, in order to convert the light output into a voltage, a light receiver 30 and a light receiving amplifier 32 are provided on the exit side of the polarizer 16 or 28, and a general measuring instrument such as an oscilloscope is used without using a special measuring instrument such as an optical power meter. It is also possible to configure so that the light output level can be observed using the voltage measuring device 34.
[0051]
Further, as in another modification shown in FIG. 7, an analog / digital (A / D) conversion unit 36 and a display unit 38 are further added to the output side of the light receiver 30 and the light receiving amplifier 32 so that they are independent and stand-alone. It can also be a mold inspection device.
[0052]
Further, a coupling lens can be added to the front side of the birefringent optical fiber 14 or the depolarizer 24 to increase the coupling efficiency when the semiconductor laser light is incident on the optical fiber.
[0053]
Specifically, in the first and second embodiments, when the optical element 12 capable of dynamically changing the polarization state has an input / output port of an optical fiber, as shown in FIG. A coupling lens 40 may be provided immediately before the element 12.
[0054]
Alternatively, when the optical element 12 capable of dynamically changing the polarization state does not have an optical fiber input / output port, as shown in FIG. 9, the optical element 12 and the birefringent optical fiber 14 or 14A (depolarizer). 24) can be provided on the entry side.
[0055]
In the third embodiment, a coupling lens 40 can be provided on the entry side of the depolarizer 24, as shown in FIG.
[0056]
Further, in order to easily adjust the optical axis of the semiconductor laser light inspection apparatus, a biaxial microstage can be added. Specifically, as shown in FIG. 11, the semiconductor laser 8 can be placed on the stage portion 54 of the microstage 50 and the laser beam can be positioned with respect to the inspection apparatus 10.
[0057]
The stage portion 54 of the microstage 50 is provided for two axes, and a hole for allowing light to pass through is formed in the center.
[0058]
In the figure, 11 is a housing of the inspection apparatus, 52 is a base portion of the microstage 50, and 56 is a gripping portion of the semiconductor laser 8.
[0059]
The application object of the present invention is not limited to the laser scanning type outer diameter measuring instrument and the optical displacement meter, but is similarly applied to an optical pickup such as a CD / DVD and other measurement systems having an output depending on the optical wavelength. Obviously we can do it.
[0060]
【The invention's effect】
According to the present invention, operation and adjustment are simplified, and the inspection time can be shortened. Further, it can be reduced in size, weight, and portability. Further, it is possible to reduce the price and reduce the power consumption (the power consumption is zero if only the passive components are configured, and the power consumption can be easily reduced even if electric parts are added). Further, the configuration can be simplified and the manufacture can be facilitated. Furthermore, environmental resistance is also improved.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a first embodiment of the present invention. FIG. 2 is a block diagram showing the same second embodiment. FIG. 3 shows a configuration of a depolarizer used in the second embodiment and a change state of a light wave. FIG. 4 is a block diagram showing a third embodiment of the present invention. FIG. 5 is a line showing visibility for explaining that the first embodiment is superior to the second and third embodiments. FIG. 6 is a block diagram showing a modification of the embodiment. FIG. 7 is a block diagram showing another modification. FIG. 8 is a block diagram showing a modification using the coupling lens. FIG. FIG. 10 is a block diagram showing another modification using the coupling lens. FIG. 11 is a cross-sectional view showing a modification using the microstage. Explanation】
DESCRIPTION OF SYMBOLS 8 ... Semiconductor laser 10 ... Inspection apparatus 12 ... Optical element 14, 14A, 14B ... Birefringence optical fiber 16, 28 ... Polarizer 18 ... Optical power meter 24 ... Fiber type depolarizer 26 ... Variable type retarder 30 ... Light receiver 32 Light receiving amplifier 34 Voltage measuring device 36 A / D conversion unit 38 Display unit 40 Coupled lens 50 Micro stage 54 Stage unit

Claims (8)

An optical element capable of dynamically changing the polarization state of the semiconductor laser;
A birefringent optical fiber into which light emitted from the optical element is incident;
A light incident from the birefringent optical fiber, and having a transmission axis rotated by 45 ° with respect to the main axis of the birefringent optical fiber,
The optical path difference between the orthogonal polarization modes generated in the birefringent optical fiber is longer than the coherence length of the semiconductor laser light during the high frequency superimposing operation and shorter than the coherence length of the semiconductor laser light when the high frequency superimposing operation is not performed. Set to be
A high-frequency superimposing operation inspection apparatus for a semiconductor laser, wherein the presence or absence of a high-frequency superimposing operation is inspected based on a change in light output of the polarizer when the polarization state of the semiconductor laser light is changed by the optical element. An optical element capable of dynamically changing the polarization state of the semiconductor laser;
A fiber type depolarizer into which light emitted from the optical element is incident;
A polarizer on which light emitted from the depolarizer is incident,
The optical path difference between the orthogonal polarization modes generated in the depolarizer is longer than the coherence length of the semiconductor laser light during the high frequency superimposing operation and shorter than the coherence length of the semiconductor laser light when the high frequency superimposing operation is not performed. Set
A high-frequency superimposing operation inspection apparatus for a semiconductor laser, wherein the presence or absence of a high-frequency superimposing operation is inspected based on a change in light output of the polarizer when the polarization state of the semiconductor laser light is changed by the optical element. A fiber type depolarizer to which a semiconductor laser beam is incident;
A variable retarder for making the output light in a state where the high frequency of the depolarizer is not superimposed into a linearly polarized state;
A rotatable polarizer on which light emitted from the retarder is incident,
The optical path difference between the orthogonal polarization modes generated in the depolarizer is longer than the coherence length of the semiconductor laser light during the high frequency superimposing operation and shorter than the coherence length of the semiconductor laser light when the high frequency superimposing operation is not performed. Set
A high-frequency superimposing operation inspection apparatus for a semiconductor laser, wherein the presence or absence of a high-frequency superimposing operation is inspected based on a change in light output of the polarizer when the polarization state of the semiconductor laser light is changed by the polarizer. 3. The semiconductor laser high-frequency superimposing operation inspection apparatus according to claim 1, wherein the optical element is a variable retarder or a polarization rotator. 5. A high-frequency superimposing operation inspection apparatus for a semiconductor laser according to claim 1, wherein a coupling lens is provided on an entrance side of the optical element, the birefringent optical fiber, or the depolarizer. 4. The semiconductor laser high-frequency superimposing operation inspection device according to claim 1, further comprising a microstage for adjusting an incident position of the semiconductor laser light. 4. The high-frequency superimposing operation inspection apparatus for a semiconductor laser according to claim 1, wherein a light receiver and a light receiving amplifier are provided on the exit side of the polarizer. 8. The high-frequency superimposing operation inspection apparatus for a semiconductor laser according to claim 7, wherein an analog / digital conversion unit and a display unit are provided on the output side of the light receiving amplifier.

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