JP2009068911A - Wavelength displacement detection device, laser light source device, and control method of laser light source device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wavelength displacement detection device capable of detecting wavelength displacement on laser light emitted from an external resonator type semiconductor laser by a simple system. <P>SOLUTION: This wavelength displacement detection device includes a condensing means for condensing light reflected by the front surface and light reflected by the rear surface of light entering a parallel flat plate. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a laser light source device including an external resonator type semiconductor laser, and in particular, a wavelength displacement detection device, a laser light source device, and a control method for the laser light source device that detect a wavelength shift of an external resonator type semiconductor laser. About.

  Hologram recording in which information is recorded on an information recording medium using a hologram is generally performed by superimposing object light containing image information and recording reference light inside the information recording medium, and creating an interference pattern that can be generated at that time. This is done by writing to an information recording medium.

  At the time of reproducing the information recorded on the information recording medium, image information is reproduced by diffracting by the interference pattern by irradiating the information recording medium with a reference light for reproduction.

  In recent years, volume holograms, particularly digital volume holograms, have been developed in practical use for ultra-high density optical recording, and are attracting attention. The volume hologram is a hologram recording method in which an interference pattern is three-dimensionally written on an information recording medium by actively utilizing the thickness direction of the information recording medium.

  Volume holograms are characterized in that the diffraction efficiency can be increased by increasing the thickness of the information recording medium, and the recording capacity can be increased by using multiple recording.

  The digital volume hologram is a computer-oriented hologram recording method in which image information to be recorded is limited to a binarized digital pattern while using the same information storage medium and recording method as the volume hologram.

  In the digital volume hologram, for example, image information such as an analog picture is once digitized and developed into two-dimensional digital pattern information, which is recorded as image information.

  However, in the case of holographic recording, the interference pattern between the object beam and the recording reference beam is recorded during recording, and the diffraction between the reproduction reference beam and the interference pattern is reproduced during reproduction. For this reason, in the case of holographic recording, the reference light is accurately incident at the same angle at the time of recording and at the time of reproduction, and the diffraction efficiency is improved unless the light source wavelength is reproduced at the same wavelength as at the time of recording. There has been a problem that the reproduction is difficult due to the extreme decrease.

  Therefore, when holographic recording is put to practical use, means for correcting the relative inclination and position of the information recording medium and the reference light and accurately controlling the oscillation wavelength of the laser light are important. .

  On the other hand, a gas laser or SHG laser (second harmonic generation laser), which is a single mode light source, is often used as a light source for hologram recording and reproduction. It has been proposed that a single mode is obtained by combining with an external resonator and used as a light source for small hologram recording and reproduction.

  FIG. 11 is a diagram for explaining an external resonator type semiconductor laser 1 called a Littrow type.

  Referring to FIG. 11, external resonator type semiconductor laser 1 includes a semiconductor laser 2, a collimator lens 3, and a diffraction grating 4.

  Multi-mode laser light 12 </ b> A emitted from the semiconductor laser 2 is collimated by the collimator lens 3 and is incident on the reflective diffraction grating 4.

  The diffraction grating 4 generates first-order diffracted light of each mode, and specific first-order diffracted light 12B is fed back to the semiconductor laser 2 via the collimator lens 3 according to the arrangement angle.

  As a result, the first-order diffracted light 12B fed back to the semiconductor laser 2 resonates to emit single-mode light, and the wavelength of the light is the same as the wavelength of the first-order diffracted light 12B returned from the diffraction grating 4. Become.

  Further, the 0th-order light (reflected light) 13 of the laser light incident on the diffraction grating 4 is emitted to the outside and becomes the emitted light of the external resonator type semiconductor laser 1 of the hologram recording / reproducing apparatus.

  The principle of variable wavelength in this Littrow type external cavity semiconductor laser will be described.

The diffraction grating 4 has a structure that can rotate in a plane perpendicular to the grating groove direction.
The wavelength of the light fed back to the semiconductor laser 2 can be adjusted by changing the arrangement angle (rotation angle) of the rotatable diffraction grating 4.

  By applying such an external resonator type semiconductor laser to a hologram recording / reproducing apparatus, it is possible to finely adjust the oscillation wavelength optimal for recording or reproduction.

  FIG. 12 is a diagram for explaining the relationship between the laser power and the wavelength of the laser light output from the external resonator type semiconductor laser described in FIG.

The horizontal axis of the graph shown in FIG. 12 indicates the laser power, and the vertical axis indicates the wavelength.
Here, the wavelength of the laser beam shows a saw-like change when schematically represented. In an external cavity semiconductor laser, the laser power increases, that is, the region where the wavelength of the emitted laser light gradually increases as the drive current increases, and the laser chip in the semiconductor laser when the laser power increases. There is a region where the wavelength of the laser beam is rapidly reduced by the mode hop.

For this reason, the wavelength of the laser light changes discretely to some extent as the laser power increases.
In the region where the wavelength of the laser light gradually increases, for example, a single mode that is a complete single wavelength, and a mode with a wavelength interval of about 0.005 nm determined by the distance between the external resonator and the laser chip. A multi-mode state occurs in which two to three occur.

  Further, in a region where the wavelength of the laser beam is rapidly reduced, a multi-mode state is generated in which different modes having a large wavelength interval of about 0.04 nm are generated.

  In the former case, since the multi-mode interval is small, the hologram recording / reproduction characteristics are not greatly affected, and the usable area, whereas in the latter case, the multi-mode interval is large and the recording / reproduction characteristics are adversely affected. Therefore, it becomes an unusable area to avoid use.

  Since the relationship between the laser power and the wavelength of the laser light varies depending on the temperature in the external cavity semiconductor laser, the position of the laser power that becomes the unusable region changes due to changes in the environmental temperature or heat generated by the laser drive current. To do.

  As a method for avoiding the unusable region, for example, in Patent Document 1, the oscillation mode is determined using a two-divided detector, the laser drive current is corrected, and the laser power that effectively belongs to the unusable region is used. A method of avoiding the problem has been proposed.

  This is to detect the wavelength of the laser beam emitted from the external resonator type semiconductor laser using a wedge prism, and based on the detection result, the external resonator type semiconductor laser is not emitted so that the laser beam in the unusable region is not emitted. The power of the semiconductor laser is to be controlled.

FIG. 13 is a diagram for explaining a state in which laser light is incident on the wedge prism 17.
Referring to FIG. 13, the laser light is reflected by wedge prism 17 and is incident on glass 2. The z-axis direction is a direction from the front side of the drawing surface or display surface of FIG. The x-axis direction is a direction parallel to the front and back surfaces of the wedge prism 17 and perpendicular to the y-axis, and the y-axis direction is a direction orthogonal to the x-axis and the z-axis.

  FIG. 14 is a side view of the case where the wedge prism 17 is disposed along the z-axis direction.

  As shown in FIG. 14, it has an inclination of an angle α, and is formed such that the thickness d decreases as the coordinate advances in the z-axis direction.

FIG. 15 is a diagram illustrating an example of an interference fringe pattern 25 on the light receiving surface of the photodetector 8.
Referring to FIG. 15, a bright and dark interference fringe pattern 25 is shown. For detecting the interference fringe pattern 25, a two-divided detector that is at least two detectors is used.

  FIG. 16 is a diagram for explaining the intensity pattern of the interference fringes of the light reflected by the wedge prism 17 when the thickness d and the angle α are set to certain values.

  In FIG. 16, the horizontal axis represents the position in the z direction on the light receiving surface of the two-divided detector, and the vertical axis represents the interference of wavelength λ when the relative intensity value is 1 when the light and dark peak value of the interference fringes is 1. A fringe intensity pattern waveform 81 is shown.

  FIG. 17 is a diagram illustrating an interference fringe intensity pattern of another light reflected by the wedge prism 17 when the thickness d is selected to be a predetermined value.

  In FIG. 17, here, an intensity pattern waveform 82 of interference fringes when the wavelength is longer by about 0.04 nm than the intensity pattern waveform 81 of wavelength λ described in FIG. Specifically, an intensity pattern waveform 82 in which the phase of the intensity pattern waveform 81 is substantially inverted is shown.

  The intensity pattern waveform of these interference fringes shifts in the x (+) direction as the wavelength λ increases, and the pitch decreases as the angle α increases.

  Here, referring again to FIG. 12, the lower limit of the wavelength in the wavelength change of the laser light that repeats the sawtooth periodic wavelength change in the external resonator type semiconductor laser is λA (409.72 nm as an example), and the upper limit is λB (409.76 nm). The difference is 0.04 nm.

  When the laser power is increased by increasing the drive current of the external cavity semiconductor laser, an intensity pattern waveform 81 due to the reflected light of the wavelength λA appears first, and then the laser power of the external cavity semiconductor laser. As the wavelength increases, the wavelength gradually changes from λA to λB and approaches the intensity pattern waveform 82. Thereafter, when the laser power is further increased, intensity pattern waveforms of both wavelength λA and wavelength λB exist.

  After that, only the intensity pattern waveform 81 of the wavelength λA is obtained. Thereafter, such an interference fringe intensity pattern waveform appears periodically.

  Next, a method of determining the mode state of the wavelength from the intensity pattern waveform (intensity distribution change) of the interference fringes in Patent Document 1 will be described.

  FIG. 18 is a diagram for explaining an intensity pattern waveform when the wavelength is changed from wavelengths λA to λB. The lower limit wavelength λA is 409.72 nm (intensity pattern waveform 81), the upper limit wavelength λB is 409.76 nm (intensity pattern waveform 82), the intermediate wavelength λC is 409.73 nm (intensity pattern waveform 83), and the wavelength λD is 409.74 nm. (Intensity pattern waveform 84), a waveform curve of 409.75 nm (intensity pattern waveform 85) is shown as wavelength λE.

  Then, as shown in FIG. 18, the first light receiving element 8b and the second light receiving element 8c constituting the two-divided detector are arranged before and after a certain position, respectively, and thereby the difference in the detected light intensity. Is generated.

  The position of light detected by the light receiving element 8b is indicated by an arrow PB, and the position of light detected by the light receiving element 8c is indicated by an arrow PC.

  As described above, the wavelength changes from λA to λB with the increase of the laser power of the external cavity semiconductor laser, that is, with the increase of the drive current, but when it further increases, the wavelength changes abruptly and returns to λA again. This change is repeated periodically.

  Further, when this abrupt change occurs, light having a wavelength in the vicinity of the wavelength λA and light having a wavelength in the vicinity of λB are mixed and become light unsuitable for hologram recording or the like (light in an unusable region).

  FIG. 19 is a diagram illustrating a case where light having a wavelength near the wavelength λA and light having a wavelength near the wavelength λB are mixed.

  As shown in FIG. 19, in this case, intensity pattern waveforms 81A and 82B of wavelengths λA and λB in which two phases are substantially inverted are generated. That is, since interference fringes overlap, a light and dark pattern with a small contrast close to a uniform distribution is obtained.

  Accordingly, the difference value indicating the difference in light intensity when detected by the two-divided detector described in FIG. 18 is a value close to zero.

On the other hand, when the wavelength of the laser light emitted from the external cavity semiconductor laser monotonously increases (usable region) in accordance with the increase of the laser power, the single mode, or the two modes with very close wavelengths, or 3 In the case of mode light, it is assumed that single mode light having a wavelength constituting a typical peak is emitted.

  Referring to FIG. 18 again, when the wavelength is 409.73 nm (intensity pattern waveform 83), there is a large difference in the amount of light detected by the light receiving element 8b and the light receiving element 8c constituting the two-divided detector. Yes, the difference value is also a large value. For the wavelength 409.74 nm (intensity pattern waveform 84), a difference value close to 0 is obtained without much difference in the amount of light.

FIG. 20 is a diagram illustrating a difference value with respect to a wavelength change.
As shown in FIG. 20, it is shown that the absolute value of the difference value in the unusable area is larger than the absolute value of the difference value obtained in the usable area. Therefore, in Patent Document 1, based on the difference value, it is detected that the wavelength λA of the laser light has approached the lower limit of 409.72 nm (or the upper limit of 409.76 nm, which is the wavelength λB), and the mode state is determined. ing. In this case, a method has been proposed in which the laser power of the semiconductor laser is changed by a predetermined value to control a mode in which light of these wavelengths coexists, that is, an unusable region.
JP-A-2005-340783

  However, as in Patent Document 1, when performing wavelength determination using a two-divided detector from the interference fringes of the reflected light of the wedge prism, in order to stably detect the difference value of the two-divided photodetector, the wedge It is necessary to make the wedge angle tolerance of the prism extremely small.

  When the angle tolerance is large, that is, when the pitch of the formed interference fringes deviates from the design value, there is a possibility that a normal difference value signal cannot be obtained.

In addition, severe restrictions are imposed on the shape, arrangement, and position tolerance of the two-divided detector.
An object of the present invention is to solve the above-described problem, and a wavelength displacement detection that detects a wavelength displacement of a laser beam emitted from an external cavity semiconductor laser by a simple method. An apparatus, a laser light source device, and a control method of the laser light source device are provided.

  The wavelength displacement detection device according to the present invention includes light that is reflected on the front surface of light incident on a parallel plate, light collecting means that collects light reflected on the back surface, and light that is collected by the light collecting means. And a light detection means for detecting.

  Preferably, the parallel plate thickness t is m × λ = 2 × n × t × cos (θ #) and sin (θ #) = sin (θ) with respect to the center value λ of the wavelength range to be detected. ) / N (where m is an integer, n is the refractive index of the parallel plate material, and θ is the incident angle of light).

In particular, the thickness t of the parallel plate is λ ² = 2 × n × t × cos (θ #) × H and sin (θ #) = sin (θ) / n with respect to the wavelength range H to be detected. Where λ is the wavelength of light, n is the refractive index of the parallel plate material, and θ is the incident angle of light.

  The laser light source device according to the present invention includes an external resonator type semiconductor laser, a parallel plate that receives a part of laser light emitted from the external resonator type semiconductor laser, and a surface that reflects light incident on the parallel plate. And a light collecting means for collecting the light reflected by the back surface, and a light detecting means for detecting the light collected by the light collecting means.

  Preferably, the thickness t of the parallel plate is m × W = 2 × n × t × cos (θ #) and sin (θ #) = sin (θ) / n (wherein the oscillation center wavelength W is , M is an integer, n is a refractive index of a parallel plate material, and θ is an incident angle of light).

In particular, the thickness t of the parallel plate is such that W ² = 2 × n × t × cos (θ #) × L and sin (θ #) = with respect to the longitudinal mode oscillation interval L of the external cavity semiconductor laser. sin (θ) / n
Where λ is the wavelength of light, n is the refractive index of the parallel plate material, and θ is the incident angle of light.

  The laser light source device according to the present invention includes an external resonator type semiconductor laser, a parallel plate that receives a part of laser light emitted from the external resonator type semiconductor laser, and a surface that reflects light incident on the parallel plate. And a light collecting means for collecting the reflected light and the light reflected from the back surface, a light detecting means for detecting the light collected by the light collecting means, and a drive current of the external resonator type semiconductor laser at a predetermined value. Displacement detecting means for detecting the displacement of the oscillation wavelength of the external resonator type semiconductor laser based on the difference value of the light detection signal based on the light intensity detected by the light detecting means before and after the increase or decrease And adjusting means for adjusting the drive current of the external resonator type semiconductor laser based on the detection result of the displacement detecting means.

  The laser light source device control method according to the present invention includes an external resonator type semiconductor laser, a parallel plate that receives a part of laser light emitted from the external resonator type semiconductor laser, and light incident on the parallel plate. The light collecting means for collecting the light reflected on the front surface and the light reflected on the back surface, the light detecting means for detecting the light collected by the light collecting means, and the drive current of the external resonator type semiconductor laser A method for controlling a laser light source device including an adjusting means for adjusting, the step of driving with a drive current obtained by increasing a drive current of an external cavity semiconductor laser by a predetermined amount, and a drive current of an external cavity semiconductor laser A step of measuring a light detection signal based on the light intensity detected by the light detection means when the predetermined amount is increased, and a step of driving the drive current of the external cavity semiconductor laser by a predetermined amount; Measuring the light detection signal based on the light intensity detected by the light detecting means when the drive current of the external cavity semiconductor laser is decreased by a predetermined amount, and increasing the drive current of the external resonator semiconductor laser by a predetermined amount And comparing the absolute value of the difference between the photodetection signals when driven with the reduced drive current with an allowable error, and adjusting the drive current based on the comparison.

  The wavelength displacement detection device according to the present invention includes light that is reflected on the front surface of light incident on a parallel plate, light collecting means that collects light reflected on the back surface, and light that is collected by the light collecting means. And a light detecting means for detecting. In other words, the wavelength displacement is detected using a parallel plate instead of a wedge prism, so the parallelism of the light reflecting / interfering surface can be obtained with high accuracy at low cost, enabling highly accurate wavelength displacement detection at low cost. It is.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals and description thereof will not be repeated.

  FIG. 1 is a diagram illustrating a schematic configuration of a hologram information processing apparatus according to an embodiment of the present invention.

  Referring to FIG. 1, a hologram information processing apparatus according to an embodiment of the present invention includes a tunable laser light source 1 for recording and reproduction, a laser adjustment unit 11, a first beam splitter 62, and a first reflection mirror. 63, a spatial light modulator 64, a first relay lens 65 (65a, 65b), a second reflection mirror 66, a second beam splitter 67, and a second relay lens 68 (68a, 68b). An objective lens 69, an image sensor 70, a third reflection mirror 61, a first galvanometer mirror 72, a lens system 73 (73 a, 73 b), and a second galvanometer mirror 74.

  The light emitted from the wavelength tunable laser light source 1 passes through a third beam splitter 6 of a laser adjusting unit, which will be described later, and is reflected by the first beam splitter 62 and the first reflecting mirror 63, and is reflected by the spatial light modulator 64. Reflected. Then, after passing through the first relay lens 65, it is further reflected by the second reflecting mirror 66, passed through the second beam splitter 67 and the second relay lens 68, and then holograms by the first objective lens 69. The recording medium 71 is irradiated.

  The spatial light modulator 64 is composed of, for example, a DMD (Digital Mirror Device) in which a large number of micromirrors are arranged in a grid so that the reflection surfaces thereof can be changed, and the micromirrors correspond to each bit constituting digital data, By changing the reflection angle according to the contents of the bit (“0” or “1”), an optical image corresponding to the digital data to be recorded to be irradiated onto the hologram recording medium 20 is generated.

  The focus of this light is adjusted so as to form an image on the recording layer of the hologram recording medium 71 by the objective lens 69 as recording light.

  The light transmitted through the first beam splitter 62 is further reflected by the third reflecting mirror 61, and the optical axis thereof is changed to the optical axis side of the recording light by the first galvanometer mirror 72. Then, after passing through the lens system 73, it enters the hologram recording medium 71 as a reference light at a predetermined incident angle with respect to the optical axis of the recording light.

  The recording light and the reference light incident on the hologram recording medium 71 interfere with each other to generate a hologram, and this hologram is recorded on the hologram recording medium 71.

  At the time of reproduction, the reference light is irradiated as in the case of recording, but after passing through the hologram recording medium 71, diffracted light (reproduced light) generated by light reflected by the second galvanometer mirror 74 at the same angle as the forward path is generated. Use.

  The diffracted light passes through the first objective lens 69 and the second relay lens 68 and is incident on the image sensor 70 by the second beam splitter 67.

  The image sensor 70 is an area sensor in which a plurality of photoelectric conversion elements are arranged in a grid pattern with a resolution higher than that of the spatial light modulator 64. For example, a CCD (Charge-Coupled Device) area sensor or a CMOS (Complementary Metal-Oxide-Semiconductor) is used. ) Is used.

  Then, the two-dimensional pattern information is reproduced based on the signal detected by the image sensor 70, and the recorded data recorded on the hologram recording medium is read.

FIG. 2 is a configuration diagram of a wavelength control system for recording / reproduction according to the embodiment of the present invention.
The wavelength control system includes a wavelength tunable laser light source 1 and a laser adjustment unit 11.

  The present invention receives a part of the laser light of an external cavity semiconductor laser, generates an interference pattern by a parallel plate, detects the light intensity of the interference pattern, determines the wavelength displacement of the laser light, and uses it The present invention relates to laser control for avoiding the use of laser light in a multimode state that is an unusable region.

  Referring to FIG. 2, wavelength tunable laser light source 1 according to the embodiment of the present invention constitutes an external resonator type semiconductor laser called a Littrow type in which a semiconductor laser and an external resonator are combined. Using an external resonator structure, the oscillation spectrum can be narrowed and single mode oscillation can be achieved.

  The wavelength tunable laser light source 1 includes a semiconductor laser 2, a collimator lens 3, a diffraction grating 4, a rotation mechanism 5, and a Peltier element 18.

  A laser beam 12 </ b> A emitted from the semiconductor laser 2 and oscillating in a multimode is collimated by the collimator lens 3 and is incident on the reflective diffraction grating 4.

  The diffraction grating 4 generates first-order diffracted light of each mode, and specific first-order diffracted light 12B is fed back to the semiconductor laser 2 via the collimator lens 3 according to the arrangement angle.

  As a result, the first-order diffracted light 12B fed back to the semiconductor laser 2 resonates to emit single-mode light, and the wavelength of the light is the same as the wavelength of the first-order diffracted light 12B returned from the diffraction grating 4. .

  In the wavelength tunable laser light source 1, which is a Littrow type external cavity semiconductor laser, the diffraction grating 4 is rotated in a plane perpendicular to the grating groove direction by a rotating mechanism 5, and the light fed back to the semiconductor laser 2 is reflected. By changing the wavelength, the oscillation wavelength of the laser can be adjusted.

Further, the zero-order light (reflected light) 13 of the laser light incident on the diffraction grating 4 is emitted to the outside.
The Peltier element 18 is thermally connected to the semiconductor laser 2 via a thermal conductor and is provided to adjust the temperature of the semiconductor laser 2. In this example, the Peltier element 18 is controlled by an adjustment circuit in the laser control circuit 10 as an example (not shown). Specifically, the adjustment circuit includes a temperature detector that detects the temperature and a power supply circuit that supplies power to the Peltier element, and the Peltier element 18 is changed from the power supply circuit based on the temperature detection result by the temperature detector. It is possible to adjust the temperature of the semiconductor laser 2 by adjusting the power supplied to.

  The laser adjustment unit 11 determines the wavelength displacement of the laser light based on the third beam splitter 6, the parallel plate 7, the photodetector 8 including one photoelectric element 8 a, and the detection result of the photodetector 8. A determination circuit 9, a laser control circuit 10 that controls the drive current of the semiconductor laser 2 of the wavelength tunable laser light source 1, and a condenser lens 20 are included. Although not shown, the laser control circuit 10 controls the drive current of the semiconductor laser 2 in response to an instruction from the outside and controls the drive current of the semiconductor laser 2 based on the determination result from the determination circuit 9. And

  A part of the laser light emitted from the diffraction grating 4 is reflected by the third beam splitter 6.

  Then, the light incident on the parallel plate 7 as the light reflecting means becomes light 15 reflected on the surface of the parallel plate 7 and light 16 reflected on the back surface of the parallel plate, and is emitted to the condenser lens 20.

  Then, the light is condensed on the photoelectric element 8 a of the photodetector 8 by the condenser lens 20. Since the area of the photoelectric element 8a can be reduced by condensing light by the condensing lens 15, the SN ratio of the output signal is improved. Further, since the collected light only needs to be in the photodetector 8, the accuracy of the position adjustment of the photodetector can be relaxed.

  Light 14 incident on the parallel plate 7 at an incident angle θ generates light 16 reflected on the surface of the parallel plate and light 15 reflected on the back surface. The light 15 and the light 16 are condensed on the light receiving unit 8 a of the photodetector 8 by the condenser lens 20. Since the area of the light receiving part 8a may be small by condensing with the condenser lens 20, the SN ratio of the output signal is improved. Further, since the condensed light only needs to be in the light receiving portion 8a, the accuracy of the position adjustment of the photodetector can be relaxed.

  In this example, unlike the conventional method in which two different light receiving elements separated by at least a predetermined distance are provided on the photodetector, the detection is performed by a single light receiving element 8a.

  FIG. 3 is a diagram for explaining the relationship between the drive current and the wavelength of the wavelength tunable laser light source 1 which is an external resonator type semiconductor laser. Here, an example of wavelength change when the drive current is changed, that is, the laser power is changed when the rotation angle of the diffraction grating 2 is set to a predetermined value is shown.

The horizontal axis of the graph shown in FIG. 3 indicates the drive current, and the vertical axis indicates the wavelength.
As described with reference to FIG. 12, the wavelength of the laser beam schematically shows a saw-like change. As described above, the external cavity semiconductor laser emits light when the laser power increases, that is, the region where the wavelength of the emitted laser light gradually increases as the drive current increases, and when the laser power increases. In addition, there is a region where the wavelength of the laser beam decreases rapidly.

For this reason, the wavelength of the laser light changes discretely to some extent as the laser power increases.
In this case, it is assumed that the lower limit of the wavelength in the wavelength change is λ41 and the upper limit is λ42.

  As described above, when the drive power of the external resonator type semiconductor laser is increased to increase the laser power, the wavelength of the laser light changes on average from λ41 to λ42. Since a plurality of oscillation modes by the type semiconductor laser are generated, a complete single mode or two or three multi-mode states are repeatedly generated.

FIG. 4 is a diagram for explaining the wavelength spectrum in the states P to R of FIG.
4A and 4B show the wavelength spectrums of states P and Q in a region where the wavelength of the laser light gradually increases. FIG. 4C shows the wavelength spectrum of the state R in a region where the wavelength of the laser beam is rapidly reduced.

  Even when a complete single mode (FIG. 4 (a)) or two or three multi-mode (FIG. 4 (b)) states repeatedly occur in the usable region, the mode spacing by the external resonator is generally Since it is small, there is almost no influence when used as a light source for hologram recording and reproduction.

  However, as shown in FIG. 4C, since the wavelength change is large in the region where the wavelength changes from the upper limit λ42 to the lower limit λ41, it has a great influence when used as a light source for hologram recording / reproduction.

  In particular, as shown in FIG. 4C, light having a large wavelength difference between the wavelengths λ41 and λ42 is generated simultaneously in the vicinity of the upper limit λ42 or the lower limit λ41.

In the following, a method for driving a semiconductor laser in the usable area will be described.
Specifically, using a wavelength displacement detection device having a parallel plate 7, an interference light reflected by the parallel plate 7 and interfering with it can obtain a maximum signal intensity at a preset wavelength, and an upper limit at which the wavelength varies and The parallel plate 7 is designed so that the minimum signal intensity is obtained at the lower limit wavelength.

Next, a method for designing parallel plate 7 according to the embodiment of the present invention will be described.
FIG. 5 is a diagram for explaining the reflected light of the light 14 incident on the parallel plate 7 at an incident angle θ.

  Referring to FIG. 5, since light 15 and light 16 pass through different optical paths, they have a phase difference φ proportional to the optical path length difference. The phase difference φ is expressed by the following equation as shown in “Optical Wave Electronics” (Author, Jiro Koyama, Hiroshi Nishihara, p39 (Equation 2.149)).

φ = 4π · n · t · cos (θ #) / λ (1)
sin (θ #) = sin (θ) / n (2)
Where n is the refractive index of the material of the parallel plate 7, t is the thickness of the parallel plate 7, θ is the incident angle of the light 14 with respect to the parallel plate 7, θ # is the refraction angle of the light 14 with respect to the parallel plate 7, and λ is the light Is the wavelength.

  Since the light 15 and the light 16 interfere with each other with the phase difference φ, when the wavelength λ changes, the change can be detected as a change in the output of the photodetector 8.

Specifically, the thickness t of the parallel plate 7 is set so as to satisfy the following formula.
m × λ = 2 × n × t × cos (θ #) (3)
λ ² = 2 × n × t × cos (θ #) × H (4)
sin (θ #) = sin (θ) / n (5)
However, λ is the wavelength of light to be detected, and in this case, the wavelength λ 43 is between the upper limit and the lower limit of the wavelength variation range shown in FIG. H is a wavelength range to be detected. In this example, the upper limit and the lower limit of the wavelength variation range shown in FIG. 4, that is, (wavelength λ 42 −wavelength λ 41). m is an integer, n is the refractive index of the material of the parallel plate, and θ is the incident angle of light with respect to the parallel plate.

  Expression (3) is a condition for the light 15 and the light 16 to strengthen each other, and is obtained by substituting an integer multiple of 2π, that is, 2mπ, into the phase difference φ in the expression (1).

Equation (4) is a condition for determining the period in which the light 15 and the light 16 are strengthened. When the wavelengths are (λ−H / 2) and (λ + H / 2), the phase difference φ in the equation (1) is From becoming 2mπ + π and 2mπ−π,
2mπ + π = 4π × n × t × cos (θ #) / (λ−H / 2) (6)
2mπ−π = 4π × n × t × cos (θ #) / (λ + H / 2) (7)
By transforming both formulas (6) and (7), λ−H / 2 = 4π × n × t × cos (θ #) / (2mπ + π) (8)
λ + H / 2 = 4π × n × t × cos (θ #) / (2mπ−π) (9)
Subtracting the sides of Equation (8) and Equation (9), H = 4π × n × t × cos (θ #) (1 / (2mπ−π) −1 / (2mπ + π))
= 4π × n × t × cos (θ #) (2 / ((4m ² −1) · π))
Is obtained.

Here, since the thickness t of the parallel plate 7 is sufficiently larger than the wavelength, m >> 1 and
H = 4π × n × t × cos (θ #) (2 / (4m ² · π))
= 2 × n × t × cos (θ #) / m ² Formula (10)
Is obtained.

Substituting equation (3) into equation (10) for transformation yields equation (4).
Since the expression (3) is a condition in which the light 15 and the light 16 strengthen each other, if the thickness t and the incident angle θ of the parallel plate 7 are set so as to satisfy the expressions (3) and (4), the wavelength becomes When λ43, the output of the photodetector 8 becomes the maximum value.

  Further, since the equation (3) is a condition for determining the period in which the light 15 and the light 16 are strengthened, in combination with the condition of the equation (3), the light is emitted at the lower limit wavelength λ41 and the upper limit wavelength 42. 15 and light 16 cancel each other, and the output of the photodetector 8 is minimized.

  FIG. 6 is a diagram for explaining the output waveform of the photodetector when the thickness of the parallel plate 7 is set so that the output of the photodetector 8 is maximized at the wavelength λ43.

  Referring to FIG. 6, the output of the photodetector 8 changes in a sine wave shape with respect to the wavelength change, the output of the photodetector 8 takes the maximum value at the center value wavelength λ43, and when the wavelength is displaced from the center wavelength, The output of the photodetector 8 decreases according to the amount of displacement.

  Since the wavelength to be detected is also the oscillation center wavelength W of the external resonator type laser light source, these equations are satisfied even if the wavelength λ in the equations (3) and (4) is replaced with the oscillation center wavelength W.

  4 is substantially equal to the longitudinal mode interval L of the semiconductor laser 2, these equations are satisfied even if H in Equation (4) is replaced with the longitudinal mode interval L. .

  In the wavelength displacement detection device according to the embodiment of the present invention, since a light detector having one light receiving element is used to collect light in the light detector, precise position control of the light detector is unnecessary. Thus, the assembly is facilitated and the reliability is improved.

  In addition, since a parallel plate is used instead of a wedge prism, the parallelism of the light reflecting / interfering surface can be obtained with high accuracy at low cost, so that highly accurate wavelength displacement detection can be performed at low cost.

A wavelength control method of the laser adjustment unit having the wavelength displacement detection device will be described.
Referring to FIG. 3 again, for example, consider the case where the external resonator type laser light source 1 is operating along the curve 18a. In this case, since the hologram can be recorded and reproduced most safely when the central portion of the usable area is the operating point, the operation current value (I0) is operated as an intermediate value of the driving current value causing the mode hop.

  FIG. 7 is a flowchart illustrating a wavelength control method of the laser adjustment unit according to the embodiment of the present invention.

  Referring to FIG. 7, the photodetector output X1 is measured by increasing the drive current from the operating current value (I0) by a current change (I1) (step S1).

  Next, the photodetector output X2 is measured by reducing the drive current from the operating current value (I0) by the current change (I1) (step S2).

  Next, it is determined whether or not the absolute value of the difference between the measured photodetector output X1 and the photodetector output X2 is within an allowable error G1 (step S3).

  If the absolute value of the difference between the photodetector output X1 and the photodetector output X2 is within the allowable error G1, the process is terminated.

  On the other hand, in step S3, if the absolute value of the difference between the photodetector output X1 and the photodetector output X2 is not within the allowable error G1, then the photodetector output X1 is greater than the photodetector output X2. Is determined (step S4).

  In step S4, when the photodetector output X1 is larger than the photodetector output X2, the operating current (I0) is increased (step S5). And it returns to step S1 again.

  On the other hand, when the photodetector output X1 is equal to or smaller than the photodetector output X2 in step S4, the operating current (I0) is decreased (step S6). And it returns to step S1 again.

  The current change (I1) is set so as to be within a range in which the photodetector output becomes the minimum value from the maximum value. The allowable error G1 is set in a range that does not particularly affect even when the wavelength displacement is detected.

  FIG. 8 is a diagram for explaining the relationship of the output of the photodetector when the intermediate value of the drive current value is the operating current value (I0).

  With reference to FIG. 8, for example, a case in which the operating current value (I0) is set and the light is in the vicinity of the light wavelength λ43 will be described. In this case, in the case where the photodetector 8 shows a characteristic of outputting the maximum value at the operating current value (I0), the drive current is changed from the operating current value (I0) to the current change (I1) in Step S1 and Step S2 described above. Since the values of the photodetector outputs X1 and X2 that are increased and decreased by approximately the same value, the absolute value of the difference between the photodetector output X1 and the photodetector output X2 is determined to be within the allowable error G1. .

  Therefore, in this case, the determination circuit determines that there is almost no wavelength displacement and ends the process.

  FIG. 9 is another diagram for explaining the relationship of the output of the photodetector when the intermediate value of the drive current value is the operating current value (I0).

  With reference to FIG. 9, for example, a case will be described in which the operation current value (I0) is set to be displaced from the light wavelength λ43 when operated. In this case, when the photodetector 8 outputs a value slightly displaced from the maximum value at the operating current value (I0), the drive current is changed from the operating current value (I0) in step S1 and step S2 described above. The values of the photodetector outputs X1 and X2 increased and decreased by (I1) are different from each other.

  Therefore, as the photodetector 8 is displaced from the maximum value at the operating current value (I0), the absolute value of the difference between the photodetector output X1 and the photodetector output X2 increases.

  Therefore, in step S3, if the degree of displacement is large, it is determined that it is not within the allowable error G1.

  Therefore, in this case, when the determination circuit determines that the difference is not within the allowable error G1, in step S4, the determination circuit determines which of the photodetector outputs X1 and X2 is greater.

  In this example, the case where the photodetector output X2 is larger than the photodetector output X1 is shown.

  That is, in this case, it can be determined that the set operating current value (I0) is larger than the current value at which the photodetector 8 detects the maximum value.

  Therefore, in step S6, the operating current (I0) is decreased, and the process returns to step S1 again until the absolute value of the difference between the photodetector output X1 and the photodetector output X2 is within the allowable error G1. Repeat the steps. That is, the operating current (I0) is adjusted so as to have a current value at which the photodetector 8 detects the maximum value.

  Therefore, the external resonator type semiconductor laser light source 1 can be operated in a usable area suitable for hologram recording / reproduction by the wavelength control method of the laser adjustment unit according to the above-described embodiment of the present invention.

  On the other hand, there may be a case where the oscillation wavelength of the external resonator type laser light source 1 changes due to a change in environmental temperature.

  For example, in FIG. 3, assuming that the wavelength of the external cavity laser light source 1 operates along the curve 18b due to a change in the environmental temperature, the oscillation wavelength is λ45 when the driving current remains at the operating current (I0). There is a possibility that it rises in the vicinity, becomes close to the unusable area, and cannot be used.

  FIG. 10 is a flowchart illustrating a wavelength control method of the laser adjustment unit according to the modification of the embodiment of the present invention.

  Referring to FIG. 10, the wavelength control method of the laser adjustment unit according to the modification of the embodiment of the present invention replaces steps S5 and S6 with steps S5 # and S6 #, respectively, as compared with the flowchart of FIG. Different points. Since the other points are the same, detailed description thereof will not be repeated.

  If the photodetector output X1 is larger than the photodetector output X2 in step S4, the set temperature of the Peltier element is raised (step S5 #). And it returns to step S1 again.

  On the other hand, when the photodetector output X1 is equal to or smaller than the photodetector output X2 in step S4, the set temperature of the Peltier element 18 is lowered (step S6 #). And it returns to step S1 again.

  For example, when the photodetector output X1 is larger than the photodetector output X2, the determination circuit 9 determines that the temperature of the laser light source is lower than the desired temperature, and the result is the laser control circuit 10. Is output to the adjustment circuit described above. The adjustment circuit can adjust the photodetector 8 to detect the maximum value based on the operating current value (I0) by increasing the set temperature of the Peltier element 18 based on the result. For example, the desired temperature can be set to about room temperature (25 ° C. to 30 ° C.) as an example.

  On the other hand, when the photodetector output X1 is equal to or less than the photodetector output X2, the determination circuit 9 determines that the temperature of the laser light source is higher than the desired temperature, and the result is the result of the laser control circuit 10. It outputs to the adjustment circuit mentioned above. The adjustment circuit can adjust the photodetector 8 to detect the maximum value based on the operating current value (I0) by lowering the set temperature of the Peltier element 18 based on the result.

  This method is useful, for example, when the temperature of the semiconductor laser 2 is not stabilized immediately after the external resonator type semiconductor laser light source 1 is turned on.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is shown not by the above description of the embodiments but by the scope of claims for patent, and is intended to include meanings equivalent to the scope of claims for patent and all modifications within the scope.

It is a figure explaining the schematic structure of the hologram information processing apparatus according to the embodiment of the present invention. It is a block diagram of the wavelength control system for recording / reproducing according to the embodiment of the present invention. It is a figure explaining the relationship between the drive current and wavelength of the wavelength variable laser light source 1 which is an external resonator type semiconductor laser. It is a figure explaining the wavelength spectrum in state P-R of FIG. It is a figure explaining the reflected light of the light 14 which injected into the parallel plate 7 with the incident angle (theta). It is a figure explaining the output waveform of a photodetector at the time of setting the thickness of the parallel plate 7 so that the output of the photodetector 8 may become the maximum at the wavelength (lambda) 43. It is a flowchart explaining the wavelength control method of the laser adjustment part according to embodiment of this invention. It is a figure explaining the relationship of the output of a photodetector when the intermediate value of a drive current value is made into operating current value (I0). It is another figure explaining the relationship of the output of a photodetector when the intermediate value of a drive current value is made into operating current value (I0). It is a flowchart explaining the wavelength control method of the laser adjustment part according to the modification of embodiment of this invention. It is a figure explaining the external resonator type semiconductor laser 1 called a Littrow type. It is a figure explaining the relationship between the laser power and wavelength of the laser beam output from the external resonator type semiconductor laser demonstrated in FIG. It is a figure explaining a mode that the laser beam was incident on the wedge prism. It is the figure which looked at the case where the wedge prism 7 is arrange | positioned along the z-axis direction from the side. It is a figure which shows an example of the pattern 25 of the interference fringe on the light-receiving surface of the photodetector 8. FIG. It is a figure explaining the intensity pattern of the interference fringe of the light reflected by the wedge prism 7 when thickness d and angle (alpha) are set to a certain value. It is a figure explaining the intensity pattern of the interference fringe of another light reflected by the wedge prism 7 when thickness d is selected to a predetermined value. It is a figure explaining the intensity | strength pattern waveform when a wavelength changes to wavelength (lambda) A-(lambda) B. It is a figure explaining the case where the light of the wavelength of wavelength (lambda) A vicinity and the light of the wavelength of wavelength (lambda) B vicinity are mixed. It is a figure explaining the difference value with respect to a wavelength change.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Wavelength variable laser light source, 2 Semiconductor laser, 3 Collimator lens, 4 Diffraction grating, 5 Rotation mechanism, 6 3rd beam splitter, 7 Parallel plate, 8 Photo detector, 9 Judgment circuit, 10 Laser control circuit, 11 Laser adjustment , 17 wedge prism, 20 condensing lens, 62 first beam splitter, 63 first reflecting mirror, 64 spatial light modulator, 65 first relay lens, 66 second reflecting mirror, 67 second beam Splitter, 68 Second relay lens, 69 Objective lens, 70 Image sensor, 61 Third reflection mirror, 72 First galvanometer mirror, 73 Lens system, 74 Second galvanometer mirror

Claims (8)

Condensing means for collecting the light reflected on the front surface and the light reflected on the back surface of the light incident on the parallel plate;
A wavelength displacement detection apparatus comprising: a light detection unit that detects light collected by the light collection unit. The thickness t of the parallel plate is relative to the center value λ of the wavelength range to be detected.
m × λ = 2 × n × t × cos (θ #) and sin (θ #) = sin (θ) / n
(Where m is an integer, n is the refractive index of the parallel plate material, and θ is the incident angle of light)
The wavelength displacement detection apparatus according to claim 1, wherein the wavelength displacement detection apparatus is set so as to satisfy. The thickness t of the parallel plate is relative to the wavelength range H to be detected.
λ ² = 2 × n × t × cos (θ #) × H and sin (θ #) = sin (θ) / n
(Where λ is the wavelength of light, n is the refractive index of the parallel plate material, and θ is the incident angle of light)
The wavelength displacement detection apparatus according to claim 2, wherein the wavelength displacement detection apparatus is set so as to satisfy. An external cavity semiconductor laser;
A parallel plate that receives part of the laser light emitted from the external cavity semiconductor laser;
Condensing means for condensing the light reflected on the front surface and the light reflected on the back surface of the light incident on the parallel plate;
A laser light source device comprising: a light detection unit that detects light collected by the light collection unit. The thickness t of the parallel plate is relative to the oscillation center wavelength W.
m × W = 2 × n × t × cos (θ #) and sin (θ #) = sin (θ) / n
The laser light source device according to claim 4, wherein m is an integer, n is a refractive index of a parallel plate material, and θ is an incident angle of light. The thickness t of the parallel plate is equal to the longitudinal mode oscillation interval L of the external cavity semiconductor laser.
W ² = 2 × n × t × cos (θ #) × L and sin (θ #) = sin (θ) / n
(Where λ is the wavelength of light, n is the refractive index of the parallel plate material, and θ is the incident angle of light)
The laser light source device according to claim 5, wherein the laser light source device is set to satisfy An external cavity semiconductor laser;
A parallel plate that receives part of the laser light emitted from the external cavity semiconductor laser;
Condensing means for condensing the light reflected on the front surface and the light reflected on the back surface of the light incident on the parallel plate;
A light detecting means for detecting the light collected by the light collecting means;
Based on the difference value of the photodetection signal based on the light intensity detected by the photodetection means before and after the drive current of the external cavity semiconductor laser is increased or decreased by a predetermined value, Displacement detecting means for detecting the displacement of the oscillation wavelength of the resonator type semiconductor laser;
A laser light source device comprising: adjusting means for adjusting a drive current of the external resonator type semiconductor laser based on a detection result of the displacement detecting means. An external cavity semiconductor laser, a parallel plate that receives a part of the laser light emitted from the external cavity semiconductor laser, light reflected on the surface of the light incident on the parallel plate, and reflected on the back surface A laser light source including a light collecting means for collecting the collected light, a light detection means for detecting the light collected by the light collecting means, and an adjusting means for adjusting a drive current of the external resonator type semiconductor laser An apparatus control method comprising:
Driving with a drive current obtained by increasing the drive current of the external cavity semiconductor laser by a predetermined amount;
Measuring a light detection signal based on the light intensity detected by the light detection means when the drive current of the external resonator type semiconductor laser is increased by a predetermined amount;
Driving with a drive current obtained by reducing the drive current of the external cavity semiconductor laser by a predetermined amount;
Measuring a light detection signal based on the light intensity detected by the light detection means when the drive current of the external cavity semiconductor laser is reduced by a predetermined amount;
Comparing the absolute value of the difference between the light detection signals when the drive current of the external cavity semiconductor laser is driven with a drive current increased and decreased by a predetermined amount with an allowable error;
And a step of adjusting the drive current based on the comparison.

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