JP4935083B2 - Laser light oscillation mode detector and laser apparatus using the same - Google Patents

Description

  The present invention relates to a laser light oscillation mode detector and a laser apparatus using the same.

  As a laser light source, a gas laser, a laser diode (semiconductor laser) and the like are conventionally used. There are various industrial applications of laser light sources. In particular, laser diodes are widely used in optical recording / reproducing apparatuses such as CDs and DVDs because they are compact, power-saving, and easy to handle. In recent years, as a next-generation optical recording / reproducing apparatus, a hologram recording / reproducing apparatus (hologram recording apparatus or hologram reproducing apparatus and hologram recording) that records information on a hologram recording medium (hologram memory) and reproduces information recorded from the hologram recording medium. And development of the playback device).

  In the hologram recording apparatus, signal light modulated by information to be recorded (recording data) configured in units of pages and reference light from the same laser light source that generates the signal light are generated, and the hologram recording medium Irradiate. Thereby, the signal light and the reference light interfere with each other on the hologram recording medium to form interference fringes in the hologram recording medium, and the interference fringes are formed by a diffraction grating (hologram) by changing the refractive index or transmittance of the hologram recording medium. As a result, the recording data is recorded.

  Further, in a hologram reproducing apparatus that reproduces recorded data from a diffraction grating (hologram) recorded in this way, the diffraction generated by irradiating the diffraction grating (hologram) formed on the recorded recording medium with reference light Recording data can be reproduced by detecting light (reproducing light) with a light receiving element.

As a laser light source used in such a hologram recording / reproducing apparatus, a gas laser, a SHG laser, or the like has been conventionally used since a laser light source close to a single mode is desirable. In recent years, the oscillation wavelength of laser diodes has been shortened, and blue laser diodes have also appeared. Therefore, attention is focused on external resonator laser devices that use a small, power-saving laser diode that makes the wavelength variable. Has been. For example, as shown in FIG. 9, a laser diode 101 that oscillates in a multimode is irradiated as parallel light by a lens 103 onto a diffraction grating 102, and the diffraction grating 102 is rotated by a screw 104 around a fulcrum 105. The relative angle between the grating 102 and the laser diode 101 is adjusted, and a specific wavelength equal to the laser wavelength of the primary light that is diffracted by the diffraction grating 102 and returned to the laser diode 101 is selected and oscillated to obtain zero-order light. There has also been a proposal for a Littrow type laser device (see, for example, Non-Patent Document 1).
Tomiji.Tanaka, et al. “Littrow-type blue laser for holographic data storage”, Technical digest of Optical Data Storage 2004 p311

  According to such an external resonator type laser device, a light source for hologram recording / reproduction with good coherency can be realized. However, since a normal laser diode oscillates in multimode, even in an external resonator type laser device, a mode hop phenomenon in which the frequency of longitudinal mode oscillation hops depending on the laser diode current applied to the laser diode, resulting in coherency. May deteriorate. Therefore, for stable recording / reproducing operation, when a mode hop phenomenon occurs, a detector for detecting this is required, and further, a laser device equipped with such a detector is required. However, conventionally, it has been difficult to provide a good detector for detecting such a mode hop and a laser apparatus incorporating such a detector.

  The present invention solves the above-described problems, provides a laser light oscillation mode detector for detecting what wavelength the laser light has, and further uses such a detector. An object of the present invention is to provide a laser device.

  The laser beam oscillation mode detector of the present invention is a laser beam oscillation mode detector that detects an oscillation mode of a laser beam having one or a plurality of wavelengths, and transmits the laser beam and transmits the laser beam. An optical member that guides a part in a direction different from the transmission direction with an optical path difference with respect to the predetermined direction, and an output corresponding to the luminance of the interference fringes generated in the predetermined direction by the optical path difference generated by the optical member A plurality of light receiving elements for outputting signals, a comparison signal according to a ratio of a minimum value among the output signals of each of the plurality of light receiving elements and a maximum value among the output signals of each of the plurality of light receiving elements The plurality of light receiving elements are arranged so that the values of the comparison signals differ depending on the combination of wavelengths of the laser light.

  The laser light oscillation mode detector is a laser light oscillation mode detector that detects a laser light oscillation mode having one or a plurality of wavelengths, and includes an optical member, a plurality of light receiving elements, a computing unit, Is provided. The optical member transmits the laser light and guides a part of the laser light in a direction different from the transmission direction with an optical path difference with respect to a predetermined direction. Each of the plurality of light receiving elements outputs an output signal corresponding to the luminance of the interference fringes generated in a predetermined direction due to the optical path difference generated by the optical member. The computing unit obtains a comparison signal corresponding to a ratio between the minimum value in the output signals of each of the plurality of light receiving elements and the maximum value in the output signals of each of the plurality of light receiving elements. Since the plurality of light receiving elements are arranged so that the values of the comparison signals differ depending on the combination of wavelengths of the laser light, the laser light oscillation mode can be detected.

  The laser device of the present invention is a laser device that outputs laser light having one or a plurality of wavelengths, and a laser diode having a plurality of oscillation modes that oscillate at one or a plurality of frequencies specified by an internal resonator, An external resonator that inputs a laser beam from the laser diode and selects a frequency in a specific range, a mode detector that detects an oscillation mode of the laser beam from the external resonator, and a current that flows through the laser diode. A laser diode current controller for controlling, and the mode detector transmits a laser beam from the external resonator and has an optical path difference in a direction different from the transmission direction for a part of the laser beam. And a plurality of receivers that output an output signal corresponding to the luminance of the interference fringes generated by the optical path difference caused by the optical member. An arithmetic unit for obtaining a comparison signal in accordance with a ratio between an element and a minimum value of output signals of the plurality of light receiving elements and a maximum value of output signals of the plurality of light receiving elements; The plurality of light receiving elements are arranged so that the value of the comparison signal is different according to a combination of wavelengths of the laser light, thereby detecting an oscillation mode of the laser light, The laser diode current is controlled in accordance with the detected oscillation mode of the laser beam.

  This laser device is a laser device that outputs laser light having one or a plurality of wavelengths, and includes a laser diode, an external resonator, a mode detector, and a laser diode current controller. The laser diode oscillates at one or more frequencies specified by the internal resonator. The external resonator inputs laser light from the laser diode and selects a specific range of frequencies. The mode detector detects the oscillation mode of the laser light from the external resonator. The laser diode current controller controls the current flowing through the laser diode. Here, the mode detector transmits the laser beam from the external resonator and guides a part of the laser beam in a direction different from the transmission direction with an optical path difference, and an optical path difference generated by the optical member. A plurality of light receiving elements that output an output signal corresponding to the luminance of the interference fringes generated by the plurality of light receiving elements, a minimum value among the output signals of each of the plurality of light receiving elements, and An arithmetic unit that obtains a comparison signal in accordance with the ratio to the maximum value. The plurality of light receiving elements are arranged so that the value of the comparison signal differs according to the combination of wavelengths of the laser light, thereby detecting the oscillation mode of the laser light, and the detected laser light The laser diode current is controlled according to the oscillation mode.

  ADVANTAGE OF THE INVENTION According to this invention, the oscillation mode detector of a laser beam can be provided and the laser apparatus using such a detector can be provided.

  With reference to FIG. 1, a multimode oscillation detector for laser light and a laser apparatus using the same according to an embodiment will be described.

  The laser device 10 used in the present embodiment is a Littrow external resonator type laser device, which is a laser diode 11 that oscillates in a multimode, a collimating lens 12, a diffraction grating 13, a mirror 14, a two-divided photodetector 15, and an optical. A wedge 16 and a row photodetector 17 are provided as main optical components, and an arithmetic controller 30 is further provided.

(About wavelength tuning mechanism)
First, a variable mechanism that makes the laser wavelength variable in the laser apparatus 10 shown in FIG. 1 will be described. The mirror 14 and the diffraction grating 13 are fixed, and are rotatable about a rotation axis 27 in the direction indicated by an arrow in the drawing. Here, the line where the extension lines of the diffraction grating surface 13 a and the mirror surface 14 a intersect is the center of the rotation shaft 27. In order to rotate around the rotation shaft 27, the rotation mechanism is composed of a gear mechanism 41b, a gear mechanism 41c, and a motor 41a. That is, the gear mechanism 41b is fixed to the mirror 14 and the diffraction grating 13, and the gear mechanism 41c is disposed so as to mesh with the gear mechanism 41b. The gear mechanism 41c is connected to the rotation shaft of the motor 41a. As a result, according to the rotation of the motor 41a, the angle formed between the diffraction grating surface 13a and the mirror surface 14a is kept constant, for example, a constant angle of 90 °, while the mirror 14 and the diffraction grating 13 are rotated on the rotation axis 27 Can be rotated as a center.

  The operation of the laser device 10 shown in FIG. 1 will be described. The light beam 20 emitted from the laser diode 11 is converted into parallel light by the collimator lens 12, and the light beam 21 converted into parallel light is converted by the diffraction grating 13 into primary light that is diffracted light traveling in different directions for each wavelength. To emit. The diffraction grating 13 diffracts in a different direction for each wavelength. Among these primary lights, there is also primary light that returns to the laser diode 11, and the wavelength of the primary light that returns to the laser diode 11 is diffracted. It varies depending on the angle between the grating 13 and the laser diode 11 and the distance between them. Then, the wavelength corresponding to the primary light returning to the laser diode 11 becomes dominant, and the laser diode 11 can oscillate close to the single mode at the wavelength by appropriately setting the laser diode current. It becomes. Here, the diffraction grating exhibits a basic function as an external resonator.

  On the other hand, most of the light beam from the laser diode 11 is reflected as if the diffraction grating 13 is a mirror to be a light beam (0th order light) 22, and finally as a light beam (0th order light) 25, Since the light is emitted, the light beam emitted as the 0th-order light 25 can be used in various devices such as a hologram recording / reproducing apparatus.

  When the angle of the diffraction grating 13 with respect to the laser diode 11 is changed, the wavelength of the laser beam can be changed. The characteristic that the wavelength can be varied is excellent in that the recording / reproducing characteristics can be improved by changing the wavelength of the light beam used for recording / reproducing according to the temperature of the hologram recording medium. However, if only the angle of the diffraction grating 13 with respect to the laser diode 11 is changed, the emission direction of the 0th-order light also changes, and it is difficult to apply to various devices. Therefore, in the laser device 10 of the present embodiment, as described above, the mirror 14 and the diffraction grating 13 are rotated around the rotation axis 27 while keeping the angle between the diffraction grating surface 13a and the mirror surface 14a constant. The zero-order light 22 from the diffraction grating 13 is also reflected by the mirror 14 so that the emission direction of the zero-order light 25 is constant. In this way, it can be optimal for use in a light source of various devices, particularly in a hologram recording / reproducing apparatus.

  Here, the mirror 14 is a transmission type mirror in which a part of the light beam applied to the mirror surface 14a is transmitted to the back surface of the mirror 14, and about 5% of the entire light beam energy is transmitted and divided into two. The photo detector 15 is reached. The photo detector 15 divided into two has a photo detector 15a and a photo detector 15b, and the wavelength of the zero-order light 25 can be detected by the ratio of the amount of light received by the photo detector 15a and the photo detector 15b.

  Specifically, the difference between the voltage V15a of the output signal S15a corresponding to the amount of light received by the photodetector 15a and the voltage V15b of the output signal S15b corresponding to the amount of light received by the photodetector 15b is detected, and the mirror 14 centered on the rotating shaft 27; The rotation angle with respect to the diffraction grating 13 can be detected. Further, since the rotation angle and the wavelength of the 0th-order light 25 have a one-to-one correspondence as described above, the voltage V15a and the voltage V15b are stored in advance in the storage table arranged in the arithmetic controller 30. The relationship between the difference voltage and the wavelength of the zero-order light 25 is stored, and the wavelength can be obtained from the difference voltage.

  Then, in order to obtain the 0th-order light 25 having a desired wavelength, the drive signal Smd is output from the arithmetic controller 30 to the motor 41a, and the mirror 14 is centered on the rotating shaft 27 by the gear mechanism 41b and the gear mechanism 41c. And the diffraction grating 13 can be rotated.

  The laser device 10 described above is a Littrow type external resonator laser device. However, the laser device 10 is not limited to the Littrow type but may be a Littman type external resonator laser device in combination with a mode hop detector described below. Even so, it can be used in combination as well.

  (About mode hop detector)

  The mode hop detector includes an optical wedge 16 placed in the optical path, a row photodetector 17, and an arithmetic controller 30.

  The optical wedge 16 functions as an optical member that transmits laser light and guides part of the laser light in a direction different from the transmission direction with an optical path difference with respect to a predetermined direction. Most of the light beam (0th-order light) 24 incident on the optical wedge 16 is transmitted and used as the 0th-order light 25, but the remaining part is reflected by the front and back surfaces of the optical wedge 16. Interference fringes are formed on the light receiving surface of the columnar photodetector 17 by the optical path difference between the light beam reflected on the front surface and the light beam reflected on the rear surface. When the mode changes depending on the characteristics of the laser diode 11 itself and the positional relationship between the laser diode 11 and the diffraction grating 13, the interference fringe pattern changes. By detecting the change in the interference pattern, Not only can the current mode be detected steadily, but also a mode hop where the mode changes can be detected.

  FIG. 2 schematically shows fringes (interference fringes) generated by the optical wedge 16. Here, the shape of the fringe can be determined by the angle between the thickness of the optical wedge 16 and the angle between both surfaces, and the angle between the optical wedge 16 and the row of photodetectors 17 arranged in a row in a row. In this embodiment, as an example, the thickness of the optical wedge 16 is 3.5 mm, and the angle formed by both surfaces is 90 seconds.

  Here, the oscillation mode is a single mode transmitting at a single frequency, an external resonance mode based on an external resonator, having a plurality of spectra separated by about 5 pm (picometer), and formed in a laser diode. The internal resonance mode can be classified into three types of internal resonance modes having a plurality of longitudinal mode spectra separated by a wavelength of about 40 pm. In hologram recording / reproduction, good recording / reproduction can be performed in the single mode and the external resonance mode, but it is known that the recording / reproduction characteristics deteriorate in the internal resonance mode. It is important to detect the oscillation mode.

  The columnar photodetector 17 functions as a plurality of light receiving elements that output output signals corresponding to the luminance of the interference fringes generated in a predetermined direction due to the optical path difference generated by the optical member. The row photodetector 17 is configured as two or more divided detectors in the above-described predetermined direction for detecting the change in the interference fringe pattern described above, and even if the individual photodetectors are arranged in a row, Alternatively, a plurality of integrated light receiving sensors such as a CMOS sensor may be provided. An output signal S17 is output from each of the plurality of photodetectors from the columnar photodetector 17, and is input to the arithmetic controller 30 for performing arithmetic operations to be described later. Further, in the present embodiment, the laser light is arranged so that the difference in the values of the plurality of wavelengths of the laser light is larger, that is, the value of the comparison signal described later increases in the order of the single mode, the external resonance mode, and the internal resonance mode. ing.

  Here, as long as the row photodetector 17 is arranged so that the value of the comparison signal is different depending on the combination of wavelengths of the laser light, the column detector 17 can function as a mode detector. It is. An example of how the arrangement is performed will be specifically described below along the arrangement of the present embodiment. The fringe pitch (interference fringe period) at the position where the row photodetector 17 is arranged is 290 μm (micrometer) in the optical component and optical component arrangement of this embodiment, and is 10 mm (millimeter meter) which is the optical path difference. You will get coherency.

  FIG. 3 shows an example of the row photodetector 17 suitable for detecting such a 290 μm fringe pitch. The row-shaped photodetector 17 includes a photodetector 17a to a photodetector 17f, and a combination of the photodetector 17a and the photodetector 17b, a photodetector 17c and the photodetector 17d, and a combination of the photodetector 17e and the photodetector 17f is repeatedly arranged. The width of each photodetector in the change direction of the fringe is 63 μm, the distance between each of the pairs of the photodetector 17a and the photodetector 17b, the photodetector 17c and the photodetector 17d, and the photodetector 17e and the detector 17f is 4 μm. The distance in the predetermined direction in which the fringe between the two changes was 160 μm.

  The fringe contrast (difference between brightness and darkness) is maximized when the laser oscillates in a single mode. In this case, FIG. 4 shows an output signal from each of the columnar photodetectors 17 when the fringe moves due to a wavelength change or the like. Here, it is desirable in terms of detecting a wide range of wavelengths that the phases of the output signals from each of the photodetectors 17a to 17f are uniformly shifted. As described above, this condition can be achieved by adjusting the distance in the fringe change direction of each photodetector, but in addition, the angle formed by the optical wedge 16 and the row photodetector 17 can be adjusted. This also makes it possible to uniformly shift the phase of the output signal from each of the photodetectors 17a to 17f. Furthermore, suitable conditions can be created also by adjusting the thickness of the optical wedge 16 or the angle formed by both sides.

  If the fringes do not move, the number of photodetectors in the row-shaped photodetector 17 is sufficient to be two (two photodetectors arranged at positions corresponding to the bright and dark peaks of the fringe). Regardless of whether mode hops occur or not, the mutual positions of the laser diode 11, the collimating lens 12, the diffraction grating 13, the mirror 14 and the columnar photodetector 17 which are optical components constituting the laser device 10 move due to temperature changes. As a result, the position of the fringe is also moved, and it becomes difficult to always position the two photodetectors as described above at the positions of the bright and dark peaks of the fringe.

  Therefore, according to the experimental results, about six photodetectors are required. In addition, when the coherency of the laser light is deteriorated in such an arrangement that signals that are uniformly out of phase can be obtained from the six photodetectors, a plurality of interference fringes generated by laser light of a plurality of wavelengths cancel each other. The phase difference of the output signal from each of the six photodetectors remains the same, and the amplitude is reduced evenly.

  When the zero-order light 25 is used for hologram recording / reproduction, it is rarely used at a constant light beam intensity at all times. Therefore, the output signals from the six photodetectors are not normalized so that they are not affected by the light beam intensity. must not. In the present embodiment, in order to perform standardization simply, the output signal S17b from the photo detector 17b and the photo detector 17d that are closest to the anti-phase relationship (the relationship in which the light and darkness of the interference fringes are reversed) are closest. Simple standardization (hereinafter referred to as simple standardization) was performed by dividing each of the output signals S17a to S17f, which are output signals of the respective photodetectors, by the sum of the output signals S17d. Since the sum of the output signal S17b and the output signal S17d becomes almost constant regardless of the movement of the fringe, it is suitable for standardization using this signal. However, the normalization is not limited to such a method, but the sum of the output signals from the six photodetectors 17a to 17f, which are the total light amount, and the output signal S15a and the output signal S15b from the two-divided photodetector 15 are used. It may be performed by dividing by the sum of.

  Next, the maximum value and the minimum value are found from the output signals of each of these six standardized photodetectors, and (minimum value) / (maximum value) are calculated. A signal obtained as a result of this calculation is hereinafter referred to as a comparison signal. This comparison signal is a simple index for determining the fringe contrast. That is, when the value of the comparison signal is small, the fringe contrast is large, and when the value of the comparison signal is large, the fringe contrast is small. As will be described in detail below, the value of this comparison signal serves as an index representing the laser oscillation mode. The calculation for obtaining the comparison signal described above is performed by the calculation controller 30.

  FIG. 5 shows the maximum value, the minimum value, and the comparison signal among the output signals of the six photodetectors after the simplified standardization on the vertical axis and the movement amount of the fringe on the horizontal axis. Here, the solid line indicates the comparison signal, the broken line indicates the maximum value, and the alternate long and short dash line indicates each of the minimum values. Further, although the amplitude of the simple standard signal is not uniform with respect to the position of the fringe, this does not become a major obstacle for specifying the mode. That is, in the final comparison signal used for mode detection, these amplitude fluctuations are canceled out, so that there is no problem. By the simple standardization only by the sum of the output signals from the photodetector 17b and the photodetector 17d described above, The mode hop can be detected. However, when the amplitude fluctuation is so large that a problem in accuracy occurs in the subsequent calculation process of the comparison signal, it is necessary to standardize with the total light quantity instead of the simple standard.

  Referring to FIG. 5, if the spectrum of the 0th-order light 25 is a single mode, it can be seen that the comparison signal is somewhere less than 17% of a possible value regardless of the position of the fringe. Conversely, if it is 17% or more, it can be seen that the coherency is deteriorated compared to the single mode. When the coherency is zero in the multimode, the interference fringes are canceled out, the output signals from the photo detectors 17a to 17f are all equal, and the maximum value and the minimum value are equal, so the value of the comparison signal is 1 ( 100%).

  Here, 17%, which is a threshold value for determining whether or not a single mode is used (hereinafter referred to as a single mode hop threshold value), has an optical wedge 16 and a columnar photodetector 17 having specific dimensions as described above. And a specific value of the laser device 10 having a specific internal resonance mode and external resonance mode generated by the laser diode 11 and the diffraction grating 13, and may take different values in other combinations. However, this single mode hop threshold can be determined in advance by experimental data.

  The above is a description of the two extreme cases of the single mode and the case where the coherency is zero. The relationship between the oscillation spectrum and the comparison signal in other cases was obtained by experiments. In the experiment, an optical spectrum analyzer (Advantest, Q8347) was used.

  FIG. 6A shows the coherency when the laser device 10 operates in the internal resonator (LD) mode (hereinafter referred to as an LD mode hop), and FIG. An oscillation spectrum is shown. Both show display results measured with an optical spectrum analyzer. In this case, the mode interval during the LD mode hop is 40 pm, and the coherence is 15% at 10 mm which is the optical path difference considering the refractive index of the optical wedge 16. This corresponds to the fact that the amplitude of the comparison signal is only 15% of the maximum, so if the fluctuation range of the comparison signal at the maximum is 0 to 100, then the fluctuation range is 42.5 to 57.5, The value of the comparison signal is represented by I in equation (1). Also, depending on the amount of movement of the fringe, a larger value may be taken.

  I = 42.5 / 57.5 ≒ 0.739 ... (1)

  Next, FIG. 7 shows the measurement results of the coherency and spectrum when the coherence is the highest while in the internal resonance mode hop. Since the coherence at this time is 40%, the comparison signal has a fluctuation range of 30 to 70, and the value of the comparison signal is represented by I in Expression (2). Also, depending on the amount of movement of the fringe, a larger value may be taken.

  I = 30/70 ≒ 0.429 ... (2)

  Here, the thickness and optical path difference of the optical wedge 16 will be described. The reason why the optical path difference is set to 10 mm is that the optical path difference shown in FIG. 6 and FIG. 7 at which the coherence of both spectra deteriorates to some extent is 10 mm. In a laser apparatus using an external resonator, the optical path difference may be determined in consideration of the mode hop characteristics of the LD mode hop and the external resonator mode hop while taking them into consideration.

  From the measurement results during the two mode hops described above, FIG. 8 shows a laser oscillation spectrum and a comparison signal with respect to the current of the laser diode 11. Here, the dotted line indicates the oscillation wavelength. As can be seen from FIG. 8, after repeating the external resonator mode hop in the range labeled b several times, the LD mode hop state in the range labeled c occurs. Further, in a narrow range where the current of the laser diode 11 is several mA, the single mode is in the range indicated by the symbol a. The comparison signal when the mode changes in this way is indicated by a thick line. In the single mode, the comparison signal indicates a low value of less than 17% (single mode hop threshold), and in the LD mode hop, the comparison signal indicates a high value of 43% or more.

  Here, 43%, which is a threshold value for determining whether or not an LD mode hop is used (hereinafter referred to as an LD mode hop threshold value), is the optical wedge 16 and the columnar photodetector having specific dimensions as described above. 17 and a specific value of the laser device 10 having a specific internal resonance mode and external resonance mode generated by the laser diode 11 and the diffraction grating 13, and may take different values in other combinations. However, this LD mode hop threshold can be determined in advance by experimental data. Further, the area of the external resonator mode hop is 17% (single mode hop threshold) or more and less than 43% (LD mode hop threshold).

  Therefore, in the example of this embodiment, when it is desired to detect that the laser device 10 is operating in the single mode, the comparison signal from the detector is less than 17% (single mode hop threshold). When it is determined by the arithmetic controller 30 and it is desired to detect that the laser device 10 is operating in the external resonator mode hop, the comparison signal from the detector is 17% (single mode hop threshold) or more and 43% ( When the operation controller 30 determines that the laser device 10 is operating at the LD mode hop, the comparison signal from the detector is 43% (LD mode). It may be determined by the arithmetic controller 30 that the value is equal to or greater than the (hop threshold).

(About laser devices and recording / reproducing devices)
When it is desired to operate the laser device 10 in the single mode, the current of the laser diode 11 is set so that the comparison signal is less than 17%, and the laser device 10 is operated in the single mode and the external resonator mode hop. If it is desired, the current of the laser diode 11 should be set so that the comparison signal is less than 43%. Such setting can be performed by the diode control signal Ssd from the arithmetic controller 30 to the laser diode 11.

  In a hologram recording / reproducing apparatus having such a detector, in which mode recording or reproduction is permitted depends on the characteristics of the optical system and the hologram recording medium for recording and reproducing the hologram. Therefore, it is only necessary to determine which mode is used as appropriate. Further, in a hologram recording / reproducing apparatus including a laser device having such a mode detector, it is detected in what mode hop region the laser device is operating, and in a mode hop region suitable for recording or reproduction, the laser If the current of the laser diode is appropriately controlled so that the apparatus operates, good recording / reproducing characteristics can be obtained.

  As described above, the mode hop detector of the present embodiment includes an optical wedge that functions as a fringe generation mechanism that detects each laser oscillation mode, and is arranged as a photodetector that can detect the movement of the fringe in any mode. Controller having a function of extracting a comparison signal representing the fringe contrast based on an output signal from the columnar photodetector, setting a threshold for mode discrimination obtained from a previous experimental result, and discriminating it Is provided. When another mode hop state occurs, it is possible not only to detect it instantaneously, but also to always detect in which mode hop state the laser device is operating.

  Further, as described above, the laser device of the present embodiment includes an optical wedge that functions as a fringe generation mechanism that detects each laser oscillation mode, and is a photodetector that can detect the movement of the fringe in any mode. It has a column-shaped photodetector, and has a function of extracting a comparison signal for recognizing fringe contrast based on an output signal from the column-shaped photodetector, setting a threshold for mode discrimination obtained from a previous experimental result, and discriminating it. Since it has a mode hop detector equipped with an arithmetic controller and further has an arithmetic controller that functions as a laser diode current controller for controlling the current of the laser diode, the output of the mode hop detector operates in a desired mode. If the laser device does not Not only to stop the operation for emitting, it is possible to control the current of the laser diode to operate in a desired mode. When a laser device having such a function is applied to a hologram recording / reproducing apparatus, high-performance recording / reproducing characteristics can be exhibited.

  Note that the present invention is not limited to the above-described embodiment, but extends to the scope of the disclosed technical idea. Further, the embodiment is not limited to the above-described embodiment, and it is needless to say that embodiments in which these are variously modified and combined can be implemented.

It is a figure which shows the multimode oscillation detector of embodiment, and a laser apparatus using the same. It is a figure which shows typically the fringe (interference fringe) produced with an optical wedge. It is a figure which shows the column-shaped photodetector used for the multimode oscillation detector of embodiment. It is a figure which shows the output signal from each of a column-shaped photodetector. It is a figure which shows each of the maximum value, the minimum value, and the comparison signal among the output signals of a plurality of photodetectors after the simplified standardization on the vertical axis and the movement amount of the fringe on the horizontal axis. It is a measurement result which shows a coherency in case the laser apparatus of embodiment is operate | moving by an internal resonator mode hop, and an oscillation spectrum. It is a measurement result which shows a coherency in case the laser apparatus of embodiment is operate | moving on the other conditions of internal resonance mode hop, and an oscillation spectrum. It is a figure which shows the oscillation spectrum and comparison signal of a laser with respect to the electric current of the laser diode of embodiment. It is a figure which shows background art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Laser apparatus, 11 Laser diode, 12 Collimate lens, 13 Diffraction grating, 13a Diffraction grating surface, 14 Mirror, 14a Mirror surface, 15, 15a, 15b, 17a, 17b, 17c, 17d, 17e, 17f Photo detector, 16 Optical wedge , 17 Row-shaped photodetector, 20, 21, 22, 24, 25 Light beam, 27 Rotating shaft, 30 Operation controller, 41a Motor, 41b Gear mechanism, 41c Gear mechanism, 41b Gear mechanism, 41c Gear mechanism, S15a, S15b, S17 Output signal, Smd drive signal, Ssd diode control signal

Claims (6)

A laser light oscillation mode detector for detecting a laser light oscillation mode having one or a plurality of wavelengths,
An optical member that transmits the laser light and guides a part of the laser light to a direction different from the transmission direction with an optical path difference with respect to a predetermined direction;
A plurality of light receiving elements for outputting an output signal corresponding to the luminance of the interference fringes generated in the predetermined direction by the optical path difference generated by the optical member;
An arithmetic unit that obtains a comparison signal according to a ratio between a minimum value in each output signal of the plurality of light receiving elements and a maximum value in each output signal of the plurality of light receiving elements,
The laser light oscillation mode detector, wherein the plurality of light receiving elements are arranged so that values of the comparison signal differ according to a combination of wavelengths of the laser light.   2. The laser light oscillation according to claim 1, wherein the light receiving element is arranged such that the value of the comparison signal increases as the difference in the values of the plurality of wavelengths of the laser light increases. Mode detector.   The computing unit determines a ratio of a minimum value among output signals of the plurality of light receiving elements and a maximum value among output signals of the plurality of light receiving elements according to the light intensity of the laser light. 2. The laser light oscillation mode detector according to claim 1, wherein a signal divided by the signal is used as a comparison signal. The optical member is an optical wedge;
The plurality of light receiving elements are a plurality of photodetectors in which light receiving surfaces are arranged in a row on a surface substantially parallel to a surface of the optical wedge that transmits the laser light. The laser light oscillation mode detector described. A laser device that outputs laser light having one or more wavelengths,
A laser diode having a plurality of oscillation modes that oscillate at one or more frequencies specified by an internal resonator;
An external resonator that inputs laser light from the laser diode and selects a specific range of frequencies;
A mode detector for detecting an oscillation mode of laser light from the external resonator;
A laser diode current controller for controlling a current flowing through the laser diode;
With
The mode detector is
An optical member that transmits the laser light from the external resonator and guides a part of the laser light in a direction different from the transmission direction with an optical path difference;
A plurality of light receiving elements that output an output signal corresponding to the luminance of the interference fringes generated by the optical path difference generated by the optical member;
An arithmetic unit that obtains a comparison signal according to a ratio between a minimum value in each output signal of the plurality of light receiving elements and a maximum value in each output signal of the plurality of light receiving elements;
The plurality of light receiving elements detect an oscillation mode of the laser light by being arranged so that the value of the comparison signal is different according to a combination of wavelengths of the laser light,
A laser device that controls the laser diode current in accordance with the detected oscillation mode of the laser beam. Laser light guided to the plurality of light receiving elements by the optical member is a single mode having a single wavelength, an internal resonance mode having a plurality of wavelengths determined by characteristics of an internal resonator constituting the laser diode, and the internal resonance mode. It is obtained as one of the external resonance modes having a plurality of wavelengths determined by the characteristics of the external resonator, in which the amount of wavelength difference is less than the difference in each wavelength,
6. The laser device according to claim 5, wherein the plurality of light receiving elements are arranged to detect a comparison signal that increases in the order of the single mode, the external resonance mode, and the internal resonance mode.

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