JP2008177470A - Wavelength determination equipment and laser equipment - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide small and low-cost wavelength determination equipment and laser equipment having the wavelength determination equipment. <P>SOLUTION: The wavelength determination equipment has an optical element 22 and a photodetector 23 for detecting interference light. The optical element 22 includes a primary reflective part (reflector 12) and a secondary reflective part (backside 2E) that reflect incident light, and makes the lights reflected from the primary and secondary reflective parts interfere with each other to produce interference light. The reflector 12 has multiple reflecting surfaces (front surfaces 2A to 2D) whose distances from the backside 2E are different from each other. Each of the front surfaces 2A to 2D is in parallel to the backside 2E. When forming an optical element 22, this will enable the number of constitutional members to become fewer, and the front surfaces 2A to 2D and the backside 2E to be easily formed. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a wavelength determination device and a laser device, and more particularly to a wavelength determination device for stabilizing an oscillation mode of an external resonance laser device and a laser device including the wavelength determination device.

  Research and development of hologram recording devices that record data using holography is ongoing.

  In the hologram recording device, modulated signal light (with data superimposed) and unmodulated reference light are generated from laser light, and these are irradiated to the same location on the hologram recording medium. Thereby, the signal light and the reference light interfere on the hologram recording medium, and a diffraction grating (hologram) is formed at the irradiation point. As a result, data is recorded on the hologram recording medium.

  By irradiating the recorded hologram recording medium with reference light, diffracted light (reproduced light) is generated from the diffraction grating formed during recording. Since the reproduction light includes data superimposed on the signal light at the time of recording, the recorded signal can be reproduced by receiving the data with a light receiving element.

  A hologram recording / reproducing light source requires a single-mode laser light source with extremely good coherency, such as a gas laser or a SHG (Second Harmonic Generation) laser. Since a normal laser diode is a multimode, it is not sufficient as a light source for hologram recording and reproduction in terms of coherency. However, if an external resonant laser including a laser diode is configured, a light source for hologram recording / reproduction with good coherency can be realized. Therefore, an external resonator type semiconductor laser using a blue laser diode that is small and power-saving can be used as a hologram light source.

  What is important when laser light is used for hologram recording is wavelength reproducibility. It is preferable that information recording and reproduction can be performed at an optimum wavelength in accordance with a volume change due to a temperature change of the hologram recording medium. In particular, when performing wavelength multiplexing for recording information by changing the wavelength, the wavelength of the output light must be controlled to the intended length. When wavelength multiplexing is performed, for example, a wavelength variable type external resonator laser can be used.

  As an external resonator in the external resonator laser, for example, there is a Littrow type. In such an external resonator, first, a laser beam emitted from a laser diode is converted into parallel light by a collimating lens, and is irradiated onto a reflective diffraction grating. The light beam reflected by the diffraction grating is separated into zero-order light and first-order light. The primary light passes through the incoming optical path as it is and returns to the laser diode.

  The laser beam returning to the laser diode constitutes an external cavity laser including the laser diode and the reflective diffraction grating. The laser diode oscillates at a wavelength determined by the grating shape of the reflective diffraction grating and the distance between the reflective diffraction grating and the laser diode.

  A means for determining the wavelength of such a light source is disclosed in, for example, Japanese Patent Laying-Open No. 2005-340783 (Patent Document 1). Japanese Patent Laid-Open No. 2005-340783 discloses a wavelength determination device that can determine a wavelength with a simple structure, and a wavelength determination method using the wavelength determination device.

  FIG. 16 is a diagram illustrating a configuration of a wavelength determination device disclosed in Japanese Patent Application Laid-Open No. 2005-340783. Referring to FIG. 16, laser device 150 includes an external resonator type semiconductor laser 110 and a wavelength determination device 120 that determines the oscillation wavelength of external resonator type semiconductor laser 110. The wavelength determination device 120 includes a beam splitter 121, an optical wedge 122, a photodetector 123, and a laser control unit 130. The laser control unit 130 determines the oscillation wavelength of the external resonator type semiconductor laser 110 based on the output of the photodetector 123 and controls the oscillation wavelength.

  FIG. 17 is a diagram illustrating the configuration of the external cavity semiconductor laser 110 shown in FIG. Referring to FIG. 17, external resonator type semiconductor laser 110 includes a laser diode 101, a collimating lens 102, and a diffraction grating 103. The laser diode 101 emits multimode laser light, for example, blue laser light of about 405 nm.

  The collimating lens 102 converts the laser light emitted from the laser diode 101 into parallel light. The diffraction grating 103 receives the laser light from the laser diode 101, generates 0th-order light L0 in a predetermined direction, and generates primary light L1a, L1b, L1k in different directions for each wavelength. The angle between the diffraction grating 103 and the laser diode 101 is set so that only the primary light L1k having a specific wavelength (for example, 405 nm) returns to the laser diode 101 among the primary lights L1a, L1b, and L1k. Thereby, only the wavelength component is amplified in the laser diode 101. As a result, the mode of the laser diode 101 becomes a single mode.

  FIG. 18 is a diagram showing the relationship between the power and wavelength of laser light output from the external cavity semiconductor laser 110 of FIG. In FIG. 18, the horizontal axis of the graph indicates laser power (unit: mW), and the vertical axis of the graph indicates wavelength (unit: nm). The graph of FIG. 18 shows that the wavelength of the laser light changes in a generally sawtooth shape as the laser power increases.

  In the external cavity semiconductor laser 110, the external cavity mode hop region where the wavelength of the emitted laser light gradually increases as the laser power increases and the wavelength of the laser light sharply increases when the laser power increases. Mode hop region (mode hop region by a laser chip in a semiconductor laser). Since the two mode hop regions coexist, the wavelength of the laser light changes discretely to some extent as the laser power increases.

  Since the mode hop by the laser chip is about 0.04 nm, good hologram recording is impossible. The state in which the mode hop is caused by the laser chip is also referred to as “multi-mode” or “unusable mode”. The laser device 150 needs to detect whether or not the external resonator type semiconductor laser 110 is oscillating in this “unusable mode”. Further, in the laser device 150, it is necessary to stabilize the oscillation wavelength by controlling the oscillation wavelength of the external cavity semiconductor laser 110 so that the mode hop region due to the laser chip can be effectively excluded.

  In the wavelength determination device 120, an interference fringe pattern in which the fringe moves with respect to the wavelength change is generated on the photodetector 123 using the optical wedge 122 that receives a part of the light emitted from the external cavity semiconductor laser 110. The oscillation wavelength is determined by

  FIG. 19 is a diagram for explaining the arrangement of the photodetectors 123 in detail. In FIG. 19, the intensity distribution of the interference fringes when the wavelength of the laser beam is 405 nm and 405.04 nm is represented by P405 and P405.04, respectively. As shown in FIG. 19, the photodetector 123 includes light receiving elements 123D and 123E that each detect light at at least two light receiving positions. The light receiving element 123D outputs detection signals DA and DB, and the light receiving element 123E outputs detection signals EA and EB. At the time of wavelength determination, a difference value (DA-DB) between the detection signals DA and DB and a difference value (EA-EB) between the detection signals EA and EB are obtained, and the wavelength of the laser beam is determined based on these difference values. .

  For example, as shown in FIG. 19, the phase of light with a wavelength of 405 nm is opposite to that of light with a wavelength of 405.04 nm. In a state where light with a wavelength of 405 nm and light with a wavelength of 405.04 nm are mixed, interference fringes do not appear clearly because the light intensity is averaged. In such a state, the detection signal difference values (DA-DB) and (EA-EB) are substantially zero.

  On the other hand, although depending on the arrangement of the light receiving elements, there is a wavelength where the difference value of the detection signal is almost 0 in the wavelength range of light emitted from the external cavity semiconductor laser (between the wavelength 405 nm and the wavelength 405.04 nm). there's a possibility that. For this reason, when the difference value of the detection signal becomes 0, it cannot be determined whether the reason is that the interference fringe has moved due to the change in the wavelength of the laser beam or the contrast has decreased due to the mixture of laser beams having opposite phases. Can happen.

  In order to solve such a problem, each conventional wavelength detection device includes a plurality of light receiving elements that detect light at at least two light receiving positions. In each of the plurality of light receiving elements, the interval between the two light receiving positions is set to be ¼ or less of the period of the intensity change of the reflected light. In addition, a plurality of light receiving elements are arranged so that the difference value between the two detection signals is different for each light receiving element.

Japanese Patent Laying-Open No. 2006-10499 (Patent Document 2) discloses a configuration example of a reflection unit that generates interference fringes. FIG. 20 is a diagram for explaining the configuration of the reflecting means disclosed in Japanese Patent Application Laid-Open No. 2006-10499. Referring to FIG. 20, the reflecting means is composed of two pieces of glass 140 that are inclined and stacked. This reflection means is a substitute for an optical wedge.
JP-A-2005-340783 JP 2006-10499 A

  In the conventional examples shown in FIGS. 16 to 20, wavelength determination is performed using at least two two-divided detectors that detect the reflected light of the optical wedge. In order to set the difference value of the two detection signals to be different for each of the two-divided detectors, it is necessary to strictly limit the tolerance of the wedge angle of the optical wedge. This is because the period of interference fringes is affected by the tolerance of the wedge angle of the optical wedge.

  For example, consider a case where the interval between two two-divided detectors is set to ¼ of the light intensity change period. It is necessary to increase the light receiving area of the photodetector and to increase the interval between the light receiving positions (in other words, to increase the period of the interference fringes). For example, the period must be set to 124 μm. In this case, the necessary wedge angle is 0.05 °. When a 10% periodic error is allowed, the tolerance of the wedge angle becomes as small as ± 0.005 °, which makes it difficult to manufacture an optical wedge that satisfies such specifications.

  Alternatively, although the wedge angle tolerance may be large, a method of adjusting the shape of the two-divided detector according to the change in the period of the interference fringes or adjusting the position at the time of arrangement can be considered. However, this method has a problem that the adjustment process becomes complicated.

  Japanese Laid-Open Patent Publication No. 2006-10499 proposes a plan in which two pieces of glass that are inclined and stacked are used as a substitute for an optical wedge. However, since it is necessary to adjust the angle of two glass sheets with high precision, manufacture becomes difficult. In addition, there is a high possibility that the interval or inclination between the two glasses changes due to a change with time, and thus there is a problem that the reliability is lowered.

  The present invention has been made for the purpose of solving the above problems, and an object of the present invention is to provide a compact and low-cost wavelength determination device and a laser device including the wavelength determination device.

  In summary, the present invention is a wavelength determination device including an optical element. The optical element includes first and second reflecting portions that reflect incident light, and causes interference light to be generated by causing the reflected light from the first reflecting portion and the reflected light from the second reflecting portion to interfere with each other. The first reflecting portion has a plurality of reflecting surfaces whose distances from the second reflecting portion are different from each other. The wavelength determination device further includes a photodetector that detects the interference light, and a determination unit that determines the wavelength of the incident light based on the detection result of the photodetector.

  Preferably, the second reflecting portion is formed in a planar shape. Each of the plurality of reflecting surfaces is parallel to the second reflecting portion.

  More preferably, the optical element includes a substrate having two parallel surfaces. The first reflecting portion is formed by overlapping a plurality of flat plates on one of the two surfaces.

  More preferably, the first reflecting portion is formed by etching a substrate having two parallel surfaces.

  More preferably, the optical element includes a substrate having two parallel surfaces. The first reflecting portion is formed by laminating a plurality of thin films on one of the two surfaces.

  Preferably, when viewed from a direction perpendicular to the plurality of reflection surfaces, the plurality of reflection surfaces form an arbitrary two-dimensional shape. The photodetector includes a plurality of light receiving elements that are respectively provided corresponding to the plurality of reflection surfaces and receive the reflected light from the corresponding reflection surfaces.

  Preferably, when m is an integer, the optical path difference between the first reflecting surface and the second reflecting portion of the plurality of reflecting surfaces is the second reflecting surface and the second reflecting surface among the plurality of reflecting surfaces. It differs by m / 4 times the wavelength of the incident light with respect to the optical path difference with the part.

  More preferably, the plurality of reflection surfaces include first to fourth reflection surfaces. The photodetector includes first to fourth light receiving elements which are provided corresponding to the first to fourth reflecting surfaces, respectively, and receive the reflected light from the corresponding reflecting surfaces. The determination unit determines the wavelength of the incident light based on the signal having the highest intensity among the first to fourth signals output from the first to fourth light receiving elements, respectively.

  More preferably, the photodetector includes a plurality of light receiving elements which are respectively provided corresponding to the plurality of reflection surfaces and receive the reflected light from the corresponding reflection surfaces. The determination unit determines the wavelength of the incident light based on the difference between the signal having the highest intensity among the plurality of signals output from the plurality of light receiving elements and the half value of the peak value of the interference light intensity.

More preferably, the number of the plurality of reflecting surfaces is 2 or 3.
According to another aspect of the present invention, a laser device includes any one of the wavelength determination devices described above and a laser light source that generates incident light.

  Preferably, the laser light source is an external cavity semiconductor laser.

  According to the present invention, a small and low-cost wavelength determination device can be realized.

  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.

[overall structure]
FIG. 1 is a diagram showing a schematic configuration of a laser apparatus 50 according to an embodiment of the present invention. Referring to FIG. 1, laser device 50 includes an external resonator type semiconductor laser 10, a wavelength determination device 20, and a laser control unit 30.

  FIG. 2 is a diagram showing the configuration of the external cavity semiconductor laser 10 of FIG. Referring to FIG. 2, external resonator type semiconductor laser 10 includes a laser diode 1, a collimating lens 2, and a diffraction grating 3.

  The laser diode 1 emits multimode laser light, for example, blue laser light of about 405 nm. The collimating lens 2 converts the laser light emitted from the laser diode 1 into parallel light. The diffraction grating 3 receives the laser light from the laser diode 1 and generates 0th-order light L0 in a predetermined direction and also generates primary light L1a, L1b, and L1k in different directions for each wavelength. The angle between the diffraction grating 3 and the laser diode 1 is set so that only the primary light L1k having a specific wavelength (eg, 405 nm) out of the primary lights L1a, L1b, and L1k returns to the laser diode 1. As a result, only the wavelength component is amplified in the laser diode 1. As a result, the mode of the laser diode 1 becomes a single mode.

  Returning to FIG. 1, the wavelength determination device 20 includes a half mirror 21, an optical element 22, a photodetector 23, and a wavelength determination unit 24. The half mirror 21 reflects a part of the light LI of the light emitted from the external cavity semiconductor laser 10 (for example, blue laser light of about 405 nm) in the direction of the optical element 22. Of the light emitted from the external cavity semiconductor laser 10, the light LT that has passed through the half mirror 21 becomes light emitted from the laser device 50.

  The optical element 22 receives the light LI and generates interference light. The interference light irradiates a predetermined position on the photodetector 23. The photodetector 23 detects interference light. The wavelength determination unit 24 determines the oscillation wavelength of the external resonator type semiconductor laser 10 based on the detection result of the interference light by the photodetector 23. Although details will be described later, the external cavity semiconductor laser 10 oscillates in either a single mode or a multimode. In the present embodiment, “determination of the oscillation wavelength” is to determine whether the oscillation mode is a single mode or a multimode.

  The laser control unit 30 controls the oscillation wavelength of the external resonator type semiconductor laser 10 based on the determination result of the wavelength determination unit 24.

  FIG. 3 is a diagram showing the configuration of the optical element 22 of FIG. 1 in detail. Note that the X direction, Y direction, and Z direction in FIG. 3 are the same as the X direction, Y direction, and Z direction in FIG. 1, respectively. Referring to FIG. 3, the optical element 22 has a reflecting portion 12 that reflects light. The reflection unit 12 has surfaces 2A to 2D that are reflection surfaces. The surfaces 2A and 2B are connected by a step portion 2F1. Similarly, the surfaces 2B and 2C are connected by a step 2F2, and the surfaces 2C and 2D are connected by a step 2F3.

  The optical element 22 further includes a back surface 2E that is a reflecting portion that reflects light. Each of the front surfaces 2A to 2D is parallel to the back surface 2E. The distance (thickness) from the back surface 2E to the front surface 2A is T1. The distance from the surface 2A to the surface 2B is T2. The distance from the surface 2B to the surface 2C is T3. The distance from the surface 2C to the surface 2D is T4.

  When the light LI is incident on the optical element 22, the light LI is reflected on the front surfaces 2A to 2D and the back surface 2E. The reflected light from the back surface 2E and the reflected light from the front surface 2A are rays A. The optical path difference δ1 between the reflected light from the back surface 2E and the reflected light from the front surface 2A is expressed as δ1 = Lp1−Lg1. Lp1 represents an optical distance when light passing through the front surface 2A and reflected by the back surface 2E passes through the optical element 22. Lg1 represents an optical path difference between the two light beams when the two light beams reach the surface 2A.

  Similarly, the reflected light from the back surface 2E and the reflected light from the front surface 2B are rays B. The optical path difference δ2 between the reflected light from the back surface 2E and the reflected light from the front surface 2B is expressed as δ2 = Lp2−Lg2. Lp2 represents an optical distance when light passing through the front surface 2B and reflected by the back surface 2E passes through the optical element 22. Lg2 represents an optical path difference between the two light beams when the two light beams reach the surface 2B.

  Similarly, the reflected light from the back surface 2E and the reflected light from the front surface 2C become a light ray C. The optical path difference δ3 between the reflected light from the back surface 2E and the reflected light from the front surface 2C is expressed as δ3 = Lp3−Lg3. Lp3 represents an optical distance when light passing through the front surface 2C and reflected by the back surface 2E passes through the optical element 22. Lg3 represents an optical path difference between the two light beams when the two light beams reach the surface 2C.

  Similarly, the reflected light from the back surface 2E and the reflected light from the front surface 2D are rays D. The optical path difference δ4 between the reflected light from the back surface 2E and the reflected light from the front surface 2D is expressed as δ4 = Lp4−Lg4. Lp4 represents an optical distance when light passing through the front surface 2D and reflected by the back surface 2E passes through the optical element 22. Lg4 represents an optical path difference between the two light beams when the two light beams reach the surface 2D.

  When the wavelength of the light LI is λ, the optical path difference δ2 is expressed as δ2 = δ1 + λ / 4. The optical path difference δ3 is expressed as δ3 = δ1 + (2λ / 4). The optical path difference δ4 is expressed as δ4 = (δ1 + 3λ / 4). That is, any two differences among the optical path differences δ1 to δ4 are expressed as λ / 4 × m (m is an integer).

  The photodetector 23 includes light receiving elements 23A to 23D provided corresponding to the surfaces 2A to 2D of the optical element 22, respectively. Light rays A, B, C, and D are incident on the light receiving elements 23A, 23B, 23C, and 23D, respectively.

  FIG. 4 is a diagram for explaining the optical path difference of the laser beam in the optical element 22 in more detail. Referring to FIG. 4, light beams A <b> 1 and A <b> 2 are incident on optical element 22. The light ray A1 is reflected by the front surface 2A of the optical element 22 to become the light ray A, and the light ray A2 is reflected by the back surface 2E of the optical element 22 and becomes the light ray A again. In order to simplify the explanation, it is assumed that the optical element 22 shown in FIG. 4 has only the front surface 2A and the rear surface 2E as the reflection surfaces.

  When the refractive index of the optical element 22 is n, the following equation (1) is established according to Snell's law.

sin θ / sin θ1 = n (1)
On the other hand, the length of Lg1 is represented by the following formula (2).

Lg1 = 2 × T1 × tan θ1 × sin θ (2)
The distance Lp0 that the light ray A2 passes through the optical element 22 is expressed by the following formula (3).

Lp0 = 2 × (LP0 / 2) = 2 × (T1 / cos θ1) (3)
Lp1 shown in FIG. 4 is the optical distance of LP0 and is represented by the following equation (4).

Lp1 = 2 × (T1 / cos θ1) × n (4)
The optical path difference δ1 is expressed by the following formula (5).

δ1 = Lp1-Lg1 = 2 × n × T1 / cos θ1-2 × T1 × tan θ1 × sin θ = 2 × T1 × (n / cos θ1-sin θ × tan θ1) (5)
Note that the optical path differences δ2 to δ4 can also be calculated according to the equations (1) to (5) in the same manner as the optical path difference δ1. In this case, T1 may be replaced with (T1 + T2), (T1 + T2 + T3), and (T1 + T2 + T3 + T4).

  FIG. 5 is a diagram illustrating a method for forming a step portion in the optical element 22. As shown in FIG. 5, for example, there are three methods for forming the stepped portion.

  In the first method, first, a parallel flat plate 25 having two surfaces (front surface 2D and back surface 2E) parallel to each other is prepared, and then a portion indicated by oblique lines on the front surface 2D side is polished or etched. Thus, the surfaces 2A to 2C and the stepped portions 2F1 to 2F3 are formed.

  In the second method, first, a parallel plate 25 (substrate) having two surfaces (front surface 2A and back surface 2E) parallel to each other is prepared, and then thin films 22A to 22C are laminated on the front surface 2A side. Each of the thin films 22A to 22C has two surfaces parallel to the surface 2A. The area of the surface 2A is larger than the surface of the thin film 22A (a plane parallel to the surface 2A). Furthermore, the area of the surface (surface parallel to the surface 2A) decreases in the order of the thin films 22A, 22B, and 22C. Thus, by laminating the thin films 22A to 22C on the surface 2A side, the surfaces 2A to 2D and the stepped portions 2F1 to 2F3 are formed.

  In the third method, plate-like members 22E to 22G having two parallel surfaces are used in place of the thin films 22A to 22C used in the second method.

  When the optical element 22 is formed by any one of the first to third methods, the reliability of the optical element 22 is improved because the change with time of the distance between the front surface and the back surface or the change with time of the tilt of the surface hardly occurs. Can be made. Further, according to the first method, since the optical element 22 can be formed from one parallel plate 25, the constituent members of the optical element 22 can be reduced.

  Therefore, according to the present embodiment, the number of constituent members of the optical element 22 can be reduced, and as a result, a small and low-cost wavelength determination device can be realized. Furthermore, according to the present embodiment, the laser device 50 includes the wavelength determination device 20 that can be reduced in size and cost. Therefore, according to the present embodiment, a small-sized and low-cost external resonator type semiconductor laser device can be realized.

  Each of the front surfaces 2A to 2D is parallel to the back surface 2E. According to the present embodiment, when the optical element 22 is manufactured using the first method, the manufacturing accuracy can be improved.

  FIG. 6 is a diagram illustrating the configuration of the optical element 22 and the configuration of the comparative example. Referring to FIG. 6, optical element 22 is manufactured by polishing a parallel plate. When the optical element 22 is manufactured from a parallel plate, the parallelism is enhanced by polishing and finishing the front surfaces 2A to 2D and the back surface 2E. In producing the optical element 22, it is only necessary to translate the polishing surface in the direction of the arrow, so that the optical element 22 can be processed with relatively high accuracy.

  On the other hand, the comparative example is an optical wedge. In the case of an optical wedge, the front surface 2A is slightly inclined with respect to the back surface 2E. For this reason, when the surface 2A and the back surface 2E are polished and finished, it is necessary to intentionally incline the polishing surface. However, since the inclination of the polishing surface is small (for example, 0.05 °), it is difficult to increase the adjustment accuracy of the inclination of the polishing surface. Therefore, the manufacturing accuracy can be improved in this embodiment compared with the comparative example.

[Wavelength determination method]
FIG. 7 is a diagram showing the relationship between the optical path difference δ between the two laser beams and the intensity of the interference light when the two laser beams interfere with each other by the optical element 22. In FIG. 7, the horizontal axis of the graph indicates the optical path difference, and the vertical axis of the graph indicates the intensity of the interference light. The wavelength of the laser beam is 405 nm. The intensity of the interference light changes periodically with respect to the optical path difference δ. The period of intensity change of the interference light is equal to the wavelength 405 nm of the laser light.

  In the present embodiment, the distances T1 to T4 are set based on the relationship between the optical path difference δ and the intensity of the interference light shown in FIG. That is, the distance T1 is set so that the intensity of the interference light at the optical path difference δ1 becomes the maximum value Ma. The distance T2 is set so that the intensity of the interference light at the optical path difference δ2 becomes an intermediate value Md between the maximum value Ma and the minimum value Mi. The distance T3 is set so that the intensity of the interference light at the optical path difference δ3 becomes the minimum value Mi. The distance T4 is set so that the intensity of the interference light at the optical path difference δ4 becomes the intermediate value Md.

  FIG. 8 is a diagram for explaining the oscillation spectrum of the external cavity semiconductor laser 10. When the oscillation mode is a single mode, laser light having a single wavelength (405 nm) is emitted. On the other hand, when a mode hop (about 0.04 nm) is generated by the laser chip and the multi-mode oscillation occurs, not only three modes of light are generated in the vicinity of the wavelength of 405 nm, but also three in the vicinity of the wavelength of 405.04 nm. Mode light is generated.

  FIG. 9 is a diagram showing a change in the intensity of the interference light and the intensity of the light incident on the photodetector 23 when the external cavity semiconductor laser is in single mode oscillation.

  Referring to FIG. 9, the relationship between the optical path difference δ and the intensity of the interference light when the external cavity semiconductor laser 10 is oscillating in a single mode at a wavelength of 405 nm is as follows. It is the same as the relationship with strength. In this case, the intensities of the light beams A to D incident on the light receiving elements 23A to 23D are the maximum value Ma, the intermediate value Md, the minimum value Mi, and the intermediate value Md, respectively. Therefore, the light receiving element 23A is the brightest and the light receiving element 23C is the darkest. The brightness of the light receiving elements 23B and 23D is intermediate between the brightness of the light receiving elements 23A and 23C.

  The light receiving element outputs a signal corresponding to the intensity of the incident light. Signals SA, SB, SC and SD are output from the light receiving elements 23A to 23D, respectively.

  FIG. 10 is a diagram showing a change in the intensity of the interference light and the intensity of the light incident on the photodetector 23 when the external resonator type semiconductor laser is in multimode oscillation. Referring to FIG. 10, at the time of multi-mode oscillation, almost opposite phase laser beams (wavelengths of 405 nm and 405.04 nm) are mixed. In this case, the interference light intensity of each wavelength is averaged. For this reason, as shown by the curve PM, the intensity of the interference light becomes a substantially constant value Mav irrespective of the optical path difference δ. In this case, the intensities of the light beams A to D incident on the light receiving elements 23A to 23D are substantially equal. Note that the value Mav is about half of the maximum value Ma of the intensity of the interference light in the single mode oscillation because the intensity is averaged by the antiphase interference light in the multimode oscillation.

  Next, a method for determining the wavelength using the signals SA, SB, SC, SD will be described. First, the maximum value Ma of the intensity of interference light when the external cavity semiconductor laser 10 is oscillating in a single mode, and the value of the intensity of interference light when the external cavity semiconductor laser 10 is oscillating in a multimode. Mav is obtained in advance. Next, an appropriate threshold value Mth between the obtained maximum value Ma and value Mav is determined.

  The wavelength determination unit 24 selects a signal value having the highest intensity among the signals SA to SD as the signal value Smax. For example, when the external cavity semiconductor laser 10 oscillates in a single mode, the signal value Smax is the value of the signal SA. On the other hand, when the external cavity semiconductor laser 10 is oscillating in multimode, the signal value Smax is the value Mav.

  Next, the wavelength determination unit 24 compares the signal value Smax with the threshold value Mth. When Smax> Mth, the wavelength determination unit 24 determines that the external cavity semiconductor laser 10 is oscillating in a single mode. When Smax <Mth, the wavelength determination unit 24 determines that the external cavity semiconductor laser 10 is oscillating in multimode.

  As shown in the graph of FIG. 10, when the wavelength of the external cavity semiconductor laser changes, the change in the intensity of the interference light shifts in the horizontal axis direction of the graph. In the graph of FIG. 10, when the wavelength of the interference light is 405 nm and when it is 405.04 nm, the intensity change of the interference light is shifted in the horizontal axis direction of the graph by an optical path difference corresponding to a phase of 180 °.

  In the present embodiment, the steps between the surfaces 2A to 2D of the optical element 22 are set so that an optical path difference δ corresponding to ¼ (90 °) of the intensity change period of the interference light occurs. As a result, when the external cavity semiconductor laser oscillates in a single mode, the optical path difference when the intensity of the interference light is maximum and the intensity of the interference light are included in the optical path differences δ1 to δ4 shown in FIG. There is an optical path difference when it is minimum, and an optical path difference where the intensity of the interference light is intermediate. As a result, even when the wavelength at the time of single mode oscillation is 405.04 nm, any one of the signals SA to SD is larger than the threshold value Mth, so that the wavelength can be reliably determined.

  As described above, when the external resonator type semiconductor laser oscillates in the single mode, the value of the signal from the light receiving element that becomes brightest among the four light receiving elements is equal to or greater than the threshold value Mth. On the other hand, when the external resonator type semiconductor laser oscillates in multimode, all the four light receiving elements receive the average amount of light, so that the signal output from each of the four light receiving elements has a constant value Mav. (<Mth). Therefore, according to the present embodiment, the wavelength can be determined by an easy calculation of comparing the absolute value of the output signal of each light receiving element with the threshold value.

  In the present embodiment, four surfaces 2A to 2D and stepped portions 2F1 to 2F3 are formed on the optical element 22, and the intensity of interference light generated by the back surface 2E and each surface of the surfaces 2A to 2D is set to the surface 2A to 2D. Detection is performed by four light receiving elements corresponding to each. However, the surface of the optical element (and the light receiving element) may be at least two. As a result, the number of surfaces in the optical element can be reduced and the number of light receiving elements can be reduced, so that the wavelength determination device can be further reduced in size and cost.

  FIG. 11 is a diagram illustrating another example of the configuration of the optical element 22 and the photodetector 23. Referring to FIG. 11, optical element 32 includes a reflecting portion 12A. The reflection part 12A includes surfaces 2A and 2B which are reflection surfaces. The surfaces 2A and 2B are connected by a step portion 2F1. The optical element 32 further includes a back surface 2E that is a reflecting portion. The photodetector 33 includes two light receiving elements 23A and 23B provided corresponding to the surfaces 2A and 2B, respectively.

  FIG. 12 is a diagram showing still another example of the configuration of the optical element 22 and the photodetector 23. Referring to FIG. 12, optical element 32A includes a reflecting portion 12B. The reflection part 12B includes surfaces 2A, 2B, and 2C that are reflection surfaces. The surfaces 2A and 2B are connected by a step portion 2F1. The surfaces 2B and 2C are connected by a step portion 2F2. The optical element 32A further includes a back surface 2E that is a reflecting portion. The photodetector 33A includes three light receiving elements 23A to 23C provided corresponding to the surfaces 2A to 2C, respectively. In addition, in order to prevent that a figure becomes complicated, the reflected light from the back surface 2E is not shown in FIG.11 and FIG.12.

  As shown in FIGS. 11 and 12, when the number of reflecting surfaces on one side of the optical element is two or three, the output of any light receiving element may not be larger than the threshold value Mth depending on the wavelength of the laser beam. There is.

  FIG. 13 is a diagram showing the relationship between the optical path difference δ and the intensity of interference light when the optical element 32A shown in FIG. 12 is used. Referring to FIG. 13, the external cavity semiconductor laser 10 oscillates in a single mode, but the intensity of the interference light is smaller than the threshold value Mth when the optical path difference is any of δ1 to δ3.

  The wavelength determination method in such a case is executed as follows. First, the maximum value Ma of the interference light intensity is obtained in advance. Next, the absolute value of the difference between the value of the signal from the light receiving element and Ma / 2 is set to a numerical value X for wavelength determination.

  When the external resonator type semiconductor laser oscillates in multimode, the numerical value X is almost zero. On the other hand, when the external cavity semiconductor laser 10 is oscillating in a single mode, any one of the three signals respectively output from the three light receiving elements sets the numerical value X to a certain value or more. Therefore, even when the number of light receiving elements is three, the threshold value of the numerical value X is set in advance, and the wavelength determination unit 24 compares the threshold value with the numerical value X, thereby enabling wavelength determination. If the numerical value X is larger than the threshold value, the wavelength determining unit 24 determines that the external resonator type semiconductor laser 10 is oscillating in a single mode. If the numerical value X is smaller than the threshold value, the external resonator type semiconductor laser 10 is in a multimode. Judge that it is oscillating.

  When the number of reflecting surfaces connected by a plurality of step portions in the optical element is an integer n of 5 or more and n light receiving elements corresponding to the number of reflecting surfaces are arranged, the intensity change of interference light is changed. The height of the step portion is set so that an optical path difference of δt / n obtained by equally dividing the optical path difference δt corresponding to one period into n occurs. The wavelength determination method in this case is the same as in the case where the number of light receiving elements is four. As the number of reflecting surfaces and the number of light receiving elements increase, the intensity of interference light can be observed more finely within one period of the intensity change of interference light, so that wavelength determination with higher accuracy can be performed.

  In the present embodiment, the type of the light receiving element is not particularly limited, and wavelength detection can be performed using, for example, a photodetector or a CCD.

  FIG. 14 is a diagram illustrating another configuration example of the laser apparatus according to the present embodiment. Referring to FIGS. 14 and 1, laser device 50 </ b> A is different from laser device 50 in that it includes a wavelength determination device 20 </ b> A instead of wavelength determination device 20. The wavelength determination device 20A is different from the wavelength determination device 20 in that it includes an optical element 42A instead of the optical element 22.

  The reflectance of the back surface 2EA of the optical element 42A is lower than that of the back surface 2E of the optical element 22. Laser light from the external resonator type semiconductor laser 10 is directly incident on the optical element 42A, and part of the laser light is transmitted through the back surface 2EA to become light LT (emitted light).

  FIG. 15 is a diagram illustrating another example of the optical element of the present embodiment. Referring to FIG. 15, the optical element 42 </ b> B includes a reflecting portion 12 </ b> C in which the surfaces 2 </ b> A to 2 </ b> D are arranged in a two-dimensional direction. “Y direction” and “X direction” shown in FIG. 15 are the same directions as “Y direction” and “X direction” shown in FIG. 1, respectively. In the optical element 22, the surfaces 2A to 2D are arranged stepwise along one direction. In the optical element 42, interference light having different phases is generated in two dimensions. In the present embodiment, the areas and shapes of the surfaces 2A to 2D are arbitrary, and the surfaces 2A to 2D are arranged so that the surfaces 2A to 2D form an arbitrary two-dimensional shape. Thereby, the freedom degree of arrangement | positioning of a light receiving element can be raised. Further, the tolerance of the arrangement position of the light receiving elements can be increased.

  In the present embodiment, an example is shown in which the wavelength determination device is used for wavelength determination of an external resonator type semiconductor laser. However, the wavelength determination device of the present invention is not limited to the one used for wavelength determination of an external resonator type semiconductor laser, and can be used for wavelength determination of a laser light source having a single oscillation mode and a multi oscillation mode as oscillation modes. is there. Examples of such a laser light source include a semiconductor laser having a diffraction grating formed in an optical waveguide, such as a DFB (Distributed Feed Back) laser and a DBR (Distributed Bragg Reflector) laser.

  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 which shows schematic structure of the laser apparatus 50 by embodiment of this invention. It is a figure which shows the structure of the external resonator type semiconductor laser 10 of FIG. It is a figure which shows the structure of the optical element 22 of FIG. 1 in detail. FIG. 5 is a diagram for explaining the optical path difference of a laser beam in the optical element 22 in more detail. 6 is a diagram illustrating a method for forming a stepped portion in the optical element 22. FIG. It is a figure explaining the structure of the optical element 22, and the structure of a comparative example. It is a figure which shows the relationship between the optical path difference (delta) of two laser beams, and the intensity | strength of interference light when two laser beams interfere with the optical element. 2 is a diagram for explaining an oscillation spectrum of an external resonator type semiconductor laser 10. FIG. It is a figure which shows the intensity | strength change of the interference light at the time of the single mode oscillation of an external resonator type semiconductor laser, and the intensity | strength of the light ray reflected on the photodetector 23. It is a figure which shows the intensity | strength change of the interference light at the time of multimode oscillation of an external resonator type semiconductor laser, and the intensity | strength of the light ray which reflects on the photodetector 23. It is a figure which shows another example of a structure of the optical element 22 and the photodetector 23. FIG. It is a figure which shows another example of a structure of the optical element 22 and the photodetector 23. FIG. FIG. 13 is a diagram showing a relationship between an optical path difference δ and the intensity of interference light when the optical element 32A shown in FIG. 12 is used. It is a figure explaining the other structural example of the laser apparatus of this Embodiment. It is a figure explaining the other example of the optical element of this Embodiment. It is a figure explaining the structure of the wavelength determination apparatus disclosed by Unexamined-Japanese-Patent No. 2005-340783. It is a figure explaining the structure of the external resonator type semiconductor laser 110 shown in FIG. It is a figure which shows the relationship between the power and wavelength of the laser beam output from the external resonator type semiconductor laser 110 of FIG. It is a figure explaining the arrangement | positioning of the photodetector 123 in detail. It is a figure explaining the structure of the reflection means disclosed by Unexamined-Japanese-Patent No. 2006-10499.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1,101 Laser diode, 2,102 Collimating lens, 2A-2D surface, 2E, 2EA back surface, 2F1-2F3 Stepped part, 3,103 Diffraction grating, 10,110 External cavity type semiconductor laser, 12, 12A-12C Reflection Part, 20, 20A, 120 wavelength determination device, 21 half mirror, 22, 32, 32A, 42, 42A, 42B optical element, 22A-22C thin film, 22E-22G plate member, 23, 33, 33A, 123 light detection 23A-23D, 123D, 123E Light receiving element, 24 wavelength determination unit, 25 parallel plate, 30, 130 laser control unit, 50, 50A, 150 laser device, 121 beam splitter, 122 optical wedge, 140 glass, AD , A1, A2 rays, L0 0th order light, L1a, L1b, L1 1-order light, Lp0, T1-T4 distance, δ, δ1~δ4, δt optical path difference.

Claims (12)

An optical element that includes first and second reflecting portions that reflect incident light, and that causes reflected light from the first reflecting portion and reflected light from the second reflecting portion to interfere to generate interference light. Prepared,
The first reflection unit has a plurality of reflection surfaces with different distances from the second reflection unit,
A photodetector for detecting the interference light;
A wavelength determination apparatus, further comprising: a determination unit that determines the wavelength of the incident light based on a detection result of the photodetector. The second reflecting portion is formed in a planar shape,
The wavelength determination device according to claim 1, wherein each of the plurality of reflection surfaces is parallel to the second reflection unit. The optical element includes a substrate having two parallel surfaces;
The wavelength determination apparatus according to claim 2, wherein the first reflection unit is formed by overlapping a plurality of flat plates on one of the two surfaces.   The wavelength determination apparatus according to claim 2, wherein the first reflection unit is formed by etching a substrate having two parallel surfaces. The optical element includes a substrate having two parallel surfaces;
The wavelength determination device according to claim 2, wherein the first reflecting portion is formed by laminating a plurality of thin films on one of the two surfaces. When viewed from a direction perpendicular to the plurality of reflection surfaces, the plurality of reflection surfaces form an arbitrary two-dimensional shape,
The photodetector is
The wavelength determination apparatus according to claim 1, further comprising a plurality of light receiving elements that are respectively provided corresponding to the plurality of reflection surfaces and receive reflected light from the corresponding reflection surfaces.   When m is an integer, the optical path difference between the first reflecting surface of the plurality of reflecting surfaces and the second reflecting portion is the second reflecting surface of the plurality of reflecting surfaces and the second reflecting surface. The wavelength determination device according to claim 1, wherein the wavelength determination device is different from the optical path difference with the reflection unit by m / 4 times the wavelength of the incident light. The plurality of reflective surfaces include first to fourth reflective surfaces;
The photodetector is
Including first to fourth light receiving elements respectively provided corresponding to the first to fourth reflecting surfaces and receiving reflected light from the corresponding reflecting surfaces;
The determination unit determines a wavelength of the incident light based on a signal having the highest intensity among first to fourth signals output from the first to fourth light receiving elements, respectively. The wavelength determination apparatus described in 1. The photodetector is
A plurality of light receiving elements that are respectively provided corresponding to the plurality of reflecting surfaces and receive reflected light from the corresponding reflecting surfaces;
The determination unit determines the wavelength of the incident light based on a difference between a signal having the highest intensity among a plurality of signals output from the plurality of light receiving elements and a half value of a peak value of the intensity of the interference light. The wavelength determination device according to claim 7, wherein:   The wavelength determination device according to claim 9, wherein the number of the plurality of reflection surfaces is 2 or 3. The wavelength determination device according to any one of claims 1 to 10,
And a laser light source for generating the incident light.   The laser apparatus according to claim 11, wherein the laser light source is an external cavity semiconductor laser.

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