JP2007178307A - Wavelength detector of laser light, and laser device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wavelength detector of laser light capable of detecting correctly the wavelength of the laser light. <P>SOLUTION: An optical wedge 16 is provided in an optical path where the laser light passes, and an interference fringe generated by the optical wedge 16 is received by a two-split detector 17a and a two-split detector 17b. Assuming that a refractive index of the optical wedge 16 is n, the width of the detector is a, a separation distance between the two sets of the two-split detectors is b, a light beam incident angle is ϕ, the shortest wavelength is λ, the longest wavelength is λ', the beam diameter of the light beam is W, and m is an integer, an equality: θ≤λ*sinϕ/(4*n*a) is established in the range of 0.1*(λ'*λ)/((2*n*(λ'-λ)))≤d≤W/(4*tan(Arcsin((sinϕ)/n))). <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a laser beam wavelength detector and a laser apparatus.

  Hologram recording / reproducing apparatus (hologram recording apparatus or hologram reproducing apparatus and hologram recording / reproducing apparatus) for recording information on a hologram recording medium (hologram memory) and reproducing information recorded from the hologram recording medium as a next-generation recording / reproducing apparatus ) Is under development.

  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, an external resonator type laser apparatus may be used since a laser light source close to a single mode is desirable. For example, as shown in FIG. 14, a laser diode (LD) 101 that oscillates in a multimode is irradiated as parallel light by a lens 103 to a grating 102, and the grating 102 is centered on a fulcrum 105 by a screw 104. Rotate, adjust the relative angle between the grating 102 and the laser diode 101, select a specific wavelength equal to the laser wavelength of the primary light diffracted by the grating 102 and returned to the laser diode 101, and oscillate. As a so-called external resonance type laser device for obtaining zero-order light, a so-called Littrow type laser device has also been proposed (for example, see Non-Patent Document 1). The primary light that does not return to the laser diode 101 illustrated in FIG. 14 is a laser beam having a wavelength different from the specific wavelength described above.
Tomiji.Tanaka, et al. “Littrow-type blue laser for holographic data storage”, Technical digest of Optical Data Storage 2004 p311

  In the external resonance type laser device represented by the above-described Littrow type laser device, although the wavelength of the laser beam can be changed, the oscillation mode of the light beam used for recording / reproducing is suitable for recording / reproducing with respect to the hologram recording medium. There are mode hop areas and mode hop areas that are not suitable for recording and reproduction. Therefore, in hologram recording / reproduction, it is confirmed by detecting the wavelength of the laser beam that the laser device emits a light beam in a mode suitable for recording / reproduction, and recording data on the hologram recording medium and Although recording data from the hologram recording medium has to be reproduced, it has been difficult to accurately detect the wavelength of the laser beam.

  An object of the present invention is to solve the above-described problems and to provide a laser beam wavelength detector capable of correctly detecting the wavelength of the laser beam and a laser device using such a laser beam detector.

  The wavelength detector of the laser beam of the present invention is a wavelength detector of a laser beam arranged in an optical path through which the laser beam passes, and an optical wedge that reflects the laser beam on each of the opposing front and back surfaces, A wavelength detection detector comprising two sets of two-divided detectors each having a light-receiving surface divided into two in the direction in which the light intensity of the interference fringes generated by the optical wedge changes, the refractive index of the optical wedge N, the width of each detector constituting the two-divided detector is a, the distance between the two sets of two-divided detectors is b, the angle of incidence of the light beam with respect to the optical wedge is φ, In the wavelength range of the light beam to be detected, λ is the shortest wavelength, λ ′ is the longest wavelength, W is the beam diameter of the light beam, and 0 M is a positive integer, and the thickness d between the front surface and the back surface of the optical wedge and the angle θ between the front surface and the back surface are each 0.1 * (λ ′ * λ) / ((2 * n * (λ'-λ)) ≦ d ≦ W / (4 * tan (Arcsin ((sinφ) / n)), where θ ≦ λ * sinφ / (4 * n * a ), The optical wedge is formed.

  The laser light wavelength detector according to the present invention is a laser light wavelength detector disposed in an optical path through which the laser light passes in order to detect the wavelength of the laser light, and is provided on each of the opposing front and back surfaces. An optical wedge that reflects laser light, and a wavelength detection detector that includes two sets of two-divided detectors each having a light-receiving surface that is divided into two in the direction in which the light intensity of interference fringes generated by the optical wedge changes. Prepare. The refractive index of the optical wedge is n, the width of each detector constituting the two-divided detector is a, the separation distance between the two sets of two-divided detectors is b, and the angle of incidence of the light beam with respect to the optical wedge is φ. In the wavelength range of the detected light beam, λ is the shortest wavelength, λ ′ is the longest wavelength, W is the beam diameter of the light beam, m is a positive integer including 0, and 0.1 * (λ ′ * λ) / ((2 * n * (λ'-λ)) ≤d≤W / (4 * tan (Arcsin ((sinφ) / n)) The thickness between the front and back of the optical wedge d is determined, and the angle θ between the front surface and the back surface is determined so that θ ≦ λ * sinφ / (4 * n * a), and the wavelength of the laser beam can be accurately detected.

  The laser apparatus of the present invention is a laser apparatus used in a hologram recording / reproducing apparatus for recording recording data on a hologram recording medium or reproducing the recording data recorded on the hologram recording medium, and a laser that oscillates in a multimode. A grating that returns the first-order diffracted light of the laser light emitted from the laser to the laser, a mirror that reflects the zero-order light from the grating in a predetermined direction, and a grating surface that is a light beam irradiation surface of the grating And a rotation axis arranged to rotate around the intersection of the lines where the extension lines of the grating surface and the mirror surface intersect, while maintaining a constant angle between the mirror surface and the mirror surface that is the light beam irradiation surface of the mirror And a wavelength detector for laser light arranged in an optical path through which the zero-order light from the mirror passes. The wavelength detector includes an optical wedge that reflects the laser light on each of the opposing front and back surfaces, and a light receiving surface that is divided into two in the direction in which the light intensity of interference fringes generated by the optical wedge changes. A wavelength detection detector comprising two sets of two-divided detectors, wherein the refractive index of the optical wedge is n, the width of each detector constituting the two-divided detector is a, and the two sets of two-divided detectors The distance between the detectors is b, the incident angle that is the angle of incidence of the light beam with respect to the optical wedge is φ, the shortest wavelength in the wavelength range of the light beam to be detected is λ, the most A long wavelength is λ ′, a beam diameter of the light beam is W, a positive integer including 0 is m, and the surface of the optical wedge is Each of the thickness d between the back surface and the angle θ between the front surface and the back surface is 0.1 * (λ ′ * λ) / ((2 * n * (λ′−λ)) ≦ d ≦ W / (4 * tan (Arcsin ((sinφ) / n)) range, wherein the optical wedge is formed such that θ ≦ λ * sinφ / (4 * n * a). To do.

  The laser apparatus according to the present invention is a laser apparatus used in a hologram recording / reproducing apparatus for recording recording data on a hologram recording medium or reproducing recorded data recorded on a hologram recording medium, the laser, a grating, and a mirror A rotation axis and a wavelength detector of the laser beam, the laser oscillates in multimode, the grating returns the first-order diffracted light of the laser beam emitted from the laser to the laser, and the mirror receives the zeroth-order from the grating Reflects light in a predetermined direction, and the rotation axis keeps the angle between the grating surface, which is the light beam irradiation surface of the grating, and the mirror surface, which is the light beam irradiation surface of the mirror, while maintaining a constant angle between the grating surface and the mirror surface. It is arranged to rotate around the intersection of the lines where the extension lines intersect, and it can rotate around the rotation axis. Flip was a laser beam having a wavelength can be emitted to a certain direction. The wavelength detector of the laser beam is arranged in the optical path through which the zero-order light from the mirror passes. The wavelength detector of the laser beam includes an optical wedge that reflects the laser beam on each of the opposing front and back surfaces, and a light receiving surface that is divided into two in the direction in which the light intensity of the interference fringes generated by the optical wedge changes A wavelength detection detector having two sets of two-divided detectors. The refractive index of the optical wedge is n, the width of each detector constituting the two-divided detector is a, the separation distance between the two sets of two-divided detectors is b, and the angle of incidence of the light beam with respect to the optical wedge is φ. In the wavelength range of the detected light beam, λ is the shortest wavelength, λ ′ is the longest wavelength, W is the beam diameter of the light beam, m is a positive integer including 0, and 0.1 * (λ ′ * λ) / ((2 * n * (λ'-λ)) ≤d≤W / (4 * tan (Arcsin ((sinφ) / n)) The thickness between the front and back of the optical wedge d is determined, and the angle θ between the front surface and the back surface is determined so that θ ≦ λ * sinφ / (4 * n * a), and the wavelength of the laser beam can be accurately detected.

  ADVANTAGE OF THE INVENTION According to this invention, the laser beam wavelength detector which can detect the wavelength of a laser beam correctly, and the laser apparatus using such a laser beam detector can be provided.

(Laser device and laser detector)
With reference to FIG. 1, the laser beam wavelength detector of embodiment and the laser apparatus 10 using the same are demonstrated. The laser device 10 includes a multi-mode laser diode 11, a collimating lens 12, a grating 13, a mirror 14, an optical wedge 16, a wavelength detection detector 17, and a calculator 18. Here, the laser light wavelength detector includes an optical wedge 16, a wavelength detection detector 17, and an arithmetic unit 18.

  The mirror 14 and the grating 13 are fixed, and are rotatable about a rotation shaft 27 in a direction indicated by an arrow in the drawing. Here, the rotating shaft 27 is provided at the intersection of the lines where the extension lines of the grating surface 13a and the mirror surface 14a intersect. Thus, the mirror 14 and the grating 13 can be rotated around the rotation axis 27 while keeping the angle formed by the grating surface 13a and the mirror surface 14a constant (for example, an angle of 90 °). .

  First, an outline of the functions of the optical wedge 16, the wavelength detection detector 17, and the computing unit 18 constituting the laser beam wavelength detector will be described. The light beam 26 obtained by reflecting the light beam 24 from the mirror 14 on both the front surface 16a and the back surface 16b of the optical wedge 16 is guided to the light receiving surfaces of two or more sets of photodetectors 17, and is then applied to the surface of the light receiving surface. The generated interference fringe motion is detected as an electrical signal. The property of the light beam of the 0th-order light 25 finally emitted from the laser device 10 can be known by the laser light wavelength detector acting in this way. That is, by detecting the wavelength of the laser beam, it is possible to detect whether the mode hop region is not suitable for recording / reproduction or the mode hop region suitable for recording / reproduction.

  Next, an outline of the operation of the laser apparatus 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 grating 13 into first-order diffracted light that is diffracted in different directions for each wavelength. To emit. The grating 13 is formed as a reflection type diffraction grating and receives the multimode light beam 20 from the laser diode 11 and diffracts it in different directions for each wavelength. There is also primary light returning to the laser diode 11, and for the wavelength corresponding to the primary light returning to the laser diode 11, the grating 13 and the laser diode 11 constitute a resonator. The laser diode 11 oscillates close to a single mode at that wavelength.

  Further, most of the light is reflected by the grating 13 and emitted as 0th-order light 22 and finally as 0th-order light 25. Therefore, the light beam emitted as the 0th-order light 25 is used as various devices such as holograms. It can be used for a recording / reproducing apparatus.

  In such a configuration, if the distance and angle of the grating 13 to the laser diode 11 are changed, the wavelength of the laser light can be changed. However, if this is done, the emission direction in the 0th-order light direction is also changed and applied to various devices. It is difficult to do. Therefore, as described above, the laser device 10 of the present embodiment employs the mirror 14 that moves in cooperation with the grating 13. The 0th-order light 22 from the grating 13 is also reflected by the mirror 14, so that the emission direction can be changed as the 0th-order light 24. The zero-order light 24 further passes through the optical wedge 16 and finally the zero-order light 25 is obtained.

  Here, the condition that the direction of the emitted light of the 0th-order light 25 is always constant is that the rotating shaft 27 is arranged at the intersection of the extended surface of the mirror surface 14a forming the mirror 14 and the grating surface 13a forming the grating 13. It is. That is, by rotating while maintaining the relative relationship between the mirror 14 and the grating 13 around the rotation axis 27, it is possible to function as a so-called tunable laser that resonates at a desired frequency.

  With reference to FIG. 2, the operation of the optical wedge 16 constituting the laser beam wavelength detector will be described. In FIG. 2, a line denoted by reference numeral 24 a indicates one light beam 24, a line denoted by reference numeral 25 a indicates one light beam 25, and a line denoted by reference numeral 26 a indicates the light beam 26. Is a single ray. On the light receiving surface 15 (virtual surface) of the light beam 26, interference fringes (fringe) are caused by interference between the light beam reflected by the front surface 16 a of the optical wedge 16 and the light beam reflected by the back surface 16 b of the optical wedge 16. (Interference fringes are schematically shown in FIG. 9).

Here, on the same plane as the light receiving surface, a photodetector 17 having two two-divided photodetectors of two two-divided photodetectors (PD) 17a and two-divided photodetectors (PD) 17b is provided. The two-divided photodetector 17a has a photodetector 17a ₁ , a photodetector 17a ₂ , and the two-divided photodetector 17b has a photodetector 17b ₁ and a photodetector 17b ₂ , and these four photodetectors are distributed with interference fringes. It is arranged in a row in the direction.

The photodetector 17a ₁ generates an electrical signal V17a ₁ , the photodetector 17a ₂ generates an electrical signal V17a ₂ , the photodetector 17b ₁ generates an electrical signal V17b ₁ , and the photodetector 17b ₂ generates an electrical signal V17b ₂ . The calculator 18 calculates and obtains a first normalized difference signal S17adn and a second normalized difference signal S17bdn shown in the following equations 1 and 2 based on the electrical signals from these photodetectors.

S17adn = (V17a ₁ -V17a ₂ ) / (V17a ₁ + V17a ₂ ) (Formula 1)
S17bdn = (V17b ₁ -V17b ₂ ) / (V17b ₁ + V17b ₂ ) ... (Formula 2)

  By performing the calculations shown in Equations 1 and 2, the first normalized difference signal S17adn and the second normalized difference signal S17bdn can be made into signals corresponding only to the distribution of interference fringes independent of the amount of light. Here, the operations of Expressions 1 and 2 are performed by the calculator 18. The computing unit 18 further obtains the computation results of Equations 1 and 2, and either the first normalized difference signal S17adn or the second normalized difference signal S17bdn, one of the signals with better quality, or A signal obtained by adding both the first normalized difference signal S17adn and the second normalized difference signal S17bdn is output as an output signal from the wavelength detector of the laser beam.

  Here, FIG. 3 shows the relationship between the diode current Id flowing through the laser diode 11 in the laser device 10 and the oscillation wavelength (spectrum). As can be seen from FIG. 3, one or more wavelengths correspond to one specific diode current Id. Then, the shape and position of the fringe are specified corresponding to the one or more wavelengths.

  FIG. 4 shows a diode current Id and a normalized difference signal (first normalized difference signal S17adn and second normalized difference signal S17bdn are hereinafter collectively referred to as a normalized difference signal) caused by a change in fringe shape. This shows the relationship. Here, the region marked with a in FIG. 4 is a region of mode hops (hereinafter referred to as LD mode hops) by an LD (internal resonator), and the width of the wavelength distribution of a plurality of wavelengths (hereinafter referred to as wavelength). A region having a wavelength width of 40 pm (picometer) as a width) and a symbol b is a region of a mode hop by an external resonator (hereinafter referred to as a mode hop of an external resonator), and is generally 4 pm. Or a wavelength width of 7 pm. A region denoted by reference sign c is a single mode region.

  Thus, the normalized difference signal has a characteristic waveform. Therefore, when only the mode hop of the LD is to be avoided, the value of the diode current Id is set so as not to oscillate in the vicinity where the normalized difference signal changes rapidly. The laser diode 11 is oscillated.

  Further, when oscillating in the complete single mode, a diode current Id that oscillates in the region denoted by reference character c in FIG. Whether the laser device 10 is used in the mode hop region of the external resonator or the laser device 10 in the single mode region depends on the required degree as the characteristics of hologram recording / reproduction. For example, when the optical path difference between the signal light and the reference light is close to zero, sufficient recording / reproduction characteristics can be obtained only by avoiding only the LD mode hop.

  In this way, by detecting the standardized difference signal, the laser light obtained from the laser device 10 and used for recording / reproducing is a single mode region, an external resonator mode hop region, or an LD mode hop. In principle, it is possible to detect which of the regions. However, the actual situation is that the waveform of the normalized difference signal shown in FIG. 4 varies greatly depending on the relationship between the position where the two-divided photodetector 17a or the two-divided photodetector 17b is arranged and the fringe.

With reference to FIG. 5, how the normalized difference signal changes according to the relationship between the position where the two-divided photodetector 17b is arranged and the fringe when the diode current Id is changed will be described in detail. First, when a portion having a large change in the density of the fringe comes between the photo detector 17b ₁ and the photo detector 17b ₂ constituting the two-divided photo detector 17b (see FIG. 2), it is difficult to detect a minute movement of the fringe, The normalized difference signal has a waveform as shown in FIG.

On the other hand, according to the relationship between the position where the two-divided photo detector 17a is arranged and the fringe, a portion where the change in the density of the fringe is small is placed between the photo detector 17a ₁ and the photo detector 17a ₂ constituting the two-divided photo detector 17a ( In FIG. 2, it is easy to detect a minute movement of the fringe, and the normalized difference signal has a waveform as shown in FIG.

  In the fixed wavelength type external resonator laser capable of substantially fixing the relationship between the position where the two-divided photodetector is disposed and the fringe, the amplitude change of the normalized difference signal is increased, that is, (A of FIG. The position where the two-divided photodetectors are arranged can be adjusted so as to obtain a waveform as shown in (2), so that a normalized difference signal having a good S / N (signal to noise ratio) can be detected. However, in a wavelength tunable external resonator laser such as the laser device 10 of the present embodiment, the relationship between the position where the two-divided photodetector is disposed and the fringe changes depending on the wavelength of the laser beam, so it is always good. Therefore, it is not always possible to detect a normalized difference signal having an S / N (signal-to-noise ratio).

  Therefore, in this embodiment, the S / N of the normalized difference signal is improved by using two two-divided photo detectors 17a and two-divided photo detectors 17b. That is, by using two two-divided photodetectors 17a and 17b having different positional relationships with respect to the fringe, a signal having a better S / N is used, or a combination of both signals is used to reduce the wavelength of the laser beam. Trying to detect.

  Below, the shape of the optical wedge 16 and the relationship between the arrangement of the two-divided photo detector 17a and the two-divided photo detector 17b will be considered.

  First, the thickness d (hereinafter abbreviated as thickness d) of the optical wedge 16 will be examined. A change in the optical path difference is caused by the thickness d of the optical wedge 16, and the change in the optical path difference causes a fringe. The relative positional relationship between the fringe and the two-divided photodetector changes as the wavelength of the light beam changes. Therefore, the thickness d affects the amount of fringe movement that moves according to the laser wavelength. Specifically, when the thickness d is reduced, the amount of movement of the fringe is reduced, the S / N of the normalized difference signal is deteriorated, and there is a problem that it is difficult to recognize the mode hop.

(About optical wedge thickness d and angle θ)
In the following, description will be made in order, and a desirable range between the thickness d of the optical wedge and the angle θ will be quantitatively determined.

  First, a relationship between the thickness d (see FIG. 6) and the amount of movement of the fringe is obtained. First, since the fringe appears at an integral multiple of the optical path difference, the optical path difference is important, but the optical path difference D is obtained by the following equation (3). n is the refractive index (see FIG. 6). In the following expression, the symbol * represents integration, and / represents division. Here, the thickness d is slightly different in each part of the optical wedge according to the angle θ (see FIG. 6) of the optical wedge, which will be described later, but in each of the following expressions, the thickness d is the transmission It is assumed that the thickness is at the center of the light beam.

  D = d * 2 * n (Formula 3)

  In obtaining Equation 3, it was assumed that light traveling back and forth in the optical wedge would go through the same optical path. The fringe can be changed from a zero optical path difference to a first optical path difference equal to the wavelength, and a second fringe double the wavelength optical path difference. The movement of the fringe is a phenomenon that occurs when the fringe number changes (the fringe passes) by changing the wavelength width carved for a certain optical path difference, and the two-divided photodetector detects the passage of the fringe. When this optical path difference D is expressed in units of laser wavelength, it is expressed by Equation 4. Where λ is the laser wavelength.

C = D / λ (Formula 4)
Since C in Equation 4 is a number representing the optical path difference in fringe units, it is represented by a real number including a decimal point. The number I of fringes that pass due to the change in wavelength is expressed by Equation 5. Here, λ is the shortest wavelength in the wavelength detection range of the laser beam, and λ ′ is the longest wavelength in the wavelength detection range of the laser beam.

I = D / λ−D / λ ′ (Formula 5)
Therefore, Equation 6 is obtained by combining Equations 3 and 5.

I = d * 2 * n * (λ′−λ) / (λ ′ * λ) (Formula 6)
Here, since the fringe passage amount in the chip mode needs to be at least 1/10 of the measurement noise, Equation 7 can be obtained.

  0.1 ≦ d * 2 * n * (λ′−λ) / (λ ′ * λ) (Expression 7)

When formula 7 is arranged, the condition of thickness d is expressed by formula 8.
d ≧ 0.1 * (λ ′ * λ) / ((2 * n * (λ′−λ)) (Expression 8)

  However, as the thickness d is increased, the amount of movement of the fringe can be increased, but the light reflected on both sides of the optical wedge does not overlap and interference fringes do not occur. That is, as shown in FIG. 7, each of a light beam reflected on the surface of the optical wedge (front surface reflected light) and a light beam reflected on the back surface of the optical wedge (back surface reflected light) having different optical paths is so-called Since it is a Gaussian beam and the light intensity becomes weaker at the periphery, when the wedge thickness d increases, it becomes difficult to cause the light beam to enter the optical wedge with an inclination of about 45 degrees, and to produce fringe. It becomes necessary to further tilt the incident angle of the light beam with respect to the wedge.

  When the incident angle of the light beam is further inclined with respect to the optical wedge, the deviation amount g of the light beam reflected on both surfaces of the optical wedge is expressed by Expression 9. Here, g and φ are as shown in FIG.

g = 2 * d * tan (Arcsin ((sinφ) / n)) (Formula 9)
Since the angle of both surfaces of the optical wedge is very small, Equation 9 is calculated based on Snell's law on the assumption that the optical wedge is a parallel plate.

  Here, assuming that the beam diameter of the light beam is W and the amount of deviation of the light beam is less than or equal to the radius of the light beam, the thickness d can be obtained from the following Equation 10. Note that e≈2.718, and the beam diameter W was 1 / (e * e) of the center intensity of the light beam. Generally, the value of the beam diameter W of a light beam emitted from a laser diode and converted into parallel light is about 3.5 mm. The reason why the amount of deviation of the light beam is equal to or smaller than the radius of the light beam is to ensure a sufficient S / N of the normalized difference signal so that a clear interference fringe is generated.

d ≦ W / (4 * tan (Arcsin ((sinφ) / n)) (Expression 10)
From Expression 8 and Expression 10, Expression 11 that defines the thickness d of the optical wedge is derived.

  0.1 * (λ ′ * λ) / ((2 * n * (λ′−λ)) ≦ d ≦ W / (4 * tan (Arcsin ((sinφ) / n))) (Equation 11)

  Next, the angle θ of the optical wedge (hereinafter abbreviated as the angle θ) is examined. The angle θ is related to the fringe pitch p, and the expressions 12 and 13 are collectively obtained by the expression 14. FIG. 9 schematically shows the relationship between the angle θ and the fringe pitch p.

  q = λ / (2 * n * tanθ) (Formula 12)

  p = q * sinφ ... (Formula 13)

  p = λ * sinφ / (2 * n * θ) ... (Formula 14)

  In obtaining the equation 14, the angle θ is very small, and thus approximated to tan θ = θ. Here, an appropriate fringe pitch p is defined by the width and interval of the two-divided photodetector. FIG. 10 shows the relationship between the width a of the photodetector and the fringe pitch p. In the arrangement of two two-divided photodetectors, when the angle θ is increased, the fringe pitch p is decreased. Can not.

  FIG. 11 shows the amplitude of the normalized difference signal output from the two-divided photodetector with respect to the ratio between the fringe pitch p and the width a. Here, the normalized difference signal becomes 1 in an ideal state where the entire light quantity is irradiated to one of the photodetectors constituting the two-divided photodetector and no light enters the other photodetector.

  From the graph shown in FIG. 11, the decrease in the amplitude of the normalized difference signal is suppressed to 30% or less in the case of Expression 15 in which the fringe pitch p is greater than or equal to twice the width a. From this condition, Equation 16 can be obtained.

2a ≦ p (Equation 15)
θ ≦ λ * sinφ / (4 * n * a) (16)

  Here, the decrease in the amplitude of the normalized difference signal is set to 30% or less. When the decrease in the amplitude is more than this level, it is difficult to accurately detect the laser wavelength from the viewpoint of S / N. This is because the conclusion was obtained as a result of the experiment.

  On the other hand, as the angle θ is decreased, the fringe pitch p is increased. However, if the fringe pitch p is too large with respect to the width a, the meaning of using two sets of two-divided photodetectors decreases. This is because the two sets of standardized difference signals are similar and are no different from using one set of two-divided photodetectors.

  Further, the separation distance b (see FIG. 10) of the two sets of the two-divided photodetectors means that two sets are provided unless the first standardized difference signal and the second standardized difference signal are arranged in the same or opposite phase. Will fade. This is because in such an arrangement, the information obtained from the two sets of two-divided photodetectors is substantially the same. Two or two periods at positions shifted by (1/2 * m + 1/4) periods, where m is a positive integer including 0, with respect to the period of the fringe pitch p. Ideally, the split photodetectors should be spaced apart.

However, in reality, it is difficult to set the separation distance b of the two sets of the two-divided photodetectors strictly as a target value, and the two-divided photodetectors are shifted in the direction indicated by the arrows as shown in FIG. In the example shown in FIG. 12, Expressions 17 and 18 can be obtained assuming that such a deviation from the ideal state can be allowed 10% for each of the two-divided photodetectors.
(1 / 4-0.1) * p ≦ b ≦ (1/4 + 0.1) * p (Expression 17)
(3 / 4-0.1) * p ≦ b ≦ (3/4 + 0.1) * p (Equation 18)

  The reason why the tolerance is set to 10% is that this degree of misalignment occurs when the accuracy of mounting the two-divided photodetector is taken into consideration. By generalizing Equations 17 and 18, Equation 19 is obtained.

(0.5 * m + 0.15) * p ≦ b ≦ (0.5 * m + 0.35) * p (m = 0.1.2 ...) (Equation 19)
From Expression 14 and Expression 19, Expression 20 is obtained for the angle θ.

  (0.5m + 0.15) * λ * sinφ / 2 * n * b ≦ θ ≦ (0.5m + 0.35) * λ * sinφ / 2 * n * b (Equation 20)

  Of the above equations, by using an optical wedge that satisfies all of Equation 11, Equation 16, and Equation 20, at least one of the first normalized difference signal and the second normalized difference signal generated by the two sets of two-part photodetectors. Therefore, a normalized difference signal having a good S / N representing a fringe movement signal can be obtained.

0.1 * (λ ′ * λ) / ((2 * n * (λ′−λ)) ≦ d ≦ W / (4 * tan (Arcsin ((sinφ) / n))) (Equation 11)
θ ≦ λ * sinφ / (4 * n * a) (16)
(0.5m + 0.15) * λ * sinφ / 2 * n * b ≦ θ ≦ (0.5m + 0.35) * λ * sinφ / 2 * n * b (Equation 20)

  Here, as described above, λ is the shortest wavelength of the light beam emitted from the external resonant laser device, λ ′ is the longest wavelength of the light beam emitted from the external resonant laser device, and n is The refractive index of the optical wedge, d is the thickness of the optical wedge, φ is the angle of incidence of the light beam on the optical wedge, θ is the angle of the optical wedge, a is the width of the photodetector, and b is divided into two sets of two The distance between the photodetectors, W is the diameter of the light beam, and m is a positive integer including zero.

(Specific numerical examples)
Specific examples will be shown using Equation 11, Equation 16, and Equation 20. The incident angle of the light beam on the optical wedge (optical wedge installation angle) φ is 45 ° (degrees), the photodetector width a is 60 μm (micrometer), the separation distance b between the two sets of two-part photodetectors is 320 μm, The refractive index n of the optical wedge formed by the above is 1.5, the shortest wavelength λ of the laser beam emitted from the external resonant laser device is 405 nm (nanometer), the longest wavelength λ ′ is 405.04 nm, Assuming that the diameter W is 3.5 mm (millimeters), this value is substituted into each of the above-described equations 11 and 16.

The thickness d is in the range of Formula 21 from Formula 11.
0.14 ≤ d ≤ 1.64 (mm) ... (Formula 21)
The angle θ can be expressed by Expression 22 from Expression 16.
θ ≦ 164 (Sec) (Equation 22)
In addition, from Equation 20, the angle θ is limited to the following range.
9 ≦ θ ≦ 22 (when m = 0),
40 ≦ θ ≦ 52 (when m = 1),
71 ≦ θ ≦ 83 (when m = 2),
102 ≦ θ ≦ 114 (when m = 3),
133 ≦ θ ≦ 145 (when m = 4)

  Therefore, the thickness d and the angle θ of the optical wedge in this example are within a square frame shown in FIG.

  The square frame in FIG. 13 is a range in the case where the light beam reflecting surface of the optical wedge and the light receiving surfaces of the two sets of two-divided photodetectors are parallel to each other. However, when the range of the thickness d is 0.14 mm or more and 1.64 mm or less and the angle θ is 164 Sec (seconds) or less, the light receiving surface of the two-divided photodetector, the light beam reflecting surface of the optical wedge, Are not parallel but can be tilted, and the square range shown in FIG. 13 can be further expanded in the vertical direction (angle θ direction) in FIG. 13 according to the tilt angle.

  According to the laser beam wavelength detector of the above-described embodiment, the wavelength of the laser beam can be accurately detected in the range from the predetermined shortest wavelength λ to the longest wavelength λ ′.

  According to the laser device of the above-described embodiment, the wavelength of the laser beam can be accurately detected. Therefore, in the hologram recording / reproducing device using the laser device having a laser diode that oscillates in multimode and an external resonator, hologram recording When recording data on the medium or reproducing the recording data recorded on the hologram recording medium, the laser light used for recording / reproducing is based on the detected wavelength of the laser light, and the laser light used for recording / reproduction is in a single mode region or external resonator. It is possible to detect the mode hop area or the LD mode hop area, manage the recording and reproduction, and perform highly reliable recording and reproduction.

  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 explaining the structure of the laser apparatus of embodiment. It is a figure explaining the effect | action of the optical wedge which comprises a laser beam wavelength detector. It is a figure which shows the relationship between a diode electric current and an oscillation wavelength (spectrum). It is a figure which shows the relationship between a diode electric current and a normalization difference signal. It is a figure which shows the relationship between a diode electric current and a normalization difference signal. It is a figure which shows the variable of each part of an optical wedge. It is a figure which shows the surface reflected light and back surface reflected light of an optical wedge. It is a figure which shows the deviation | shift amount of the light beam reflected on both surfaces of the optical wedge in the case of inclining the incident angle of a light beam. It is a figure which shows typically the relationship between the angle of an optical wedge, and a fringe pitch. It is a figure which shows the relationship between the width | variety of a photodetector, and a fringe pitch. It is a figure which shows the normalization difference signal with respect to the ratio of fringe pitch and width. It is a figure which shows the shift | offset | difference direction of a 2 division | segmentation photodetector. It is a figure which shows the range of the thickness and angle of the optical wedge of an Example. It is a figure explaining the structure of the laser apparatus of background art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Laser apparatus, 11 Laser diode, 12 Collimating lens, 13 Grating, 13a Grating surface, 14 Mirror, 14a Mirror surface, 15 Light-receiving surface, 16 Optical wedge, 17 Wavelength detection detector, 17a, 17b Two-part photo detector, 17a _1, 17a _2, 17b _1, 17b ₂ photodetector, 18 computing unit, 27 rotating shaft

Claims (4)

A wavelength detector for laser light arranged in an optical path through which the laser light passes,
An optical wedge that reflects the laser beam on each of the opposing front and back surfaces;
A wavelength detection detector comprising two sets of two-divided detectors having a light-receiving surface divided into two in the direction in which the light intensity of the interference fringes generated by the optical wedge changes,
The refractive index of the optical wedge is n, the width of each detector constituting the two-divided detector is a, the separation distance between the two sets of the two-divided detectors is b, and the optical beam is separated from the optical wedge. The incident angle, which is the incident angle, is φ, the shortest wavelength in the wavelength range of the light beam where the wavelength is detected is λ, the longest wavelength is λ ′, the beam diameter of the light beam is a positive value including W and 0 Let m be an integer,
Each of the thickness d between the front and back surfaces of the optical wedge and the angle θ between the front and back surfaces is 0.1 * (λ ′ * λ) / ((2 * n * (λ '-λ)) ≦ d ≦ W / (4 * tan (Arcsin ((sinφ) / n))
A laser light wavelength detector, wherein the optical wedge is formed such that θ ≦ λ * sinφ / (4 * n * a).   The front and back surfaces of the optical wedge are arranged substantially parallel to the light receiving surfaces of the two sets of the two-divided detectors, and (0.5 m + 0.15) * λ * sinφ / 2 * n * b ≦ θ ≦ (0.5 m 2. The laser according to claim 1, wherein the optical wedge is formed so that an angle θ between the front surface and the back surface is included in a range of +0.35) * λ * sinφ / 2 * n * b. Light wavelength detector. A laser device having a wavelength detector for use in a hologram recording / reproducing apparatus for recording recording data on a hologram recording medium or reproducing the recording data recorded on the hologram recording medium,
A laser that oscillates in multimode;
A grating that returns the first-order diffracted light of the laser light emitted from the laser to the laser;
A mirror that reflects zero-order light from the grating in a predetermined direction;
While maintaining a constant angle between the grating surface that is the light beam irradiation surface of the grating and the mirror surface that is the light beam irradiation surface of the mirror, the intersection of the lines where the extension lines of the grating surface and the mirror surface intersect A rotation axis arranged to rotate as a center;
A wavelength detector of laser light arranged in an optical path through which the zero-order light from the mirror passes,
The wavelength detector is
An optical wedge that reflects the laser beam on each of the opposing front and back surfaces;
A wavelength detection detector comprising two sets of two-divided detectors having a light-receiving surface divided into two in the direction in which the light intensity of the interference fringes generated by the optical wedge changes,
The refractive index of the optical wedge is n, the width of each detector constituting the two-divided detector is a, the separation distance between the two sets of the two-divided detectors is b, and the optical beam is separated from the optical wedge. The incident angle, which is the incident angle, is φ, the shortest wavelength in the wavelength range of the light beam where the wavelength is detected is λ, the longest wavelength is λ ′, the beam diameter of the light beam is a positive value including W and 0 Let m be an integer,
Each of the thickness d between the front surface and the back surface of the optical wedge, and the angle θ formed by the front surface and the back surface,
0.1 * (λ '* λ) / ((2 * n * (λ'-λ)) ≤d≤W / (4 * tan (Arcsin ((sinφ) / n))
A laser apparatus, wherein the optical wedge is formed so that θ ≦ λ * sinφ / (4 * n * a). The front and back surfaces of the optical wedge are arranged substantially parallel to the light receiving surfaces of the two sets of the two-divided detectors, and (0.5 m + 0.15) * λ * sinφ / 2 * n * b ≦ θ ≦ (0.5 m The optical wedge is formed so that an angle θ between the front surface and the back surface is included in a range of +0.35) * λ * sinφ / 2 * n * b. Laser device.

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