Frequency shifter optical modulator and laser doppler measuring device

Abstract

To provide a frequency shifter optical modulator and a laser doppler measuring device, capable of easily achieving a size reduction, high accuracy and a high frequency.SOLUTION: A frequency shifter optical modulator 12 includes an oscillator 30 and a diffraction grating 34 including a plurality of grooves 32 arranged in parallel in a displacement direction of the oscillator 30. The diffraction grating 34 is provided in the oscillator 30. Providing the diffraction grating 34 in the oscillator 30 facilitates achievement of a size reduction and high accuracy of the frequency shifter optical modulator 12, and also facilitates correspondence to a high frequency region of a MHz band, that is, achievement of a high frequency. Thus, an effect based on a combination of the oscillator 30 and the diffraction grating 34 can be efficiently obtained.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a frequency shifter type optical modulator and a laser Doppler measurement device used for grasping the movement state of a moving object.

Background Art

[0002] As a prior art document of this type of frequency shifter type optical modulator and laser Doppler measurement device, there is Japanese Patent Application Laid-Open No. 2007-285898. This document describes obtaining the amount of frequency shift by using heterodyne interference. Specifically, in paragraph 0028 of the specification, it is described that "it is desirable to use a piezo element that has a property of deforming when a voltage, magnetism, etc. is applied and whose vibration frequency can be varied by changing the voltage. Further, the vibration frequency must be a triangular wave or a sawtooth wave whose waveform rise shows linearity. The optical Doppler shift caused by the incidence of laser light at the rising edge of the sawtooth wave applied voltage or at the rising and falling edges of the triangular wave applied voltage is utilized."

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in general, vibration elements such as crystal oscillators and Si oscillators utilize single vibration drive with an extremely high Q value, which is a value indicating the sharpness of oscillation, and there is a problem that the driving method described therein is not accurate and is not practical. Further, there is no description or suggestion regarding the realization of high-frequency operation, that is, the realization of high-frequency conversion to MHz band high-frequency vibration.

Means for Solving the Problems

[0005] To solve the above problems, the present invention provides a frequency shifter type optical modulator comprising a plate-shaped vibrator that repeats a mode in which crystals are distorted and vibrate in a direction along the plane by applying a potential, and a diffraction grating provided on the surface of the vibrator, the diffraction grating being formed by a plurality of linear grooves arranged periodically. BRIEF DESCRIPTION OF THE DRAWINGS

[0006]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

[0007] First, the present invention will be schematically described. A frequency shifter type optical modulator according to a first aspect of the present invention for solving the above problems includes a vibrator and a diffraction grating including a plurality of grooves arranged in parallel in the displacement direction of the vibrator, the diffraction grating being provided on the vibrator.

[0008] According to this aspect, by providing the diffraction grating on the vibrator, it becomes easier to achieve miniaturization and high precision of the frequency shifter type optical modulator. Also, it becomes easier to achieve correspondence to a high frequency region in the MHz band, that is, high frequency operation. Further, when obtaining frequency modulation, when vibrating in the grating vector direction, the diffracted light can obtain a larger modulation frequency. According to this aspect, since the extending direction of the plurality of grooves intersects with the direction of the vibration, the effect based on the combination of the vibrator and the diffraction grating can be efficiently obtained, and thus the high-frequency operation of the frequency shifter type optical modulator can be easily realized.

[0009] A second aspect of the present invention is characterized in that, in the frequency shifter type optical modulator according to the first aspect, the diffraction grating is a blazed diffraction grating.

[0010] According to this aspect, since the diffraction grating is a blazed diffraction grating, the light utilization efficiency can be increased, and thus the miniaturization and high SN of the frequency shifter type optical modulator can be easily realized.

[0011] A third aspect of the present invention is characterized in that, in the frequency shifter type optical modulator according to the first aspect or the second aspect, the vibrator is a crystal AT vibrator that performs thickness-shear vibration in a high-frequency region of the MHz band.

[0012] According to this aspect, since the vibrator is a crystal AT vibrator that performs thickness-shear vibration in a high-frequency region of the MHz band, the miniaturization, high-precision operation, and further high-frequency operation of the frequency shifter type optical modulator can be easily realized.

[0013] The laser Doppler measurement device according to the fourth aspect of the present invention includes a light source unit that outputs laser light of a predetermined wavelength, a polarization beam splitter, a plurality of λ / 4 plates, an analyzer, a light receiving element that converts light into an electrical signal, a frequency shifter type optical modulator described in any one of the first to third aspects, and a setting unit for the object to be measured. The polarization beam splitter, the λ / 4 plate, and the setting unit are arranged on the optical axis of the laser light output from the light source unit. The λ / 4 plate and the frequency shifter type optical modulator are arranged on the optical axis of the laser light reflected by the polarization beam splitter. An analyzer is arranged between the deflection beam splitter and the light receiving element. The Doppler shift light reflected from the object to be measured and the frequency shift light reflected by the frequency shifter type optical modulator are guided to the light receiving element through the respective λ / 4 plates, the polarization beam splitter, and the analyzer. This is the feature.

[0014] According to this aspect, as a laser Doppler measurement device, since it includes a frequency shifter type optical modulator described in any one of the first to third aspects, miniaturization, high precision, and further high frequency of the laser Doppler measurement device can be easily realized.

[0015] The fifth aspect according to the present invention is characterized in that, in the laser Doppler measurement device of the fourth aspect, the frequency shift light is ±1st order diffracted light.

[0016] According to this aspect, since the frequency shifter type optical modulator is arranged such that ±1st order diffracted light is used as the frequency shift light, miniaturization and high stability of the measurement device can be realized.

[0017] The sixth aspect according to the present invention is characterized in that, in the laser Doppler measurement device of the fourth aspect, the frequency shift light is diffracted light of any of ±2nd order or higher.

[0018] According to this aspect, since the frequency shifter type optical modulator is arranged such that any diffracted light of ± second order or higher is used as the frequency shifted light, miniaturization and high frequency operation of the measurement device can be achieved.

[0019] A seventh aspect of the present invention is the laser Doppler measurement device according to the fifth or sixth aspect, wherein the frequency shifter type optical modulator is arranged such that the angle formed by the incident direction of the laser light on the frequency shifter type optical modulator and the traveling direction of the frequency shifted light reflected from the frequency shifter type optical modulator is 180 degrees. Here, the "angle is 180 degrees" in the "arrangement such that the angle formed by the incident direction of the laser light on the frequency shifter type optical modulator and the traveling direction of the frequency shifted light reflected from the frequency shifter type optical modulator is 180 degrees" includes cases where it is performed using reflection by a mirror and cases where it is performed by inclining the surface formed by the diffraction grating without using a mirror.

[0020] According to this aspect, when performing the "angle is 180 degrees" using a mirror, the modulated light undergoes Doppler shift twice, enabling further high frequency modulation. Also, when performing the "angle is 180 degrees" without using a mirror, the mirror becomes unnecessary, enabling further miniaturization.

[0021] Subsequently, a frequency shifter type optical modulator and a laser Doppler measurement device according to an embodiment of the present invention will be described with reference to FIGS. 1 to 5. In each figure, the same parts are denoted by the same reference numerals, and individual descriptions for each figure are omitted.

[0022] [Embodiment] <Laser Doppler Measurement Device> Based on FIG. 1, the configuration of a laser Doppler measurement device 1 according to an embodiment of the present invention will be described. The laser Doppler measurement device 1 according to this embodiment includes a light source unit 2 that outputs laser light of a predetermined wavelength, a polarization beam splitter 4, two λ / 4 plates 6 and 8, an analyzer 9, a light receiving element 10 that converts light into an electrical signal, a frequency shifter type optical modulator 12, and a setting unit 16 for the object to be measured 14. On the optical axis 18 of the light output from the light source unit 2, the polarization beam splitter 4, one λ / 4 plate 6, and the setting unit 16 are arranged in this order. On the other hand, on the optical axis 20 of the light reflected by the polarization beam splitter 4, the λ / 4 plate 8 and the frequency shifter type optical modulator 12 are arranged in this order. The analyzer 9 is arranged between the polarization beam splitter 4 and the light receiving element 10. And the Doppler shift light 22 reflected from the object to be measured 14 and the frequency shift light 24 reflected by the frequency shifter type optical modulator 12 are configured to be guided to the light receiving element 10 through the respective λ / 4 plates 6 and 8, the polarization beam splitter 4, and the analyzer 9. Also, the polarization beam splitter may be used as an unpolarized beam splitter. In this case, the λ / 4 is unnecessary, the number of components is reduced, and the device can be made smaller. However, when the stability of interference deteriorates, it is preferable to design considering this.

[0023] <Light source unit> Since the light source unit 2 requires coherence, a laser light source with a narrow linewidth in the MHz band is used. Specifically, gas lasers typified by HeNe lasers, and DFB-LDs and VCSELs, which are semiconductor lasers that are easy to miniaturize, can be mentioned. <Polarization beam splitter> The polarization beam splitter 4 is configured by P-polarization transmission and S-polarization reflection in this embodiment. The laser light emitted from the light source unit 2 enters the center of the polarization beam splitter 4 on the optical axis 18 with P-polarization and S-polarization at a ratio of 50%. The P-polarization passes through the polarization beam splitter 4 and goes toward the λ / 4 plate 6 on the optical axis 18. On the other hand, the S-polarization is reflected by the polarization beam splitter 4 and goes toward the λ / 4 plate 8 on the optical axis 20. <λ / 4 plate> The P polarization passes through the λ / 4 plate 6 and is converted into circular polarization, and is irradiated onto the moving object to be measured 14. The laser light irradiated onto the moving object to be measured 14 becomes Doppler-shifted light 22 Doppler-shifted by fd (Hz) and is reflected, passes through the λ / 4 plate 6 again, and becomes S polarization. On the other hand, the S polarization reflected by the polarization beam splitter 4 and directed toward the λ / 4 plate 8 on the optical axis 20 passes through the λ / 4 plate 8 and is converted into circular polarization, and enters the frequency shifter type optical modulator 12. The laser light incident on the optical modulator 12 becomes frequency-shifted light 24 that has received a frequency shift of fm and is reflected, passes through the λ / 4 plate 8 again, and becomes P polarization. <Analyzer> Since orthogonal polarizations are in independent states from each other, interference does not appear simply by overlapping them. Therefore, the synthesized light wave is passed through an analyzer 9 tilted at 45 degrees with respect to both polarizations, and then this light is detected by a light receiving element 10. By doing so, it becomes a state of components common to each other, and an interference phenomenon can be generated. <Light receiving element> The Doppler-shifted light 22 Doppler-shifted by the moving object to be measured 14 and the frequency-shifted light 24 that has received a frequency shift by the frequency shifter type optical modulator 12 are guided to the light receiving element 10 via the polarization beam splitter 4 and the analyzer 9. At the light receiving element 10, the Doppler-shifted light 22 and the frequency-shifted light 24 interfere (optical heterodyne interference), and light having a frequency of fm - fd is generated. Based on the light having a frequency of fm - fd, the movement of the object to be measured 14, that is, the speed and vibration can be obtained. As the light receiving element 10, a photodiode or the like can be used.

[0024] <Frequency shifter type optical modulator> Based on FIG. 2, the configuration of the frequency shifter type optical modulator 12 according to the embodiment of the present invention will be described. The frequency shifter type optical modulator 12 includes a plate-shaped vibrator 30 that repeats a mode in which a crystal is distorted and vibrates in a direction along the surface by applying a potential, and a diffraction grating 34 provided on the surface of the vibrator 30 and formed by periodically arranging a plurality of linear grooves 32. In FIG. 2, reference numeral 31 denotes a substrate to which the vibrator is attached. Pads 33 are provided on the substrate 31, and further pads 35 are provided on the back surface of the substrate 31. In this embodiment, the dimensions and shape of the substrate 31 are a square with a side length of 1.6 mm and a thickness of 0.35 mm. The vibrator 30 is a square with a side length of 1 mm and a thickness of 0.07 mm, and oscillates at 24 MHz. Here, an example of a vibrator with a fundamental oscillation frequency of 24 MHz is shown, but the fundamental oscillation frequency can be adjusted from 1 MHz to 300 MHz by changing the thickness of the vibrator. The diffraction grating 34 has a pitch of 1 μm and a blazed angle of 25 degrees, and is provided on the entire surface of the vibrator 30. Note that the diffraction grating 34 may be provided only on a part of the surface of the vibrator 30 instead of the entire surface.

[0025] In this embodiment, the vibrator 30 is a crystal AT vibrator that undergoes thickness shear vibration in the high-frequency region of the MHz band. And the diffraction grating 34 is a blazed diffraction grating. A blazed diffraction grating refers to one in which the cross-sectional shape of the diffraction grating is stepped. As shown in FIG. 2, the linear grooves 32 of the diffraction grating 34 are provided in a direction orthogonal to the direction 36 of vibration of the vibrator 30.

[0026] <Method of forming the diffraction grating> As a method for forming the diffraction grating 34, a method of first creating a mold using a mechanical scribing method (ruling engine) and then forming it on the electrode of the crystal AT oscillator chip by nanoimprinting can be mentioned. Here, it is formed on the electrode because, in the case of an AT oscillator, high-quality thickness-shear vibration can be generated on the electrode in principle. It is not limited to the electrode, and it can also be formed on the surface of the material of the non-electrode part. Also, in the case of an electrostatic drive type Si oscillator or a SAW device, etc., high-quality in-plane vibration can also be generated on the surface of the material of the non-electrode part, so the formation location can be appropriately selected. Also, it can be formed by a method using exposure and etching, electron beam lithography, focused ion beam processing (FIB), etc. Also, a mirror film made of a metal film or a dielectric multilayer film may be provided on the resist diffraction grating formed on the chip of the crystal AT oscillator. This is because the reflectivity of the diffraction grating 34 provided with the metal film or the mirror film increases. The chip or wafer provided with the resist diffraction grating may be processed by dry etching, and after removing the resist, the mirror film may be provided. This eliminates the influence of moisture absorption by the resist, etc., and thus the stability of the diffraction grating 34 can be increased. Also, by making the mirror film a metal such as Au or Al, it can also serve as the oscillation electrode of the oscillator. Or, a structure such as anodized alumina (porous alumina) may be used.

[0027] <Modulation frequency fm in the case of using a thickness-shear vibration element> The modulated light undergoes a Doppler shift and its frequency is modulated. As shown in FIG. 3, according to the principle of a laser Doppler, a plurality of diffracted lights Kns are generated from the incident light Ki incident on the diffraction grating 30 of the frequency shifter type optical modulator 12. Here, n is the order of the diffracted light, and n = 0, ±1, ±2, …. In addition, in FIG. 3, the diffraction grating 34 is not the blazed diffraction grating of FIG. 2, but a diffraction grating formed by repeating irregularities is shown. In actual manufacturing, the order n of the diffracted light can be appropriately selected and used. FIG. 3 shows the case where the incident light Ki is incident from a direction perpendicular to the surface of the oscillator 30. However, it is not limited to this perpendicular incident angle and may be incident obliquely, that is, the incident angle may be set as appropriate. When the incident light is oblique, the direction of the diffracted light also changes accordingly.

[0028] In the design of the diffraction grating, higher-order light with |n| ≥ 2 may not appear. Therefore, in order to stably obtain a modulation signal, it is desirable to set |n| = 1. That is, in the laser Doppler measurement device 1 of FIG. 1, the frequency shifter type optical modulator 12 is preferably arranged such that the ±1st order diffracted light is used as the frequency-shifted light 24. With this arrangement, miniaturization and high stability of the laser Doppler measurement device 1 can be achieved.

[0029] Also, when the diffraction grating 34 has higher-order light with |n| ≥ 2 appearing, the frequency shifter type optical modulator 12 may be arranged in the laser Doppler measurement device 1 of FIG. 1 such that any diffracted light of ±2nd order or higher is used as the frequency-shifted light 24. Thereby, since higher-order diffracted light can be utilized, high-frequency operation and miniaturization of the laser Doppler measurement device 1 can be achieved.

[0030] In the present embodiment, the frequency shifter type optical modulator 12 is arranged such that the angle formed by the incident direction of the laser light to the frequency shifter type optical modulator 12 and the traveling direction of the frequency-shifted light 24 reflected from the frequency shifter type optical modulator 12 is 180 degrees. The figure located above FIG. 4 shows the case where a mirror 37 is used to achieve the 180 degrees. The figure located at the center of FIG. 4 shows the case where the oscillator 30 is tilted and installed to achieve the 180 degrees. The figure located below FIG. 4 shows that the diffraction grating 34 is a blazed diffraction grating, and the blazed angle θ B is used. The combination of the blazed angle θ B and the incident angle β of the incident light Ki achieves the 180 degrees.

[0031] Thus, when realizing the "180 degrees" using the mirror 37, the modulated light will undergo Doppler shift twice, enabling further high-frequency modulation to be realized. Also, when realizing the "180 degrees" by tilting the vibrator 30 without using a mirror, the mirror becomes unnecessary, enabling further miniaturization to be realized. When the diffraction grating 34 is a blazed diffraction grating, the "180 degrees" is realized by the combination of the blaze angle θ B and the incident angle β of the incident light Ki, enabling further miniaturization and high-frequency operation to be realized.

[0032] <Mounting Structure of Laser Doppler Measuring Device> FIG. 5 is a schematic configuration diagram in which the light source unit 2, the frequency shifter type optical modulator 12, and the light receiving element 10 are mounted on a single substrate 39. In FIG. 5, the light source unit 2 is mounted at the center, the frequency shifter type optical modulator 12 is mounted at the lower position, and the light receiving element 10 is mounted at the upper position on the substrate 39, but it is of course not limited to this arrangement. By mounting the light source unit 2, the frequency shifter type optical modulator 12, and the light receiving element 10 on the substrate 39, prisms 40 and 42 are provided on the optical axis 20. Further, a convex lens 44 is disposed between the light source unit 2 and the polarization beam splitter 4, and is configured to effectively utilize the light emitted from the light source unit 2. In the mounting structure shown in FIG. 5, the light receiving element 10 is a photodiode with a size of 0.1 mm square, the light source unit 2 is a VCSEL light source with a size of 10 μm square, and the frequency shifter type optical modulator 12 has a size of 1 mm square. In this way, the module can be miniaturized to a size of several mm square.

[0033] <Explanation of Effects of Embodiment> According to the present embodiment, by providing the diffraction grating 34 on the vibrator 30, it is possible to promote the miniaturization and high-precision of the frequency shifter type optical modulator 12. Also, it is possible to promote the correspondence to the high-frequency region in the MHz band, that is, the realization of high-frequency operation.

[0034] In addition, the linear grooves 32 of the diffraction grating 34 are configured such that the direction of the straight line intersects the vibration direction 36 of the vibrator 30, so that the effect based on the combination of the vibrator 30 and the diffraction grating 34 can be efficiently obtained, and thus the high-frequency operation of the frequency shifter type optical modulator 12 can be easily realized. Moreover, in the present embodiment, since the diffraction grating 34 is a blazed diffraction grating, the light utilization efficiency can be increased, and thus the miniaturization and high SN of the frequency shifter type optical modulator 12 can be easily realized. In the present embodiment, the vibrator 30 is a crystal AT vibrator that vibrates in a thickness-shear mode in a high-frequency region of the MHz band. Thereby, the miniaturization, high-precision, and further high-frequency operation of the frequency shifter type optical modulator 12 can be easily realized.

[0035] In addition, since the laser Doppler measurement device of the present embodiment includes the frequency shifter type optical modulator 12 according to the present embodiment, the miniaturization, high-precision, and further high-frequency operation of the laser Doppler measurement device 1 can be easily realized.

[0036] [Other Embodiments] The frequency shifter type optical modulator and the laser Doppler measurement device according to the present invention are basically configured as described above, but it is of course possible to make partial configuration changes, omissions, etc. within the scope that does not deviate from the gist of the present invention.

[0037] In the above embodiment, the case where the diffraction grating 34 uses a crystal AT vibrator has been described, but any element that starts thickness-shear vibration and includes in-plane vibration can be appropriately used. For example, other than the thickness-shear vibration element, an electrostatically driven Si-MEMS vibrator, a vibrator using a piezoelectric element such as a piezo, can be mentioned. Also, a surface acoustic wave (SAW device) vibrator can be used.

Description of Reference Numerals

[0038] 1... Laser Doppler measurement device, 2... Light source unit, 4... Polarizing beam splitter, 6… λ / 4 plate, 8… λ / 4 plate, 9… photon detector, 10… light receiving element, 12… frequency shifter type optical modulator, 14… object to be measured, 16… setting section, 18… optical axis, 20… optical axis, 22… Doppler shifted light, 24… frequency shifted light, 30… oscillator, 31… substrate, 32… groove, 33… pad, 34… diffraction grating, 35… pad, 36… vibration direction of oscillator, 37… mirror, 39… base, 40… prism, 42… prism, 44… convex lens, θ B … blaze angle, Ki… incident light

Claims

1. A vibrator; a diffraction grating including a plurality of grooves arranged in parallel in the displacement direction of the transducer; The diffraction grating is provided on the transducer, the direction in which the grooves of the diffraction grating extend intersects with a direction of vibration that is a displacement direction of the vibrator, The transducer has an electrode, The diffraction grating is disposed on the electrode.

1. A frequency shifter type optical modulator comprising:

2. 3. The frequency shifter type modulator according to claim 2, The oscillator oscillates when a potential difference is applied by the electrodes.

1. A frequency shifter type modulator comprising:

3. 3. The frequency shifter type optical modulator according to claim 1, The diffraction grating is a blazed diffraction grating.

1. A frequency shifter type optical modulator comprising:

4. 4. The frequency shifter type optical modulator according to claim 1, The vibrator is a quartz crystal AT vibrator that vibrates in a thickness-shear mode in a high frequency range of the MHz band.

1. A frequency shifter type modulator comprising:

5. a light source unit that outputs a laser beam of a predetermined wavelength, a polarizing beam splitter, a plurality of λ / 4 plates, an analyzer, a light receiving element that converts light into an electric signal, the frequency shifter type optical modulator according to any one of claims 1 to 4, and a setting unit for an object to be measured; the polarizing beam splitter, the λ / 4 plate, and the setting unit are disposed on an optical axis of light output from the light source unit, the λ / 4 plate and the frequency shifter type optical modulator are disposed on an optical axis of the light reflected by the polarizing beam splitter, an analyzer is disposed between the polarizing beam splitter and the light receiving element; the Doppler shifted light reflected from the object to be measured and the frequency shifted light reflected by the frequency shifter-type optical modulator are guided to the light receiving element through the λ / 4 plates, the polarizing beam splitter, and the analyzer; A laser Doppler measurement device characterized by:

6. 6. The laser Doppler measurement device according to claim 5, The frequency-shifted light is ±1st-order diffracted light. A laser Doppler measurement device characterized by:

7. 6. The laser Doppler measurement device according to claim 5, The frequency-shifted light is any one of ±2nd order or higher diffracted light. A laser Doppler measurement device characterized by:

8. 8. The laser Doppler measurement device according to claim 6, the frequency shifter-type optical modulator is arranged such that an angle between a direction in which the laser light enters the frequency shifter-type optical modulator and a direction in which the frequency shifted light reflected from the frequency shifter-type optical modulator travels is 180 degrees; A laser Doppler measurement device characterized by:

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