Frequency shifter type optical modulator and laser Doppler measurement 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 to grasp the state of motion of a moving object. [Background technology]

[0002] A prior art document relating to this type of frequency shifter-type optical modulator and laser Doppler measurement device is Japanese Patent Application Laid-Open No. 2007-285898, which describes obtaining the amount of frequency shift using heterodyne interference. Specifically, paragraph 0028 of the specification states, "It is desirable to use a piezoelectric element as the vibration element, which has the property of deforming when voltage, magnetism, etc. are applied, and the vibration frequency can be varied by changing the voltage. Furthermore, the vibration frequency must be a triangular wave or a sawtooth wave whose waveform rise is linear. The optical Doppler shift caused by the incidence of laser light at the rise of the sawtooth wave applied voltage or at the rise and fall of the triangular wave applied voltage is utilized." [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-285898 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in general, oscillator elements such as quartz oscillators and silicon oscillators use simple harmonic drive with an extremely high Q value, which indicates the sharpness of oscillation, and the drive method described therein is problematic in that it does not provide sufficient precision and is not practical.Furthermore, there is no mention or suggestion whatsoever about compatibility with high frequency oscillations in the MHz band, i.e., the realization of higher frequencies. [Means for solving the problem]

[0005] In order to solve the above problem, the present invention provides a frequency shifter-type optical modulator, characterized by comprising: a plate-shaped vibrator that repeats a mode in which the crystal is distorted in a direction along the surface by applying an electric potential, and a diffraction grating provided on the surface of the vibrator and consisting of a plurality of linear grooves arranged periodically. [Brief explanation of the drawings]

[0006] [Figure 1] 1 is a schematic diagram of a laser Doppler measurement device according to an embodiment of the present invention; [Figure 2] FIG. 2 is a schematic perspective view of a frequency shifter type optical modulator according to the embodiment. [Figure 3] 4A and 4B are diagrams for explaining how a plurality of diffracted beams are generated from incident light in the optical modulator of the embodiment. [Figure 4] FIG. 2 is a diagram illustrating the optical paths of incident light and diffracted light in the optical modulator according to the embodiment. [Figure 5] FIG. 10 is a schematic configuration diagram of the embodiment in which a light source unit, a frequency shifter-type optical modulator, and a light receiving element are mounted on one board. DETAILED DESCRIPTION OF THE INVENTION

[0007] First, the present invention will be briefly described. In order to solve the above problem, a frequency shifter-type optical modulator according to a first aspect of the present invention comprises an oscillator and a diffraction grating including a plurality of grooves arranged in parallel in the displacement direction of the oscillator, and the diffraction grating is provided on the oscillator.

[0008] According to this aspect, by providing the diffraction grating on the oscillator, it becomes easier to realize a miniaturized and highly accurate frequency shifter type optical modulator, and it also becomes easier to realize a high frequency range in the MHz band, i.e., to realize a high frequency range. Furthermore, when frequency modulation is to be obtained, the diffracted light can have a higher modulation frequency if it is vibrated in the direction of the lattice vector. According to this aspect, the extension direction of the multiple grooves intersects with the direction of the vibration, so that the effect based on the combination of the vibrator and the diffraction grating can be efficiently obtained, thereby easily realizing a higher frequency for the frequency shifter type optical modulator.

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

[0010] According to this aspect, the diffraction grating is a blazed diffraction grating, which can improve the light utilization efficiency, thereby easily realizing a miniaturized frequency shifter-type optical modulator with a high SNR.

[0011] A third aspect of the present invention is characterized in that, in the frequency shifter type optical modulator of the first or second aspect, the oscillator is a quartz crystal AT oscillator that vibrates in thickness-shear mode in a high frequency region in the MHz band.

[0012] According to this aspect, the vibrator is a quartz crystal AT vibrator that vibrates in thickness-shear mode in the high frequency range of the MHz band, so that the frequency shifter type optical modulator can be easily made smaller, more accurate, and operate at higher frequencies.

[0013] A laser Doppler measurement device according to a fourth aspect of the present invention comprises a light source unit that outputs laser light 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 the first to third aspects, and a setting unit for an object to be measured, wherein the polarizing 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 polarizing beam splitter, an analyzer is arranged between the polarizing beam splitter and the light receiving element, and the Doppler-shifted light reflected by 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.

[0014] According to this aspect, the laser Doppler measurement device is equipped with a frequency shifter-type optical modulator described in any one of the first to third aspects, so that the laser Doppler measurement device can be easily made smaller, more accurate, and operate at a higher frequency.

[0015] A fifth aspect of the present invention is characterized in that, in the laser Doppler measurement device of the fourth aspect, the frequency-shifted light is ±1st-order diffracted light.

[0016] According to this aspect, the frequency shifter-type optical modulator is arranged so that ±1st-order diffracted light is used as the frequency shifted light, thereby making it possible to achieve a compact and highly stable measurement device.

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

[0018] According to this aspect, the frequency shifter-type optical modulator is arranged so that any of the ±2nd order or higher diffracted light is used as the frequency shifted light, thereby making it possible to reduce the size of the measurement device and increase the frequency.

[0019] A seventh aspect of the present invention is characterized in that, in the laser Doppler measurement device of the fifth or sixth aspect, the frequency shifter-type optical modulator is arranged so that the angle formed between the direction of incidence of the laser light into the frequency shifter-type optical modulator and the direction of propagation of the frequency-shifted light reflected from the frequency shifter-type optical modulator is 180 degrees. Here, the "angle of 180 degrees" in "arranged so that the angle formed between the direction of incidence of the laser light into the frequency shifter-type optical modulator and the direction of propagation of the frequency-shifted light reflected from the frequency shifter-type optical modulator is 180 degrees" can be achieved by using reflection by a mirror or by installing the surface formed by the diffraction grating at an angle without using a mirror.

[0020] According to this aspect, when the "angle of 180 degrees" is achieved using a mirror, the modulated light undergoes Doppler shift twice, enabling even higher frequency modulation. Furthermore, when the "angle of 180 degrees" is achieved without using a mirror, the mirror becomes unnecessary, enabling even greater miniaturization.

[0021] Next, 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 Figures 1 to 5. In each figure, the same parts are given the same reference numerals, and individual descriptions for each figure will be omitted.

[0022] [Embodiment] <Laser Doppler measurement device> The configuration of a laser Doppler measurement device 1 according to an embodiment of the present invention will be described with reference to FIG. 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 polarizing 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 an object to be measured 14. The polarizing beam splitter 4, one λ / 4 plate 6, and the setting unit 16 are arranged in this order on an optical axis 18 of the light output from the light source unit 2. Meanwhile, the λ / 4 plate 8 and the frequency shifter-type optical modulator 12 are arranged in this order on an optical axis 20 of the light reflected by the polarizing beam splitter 4. The analyzer 9 is arranged between the polarizing beam splitter 4 and the light-receiving element 10. The Doppler-shifted light 22 reflected from the object to be measured 14 and the frequency-shifted light 24 reflected by the frequency shifter-type optical modulator 12 are guided to the light-receiving element 10 through the λ / 4 plates 6 and 8, the polarizing beam splitter 4, and the analyzer 9. A polarizing beam splitter may also be used as a non-polarizing beam splitter. In this case, the λ / 4 is unnecessary, the number of parts is reduced, and the system can be made smaller. However, if the stability of the interference deteriorates, it is preferable to take this into account when designing.

[0023] <Light source section> A narrow-linewidth, MHz-band laser light source is used for the light source unit 2 because coherence is required. Specifically, examples include gas lasers such as HeNe lasers, and semiconductor lasers such as DFB-LD and VCSEL, which are easy to miniaturize. <Polarizing beam splitter> In this embodiment, the polarizing beam splitter 4 is configured to transmit P polarized light and reflect S polarized light. The laser light emitted from the light source unit 2 is incident on the center of the polarizing beam splitter 4 located on the optical axis 18, with the ratio of P polarized light and S polarized light being 50%. The P polarized light passes through the polarizing beam splitter 4 and heads toward the λ / 4 plate 6 on the optical axis 18. On the other hand, the S polarized light is reflected by the polarizing beam splitter 4 and heads toward the λ / 4 plate 8 on the optical axis 20. <λ / 4 plate> The P-polarized light passes through the λ / 4 plate 6 and is converted into circular polarization, which is then irradiated onto the moving object under test 14. The laser light irradiated onto the moving object under test 14 is Doppler shifted by fd (Hz) and reflected as Doppler-shifted light 22, which passes through the λ / 4 plate 6 again and becomes S-polarized. On the other hand, the S-polarized light reflected by the polarizing beam splitter 4 and directed toward the λ / 4 plate 8 on the optical axis 20 passes through the λ / 4 plate 8, where it is converted into circularly polarized light, and then enters the frequency shifter-type optical modulator 12. The laser light that entered the optical modulator 12 is reflected as frequency-shifted light 24 that has been subjected to a frequency shift fm, and passes through the λ / 4 plate 8 again to become P-polarized light. <Analyzer> Since the orthogonal polarized lights are independent of each other, simply superimposing them will not cause interference. Therefore, the combined light wave is passed through an analyzer 9 tilted at 45 degrees to both polarized lights, and then this light is detected by a light receiving element 10. This causes the lights to have common components, which can cause interference. <Photodetector> Doppler-shifted light 22, which has been Doppler-shifted by the moving object under test 14, and frequency-shifted light 24, which has been frequency-shifted by the frequency shifter-type optical modulator 12, are guided to the light-receiving element 10 via the polarizing beam splitter 4 and analyzer 9. In the light-receiving element 10, the Doppler-shifted light 22 and the frequency-shifted light 24 interfere (optical heterodyne interference), generating light with a frequency of fm-fd. Based on this light with a frequency of fm-fd, the movement of the object under test 14, i.e., its speed and vibration, can be determined. A photodiode or the like can be used for the light-receiving element 10.

[0024] <Frequency shifter type optical modulator> The configuration of a frequency shifter type optical modulator 12 according to an embodiment of the present invention will be described with reference to FIG. The frequency shifter-type optical modulator 12 comprises a plate-shaped vibrator 30 that repeats a mode in which the crystal is distorted in a direction along the surface by applying an electric potential, and a diffraction grating 34 that is provided on the surface of the vibrator 30 and has a plurality of linear grooves 32 arranged periodically. In Fig. 2, reference numeral 31 denotes a substrate on which the vibrator is attached, and a pad 33 is provided on the substrate 31, and a pad 35 is further provided on the back surface of the substrate 31. In this embodiment, the dimensions and shape of the substrate 31 are a square with sides of 1.6 mm and a thickness of 0.35 mm. The vibrator 30 is a square with sides of 1 mm and a thickness of 0.07 mm, and oscillates at 24 MHz. Note that while an example of a vibrator with a fundamental oscillation frequency of 24 MHz is shown here, 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 portion of the surface of the vibrator 30, rather than on the entire surface.

[0025] In this embodiment, the resonator 30 is a quartz crystal AT resonator that vibrates in a thickness-shear mode in the high frequency range of the MHz band. The diffraction grating 34 is a blazed diffraction grating. A blazed diffraction grating is a diffraction grating whose cross section has a stepped shape. As shown in FIG. 2, the linear grooves 32 of the diffraction grating 34 are provided so that the direction of the linear grooves is perpendicular to the direction 36 of vibration of the vibrator 30.

[0026] <Method for forming a diffraction grating> One method for forming the diffraction grating 34 is to first create a mold using a mechanical ruling engine, and then form it on the electrodes of a quartz AT resonator chip using nanoimprinting. Here, we chose to form it on the electrodes because, in principle, high-quality thickness-shear vibration can be generated on the electrodes of an AT resonator. It is not limited to being formed on the electrodes; it can also be formed on the surface of a non-electrode material. Furthermore, electrostatically driven Si resonators and SAW devices can generate high-quality in-plane vibration even on the surface of a non-electrode material, so the formation location can be selected appropriately. It can also be formed using exposure and etching, electron beam lithography, focused ion beam processing (FIB), etc. Furthermore, a metal film or a mirror film made of a dielectric multilayer film may be provided on the resist diffraction grating formed on the chip of the quartz crystal AT resonator, because the reflectance of the diffraction grating 34 provided with a metal film or a mirror film increases. The chip or wafer on which the resist diffraction grating is formed may be processed by dry etching, and the resist may be removed before the mirror film is formed. This eliminates the effects of moisture absorption by the resist, thereby increasing the stability of the diffraction grating 34. Furthermore, by using a metal such as Au or Al for the mirror film, it can also function as an oscillation electrode for the vibrator. Alternatively, a structure such as anodized alumina (porous alumina) may be used.

[0027] <Modulation frequency fm when using a thickness shear vibration element> The modulated light undergoes a Doppler shift, resulting in frequency modulation. As shown in Figure 3, according to the principle of laser Doppler, multiple diffracted lights Kns are generated from incident light Ki that is 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 Fig. 3, the diffraction grating 34 is a diffraction grating with repeated concave and convex portions, rather than the blazed diffraction grating of Fig. 2. In actual manufacturing, the order n of the diffracted light can be appropriately selected. 3 shows the case where the incident light Ki is incident from a direction perpendicular to the surface of the vibrator 30, but the incident angle is not limited to this perpendicular angle and may be oblique, i.e., the incident angle may be set appropriately. When the incident light is oblique, the direction of the diffracted light also changes accordingly.

[0028] In designing a diffraction grating, higher-order light of |n| ≥ 2 may not appear. Therefore, to obtain a stable modulated signal, it is desirable to set |n| = 1. That is, in the laser Doppler measurement device 1 shown in Figure 1, the frequency shifter-type optical modulator 12 is preferably arranged so that ±1st-order diffracted light is used as the frequency-shifted light 24. This arrangement allows the laser Doppler measurement device 1 to be made compact and highly stable.

[0029] Furthermore, when the diffraction grating 34 emits high-order light of |n|≧2, the frequency shifter-type optical modulator 12 may be arranged in the laser Doppler measurement device 1 of FIG. 1 so that any of the ±2nd-order or higher diffracted light beams is used as the frequency-shifted light 24. This allows for the use of high-order diffracted light, making it possible to achieve a higher frequency and a smaller size for the laser Doppler measurement device 1.

[0030] In this embodiment, the frequency shifter-type optical modulator 12 is positioned so that the angle formed between the direction of entry of the laser light into the frequency shifter-type optical modulator 12 and the direction of travel of the frequency-shifted light 24 reflected from the frequency shifter-type optical modulator 12 is 180 degrees. The diagram at the top of FIG. 4 shows a case where a mirror 37 is used to achieve the 180 degrees. The diagram in the center of FIG. 4 shows a case where the transducer 30 is installed at an angle to achieve the 180 degrees. The diagram at the bottom of FIG. 4 shows that the diffraction grating 34 is a blazed diffraction grating, and in order to achieve the 180 degrees, the blazed angle θ B This is the case when the blazed angle θ B The above 180 degrees is realized by the combination of the incident angle β of the incident light Ki and the incident angle β of the incident light Ki.

[0031] As a result, when the "angle of 180 degrees" is realized using the mirror 37, the modulated light undergoes Doppler shift twice, making it possible to achieve even higher frequency modulation. Furthermore, if the "angle of 180 degrees" is achieved by tilting the vibrator 30 without using a mirror, the mirror becomes unnecessary, making it possible to achieve even greater miniaturization. If the diffraction grating 34 is a blazed diffraction grating, the blazed angle θ B and the incident angle β of the incident light Ki, the 180 degrees is realized, so that miniaturization and higher frequencies can be further realized.

[0032] <Laser Doppler measurement device mounting structure> Fig. 5 is a schematic diagram showing the configuration in which the light source unit 2, frequency shifter-type optical modulator 12, and light receiving element 10 are mounted on one substrate 39. In Fig. 5, the light source unit 2 is mounted in the center, the frequency shifter-type optical modulator 12 in the lower position, and the light receiving element 10 in the upper position on the substrate 39, but it goes without saying that the arrangement is not limited to this. By mounting the light source unit 2, frequency shifter-type optical modulator 12, and light receiving element 10 on a substrate 39, prisms 40 and 42 are provided on the optical axis 20. Furthermore, a convex lens 44 is disposed between the light source unit 2 and the polarizing beam splitter 4, and the light emitted from the light source unit 2 is configured to be used effectively. 5, the light receiving element 10 is a 0.1 mm square photodiode, the light source unit 2 is a 10 μm square VCSEL light source, and the frequency shifter type optical modulator 12 is 1 mm square. In this way, the module can be miniaturized to a size of several mm square.

[0033] <Explanation of Effects of the Embodiment> According to this embodiment, by providing the diffraction grating 34 on the oscillator 30, it is possible to achieve a smaller size and higher precision for the frequency shifter-type optical modulator 12. It is also possible to achieve compatibility with high frequency regions in the MHz band, i.e., higher frequencies.

[0034] Furthermore, by configuring the linear grooves 32 of the diffraction grating 34 so that the direction of the straight lines intersects with the vibration direction 36 of the vibrator 30, the effect based on the combination of the vibrator 30 and the diffraction grating 34 can be efficiently obtained, thereby easily realizing a higher frequency for the frequency shifter-type optical modulator 12. Furthermore, in this embodiment, the diffraction grating 34 is a blazed diffraction grating, which can improve the light utilization efficiency, thereby easily achieving a small size and high SNR for the frequency shifter type optical modulator 12. In this embodiment, the vibrator 30 is a quartz crystal AT vibrator that vibrates in a thickness-shear mode in the high frequency range of the MHz band, which makes it easy to achieve a smaller, more accurate, and higher frequency frequency for the frequency shifter-type optical modulator 12.

[0035] Furthermore, since the laser Doppler measurement device of this embodiment includes the frequency shifter type optical modulator 12 according to this embodiment, the laser Doppler measurement device 1 can be easily made smaller, more accurate, and operate at a higher frequency.

[0036] [Other embodiments] The frequency shifter-type optical modulator and laser Doppler measurement device according to the present invention are based on the configuration described above, but it is of course possible to modify or omit parts of the configuration within the scope of the gist of the present invention.

[0037] In the above embodiment, the diffraction grating 34 is described as using a quartz crystal AT resonator, but any element that vibrates in-plane, such as thickness-shear vibration, can be used as appropriate. For example, in addition to thickness-shear vibration elements, electrostatically driven Si-MEMS resonators and resonators using piezoelectric elements such as piezo elements can be used. Surface acoustic wave (SAW) resonators can also be used. [Explanation of symbols]

[0038] 1...laser Doppler measurement device, 2...light source unit, 4...polarized beam splitter, 6...λ / 4 plate, 8...λ / 4 plate, 9...analyzer, 10...light receiving element, 12... frequency shifter type optical modulator, 14... object to be measured, 16... setting unit, 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 vibrator, 37...mirror, 39...base, 40...Prism, 42...Prism, 44...Convex lens, θ B …blazed angle, Ki…incident light

Claims

1. A vibrator and a diffraction grating including a plurality of grooves arranged in parallel in the displacement direction of the vibrator, the diffraction grating is provided on the oscillator, the direction in which the grooves of the diffraction grating extend intersects with a vibration direction that is a displacement direction of the vibrator, The vibrator has an electrode, the diffraction grating is formed on the electrode; A frequency shifter type optical modulator characterized by:

2. 2. The frequency shifter type modulator according to claim 1, The vibrator oscillates when a potential difference is applied by the electrodes. A frequency shifter type modulator characterized by:

3. 3. The frequency shifter type optical modulator according to claim 1, the diffraction grating is a blazed diffraction grating; A frequency shifter type optical modulator characterized by:

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. A frequency shifter type modulator characterized by:

5. a light source unit that outputs laser light 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 the 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 of ±2nd-order or higher-order diffracted light, A laser Doppler measurement device characterized by:

8. 8. The laser Doppler measurement device according to claim 6 or 7, the frequency shifter-type optical modulator is arranged so that an angle formed 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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