JP2021140193A - Method for controlling resonance length of optical resonator, optical comb generator, and optical comb generation device - Google Patents

Abstract

To stabilize resonator length control by using a dither signal by inputting laser beams in which modulation of amplitude smaller than that of FSR of an optical resonance mode is applied to a laser frequency by a dither signal to an optical comb generator.SOLUTION: An error signal corresponding to deviation between a laser frequency of laser beams applied to a low-power type optical comb module 100A and a resonance frequency of the optical comb module 100A is generated by a control circuit 113 from a light detection signal obtained by detecting one portion of an optical comb emitted from the optical comb module 100A by a photodetector 112, and a dither signal, resonator length of the optical comb module 100A is controlled by the error signal, and control is performed such that a resonance frequency of the optical comb module 100A follows the laser frequency.SELECTED DRAWING: Figure 1

Description

The present invention is an optical resonator applied to a field that requires a standard light source having high coherence at multiple wavelengths such as optical communication, optical CT, and optical frequency standard, or a light source that can also utilize coherence between each wavelength. The present invention relates to a method for controlling a resonance length, an optical comb generator, and an optical comb generator.

With the development of optoelectronics in recent years, a waveguide type optical resonator that resonates light confined in an optical waveguide in order to meet the demand for laser light control for frequency multiplex communication and frequency measurement of absorption lines distributed over a wide range. Has come to be used frequently.

When measuring the optical frequency with high accuracy, heterodyne detection is performed by interfering the measured light with other light and detecting the electric signal of the generated optical beat frequency. The band of light that can be measured in this heterodyne detection is limited to the band of the light receiving element used in the detection system, and is about several tens of GHz.

On the other hand, in order to perform optical control for frequency multiplexing communication and frequency measurement of absorption lines distributed over a wide range, it is necessary to further expand the measurable band of light.

In order to meet the demand for expanding the measurable band, a broadband heterodyne detection system using an optical comb generator has been conventionally proposed (see, for example, Patent Document 1). This optical comb generator generates comb-shaped sidebands arranged at equal intervals on the frequency axis over a wide band, and the frequency stability of this sideband is almost the same as the frequency stability of incident light. be. By heterodyne detection of the generated sideband and the light to be measured, it is possible to construct a wideband heterodyne detection system over several THz.

Further, an optical comb generator and an optical modulator in which phase modulation is applied not only to the light propagating in the optical waveguide in the outward path direction but also to the light propagating in the return path direction have been proposed (see, for example, Patent Document 2). ).

Furthermore, by suppressing chipping and rounding during machining of the corners of the waveguide end face and stably adhering each reflective film without peeling off at the uppermost corner of the end face, the reflectance of the reflective film and the optical cavity We have proposed an optical resonator, an optical modulator, an optical comb generator, and an optical oscillator with improved finesse and enhanced functions of the device itself (see, for example, Patent Document 3).

That is, a member having the same hardness as the substrate for forming the optical waveguide from the upper surface, at least one end surface thereof forms the same plane as the end surface of the substrate including the light incident end or the light emitting end in the optical waveguide. The incident side reflective film and the outgoing side reflective film constituting the resonance means are covered on the plane formed by polishing the end face of the member and the end face of the substrate. Since it is attached, chipping and rounding during processing of the corners of the end face of the waveguide can be suppressed, and each reflective film can be stably attached without peeling off at the uppermost corner of the end face, and the reflectance of the reflective film and It is possible to improve the finesse of the optical resonator and enhance the function of the device itself.

Japanese Unexamined Patent Publication No. 2003-202609 Japanese Patent No. 3891977 Japanese Patent No. 4781648

By the way, in the optical comb generator, when the optical comb is used for measurement, a part of the light emitted from the optical resonator is detected by the photodetector in order to obtain a stable output, and a predetermined resonance length is obtained. It is necessary to feedback control the resonance length of the optical resonator so as to be.

Therefore, an object of the present invention is to input a laser beam to the optical comb generator in which a modulation having a smaller amplitude than that of the FSR in the resonance mode is applied to the laser frequency by a dither signal in view of the conventional situation as described above. The purpose is to be able to stabilize the resonator length control using a laser signal.

Further, another object of the present invention is to make it possible to stabilize the optical comb generator, improve the accuracy of the measuring device including the optical comb, reduce the error, and the like.

Other objects of the present invention, the specific advantages obtained by the present invention, will be further clarified from the description of the embodiments described below.

In the present invention, the phase of the laser light resonated by the optical resonator is modulated according to the modulated signal, so that the side band centered on the frequency of the laser light incident on the optical resonator is the frequency of the modulated signal. It is a method of controlling the resonance length of the optical cavity in the optical comb generator generated at the interval of The laser frequency and the light of the laser light incident on the optical cavity from the light detection signal and the dither signal that are incident on the resonator and detect a part of the transmitted light or the reflected light emitted from the optical resonator. An error signal corresponding to the deviation of the resonance frequency of the resonator is generated, and the resonance length of the optical resonator is controlled by using this error signal to control the resonance frequency of the optical resonator to follow the laser frequency. It is characterized by doing.

Further, in the present invention, the resonance length of the optical resonator in an optical comb generator including a plurality of optical comb generators in which the laser light emitted from one laser light source is separated into a plurality of and incident on each optical resonator. In the control method, laser light in which a modulation having a smaller amplitude than that of the FSR in the resonance mode is applied to the laser frequency by a dither signal is incident on each of the optical cavities from the one laser light source, and the respective optical cavities are resonated. The laser frequency of the laser beam incident on each optical cavity and the resonance frequency of each optical cavity from each light detection signal that detects a part of the transmitted light or the reflected light emitted from the device and the dither signal. Control to generate each error signal according to the deviation of, control the resonance length of each optical resonator using the generated error signal, and make the resonance frequency of each optical resonator follow the laser frequency. It is characterized by performing.

The method for controlling the resonance length of the optical resonator according to the present invention can generate the error signal by mixing the optical detection signal and the synchronization signal.

Further, in the method for controlling the resonance length of the optical resonator according to the present invention, an error signal can be obtained by a product-sum calculation of digital signal processing by using the light detection signal and the synchronization signal.

The present invention is an optical comb generator, a light source containing a single frequency component, a laser light source in which the oscillation frequency is modulated by a dither signal given from the outside, and a modulated signal having a predetermined frequency. An oscillator that oscillates a modulated signal that oscillates, and an optical resonator in which laser light is incident from the laser light source are provided, and the phase of the light resonated by the optical resonator according to the modulated signal supplied from the oscillator is adjusted. An optical comb generating means that modulates and generates a side band centered on the frequency of the incident light at the frequency interval of the modulated signal, a control unit that controls the resonance length of the optical resonator, and the control unit. The control unit includes a part of transmitted light or reflected light emitted from the optical resonator of the optical comb generating means. The laser frequency of the light incident on the optical resonator of the optical comb generating means from the laser light source and the optical resonator by the optical detection signal by the optical detector and the synchronization signal are provided. An error signal corresponding to the deviation of the resonance frequency of the above is generated, and the error signal is used to control the resonance frequency of the optical resonator to follow the laser frequency.

The present invention is an optical comb generator, which is a light source containing a single frequency component, and a laser light source whose oscillation frequency is modulated by a dither signal given from the outside, and laser light from the laser light source. The phase of the light resonated by the optical resonator whose resonance length is branched and incident is modulated according to the modulation signals having different modulation frequencies, and the side band centered on the frequency of the incident light. Two optical comb generators that generate the above-mentioned modulation signals at intervals of different modulation frequencies, and a synchronization signal source that gives a synchronization signal to the above two optical comb generators and outputs a dither signal to be given to the laser light source. The plurality of optical comb generators are provided with light incident from the laser light source by the light detection signal for detecting a part of the transmitted light or the reflected light emitted from the optical comb generator and the synchronization signal. It is characterized in that an error signal corresponding to the deviation between the laser frequency of the above and the resonance frequency of the optical resonator is generated to control the resonance frequency of the optical resonator to follow the laser frequency.

In the present invention, a laser beam whose amplitude is smaller than that of the FSR in the resonance mode is applied to the laser frequency by the dither signal is input to the optical comb generator, and the dither signal is used to stabilize the resonator control. The resonance frequency of the optical resonator can be made to follow the laser frequency.

It is a perspective view which shows the structure of an optical modulator. It is a side view of the said optical modulator. It is a vertical sectional view of the main part in each step for explaining the manufacturing method of the said optical modulator. It is a perspective view of the substrate provided with three optical waveguides produced for measuring the end face reflectance of an optical modulator. It is a front view which shows the incident side end face of the said substrate. It is a top view of the said substrate. It is a figure which shows the structure of the reciprocating modulation type optical modulator. It is a figure which shows the intensity distribution at each frequency (wavelength) of a side band in an optical comb generator. It is a perspective view which shows the structure of the optical comb generator. It is a side view of the said optical comb generator. It is a front view which shows the plane in which the incident side reflection film is formed in the said optical comb generator. It is a perspective view which shows the structure of the optical modulator (optical comb generator) which has an electrode which has a ridge structure. It is a vertical sectional view of the main part in each process for explaining the manufacturing method of the said optical modulator (optical comb generator). It is a perspective view which shows the substrate which forms the electrode which has the ridge structure of the light modulator (optical comb generator). It is a vertical sectional front view of a main part which shows the electrode which has the ridge structure of the said optical modulator (optical comb generator). It is a figure for demonstrating the result of having measured the change of the drive voltage (AC Vpi) at 25 GHz with and without the ridge structure of an electrode about an optical modulator. It is a figure for demonstrating the result of having measured the change of the direct current drive voltage (DC Vpi) with and without the ridge structure of an electrode about an optical modulator. It is a block diagram which shows the structural example of the optical comb generator using the optical comb module to which this invention is applied. It is a block diagram which shows the other configuration example of the optical comb generator using the optical comb module to which this invention is applied. It is a block diagram which shows the structural example of the optical comb generator constructed by using the optical comb module to which this invention is applied. It is a characteristic diagram which shows the transmission mode waveform without deformation obtained when the resonance length of an optical resonator is feedback-controlled in the optical comb generator using the optical waveguide which passes only a single polarization component. A characteristic diagram showing deformation of the transmission mode waveform due to the orthogonal polarization component generated when the resonance length of the optical resonator is feedback-controlled in a conventional optical comb generator using an optical waveguide that transmits a polarization component in which orthogonal modes are mixed. Is.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The common components will be described with reference numerals in the drawings. Further, the present invention is not limited to the following examples, and it goes without saying that the present invention can be arbitrarily modified without departing from the gist of the present invention.

FIG. 1 is a perspective view showing the configuration of the light modulator 8, and FIG. 2 is a side view of the light modulator 8.

The waveguide type optical modulator 8 includes a substrate 11, an optical waveguide 12 that modulates the phase of a single polarization component of light that is formed and propagates on the substrate 11, and an optical waveguide 12 on the substrate 11. The buffer layer 14 laminated so as to cover the light, the electrode 83 provided on the upper surface of the optical waveguide 12 so that the direction of the modulation electric field is substantially perpendicular to the light propagation direction, and the optical waveguide 12 are provided with each other. A first protective material 86 arranged on the upper part of the optical waveguide 12 so as to form the same plane as the first end face 84 and the second end face 85 provided so as to face each other. A second protective material 87 arranged on the upper part of the optical waveguide 12 so as to form the same plane as the second end surface 85, and an end surface 86a of the first end surface 84 and the first protective material 86. Adhered on the plane 92 formed between the incident side antireflection film 63 adhered on the plane 91 formed between the two, and the end surface 87a of the second end surface 85 and the second protective material 87. It includes an emission-side antireflection film 64, an oscillator 16 arranged on one end side of the electrode 83 and oscillating a modulation signal having a frequency fm, and a termination resistor 18 arranged on the other end side of the electrode 83. ..

The substrate 11 is obtained by cutting out a large crystal such as _{LiNbO 3} or GaAs having a diameter of 3 to 4 inches grown by a pulling method into a wafer shape. On the cut-out substrate 11, a ridge portion 20 for forming the electrode 83 is provided by performing a treatment such as mechanical polishing or chemical polishing.

The optical waveguide 12 has a waveguide mode only for a single polarization component on the substrate 11 having at least an electro-optical effect so as to penetrate from the incident side antireflection film 63 to the exit side antireflection film 64. It is formed as a region.

The light incident on the optical waveguide 12 via the antireflection film 63 on the incident side propagates while only a single polarization component is totally reflected at the boundary surface of the waveguide 12.

Here, the optical waveguide 12 that allows only a single polarizing component to pass through is single on the substrate 11 having an electro-optical effect by an optical waveguide forming method that causes a change in the refractive index only for a specific polarizing component, for example, a proton exchange method. It can be formed as a region having a high refractive index only with respect to the polarization component of.

The optical waveguide 12 can be formed, for example, on _{a substrate 11 made of LiNbO 3} or the like as a region in which a waveguide mode exists only for a single polarization component by a proton exchange method.

Further, when the optical waveguide 12 is manufactured by diffusing Ti atoms in the substrate 11 or by epitaxially growing it on the substrate 11, the waveguide mode is changed to single polarized light by devising the refractive index distribution. It can be formed as a limited area. For this optical waveguide 12, for example, a LiNbO ₃ crystal optical waveguide can be used, and it can be formed by diffusing Ti on the surface of a substrate 11 _{made of LiNbO 3 or the like.} The refractive index of this Ti-diffused region is higher than that of other regions, and the light of a single polarization component can be confined. Therefore, the light capable of propagating the light of a single polarization component can be propagated. The waveguide 12 can be formed. The refractive index is high for both orthogonal polarization components, but there is also a condition that the waveguide mode is established only for a single polarization component.

_{The LiNbO 3} crystal type optical waveguide 12 produced based on such a method has electricity such as the Pockels effect in which the refractive index changes in proportion to the electric field and the Kerr effect in which the refractive index changes in proportion to the self-power of the electric field. Since it has an optical effect, it is possible to modulate the light of a single polarization component by utilizing such a physical phenomenon.

The buffer layer 14 covers the optical waveguide 12 in order to suppress the propagation loss of light of a single polarization component. By the way, if the film thickness of the buffer layer 14 is made too thick, the electric field strength is lowered and the modulation efficiency is lowered. Therefore, the film thickness is set as thin as possible within a range in which the light propagation loss of a single polarization component does not increase. You may try to do it.

The electrode 83 is made of a metal material such as Ti, Pt, or Au, and is phase-modulated to light propagating in the optical waveguide 12 by driving and inputting a modulation signal having a frequency fm supplied from the oscillator 16 to the optical waveguide 12. multiply.

The first protective material 86 and the second protective material 87 are each composed of members corresponding to the material of the substrate 11. The first protective material 86 and the second protective material 87 may be made of the same material as the substrate 11. Further, the end face 86a and the first end face 84 of the first protective material 86 forming the plane 91 may be processed so as to have the same crystal orientation as each other, and similarly, the plane 92 is formed. The end face 87a of the protective material 87 and the second end face 85 may be processed so as to have the same crystal orientation as each other.

The incident-side antireflection film 63 is adhered on a flat surface 91 formed between the first end surface 84 and the end surface 86a of the first protective material 86. The exit-side antireflection film 64 is adhered on a flat surface 91 formed between the second end surface 85 and the end surface 87a of the second protective material 87. These antireflection films 63 and 64 may be made of a low reflection film, or may be made of an uncoated film so as to obtain the same effect as that of a low reflection film. ..

The terminating resistor 18 is a resistor attached to the end of the electrode 83, and prevents the disturbance of the waveform by preventing the reflection of the electric signal at the end.

Next, a method of manufacturing the light modulator 8 will be described with reference to FIG.

First, in step S11, a photoresist pattern 13 is produced on the surface of the substrate 11 made of _{LiNbO 3 crystals.}

Next, the process proceeds to step S12, and _{the substrate 11 of the LiNbO 3} crystal having the photoresist pattern 13 formed on the surface is heated in a state of being immersed in a proton exchange solution such as benzoic acid to heat the Li of the surface layer portion of the substrate 11. The optical waveguide 12 is formed as a region in which the waveguide mode exists only for a single polarization component by the proton exchange method of substituting with H +.

In the manufacturing process of the optical waveguide 12 in this step S11 and S12, is not limited to a proton exchange method, for example, in step S11, the photoresist pattern 13 on the surface of the substrate 11 made of LiNbO ₃ crystal Ti is vapor-deposited on the surface of the substrate 11 made of LiNbO ₃ crystals, and the photoresist is removed to prepare a thin wire of Ti having a width of a micron size. In the next step S12, this is prepared. By heating the substrate 11 on which the thin wire of Ti is formed, the Ti atom is thermally diffused in the substrate 11, and the optical waveguide 12 is provided as a region in which the waveguide mode exists only for a single polarization component. This may be replaced by the Ti diffusion method to be formed.

Next, the process proceeds to step S13, the resist pattern 13 is removed, and a SiO ₂ thin film as the buffer layer 14 is deposited on the surface of the substrate 11. In this step S13, the buffer layer 14 may be formed by a method of attaching _{the SiO 2 wafer to the surface of the substrate 11.} In such a case, the film thickness may be controlled to an appropriate level by polishing the vapor-deposited buffer layer 14 in consideration of the electrode mounting region in the next step S14.

Next, the process proceeds to step S14, and the electrode 83 is formed on the buffer layer 14.

Next, the process proceeds to step S15, and the protective materials 86 and 87 are adhered to the upper part of the optical waveguide 12. As for the method of adhering the protective materials 86 and 87, they may be attached with an adhesive or may be directly bonded based on another method. When the substrate 11 is _{composed of LiNbO 3} crystals, the protective materials 86 and 87 may be composed _{of LiNbO 3} as the same material. In step S15, the end faces 86a and 87a of the attached protective materials 86 and 87 can form planes 91 and 92 between the first end face 84 and the second end face 85, respectively. And trim it.

Finally, the process proceeds to step S16, and the obtained planes 91 and 92 are polished. Then, the incident side antireflection film 63 and the exit side antireflection film 64 are formed on the polished flat surfaces 91 and 92, respectively, over one surface thereof. Incidentally, in this step S16, the incident side antireflection film 63 and the exit side antireflection film 64 may be formed first on the planes 91 and 92, and then polished.

As described above, in the light modulator 8, since the protective materials 86 and 87 are attached to each end portion, the end surface of the optical waveguide 12 conventionally located at the uppermost corner of the end surface is a flat surface 91 (92). ) Moves to the central part. As a result, even when the corner of the plane 91 (92) is chipped during polishing in step S16, the end face of the optical waveguide 12 is not chipped. That is, it is possible to configure the optical waveguide 12 so that the end face itself is less likely to be chipped. This makes it possible to suppress the light loss from each end face of the optical waveguide 12 as much as possible.

Further, by configuring the protective materials 86 and 87 with the optimum material corresponding to the material of the substrate 11, the polishing speed in step S16 is set to the first end face 84 and the second end face 85 to the end face 86a of the substrate 11. It can be made uniform over 87a. As a result, the end face of the optical waveguide 12 is not rounded during processing, flat surfaces 91 and 92 made of a flat polished surface can be obtained, and the reflection loss on the end face of the optical waveguide 12 can be minimized. Become. Further, by making the crystal orientations of the end faces constituting the planes 91 and 92 the same, it is possible to further suppress the reflection loss.

Further, by intentionally providing the protective materials 86 and 87, the accuracy of polishing in step S16 is improved, and the verticality of the obtained plane 91 (92) with respect to the optical waveguide 12 is also improved. As a result, it is possible to minimize the light loss due to the deviation of the verticality.

Further, the incident side antireflection film 63 and the exit side antireflection film 64 are very stable because they are formed over a wide range from the first end face 84 and the second end face 85 to the end faces 86a and 87a of the substrate 11. Therefore, it is difficult to peel off, and it is possible to improve the reproducibility of the film formation.

In order to experimentally verify the effect of providing the protective material 86,87, the flat surface 91 (92) after the protective material 86,87 was attached was polished, and the end face of the optical waveguide 12 was actually polished. It can be confirmed that no chipping or bending occurs in the portion, and that flat optical polishing suitable for adhesion of the incident side antireflection film 63 and the exit antireflection film 64 is performed.

In particular, the first protective material 86 and the second protective material 87 are made of the same material as the substrate 11, and the end faces 86a, 87a and the first end face 84 of the protective materials 86, 87 forming the flat surfaces 91, 92. By processing the second end face 85 so as to have the same crystal orientation as each other, the hardness of the crystals becomes the same between the two, so that the planes 91 and 92 do not tilt due to the difference in polishing speed.

In the light modulator 8 having such a configuration, only a single polarization component that is incident through the incident-side antireflection film 63 and propagates through the optical waveguide 12 is a modulated signal having a frequency fm supplied from the oscillator 16. Is phase-modulated and is emitted via the emission-side antireflection film 64. Moreover, in the optical modulator 8, the end face of the optical waveguide 12 can be moved to a substantially central portion of the plane 91 (92) by attaching protective materials 86 and 87 at each end portion, so that the optical waveguide 12 can be moved. It is possible to chip or round the end face, secure the verticality between the optical waveguide 12 and the flat surfaces 91, 92, improve the polishing accuracy on the flat surfaces 91, 92, and improve the yield.

Here, both the single-polarized optical waveguide produced by the proton exchange method in which only a single polarization component of the incident light is propagated and only the orthogonal polarization component of the incident light are propagated. As shown in FIGS. 4, 5, and 6, the orthogonally polarized optical waveguide produced by the Ti diffusion method has a width W1 = 1.9 [mm], a length L1 = 27.4 [mm], and a thickness T1. _{Three optical waveguides 12A, 12B, 12C are formed on the LiNbO 3} crystal substrate 11 of = 0.5 [mm], and the width W2 = 1.9 [mm] and the length above the optical waveguides 12A, 12B, 12C. By adhering protective materials 86,87 having L2 = 1.5 [mm] and thickness T2 = 0.5 [mm] and polishing the end face of _{the protective material 86,87 and the end face of the LiNbO 3 crystal substrate 11. As a LiNbO 3} crystal substrate block in which the entrance surface and the exit surface of the three optical waveguides 12A, 12B and 12C are finished flat, the optical path width W3 of the three optical waveguides 12A, 12B and 12C is 6.0 [μm], respectively. , 6.3 [μm], 6.6 [μm], 6.9 [μm], 7.2 [μm], 7.5 [μm], 10 samples of 6 types were prepared, and 3 samples were prepared. When the reflectances of the incident surfaces A1, B1, C1 and the exit surfaces A2, B2, and C2 of the optical waveguides 12A, 12B, and 12C were measured and the finesse and transmittance of each sample were obtained, the following results were obtained. rice field.

That is, the finesse of the orthogonally polarized optical waveguide was about 30 to 45, whereas the finesse of the single polarized optical waveguide was about 50 to 65. Further, the transmittance of the orthogonally polarized optical waveguide was about 12.5 to 25 [%], whereas the transmittance of the single polarized optical waveguide was about 20 to 32.5 [%]. Has been done.

Next, the reciprocating modulation type optical modulator 51 as shown in FIG. 7 will be described. Regarding the same configuration and elements as the above-mentioned optical modulator 8 in the light modulator 51, the explanations in FIGS. 1 and 2 will be cited, and the description thereof will be omitted here.

As shown in FIG. 7A, the light modulator 51 is formed and propagates on the substrate 11 and the substrate 11 as a region in which the waveguide mode exists only for a single polarization component. The optical waveguide 12 that modulates the phase of the light to be generated, the buffer layer 14 that is laminated so as to cover the optical waveguide 12 on the substrate 11, and the direction of the modulation electric field are substantially perpendicular to the light propagation direction. An electrode 83 provided on the upper surface of the optical waveguide 12, a first protective material 86 and a second protective material 87 arranged on the upper part of the optical waveguide 12, and an antireflection film 63 adhered on a flat surface 91. And an emission side reflective film 94 to be adhered on the flat surface 92.

Further, when the optical modulator 51 is actually used, as shown in FIG. 7B, the input light from a light source (not shown) is transmitted or the output light output from the optical modulator 51 is transmitted. An optical system consisting of an optical transmission path 23 composed of an optical fiber or the like for transmitting to the outside, an optical circulator 21 for separating the input light and the output light, and a focuser 22 optically connected to the optical circulator 21. Is mounted, and an oscillator 16 arranged on one end side of the electrode 83 to oscillate a modulated signal having a frequency fm and a termination resistor 18 arranged on the other end side of the electrode 83 are further arranged.

The antireflection film 63 is adhered on a flat surface 91 formed between the first end surface 84 and the end surface 86a of the first protective material 86. The antireflection film 63 may be made of a low-reflection film, or may be made of an uncoated film so as to obtain the same effect as that of a low-reflection film.

The focuser 22 focuses the input light that has passed through the optical circulator 21 to the end of the optical waveguide 12, and also collects the output light that has passed through the antireflection film 63 from the end of the optical waveguide 12 and collects the output light, which is collected by the optical circulator 21. Send to. The focuser 22 may be composed of a lens or the like for optical coupling of input light so that the spot diameter corresponds to the diameter of the optical waveguide 12.

The light modulator 51 having such a configuration emits light as a highly reflective film at one end of the optical waveguide 12 formed as a region in which the waveguide mode exists only for a single polarization component. By providing the side reflective film 94 and providing the antireflection film 63 at the other end portion, the light modulator operates as a so-called reciprocating modulation type optical modulator. The input light incident on the optical waveguide 12 is modulated while propagating only a single polarization component through the optical waveguide 12, is reflected by the emission side reflective film 94 on the end face, and then propagates through the optical waveguide 12 again. It passes through the antireflection film 63 and is emitted to the focuser 22 side to be output light having only a single polarization component. At the same time, the electric signal having a frequency of fm supplied from the oscillator 16 propagates on the electrode 83 while modulating the input light, and then is absorbed by the terminating resistor 18.

Further, as shown in FIG. 7C, the light modulator 51 is provided with an oscillator 25 and a terminating resistor 27 on one end side of the electrode 83, and propagates an electric signal supplied from the oscillator 25 on the electrode 83. After that, this may be reflected on the other end side of the electrode 83. At this time, an isolator 26 for separating the electric signal supplied from the oscillator 25 and the electric signal reflected by the other end side of the electrode 83 may be provided. Further, in the light modulator 51, an incident side reflective film 93 having a high reflectance is adhered. As a result, light can be resonated inside the optical waveguide 12. Further, as an alternative to the incident side reflective film 93, the above-mentioned low reflectance antireflection film 63 may be adhered. This makes it possible to perform phase modulation while reciprocating light only once in the optical waveguide 12.

In the light modulator 51, the phase of light can be modulated by each of the electric signals reciprocating on the electrode 83 by adjusting the reflection phase of the electric signal according to the phase of the light reflected by the light emitting side reflecting film 94. Therefore, the modulation efficiency can be increased. In particular, by attaching the protective materials 86 and 87, it is possible to further increase the optical modulation efficiency in the optical modulator 51 in which the peeling and chipping of the films 63 and 94 are suppressed and the finesse is further improved as described above. It becomes.

Further, when these light modulators 51 are applied to an optical comb generator, it is possible to perform reciprocating modulation on the light resonating in the optical waveguide 12 by the electric signal reciprocating between the electrodes. In such a case, the intensity distribution at each frequency (wavelength) of the generated sideband is when the modulation frequency of the electric signal applied to the electrode 83 is 25 GHz and the power is 0.5 W, as shown in FIG. In, the modulation index expressed as the magnitude of the modulation applied in the optical waveguide 12 is π-radian per propagation direction. From this result, it can be seen that the half-wave voltage Vπ defined as the voltage required to move the phase by half a wavelength is 7.1 V.

Instead of reflecting the electric signal, the light modulator 51 may divide the output of the oscillator 16 as a signal source to separately drive and input the electric signal from both ends of the electrode 83. , This may be performed by connecting different oscillators 16 to both ends of the electrode 83.

Here, in the light modulators 8 and 51, the electrode 83 provided on the upper surface of the optical waveguide 12 to which the modulation signal is supplied has a ridge structure, so that the light modulation efficiency can be further improved. ..

Here, in the optical modulators 8 and 51 produced in this manner, as shown in FIGS. 9 and 10, in step S16, the planes 91 and 92 are polished in parallel with each other, and the polished planes are polished. An optical comb is generated by forming the incident side antireflection film 93 and the exit side antireflection film 94 on the 91 and 92 in place of the incident side antireflection film 63 and the emission side antireflection film 64, respectively, over one surface thereof. Functions as vessel 1.

That is, in the optical comb generator 1, the incident side reflective film 93 and the outgoing side reflective film 94 are provided so as to be parallel to each other in order to resonate the light incident on the optical waveguide 12. The optical resonator 5 is configured to resonate by reciprocating the light passing through the light.

Light having a frequency ν1 is incident on the incident side reflective film 93 from a light source (not shown). Further, the incident-side reflective film 93 reflects light of a single polarized component that is reflected by the outgoing-side reflective film 94 and has passed through the optical waveguide 12. The exit-side reflective film 94 reflects light of a single polarized component that has passed through the optical waveguide 12. Further, the exit-side reflective film 94 emits light of a single polarized component that has passed through the optical waveguide 12 to the outside at a constant ratio.

The incident side reflective film 93 and / or the outgoing side reflective film 94 may be formed over one plane 91 and 92, respectively, but are formed so as to cover only the end portion of the optical waveguide 12 at a minimum. It suffices if it is done.

The terminating resistor 18 is a resistor attached to the end of the electrode 83, and prevents the disturbance of the waveform by preventing the reflection of the electric signal at the end.

FIG. 11 shows the plane 91 on which the incident side reflective film 93 is formed from the direction A in FIG.

The same plane 91 is formed by the first end surface 84 including the light incident end of the optical waveguide 12 and the end surface 86a of the protective material 86. The formed plane 91 has an inclination of 0.05 ° or less. Relative to the plane 91 of the slope 0.05 °, when calculating the loss in the case where the light of the 1 / _{e 2} beam diameter 10μm is reflected at the end face of the slope 0.05 °, a 4 × ^{10 -4,} the incident It is negligibly small compared to the reflectance of the side reflective film 93.

By forming the first end face 84 and the second end face 85 substantially perpendicular to the optical waveguide 12 in this way, the incident side reflective film 93 and the outgoing side reflective film 94 adhered thereto are single. The light of the polarized component can be efficiently resonated.

In the optical comb generator 1 having the above-described configuration, the light incident from the outside through the incident side reflective film 93 is such that the light of a single polarization component propagates in the optical waveguide 12 in the outward path direction and is emitted from the exit side. It is reflected by the reflective film 94 and partially penetrates to the outside. The light of a single polarized wave component reflected by the emitting side reflecting film 94 propagates in the optical waveguide 12 in the return path direction and is reflected by the incident side reflecting film 93. By repeating this, the light of a single polarized wave component resonates in the optical waveguide 12.

Further, by using an electric signal synchronized with the time when the light of a single polarization component reciprocates in the optical waveguide 12 as a drive input via the electrode 83, the light of a single polarization component is converted into this light modulator. It is possible to apply a deep phase modulation of several tens of times or more as compared with the case of passing through the inside of 8 only once. In addition, hundreds of sidebands can be generated over a wide band around the frequency ν1 of the incident light. Incidentally, the frequency interval of each of the generated side bands is equivalent to the frequency fm of the input electric signal. Therefore, the optical modulator 8 is a single bias composed of a large number of side bands by replacing the incident side antireflection film 63 and the exit side antireflection film 64 with the incident side antireflection film 93 and the exit side antireflection film 94. It functions as an optical comb generator 1 that generates optical combs of wave components.

The optical comb generator 1 is waveguideed only to a single polarization component on a substrate 11 having at least an electro-optical effect so as to penetrate from the incident-side reflective film 93 constituting the resonance means to the outgoing-side reflective film 94. Since the optical waveguide 12 formed as a region where the mode exists is provided, only a single polarization component of the light incident through the incident side reflective film 93 is propagated through the optical waveguide 12. An optical comb can be generated as an optical modulation output of only a single polarization component via the light emitting side reflective film 94.

As described above, in the light modulators 8 and 51, the protective materials 86 and 87 are attached to each end, so that the end face of the optical waveguide 12 conventionally located at the uppermost corner of the end face is a flat surface 91. As a result of moving to the substantially central portion of (92), even if the corner of the plane 91 (92) is chipped during polishing in step S17, the end face of the optical waveguide 12 is not chipped. That is, it is possible to configure the optical waveguide 12 so that the end face itself is less likely to be chipped. This makes it possible to suppress light loss from each end face of the optical waveguide 12 as much as possible. Moreover, by providing the protective materials 86 and 87, it is possible to suppress a change in the film thickness due to the incident side reflective film 93 and the outgoing side reflective film 94 to be adhered wrapping around from the planes 91 and 92 to other side surfaces. .. Therefore, the film thickness in the vicinity of the end face of the optical waveguide 12, which is important for ensuring the reflectance, can be optimized, and the reflectance can be further improved.

Further, the incident side reflective film 93 and the outgoing side reflective film 94 are very stable because they are formed over a wide range from the first end face 84 and the second end face 85 to the end faces 86a and 87a on the substrate 11. It is difficult to peel off, and it is possible to improve the reproducibility of the film formation.

In order to experimentally verify the effect of providing the protective material 86,87, the flat surface 91 (92) after the protective material 86,87 was attached was polished, and the end face of the optical waveguide 12 was actually polished. It can be confirmed that no chipping or bending occurs in the portion, and that flat optical polishing suitable for adhesion of the incident side reflective film 93 and the outgoing side reflective film 94 is performed.

In particular, the first protective material 86 and the second protective material 87 are made of the same material as the substrate 11, and the end faces 86a, 87a and the first end face 84 of the protective materials 86, 87 forming the flat surfaces 91, 92. By processing the second end face 85 so as to have the same crystal orientation as each other, the hardness of the crystals becomes the same between the two, so that the planes 91 and 92 do not tilt due to the difference in polishing speed.

As described above, in the optical modulator 8 and the optical comb generator 1, the end faces of the optical waveguide 12 are abbreviated as the flat surface 91 (92) as shown in FIG. 11 by attaching the protective materials 86 and 87 to the respective end portions. Since it can be moved to the central part, the end face of the optical waveguide 12 is chipped or rounded, the verticality between the optical waveguide 12 and the flat surfaces 91 and 92 is ensured, the polishing accuracy on the flat surfaces 91 and 92 is improved, and the incident side reflective film 93 is used. In addition, it is possible to suppress peeling and wraparound of the exit side reflective film 94, improve the reflectance of the incident side reflective film 93 and the outgoing side reflective film 94, realize the designed reflection characteristics, and improve the performance reproducibility of the reflective film. As a result, the finesse of the optical resonator 5 composed of the incident side reflective film 93 and the outgoing side reflective film 94 can be improved, and a high-performance optical modulator and optical comb generator can be manufactured with good reproducibility. It becomes possible, and it becomes possible to improve the yield.

Further, a waveguide type optical modulator 8A (optical comb generator 1A) having a configuration as shown in FIG. 12 will be described.

The waveguide type optical modulator 8A (optical comb generator 1A) has a ridge structure of an electrode 83 in the waveguide type optical modulator 8 (optical comb generator 1A) shown in FIGS. 1 and 2 (FIGS. 9 and 10). For the same configuration and elements as the above-mentioned optical modulator 8 (optical comb generator 1), the explanations in FIGS. 1 and 2 (FIGS. 9 and 10) are cited here. The explanation of is omitted.

In this waveguide type optical modulator 8A (optical comb generator 1A), the substrate 11 is formed by cutting out a large crystal such as _{LiNbO 3 or GaAs having a diameter of 3 to 4 inches grown by a pulling method into a wafer shape.} be. A ridge portion 20 for forming an electrode 83A having a ridge structure is provided on the cut-out substrate 11 by subjecting it to a process such as mechanical polishing or chemical polishing.

The optical waveguide 12 is formed by a proton exchange method or a Ti diffusion method so as to penetrate from an incident end to an emitted end, and guides light of a single polarized component only to a single polarized component in order to propagate light. It is formed as a region where the wave mode exists.

The refractive index of the layers constituting the optical waveguide 12 is set higher than that of other layers such as the substrate 11 only for a single polarization component. The light incident on the optical waveguide 12 propagates while only a single polarization component is totally reflected at the boundary surface of the optical waveguide 12.

_{The LiNbO 3} crystal type optical waveguide 12 produced based on such a method has electricity such as the Pockels effect in which the refractive index changes in proportion to the electric field and the Kerr effect in which the refractive index changes in proportion to the self-power of the electric field. Since it has an optical effect, it is possible to modulate the light of a single polarization component by utilizing such a physical phenomenon.

The electrode 83A having a ridge structure has a main electrode formed on the ridge portion 20, and is made of a metal material such as Ti, Pt, or Au. The electrode 83A having a ridge structure in which the main electrode is formed on the ridge portion 20 propagates in the optical waveguide 12 by using a modulation signal having a frequency fm supplied from the oscillator 16 as a drive input to the optical waveguide 12. Phase modulation the light.

A method of manufacturing the waveguide type optical modulator 8A (optical comb generator 1A) having such a structure will be described with reference to FIG.

That is, first, in step S21, a photoresist pattern 13 is produced on the surface of the substrate 11 made of _{LiNbO 3 crystals.}

Next, the process proceeds to step S22, and _{the substrate 11 of the LiNbO 3} crystal having the photoresist pattern 13 formed on the surface is heated in a state of being immersed in a proton exchange solution such as benzoic acid to heat the Li of the surface layer portion of the substrate 11. The optical waveguide 12 is formed as a region in which the waveguide mode exists only for a single polarization component by the proton exchange method of substituting with H +.

In the manufacturing process of the optical waveguide 12 in this step S21 and S22, is not limited to a proton exchange method, for example, in step S21, the photoresist pattern 13 on the surface of the substrate 11 made of LiNbO ₃ crystal Ti is vapor-deposited on the surface of the substrate 11 made of LiNbO ₃ crystals, and the photoresist is removed to prepare a thin wire of Ti having a width of a micron size. In the next step S22, this is prepared. By heating the substrate 11 on which the thin wire of Ti is formed, the Ti atom is thermally diffused in the substrate 11, and the optical waveguide 12 is provided as a region in which the waveguide mode exists only for a single polarization component. This may be replaced by the Ti diffusion method to be formed.

Next, the process proceeds to step S23, the resist pattern 13 of the substrate 11 on which the optical waveguide 12 is formed is removed, and the electrode 83A having a ridge structure is further processed by mechanical polishing, chemical polishing, or the like, as shown in FIG. The ridge portion 20 for forming the above is provided.

Next, the process proceeds to step S24, and the SiO ₂ thin film as the buffer layer 14 is vapor-deposited on the surface of the substrate 11. In this step S24, the buffer layer 14 may be formed by a method of attaching _{the SiO 2 wafer to the surface of the substrate 11.} In such a case, the film thickness may be controlled to an appropriate level by polishing the vapor-deposited buffer layer 14 in consideration of the electrode mounting region in step S25 described later.

Next, the process proceeds to step S25, and as shown in the vertical cross section of the main part of FIG. 15, an electrode 83A having a ridge structure is formed on the buffer layer 14 of the substrate 11.

Next, the process proceeds to step S26, and the protective materials 86 and 87 are adhered to the upper part of the optical waveguide 12. As for the method of adhering the protective materials 86 and 87, they may be attached with an adhesive or may be directly bonded based on another method. When the substrate 11 is _{composed of LiNbO 3} crystals, the protective materials 86 and 87 may be composed _{of LiNbO 3} as the same material. In step S26, the end faces 86a and 87a of the attached protective materials 86 and 87 can form planes 91 and 92 with the first end face 84 and the second end face 85, respectively. And trim it.

In the light modulator 8A, in the final step S27, the obtained planes 91 and 92 are polished, and the incident side antireflection film 63 and the exit side antireflection film 64 are respectively formed on the polished planes 91 and 92. It is formed over one surface.

Further, in the optical comb generator 1A, in step S27, the flat surfaces 91 and 92 are polished in parallel with each other, and the incident side antireflection film 63 and the outgoing side antireflection film 64 are polished on the polished flat surfaces 91 and 92. Instead, the incident side reflective film 93 and the outgoing side reflective film 94 are formed over one surface of each.

In the optical modulator 8A and the optical comb generator 1A having such a configuration, the oscillator 16 is connected to the electrode 83A having a ridge structure with respect to light of only a single polarization component incident from the incident end and propagating through the optical waveguide 12. The modulation signal of the frequency fm supplied from the above can efficiently perform phase modulation.

Moreover, in the optical modulator 8A and the optical comb generator 1A, the end face of the optical waveguide 12 can be moved to a substantially central portion of the plane 91 (92) by attaching protective materials 86 and 87 at each end. Therefore, the end face of the optical waveguide 12 is chipped or rounded, the verticality between the optical waveguide 12 and the planes 91 and 92 can be ensured, the polishing accuracy on the planes 91 and 92 can be improved, and the yield can be improved.

Further, in the optical comb generator 1A, the light incident from the outside through the incident side reflective film 93 propagates only a single polarization component in the outward path direction in the optical waveguide 12 and is reflected by the outgoing side reflective film 94. At the same time, it partially penetrates to the outside. The light of a single polarized wave component reflected by the emitting side reflecting film 94 propagates in the optical waveguide 12 in the return path direction and is reflected by the incident side reflecting film 93. By repeating this, the light of a single polarized wave component resonates in the optical waveguide 12.

Further, by using an electric signal synchronized with the time when the light of a single polarization component reciprocates in the optical waveguide 12 as a drive input via the electrode 83A, the light of a single polarization component is converted into this light modulator. It is possible to apply a deep phase modulation of several tens of times or more as compared with the case of passing through the inside of 8 only once. Further, hundreds of side bands can be generated over a wide band around the frequency ν1 of the incident light. Incidentally, the frequency interval of each of the generated side bands is equivalent to the frequency fm of the input electric signal. Therefore, the light modulator 8A functions as an optical comb generator 1A that generates an optical comb of a single polarization component composed of a large number of side bands.

As described above, in the optical comb generator 1A, with respect to a single polarization component on a substrate having at least an electro-optical effect so as to penetrate from the incident side reflective film 93 constituting the resonance means to the outgoing side reflective film 94. Since the optical waveguide 12 is provided as a region where only the waveguide mode exists, only a single polarization component of the light incident through the incident side reflective film 93 propagates through the optical waveguide 12. Therefore, an optical comb can be generated as an optical waveguide output of only a single polarization component via the light emitting side reflective film 94, and protective materials 86 and 87 are attached to each end portion. The end face of the optical waveguide, which was conventionally located at the uppermost corner of the end face, moves to the substantially central portion of the plane 91 (92). As a result, even when the corner of the plane 91 (92) is chipped during polishing in step S27, the end face of the optical waveguide 12 is not chipped. That is, it is possible to configure the optical waveguide 12 so that the end face itself is less likely to be chipped. This makes it possible to suppress the light loss from each end face of the optical waveguide 12 as much as possible.

Moreover, since the optical modulator 8A includes an electrode 83A having a ridge structure formed on the buffer layer 14 of the substrate 11, as shown in the vertical cross section of the main part of FIG. 15, the modulation efficiency is further improved. Can be made to.

Here, in this light modulator 8A, the ridge width RW of the electrode 83A having a ridge structure formed on the buffer layer 14 of the substrate 11 is 10, 12, 14, 16, and 18 [μm], and the average of the ridge grooves. Samples of the light modulator 8A with depths AVD (Average depths) of 3.3, 2.96, 4.79, and 4.72 [μm] were prepared, and the drive voltage (AC Vpi) and direct current at 25 GHz were prepared. The driving voltage (DC Vpi) was actually measured and the results are shown in FIGS. 16 and 17. Vpi is the voltage required to π-radian the phase.

That is, in the conventional optical modulator having an electrode structure having no ridge structure, the drive voltage (AC Vpi) at 25 GHz is about 8 to 10 V, and the DC drive voltage (DC Vpi) is about 6 to 6.5 V. On the other hand, in the optical modulator 8A provided with the electrode 83A having a ridge structure, the drive voltage (AC Vpi) at 25 GHz is about 3.5 to 7.5 V, and the DC drive voltage (DC Vpi) is 5 to 6 V. It has become a degree.

By providing the ridge structure in this way, the average voltage of the drive voltage (AC Vpi) at 25 GHz is reduced to about 70% of the original voltage as compared with the case without the ridge structure, and the electric power is about 50%. Corresponds to a decrease in. Further, as for the direct current drive voltage (DC Vpi), the average voltage is reduced to about 80% of the original voltage as compared with the case without the ridge structure, which corresponds to a decrease of about 50% in electric power.

That is, the optical modulator 8A and the optical comb generator 1A are composed of members having the same hardness as the substrate 11 of the optical waveguide 12, and at least one end face of the member is the light incident end or light in the optical waveguide 12. A first protective material 86 and a second protective material 87 arranged on the upper part of the optical waveguide 12 so as to form the same plane as the end surface of the substrate 11 including the emission end are provided, and the end surface of the member Since the incident side antireflection film 63 or the incident side antireflection film 93 and the emission side antireflection film 63 or the emission side antireflection film 94 are respectively adhered to the plane formed by polishing the end surface of the substrate, the end surface of the optical waveguide The finesse of the optical resonator 5 composed of the incident side reflective film 93 and the outgoing side reflective film 94 can be improved by preventing the occurrence of chipping and stabilizing the attachment of the high reflective film. The driving power can be reduced by providing the electrode 83A having a ridge structure.

Therefore, the optical modulators optical modulators 8 and 8A and the optical modulator 51 having the above-described configuration include at least one of the members 86 and 87 having the same hardness as the substrate 11 for forming the optical waveguide 12 from the upper surface. Is arranged on the upper part of the optical waveguide 12 so that the end surface of the optical waveguide 12 forms the same plane as the end surface of the substrate 11 including the light incident end or the light emitting end of the optical waveguide 12, and is arranged with the end faces of the members 86 and 87. Since the incident side reflective film 93 and the outgoing side reflective film 94 constituting the resonance means are adhered on the plane formed by polishing the end surface of the substrate 11, chipping during processing of the corners of the waveguide end surface may occur. It suppresses rounding, and each reflective film can be stably adhered without peeling off at the uppermost corner of the end face, improving the reflectance of the reflective film and the finesse of the optical resonator, and enhancing the function of the device itself. The waveguide mode exists only for a single polarization component on the substrate 11 having at least an electro-optical effect so as to penetrate from the incident-side reflective film 93 constituting the resonance means to the outgoing-side reflective film 94. By providing the optical waveguide 12 formed as the region, only a single polarization component of the light incident through the incident side reflective film 93 is propagated through the optical waveguide 12 and the exit side reflective film. An optical comb capable of generating a stable optical comb as an optical modulation output of only a single polarization component via 94 functions as generators 1 and 1A.

The optical waveguide 12 in the optical modulators 8 and 8A and the optical modulator 51 as described above has a single polarization on the substrate 11 having at least an electro-optical effect so as to penetrate from the incident side reflective film 93 to the outgoing side reflective film 94. A low-power laser light source or optical comb that can output laser light or optical comb with only a single polarization component by being formed as a region where the waveguide mode exists only for the wave component. You can build a generator.

Next, the configuration of the optical comb generator 110 using the optical comb module to which the present invention is applied is shown in the block diagram of FIG.

The optical comb generator 110 includes an optical coupler 111 that branches a part of the optical comb output from the optical comb module 100A, an optical detector 112 that detects the light branched by the optical cup 111, and the photodetector. A control circuit 113 or the like to which the light detection signal obtained by 112 is supplied is provided.

In the optical comb module 100A, the laser light is incident from the laser light source 131, and the RF modulation signal is input via the bias tee 114, so that the incident laser light has a single polarization component. By applying phase modulation with an RF modulated signal, an optical comb is generated and output. In this optical comb module 100A, the resonance length of the resonance means by the incident side reflective film and the outgoing side reflective film provided in the optical waveguide is controlled by the temperature control by the temperature control circuit 119.

The control circuit 113 obtains an error with respect to the control target from the light detection signal, generates a control signal such that the error becomes zero, and supplies the control signal to the bias tee 114.

By applying it to the DC bias of the optical comb module 100A, the resonance frequency of the optical comb module 100A can be made to follow the input laser frequency.

The control circuit 113 may be a single printed circuit board or a combination of an RF mixer or an isolator and a printed circuit board. By mixing the photodetector signal and the synchronization signal of the photodetector 112, a control signal corresponding to the amount of error from the control target is created.

Part of the output of the RF modulated signal source can be used as the sync signal. In that case, the operating band of the photodetector 112 needs to be equal to or higher than the RF drive frequency.

In the control circuit 113, the low frequency component of the signal obtained by inputting the photodetector signal and the synchronization signal to the mixer via the phase adjuster is taken as an error signal. Alternatively, a modulated signal (dither signal) given by a synchronization signal source 140 different from the RF drive signal source can be used as the synchronization signal. For example, an error signal can be generated by applying modulation having a smaller amplitude than the FSR in the resonance mode to the laser frequency and mixing the output signal of the photodetector 112 with the synchronization signal. Further, if the dither signal frequency is low, it is possible to generate an error signal by the product-sum calculation of digital signal processing after converting the optical detection signal into a digital signal by an analog-digital converter.

The frequency characteristic of the error signal is adjusted and applied as a control signal to the DC bias of the optical comb module 100A via the bias tee 114. Generally, the error signal is input to a circuit having proportional, integral, and differential functions, the frequency characteristics of the control loop are determined by adjusting the amplitude of those components, and the resonance frequency of the optical comb module 100A is the input laser. It is controlled to follow the oscillation frequency.

Further, the block diagram of FIG. 19 shows the configuration of the optical comb generator 120 using the optical comb module to which the present invention is applied.

The optical comb generator 120 controls the resonator by using the reflected light of the optical comb module 100A, and a part of the reflected light of the optical comb module 100A is branched by the optical coupler 111 to the photodetector 112. It is designed to be incident.

Each component of the optical comb generator 120 is the same as the component of the optical comb generator 110 shown in FIG. 18, and the corresponding components are designated by the same reference numerals in FIG. 19 and detailed description thereof will be omitted. do.

The control circuit 113 obtains an error with respect to the control target from the light detection signal obtained by the photodetector 112, and outputs a control signal such that the error becomes zero. By applying the control signal to the DC bias of the optical comb module, the resonance frequency of the optical comb module 100A can be made to follow the input laser frequency.

That is, in the control circuit 113 in the optical comb generators 110 and 120, the laser beam in which the modulation having a smaller amplitude than the FSR in the resonance mode is applied to the laser frequency by the dither signal is applied to the optical resonator of the optical comb module 100A. Laser of laser light incident on the optical resonator from the light detection signal obtained by the light detector 112 and a part of the transmitted light or reflected light emitted from the optical resonator due to the incident and the dither signal. An error signal is generated according to the deviation between the frequency and the resonance frequency of the optical resonator, and the resonance length of the optical resonator is controlled by using this error signal to set the resonance frequency of the optical resonator to the laser frequency. It is possible to control to follow.

Further, the optical comb generators 110 and 120 include, for example, a plurality of optical comb modules in which the laser light emitted from one laser light source 131 is separated into a plurality of light sources and incident, as shown in FIG. 20, for example. The optical comb generator 130 having the configuration can be constructed.

The optical comb generator 130 is a separation optical system 132 such as a laser light source 131 for single frequency oscillation and an optical coupler or an optical beam splitter that separates a single frequency laser light emitted from the laser light source 131 into two laser lights. , A frequency shifter 135 that shifts the frequency of one of the laser beams separated by the separation optical system 132, and two optical comb generators (OFCG1, OFCG2) 130A, 130B, etc., each using an optical comb module.

In this optical comb light source 130, the laser light emitted from one single frequency oscillation laser light source 131 is separated into two laser lights by the separation optical system 132, and two optical comb generators (OFCG1 and OFCG2) are used. It is input to 130A and 130B, a synchronization signal is given to each optical comb generator 130A and 130B from the synchronization signal source 140, and a dither signal is given to the laser light source 131. The length of each resonator of 130A and 130B is controlled.

The two optical comb generators 130A and 130B are driven by oscillators 133A and 133B that oscillate at different frequencies fm + Δfm and frequency fm. The respective oscillators 133A and 133B are phase-locked by a common reference oscillator 134, so that the relative frequencies of fm + Δfm and fm become stable. In front of the optical comb generator (OFCG1) 130A, a frequency shifter 135 such as an acoustic optical frequency shifter (AOFS) is provided so that the input laser light is given an optical frequency shift of frequency fa by this frequency shifter 135. It has become. As a result, the beat frequency between the carrier frequencies becomes an AC signal having a frequency fa instead of a DC signal. As a result, the beat signal of the high frequency side band of the carrier frequency and the beat signal of the low frequency side side band are generated in the frequency regions opposite to each other with the beat frequency fa between the carrier frequencies of the beat signal, which is convenient for phase comparison. ..

The two optical comb generators (OFCG1 and OFCG2) 130A and 130B are each composed of a low-power optical comb module, and by phase-modulating only a single polarization component of the input laser light. It is possible to output an optical comb with a single polarization component.

This optical comb generator 130 shares one single frequency oscillation laser light source 131, and two optical combs having different center frequencies and frequency intervals from the two optical comb generators (OFCG1 and OFCG2) 130A and 130B. For example, the first and second light sources in the distance meter and the optical three-dimensional shape measuring machine according to the patent No. 5231883 previously proposed by the present inventor, that is, the intensity or the intensity or each of them is periodically generated. Two optical comb generators (OFCG1, OFCG2) are used as the first and second light sources that emit interfering reference light and measurement light whose phases are modulated and whose modulation periods are different from each other. ) The optical comb output for measuring a single polarization component emitted from 130A and 130B is irradiated to the surface of the measurement target while scanning, and the reflected light from the surface is detected at each irradiation point. By calculating the distance (height), it is possible to construct a measurement system of a distance meter or an optical three-dimensional shape measuring machine that performs stable measurement operation. The surface shape of the object can be obtained from the distribution of scan coordinates and distance (height). There are various forms of scanner optics. Telecentric optics can be used to ensure that light is incident almost perpendicular to the object within the measurement range.

Further, for example, it is a light source in the vibration measuring device according to Patent No. 5336921 and Patent No. 5363231 proposed by the present inventor, that is, a spectrum having a predetermined frequency interval, and the modulation frequencies and center frequencies are different from each other. By using the optical comb light source 130 as a light source unit that emits phase-synchronized and interfering reference light and measurement light, a single light comb generator (OFCG1, OFCG2) 130A, 130B is emitted. It is possible to construct a measurement system of a vibration measuring device that performs stable multipoint vibration measurement operation by irradiating different places depending on the wavelength through an element that divides the optical comb of the polarization component for each frequency.

Here, in a measuring device using an optical comb obtained by an optical comb generator using an optical waveguide that transmits a polarized component in which orthogonal modes are mixed, as shown by a circle in FIG. 22, the orthogonal polarized component is used. The transmission mode waveform may be deformed, and the locations where it occurs (relative to the main mode) are different, and there are multiple minimum parts, which causes instability in control, but only a single polarization component. By using the optical waveguide through which the light is passed, as shown in FIG. 21, the transmission mode waveform is not deformed, the stabilization as an optical comb generator, the improvement of the accuracy of the measuring device including the optical comb, the reduction of error, and the like. Can be planned.

That is, the polarization component orthogonal to the generation of the optical comb causes measurement errors in distance and height when the optical comb is used for measurement, and the polarization component orthogonal to the generation of the optical comb is the cause of the optical comb generator. The control for matching the resonance frequency to the laser frequency may become unstable, causing the control point to shift and the control to oscillate. Also, when using the optical comb for measurement, the distance and height are measured. Although it was a factor of error, by generating optical combs using an optical waveguide that allows only a single polarizing component to pass through, the output of orthogonally polarized light components that do not contribute to the generation of optical combs is suppressed, and the polarization of the optical comb output is suppressed. The extinction ratio can be improved, the degree of single polarization can be increased, the resonator control is stabilized, unnecessary interference signals are eliminated, and measurement errors in distance measurement and shape measurement using optical combs are eliminated. It is possible to improve the measurement accuracy and the reliability of the entire system.

1,1A, 110, 120, 130A, 130B Optical comb generator, 8,51 Optical modulator, 11 substrate, 12 Optical waveguide, 14 Buffer layer, 16 Oscillator, 18 Termination resistor, 20 Convex part, 21 Optical circulator, 22 Focuser, 63 Antireflection film, 83,83A Electrode, 84 First end face, 85 Second end face, 86 First protective material, 86a, 87a End face, 87 Second protective material, 91, 92 flat surface, 93 Incident side reflective film, 94 Exit side reflective film, 100A optical comb module, 111 optical coupler, 112 optical detector, 113 control circuit, 114 bias tee, 130 optical comb generator, 131 laser light source, 132 separation optical system, 135 Frequency shifter, 140 synchronous signal source

Claims (6)

By modulating the phase of the laser light resonated by the optical resonator according to the modulated signal, a side band centered on the frequency of the laser light incident on the optical resonator is generated at intervals of the frequencies of the modulated signal. This is a method for controlling the resonance length of the optical cavity in the optical comb generator.
A laser beam whose amplitude is smaller than that of the FSR in resonance mode and which is applied to the laser frequency by a dither signal is incident on the optical resonator.
From the light detection signal that detects a part of the transmitted light or reflected light emitted from the optical resonator and the dither signal, the laser frequency of the laser light incident on the optical resonator and the resonance frequency of the optical resonator It is characterized in that an error signal corresponding to the deviation is generated, the resonance length of the optical resonator is controlled by using this error signal, and the resonance frequency of the optical resonator is controlled to follow the laser frequency. How to control the resonance length of an optical cavity. A method for controlling the resonance length of an optical resonator in an optical comb generator including a plurality of optical comb generators in which laser light emitted from one laser light source is separated into a plurality of laser beams and incident on each optical resonator.
From the above-mentioned one laser light source, a laser beam in which a modulation having a smaller amplitude than that of the FSR in the resonance mode is applied to the laser frequency by a dither signal is incident on each of the above-mentioned optical cavities.
From each light detection signal that detects a part of the transmitted light or reflected light emitted from each of the optical cavities and the dither signal, the laser frequency of the laser light incident on each of the optical cavities and the respective optical resonances. Each error signal is generated according to the deviation of the resonance frequency of the instrument, the resonance length of each optical cavity is controlled by using each generated error signal, and the resonance frequency of each optical cavity is set to the laser frequency. A method for controlling the resonance length of an optical cavity, which comprises controlling the frequency to follow. The method for controlling the resonance length of an optical resonator according to claim 1 or 2, wherein the error signal is generated by mixing the optical detection signal and the synchronization signal. The method for controlling the resonance length of an optical resonator according to claim 1 or 2, wherein an error signal is obtained by a product-sum calculation of digital signal processing from the optical detection signal and the synchronization signal. By modulating the phase of the laser light resonated by the optical resonator according to the modulated signal, a side band centered on the frequency of the laser light incident on the optical resonator is generated at intervals of the frequencies of the modulated signal. Optical frequency generator
A laser light source that contains a single frequency component and emits laser light that is applied to the laser frequency by a dither signal that is modulated with a smaller amplitude than the FSR in resonance mode.
An oscillator that oscillates a modulated signal that oscillates a modulated signal of a predetermined frequency,
It is provided with an optical resonator in which laser light is incident from the laser light source, and the phase of the light resonated by the optical resonator is modulated according to the modulation signal supplied from the oscillator, and the frequency of the incident light is An optical comb generating means that generates a side band centered on the above at intervals of the frequencies of the modulated signals, and
A control unit that controls the resonance length of the optical resonator,
It is provided with a synchronization signal source that gives a synchronization signal to the control unit and outputs a dither signal to be applied to the laser light source.
The control unit includes an optical detector that detects a part of transmitted light or reflected light emitted from the optical resonator of the optical comb generating means, and uses the optical detection signal by the optical detector and the synchronization signal. An error signal is generated from the laser light source according to the deviation between the laser frequency of the light incident on the optical resonator of the optical comb generating means and the resonance frequency of the optical resonator, and the error signal is used to generate the light. An optical comb generator characterized in that the resonance frequency of the resonator is controlled to follow the laser frequency. An optical comb generator including a plurality of optical comb generators in which laser light emitted from one laser light source is separated into a plurality of laser beams and incident on each optical resonator.
A laser light source that contains a single frequency component and emits laser light that is applied to the laser frequency by a dither signal that is modulated with a smaller amplitude than the FSR in resonance mode.
The laser light is branched and incident from the laser light source, and the phase of the laser light resonated by each optical resonator whose resonance length is controlled is modulated according to modulation signals having different modulation frequencies and then incident. A plurality of optical comb generators that generate side bands centered on the frequency of the laser light at intervals of different modulation frequencies of the above-mentioned modulated signals.
It is equipped with a synchronization signal source that gives a synchronization signal to the plurality of optical comb generators and outputs a dither signal to be applied to the laser light source.
The plurality of optical comb generators are lasers of light incident from the laser light source by the light detection signal for detecting a part of the transmitted light or the reflected light emitted from the optical comb generator and the synchronization signal, respectively. An optical comb generator characterized in that an error signal corresponding to a deviation between a frequency and a resonance frequency of the optical resonator is generated to control the resonance frequency of the optical resonator to follow the laser frequency.

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