Optical comb generator control arrangement

Abstract

Provided is an optical comb generator control device that, even if the method of generating an optical comb is not entirely single mode, or even if the optical comb generator is one in which the modulation frequency is switched, controls the optical resonance length or the light source frequency in such a way that the control point is located at a position at which the widest optical comb is generated, thereby enabling stable output. The carrier frequency component of the optical comb emitted from the optical resonator of the optical comb generator (2) as transmitted light or reflected light is attenuated by an optical filter (5), the optical frequency component extracted by the optical filter (5) is received by a photodetector (6) and the light intensity is detected, and the optical resonance length of the optical resonator or the light source frequency of the incident light is feedback controlled by a resonance control section (8) using the detection signal obtained by the photodetector (6).

Description

Technical Field

[0001] This invention relates to a control device for an optical comb generator used in optical communication systems and optical measurement systems. This application claims priority based on Japanese Patent Application No. 2021-074337, filed on April 26, 2021, which is incorporated herein by reference. Background Technology

[0002] Previously, optical frequency comb generators were used, for example, in cases requiring high-precision measurement of optical frequencies. Specifically, when determining the difference frequency between two laser beams using heterodyne detection, the bandwidth is limited by the optical element's bandwidth, typically around tens of GHz. Therefore, optical comb generators are used to construct broadband heterodyne detection systems. The optical comb generator produces hundreds of sidebands of the incident laser at equal frequency intervals, and the frequency stability of these sidebands is approximately equal to that of the original laser. Therefore, by heterodyne detecting these sidebands and the laser being measured, a broadband heterodyne detection system covering several THz or higher can be constructed.

[0003] Optical combs can be used, for example, in optical communications to transmit large amounts of data at high speeds. Furthermore, by applying optical combs to the field of distance measurement, it is possible to perform distance measurements with high precision, ranging from micrometers to kilometers (km).

[0004] Regarding an optical comb generator that has an optical modulator that modulates incident light within an optical resonator, if the optical resonant length of the optical resonator varies, a stable optical comb output cannot be obtained.

[0005] Therefore, in order to obtain a stable optical comb output, the intensity of the optical comb output taken from the optical comb generator as transmitted or reflected light is detected, and the optical resonant length is controlled by feedback (for example, see Patent Documents 1 and 2).

[0006] Furthermore, the inventors of this case previously proposed an optical comb rangefinder (see, for example, Patent Documents 3 and 4), which includes two optical comb generators that emit interferometric reference light and measurement light, respectively with periodically modulated intensity or phase and different modulation periods. The interference light between the reference light illuminating a reference surface and the measurement light illuminating a measurement surface is detected by a reference light detector, and the interference light between the reference light reflected from the reference surface and the measurement light reflected from the measurement surface is detected by a measurement light detector. Based on the time difference between the two interference signals obtained by the reference light detector and the measurement light detector, the difference between the distance to the reference surface and the distance to the measurement surface is calculated, thereby enabling high-precision measurement in a short time.

[0007] That is, by using two optical comb generators driven by two modulation signals of different frequencies, interferometric reference light and measurement light are emitted. This allows for frequency analysis of the interference signals obtained by the reference photodetector (hereinafter referred to as the reference signal) and the measurement photodetector (hereinafter referred to as the measurement signal). The mode number starting from the center frequency of the optical comb is set as N. The phase difference between the Nth mode of the reference signal and the measurement signal is calculated to cancel the optical phase difference during the optical comb generation and transmission process from the optical comb generator to the reference point. Then, the phase difference between the measurement signal pulse and the reference signal pulse is obtained by calculating the increment of the phase difference at each frequency axis. This allows the calculation of the distance from the reference point to the measurement surface.

[0008] Here, the distance measured using reference and measurement light from two optical comb generators driven by a pair of modulation signals with a modulation frequency fm (e.g., 25 GHz) and a frequency difference Δf (e.g., 500 kHz) within the microwave band is the remainder after subtracting an integer multiple of half the wavelength of the modulation frequency fm from the total distance from the reference point to the measurement surface (called the absolute distance). The interference signal has a periodicity of Δf, allowing the determination of the phase difference between the closest reference signal and the measurement signal. When measuring distances exceeding half the wavelength, the phase obtained by multiplying 2π by an integer inherently exists as a phase corresponding to the time difference from the reference time to the compared reference signal. This integer value cannot be determined using a single set of frequency settings. By performing distance measurements multiple times with slight variations in fm, this integer value, matching multiple measurement conditions, can be calculated.

[0009] Existing technical documents

[0010] Patent documents

[0011] Patent Document 1: Japanese Patent No. 3976756

[0012] Patent Document 2: Japanese Patent Application Publication No. 2006-337832

[0013] Patent Document 3: Japanese Patent No. 5231883

[0014] Patent Document 4: Japanese Patent Application Publication No. 2020-12641 Summary of the Invention

[0015] The problem the invention aims to solve

[0016] In an optical comb generator, the frequency of the input light is matched with the frequency of any mode of the optical resonator to generate the optical comb. Therefore, in order to generate the optical comb efficiently, it is necessary to control the optical resonant length to match the frequency of the input light with the frequency of a specific mode of the optical resonator.

[0017] Previously, the optical resonant length was controlled by feedback by detecting the intensity of the optical comb output taken from the optical comb generator as transmitted or reflected light, thereby obtaining a stable optical comb output. However, there are multiple resonant modes of the optical resonator in the optical comb generator, and there is also a mode that does not generate an optical comb (with a large input wavelength component). In this mode, there is a problem that the optical comb output cannot be stabilized.

[0018] For example, in the case of an electro-optic modulator, there are two polarization modes (TE, TM, etc.) in the optical resonator. The FSR of the polarization mode that generates the optical comb (one of which is selected) is set to an integer fraction of the modulation frequency, resulting in a large electro-optic constant and high comb generation efficiency. However, other polarization modes also have different FSRs and electro-optic constants, thus preventing the formation of an optical comb with a wide frequency range.

[0019] In optical comb generators using the electro-optic effect, polarization modes other than the dominant mode have different effective refractive indices, resulting in different FSRs and smaller electro-optic constants. Therefore, modes other than the dominant polarization mode are almost impossible to generate an optical comb; even if some are generated, they are primarily carrier components. Although these modes are removed by polarizers, depending on the extinction ratio limits of the polarizers or the assembly precision, they may sometimes mix with the output light, adversely affecting the control of the optical resonant length. This also results in the inability to relatively control the resonant frequencies of non-dominant polarization modes relative to the resonant frequency of the dominant mode.

[0020] Furthermore, the presence of transverse modes can also lead to mixing with the output light, negatively impacting the control of the optical resonant length. This occurs when the optical comb generation method is not entirely single-mode, and cannot be avoided even in optical comb generation methods other than those involving electro-optic effects.

[0021] Here, Figure 1 The characteristic diagram is obtained by plotting the output light intensity under DC bias when the optical resonant length is scanned using the bias voltage T of the modulation input of the optical comb module. Figure 1 The locations where polarization modes overlap are indicated by circular markers. Figure 1 In this process, multiple modes generate the optical comb, including some that are problem-free but show deformation at the zero mark. The electro-optic constants of the polarization modes differ, making it impossible to predict whether mode overlap will occur. Furthermore, it is also affected by slight variations in the input light frequency and environmental changes. The shape is also unstable, possibly due to interference with the carrier wave of the optical comb. Therefore, devices containing such modes cannot be used in products, as it will reduce product yield.

[0022] As described above, in optical comb generation using a waveguide-type optical resonator that resonates light confined in an optical waveguide, orthogonal polarization components can sometimes cause instability in the control used to make the resonant frequency of the optical comb generator match the laser frequency, thus becoming the cause of control point offset, control oscillation, etc. When the optical comb is used, for example, in a measuring device to measure the distance or height to the object being measured, orthogonal polarization components become the main cause of measurement error.

[0023] In addition, when using the optical comb rangefinder as an absolute rangefinder, the modulation frequency of the optical comb generator is switched while the object to be measured is fixed, and the distance, which is an integer multiple of half a wavelength, is calculated based on the phase change of the interference signal, thereby eliminating the ambiguity of the distance, which is an integer multiple of half a wavelength. However, there is a problem that if the switching takes time, the ambiguity cannot be fully eliminated.

[0024] That is, if the effects of vibration, air refractive index change, etc. are mixed in during the modulation frequency switching, it is difficult to determine whether the phase difference is measured due to the modulation frequency switching or due to vibration or refractive index change.

[0025] Therefore, it is important to switch the modulation frequency as quickly as possible within a short timeframe. Microwave switches are used for modulation frequency switching, with a short transition period (approximately 30 ns), but... Figure 2 As shown, a situation occurs where no modulation signal is output, and a situation occurs where the modulation frequency shift is large even though the duration is short.

[0026] Figure 2 This is a waveform diagram showing the modulated signal with an unmodulated interval generated when the modulation frequency of the optical comb generator is switched.

[0027] Without outputting a modulation signal, at this instant, the input light is not modulated and therefore does not become comb-shaped light, thus outputting light whose main component is the input wavelength.

[0028] Furthermore, the same applies when the modulation frequency shift is large. Because the optical resonator and electrode structure of the optical comb generator are designed to have high modulation efficiency for a specific modulation frequency, the modulation efficiency for the shifted frequency is low. As a result, the output light is mainly composed of the input wavelength.

[0029] Compared to the power of the modulated sideband (optical comb other than the input wavelength), the output wavelength power of the unmodulated, very weakly modulated state is higher, so even a short-term phenomenon has a significant impact.

[0030] Furthermore, in the technology disclosed in Patent Document 1, in an optical comb generator that includes an optical modulator composed of an electro-optic crystal through which light to be modulated passes, the optical resonant length of the optical resonator is feedback-controlled based on a detection signal from an optical detector that receives the optical frequency components extracted from the optical comb emitted from the electro-optic crystal by an optical filter and detects the light intensity. However, the optical filter extracts the optical frequency components near the end of the optical comb that attenuates sharply, far from the center frequency of the optical comb, instead of attenuating the carrier component of the center frequency.

[0031] Therefore, in view of the prior art as described above, the object of the present invention is to provide a comb generator control device that can control the optical resonant length or the light source frequency in such a way that the control point is located at the position where the widest comb is generated, even when the comb generation method is not completely single-mode, thereby achieving stable output.

[0032] Another object of the present invention is to provide a comb generator control device that controls the optical resonant length or the light source frequency in such a way that the control point is located at the position that generates the widest comb, even when the modulation frequency of the comb generator is switched, thereby achieving a stable output.

[0033] Other objects of the present invention and specific advantages obtained by the present invention will become more apparent from the following description of the embodiments.

[0034] Solution for solving the problem

[0035] In this invention, the optical resonant length of the optical resonator or the light source frequency of the incident light is controlled by detecting the light intensity of the light frequency component extracted by an optical filter that attenuates the carrier frequency component of the optical comb emitted from an optical comb generator equipped with an optical modulator that modulates the incident light within the optical resonator, thereby stabilizing the optical comb.

[0036] That is, the present invention is a control device for an optical comb generator, which is a control device for an optical comb generator. The optical comb generator has an optical modulator within an optical resonator for optical modulation of incident light. The control device for the optical comb generator is characterized by comprising: an optical filter that attenuates the carrier frequency component of the optical comb emitted from the optical resonator as transmitted or reflected light; a photodetector that receives the optical frequency component extracted from the optical comb by means of the optical filter and detects the light intensity; and a resonance control unit that is supplied with a detection signal to the photodetector, wherein the photodetector detects the light intensity of the optical comb after the carrier frequency component is attenuated by the optical filter, and the resonance control unit performs feedback control on the optical resonance length of the optical resonator or the light source frequency of the incident light.

[0037] The optical comb generator control device of the present invention can be configured to further include a bottom position discriminator, which is used to discriminate the bottom position of the signal level of the detection signal of the photodetector. The resonance control unit performs feedback control to set the bottom position of the signal level of the detection signal of the photodetector to a stable point based on the discriminator's output.

[0038] Furthermore, the optical comb generator control device according to the present invention can be configured to switchly supply multiple modulation signals with different modulation frequencies to the optical modulator.

[0039] Furthermore, in the optical comb generator control device according to the present invention, the optical filter can be configured to be a notch filter that attenuates the carrier frequency component of the optical comb.

[0040] Furthermore, in the optical comb generator control device according to the present invention, the optical filter can be configured to be a high-pass filter having a cutoff frequency near the carrier frequency of the optical comb.

[0041] Furthermore, in the optical comb generator control device according to the present invention, the optical filter can be configured to be a low-pass filter having a cutoff frequency near the carrier frequency component of the optical comb.

[0042] Furthermore, in the optical comb generator control device according to the present invention, the optical filter can be configured to be a bandpass filter having a cutoff frequency near one of the carrier frequency components of the optical comb.

[0043] The effects of the invention

[0044] In the optical comb generator control device according to the present invention, the optical resonant length of the optical resonator is controlled by detecting the light intensity of the light frequency component extracted by means of an optical filter that attenuates the carrier frequency component of the optical comb emitted from the optical comb generator, which has an optical modulator that performs optical modulation of the incident light in the optical resonator, thereby stabilizing the optical comb. Therefore, it is possible to provide an optical comb generator control device that can control the optical resonant length in a way that places the control point at the position where the widest optical comb is generated, even when the optical comb generation method is not completely single-mode, thereby obtaining a stable output.

[0045] Furthermore, in this invention, the optical resonant length of the optical resonator is controlled by detecting the light intensity of the light frequency component extracted by an optical filter that attenuates the carrier frequency component of the optical comb emitted from an optical comb generator equipped with an optical modulator that modulates the incident light within the optical resonator. This stabilizes the optical comb. Therefore, it is possible to provide an optical comb generator control device that controls the optical resonant length in such a way that the control point is located at the position where the widest optical comb is generated, even if the modulation frequency of the optical comb generator is switched, thereby obtaining a stable output. Attached Figure Description

[0046] Figure 1 The characteristic diagram is obtained by plotting the output light intensity when the optical resonant length is scanned with DC bias using the bias voltage T of the modulation input of the optical comb module.

[0047] Figure 2 This is a waveform diagram showing the modulated signal with an unmodulated interval generated when the modulation frequency of the optical comb generator is switched.

[0048] Figure 3 This is a block diagram illustrating the basic structure of a comb generator control device for applying the present invention to a comb generating apparatus.

[0049] Figure 4 This is a schematic cross-sectional view showing the structure of the optical comb generator in the aforementioned optical comb generating device.

[0050] Figures 5(A), 5(B), and 5(C) are characteristic diagrams of the transmitted light power of the optical comb generator obtained by plotting the scanning light resonant length for the optical comb generator described above. Figure 5(A) shows the light output in the mode without forming an optical comb, Figure 5(B) shows the light output in the mode that operates as an optical comb, and Figure 5(C) shows the light output when all modes are mixed.

[0051] Figures 6(A), 6(B), and 6(C) are spectral diagrams showing the light output spectrum of the above-mentioned optical comb generator and the light output spectrum obtained by means of an optical filter. Figure 6(A) shows the light output spectrum when the above modes are mixed. Figure 6(B) shows the light output spectrum after the carrier periphery is removed by a notch filter. Figure 6(C) shows the light output spectrum after the carrier periphery is removed by a bandpass filter, a high-pass filter, or a low-pass filter.

[0052] Figure 7 It is a characteristic diagram of the transmitted light power that can be obtained from the above-mentioned optical comb generator with the aid of an optical filter when the optical resonant length is scanned.

[0053] Figure 8This is a block diagram illustrating a structural example of a comb generating apparatus having two comb generators used in a comb rangefinder for absolute distance measurement, incorporating the comb generator control device according to the present invention.

[0054] Figure 9 This is a state transition diagram showing the state transitions of the drive signals supplied to the two optical comb generators in the aforementioned optical comb generating apparatus.

[0055] Figure 10 This is a diagram showing the observation results of the optical comb generator being driven by the aforementioned driving signal and the detection signal obtained by the photodetector receiving the output of the optical comb without inserting an optical filter.

[0056] Figure 11 This is a diagram showing the observation results of the optical comb generator being driven by the aforementioned driving signal and the detection signal obtained by the photodetector receiving the output of the optical comb via an optical filter.

[0057] Figures 12(A) and 12(B) are schematic block diagrams showing structural examples of optical comb generating apparatus. Figure 12(A) shows an optical comb generating apparatus equipped with an optical comb generator that extracts optical comb as reflected light, and Figure 12(B) shows an optical comb generating apparatus equipped with an annular optical comb generator.

[0058] Figures 13(A) and 13(B) are characteristic diagrams of the optical output power of the optical comb generator in the reflection mode and the optical output power of the reflection mode obtained by means of an optical filter. Figure 13(A) shows the characteristics of the optical output power of the optical comb generator in the reflection mode, and Figure 13(B) shows the characteristics of the optical output power of the above reflection mode after the carrier component is removed by the optical filter.

[0059] Figure 14 This is a block diagram illustrating an example of the structure of a light comb generator control device that uses a light comb, which is emitted from the light comb generator as transmitted light, to control the optical resonance length.

[0060] Figure 15 (A) Figure 15 (B) is a waveform diagram illustrating the operation of the lock-in amplifier used in the above-mentioned optical comb generator control device. Figure 15 (A) shows the signal obtained by normalizing the transmitted light intensity when the power of the input light changes the optical resonant length or laser frequency. Figure 15 (B) shows the differential signal of the above-mentioned standardized light intensity signal.

[0061] Figure 16 This is a block diagram illustrating another structural example of a comb generator control device that uses a comb, which is emitted from the comb generator as transmitted light, to control the optical resonance length.

[0062] Figure 17 This is a block diagram illustrating an example of the structure of a comb generator control device that uses a comb emitted from the comb generator as reflected light to control the optical resonance length.

[0063] Figure 18 This is a block diagram illustrating another structural example of a comb generator control device that uses a comb emitted from the comb generator as reflected light to control the optical resonance length. Detailed Implementation

[0064] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings. Furthermore, common indicator marks are used to illustrate common constituent elements in the drawings. It is self-evident that the present invention is not limited to the examples below, and various modifications can be made without departing from the spirit of the invention.

[0065] like Figure 3 As shown in the block diagram, the basic structure of the optical comb generator control device 10 of the present invention is applied to an optical comb generating device 3 equipped with an optical comb generator 2, which receives incident light from a light source 1 and generates an optical comb by optically modulating the incident light.

[0066] For example, such as Figure 4 As shown, the optical comb generator 2 is formed by inserting an optical phase modulator 22 inside an optical resonator 21, which is composed of a pair of mirrors 21A and 21B. When a continuous wave of light of a single frequency is input and the optical phase modulator 22 is driven at a frequency that is an integer multiple of the free spectral range (FSR) of the optical resonator 21, the multiple reciprocating cycles within the optical resonator 21 can be synchronized with the modulation signal period. Therefore, compared with an optical phase modulator without a resonator, it can perform highly efficient modulation, with hundreds to thousands of sidebands, and can obtain an optical frequency comb with a spectral width of several megahertz as the output. The optical comb generator 2 can generate short pulses, with a pulse width of less than 1 picosecond. The output of the optical comb generator 2 is a comb-shaped light with a center frequency equal to the input frequency and a frequency interval equal to the modulation frequency. On the time axis, it is a pulse train with the modulation frequency as the repetition frequency. The higher the modulation index to increase the spectral width, the shorter the pulse width.

[0067] That is, the optical comb generator control device 10 is a control device for the optical comb generator 2, which has an optical modulator that performs optical modulation of incident light in the optical resonator 21. The optical comb generator control device 10 includes: an optical filter 5, which is a optical comb emitted from the optical resonator 21 as transmitted or reflected light, and is branched by a coupler 4 such as an optical coupler or a circulator, so that a part of the optical comb is incident on the optical filter 5; a photodetector 6, which receives a part of the optical comb by means of the optical filter 5 and detects the light intensity; and a resonance control unit 8, which is supplied with a detection signal from the photodetector 6, wherein the resonance control unit 8 performs feedback control on the optical resonance length of the optical resonator 21 or the light source frequency of the incident light.

[0068] The optical filter 5 of the optical comb generator control device 10 has optical characteristics that attenuate the carrier frequency component of the optical comb incident by means of the coupler 4.

[0069] Here, Figures 5(A), 5(B), and 5(C) are characteristic diagrams of the transmitted light power of the optical comb generator 2 obtained by plotting the scanning light resonant length. In the optical comb generator 2, the optical comb generation mode of the optical resonator 21 that generates the optical comb is only a specific mode. As shown in Figure 5(A), when plotting the transmitted light power of the mode without forming an optical comb by plotting the scanning light resonant length, the carrier frequency component, which is the center frequency of the incident light, is the main component of the transmitted light power in the mode without forming an optical comb. In contrast, as shown in Figure 5(B), when plotting the transmitted light power of the mode in which the optical comb operates by plotting the scanning light resonant length, in the mode in which the optical comb operates, by controlling the light resonant length to the bottom position b0 between the two peaks existing on both sides of the center frequency, the optical comb can be generated over the maximum range. However, as shown in Figure 5(C), when the transmitted light power is plotted by scanning the optical resonance length in the case where the above-mentioned mode of not forming an optical comb exists in combination with the above-mentioned mode of operating as an optical comb, there will be multiple bottom positions b1, b2 between the two peaks, resulting in multiple control points for the optical resonance length and making it impossible to obtain a stable optical comb.

[0070] Figures 6(A), 6(B), and 6(C) are spectral diagrams showing the light output spectrum of the optical comb generator 2 and the light output spectrum obtained by means of the optical filter 5. That is, when the above-mentioned mode of not forming an optical comb and the above-mentioned mode of operating as an optical comb exist in a mixed state, the spectrum of the optical comb emitted from the optical comb generator 2, as shown by the thick line in Figure 6(A), contains a component near the carrier frequency that is close to the unmodulated state in the above-mentioned mode of not forming an optical comb. However, in the optical comb generator control device 10, for example by using a notch filter as the optical filter 5, the component near the carrier frequency can be removed as shown in Figure 6(B).

[0071] Alternatively, it can be configured to remove the single-sideband component along with the carrier frequency component by using a high-pass filter, a low-pass filter, or a band-pass filter as the aforementioned optical filter 5, as shown in FIG6(C).

[0072] That is, the optical filter 5 provided by the optical comb generator control device 10 can be configured as a notch filter that attenuates the carrier frequency component of the optical comb.

[0073] In addition, the optical filter 5 can be configured to be a high-pass filter with a cutoff frequency near the carrier frequency of the optical comb.

[0074] Alternatively, the optical filter 5 can be configured to be a low-pass filter having a cutoff frequency near the carrier frequency component of the optical comb.

[0075] Furthermore, the aforementioned optical filter 5 can be configured as a bandpass filter having a cutoff frequency near one of the carrier frequency components of the aforementioned optical comb.

[0076] In this optical comb generator control device 10, the light intensity of the optical comb after attenuation of the carrier frequency component is detected by a photodetector 6, which receives a portion of the optical comb by means of the optical filter 5 having optical characteristics that attenuate the carrier frequency component of the incident optical comb. Therefore, even when the mode of not forming an optical comb and the mode of operating as an optical comb coexist, the detection signal of the photodetector 6 will not have multiple bottoms between the two peaks, and... Figure 7 As shown, it becomes a bottom position b0. Therefore, the resonance control unit 8 uses the detection signal of the photodetector 6 to perform feedback control on the optical resonance length of the optical resonator 21 or the light source frequency of the incident light, thereby enabling the optical comb generator 2 to emit a stable optical comb.

[0077] Furthermore, the optical comb generator control device 10 includes a bottom position discriminator 7, which is used to discriminate the bottom position of the signal level of the detection signal of the photodetector 6. The resonance control unit 8 performs feedback control to set the bottom position b0 of the signal level of the detection signal of the photodetector 6 to a stable point based on the discriminator output of the bottom position discriminator 7.

[0078] In this way, even when the above-mentioned mode of not forming a light comb and the above-mentioned mode of operating as a light comb coexist in the light comb generator control device 10, a stable light comb can be obtained from the light comb generator 2 by the resonance control unit 8 using the detection signal of the photodetector 6 to perform feedback control on the light resonance length of the light resonator 21 or the light source frequency of the incident light. Therefore, for example, by using a light comb rangefinder that performs absolute distance measurement by switching the modulation frequencies of the two light comb generators, the situation where the light resonance length control is disturbed by the influence of the no-modulation period generated when the modulation frequency is switched can be eliminated, thereby enabling the use of two stable light combs to perform absolute distance measurement.

[0079] Figure 8 This is a block diagram showing the structure of an optical comb generating device 50 equipped with two optical comb generators 15A and 15B used in an optical comb rangefinder for absolute distance measurement.

[0080] The optical comb generating device 50 supplies two frequency signals with cyclically switched modulation periods and different periods to the first frequency converter 14A and the second frequency converter 14B by cyclically switching four frequency signals output from the synthesizer circuit 53 via the switching circuit 54.

[0081] The optical comb generating device 50 includes, for example, a synthesizer circuit 53 that outputs four independent frequency signals (F1: 1000MHz, F2: 1010MHz, F3: 1000.5MHz, F4: 1010.5MHz) with a difference frequency of 500kHz, and a 4-input 2-output switching circuit 54 that receives the four frequency signals from the synthesizer circuit 53 via isolators 57A, 57B, 57C, and 57D respectively. The first frequency converter 14A and the second frequency converter 14B are connected to the two output terminals of the switching circuit 54.

[0082] In the optical comb generating device 50, the first oscillator 13 supplies a frequency signal with a phase synchronized with the phase of the reference frequency signal FREF supplied from the reference oscillator 11 via the 5-branch power divider 52 and with a fixed oscillation phase, to the first frequency converter 14A and the second frequency converter 14B via the 2-branch power divider 12 and isolators 17A and 17B.

[0083] The synthesizer circuit 53 described above has four oscillators 53A, 53B, 53C, and 53D that generate four different frequency signals F1, F2, F3, and F4, which are synchronized with the phase of the reference frequency signal FREF supplied from the reference oscillator 11 via the 5-branch power divider 52 and whose frequencies are fixed.

[0084] The second oscillator 53A generates a first frequency signal whose phase is synchronized with the phase of the reference frequency signal FREF generated by the reference oscillator 11 and fixed to the first frequency F1 (1000MHz) through the PLL circuit.

[0085] In addition, the third oscillator 53B generates a second frequency signal whose phase is synchronized with the phase of the reference frequency signal FREF generated by the reference oscillator 11 and is fixed at the second frequency F2 (1010MHz) through the PLL circuit.

[0086] In addition, the third oscillator 53C generates a third frequency signal F3, which is phase-synchronized with the reference frequency signal FREF generated by the aforementioned reference oscillator 11 through the PLL circuit and whose frequency is fixed at the third frequency of 1000.5MHz.

[0087] Furthermore, the fourth oscillator 53D generates a fourth frequency signal whose phase is synchronized with the reference frequency signal FREF generated by the aforementioned reference oscillator 11 through the PLL circuit and whose frequency is fixed at the fourth frequency F4 (1010.5MHz).

[0088] The aforementioned switching circuit 54 alternately outputs four frequency signals in the 1GHz band input from the synthesizer circuit 53 via isolators 57A, 57B, 57C, and 57D from its two output terminals in a cyclic switching manner. That is, the switching circuit 54 functions as a 4-input, 2-output selection switch that cyclically switches between the four frequency signals in the 1GHz band supplied by the first frequency converter 14A and the second frequency converter 14B connected to the two output terminals.

[0089] Here, by inserting isolators 57A, 57B, 57C, and 57D between the synthesizer circuit 53 and the switch circuit 54, and by inputting frequency signals from the synthesizer circuit 53 to the switch circuit 54 via isolators 57A, 57B, 57C, and 57D, it is possible to prevent the signal source from becoming unstable due to load fluctuations caused by the cutting off or releasing of circuits after the switch circuit 54.

[0090] For the aforementioned isolators 57A, 57B, 57C, and 57D, isolator components such as microwave amplifiers with high reverse isolation, PI-type resistor attenuators, resistor attenuators, and microwave isolators using ferrites can be used, as well as isolator circuits composed of variable attenuators and bandpass filters, and isolator circuits composed of isolation amplifiers, resistor attenuators, and bandpass filters.

[0091] Furthermore, the first frequency converter 14A and the second frequency converter 14B obtain a first modulation signal Fma and a second modulation signal Fmb based on the frequency signal F0 (e.g., 24GHz) supplied from the first oscillator 13 and the frequency signals F1, F2, F3, F4 of the 1GHz band that are cyclically and alternately output from the switching circuit 54. The frequency signals F1, F2, F3, F4 of the 1GHz band that are cyclically switched by the switching circuit 54 are frequency-converted into four modulation frequencies Fm1, Fm2, Fm3, Fm4 of the 25GHz band. These signals are then used as driving signals and supplied to the first optical comb generator 15A and the second optical comb generator 15B via bandpass filters 16A and 16B.

[0092] That is, the first frequency converter 14A and the second frequency converter 14B above function as upconverters that convert the frequency signal of the 1GHz band into the first modulation signal Fma and the second modulation signal Fmb of the 25GHz band, which are supplied as driving signals to the first optical comb generator 15A and the second optical comb generator 15B.

[0093] Here, the optical comb generating device 50 generates two optical combs as reference light and measuring light for absolute distance measurement requiring frequency switching in optical comb rangefinders and three-dimensional shape measuring machines described in Patent Documents 1 and 2. The first modulation signal Fma and the second modulation signal Fmb are obtained by cyclically switching the frequency signals of the four frequencies F1, F2, F3, and F4 of the 1GHz band by the switching circuit 54 and upconverting them by the first frequency converter 14A and the second frequency converter 14B to the four modulation frequencies Fm1, Fm2, Fm3, and Fm4 of the 25GHz band, and supplying them as driving signals to the first optical comb generator 15A and the second optical comb generator 15B. The first optical comb generator 15A and the second optical comb generator 15B output two optical combs with cyclically switching modulation periods as shown in Table 1, and the modulation periods are different from each other.

[0094] [Table 1]

[0095]

[0096] Table 1 shows the transition states OFCG1 / OFCG2 and phase difference of the drive signals of the first optical comb generator 15A and the second optical comb generator 15B in settings #1 to #4. The frequencies of the drive signals are Δf = 500kHz, Δfm = 10MHz, fm = Fm1 (25000MHz), fm+Δfm = Fm2 (25010MHz), fm+Δf = Fm3 (25000.5MHz), and fm+Δfm+Δf = Fm4 (25010.5MHz). Figure 9 This is a state transition diagram showing the state transitions of the drive signals supplied to the two optical comb generators 15A and 15B in the optical comb generating device 50.

[0097] In this optical comb rangefinder, in principle, by using two optical comb generators driven by two modulation signals with different frequencies to emit interferometric reference light and measurement light, frequency analysis is performed in the signal processing unit on the interference signal obtained by the reference photodetector (hereinafter referred to as the reference signal) and the interference signal obtained by the measurement photodetector (hereinafter referred to as the measurement signal). The mode number starting from the center frequency of the optical comb is set as N. The phase difference between the Nth mode of the reference signal and the measurement signal is calculated to cancel the optical phase difference during the optical comb generation and transmission process from the optical comb generator to the reference point. Then, the phase difference of the signal pulse is obtained by calculating the increment of the phase difference at each time on the frequency axis, thereby calculating the distance from the reference point to the measurement surface.

[0098] Furthermore, when the measured distance exceeds half the wavelength of the modulation frequency fm, the distance that is an integer multiple of the half wavelength is unclear due to the periodicity of the object light, and the distance cannot be uniquely determined. Therefore, four measurements are performed using a reference light and a measurement light with four modulation frequencies set as shown in Table 1. In the signal processing unit, the phase differences obtained by performing the same processing are used to calculate the distance that exceeds the ambiguous distance (La = c / 2fm, c: speed of light) equivalent to half the wavelength.

[0099] That is, regarding the phase difference between the reference signal and the measurement signal obtained by measuring the four modulation frequencies shown in Table 1, the phase difference is -2πfmT when the modulation frequency of the modulation signal used to drive the two optical comb generators (OFCG1, OFCG2) is set to fm and fm+Δf #1, the phase difference is -2π(fm+Δfm)T when the modulation frequency of the modulation signal is set to fm+Δfm and fm+Δfm+Δf #2, the phase difference is -2π(fm+Δfm)T when the modulation frequency of the modulation signal is set to fm+Δf and fm #3, and the phase difference is -2π(fm+Δfm+Δf)T when the modulation frequency of the modulation signal is set to fm+Δfm+Δf and fm+Δfm #4.

[0100] When the distance is longer than the distance (La = c / 2fm, c: speed of light), the phase difference (-2πfmT) between the reference signal and the measured signal is in the form of φ + 2mπ when m is set to an integer. Only the part of φ can be calculated, and the integer value m is unknown.

[0101] On the other hand, the phase difference between the reference signal and the measured signal under setting #1, -2πfmT, is 2πΔfmT, which is the same as the phase difference between the reference signal and the measured signal under setting #2, -2π(fm+Δfm)T. In addition, the phase difference between the reference signal and the measured signal under setting #3, -2π(fm+Δf)T, is 2πΔfmT, which is the same as the phase difference between the reference signal and the measured signal under setting #4, -2π(fm+Δfm+Δf)T. If the phase difference is within half the wavelength of Δfm (La is 15m if Δfm = 10MHz), then the phase is uniquely determined.

[0102] Furthermore, the integer m can be determined by multiplying the phase by fm / Δfm and comparing it with the phase difference of #1.

[0103] Furthermore, based on the difference between the phase difference -2πfmT under setting #1 and the phase difference -2π(fm+Δf)T under setting #3 in Table 1, 2πΔf can be obtained.

[0104] Here, with fm = 25 GHz, Δf = 500 kHz, and Δfm = 10 MHz, distance measurements within La = 300 m are possible because Δf = 500 kHz.

[0105] In the optical comb rangefinder equipped with the optical comb generating device 50, absolute distance measurement is performed using a reference signal and a measurement signal obtained by measuring four modulation frequencies set as shown in Table 1. That is, after maintaining one state for a fixed time, the rangefinder transitions to another state, measures the signal phase of that state within a fixed interval, and uses the phase of the set states #1, #2, #3, and #4 to perform the calculation of the absolute distance.

[0106] Regarding the measurement speed in the optical comb rangefinder, it is equal to 500kHz for relative distance measurements within 6mm. In contrast, for absolute distance measurements that require frequency switching, the time for frequency switching and the time for absolute distance calculation are included.

[0107] In the optical comb generating apparatus 50 described above, isolators 57A, 57B, 57C, and 57D are inserted between the synthesizer circuit 53 and the switching circuit 54. Therefore, the operation of the synthesizer circuit 53 will not become unstable due to momentary load fluctuations caused by the instantaneous switching of the four frequency signals F1, F2, F3, and F4 via the switching circuit 54. This allows for rapid switching of the drive signals of the optical comb generators 15A and 15B to change their drive states. In other words, the drive states of the first optical comb generator 15A and the second optical comb generator 15B can be rapidly changed by cyclically switching the four modulation frequencies Fm1, Fm2, Fm3, and Fm4 via the switching circuit 54. By using the modulation frequencies used to switch the reference signal and the measurement signal for absolute distance measurement, the measurement time for absolute distance can be shortened.

[0108] Furthermore, if only distance measurement is required, it can be performed using only settings #1 and #2, or only settings #3 and #4. However, by setting #1, #2, #3, and #4 as described above, that is, by cyclically switching the four modulation frequencies Fm1, Fm2, Fm3, and Fm4 using the aforementioned switch circuit 54, phase shifts caused by signal transmission paths other than the object being measured can be corrected, thereby obtaining a high-precision absolute distance result. That is, when the modulation frequencies of the two optical comb generators (OFCG1 and OFCG2) are changed, the absolute value of the phase caused by the distance to the object being measured remains unchanged, but the sign is reversed. On the other hand, the sign of the shift caused by the cable length interfering with the signal transmission path remains unchanged and is a fixed value. Therefore, by subtracting the results of the two phase measurements and dividing by 2, the phase value that has eliminated the shift can be obtained.

[0109] Here, in the optical comb generating device 50, isolators 57A, 57B, 57C, and 57D are inserted between the synthesizer circuit 53 and the switching circuit 54 as described above to prevent the operation of the synthesizer circuit 53 from becoming unstable due to load fluctuations during the instantaneous switching of the frequency signals of the four frequencies F1, F2, F3, and F4 via the switching circuit 54. However, during the switching operation of the switching circuit 54, although it is a very short time, a no-modulation state is generated each time the circuit is cut off.

[0110] Therefore, in this optical comb generating device 50, the optical combs emitted from the first optical comb generator 15A and the second optical comb generator 15B are separated by couplers 4A and 4B, so that a portion of each optical comb is incident on the optical comb generator control device 10A and 10B involved in the present invention, thereby performing feedback control on the optical resonance length of each optical resonator of the first optical comb generator 15A and the second optical comb generator 15B.

[0111] For the aforementioned optical comb generator control devices 10A and 10B, the application is... Figure 3 The block diagram shows the optical comb generator control device 10 of the optical comb generating device 3.

[0112] In the optical comb generating apparatus 3, where incident light of 192.8 THz is incident from light source 1 onto optical comb generator 2, the optical comb generator 2 is driven by a drive signal supplied to the first optical comb generator 15A, and the detection signal obtained by the photodetector 6 detects a frequency component lower than the switching frequency of the switching circuit 54 after passing through a low-pass filter. If the optical filter 5 is not inserted before the photodetector 6, then... Figure 10 As shown, the generation of the smaller peak in the middle of the double peaks is confirmed. In this state, continuous SW switching will hinder resonance control. However, if the optical filter 5 is inserted before the aforementioned photodetector 6, then... Figure 11 As shown, the generation of the smaller peak in the middle of the double peaks was not confirmed, thus enabling stable resonance control while continuously performing SW switching.

[0113] Since there are commercially available WDM filters for DWM with a bandwidth of 100 GHz centered at 190 THz and CH numbers specified every 100 GHz, when attempting to use WDM filters CH27 (192.7 THz 1555.75 nm), CH28 (192.8 THz 1554.94 nm), CH29 (192.7 THz 1555.75 nm), and CH30 (193.0 THz 1553.33 nm) as the aforementioned optical filter 5, for WDM filters other than the input laser wavelength CH28 (192.8 THz 1554.94 nm), the small peak during the SW transition disappears, and the optical resonant length of the optical resonator 21 can be locked by applying dithering. Furthermore, no problems were found in the transmission mode with the switch selection fixed.

[0114] Furthermore, the signal level decreases in the transmission mode, but this is due to the narrow bandwidth of the WDM filter used as optical filter 5. The optical filter 5 described above can also extend the bandwidth if it possesses characteristics other than those near the wavelength of the input laser. Alternatively, if it possesses characteristics other than those near the wavelength of the input laser, it can also be a notch filter.

[0115] Here, Figures 12(A) and 12(B) are schematic block diagrams showing an example of the structure of an optical comb generating device. Figures 13(A) and 13(B) are characteristic diagrams of the optical output power in the reflection mode of the optical comb generator and the optical output power in the reflection mode obtained by means of an optical filter. That is, for the optical comb generator 2 in the optical comb generating device 3 described above, in addition to being a optical comb generator that emits optical comb from the optical resonator 21 as transmitted light, it is also possible to use an optical comb generator like the optical comb generator 2B shown in Figure 12(A) that emits optical comb from the optical resonator 21 by means of a coupler 4C as reflected light, and a ring-shaped optical comb generator 2C shown in Figure 12(B). In these configurations, the uncoupled carrier component in the optical resonator is strong, and when the average value of the output power is measured, it exhibits characteristics as shown in Figure 13(A), with the optical resonant length or laser frequency as the horizontal axis. Even in this case, if an optical filter 5 for carrier removal is used, the measured output is as shown in Figure 13(B), and the same signal as described above can be obtained. In addition, there are various modulation methods, such as electro-optic modulation and Kerr effect modulation, but the present invention can be applied to optical comb generators of various types regardless of the modulation method.

[0116] Next, use Figure 14 The block diagram illustrates the optical comb generator control device 110A according to the present invention, which uses an optical comb emitted from an optical comb generator 2 as a light source to control the optical resonance length.

[0117] Should Figure 14 The optical comb generator control device 110A shown includes: a first photodetector 116A, which receives a portion of the light beam incident from the light source 1 of the optical comb generator 3A to the optical comb generator 2 via an incident-side coupler 114A; an optical filter 5, which receives a portion of the optical comb after it has been branched by the exit-side coupler 114B and emitted from the optical comb generator 2; and a second photodetector 116B, which receives a portion of the optical comb via the optical filter 5.

[0118] In this optical comb generator control device 110A, the modulation signal S generated by the oscillator 122 of the resonance control unit 120A is used. D The light source frequency of light source 1 is modulated, and the optical resonant length control signal S generated by the control signal generation unit 123A is used. CTL To control the optical resonant length of the optical comb generator 2, thereby controlling the frequency difference between the laser frequency of the optical comb generator 2 and the mode of the optical resonator.

[0119] The first photodetector 116A receives a portion of the light beam via the incident-side coupler 114A to obtain an incident light detection signal S representing the light intensity A of the light beam emitted from the light source 1 and incident on the light comb generator 2. A And the incident light detection signal S A A lock-in amplifier 121 is supplied to the resonant control unit 120A.

[0120] Furthermore, the second photodetector 116B receives a portion of the optical comb via the emission-side coupler 114B to obtain an emitted light detection signal S representing the light intensity B of the optical comb emitted from the optical comb generator 2 as transmitted light. B And detect the emitted light signal S B It is supplied to the aforementioned lock-in amplifier 121.

[0121] Figure 15 (A) Figure 15 (B) is a waveform diagram illustrating the operation of the lock-in amplifier 121 used in the optical comb generator control device 110A. Specifically, in the resonance control unit 120A of the optical comb generator control device 110A, the light intensity A of the light beam emitted from the light source 1 of the optical comb generator 3A varies depending on the modulation input and environmental changes. Therefore, in order to remove this influence from the signal, the incident light detection signal S... A The light intensity A of the aforementioned beam and the signal S detected by the emitted light represent the light intensity A and the light intensity S of the emitted light. B The light intensity B of the optical comb is obtained by the lock-in amplifier 121 through digital processing, etc., resulting in B' = B / A, and thus... Figure 15 (A) represents the light intensity signal S of the optical comb, which is independent of the power variation of the input light, representing the light intensity B' of the optical comb. B’ Based on the modulation signal S generated by modulating the light source frequency of the light source 1 of the aforementioned optical comb generating device 3A. D The oscillator 122 provides a synchronization signal, and the output is as follows: Figure 15 As shown in (B), the light intensity signal S represents the light intensity B' of the aforementioned optical comb. B’ The differential signal S C .

[0122] like Figure 15 As shown in (B), in the differential signal C, Figure 15 (A) shows the bottom position of the light intensity signal representing the light intensity B' of the optical comb, where the sign is reversed.

[0123] Furthermore, the control signal generation unit 123A, according to Figure 15 (A) The predefined range of ef is used to determine the light intensity signal S representing the light intensity B' of the aforementioned optical comb. B’The optical comb is generated efficiently over a range including the bottom between the two peaks, from the differential signal S. C Among multiple sign inversion points, the sign inversion point that matches the point that efficiently generates the optical comb is identified, and an optical resonance length control signal S is generated to control the optical resonance length of the optical comb generator 2 in a manner that serves as a sign inversion point. CTL The optical resonant length control signal S CTL This is used to control the optical resonance length of the optical comb generator 2. That is, the control signal generation unit 123A has the function of the bottom position detector 7, and generates an optical intensity signal S representing the optical intensity B' of the optical comb. B’ The feedback control sets the bottom position of the signal level to the stable point. The feedback control of the optical resonant length of the optical comb generator 2, performed by the control signal generation unit 123A, can also be achieved by using the optical intensity signal S, which represents the optical intensity B' of the optical comb. B’ The position obtained by applying an offset to the bottom position of the signal level is set as the stable point.

[0124] Furthermore, here, the modulation signal S is used. D The frequency of the light source 1 is modulated and the frequency difference between the laser frequency of the optical comb generator 2 and the mode of the optical resonator is controlled by controlling the optical resonant length. However, the optical resonant length control of the optical comb generator 2 can also be achieved by changing the DC bias voltage through the bias voltage T of the modulation input of the optical comb module.

[0125] Next, use Figure 16 The block diagram illustrates the optical comb generator control device 120B of the present invention, which uses an optical comb emitted from an optical comb generator 2 as a light source to control the frequency of the light source.

[0126] The optical resonant length control signal S in the optical comb generator control device 110A described above is replaced by the optical resonant length control signal S. CTL The control signal generation unit 123A, Figure 16 The optical comb generator control device 110B shown obtains a resonant control signal S for controlling the frequency difference between the laser frequency of the optical comb generator 2 and the mode of the optical resonator by controlling the light source frequency of the light source 1 of the optical comb generator 3A through the control signal generation unit 123B. CTF This controls the light source frequency of light source 1.

[0127] Furthermore, in this optical comb generator control device 110B, for the same constituent elements as those in the optical comb generator control device 110A described above, in Figure 16 The same reference numerals are used in the accompanying drawings, and their detailed descriptions are omitted.

[0128] The resonant control unit 120B in the optical comb generator control device 110B uses a lock-in amplifier 121 to control the incident light detection signal S obtained by the first photodetector 116A. A and the emitted light detection signal S obtained by the second photodetector 116B mentioned above. B To obtain such Figure 15 (A) represents the light intensity signal S of the optical comb, which is independent of the power variation of the input light, representing the light intensity B' of the optical comb. B’ and output as follows Figure 15 (B) represents the light intensity signal S of the aforementioned optical comb, which indicates the light intensity B'. B’ The differential signal S C .

[0129] Furthermore, the control signal generation unit 123B according to Figure 15 (A) The predefined range of ef is used to determine the light intensity signal S representing the light intensity B' of the aforementioned optical comb. B’ The optical comb is generated efficiently over a range including the bottom between the two peaks, from the differential signal S. C Among multiple sign inversion points, the sign inversion point that coincides with the point that efficiently generates the optical comb is identified, and a resonant control signal S is generated to control the light source frequency of the aforementioned light source 1 in a manner that serves as a sign inversion point. CTF The modulating signal S generated by the oscillator 122 for modulating the light source frequency of the light source 1 is converted by the adder 124. D Adding the aforementioned resonance control signal to the light source 1, feedback control is performed on the light source frequency of the light source 1. That is, the control signal generation unit 123B has the function of the bottom position detector 7, and generates a light intensity signal S representing the light intensity B' of the optical comb. B’ The bottom position of the signal level is set as the feedback control of the stable point.

[0130] Next, use Figure 17 The block diagram illustrates the optical comb generator control device 110C of the present invention, which uses an optical comb emitted from an optical comb generator 2B as reflected light to control the optical resonance length.

[0131] Instead of the aforementioned optical comb generator control device 110A, which branches the optical comb emitted from the optical comb generator 2 of the optical comb generating device 3A as transmitted light and allows the second photodetector 116B to receive a portion of the optical comb via the optical filter 5, this... Figure 17 The optical comb generator control device 110C shown is configured to branch the optical comb emitted from the optical comb generator 2B of the optical comb generator 3B as reflected light by the incident side coupler 114, and to receive a portion of the optical comb emitted as reflected light by the second photodetector 116B with the aid of the optical filter 5.

[0132] Furthermore, in this optical comb generator control device 110C, for the same constituent elements as those in the optical comb generator control device 110A described above, in Figure 17 The same reference numerals are used in the accompanying drawings, and their detailed descriptions are omitted.

[0133] In the optical comb generator control device 110C, the incident-side coupler 114 of the optical comb generator 3B has the following functions: branching the optical comb emitted from the optical comb generator 2B as reflected light, and receiving a portion of the optical comb emitted as reflected light by means of the optical filter 5, and branching the light beam incident from the light source 1 to the optical comb generator 2B so that the first photodetector 116A receives a portion of the light beam.

[0134] In the resonance control unit 120C of the optical comb generator control device 110C, the incident light detection signal S obtained by the first photodetector 116A is... A The light intensity A of the aforementioned light beam and the reflected light detection signal S obtained by the second photodetector 116B are represented. B The light intensity B of the optical comb is obtained by the lock-in amplifier 121 through digital processing, etc., resulting in B' = B / A, and thus the following is obtained: Figure 15 (A) represents the light intensity signal S of the optical comb, which is independent of the power variation of the input light, representing the light intensity B' of the optical comb. B’ Based on the modulation signal S generated by modulating the light source frequency of the light source 1 of the aforementioned optical comb generating device 3B. D The synchronization signal provided by the oscillator 122 comes from... Figure 15 As shown in (B), the output signal S represents the light intensity B' of the aforementioned optical comb. B’ The differential signal S C .

[0135] Furthermore, the control signal generation unit 123C, according to Figure 15 (A) The predefined range of ef is used to determine the light intensity signal S representing the light intensity B' of the aforementioned optical comb. B’ The optical comb is generated efficiently over a range including the bottom between the two peaks, from the differential signal S. C Among multiple sign inversion points, the sign inversion point that coincides with the point that efficiently generates the optical comb is identified, and an optical resonance length control signal S is generated to control the optical resonance length of the optical comb generator 2B in a manner that serves as a sign inversion point. CTL The optical resonant length control signal S CTLFeedback control is performed on the optical resonant length of the optical comb generator 2B. That is, the control signal generation unit 123C has the function of the bottom position detector 7, and generates a light intensity signal S representing the light intensity B' of the optical comb. B’ The bottom position of the signal level is set as the feedback control of the stable point.

[0136] And, using Figure 18 The block diagram illustrates the optical comb generator control device 110D of the present invention, which uses an optical comb emitted from an optical comb generator 2A as reflected light to control the laser frequency.

[0137] The optical resonant length control signal S in the optical comb generator control device 110C described above is replaced by the optical resonant length control signal S. CTL The control signal generation unit 123C, Figure 18 The optical comb generator control device 110D shown obtains a resonant control signal S for controlling the frequency difference between the laser frequency of the optical comb generator 2 and the mode of the optical resonator by controlling the light source frequency of the light source 1 of the optical comb generator 3B by the control signal generation unit 123D. CTF This controls the light source frequency of light source 1.

[0138] Furthermore, in this optical comb generator control device 110D, for the same constituent elements as those in the optical comb generator control device 110C described above, in Figure 18 The same reference numerals are used in the accompanying drawings, and their detailed descriptions are omitted.

[0139] The resonant control unit 120D in the optical comb generator control device 110D uses a lock-in amplifier 121 to detect the incident light signal S obtained by the first photodetector 116A. A and the reflected light detection signal S obtained by the second photodetector 116B mentioned above. B To obtain such Figure 15 (A) represents the light intensity signal S of the optical comb, which is independent of the power variation of the input light, representing the light intensity B' of the optical comb. B’ and output as follows Figure 15 (B) represents the light intensity signal S of the aforementioned optical comb, which indicates the light intensity B'. B’ The differential signal S C .

[0140] Furthermore, the control signal generation unit 123D, according to Figure 15 (A) The predefined range of ef is used to determine the light intensity signal S representing the light intensity B' of the aforementioned optical comb. B’ The optical comb is generated efficiently over a range including the bottom between the two peaks, from the differential signal S. CAmong multiple sign inversion points, the sign inversion point that coincides with the point that efficiently generates the optical comb is identified, and a resonant control signal S is generated to control the light source frequency of the aforementioned light source 1 in a manner that serves as a sign inversion point. CTF The modulating signal S generated by the oscillator 122 for modulating the light source frequency of the light source 1 is converted by the adder 124. D Adding the aforementioned resonance control signal to the light source 1, feedback control is performed on the light source frequency of the light source 1. That is, the control signal generation unit 123D has the function of the bottom position detector 7, and generates a light intensity signal S representing the light intensity B' of the optical comb. B’ The bottom position of the signal level is set as the feedback control of the stable point.

[0141] Explanation of reference numerals in the attached figures

[0142] 1: Light source; 2, 2A, 2B, 2C: Optical comb generator; 3, 3A, 3B: Optical comb generating device; 4, 4A, 4B, 114A, 114B: Coupler; 5: Optical filter; 6, 116A, 116B: Photodetector; 7: Bottom position detector; 8, 120A, 120B, 120C, 120D: Resonance control unit; 10, 10A, 10B, 110A, 110B, 110C, 110D: Optical comb generator control device; 11: Reference oscillator; 12, 52: Power divider; 13, 53 A, 52B, 53C, 53D: Oscillators; 14A, 14B: Frequency converters; 15A; 15B: Optical comb generators; 16A, 16B: Bandpass filters; 17A, 17B, 57A, 57B, 57C, 57D: Isolators; 21: Optical resonator; 21A, 21B: Mirrors; 22: Optical phase modulator; 53: Synthesizer circuit; 54: Switching circuit; 121: Lock-in amplifier; 122: Oscillator; 123A, 123B, 123C, 123D: Control signal generation unit; 124: Adder.

Claims

1. A control device for an optical comb generator, comprising a control device for an optical comb generator, wherein the optical comb generator includes an optical modulator within an optical resonator for optical modulation of incident light, and the control device for the optical comb generator is characterized by comprising: An optical filter that attenuates the carrier frequency component of an optical comb emitted from the optical resonator as transmitted or reflected light; A photodetector receives the light frequency components extracted from the optical comb by means of the optical filter and detects the light intensity; and The resonant control unit is supplied with the detection signal from the photodetector. wherein The light intensity of the optical comb after the carrier frequency component is attenuated by the optical filter is detected by the photodetector, and the light source frequency of the incident light is controlled by the resonance control unit. It also includes a bottom position discriminator, which is used to discriminate the bottom position of the signal level of the detection signal of the photodetector. The resonance control unit performs feedback control based on the discriminator output to set the bottom position of the signal level of the detection signal of the photodetector as a reference stable point.

2. The optical comb generator control device of claim 1, wherein, Multiple modulation signals with different modulation frequencies are switched and supplied to the optical modulator.

3. The optical comb generator control device according to any one of claims 1 to 2, characterized in that, The optical filter is a notch filter that attenuates the carrier frequency component of the optical comb.

4. The optical comb generator control device according to any one of claims 1 to 2, characterized in that, The optical filter is a high-pass filter with a cutoff frequency near the carrier frequency of the optical comb.

5. The optical comb generator control apparatus of any one of claims 1 to 2, wherein, The optical filter is a low-pass filter with a cutoff frequency near the carrier frequency component of the optical comb.

6. The optical comb generator control device according to any one of claims 1 to 2, characterized in that, The optical filter is a bandpass filter having a cutoff frequency near one of the carrier frequency components of the optical comb.

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