Optical comb generation device

Abstract

Isolator elements (12A, 12B) are inserted between the synthesizer circuit (11) and the switching circuit (13) to input a modulation signal from the synthesizer circuit (11) to the switching circuit (13) via the isolator elements (12A, 12B). This prevents the operation of the synthesizer circuit (11) from becoming unstable due to load changes caused by the cutting off or releasing of the circuit after the switching circuit (13). Thus, there is no problem of the synthesizer circuit (11) becoming unstable due to the instantaneous load change when the drive signal supplied to the optical comb generator (14A, 14B) is switched through the switching circuit (13). The drive state is changed by quickly switching the drive signal of the optical comb generator (14A, 14B).

Description

Technical Field

[0001] This invention relates to an optical comb generating apparatus, such as an optical comb rangefinder, for measuring distance based on the time difference between the interference signal of a measured light and the interference signal of a reference light. This application claims priority based on Japanese Patent Application No. 2021-010486, filed January 26, 2021, which is incorporated herein by reference. Background Technology

[0002] Conventional active distance measurement methods capable of precise point distance measurement include optical distance measurement using lasers. In laser rangefinders that use lasers to determine the distance to an object, the distance is calculated based on the difference between the laser emission time and the time when the laser light reflected back from the object is detected by a light-receiving element (see, for example, Patent Document 1). Alternatively, for example, modulation of the driving current of a semiconductor laser using a triangular wave or similar modulator is applied, and a photodiode embedded in the semiconductor laser element receives the reflected light from the object. Distance information is obtained based on the dominant wavenumber of the sawtooth wave appearing in the output current of the photodiode.

[0003] Laser rangefinders are known devices for measuring the absolute distance from a point to a measurement point with high precision. For example, Patent Document 1 describes a rangefinder that measures distance based on the time difference between the interference signal of the measurement light and the interference signal of the reference light.

[0004] In the past, it was difficult to achieve a practical absolute distance measuring instrument that could measure long distances with high precision. In order to obtain high resolution, the only method required returning to the origin, such as a laser displacement meter, but this method is not suitable for absolute distance measurement.

[0005] The inventors of this case previously proposed an optical comb rangefinder (see, for example, Patent Document 2). This optical comb rangefinder has two optical comb generators that pulse-emit reference light and measurement light, which are interferometrically modulated in intensity or phase and have different modulation periods. The interference light between the reference light pulse illuminating the reference surface and the measurement light pulse illuminating the measurement surface is detected by a reference light detector, and the interference light between the reference light pulse reflected from the reference surface and the measurement light pulse reflected from the measurement surface is detected by a measurement light detector. The difference between the distance to the reference surface and the distance to the measurement surface is calculated based on the time difference between the two interference signals obtained by the reference light detector and the measurement light detector, thereby enabling high-precision measurement in a short time.

[0006] In addition, a comb rangefinder has been proposed before (for example, see Patent Document 3): the reference point position for the distance to the measurement surface is specified by a reference optical path, thereby enabling high-precision and short-time long-distance measurement.

[0007] In principle, an optical comb rangefinder uses two optical comb generators, driven by two modulation signals with different frequencies, to pulse interferometrically emitted reference and measurement light pulses. The signal processing unit performs frequency analysis on the interference signals obtained from 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 designated N. The phase difference between the Nth mode of the reference signal and the measurement signal is calculated to compensate for the phase difference in the optical comb generation and transmission process from the optical comb generator to the reference point. Then, the increment of the phase difference on the frequency axis is calculated to determine the phase difference between the measurement signal pulse and the reference signal pulse, thereby calculating the distance from the reference point to the measurement surface.

[0008] Here, the distance measured using reference and measurement light pulses 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 a wavelength, the phase obtained by multiplying 2π by an integer inherently exists as the 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] That is, in absolute distance measurement that requires frequency switching, the time required for measurement includes the frequency switching time, the measurement time, and the absolute distance calculation time.

[0010] For example, the frequency of the modulation signal used to drive two optical comb generators can be switched by using a modulation signal generator whose frequency can be set via a PLL (Phase-Locked Loop).

[0011] The desired outcome is to minimize the phase noise of the signal used to drive the optical comb generator. When synchronizing the VCO with low phase noise with an external reference signal, the following process is performed: instead of unnecessarily expanding the control frequency range, the control bandwidth is limited to a frequency range where the phase noise of the VCO is lower than that of the reference signal, so that the characteristics of the VCO are presented as is.

[0012] It is sufficient to consider that the time until the PLL frequency stabilizes (setup time) is roughly proportional to the reciprocal of the control bandwidth. When the control bandwidth is narrowed for purification, the setup time becomes longer. When the control bandwidth is widened to shorten the setup time, the phase noise in the high-frequency band increases, and the level of spurious signal interference associated with the comparison frequency increases.

[0013] In a reference related to PLLs (Non-Patent Document 1), the following design example is described: with the bandwidth of the low-pass filter set to approximately 207 kHz, the frequency is locked within an error range of 1 kHz within approximately 51 microseconds. However, with the frequency divider circuit set in units of 500 kHz, the bandwidth of the low-pass filter must be further reduced to avoid spurious interference, and the settling time is expected to be several times longer than 51 microseconds.

[0014] Therefore, in the two optical comb generators of the optical comb rangefinder, multiple modulation signals that are phase-synchronized with the reference frequency signal through the PLL circuit and have a fixed frequency are supplied as driving signals by switching circuits.

[0015] Existing technical documents

[0016] Patent documents

[0017] Patent Document 1: Japanese Patent Application Publication No. 2001-343234

[0018] Patent Document 2: Japanese Patent No. 5231883

[0019] Patent Document 3: Japanese Patent Application Publication No. 2020-12641

[0020] Non-patent literature

[0021] Non-Patent Literature 1: Analog Dialogue, 52-07, July 2018, Fundamentals of Phase-Locked Loops (PLLs), Author: Ian Collins Summary of the Invention

[0022] The problem the invention aims to solve

[0023] As described above, in the two optical comb generators of the conventional optical comb rangefinder, multiple modulation signals that are phase-synchronized with the frequency signal of the reference through the PLL circuit and have a fixed frequency are supplied as drive signals by switching circuits. However, there is a problem that the operation becomes unstable due to the load change of the modulation signal generator at the moment of switching the drive signal through the switching circuit.

[0024] Therefore, in the process of such Figure 7 When the first experimental circuit 100 with the structure shown was used to analyze the transient response, the following results were obtained: Figure 8 The analysis results are as shown.

[0025] Figure 7 The first experimental circuit 100 shown includes a synthesizer circuit 101 that outputs two independent frequency signals (FM1: 1000MHz, FM2: 1000.5MHz) with a difference frequency of 500kHz, a 4-input 2-output switching circuit 102, and a double-balanced modulator (DBM) 103. A 1000.5MHz frequency signal is input from the FM2 terminal of the synthesizer circuit 101 to the RF input terminal of the DBM 103. At the instant the 4-input 2-output switching circuit 102 is switched on and off, a 1000MHz frequency signal is input from the FM1 terminal of the synthesizer circuit 101 to the LO input terminal of the DBM 103 via the 4-input 2-output switching circuit 102 with a delay of nanoseconds.

[0026] In the first experimental circuit 100, during the period when the 4-input 2-output switching circuit 102 is turned on, a waveform signal with a frequency difference of 500kHz between the 1000MHz frequency signal input to the LO input terminal and the 1000.5MHz frequency signal input to the RF input terminal is output at the IF output terminal of the DBM 103.

[0027] Figure 8 (A) is a waveform diagram obtained by plotting the time waveform of the 500kHz waveform signal obtained at the IF output terminal of the DBM 103 above, with the voltage value acquired at 200Msample / s (with a period of 5ns). It shows the case where the switching circuit 101 is turned on near 5000 points and a 500kHz waveform signal is output from the IF output terminal.

[0028] Figure 8(B) is a characteristic diagram showing the phase of the 500kHz component obtained at the IF output terminal of the DBM 103 mentioned above, showing a phase change of 30rad every 280μs, which results in a frequency deviation of about 17kHz.

[0029] Figure 8 (C) is a characteristic graph showing the power ratio of the 500kHz waveform signal obtained at the IF output terminal of the DBM 103 as determined by FFT analysis. Figure 8 (D) is an enlarged view of its vertical axis.

[0030] Regarding the power ratio, at first glance it appears to stabilize immediately after the switch, but... Figure 8 As shown in (D), residual vibrations remain in the 250 kHz range, and there is no stability at the 500 kHz frequency.

[0031] In addition, through such Figure 9 When the second experimental circuit 110, with the structure shown, was used to analyze the transient response, the following results were obtained: Figure 10 The analysis results are as shown.

[0032] exist Figure 9 In the second experimental circuit 110 shown, an isolator circuit 105 is inserted between the synthesizer circuit 101 and the 4-input 2-output switch circuit 102 in the first experimental circuit 100 for isolation. The other structures are the same as those in the first experimental circuit 100. The isolator circuit 105 consists of an RF amplifier with reverse isolation of 30dB or more and an attenuator with reverse isolation of 15dB.

[0033] In the second experimental circuit 110, at the instant the switching circuit 102 with the above-mentioned 4 inputs and 2 outputs is switched on, a frequency signal of 1000MHz is input from the FM1 terminal of the synthesizer circuit 101 to the LO input terminal of the DBM 103 via the isolator circuit 105 with a delay of nanoseconds through the switching circuit 102 with the above-mentioned 4 inputs and 2 outputs.

[0034] During the period when the aforementioned 4-input 2-output switching circuit 102 is turned on, at the IF output terminal of the aforementioned DBM 103, a waveform signal with a frequency difference of 500kHz between the 1000MHz frequency signal input to the LO input terminal via the isolator circuit 105 and the 1000.5MHz frequency signal input to the RF input terminal is output.

[0035] Figure 10(A) is a waveform diagram obtained by plotting the time waveform of the 500kHz waveform signal obtained at the IF output terminal of the DBM 103 above, with the voltage value acquired at 200Msample / s (with a period of 5ns). It shows the case where the switching circuit 102 is turned on near 5000 points and a 500kHz waveform signal is output from the IF output terminal.

[0036] Figure 10 (B) is a characteristic diagram showing the phase of the 500kHz component obtained at the IF output terminal of the DBM 103 mentioned above, showing a phase change of 0.1 rad every 200 μs, which results in a frequency deviation of about 80 Hz.

[0037] Figure 10 (C) is a characteristic graph showing the power ratio of the 500kHz waveform signal obtained at the IF output terminal of the DBM 103 as determined by FFT analysis. Figure 10 (D) is its magnified view. The power ratio stabilizes immediately after the switch, as shown. Figure 10 As shown in (D), there is no vibration and the frequency is stable.

[0038] In view of the actual situation described above, the object of the present invention is to provide an optical comb generating device that can instantaneously switch the drive frequency by avoiding unstable operating states associated with the switching of the drive frequency.

[0039] Furthermore, the present invention aims to provide an optimal optical comb generating device for measuring distance using an optical comb rangefinder or similar device based on the time difference between the interference signal of the measured light and the interference signal of the reference light.

[0040] 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.

[0041] Solution for solving the problem

[0042] In this invention, an isolator is inserted between the signal source and the switching circuit. The modulation signal is input from the signal source to the switching circuit through the isolator. This prevents the operation of the signal source from becoming unstable due to load changes caused by the cutting off or releasing of the circuit after the switching circuit. Thus, the problem of unstable operation caused by the instantaneous load change when the drive signal supplied to the optical comb generator is switched through the switching circuit is eliminated, and the drive signal of the optical comb generator is quickly switched to change the drive state.

[0043] That is, the present invention is an optical comb generating device, characterized in that it comprises: N signal sources, the N signal sources outputting N modulation signals with different frequencies, wherein N is an integer greater than or equal to 2; at least two isolators connected to the N signal sources; a switching circuit with N inputs and M outputs, wherein the N modulation signals are input to the switching circuit via the at least two isolators, wherein M is a positive integer; and M optical comb generators, wherein the N modulation signals are selectively input to the M optical comb generators via the N isolators and the switching circuit, wherein the M optical comb generators use at least two of the N modulation signals to output M optical combs with periodically modulated intensity or phase and different modulation periods.

[0044] The optical comb generating apparatus of the present invention can be configured to include M optical comb generators, where M is 2 or more, and to cyclically input the N types of modulation signals, which are input to the switching circuit via the N isolators connected to the N signal sources, to the M optical comb generators, and output M types of optical combs with different modulation periods that have been cyclically switched from the M optical comb generators.

[0045] In addition, the optical comb generating device involved in the present invention can be configured such that N=4 and M=2, and four modulation signals are cyclically input to two optical comb generators via the switching circuit, thereby outputting two optical combs with cyclically switched modulation periods and different modulation periods.

[0046] Furthermore, the optical comb generating device involved in this invention can be configured such that N=M=2, and two optical combs with different modulation periods are alternately output from two optical comb generators.

[0047] Furthermore, the optical comb generating device according to the present invention can be configured such that M=1, and the N types of modulation signals are cyclically input via the switching circuit, thereby outputting an optical comb with a cyclically switched modulation period from one optical comb generator.

[0048] Furthermore, in the optical comb generating apparatus of the present invention, it can be configured such that the N signal sources respectively generate N modulation signals whose phases are synchronized with the phase of the reference frequency signal through the PLL circuit and whose frequencies are fixed.

[0049] The effects of the invention

[0050] In this invention, an isolator is inserted between the signal source and the switching circuit. The modulation signal is input from the signal source to the switching circuit through the isolator. This prevents the operation of the signal source from becoming unstable due to load changes caused by the cutting off or releasing of the circuit after the switching circuit. Thus, the problem of unstable operation caused by the instantaneous load change when the drive signal supplied to the optical comb generator is switched through the switching circuit is eliminated, and the drive signal of the optical comb generator is quickly switched to change the drive state.

[0051] That is, in this invention, an optical comb generating device is provided that can instantaneously switch the drive frequency by avoiding unstable operating states associated with the switching of the drive frequency.

[0052] Furthermore, the present invention provides an optimal optical comb generating device for measuring distance using an optical comb rangefinder or similar device based on the time difference between the interference signal of the measuring light and the interference signal of the reference light. Attached Figure Description

[0053] Figure 1 This is a block diagram illustrating a structural example of a comb generating apparatus to which the present invention is applied.

[0054] Figure 2 This is a block diagram illustrating other structural examples of the optical comb generating apparatus to which the present invention is applied.

[0055] Figure 3 This is a state transition diagram showing the state transitions of the drive signals supplied to the two optical comb generators in the optical comb generating apparatus of the other structural examples described above.

[0056] Figure 4 This is a block diagram illustrating a specific structural example of the switching circuit in the optical comb generating device described in the other structural examples above.

[0057] Figure 5 This is a block diagram illustrating other structural examples of the optical comb generating apparatus to which the present invention is applied.

[0058] Figure 6 This is a block diagram illustrating yet another structural example of the optical comb generating apparatus to which the present invention is applied.

[0059] Figure 7 This is a block diagram showing the structure of a first experimental circuit used in an experiment to analyze the transient response of a drive signal whose phase has been synchronized by switching a drive signal through a PLL circuit via a switching circuit.

[0060] Figure 8 (A) Figure 8 (B) Figure 8 (C) Figure 8 (D) is a diagram showing the experimental results obtained through the first experimental circuit described above. Figure 8 (A) is a waveform diagram showing the 500kHz waveform signal obtained at the IF output terminal of the DBM. Figure 8 (B) is a characteristic diagram showing the phase of the aforementioned 500kHz component. Figure 8 (C) is a characteristic graph showing the power ratio of the aforementioned 500kHz waveform signal obtained through FFT analysis. Figure 8 (D) is its enlarged view.

[0061] Figure 9 This is a block diagram showing the structure of the second experimental circuit used in an experiment to analyze the transient response of a drive signal whose phase was synchronized by switching the PLL circuit via a switching circuit.

[0062] Figure 10 (A) Figure 10 (B) Figure 10 (C) Figure 10 (D) is a diagram showing the experimental results obtained through the second experimental circuit described above. Figure 10 (A) is a waveform diagram showing the 500kHz waveform signal obtained at the IF output terminal of the DBM. Figure 10 (B) is a characteristic diagram showing the phase of the aforementioned 500kHz component. Figure 10 (C) is a characteristic graph showing the power ratio of the aforementioned 500kHz waveform signal obtained through FFT analysis. Figure 10 (D) is its enlarged view. Detailed Implementation

[0063] 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.

[0064] The present invention is applied to a light comb generating apparatus 10, which, for example... Figure 1 As shown in the block diagram, it has two optical comb generators 14A and 14B that emit interferometric measurement light and reference light, which are periodically modulated in intensity or phase and have different modulation periods.

[0065] The optical comb generating device 10 is used, for example, in optical comb rangefinders and three-dimensional shape measuring machines that measure distance based on the time difference between the interference signal of the measurement light and the interference signal of the reference light, as described in Patent Documents 2 and 3, to emit interferometric measurement light and reference light that are periodically modulated in intensity or phase and have different modulation periods.

[0066] The optical comb generating device 10 includes a synthesizer circuit 11 that outputs two independent frequency signals (Fm1: 1000MHz, Fm2: 1000.5MHz) with a difference frequency of 500kHz, a first isolator 12A connected to the Fm1 terminal of the synthesizer circuit 11, a second isolator 12B connected to the Fm2 terminal of the synthesizer circuit 11, a 2-input 2-output switching circuit 13, a first optical comb generator 14A, and a second optical comb generator 14B. The Fm1 and Fm2 terminals of the synthesizer circuit 11 are connected to the two input terminals of the switching circuit 13 via the first isolator 12A and the second isolator 12B. The first optical comb generator 14A and the second optical comb generator 14B are connected to the two output terminals of the switching circuit 13.

[0067] The synthesizer circuit 11 described above includes: a reference frequency signal generator 11R, which generates a 10MHz reference frequency signal F. REF ; and generate a reference frequency signal F whose phase is respectively different from that generated by the reference frequency signal generator 11R. REF A first modulation signal generator 11A and a second modulation signal generator 11B with phase-synchronized and fixed-frequency first modulation signal Fm1 and second modulation signal Fm2 of different frequencies.

[0068] The first modulation signal generator 11A generates a reference frequency signal F that is phased with the reference frequency signal generated by the reference frequency signal generator 11R via a PLL circuit. REF The first modulation signal Fm1 is phase-synchronized and has a fixed frequency of 1000MHz.

[0069] In addition, the second modulation signal generator 11B generates a reference frequency signal F that is phased with the reference frequency signal generator 11R by the PLL circuit. REF The second modulation signal Fm2 is phase-synchronized and has a fixed frequency of 1000.5MHz.

[0070] The first modulation signal Fm1 obtained by the first modulation signal generator 11A is input to one input terminal of the switch circuit 13 via the first isolator 12A connected to the Fm1 terminal of the synthesizer circuit 11, and is selectively supplied to the first optical comb generator 14A and the second optical comb generator 14B connected to the two output terminals of the switch circuit 13.

[0071] In addition, the second modulation signal Fm2 obtained by the second modulation signal generator 11B is input to another input terminal of the switch circuit 13 via the second isolator 12B connected to the Fm2 terminal of the synthesizer circuit 11, and is selectively supplied to the first optical comb generator 14A and the second optical comb generator 14B connected to the two output terminals of the switch circuit 13.

[0072] The aforementioned switching circuit 13 functions as a 2-input 2-output selection switch, alternately outputting the first modulation signal Fm1 and the second modulation signal Fm2, which are input to the two input terminals, from the two output terminals to switch the first modulation signal Fm1 and the second modulation signal Fm2 supplied as driving signals to the first optical comb generator 14A and the second optical comb generator 14B connected to the two output terminals.

[0073] In the optical comb generating device 10, the first modulation signal Fm1 and the second modulation signal Fm2 are switched as driving signals through the switching circuit 13 and supplied to the first optical comb generator 14A and the second optical comb generator 14B, thereby enabling the first optical comb generator 14A and the second optical comb generator 14B to alternately output two optical combs with different modulation periods.

[0074] Furthermore, in this optical comb generating device 10, the first modulation signal Fm1 and the second modulation signal Fm2 are input to the switching circuit 13 via the first isolator 12A and the second isolator 12B inserted between the synthesizer circuit 11 and the switching circuit 13. Therefore, similar to the experimental results obtained through the second experimental circuit 110, the problem of unstable operation of the first modulation signal generator 11A and the second modulation signal generator 11B due to the instantaneous load change when the drive signals of the first optical comb generator 14A and the second optical comb generator 14B are switched through the switching circuit 13 can be avoided, and the drive signals of the first optical comb generator 14A and the second optical comb generator 14B can be quickly switched to change the drive state.

[0075] Furthermore, in the case of absolute distance measurement requiring frequency switching in the optical comb rangefinder and three-dimensional shape measuring machine described in Patent Documents 2 and 3, the time including the frequency switching time and the absolute distance calculation time becomes the measurement time. However, by inserting the first isolator 12A and the second isolator 12B between the synthesizer circuit 11 and the switch circuit 13 as described above, the optical comb generating device 10 can quickly switch the drive signals of the first optical comb generator 14A and the second optical comb generator 14B to change the drive state. Therefore, by using the optical comb generating device 10 as two optical comb light sources for absolute distance measurement by switching the modulation frequency of the reference signal and the measurement signal, the absolute distance measurement time can be shortened.

[0076] Here, the optical comb generating device 10 alternately outputs two optical combs with different modulation periods from two optical comb generators 14A and 14B. However, the present invention is not limited to the optical comb generating device 10 that alternately outputs two optical combs. It can be any structure that includes: N signal sources, the N signal sources outputting N modulation signals with different frequencies, where N is an integer of 2 or more; at least two isolators connected to the N signal sources; a switching circuit with X (an integer of 2 or more) input and Y (a positive integer) output, whereby the N modulation signals are input to the switching circuit via the at least two isolators; and M optical comb generators, whereby the N modulation signals are selectively input to the M optical comb generators via the N isolators and the switching circuit, where M is a positive integer. The M optical comb generators use at least two of the N modulation signals to output M optical combs with periodically modulated intensity or phase and different modulation periods. Furthermore, N≠X≠Y≠M is also acceptable. Unused output terminals of the switching circuit should be pre-connected with terminating resistors.

[0077] This invention can be applied to optical comb generating devices with various structures, such as: Figure 2 The optical comb generating device 20 shown is configured with N=4, X=4, Y=2, M=2, and cyclically outputs four modulation signals with different modulation periods to two optical comb generators 24A and 24B; as shown Figure 5 The optical comb generating device 30 shown has a structure in which N different modulation periods of M optical combs are cyclically switched from the outputs of M optical comb generators 34A, 34B, ..., 34M; as shown... Figure 6 The optical comb generating device 40 shown is configured such that N=X=Y, M=1, and outputs N different modulation periods of optical combs that are cyclically switched from one optical comb generator 44.

[0078] Figure 2The optical comb generating device 20 shown is obtained by applying the present invention to an optical comb generating device in which N=4, M=2, and four modulation signals output from synthesizer circuit 21 are cyclically input to two optical comb generators 24A and 24B via switch circuit 23 to output two optical combs with cyclically switched modulation periods and different modulation periods. The optical comb generating device 20 includes: a synthesizer circuit 21 that outputs four independent frequency signals (Fm1: 1000MHz, Fm2: 1010MHz, Fm3: 1000.5MHz, Fm4: 1010.5MHz) with a difference frequency of 500kHz; a first isolator 22A connected to the Fm1 terminal of the synthesizer circuit 21; and a first isolator 22A connected to the synthesizer circuit 21. The circuit includes a second isolator 22B connected to the Fm2 terminal of synthesizer circuit 21; a third isolator 22C connected to the Fm3 terminal of synthesizer circuit 21; a fourth isolator 22D connected to the Fm4 terminal of synthesizer circuit 21; a 4-input 2-output switching circuit 23; a first optical comb generator 24A; and a second optical comb generator 24B. The Fm1, Fm2, Fm3, and Fm4 terminals of synthesizer circuit 21 are connected to the four input terminals of switching circuit 23 via the first isolator 22A, the second isolator 22B, the third isolator 22C, and the fourth isolator 22D. The first optical comb generator 24A and the second optical comb generator 24B are connected to the two output terminals of switching circuit 23.

[0079] The synthesizer circuit 21 described above includes: a reference frequency signal generator 21R, which generates a 10MHz reference frequency signal F. REF ; and generate a reference frequency signal F whose phase is respectively different from that generated by the reference frequency signal generator 21R. REF Four modulation signal generators 21A, 21B, 21C, and 21D are used to generate four modulation signals Fm1, Fm2, Fm3, and Fm4 with different frequencies and phase synchronization and fixed frequency.

[0080] The first modulation signal generator 21A generates a reference frequency signal F that is phased with the reference frequency signal generated by the aforementioned reference frequency signal generator 21R via a PLL circuit. REF The first modulation signal Fm1 is phase-synchronized and has a fixed frequency of 1000MHz.

[0081] In addition, the second modulation signal generator 21B generates a reference frequency signal F that is phased with the reference frequency signal generator 21R by the PLL circuit. REF The second modulation signal Fm2 is phase-synchronized and has a fixed frequency of 1010MHz.

[0082] In addition, the third modulation signal generator 21C generates a reference frequency signal F that is phased with the reference frequency signal generator 21R by the PLL circuit. REF The third modulation signal Fm3 is phase-synchronized and has a fixed frequency of 1000.5MHz.

[0083] Furthermore, the fourth modulation signal generator 21D generates a reference frequency signal F that is phased with the reference frequency signal generated by the aforementioned reference frequency signal generator 21R via a PLL circuit. REF The fourth modulation signal Fm4 is phase-synchronized and has a fixed frequency of 1010.5MHz.

[0084] Furthermore, the aforementioned switch circuit 23 functions as a 4-input 2-output selection switch, cyclically switching the output of the four modulation signals Fm1, Fm2, Fm3, and Fm4, which are input from the synthesizer circuit 21 to the four input terminals via the first isolator 22A, the second isolator 12B, the third isolator 22C, and the fourth isolator 22D, from the two output terminals. This cyclically switches the output of the four modulation signals Fm1, Fm2, Fm3, and Fm4 supplied as drive signals to the first optical comb generator 24A and the second optical comb generator 24B connected to the two output terminals.

[0085] like Figure 3 As shown in the diagram, the switching circuit 23 cyclically switches between four modulation signals Fm1, Fm2, Fm3, and Fm4 supplied to the first optical comb generator 24A and the second optical comb generator 24B as drive signals, just as the driving signal switching state is shown in the diagram.

[0086] Here, as a reference light pulse and a measurement light pulse for absolute distance measurement requiring frequency switching in the optical comb rangefinder and three-dimensional shape measuring machine described in Patent Documents 2 and 3, the optical comb generating device 20 generates two types of optical combs, as shown in Table 1. The above four modulation signals Fm1, Fm2, Fm3, and Fm4 are cyclically switched as driving signals and supplied to the first optical comb generator 24A and the second optical comb generator 24B through the above switching circuit 23. As a result, two types of optical combs with cyclically switched modulation periods and different modulation periods are output from the first optical comb generator 24A and the second optical comb generator 24B.

[0087] [Table 1]

[0088]

[0089] Table 1 shows the transition states OFCG1 / OFCG2 of the drive signals of the first optical comb generator 24A and the second optical comb generator 24B in settings #1 to #4, and their phase difference, which is the phase obtained by correcting the sign that is reversed depending on the magnitude relationship between the drive frequencies of the OFCG1 side and the OFCG2 side. The frequencies of the drive signals are, for example, Δf = 500kHz, Δfm = 10MHz, fm = Fm1 (1000MHz), fm + Δfm = Fm2 (1010MHz), fm + Δf = Fm3 (1000.5MHz), and fm + Δfm + Δf = Fm4 (1010.5MHz). Furthermore, in the frequency settings #1 to #4 of the drive signals in the first optical comb generator 24A and the second optical comb generator 24B installed in the optical comb rangefinder, the synthesizer circuit 21 of the 1GHz band is combined with the upconverter, thereby setting Δf = 500kHz, Δfm = 10MHz, fm = Fm1 (25000MHz), fm + Δfm = Fm2 (25010MHz), fm + Δf = Fm3 (25000.5MHz), and fm + Δfm + Δf = Fm4 (25010.5MHz).

[0090] In this optical comb rangefinder, in principle, two optical comb generators driven by two modulation signals of different frequencies pulse with interferometric characteristics, namely, a reference light pulse and a measurement light pulse, are pulsed out. The signal processing unit performs frequency analysis on the interference signal obtained from the reference photodetector (hereinafter referred to as the reference signal) and the interference signal obtained from the measurement photodetector (hereinafter referred to as the measurement signal). The mode number, starting from the center frequency of the optical comb, is set to 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 determined by calculating the increment of the phase difference at each step on the frequency axis, thereby calculating the distance from the reference point to the measurement surface.

[0091] 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 becomes ambiguous due to the periodicity of the object light, making it impossible to uniquely determine the distance. Therefore, four measurements are performed using a reference light pulse and a measurement light pulse set to the four modulation frequencies shown in Table 1. In the signal processing unit, the phase differences obtained by performing the same processing are used to calculate the ambiguous distance (L) exceeding half the wavelength. a =c / 2fm, where c is the speed of light) is the distance.

[0092] 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.

[0093] The distance measured under setting #1 is equivalent to the distance (L) that is half the wavelength of the modulation frequency fm. a =c / 2fm, where c is the speed of light. When the length is long, 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 of m is unknown.

[0094] On the other hand, the difference between the phase difference -2πfmT between the reference signal and the measurement signal under setting #1 and the phase difference -2π(fm+Δfm)T between the reference signal and the measurement signal under setting #2 is 2πΔfmT. In addition, the difference between the phase difference -2π(fm+Δf)T between the reference signal and the measurement signal under setting #3 and the phase difference -2π(fm+Δfm+Δf)T between the reference signal and the measurement signal under setting #4 is 2πΔfmT. If it is within a distance equivalent to half the wavelength of Δfm (La is 15m if Δfm=10MHz), then the phase is uniquely determined.

[0095] Furthermore, by multiplying this phase by fm / Δfm and comparing it with the phase difference of #1, the integer m can be determined.

[0096] 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.

[0097] Here, with fm = 25 GHz, Δf = 500 kHz, and Δfm = 10 MHz, distance measurements within La = 300 m are possible because Δf = 500 kHz.

[0098] In the optical comb rangefinder equipped with the optical comb generating device 20, 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 phases of the set states #1, #2, #3, and #4 to perform the calculation of the absolute distance.

[0099] 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.

[0100] In the optical comb generating device 20 described above, by inserting isolators 22A, 22B, 22C, and 22D between the synthesizer circuit 21 and the switching circuit 23, the four modulation signals Fm1, Fm2, Fm3, and Fm4 can be cyclically switched by the switching circuit 23 to quickly change the driving state of the first optical comb generator 24A and the second optical comb generator 24B. As a result, the two optical comb light sources used as modulation frequencies for switching reference signals and measurement signals to perform absolute distance measurement can shorten the measurement time of absolute distance.

[0101] 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 signals Fm1, Fm2, Fm3, and Fm4 through the aforementioned switch circuit 23, the phase shift caused by the signal return path other than the measured object can be corrected, thereby obtaining the absolute distance result with high accuracy. 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 measured object remains unchanged, but the sign is reversed. On the other hand, the sign of the shift caused by the cable length of the interference 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.

[0102] Here, Figure 4 This is a block diagram showing a specific structural example of the 4-input, 2-output switching circuit 23 of the aforementioned optical comb generating device 20.

[0103] That is, regarding the switching circuit 23, such as Figure 4As shown in the block diagram, four switching circuits 23, each with one input and two outputs, are connected to the primary of the four modulation signals Fm1, Fm2, Fm3, and Fm4 generated by the modulation signal generators 21A, 21B, 21C, and 21D of the synthesizer circuit 21 via isolators 22A, 22B, 22C, and 22D connected to the terminals Fm1, Fm2, Fm3, and Fm4 of the synthesizer circuit 21. 1A ,twenty three 1B ,twenty three 1C ,twenty three 1D via the aforementioned primary switching circuit 23 1A ,twenty three 1B ,twenty three 1C ,twenty three 1D The four modulation signals Fm1, Fm2, Fm3, and Fm4 input to the circuit are set in the next stage as two switching circuits with two inputs and one output, respectively. 2A ,twenty three 2B , and the above two switching circuits 23 2A ,twenty three 2B Each output terminal is connected to the next level of two 1-input, 2-output switching circuits 23 3A ,twenty three 3B , and the above two switching circuits 23 3A ,twenty three 3B The final stage of the connection consists of two switching circuits with 2 inputs and 1 output, respectively. 4A ,twenty three 4B The control logic 23C is used to interact with the 10MHz reference signal F. REF The switching control is performed synchronously, thus... Figure 3 The diagram shows the switching states of the drive signals in the first optical comb generator 14A and the second optical comb generator 14B, where the four modulation signals Fm1, Fm2, Fm3, and Fm4 supplied to the first optical comb generator 14A and the second optical comb generator 14B as drive signals are cyclically switched.

[0104] In this switching circuit 23, the primary four switching circuits 23 1A ,twenty three 1B ,twenty three 1C ,twenty three 1D One of each of the two output terminals is connected to the two switching circuits of the next stage 23. 2A ,twenty three 2B One input terminal is connected, while the other output terminal is terminated by a terminating resistor.

[0105] In addition, Figure 4In a specific example of the switching circuit 23 shown in the block diagram, four switching circuits 23 are connected to the primary side via first to fourth isolators 22A, 22B, 22C, and 22D, which are respectively composed of isolator circuits combining a variable attenuator and a bandpass filter. 1A ,twenty three 1B ,twenty three 1C ,twenty three 1D Input the above four modulation signals Fm1, Fm2, Fm3, and Fm4, and from the two switching circuits 23 in the final stage... 4A ,twenty three 4B The output terminals output the four cyclically switched modulation signals Fm1, Fm2, Fm3, and Fm4 via the first and second isolators 23A and 23B, which are composed of isolator circuits that combine isolation amplifiers and bandpass filters, respectively.

[0106] in addition, Figure 5 The optical comb generating device 30 shown enables M optical combs with different modulation periods that are cyclically switched from the outputs of M optical comb generators 34A, 34B, ..., 34M. It includes a synthesizer circuit 31 with N independent frequency signals Fm1, Fm2, ..., FmN with an output difference frequency of Δf, N isolators 32A, 32B, ..., 32N connected to the N output terminals of the synthesizer circuit 31, a switch circuit 33 with N inputs and M outputs, and M optical comb generators 34A, 34B, ..., 34M. The N output terminals of the synthesizer circuit 31 are connected to the N input terminals of the switch circuit 33 via the N isolators 32A, 32B, ..., 32N. The M optical comb generators 34A, 34B, ..., 34M are connected to the M output terminals of the switch circuit 33.

[0107] The synthesizer circuit 31 described above is equipped with a reference frequency signal F that generates 10MHz. REF The reference frequency signal generator 31R, and the reference frequency signal F generated by the reference frequency signal generator 31R with phases respectively. REF N modulation signal generators 31A, 31B, ..., 31N, which generate N different modulation signals Fm1, Fm2, ..., FmN with phase synchronization and fixed frequency.

[0108] Furthermore, the aforementioned switching circuit 33 functions as an N-input M-output selection switch, cyclically switching the N modulation signals Fm1, Fm2, ..., FmN input from the synthesizer circuit 31 via the N isolators 32A, 32B, ..., 32N to the N input terminals and cyclically outputting them from the M output terminals, thereby cyclically switching the N modulation signals Fm1, Fm2, ..., FmN supplied as drive signals to the M optical comb generators 34A, 34B, ..., 34M connected to the M output terminals.

[0109] In this optical comb generating device 30, a combination of one or more optical combs can be provided to the optical comb interferometer system in the optical comb rangefinder. Since N isolators 32A, 32B, ..., 32N are inserted between the synthesizer circuit 31 and the switching circuit 33, the frequencies of the N modulation signal generators 31A, 31B, ..., 31N constituting the synthesizer circuit 31 can remain stable without being affected by load fluctuations caused by open circuits or short circuits in the circuit after the switching circuit 33. Thus, the drive signals supplied to the M optical comb generators 34A, 34B, ..., 34M can be cyclically switched by the switching circuit 33, and phase measurement can be performed immediately after switching.

[0110] Furthermore, in this optical comb generating device 30, N and M are 2 or more, but are not limited to N=M. Generally, N≥M. When N<M, a power divider is used for signal supply, or an optical comb generator that is not supplied with a signal may exist.

[0111] and, Figure 6 The optical comb generating device 40 shown is configured with M=1. It outputs an optical comb with N modulation periods that are cyclically switched from one optical comb generator 44. It includes a synthesizer circuit 41 that outputs N independent frequency signals Fm1, Fm2, ..., FmN, N isolators 42A, 42B, ..., 42N connected to the N output terminals of the synthesizer circuit 41, a switch circuit 43 with N inputs and 1 output, and one optical comb generator 44. The N output terminals of the synthesizer circuit 41 are connected to the N input terminals of the switch circuit 43 via the N isolators 42A, 42B, ..., 42N. The optical comb generator 44 is connected to the output terminals of the switch circuit 43.

[0112] The synthesizer circuit 41 described above is equipped with a function to generate a reference frequency signal F. REF The reference frequency signal generator 41R, and the reference frequency signal F generated by the reference frequency signal generator 41R with phases respectively. REFN modulation signal generators 41A, 41B, ..., 41N, which generate N different modulation signals Fm1, Fm2, ..., FmN with phase synchronization and fixed frequency.

[0113] Furthermore, the aforementioned switching circuit 43 functions as an N-input, 1-output selection switch, cyclically switching the N modulation signals Fm1, Fm2, ..., FmN input from the synthesizer circuit 41 via the N isolators 42A, 42B, ..., 42N to the N input terminals and cyclically outputting them from the output terminals, thereby cyclically switching the N modulation signals Fm1, Fm2, ..., FmN supplied as drive signals to the optical comb generator 44 connected to the aforementioned output terminals.

[0114] In this optical comb generating device 40, since N isolators 42A, 42B, ..., 42N are inserted between the synthesizer circuit 41 and the switching circuit 43, the frequencies of the N modulation signal generators 41A, 41B, ..., 41N constituting the synthesizer circuit 41 can remain stable without being affected by load fluctuations caused by open circuits or short circuits in the circuits after the switching circuit 43. Thus, the N modulation signals Fm1, Fm2, ..., FmN can be cyclically switched and supplied to the optical comb generator 44 as driving signals through the switching circuit 43.

[0115] The optical comb generating device 40 is used as a separate optical comb light source that emits an optical comb from an optical comb generator 44 that operates by using N modulation signals Fm1, Fm2, ..., FmN that are cyclically switched by the aforementioned switching circuit 43 as driving signals.

[0116] Furthermore, in the aforementioned optical comb generating devices 10, 20, 30, and 40, isolation elements such as microwave amplifiers with high reverse isolation, PI-type resistor attenuators, T-type resistor attenuators, and microwave isolators using ferrite, as well as isolator circuits combining variable attenuators and bandpass filters, and isolator circuits combining isolation amplifiers, resistor attenuators, and bandpass filters, can be inserted into isolators 12A, 12B, 22A-22D, 32A-32N, and 42A-42N between synthesizer circuits 11, 21, 31, and 41 and switch circuits 13, 23, 33, and 43. Additionally, the isolators can exist independently of the synthesizer circuits and switch circuits as separate housings, or they can be installed as part of the output section of the synthesizer circuit and the input section of the switch circuit.

[0117] Explanation of reference numerals in the attached figures

[0118] 10, 20, 30, 40: Optical comb generating device; 11, 21, 31, 41: Synthesizer circuit; 13, 23, 231A ,twenty three 1B ,twenty three 1C ,twenty three 1D ,twenty three 2A ,twenty three 2B ,twenty three 3A ,twenty three 3B ,twenty three 4A ,twenty three 4B 1, 33, 43: Switching circuits; 12A, 12B, 22A~22D, 32A~32N, 42A~42N: Isolators; 14A, 14B, 24A, 24B, 34A~34M, 44: Comb generators; 23C: Control logic.

Claims

1. A light comb generating device, characterized in that, have: There are N signal sources, and the N signal sources output N different modulation signals with different frequencies, where N is an integer greater than 2; At least two isolators are connected to the N signal sources; A switching circuit with X input and Y output, wherein the N types of modulation signals are input to the switching circuit via the at least two isolators, wherein X is an integer greater than or equal to 2, and Y is a positive integer; and M optical comb generators are used, and the N modulation signals are selectively input to the M optical comb generators via the at least two isolators and the switching circuit, where M is a positive integer. The M optical comb generators utilize at least two of the N modulation signals to output M optical combs whose intensity or phase is periodically modulated and whose modulation periods are different from each other.

2. The optical comb generating apparatus according to claim 1, characterized in that, The system comprises M optical comb generators, where M is 2 or more, and inputs N types of modulation signals, which are connected to the N signal sources and then to the switching circuit, to the M optical comb generators in a cyclic switching manner via the switching circuit. The M optical combs are output from the M optical comb generators, and the N modulation periods of each optical comb are cyclically switched.

3. The optical comb generating apparatus according to claim 2, characterized in that, N=4, M=2, By cyclically inputting four modulation signals into two optical comb generators via the switching circuit, two optical combs with cyclically switched modulation periods and different modulation periods are output.

4. The optical comb generating apparatus according to claim 1, characterized in that, N = M = 2, Two optical combs with different modulation periods are alternately output from two optical comb generators.

5. The optical comb generating apparatus according to claim 1, characterized in that, M＝1， By cyclically inputting the N types of modulation signals into the switching circuit via the N isolators connected to the N signal sources, an optical comb with a cyclically switched modulation period is output from one optical comb generator.

6. The optical comb generating apparatus according to any one of claims 1 to 5, characterized in that, The N signal sources generate N modulation signals that are phase-synchronized with the reference frequency signal and have a fixed frequency through a phase-locked loop circuit.

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