Low relative phase noise optical comb generation apparatus

Abstract

The present invention provides a low relative phase noise optical comb generator device, which stabilizes the repetition frequency of an interference signal of an optical comb by reducing the relative phase noise of driving signals of different frequencies for driving a plurality of optical comb generators in an optical comb range finder. It is provided with: at least three oscillators (13, 13A, 13B) that generate frequency signals whose phases are synchronized with the phase of a reference frequency signal (F REF ) provided by a reference oscillator (11) and whose frequencies are different from each other; and at least two frequency converters (14A, 14B) that are input with a frequency signal obtained by one of the three oscillators (13) and each frequency signal obtained by each of the oscillators (13A, 13B) other than the one oscillator, wherein the at least two frequency converters (14A, 14B) respectively supply, as driving signals, at least two kinds of modulation signals of a sum frequency signal or a difference frequency signal of the frequency signal of the one oscillator and the frequency signal of the other oscillator whose phase noise is smaller than that of the frequency signal of the one oscillator, whose relative phase noise is reduced, to at least two optical comb generators (15A, 15B).

Description

Technical Field

[0001] This invention relates to a low relative phase noise optical comb generating apparatus for use in optical comb rangefinders and the like, which generates two or more optical combs and is capable of measuring distance based on the time difference between the interference signal of the measured light and the interference signal of the reference light. This application claims priority based on Japanese Patent Application No. 2021-026718, filed February 22, 2021, which is incorporated herein by reference. Background Technology

[0002] Previously, as an active distance measurement method capable of precise point-to-point distance measurement, optical distance measurement using lasers is known. In laser rangefinders that use lasers to determine the distance to an object, the distance to the object is calculated based on the difference between the laser emission time and the time when the laser light reflected back after illuminating the object is detected by a light-receiving element. Alternatively, for example, modulation of the driving current of a semiconductor laser using a triangular wave or the like 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 another point with high precision.

[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 interferometer, but this method was not suitable for absolute distance measurement.

[0005] The inventors of this case previously proposed a rangefinder, a distance measurement method, and an optical three-dimensional shape measuring machine (see, for example, Patent Document 1). This machine comprises two optical comb generators that pulse-emit reference light and measurement light, respectively, whose intensity or phase is periodically modulated and whose modulation periods are different. A reference light detector detects the interference light between a reference light pulse illuminating a reference surface and a measurement light pulse illuminating a measurement surface. A measurement light detector detects the interference light between a reference light pulse reflected from the reference surface and a measurement light pulse reflected from the measurement surface. 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.

[0006] In addition, a rangefinder, distance measurement method, and optical three-dimensional shape measuring machine capable of measuring the dynamic range of reflected light levels from low-reflectivity materials to high-reflectivity materials have been proposed previously (see, for example, Patent Document 2).

[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] As described in Patent Document 1, in order to ensure the relative stability of the frequency fm and the frequency fm+Δf signals of the two optical comb generators in the optical comb rangefinder, it is important to keep the repetition frequency of the pulses relatively fixed.

[0009] Furthermore, in optical comb rangefinders, if additional phase fluctuations or phase noise exist besides the periodic phase rotation caused by the frequency difference between frequency fm and frequency fm+Δf, and the phase delay caused by the reciprocating motion of the measurement light pulse within the measurement interval, these will become the cause of measurement deviations. Regarding the interference signal, both the reference signal and the measurement signal are generated with a period of 1 / Δf. By calculating the delay time or phase of the measurement signal based on the generation time of the reference signal, phase fluctuations and phase noise at frequencies much lower than Δf (with periods longer than 1 / Δf) can be eliminated to some extent. However, there is no correction method for phase fluctuations with periods shorter than the reciprocal of Δf; they directly appear as deviations in the measurement results. To reduce the deviation of the measurement value within a period of Δf (the shortest measurement time), short-term stability of the relative phase between fm and fm+Δf is required.

[0010] The modulation signals used to drive the two optical comb generators can be obtained, for example, by a modulation signal generator that is configured to have a frequency set by a PLL (Phase-Locked Loop).

[0011] However, even when using the same reference oscillator, phase jitter accumulates when the frequency of the low-frequency reference signal is increased to the microwave band drive frequency via a PLL. In the short term, this adds jitter to the relative phase. In this case, measurement accuracy decreases for short-duration measurements. Therefore, to shorten measurement time, a pair of oscillators is needed to achieve a wide bandwidth for the phase-locked loop.

[0012] In addition, in the optical comb rangefinder, a peak detection circuit can be used in the signal processing unit to determine the time difference of the signal peak, or a high-speed Fourier transform can be performed on the signal to determine the relationship between frequency and phase. Since the signal repeats quickly, distance measurement can be performed in a short time.

[0013] In the technology disclosed in Patent Document 2, the orthogonal phase component of the original waveform is obtained by Hilbert transforming the value of the low-gain second interference signal or the high-gain second interference signal, and the maximum intensity of the envelope of the AC signal is obtained by the peak value of the square waveform of the envelope obtained by the sum of the squares of the original waveform and the Hilbert-transformed waveform.

[0014] This means that it is important to determine the envelope peak of the interference waveform in the shape measuring device and rangefinder of Patent Document 1 and Patent Document 2. For example, in Patent Document 1... Figure 2 As shown, the envelope peak is determined by the overlap between the reference pulse sequence and the measurement pulse sequence. The jitter of the reference pulse sequence and the measurement pulse sequence is determined by the phase noise of the signals fm and fm+Δf. Therefore, the jitter of the envelope peak of the interference signal of the optical combs fm and fm+Δf is determined by the relative phase noise of the drive signals of the optical combs fm and fm+Δf.

[0015] Here, in Figure 1 The image shows an example of a conventional optical comb drive circuit. It is derived from Patent Document 1. Figure 6 This is obtained by extracting the part to be explained here from the light source 100 shown.

[0016] Oscillators 103A and 103B are phase-synchronized with a common reference oscillator 104. Typically, a 10MHz OCXO, or a 10MHz OCXO synchronized with a rubidium atomic oscillator or a cesium atomic oscillator, is used in reference oscillator 104. The absolute frequency accuracy of the reference frequency is selected as needed, depending on the measurement distance and required precision. Regarding optical comb generators 120A and 120B, they can be considered as devices that generate a sequence of optical pulses when the AC signal of oscillators 103A and 103B, which generate periodic drive signals, crosses zero. The jitter of each pulse is determined by the phase noise of oscillators 103A and 103B.

[0017] Here, Figure 2 Reference to patent document 1 Figure 8 For example waveforms, the peak times of each envelope are represented as t0, t1, t2, t3, ..., t n The two optical comb generators 120A and 120B have driving signal frequencies of fm+Δf and fm, respectively. Therefore, the envelope peaks of the interference waveform are synchronized with the difference frequency Δf of the driving signal, and the envelope peak interval is Tb = 1 / Δf.

[0018] Regarding the timing of the envelope peak of the interference waveform, as mentioned in paragraph

[0045] of Patent Document 1, the peak is formed when the timing of the optical pulses output by the optical comb generators 120A and 120B is consistent. Therefore, there is jitter in the relative phase noise of the two oscillators 103A and 103B that drive the optical comb generators 120A and 120B.

[0019] A distance measuring instrument using optical comb interferometry and an optical three-dimensional shape measuring machine calculates the distance to the object based on the peak of the envelope of a reference signal and the time taken to reach the peak of the envelope of the measurement signal. Its measurement accuracy is determined by the jitter (the deviation t) of the peak of the envelope from the periodic phase. n The relative phase noise of the two oscillators 103A and 103B is determined by the RMS value of the jitter (-nTb). When the phase noise of the two oscillators 103A and 103B is set to φA(t) and φB(t) respectively, the jitter of the envelope peak (the deviation t from the periodic phase) is determined. n The RMS value of the jitter of -nTb is represented by the following formula (1).

[0020]

[0021] Here, <> represents the time average.

[0022] Even if the reference oscillator 104 is assumed to be a noise-free ideal oscillator, uncorrelated phase noise will still remain due to the noise of the phase comparator and the limitation of the control band for phase synchronization. When the phase noises of the two oscillators 103A and 103B are set as φA(t) and φB(t) respectively, since φA(t) and φB(t) are uncorrelated, therefore t n The RMS value of the jitter of -nTb is given by the following formula (2).

[0023]

[0024] Here, as a typical example of phase noise in oscillators, regarding the UltraHerley Series PCRO, UltraHerley Series PDRO, and UltraHerley Series PXS manufactured by UltraHerley, in... Figure 3 The diagram shows the phase noise characteristics of each oscillator plotted using data cited in Non-Patent Document 1. Oscillators with very low phase noise are selected when designing high-performance systems, and this product is an oscillator with very low phase noise.

[0025] When fm = 25 GHz, the phase noise of oscillators 103A and 103B is similar to... Figure 3 The phase noise of the 26 GHz oscillator (Ultra Herley Series PDRO) shown is comparable. This phase noise is above -110 dBc / Hz at least up to 100 kHz. When assumed to be uncorrelated, the relative phase noise of fm and fm+Δfm can be considered to be approximately +3 dB as a characteristic of the PCRO.

[0026] Even if the RMS value of the phase noise of the phase difference is calculated by considering only the cumulative phase noise up to 100kHz as shown in Equation (3) below, it is -57dBc.

[0027] -110dBc / Hz +3dB +10Log 10 (100kHz)

[0028] = -57dBc (3)

[0029] The RMS value of the phase noise of the aforementioned phase difference is -57 dBc. As the jitter of the interference waveform under the condition of Tb = 10 μs, it is equivalent to 14 ns in terms of RMS value. When converted to distance measurement under the condition of fm = 25 GHz, it is equivalent to 6 μm.

[0030] Therefore, in order to reduce the measurement deviation in the optical comb rangefinder, it is essentially necessary to reduce φA(t)-φB(t) as shown in equation (1).

[0031] In the technologies disclosed in Patent Documents 1 and 2, the influence of phase noise is reduced by using a system that uses an interferometer with a reference plane and a reference photodetector to measure the envelope peak of the interference waveform, and a system that uses an interferometer containing the measured distance and a measuring photodetector to measure the envelope peak of the interference waveform.

[0032] Existing technical documents

[0033] Patent documents

[0034] Patent Document 1: Japanese Patent No. 5231883

[0035] Patent Document 2: Japanese Patent Application Publication No. 2020-008357

[0036] Non-patent literature

[0037] Non-patent literature 1: Ultra Herley homepage https: / / www.ultra-herley.com / uploads / herley / datasheets / cti / Ultra%20Herley%20Series%20PDRO.pdf https: / / www.ultra-herley.com / uploads / herley / datasheets / cti / Ultra%20Herley%20Series%20PCRO.pdf https: / / www.ultra-herley.com / uploads / herley / datasheets / cti / Ultra%20Herley%20Series%20PXS.pdf

[0038] Non-patent literature 2: Analog devices homepage https: / / www.analog.com / media / jp / analog-dialogue / volume-51 / number-3 / articles / improved-dac-phase-noise-measurements-enable-ultra-low-phase-noise-dds-applications_jp.pdf Summary of the Invention

[0039] The problem the invention aims to solve

[0040] In the optical comb rangefinder disclosed in Patent Document 1, the frequencies of the two oscillators 103A and 103B are f. m High frequencies like 25 GHz. In phase synchronization of oscillators at such high frequencies using low-frequency (e.g., 10 MHz) reference signals, the phase comparator has a large division ratio, resulting in high noise. Furthermore, even if the noise level of the phase comparator is controlled, the phase noise of the two oscillators is also uncorrelated because the noise between the phase comparators is uncorrelated. Additionally, controlling for a wide bandwidth can conversely increase the phase noise of the oscillator; therefore, there is an optimal control bandwidth.

[0041] However, in Patent Document 1, although it is strongly recognized that the relative stability of the drive signal sources of multiple optical combs is important, it only describes the need for a pair of oscillators to achieve a wide bandwidth for the phase-locked loop, and does not show a scheme to achieve the relative stability of the drive signal sources of multiple optical combs.

[0042] Therefore, in view of the conventional realities described above, the object of the present invention is to provide a low relative phase noise optical comb generating apparatus that stabilizes the repetition frequency of the interference signal of the optical comb by reducing the relative phase noise of the drive signals of different frequencies used to drive multiple optical comb generators in an optical comb rangefinder.

[0043] In addition, other objects of the present invention are to reduce the measurement deviation of rangefinders, shape measuring machines, etc., and to enable high-speed measurement.

[0044] 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.

[0045] Solution for solving the problem

[0046] In this invention, the repetition frequency of the interference signal of the optical comb is stabilized by reducing the relative phase noise of the drive signals of different frequencies used to drive multiple optical comb generators in the optical comb rangefinder.

[0047] That is, the present invention is a low relative phase noise optical comb generating device, characterized in that it comprises: at least three oscillators that generate frequency signals with different frequencies; at least two frequency converters that are input with a frequency signal obtained from one of the three oscillators and frequency signals obtained from each of the other oscillators; and at least two optical comb generators that are supplied with at least two modulation signals with different frequencies obtained by frequency conversion by the at least two frequency converters, wherein the at least two frequency converters respectively supply the at least two modulation signals with reduced relative phase noise, which are the sum or difference frequency signals of the frequency signal obtained from the one oscillator and the frequency signals obtained from the other oscillators, to the at least two optical comb generators as driving signals.

[0048] In the low relative phase noise optical comb generating apparatus of the present invention, it can be configured such that the phase noise of each of the at least three oscillators, excluding the first oscillator, is lower than the phase noise of the first oscillator.

[0049] In the low relative phase noise optical comb generating device of the present invention, it can be configured such that the at least three oscillators generate frequency signals whose phases are synchronized with the phase of the reference frequency signal provided by the reference oscillator but whose frequencies are different from each other.

[0050] In the low relative phase noise optical comb generating device of the present invention, it can be configured such that the at least two frequency converters are mixers.

[0051] Furthermore, in the low relative phase noise optical comb generating device of the present invention, it can be configured such that the at least two frequency converters are respectively composed of a mixer, a phase comparator and a voltage-controlled oscillator.

[0052] Furthermore, in the low relative phase noise optical comb generating apparatus of the present invention, it can be configured such that the modulation signal is supplied as a driving signal from the at least two frequency converters via bandpass filters to the at least two optical comb generators.

[0053] Furthermore, in the low relative phase noise optical comb generating apparatus of the present invention, it can be configured such that the frequency signal obtained by the one oscillator is input to the at least two frequency converters via isolators.

[0054] Furthermore, in the low relative phase noise optical comb generating apparatus of the present invention, it can be configured such that the at least three oscillators generate at least three frequency signals whose phases are synchronized with the phase of a reference frequency signal through a PLL circuit and whose frequencies are fixed.

[0055] Furthermore, in the low relative phase noise optical comb generating apparatus according to the present invention, each of the at least three oscillators, except for the first oscillator, is a direct digital synthesizer (DDS) driven by a clock whose phase is synchronized with the phase of the reference frequency signal.

[0056] Furthermore, the low relative phase noise optical comb generating device involved in the present invention can be configured to include three oscillators, two frequency converters, and two optical comb generators. The three oscillators generate frequency signals whose phases are synchronized with the phase of the reference frequency signal but whose frequencies are different from each other. The two modulation signals with reduced relative phase noise obtained by the two frequency converters are supplied as driving signals to the two optical comb generators.

[0057] Furthermore, the low relative phase noise optical comb generating device according to the present invention can be configured such that, when X is set to an integer of 1 or more and Y is set to an integer of 2 or more, it comprises (X+Y) oscillators, XY frequency converters and Y optical comb generators, wherein the (X+Y) oscillators generate frequency signals whose phases are synchronized with the phase of the reference frequency signal but whose frequencies are different from each other, and the low relative phase noise optical comb generating device is formed by re-enhancing the basic structure X consisting of (Y+1) oscillators, Y frequency converters and Y optical comb generators.

[0058] The effects of the invention

[0059] In this invention, a low relative phase noise optical comb generating apparatus is provided in which a modulated signal with reduced relative phase noise, consisting of a frequency signal obtained from one oscillator and a frequency signal obtained from another oscillator with a lower phase noise than the frequency signal, is supplied as a driving signal to two optical comb generators, thereby stabilizing the repetition frequency of the interference signal of the optical comb.

[0060] In this invention, the relative phase noise of the drive signals of different frequencies used to drive multiple optical comb generators can be reduced to stabilize the repetition frequency of the interference signal of the optical comb, thereby reducing the measurement deviation of rangefinders, shape measuring machines, etc. and speeding up the measurement. Attached Figure Description

[0061] Figure 1 This is a block diagram showing the structure of a conventional optical comb drive circuit.

[0062] Figure 2 It is a waveform diagram of the detection output obtained by interfering the output of the optical comb generator driven by the optical comb drive circuit and detecting it by the photodetector.

[0063] Figure 3 This is a characteristic diagram of the phase noise of each oscillator obtained by plotting data cited in Patent Document 1.

[0064] Figure 4 This is a block diagram illustrating a structural example of a low relative phase noise optical comb generating apparatus to which the present invention is applied.

[0065] Figure 5 This is a block diagram illustrating a structural example of the frequency converter in the aforementioned optical comb generating apparatus.

[0066] Figure 6 This is a block diagram illustrating other structural examples of the low relative phase noise optical comb generating apparatus to which the present invention is applied.

[0067] Figure 7 This is a block diagram illustrating other structural examples of the low relative phase noise optical comb generating apparatus to which the present invention is applied.

[0068] Figure 8 This is a block diagram illustrating other structural examples of the low relative phase noise optical comb generating apparatus to which the present invention is applied.

[0069] Figure 9 This is a block diagram illustrating yet another structural example of a low relative phase noise optical comb generating apparatus to which the present invention is applied.

[0070] Figure 10 This is a state transition diagram showing the state transitions of the drive signals supplied to the two optical comb generators in the aforementioned low relative phase noise optical comb generating device. Detailed Implementation

[0071] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings. Furthermore, common structural elements are indicated by common reference numerals in the drawings. It is self-evident that the present invention is not limited to the examples below, and modifications can be made freely without departing from the spirit of the invention.

[0072] The present invention is applied to a low relative phase noise optical comb generating device 10, which, for example... Figure 4 As shown in the block diagram, it has two optical comb generators 15A and 15B, which emit interferometric measurement light and reference light with periodically modulated intensity or phase and different modulation periods.

[0073] The low relative phase noise optical comb generating device 10 is used as a light source to emit interferometric measurement light and reference light that are periodically modulated in intensity or phase and have different modulation periods 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 1 and 2, for example.

[0074] The low relative phase noise optical comb generating device 10 is supplied with a reference frequency signal F from a reference oscillator 11 via a three-branch power divider 12A. REF The system comprises three oscillators 13A, 13B, and 13, and two frequency converters 14A and 14B that receive a third frequency signal from the third oscillator 13 via a two-branch power divider 12B. By supplying a first frequency signal from the first oscillator 13A to the first frequency converter 14A and a second frequency signal from the second oscillator 13B to the second frequency converter 14B, a first modulation signal is obtained, which is the sum of the frequencies f1 and f3 of the first frequency signal obtained by frequency conversion by the first frequency converter 14A. A second modulation signal is obtained, which is the sum of the frequencies f2 and f3 of the second frequency signal obtained by frequency conversion by the second frequency converter 14B. The first modulation signal obtained by the first frequency converter 14A is supplied to the first optical comb generator 15A, and the second modulation signal obtained by the second frequency converter 14B is supplied to the second optical comb generator 15B.

[0075] The three oscillators 13A, 13B, and 13 mentioned above generate, for example, signals whose phase is similar to the reference frequency signal F provided by the reference oscillator 11, through a PLL (Phase-Locked Loop). REFThree frequency signals with phase synchronization and different frequencies: fb, fb+Δf, and fm-fb.

[0076] In this way, when the frequency f1 of the first frequency signal output by the first oscillator 13A is set to fb, the frequency f2 of the second frequency signal output by the second oscillator 13B is set to fb+Δf, and the frequency f3 of the third frequency signal output by the third oscillator 13B is set to fm-fb, the sum frequency f1+f3 obtained by the first frequency converter 14A based on the first frequency signal and the third frequency signal is fm, thereby obtaining a first modulation signal with a frequency of fm by the first frequency converter 14A. In addition, the sum frequency f2+f3 based on the second frequency signal and the third frequency signal is fm+Δf, thereby obtaining a second modulation signal with a frequency of fm+Δf by the second frequency converter 14B.

[0077] Of the three oscillators 13A, 13B, and 13 mentioned above, the phase noise of the two oscillators 13A and 13B other than the third oscillator 13 is smaller than that of the third oscillator 13.

[0078] The phase noises of the three oscillators 13A, 13B, and 13 are denoted as φa(t), φb(t), and φC(t), respectively. Here, the average value is 0. It is assumed that they are all uncorrelated.

[0079] The frequency converters 14A and 14B described above have the following functions: generating a sum-frequency signal or difference-frequency signal of the input third frequency signal and other frequency signals whose phase noise is smaller than that of the third frequency signal, that is, performing frequency addition or subtraction.

[0080] Furthermore, when frequency converters 14A and 14B are adders, the phase noise φA(t) of the signal with frequency fm obtained by the first frequency converter 14A is given by the following equation (4).

[0081] φA(t)=φC(t)+φa(t) (4)

[0082] In addition, the phase noise φB(t) of the signal with frequency fm+Δf obtained by the second frequency converter 14B is given by the following equation (5).

[0083] φB(t)=φC(t)+φb(t) (5)

[0084] When t is substituted into equation (1) above, n The RMS value of the jitter of -nTb is given by the following formula (6).

[0085]

[0086] Equation (6) above is the calculation when the measured distance is 0. It can also be considered as the calculation of a system that uses the reference photodetector disclosed in Patent Documents 1 and 2 to measure the envelope peak. In a system that uses an interferometer containing the measured distance and a measuring photodetector to detect the envelope peak of the interference waveform, the time delay τ of the measured distance must be considered. When the time delay of the measured distance is set to τ, equation (6) is rewritten as equation (7) below.

[0087]

[0088] Here, <φC(t)φC(t-τ)> is the autocorrelation function of the phase noise of oscillator 13. Within the range where the autocorrelation is large (τ), 2<φC(t) 2 The term >-2<φC(t)φC(t-τ)> can take a smaller value. In the following discussion, τ will be approximated as 0.

[0089] That is, the jitter is determined by the phase noise φa(t) of the first frequency signal with frequency fb and the phase noise φb(t) of the second frequency signal with frequency fb+Δf, and is determined by the phase noise φa(t) of the first frequency signal with frequency f1(fb) output by the first oscillator 13A and the phase noise φb(t) of the second frequency signal with frequency f2(fb+Δf) output by the second oscillator 13B.

[0090] Therefore, even if fm = 25 GHz, as long as fb and fb + Δf are around 1 GHz, the phase noise levels of φa(t) and φb(t) are reduced to a level similar to... Figure 3 The properties of the shown PCRO are at a comparable level, thus in Figure 3 An improvement of approximately 20 dB in relative phase noise can be expected within a 1 MHz offset frequency range.

[0091] Furthermore, when fb and fb+Δf are set to approximately 100MHz, it becomes similar to... Figure 3 The XPS characteristics shown are quite good, and an improvement of more than 40 dB in relative phase noise can be expected.

[0092] Here, the low relative phase noise optical comb generating apparatus 10 described above is explained as performing frequency addition using frequency converters 14A and 14B, but frequency subtraction can also be performed. In this case, the frequency f3 of the third frequency signal generated by the third oscillator 13 is set to fm+fb, the frequency f1 of the first frequency signal generated by the first oscillator 13A is set to fb, and the frequency f2 of the second frequency signal generated by the second oscillator 13B is set to fb-Δf. Thus, by performing frequency subtraction in frequency converters 14A and 14B, a first modulation signal with frequency fm is obtained by the first frequency converter 14A. Furthermore, based on the sum frequency f2-f3 of the second and third frequency signals being fm+Δf, a second modulation signal with frequency fm+Δf is obtained by the second frequency converter 14B.

[0093] In the aforementioned low relative phase noise optical comb generating device 10, a mixer such as a diode, a double-balanced mixer, or an IQ mixer, or for example... Figure 5 The structure shown utilizes a phase-synchronized frequency converter 14 for frequency converters 14A and 14B.

[0094] Here, when using mixers such as diodes, double-balanced mixers, and IQ mixers in the aforementioned frequency converters 14A and 14B, since the mixer is a non-linear element, it will generate frequency components other than the desired frequency components, thus... Figure 4 As shown in the dashed box, bandpass filters 16A and 16B are inserted at the output sides of frequency converters 14A and 14B respectively to supply only the required frequency components as drive signals to optical comb generators 15A and 15B.

[0095] For example, in the first frequency converter 14A using a mixer, not only is the desired frequency component fm generated, but also unwanted frequency components fm+Mfb (except when M=0) are generated as spurious signals. Here, M is an integer. When these frequency components are mixed into the first modulation signal supplied to the first optical comb generator 15A as a drive signal, they may sometimes become spurious signals during the optical comb generation of the first optical comb generator 15A and affect the measurement value. To eliminate this effect, a bandpass filter 16A is used to allow only the desired frequency component fm to pass through, attenuating other frequency components to a level that does not affect the measurement specifications.

[0096] Furthermore, the unwanted frequency component fm+Mfb generated by the first frequency converter 14A using a mixer propagates towards the power divider 12B on the input side. Since the power divider 12B also has imperfect characteristics, it reaches the second frequency converter 14B. By performing frequency transformation on the aforementioned unwanted frequency component fm+Mfb that reaches the second frequency converter 14B, a frequency component of fm+Mfb+M'(fb+Δf) will be mixed into the output of the second frequency converter 14B.

[0097] Here, M' is an integer. Since frequency components other than M+M'=0 fall outside of fm+fb or fm-fb, they can be attenuated by using a bandpass filter 16A that allows the desired fm+Δf frequency to pass through. However, the frequency component where M+M'=0 is fm+M'Δf, which is very close to the desired fm+Δfm for M'=1, is difficult to remove using the bandpass filter 16A. Therefore, as... Figure 4 As shown in the dotted-line box, the reflected components of frequency converters 14A and 14B can be attenuated by inserting isolators 17A and 17B respectively on the input side.

[0098] For the aforementioned isolators 17A and 17B, isolator components such as microwave amplifiers with high reverse isolation, PI-type resistor attenuators, T-type 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.

[0099] In addition, by changing the frequency f3 of the third frequency signal output by the third oscillator 13 to (fm-fb) / P and replacing the aforementioned isolators 17A and 17B with a frequency multiplier of P inserted into the input side of the frequency converters 14A and 14B, the influence of the reflection components of the frequency converters 14A and 14B can be reduced.

[0100] That is, the frequency (fm-fb) / P of the third frequency signal supplied from the third oscillator 13 via the power divider 12B is multiplied by P by each frequency multiplier and then input to the frequency converters 14A and 14B. Thus, the first frequency converter 14A obtains a first modulation signal with a frequency of fm, and the second frequency converter 14B obtains a second modulation signal with a frequency of fm+Δf.

[0101] Even if the unwanted frequency component fm+Mfb generated by the second frequency converter 14B is propagated, the amount of frequency components close to (fm-fb) / P reaching the first frequency converter 14A via the bandpass filter will be very small because the multiplier has a small effect as a frequency divider, i.e., a high isolation effect.

[0102] Furthermore, the influence of the reflection components generated by the frequency converters 14A and 14B can be reduced by inserting a bandpass filter that allows only the frequency f3 component of the third frequency signal generated by the third oscillator 13 to pass through the output side of the power divider 12B.

[0103] That is, a bandpass filter can be used to allow only the frequency component f3 of the third frequency signal generated by the third oscillator 13, i.e., the frequency component of (fm-fb), to pass through. When using a frequency multiplier, a bandpass filter can be used to allow only the frequency component of (fm-fb) / P to pass through, while attenuating other frequency components to a level that does not affect the measurement specifications. For example, even if the frequency component of (fm-fb) or the frequency component of (fm-fb) / P reaches the first frequency converter 14A, since it is the same as the frequency f3 of the third frequency signal generated by the third oscillator 13, no unwanted spurious components will be generated.

[0104] In the aforementioned low relative phase noise optical comb generating device 10, the optimal structure is achieved by combining these components in practical use, thus improving performance.

[0105] Furthermore, in the aforementioned low relative phase noise optical comb generating apparatus 10, it is assumed that when fb and fb+Δf are set to around 100MHz, an improvement of more than 40dB in relative phase noise can be expected. However, when fm = 25GHz and fb = 100MHz, filters with extremely high Q values ​​of 2500 or more are required for bandpass filters 16A and 16B to reduce spurious emissions from fm+fb or fm-fb.

[0106] Here, for the aforementioned frequency converters 14A and 14B, it is also possible to use, as shown in the example above. Figure 5 The structure shown utilizes a phase-synchronized frequency converter 14 instead of a diode, a double-balanced mixer, an IQ mixer, or other mixers.

[0107] The frequency converter 14 includes a phase comparator 141, a voltage-controlled oscillator 142, and a mixer 143. The phase comparator 141 controls the oscillation phase of the voltage-controlled oscillator 142, and the frequency signal output from the voltage-controlled oscillator 142 is branched and input to the mixer 143.

[0108] In this frequency converter 14, in the mixer 143, the third frequency signal with frequency f3 (fm-fb) obtained by the third oscillator 13 is input to the mixer 143. For example, when used as the frequency converter 14A, the first frequency signal with frequency f1 (fb) obtained by the first oscillator 13A is input to the phase comparator 141, and the fourth frequency signal with frequency f4 (fm) is output from the voltage-controlled oscillator 142.

[0109] The mixer 143 inputs the frequency signal f4-f3=fb, which is the difference between the frequency f4 (fm) of the fourth frequency signal and the frequency f3 (fm-fb) of the third frequency signal, to the phase comparator 141.

[0110] In the phase comparator 141, the phase comparison between the frequency signal of the difference frequency fb and the first frequency signal of the frequency f1 (i.e., fb) is performed and fed back to the voltage-controlled oscillator 142 to control the oscillation phase of the voltage-controlled oscillator 142. As a result, the voltage-controlled oscillator 142 obtains the fourth frequency signal of the frequency f4 (i.e., fm) whose phase is synchronized with the phase of the third frequency signal of the frequency f3 (i.e., fm-fb).

[0111] Furthermore, when the frequency converter 14 is used as the frequency converter 14B, it inputs a second frequency signal, f2 (fb+Δf), obtained from the second oscillator 13B, to the phase comparator 141, thereby controlling the oscillation phase of the voltage-controlled oscillator 142, so that the difference frequency f5f3 between the frequency f5 of the fifth frequency signal obtained from the voltage-controlled oscillator 142 and the frequency f3 of the third frequency signal is consistent with the frequency f2 of the second frequency signal.

[0112] f5 = f2 + f3

[0113] = (fb + Δf) + (fm - fb)

[0114] =fm+Δf

[0115] Thus, the voltage-controlled oscillator 142 obtains a fifth frequency signal, f5 (fm+Δf), whose phase is synchronized with the phase of the third frequency signal, f3 (fm-fb).

[0116] Here, phase comparator 141 is a phase comparator such as a double-balanced mixer, which has low noise because it performs phase comparisons between signals at the same frequency. Furthermore, since the frequency comparison is performed at frequency fb, the control bandwidth can be large, for example, 10MHz or more. Therefore, the relative phase noise of the outputs of frequency converters 14A and 14B is equal to the relative phase noise of the fourth frequency signal at frequency fb and the fifth frequency signal at frequency fb+Δf.

[0117] Furthermore, since the output of the frequency converter 14 is sufficiently large compared to the control band of the fourth frequency signal at frequency fb or the fifth frequency signal at frequency fb+Δf, the spurious fm+fb or fm of the voltage-controlled oscillator 142 can be reduced.

[0118] Therefore, by using the frequency converter 14 with phase synchronization as the frequency converters 14A and 14B respectively, the output bandpass filters 16A and 16B are not required, or the specifications of the output bandpass filters 16A and 16B can be reduced.

[0119] In addition, by making the above Figure 4 The block diagram shows that the low relative phase noise optical comb generating device 10 has a dual structure to reduce relative phase noise during the generation stages of frequency fb and frequency fb+Δf, which can reduce the specifications of bandpass filters 16A and 16B.

[0120] Figure 6 The low relative phase noise optical comb generating device 20 shown in the block diagram includes four oscillators 13, 23, 23A, 23B, four frequency converters 14A, 14B, 24A, 24B, and two optical comb generators 15A, 15B. The four oscillators 13, 23, 23A, 23B generate frequency signals whose phases are synchronized with the reference frequency signal provided by the reference oscillator 11, but whose frequencies are different from each other. The low relative phase noise optical comb generating device 20 is formed by dualizing the structure of the low relative phase noise optical comb generating device 10.

[0121] Furthermore, in this low relative phase noise optical comb generating device 20, the same reference numerals are used for structural elements that are the same as those in the low relative phase noise optical comb generating device 10 described above, and detailed descriptions are omitted.

[0122] That is, the low relative phase noise optical comb generating device 20 includes four oscillators 23A, 23B, 13, 23 supplied via a four-branch power divider 22A, a first frequency converter 14A and a second frequency converter 14B supplied with a third frequency signal from the third oscillator 13 via a two-branch power divider 12B, and a third frequency converter 24A and a fourth frequency converter 24B supplied with an eighth frequency signal from the eighth oscillator 23 via a two-branch power divider 22B.

[0123] Furthermore, in this low relative phase noise optical comb generating device 20, a sixth frequency signal is supplied from the fourth oscillator 23A to the third frequency converter 24A, and a seventh frequency signal is supplied from the fifth oscillator 23B to the fourth frequency converter 24B. Thus, the third frequency converter 24A obtains a ninth frequency signal, which is the sum of the frequencies f6 (which are the sixth frequency signal after frequency conversion) and f8 (which are the eighth frequency signal), and the fourth frequency converter 24B obtains a tenth frequency signal, which is the sum of the frequencies f7 (which are the seventh frequency signal) and f8 (which are the eighth frequency signal), and the fourth frequency converter 24B obtains a tenth frequency signal, which is the sum of the frequencies f7 (which are the seventh frequency signal) and f8 (which are the eighth frequency signal), and the fourth frequency converter 24B obtains a tenth frequency signal, which is the sum of the frequencies f7 (which are the seventh frequency signal) and f8 (which are the eighth frequency signal), and the tenth frequency signal obtained from the fourth frequency converter 24B is supplied to the second frequency converter 14B.

[0124] Here, the phases of the four oscillators 23A, 23B, 13, and 23 are synchronized with the reference frequency signal F, for example, 10MHz, provided by the reference oscillator 11. REF The phases are synchronized and the oscillation frequency is fixed. The third oscillator 13 outputs a third frequency signal with frequency f3 = fm - fb, the eighth oscillator 23 outputs an eighth frequency signal with frequency f8 = fb - fc, the fourth oscillator 23A outputs a sixth frequency signal with frequency f6 = fc, and the fifth oscillator 23B outputs a seventh frequency signal with frequency fT = fc + Δf.

[0125] In this low relative phase noise optical comb generating apparatus 20, the first frequency converter 14A is supplied from the third frequency converter 24A.

[0126] f9 = f6 + f8

[0127] =fb-fc+fc

[0128] =fb

[0129] The ninth frequency signal f9, which is the ninth frequency, is used to sum the frequency f3 = fm - fb of the third frequency signal provided by the third oscillator 13 with the frequency f9 of the aforementioned ninth frequency signal, resulting in a frequency f4.

[0130] f4 = f3 + f9

[0131] =fm-fb+fb

[0132] =fm

[0133] The first modulation signal is supplied as a drive signal to the first optical comb generator 15A.

[0134] In addition, the second frequency converter 14B is supplied with frequency f10 from the fourth frequency converter 24B.

[0135] f10 = f7 + f8

[0136] = (fc + Δf) + (fb - fc)

[0137] =fb+Δf

[0138] The tenth frequency signal, thereby the sum of the frequency f3 = fm - fb of the third frequency signal provided by the third oscillator 13 and the frequency f10 of the aforementioned tenth frequency signal, is f5.

[0139] f5 = f3 + f10

[0140] =(fm-fb)+(fb+Δf)

[0141] =fm+Δf

[0142] The second modulation signal is supplied as a drive signal to the second optical comb generator 15B.

[0143] Here, the frequency f10, i.e. fb+Δf, of the tenth frequency signal supplied from the fourth frequency converter 24B to the second frequency converter 14B is 1GHz+500kHz, and fc is set to 100MHz.

[0144] In this low relative phase noise optical comb generating device 20, the relative phase noise of the driving signal is determined by the fourth oscillator 23A and the fifth oscillator 23B that supply the driving signal to the first optical comb generator 15A and the second optical comb generator 15B, and is no longer determined by the first oscillator 13A and the second oscillator 13B in the low relative phase noise optical comb generating device 10. Even if fb is 1GHz, since fc is 100MHz, the ratio is about 10, so it is possible to easily remove unwanted frequency components by inserting bandpass filters 26A and 26B into the third frequency converter 24A and the fourth frequency converter 24B.

[0145] Let the phase noise of the four oscillators 23A, 23B, 13, and 23 be φD(t), φc(t), and φd(t), respectively. Here, the average value is 0. It is assumed that they are all uncorrelated.

[0146] When the fourth frequency converter 24A and the fifth frequency converter 24B are adders, the phase noise φa(t) of the ninth frequency signal with frequency fb obtained by the fourth frequency converter 24A and the phase noise φb(t) of the tenth frequency signal with frequency fb+Δf obtained by the fifth frequency converter 24B are respectively given by Equations (8) and (9) below.

[0147] φa(t)=φD(t)+φc(t) (8)

[0148] φb(t)=φD(t)+φd(t) (9)

[0149] When substituted into the above equation (6), the RMS value of the jitter of tn-nTb is given by the following equation (10).

[0150]

[0151] Here, the phase noise of φc(t) and φd(t) is... Figure 3 The 100MHz oscillator XPS shown exhibits characteristics comparable to those of the 25GHz oscillator, with an expected improvement of over 40dB in relative phase noise compared to the 25GHz oscillator.

[0152] The aforementioned low relative phase noise optical comb generating device 20 is for making the aforementioned Figure 4 The basic structure of the low relative phase noise optical comb generating device 10 shown in the block diagram is obtained by doubling the structure. However, the optical comb generator can also be set to three or more. In addition, when X is set to an integer of 1 or more and Y is set to an integer of 2 or more, the basic structure X consisting of (Y+1) oscillators, Y frequency converters and Y optical comb generators can be duplicated by having (X+Y) oscillators, XY frequency converters and Y optical comb generators, and the (X+Y) oscillators generating frequency signals whose phases are synchronized with the phase of the above-mentioned reference frequency signal and whose frequencies are different from each other.

[0153] For example, if X=3 and Y=3, a low relative phase noise optical comb generating device can be constructed by having 6 oscillators, 9 frequency converters and 3 optical comb generators, which triples the basic structure consisting of 4 oscillators, 3 frequency converters and 3 optical comb generators.

[0154] Furthermore, in the low relative phase noise optical comb generating apparatus obtained by X refactoring, if oscillators with the same oscillation frequency are provided, for example, in the low relative phase noise optical comb generating apparatus 20 described above, if the third oscillator 13 and the eighth oscillator 23 have the same oscillation frequency, the third oscillator 13 can also be used as the fourth oscillator 23, thereby reducing the number of oscillators.

[0155] Here, in the above Figure 4 In the low relative phase noise optical comb generating device 10 shown in the block diagram, a reference frequency signal F, which is phased with the reference frequency signal provided by the reference oscillator 11, is generated in the first oscillator 13A and the second oscillator 13B and passed through a PLL. REF The first frequency signal and the second frequency signal are phase-synchronized and have different frequencies, but DDS (Direct Digital Synthesizer) can also be used.

[0156] A DDS (Distributed Signal Generator) is a high-speed signal generator that operates at a system clock frequency higher than its output frequency. According to the Nyquist theorem, it can theoretically output at frequencies less than half the system clock frequency. A DDS can be considered a frequency divider, and the division ratio does not need to be an integer value. The phase noise of the DDS output is determined by the system clock and is reduced by a factor corresponding to the division ratio. The relative phase noise of multiple DDS outputs operating under the same system clock is smaller than the absolute phase noise (see, for example, Non-Patent Document 2). Relative phase noise can be reduced by using such a DDS oscillator operating under the same system clock as a signal source of fm, fm+Δf, fb, fb+Δf or fc, fc+Δf.

[0157] Figure 7 The low relative phase noise optical comb generating device 30 shown is obtained by replacing the first oscillator 13A and the second oscillator 13B in the aforementioned low relative phase noise optical comb generating device 10 with DDS oscillators 33A and 33B, and has a phase that is synchronized with the reference frequency signal F provided from the reference oscillator 11 via a two-branch power divider 32A through a PLL. REF An oscillator 33 generates the system clock in phase synchronization with the reference frequency signal F. The system clock obtained from this oscillator 33 is supplied to DDS oscillators 33A and 33B via a two-branch power divider 32B. The aforementioned DDS oscillators 33A and 33B are synchronized with the aforementioned reference frequency signal F in phase. REF Driven by the same system clock that is phase-synchronized, it is able to generate a first modulation signal of frequency fm and a second modulation signal of frequency fm+Δf that reduce the relative phase noise by an amount equivalent to or greater than that of the aforementioned low relative phase noise optical comb generator 10, and supply them to the two optical comb generators 15A and 15B.

[0158] Furthermore, in this low relative phase noise optical comb generating device 30, the same reference numerals are used for structural elements that are the same as those in the low relative phase noise optical comb generating device 10 described above, and detailed descriptions are omitted.

[0159] in addition, Figure 8The low relative phase noise optical comb generating device 40 shown is obtained by replacing the fourth oscillator 23A and the fifth oscillator 23B in the aforementioned low relative phase noise optical comb generating device 20 with DDS oscillators 43A and 43B, and has the capability to synchronize the phase of the reference frequency signal F provided from the reference oscillator 11 via the three-branch power divider 12A with the phase of the PLL. REF An oscillator 43 generates the system clock in phase synchronization with the reference frequency signal F. The system clock obtained by the oscillator 43 is supplied to DDS oscillators 43A and 43B via a two-branch power divider 44. The aforementioned DDS oscillators 43A and 43B are synchronized with the aforementioned reference frequency signal F in phase. REF Driven by the same system clock that is phase-synchronized, it is able to generate a sixth frequency signal fc and a seventh frequency signal fc+Δf, which reduce the relative phase noise by an amount comparable to or greater than that of the aforementioned low relative phase noise optical comb generating device 20.

[0160] Furthermore, in this low relative phase noise optical comb generating device 40, the same reference numerals are used for structural elements that are the same as those in the low relative phase noise optical comb generating device 20 described above, and detailed descriptions are omitted.

[0161] The fourth frequency converter 24A and the fifth frequency converter 24B supply the sixth frequency signal with reduced relative phase noise (frequency fc) and the seventh frequency signal with frequency fc+Δf obtained from the aforementioned DDS oscillators 43A and 43B, along with the eighth frequency signal with frequency f8=fb-fc provided by the eighth oscillator 23, to the first frequency converter 14A and the second frequency converter 14B via bandpass filters 26A and 26B.

[0162] In the first frequency converter 14A and the second frequency converter 14B described above, a first modulation signal with frequency fm, which is the sum of the frequency signal with frequency f3 = fm - fb provided by the third oscillator 13 and the ninth and tenth frequency signals supplied from the fourth frequency converter 24A and the fifth frequency converter 24B via the bandpass filters 26A and 26B, can be supplied from the first frequency converter 14A to the first optical comb generator 15A as a driving signal, and a second modulation signal with frequency fm + Δf can be supplied from the second frequency converter 14B to the second optical comb generator 15B as a driving signal.

[0163] Figure 9The low relative phase noise optical comb generating device 50 shown replaces the first oscillator 13A and the second oscillator 13B in the low relative phase noise optical comb generating device 10 above, and inputs four frequency signals output from the synthesizer circuit 53 via the switching circuit 54 in a cyclic switching manner, so as to supply two frequency signals with cyclically switched modulation periods and different periods to the first frequency converter 14A and the second frequency converter 14B.

[0164] Furthermore, in this low relative phase noise optical comb generating device 50, the same reference numerals are used for structural elements that are the same as those in the low relative phase noise optical comb generating device 10 described above, and detailed descriptions are omitted.

[0165] The low relative phase noise 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.

[0166] In this low relative phase noise optical comb generating device 50, the first oscillator 13 supplies the first frequency converter 14A and the second frequency converter 14B with a reference frequency signal F whose phase is the same as that supplied from the reference oscillator 11 via the 5-branch power divider 52. REF A frequency signal with a phase-synchronized and oscillating phase of fixed frequency F0 (e.g., 24 GHz).

[0167] The synthesizer circuit 53 described above is equipped with a phase-dependent reference frequency signal F supplied from the reference oscillator 11 via a 5-branch power divider 52. REF Four oscillators 53A, 53B, 53C, and 53D are used to generate four different frequency signals F1, F2, F3, and F4, which are synchronized in phase and have fixed frequencies.

[0168] The second oscillator 53A generates a reference frequency signal F that is phased with the reference frequency signal generated by the aforementioned reference oscillator 11 via a PLL circuit. REF The phase is synchronized and the first frequency signal is fixed at the first frequency F1 (1000MHz).

[0169] In addition, the third oscillator 53B generates a reference frequency signal F that is phased with the reference frequency signal generated by the aforementioned reference oscillator 11 through the PLL circuit. REF The phase is synchronized and the second frequency signal is fixed at the second frequency F2 (1010MHz).

[0170] In addition, the third oscillator 53C generates a reference frequency signal F that is phased with the reference frequency signal generated by the aforementioned reference oscillator 11 through the PLL circuit. REF The phase is synchronized and the frequency is fixed at the third frequency of 1000.5MHz, which is the third frequency signal F3.

[0171] Furthermore, the fourth oscillator 53D generates a reference frequency signal F that is phased with the reference frequency signal generated by the aforementioned reference oscillator 11 via the PLL circuit. REF The phase is synchronized and the frequency is fixed at the fourth frequency F4 (1010.5MHz).

[0172] 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.

[0173] 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.

[0174] For the aforementioned isolators 57A, 57B, 57C, and 57, 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.

[0175] Furthermore, the first frequency converter 14A and the second frequency converter 14B obtain a first modulation signal Fma and a second modulation signal Fmb of four modulation frequencies Fm1, Fm2, Fm3, and Fm4 of the 1GHz band based on the frequency signal F0 (e.g., 24GHz) supplied from the first oscillator 13 and the frequency signals F1, F2, F3, and F4 of the 1GHz band that are cyclically switched and alternately output from the switching circuit 54. These signals are then frequency-converted to the four modulation frequencies Fm1, Fm2, Fm3, and Fm4 of the 25GHz band and supplied as drive signals to the first optical comb generator 15A and the second optical comb generator 15B.

[0176] That is, the first frequency converter 14A and the second frequency converter 14B function as upconverters that convert the 1GHz frequency signal into the first modulation signal Fma and the second modulation signal Fmb of the 25GHz frequency band, which are supplied as driving signals to the first optical comb generator 15A and the second optical comb generator 15B.

[0177] Here, the low relative phase noise optical comb generating device 50 generates two optical combs as reference light pulses and measurement light pulses 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 to the four modulation frequencies Fm1, Fm2, Fm3, and Fm4 of the 25GHz band by the first frequency converter 14A and the second frequency converter 14B, respectively, and are supplied 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.

[0178] [Table 1]

[0179]

[0180] 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 10 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 low relative phase noise optical comb generating device 50.

[0181] In this optical comb rangefinder, in principle, by using two optical comb generators driven by two modulation signals of different frequencies to pulse interferometrically emitted reference light pulses and measurement light pulses, the signal processing unit performs frequency analysis on 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 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.

[0182] 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 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 distance that exceeds the ambiguous distance (La = c / 2fm, c: speed of light) equivalent to half the wavelength.

[0183] 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.

[0184] When the distance (La = c / 2fm, c: speed of light) is also 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 m is unknown.

[0185] 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 a distance equivalent to the wavelength of 1 / Δfm (La is 15m if Δfm = 10MHz), then the phase is uniquely determined.

[0186] Furthermore, the integer m can be determined by multiplying the phase by fm / Δfm and comparing it with the phase difference of #1.

[0187] 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.

[0188] Here, with fm = 25 GHz, Δf = 500 kHz, and Δfm = 10 MHz, distance measurements within La = 300 m are possible because Δf = 500 kHz.

[0189] In the optical comb rangefinder equipped with the low relative phase noise optical comb generating device 50, absolute distance measurement is performed using a reference signal and a measurement signal obtained by measuring at the four modulation frequencies shown in Table 1. That is, after maintaining one state for a fixed time, the rangefinder transitions to another state, and the signal phase of that state is measured within a fixed interval. The phase of the set states #1, #2, #3, and #4 is used to perform the absolute distance calculation.

[0190] 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.

[0191] In the aforementioned low relative phase noise optical comb generating device 50, 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 instantaneous load fluctuations during the cyclic 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.

[0192] 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 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.

[0193] Explanation of reference numerals in the attached figures

[0194] 10, 20, 30, 40, 50: Low relative phase noise optical comb generator; 11: Reference oscillator; 12A, 12B, 22A, 22B, 32, 44, 52: Power divider; 13, 13A, 13B, 23, 23A, 23B, 33, 33A, 33B, 43, 43A, 43B, 53A, 53B, 53C, 53D: Oscillator; 14, 14A, 14B, 24A, 24B, 143: Frequency converter; 15A, 15B: Optical comb generator; 16A, 16B, 26A, 26B: Bandpass filter; 17A, 17B, 57A, 57B, 57C, 57D: Isolator; 53: Synthesizer circuit; 54: Switching circuit; 141: Phase comparator; 142: Voltage-controlled oscillator.

Claims

1. A low relative phase noise optical comb generator comprising: at least three oscillators generating frequency signals different in frequency from each other; at least two frequency converters inputting a frequency signal from one of the three oscillators and each frequency signal from each of the oscillators other than the one oscillator; and at least two optical comb generators supplied with at least two kinds of modulation signals different in frequency from each other, which are frequency-converted by the at least two frequency converters, wherein the at least two frequency converters supply, as driving signals to the at least two optical comb generators, the at least two kinds of modulation signals, which are reduced in relative phase noise, as sum or difference frequency signals of the frequency signal from the one oscillator and each of the frequency signals from the other oscillators.

2. The low relative phase noise optical comb generator according to claim 1, wherein phase noises of each of the oscillators other than the one oscillator are smaller than a phase noise of the one oscillator.

3. The low relative phase noise optical comb generator according to claim 1 or 2, wherein the at least three oscillators generate frequency signals different in frequency from each other in phase with a reference frequency signal provided from a reference oscillator.

4. The low relative phase noise optical comb generator according to claim 1, wherein the at least two frequency converters are each a frequency mixer; or the at least two frequency converters are each constituted by a frequency mixer, a phase comparator and a voltage controlled oscillator.

5. The low relative phase noise optical comb generator according to claim 3, wherein the at least two frequency converters are each a frequency mixer; or the at least two frequency converters are each constituted by a frequency mixer, a phase comparator and a voltage controlled oscillator.

6. The low relative phase noise optical comb generator according to claim 1, wherein the modulation signals are each supplied as a driving signal to the at least two optical comb generators via a band pass filter from the at least two frequency converters.

7. The low relative phase noise optical comb generator according to claim 1, wherein the frequency signals from the one oscillator are each input to the at least two frequency converters via an isolator.

8. The low relative phase noise optical comb generator according to claim 3, wherein the at least three oscillators generate at least three kinds of frequency signals different in frequency from each other in phase with the reference frequency signal by a phase locked loop circuit.

9. The low relative phase noise optical comb generator according to claim 3, wherein each of the oscillators other than the one oscillator is a digital direct synthesis oscillator driven by a clock in phase with the reference frequency signal. ​ 10. The low relative phase noise optical comb generator according to claim 3, wherein three oscillators which generate frequency signals whose phases are synchronized with the phase of the reference frequency signal and whose frequencies are different from each other, two frequency converters which generate two kinds of modulation signals whose relative phase noises are reduced by frequency conversion of the frequency signals from the three oscillators, 11. The low relative phase noise optical comb generator according to claim 3, wherein when X is set to an integer of 1 or more and Y is set to an integer of 2 or more, (X+Y) oscillators which generate frequency signals whose phases are synchronized with the phase of the reference frequency signal and whose frequencies are different from each other, the low relative phase noise optical comb generator is formed by repeating the basic structure of (Y+1) oscillators, Y frequency converters, and Y optical comb generators X times.

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