Optical comb generation device and optical comb spectroscopic measurement device

Abstract

This optical comb generation device (1) comprises: an optical comb output unit (10) that outputs a first optical comb (L1) and a second optical comb (L2) that have a plurality of frequency modes arranged in a comb shape on a frequency axis; and a wavelength conversion unit (3) that uses soliton self-frequency shift to convert the wavelengths of the first optical comb (L1) and the second optical comb (L2) output from the optical comb output unit (10).

Description

Optical comb generator and optical comb spectroscopic measurement device

[0001] The present disclosure relates to an optical comb generator and an optical comb spectroscopy measurement device.

[0002] Optical combs are known, which are ultrashort pulse laser beams having multiple frequency modes (longitudinal modes) arranged at equal intervals like a comb on the frequency axis. Optical combs can be used for precise spectroscopic measurements. For example, Non-Patent Document 1 describes a technology that realizes high-speed spectroscopic measurements by using two optical combs (a first optical comb and a second optical comb).

[0003] Sho Okubo, et al, “Ultra-broadband dual-comb spectroscopy across 1.0-1.9μm”, Applied Physics Express 082402 (2015), published online July 14, 2015, The Japan Society of Applied Physics

[0004] In the above-described technology, spectroscopic measurement over a wide measurement band is sometimes desired. However, there is a trade-off between the measurement band and the measurement time, and broadening the measurement band results in a longer measurement time. Furthermore, broadening the measurement band may result in a deterioration in the S / N ratio, necessitating stabilization control. In this regard, it has been discovered that by varying the wavelengths of the first and second optical combs to generate multiple spectra with different narrow bands, high-speed spectroscopic measurement over a wide measurement band becomes possible.

[0005] The present disclosure has been made in consideration of the above-described circumstances, and aims to provide an optical comb generator and an optical comb spectroscopic measurement device in which the wavelengths of the first optical comb and the second optical comb are tunable.

[0006] The optical comb generator disclosed herein is [1] "an optical comb generator comprising: an optical comb output unit that outputs a first optical comb and a second optical comb having a plurality of frequency modes arranged in a comb-like pattern on the frequency axis; and a wavelength conversion unit that converts the wavelengths of the first optical comb and the second optical comb output from the optical comb output unit by utilizing soliton self-frequency shift."

[0007] In this optical comb generator, the wavelengths of the first and second optical combs can be converted by utilizing the soliton self-frequency shift, which means that an optical comb generator in which the wavelengths of the first and second optical combs are tunable can be realized.

[0008] The optical comb generator of the present disclosure may be [2] "the optical comb generator according to [1], wherein the optical comb output unit is a dual comb laser light source that outputs the first optical comb and the second optical comb, the time interval of which is different from that of the first optical comb." In this case, it is possible to generate the first optical comb and the second optical comb using a dual comb laser light source.

[0009] The optical comb generator of the present disclosure may be [3] "the optical comb generator according to [1] or [2], including an optical amplifier that broadens the spectrum of the first optical comb and the second optical comb output from the optical comb output unit before wavelength conversion by the wavelength converter." As a result of extensive research, the present inventors have found that multi-soliton generation can be suppressed by broadening the spectrum of the optical comb before wavelength conversion using soliton self-frequency shift. Therefore, according to the present disclosure, it is possible to suppress multi-soliton generation.

[0010] The optical comb generator of the present disclosure may be [4] "the optical comb generator according to [3], in which the optical amplifier broadens the spectrum of the first optical comb and the second optical comb by similariton amplification." In this case, the optical amplifier can suppress stretching of the first optical comb and the second optical comb, and can effectively realize wavelength conversion using soliton self-frequency shift.

[0011] The optical comb generator of the present disclosure may be [5] "the optical comb generator according to [3] or [4], in which the optical amplifier controls the intensities of the first optical comb and the second optical comb." In this case, by controlling the intensities of the first optical comb and the second optical comb, it is possible to vary the wavelengths of the first optical comb and the second optical comb, for example, for each pulse.

[0012] The optical comb spectroscopy measurement device of the present disclosure is [6] "an optical comb spectroscopy measurement device that includes an optical comb generator according to any one of [1] to [6], and performs spectroscopic measurement of a sample using the first optical comb and the second optical comb generated by the optical comb generator, the optical comb spectroscopy measurement device including: a multiplexing unit that multiplexes the first optical comb and the second optical comb; an optical detection unit that detects the first optical comb and the second optical comb multiplexed by the multiplexing unit; and an analysis unit that performs analysis related to the spectroscopic measurement based on the detection results of the optical detection unit."

[0013] This optical comb spectroscopic measurement device includes the optical comb generator, and is therefore capable of varying the wavelengths of the first and second optical combs generated by the optical comb generator. By varying the wavelengths of the first and second optical combs, it is possible to generate multiple spectra with different narrow bands, enabling high-speed spectroscopic measurement over a wide measurement band.

[0014] The optical comb spectroscopy measurement device of the present disclosure may be [7] "the optical comb spectroscopy measurement device according to [6], in which the sample can be placed on the optical path of either the first optical comb or the second optical comb before being multiplexed by the multiplexing unit." In this case, the complex refractive index of the sample, that is, the absorption coefficient and refractive index (complex transmittance and complex reflectance), can be determined.

[0015] The optical comb spectroscopy measurement device of the present disclosure may be [8] "the optical comb spectroscopy measurement device according to [6], in which the sample can be placed on the optical paths of the first optical comb and the second optical comb after multiplexing in the multiplexing unit." In this case, the device configuration can be simplified.

[0016] The optical comb spectroscopy measurement device of the present disclosure may be [9] "the optical comb spectroscopy measurement device according to any one of [6] to [8], including a wavelength selector arranged between the optical comb generator and the multiplexer in the optical paths of the first optical comb and the second optical comb, and transmitting the first optical comb and the second optical comb of a predetermined wavelength band." In this case, the measurement band of the spectroscopic measurement can be appropriately set by the wavelength selector to match, for example, the sample.

[0017] The optical comb spectroscopy measurement device disclosed herein may be the optical comb spectroscopy measurement device described in [9],

[10] in which "the wavelength selection unit further includes a trigger signal acquisition unit that reflects the first optical comb and the second optical comb having wavelengths other than the predetermined band and acquires a trigger signal based on the difference frequency light of the first optical comb and the second optical comb reflected by the wavelength selection unit, and the analysis unit performs the analysis based on the trigger signal acquired by the trigger signal acquisition unit and the detection result of the light detection unit." In this case, trigger timing for analysis by the analysis unit can be obtained without using output for spectroscopy measurement (the first optical comb and the second optical comb having wavelengths in the predetermined band). Furthermore, by using difference frequency light, spectroscopy measurement can be performed based on the overlap of the pulses of the first optical comb and the second optical comb, thereby improving the S / N ratio.

[0018] The optical comb spectroscopy measurement device of the present disclosure may be the optical comb spectroscopy measurement device described in [9],

[11] in which "the wavelength selection unit further includes a trigger signal acquisition unit that reflects the first optical comb or the second optical comb of a wavelength other than the predetermined band and acquires a trigger signal based on the first optical comb or the second optical comb reflected by the wavelength selection unit, and the analysis unit performs the analysis based on the trigger signal acquired by the trigger signal acquisition unit and the detection result of the optical detection unit." In this case, the trigger timing for the analysis by the analysis unit can be obtained without using output for spectroscopic measurement. Furthermore, the device configuration can be simplified.

[0019] The optical comb spectroscopy measurement device of the present disclosure may be the optical comb spectroscopy measurement device according to any one of [6] to

[11] , including a control unit that is capable of performing a first process of converting the wavelengths of the first optical comb and the second optical comb to a predetermined wavelength in the wavelength conversion unit, a second process of acquiring a spectrum in the analyzer based on an interference waveform acquired by detecting the first optical comb and the second optical comb with the optical detection unit when the wavelengths of the first optical comb and the second optical comb are converted to the predetermined wavelength by the first process, and a third process of repeatedly performing the first process and the second process by switching the predetermined wavelength among a plurality of different wavelengths." In this case, the process of generating a plurality of different narrowband spectra by varying the wavelengths of the first optical comb and the second optical comb can be realized by controlling the control unit.

[0020] The optical comb spectroscopic measurement device of the present disclosure may be

[13] "the optical comb spectroscopic device according to

[12] , wherein the controller switches the wavelengths of the first optical comb and the second optical comb at a time interval of 1 / Δfrep in the third process." In this case, it is possible to prevent wavelength switching during acquisition of an interference waveform, thereby preventing wavelength components before and after the switching from being mixed in the interference waveform.

[0021] According to the present disclosure, it is possible to provide an optical comb generator and an optical comb spectroscopic measurement device in which the wavelengths of the first optical comb and the second optical comb are tunable.

[0022] FIG. 1 is a diagram illustrating the configuration of an optical comb spectroscopy measurement device according to a first embodiment. FIG. 2 is a graph illustrating a first optical comb, a second optical comb, and an interference waveform on the time axis. FIG. 3 is a diagram illustrating the configuration of an optical comb output unit of FIG. 1. FIG. 4(a) is a diagram illustrating the peripheral configuration of a multiplexing unit when a sample is placed on the optical path of the first optical comb. FIG. 4(b) is a diagram illustrating the peripheral configuration of a multiplexing unit when samples are placed on the optical paths of the first optical comb and the second optical comb. FIG. 4(c) is a diagram illustrating the peripheral configuration of a multiplexing unit when no sample is placed. FIG. 5 is a flowchart illustrating an example of spectroscopic measurement using the optical comb spectroscopy measurement device of FIG. 1. FIG. 6 is a flowchart illustrating an example of measuring the transmittance of a sample using the optical comb spectroscopy measurement device of FIG. 1. FIG. 7(a) is a graph illustrating an example of an acquired spectrum. FIG. 7(b) is a graph illustrating an example of a calculated transmittance. FIG. 8 is a flowchart illustrating an example of measuring the complex transmittance and complex reflectance of a sample using the optical comb spectroscopy measurement device of FIG. 1. FIG. 9 is a diagram illustrating the configuration of an optical comb spectroscopy measurement device according to a second embodiment. Fig. 10(a) is a diagram showing another configuration around the wavelength selection unit, Fig. 10(b) is a diagram showing yet another configuration around the wavelength selection unit, and Fig. 10(c) is a diagram showing yet another configuration around the wavelength selection unit.

[0023] Hereinafter, the embodiments will be described in detail with reference to the drawings. In each drawing, the same or corresponding parts are designated by the same reference numerals, and duplicated explanations will be omitted.

[0024] 1 , an optical comb spectroscopy measurement device 100 according to a first embodiment is a device that performs spectroscopic measurement of a sample S using a first optical comb L1 and a second optical comb L2. The optical comb spectroscopy measurement device 100 includes an optical comb generator 1, a wavelength selector 4, a multiplexer 5, a light detector 6, a trigger signal acquirer 7, an analyzer 8, and a controller 9. The sample S is not particularly limited and may be any of various measurement targets. The optical comb spectroscopy measurement device 100 is used, for example, to evaluate optical atomic clocks.

[0025] The first optical comb L1 and the second optical comb L2 are frequency-controlled ultrashort pulse laser beams (mode-locked laser beams). When viewed on the time axis (time domain), the first optical comb L1 and the second optical comb L2 are represented as ultrashort pulse trains (see FIG. 2). By performing a Fourier transform on the ultrashort pulse trains constituting the first optical comb L1 and the second optical comb L2, an optical spectrum with equally spaced frequency modes (longitudinal modes) is obtained. That is, the first optical comb L1 and the second optical comb L2 are represented as optical spectra with multiple frequency modes arranged in a comb-like pattern on the frequency axis (frequency domain). The first optical comb L1 is represented by two parameters: a repetition frequency frep1 and an offset frequency fCEO1. The second optical comb L2 is represented by two parameters: a repetition frequency frep2 and an offset frequency fCEO2. The second optical comb L2 has a slightly different time interval from the first optical comb L1.

[0026] The optical comb generator 1 includes an optical comb output unit 10 that outputs a first optical comb L1 and a second optical comb L2, an optical amplifier unit 2 that broadbandizes the spectra of the first optical comb L1 and the second optical comb L2, and a wavelength converter 3 that converts the wavelengths of the first optical comb L1 and the second optical comb L2 using soliton self-frequency shift.

[0027] As shown in FIG. 3 , the optical comb output unit 10 is a bidirectional oscillation dual comb laser light source that outputs a first optical comb L1 and a second optical comb L2. The optical comb output unit 10 outputs the first optical comb L1, which oscillates clockwise (CW), and the second optical comb L2, which oscillates counterclockwise (CCW). In the optical comb output unit 10, light from a light source 11, such as a laser diode, is sent to a doped fiber 12, such as an erbium-doped fiber, and amplified. The amplified light circulates in two different directions, clockwise and counterclockwise, within a loop optical path 13. The loop optical path 13 is provided with a nonlinear polarization rotator 15 that changes the polarization state of the light to control the intensity and phase of the light, and a semiconductor saturable absorber mirror 16, which is a device for generating optical pulses. A portion of the light circulating clockwise within the loop optical path 13 is extracted by a coupler 17 and output as the first optical comb L1. A portion of the light circulating counterclockwise in the loop optical path 13 is extracted by the coupler 18 and output as a second optical comb L2.

[0028] 1 , the optical amplifier 2 broadens the spectrum of the first optical comb L1 and the second optical comb L2 that are output from the optical comb output unit 10 and before their wavelengths are converted by the wavelength converter 3. The optical amplifier 2 includes acousto-optic modulators 21 and 22 and fiber amplifiers 23 and 24.

[0029] The acousto-optic modulators 21 and 22 are devices that perform modulation using acoustic (sound wave) power and are called AOMs (Acousto-Optic Modulators). The acousto-optic modulator 21 controls the intensity of the first optical comb L1 for each pulse. The acousto-optic modulator 21 is disposed between the optical comb output unit 10 and the fiber amplifier 23 in the optical path of the first optical comb L1. The acousto-optic modulator 22 controls the intensity of the second optical comb L2 for each pulse. The acousto-optic modulator 22 is disposed between the optical comb output unit 10 and the fiber amplifier 24 in the optical path of the second optical comb L2. The acousto-optic modulators 21 and 22 may be disposed anywhere between the optical comb output unit 10 and the wavelength converter 3.

[0030] The fiber amplifier 23 broadens the spectrum of the first optical comb L1. Specifically, the fiber amplifier 23 broadens the spectrum of the first optical comb L1 and increases the output power of the first optical comb L1 through similariton amplification. The fiber amplifier 23 is disposed in the optical path of the first optical comb L1 between the acousto-optic modulator 21 and the wavelength converter 3. The fiber amplifier 24 broadens the spectrum of the second optical comb L2. Specifically, the fiber amplifier 24 broadens the spectrum of the second optical comb L2 and increases the output power of the second optical comb L2 through similariton amplification. The fiber amplifier 24 is disposed in the optical path of the second optical comb L2 between the acousto-optic modulator 22 and the wavelength converter 3.

[0031] The fiber amplifiers 23 and 24 each include a normal dispersion fiber and a pumping light source. The normal dispersion fiber is a double-clad fiber co-doped with erbium and ytterbium. That is, the fiber amplifiers 23 and 24 perform amplification while generating a nonlinear effect using the normal dispersion double-clad fiber to prevent stretching, and obtain the first optical comb L1 and the second optical comb L2 as broadband amplified light. The normal dispersion fiber is a fiber with a negative dispersion parameter D (ps / nm / km). The dopant used in the fiber amplifiers 23 and 24 is not particularly limited, and various dopants may be used. The fiber amplifiers 23 and 24 may also broaden the spectrum so that the spectral width of the first optical comb L1 and the second optical comb L2 is 100 nm or more, for example.

[0032] The wavelength converter 3 converts the wavelengths of the first optical comb L1 and the second optical comb L2 output from the optical comb output unit 10 by using soliton self-frequency shift. The wavelength converter 3 includes Raman shift fibers 31 and 32.

[0033] The Raman shift fiber 31 tunes the wavelength of the first optical comb L1, which has been spectrum-broadened and output-enhanced by the fiber amplifier 23, by using soliton self-frequency shift (Raman soliton shift). The Raman shift fiber 31 tunes the wavelength of the first optical comb L1 to generate solitons. The Raman shift fiber 31 is disposed between the fiber amplifier 23 and the wavelength selector 4 in the optical path of the first optical comb L1. The Raman shift fiber 32 tunes the wavelength of the second optical comb L2, which has been spectrum-broadened and output-enhanced by the fiber amplifier 24, by using soliton self-frequency shift. The Raman shift fiber 32 tunes the wavelength of the second optical comb L2 to generate solitons. The Raman shift fiber 32 is disposed between the fiber amplifier 24 and the wavelength selector 4 in the optical path of the second optical comb L2.

[0034] The Raman shifted fibers 31 and 32 can be, for example, single-mode anomalous dispersion fibers that exhibit anomalous dispersion in the wavelength bands of the first optical comb L1 and the second optical comb L2 generated by the fiber amplifiers 23 and 24. The Raman shifted fibers 31 and 32 can output the first optical comb L1 and the second optical comb L2 (solitons) in a wavelength band of, for example, 1600 nm to 2000 nm. Note that the first optical comb L1 and the second optical comb L2 modulated by the soliton self-frequency shift contain non-soliton components (components that do not become solitons).

[0035] The wavelength selector 4 transmits the first optical comb L11 and the second optical comb L21 with wavelengths in a predetermined band, and reflects the first optical comb L12 and the second optical comb L22 with wavelengths outside the predetermined band. The predetermined band is, for example, 1600 nm to 2000 nm. The wavelengths outside the predetermined band are, for example, 1550 nm or shorter. The wavelength selector 4 is disposed between the optical comb generator 1 and the multiplexer 5 in the optical paths of the first optical comb L1 and the second optical comb L2. The wavelength selector 4 includes reflective long-pass filters 41 and 42.

[0036] The long-pass filter 41 is disposed in the optical path of the first optical comb L1 between the Raman-shifted fiber 31 and the multiplexer 5. The long-pass filter 41 transmits the long-wavelength component of the first optical comb L1 and reflects components other than the long-wavelength component of the first optical comb L1. The long-pass filter 42 is disposed in the optical path of the second optical comb L2 between the Raman-shifted fiber 32 and the multiplexer 5. The long-pass filter 42 transmits the long-wavelength component of the second optical comb L2 and reflects components other than the long-wavelength component of the second optical comb L2.

[0037] The multiplexing unit 5 multiplexes the first optical comb L11 and the second optical comb L21. The multiplexing unit 5 is disposed between the wavelength selection unit 4 and the optical detection unit 6 in the optical paths of the first optical comb L1 and the second optical comb L2. In the illustrated example, the multiplexing unit 5 includes mirrors 51 and 52. The mirrors 51 and 52 transmit the first optical comb L11 and reflect the second optical comb L21 so that the first optical comb L11 and the second optical comb L21 are multiplexed on the same optical axis.

[0038] The optical detection unit 6 detects the first optical comb L11 and the second optical comb L21 combined by the multiplexing unit 5. The optical detection unit 6 is configured with, for example, a photodetector. The optical detection unit 6 detects the first optical comb L11 and the second optical comb L21 combined by the multiplexing unit 5 to obtain an interference waveform between the first optical comb L11 and the second optical comb L21 (hereinafter simply referred to as "interference waveform") (see FIG. 2). The optical detection unit 6 outputs the obtained interference waveform to the analysis unit 8.

[0039] The trigger signal acquirer 7 acquires a trigger signal based on the difference frequency light LS of the first optical comb L12 and the second optical comb L22 reflected by the wavelength selector 4. In the example shown, the trigger signal acquirer 7 reflects the first optical comb L12 and the second optical comb L22, which have wavelengths outside the predetermined band, off mirrors 71 and 72 and causes them to enter a nonlinear optical medium 74 via a lens 73. This utilizes difference frequency generation to generate difference frequency light LS corresponding to the difference frequency. The trigger signal acquirer 7 then acquires the trigger signal by detecting the difference frequency light LS with a light detector 75.

[0040] The analysis unit 8 performs analysis related to the spectroscopic measurement based on the detection results of the light detection unit 6. The analysis unit 8 is physically configured to include memories such as RAM and ROM, a processor (arithmetic circuit) such as a CPU, a communication interface, and a storage unit such as a hard disk. Examples of the analysis unit 8 include a personal computer, a cloud server, and a smart device (smartphone, tablet terminal, etc.). The analysis unit 8 functions by executing a program stored in the memory with the CPU of the computer system.

[0041] The analysis unit 8 performs analysis related to spectroscopic measurement based on the trigger signal acquired by the trigger signal acquisition unit 7 and the interference waveform acquired by the light detection unit 6. The analysis unit 8 acquires the interference waveform (interference waveform signal) in synchronization with the trigger signal. The analysis unit 8 performs a Fourier transform on the acquired interference waveform to acquire a spectrum. The analysis unit 8 calculates the transmittance of the sample S based on the acquired spectrum. The analysis unit 8 refers to the complex transmittance and complex reflectance of the sample S based on the acquired interference waveform. The calculation of the transmittance, complex transmittance, and complex reflectance will be described in detail below.

[0042] The control unit 9 controls various operations of the optical comb spectroscopy measurement device 100. The control unit 9 is physically configured in the same manner as the analysis unit 8. The control unit 9 may be formed integrally with the analysis unit 8 or may be formed separately. The control unit 9 controls the output of the first optical comb L1 and the second optical comb L2 from the optical comb output unit 10.

[0043] The control unit 9 is capable of executing a first process of controlling the acousto-optical modulators 21, 22 to adjust the intensities of the first optical comb L1 and the second optical comb L2 (i.e., adjusting the excitation laser output of the optical amplification unit 2) and converting the wavelengths of the first optical comb L1 and the second optical comb L2 to predetermined wavelengths in the wavelength conversion unit 3; a second process of acquiring a spectrum based on the interference waveform in the analysis unit 8 when the wavelengths of the first optical comb L1 and the second optical comb L2 have been converted to the predetermined wavelengths by the first process; and a third process of repeatedly executing the first process and the second process by switching the predetermined wavelength among a plurality of different wavelengths.

[0044] In the third process, the control unit 9 converts the wavelengths of the first optical comb L1 and the second optical comb L2 by a time width of 1 / Δfrep. Δfrep is the measurement time, which is the difference between the repetition frequency frep1 of the first optical comb L1 and the repetition frequency frep2 of the second optical comb L2 (Δfrep = frep1 - frep2). More preferably, the control unit 9 may switch the wavelengths when 1 / (2 × Δfrep) has elapsed from the peak of the interference waveform. Note that since the measurement bandwidth Δν can be expressed as (frep1 × frep2) / (2 × Δfrep), Δfrep may be calculated from the measurement bandwidth Δν.

[0045] As shown in FIGS. 1 and 4A, in the optical comb spectroscopy measurement device 100, the sample S can be placed on the optical path of the first optical comb L1 before being multiplexed in the multiplexing unit 5. As shown in FIG. 4B, in the optical comb spectroscopy measurement device 100, the sample S can be placed on the optical paths of the first optical comb L1 and the second optical comb L2 after being multiplexed in the multiplexing unit 5. Note that the sample S may also be placed on the optical path of the second optical comb L2 before being multiplexed in the multiplexing unit 5. As shown in FIG. 4C, in the optical comb spectroscopy measurement device 100, the sample S may not be placed on the optical paths of the first optical comb L1 and the second optical comb L2.

[0046] Next, an example of spectrometry using the optical comb spectrometry device 100 will be described with reference to the flowchart of FIG.

[0047] First, the control unit 9 controls the optical comb output unit 10 to output the first optical comb L1 and the second optical comb L2, which are then input to the optical amplifier 2 (step S1). The acousto-optic modulators 21 and 22 adjust the intensities of the first optical comb L1 and the second optical comb L2, and the fiber amplifiers 23 and 24 broaden the spectra of the first optical comb L1 and the second optical comb L2. Then, the wavelength converter 3 converts the wavelengths of the first optical comb L1 and the second optical comb L2 using soliton self-frequency shift (step S2).

[0048] In step S2, the control unit 9 controls the acousto-optic modulators 21 and 22 to adjust the intensities of the first optical comb L1 and the second optical comb L2 so that the wavelengths of the first optical comb L1 and the second optical comb L2 are converted to a predetermined wavelength by the wavelength conversion unit 3. The predetermined wavelength is, for example, a wavelength in the range of 1600 to 2000 nm, and is 1600 nm in this example.

[0049] Next, the wavelength selector 4 transmits the first optical comb L11 and the second optical comb L21 with wavelengths in the predetermined band and extracts the first optical comb L11 and the second optical comb L21 with wavelengths in the predetermined band (step S3). At the same time, the wavelength selector 4 reflects the first optical comb L12 and the second optical comb L22 with wavelengths outside the predetermined band and extracts the first optical comb L12 and the second optical comb L22 with wavelengths outside the predetermined band (step S4).

[0050] The multiplexer 5 multiplexes the first optical comb L11 and the second optical comb L21, each having a wavelength in a predetermined band (step S5). The optical detector 6 detects the multiplexed first optical comb L11 and the second optical comb L21, and acquires an interference waveform (step S6). Meanwhile, the trigger signal acquirer 7 separately converts the wavelengths of the first optical comb L12 and the second optical comb L22, each having a wavelength outside the predetermined band, using difference frequency generation to generate difference frequency light LS (step S7). The optical detector 75 detects the difference frequency light LS, and acquires a trigger signal (step S8).

[0051] The analyzer 8 acquires an interference waveform signal in synchronization with the trigger signal (step S9). The analyzer 8 performs a Fourier transform on the interference waveform acquired in step S9 to obtain a spectrum (step S10). It is determined whether multiple spectra across a wide band have been acquired (step S11). In step S11, it is determined whether, for example, five spectra across 1600 to 2000 nm have already been acquired. If the answer is NO in step S11, the wavelength shift amount of the predetermined wavelength, which is the target wavelength for wavelength conversion in step S2, is changed (for example, increased by 100 nm), and the process returns to step S2 (step S12). If the answer is YES in step S11, the multiple spectra acquired in step S10 are connected, and the process ends, assuming that a wideband spectrum has been obtained.

[0052] Next, an example of measuring the transmittance of the sample S using the optical comb spectroscopic measurement device 100 will be described with reference to the flowchart of FIG.

[0053] First, the sample S is not placed on the optical path of the first optical comb L1 and the second optical comb L2 (see FIG. 4(c)). In this state, the processes described above and shown in FIG. 5 are performed to obtain the spectrum Iref(λ) (step S21: see FIG. 7(a)). Next, the sample S is placed on the optical path of the first optical comb L1 and the second optical comb L2 after multiplexing in the multiplexer 5 (see FIG. 4(b)). In this state, the processes described above and shown in FIG. 5 are performed to obtain the spectrum Isam(λ) (step S22: see FIG. 7(a)). Then, the transmittance of the sample S is calculated from the ratio of the spectra Iref(λ) and Isam(λ) (step S23: see FIG. 7(b)).

[0054] Next, an example of measuring the complex transmittance and complex reflectance of the sample S using the optical comb spectroscopic measurement device 100 will be described with reference to the flowchart of FIG.

[0055] First, the sample S is not placed on the optical path of the first optical comb L1 or the second optical comb L2 (see FIG. 4(c)). In this state, the processes of steps S1 to S9 described above are performed, and the interference waveform IGMref(t) is acquired (step S31). The sample S is placed on the optical path of the first optical comb L1 or the second optical comb L2 before being combined in the combiner 5 (see FIG. 4(a)). In this state, the processes of steps S1 to S9 described above are performed, and the interference waveform IGMsam(t) is acquired (step S32). Then, the complex transmittance T(ω) is obtained according to the following equation (1), and the complex reflectance R(ω) is obtained according to the following equation (2) (step S33). In the following, F[ ] denotes the Fourier transform, and ω is the angular frequency. T(ω) = F[IGMsam(t)] / F[IGMref(t)] (1) R(ω) = 1 - T(ω) (2)

[0056] T(ω) can also be expressed by the following equation (3): ～ (=n+ik) is the complex refractive index of the sample, n ａｉｒis the refractive index of air, i is an imaginary number, d is the thickness of the sample S, n is the refractive index of the sample, and k is the extinction coefficient. n and k can be obtained by separating the real and imaginary components of the following equation (3). T(ω) = ((4 × n ～ ×n ａｉｒ ) / (n ～ +n ａｉｒ ) ２ ) exp[i×((n ～ -n ａｉｒ )×d / c)]…(3)

[0057] R(ω) can also be expressed by the following equation (4). Here, Δ and Ψ are ellipsometric parameters, respectively. Δ and Ψ can be obtained by separating the real and imaginary components of the following equation (4). Incidentally, the above steps S31 to S33 may be repeated by changing the predetermined wavelength, which is the target wavelength for wavelength conversion in the above step S2. R(ω) = tan Ψ × exp(iΔ) (4)

[0058] As described above, the optical comb generator 1 can convert the wavelengths of the first optical comb L1 and the second optical comb L2 by using the soliton self-frequency shift. That is, it is possible to realize an optical comb generator 1 in which the wavelengths of the first optical comb L1 and the second optical comb L2 are tunable.

[0059] Furthermore, in dual-comb spectroscopy using the first optical comb L1 and the second optical comb L2, there is a trade-off between the measurement time (difference in repetition frequency) and the measurement bandwidth (Nyquist spectrum bandwidth). If the measurement time is shortened, the measurement bandwidth narrows. Furthermore, changing the measurement bandwidth requires multiple bandpass filters. Widening the measurement bandwidth also requires stabilization control to prevent degradation of the S / N ratio. In this regard, this embodiment achieves high-speed, wide-band dual-comb spectroscopy because it can tune the wavelength while maintaining a narrow spectrum (bandwidth of up to 10 nm).

[0060] In the optical comb generator 1, the optical comb output unit 10 is a dual comb laser light source that outputs a first optical comb L1 and a second optical comb L2. In this case, the first optical comb L1 and the second optical comb L2 can be generated using the dual comb laser light source. Using the first optical comb L1 and the second optical comb L2 of the dual comb laser light source enables high-speed, high-resolution spectroscopic measurements. For example, interference waveforms can be sampled in subnanoseconds, enabling measurements using interference waveforms to be performed in less than 1 ms. In particular, in this embodiment, the optical comb output unit 10 is a bidirectional oscillation dual comb laser light source, which allows the optical comb output unit 10 to be configured inexpensively and simply, eliminating the need for complex control.

[0061] As a result of extensive research, the present inventors have discovered that multi-soliton generation can be suppressed by broadening the spectrum of an optical comb before wavelength conversion using soliton self-frequency shift. Multi-soliton generation occurs when, for example, an optical comb is split into multiple optical combs by modulation. Applications in which multiple optical combs are simultaneously used as desired wavelength bands are extremely rare, and such multi-soliton generation is often undesirable from a practical standpoint, such as the need to remove unnecessary optical combs. Therefore, in the optical comb generator 1, the optical amplifier 2 broadens the spectrum of the first optical comb L1 and the second optical comb L2. This makes it possible to suppress multi-soliton generation.

[0062] In the optical comb generator 1, the optical amplifier 2 broadens the spectrum of the first optical comb L1 and the second optical comb L2 by similariton amplification. In this case, the optical amplifier 2 can suppress stretching of the first optical comb L1 and the second optical comb L2, and can effectively realize wavelength conversion using soliton self-frequency shift.

[0063] In the optical comb generator 1, the optical amplifier 2 controls the intensities of the first optical comb L1 and the second optical comb L2. By controlling the intensities of the first optical comb L1 and the second optical comb L2, it becomes possible to change the wavelengths of the first optical comb L1 and the second optical comb L2, for example, for each pulse.

[0064] The optical comb spectroscopy measurement device 100 includes the optical comb generator 1, and is therefore capable of varying the wavelengths of the first optical comb L1 and the second optical comb L2 generated by the optical comb generator 1. By varying the wavelengths of the first optical comb L1 and the second optical comb L2, it is possible to generate multiple spectra with different narrow bands, enabling high-speed, wide-band spectroscopy measurement.

[0065] In the optical comb spectroscopic measurement device, the sample S can be placed on the optical path of either the first optical comb L1 or the second optical comb L2 before being multiplexed in the multiplexer 5. In this case, the complex refractive index of the sample S, that is, the absorption coefficient and refractive index (complex transmittance and complex reflectance), can be determined.

[0066] In the optical comb spectroscopy measurement device 100, the sample S can be placed on the optical path of the first optical comb L1 and the second optical comb L2 after being multiplexed by the multiplexer 5. In this case, the device configuration can be simplified. Also, because the first optical comb L1 and the second optical comb L2 interfere with each other with the same wavefront, degradation of the S / N ratio can be suppressed.

[0067] The optical comb spectroscopy measurement device 100 includes a wavelength selector 4 that transmits the first optical comb L1 and the second optical comb L2 having wavelengths in a predetermined band. In this case, the measurement band of the spectroscopic measurement can be appropriately set by the wavelength selector 4 to match, for example, the sample.

[0068] The optical comb spectroscopy measurement device 100 further includes a trigger signal acquisition unit 7 that acquires a trigger signal based on the difference frequency light LS between the first optical comb L12 and the second optical comb L22 reflected by the wavelength selection unit 4. The analysis unit 8 performs analysis based on the trigger signal acquired by the trigger signal acquisition unit 7 and the detection result of the light detection unit 6. In this case, trigger timing for analysis by the analysis unit 8 can be obtained without using the output for spectroscopic measurement (the first optical comb L11 and the second optical comb L21 of a predetermined wavelength band). Furthermore, because the difference frequency light LS is used, spectroscopic measurement can be performed based on the overlap of the pulses of the first optical comb L1 and the second optical comb L2, thereby improving the S / N ratio.

[0069] In the optical comb spectroscopy measurement device 100, the control unit 9 can execute the following operations: a first process in which the wavelength converter 3 converts the wavelengths of the first optical comb L1 and the second optical comb L2 to predetermined wavelengths; a second process in which the analyzer 8 acquires a spectrum based on the interference waveform when the wavelengths of the first optical comb L1 and the second optical comb L2 are converted to the predetermined wavelengths by the first process; and a third process in which the first and second processes are repeatedly executed by switching the predetermined wavelength among a plurality of different wavelengths. In this case, the process of generating a plurality of different narrowband spectra by varying the wavelengths of the first optical comb L1 and the second optical comb L2 can be realized by control of the control unit 9.

[0070] In the optical comb spectroscopy measurement device 100, the control unit 9 switches the wavelengths of the first optical comb L1 and the second optical comb L2 at a time interval of 1 / Δfrep in the third process. This prevents the wavelength from being switched while the interference waveform is being acquired, resulting in the mixing of wavelength components before and after the switch in the interference waveform. Note that when integrating the interference waveform n times, the wavelength may be switched at an interval of n / Δfrep. Alternatively, the wavelength may be tuned from 1600 to 2000 nm, and then the measurement may be repeated n times.

[0071] In the above, step S2 constitutes the first process, steps S3 to S10 constitute the second process, and steps S11 and S12 constitute the third process.

[0072] Second Embodiment Next, a second embodiment will be described. An optical comb spectroscopy measurement device 200 according to the second embodiment shown in Fig. 9 differs from the first embodiment in that it includes a trigger signal acquirer 207 instead of the trigger signal acquirer 7 (see Fig. 1).

[0073] The trigger signal acquirer 207 acquires a trigger signal based on the second optical comb L2 reflected by the wavelength selector 4. In the example shown, the trigger signal acquirer 207 acquires the trigger signal by reflecting the second optical comb L21, which has a wavelength outside the predetermined band, by the mirror 72 and detecting it with the optical detector 75.

[0074] As described above, the optical comb spectroscopy measurement device 200 can also realize an optical comb generator 1 in which the wavelengths of the first optical comb L1 and the second optical comb L2 can be tunable. The optical comb spectroscopy measurement device 200 further includes a trigger signal acquirer 207 that acquires a trigger signal based on the second optical comb L22 reflected by the wavelength selector 4. In this case, the trigger timing for analysis by the analyzer 8 can be obtained without using the output for spectroscopic measurement. The device configuration of the optical comb spectroscopy measurement device 200 can also be simplified. The trigger signal acquirer 207 may acquire the trigger signal based on the first optical comb L1 reflected by the wavelength selector 4.

[0075] As described above, one aspect of the present disclosure is not limited to the above embodiment.

[0076] In the above embodiment, as shown in FIG. 10A, a nonlinear optical crystal (second harmonic crystal) 340 may be disposed downstream of the long-pass filter 41. The nonlinear optical crystal 340 includes beta barium borate (BBO), periodically poled lithium niobate (PPLN), and potassium titanyl phosphate (KTP). In this case, for example, the first optical comb L12 having a wavelength of 1600 to 2000 nm is further transmitted through the nonlinear optical crystal 340 and wavelength-converted (optical parametrically generated) to the first optical comb L13 having a wavelength of 800 to 1000 nm. This allows the wavelength band of the first optical comb L1 to be controlled toward shorter wavelengths. Note that this wavelength band control also applies to the second optical comb L2.

[0077] As shown in FIG. 10B, the above embodiment may include a short-pass filter 341 instead of the long-pass filter 41 (see FIG. 1). In this case, the short-pass filter 341 transmits the first optical comb L12, which has a wavelength of 800 to 1000 nm, for example. This allows the wavelength band of the first optical comb L1 to be controlled toward shorter wavelengths. This wavelength band control also applies to the second optical comb L2.

[0078] In the above embodiment, as shown in FIG. 10C , a nonlinear optical crystal 350, such as a GaSe crystal, may be disposed downstream of the short-pass filter 341. In this case, for example, the first optical comb L12 having a wavelength of 800 nm is further transmitted through the nonlinear optical crystal 350 and wavelength-converted (optical parametric generation) to the first optical comb L13 having a wavelength of 10 μm. This allows the wavelength band of the first optical comb L1 to be controlled toward the higher wavelength side, thereby generating, for example, mid-infrared light. Note that this wavelength band control is similarly applied to the second optical comb L2.

[0079] In the above embodiment, the nonlinear polarization rotator 15 (see FIG. 2 ) was used as the mode locking technique in the optical comb output unit 10, but this is not limited to this. For example, a nonreciprocal phase shifter, a nonlinear loop mirror, or a saturable absorber that absorbs only continuous light and has high transmittance for pulsed light may also be used as the mode locking technique. Preferably, a nonreciprocal phase shifter or a saturable absorber using a polarization-maintaining fiber that is robust against external disturbances may also be used as the mode locking technique. The gain medium for the laser light in the optical comb output unit 10 is not particularly limited and may be, for example, erbium, ytterbium, thulium, niodymium, or the like.

[0080] In the above embodiment, a bidirectional oscillation type dual-comb laser light source is used as the optical comb output unit 10. However, the optical comb output unit 10 is not particularly limited in configuration and type as long as it can output the first optical comb L1 and the second optical comb L2. For example, the optical comb output unit 10 may be a dual-comb laser light source synchronized with two lasers. In this case, noise can be suppressed. For example, the technology described in the following document 1 may be used as the dual-comb laser light source synchronized with two lasers. Document 1: Sho Okubo, et al., "Ultra-broadband dual-comb spectroscopy across 1.0-1.9 μm," Applied Physics Express 8, 082402 (2015), published online July 14, 2015, The Japan Society of Applied Physics, pp. 082402-1-82402-05

[0081] Furthermore, for example, the optical comb output unit 10 may be a mechanically shared dual-comb laser light source. For example, the technology described in the following document 2 may be adopted as the mechanically shared dual-comb laser light source. Document 2: TAKUMI YUMOTO, et al., "All-polarization-maintaining dual-comb fiber laser with mechanically shared cavity configuration and micro-optic component," Optics Continuum, Vol. 2, No. 8 / 15, August 2023, pp. 1867-1874

[0082] Furthermore, for example, the optical comb output unit may be a multi-polarization dual-comb laser light source. For example, the technology described in the following document 3 may be adopted as the multi-polarization dual-comb laser light source. Document 3: YOSHIAKI NAKAJIMA, et al., "All-polarization-maintaining, polarization-multiplexed, dual-comb fiber laser with a nonlinear amplifying loop mirror," OPTICS EXPRESS, Vol. 27, No. 10, 13 May 2019, pp. 14648-14656

[0083] Furthermore, for example, the optical comb output unit may be a microcomb-type dual-comb laser light source. For example, the technology described in the following document 4 may be adopted as the microcomb-type dual-comb laser light source. Document 4: Nikita Yu. Dmitriev, et al., "A hybrid integrated dual-microcomb source," physics.optics, arXiv:2112.07398v1, December 14, 2021, pp. 1-5

[0084] In the above embodiment, fiber amplifiers using double-clad normal dispersion fiber are used as the fiber amplifiers 23 and 24. However, fiber amplifiers using single-clad normal dispersion fiber (e.g., erbium-doped) may be used instead. Even in this case, the spectrum of the first optical comb L1 and the second optical comb L2 can be broadened.

[0085] In the above embodiment, the optical amplifier 2 may use an electro-optic modulator instead of the acousto-optic modulators 21 and 22, or may modulate the intensity through current modulation of the pump laser. In the above embodiment, the wavelength converter 3 may use anti-Stokes to change the wavelengths of the first optical comb L1 and the second optical comb L2.

[0086] The respective components in the above-described embodiment and modified examples are not limited to the materials and shapes described above, and various materials and shapes can be applied. Furthermore, the respective components in the above-described embodiment and modified examples can be arbitrarily applied to the respective components in other embodiments or modified examples.

[0087] 1...optical comb generator, 2...optical amplifier, 3...wavelength conversion unit, 4...wavelength selection unit, 5...combining unit, 6...optical detection unit, 7...trigger signal acquisition unit, 8...analysis unit, 9...control unit, 10...optical comb output unit, 100, 200...optical comb spectroscopy measurement device, L1, L11, L12, L13...first optical comb, L2, L21, L22...second optical comb, LS...difference frequency light, S...sample.

Claims

1. An optical comb generator comprising: an optical comb output unit that outputs a first optical comb and a second optical comb having a plurality of frequency modes arranged in a comb-like pattern on the frequency axis; and a wavelength conversion unit that converts the wavelengths of the first optical comb and the second optical comb output from the optical comb output unit by utilizing soliton self-frequency shift.

2. The optical comb generator according to claim 1, wherein the optical comb output unit is a dual comb laser light source that outputs the first optical comb and the second optical comb, the second optical comb having a different time interval from the first optical comb.

3. An optical comb generator according to claim 1, further comprising an optical amplifier section that broadens the spectrum of the first optical comb and the second optical comb output from the optical comb output section before the wavelengths are converted by the wavelength conversion section.

4. The optical comb generator according to claim 3, wherein the optical amplifier broadens the spectrum of the first optical comb and the second optical comb by similariton amplification.

5. The optical frequency comb generator according to claim 3 or 4, wherein the optical amplifier controls the intensities of the first optical frequency comb and the second optical frequency comb.

6. A spectroscopic measurement device equipped with the optical comb generator according to claim 1 or 2, which performs spectroscopic measurement of a sample using the first optical comb and the second optical comb generated by the optical comb generator, comprising: a multiplexing unit that multiplexes the first optical comb and the second optical comb; an optical detection unit that detects the first optical comb and the second optical comb multiplexed by the multiplexing unit; and an analysis unit that performs analysis related to the spectroscopic measurement based on the detection results of the optical detection unit.

7. The optical comb spectroscopy measurement device according to claim 6, wherein the sample can be placed on the optical path of either the first optical comb or the second optical comb before being multiplexed in the multiplexing section.

8. The optical frequency comb spectroscopy measurement device according to claim 6, wherein the sample can be placed on the optical path of the first optical comb and the second optical comb after being multiplexed in the multiplexing section.

9. An optical comb spectroscopy measurement device as described in claim 6, further comprising a wavelength selection unit arranged between the optical comb generator and the multiplexing unit in the optical paths of the first optical comb and the second optical comb, and transmitting the first optical comb and the second optical comb of a predetermined wavelength band.

10. The optical comb spectroscopy measurement device of claim 9, wherein the wavelength selection unit reflects the first optical comb and the second optical comb of wavelengths other than the predetermined band, and further comprises a trigger signal acquisition unit that acquires a trigger signal based on the difference frequency light of the first optical comb and the second optical comb reflected by the wavelength selection unit, and the analysis unit performs the analysis based on the trigger signal acquired by the trigger signal acquisition unit and the detection result of the optical detection unit.

11. The optical comb spectroscopy measurement device of claim 9, wherein the wavelength selection unit reflects the first optical comb or the second optical comb of a wavelength other than the predetermined band, and further comprises a trigger signal acquisition unit that acquires a trigger signal based on the first optical comb or the second optical comb reflected by the wavelength selection unit, and the analysis unit performs the analysis based on the trigger signal acquired by the trigger signal acquisition unit and the detection result of the optical detection unit.

12. An optical comb spectroscopy measurement device as described in claim 6, comprising a control unit, the control unit being capable of performing the following: a first process of converting the wavelengths of the first optical comb and the second optical comb to a predetermined wavelength in the wavelength conversion unit; a second process of acquiring a spectrum in the analysis unit based on an interference waveform acquired by detecting the first optical comb and the second optical comb with the optical detection unit when the wavelengths of the first optical comb and the second optical comb are converted to the predetermined wavelength by the first process; and a third process of repeatedly performing the first process and the second process by switching the predetermined wavelength among a plurality of wavelengths different from each other.

13. The optical frequency comb spectroscopic measurement device according to claim 12, wherein the control unit switches the wavelengths of the first optical comb and the second optical comb at a time interval of 1 / Δfrep in the third process.

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