Optical comb generator and optical comb spectroscopy measuring device

Abstract

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) having multiple frequency modes arranged in a comb-like manner on the frequency axis, and a wavelength conversion unit (3) that converts the wavelengths of the first optical comb (L1) and the second optical comb (L2) output from the optical comb output unit (10) using soliton self-frequency shift.

Description

[Technical Field]

[0001] This disclosure relates to an optical comb generator and an optical comb spectroscopy measuring device. [Background technology]

[0002] Optical combs are known as ultrashort pulse laser beams that have multiple frequency modes (longitudinal modes) arranged in a comb-like fashion at equal intervals along the frequency axis. Optical combs can be used for precise spectroscopic measurements and other applications. For example, Non-Patent Document 1 describes a technique for achieving high-speed spectroscopic measurements using two optical combs (a first optical comb and a second optical comb). [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] 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 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] In the techniques described above, a broadband spectral measurement is sometimes desired. However, there is a trade-off between the measurement bandwidth and the measurement time; broadening the measurement bandwidth slows down the measurement time. Furthermore, broadening the measurement bandwidth may degrade the signal-to-noise ratio, requiring stabilization control. In this regard, it has been found that by varying the wavelengths of the first and second optical combs and generating multiple spectra of different narrowbands, it becomes possible to perform high-speed spectral measurements with a broadband measurement bandwidth.

[0005] This disclosure is made in view of the above circumstances and aims to provide an optical comb generator and an optical comb spectrometer in which the wavelengths of the first and second optical combs are variable. [Means for solving the problem]

[0006] The optical comb generator of the present disclosure is an optical comb generator comprising: [1] 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 manner 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 using soliton self-frequency shift.

[0007] This optical comb generator utilizes soliton self-frequency shifting to enable wavelength conversion of the first and second optical combs. In other words, it is possible to realize an optical comb generator in which the wavelengths of the first and second optical combs are variable.

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

[0009] The optical comb generator of the present disclosure may also be [3] "an optical comb generator according to [1] or [2], comprising an optical amplification unit that broadens the spectrum of the first optical comb and the second optical comb before the wavelength is converted by the wavelength conversion unit that is output from the optical comb output unit." As a result of diligent research, the Disclosers have found that multi-solitonization 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-solitonization.

[0010] The optical comb generator of the present disclosure may also be the optical comb generator according to [3], wherein the optical amplification unit broadens the spectrum of the first optical comb and the second optical comb by simillariton amplification. In this case, the optical amplification unit can suppress the stretching of the first optical comb and the second optical comb, and wavelength conversion using soliton self-frequency shift can be effectively realized.

[0011] The optical comb generator of the present disclosure may also be the optical comb generator according to [3] or [4], wherein the optical amplification unit controls the intensity of the first optical comb and the second optical comb. In this case, by controlling the intensity of the first optical comb and the second optical comb, it becomes possible to vary the wavelengths of the first optical comb and the second optical comb, for example, on a pulse-by-pulse basis.

[0012] The optical comb spectrometer of the present disclosure is [6] "an optical comb spectrometer comprising an optical comb generator as described in any of [1] to [6], 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 wave combining unit for combining the first optical comb and the second optical comb; a photodetector for detecting the first optical comb and the second optical comb combined by the wave combining unit; and an analysis unit for performing analysis related to spectroscopic measurement based on the detection result of the photodetector."

[0013] Since this optical comb spectrometer is equipped with the optical comb generator described above, the wavelengths of the first and second optical combs generated by the optical comb generator can be varied. By varying the wavelengths of the first and second optical combs, multiple spectra of different narrowbands can be generated, enabling high-speed and wideband spectroscopic measurements.

[0014] The optical comb spectroscopy apparatus of the present disclosure may also be the optical comb spectroscopy apparatus described in [6], wherein the sample can be placed on the optical path of either the first optical comb or the second optical comb before it is combined in the multiplexer. 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 apparatus of the present disclosure may also be the optical comb spectroscopy apparatus described in [6], wherein the sample can be placed on the optical paths of the first optical comb and the second optical comb after they have been combined in the multiplexer. In this case, the apparatus configuration can be simplified.

[0016] The optical comb spectrometer according to the present disclosure may also be [9] "an optical comb spectrometer according to any one of [6] to [8], comprising a wavelength selection unit disposed 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 wavelengths of the first optical comb and the second optical comb within a predetermined band." In this case, the measurement band of the spectroscopic measurement can be appropriately set by the wavelength selection unit, for example, to match the sample.

[0017] The optical comb spectrometer according to the present disclosure may also be

[10] "the optical comb spectrometer according to [9], wherein the wavelength selection unit further comprises a trigger signal acquisition unit that reflects the first optical comb and the second optical comb of 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 photodetector." In this case, the trigger timing for the analysis in the analysis unit can be obtained without using the output for spectroscopic measurement (the first optical comb and the second optical comb of wavelengths in the predetermined band). Furthermore, since difference frequency light is used, spectroscopic measurement can be performed based on the time when the pulses of the first optical comb and the second optical comb overlap, thereby improving the signal-to-noise ratio.

[0018] The optical comb spectroscopy apparatus of the present disclosure may also be

[11] "the optical comb spectroscopy apparatus according to [9], wherein the wavelength selection unit further comprises 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 photodetector." In this case, the trigger timing in the analysis of the analysis unit can be obtained without using the output for spectroscopic measurement. Furthermore, the apparatus configuration can be simplified.

[0019] The optical comb spectroscopy apparatus of the present disclosure may be

[12] "an optical comb spectroscopy apparatus according to any one of [6] to

[11] , which includes a control unit, wherein the control unit 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 obtaining a spectrum in the analysis unit based on an interference waveform obtained by detecting the first optical comb and the second optical comb with the photodetector unit when the wavelengths of the first optical comb and the second optical comb have been 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 between a plurality of wavelengths that are different from each other." In this case, the process of generating a plurality of spectra of different narrowbands by varying the wavelengths of the first optical comb and the second optical comb can be realized by the control unit.

[0020] The optical comb spectrometer of the present disclosure may also be the optical comb spectrometer according to

[12] , wherein the control unit switches the wavelengths of the first optical comb and the second optical comb in the third process with a time width of 1 / Δfrep. In this case, it is possible to prevent the wavelength from being switched during the acquisition of the interference waveform, which would result in the wavelength components before and after the switch being mixed in the interference waveform. [Effects of the Invention]

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

Brief Description of the Drawings

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

Embodiments for Carrying Out the Invention

[0023] The embodiments will be described in detail below with reference to the drawings. In each drawing, the same or corresponding parts are denoted by the same reference numerals, and redundant descriptions are omitted.

[0024] [First Embodiment] As shown in Figure 1, the optical comb spectrometer 100 according to the first embodiment is a device that performs spectroscopic measurements of a sample S using a first optical comb L1 and a second optical comb L2. The optical comb spectrometer 100 comprises an optical comb generator 1, a wavelength selection unit 4, a multiplexing unit 5, a photodetector 6, a trigger signal acquisition unit 7, an analysis unit 8, and a control unit 9. The sample S is not particularly limited and may be various objects to be measured. The optical comb spectrometer 100 can be used, for example, to evaluate an atomic clock.

[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 in the time domain, the first optical comb L1 and the second optical comb L2 are represented as ultrashort pulse trains (see Figure 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 is obtained in which frequency modes (longitudinal modes) are arranged at equal intervals. That is, the first optical comb L1 and the second optical comb L2 are represented as optical spectra having multiple frequency modes arranged in a comb-like pattern in the frequency domain. The first optical comb L1 is represented by two parameters: repetition frequency frep1 and offset frequency fCEO1. The second optical comb L2 is represented by two parameters: repetition frequency frep2 and offset frequency fCEO2. The time interval of the second optical comb L2 is slightly different from that of 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 amplification unit 2 that broadens the spectrum of the first optical comb L1 and the second optical comb L2, and a wavelength conversion unit 3 that converts the wavelengths of the first optical comb L1 and the second optical comb L2 using soliton self-frequency shifting.

[0027] As shown in Figure 3, the optical comb output unit 10 is a bidirectional oscillation type 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 a first optical comb L1 that oscillates clockwise (CW) and a second optical comb L2 that 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 the loop optical path 13. The loop optical path 13 is provided with a nonlinear polarization rotation unit 15 that controls the intensity and phase of light by changing the polarization state of the light, and a semiconductor saturation absorption mirror 16 which is a device for generating light pulses. A portion of the light circulating clockwise within the loop optical path 13 is taken out by a coupler 17 and output as the first optical comb L1. A portion of the light circulating counterclockwise within the loop optical path 13 is taken out by a coupler 18 and output as the second optical comb L2.

[0028] Returning to Figure 1, the optical amplification unit 2 broadens the spectrum of the first optical comb L1 and the second optical comb L2, which are output from the optical comb output unit 10 and whose wavelengths are converted by the wavelength conversion unit 3. The optical amplification unit 2 includes acousto-optic modulators 21 and 22 and fiber amplifiers 23 and 24.

[0029] Acousto-optic modulators 21 and 22 are devices that perform modulation using the power of sound (sound waves), and are called AOMs (Acousto Optic Modulators). Acousto-optic modulator 21 is the first optical comb L The intensity of 1 is controlled pulse by pulse. Acousto-optic modulator 21 is located between the optical comb output unit 10 and the fiber amplifier 23 in the optical path of the first optical comb L1. Acousto-optic modulator 22 controls the intensity of the second optical comb L2 pulse by pulse. Acousto-optic modulator 22 is located between the optical comb output unit 10 and the fiber amplifier 24 in the optical path of the second optical comb L2. Note that acousto-optic modulators 21 and 22 may be located at any position between the optical comb output unit 10 and the wavelength conversion unit 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 by similariton amplification. The fiber amplifier 23 is positioned between the acousto-optic modulator 21 and the wavelength conversion unit 3 in the optical path of the first optical comb L1. 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 by similariton amplification. The fiber amplifier 24 is positioned between the acousto-optic modulator 22 and the wavelength conversion unit 3 in the optical path of the second optical comb L2.

[0031] The fiber amplifiers 23 and 24 are composed of a normally dispersed fiber and an excitation light source. The normally dispersed fiber is a double-clad fiber co-doped with erbium and ytterbium. That is, the fiber amplifiers 23 and 24 amplify the signal while causing a nonlinear effect using the normally dispersed double-clad fiber without stretching, thereby obtaining the first optical comb L1 and the second optical comb L2 as broadband amplifier light. The normally dispersed fiber is a fiber with a negative dispersion parameter D (ps / nm / km). The dopants used in the fiber amplifiers 23 and 24 are not particularly limited, and various dopants may be used. The fiber amplifiers 23 and 24 may broaden their spectra so that, for example, the spectral widths of the first optical comb L1 and the second optical comb L2 are 100 nm or more.

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

[0033] The Raman shift fiber 31 varies the wavelength of the first optical comb L1, whose output power has been increased while its spectrum has been broadened by the fiber amplifier 23, using soliton self-frequency shift (Raman soliton shift). The Raman shift fiber 31 varies the wavelength of the first optical comb L1 and generates solitons. The Raman shift fiber 31 is positioned between the fiber amplifier 23 and the wavelength selection unit 4 in the optical path of the first optical comb L1. The Raman shift fiber 32 varies the wavelength of the second optical comb L2, whose output power has been increased while its spectrum has been broadened by the fiber amplifier 24, using soliton self-frequency shift. The Raman shift fiber 32 varies the wavelength of the second optical comb L2 and generates solitons. The Raman shift fiber 32 is positioned between the fiber amplifier 24 and the wavelength selection unit 4 in the optical path of the second optical comb L2.

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

[0035] The wavelength selection unit 4 transmits wavelengths within a predetermined band for the first optical comb L11 and the second optical comb L21, while reflecting wavelengths outside the predetermined band for the first optical comb L12 and the second optical comb L22. The predetermined band is, for example, 1600 nm to 2000 nm. Wavelengths outside the predetermined band are, for example, 1550 nm or less. The wavelength selection unit 4 is positioned between the optical comb generator 1 and the multiplexing unit 5 in the optical paths of the first optical comb L1 and the second optical comb L2. The wavelength selection unit 4 includes reflective long-pass filters 41 and 42.

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

[0037] The multiplexer 5 combines the first optical comb L11 and the second optical comb L21. The multiplexer 5 is positioned between the wavelength selection unit 4 and the photodetector 6 in the optical paths of the first optical comb L1 and the second optical comb L2. In the illustrated example, the multiplexer 5 has mirrors 51 and 52. The mirrors 51 and 52 transmit light from the first optical comb L11 and reflect light from the second optical comb L21 so that the first optical comb L11 and the second optical comb L21 combine on the same optical axis.

[0038] The photodetector 6 detects the first optical comb L11 and the second optical comb L21, which are combined in the multiplexing unit 5. The photodetector 6 is composed of, for example, a photodetector. By detecting the first optical comb L11 and the second optical comb L21, which are combined in the multiplexing unit 5, the photodetector 6 acquires the interference waveform between the first optical comb L11 and the second optical comb L21 (hereinafter also simply referred to as the "interference waveform") (see Figure 2). The photodetector 6 outputs the acquired interference waveform to the analysis unit 8.

[0039] The trigger signal acquisition unit 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 selection unit 4. In the illustrated example, the trigger signal acquisition unit 7 reflects the first optical comb L12 and the second optical comb L22, which have wavelengths other than the predetermined band, using mirrors 71 and 72 and causes them to enter the nonlinear optical medium 74 via lens 73. This generates difference frequency light LS corresponding to the difference frequency by utilizing difference frequency generation. The trigger signal acquisition unit 7 then acquires a trigger signal by detecting this difference frequency light LS with an optical detection unit 75.

[0040] The analysis unit 8 performs analysis related to spectroscopic measurement based on the detection results of the photodetector unit 6. Physically, the analysis unit 8 is composed of memory 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 personal computers, cloud servers, and smart devices (smartphones, tablet terminals, etc.). The analysis unit 8 functions by executing a program stored in memory using 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 photodetector unit 6. The analysis unit 8 acquires the interference waveform (the signal of the interference waveform) in synchronization with the trigger signal. The analysis unit 8 performs a Fourier transform on the acquired interference waveform and acquires a spectrum. Based on the acquired spectrum, the analysis unit 8 calculates the transmittance of sample S. Based on the acquired interference waveform, the analysis unit 8 refers to the complex transmittance and complex reflectance of sample S. The calculation of transmittance, complex transmittance, and complex reflectance will be described in detail later.

[0042] The control unit 9 controls various operations of the optical comb spectrometer 100. Physically, the control unit 9 is configured similarly to the analysis unit 8. The control unit 9 may be formed integrally with the analysis unit 8 or as a separate unit. The control unit 9 controls the outputs of the first optical comb L1 and the second optical comb L2 in the optical comb output unit 10.

[0043] The control unit 9 can perform the following: a first process in which it controls the acousto-optic modulators 21 and 22 to adjust the intensity of the first optical comb L1 and the second optical comb L2 (i.e., adjust the excitation laser output of the optical amplification unit 2) and converts the wavelengths of the first optical comb L1 and the second optical comb L2 to a predetermined wavelength in the wavelength conversion unit 3; a second process in which, when the wavelengths of the first optical comb L1 and the second optical comb L2 have been converted to a predetermined wavelength by the first process, it causes the analysis unit 8 to acquire a spectrum based on the interference waveform; and a third process in which it repeatedly performs the first and second processes by switching the predetermined wavelength between a plurality of different wavelengths.

[0044] In the third processing, the control unit 9 converts the wavelengths of the first optical comb L1 and the second optical comb L2 with a time width of 1 / Δfrep. Δfrep is the measurement time and 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 controls the peak time of the interference waveform. The wavelength may be switched after 1 / (2×Δfrep) has elapsed from the point. Furthermore, since the measurement bandwidth Δν can be expressed as (frep1×frep2) / (2×Δfrep), Δfrep may also be calculated from the measurement bandwidth Δν.

[0045] As shown in Figures 1 and 4(a), in the optical comb spectroscopy apparatus 100, the sample S can be placed on the optical path of the first optical comb L1 before it is combined in the multiplexing unit 5. As shown in Figure 4(b), in the optical comb spectroscopy apparatus 100, the sample S can be placed on the optical paths of the first optical comb L1 and the second optical comb L2 after they have been combined in the multiplexing unit 5. The sample S may also be placed on the optical path of the second optical comb L2 before it is combined in the multiplexing unit 5. As shown in Figure 4(c), in the optical comb spectroscopy apparatus 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 a spectroscopic measurement using the optical comb spectrometer 100 will be explained with reference to the flowchart in Figure 5.

[0047] First, the control unit 9 controls the optical comb output unit 10, outputting the first optical comb L1 and the second optical comb L2 from the optical comb output unit 10 and directing them into the optical amplification unit 2 (step S1). Acousto-optic modulators 21 and 22 adjust the intensity of the first optical comb L1 and the second optical comb L2, and fiber amplifiers 23 and 24 broaden the spectrum of the first optical comb L1 and the second optical comb L2. Then, the wavelength conversion unit 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 and adjusts the intensity 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 in this case, it is 1600 nm.

[0049] Next, the wavelength selection unit 4 transmits the first optical comb L11 and the second optical comb L21 with wavelengths in a predetermined band, and extracts the first optical comb L11 and the second optical comb L21 with wavelengths in a predetermined band (step S3). At the same time, the wavelength selection unit 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 combined wave unit 5 combines the first optical comb L11 and the second optical comb L21, which have wavelengths within a predetermined band (step S5). The combined first optical comb L11 and the second optical comb L21 are detected by the photodetector 6, and the interference waveform is acquired (step S6). Meanwhile, the trigger signal acquisition unit 7 separately converts the wavelengths of the first optical comb L12 and the second optical comb L22, which have wavelengths outside the predetermined band, using difference frequency generation to generate difference frequency light LS (step S7). The difference frequency light LS is detected by the photodetector 75, and the trigger signal is acquired (step S8).

[0051] The analysis unit 8 acquires the interference waveform signal in synchronization with the trigger signal (step S9). The analysis unit 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 over a wide bandwidth have already been acquired (step S11). In step S11, for example, it is determined whether five spectra have already been acquired over 1600 to 2000 nm. If the answer in step S11 is NO, the predetermined wavelength, which is the target wavelength for wavelength conversion in step S2, is changed by a wavelength shift amount (for example, by increasing it by 100 nm), and the process returns to step S2 (step S12). If the answer in step S11 is YES, the multiple spectra obtained 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 sample S using the optical comb spectrometer 100 will be explained with reference to the flowchart in Figure 6.

[0053] First, the sample S is not placed on the optical paths of the first optical comb L1 and the second optical comb L2 (see Figure 4(c)). In this state, the processes described above as shown in Figure 5 are performed to obtain the spectrum Iref(λ) (step S21: see Figure 7(a)). Next, the sample S is placed on the optical paths of the first optical comb L1 and the second optical comb L2 after they have been combined in the multiplexing unit 5 (see Figure 4(b)). In this state, the processes described above as shown in Figure 5 are performed to obtain the spectrum Isam(λ) (step S22: see Figure 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 Figure 7(b)).

[0054] Next, an example of measuring the complex transmittance and complex reflectance of sample S using the optical comb spectrometer 100 will be explained with reference to the flowchart in Figure 8.

[0055] First, do not place the sample S on the optical paths of the first optical comb L1 and the second optical comb L2 (see Fig. 4(c)). In this state, perform the processes of steps S1 to S9 described above, and capture the interference waveform IGMref(t) (step S31). Place the sample S on the optical path of the first optical comb L1 or the second optical comb L2 before combining at the multiplexer 5 (see Fig. 4(a)). In this state, perform the processes of steps S1 to S9 described above, and capture the interference waveform IGMsam(t) (step S32). Then, obtain the complex transmittance T(ω) according to the following formula (1), and obtain the complex reflectance R(ω) according to the following formula (2) (step S33). Hereinafter, F[] represents Fourier transform, and ω is the angular frequency. T(ω)=F[IGMsam(t)] / F[IGMref(t)]…(1) R(ω)=1-T(ω)…(2)

[0056] Note that T(ω) can also be expressed by the following formula (3). Here, n ~ (=n+ik) is the complex refractive index of the sample, n air is 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 attenuation coefficient. Separating the real and imaginary components of the following formula (3), n and k can be obtained. 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 formula (4). Here, Δ and Ψ are the ellipsometric parameters, respectively. Separating the real and imaginary components of the following formula (4), Δ and Ψ can be obtained. Incidentally, steps S31 to S33 described above may be repeatedly performed by changing the predetermined wavelength, which is the target wavelength of the wavelength conversion in step S2 described above. R(ω)=tanΨ×exp(iΔ)…(4)

[0058] In summary, the optical comb generator 1 can convert the wavelengths of the first optical comb L1 and the second optical comb L2 by utilizing soliton self-frequency shifting. In other words, 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 variable.

[0059] Furthermore, in dual-comb spectroscopic measurements using the first optical comb L1 and the second optical comb L2, there is a trade-off relationship between the measurement time (difference in repetition frequency) and the measurement bandwidth (Nyquist spectral bandwidth). If the measurement time is increased, the measurement bandwidth narrows. Also, if the measurement bandwidth is changed, multiple bandpass filters are required. If the measurement bandwidth is widened, stabilization control is required due to the degradation of the signal-to-noise ratio. In this embodiment, however, since the spectrum is narrowband (bandwidth ~10nm) yet wavelength tunable, high-speed and wideband dual-comb spectroscopic measurements can be realized.

[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, it is possible to generate the first optical comb L1 and the second optical comb L2 using the dual-comb laser light source. By using the first optical comb L1 and the second optical comb L2 of the dual-comb laser light source, high-speed and high-resolution spectroscopic measurements become possible. For example, interference waveforms can be sampled in sub-nanoseconds, and measurements using interference waveforms can be performed in less than 1 ms. In particular, in this embodiment, since the optical comb output unit 10 is a bidirectional oscillating dual-comb laser light source, the optical comb output unit 10 can be configured inexpensively and simply, and complicated control is unnecessary.

[0061] As a result of diligent research, the disclosers have found that multi-solitonization can be suppressed by broadening the bandwidth of the optical comb spectrum before wavelength conversion using soliton self-frequency shifting. Multi-solitonization occurs, for example, when an optical comb splits due to modulation, forming multiple optical combs. Since it is very rare for all of the multiple optical combs to be used simultaneously in the desired wavelength band, such multi-solitonization is often undesirable from a practical standpoint, as it is necessary to remove unnecessary optical combs. Therefore, in the optical comb generator 1, the optical amplification unit 2 broadens the bandwidth of the spectra of the first optical comb L1 and the second optical comb L2. This makes it possible to suppress multi-solitonization.

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

[0063] In the optical comb generator 1, the optical amplification unit 2 controls the intensity of the first optical comb L1 and the second optical comb L2. By controlling the intensity of the first optical comb L1 and the second optical comb L2, it becomes possible to vary the wavelengths of the first optical comb L1 and the second optical comb L2, for example, on a pulse-by-pulse basis.

[0064] Since the optical comb spectrometer 100 is equipped with an optical comb generator 1, the wavelengths of the first optical comb L1 and the second optical comb L2 generated by the optical comb generator 1 can be varied. By varying the wavelengths of the first optical comb L1 and the second optical comb L2, multiple spectra of different narrowbands can be generated, enabling high-speed and wideband spectroscopic measurements.

[0065] In an optical comb spectrometer, the sample S can be placed on either the optical path of the first optical comb L1 or the second optical comb L2 before being combined in the multiplexing section 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 spectrometer 100, the sample S can be placed on the optical paths of the first optical comb L1 and the second optical comb L2 after they have been combined in the multiplexing section 5. In this case, the device configuration can be simplified. Also, since the first optical comb L1 and the second optical comb L2 interfere with the same wavefront, degradation of the signal-to-noise ratio can be suppressed.

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

[0068] The optical comb spectrometer 100 further includes a trigger signal acquisition unit 7 that 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 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 photodetection unit 6. In this case, the trigger timing for the analysis in 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 with wavelengths in a predetermined band). Furthermore, since difference frequency light LS is used, spectroscopic measurement can be performed based on the point when the pulses of the first optical comb L1 and the second optical comb L2 overlap, thereby improving the signal-to-noise ratio.

[0069] In the optical comb spectrometer 100, the control unit 9 can perform a first process in which the wavelength conversion unit 3 converts the wavelengths of the first optical comb L1 and the second optical comb L2 to a predetermined wavelength; a second process in which, when the wavelengths of the first optical comb L1 and the second optical comb L2 have been converted to a predetermined wavelength by the first process, the analysis unit 8 acquires a spectrum based on the interference waveform; and a third process in which the first and second processes are repeatedly performed by switching the predetermined wavelength between a plurality of different wavelengths. In this case, the process of generating multiple spectra of different narrowbands by varying the wavelengths of the first optical comb L1 and the second optical comb L2 can be realized by the control of the control unit 9.

[0070] In the optical comb spectrometer 100, the control unit 9 switches the wavelengths of the first optical comb L1 and the second optical comb L2 in the third process with a time width of 1 / Δfrep. In this case, it is possible to prevent the wavelength from being switched during the acquisition of the interference waveform, which would result in the wavelength components before and after the switch being mixed into the interference waveform. Note that if the interference waveform is integrated n times, the wavelength switching may be set to 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 to S12 constitute the third process.

[0072] [Second Embodiment] Next, a second embodiment will be described. The optical comb spectrometer 200 according to the second embodiment shown in Figure 9 differs from the first embodiment in that it is equipped with a trigger signal acquisition unit 207 instead of a trigger signal acquisition unit 7 (see Figure 1).

[0073] The trigger signal acquisition unit 207 acquires a trigger signal based on the second optical comb L2 reflected by the wavelength selection unit 4. In the illustrated example, the trigger signal acquisition unit 207 acquires a trigger signal by reflecting the second optical comb L21 of a wavelength other than the predetermined band with the mirror 72 and detecting it with the photodetector 75.

[0074] As described above, the optical comb spectroscopy apparatus 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 are variable. Furthermore, the optical comb spectroscopy apparatus 200 is further equipped with a trigger signal acquisition unit 207 that acquires a trigger signal based on the second optical comb L22 reflected by the wavelength selection unit 4. In this case, the trigger timing for the analysis of the analysis unit 8 can be obtained without using the output for spectroscopic measurement. In addition, the device configuration of the optical comb spectroscopy apparatus 200 can be simplified. Note that the trigger signal acquisition unit 207 may acquire a trigger signal based on the first optical comb L1 reflected by the wavelength selection unit 4.

[0075] The embodiments described above are not limited to the above-described embodiments.

[0076] In the above embodiment, as shown in Figure 10(a), a nonlinear optical crystal (second harmonic crystal) 340 may be placed on the downstream side of the long-pass filter 41. The nonlinear optical crystal 340 includes BBO (Beta Barium Borate), PPLN (Periodically Poled Lithium Niobate), and KTP (Potassium Titanyl Phosphate). In this case, for example, the wavelength is The first optical comb L12, with a wavelength of 1600-2000 nm, is further transmitted through the nonlinear optical crystal 340, resulting in wavelength conversion (optical parametric generation) to the first optical comb L13, with a wavelength of 800-1000 nm. This allows the wavelength band of the first optical comb L1 to be controlled towards lower wavelengths. The same wavelength band control applies to the second optical comb L2.

[0077] The above embodiment may also include a short-pass filter 341 instead of the long-pass filter 41 (see Figure 1), as shown in Figure 10(b). In this case, the short-pass filter 341 transmits, for example, the first optical comb L12 with a wavelength of 800 to 1000 nm. This allows the wavelength band of the first optical comb L1 to be controlled towards lower wavelengths. The same applies to the second optical comb L2 in this manner.

[0078] In the above embodiment, as shown in Figure 10(c), a nonlinear optical crystal 350, such as a GaSe crystal, may be placed downstream of the short-pass filter 341. In this case, for example, the first optical comb L12 with a wavelength of 800 nm is further transmitted through the nonlinear optical crystal 350 and wavelength-converted (optically parametrically generated) to the first optical comb L13 with a wavelength of 10 μm. This controls the wavelength band of the first optical comb L1 towards higher wavelengths, enabling the generation of, for example, mid-infrared light. The same applies to the second optical comb L2 in this control of the wavelength band.

[0079] In the above embodiment, a nonlinear polarization rotation unit 15 (see Figure 2) was used as the mode-synchronization method in the optical comb output unit 10, but it is not limited thereto. For example, a non-reciprocal phase shifter, a nonlinear loop mirror, and a saturable absorber that absorbs only continuous light and has high transmittance of pulsed light may be used as the mode-synchronization method. Preferably, a non-reciprocal phase shifter or a saturable absorber using a polarization-maintaining fiber that is robust to disturbances may be used as the mode-synchronization method. The gain medium for the laser light in the optical comb output unit 10 is not particularly limited and may be any of the following: erbium, yttrivium, thulium, and niodium.

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

[0081] For example, the optical comb output unit 10 may be a mechanically shared dual-comb laser light source. As a mechanically shared dual-comb laser light source, for example, the technology described in reference 2 below may be adopted. Reference 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, Aug 2023, pp.1867-1874

[0082] For example, the optical comb output section may be a multi-polarized dual-comb laser light source. As a multi-polarized dual-comb laser light source, for example, the technology described in reference 3 below may be adopted. Reference 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] For example, the optical comb output section may be a microcomb type dual-comb laser light source. As a microcomb type dual-comb laser light source, for example, the technology described in reference 4 below may be adopted. Document 4: Nikita Yu. Dmitriev, et al, “A hybrid integrated dual-microcomb source”, physics.optics, arXiv:2112.07398v1, 14 December, 2021, pp.1-5

[0084] In the above embodiment, fiber amplifiers 23 and 24 were made of normally dispersed, double-clad fibers. However, fiber amplifiers made of normally dispersed, single-clad fibers (e.g., erbium-doped fibers) may be used instead. In this case as well, broadening of the spectra of the first optical comb L1 and the second optical comb L2 is still possible.

[0085] In the above embodiment, the optical amplification unit 2 may use an electro-optic modulator instead of the acousto-optic modulators 21 and 22, or it may modulate the intensity via current modulation of the excitation laser. In the above embodiment, the wavelength conversion unit 3 may change the wavelengths of the first optical comb L1 and the second optical comb L2 using anti-Stokes.

[0086] The components in the above embodiments and modifications are not limited to the materials and shapes described above, and various materials and shapes can be applied. Furthermore, the components in the above embodiments and modifications can be arbitrarily applied to the components in other embodiments or modifications. [Explanation of Symbols]

[0087] 1...Optical comb generator, 2...Optical amplification unit, 3...Wavelength conversion unit, 4...Wavelength selection unit, 5...Multiplier 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 spectrometer, L1, L11, L12, L13...First optical comb, L2, L21, L22...Second optical comb, LS...Difference frequency light, S...Sample.

Claims

1. An optical comb output unit that outputs a first optical comb and a second optical comb having multiple frequency modes arranged in a comb-like manner on the frequency axis, An optical amplification unit that broadens the bandwidth of the spectra of the first optical comb and the second optical comb output from the optical comb output unit, The system includes a wavelength conversion unit that converts the wavelengths of the first optical comb and the second optical comb output from the optical amplification unit using soliton self-frequency shift, The optical amplification unit is an optical comb generator that broadens the spectrum of the first optical comb and the second optical comb by similariton amplification.

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

3. The optical comb generator according to claim 1 or 2, wherein the optical amplification unit controls the intensity of the first optical comb and the second optical comb.

4. A spectroscopic measuring apparatus comprising an optical comb generator according to claim 1 or 2, wherein the apparatus performs a spectroscopic measurement of a sample using the first optical comb and the second optical comb generated by the optical comb generator, A combined wave unit that combines the first optical comb and the second optical comb, An optical detection unit for detecting the first optical comb and the second optical comb combined in the above-mentioned multiplexing unit, An optical comb spectroscopy apparatus comprising: an analysis unit that performs analysis related to spectroscopic measurement based on the detection results of the aforementioned photodetector; and

5. The optical comb spectroscopy apparatus according to claim 4, wherein the sample can be placed on the optical path of either the first optical comb or the second optical comb before being combined in the multiplexing section.

6. The optical comb spectroscopy apparatus according to claim 4, wherein the sample can be placed on the optical paths of the first optical comb and the second optical comb after they have been combined in the multiplexing section.

7. The optical comb spectrometer according to claim 4, further comprising a wavelength selection unit disposed between the optical comb generator and the multiplexing unit in the optical paths of the first optical comb and the second optical comb, which transmits wavelengths of the first optical comb and the second optical comb within a predetermined band.

8. The wavelength selection unit reflects wavelengths other than the predetermined band from the first optical comb and the second optical comb. The system further includes 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, The optical comb spectrometer according to claim 7, wherein the analysis unit performs the analysis based on the trigger signal acquired by the trigger signal acquisition unit and the detection result of the photodetector.

9. The wavelength selection unit reflects the first optical comb or the second optical comb at wavelengths other than the predetermined band. The system further includes 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, The optical comb spectrometer according to claim 7, wherein the analysis unit performs the analysis based on the trigger signal acquired by the trigger signal acquisition unit and the detection result of the photodetector.

10. Equipped with a control unit, The control unit, The wavelength conversion unit performs a first process to convert the wavelengths of the first optical comb and the second optical comb to a predetermined wavelength, When the wavelengths of the first optical comb and the second optical comb are converted to the predetermined wavelength by the first process, the second process involves the analysis unit acquiring a spectrum based on the interference waveform obtained by detecting the first optical comb and the second optical comb with the light detection unit, The optical comb spectrometer according to claim 4, which is capable of performing a third process in which the first process and the second process are repeatedly performed by switching the predetermined wavelength between a plurality of wavelengths that are different from each other.

11. The optical comb spectrometer according to claim 10, wherein the control unit switches the wavelengths of the first optical comb and the second optical comb in the third process with a time width of 1 / Δfrep.

Images