WO2018159445A1

WO2018159445A1 - Optical comb control method and optical comb control device

Info

Publication number: WO2018159445A1
Application number: PCT/JP2018/006447
Authority: WO
Inventors: 薫美濃島; 彰文浅原
Original assignee: 国立大学法人電気通信大学
Priority date: 2017-02-28
Filing date: 2018-02-22
Publication date: 2018-09-07
Also published as: JPWO2018159445A1; JP6963322B2

Abstract

An optical comb control method to which the present invention is applied comprises a control step of controlling a difference between a first frequency offset and a second frequency offset using: a first optical comb which has a first frequency mode having the first frequency offset with respect to zero on a frequency axis, and a plurality of third frequency modes arranged at an interval of an integer multiple of a first frequency interval with respect to the first frequency mode on the frequency axis; and a second optical comb which has a second frequency mode having the second frequency offset with respect to zero on the frequency axis, and a plurality of fourth frequency modes arranged at an interval of an integer multiple of a second frequency interval with respect to the second frequency mode on the frequency axis.

Description

Optical comb control method and optical comb control device

The present invention relates to an optical comb control method and an optical comb control device. This application claims priority based on Japanese Patent Application No. 2017-037766 filed in Japan on February 28, 2017, the contents of which are incorporated herein by reference.

The light having a spectrum in which the optical frequency mode is precisely distributed at regular intervals on the frequency axis is called an optical comb or an optical frequency comb. In this specification, the optical frequency mode may be simply referred to as “frequency mode”. The optical comb is emitted from an ultrashort pulse laser such as a mode-locked laser. As an example of a mode-locked laser that emits an optical comb, Non-Patent Document 1 discloses a car lens mode-locked Ti: sapphire laser (Kerr-lens mode-locked Ti: sapphire laser). As disclosed in Non-Patent Document 1, an optical comb having a comb-like spectrum is widely used as a precise measure of time, space, and frequency.

The optical comb is a low-jitter coherent pulse train in the time domain, and is a comb-like spectrum that is accurately mode-resolved in the frequency domain. As disclosed in Non-Patent Document 1, the optical comb is used as a light source stabilized with high accuracy. However, conventionally, the offset frequency of the optical comb, the repetition frequency, the offset frequency difference when using two optical combs, and the repetition frequency difference are treated as fixed parameters.

As shown in FIG. 2, the present inventor found that the difference in the relative carrier envelope phase (Carrier-Envelope offset Phase: CEP) between the two optical combs was made evenly little by little according to the offset frequency difference. We focused on the essence of deviation and the essence that the relative CEP difference period is represented by the reciprocal of the offset frequency difference, as shown in FIG. Based on the essence of the optical comb, the inventor has obtained new knowledge that the relative CEP can be arbitrarily controlled by controlling the offset frequency difference, and has completed the present invention.

The present invention provides an optical comb control method and an optical comb control apparatus that actively utilize the degree of freedom of arbitrary control of relative CEP between two optical combs.

An optical comb control method according to the present invention includes a first frequency mode having a first frequency offset with respect to zero on a frequency axis and an integer of a first frequency interval with respect to the first frequency mode on the frequency axis. A first optical comb having a plurality of third frequency modes arranged at double intervals, a second frequency mode having a second frequency offset with respect to zero on the frequency axis, and the frequency axis A second optical comb having a plurality of fourth frequency modes arranged at intervals of an integral multiple of the second frequency interval with respect to the second frequency mode, and using the first frequency offset and A control step of controlling a difference from the second frequency offset;

In the optical comb control method of the present invention, in the control step, the first frequency offset and the second frequency offset are set such that a difference between the first frequency offset and the second frequency offset satisfies the expression (1). A difference from the second frequency offset may be set.

In the equation (1), Δf _CEO represents the difference between the first frequency offset and the second frequency offset. Δf _rep represents the difference between the first frequency interval and the second frequency interval. K and N represent arbitrary natural numbers.

In the optical comb control method of the present invention, in the control step, a difference between the first frequency offset and the second frequency offset may be set so as to satisfy Equation (2).

In the equation (2), Δf _CEO represents the difference between the first frequency offset and the second frequency offset. f _rep represents the first frequency interval or the second frequency interval. N is represented by an arbitrary natural number.

In the optical comb control method of the present invention, in the control step, the difference between the first frequency offset and the second frequency offset may be set to satisfy the expression (3).

In the equation (3), Δf _CEO is expressed by the first frequency offset and the second frequency offset. f _CEO represents the first frequency interval or the second frequency interval. N represents an arbitrary natural number.

An optical comb control device according to the present invention includes a first frequency mode having a first frequency offset with respect to zero on a frequency axis and an integer of a first frequency interval with respect to the first frequency mode on the frequency axis. Controlling the first frequency offset and the first frequency interval in a first optical comb having a plurality of third frequency modes arranged at double intervals, and with respect to zero on the frequency axis A second frequency mode having a second frequency offset, and a plurality of fourth frequency modes arranged at intervals of an integer multiple of the second frequency interval with respect to the second frequency mode on the frequency axis. A frequency control mechanism for controlling the second frequency offset and the second frequency interval in the second optical comb is provided. The control mechanism includes the first frequency offset, the second frequency offset, the difference between the first frequency offset and the second frequency offset, the first frequency interval, the second frequency interval, and The first frequency offset, the second frequency offset, the first frequency interval, and the first frequency offset so that a difference between the first frequency interval and the second frequency interval is expressed by an integer ratio. Two frequency intervals can be controlled.

According to the present invention, the relative CEP between the two optical combs can be arbitrarily controlled by controlling the offset frequency difference between the two optical combs.

It is a schematic diagram of two optical combs Comb1 and Comb2 in each of a time domain and a frequency domain. It is a schematic diagram which shows the relative relationship of the repetition period of the pulse train of two optical combs Comb1 and Comb2 in a time domain. It is a schematic diagram which shows the relative relationship of the phase of the pulse train of two optical combs Comb1 and Comb2 in a time domain. It is the schematic which shows an example of the optical comb output mechanism comprised so that control of the offset frequency and repetition frequency of an optical comb was possible. It is the schematic which shows one Embodiment of the control apparatus of the optical comb based on this invention. It is a schematic diagram which shows the optical behavior of the acoustooptic device for controlling the offset frequency of an optical comb. FIG. 3 is a schematic diagram illustrating a relationship between waveforms of two optical combs Comb1 and Comb2 and interference signals in the first embodiment. 6 is a graph showing a measurement result of an interference signal when an offset frequency difference Δf _CEO between two optical combs Comb1 and Comb2 is set to 0 in the first embodiment. 6 is a graph showing a measurement result of an interference signal when an offset frequency difference Δf _CEO between two optical combs Comb1 and Comb2 is Δf _rep / 2 in Example 1. 6 is a graph showing a measurement result of an interference signal when an offset frequency difference Δf _CEO between two optical combs Comb 1 and Comb 2 is Δf _rep / 3 in Example 1. 6 is a graph showing an IGM obtained by averaging the differences of IGMs that are inverted with each other when the offset frequency difference Δf _CEO between two optical combs Comb 1 and Comb 2 is Δf _rep / 2 in Example 1. FIG. In Example 2, it is the schematic which shows the structure of the apparatus for controlling offset frequency difference (DELTA) f _CEO of the 0th-order diffracted light and 1st-order diffracted light of optical comb Comb1. It is a schematic diagram which shows the spectrum of the 0th-order diffracted light and 1st-order diffracted light of optical comb Comb1 in the acoustooptic device of the apparatus shown to FIG. It is a figure which shows the relative CEP of the 0th-order diffracted light and 1st-order diffracted light of optical comb Comb1. It is a graph which shows the relationship between elapsed time and relative CEP in Example 2. FIG. 11B is a graph showing the polarization state of the optical comb Comb1 and the measurement result of the intensity of interference light according to the relative change in CEP shown in FIG. 11A. It is a graph which shows the waveform of the interference light in the polarization state shown to FIG. 11B. FIG. 11B is another graph showing the polarization state of the optical comb Comb1 and the measurement result of the intensity of the interference light according to the relative CEP change shown in FIG. 11A. It is a graph which shows the waveform of the interference light in the polarization state shown to FIG. 11D.

Hereinafter, an embodiment of an optical comb control method and an optical comb control apparatus according to the present invention will be described with reference to the drawings. In addition, the ratio of the length of each component of the drawing referred in the following description, the width | variety, thickness, etc. are typical.

A mode-locked laser outputs a periodic pulse train in the time domain. The pulse train output from the mode-locked laser can be illustrated by a function of an electric field (that is, carrier) oscillating at high speed and an envelope. The propagation speed of the envelope and the group velocity v _g, the propagation speed of the carrier and phase velocity v _p, holds the relationship expressed by the equation (4) and (5).

In equation (4), n represents the refractive index of the medium through which the optical comb propagates. λ represents the center wavelength of the optical comb.

Since the group velocity v _g and the phase velocity v _p are different, the pulse envelope and the carrier peak gradually shift as time advances. The phase shift (that is, the phase difference) is called a carrier-envelope offset phase (CEP). The relative CEP difference Δφ _CEP between adjacent pulse trains on the time axis is periodically shifted between pulses, and has a constant period _TCEO . The inverse of the period _{T CEO,} the carrier envelope offset frequency of the frequency domain (also simply referred to as the offset _frequency) corresponding to _{f CEO.} When the optical comb output from the mode-locked laser is observed in the frequency domain, the longitudinal mode of the mode-locked laser is very uniformly distributed at intervals of the repetition frequency (frequency interval) f _rep that is the reciprocal of the pulse repetition period T _rep. is doing. The relationship between the offset frequency f _CEO and the repetition frequency f _rep is expressed as shown in equation (6) using a relative CEP difference Δφ _CEP .

FIG. 1 schematically shows a waveform in the time domain and a spectrum in the frequency domain of each of the two optical combs Comb1 and Comb2. As shown in FIG. 1, the optical comb (first optical comb) Comb1 has an optical frequency mode (first frequency mode) FM1 having an offset frequency (first frequency offset) _fCEO1 with respect to zero on the frequency axis. And a plurality of optical frequency modes (third optical frequency modes) FM3 arranged at intervals of an integral multiple of the repetition frequency (first frequency interval) f _rep1 with respect to the optical frequency mode FM1 on the frequency axis. . The optical comb (second optical comb) Comb2 includes an optical frequency mode (second frequency mode) FM2 having an offset frequency (second frequency offset) f _CEO2 with respect to zero on the frequency axis, and an optical frequency on the frequency axis. A plurality of optical frequency modes (fourth optical frequency mode) _FM4 arranged at intervals of an integer multiple of the repetition frequency (second frequency interval) f _rep2 with respect to the mode FM2.

From Expression (6), Expression (7) is established as a correspondence relationship between the offset frequency f _CEO1 and the repetition frequency f _rep1 in the optical comb Comb1.

Similarly, from the equation (6), the relationship of the equation (8) is established as a correspondence relationship between the offset frequency f _CEO2 and the repetition frequency f _rep2 in the optical comb Comb2.

In the equation (7), Δφ _CEP1 represents a relative CEP of the optical comb Comb1. In the equation (8), Δφ _CEP2 represents the relative CEP of the optical comb Comb2.

(7) and (8) As shown in equation, the offset frequency _f _{CEO1, f CEO2} the change amount of the CEP between pulses, i.e. relative CEPderutafai _CEP1, is associated with Δφ _CEP2. In other words, by controlling the offset frequency _f _{CEO1, f CEO2,} you can control the coherence of each pulse train of optical frequency comb COMB1, COMB2.

2 and 3 schematically show waveforms when the two optical combs Comb1 and Comb2 are viewed on a common time axis. As shown in FIG. 2, each waveform when the positions of the waveforms of the two optical combs Comb1 and Comb2 are aligned on the time axis is referred to as a first waveform. The difference on the time axis between the second waveforms that have advanced on the time axis from the position of the first waveform of each of the two optical combs Comb 1 and Comb 2 is defined as a time difference ΔT _rep . The difference on the time axis between the third waveforms of the two optical combs Comb 1 and Comb 2 is the time difference (2 × ΔT _rep ). That is, the difference on the time axis between the (M + 1) th waveforms of the two optical combs Comb1 and Comb2 is the time difference (M × ΔT _rep ). M represents an arbitrary natural number. That is, in the time domain, the waveforms of the two optical combs Comb1 and Comb2 are shifted evenly little by little according to the repetition frequency difference Δf _rep . The repetition frequency difference Δf _rep is a frequency difference | f _rep1 −f _rep2 |.

When time (1 / Δf _rep ) elapses from the position on the time axis of the first waveform of the two optical combs Comb1 and Comb2, the positions on the time axis of the waveforms of the two optical combs Comb1 and Comb2 are aligned again. In other words, the timing of each pulse of the two optical combs Comb1 and Comb2 changes with a period of (1 / Δf _rep ). The waveforms after the time (1 / Δf _rep ) has elapsed from the position on the time axis of the first waveform are respectively the (M + 2) th waveform of the optical comb Comb1 and the (M + 1) th waveform of the optical comb Comb2. Then, the relationship of (9) Formula and (10) Formula is formed.

On the other hand, the CEPs of the two optical combs Comb1 and Comb2 have a relative relationship at a common position on the time axis. As shown in FIG. 3, when each waveform having a relative relationship (ie, CEP difference) is the first waveform, the first waveform CEP and the second waveform of each of the two optical combs Comb 1 and Comb 2 are used. The phase difference between the waveform and CEP is Δφ _CEP1 and Δφ _CEP2 . The phase difference between the first waveform CEP and the third waveform CEP of each of the two optical combs Comb1 and Comb2 is 2 × Δφ _CEP1 and 2 × Δφ _CEP2 . That is, the difference on the time axis between the (M + 1) -th waveforms of the two optical combs Comb1 and Comb2 is the time difference (M × Δφ _CEP1 ) and (M × Δφ _CEP2 ). In other words, in the time domain, the relative CEP of the two optical combs Comb1 and Comb2 is shifted little by little according to the offset frequency difference Δf _CEO . The offset frequency difference Δf _CEO is the frequency difference | f _CEO1 −f _CEO2 |).

Accordingly, after the time (1 / Δf _CEO ) has elapsed from the position on the time axis of the first waveform of each of the two optical combs Comb 1 and Comb 2, the difference between the phase difference Δφ _CEP1 and the phase difference Δφ _CEP2 becomes 2π. Become. At this time, a pulse train having a waveform having the same relationship as the relative CEP relationship between the waveforms of the first pulse trains of the two optical combs Comb 1 and Comb 2 appears. In FIG. 3, it is assumed that the repetition frequency difference Δf _rep between the two optical combs Comb 1 and Comb 2 is 0 for easy understanding. Therefore, the positions on the time axis of the pulse trains having the same waveform as the relative CEP relationship between the first pulse trains of the two optical combs Comb 1 and Comb 2 are aligned. That is, the relative CEP relationship between the two optical combs Comb1 and Comb2 changes with a period of (1 / Δf _CEO ).

(Control method of optical comb)
As described above, the repetition frequency f _rep and the offset frequency f _CEO of the optical comb are important frequency parameters indicating the characteristics of the pulse train. Considering the relative relationship between the two optical combs, in addition to the repetition frequency f _rep and the offset frequency f _CEO of each optical comb, the repetition frequency difference Δf _rep and the offset frequency difference Δf _CEO are important frequency parameters indicating the characteristics of the pulse train. It is. Conventionally, the offset frequency difference Δf _CEO has been treated as a fixed parameter. However, the CEP can be arbitrarily controlled by controlling the offset frequency difference Δf _CEO . In the present invention, the degree of freedom in controlling the offset frequency difference Δf _CEO is positively utilized.

The optical comb control method according to the present invention uses an optical comb Comb1 having an optical frequency mode FM1 and a plurality of optical frequency modes FM3, and an optical comb Comb2 having an optical frequency mode FM2 and a plurality of optical frequency modes FM4. And a control step of controlling the difference between the offset frequencies f _CEO1 and f _CEO2 (offset frequency difference, difference between the first frequency offset and the second frequency offset) Δf _CEO .

The offset frequency difference Δf _CEO is the difference between the offset frequencies f _CEO1 and f _CEO2 of the two optical combs Comb1 and Comb2. As shown by the equations (7) and (8), the repetition frequencies f _rep1 and f _rep2 of the two optical combs Comb1 and Comb2 have a relative relationship with the offset frequencies f _CEO1 and f _CEO2 , respectively. Therefore, in order to control the offset frequency difference Delta] f _CEO, two offset frequency difference Delta] f _CEO optical comb COMB1, COMB2, offset frequency _{f CEO1} optical comb COMB1, offset frequency _{f CEO2} optical comb COMB2, two optical comb COMB1 , Comb2 repetition frequency difference Δf _rep , optical comb Comb1 offset frequency f _rep1 , optical comb Comb2 offset frequency f _rep2 is preferably controlled so that the six parameters are expressed by an arbitrary integer ratio. . Specifically, the relative relationship (predetermined condition) in which the above six parameters are represented by an arbitrary integer ratio is the control of four parameters of the offset frequencies f _CEO1 and f _CEO2 and the offset frequencies f _rep1 and f _rep2. It is preferable that

For example, setting the above parameters other than the offset frequency difference Delta] f _CEO to meet the offset frequency difference Delta] f _CEO is (1) (3) at least one or more expression of expression from the equation. Can set the parameters other than the offset frequency difference Delta] f _CEO to meet the offset frequency difference Delta] f _CEO from (1) reacting a (3) any two equations of the type at the same time. Offset frequency difference Delta] f _CEO may set the parameters other than the offset frequency difference Delta] f _CEO to satisfy all the above-mentioned equation (1) (3).

(Optical Com control device)
First, an optical comb output mechanism used in an optical comb control apparatus applicable to the optical comb control method of the present invention will be described. The optical comb output mechanism detects an interference signal between modes of two optical combs satisfying a predetermined relationship (so-called 1f-2f relationship) in the control step of the optical comb control method of the present invention. This interference signal is a beat signal and is based on the frequency difference between the modes of the two optical combs. The optical comb output mechanism is configured to be able to control the offset frequency and the repetition frequency by detecting an interference signal between modes of two optical combs.

As shown in FIG. 4, the optical comb output mechanism 10 includes an optical comb light source 12, an optical interference unit 14, a beat signal detection unit 16, an offset frequency control unit 18, an optical comb output unit 20, and a repetition frequency control unit 22. ing.

The optical comb light source 12 is configured as a loop type fiber laser. The optical comb light source 12 includes an erbium-doped optical fiber (EDF) 24 and a semiconductor laser (hereinafter referred to as pumping LD) 26 that pumps the EDF 24 by supplying pumping light to the EDF 24 via an optical coupler 25. And comprising. An optical isolator 34, an optical coupler 32, a piezo (PZT) element 30 capable of changing the resonator length of the above-described fiber laser, and a polarization along the emission direction of light from the EDF 24 (the clockwise direction in FIG. 4). The wave controller 28 is connected by the EDF 24. The configuration of the optical comb light source 12 is not limited to the above configuration as long as it can emit the optical comb.

The optical comb emitted from the optical coupler 32 is supplied to the optical interference unit 14 and the optical comb output unit 20. Between the optical coupler 32, the optical interference unit 14, and the optical comb output unit 20, a polarization controller 38 and an EDF amplifier 40 are provided in order from the side closer to the optical coupler 32. The EDF amplifier 40 includes an EDF 39, a pumping LD 41, and an optical coupler 43. Each configuration up to the optical coupler 32 and the optical interference unit 14 and each configuration up to the optical coupler 32 and the output unit 20 are connected by an optical fiber 36.

A high-nonlinear optical fiber (HNLF) 42 is disposed between the EDF amplifier 40A and the optical interference unit 14. The optical comb whose polarization is controlled and amplified by the polarization controller 38 </ b> A and the EDF amplifier 40 </ b> A is emitted by the HNLF 42 as a wider-band optical comb than before entering the HNLF 42.

The optical interference unit 14 includes, in order from the side closer to the optical comb light source 12, a fiber collimator 44, a condenser lens 46, a λ / 2 wavelength plate 48, a periodically-poled lithium niobate (PPLN) 50, light A band pass filter 52 is provided. The broadband optical comb emitted from the HNLF 42 enters the optical interference unit 14 and is focused on the PPLN 50. From PPLN50, second harmonic _{W S} broadband optical comb is emitted. That is, the broadband optical comb component (2f) and the second harmonic component (2 × 1f) newly generated by the PPLN 50 are emitted from the PPLN 50 in an overlapping manner.

In the broadband optical comb, the frequency f _B of the zeroth to n-th mode on the frequency axis is expressed as shown in Equation (11).

(N) in equation (11) is an arbitrary natural number.

For the frequency of the nth mode of the broadband optical comb, the frequency f _S of the spectrum of the second harmonic wave W _S is expressed as in equation (12).

The frequency f _W of the 2n-th mode adjacent to the mode of the frequency f _S in the broadband optical comb represented by the equation (12) on the low frequency side is represented by the equation (13).

The broadband optical comb and the second harmonic interfere with each other at the beat signal detector 16. The beat signal detector 16 detects beat signals of a broadband optical comb and second harmonics. The offset frequency difference f _CEO is detected by detecting the beat signal between the spectrums of the frequencies represented by the equations (12) and (13). In other words, the beat signals of the frequencies having the frequencies represented by the equations (12) and (13) are emitted to the beat signal detector 16 as interference light. The beat signal detection unit 16 includes a photo detector 54. The light emitted from the PPLN 50 is detected by the photodetector 54. The intensity of the light emitted from the PPLN 50 is converted into the magnitude of an electric signal.

The electric signal output from the photodetector 54 is transmitted to the offset frequency control unit 18 through the electric cable 56 and is repeatedly transmitted to the frequency control unit 22 through the electric cable 58.

The offset frequency control unit 18 includes a high-frequency bandpass filter 61, a high-frequency amplifier 62, a function generator (FG) 64, a frequency converter (Double Balanced Mixer: DBM) 66, and a loop filter 68. The high-frequency bandpass filter 61 extracts an offset frequency component from the electrical signal output from the photodetector 54. The high frequency amplifier 62 amplifies the electric signal emitted from the high frequency band pass filter 61. The FG 64 can transmit a reference signal of the frequency by setting a desired frequency. The DBM 66 mixes the amplified electrical signal and the reference signal transmitted from the FG 64. The loop filter 68 applies feedback to the current applied to the pumping LD 26 of the optical comb light source 12 in accordance with the mixed electric signal.

In the offset frequency control unit 18, when the frequency of the reference signal transmitted from the FG 64 is changed, feedback is applied to the current applied to the excitation LD 26 of the optical comb light source 12 by the loop filter 68. The applied current of the pumping LD 26 is changed, and the offset frequency f _{CEO of the} optical comb emitted from the optical comb light source 12 is aligned with the frequency of the reference signal transmitted from the FG 64. In other words, the offset frequency f _CEO of the optical comb emitted from the optical comb light source 12 can be controlled by controlling the frequency of the reference signal transmitted from the FG 64.

The repetitive frequency control unit 22 includes a high frequency band pass filter 71, a high frequency amplifier 72, an FG 74, a DBM 76, and a loop filter 78. The high-frequency bandpass filter 71 repeatedly extracts frequency components from the electrical signal output from the photodetector 54. The high frequency amplifier 72 amplifies the electric signal from the high frequency band pass filter 71. The FG 74 can transmit a reference signal of the frequency by setting a desired frequency. The DBM 76 mixes the amplified electrical signal and the reference signal transmitted from the FG 74. The loop filter 78 applies feedback to the voltage applied to the PZT element 30 of the optical comb light source 12 in accordance with the mixed electric signal.

In the repetition frequency control unit 22, when the frequency of the reference signal transmitted from the FG 74 is changed, the loop filter 78 applies feedback to the PZT element 30 of the optical comb light source 12 as described above. Thereby, the resonator length of the fiber laser of the optical comb light source 12 is changed, and the repetition frequency f _{rep of the} optical comb emitted from the optical comb light source 12 is aligned with the frequency of the reference signal transmitted from the FG 74. In other words, the repetition frequency f _rep of the optical comb emitted from the optical comb light source 12 can be controlled by controlling the frequency of the reference signal transmitted from the FG 74.

Next, the configuration and control procedure of the control device for the optical comb of the present invention will be described.

As shown in FIG. 5, the control device (optical comb control device) 100 of the present invention includes optical

comb output mechanisms

10A and 10B, a frequency control mechanism 90, a continuous wave laser (hereinafter referred to as CW laser) 92, and a frequency. A stabilizing mechanism 94. The optical comb output mechanism 10A is provided for emitting the optical comb Comb1, and the optical comb output mechanism 10B is provided for emitting the optical comb Comb2. The frequency control mechanism 90 inputs a reference signal for controlling the offset frequency difference Δf _CEO to each of the optical

comb output mechanisms

10A and 10B. The CW laser 92 synchronizes the phases of the two optical combs Comb1 and Comb2. The frequency stabilization mechanism 94 controls the beat signal between the continuous wave light (hereinafter, CW light) emitted from the CW laser 92 and each of the two optical combs Comb1 and Comb2.

In FIG. 5, main parts such as the optical comb output unit 20 of the optical

comb output mechanisms

10A and 10B, the FG 64 of the offset frequency control unit 18, the FG 74 of the repetition frequency control unit 22, and the PZT 12 are illustrated. Is omitted.

The control device 100 includes two FG 64 and FG 74 by providing two optical

comb output mechanisms

10A and 10B. That is, the control apparatus 100 has two FGs 64 and FG74, each of which has a configuration capable of controlling a total of four parameters of the offset frequencies f _CEO1 and f _CEO2 and the repetition frequencies f _rep1 and f _rep2 of the two optical combs Comb1 and Comb2. And a frequency control mechanism 90 that transmits a reference signal to the two FGs 64.

The frequency stabilization mechanism 94 includes a frequency control mechanism 90 formed of a program or the like built in the computer,

FGs

130 and 132,

DBMs

108 and 118, and

PID controllers

110 and 120. The optical comb Comb1 emitted from the optical comb output unit 20 of the optical comb output mechanism 10A enters the optical coupler 104 via the optical coupler 102. CW light emitted from the CW laser 92 enters the optical coupler 104 via the optical coupler 112. The optical comb Comb1 and the CW light combined by the optical coupler 104 are received by a light receiving unit 106 such as a photodetector and converted into an electric signal. The electrical signal emitted from the light receiving unit 106 is input to the DBM 108 and is combined with the reference signal from the FG 130. An output from the DBM 108 is input to the PID controller 110. The output from the PID controller 110 is fed back to the input current value to the CW laser 92.

The CW light emitted from the CW laser 92 enters the optical coupler 114 via the optical coupler 112. The optical comb Comb2 emitted from the optical comb output unit 20 of the optical comb output mechanism 10B enters the optical coupler 114 via the optical coupler 122. The optical comb Comb2 and the CW light combined by the optical coupler 114 are received by a light receiving unit 116 such as a photodetector and converted into an electric signal. The electrical signal emitted from the light receiving unit 116 is input to the DBM 118 and is combined with the reference signal from the FG 132. An output from the DBM 118 is input to the PID controller 120. The output from the PID controller 110 is fed back to the displacement amount of the PZT 30 in the optical comb light source 12 of the optical comb output mechanism 10B.

Next, an example of a control procedure of the offset frequency difference Δf _CEO using the control device 100 will be described.

First, the repetition frequency f _rep1 and offset frequency f _CEO1 of the optical comb Comb1 are stabilized according to the frequency of the reference signal transmitted from the

FGs

64 and 74 of the optical comb output mechanism 10A according to the control procedure described with reference to FIG. Make it.

Next, a beat signal between the optical comb Comb1 and the CW light output from the optical comb output unit 20 of the optical comb output mechanism 10A is detected. The detected beat signal is stabilized with respect to the reference signal from the FG 130. By such a procedure, the frequency of the CW light is made to follow the repetition frequency f _rep1 and the offset frequency f _CEO1 of the optical comb Comb1.

Next, after stabilizing the offset frequency f _CEO2 of the optical comb Comb2 output from the optical comb output unit 20 of the optical comb output mechanism 10B by the control procedure described with reference to FIG. A beat signal with the comb Comb 2 is detected, and the detected beat signal is stabilized with respect to the reference signal from the FG 118. By such a procedure, the repetition frequency f _rep2 of the optical comb Comb2 is made to follow the frequency of the CW light. As a result, the comb 2 follows the optical comb Comb1, and a dual-comb light source with phase synchronization is obtained.

The FG 64 of the optical comb output mechanism 10A _issues a reference signal of the offset frequency f _CEO1 of the optical comb Comb1. The FG 74 of the optical comb output mechanism 10A _generates a reference signal having a repetition frequency f _rep1 of the optical comb Comb1. The FG 132 emits a reference signal having a repetition frequency f _rep2 of the optical comb Comb2. By controlling the respective settings of FG64, FG64, and FG132 by the frequency control mechanism 90, an arbitrary repetition frequency difference or offset frequency difference can be obtained. In order to control the offset frequency difference Δf _CEO , the frequency control mechanism 90 includes _six offset frequency differences Δf _CEO , offset frequency f _CEO1 , offset frequency f _CEO2 , repetition frequency difference Δf _rep , offset frequency f _rep1 , and offset frequency f _rep2 . The four parameters of the offset frequencies f _CEO1 and f _CEO2 and the offset frequencies f _rep1 and f _rep2 are controlled so that a relative relationship (predetermined condition) expressed by an arbitrary integer ratio between the two parameters is established. That is, the control device 100 arbitrarily controls the offset frequency difference Δf _CEO between the two optical combs Comb1 and Comb2.

According to the above control procedure, optical combs Comb1 and Comb2 having a desired offset frequency difference Δf _CEO and phase-controlled with each other are output from the optical

comb output mechanisms

10A and 10B.

As described above, in the optical comb control method of the present invention, the CEP of the two optical combs Comb1 and Comb2 is arbitrarily set by actively controlling the offset frequency difference Δf _CEO between the two optical combs Comb1 and Comb2. Can be controlled. In the method for controlling an optical comb according to the present invention, since the correspondence between important parameters that determine the characteristics of the pulse train of the optical comb can be appropriately controlled, arbitrary coherent modulation can be performed. Examples of applications that perform coherent modulation include, but are not limited to, coherent spectroscopy, coherent physical property analysis, and coherent time-resolved measurement.

The control device 100 of the present invention inputs a reference signal of an arbitrary frequency from the frequency control mechanism 90 to each of the

FGs

64 and 74 of the optical comb output mechanism 10A, the

FGs

130 and 132, and the FG 64 of the optical comb output mechanism 10B. The optical combs Comb1 and Comb2 that are phase-controlled with each other can be generated. As described above, the reference signal output from the FG 132 is indirectly input to the PZT 30 of the optical comb output mechanism 10B.

The optical comb control method and control apparatus according to the present invention can be applied in a wide field of performing arbitrary coherent modulation. In the control method of the optical comb of the present invention, by positively controlling the offset frequency difference Delta] f _CEO, it can control the relative phase of the optical comb COMB1, COMB2. Therefore, the optical comb control method and control apparatus of the present invention can be applied to highly efficient signal detection and signal control in observation of a phenomenon depending on relative CEP. Phenomena that depend on relative CEP are, for example, interference and nonlinear optical phenomena.

The preferred embodiments of the present invention have been described in detail above. The present invention is not limited to the above-described embodiment, and may be appropriately modified and changed within the scope of the gist of the present invention described in the claims.

As an example, the optical comb output mechanism 10 may be configured by using an acousto-optic device (Acoustic Optical Modulator: AOM) 120 shown in FIG. In the optical comb output mechanism 10 shown in FIG. 4, a fiber collimator may be disposed on the input side of the optical comb output unit 20, and the AOM 120 may be disposed on the output side of the fiber collimator. When the AOM 120 is used, the frequency of the first-order diffracted light diffracted from the _AOM 120 is shifted by the modulation frequency f _AOM of the sound wave A applied to the _AOM 120. Based on this principle, the offset frequency difference Δf _CEO can be positively controlled by appropriately setting the modulation frequency f _AOM .

Hereinafter, the present invention will be specifically described by way of examples and comparative examples, but the present invention is not limited to the following examples.

Example 1
As shown in FIG. 5, the optical comb Comb1 emitted from the optical comb output unit 20 of the optical comb output mechanism 10A of the control device 100 and the light emitted from the optical comb output unit 20 of the optical comb output mechanism 10B of the control device 100 Comb 2 was combined and interfered, and an interference signal (Interferogram: IGM) was received by high-speed detector 96. The IGM received by the high-speed detector 96 is post-processed by the data processing unit 98 to visualize the IGM on a display (not shown).

As shown in FIG. 7, the repetition frequency difference Δf _rep between the two optical combs Comb1 and Comb2 affects the period in which the IGM is detected. The period in which IGM is detected corresponds to the interval on the time axis between plots of IGM shown in FIG. The time period T _IGM = (1 / Δf _rep ) elapses from the plot of the IGM caused by the first waveform of the two optical combs Comb 1 and Comb 2 when the positions on the time axis are aligned, and the timing of the first waveform The IGM plots resulting from the (M + 2) th waveform of the optical comb Comb1 and the (M + 1) th waveform of the optical comb Comb2 overlap. The offset frequency difference Δf _CEO between the two optical combs Comb 1 and Comb 2 affects the phase of the IGM. The phase of the IGM is indicated by the shape of the IGM waveform shown in FIG.

By performing Fourier analysis on the IGM, for example, if a sample S or a laser medium is arranged in front of the emission direction of the optical comb Comb1, a spectrum including information such as optical characteristics of the sample S can be acquired. The sample S is a semiconductor such as silicon or gallium arsenide. The laser medium is, for example, Er: YAG, Nd: YAG, or the like. Various information of the sample S can be obtained from optical characteristics included in the acquired spectrum.

In this example, the repetition frequency Δf _rep = 120.6 Hz, the offset frequency Δf _CEO was controlled to be an inverse multiple of the integer of the repetition frequency difference Δf _rep , and IGM was measured. 8A, 8B, and 8C, the upper graph shows the IGM after a predetermined time on the time axis, and the lower graph shows the IGM after the predetermined time on the page. It is displayed by shifting in the vertical direction.

As shown in FIG. 8A, when the offset frequency Δf _CEO is set to 0, the IGM waveform does not change over time, and the offset frequency difference Δf _CEO between the two optical combs Comb 1 and Comb 2 becomes the IGM waveform. It clearly shows that it affects.

As shown in FIG. 8B, when the offset frequency Δf _CEO is set to (Δf _rep / 2), the IGM waveform is inverted when a predetermined time has elapsed, and a phase shift of 2π / 2 = π has occurred. Recognize. By controlling the IGM so that a phase shift of π occurs, and taking the average of the differences between the IGMs obtained continuously and having the distributions inverted from each other, as shown in FIG. Bias can be canceled. From the measurement results shown in FIGS. 8B and 9, it was confirmed that IGM robust to environmental fluctuations can be detected by controlling the offset frequency Δf _{CEO to} be (Δf _rep / 2).

As shown in FIG. 8C, when the offset frequency Δf _CEO is set to Δf _rep / 3, it can be seen that the IGM waveform is repeated in three patterns as a predetermined time elapses, resulting in a phase shift of 2π / 3. . In FIG. 8C, three patterns of IGM are illustrated by a solid line, a broken line, and an alternate long and short dash line, respectively.

As shown in FIGS. 8A to 8C, by controlling the offset frequency Δf _{CEO to} be (1 / N) of the repetition frequency difference _Δrep , a phase shift of (2π / N) is generated for each IGM, It was confirmed that coherent waveform control was possible. N is an arbitrary natural number.

(Example 2)
As shown in FIG. 10A, in Example 2, an optical comb light source 110 in which relative CEP between pulse trains was stabilized was prepared. The optical comb Comb1 emitted from the optical comb light source 110 was made incident on the AOM 120, and the offset frequency difference Δf _CEO between the 0th-order diffracted light and the first-order diffracted light of the optical comb Comb1 was controlled. In addition, in order to perform so-called dual comb spectroscopy, an optical comb light source 112 different from the optical comb light source 110 was prepared. The center wavelength λ of the optical comb

light sources

110 and 112 was set to 1560 nm. The repetition frequency f _rep1 of the optical comb Comb 1 emitted from the optical comb light source 110 was _set to 56.5 MHz. The repetition frequency difference Δf _rep between the optical comb Comb1 and the optical comb Comb2 emitted from the optical comb light source 112 was set to 120.6 Hz.

A (1/4) wavelength plate 116 and a (1/2) wavelength plate 118 for converting the optical comb Comb1 into linearly polarized light in a predetermined direction are disposed in front of the direction in which the optical comb Comb1 is emitted from the optical comb light source 110. did. (1/2) The AOM 120 is arranged in front of the emission direction of the optical comb Comb1 from the wave plate 118. From the AOM 120, the 0th order diffracted light (0 ^th shown in FIG. 10A) and the 1st order diffracted light (1 ^st shown in FIG. 10A) of the optical comb Comb1 are emitted at different angles in plan view. As shown in FIG. 10B, by applying modulation to the _AOM 120 at the frequency f _AOM , the offset frequency difference Δf _CEO between the 0th-order diffracted light and the 1st-order diffracted light of the optical comb Comb1 is expressed as shown in Equation (14).

As shown in FIG. 10A, the optical path length changing unit 122 is disposed in front of the emission direction of the 0th-order diffracted light of the optical comb Comb1. A (1/2) wavelength plate 124 is arranged in front of the emission direction of the first-order diffracted light of the optical comb Comb1. With this arrangement, the direction of linearly polarized light of the first-order diffracted light of the optical comb Comb1 was made orthogonal to the direction of linearly polarized light of the 0th-order diffracted light of the optical comb Comb1. The zero-order diffracted light and the first-order diffracted light were combined by a polarizing beam splitter (PBS) 126 to generate a coherently controlled and polarization-modulated light pulse. As shown in FIG. 10C, in the optical pulse subjected to coherent control and polarization modulation, the vibration direction of the pulse train of the 0th-order diffracted light of the optical comb Comb1 and the vibration direction of the pulse train of the 1st-order diffracted light are orthogonal to each other. Relative CEP between 0-order diffracted light and 1-order diffracted light of the optical comb Comb1: Δφ _CEP' occurs, (15) it holds.

T in equation (15) represents time.

As shown in FIG. 10A, a (1/4) wavelength plate 130 for converting the optical comb Comb2 into linearly polarized light in a predetermined direction ahead of the emission direction of the optical comb Comb2 from the optical comb light source 112, (1 / 2) The wave plate 132 and the polarizing plate 134 were disposed. The optical comb Comb2 having a predetermined linear polarization was combined with the 0th-order diffracted light and the first-order diffracted light of the optical comb Comb1 coherently controlled and polarization-modulated as described above by a beam splitter (Beam （Splitter: BS) 138. The photodetector 140 detects the interference light of the two optical combs Comb 1 and Comb 2. In this configuration, the optical comb Comb2 functions as a local oscillator (LO) signal. The interference light received by the photo detector 140 is processed by a digitizer (data processing unit) 142 to visualize the interference light on a display (not shown). An optical comb Comb2 emitted from the optical comb light source 112 and not passing through the (1/4) wave plate 130, the (1/2) wave plate 132, and the polarizing plate 134 is supplied to the digitizer 142 as a reference clock signal. I input it.

Using the optical path length changing unit 122, the timing of the 0th-order diffracted light and the 1st-order diffracted light of the optical comb Comb1 was matched. As shown in FIG. 11A, the relative CEP between 0-order diffracted light and 1-order diffracted light of the optical comb COMB1: the Δφ _CEP', it was varied under the conditions of _{_{Δf CEO = (Δf rep / 4}} ).

As shown in FIGS. 11B and 11C, when the direction of the linearly polarized light of the optical comb Comb2 as the LO signal is slanted to the right on the paper, the direction of the linearly polarized light of the optical comb Comb1 is slanted to the right on the paper. The directions of the linearly polarized light of the optical comb Comb2 became the same as each other, and a strong interference signal was generated. When the optical comb Comb1 becomes counterclockwise or clockwise circularly polarized light on the paper, the optical comb Comb1 partially includes the linearly polarized light of the optical comb Comb2. At this time, a weak interference signal was generated as compared with the case where the direction of the linearly polarized light of the optical comb Comb1 is slantingly downward on the paper surface. When the direction of the linearly polarized light of the optical comb Comb1 is slanting downward to the left and is orthogonal to the direction of the linearly polarized light of the optical comb Comb2, the optical combs Comb1 and Comb2 do not interfere with each other, and the interference signal is smaller than in other cases. It became very weak.

As shown in FIGS. 11D and 11E, when the direction of the linearly polarized light of the optical comb Comb2 as the LO signal is slanted to the left on the paper, the direction of the linearly polarized light of the optical comb Comb1 is slanted to the left on the paper. The directions of the linearly polarized light of the optical comb Comb2 became the same as each other, and a strong interference signal was generated. When the optical comb Comb1 becomes counterclockwise or clockwise circularly polarized light on the paper surface, the optical comb Comb1 partially includes the linearly polarized light of the optical comb Comb2. At this time, a weak interference signal was generated as compared with the case where the direction of the linearly polarized light of the optical comb Comb1 is slantingly downward on the paper. When the optical comb Comb1 is slanted to the right and is orthogonal to the direction of linear polarization of the optical comb Comb2, the optical combs Comb1 and Comb2 do not interfere with each other, and a very weak interference signal is generated compared to the other cases. .

As can be seen from the results shown in FIG. 11A to FIG. 11E, the offset frequency difference Δf _CEO between the 0th-order diffracted light and the 1st-order diffracted light of Comb1 is set to a desired condition (in this example, Δf _CEO = It can be controlled to satisfy Δf _rep / 4). Further, in this example, the 0th-order diffracted light and the 1st-order diffracted light of Comb 1 were orthogonal to each other and overlapped spatially and temporally. At this time, a polarization interference waveform modulated into linearly polarized light, elliptically polarized light, circularly polarized light, or the like was generated according to the desired conditions described above. By applying the present invention, it was confirmed that relative CEP high-speed modulation and coherent detection by polarization interference becomes possible.

90 ... frequency control mechanism 100 ... control device (control device for optical comb)
Comb1 ... optical comb (first optical comb)
Comb2 ... optical comb (second optical comb)
f _CEO1 Offset frequency (first frequency offset)
f _rep1 ... repetition frequency (first frequency interval)
f _CEO2 ... offset frequency (second frequency offset)
f _rep2 ... repetition frequency (second frequency interval)
FM1 ... optical frequency mode (first frequency mode)
FM3 ... optical frequency mode (third optical frequency mode)
FM2 ... Optical frequency mode (second frequency mode)
FM4 ... optical frequency mode (fourth optical frequency mode)

Claims

A first frequency mode having a first frequency offset with respect to zero on the frequency axis and a plurality of second frequency lines arranged at an integer multiple of the first frequency interval with respect to the first frequency mode on the frequency axis. A first optical comb having three frequency modes;
A second frequency mode having a second frequency offset with respect to zero on the frequency axis and a plurality of lines arranged with an interval that is an integral multiple of a second frequency interval with respect to the second frequency mode on the frequency axis. Using a second optical comb having a fourth frequency mode,
An optical comb control method comprising a control step of controlling a difference between the first frequency offset and the second frequency offset.
In the control step,
2. The optical comb control method according to claim 1, wherein a difference between the first frequency offset and the second frequency offset is set to satisfy the expression (1). 3.

In the equation (1), Δf CEO represents the difference between the first frequency offset and the second frequency offset. Δf rep represents the difference between the first frequency interval and the second frequency interval. K and N represent arbitrary natural numbers.
In the control step,
3. The optical comb control method according to claim 1, wherein a difference between the first frequency offset and the second frequency offset is set so as to satisfy the expression (2).

In the equation (2), Δf CEO represents the difference between the first frequency offset and the second frequency offset. f rep represents the first frequency interval or the second frequency interval. N represents an arbitrary natural number.
In the control step,
4. The optical comb according to claim 1, wherein a difference between the first frequency offset and the second frequency offset is set to satisfy the expression (3). 5. Control method.

In the formula (3), Δf CEO represents the difference between the first frequency offset and the second frequency offset. f CEO represents the first frequency interval or the second frequency interval. N represents an arbitrary natural number.
A first frequency mode having a first frequency offset with respect to zero on the frequency axis and a plurality of second frequency lines arranged at an integer multiple of the first frequency interval with respect to the first frequency mode on the frequency axis. Controlling the first frequency offset and the first frequency interval in a first optical comb having three frequency modes;
A second frequency mode having a second frequency offset with respect to zero on the frequency axis and a plurality of lines arranged with an interval that is an integral multiple of a second frequency interval with respect to the second frequency mode on the frequency axis. A frequency control mechanism for controlling the second frequency offset and the second frequency interval in a second optical comb having a fourth frequency mode;
The control mechanism includes the first frequency offset, the second frequency offset, the difference between the first frequency offset and the second frequency offset, the first frequency interval, the second frequency interval, And the first frequency offset, the second frequency offset, the first frequency interval, and the difference between the first frequency interval and the second frequency interval are expressed as an integer ratio to each other. An optical comb control device configured to be able to control the second frequency interval.