New method for generating and detecting spectrum of high-spectral-density frequency-agile broadband terahertz optical comb

Abstract

The invention provides a novel spectrum generation and detection method for a high-spectral-density frequency agility broadband terahertz optical comb, belongs to the technical field of terahertz frequency combs, and particularly provides a femtosecond pulse width electric modulation optical comb with the frequency modulation precision at the mHz magnitude, the repetition frequency tuning range covering the kHz-GHz range and capable of being synchronously and rapidly tuned. The high-coherence electro-optical frequency comb is used as a broadband terahertz optical comb pumping source, and kHz-magnitude narrow-linewidth terahertz detection is realized by combining an asynchronous sampling technology; the spectrum sampling density is improved by simultaneously tuning pumping and detecting the repetition frequency of the electro-optical frequency comb at high precision, and the terahertz spectrum interval is further reduced to the magnitude of kHz, so that the terahertz optical comb spectrum with high resolution, high precision, narrow linewidth and wide spectrum coverage range is realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of terahertz frequency combs, and in particular relates to a high-precision terahertz measurement technology based on high spectral density and wide spectrum terahertz optical comb generation and agile frequency asynchronous electro-optical sampling based on a femtosecond electro-optical frequency comb. Background Art

[0002] Terahertz waves are an electromagnetic band between infrared and microwaves. Compared with the infrared band, they have lower photon energy and are used in biomedicine, material analysis, non-destructive testing and other fields with better safety and stronger penetration. Compared with the microwave band, terahertz waves have larger bandwidth, higher carrier frequency and faster update rate, providing the possibility for the development of large-capacity communications in the terahertz band.

[0003] The existing terahertz technologies mainly include low-frequency electronic methods and wide-band photonic methods.

[0004] The electronic method mainly converts the terahertz signal to the high-frequency microwave band for analysis by electrical means. However, due to the narrow working bandwidth of electrical devices and harmonic interference, it mainly works in the frequency band below 0.3 THz, making it difficult to achieve high-frequency terahertz generation.

[0005] Although commonly used photonic methods can cover a wide band, traditional terahertz time-domain spectroscopy technology is limited by the sampling method and cannot achieve high-precision narrow-linewidth spectrum measurement; although terahertz asynchronous sampling technology based on precise control of two mode-locked lasers can achieve high-precision spectrum measurement, and the spectrum interval depends on the laser repetition frequency, it is limited by the cavity length tuning accuracy and range, and it is difficult to achieve high-precision synchronous tuning of two lasers, and therefore cannot achieve terahertz wave generation and detection covering a wide band and high spectrum density; traditional electro-optic frequency combs based on electro-optic modulation achieve wide-range tuning by using electro-optic modulators instead of cavity length tuning, but commonly used electro-optic combs can only achieve picosecond pulse output, making it difficult to achieve the generation of broadband terahertz frequency combs.

[0006] Therefore, there is a need for a method that can achieve the generation of a broadband terahertz frequency comb, as well as the generation and detection of terahertz waves covering a wide band and with high spectral density. Summary of the invention

[0007] The purpose of the present invention is to provide a method for generating and detecting a wide-spectrum terahertz optical comb based on a femtosecond electro-optical frequency comb, which has the characteristics of high spectral resolution accuracy and high spectral density, and can realize high-precision broadband terahertz spectrum generation and detection with narrow linewidth, resolvable comb teeth, and continuously tunable spectrum.

[0008] Specifically, a method for generating a wide-spectrum terahertz optical comb based on a femtosecond electro-optical frequency comb comprises the following steps:

[0009] Step 1: The signal generator outputs a MHz-level sinusoidal signal, which is modulated into a ps-level electrical pulse by an electrical pulse generator, and used to drive the intensity modulator; the intensity modulator modulates the continuous laser to generate a ps optical pulse with a repetition frequency consistent with the frequency of the sinusoidal signal. The ps optical pulse is amplified by an optical fiber amplifier, and then the spectrum is broadened by a nonlinear optical fiber, and finally the pulse width is compressed by an ordinary single-mode optical fiber to obtain a pulse output with a pulse width of the fs level. The repetition frequency tuning of the fs electro-optical frequency comb can be achieved by adjusting the frequency of the sinusoidal signal output by the signal generator. The repetition frequency tuning range determines the frequency interval of the terahertz optical comb spectrum, and a large tuning range from kHz to GHz can be achieved. The fs pulse width determines that its spectrum coverage range reaches 4-6THz.

[0010] Step 2: Use the fs electrically modulated optical comb generated in step 1 as a wide-spectrum terahertz optical comb pump light source, input it into a photoconductive antenna or an electro-optical crystal, etc., and generate terahertz pulses through antenna radiation or optical rectification effect. Since the generation of terahertz pulses depends on the envelope of the pump pulse rather than the phase, the generated terahertz pulses are frequency-stable terahertz optical combs, and their repetition frequency is consistent with the repetition frequency of the pump electrically modulated optical comb, that is, f THz =mf r1 , where m is the order of the mth terahertz frequency.

[0011] A narrow linewidth terahertz optical comb detection method, using a wide spectrum terahertz optical comb generation method, two electro-optical frequency combs with pulse widths of the order of fs are generated by signal generator A and signal generator B, and the repetition frequencies are f r1 and f r2 Detection is performed, where Δf = |f r1 -f r2 |. The specific steps include:

[0012] Step 1: Set the repetition frequency to f r1 The electro-optic frequency comb is used as the signal light comb and input into the photoconductive antenna to generate a wide spectrum terahertz light comb. The repetition frequency of the generated terahertz light comb is consistent with the repetition frequency of the signal light comb, that is, f THz =mf r1 , m is the order. The terahertz light comb is reflected by off-axis parabolic mirror A and off-axis parabolic mirror B and then focused on the sample to be tested; the terahertz light comb carrying the information of the sample to be tested is reflected by off-axis parabolic mirror C and off-axis parabolic mirror D and then focused on the electro-optical crystal ZnTe.

[0013] Step 2: Use frequency-agile asynchronous electro-optical sampling technology to set the repetition frequency to f r2The electro-optical frequency comb is used as the sampling optical comb, and the square wave signal output by the signal generator A of the signal optical comb is connected to the sweep trigger port of the signal generator B of the sampling optical comb, so that the sampling optical comb can be quickly swept according to the pulse period.

[0014] Furthermore, the repetition frequency of the sampled electrically modulated optical comb is (f r1 ±Δf) range, so that the sampling pulse only scans the signal part within the terahertz pulse period. When the repetition frequency f of the sampling electrically modulated optical comb is r2 When fixed, the scanning time period of a single cycle is 1 / Δf; when f r2 When scanning the terahertz pulse back and forth, the scanning time of a single cycle will be shortened by a factor of 1 / f of the entire terahertz pulse period. r1 The method greatly improves the scanning speed of terahertz pulses because of the ratio of the terahertz pulse width (signal part).

[0015] Step 3: The terahertz optical comb and the sampling optical comb in the rapid frequency sweep are injected into ZnTe together. The polarization change of the sampling optical comb reflects the change of the terahertz electric field. The resolution of the terahertz spectrum obtained at this time is the repetition frequency f of the signal optical comb 2. r1 , the comb line width is f r1 / N, where N is the number of terahertz pulses sampled continuously.

[0016] Furthermore, using the repetition frequency f r2 The electrically modulated optical comb light source is used as the detection light, which beats with the generated terahertz pulse. At this time, a periodic beat signal with a time period of 1 / Δf is obtained. This RF signal is the down-converted RF signal of the terahertz time domain signal, and the time magnification factor is f. r1 / Δf, and broadband terahertz spectrum information is obtained through Fourier transform. The comb line width of the collected terahertz spectrum is determined by the number of continuous sampling pulses and the repetition frequency, which is f r1 / N. This asynchronous sampling method replaces the traditional time domain scanning, overcomes the limitation of the spatial delay distance on the comb line width, and realizes the narrow line width terahertz spectroscopy measurement.

[0017] A method for detecting terahertz optical comb spectra with high resolution, high precision, narrow linewidth and wide spectral coverage is based on the above narrow linewidth terahertz optical comb detection method, specifically by adjusting the repetition frequency of the signal optical comb and the sampling optical comb at the same time, and keeping the repetition frequency difference Δf unchanged. Assume that the change of the signal optical comb and the sampling optical comb each time is δf r1 , then, the m-order offset is mδf r1 , note that it needs to be repeated at intervals of the comb line width, that is, mδf r1 =f r1 / N, we can get the change δf r1 =f r1(mN), and all generated terahertz comb spectra overlap in the frequency domain, then the frequency resolution is f r1 The gaps in the optical comb spectrum can be completely removed; in order to fill the frequency gaps, this tuning must be repeated M times (M = f r1 / (mδf r1 )=N) times, thus high-density, high-precision, narrow-linewidth detection of the sample to be tested can be achieved.

[0018] Compared with the prior art, the beneficial effects of the present invention are mainly reflected in:

[0019] 1. The present invention realizes the generation of an electro-optical frequency comb with a pulse width of the order of fs through spectrum broadening and dispersion control, a repetition frequency of the order of MHz, and an output power of the order of hundreds of mW.

[0020] 2. The present invention adopts a frequency-agile and frequency-locked electro-optical comb light source as the signal and sampling optical comb. Compared with the fiber laser, it has higher repetition frequency stability, lower phase noise and a simpler feedback system, which can further improve the measurement accuracy of the terahertz optical comb spectrum.

[0021] 3. Compared with traditional terahertz asynchronous sampling, the present invention incorporates the frequency-agile asynchronous electro-optical sampling technology, that is, by simultaneously tuning the signal and sampling optical comb repetition frequency, the frequency gap is filled to achieve high-density, high-precision, and narrow-linewidth detection of the terahertz optical comb spectrum. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Schematic diagram of sample detection of the present invention.

[0023] Figure 2 This is a schematic diagram of the high spectral density agile frequency terahertz optical comb spectrum detection principle of the present invention.

[0024] In the figure, 201 is a signal generator A; 202 is a signal light comb; 203 is a signal generator B; 204 is a sampling light comb; 205 is a photoconductive antenna; 206 is an off-axis parabolic mirror A; 207 is an off-axis parabolic mirror B; 208 is a sample to be tested; 209 is an off-axis parabolic mirror C; 210 is an off-axis parabolic mirror D. DETAILED DESCRIPTION

[0025] The following is a more detailed description of an ultra-wideband antenna design based on characteristic mode analysis of the present invention in conjunction with a schematic diagram, in which a preferred embodiment of the present invention is shown. It should be understood that those skilled in the art can modify the present invention described herein and still achieve the beneficial effects of the present invention. Therefore, the following description should be understood as being widely known to those skilled in the art and not as a limitation to the present invention.

[0026] Example

[0027] The present invention is further described below in conjunction with the accompanying drawings.

[0028] like Figure 1 As shown in the figure, a new method for generating and detecting high spectral density frequency-agile broadband terahertz optical comb spectrum is described as follows:

[0029] Signal generator A201 and signal generator B203 respectively output MHz level and the frequency difference is Δf=|f r1 -f r2 | A sinusoidal signal is modulated into an electrical pulse of the order of ps by an electrical pulse generator, and used to drive an intensity modulator; the intensity modulator modulates the continuous laser to generate a ps optical pulse with a repetition frequency consistent with the frequency of the sinusoidal signal. The ps optical pulse is power-amplified by an optical fiber amplifier, and then the spectrum is broadened by a nonlinear optical fiber, and finally the pulse width is compressed by an ordinary single-mode optical fiber, thereby obtaining a pulse output with a pulse width of the order of fs. The repetition frequency tuning of the fs electro-optical frequency comb can be achieved by adjusting the frequency of the sinusoidal signal output by the signal generator. Two fs electro-optical frequency combs are used as the signal optical comb 202 and the sampling optical comb 204, respectively.

[0030] The repetition frequencies are f r1 and f r2 (Δf=|f r1 -f r2 |). The signal light comb 202 is used as a wide spectrum terahertz light comb pump light source and input into the photoconductive antenna 205, thereby generating a wide spectrum terahertz light comb. The repetition frequency of the generated terahertz light comb is consistent with the repetition frequency of the signal light comb 202, that is, f THz =mf r1 , m is the order.

[0031] The terahertz optical comb is focused on the sample 208 to be tested after being reflected by the off-axis parabolic mirror A206 and the off-axis parabolic mirror B207; the terahertz optical comb carrying the information of the sample 208 to be tested is focused on the electro-optical crystal ZnTe after being reflected by the off-axis parabolic mirror C209 and the off-axis parabolic mirror D210, so as to facilitate the frequency-agile asynchronous electro-optical sampling. The square wave signal output by the signal generator 201 of the signal optical comb 202 is connected to the frequency sweep trigger port of the signal generator 203 of the sampling optical comb 204, so that the sampling optical comb 204 can quickly sweep the frequency according to the pulse period; the terahertz optical comb and the sampling optical comb 204 in the frequency sweep are jointly injected into ZnTe, thereby realizing the kHz-level narrow linewidth terahertz detection. Among them, the polarization change of the sampling optical comb 204 reflects the change of the terahertz electric field. At this time, the resolution of the terahertz spectrum obtained is the repetition frequency f of the signal optical comb 202. r1 The comb line width of the terahertz spectrum is f r1 / N, where N is the number of terahertz pulses sampled continuously.

[0032] refer to Figure 2 Finally, the repetition frequencies of the signal light comb 202 and the sampling light comb 204 are adjusted simultaneously, and the repetition frequency difference Δf is kept constant. Assume that the change amount of the signal light comb 202 and the sampling light comb 204 each time is δf r1 , then, the m-order offset is mδf r1 , note that it needs to be repeated at intervals of the comb line width, that is, mδf r1 =f r1 / N, we can get the change δf r1 =f r1 (mN), and all generated terahertz comb spectra overlap in the frequency domain, then the frequency resolution is f r1 The gaps in the optical comb spectrum can be completely removed; in order to fill the frequency gaps, this tuning must be repeated M times (M = f r1 / (mδf r1 )=N) times. In this way, high-density, high-precision, and narrow-linewidth detection of the sample 208 to be tested can be achieved.

[0033] The above is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any technician in the relevant technical field, without departing from the scope of the technical solution of the present invention, makes any form of equivalent replacement or modification to the technical solution and technical content disclosed in the present invention, which does not depart from the content of the technical solution of the present invention and still falls within the protection scope of the present invention.

Claims

1. A method for generating a wide spectrum terahertz light comb, characterized in that: The steps include: SA1: Signal generator A outputs a sinusoidal signal of the MHz order. The electric pulse generator modulates the sinusoidal signal into ps electric pulses to drive the intensity modulator. The intensity modulator modulates the continuous laser to generate ps optical pulses with a repetition frequency consistent with the frequency of the sinusoidal signal. The optical fiber amplifier amplifies the power of the ps optical pulses and then broadens the spectrum through a nonlinear optical fiber. The single-mode optical fiber compresses the pulse width of the broadened spectrum to obtain an electro-optical frequency comb with a pulse width of the fs order. SA2: Use a fs-level electro-optic frequency comb as a wide-spectrum terahertz optical comb pump light source and input it into a photoconductive antenna to generate a wide-spectrum terahertz optical comb.

2. The method for generating a wide spectrum terahertz optical comb according to claim 1, characterized in that: In the SA1, the repetition frequency of the electro-optical frequency comb with a pulse width of the order of fs is achieved by adjusting the frequency of the output sinusoidal signal of the signal generator. The tuning range of the repetition frequency determines the frequency interval of the wide-spectrum terahertz optical comb, specifically kHz-GHz; the fs pulse width determines the coverage range of the wide-spectrum terahertz optical comb spectrum, specifically 4-6THz width.

3. The method for generating a wide spectrum terahertz optical comb according to claim 1, characterized in that: In the SA2, the generated wide spectrum terahertz optical comb has the same repetition frequency as the electro-optic frequency comb, i.e., f THz =mf r1 , where m is the order.

4. A narrow linewidth terahertz optical comb detection method, using the wide spectrum terahertz optical comb generated as claimed in claim 1, characterized in that: The steps include: SC1: Based on SA1, signal generator A and signal generator B respectively output sinusoidal signals with a frequency difference of Δf, and finally obtain a repetition frequency of f r1 The signal light comb and repetition frequency are f r2 The sampling light comb, where Δf = |f r1 -f r2 |; SC2: The signal light comb passes through the photoconductive antenna, off-axis parabolic mirror A and off-axis parabolic mirror B in sequence and is focused to the sample to be tested. The terahertz light comb carrying the information of the sample to be tested passes through the off-axis parabolic mirror C and off-axis parabolic mirror D in sequence and is focused to the electro-optical crystal ZnTe. SC3: Using the agile frequency sampling technology, the output square wave signal of signal generator A is connected to the sweep trigger port of signal generator B, so that the sampling optical comb can quickly sweep the pulse period; SC4: The sampling light comb in the frequency sweep and the terahertz light comb in SC2 are simultaneously injected into ZnTe to achieve narrow linewidth detection of the sample to be tested.

5. The narrow linewidth terahertz optical comb detection method according to claim 4, characterized in that: In the SC4, when the terahertz optical comb and the sampling optical comb in the frequency sweep are jointly injected into ZnTe, the polarization change of the sampling optical comb reflects the change of the terahertz electric field. At this time, the resolution of the terahertz spectrum obtained is the repetition frequency f of the signal optical comb. r1 ; The comb line width of the terahertz spectrum is f r1 / N, where N is the number of terahertz pulses sampled continuously.

6. A method for detecting a terahertz optical comb spectrum with high resolution, high precision, narrow line width and wide spectral coverage, based on the narrow line width terahertz optical comb detection method as claimed in claims 4-5, characterized in that: Based on the fact that the repetition frequency difference Δf remains unchanged, the repetition frequencies of the signal light comb and the sampling light comb are adjusted to a certain number of times at the same time, so that the frequency gap is filled, thereby realizing the detection of the high spectral density agile frequency terahertz light comb spectrum.

7. The method for detecting terahertz optical comb spectrum according to claim 6, characterized in that: The repetition frequency of the signal light comb and the sampling light comb is adjusted at the same time: the change of the signal light comb and the sampling light comb each time is set to δf r1 , then, the m-order offset is mδf r1 , repeated at intervals of the comb line width, i.e. mδf r1 =f r1 / N, and the change δf is obtained r1 =f r1 (mN), and all generated terahertz comb spectra overlap in the frequency domain, then the frequency resolution is f r1 The optical comb spectrum gap is completely removed; a certain number of times is M (M = f r1 / (mδf r1 )=N) times.

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