Near-field imaging-oriented self-detection terahertz double-optical comb spectrum information high-speed extraction method

Abstract

The invention relates to a near-field imaging-oriented self-detection terahertz double-optical comb spectrum information high-speed extraction method. The method comprises the following steps of down-converting a certain comb tooth signal of double optical combs to a set frequency band to serve as a first processing signal; collecting time domain information of the first processing signal in a first set time, and analyzing the first time domain information to obtain comb tooth information of any comb tooth; a set frequency is adopted to modulate a light source laser so as to simulate modulation of scattering light intensity by probe vibration under near-field configuration, and an alternating current signal of the self-detection terahertz double-optical comb after modulation is obtained to serve as a second processing signal; and time domain information of the second processing signal in a second set time is collected to obtain second time domain information, and the second time domain information is analyzed to obtain sideband information of the self-detection terahertz double-optical comb as near-field imaging information. According to the invention, the spectrum change information of the terahertz self-detection double-optical-comb signal under the microsecond scale can be extracted and processed at a high speed, and support is provided for near-field self-detection double-optical-comb imaging application.

Description

Technical Field

[0001] The present invention relates to the technical field of self-detected terahertz dual-comb near-field imaging, and in particular to a method for high-speed extraction of self-detected terahertz dual-comb spectrum information for near-field imaging. Background Art

[0002] Different from the laser plus detector configuration of a single-mode laser in traditional near-field imaging, the self-detecting dual-comb mixes the light emitted by a laser as a light source directly in the cavity of another laser as a detector, converting it from the optical frequency band to the microwave band. This technology has great potential in near-field imaging applications and is expected to achieve broadband spectral high-resolution characterization of materials in the terahertz band.

[0003] The single-mode laser plus detector configuration of the traditional near-field imaging system uses a monochromatic light source and cannot be used for optical comb imaging. Its general usage is to use a reference signal to control the atomic force microscope probe to vibrate at a fixed frequency. During the vibration process, the distance between the probe and the sample changes continuously, causing the intensity of the scattered light from the probe tip to change continuously. The frequency is the same as the reference light, which is equivalent to the scattered light being modulated at the reference frequency. The detector receives the scattered signal and uses a phase-locked amplifier to demodulate it, and the demodulation frequency is the above reference frequency. This configuration has two characteristics. First, a reference frequency can be used as both the modulation and demodulation frequency; second, the laser will separate a reference light that does not pass through the sample and give it to the detector.

[0004] These two characteristics cannot be met in the self-detection dual optical comb. First, the signal from the detector can be regarded as a DC signal with a zero-frequency carrier. The result of its modulation is to transfer the carrier to the reference frequency. Therefore, demodulation only needs to demodulate the reference frequency. In the self-detection configuration, the signal from the laser used as a mixer is a GHz-band signal. The comb teeth are not at zero frequency and the frequency is unstable. The result of modulation is only to produce sidebands on both sides of the peak, which is difficult to measure directly using a phase-locked amplifier. Second, each pair of dual-comb lasers has specificity, and it is impossible to set up a mixer to mix the reference light. In addition, the current QCL intensity is not enough to support dual optical paths, so it is impossible to set a reference light.

[0005] The extraction of dual-comb signals is generally done by directly reading them through a spectrum analyzer. For imaging applications that require automated continuous measurement and data storage, this can be achieved through software remote control of the spectrum analyzer. However, the measurement speed of the spectrum analyzer is slow. Under conventional scanning parameter settings, the time to read the peak of a single comb tooth is close to 100mS, which is unacceptable for some applications that are sensitive to measurement time, especially near-field imaging applications of high-frequency self-detecting dual-combs.

[0006] There are two technical routes for near-field imaging: the first is the peak force scanning near-field microscope, which requires the probe to be pressed down onto the sample at an extremely slow speed, and the intensity of the scattered signal is continuously recorded during the pressing process. Finally, a curve of the change of the scattered signal with the height of the probe sample surface is drawn, and the near-field signal is extracted according to the trend of the curve change. The advantage of this technical route is that it does not require high signal intensity itself, but it has high requirements for measurement speed. In this application scenario, the near-field interaction only occurs within a range of tens of nanometers from the probe to the sample, and the corresponding time window is only more than one hundred milliseconds at most. In this time window, as much near-field imaging data of the self-detection dual-optical comb as possible needs to be recorded, and the measurement time must be at least in the microsecond level, which is obviously difficult for the spectrum analyzer to measure. The second is that the probe mentioned above vibrates at high speed on the sample surface and modulates the scattered light, and then demodulates according to the vibration frequency of the probe. Correspondingly, for the spectrum analyzer, the sideband intensity is extracted. This method does not have high requirements for measurement speed, but generally, the high-order sidebands of the signal need to be extracted, and the intensity of the self-detection dual-optical comb signal may be difficult to meet the requirements.

[0007] Therefore, a method is needed to collect dual-comb signals at high speed and then process them to extract the corresponding comb tooth intensity to obtain near-field signals, which is of great significance in the field of near-field terahertz imaging. Summary of the invention

[0008] The technical problem to be solved by the present invention is to provide a method for high-speed extraction of self-detected terahertz dual-comb spectral information for near-field imaging, which can extract terahertz self-detected dual-comb signals on a microsecond scale and further process them to obtain spectral information.

[0009] The technical solution adopted by the present invention to solve the technical problem is: to provide a method for high-speed extraction of self-detected terahertz dual-comb spectral information for near-field imaging, comprising the following steps:

[0010] generating a first optical frequency comb using a first laser and coupling the first optical frequency comb to a second laser;

[0011] Generate a second optical frequency comb using the second laser, and beat the second optical frequency comb with the received first optical frequency comb through the second laser to generate a self-detection terahertz dual optical comb;

[0012] Selecting the first extraction method or the second extraction method according to different near-field imaging application scenarios to obtain the spectrum information of the self-detection terahertz dual-comb;

[0013] The first extraction method comprises the following steps:

[0014] Acquire the AC signal of the self-detected terahertz dual optical comb and extract one of the comb teeth and down-convert it to a set frequency band to obtain a first processed signal of the comb tooth;

[0015] Collecting time domain information of the first processed signal within a first set time to obtain first time domain information;

[0016] Analyzing the first time domain information to obtain comb tooth information of the current comb teeth;

[0017] Acquiring the comb tooth information of all the comb teeth of the self-detection terahertz dual-light comb as near-field imaging information;

[0018] The second extraction method comprises the following steps:

[0019] Modulating the first laser with a set frequency to simulate modulation of scattered light by probe vibration in a near-field configuration;

[0020] Acquiring the modulated alternating current signal of the self-detecting terahertz dual-optical comb as a second processing signal;

[0021] Collecting time domain information of the second processed signal within a second set time to obtain second time domain information;

[0022] The second time domain information is analyzed to obtain the sideband information of the self-detected terahertz dual-comb as near-field imaging information.

[0023] Furthermore, the method of selecting the first extraction method or the second extraction method for different near-field imaging application scenarios to obtain self-detected terahertz dual-light comb spectrum information includes:

[0024] For the application scenario of peak force scanning near-field imaging, the first extraction method is selected, otherwise the second extraction method is selected to obtain self-detected terahertz dual-comb spectrum information.

[0025] Further, the analyzing the first time domain information to obtain comb tooth information of the current comb tooth includes:

[0026] Performing a short-time Fourier transform on the first time domain information to obtain a first Fourier transform result of a plurality of time blocks;

[0027] The peak frequency in each of the first Fourier transform results and the time starting point of the corresponding time block are obtained to obtain the frequency change information and the intensity change information of the current comb tooth.

[0028] Furthermore, it also includes the step of obtaining the comb tooth information of all the comb teeth of the self-detection terahertz dual-light comb to form a comb spectrum.

[0029] Furthermore, the analyzing the second time domain information to obtain the sideband information of the self-detected terahertz dual-comb includes:

[0030] Performing a short-time Fourier transform on the second time domain information to obtain a second Fourier transform result of a plurality of time blocks;

[0031] The peak frequency in each of the second Fourier transform results and the time starting point of the corresponding time block are obtained, and the sideband peak is found at a position where the peak frequency is the set frequency to obtain the sideband information of the self-detected terahertz dual-light comb.

[0032] Furthermore, both the first time domain information and the second time domain information are collected by a high-speed oscilloscope.

[0033] Furthermore, the AC signal is obtained by using a bias device to block the DC component in the terahertz dual-comb signal.

[0034] Furthermore, before the step of collecting time domain information of the first processed signal within a first set time, the step of amplifying the first processed signal is also included; before the step of collecting time domain information of the second processed signal within a second set time, the step of amplifying the second processed signal is also included.

[0035] Furthermore, both the first laser and the second laser are terahertz quantum cascade lasers.

[0036] Beneficial Effects

[0037] Due to the adoption of the above technical scheme, the present invention has the following advantages and positive effects compared with the prior art: the existing dual-comb imaging technology generally relies on a high-speed mixer, and the optical comb frequency is mostly in the infrared band. There is a lack of a means to directly obtain the dual-comb spectrum information for imaging in the terahertz range. The present invention adopts two QCLs for self-detection, in which one laser is used as a light source, and the other is used as a mixer and detector to down-convert the signal from the optical frequency band to the microwave band. The down-converted time domain signal is given to a high-speed oscilloscope for recording. The data is processed by short-time Fourier transform and frequency domain signal extraction to obtain information on the changes of the comb teeth and its sidebands over time. No additional detector or optical mixer is required. While avoiding the problem of lack of effective optical mixers in the 2-5THz range, the experimental device is simplified and the measurement time is greatly shortened. It is expected to be subsequently applied to near-field terahertz self-detection imaging research. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is a schematic diagram of a flow chart of an embodiment of the present invention;

[0039] Figure 2 is a schematic diagram of a measurement scheme for the first scenario of an implementation manner of the present invention;

[0040] Figure 3is a schematic diagram of a measurement scheme for a second scenario in an implementation manner of the present invention;

[0041] Figure 4 It is a schematic diagram of the principle of an embodiment of the present invention;

[0042] Figure 5 It is a schematic diagram of some results of an implementation mode of the present invention. DETAILED DESCRIPTION

[0043] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall within the scope limited by the appended claims of the application equally.

[0044] The embodiments of the present invention relate to a high-speed extraction method of self-detected terahertz dual-comb spectrum information for near-field imaging, such as Figure 1 As shown, the following steps are included:

[0045] generating a first optical frequency comb using a first laser and coupling the first optical frequency comb to a second laser;

[0046] A second optical frequency comb is generated by using a second laser, and the second optical frequency comb is beat with the received first optical frequency comb by the second laser to generate a self-detection terahertz dual optical comb;

[0047] Selecting the first extraction method or the second extraction method to obtain the spectrum information of the self-detection terahertz dual-comb according to different near-field imaging application scenarios;

[0048] Wherein, the first extraction method comprises the following steps:

[0049] Acquire an AC signal from a self-detecting terahertz dual optical comb and extract one of the comb teeth and down-convert it to a set frequency band to obtain a first processed signal of the comb tooth;

[0050] Collecting time domain information of the first processed signal within a first set time to obtain first time domain information;

[0051] Analyze the first time domain information to obtain comb tooth information of the current comb tooth;

[0052] Acquire the comb tooth information of all the comb teeth of the self-detecting terahertz dual-light comb as near-field imaging information;

[0053] The second extraction method comprises the following steps:

[0054] The first laser is modulated with a set frequency to simulate the modulation of the scattered light by the vibration of the probe in the near-field configuration;

[0055] Acquiring the modulated alternating current signal of the self-detecting terahertz dual-optical comb as a second processing signal;

[0056] Collecting time domain information of the second processed signal within a second set time to obtain second time domain information;

[0057] The second time domain information is analyzed to obtain the sideband information of the self-detected terahertz dual-comb as near-field imaging information.

[0058] The following is a further description of the measurement system and principle of this embodiment. It is worth mentioning that a person skilled in the art can make various changes or modifications to this embodiment, including but not limited to changing the laser type (such as gas laser, semiconductor laser, etc.), changing the optical path design, changing the working band, etc. Such changes will be regarded as an equivalent form of this embodiment and also fall within the scope defined by the claims attached to this application.

[0059] The device used in this embodiment includes:

[0060] 2 quantum cascade lasers, both operating in optical frequency comb mode;

[0061] 2 laser power supplies, which are DC power supplies, are used to power two lasers. Both use high-stability constant current sources to achieve stable current output, and each power supply has a modulation function;

[0062] 1 T-type bias tee can realize the input and output of DC and AC signals respectively. Here, its function is to block the DC component of the RF signal from the microstrip line, leaving only the AC signal for the subsequent signal processing process;

[0063] 1 low noise amplifier, microwave low noise amplifier, the input end is connected to the AC port of the T-type bias device, and the output end is connected to the mixer, which is used to amplify the dual optical comb signal from the T-type bias device. The amplifier provides a signal gain of 30dB, and its bandwidth includes the dual optical comb bandwidth, thereby achieving the overall amplification effect of the dual optical comb signal, meeting the requirements of oscilloscope measurement and spectrum analyzer observation;

[0064] 1 RF source, which can generate RF signals of different powers and frequencies, and mix them with the dual-comb signal in the mixer to down-convert it to the MHz range;

[0065] 1 mixer, whose operating range includes the frequency range of the dual optical comb and the radio frequency signal, and is used to mix the amplified dual optical comb signal with the radio frequency signal and output the difference frequency signal between the two;

[0066] 1 high-speed oscilloscope, used to measure and record the time domain information of the filtered dual-optical comb teeth for further processing;

[0067] 1 spectrum analyzer, used to monitor the dual-comb signal status and ensure the validity of the measurement results.

[0068] With respect to the two technical routes in near-field imaging, this implementation includes the following two scenarios.

[0069] First, when faced with a peak force scanning near-field microscope configuration, it is necessary to obtain the signal changes over a long period of time, such as Figure 2 As shown, the light emitted by laser 1 hits the end face of laser 2, and mixing occurs in the cavity of laser 2 to generate a down-converted RF dual-comb signal. The RF signal is derived from laser 2 through a microstrip line, amplified by an amplifier after passing through a T-type bias device (bias-T), and then mixed with the RF signal emitted by the RF signal source and the signal is down-converted to about 100MHz. The signal is observed by a spectrum analyzer, and its frequency and size are confirmed before being sent to an oscilloscope to record the time domain information. After post-processing of the recorded data, the spectrum information of a single pixel point is obtained.

[0070] Second, when faced with the traditional probe vibration modulation scanning near-field microscope configuration, the measurement process is not time-sensitive and requires measuring the comb sideband information, such as Figure 3 As shown, no need Figure 2 The secondary down-conversion link in the scheme (i.e., without the need for a RF source and a mixer) directly obtains its time domain information at a high sampling rate and performs subsequent processing.

[0071] like Figure 4 As shown in the figure, the short-time Fourier transform is to divide the time domain signal into blocks and then perform the Fourier transform. Each block represents a measurement, and the block size determines the spectrum resolution after the Fourier transform. In order to ensure the accuracy of the measurement, the spectrum resolution cannot be too low, so the single time block cannot be too small. Here, a block size of 140uS is selected to achieve a spectrum resolution of 10kHz.

[0072] The sampling rate of the oscilloscope is directly related to the frequency range of the measurement, and its maximum measurable frequency is half of the sampling rate. The sampling rate of the oscilloscope is also related to the maximum single measurement time. The higher the sampling rate, the shorter the maximum single measurement time; conversely, the single measurement time of the oscilloscope decreases as the sampling rate increases. For the first scenario where continuous measurement is required, a single measurement time as large as possible is required. At this time, the sampling rate of the oscilloscope is low, so the dual-optical comb signal needs to be down-converted. Because the sampling rate of the oscilloscope is low, the comb teeth with higher frequencies in the dual-optical comb signal may not be detected, so each comb signal needs to be measured separately. In this embodiment, the comb teeth of interest in the dual-optical comb signal are down-converted to about 100MHz, so that only a sampling rate of 250M is required to obtain a sampling time of 128mS. Fourier transform is performed on each time domain block to obtain its frequency domain diagram, and the peak value of interest in each block is extracted in the frequency domain as a single measurement result, thereby obtaining a continuous change in the comb signal strength within 128mS. Repeating this process for multiple comb teeth will yield a complete comb spectrum. For the second case, we do not need to know the continuous changes of the signal over a longer time range, but the optical signal is modulated, so what is measured is the intensity of the comb sidebands. In this case, we do not need to down-convert the dual-optical comb signal and directly use a 20G sampling rate to obtain the overall dual-optical comb spectrum. We obtain the complete spectrum for each time domain block, and then we can extract the sideband intensity in each block.

[0073] The present invention is further illustrated by specific data below.

[0074] The lasing spectra of the two terahertz quantum cascade lasers are both 4.1~4.3THz, and the carriers of the laser mixing signals are both 6.9GHz; the working range of the T-type bias device (Bias-T) is DC-18GHz, and the AC port is linked to the microstrip line output port of the laser as a detector, which is used to block the DC signal and transmit the AC signal; the RF source has an output frequency that is 100MHz below the selected comb frequency; the mixer mixes the amplified dual optical comb with the RF signal provided by the RF source, and converts the comb signal of interest down to about 100MHz to obtain a longer measurement time; the high-speed oscilloscope has a maximum sampling rate of 40G.

[0075] For the first case, a high-speed oscilloscope is used to collect and save 128mS time domain data at a sampling rate of 250M. The collected data is then processed on a PC, and the time domain data is subjected to a short-time Fourier transform to analyze the changes in the comb tooth frequency and intensity over time. When performing a short-time Fourier transform, the data is first divided into blocks, and the length of the time block is set to 140uS to obtain a spectral resolution of 10kHz. The data of each time block is then multiplied by a hanning window function to avoid spectrum leakage. The peak value corresponding to the comb tooth is extracted from the Fourier transform result of each block and saved as an array. Each element in the array corresponds to the time starting point of the block, so that the changes in the comb tooth frequency and intensity over time are obtained.

[0076] For the second case, firstly, a 1MHz modulation is added to the QCL as the light source through a current source to simulate the influence of the probe vibration, so that sideband signals with a frequency of 1MHz away from the center frequency of the comb teeth are generated on the left and right sides of each dual-comb comb tooth. Then, the RF source and mixer are removed, and the amplified modulated dual-comb signal is directly fed to the high-speed oscilloscope. A 20G sampling rate is used to measure the high-frequency dual-comb comb signal and its sidebands. The measurement time is 1.6mS, and the measured data is saved. The measured data is processed on the PC, and the short-time Fourier transform is performed using the same settings as the previous single-comb measurement. The sideband peak is found at a frequency position 1MHz away from the peak frequency, thereby obtaining the change of the sideband intensity over time.

[0077] It is worth noting that in the second case, it is not necessary to know the long-term changes in the sideband values, but only to average their changes. Since this method obtains the complete spectrum information in each short-time Fourier transform time block, all comb teeth and their sidebands can be processed and collected at the same time, thus obtaining the spectrum information of the entire dual-light comb spectrum through a single measurement.

[0078] Specific results such as Figure 5 As shown, the information of the selected comb strength changing with time within the 128mS time range is obtained (such as Figure 5 (a)), and the information about the center frequency changing with time (e.g. Figure 5 (b) shows that the time resolution is 140uS. Another method is used to measure the sideband intensity information of the comb teeth. We take one of the comb teeth as an example. Figure 5 (c) is a spectrum diagram containing only a single comb tooth and its sideband in a single time block. By obtaining the sideband peak, the change information of the sideband intensity within 1.6mS is obtained ( Figure 5 (d)), and the time average can be used as the imaging information of the corresponding pixel. The experimental results show that this method can be used for high-speed extraction and processing of terahertz self-detection dual-comb signals, providing support for further realization of terahertz near-field self-detection imaging.

Claims

1. A method for high-speed extraction of self-detected terahertz dual-comb spectrum information for near-field imaging, characterized in that: The following steps are involved: generating a first optical frequency comb using a first laser and coupling the first optical frequency comb to a second laser; Generate a second optical frequency comb using the second laser, and beat the second optical frequency comb with the received first optical frequency comb through the second laser to generate a self-detection terahertz dual optical comb; Selecting the first extraction method or the second extraction method according to different near-field imaging application scenarios to obtain the spectrum information of the self-detection terahertz dual-comb; The first extraction method comprises the following steps: Acquire the AC signal of the self-detected terahertz dual optical comb and extract one of the comb teeth and down-convert it to a set frequency band to obtain a first processed signal of the comb tooth; Collecting time domain information of the first processed signal within a first set time to obtain first time domain information; Analyzing the first time domain information to obtain comb tooth information of the current comb teeth; Acquiring the comb tooth information of all the comb teeth of the self-detection terahertz dual-light comb as near-field imaging information; The second extraction method comprises the following steps: Modulating the first laser with a set frequency to simulate modulation of scattered light by probe vibration in a near-field configuration; Acquiring the modulated alternating current signal of the self-detecting terahertz dual-optical comb as a second processing signal; Collecting time domain information of the second processed signal within a second set time to obtain second time domain information; The second time domain information is analyzed to obtain the sideband information of the self-detected terahertz dual-comb as near-field imaging information.

2. The method according to claim 1, characterized in that: The method of selecting the first extraction method or the second extraction method for different near-field imaging application scenarios to obtain self-detected terahertz dual-light comb spectrum information includes: For the application scenario of peak force scanning near-field imaging, the first extraction method is selected, otherwise the second extraction method is selected to obtain self-detected terahertz dual-comb spectrum information.

3. The method according to claim 1, characterized in that The analyzing the first time domain information to obtain comb tooth information of the current comb tooth includes: Perform a short-time Fourier transform on the first time domain information to obtain first Fourier transform results of multiple time blocks; obtain the peak frequency in each of the first Fourier transform results and the time starting point of the corresponding time block to obtain the frequency change information and intensity change information of the current comb tooth.

4. The method according to claim 1, characterized in that: The method also includes the step of obtaining the comb tooth information of all the comb teeth of the self-detection terahertz dual-light comb to form a comb spectrum.

5. The method according to claim 1, characterized in that The step of analyzing the second time domain information to obtain the sideband information of the self-detected terahertz dual-light comb includes: Perform a short-time Fourier transform on the second time domain information to obtain second Fourier transform results of multiple time blocks; obtain the peak frequency in each of the second Fourier transform results and the time starting point of the corresponding time block, and search for the sideband peak at a position where the peak frequency is the set frequency to obtain the sideband information of the self-detected terahertz dual-light comb.

6. The method according to claim 1, characterized in that The first time domain information and the second time domain information are both collected by a high-speed oscilloscope.

7. The method according to claim 1, characterized in that The AC signal is obtained by using a bias device to block the DC component in the terahertz dual-comb signal.

8. The method according to claim 1, characterized in that Before the step of collecting the time domain information of the first processed signal within the first set time, it also includes a step of amplifying the first processed signal; before the step of collecting the time domain information of the second processed signal within the second set time, it also includes a step of amplifying the second processed signal.

9. The system according to claim 1, characterized in that The first laser and the second laser are both terahertz quantum cascade lasers.

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