Photoelectric terahertz hyperspectral imaging system and method based on swept-frequency dual-comb

Abstract

Disclosed in the present invention are a photoelectric terahertz hyperspectral imaging system and method based on a swept-frequency dual-comb. The system comprises: a swept-frequency laser, a distributed feedback laser, a dual-comb generation module, a coupling beam splitter, a spectrum analyzer, a variable optical attenuator, a terahertz mixer, an off-axis parabolic mirror group, a zero bias detector, an electric amplifier and a signal analyzer, wherein the tunable distributed feedback laser is mixed with electro-optical dual-comb light to generate a swept-frequency terahertz dual-comb signal, point-by-point imaging of an object is realized by means of grid scanning, the swept-frequency dual-comb breaks the trade-off between a spectral bandwidth and a frequency resolution in a dual-comb system, and a frequency resolution of 50 MHz is achieved under a spectral bandwidth ranging from 220 GHz to 320 GHz. Scanning dual-comb terahertz imaging has a higher data collection rate, a more flexible spectral tuning range and a higher frequency resolution at room temperature, and the system is flexible and configurable, and has a simple structure and a high detection efficiency.

Description

A photoelectric terahertz hyperspectral imaging system and method based on swept-frequency dual-comb Technical Field

[0001] The present invention relates to the field of terahertz imaging systems, and in particular to an optoelectronic terahertz hyperspectral imaging system and method based on a frequency-sweeping dual-comb. Background Art

[0002] Terahertz (THz) waves are a form of electromagnetic radiation with frequencies between 0.1 THz and 10 THz. Due to their low attenuation rate when penetrating optically opaque materials and their low energy, non-ionizing properties, THz imaging technology has attracted widespread attention from both the scientific and industrial communities for its potential in nondestructive testing and safety imaging. Furthermore, most molecular bonds, as well as vibrational and rotational energy levels, lie within the THz frequency range. This remarkable phenomenon endows THz spectroscopy with inherent molecular fingerprinting capabilities, enabling the specific identification of resonant peaks and chemical bonds within molecular compounds. Due to the aforementioned unique advantages of THz waves in imaging and spectroscopy, THz hyperspectral imaging has found widespread application in fields such as chemical identification, biomedicine, and molecular dynamics.

[0003] In the development of terahertz hyperspectral imaging systems, the pursuit of higher performance and enhanced stability of imaging systems has always been the focus of the scientific community. Among them, dual-comb hyperspectral imaging (DCHI) technology has attracted much attention due to its advantages such as high frequency stability and fast signal reconstruction speed. In DCHI, an optical frequency comb with very small comb line spacing difference is generated, which enables the emission of THz comb signals and high-frequency precision multi-heterodyne detection. This method does not require any mechanical scanning equipment, thus achieving microsecond acquisition speed and convenience. In recent years, hyperspectral imaging systems based on different THz dual-comb generation technologies have been proven to be feasible, such as architectures based on mode-locked lasers, quantum cascade lasers (QCLs), and photonic THz systems based on electro-optical modulation.

[0004] In DCHI based on mode-locked lasers, two synchronized femtosecond lasers with slightly different repetition rates are used for spectral detection and optical delay sampling. This approach reduces sampling time while maintaining a broadband spectrum. However, mode-locked lasers rely on additional high-precision synchronization control units and adaptive sampling algorithms to address the asynchrony in femtosecond pulses, which incurs additional system overhead.

[0005] The main advantages of QCL dual-comb hyperspectral imaging lie in its compact design and the high optical power, typically tens of μW, of each mode generated by the QCL at high frequencies. However, due to current technological and process limitations, QCLs currently have a limited tuning range and operate at cryogenic temperatures, requiring additional mechanical cooling equipment and reducing long-term stability.

[0006] Photonic terahertz sources based on electro-optical modulation offer unique advantages in the field of hyperspectral imaging. Electro-optical modulation provides absolute frequency precision and highly stable optical combs. The use of a single laser to generate a dual-frequency comb provides high coherence for down-converting the dual-comb signals to the radio frequency region. Notably, EO-DCHI allows for flexible adjustment of the terahertz frequency and comb spacing, making it easy to increase spectral coverage or comb precision without additional design modifications. However, in previous dual-comb systems, due to limitations in device power and generation methods for the terahertz dual combs, there is an inverse relationship between spectral bandwidth and frequency resolution, resulting in a trade-off between wide-spectrum imaging and high-precision spectroscopy.

[0007] Summary of the Invention

[0008] In light of the foregoing, the present invention aims to provide an electro-optical terahertz hyperspectral imaging system based on a swept dual-comb optical system. This system utilizes a tunable DFB laser and an electro-optical dual-comb optical hybrid to generate a swept dual-comb terahertz signal. This system then uses raster scanning to achieve point-by-point imaging of an object. This groundbreaking approach overcomes the trade-off between spectral bandwidth and frequency resolution in conventional dual-comb systems, achieving a frequency resolution of 50 MHz within a spectral bandwidth of 220-320 GHz. This system offers faster data acquisition rates, a more flexible spectral tuning range, and higher frequency resolution at room temperature. The system is flexible and configurable, with a simple structure and high detection efficiency.

[0009] To achieve the above-mentioned purpose, the present invention provides an optoelectronic terahertz hyperspectral imaging system based on a swept dual-comb optical system, comprising: a swept laser, a distributed feedback laser, a dual-comb optical generation module, a coupled beam splitter, a spectrum analyzer, an adjustable optical attenuator, a terahertz mixer, an off-axis parabolic mirror assembly, a zero-bias detector, an electrical amplifier, and a signal analyzer.

[0010] The continuous wave laser signal emitted by the frequency-sweeping laser is injected into the dual-comb generation module to generate a dual-comb signal;

[0011] The tunable laser signal emitted by the distributed feedback laser and the dual-comb signal are coupled in a coupling beam splitter to form two output signals. One output signal is subjected to spectral analysis by a spectrum analyzer, and the other output signal is output to a terahertz mixer after the optical signal power is controlled by an adjustable optical attenuator. In the terahertz mixer, photoelectric mixing is performed to form a terahertz dual-comb signal of optical mixing, which is then input to an off-axis parabolic mirror assembly.

[0012] The first half of the off-axis parabolic mirror assembly collimates and focuses the input terahertz dual-comb signal onto the sample, and the second half of the mirror assembly collimates and focuses the sample-modulated signal onto a zero-bias detector.

[0013] The zero-bias detector mixes the input terahertz dual-comb signal into an intermediate frequency dual-comb signal, which is then amplified by an electrical amplifier and then sampled and recorded by a signal analyzer.

[0014] Preferably, the dual optical comb generation module includes a beam splitter, a first phase modulator, a second phase modulator, a first radio frequency source, a second radio frequency source, a first tunable optical filter, a second tunable optical filter, a coupler, and an erbium-doped fiber amplifier;

[0015] The continuous wave laser signal emitted by the swept laser is split into two beams by a beam splitter. The two beams are modulated by a first phase modulator driven by a first RF source and a second phase modulator driven by a second RF source to generate two coherent optical frequency comb signals. The two coherent optical frequency comb signals are filtered out by a first tunable optical filter and a second tunable optical filter to obtain the positive 1st to 5th order optical combs, and then coupled and amplified by a coupler and an erbium-doped fiber amplifier to output dual optical comb signals. The RF frequencies of the first and second RF sources differ, with the difference ranging from 100 to 500 kHz.

[0016] Preferably, the off-axis parabolic mirror group includes a first off-axis parabolic mirror, a second off-axis parabolic mirror, a third off-axis parabolic mirror, and a fourth off-axis parabolic mirror arranged in sequence, wherein the first off-axis parabolic mirror and the second off-axis parabolic mirror constitute the front half of the mirror group, and the third off-axis parabolic mirror and the fourth off-axis parabolic mirror constitute the back half of the mirror group.

[0017] Preferably, the parent focal length of each off-axis parabolic mirror in the off-axis parabolic mirror group is 50.8 mm, the reflection effective focal length and the center offset are both 101.6 mm, and the device diameter is 50.8 mm.

[0018] Preferably, the frequency sweep laser is a narrow linewidth external cavity laser with a frequency sweep function;

[0019] The distributed feedback laser has a tunable function.

[0020] Preferably, the terahertz mixer is a single-row carrier photodiode.

[0021] Preferably, the coupling beam splitter is a 1:1 power coupling beam splitter.

[0022] Preferably, the spectrum analyzer is a spectrum analyzer with a maximum resolution of 150 MHz;

[0023] Preferably, the electrical amplifier is a 20dB gain amplifier.

[0024] To achieve the above-mentioned object of the invention, an embodiment of the present invention further provides a photoelectric terahertz hyperspectral imaging method, which uses the above-mentioned photoelectric terahertz hyperspectral imaging system and includes the following steps:

[0025] The sample is placed in an optoelectronic terahertz hyperspectral imaging system, and a raster scanning mechanical stage is used to move the sample along the x and y directions to obtain dual-comb spectra of the sample at different positions.

[0026] Based on dual-comb spectral analysis, the peak beat note power array corresponding to each pixel is obtained. Each peak beat note power array includes multiple 5th-order RF comb spectra, thereby realizing optoelectronic terahertz hyperspectral imaging.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] (1) This paper is the first attempt to extend terahertz dual-comb technology to a swept-frequency mode, achieving a frequency resolution of up to 50 MHz within a bandwidth of 100 GHz and applying it to the field of terahertz hyperspectral imaging. This breakthrough may stimulate further research on higher precision and tuning freedom in terahertz imaging, potentially promoting the technology towards industrial applications.

[0029] (2) This invention breaks through the inverse relationship between spectral bandwidth and frequency resolution in dual-comb spectroscopy systems. By adjusting the modulation frequency of the swept laser and the scanning accuracy of the tunable distributed feedback laser, it is easy to simultaneously improve the spectral bandwidth and frequency resolution, providing a new direction for the research of broadband precision spectroscopy and hyperspectral imaging.

[0030] (3) The present invention can reduce system costs. The distributed feedback laser used in the present invention can be mass-produced at a low cost, and the system architecture is simpler, which is conducive to its application in various fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0032] FIG1 is a schematic structural diagram of a photoelectric terahertz hyperspectral imaging system provided in an embodiment of the present invention;

[0033] FIG2 is a schematic structural diagram of a dual optical comb generation module provided in an embodiment of the present invention;

[0034] FIG3 is a test diagram of a dual optical comb signal provided by an embodiment of the present invention;

[0035] FIG4 is an imaging effect diagram provided by an embodiment of the present invention;

[0036] Among them, 1- swept laser, 2- distributed feedback laser, 3- dual optical comb generation module, 4- coupled beam splitter, 5- spectrum analyzer, 6- adjustable optical attenuator, 7- terahertz mixer, 8- first off-axis parabolic mirror, 9- second off-axis parabolic mirror, 10- sample, 11- third off-axis parabolic mirror, 12- fourth off-axis parabolic mirror, 13- zero bias detector, 14- electrical amplifier, 15- signal analyzer, 31- beam splitter, 32- first phase modulator, 33- second phase modulator, 34- first RF source, 35- second RF source, 36- first tunable optical filter, 37- second tunable optical filter, 38- coupler, 39- erbium-doped fiber amplifier. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and do not limit the scope of protection of the present invention.

[0038] The technical concept of this invention is as follows: The present invention provides, for the first time, an optoelectronic terahertz hyperspectral imaging system based on a swept dual-comb, utilizing electro-optical modulation and a tunable distributed feedback laser. This system not only offers a high degree of dual-comb tuning flexibility, but also improves spectral coverage and frequency accuracy through the swept laser. This system strikes a balance between spectral bandwidth and frequency resolution, achieving 50 MHz spectral accuracy and over 100 GHz spectral coverage near a central wavelength of 270 GHz. This limitation is primarily due to the bandwidth of the zero-bias detector, which can be improved by using a higher-frequency, wider-bandwidth ZBD.

[0039] Based on the above technical concept, as shown in Figure 1, an embodiment of the present invention provides an optoelectronic terahertz hyperspectral imaging system including a swept laser 1, a distributed feedback laser 2, a dual-comb generation module 3, a coupled beam splitter 4, a spectrum analyzer 5, an adjustable optical attenuator 6, a terahertz mixer 7, an off-axis parabolic mirror group, a zero-bias detector 13, an electrical amplifier 14, and a signal analyzer 15, wherein the off-axis parabolic mirror group includes a first off-axis parabolic mirror 8, a second off-axis parabolic mirror 9, a third off-axis parabolic mirror 11, and a fourth off-axis parabolic mirror 12.

[0040] The swept laser 1 can use an external cavity laser (ECL). By introducing optical feedback elements such as diffraction gratings, the ECL filters the laser's output frequency and simultaneously narrows the linewidth from MHz to kHz. The continuous wave (CW) optical signal emitted from the ECL is injected into the dual comb generator (DCG) 3 to generate a dual comb signal.

[0041] As shown in Figure 2, the dual-comb generation module 3 includes a beam splitter (OS) 31, a first phase modulator (PM1) 32, a second phase modulator (PM2) 33, a first RF source (RF1) 34, a second RF source (RF2) 35, a first tunable optical filter (OBPF1) 36, a second tunable optical filter (OBPF2) 37, a coupler (OC) 38, and an erbium-doped fiber amplifier (EDFA) 39. In the dual-comb generation module 3, the continuous-wave laser signal emitted by the swept laser 1 is split into two beams by the beam splitter 31. The two beams are then input into the first phase modulator 32 and the second phase modulator 33, respectively, to generate two coherent optical frequency comb signals. The frequency spacing of the optical frequency comb signals is determined by the RF source. The first phase modulator 32 is driven by a 20 GHz RF signal emitted by a first RF source 34, while the second phase modulator 33 is driven by a 19.9999 GHz RF signal emitted by a second RF source 35. Both RF signals are generated by the RF source and amplified by an electronic amplifier with a 38 dB gain. The two coherent optical frequency comb signals are filtered out by a first tunable optical filter 36 and a second tunable optical filter 37, respectively, to remove the first to fifth harmonics. The two combs are then coupled together via a coupler 38 to form a dual-comb signal.

[0042] The frequency spacing of the optical comb teeth in the dual-comb signal is equal to the frequency of the RF signal. Since the frequencies of the two RF signals differ slightly, the optical combs generated by the first phase modulator 32 and the second phase modulator 33 also have the same frequency difference. The electro-optical modulation-based approach provides absolute comb frequency accuracy and tunable comb spacing, representing an ideal architecture for high-spectral-resolution imaging. The dual-comb signal is then amplified by an erbium-doped fiber amplifier (EDFA) 39 and coupled to another continuous laser signal emitted by a distributed feedback laser (DFB) 2 via a coupling beam splitter 4.

[0043] The coupled beam splitter 4 couples and splits the beams in a 1:1 power ratio. One output signal is fed through a variable optical attenuator (VOA) to control the optical signal power before being fed to a terahertz mixer 7 for photoelectric mixing, generating a terahertz dual-comb signal. The frequency of the dual-comb signal is determined by the difference frequency between the swept laser 1 and the distributed feedback laser 2. The terahertz mixer 7 utilizes a single-line carrier photodiode (UTC-PD). The other output signal is analyzed by an optical spectrum analyzer 5 with a maximum resolution of 150 MHz. Figure 3(a) shows the resulting dual-comb spectrum.

[0044] In the dual-comb spectrum, the terahertz dual-comb signal generated by electro-optical modulation and the continuous-wave laser signal generated by the distributed feedback laser 2 are clearly visible. However, due to the limitation of the resolution (150 MHz) of the spectrum analyzer 5, the two optical combs with a frequency difference of 0.1 MHz cannot be distinguished in the spectrum. In the specific experiment, power matching was performed between the terahertz dual-comb signal and the continuous-wave laser signal to ensure the maximum photoelectric mixing efficiency of the UTC-PD. The frequency of the terahertz dual-comb signal emitted by the UTC-PD is equal to the frequency difference between the dual-comb signal and the continuous-wave laser signal output by the swept-frequency laser 1. Since the wavelength of the distributed feedback laser 2 can be flexibly adjusted with MHz accuracy by the temperature electronic control unit, the frequency of the terahertz signal can cover a wide spectrum range of 800 GHz.

[0045] After being emitted from the UTC-PD, the terahertz dual-comb signal is collimated by the first off-axis parabolic mirror (OPM1) 8 and focused onto the sample 10 by the second off-axis parabolic mirror (OPM2) 9. When the terahertz dual-comb signal is transmitted to the sample 10, it undergoes amplitude and phase changes caused by the sample 10. It is then realigned by the third off-axis parabolic mirror (OPM3) 11. The fourth off-axis parabolic mirror (OPM3) 12 then refocuses the terahertz dual-comb signal onto the zero-bias detector (ZBD) 13, enabling square-law detection. In the ZBD 13, the identical harmonics of the two terahertz comb signals interfere with each other and are ultimately down-converted to an intermediate-frequency dual-comb signal. The intermediate-frequency dual-comb signal is amplified by the electrical amplifier (EA) 14, and the dual-comb spectrum is sampled and recorded by the signal analyzer (FA) 15, as shown in Figure 3(b). The electrical amplifier can be a 20dB gain amplifier.

[0046] It should be noted that the parent focal length of each off-axis parabolic mirror in the off-axis parabolic mirror group is 50.8 mm, the effective reflection focal length and the center offset are both 101.6 mm, and the device diameter is 50.8 mm. The present invention does not limit the parameter selection of the off-axis parabolic mirror. The main reason for using this parameter for configuration is to ensure sufficient space for placing the attenuated total reflection prism 11 and to achieve a smaller terahertz spot diameter.

[0047] The present embodiment also provides a photoelectric terahertz hyperspectral imaging method utilizing the above-mentioned photoelectric terahertz hyperspectral imaging system. During imaging, a sample 10 is moved in the x and y directions using a raster scanning mechanical stage. Dual-comb spectra are obtained at different positions of the sample 10. Based on dual-comb spectral analysis, the peak beat note power array corresponding to each image is obtained and recorded as a DFB laser scan with a custom-set frequency interval of 2 GHz, typically between 0.1 and 10 GHz. Within the context of the above-mentioned imaging steps, a THz hyperspectral image cube containing 50 frequency points is acquired and normalized and denoised using a Gaussian filter. The number of pixels in the image depends on the resolution of the raster scan and the size of the sample. Specifically, imaging tests were conducted on a 4.5 × 5.5 cm standard resolution test plate manufactured in 1951, with the results shown in Figure 4. These results demonstrate that S-DCHI offers a promising avenue for the future application of terahertz hyperspectral imaging systems in security screening, chemical testing, and biomedical research.

[0048] In summary, a tunable distributed feedback laser is combined with an electro-optical dual-comb to generate a swept terahertz dual-comb signal, which is then used to image an object point by point through raster scanning. This sweeping dual-comb breaks the trade-off between spectral bandwidth and frequency resolution in dual-comb systems, achieving a frequency resolution of 50 MHz within a spectral bandwidth of 220-320 GHz. Scanning dual-comb terahertz imaging offers faster data acquisition rates, a more flexible spectral tuning range, and higher frequency resolution at room temperature. The system is flexible and configurable, with a simple structure and high detection efficiency.

[0049] The specific implementation methods described above provide a detailed description of the technical solutions and beneficial effects of the present invention. It should be understood that the above is only the most preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, supplements and equivalent substitutions made within the scope of the principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. An optoelectronic terahertz hyperspectral imaging system based on a swept frequency dual optical comb, characterized in that: include: Sweep laser, distributed feedback laser, dual optical comb generation module, coupled beam splitter, spectrum analyzer, adjustable optical attenuator, terahertz mixer, off-axis parabolic mirror set, zero bias detector, electrical amplifier, signal analyzer, The continuous wave laser signal emitted by the frequency sweeping laser is injected into the dual optical comb generation module to generate a dual optical comb signal; The tunable laser signal emitted by the distributed feedback laser and the dual optical comb signal are coupled in a coupling beam splitter to form two output signals, one output signal is subjected to spectrum analysis by a spectrum analyzer, and the other output signal is output to a terahertz mixer after the optical signal power is controlled by an adjustable optical attenuator, where optical-electrical mixing is performed to form a terahertz dual-comb signal of optical mixing, and then input to an off-axis parabolic mirror group; The first half of the off-axis parabolic mirror group collimates and focuses the input terahertz dual-comb signal onto the sample, and the second half of the mirror group collimates and focuses the sample modulated signal onto the zero-bias detector; The zero-bias detector mixes the input terahertz dual-comb signal into an intermediate frequency dual-comb signal, and then amplifies the power through an electrical amplifier, and then the dual-comb spectrum is sampled and recorded by a signal analyzer.

2. The optoelectronic terahertz hyperspectral imaging system based on swept frequency dual optical comb according to claim 1, characterized in that: The dual optical comb generation module includes a beam splitter, a first phase modulator, a second phase modulator, a first radio frequency source, a second radio frequency source, a first tunable optical filter, a second tunable optical filter, a coupler, and an erbium-doped fiber amplifier; The continuous wave laser signal emitted by the frequency sweeping laser is divided into two beams by a beam splitter, and then modulated by a first phase modulator driven by a first radio frequency source and a second phase modulator driven by a second radio frequency source to generate two coherent optical frequency comb signals. The two coherent optical frequency comb signals are filtered out by a first tunable optical filter and a second tunable optical filter to obtain positive 1st to 5th order optical combs, and then coupled and amplified by a coupler and an erbium-doped fiber amplifier to output dual optical comb signals. The radio frequency frequencies of the first radio frequency source and the second radio frequency source are different, and the difference ranges from 100 to 500 kHz.

3. The optoelectronic terahertz hyperspectral imaging system based on swept frequency dual optical comb according to claim 1, characterized in that: The off-axis parabolic mirror group includes a first off-axis parabolic mirror, a second off-axis parabolic mirror, a third off-axis parabolic mirror, and a fourth off-axis parabolic mirror arranged in sequence, wherein the first off-axis parabolic mirror and the second off-axis parabolic mirror form a front half of the mirror group, and the third off-axis parabolic mirror and the fourth off-axis parabolic mirror form a rear half of the mirror group.

4. The optoelectronic terahertz hyperspectral imaging system based on swept frequency dual optical comb according to claim 3, characterized in that: The parent focal length of each off-axis parabolic mirror in the off-axis parabolic mirror group is 50.8 mm, the reflection effective focal length and the center offset are both 101.6 mm, and the device diameter is 50.8 mm.

5. The optoelectronic terahertz hyperspectral imaging system based on swept frequency dual optical comb according to claim 1, characterized in that: The frequency sweeping laser is a narrow line width external cavity laser with frequency sweeping function; The distributed feedback laser has a tunable function.

6. The optoelectronic terahertz hyperspectral imaging system based on swept frequency dual optical comb according to claim 1, characterized in that: The terahertz mixer is a single-row carrier photodiode.

7. The optoelectronic terahertz hyperspectral imaging system based on swept frequency dual optical comb according to claim 1, characterized in that: The coupling beam splitter is a coupling beam splitter with a power of 1:

1.

8. The optoelectronic terahertz hyperspectral imaging system based on swept frequency dual optical comb according to claim 1, characterized in that: The spectrum analyzer has a maximum resolution of 150 MHz.

9. The optoelectronic terahertz hyperspectral imaging system based on swept frequency dual optical comb according to claim 1, characterized in that: The electrical amplifier is a 20dB gain amplifier.

10. A photoelectric terahertz hyperspectral imaging method, characterized in that: Using the photoelectric terahertz hyperspectral imaging system according to any one of claims 1 to 9 comprises the following steps: The sample is placed in an optoelectronic terahertz hyperspectral imaging system, and a grating scanning mechanical stage is used to move the sample along the x and y directions to obtain a swept frequency dual-comb spectrum of the sample at different positions. Based on dual-comb spectral analysis, the peak beat note power array corresponding to each pixel is obtained. Each peak beat note power array includes multiple 5th-order RF comb spectra, thereby realizing optoelectronic terahertz hyperspectral imaging.

Images