JP7745938B2 - Optoelectronic terahertz hyperspectral imaging system and method based on frequency-swept dual optical comb - Google Patents

Description

The present invention relates to the field of terahertz imaging systems, and more particularly to an optoelectronic terahertz hyperspectral imaging system and method based on a frequency-swept dual optical comb.

Terahertz (THz) waves are a type of electromagnetic radiation with frequencies between 0.1 THz and 10 THz. Due to their low attenuation rate when passing through optically opaque materials, low energy, and non-ionizing properties, THz imaging technology has attracted widespread attention from the scientific and industrial communities for its potential in nondestructive testing and security imaging. Furthermore, most molecular bonds and vibrational and rotational energy levels are located in the terahertz frequency range. This remarkable phenomenon endows terahertz spectra with unique molecular fingerprinting capabilities, which can specifically recognize the resonance peaks and chemical bonds of molecular compounds. Due to the unique advantages of terahertz wave imaging and spectroscopy, terahertz hyperspectral imaging has been widely used in fields such as chemical recognition, biomedicine, and molecular dynamics.

In the development of terahertz hyperspectral imaging systems, the pursuit of higher performance and improved stability has always been a focus of the scientific community. Among these, dual-comb hyperspectral imaging (DCHI) technology has attracted attention due to its advantages, such as high frequency stability and fast signal reconstruction speed. DCHI generates an optical frequency comb with a small spacing difference between the comb lines, enabling the transmission of THz comb signals and multi-heterodyne detection with high frequency precision. This method eliminates the need for any mechanical scanning devices and achieves microsecond-level acquisition speed and convenience. In recent years, the feasibility of hyperspectral imaging systems based on different THz dual-comb generation technologies has been proven, including mode-locked lasers, quantum cascade laser (QCL)-based architectures, and photon THz systems based on electro-optic modulation.

DCHI, based on a mode-locked laser, uses two synchronized femtosecond lasers with slightly different repetition rates to perform spectral detection and optical delay sampling. This method shortens the sampling time while maintaining a broadband spectrum. However, mode-locked lasers rely on additional high-precision synchronization control and self-adaptive sampling algorithms to solve the asynchrony problem of femtosecond pulses, resulting in additional system overhead.

The main advantages of QCL dual-comb hyperspectral imaging are: they have a compact design and operate at high frequencies; each mode generated by a QCL typically has high optical power, often reaching tens of μW. However, due to current technology and process limitations, QCLs have a limited tuning range and operate at low temperatures, requiring additional mechanical cooling equipment and reducing long-term stability.

Photon terahertz sources based on electro-optic modulation offer unique advantages in the field of hyperspectral imaging. Electro-optic modulation enables absolute frequency accuracy and high stability of the optical comb. A solution that generates a dual frequency comb using a single laser provides high coherence for downconverting the dual comb signal to the radio frequency domain. Furthermore, EO-DCHI allows for flexible adjustment of the terahertz frequency and comb tooth spacing, making it easy to improve spectral coverage and comb tooth accuracy without additional design changes. However, in previous dual comb systems, limitations in the device power and generation method of terahertz dual combs resulted in an inverse relationship between spectral bandwidth and frequency resolution, resulting in a trade-off between broad-spectrum imaging and high-precision spectrum.

In view of the above, the objective of the present invention is to provide an optoelectronic terahertz hyperspectral imaging system based on a frequency-swept dual optical comb. This system optically mixes a tunable DFB laser with an electro-optical dual comb to generate a frequency-swept terahertz dual comb signal, enabling point-by-point imaging of an object through raster scanning. This pioneering method breaks the trade-off between spectral bandwidth and frequency resolution in conventional dual comb systems, achieving a frequency resolution of 50 MHz with a spectral bandwidth of 220-320 GHz. Scanning dual comb terahertz imaging offers faster data acquisition speeds at room temperature, a more flexible spectral tuning range, and higher frequency resolution. The system is flexible and configurable, has a simple structure, and high detection efficiency.

In order to achieve the above object of the invention, an embodiment of the present invention provides an optoelectronic terahertz hyperspectral imaging system based on a frequency-swept dual optical comb, the system including: a frequency-swept laser, a distributed feedback laser, a dual optical comb generation module, a coupling beam splitter, a spectrum analyzer, an adjustable optical attenuator, a terahertz mixer, an off-axis parabolic mirror group, a zero-bias detector, a power amplifier, and a signal analyzer;
The continuous wave laser signal emitted by the frequency swept laser is injected into a dual optical comb generation module to generate a dual optical comb signal;
The tunable laser signal and the dual optical comb signal emitted from the distributed feedback laser are combined by a coupling beam splitter to form two output signals, one of which is subjected to spectrum analysis through a spectrum analyzer, and the other is subjected to optical signal power control through an adjustable optical attenuator, and then output to a terahertz mixer, which mixes light and electricity to generate an optically mixed terahertz dual comb signal, which is input to an off-axis parabolic mirror group;
The first half of the off-axis parabolic mirror group collimates the input terahertz dual comb signal and focuses it on the sample, and the second half of the mirror group collimates the signal modulated by the sample and focuses it on the zero-bias detector;
The zero-bias detector mixes the input terahertz dual comb signal into an intermediate frequency dual comb signal, amplifies the power using a power amplifier, and then samples and records the dual comb spectrum using a signal analyzer.

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 optical fiber amplifier;
The continuous wave laser signal emitted by the frequency swept laser is split into two beams through 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, respectively, to generate two coherent optical frequency comb signals. The two coherent optical frequency comb signals pass through a first tunable optical filter and a second tunable optical filter, respectively, to filter out the positive first to fifth order optical combs, and then are combined and amplified by a coupler and an erbium-doped optical fiber amplifier to output a dual optical comb signal, and the first radio frequency source and the second radio frequency source have a radio frequency difference, which ranges from 100 to 500 kHz.

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 order, the first off-axis parabolic mirror and the second off-axis parabolic mirror constituting the first half of the mirror group, and the third off-axis parabolic mirror and the fourth off-axis parabolic mirror constituting the second half of the mirror group.

Preferably, the parent focal length of each off-axis parabolic mirror in the group of off-axis parabolic mirrors is 50.8 mm, the reflective effective focal length and center offset are both 101.6 mm, and the element diameter is 50.8 mm.

Preferably, the frequency swept laser is a narrow linewidth external cavity laser with frequency sweep capability;
The distributed feedback laser is tunable.

Preferably, the terahertz mixer is a uni-traveling carrier photodiode.

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

Preferably, the spectrum analyzer has a resolution of up to 150 MHz.

Preferably, the power amplifier is a 20 dB gain amplifier.

In order to achieve the above object of the invention, an embodiment of the present invention further provides an optoelectronic terahertz hyperspectral imaging method using the above optoelectronic terahertz hyperspectral imaging system, the method including:
placing the sample in an optoelectronic terahertz hyperspectral imaging system and using a mechanical raster scanning stage to move the sample along x and y directions to acquire dual comb spectra of the sample at different positions;
and obtaining a beat peak power sequence corresponding to each pixel and including multiple fifth-order radio frequency comb spectra based on the dual comb spectrum analysis, thereby realizing optoelectronic terahertz hyperspectral imaging.

Compared with the prior art, the beneficial effects of the present invention include at least the following:
(1) This invention extends terahertz dual-comb technology to a frequency sweep mode, achieving a frequency resolution of 50 MHz within a 100 GHz bandwidth, and applying it to the field of terahertz hyperspectral imaging for the first time. This breakthrough may encourage further research to improve the precision and tuning flexibility of terahertz imaging, potentially propelling this technology toward industrial applications.
(2) This invention breaks the inverse relationship between spectral bandwidth and frequency resolution in dual-comb spectrum systems. By adjusting the modulation frequency of the frequency-swept laser and the scanning accuracy of the tunable distributed feedback laser, it is easy to simultaneously improve spectral bandwidth and frequency resolution, bringing new directions to the research of broadband high-precision spectroscopy and hyperspectral imaging.
(3) The present invention can reduce system costs. The distributed feedback laser used in the present invention can be batch-produced, is low-cost, and has a simpler system architecture, which contributes to its widespread use and application in various fields.

In order to more clearly explain the technical solutions in the embodiments of the present invention or the prior art, the drawings that need to be used in the description of the embodiments or the prior art are briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention, and those skilled in the art can derive other drawings based on these drawings without any creative efforts.

FIG. 1 is a schematic structural diagram of an optoelectronic terahertz hyperspectral imaging system provided by an embodiment of the present invention. FIG. 2 is a schematic structural diagram of a dual optical comb generation module provided by an embodiment of the present invention. FIG. 3 is a test chart of a dual optical comb signal provided by an embodiment of the present invention. FIG. 4 is an imaging effect diagram provided by an embodiment of the present invention.

1. Frequency-swept laser, 2. Distributed feedback laser, 3. Dual optical comb generation module, 4. Coupling 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. Power amplifier, 15. Signal analyzer, 31. Beam splitter, 32. First phase modulator, 33. Second phase modulator, 34. First radio frequency source, 35. Second radio frequency source, 36. First tunable optical filter, 37. Second tunable optical filter, 38. Coupler, 39. Erbium-doped fiber amplifier.

In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be described in more detail below with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are only used to illustrate the present invention and are not intended to limit the scope of protection of the present invention.

The technical concept of the present invention is as follows: An embodiment of the present invention provides, for the first time, an optoelectronic terahertz hyperspectral imaging system based on a frequency-swept dual optical comb, realized using electro-optic modulation and a tunable distributed feedback laser. The system not only achieves high flexibility in dual-comb tuning, but also further improves spectral coverage and frequency accuracy through the use of frequency-swept lasers. The system breaks the trade-off between spectral bandwidth and frequency resolution, achieving 50 MHz spectral accuracy around a center wavelength of 270 GHz and spectral coverage exceeding 100 GHz, which is primarily limited by the bandwidth of the zero-bias detector and can be improved by using a higher-frequency, wider-bandwidth ZBD.

Based on the above technical concept, as shown in FIG. 1, an optoelectronic terahertz hyperspectral imaging system provided by an embodiment of the present invention includes a frequency-swept laser 1, a distributed feedback laser 2, a dual optical comb generation module 3, a coupling 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, a power amplifier 14, and a signal analyzer 15. 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.

The frequency-swept laser 1 can be an external cavity laser (ECL). By incorporating an optical feedback element such as a diffraction grating, the external cavity laser (ECL) can select the laser's output frequency and narrow the linewidth from MHz to kHz. The continuous wave (CW) optical signal emitted by the external cavity laser is injected into a dual optical comb generation module (DCG) 3 to generate a dual optical comb signal.

2, the dual optical comb generation module 3 includes a beam splitter (OS) 31, a first phase modulator (PM1) 32, a second phase modulator (PM2) 33, a first radio frequency source (RF1) 34, a second radio frequency 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 optical fiber amplifier (EDFA) 39. In the dual optical comb generation module 3, the continuous-wave laser signal emitted by the frequency-swept laser 1 is split into two beams through the beam splitter 31, which are then input to the first phase modulator 32 and the second phase modulator 33, respectively, to generate two coherent optical frequency comb signals, the frequency spacing of which is determined by the radio frequency source. The first phase modulator 32 is driven by a 20 GHz radio frequency signal emitted by a first radio frequency source 34, and the second phase modulator 33 is driven by a 19.9999 GHz radio frequency signal emitted by a second radio frequency source 35. Both RF signals are generated by the radio frequency source and amplified by a power amplifier with 38 dB gain. The two coherent optical frequency comb signals pass through a first tunable optical filter 36 and a second tunable optical filter 37, respectively, to filter out the positive first through fifth harmonics, after which the two combs are combined together by a coupler 38 to form a dual optical comb signal.

The frequency spacing of the optical comb teeth in the dual optical comb signal is equal to the frequency of the radio frequency signal. Because there is a small difference between the frequencies of the two radio frequency signals, there is also the same frequency difference between the optical combs generated by the first phase modulator 32 and the second phase modulator 33. The electro-optic modulation-based method achieves absolute frequency precision and tunable comb tooth spacing, representing an ideal architecture for hyperspectral resolution imaging. The dual optical comb signal is then amplified by an erbium-doped fiber amplifier (EDFA) 39 and combined with another continuous laser signal emitted by a distributed feedback laser (DFB) 2 via a coupling beam splitter 4.

The coupling beam splitter 4 combines and splits the power at a 1:1 ratio. A portion of the split output signal is output to the terahertz mixer 7, where the optical signal power is controlled by a variable optical attenuator (VOA), and the output signal is mixed optically and electrically to produce a terahertz dual comb signal. The frequency of the terahertz dual comb signal is determined by the difference frequency between the frequency-swept laser 1 and the distributed feedback laser 2. The terahertz mixer 7 uses a uni-traveling-carrier photodiode (UTC-PD). The remaining portion of the output signal is spectrally analyzed by the spectrum analyzer 5, specifically, a spectrum analyzer with a maximum resolution of 150 MHz. Figure 3(a) shows the analyzed dual comb spectrum.

The dual-comb spectrum clearly shows the terahertz dual-comb signal generated by electro-optic modulation and the continuous-wave laser signal generated by the distributed feedback laser 2. However, due to the limited resolution (150 MHz) of the spectrum analyzer 5, it is impossible to distinguish the two optical combs, which have a frequency difference of 0.1 MHz, in the spectrum. During the specific experiment, power matching was performed between the terahertz dual-comb signal and the continuous-wave laser signal to ensure 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 from the distributed feedback laser 2. The wavelength of the distributed feedback laser 2 can be flexibly adjusted with MHz accuracy using a temperature electronic control unit, allowing the terahertz signal frequency to cover a wide spectral range of 800 GHz.

After being emitted from the UTC-PD, the terahertz dual comb signal is collimated by a first off-axis parabolic reflector (OPM1) 8 and focused onto a sample 10 by a second off-axis parabolic reflector (OPM2) 9. When the terahertz dual comb signal is transmitted to the sample 10, it experiences amplitude and phase changes induced by the sample 10. It is then recollimated by a third off-axis parabolic reflector (OPM3) 11 and subsequently refocused by a fourth off-axis parabolic reflector (OPM3) 12 onto a zero-bias detector (ZBD) 13, achieving square-law detection. In the zero-bias detector 13, the same harmonics of the two terahertz comb signals interfere with each other and are ultimately downconverted to an intermediate-frequency dual comb signal. As shown in Figure 3(b), the intermediate frequency dual comb signal is amplified by a power amplifier (EA) 14, and then the dual comb spectrum is sampled and recorded by a signal analyzer (FA) 15. The power amplifier may be a 20 dB gain amplifier.

The parent focal length of each off-axis parabolic mirror in the off-axis parabolic mirror group is 50.8 mm, the reflective effective focal length and center offset are both 101.6 mm, and the element diameter is 50.8 mm. The present invention does not limit the parameter selection of the off-axis parabolic mirror, and the main reason for using this parameter in the configuration is to ensure enough space to arrange an attenuated total reflection prism and achieve a smaller terahertz spot diameter.

This example further provides a photoelectron terahertz hyperspectral imaging method using the above-described photoelectron terahertz hyperspectral imaging system. In specific imaging, a mechanical raster-scanning worktable is used to move the sample 10 along the x and y directions to acquire dual-comb spectra of the sample 10 at different positions. Based on the dual-comb spectral analysis, a beat peak power array corresponding to each pixel is obtained and recorded as a 2 GHz DFB laser scan with a self-configured frequency interval. The frequency interval can be set between 0.1 and 10 GHz. In connection with the above imaging step, a THz hyperspectral image cube containing 50 frequency points is acquired, and normalization and noise removal are performed using a Gaussian filter. The number of pixels in the image depends on the raster-scan resolution and the size of the sample. Specifically, an imaging test was conducted on a 4.5 x 5.5 cm standard resolution test chart made in 1951. The results are shown in Figure 4. As can be seen from the results, S-DCHI provides a promising approach for future applications of terahertz hyperspectral imaging systems in security screening, chemical substance detection, and biomedical research.

As described above, a tunable distributed feedback laser is optically mixed with an electro-optical dual comb to generate a frequency-swept terahertz dual comb signal, enabling point-by-point imaging of an object through raster scanning. The frequency-swept dual comb breaks the trade-off between spectral bandwidth and frequency resolution in dual comb systems, achieving a frequency resolution of 50 MHz over a spectral bandwidth of 220-320 GHz. Scanning dual comb terahertz imaging offers faster data acquisition speeds at room temperature, a more flexible spectral tuning range, and higher frequency resolution. The system is flexible and configurable, has a simple structure, and high detection efficiency.

The above-mentioned specific embodiments illustrate in detail the technical solutions and beneficial effects of the present invention, but the above are merely the most preferred embodiments of the present invention and do not limit the present invention. Any modifications, additions, equivalent substitutions, etc. made within the scope of the principles of the present invention shall be included in the protection scope of the present invention.

Claims (10)

1. A frequency-swept dual optical comb-based optoelectronic terahertz hyperspectral imaging system, comprising:
The present invention includes a frequency swept laser, a distributed feedback laser, a dual optical comb generation module, a coupling beam splitter, a spectrum analyzer, an adjustable optical attenuator, a terahertz mixer, an off-axis parabolic mirror group, a zero bias detector, a power amplifier, and a signal analyzer.
The continuous wave laser signal emitted by the frequency swept laser is injected into a dual optical comb generation module to generate a dual optical comb signal;
The tunable laser signal and the dual optical comb signal emitted from the distributed feedback laser are combined by a coupling beam splitter to form two output signals, one of which is subjected to spectrum analysis through a spectrum analyzer, and the other is subjected to optical signal power control through an adjustable optical attenuator, and then output to a terahertz mixer, which mixes light and electricity to generate an optically mixed terahertz dual comb signal, which is input to an off-axis parabolic mirror group;
The first half of the off-axis parabolic mirror group collimates the input terahertz dual comb signal and focuses it on the sample, and the second half of the mirror group collimates the signal modulated by the sample and focuses it on the zero-bias detector;
The zero-bias detector mixes the input terahertz dual comb signal into an intermediate frequency dual comb signal, amplifies the power of the signal using a power amplifier, and then samples and records the dual comb spectrum using a signal analyzer. 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 optical fiber amplifier;
2. The frequency-swept dual optical comb-based optoelectronic terahertz hyperspectral imaging system of claim 1, wherein the continuous-wave laser signal emitted by the frequency-swept laser is split into two beams through 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, respectively, to generate two coherent optical frequency comb signals. The two coherent optical frequency comb signals pass through a first tunable optical filter and a second tunable optical filter, respectively, to filter out the positive first to fifth order optical combs, and are then combined and amplified by a coupler and an erbium-doped optical fiber amplifier to output a dual optical comb signal. The first and second radio frequency sources have a radio frequency difference ranging from 100 kHz to 500 kHz. The optoelectronic terahertz hyperspectral imaging system based on a frequency-swept dual optical comb described in 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 order, the first off-axis parabolic mirror and the second off-axis parabolic mirror forming the first half of the mirror group, and the third off-axis parabolic mirror and the fourth off-axis parabolic mirror forming the second half of the mirror group. The optoelectronic terahertz hyperspectral imaging system based on a frequency-swept dual optical comb described in claim 3, characterized in that the parent focal length of each off-axis parabolic mirror in the group of off-axis parabolic mirrors is 50.8 mm, the reflective effective focal length and center offset are both 101.6 mm, and the element diameter is 50.8 mm. the frequency swept laser is a narrow linewidth external cavity laser with frequency sweep capability;
The frequency-swept dual optical comb-based optoelectronic terahertz hyperspectral imaging system of claim 1 , wherein the distributed feedback laser is tunable. The optoelectronic terahertz hyperspectral imaging system based on a frequency-swept dual optical comb described in claim 1, characterized in that the terahertz mixer is a uni-traveling-carrier photodiode. The optoelectronic terahertz hyperspectral imaging system based on a frequency-swept dual optical comb described in claim 1, characterized in that the coupling beam splitter is a 1:1 power coupling beam splitter. The optoelectronic terahertz hyperspectral imaging system based on a frequency-swept dual optical comb described in claim 1, characterized in that the spectrum analyzer is a spectrum analyzer with a resolution of up to 150 MHz. The optoelectronic terahertz hyperspectral imaging system based on a frequency-swept dual optical comb described in claim 1, characterized in that the power amplifier is a 20 dB gain amplifier. 10. An optoelectronic terahertz hyperspectral imaging method using an optoelectronic terahertz hyperspectral imaging system according to any one of claims 1 to 9, the method comprising:
placing the sample in an optoelectronic terahertz hyperspectral imaging system and moving the sample along x and y directions using a mechanical raster scanning stage to acquire frequency-swept dual-comb spectra of the sample at different positions;
and obtaining a beat peak power array corresponding to each pixel and including a plurality of fifth-order radio frequency comb spectra based on the dual comb spectrum analysis, thereby realizing photonic terahertz hyperspectral imaging.