Optical coherence tomography system based on single-cavity dual-frequency-comb Ytterbium-doped fiber mode-locked laser

Abstract

The invention provides an optical coherence tomography system based on a single-cavity dual-frequency comb ytterbium-doped fiber mode-locked laser, belonging to the technical field of optical coherence tomography; the invention solves the problems of great technical difficulty and high cost in the existing dual-frequency comb for optical coherence tomography; the invention comprises: a single-cavity dual-frequency comb ytterbium-doped fiber mode-locked laser module, an amplification and sample scanning module and a signal acquisition module; in the single-cavity dual-frequency comb ytterbium-doped fiber mode-locked laser module, a polarization-maintaining fiber and a single-mode fiber with a certain amount of polarization birefringence are introduced into a single fiber ring cavity to enhance the intracavity birefringence and nonlinear effects, realize gain spectrum filtering under different wavelength windows, and use a semiconductor saturable absorption mirror to realize passive mode locking of the ytterbium-doped fiber laser, thereby obtaining single-cavity full-positive dispersion dual-frequency comb pulse mode locking; the invention is used for dual-frequency comb acquisition of optical coherence tomography.

Description

Technical Field

[0001] The invention provides an optical coherence tomography system based on a single-cavity dual-optical frequency comb ytterbium-doped fiber mode-locked laser, belonging to the technical field of optical coherence tomography. Background Art

[0002] A dual optical frequency comb refers to two optical frequency combs with a small repetition frequency difference. Generally, two lasers generate one comb each. Since the dual optical frequency combs generated by the two lasers need an external frequency stabilization system for frequency locking, the coherence and stability are poor. If the technology of generating dual optical frequency combs in one cavity is discovered, this single-cavity dual optical frequency comb technology can maintain the high coherence between the two sets of optical frequency combs and greatly simplify the complexity of the dual optical frequency comb system.

[0003] The beat frequencies of dual optical frequency combs can be described in the frequency and time domains respectively and can be measured using narrowband detectors and electronics, which makes dual optical frequency combs very promising in various applications, such as spectroscopy, nonlinear and linear microscopy, distance measurement and optical coherence tomography.

[0004] Optical coherence tomography (OCT) is a well-recognized label-free optical imaging method that can perform high-resolution, non-invasive tomographic imaging of biological tissues and materials. It can be used to visualize multidimensional tissue structures with micron-level resolution. Currently, two main types of optical coherence tomography systems have been developed, namely time-domain optical coherence tomography (TD-OCT) and Fourier domain optical coherence tomography (FD-OCT). Time-domain optical coherence tomography is generally based on a Michelson interferometer, and a single-point detector is used to record the time-domain signal of low-coherence interference of a broadband light source. Interference signals can only be generated when the optical path of the reference arm and the sample arm are completely matched. Through the periodic scanning of the reference arm, the longitudinal information (depth direction) inside the sample is acquired point by point. The mechanical scanning speed and range determine the axial line scanning speed (2-4kHz) and imaging depth (<3mm) of time-domain optical coherence tomography. In Fourier domain optical coherence tomography, the scanned image can be obtained by Fourier transforming the interference fringes. According to different spectral detection methods, Fourier domain optical coherence tomography can be divided into spectral domain optical coherence tomography (SD-OCT) and swept-frequency optical coherence tomography (SS-OCT). In spectral domain optical coherence tomography and swept-frequency optical coherence tomography, the density of interference fringes increases monotonically with the imaging depth. Therefore, a high-resolution camera or a broadband photodetector should be equipped to resolve the dense interference fringes to achieve a larger imaging depth. These broadband electronic devices require the support of high-performance computing platforms, such as graphics processing units (GPUs) or field-programmable gate arrays (FPGAs), to process large amounts of data in real time to achieve video-rate imaging. Broadband detectors and high-performance computing platforms increase system costs. To solve this problem, some researchers have begun to explore the application of dual-optical comb technology to optical coherence tomography. However, most optical coherence tomography currently uses single-pulse lasers and relies on mechanical delay scanning sampling, resulting in slow system scanning speeds and poor stability. Some researchers have proposed that using two sets of lasers to generate two sets of optical frequency combs respectively, combined with dual optical frequency comb sampling technology, can greatly improve the stability and sampling speed of the optical coherence tomography system. However, this configuration requires a complex feedback control system to eliminate the randomness of the output frequencies of the two sets of lasers. This not only increases the technical difficulty, but also leads to a significant increase in costs, limiting the widespread application of optical coherence tomography outside the laboratory. Summary of the invention

[0005] In order to solve the above technical problems, the present invention proposes an optical coherence tomography system based on a single-cavity dual-frequency-comb ytterbium-doped fiber mode-locked laser. The system does not require two sets of mode-locked lasers and mechanical delay scanning devices, does not require broadband detectors and high-performance computing platforms, and uses a single-cavity dual-frequency-comb laser to replace two independent lasers. At the same time, ordinary detectors can be used for data measurement, which greatly reduces the system complexity and cost.

[0006] The technical solution adopted by the present invention is: an optical coherence tomography system based on a single-cavity dual-frequency comb ytterbium-doped fiber mode-locked laser, comprising a single-cavity dual-frequency comb ytterbium-doped fiber mode-locked laser module, an amplification and sample scanning module and a signal acquisition module, wherein the single-cavity dual-frequency comb ytterbium-doped fiber mode-locked laser module comprises a pump source, a wavelength division multiplexer, an ytterbium-doped fiber, a first coupler, a polarization controller, a polarization-maintaining fiber, a first circulator and a semiconductor saturable absorber mirror connected in sequence by fiber fusion splicing, a first port pigtail of the first circulator is fused with an output end pigtail of the polarization-maintaining fiber, a second port pigtail of the first circulator is fused with a pigtail of the semiconductor saturable absorber, and a third port pigtail of the first circulator is fused with a straight-through end pigtail of the wavelength division multiplexer to form a fiber ring laser resonant cavity;

[0007] The dual optical frequency comb generated by the single-cavity dual optical frequency comb ytterbium-doped fiber mode-locked laser module is separated into two optical frequency combs after filtering by the fiber Bragg grating and respectively input into the amplification and sample scanning module for beating frequency to obtain interference signals, and the signal acquisition module processes the signals.

[0008] Furthermore, the polarization controller is used to optimize the polarization state and loss of the single-cavity dual-optical frequency comb ytterbium-doped fiber mode-locked laser module.

[0009] Furthermore, by using polarization-maintaining fiber and single-mode fiber to enhance intracavity birefringence and nonlinear effects, gain spectrum filtering in different wavelength windows is achieved, so that the single-cavity dual-optical frequency comb ytterbium-doped fiber mode-locked laser module can produce dual-optical frequency combs.

[0010] Furthermore, the semiconductor saturable absorber mirror is used as an intracavity mode-locking element of a resonant cavity to obtain a mode-locked fiber laser.

[0011] Furthermore, the amplification and sample scanning module includes a reference arm, a sample arm, a sample and a second coupler. The dual optical frequency combs generated by the single-cavity dual optical frequency comb ytterbium-doped fiber mode-locked laser module are spectrally broadened and filtered by the reference arm and the sample arm respectively, and then one optical frequency comb is emitted into the sample arm to scan the sample and then reflected, and then beat with the other optical frequency comb in the second coupler.

[0012] Furthermore, the sample arm is composed of a first ytterbium-doped fiber amplifier, a first bandpass filter, a collimator, a galvanometer and a scanning lens.

[0013] Furthermore, the first bandpass filter is connected to the sample arm through a second circulator, the first port pigtail of the second circulator is fused with the pigtail of the first bandpass filter, the second port pigtail of the second circulator is connected to the collimator, and the third port pigtail of the second circulator is connected to the second coupler.

[0014] Furthermore, the reference arm includes a second ytterbium-doped fiber amplifier and a second bandpass filter. The second ytterbium-doped fiber amplifier performs spectrum broadening on one optical frequency comb, and then filters it through the second bandpass filter and then inputs it into the second coupler.

[0015] Furthermore, the signal acquisition module includes a balanced photodetector, and the balanced photodetector is used to perform photoelectric conversion on the interference signal.

[0016] Furthermore, the dual optical frequency comb generated by the single-cavity dual optical frequency comb ytterbium-doped fiber mode-locked laser module generates an optical coherence tomography interference signal when the optical path of the reference arm and the sample arm are fully matched.

[0017] The beneficial effects of the present invention compared with the prior art are as follows:

[0018] 1. Through the ring cavity design, the semiconductor saturable absorber mirror is used to realize the passive mode locking of the ytterbium-doped fiber laser, and the birefringence and nonlinear effects in the cavity are enhanced by introducing polarization-maintaining fiber and single-mode fiber, so as to obtain full positive dispersion dual-wavelength mode locking. This single-cavity dual-optical frequency comb technology not only maintains the high coherence between the two sets of optical frequency combs, but also greatly simplifies the complexity of the dual-optical frequency comb system.

[0019] 2. The single-cavity dual-frequency comb ytterbium-doped fiber mode-locked laser proposed in the present invention adopts dissipative soliton dual-wavelength mode-locking under full positive dispersion, which not only avoids the trouble of intracavity dispersion compensation and thus simplifies the laser structure, but also allows higher single pulse energy without pulse splitting and produces a flat-top spectrum, which is an extremely attractive feature for dual-frequency comb light sources.

[0020] 3. The present invention proposes an optical coherence tomography system based on a single-cavity dual-frequency-comb ytterbium-doped fiber mode-locked laser. The system does not require two sets of mode-locked lasers and mechanical delay scanning devices, nor does it require broadband detectors and high-performance computing platforms. A single-cavity dual-frequency-comb ytterbium-doped fiber mode-locked laser is used to replace two independent lasers. At the same time, narrow-band detectors and electronic equipment can be used to measure data, which significantly improves the imaging speed and resolution and greatly reduces the system cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The present invention will be further described below in conjunction with the accompanying drawings:

[0022] Figure 1 The schematic diagram of the optical coherence tomography system based on the single-cavity dual-frequency-comb ytterbium-doped fiber mode-locked laser.

[0023] Figure 2 This is the mode-locked output spectrum of the single-cavity dual-frequency comb ytterbium-doped fiber mode-locked laser module when the pump power is 150mW;

[0024] Figure 3 This is the RF spectrum diagram of the single-cavity dual-frequency comb ytterbium-doped fiber mode-locked laser module when the pump power is 150mW;

[0025] Figure 4 This is the pulse sequence diagram of the mode-locked output of the single-cavity dual-frequency comb ytterbium-doped fiber mode-locked laser module at a pump power of 150mW;

[0026] Figure 5 This is the interference signal diagram of the dual optical frequency comb within 4ms;

[0027] Figure 6 A magnified detail of the dual-optical frequency comb interferometer signal;

[0028] Figure 7 is the transmittance and transmission wavelength of the birefringence filter effect λ Relationship diagram;

[0029] In the figure: 101 is a single-cavity dual-optical frequency comb ytterbium-doped fiber mode-locked laser module, 102 is an amplification and sample scanning module, and 103 is a signal acquisition module;

[0030] 1 is a pump source, 2 is a wavelength division multiplexer, 3 is an ytterbium-doped fiber, 4 is a first circulator, 5 is a semiconductor saturable absorber, 6 is a polarization-maintaining fiber, 7 is a polarization controller, 8 is a first coupler, 9 is a fiber Bragg grating, 10 is a first ytterbium-doped fiber amplifier, 11 is a first bandpass filter, 12 is a second circulator, 13 is a collimator, 14 is a galvanometer, 15 is a scanning lens, 16 is a sample, 17 is a second ytterbium-doped fiber amplifier, 18 is a second bandpass filter, 19 is a second coupler, and 20 is a balanced photodetector. DETAILED DESCRIPTION

[0031] like Figures 1 to 7 As shown, the present invention provides an optical coherence tomography system based on a single-cavity dual-frequency comb ytterbium-doped fiber mode-locked laser, comprising: a single-cavity dual-frequency comb ytterbium-doped fiber mode-locked laser module 101, an amplification and sample scanning module 102 and a signal acquisition module 103. In the single-cavity dual-frequency comb ytterbium-doped fiber mode-locked laser module 101, a pump source 1, a wavelength division multiplexer 2, an ytterbium-doped fiber 3, a first coupler 8, a polarization controller 7, a polarization-maintaining fiber 6, a first circulator 4 and a semiconductor saturable absorber 5 are sequentially connected by fiber fusion. The third port pigtail of the first circulator 4 is fused with the straight-through end pigtail of the wavelength division multiplexer 2 to form a fiber ring laser resonant cavity.

[0032] In the amplification and sample scanning module 102, the dual optical frequency comb generated by the single-cavity dual optical frequency comb ytterbium-doped fiber mode-locked laser module 101 is separated into two optical frequency combs after filtering by the fiber Bragg grating 9. The two optical frequency combs are respectively spectrally broadened by the first ytterbium-doped fiber amplifier 10 and the second ytterbium-doped fiber amplifier 17, and then filtered by the first bandpass filter 11 and the second bandpass filter 18 to ensure good mutual coherence. After passing through the first bandpass filter 11 and the second bandpass filter 18, the output pulse power of the two optical frequency combs is about 10mW. Then, one optical frequency comb is emitted to the collimator 13, the galvanometer 14 and the scanning lens 15 of the sample arm, and after being reflected back by the scanning sample 16, it beats with the other optical frequency comb in the 50:50 second coupler 19 to obtain an interference signal. Then, in the signal acquisition module 103, the collected signal is photoelectrically converted through the balanced photodetector 20 to further process the data. The two optical frequency combs generated by the single-cavity dual-optical frequency comb ytterbium-doped fiber mode-locked laser module 101 can generate optical coherence tomography interference signals only when the optical path lengths of the reference arm and the sample arm are completely matched.

[0033] In the single-cavity dual-optical frequency comb ytterbium-doped fiber mode-locked laser module 101, the pump source 1 is used to provide energy for the laser working material to achieve particle number inversion; the wavelength division multiplexer 2 is used to input multiple modulated optical signals of different wavelengths or frequencies into the optical fiber transmission link together; the ytterbium-doped optical fiber 3 amplifies the power of the signal light under the action of the pump light to generate a signal laser; the first coupler 8 is used to divide the optical signal from one optical fiber to multiple optical fibers; the polarization state and loss of the single-cavity dual-optical frequency comb ytterbium-doped fiber mode-locked laser module 101 are optimized by adjusting the state of the polarization controller 7; the polarization-maintaining optical fiber 6 and the single-mode optical fiber , which is used to enhance the intracavity birefringence and nonlinear effects and realize gain spectrum filtering under different wavelength windows, so that the single-cavity dual-frequency comb ytterbium-doped fiber mode-locked laser module 101 can generate a dual-frequency comb; the first port pigtail of the first circulator 4 is fused with the output end pigtail of the polarization-maintaining fiber 6, the second port pigtail of the first circulator 4 is fused with the pigtail of the semiconductor saturable absorber 5, and the third port pigtail of the first circulator 4 is fused with the straight-through end pigtail of the wavelength division multiplexer 2 to form a fiber ring laser resonant cavity; the semiconductor saturable absorber 5 is used as an intracavity mode-locking element of the resonant cavity to obtain a mode-locked fiber laser.

[0034] In the amplification and sample scanning module 102, the fiber Bragg grating 9 is used as a filtering device to separate the dual optical frequency comb generated by the single-cavity dual optical frequency comb ytterbium-doped fiber mode-locked laser module 101 into two optical frequency combs; the first ytterbium-doped fiber amplifier 10 and the second ytterbium-doped fiber amplifier 17 respectively perform spectral broadening on the two optical frequency combs; the first bandpass filter 11 and the second bandpass filter 18 respectively filter the two broadened optical frequency combs to ensure good mutual coherence.

[0035] Figure 2 This is the mode-locked output spectrum of the single-cavity dual-optical frequency comb ytterbium-doped fiber mode-locked laser module when the pump power is 150mW; the results show that the central wavelengths of the dual-optical frequency combs are in the range of 1060.14nm and 1069.73nm, and the 3dB bandwidths are 3.15nm and 3.11nm respectively.

[0036] Figure 3 This is the RF spectrum diagram of the mode-locked output of the single-cavity dual-optical frequency comb ytterbium-doped fiber mode-locked laser module at a pump power of 150mW; the results show that the repetition frequencies of the dual-optical frequency comb are in the range of 42.786845MHz and 42.783759MHz, respectively, and the repetition frequency difference is 3086Hz.

[0037] The beat frequency of the dual optical frequency comb can be described in the frequency domain and time domain respectively. In the frequency domain, the frequencies of the mth comb teeth of optical frequency comb 1 and optical frequency comb 2 are: , ,in and are the repetition frequencies of frequency comb 1 and frequency comb 2, and are the carrier envelope offset frequencies of optical frequency comb 1 and optical frequency comb 2 respectively. The beat frequency of the two is obtained ,in , Therefore, the two sets of optical frequency combs beat each other. The proportional relationship forms a series of intervals in the RF domain. The RF frequency comb can be used to obtain the optical frequency comb information in the optical frequency domain by detecting the comb teeth information in the RF domain. In the time domain, it can be understood as linear asynchronous optical sampling between two pulse trains with different repetition rates. The sampling is performed at time intervals to obtain a period of The time domain interference signal of the periodic time domain interference signal can be Fourier transformed to obtain the RF frequency comb.

[0038] The detected signal is passed through a low-pass filter using a photodetector to obtain the following Figure 5 The interference pattern shown. Figure 6 This is an enlarged detail of the dual-optical frequency comb interference signal.

[0039] The birefringence filtering effect based on polarization-maintaining fiber and ordinary single-mode fiber can be expressed as:

[0040] ;

[0041] in: , are the angles between the polarization directions of the input pulse, the polarization controller and the optical fiber fast axis, respectively. , are the linear and nonlinear phase delays, which can be expressed as:

[0042] ;

[0043] ;

[0044] in: , , , and They are polarization-maintaining fiber length, transmission wavelength, nonlinear coefficient, pulse instantaneous power and effective core area. is the refractive index difference between the two polarization axes of the polarization-maintaining fiber. =π / 8, =π / 8, =π / 2, , the transmittance and transmission wavelength of the birefringence filter effect The relationship is as Figure 7 As shown, it can be seen that the birefringence filtering effect has a comb filtering function.

[0045] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An optical coherence tomography system based on a single-cavity dual-frequency-comb ytterbium-doped fiber mode-locked laser, characterized in that: The invention comprises a single-cavity dual-frequency comb ytterbium-doped fiber mode-locked laser module (101), an amplification and sample scanning module (102) and a signal acquisition module (103), wherein the single-cavity dual-frequency comb ytterbium-doped fiber mode-locked laser module (101) comprises a pump source (1), a wavelength division multiplexer (2), an ytterbium-doped fiber (3), a first coupler (8), a polarization controller (7), a polarization-maintaining fiber (6), a first circulator (4) and a semiconductor saturable absorber (5) which are sequentially connected by means of fiber fusion splicing, wherein the first port pigtail of the first circulator (4) is fusion spliced ​​with the output end pigtail of the polarization-maintaining fiber (6), the second port pigtail of the first circulator (4) is fusion spliced ​​with the pigtail of the semiconductor saturable absorber (5), and the third port pigtail of the first circulator (4) is fusion spliced ​​with the straight-through end pigtail of the wavelength division multiplexer (2), thereby forming a fiber ring laser resonant cavity; The dual optical frequency comb generated by the single-cavity dual optical frequency comb ytterbium-doped fiber mode-locked laser module (101) is separated into two optical frequency combs after filtering by the fiber Bragg grating (9), and respectively input into the amplification and sample scanning module (102) for beating to obtain interference signals, and the signal acquisition module (103) processes the signals; The amplification and sample scanning module (102) comprises a reference arm, a sample arm, a sample (16) and a second coupler (19); the dual optical frequency combs generated by the single-cavity dual optical frequency comb ytterbium-doped fiber mode-locked laser module (101) are spectrally broadened and filtered in the reference arm and the sample arm respectively, and then one optical frequency comb is emitted to the sample arm to scan the sample (16) and then reflected, and then beat with the other optical frequency comb in the second coupler (19); The sample arm is composed of a first ytterbium-doped optical fiber amplifier (10), a first bandpass filter (11), a collimator (13), a galvanometer (14), and a scanning lens (15); The first bandpass filter (11) is connected to the sample arm via a second circulator (12); the first port pigtail of the second circulator (12) is fused to the pigtail of the first bandpass filter (11); the second port pigtail of the second circulator (12) is connected to a collimator (13); and the third port pigtail of the second circulator (12) is connected to a second coupler (19); The reference arm comprises a second ytterbium-doped fiber amplifier (17) and a second bandpass filter (18). The second ytterbium-doped fiber amplifier (17) performs spectrum broadening on an optical frequency comb, and then filters the spectrum through the second bandpass filter (18) before inputting the spectrum into the second coupler (19).

2. The optical coherence tomography system based on a single-cavity dual-frequency-comb ytterbium-doped fiber mode-locked laser according to claim 1, characterized in that: The polarization controller (7) is used to optimize the polarization state and loss of a single-cavity dual-optical frequency comb ytterbium-doped fiber mode-locked laser module (101).

3. The optical coherence tomography system based on a single-cavity dual-frequency-comb ytterbium-doped fiber mode-locked laser according to claim 1, characterized in that: By enhancing the intracavity birefringence and nonlinear effects through polarization-maintaining optical fiber (6) and single-mode optical fiber, gain spectrum filtering in different wavelength windows is achieved, so that the single-cavity dual-optical frequency comb ytterbium-doped optical fiber mode-locked laser module (101) generates a dual-optical frequency comb.

4. The optical coherence tomography system based on a single-cavity dual-frequency-comb ytterbium-doped fiber mode-locked laser according to claim 1, characterized in that: The semiconductor saturable absorption mirror (5) is used as an intracavity mode-locking element of a resonant cavity to obtain a mode-locked fiber laser.

5. The optical coherence tomography system based on a single-cavity dual-frequency-comb ytterbium-doped fiber mode-locked laser according to claim 1, characterized in that: The signal acquisition module (103) comprises a balanced photoelectric detector (20), and the balanced photoelectric detector (20) is used to perform photoelectric conversion on the interference signal.

6. The optical coherence tomography system based on a single-cavity dual-frequency-comb ytterbium-doped fiber mode-locked laser according to claim 1, characterized in that: The dual optical frequency comb generated by the single-cavity dual optical frequency comb ytterbium-doped fiber mode-locked laser module (101) generates an optical coherence tomography interference signal when the optical path of the reference arm and the sample arm are completely matched.

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