CN112436367B - Sweep frequency light source applying NPR mode locking and OCT imaging system - Google Patents
Sweep frequency light source applying NPR mode locking and OCT imaging system Download PDFInfo
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Abstract
The embodiment of the invention is suitable for the technical field of modern optical communication, and comprises an NPR mode locking pulse output unit, an optical circulator, a dispersion medium and a sweep frequency laser output end, wherein the NPR mode locking pulse output unit is used for outputting pulse laser, a first end of the optical circulator is connected with the NPR mode locking pulse output unit, a second end of the optical circulator is connected with the dispersion medium, a third end of the optical circulator is connected with the sweep frequency laser output end, the pulse laser enters from the first end of the optical circulator and is transmitted into the dispersion medium through the second end of the optical circulator, the pulse laser forms long pulse after being subjected to dispersion Fourier transformation in the dispersion medium, and the long pulse passes through the second end of the optical circulator and the third end of the optical circulator and is output from the sweep frequency laser output end. The invention aims to solve the problem that the sweep speed of the sweep light source in the related art is not fast enough.
Description
Technical Field
The invention belongs to the technical field of modern optical communication, and particularly relates to a sweep frequency light source applying NPR mode locking and an OCT imaging system.
Background
The optical coherence tomography (Optical coherence tomography, OCT) is a non-invasive, non-contact optical tomography technique with extremely high resolution. The sweep OCT technology belongs to the OCT technology of the third generation, and the sensitivity and the signal to noise ratio of the sweep OCT technology are obviously superior to those of the traditional OCT technology; and the depth information acquisition process of the sweep OCT technology does not need axial mechanical scanning, so that the imaging speed of an OCT system can be remarkably improved, and the stability of the system is enhanced. The sweep OCT system performs intensity detection on interference signals of wavelengths through fast wavelength scanning of a sweep laser and using a point detector, and finally obtains microstructure information of an object through Fourier transformation of interference spectrum signals to obtain a tomographic image of a sample to be detected. The axial scanning speed of the system depends on the wavelength scanning speed of the sweep laser, so that the imaging speed of the system can be greatly improved.
The related art swept OCT system mostly adopts a polygon mirror tuning type swept light source, a Fourier domain swept light source or a Mems swept light source, the polygon mirror tuning type swept light source adopts a traditional mechanical structure to carry out wavelength tuning, and the tuning speed is the lowest and is about 10 1 kHz; the Fourier domain sweep frequency light source adopts piezoelectric ceramics as a Fabry-Perot resonant cavity, the cavity length is adjusted to tune by loading periodically-changing electric signals, the tuning speed depends on the response speed of the piezoelectric ceramics to the electric signals, which is generally 10 2 kHz, and after the piezoelectric ceramics are added into the cavity buffer structure, the sweep frequency speed can reach 10 3 kHz; the Mems sweep frequency light source obtains sweep frequency output by changing the length of a vertically arranged Fabry-Perot resonant cavity through a micro motor, the sweep frequency speed is limited by the speed of the motor for adjusting the cavity length and is 10 2～103 kHz, and the defects of insufficient sweep frequency speed exist in a polygon mirror tuning type sweep frequency light source, a Fourier domain sweep frequency light source or a Mems sweep frequency light source.
Disclosure of Invention
The technical problem to be solved by the embodiment of the invention is to provide a sweep frequency light source applying NPR mode locking, which aims to solve the problem that the sweep frequency speed of the sweep frequency light source in the related art is not fast enough.
The embodiment of the invention is realized by the adoption of the NPR mode locking sweep frequency light source, which comprises an NPR mode locking pulse output unit, an optical circulator, a dispersion medium and a sweep frequency laser output end, wherein the NPR mode locking pulse output unit is used for outputting pulse laser, the first end of the optical circulator is connected with the NPR mode locking pulse output unit, the second end of the optical circulator is connected with the dispersion medium, the third end of the optical circulator is connected with the sweep frequency laser output end, the pulse laser enters from the first end of the optical circulator and is transmitted into the dispersion medium through the second end of the optical circulator, the pulse laser forms long pulse after being subjected to dispersion Fourier transform in the dispersion medium, and the long pulse passes through the second end of the optical circulator and the third end of the optical circulator and is output from the sweep frequency laser output end.
Further, the NPR mode-locked pulse output unit includes a light source, a wavelength division multiplexer, a C-band erbium-doped fiber, an L-band erbium-doped fiber, a polarization controller, a polarization-related optical isolator, a first single-mode fiber and an optical coupler, wherein an output end of the light source is connected with a first input end of the wavelength division multiplexer, an output end of the wavelength division multiplexer is connected with one end of the C-band erbium-doped fiber, the other end of the C-band erbium-doped fiber is connected with one end of the L-band erbium-doped fiber, the other end of the L-band erbium-doped fiber is connected with an input end of the polarization controller, an output end of the polarization-related optical isolator is connected with one end of the first single-mode fiber, the other end of the first single-mode fiber is connected with a first input end of the optical coupler, a first output end of the optical coupler is connected with a second input end of the wavelength division multiplexer, and a second output end of the optical coupler outputs the pulse.
Further, the L-band erbium-doped fiber is surrounded inside the polarization controller.
Further, a first erbium-doped fiber amplifier for amplifying the energy of the pulse laser and fine tuning the spectral shape is also connected between the second output end of the optical coupler and the first end of the optical circulator.
Further, the dispersion medium is a linearly chirped Bragg grating, the grating spacing from the entrance to the tail end of the linearly chirped Bragg grating is gradually increased, and the dispersion value is correspondingly and gradually increased.
Further, a repetition frequency doubling structure is further arranged between the third end of the optical circulator and the sweep laser output end.
Further, the repetition frequency doubling structure comprises a first coupler, a second single-mode fiber, a second coupler, a third single-mode fiber and a third coupler, wherein the input end of the first coupler is connected with the third end of the optical circulator, the first output end of the first coupler is connected with one end of the second single-mode fiber, the other end of the second single-mode fiber is connected with the first input end of the second coupler, the second output end of the first coupler is connected with the second input end of the second coupler, the first output end of the second coupler is connected with one end of the third single-mode fiber, the other end of the third single-mode fiber is connected with the first input end of the third coupler, the second output end of the second coupler is connected with the second input end of the third coupler, and the output end of the third coupler is connected with the sweep laser output end.
Further, a second erbium-doped fiber amplifier is further connected between the output end of the third coupler and the sweep frequency laser output end, and the second erbium-doped fiber amplifier is a boosting-stage erbium-doped fiber amplifier.
Further, the first coupler is a 1x2 coupler with a 50:50 splitting ratio, the second single-mode fiber is a 1/2 cavity length single-mode fiber, the second coupler is a 2x2 coupler with a 50:50 splitting ratio, the third single-mode fiber is a 1/4 cavity length single-mode fiber, and the third coupler is a 2x1 coupler.
Further, an OCT imaging system is provided that includes a swept light source as described above that applies NPR mode locking.
Compared with the prior art, the embodiment of the invention has the beneficial effects that: the pulse laser output by the NPR mode-locked pulse output unit is input into a dispersive medium for dispersion Fourier transformation, namely, the spectrum information of a frequency domain is mapped onto time domain long pulses with sequentially arranged frequencies in a time stretching mode, so that the repetition frequency of a light source pulse is equivalent to the line scanning frequency of an optical coherence tomography system and can reach 10 1～102 MHz, and the problem that the sweep speed of a sweep frequency light source in the related art is not fast is solved.
Drawings
FIG. 1 is a schematic diagram of the overall structure of a swept source employing NPR mode locking according to an embodiment of the present invention;
FIG. 2 is a graph of a dissipative soliton mode locking spectral generated by NPR mode locking in accordance with the invention;
FIG. 3 is a long pulse plot obtained after time stretching in accordance with an embodiment of the present invention;
Fig. 4 is a schematic diagram of the change in spectral shape and pulse before and after time-frequency mapping.
In the drawings, each reference numeral denotes:
11. A light source; 12. a wavelength division multiplexer; 13. c-band erbium-doped fiber; 14. l-band erbium-doped fiber; 15. a polarization controller; 16. a polarization dependent optical isolator; 17. a first single mode optical fiber; 18. an optical coupler; 2. an optical circulator; 3. a dispersive medium; 41. a first coupler; 42. a second single mode optical fiber; 43. a second coupler; 44. a third single mode optical fiber; 45. a third coupler; 5. a first erbium-doped fiber amplifier; 6. a second erbium-doped fiber amplifier; 7. and the laser output end is swept.
Detailed Description
The present invention will be described in further detail with reference to the drawings and examples, in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention.
As shown in fig. 1, the swept source applying NPR mode locking according to the embodiment of the present invention includes an NPR mode locking pulse output unit, an optical circulator 2, a dispersive medium 3, and a swept laser output end 7, where the NPR mode locking pulse output unit is configured to output pulse laser, a first end of the optical circulator 2 is connected to the NPR mode locking pulse output unit, a second end of the optical circulator 2 is connected to the dispersive medium 3, a third end of the optical circulator 2 is connected to the swept laser output end 7, pulse laser enters from the first end of the optical circulator 2 and is transmitted to the dispersive medium 3 through the second end of the optical circulator 2, after performing dispersion fourier transformation in the dispersive medium 3, the pulse laser forms a long pulse, and the long pulse passes through the second end of the optical circulator 2 and the third end of the optical circulator 2 and is output from the swept laser output end 7.
The pulse laser output by the NPR mode-locked pulse output unit is input into the dispersive medium 3 for dispersion Fourier transformation, namely, the spectrum information of the frequency domain is mapped onto the time domain long pulses with the frequencies being sequentially arranged in a time stretching mode, so that the repetition frequency of the light source pulse is equivalent to the line scanning frequency of an optical coherence tomography system and can reach 10 1～102 MHz, and the problem that the sweep speed of a sweep frequency light source in the related art is not fast is solved.
In this embodiment, the NPR mode-locked pulse output unit includes a light source 11, a wavelength division multiplexer 12, a C-band erbium-doped fiber 13, an L-band erbium-doped fiber 14, a polarization controller 15, a polarization-related optical isolator 16, a first single-mode fiber 17, and an optical coupler 18, where the output end of the light source 11 is connected to the first input end of the wavelength division multiplexer 12, the output end of the wavelength division multiplexer 12 is connected to one end of the C-band erbium-doped fiber 13, the other end of the C-band erbium-doped fiber 13 is connected to one end of the L-band erbium-doped fiber 14, the other end of the L-band erbium-doped fiber 14 is connected to the input end of the polarization controller 15, the output end of the polarization controller 15 is connected to the input end of the polarization-related optical isolator 16, the output end of the polarization-related optical isolator 16 is connected to one end of the first single-mode fiber 17, the other end of the first single-mode fiber 17 is connected to the first input end of the optical coupler 18, the first output end of the optical coupler 18 is connected to the second input end of the wavelength division multiplexer 12, and the second output end of the optical coupler 18 outputs pulse laser, and the NPR mode-locked mode fiber 13 can generate a high-flatness pulse energy under the conditions of the whole-color and a high-second pulse dispersion time. In the aspect of imaging quality, the spectrum flat mode locking seed can improve the energy balance degree of each wavelength of the time stretching type sweep frequency light source 11, so that the relative intensity noise of OCT imaging is reduced, and the imaging quality is improved.
The wavelength division multiplexer 12, the C-band erbium doped fiber 13, the L-band erbium doped fiber 14, the polarization controller 15, the polarization dependent optical isolator 16, the first single mode fiber 17, and the optical coupler 18 form a closed ring resonator. Specifically, the light source 11 is a semiconductor laser diode, the semiconductor laser diode emits 980nm stimulated radiation background light, the C-band erbium-doped optical fiber 13 is pumped forward through the wavelength division multiplexer 12, stimulated radiation of 1530-1565nm is excited and then is partially absorbed by the L-band erbium-doped optical fiber 14, stimulated radiation of 1565-1625nm is excited, at this time, the stimulated radiation of the C-band and the L-band are connected into a stimulated radiation spectrum with a larger bandwidth than that of a single band in a frequency domain, and the polarization controller 15 adjusts the polarization states of the stimulated radiation background light from the C-band erbium-doped optical fiber 13 and the L-band erbium-doped optical fiber 14 to become linearly polarized light which is matched with a preset angle of the polarization-dependent optical isolator 16; the single-mode fiber can adjust and obtain the condition of micro positive dispersion of the whole cavity by changing the length of the single-mode fiber, thereby providing possibility for obtaining the dissipative soliton spectrum. The optical coupler 18 is an optical coupler 18 with a coupling ratio of 90:10, which is capable of receiving light from the first single mode fiber 17 and returning 90% of the light to the wavelength division multiplexer 12 for intra-cavity recycling while 10% of the pulsed laser light is output out of the cavity.
The dispersion value of the C-band erbium-doped fiber 13 is about 15.5ps 2/nm, the C-band stimulated radiation can be generated when the C-band erbium-doped fiber 13 is excited by 980nm background light, the L-band erbium-doped fiber 14 can absorb part of the C-band radiation from the C-band erbium-doped fiber 13 and generate L-band stimulated radiation, the C-band erbium-doped fiber 13 and the L-band erbium-doped fiber 14 can be matched in length to generate stimulated radiation background light with large bandwidth and flat spectrum, and the possibility is provided for realizing flat mode-locking spectrum (3 dB bandwidth 71 nm) subsequently. Preferably, the L-band erbium-doped fiber 14 of the present embodiment surrounds the inside of the polarization controller 15, that is, the L-band erbium-doped fiber 14 is wound around two paddles of the polarization controller 15, so as to serve as a torsion fiber medium, so as to shorten the cavity length of the ring resonator, thereby improving the repetition frequency and being beneficial to gain in the soliton state.
It should be noted that, the dispersion value of the first single-mode fiber 17 is about-22 ps 2/nm, the first single-mode fiber 17 is mainly used for whole cavity dispersion value allocation, if the accurate dispersion value of the L-band erbium-doped fiber 14 cannot be known, the mode can be locked from the traditional soliton representing the whole cavity tiny negative dispersion, and the first single-mode fiber 17 with negative dispersion is cut short to approach the whole cavity tiny positive dispersion until the dissipative soliton spectrum representing the whole cavity positive dispersion appears.
Alternatively, the polarization controller 15 may use a more sensitive squeeze polarization controller 15, and the in-line polarization state adjustment method thereof does not introduce extra length of cavity length, which is beneficial to the realization of high repetition frequency. In addition, polarization dependent optical isolator 16 may be selected to be a primary polarization isolation device, or a dual-stage polarization isolation device, a tertiary dual-stage polarization isolation device, with a higher polarization isolation level meaning a narrower pulse width and more balanced pulse energy.
Preferably, a first erbium-doped fiber amplifier 5 for amplifying the energy of the pulsed laser and fine-tuning the spectral shape is also connected between the second output of the optical coupler 18 and the first end of the optical circulator 2.
The dispersive medium 3 in this embodiment is a linearly chirped bragg grating, and the grating pitch gradually increases from the entrance to the end of the linearly chirped bragg grating, and the dispersion value also gradually increases accordingly. According to equation 1-1, different Bragg wavelengths are reflected at different depth positions of the grating, and short waves are reflected first, the linear relationship of wavelength to reflection time can be achieved by a linearly varying grating period.
λB＝2neffΛ (1-1)
Where lambda B is the Bragg wavelength, n eff is the effective refractive index of the grating, lambda is the grating period;
Meanwhile, the linear chirped Bragg grating has a large second-order dispersion value, acts on the narrow pulse and is equivalent to performing approximate Fourier transform (dispersion Fourier transform), and wavelength sequencing on a spectrum can be mapped from a frequency domain to a time domain long pulse, so that sweep frequency output in popular sense is completed. As shown in fig. 4, the spectral shape is unchanged before and after time-frequency mapping, but the narrow pulse in the time domain is stretched into a long pulse approximating the spectral shape. Alternatively, in other possible embodiments, the dispersive medium 3 may be a dispersion compensating fiber with a dispersion coefficient close to linear, and the present embodiment does not limit the type of the dispersive medium 3.
In this embodiment, a repetition frequency doubling structure is further disposed between the third end of the optical circulator 2 and the swept laser output end 7, and the repetition frequency doubling structure can double the repetition frequency and amplify the energy of the swept laser to form a swept laser source suitable for the OCT imaging system. Specifically, the repetition frequency doubling structure includes a first coupler 41, a second single-mode fiber 42, a second coupler 43, a third single-mode fiber 44, and a third coupler 45, where an input end of the first coupler 41 is connected to a third end of the optical circulator 2, a first output end of the first coupler 41 is connected to one end of the second single-mode fiber 42, another end of the second single-mode fiber 42 is connected to a first input end of the second coupler 43, a second output end of the first coupler 41 is connected to a second input end of the second coupler 43, a first output end of the second coupler 43 is connected to one end of the third single-mode fiber 44, another end of the third single-mode fiber 44 is connected to a first input end of the third coupler 45, and an output end of the third coupler 45 is connected to the swept laser output end 7.
Further, the first coupler 41 is a 1×2 coupler with a 50:50 splitting ratio, the first coupler 41 distributes 50% of light to the second single-mode fiber 42, and the rest 50% is transmitted forward, and the second single-mode fiber 42 is a single-mode fiber with a 1/2 cavity length, which acts as a delay line, so that the delayed optical pulse sequence is slower than the undelayed pulse sequence by half an intracavity cycle period. The second coupler 43 has a spectral ratio of 50:50, which is operative to combine the delayed and undelayed pulse trains to increase the pulse repetition frequency by a factor of 2. The third single mode fiber 44 is a single mode fiber with a 1/4 cavity length, the third coupler 45 is a 2x1 coupler, the third single mode fiber 44 is a delay line which is the same as the second single mode fiber 42, and after one row of pulses are delayed and the other row of pulses are combined in the third coupler 45, the repetition frequency of the sweep frequency light source is increased to 4 times of the initial repetition frequency. It should be noted that whether or not to increase the repetition frequency doubling structure may be determined according to the magnitude of the pulse duty of the light source 11, and although the duty may reach 100% in theory, in actual operation, a duty of 100% cannot be used in order to avoid aliasing of adjacent long pulses.
Preferably, a second erbium-doped fiber amplifier 6 is further connected between the output end of the third coupler 45 and the swept laser output end 7, the second erbium-doped fiber amplifier 6 is a boosting-stage erbium-doped fiber amplifier, and the second erbium-doped fiber amplifier 6 can perform gain on the pulse which is stretched in the front and doubled in the heavy frequency, so as to achieve the spectral energy which is most suitable for the requirement of the subsequent interference imaging, and can also properly fine tune the spectral shape.
As shown in fig. 2 and 3, fig. 2 is a graph of a dissipative soliton mode locking spectrum generated by NPR mode locking according to the present invention, and fig. 3 is a long pulse graph obtained after time stretching. It can be seen that the invention uses the dissipative soliton mode locking spectrum generated by NPR mode locking, the 3dB bandwidth reaches 71nm, the long pulse width after time stretching is 12.4ns, the duty cycle is 39%, and the wavelengths are linearly arranged in each long pulse in fig. 2.
The other embodiment of the invention also provides an OCT imaging system, which comprises the sweep frequency light source applying NPR mode locking in the technical scheme.
In summary, the pulse laser output by the NPR mode-locked pulse output unit of the present invention is input into the dispersive medium 3 to perform dispersion fourier transform, that is, the spectrum information of the frequency domain is mapped onto the time domain long pulses with sequentially arranged frequencies in a time stretching manner, so that the repetition frequency of the light source pulse is equivalent to the line scanning frequency of the optical coherence tomography system, and can reach 10 1～102 MHz, thereby solving the problem of insufficient frequency sweep speed of the sweep frequency light source of the related art. In the aspect of imaging quality, the spectrum flat mode locking seed can improve the energy balance degree of each wavelength of the time stretching type sweep frequency light source, thereby reducing the relative intensity noise of OCT imaging and improving the imaging quality.
The foregoing description of the preferred embodiments of the invention is not intended to be limiting, but rather is intended to cover all modifications, equivalents, and alternatives falling within the spirit and principles of the invention.
Claims (6)
1. A swept source employing NPR mode locking, comprising an NPR mode locking pulse output unit, an optical circulator (2), a dispersive medium (3) and a swept laser output end (7), wherein the NPR mode locking pulse output unit is used for outputting pulse laser, a first end of the optical circulator (2) is connected with the NPR mode locking pulse output unit, a second end of the optical circulator (2) is connected with the dispersive medium (3), a third end of the optical circulator (2) is connected with the swept laser output end (7), the pulse laser enters from the first end of the optical circulator (2) and is transmitted into the dispersive medium (3) through the second end of the optical circulator (2), and long pulses are formed after being subjected to dispersion fourier transform in the dispersive medium (3), pass through the second end of the optical circulator (2) and the third end of the optical circulator (2) and are output from the swept laser output end (7);
The NPR mode-locked pulse output unit comprises a light source (11), a wavelength division multiplexer (12), a C-band erbium-doped optical fiber (13), an L-band erbium-doped optical fiber (14), a polarization controller (15), a polarization-related optical isolator (16), a first single-mode optical fiber (17) and an optical coupler (18), wherein the output end of the light source (11) is connected with the first input end of the wavelength division multiplexer (12), the output end of the wavelength division multiplexer (12) is connected with one end of the C-band erbium-doped optical fiber (13), the other end of the C-band erbium-doped optical fiber (13) is connected with one end of the L-band erbium-doped optical fiber (14), the other end of the L-band erbium-doped optical fiber (14) is connected with the input end of the polarization controller (15), the output end of the polarization-related optical isolator (16) is connected with one end of the first single-mode optical fiber (17), and the other end of the polarization-related optical isolator (16) is connected with the first single-mode optical fiber (18), and the other end of the first optical isolator (17) is connected with the first input end of the second single-mode optical fiber (18) and the second optical coupler (18);
The L-band erbium-doped fiber (14) surrounds the inside of the polarization controller (15);
a repetition frequency doubling structure is further arranged between the third end of the optical circulator (2) and the sweep frequency laser output end (7);
the repetition frequency doubling structure comprises a first coupler (41), a second single-mode fiber (42), a second coupler (43), a third single-mode fiber (44) and a third coupler (45), wherein the input end of the first coupler (41) is connected with the third end of the optical circulator (2), the first output end of the first coupler (41) is connected with one end of the second single-mode fiber (42), the other end of the second single-mode fiber (42) is connected with the first input end of the second coupler (43), the second output end of the first coupler (41) is connected with the second input end of the second coupler (43), the first output end of the second coupler (43) is connected with one end of the third single-mode fiber (44), the other end of the third single-mode fiber (44) is connected with the first input end of the third coupler (45), the second output end of the second coupler (43) is connected with the second input end of the third coupler (45), and the output end of the third coupler (7) is connected with the output end of the laser.
2. A swept optical source employing NPR mode locking as claimed in claim 1, wherein a first erbium doped fibre amplifier (5) for amplifying the energy of the pulsed laser and trimming the spectral shape is further connected between the second output of the optical coupler (18) and the first end of the optical circulator (2).
3. A swept optical source employing NPR mode locking as claimed in claim 1, wherein the dispersive medium (3) is a linearly chirped bragg grating, and the dispersion value increases gradually from the entrance to the end grating pitch of the linearly chirped bragg grating.
4. A swept optical source employing NPR mode locking as claimed in claim 1, wherein a second erbium-doped fiber amplifier (6) is further connected between the output of the third coupler (45) and the swept laser output (7), the second erbium-doped fiber amplifier (6) being a boost stage erbium-doped fiber amplifier.
5. The swept optical source employing NPR mode locking of claim 4, wherein the first coupler (41) has a spectral ratio of 50:50, the second single mode fiber (42) is a single mode fiber with a 1/2 cavity length, and the second coupler (43) is a single mode fiber with a spectral ratio of 50:50, the third single mode fiber (44) is a 1/4 cavity length single mode fiber, and the third coupler (45) is a 2x1 coupler.
6. An OCT imaging system comprising a swept source of any of claims 1 to 5 employing NPR mode locking.
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| WO2017053592A1 (en) * | 2015-09-23 | 2017-03-30 | The Regents Of The University Of California | Deep learning in label-free cell classification and machine vision extraction of particles |
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| CN102223183A (en) * | 2011-08-05 | 2011-10-19 | 中国电子科技集团公司第三十四研究所 | Non-relay optical fiber transmission system and method |
| CN107800024A (en) * | 2017-11-22 | 2018-03-13 | 西南交通大学 | Tunable pulse-position modulation signal generation device based on spectral structure |
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