CN214013389U - Frequency sweep light source applying NPR mode locking and OCT imaging system - Google Patents
Frequency sweep light source applying NPR mode locking and OCT imaging system Download PDFInfo
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Abstract
The embodiment of the utility model is suitable for the technical field of modern optical communication, in particular to a swept-frequency light source and an OCT imaging system applying NPR mode locking, which comprises an NPR mode locking pulse output unit, an optical circulator, a dispersion medium and a swept-frequency laser output end, 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 swept 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, and the pulse laser forms long pulse after carrying out 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 laser output end. The utility model discloses the not fast enough problem of sweep frequency light source frequency sweep speed who aims at solving correlation technique.
Description
Technical Field
The utility model belongs to the technical field of modern optical communication, especially, relate to a sweep frequency light source and OCT imaging system of applied NPR mode locking.
Background
Optical Coherence Tomography (OCT) is a non-invasive, non-contact Optical tomography with extremely high resolution. The frequency-sweeping OCT technology belongs to the third generation OCT technology, and the sensitivity and the signal-to-noise ratio of the frequency-sweeping OCT technology are obviously superior to those of the traditional OCT technology; and the depth information acquisition process of the frequency-sweeping OCT technology does not need axial mechanical scanning, so that the imaging speed of the OCT system can be obviously improved, and the stability of the system is enhanced. The frequency-sweeping OCT system scans the fast wavelength of a frequency-sweeping laser, detects the intensity of an interference signal with the wavelength by using a point detector, and finally obtains the microstructure information of an object by Fourier transform of the interference spectrum signal to obtain a chromatographic image of a sample to be detected. The axial scanning speed of the system depends on the wavelength scanning speed of the frequency-scanning laser, so that the imaging speed of the system can be greatly improved.
In the related art, the frequency-sweep OCT system mostly adopts a multi-mirror tuned frequency-sweep light source, a Fourier domain frequency-sweep light source or a Mems frequency-sweep light source, and the multi-mirror tuned frequency-sweep light source adopts a traditional mechanical structure to carry out traveling waveLong tuning, the lowest tuning speed, about 101kHz; the Fourier domain sweep frequency light source adopts piezoceramics as a Fabry-Perot resonant cavity, the cavity length is adjusted by loading a periodically changed electric signal for wavelength tuning, the tuning speed depends on the response speed of the piezoceramics to the electric signal, and is generally 102kHz, after the buffer structure in the cavity is added, the sweep frequency speed can reach 103kHz; 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, and the sweep frequency speed is limited by the adjusting speed of the motor to the cavity length and is 102～103kHz whether a polygon mirror tuning type frequency sweep light source, a Fourier domain frequency sweep light source or a Mems frequency sweep light source has the defect that the frequency sweep speed is not high enough.
SUMMERY OF THE UTILITY MODEL
The embodiment of the utility model provides a technical problem that will solve provides a frequency sweep light source of using NPR mode locking, aims at solving the not fast enough problem of frequency sweep light source frequency sweep speed of correlation technique.
The embodiment of the utility model is realized in such a way that a sweep frequency light source applying NPR mode locking comprises an NPR mode locking pulse output unit, an optical circulator, a dispersion medium and a sweep frequency laser output end, 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 swept 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, and the pulse laser forms long pulse after carrying out 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 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-dependent optical isolator, a first single-mode fiber, and an optical coupler, an output end of the light source is connected to a first input end of the wavelength division multiplexer, an output end of the wavelength division multiplexer is connected to one end of the C-band erbium-doped fiber, another end of the C-band erbium-doped fiber is connected to one end of the L-band erbium-doped fiber, another end of the L-band erbium-doped fiber is connected to an input end of the polarization controller, an output end of the polarization controller is connected to an input end of the polarization-dependent optical isolator, an output end of the polarization-dependent optical isolator is connected to one end of the first single-mode fiber, another end of the first single-mode fiber is connected to a first input end of the optical coupler, and the first output end of the optical coupler is connected with the second input end of the wavelength division multiplexer, and the second output end of the optical coupler outputs the pulse laser.
Further, the L-band erbium-doped optical fiber is surrounded inside the polarization controller.
Further, a first erbium-doped fiber amplifier for amplifying the energy of the pulse laser and finely adjusting the spectral shape is connected between the second output end of the optical coupler and the first end of the optical circulator.
Further, the dispersion medium is a linear chirped bragg grating, the grating grid distance from the entrance of the linear chirped bragg grating to the tail end is gradually increased, and the dispersion value is correspondingly gradually increased.
Furthermore, a repetition frequency doubling structure is arranged between the third end of the optical circulator and the output end of the sweep laser.
Further, the repetition frequency doubling structure includes a first coupler, a second single mode fiber, a second coupler, a third single mode fiber, and a third coupler, 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 a first input end of the second coupler, a second output end of the first coupler is connected with a 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, and a second output end of the second coupler is connected with a second input end of the third coupler, and an output end of the third coupler is connected with the swept laser output end.
Further, a second erbium-doped fiber amplifier is connected between the output end of the third coupler and the sweep laser output end, and the second erbium-doped fiber amplifier is a boost-level erbium-doped fiber amplifier.
Further, the first coupler has a splitting ratio of 50: 50, the second single-mode fiber is a 1/2 cavity length single-mode fiber, and the second coupler is a 1x2 coupler with a splitting ratio of 50: a 50 x2 coupler, the third single mode fiber being a 1/4 cavity length single mode fiber, the third coupler being a 2x1 coupler.
Further, an OCT imaging system is provided, comprising a swept optical source applying NPR mode locking as described above.
Compared with the prior art, the embodiment of the utility model, beneficial effect lies in: the utility model discloses a carry out dispersion Fourier transform in the pulse laser input dispersion medium of NPR mode locking pulse output unit output, through the mode of time stretching promptly, map the spectral information of frequency domain to the long pulse of time domain that the frequency was arranged in proper order on for the repetition frequency of light source pulse is equal to the line scanning frequency of optical coherence tomography imaging system, can reach 101～102MHz, thereby solving the problem that the sweep frequency speed of the sweep frequency light source in the related technology is not fast enough.
Drawings
Fig. 1 is a schematic view of an overall structure of a swept-frequency light source using NPR mode locking provided by an embodiment of the present invention;
FIG. 2 is a graph of a dissipative soliton mode-locking spectrum generated by NPR mode-locking of the present invention;
FIG. 3 is a long pulse plot obtained after time stretching of an embodiment of the present invention;
FIG. 4 is a diagram showing changes 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. a C-band erbium-doped fiber; 14. an 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 a swept laser output end.
Detailed Description
In order to make the objects, 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 merely illustrative of the invention and are not intended to limit the invention.
As shown in fig. 1, it is a frequency sweep light source of application NPR mode locking that the embodiment of the utility model provides an, including NPR mode locking pulse output unit, optical circulator 2, dispersion medium 3 and frequency sweep laser output end 7, NPR mode locking pulse output unit is used for output pulse laser, optical circulator 2's first end and NPR mode locking pulse output unit are connected, optical circulator 2's second end and dispersion medium 3 are connected, optical circulator 2's third end and frequency sweep laser output end 7 are connected, pulse laser gets into and transmits to dispersion medium 3 through optical circulator 2's second end from optical circulator 2's first end, pulse laser forms long pulse after carrying out dispersion fourier transform in dispersion medium 3, long pulse passes through optical circulator 2's second end and optical circulator 2's third end and outputs from frequency sweep laser output end 7.
The utility model discloses a carry out dispersion Fourier transform in the pulse laser input dispersion medium 3 of NPR mode locking pulse output unit output, through the mode of time stretching promptly, map the spectral information of frequency domain to the long pulse of time domain that the frequency was arranged in proper order on for the repetition frequency of light source pulse is equal to the line scanning frequency of optical coherence tomography imaging system, can reach 101～102MHz, thereby solving the problem of sweep source scanning in the related artThe frequency speed is not fast enough.
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, an output end of the light source 11 is connected to a first input end of the wavelength division multiplexer 12, an output end of the wavelength division multiplexer 12 is connected to one end of the C-band erbium-doped fiber 13, another end of the C-band erbium-doped fiber 13 is connected to one end of the L-band erbium-doped fiber 14, another end of the L-band erbium-doped fiber 14 is connected to an input end of the polarization controller 15, an output end of the polarization controller 15 is connected to an input end of the polarization-related optical isolator 16, an output end of the polarization-related optical isolator 16 is connected to one end of the first single-mode fiber 17, another end of the first single-mode optical isolator 17 is connected to a first input end of the optical coupler 18, a first output end of the optical coupler 18 is connected to a second input end of the wavelength division multiplexer 12, the second output end of the optical coupler 18 outputs pulse laser, and by using NPR mode locking, a dissipative soliton spectrum and a femtosecond pulse with high flatness can be generated under the condition of whole-cavity small positive dispersion to improve the balance of energy of each wavelength in the long pulse after time stretching. In the aspect of imaging quality, the mode-locked seed with flat spectrum can improve the energy balance degree of each wavelength of the time stretching type sweep-frequency light source 11, thereby reducing the relative intensity noise of OCT imaging and improving the imaging quality.
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 fiber 13 is pumped forwards through the wavelength division multiplexer 12, stimulated radiation of 1530-1565nm is excited, the stimulated radiation is partially absorbed by the L-band erbium-doped fiber 14, stimulated radiation of 1565-1625nm is excited, the stimulated radiation of the C-band and the L-band is connected in a frequency domain to form a stimulated radiation spectrum with a bandwidth larger than that of a single band, and the polarization controller 15 adjusts the polarization state of the stimulated radiation background light from the C-band erbium-doped fiber 13 and the L-band erbium-doped fiber 14 to enable the stimulated radiation background light to become linearly polarized light fitting a preset angle of the polarization-related optical isolator 16; the single-mode fiber can adjust and obtain the condition of the whole cavity tiny positive dispersion by changing the length of the single-mode fiber, and provides possibility for obtaining a dissipative soliton spectrum. The optical coupler 18 has a coupling ratio of 90: the optical coupler 18 of 10 can receive the light from the first single mode optical fiber 17 and return 90% of the light to the wavelength division multiplexer 12 to complete the intracavity cycle, and simultaneously 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 15.5ps2The excitation radiation source is/nm and excited by 980nm background light to generate C-band excitation radiation, the L-band erbium-doped fiber 14 can absorb part of C-band radiation from the C-band erbium-doped fiber 13 to generate L-band excitation radiation, the excited radiation background light with larger bandwidth and flat spectrum can be generated by matching the lengths of the C-band erbium-doped fiber 13 and the L-band erbium-doped fiber 14, and the possibility of realizing flat mode-locking spectrum (3dB bandwidth 71nm) is provided for the follow-up. Preferably, the L-band erbium-doped fiber 14 of this embodiment is wound inside the polarization controller 15, that is, the L-band erbium-doped fiber 14 is wound inside two paddles of the polarization controller 15, and as a twisted fiber medium, the cavity length of the ring resonator can be shortened, so as to improve the repetition frequency, and at the same time, gain in a soliton state is facilitated.
It should be noted that the dispersion value of the first single mode fiber 17 is about-22 ps2And/nm, the first single-mode fiber 17 is mainly used for adjusting the dispersion value of the whole cavity, and if the accurate dispersion value of the L-band erbium-doped fiber 14 cannot be obtained, the traditional soliton mode locking representing the small negative dispersion of the whole cavity can be used for approaching the small positive dispersion of the whole cavity by cutting short the first single-mode fiber 17 with negative dispersion until a dissipative soliton spectrum representing the positive dispersion of the whole cavity appears.
Optionally, the polarization controller 15 may use a higher-sensitivity extrusion-type polarization controller 15, and the online polarization state adjustment method does not introduce an extra-length cavity length, which is beneficial to implementation of high repetition frequency. In addition, the polarization dependent optical isolator 16 can be selected to be a one-stage polarization isolation device, or a two-stage polarization isolation device, or a three-stage two-stage polarization isolation device, with a higher number of polarization isolation stages meaning a narrower pulse width and more uniform pulse energy.
Preferably, a first erbium-doped fiber amplifier 5 for amplifying the energy of the pulsed laser and finely adjusting the spectral shape is further connected between the second output end of the optical coupler 18 and the first end of the optical circulator 2.
The dispersion medium 3 of this embodiment is a linear chirped bragg grating, and the grating grid distance gradually increases from the entrance of the linear chirped bragg grating to the end, 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, and the linear relationship between the wavelength and the reflection time can be realized by the grating period which changes linearly.
λB＝2neffΛ (1-1)
Wherein λBIs the Bragg wavelength, neffIs the effective index of refraction of the grating, Λ is the grating period;
meanwhile, the linear chirped Bragg grating has a large second-order dispersion value, acts on the narrow pulse, is equivalent to performing approximate Fourier transform (dispersion Fourier transform), can map wavelength sequencing on a spectrum from a frequency domain into a time domain long pulse, and completes popular frequency sweep output. As shown in fig. 4, the spectral shape before and after time-frequency mapping is unchanged, but the time-domain narrow pulses are broadened to be long pulses that approximate the spectral shape. Optionally, in other possible embodiments, the dispersion medium 3 may also be a dispersion compensation fiber with a dispersion coefficient close to linearity, and this embodiment does not limit the kind of the dispersion 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 perform repetition frequency doubling and energy amplification on 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, 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, the other 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, the other end of the third single-mode fiber 44 is connected to a first input end of the third coupler 45, a second output end of the second coupler 43 is connected to a second input end of the third coupler 45, and an output end of the third coupler 45 is connected to the sweep laser output end 7.
Further, the first coupler 41 has a splitting ratio of 50: a 50 1x2 coupler, the first coupler 41 splits 50% of the light into a second single mode fiber 42, the remaining 50% being transmitted forward, the second single mode fiber 42 being a 1/2 cavity length single mode fiber that acts as a delay line such that the delayed optical pulse train is one half of the intracavity cycle period slower than the undelayed pulse train. The second coupler 43 has a splitting ratio of 50: the 50 x 22 coupler is used to combine two pulse trains, delayed and undelayed, to increase the pulse repetition frequency by a factor of 2. The third single-mode fiber 44 is a single-mode fiber with a cavity length of 1/4, the third coupler 45 is a 2x1 coupler, the third single-mode fiber 44 is a delay line similar to the second single-mode fiber 42, and after one pulse is delayed and the other pulse is combined by the third coupler 45, the repetition frequency of the swept-frequency light source is increased to 4 times of the initial frequency. It should be noted that whether to increase the repetition frequency doubling structure may be determined according to the duty ratio of the pulses of the light source 11, and although the duty ratio may reach 100% theoretically, in actual operation, in order to avoid aliasing of adjacent long pulses, a duty ratio of 100% cannot be used.
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 booster-stage erbium-doped fiber amplifier, and the second erbium-doped fiber amplifier 6 can gain the forward pulse subjected to time stretching and repetition frequency doubling so as to achieve the spectral energy most suitable for the subsequent interference imaging requirement, and meanwhile, the spectral shape can be properly fine-tuned.
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 graph of a long pulse obtained after time stretching. It can be seen that the utility model discloses the dissipation soliton mode-locking spectrum that utilizes NPR mode locking to produce, the 3dB bandwidth reaches 71nm, and the long pulse width after the time stretching is 12.4ns, and the duty cycle is 39%, and each wavelength is linear arrangement in every long pulse in figure 2.
The utility model discloses another embodiment still provides an OCT imaging system, and OCT imaging system includes the sweep frequency light source of above-mentioned technical scheme's application NPR mode locking.
To sum up, the utility model discloses a carry out dispersion Fourier transform in the pulse laser input dispersion medium 3 of NPR mode locking pulse output unit output, through the mode of time stretching promptly, map the spectral information of frequency domain to the long pulse of time domain that the frequency was arranged in proper order on for the repetition frequency of light source pulse is equal to the line scanning frequency of optical coherence tomography imaging system, can reach 101～102MHz, thereby solving the problem that the sweep frequency speed of the sweep frequency light source in the related technology is not fast enough. In the aspect of imaging quality, the mode-locked seed with flat spectrum 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 above description is only exemplary of the present invention and should not be taken as limiting the scope of the present invention, as any modifications, equivalents, improvements and the like made within the spirit and principles of the present invention are intended to be included within the scope of the present invention.
Claims (10)
1. A swept-frequency light source applying NPR mode locking is characterized by comprising an NPR mode-locking pulse output unit, an optical circulator (2), a dispersion medium (3) and a swept-frequency laser output end (7), wherein the NPR mode-locking pulse output unit is used for outputting pulse laser, the first end of the optical circulator (2) is connected with the NPR mode-locking pulse output unit, the second end of the optical circulator (2) is connected with the dispersion medium (3), the third end of the optical circulator (2) is connected with the swept-frequency laser output end (7), the pulse laser enters from the first end of the optical circulator (2) and is transmitted into the dispersion medium (3) through the second end of the optical circulator (2), the pulse laser forms long pulses after carrying out Fourier transform in the dispersion medium (3), and the long pulses pass through the second end of the optical circulator (2) and the third end of the optical circulator (2) and are transmitted from the swept-frequency laser output end (7) And outputting by a laser output end (7).
2. A swept-frequency light source applying NPR mode locking as claimed in claim 1, wherein the NPR mode-locked pulse output unit comprises 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-dependent optical isolator (16), a first single-mode fiber (17), and an optical coupler (18), wherein an output end of the light source (11) is connected with a first input end of the wavelength division multiplexer (12), an output end of the wavelength division multiplexer (12) is connected with one end of the C-band erbium-doped fiber (13), the other end of the C-band erbium-doped fiber (13) is connected with one end of the L-band erbium-doped fiber (14), the other end of the L-band erbium-doped fiber (14) is connected with an input end of the polarization controller (15), an output end of the polarization controller (15) is connected with an input end of the polarization-dependent optical isolator (16), the output end of the polarization-dependent optical isolator (16) is connected with one end of the first single-mode optical fiber (17), the other end of the first single-mode optical fiber (17) is connected with the first input end of the optical coupler (18), the first output end of the optical coupler (18) is connected with the second input end of the wavelength division multiplexer (12), and the second output end of the optical coupler (18) outputs the pulse laser.
3. A swept-frequency optical source using NPR mode-locking as claimed in claim 2, wherein the L-band erbium-doped fiber (14) is surrounded inside the polarization controller (15).
4. A swept-frequency optical source using NPR mode-locking according to claim 2, wherein a first erbium-doped fiber amplifier (5) for amplifying the energy of the pulsed laser and trimming the spectral shape is further connected between the second output terminal of the optical coupler (18) and the first end of the optical circulator (2).
5. A swept-frequency light source using NPR mode-locking according to claim 1, wherein the dispersive medium (3) is a linearly chirped bragg grating, and the grating grid distance gradually increases from the entrance to the end of the linearly chirped bragg grating, and the dispersion value also gradually increases accordingly.
6. A swept-frequency optical source using NPR mode-locking as defined in claim 1, wherein a repetition frequency doubling structure is further provided between the third end of the optical circulator (2) and the swept-frequency laser output end (7).
7. A swept-frequency light source applying NPR mode locking according to claim 6, wherein 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 an input end of the first coupler (41) is connected with a third end of the optical circulator (2), a 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 a first input end of the second coupler (43), a second output end of the first coupler (41) is connected with a second input end of the second coupler (43), a first output end of the second coupler (43) is connected with one end of the third single-mode fiber (44), and the other end of the third single-mode fiber (44) is connected with a first input end of the third coupler (45), and a second output end of the second coupler (43) is connected with a second input end of the third coupler (45), and an output end of the third coupler (45) is connected with the swept-frequency laser output end (7).
8. A swept-frequency optical source using NPR mode locking according to claim 7, wherein a second erbium-doped fiber amplifier (6) is further connected between the output end of the third coupler (45) and the swept-frequency laser output end (7), and the second erbium-doped fiber amplifier (6) is a booster-stage erbium-doped fiber amplifier.
9. A swept optical source using NPR mode locking according to claim 7, wherein the first coupler (41) is a 50: 50, the second single-mode fiber (42) is a 1/2 cavity length single-mode fiber, and the second coupler (43) is a fiber splitting ratio of 50: a 50 x2 coupler, the third single mode fiber (44) being a 1/4 cavity length single mode fiber, and the third coupler (45) being a 2x1 coupler.
10. An OCT imaging system comprising a swept source using NPR mode locking as claimed in any one of claims 1 to 9.
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