All-fiber flat high-coherence supercontinuum light source and working method thereof

Abstract

The present invention discloses an all-fiber flat high-coherence supercontinuum light source and a working method thereof, which can realize a cross-octave flat high-coherence supercontinuum spectrum covering the near-infrared long-wave band. The light source solves the problems of complex structure, low integration, and the need for regular maintenance of the traditional supercontinuum light source system, and the difficulty of the generated supercontinuum spectrum to be compatible with good spectral flatness and time coherence, and has the advantages of all-fiber, easy assembly and operation. It includes: an all-fiber femtosecond pulse mode-locked oscillator, a laser pulse stretcher, a first-stage preamplifier, a second-stage main amplifier, a laser pulse compressor, and a nonlinear medium arranged in sequence along the optical path, wherein the nonlinear medium is a dispersion-flattened high-nonlinear single-mode quartz optical fiber with a nominal dispersion of zero at 1550 nm, and is connected to the laser pulse compressor by optical fiber fusion splicing.

Description

Technical Field

[0001] The present invention relates to the technical field of lasers, and in particular to an all-fiber flat high-coherence supercontinuum light source, and a working method of the all-fiber flat high-coherence supercontinuum light source. Background Art

[0002] Supercontinuum light sources have unique application advantages in the fields of spectroscopy, optical communications, atmospheric science, optical coherence tomography, environmental monitoring, life sciences, and national defense, as they have the wide spectrum characteristics of ordinary light sources and the high brightness characteristics of laser light sources, and have attracted widespread attention. In the early research on supercontinuum generation, the two key nodes were the acquisition of fiber-based supercontinuum light sources in 1976 and the invention of photonic crystal fiber (PCF) in 1996. The former solved the problem of nonlinear action distance and energy density, and the latter provided an effective means of generating supercontinuum light sources due to the high nonlinear coefficient and freely adjustable dispersion of photonic crystal fiber. Since then, the development of fiber-based supercontinuum light sources has been on the right track and has entered a rapid development stage in the past 20 years. Especially in terms of spectral flatness and coherence, in recent years, the importance of flatness and temporal coherence of supercontinuum light sources has become increasingly prominent in applications such as biological microscopy and optical coherence tomography, because in practical applications, a flat supercontinuum light source can not only improve the accuracy of measurement, but also reduce the difficulty of power balancing; the coherence of supercontinuum light sources has become the main factor limiting acquisition speed, sensitivity and resolution due to the continuous improvement of the sensitivity and speed of spectral detection and imaging technology.

[0003] After exploring the generation mechanism of supercontinuum spectrum, people found that using picosecond or longer pump pulses to pump nonlinear optical fibers can obtain flat supercontinuum light sources. Since the soliton order of picosecond pulses is very high, during the nonlinear optical fiber transmission process, the high-order soliton splitting will produce many basic soliton clusters. The basic soliton clusters are superimposed in the frequency domain to achieve the high flatness effect of the supercontinuum light source. However, since the pulse width of picosecond or longer will cause nonlinear amplification of the noise signal in the nonlinear optical fiber, the spectral amplitude and phase of these supercontinuum light sources have large inter-pulse fluctuations, and can only provide poor temporal coherence. Using femtosecond pulses with a smaller soliton order as a pump source can avoid the degradation of the temporal coherence of the supercontinuum light source, but the spectral flatness of these supercontinuum light sources is poor.

[0004] Therefore, although some progress has been made in generating supercontinuum light sources with high flatness and high temporal coherence, there are the following problems with this technology: first, most of the current supercontinuum light sources are difficult to have both good spectral flatness and temporal coherence; second, quartz fiber is used in erbium-doped or thulium-doped femtosecond fiber lasers as pump sources for generating supercontinuum, while the nonlinear medium for generating supercontinuum is usually special optical fibers such as photonic crystal fiber or soft glass fiber. The physical properties of these optical fibers are quite different from those of quartz fiber, which makes it difficult to connect them by fusion splicing or the fusion splicing efficiency is very low. Therefore, the connection between the nonlinear optical fiber and the pump source in the supercontinuum light source is achieved by spatial docking, which results in the need for regular optical path adjustment and dust removal maintenance for these supercontinuum light sources, low long-term working stability of the system, and poor environmental adaptability. In addition, the use of special optical fibers as nonlinear media to achieve supercontinuum generation of cross-octave spectrum requires high pump power, usually requiring the average power of the laser to reach the watt level or even tens of watts. At this time, the heat dissipation of the laser becomes an issue that cannot be ignored. In order to ensure good heat dissipation, the actual structure of these supercontinuum light sources is often large and complex. Finally, at present, this type of special optical fiber is usually designed and prepared in the laboratory. Whether from the perspective of cost or quality control, there is a certain transformation period from engineering application to mass production.

[0005] Therefore, in response to the above problems, it is of great significance to develop a supercontinuum light source that has both high spectral flatness and high temporal coherence, simple system structure, high integration, good stability, strong environmental adaptability, and is conducive to mass production and cost control. Summary of the invention

[0006] In order to overcome the defects of the prior art, the technical problem to be solved by the present invention is to provide an all-fiber flat high-coherence supercontinuum light source, which can realize a cross-octave flat high-coherence supercontinuum spectrum covering the near-infrared long-wave band. The light source solves the problems of complex structure, low integration and regular maintenance of traditional supercontinuum light source systems, and the difficulty of the generated supercontinuum spectrum to be compatible with good spectral flatness and temporal coherence. It has the advantages of all-fiber and easy assembly and operation.

[0007] The technical solution of the present invention is: the all-fiber flat high-coherence supercontinuum light source comprises: an all-fiber femtosecond pulse mode-locked oscillator, a laser pulse stretcher, a first-stage preamplifier, a second-stage main amplifier, a laser pulse compressor, and a nonlinear medium arranged in sequence along the optical path;

[0008] The all-fiber femtosecond pulse mode-locked oscillator generates a stable mode-locked laser output; the laser pulse stretcher stretches the pulse width of the output pulse of the all-fiber femtosecond pulse mode-locked oscillator, and the first-stage pre-amplifier pre-amplifies the laser power output by the laser pulse stretcher to provide a pulsed laser light source for the second-stage main amplifier; the second-stage main amplifier amplifies the weak signal light of the previous stage, and the laser pulse compressor compresses the pulse width of the signal light. After compression, a high-power femtosecond pulse laser with a pulse width of sub-hundred femtoseconds is used to pump the nonlinear medium, and the spectrum is broadened by the nonlinear effects of high-order soliton splitting, soliton self-frequency shift and dispersive waves, so as to obtain a cross-octave flat high-coherence supercontinuum spectrum covering the near-infrared long-wave band; the nonlinear medium is a dispersion-flattened, highly nonlinear single-mode quartz fiber with a nominal dispersion of zero at 1550 nm, and the fiber is connected to the laser pulse compressor by fiber fusion splicing.

[0009] The nonlinear medium of the present invention is a dispersion-flattened high-nonlinear single-mode quartz optical fiber with a nominal dispersion of zero at 1550 nm. The nonlinear medium is connected to a laser pulse compressor by optical fiber fusion splicing. No spatial optical device is required in the system, and the system is a fully optical fiber structure based entirely on single-mode quartz optical fiber. As a cross-octave flat high-coherence supercontinuum light source covering the near-infrared long-wave band, all devices used in the system are commercial communication band single-mode optical fiber devices, and the light source has good beam quality. Therefore, the device of the present invention has a simple structure and a high degree of integration, and can realize a cross-octave flat high-coherence supercontinuum covering the near-infrared long-wave band. The light source solves the problems of complex structure, low degree of integration, need for regular maintenance, and difficulty in compatibility of the generated supercontinuum with good spectral flatness and time coherence of traditional supercontinuum light source systems, and has the advantages of full fiberization, easy assembly and operation.

[0010] The working method of the all-fiber flat high-coherence supercontinuum light source is also provided. The passive mode locking technology is used to realize the self-starting mode locking of the laser pulse. The all-fiber pulse mode-locked oscillator generates a stable high-repetition-rate femtosecond pulse mode-locked laser output. After the seed light pulse width is widened to the picosecond level by the pulse stretcher, the optical power is pre-amplified by the first-stage pre-amplifier to provide a high-quality pulse laser light source for the second-stage main amplifier to ensure that the main amplifier has sufficient energy amplification. The second-stage main amplifier amplifies the weak signal light of the previous stage to obtain a high-power pulse laser output. The pulse width of the high-power pulse is compressed to sub-hundred femtoseconds, and finally the high-power femtosecond laser is used to pump a highly nonlinear single-mode quartz fiber. The spectrum is broadened by nonlinear effects such as high-order soliton splitting, soliton self-frequency shift and dispersive wave generation, to obtain a cross-octave flat high-coherence supercontinuum spectrum covering the near-infrared long-wave band. The high-nonlinear fiber has the characteristics of low dispersion slope and near-zero dispersion value at the pump wavelength, which makes the supercontinuum spectrum have good flatness. The temporal coherence degradation of the supercontinuum spectrum is avoided by using pump pulses with a pulse width of less than 100fs. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 It is a schematic structural diagram of the all-fiber flat high-coherence supercontinuum light source of the present invention.

[0012] Figure 2 Schematic diagram of the structure of the all-fiber pulse mode-locked oscillator 10 of the present invention.

[0013] Figure 3 FIG. 4 is a schematic diagram of the structure of the first stage pre-amplifier 30 of the present invention.

[0014] Figure 4 FIG. 4 is a schematic diagram of the structure of the second-stage main amplifier 40 of the present invention. DETAILED DESCRIPTION

[0015] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0016] It should be noted that the term "comprises" and any variations in the specification and claims of the present invention and the above-mentioned drawings are intended to cover non-exclusive inclusions. For example, a process, method, apparatus, product or equipment that includes a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or are inherent to these processes, methods, products or equipment.

[0017] like Figure 1 As shown, the all-fiber flat high-coherence supercontinuum light source comprises: an all-fiber femtosecond pulse mode-locked oscillator 10, a laser pulse stretcher 20, a first-stage pre-amplifier 30, a second-stage main amplifier 40, a laser pulse compressor 50, and a nonlinear medium 60 arranged in sequence along the optical path;

[0018] The all-fiber femtosecond pulse mode-locked oscillator generates a stable mode-locked laser output; the laser pulse stretcher stretches the pulse width of the output pulse of the all-fiber femtosecond pulse mode-locked oscillator, and the first-stage pre-amplifier pre-amplifies the laser power output by the laser pulse stretcher to provide a pulsed laser light source for the second-stage main amplifier; the second-stage main amplifier amplifies the weak signal light of the previous stage, and the laser pulse compressor compresses the pulse width of the signal light. After compression, a high-power femtosecond pulse laser with a pulse width of sub-hundred femtoseconds (i.e., 60-99 femtoseconds) is used to pump the nonlinear medium, and the spectrum is broadened by nonlinear effects such as high-order soliton splitting, soliton self-frequency shift, and dispersion wave generation, to obtain a cross-octave flat high-coherence supercontinuum spectrum that can cover the near-infrared long-wave band. The pump pulse with a pulse width of less than 100 fs after compression can effectively avoid the temporal coherence degradation of the supercontinuum spectrum.

[0019] The nonlinear medium is a dispersion-flattened highly nonlinear single-mode quartz fiber with a nominal dispersion of zero at 1550 nm. The fiber has the characteristics of close to zero group velocity dispersion and low dispersion slope at the pump wavelength. In the process of spectrum broadening, it supports more phase matching conditions to produce a supercontinuum spectrum with good flatness. The flatness and low dispersion of the fiber also help the supercontinuum spectrum to have good temporal coherence. The fiber is connected to the laser pulse compressor by fiber fusion splicing. No spatial optical devices are required in the system. It is an all-fiber structure based entirely on single-mode quartz fiber.

[0020] The nonlinear medium of the present invention is a dispersion-flattened high-nonlinear single-mode quartz optical fiber with a nominal dispersion of zero at 1550 nm. The nonlinear medium is connected to a laser pulse compressor by optical fiber fusion splicing. No spatial optical device is required in the system, and the system is a fully optical fiber structure based entirely on single-mode quartz optical fiber. As a cross-octave flat high-coherence supercontinuum light source covering the near-infrared long-wave band, all devices used in the system are commercial communication band single-mode optical fiber devices, and the light source has good beam quality. Therefore, the device of the present invention has a simple structure and a high degree of integration, and can realize a cross-octave flat high-coherence supercontinuum covering the near-infrared long-wave band. The light source solves the problems of complex structure, low degree of integration, need for regular maintenance, and difficulty in compatibility of the generated supercontinuum with good spectral flatness and time coherence of traditional supercontinuum light source systems, and has the advantages of full fiberization, easy assembly and operation.

[0021] Preferably, the all-fiber femtosecond pulse mode-locked oscillator comprises: a first single-mode pigtail output laser diode 11, and a first wavelength division multiplexer 12, a first highly doped single-mode erbium-doped fiber 13, a first high-power optical isolator 14, a wide-band saturable absorber 15 and a first fiber coupler output 16 arranged in sequence along the optical path;

[0022] In the ring cavity, the first single-mode pigtail output laser diode is connected to the pump end of the first wavelength division multiplexer through the optical fiber pigtail, the common end pigtail of the first wavelength division multiplexer is fused with the first highly doped single-mode erbium-doped optical fiber, and then fused with the output end of the first high-power optical isolator; the input end of the first high-power optical isolator is connected to the wide-band saturable absorber through the optical fiber pigtail; the wide-band saturable absorber is connected to one end of the output end of the first optical fiber coupler output device through the optical fiber pigtail; the coupling input end of the first optical fiber coupler output device is fused with the signal end of the first wavelength division multiplexer, and the other end is used as the output end of the oscillator to realize the output of mode-locked laser pulses.

[0023] Preferably, the all-fiber femtosecond pulse mode-locked oscillator is a ring cavity, a linear cavity non-polarization-maintaining mode-locked oscillator, a polarization-maintaining mode-locked oscillator, a nonlinear polarization rotation mode-locked fiber oscillator, or a nonlinear ring mirror mode-locked fiber oscillator based on a broadband saturable absorber; the wide-band saturable absorber is a carbon nanotube, graphene or a topological insulator; the central wavelength of the seed light output by the oscillator is 1560nm, and the pulse width is on the order of hundreds of femtoseconds.

[0024] Preferably, the laser pulse stretcher comprises: a pulse stretching optical fiber with a positive group velocity dispersion at a wavelength of 1560 nm, or a broadband fiber Bragg grating with a positive group delay dispersion at a wavelength of 1560 nm, and the laser pulse stretcher is a non-polarization-maintaining single-mode optical fiber compressor or a polarization-maintaining single-mode optical fiber compressor.

[0025] Preferably, the first stage preamplifier comprises: a second high-power optical isolator 31, a second single-mode pigtail output laser diode 32, and a second wavelength division multiplexer 33 and a second highly doped single-mode erbium-doped optical fiber 34 sequentially arranged along the optical path;

[0026] The input end of the second high-power optical isolator is connected to the output end of the laser pulse stretcher; the second wavelength division multiplexer is placed on the optical path between the second high-power optical isolator and the second highly doped single-mode erbium-doped optical fiber, and the signal end of the second wavelength division multiplexer is fused to the output end of the second high-power optical isolator; the second single-mode pigtail output laser diode is connected to the second wavelength division multiplexer through the optical fiber pigtail; one end of the second highly doped single-mode erbium-doped optical fiber is fused to the common end of the second multiplexer, and the other end serves as the output end of the first stage pre-amplifier;

[0027] The second highly doped single-mode erbium-doped optical fiber is a non-polarization-maintaining highly doped single-mode erbium-doped optical fiber or a polarization-maintaining highly doped single-mode erbium-doped optical fiber. The optical fiber devices and optical fibers other than the laser diode are both polarization-maintaining or non-polarization-maintaining structures.

[0028] Preferably, the second stage main amplifier comprises: a third high power optical isolator 41, a third single mode pigtail output laser diode 42, and a third wavelength division multiplexer 43 and a third highly doped single mode erbium doped optical fiber 44 sequentially arranged along the optical path;

[0029] The input end of the third high-power optical isolator is connected to the output end of the first-stage pre-amplifier; the third wavelength division multiplexer is placed on the optical path between the third high-power optical isolator and the third highly doped single-mode erbium-doped optical fiber, and the signal end of the third wavelength division multiplexer is fused to the output end of the third high-power optical isolator; the third single-mode pigtail output laser diode is connected to the third wavelength division multiplexer through the optical fiber pigtail; one end of the third highly doped single-mode erbium-doped optical fiber is fused to the common end of the third multiplexer, and the other end serves as the output end of the second-stage main amplifier.

[0030] Preferably, the third highly doped single-mode erbium-doped optical fiber is a non-polarization-maintaining highly doped single-mode erbium-doped optical fiber or a polarization-maintaining highly doped single-mode erbium-doped optical fiber. Fiber optic devices and optical fibers other than laser diodes are both polarization-maintaining or non-polarization-maintaining structures.

[0031] Preferably, the laser pulse compressor comprises: a single-mode optical fiber with a negative group velocity dispersion at a wavelength of 1560 nm or a broadband fiber Bragg grating with a negative group delay dispersion at a wavelength of 1560 nm;

[0032] The laser pulse compressor is connected to the output end of the second-stage main amplifier. According to the laser pulse parameters output by the second-stage main amplifier, the laser pulse width after passing through the fiber compressor is compressed to sub-hundred femtoseconds by controlling the length of the single-mode fiber or the group delay dispersion of the broadband fiber Bragg grating, so as to generate a supercontinuum spectrum with high coherence. The laser pulse compressor is a non-polarization-maintaining single-mode fiber compressor or a polarization-maintaining single-mode fiber compressor.

[0033] Preferably, the high nonlinear single-mode quartz optical fiber is a non-polarization-maintaining high nonlinear single-mode quartz optical fiber or a polarization-maintaining high nonlinear single-mode quartz optical fiber.

[0034] The working method of the all-fiber flat high-coherence supercontinuum light source is also provided. The passive mode locking technology is used to realize the self-starting mode locking of the laser pulse. The all-fiber pulse mode-locked oscillator generates a stable high-repetition-rate femtosecond pulse mode-locked laser output. After the seed light pulse width is widened to the picosecond level by the pulse stretcher, the optical power is pre-amplified by the first-stage pre-amplifier to provide a high-quality pulse laser light source for the second-stage main amplifier to ensure that the main amplifier has sufficient energy amplification. The second-stage main amplifier amplifies the weak signal light of the previous stage to obtain a high-power pulse laser output. The pulse width of the high-power pulse is compressed to sub-hundred femtoseconds, and finally the high-power femtosecond laser is used to pump a highly nonlinear single-mode quartz fiber. The spectrum is broadened by nonlinear effects such as high-order soliton splitting, soliton self-frequency shift and dispersive wave generation, to obtain a cross-octave flat high-coherence supercontinuum spectrum covering the near-infrared long-wave band. The high-nonlinear fiber has the characteristics of low dispersion slope and near-zero dispersion value at the pump wavelength, which makes the supercontinuum spectrum have good flatness. The temporal coherence degradation of the supercontinuum spectrum is avoided by using pump pulses with a pulse width of less than 100fs.

[0035] The following content will explain the implementation process of the entire scheme from the mode-locked oscillator to the step-by-step amplification.

[0036] Figure 1-4 The functions of each component are as follows:

[0037] An all-fiber pulse mode-locked oscillator 10, used to generate a stable mode-locked pulse laser;

[0038] The first single-mode pigtail output laser diode 11 is used to pump the gain optical fiber to generate gain;

[0039] A first wavelength division multiplexer 12, used for coupling the pump light into the gain fiber and forming a resonant cavity;

[0040] A first highly doped single-mode erbium-doped optical fiber 13, used to provide a gain medium for the oscillator;

[0041] The first high-power optical isolator 14 realizes the isolation of reverse light to ensure the unidirectional transmission of laser light in the resonant cavity;

[0042] A wide-band saturable absorber 15, used for generating a mode-locked pulse laser;

[0043] The first optical fiber coupler output device 16 is used to keep part of the seed light in the resonant cavity to continue oscillating and transmitting, and couple out another part of the seed light to provide signal light for the next stage of amplification;

[0044] A laser pulse stretcher 20, used to stretch the pulse width of the mode-locked laser output by the oscillator;

[0045] The first stage pre-amplifier 30 is used to amplify the broadened mode-locked pulse laser;

[0046] The second high-power optical isolator 31 realizes the isolation of reverse light to avoid affecting the normal operation of the previous stage device;

[0047] The second single-mode pigtail output laser diode 32 is used to provide pump light for the first stage pre-amplifier;

[0048] A second wavelength division multiplexer 33, used for coupling the pump light into the gain optical fiber;

[0049] A second highly doped single-mode erbium-doped optical fiber 34 is used to provide a gain medium to generate gain;

[0050] The second stage main amplifier 40 is used to amplify the previous stage signal light to achieve high power pulse laser output;

[0051] A third high-power optical isolator 41 is used to isolate the reverse light and avoid damaging the previous stage device;

[0052] The third single-mode pigtail output laser diode 42 is used to provide pump light for the second-stage main amplifier;

[0053] A third wavelength division multiplexer 43, used for coupling the pump light output by the single-mode laser diode into the high-doped gain optical fiber;

[0054] A highly doped erbium-doped optical fiber 44, used to provide a gain medium for the second-stage main amplifier;

[0055] A laser pulse compressor 50, used to compress the pulse width of the amplified high-power laser to tens of femtoseconds;

[0056] The nonlinear medium 60 provides a high nonlinear coefficient for realizing the generation of a flat high-coherence supercontinuum spectrum across an octave.

[0057] The connection relationship between the above components is:

[0058] The all-fiber pulse mode-locked oscillator 10 of the present invention comprises a first single-mode pigtail output laser diode 11, a first wavelength division multiplexer 12, a first highly doped single-mode erbium-doped fiber 13, a first high-power optical isolator 14, a wide-band saturable absorber 15, and a first fiber coupler output device 16, which are sequentially arranged along the optical path;

[0059] In the ring cavity, by means of core pumping, the first single-mode pigtail output laser diode 11 pumps the first highly doped single-mode erbium-doped optical fiber 13 via coupling of the first wavelength division multiplexer 12 to generate gain, and a first high-power optical isolator 14 and a wide-band saturable absorber 15 are used to achieve a stable mode-locked pulse sequence output, and part of the seed light is coupled out via the first optical fiber coupler output device 16;

[0060] The laser pulse stretcher 20 of the present invention includes a pulse stretching optical fiber with a positive group velocity dispersion at a wavelength of 1560nm or a broadband fiber Bragg grating with a positive group delay dispersion at a wavelength of 1560nm. The input end of the stretcher 20 is connected to the output end of the oscillator 10 to stretch the pulse width of the mode-locked pulse output by the oscillator to provide signal light for the next-stage amplifier.

[0061] The first stage preamplifier 30 of the present invention comprises a second high-power optical isolator 31, a second wavelength division multiplexer 33, a second highly doped single-mode erbium-doped optical fiber 34, and a second single-mode pigtail output laser diode 32 connected to the second wavelength division multiplexer 33 via an optical fiber pigtail, which are sequentially arranged along the optical path. The input end of the second high-power optical isolator 31 is connected to the output end of the expander 20 to prevent the adverse effect of the backward feedback light on the previous stage system. The second single-mode pigtail output laser diode 32 pumps the second highly doped single-mode erbium-doped optical fiber 34 via the coupling of the second wavelength division multiplexer 33. The signal light output by the second high-power optical isolator 31 is coupled into the second highly doped single-mode erbium-doped optical fiber 34 via the second wavelength division multiplexer 33 to achieve amplification of the weak signal light and then enters the next stage amplifier.

[0062] The second stage main amplifier 40 of the present invention comprises a third high power optical isolator 41, a third wavelength division multiplexer 43, a third highly doped single mode erbium doped optical fiber 44 and a third single mode pigtail output laser diode 42 connected to the third wavelength division multiplexer 43 via an optical fiber pigtail, which are sequentially arranged along the optical path. The input end of the third high power optical isolator 41 is connected to the output end of the stretcher 30 to prevent the backward feedback light from causing adverse effects on the previous stage system. The third wavelength division multiplexer 43 couples the pump light output by the third single mode pigtail output laser diode 42 into the third highly doped single mode erbium doped optical fiber 44 to generate gain to achieve power amplification of the incident laser at the previous stage.

[0063] The laser pulse compressor 50 of the present invention includes a pulse stretching optical fiber with a negative group velocity dispersion at a wavelength of 1560nm or a broadband fiber Bragg grating with a negative group delay dispersion at a wavelength of 1560nm. The input end of the stretcher 50 is connected to the output end of the main amplifier 40 to compress the pulse width of the high-power laser pulse output by the amplifier, thereby achieving stable high-power femtosecond laser operation and serving as a high-performance pump source for the next-level nonlinear medium.

[0064] The nonlinear medium 60 for generating a supercontinuum spectrum of the present invention is a dispersion-flattened highly nonlinear single-mode quartz optical fiber with a nominal dispersion of zero at 1550 nm, and the optical fiber has a high nonlinear coefficient and a low dispersion slope. The input end of the highly nonlinear single-mode quartz optical fiber 60 is connected to the output end of the laser pulse compressor 50, and a cross-octave flat highly coherent supercontinuum spectrum covering the near-infrared long-wave band is generated by utilizing the nonlinear effect.

[0065] The specific working process of an embodiment of this system is as follows:

[0066] In the ring cavity, the first wavelength division multiplexer 12 is used to couple the pump light output by the first single-mode pigtail output laser diode 11 into the core of the first highly doped single-mode erbium-doped optical fiber 13 to ensure sufficient gain in the cavity. The generated continuous light continuously passes through the wide-band saturable absorber 15 in the ring cavity composed of the first wavelength division multiplexer 12 and the first high-power optical isolator 14, and finally a stable self-starting mode-locked pulse is realized in the cavity. Part of the laser in the cavity remains in the cavity and continues to transmit, and the remaining pulse light is coupled out through the first optical fiber coupler output device 16.

[0067] The mode-locked pulse generated by the all-fiber pulse mode-locked oscillator 10 enters the laser pulse stretcher 20. The stretcher 20 is composed of a stretching optical fiber with a positive group velocity dispersion at a wavelength of 1560nm. The pulse width of the mode-locked pulse generated by the all-fiber pulse mode-locked oscillator 10 is stretched to the picosecond level as the signal light used by the next-stage amplifier.

[0068] The signal light output by the laser pulse stretcher 20 enters the first-stage pre-amplifier 30, and the second single-mode pigtail output laser diode 32 is coupled into the second highly doped single-mode erbium-doped optical fiber 34 through the second wavelength division multiplexer 33 to pre-amplify the weak signal light, thereby improving the energy of the picosecond pulse laser while avoiding the occurrence of nonlinear distortion in the output pulse spectrum of the pre-amplifier, and keeping the amplification of spontaneous emission (ASE) level at the lowest possible level, thereby providing signal light for the next stage of amplification, wherein the second high-power optical isolator 31 is used to prevent the backward feedback light from causing adverse effects on the previous stage system.

[0069] The peak power of the laser pulse output by the first-stage pre-amplifier 30 is relatively low, and effective energy amplification can be obtained. Therefore, it enters the second-stage main amplifier 40 for high-power amplification of the signal light. The third wavelength division multiplexer 43 couples the pump light output by the third single-mode pigtail output laser diode 42 into the third highly doped single-mode erbium-doped optical fiber 44 to generate gain, and amplifies the power of the previous-stage incident laser to the order of hundreds of milliwatts, thereby achieving high average power laser output. The third high-power optical isolator 41 is used to prevent the backward feedback light from causing adverse effects on the previous-stage system.

[0070] The high-power laser pulse generated by the second main amplifier 40 is compressed via a laser pulse compressor 50. The compressor 50 is composed of a single-mode optical fiber with a negative group velocity dispersion at a wavelength of 1560nm. By optimizing the length of the single-mode optical fiber, the pulse width of the high-average-power laser output by the second main amplifier 40 is compressed to tens of femtoseconds. The obtained high-average-power femtosecond laser pulse is used as a pump light source for generating a flat and highly coherent supercontinuum spectrum.

[0071] The average power femtosecond laser pulses output by the laser pulse compressor are pumped into the nonlinear medium 60 for generating the supercontinuum spectrum, and the nonlinear effect is used to generate a cross-octave flat high-coherence supercontinuum spectrum covering the near-infrared long-wave band. The nonlinear medium 60 is a highly nonlinear single-mode quartz fiber with a nominal dispersion of zero at 1550 nm, a high nonlinear coefficient and a low dispersion slope. The fiber length is usually several tens of meters. By controlling the length of the fiber, the width and flatness of the generated supercontinuum spectrum can be optimized to a certain extent.

[0072] The present invention is a fully fiber-optic structure based entirely on single-mode quartz optical fiber. The erbium-doped femtosecond laser and the highly nonlinear optical fiber used as the pump source are connected by high-efficiency fusion splicing, and no spatial optical devices are required in the system. As a cross-octave flat high-coherence supercontinuum light source covering the near-infrared long-wave band, all devices used in the system are commercial communication band single-mode optical fiber devices, and the light source has good beam quality. The patented device of the present invention has a simple structure, high integration, good stability, and is easy to maintain. It is easy to realize the miniaturization and portability of the system, adapt to harsh working environments, and is conducive to mass production and cost control in actual engineering.

[0073] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Any simple modification, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention are still within the protection scope of the technical solution of the present invention.

Claims

1. All-fiber flat high-coherence supercontinuum light source, characterized by: It includes: An all-fiber femtosecond pulse mode-locked oscillator (10), a laser pulse stretcher (20), a first-stage preamplifier (30), a second-stage main amplifier (40), a laser pulse compressor (50), and a nonlinear medium (60) are sequentially arranged along the optical path; The all-fiber femtosecond pulse mode-locked oscillator generates a stable mode-locked laser output; the laser pulse stretcher stretches the pulse width of the output pulse of the all-fiber femtosecond pulse mode-locked oscillator, and the first-stage pre-amplifier pre-amplifies the laser power output by the laser pulse stretcher to provide a pulsed laser light source for the second-stage main amplifier; The second-stage main amplifier amplifies the weak signal light of the previous stage, and the laser pulse compressor compresses the pulse width of the signal light. The high-power femtosecond pulse laser with a pulse width of sub-hundred femtoseconds after compression pumps the nonlinear medium, and uses nonlinear effects such as high-order soliton splitting, soliton self-frequency shift and dispersive wave generation to broaden the spectrum, thereby obtaining a cross-octave flat high-coherence supercontinuum spectrum covering the near-infrared long-wave band; the nonlinear medium is a dispersion-flattened, highly nonlinear single-mode quartz optical fiber with a nominal dispersion of zero at 1550nm, and the optical fiber is connected to the laser pulse compressor by fiber fusion splicing.

2. The all-fiber flat high-coherence supercontinuum light source according to claim 1, characterized in that: The all-fiber femtosecond pulse mode-locked oscillator comprises: a first single-mode pigtail output laser diode (11), and a first wavelength division multiplexer (12), a first highly doped single-mode erbium-doped optical fiber (13), a first high-power optical isolator (14), a wide-band saturable absorber (15) and a first optical fiber coupler output device (16) arranged in sequence along an optical path; In the ring cavity, the first single-mode pigtail output laser diode is connected to the pump end of the first wavelength division multiplexer through the optical fiber pigtail, the common end pigtail of the first wavelength division multiplexer is fused with the first highly doped single-mode erbium-doped optical fiber, and then fused with the output end of the first high-power optical isolator; the input end of the first high-power optical isolator is connected to the wide-band saturable absorber through the optical fiber pigtail; the wide-band saturable absorber is connected to one end of the output end of the first optical fiber coupler output device through the optical fiber pigtail; the coupling input end of the first optical fiber coupler output device is fused with the signal end of the first wavelength division multiplexer, and the other output end is used as the output end of the oscillator to realize the output of mode-locked laser pulses.

3. The all-fiber flat high-coherence supercontinuum light source according to claim 1, characterized in that: The all-fiber femtosecond pulse mode-locked oscillator is a ring cavity, a linear cavity non-polarization-maintaining mode-locked oscillator, a polarization-maintaining mode-locked oscillator, a nonlinear polarization rotation mode-locked fiber oscillator, or a nonlinear ring mirror mode-locked fiber oscillator based on a broadband saturable absorber; the broadband saturable absorber is a carbon nanotube, graphene or a topological insulator; the central wavelength of the seed light output by the oscillator is 1560nm, and the pulse width is on the order of hundreds of femtoseconds.

4. The all-fiber flat high-coherence supercontinuum light source according to claim 3, characterized in that: The laser pulse stretcher comprises: a pulse stretching optical fiber with a positive group velocity dispersion at a wavelength of 1560nm, or a broadband optical fiber Bragg grating with a positive group delay dispersion at a wavelength of 1560nm. The laser pulse stretcher is a non-polarization-maintaining single-mode optical fiber compressor or a polarization-maintaining single-mode optical fiber compressor.

5. The all-fiber flat high-coherence supercontinuum light source according to claim 4, characterized in that: The first-stage preamplifier comprises: a second high-power optical isolator (31), a second single-mode pigtail output laser diode (32), and a second wavelength division multiplexer (33) and a second highly doped single-mode erbium-doped optical fiber (34) sequentially arranged along the optical path; The input end of the second high-power optical isolator is connected to the output end of the laser pulse stretcher; the second wavelength division multiplexer is placed on the optical path between the second high-power optical isolator and the second highly doped single-mode erbium-doped optical fiber, and the signal end of the second wavelength division multiplexer is fused to the output end of the second high-power optical isolator; the second single-mode pigtail output laser diode is connected to the second wavelength division multiplexer through the optical fiber pigtail; one end of the second highly doped single-mode erbium-doped optical fiber is fused to the common end of the second multiplexer, and the other end serves as the output end of the first stage pre-amplifier; The second highly doped single-mode erbium-doped optical fiber is a non-polarization-maintaining highly doped single-mode erbium-doped optical fiber or a polarization-maintaining highly doped single-mode erbium-doped optical fiber.

6. The all-fiber flat high-coherence supercontinuum light source according to claim 5, characterized in that: The second-stage main amplifier comprises: a third high-power optical isolator (41), a third single-mode pigtail output laser diode (42), and a third wavelength division multiplexer (43) and a third highly doped single-mode erbium-doped optical fiber (44) arranged in sequence along the optical path; The input end of the third high-power optical isolator is connected to the output end of the first-stage pre-amplifier; the third wavelength division multiplexer is placed on the optical path between the third high-power optical isolator and the third highly doped single-mode erbium-doped optical fiber, and the signal end of the third wavelength division multiplexer is fused to the output end of the third high-power optical isolator; the third single-mode pigtail output laser diode is connected to the third wavelength division multiplexer through the optical fiber pigtail; one end of the third highly doped single-mode erbium-doped optical fiber is fused to the common end of the third multiplexer, and the other end serves as the output end of the second-stage main amplifier.

7. The all-fiber flat high-coherence supercontinuum light source according to claim 6, characterized in that: The third highly doped single-mode erbium-doped optical fiber is a non-polarization-maintaining highly doped single-mode erbium-doped optical fiber or a polarization-maintaining highly doped single-mode erbium-doped optical fiber.

8. The all-fiber flat high-coherence supercontinuum light source according to claim 7, characterized in that: The laser pulse compressor comprises: a single-mode optical fiber with a negative group velocity dispersion at a wavelength of 1560 nm or a broadband optical fiber Bragg grating with a negative group delay dispersion at a wavelength of 1560 nm; The laser pulse compressor is connected to the output end of the second-stage main amplifier. According to the laser pulse parameters output by the second-stage main amplifier, the laser pulse width after passing through the fiber compressor is compressed to sub-hundred femtoseconds by controlling the length of the single-mode fiber or the group delay dispersion of the broadband fiber Bragg grating, so as to generate a supercontinuum spectrum with high coherence. The laser pulse compressor is a non-polarization-maintaining single-mode fiber compressor or a polarization-maintaining single-mode fiber compressor.

9. The all-fiber flat high-coherence supercontinuum light source according to claim 8, characterized in that: The high nonlinear single-mode quartz optical fiber is a non-polarization-maintaining high nonlinear single-mode quartz optical fiber or a polarization-maintaining high nonlinear single-mode quartz optical fiber.

10. The operating method of the all-fiber flat high-coherence supercontinuum light source according to claim 9, characterized in that: Passive mode locking technology is used to achieve self-starting mode locking of laser pulses. The all-fiber pulse mode-locked oscillator generates stable high-repetition-rate femtosecond pulse mode-locked laser output. After the seed light pulse width is widened to the picosecond level by a pulse stretcher, the optical power is pre-amplified by the first-stage pre-amplifier to provide a high-quality pulse laser light source for the second-stage main amplifier to ensure that the main amplifier has sufficient energy amplification. The second-stage main amplifier amplifies the weak signal light of the previous stage. After obtaining high-power pulse laser output, the pulse compressor is used to compress the pulse width of the high-power pulse to sub-hundred femtoseconds. Finally, this high-power femtosecond laser is used to pump a highly nonlinear single-mode quartz fiber. The spectrum is widened by nonlinear effects such as high-order soliton splitting, soliton self-frequency shift and dispersive wave generation to obtain a cross-octave flat high-coherence supercontinuum spectrum covering the near-infrared long-wave band. The high-nonlinear fiber has a low dispersion slope and a dispersion value of nearly zero at the pump wavelength, which makes the supercontinuum spectrum have good flatness. The temporal coherence degradation of the supercontinuum spectrum is avoided by using a pump pulse with a pulse width of less than 100fs.

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