A fiber laser for generating coherent supercontinuum spectrum

Abstract

The present invention proposes a fiber laser for generating a coherent supercontinuum spectrum, which relates to the technical field of fiber lasers. The fiber laser for generating a coherent supercontinuum spectrum comprises a pump source, a wavelength division multiplexer, a first gain fiber, a second gain fiber, a dispersion delay line, a coupler, a saturable absorber, a high nonlinear fiber, and a filter. The fiber laser for generating a coherent supercontinuum spectrum proposed by the present invention utilizes high-order soliton dynamics to directly generate a highly coherent supercontinuum spectrum in a cavity, has the advantages of small size and low cost, and is easy to apply.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber lasers, and more particularly to an optical fiber laser for generating a coherent supercontinuum spectrum. Background Art

[0002] Supercontinuum spectroscopy has important applications in basic scientific research, frequency metrology, spectroscopy, and biomedicine. The generation of supercontinuum spectroscopy has been one of the hot research topics in optics in recent years. At present, the generation of supercontinuum spectroscopy is usually achieved outside the laser cavity by pumping a section of highly nonlinear optical fiber with high-power laser output from a laser amplifier.

[0003] In addition, directly generating ultra-wideband spectra from the fiber laser oscillator cavity is also an important means to achieve supercontinuum spectrum generation. This method has the advantages of more compact structure and has recently attracted widespread research interest.

[0004] At present, the supercontinuum spectrum generated from the fiber laser oscillator cavity is mainly in the "noise-like pulse" state. Due to the dual time scale structure of its time domain pulse and the obvious fluctuations between the supercontinuum spectra generated by different intracavity cycles, it has poor temporal coherence. These characteristics have severely limited the application scope of the supercontinuum spectrum in the "noise-like pulse" state. Summary of the invention

[0005] In view of the problems in the background technology, the present invention provides a fiber laser for generating a coherent supercontinuum spectrum, which solves the defects of the fiber laser for generating a coherent supercontinuum spectrum in the prior art.

[0006] The technical solution of the present invention to solve the above technical problems is as follows:

[0007] A fiber laser for generating a coherent supercontinuum spectrum, comprising: a pump source, a wavelength division multiplexer, a first gain fiber, a second gain fiber, a dispersion delay line, a coupler, a saturable absorber, a highly nonlinear fiber, and a filter;

[0008] The output end of the pump source is connected to the first input end of the wavelength division multiplexer, the output end of the wavelength division multiplexer is connected to the input end of the first gain fiber, the output end of the first gain fiber is connected to the input end of the second gain fiber, the output end of the second gain fiber is connected to the input end of the dispersion delay line, the output end of the dispersion delay line is connected to the input end of the coupler, the first output end of the coupler is connected to the input end of the saturable absorber, the output end of the saturable absorber is connected to the input end of the high nonlinear fiber, the output end of the high nonlinear fiber is connected to the input end of the filter, and the output end of the filter is connected to the second input end of the wavelength division multiplexer;

[0009] The first gain fiber has anomalous dispersion, and the second gain fiber has normal dispersion;

[0010] The dispersion delay line performs dispersion compensation on the pulse with positive chirp output by the second gain optical fiber, thereby compressing the pulse width;

[0011] The highly nonlinear optical fiber has anomalous dispersion, and high-order solitons are formed in the highly nonlinear optical fiber. The high-order solitons undergo a high-order soliton self-compression process in the highly nonlinear optical fiber, thereby generating a coherent supercontinuum spectrum with a spectrum width covering one octave in the cavity;

[0012] The filter is used to make the pulse evolve self-consistently in the cavity, and after each cycle, the pulse completely repeats the spectrum and time domain intensity waveform of the previous cycle.

[0013] Preferably, the angular frequency bandwidth corresponding to the filter is for: ,in is the central wavelength of the filter, is the full width at half maximum bandwidth of the filter, c is the speed of light in vacuum; the energy of the input pulse entering the first gain fiber after filtering Given by: ,in is the peak power of the input pulse entering the first gain fiber after filtering.

[0014] Preferably, the soliton order of the high-order soliton is N The calculation formula is:

[0015] ,

[0016] in, is the peak power of the input pulse entering the first gain fiber after filtering, is the energy of the input pulse entering the first gain fiber after filtering, is the sum of the first gain fiber length and the second gain fiber length, is the average small signal gain of the first gain fiber and the second gain fiber, is the average second-order dispersion of the first gain fiber and the second gain fiber, is the average nonlinear parameter of the first gain fiber and the second gain fiber, is the spectral bandwidth reduction coefficient, is the dispersion parameter of the dispersive delay line, is the equivalent transmittance, is the second-order dispersion of the highly nonlinear optical fiber, is the nonlinear parameter of the highly nonlinear optical fiber.

[0017] The beneficial effects of the present invention are:

[0018] (1) The first gain fiber has anomalous dispersion; the second gain fiber has normal dispersion. The pulse transmission in it has characteristics similar to self-similar evolution, forming a nonlinear attractor with strong anti-interference performance; after the pulse is output from the second gain fiber, it has a large positive chirp and a wide time domain pulse width;

[0019] (2) The dispersion delay line performs dispersion compensation on the pulse with positive chirp output from the second gain fiber, compresses the pulse width, and increases the peak power;

[0020] (3) By adding a highly nonlinear optical fiber with anomalous dispersion after the dispersion delay line, high-order solitons are formed in the highly nonlinear optical fiber. During the transmission of the high-order solitons, the negative chirp generated by the anomalous dispersion compensates for the positive chirp generated by the self-phase modulation effect. As a result, the pulse width of the high-order soliton continues to narrow, and the corresponding peak power continues to increase, resulting in the continuous enhancement of the nonlinear spectral broadening. Thus, significant nonlinear spectral broadening can be achieved using a lower incident peak power.

[0021] (4) A bandwidth-adjustable filter is set behind the highly nonlinear optical fiber for filtering. The design ensures that the pulse evolves self-consistently in the laser cavity. After each cycle, the pulse completely repeats the spectrum and time-domain intensity waveform of the previous cycle, achieving high coherence of the supercontinuum spectrum.

[0022] (5) The order of high-order solitons is designed by coordinating the first and second gain fibers, filters, dispersion delay lines, and high nonlinear fibers as a whole, and the soliton order of high-order solitons is obtained. N The expression of makes the supercontinuum spectrum generated in the highly nonlinear fiber have high coherence. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to make the present invention more easily understood, the present invention will be described in more detail by referring to the specific embodiments shown in the accompanying drawings. These drawings only depict typical embodiments of the present invention and should not be considered as limiting the scope of protection of the present invention.

[0024] Figure 1 It is a schematic diagram of the structure of a fiber laser for generating a coherent supercontinuum spectrum provided by an embodiment of the present invention.

[0025] Reference numerals:

[0026] 1- pump source, 2- wavelength division multiplexer, 3- first gain fiber, 4- second gain fiber, 5- dispersion delay line, 6- coupler, 7- saturable absorber, 8- high nonlinear fiber, 9- filter. DETAILED DESCRIPTION

[0027] The following describes the embodiments of the present invention with reference to the accompanying drawings so that those skilled in the art can better understand the present invention and implement it, but the enumerated embodiments are not intended to limit the present invention. In the absence of conflict, the following embodiments and the technical features in the embodiments can be combined with each other, wherein the same components are represented by the same reference numerals. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0028] Please refer to Figure 1 , Figure 1 A schematic structural diagram of a fiber laser for generating a coherent supercontinuum spectrum provided by an embodiment of the present invention.

[0029] See also Figure 1 In an embodiment of the present invention, a fiber laser for generating a coherent supercontinuum spectrum includes: a pump source 1, a wavelength division multiplexer 2, a first gain fiber 3, a second gain fiber 4, a dispersion delay line 5, a coupler 6, a saturable absorber 7, a highly nonlinear fiber 8, and a filter 9;

[0030] The output end of the pump source 1 is connected to the first input end of the wavelength division multiplexer 2, the output end of the wavelength division multiplexer 2 is connected to the input end of the first gain fiber 3, the output end of the first gain fiber 3 is connected to the input end of the second gain fiber 4, the output end of the second gain fiber 4 is connected to the input end of the dispersion delay line 5, the output end of the dispersion delay line 5 is connected to the input end of the coupler 6, the first output end of the coupler 6 is connected to the input end of the saturable absorber 7, the output end of the saturable absorber 7 is connected to the input end of the high nonlinear fiber 8, the output end of the high nonlinear fiber 8 is connected to the input end of the filter 9, and the output end of the filter 9 is connected to the second input end of the wavelength division multiplexer 2.

[0031] The other output end of the coupler 6 is generally connected to a detector for measuring the output power and spectrum of the pulse, which belongs to the prior art.

[0032] The present invention uses a pump source 1 to pump a two-stage gain fiber, and the pump light is coupled to the first gain fiber 3 through a wavelength division multiplexer 2 to provide gain amplification for the pulse. The first gain fiber 3 is a pre-amplification stage, and the second gain fiber 4 is a main amplification stage. The first gain fiber 3 has anomalous dispersion; the second gain fiber 4 has normal dispersion. The pulse transmission in the two-stage gain fiber has a characteristic similar to self-similar evolution, forming a nonlinear attractor with good anti-interference performance. Since the second gain fiber 4 has a large normal dispersion, the pulse has a large positive chirp after being output from the second gain fiber 4, so the pulse width is wide.

[0033] The present invention utilizes the dispersion delay line 5 to perform dispersion compensation on the pulse with positive chirp outputted from the second gain optical fiber 4, thereby compressing the pulse width and increasing its peak power. The dispersion-compensated pulse is inputted into the high nonlinear optical fiber 8 with anomalous dispersion. Due to the dispersion compensation, the pulse inputted into the high nonlinear optical fiber 8 has a narrow pulse width and a high peak power. Thus, high-order solitons are formed in the high nonlinear optical fiber 8. When the pulse is transmitted in the high nonlinear optical fiber 8, the self-phase modulation effect is the main nonlinear effect, which causes the pulse to obtain a nonlinear phase related to its intensity, resulting in its spectrum broadening. The high-order solitons undergo a high-order soliton self-compression process in the high nonlinear optical fiber 8, and its pulse width is significantly reduced to 2.7 optical cycles. At the same time, a highly coherent supercontinuum spectrum with a spectrum width covering one octave (-60dB) is generated in the laser cavity.

[0034] It is worth noting that if a high nonlinear optical fiber with normal dispersion is used after the dispersion delay line, since both normal dispersion and self-phase modulation effects cause the pulse to produce positive chirp, the pulse width increases continuously during the transmission process and the peak power decreases continuously. This means that if significant nonlinear spectrum broadening is to be obtained, the incident pulse needs to have a higher peak power. However, the present invention forms high-order solitons in the high nonlinear optical fiber by adding a high nonlinear optical fiber with anomalous dispersion after the dispersion delay line. During the transmission of the high-order solitons, the negative chirp produced by the anomalous dispersion compensates for the positive chirp produced by the self-phase modulation effect, so that the pulse width of the high-order soliton becomes narrower and the corresponding peak power increases continuously, resulting in the continuous enhancement of the nonlinear spectrum broadening, thereby achieving significant nonlinear spectrum broadening using a lower incident peak power.

[0035] In the present invention, the supercontinuum spectrum output by the highly nonlinear optical fiber 8 is filtered by a bandwidth-adjustable filter 9, the bandwidth of which is designed to ensure that the pulse evolves self-consistently in the laser cavity, and after each cycle the pulse completely repeats the spectrum and time-domain intensity waveform of the previous cycle.

[0036] It is worth noting that in the existing technology of directly generating supercontinuum spectrum from the laser oscillator cavity, filters are often not used or the bandwidth of the filters is not carefully optimized. Generally, Q-switching technology and noise-like pulse mode-locking technology are used to generate supercontinuum spectrum, and the spectrum output between different cycles has a large difference, so the coherence is poor. However, the supercontinuum spectrum generated by the present invention uses a filter, and after each cycle, the pulse completely repeats the spectrum and time-domain intensity waveform of the previous cycle, thereby ensuring the high coherence of the generated supercontinuum spectrum.

[0037] The pulse after filtering by the filter 9 of the present invention has very small chirp, and the angular frequency bandwidth corresponding to the filter 9 is for: ,in is the central wavelength of filter 9, is the full width at half maximum bandwidth of filter 9, c is the speed of light in vacuum; the energy of the input pulse entering the first gain fiber 3 after filtering Given by: ,in is the peak power of the input pulse entering the first gain fiber 3 after filtering.

[0038] The present invention compresses the picosecond pulses output by the two-stage gain optical fiber to the order of hundreds of femtoseconds through the dispersion delay line 5, and can form high-order solitons with an order of less than 10, so that the supercontinuum spectrum generated in the high nonlinear optical fiber has high coherence.

[0039] The invention designs the order of high-order solitons by coordinating two-stage gain optical fiber, filter, dispersion delay line and high nonlinear optical fiber as a whole to obtain the soliton order of high-order solitons. N The expression is:

[0040] ;

[0041] in, is the peak power of the input pulse entering the first gain fiber 3 after filtering, is the energy of the input pulse entering the first gain fiber 3 after filtering, is the sum of the length of the first gain fiber 3 and the length of the second gain fiber 4, is the average small signal gain of the first gain fiber 3 and the second gain fiber 4, is the average second-order dispersion of the first gain fiber 3 and the second gain fiber 4, is the average nonlinear parameter of the first gain fiber 3 and the second gain fiber 4, is the spectral bandwidth reduction coefficient, is the dispersion parameter of dispersion delay line 5, is the equivalent transmittance, is the second-order dispersion of the highly nonlinear optical fiber 8, is the nonlinear parameter of the highly nonlinear optical fiber 8.

[0042] In another embodiment of the present invention: the first gain fiber 3 with a first length of 1.5 m performs pre-amplification processing on the pulse to reduce the influence of spontaneous radiation noise. After output, it enters the second gain fiber 4 with a length of 10.5 m to evolve into a pulse similar to a self-similar sub, which improves the stability of the laser. Then, it passes through the dispersion delay line 5 for compression to obtain a pulse with a power of about 5485W. The coupler 6 and the saturable absorber 7 are placed in sequence between the dispersion delay line 5 and the high nonlinear fiber 8. The coupling ratio of the coupler 6 is 19.7%, the modulation depth of the saturable absorber 7 is 0.9, and the saturation power is 150 W. Then, the pulse enters a 1.1 m high nonlinear fiber 8 for high-order soliton self-compression evolution, generates a coherent supercontinuum spectrum, and then enters the filter 9 with a filtering bandwidth of 5.6 nm for filtering, ensuring that the pulse evolves self-consistently in the cavity. Under this condition, MATLAB is run to simulate the dissipative soliton mode-locked laser. The duration of the input and output pulses of the first gain fiber 3 is 726 fs and 702 fs respectively, and the output of the second gain fiber 4 is widened to 4.836 ps. After passing through the dispersion delay line 5, the pulse power reaches 5485 W. The spectral bandwidth of the first gain fiber 3 has only a small change. In the second gain fiber 4, the spectral bandwidth increases to 39 nm, and its profile is similar to the self-similar. After passing through the saturable absorber 7, the peak power of the pulse is reduced to 765W. The self-phase modulation effect in the highly nonlinear fiber 8 gradually broadens the spectrum. At the same time, the anomalous dispersion causes negative chirp, which compensates for the positive chirp caused by the self-phase modulation, so the peak power of the pulse increases rapidly. The pulse width corresponding to the maximum compression in the cavity is 14.5 fs, and the power is 3821W. At this time, the spectral bandwidth of the corresponding spectrum at -60 dB is 1170.4 nm.

[0043] 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. A fiber laser for generating a coherent supercontinuum spectrum, characterized in that: include: Pump source, wavelength division multiplexer, first gain fiber, second gain fiber, dispersion delay line, coupler, saturable absorber, high nonlinear fiber, filter; The output end of the pump source is connected to the first input end of the wavelength division multiplexer, the output end of the wavelength division multiplexer is connected to the input end of the first gain fiber, the output end of the first gain fiber is connected to the input end of the second gain fiber, the output end of the second gain fiber is connected to the input end of the dispersion delay line, the output end of the dispersion delay line is connected to the input end of the coupler, the first output end of the coupler is connected to the input end of the saturable absorber, the output end of the saturable absorber is connected to the input end of the high nonlinear fiber, the output end of the high nonlinear fiber is connected to the input end of the filter, and the output end of the filter is connected to the second input end of the wavelength division multiplexer; The first gain fiber has anomalous dispersion, and the second gain fiber has normal dispersion; The dispersion delay line performs dispersion compensation on the pulse with positive chirp output by the second gain optical fiber to compress the pulse width; The highly nonlinear optical fiber has anomalous dispersion, and high-order solitons are formed in the highly nonlinear optical fiber. The high-order solitons undergo a high-order soliton self-compression process in the highly nonlinear optical fiber, thereby generating a coherent supercontinuum spectrum with a spectrum width covering one octave in the cavity; The filter is used to make the pulse evolve self-consistently in the cavity, and after each cycle, the pulse completely repeats the spectrum and time domain intensity waveform of the previous cycle.

2. The fiber laser for generating a coherent supercontinuum spectrum according to claim 1, characterized in that: The filter corresponds to the angular frequency bandwidth for: ,in is the central wavelength of the filter, is the full width at half maximum bandwidth of the filter, c is the speed of light in a vacuum; The energy of the input pulse entering the first gain fiber after filtering Given by: ,in is the peak power of the input pulse entering the first gain fiber after filtering.

3. The fiber laser for generating a coherent supercontinuum spectrum according to claim 2, characterized in that: The soliton order of the higher-order soliton N The calculation formula is: ， in, is the peak power of the input pulse entering the first gain fiber after filtering, is the energy of the input pulse entering the first gain fiber after filtering, is the sum of the first gain fiber length and the second gain fiber length, is the average small signal gain of the first gain fiber and the second gain fiber, is the average second-order dispersion of the first gain fiber and the second gain fiber, is the average nonlinear parameter of the first gain fiber and the second gain fiber, is the spectral bandwidth reduction coefficient, is the dispersion parameter of the dispersive delay line, is the equivalent transmittance, is the second-order dispersion of the highly nonlinear optical fiber, is the nonlinear parameter of the highly nonlinear optical fiber.

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