CN112255858A - Narrow-linewidth ultrashort pulse generation device and method with wide-range wavelength tuning - Google Patents
Narrow-linewidth ultrashort pulse generation device and method with wide-range wavelength tuning Download PDFInfo
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Abstract
The invention discloses a narrow linewidth ultrashort pulse generating device and method with a large-range wavelength tuning function, wherein the device comprises an ultrashort pulse laser (1), an electric control variable optical attenuator (2), a signal generator (3), a beam expander (4), an optical fiber coupling mirror (5), a high nonlinear photonic crystal optical fiber (6), a multimode optical fiber (7), an electric control optical fiber telescopic platform (8) and a single-mode optical fiber (9); the device utilizes soliton self-frequency shift effect generated when femtosecond pulses are transmitted in the high nonlinear photonic crystal fiber to generate optical solitons with continuously tunable wavelength in a large range, and uses a multimode-single mode fiber cascade structure to realize soliton spectrum compression based on self-imaging effect, so that narrow-linewidth ultrashort pulse output with widely tunable wavelength is realized. The ultra-short pulse wavelength tuning device has the advantages of large ultra-short pulse wavelength tuning range, continuously tunable wavelength, narrow ultra-short pulse spectral line width and single output spectrum, and is convenient to integrate with other optical fiber application systems.
Description
Technical Field
The invention relates to the field of nonlinear optics and fiber optics, in particular to a narrow-linewidth ultrashort pulse generating device and method.
Background
With the continuous development of optical technology, ultrashort pulses (pulse width less than 100ps) have since appeared, and due to the advantages of high instantaneous power, low duration, low average power and the like, the ultrashort pulses have important applications in the fields of nonlinear optics, biomedicine, raman detection, laser processing and the like. However, the wavelength tuning range of the conventional titanium sapphire femtosecond laser is small and is only dozens of nanometers, so that the wide application of ultrashort pulses is limited. For example, in coherent anti-Stokes Raman Scattering (CARS) technology, to detect 3000cm-1The above raman resonance peak requires that the wavelength tuning range of the ultrashort pulse is above 300nm, and requires that the wavelength of the ultrashort pulse can be continuously tuned within such a large range to realize complete detection of the raman resonance spectrum. In addition, in order to realize high-resolution detection of raman resonance spectrum in CARS, it is necessary to use ultra-short pulses of narrow line width as the pump light pulse and the stokes light pulse. Therefore, ultrashort pulses with narrow line width and wide wavelength range tuning have important application value in CARS, and thus become a research hotspot of people.
Disclosure of Invention
The invention provides a narrow-linewidth ultrashort pulse generating device and method with a large-range wavelength tuning function, aiming at overcoming the problem that the wavelength tuning range of the existing narrow-linewidth ultrashort pulse is small, the device and method are used for generating optical solitons with wavelengths capable of being continuously tuned in a large range based on the soliton self-frequency shift effect when femtosecond pulses are transmitted in a high nonlinear photonic crystal fiber, and a multimode-single mode fiber cascade structure is used for realizing soliton spectrum compression based on the self-imaging effect, so that the narrow-linewidth ultrashort pulse output with the large-range wavelength tuning function of a system is realized.
The invention provides a narrow linewidth ultrashort pulse generating device with wide-range wavelength tuning, which comprises an ultrashort pulse laser 1, an electrically-controlled adjustable optical attenuator 2, a signal generator 3, a beam expander 4, an optical fiber coupling mirror 5, a high nonlinear photonic crystal optical fiber 6, a multimode optical fiber 7, an electrically-controlled optical fiber telescopic platform 8 and a single-mode optical fiber 9, wherein the ultrashort pulse laser 1, the electrically-controlled adjustable optical attenuator 2, the beam expander 4, the optical fiber coupling mirror 5, the high nonlinear photonic crystal optical fiber 6, the multimode optical fiber 7 and the single-mode optical fiber 9 are sequentially connected, the multimode optical fiber 7 is arranged on the electrically-controlled optical fiber telescopic platform 8 and can change the optical fiber length along with the telescopic change of the electrically-controlled optical fiber telescopic platform 8, the signal generator 3 is connected across the electrically-controlled adjustable optical attenuator 2 and the electrically-controlled optical fiber telescopic platform 8, wherein:
the ultra-short pulse laser 1 outputs single-frequency femtosecond pulses, the single-frequency femtosecond pulses are transmitted by the electric control adjustable optical attenuator 2 and then are incident to the beam expander 4, and the electric control adjustable optical attenuator 2 is controlled by an electric signal output by the signal generator 3, so that the femtosecond pulse optical power incident to the beam expander 4 is continuously adjusted;
the femtosecond pulse output by the beam expander 4 enters the high nonlinear photonic crystal fiber 6 through the fiber coupling mirror 5; when the femtosecond pulse is transmitted in the high nonlinear photonic crystal fiber 6, soliton self-frequency shift effect is generated, optical solitons with the wavelength moving to the long wavelength direction are generated, and the wavelength offset is increased along with the increase of the power of the incident femtosecond pulse light; the high nonlinear photonic crystal fiber 6 outputs optical solitons to the multimode fiber 7; one section of the multimode optical fiber 7 is fixed on the electric control optical fiber telescopic platform 8, and the signal generator 3 adjusts the electric control optical fiber telescopic platform 8 through electric signals so as to change the length of the multimode optical fiber 7 fixed on the electric control optical fiber telescopic platform 8; the multimode optical fiber 7 outputs the light beam to a single mode optical fiber 9; the light beam output by the single mode fiber 9 is a narrow linewidth ultrashort pulse with a tunable wavelength in a large range.
The invention provides a method for generating a narrow-linewidth ultrashort pulse with a large-range wavelength tuning, which mainly comprises the following steps:
the first step is as follows: the single-frequency femtosecond pulse output by the ultrashort pulse laser is incident to the electric control variable optical attenuator, the electric control variable optical attenuator continuously adjusts the optical power of the output femtosecond pulse through an external electric signal, wherein the external electric signal used for adjustment is generated by a signal generator;
the second step is that: the power variable femtosecond pulse output by the electrically controlled adjustable optical attenuator is expanded by the beam expander and output and enters the optical fiber coupling mirror, and the femtosecond pulse realizes the convergence of light beams by the optical fiber coupling mirror and is coupled into the high nonlinear photonic crystal fiber; the beam expander expands the diameter of the femtosecond pulse beam which is transmitted in space to fill the entrance pupil of the fiber coupling mirror, thereby realizing the high-efficiency coupling of the femtosecond pulse focused by the fiber coupling mirror to the high-nonlinear photonic crystal fiber;
the third step: when the femtosecond pulse is transmitted in the high nonlinear photonic crystal fiber, optical solitons with the wavelength moving towards the long wavelength direction are generated, and the wavelength of the optical solitons is increased along with the increase of the power of the femtosecond pulse light incident to the high nonlinear photonic crystal fiber; the wavelength range of the anomalous dispersion region of the high nonlinear photonic crystal fiber determines the wavelength tuning range of the ultrashort pulse;
the fourth step: the light beam which is output by the high nonlinear photonic crystal fiber and transmitted by the multimode fiber directly enters the single mode fiber; when light beams are transmitted in the multimode optical fiber, high-order guided modes in the multimode optical fiber are excited, the high-order guided modes interfere with each other, and images of an incident light field can be repeated at specific periodic positions of the multimode optical fiber, namely, a self-imaging phenomenon occurs; the length of the multimode fiber is integral multiple of a self-imaging period corresponding to the central wavelength of the transmission ultra-short pulse, so that the central wavelength of the ultra-short pulse transmitted by the multimode fiber is converged at the emergent end face of the multimode fiber and then coupled into the fiber core of the single mode fiber, and partial light beams with other wavelengths around the central wavelength of the ultra-short pulse are coupled into the cladding of the single mode fiber, so that the ultra-short pulse is rapidly lost; the multi-mode and single-mode fiber cascade structure is utilized to realize selective transmission of the ultra-short pulse spectrum, thereby realizing the compression of the line width of the ultra-short pulse spectrum;
the fifth step: the electric control optical fiber expansion platform controls the expansion amount of the multimode optical fiber by the electric signal generated by the signal generator, so that the transmission wavelength of the multimode-single mode optical fiber cascade structure is changed, and the electric control tuning of the narrow-linewidth spectral center wavelength output by the multimode-single mode optical fiber cascade structure is realized;
and a sixth step: the signal generator controls the electrically controlled adjustable optical attenuator to change the femtosecond pulse optical power incident to the high nonlinear photonic crystal fiber, so as to change the optical soliton wavelength output by the high nonlinear photonic crystal fiber; the electric control optical fiber telescopic platform realizes the ultra-short pulse wavelength tuning and the spectral line width compression output by the multimode-single mode optical fiber cascade structure by changing the length of the multimode optical fiber; the signal generator simultaneously controls the electric control variable optical attenuator and the electric control optical fiber telescopic platform to enable the electric control variable optical attenuator and the electric control optical fiber telescopic platform to be matched with each other, and the attenuation of each electric control variable optical attenuator corresponds to the telescopic quantity of one electric control optical fiber telescopic platform, so that tuning of output optical soliton wavelength of the high nonlinear photonic crystal optical fiber and change of transmission filtering center wavelength of the multimode-single mode optical fiber structure along with the optical soliton center wavelength are realized; the multimode-single mode fiber cascade structure carries out spectrum filtering on the central wavelength of the optical solitons and eliminates the influence of residual pump femtosecond pulses in the output beam of the high nonlinear photonic crystal fiber;
therefore, the large-range continuous tuning of the ultra-short pulse wavelength and the compression of the spectral line width of the ultra-short pulse with the corresponding wavelength are realized.
Compared with the traditional ultrashort pulse generation mode, the narrow linewidth ultrashort pulse generation device and method with the wavelength tuned in a large range, provided by the invention, have the advantages of large tuning range of ultrashort pulse wavelength, continuous tuning of wavelength and narrow linewidth of ultrashort pulse spectrum; meanwhile, the multi-mode-single-mode fiber cascade structure is utilized to realize the line width compression of the ultrashort pulse spectrum, and simultaneously, the influence of the residual pump femtosecond pulse in the output beam of the high nonlinear photonic crystal fiber is filtered, the filtering structure in the traditional ultrashort pulse generating mode is simplified, and the spectrum quality of the ultrashort pulse output by the system is optimized; moreover, the mode of single-mode fiber output enables the ultrashort pulse generating device provided by the invention to have the property of being compatible with the nature of the fiber, and is convenient to integrate with other fiber application systems.
Drawings
FIG. 1 is a schematic structural diagram of a narrow linewidth ultra-short pulse generating device with widely tunable wavelength according to the present invention;
FIG. 2 is a graph of tuning measurement results of output optical soliton wavelength of a highly nonlinear photonic crystal fiber;
fig. 3 is a graph showing the results of optical soliton spectral compression measurements at different wavelengths.
1. The system comprises an ultra-short pulse laser, 2, an electrically-controlled adjustable optical attenuator, 3, a signal generator, 4, a beam expander, 5, an optical fiber coupling mirror, 6, a high nonlinear photonic crystal optical fiber, 7, a multimode optical fiber, 8, an electrically-controlled optical fiber telescopic platform, 9 and a single-mode optical fiber.
Detailed Description
The present invention will be described in further detail with reference to specific embodiments;
fig. 1 is a schematic structural diagram of a narrow linewidth ultra-short pulse generating device with a wide-range wavelength tuning proposed by the present invention. The device mainly comprises an ultrashort pulse laser 1, an electric control variable optical attenuator 2, a signal generator 3, a beam expander 4, an optical fiber coupling mirror 5, a high nonlinear photonic crystal fiber 6, a multimode fiber 7, an electric control fiber telescopic platform 8 and a single-mode fiber 9.
The ultra-short pulse laser 1 outputs single-frequency femtosecond pulses, the single-frequency femtosecond pulses are transmitted by the electric control adjustable optical attenuator 2 and then are incident to the beam expander 4, and the electric control adjustable optical attenuator 2 is controlled by an electric signal output by the signal generator 3, so that the femtosecond pulse optical power incident to the beam expander 4 is continuously adjusted. The femtosecond pulse output by the beam expander 4 enters the high nonlinear photonic crystal fiber 6 through the fiber coupling mirror 5. When the femtosecond pulse is transmitted in the high nonlinear photonic crystal fiber 6, soliton self-frequency shift effect is generated, optical solitons with the wavelength moving to the long wavelength direction are generated, and the wavelength offset is increased along with the increase of the power of the incident femtosecond pulse light. The output end of the high nonlinear photonic crystal fiber 6 is connected with a multimode fiber 7, one section of the multimode fiber 7 is fixed on an electric control fiber telescopic platform 8, and the signal generator 3 adjusts the electric control fiber telescopic platform 8 through electric signals so as to change the length of the multimode fiber 7 fixed on the electric control fiber telescopic platform 8. The output end of the multimode optical fiber 7 is connected with a single-mode optical fiber 9. The light beam output by the single mode fiber 9 is a narrow linewidth ultrashort pulse with a tunable wavelength in a large range.
The ultrashort pulse laser 1 generates a single frequency femtosecond pulse and its center wavelength is located in an anomalous dispersion region of the highly nonlinear photonic crystal fiber 6.
The electrically controlled variable optical attenuator 2 is used for continuously adjusting femtosecond pulse optical power and can be controlled by an external electric signal.
The signal generator 3 is used for generating electric signals for controlling the electric control adjustable optical attenuator 3 and the electric control optical fiber telescopic platform 8 and synchronously adjusting the electric control adjustable optical attenuator and the electric control optical fiber telescopic platform.
The beam expander 4 is used for expanding the diameter of the spatially transmitted femtosecond pulse beam so as to fill the entrance pupil of the fiber coupling mirror 5.
The fiber coupling mirror 5 is used for coupling the spatially transmitted femtosecond pulses into the highly nonlinear photonic crystal fiber 6.
The highly nonlinear photonic crystal fiber 6 has the characteristic of high nonlinearity, when the femtosecond pulse is transmitted in the highly nonlinear photonic crystal fiber, optical solitons with the wavelength moving to the long wavelength direction are generated, and the central wavelength of the optical solitons is increased along with the increase of the power of incident femtosecond pulse light.
The combination of the multimode fiber 7 and the single mode fiber 9 is used for realizing spectral filtering based on a self-imaging effect, so that only narrow-band spectrum near the transmission wavelength is output, and other spectral components are filtered, thereby realizing the line width compression of the ultra-short pulse spectrum transmitted by the fiber structure.
The electric control optical fiber telescopic platform 8 is used for adjusting the length of the multimode optical fiber 7 fixed on the electric control optical fiber telescopic platform through an external electric signal, so that the tuning of the transmission center wavelength of the multimode-single mode optical fiber structure is realized.
The invention relates to a method for generating a narrow-linewidth ultrashort pulse with a large-range wavelength tuning, which mainly comprises the following steps:
the first step is as follows: the single-frequency femtosecond pulse output by the ultrashort pulse laser is incident to the electric control variable optical attenuator, the electric control variable optical attenuator can continuously adjust the optical power of the output femtosecond pulse through an external electric signal, wherein the external electric signal used for adjustment is generated by a signal generator;
the second step is that: the power variable femtosecond pulse output by the electrically controlled adjustable optical attenuator is expanded by the beam expander and output and then enters the optical fiber coupling mirror, and the femtosecond pulse realizes the convergence of light beams by the optical fiber coupling mirror and is coupled into the high nonlinear photonic crystal fiber. The beam expander is used for expanding the diameter of the femtosecond pulse beam transmitted in space to fill the entrance pupil of the optical fiber coupling mirror, so that the femtosecond pulse focused by the optical fiber coupling mirror is efficiently coupled to the high-nonlinearity photonic crystal fiber;
the third step: when the femtosecond pulse is transmitted in the high nonlinear photonic crystal fiber, soliton self-frequency shift phenomenon is generated under the influence of fiber dispersion effect, nonlinear effect and intra-pulse Raman scattering effect, optical solitons with the wavelength moving towards the long wavelength direction are generated, and the wavelength of the optical solitons is increased along with the increase of the power of the femtosecond pulse light which is incident to the high nonlinear photonic crystal fiber. The wavelength range of the anomalous dispersion region of the high nonlinear photonic crystal fiber determines the wavelength tuning range of the ultrashort pulse provided by the invention; the soliton self-frequency shift effect can be easier to occur by using the femtosecond pulse to be incident to the high nonlinear photonic crystal fiber, and the wavelength offset of optical solitons under the same incident light power is improved; when the femtosecond pulse is used for being incident to the high nonlinear photonic crystal fiber, the optical solitons generated by the soliton self-frequency shift effect are also the femtosecond pulse, and because the optical solitons have no chirp property essentially, the spectrum of the optical solitons is wider and is generally larger than 10 nm;
the fourth step: the output end of the high nonlinear photonic crystal fiber is directly connected with the multimode fiber, and the light beam transmitted through the multimode fiber directly enters the single-mode fiber. When light beams are transmitted in the multimode optical fiber, high-order guided modes in the multimode optical fiber are excited, the high-order guided modes interfere with each other, and images of incident light fields are repeated at specific periodic positions of the multimode optical fiber, namely, a self-imaging phenomenon occurs. The period of the fiber length corresponding to the self-imaging phenomenon in the multimode fiber is related to the transmission wavelength. When the length of the multimode fiber is an integral multiple of a self-imaging period corresponding to the central wavelength of the transmission ultra-short pulse, the central wavelength of the ultra-short pulse transmitted by the multimode fiber can be converged at the emergent end face of the multimode fiber and then coupled into the fiber core of the single mode fiber, and other wavelength parts around the central wavelength of the ultra-short pulse cannot be converged at the output end face of the multimode fiber and are coupled into the cladding part of the single mode fiber, so that the ultra-short pulse is rapidly lost. Therefore, the multimode and single-mode fiber cascade structure can realize selective transmission of the ultra-short pulse spectrum, thereby realizing the compression of the line width of the ultra-short pulse spectrum. In addition, the tuning of the transmission wavelength of the multimode-single mode fiber cascade structure can be realized by controlling the length of the multimode fiber;
the fifth step: the middle part of the multimode optical fiber is fixed on the electric control optical fiber telescopic platform, and the electric control optical fiber telescopic platform controls the telescopic amount of the multimode optical fiber by an electric signal generated by a signal generator, so that the length of the multimode optical fiber fixed on the electric control optical fiber telescopic platform is changed, and the transmission wavelength of the multimode-single mode optical fiber structure can be changed due to the influence of the self-imaging effect in the multimode optical fiber, thereby realizing the electric control tuning of the narrow-linewidth spectrum center wavelength output by the multimode-single mode optical fiber cascade structure;
and a sixth step: the signal generator simultaneously controls the electric control variable optical attenuator and the electric control optical fiber telescopic platform, so that the electric control variable optical attenuator and the electric control optical fiber telescopic platform are matched with each other, and the large-range tuning of the output ultrashort pulse wavelength of the system and the compression of the spectrum are realized. The electrically-controlled adjustable optical attenuator changes the femtosecond pulse optical power incident to the high nonlinear photonic crystal fiber so as to change the optical soliton wavelength output by the high nonlinear photonic crystal fiber, and due to the influence of the soliton self-frequency shift effect, the femtosecond pulse optical power incident to the high nonlinear photonic crystal fiber and the optical soliton center wavelength output by the high nonlinear photonic crystal fiber are in one-to-one correspondence; the electric control optical fiber telescopic platform can realize the ultra-short pulse wavelength tuning and the spectral line width compression output by the multimode-single mode optical fiber cascade structure by changing the length of the multimode optical fiber; the signal generator simultaneously controls the electric control variable optical attenuators and the electric control optical fiber telescopic platform, so that the attenuation of each electric control variable optical attenuator corresponds to the telescopic quantity of one electric control optical fiber telescopic platform, and therefore tuning of output optical soliton wavelength of the high nonlinear photonic crystal optical fiber and change of transmission filtering center wavelength of the multimode-single mode optical fiber structure along with the optical soliton center wavelength are achieved. In addition, the multimode-single mode fiber cascade structure can eliminate the influence of residual pump femtosecond pulses in the output beam of the high nonlinear photonic crystal fiber while performing spectrum filtering on the central wavelength of the optical solitons. Based on the process, the large-range continuous tuning of the ultra-short pulse wavelength and the compression of the line width of the ultra-short pulse spectrum with the corresponding wavelength are realized.
In summary, the narrow-linewidth ultrashort pulse generating device and method with widely tunable wavelength provided by the present invention can realize the wide tuning of ultrashort pulse wavelength and the synchronous compression of ultrashort pulse spectrum. In the invention, the soliton self-frequency shift effect generated when femtosecond pulses are transmitted in the photonic crystal fiber generates optical solitons with the wavelength capable of moving in a large range, and the soliton wavelength is adjusted by a signal generator; based on the self-imaging effect in the multimode fiber, the line width compression of the ultrashort pulse spectrum is realized by using a multimode-single mode fiber cascade structure; in addition, the signal generator is used for adjusting the electric control optical fiber telescopic platform, so that the movement of the narrow-linewidth spectrum center wavelength output by the multimode-single-mode optical fiber cascade structure can be realized.
Claims (2)
1. The device is characterized by comprising an ultrashort pulse laser (1), an electric control adjustable optical attenuator (2), a signal generator (3), a beam expander (4), an optical fiber coupling mirror (5), a high nonlinear photonic crystal fiber (6), a multimode fiber (7), an electric control fiber telescopic platform (8) and a single-mode fiber (9), wherein the ultrashort pulse laser (1), the electric control adjustable optical attenuator (2), the beam expander (4), the optical fiber coupling mirror (5), the high nonlinear photonic crystal fiber (6), the multimode fiber (7), the electric control fiber telescopic platform (8) and the single-mode fiber (9) are sequentially connected, the signal generator (3) is respectively connected with the electric control adjustable optical attenuator (2) and the electric control fiber telescopic platform (8), wherein:
the ultrashort pulse laser (1) outputs single-frequency femtosecond pulses, the single-frequency femtosecond pulses are transmitted by the electric control variable optical attenuator (2) and then are incident to the beam expander (4), and the electric control variable optical attenuator (2) is controlled by an electric signal output by the signal generator (3), so that the power of the femtosecond pulse light incident to the beam expander (4) is continuously adjusted;
the femtosecond pulse output by the beam expander (4) enters the high nonlinear photonic crystal fiber (6) through the fiber coupling mirror (5); when the femtosecond pulse is transmitted in the high nonlinear photonic crystal fiber (6), soliton self-frequency shift effect is generated, optical solitons with the wavelength moving to the long wavelength direction are generated, and the wavelength offset is increased along with the increase of the power of the incident femtosecond pulse light; the highly nonlinear photonic crystal fiber (6) outputs optical solitons to the multimode fiber (7); one section of the multimode optical fiber (7) is fixed on the electric control optical fiber telescopic platform (8), and the signal generator (3) adjusts the electric control optical fiber telescopic platform (8) through electric signals so as to change the length of the multimode optical fiber (7) fixed on the electric control optical fiber telescopic platform (8); the multimode optical fiber (7) outputs a light beam to a single mode optical fiber (9); the light beam output by the single mode fiber (9) is a narrow linewidth ultrashort pulse with the wavelength capable of being tuned in a large range.
2. The method for generating a narrow-linewidth ultrashort pulse with a wide-range wavelength tuning, which is implemented by using the narrow-linewidth ultrashort pulse generating device with a wide-range wavelength tuning as claimed in claim 1, mainly comprises the following steps:
the first step is as follows: the single-frequency femtosecond pulse output by the ultrashort pulse laser is incident to the electric control variable optical attenuator, the electric control variable optical attenuator continuously adjusts the optical power of the output femtosecond pulse through an external electric signal, wherein the external electric signal used for adjustment is generated by a signal generator;
the second step is that: the power variable femtosecond pulse output by the electrically controlled adjustable optical attenuator is expanded by the beam expander and output and enters the optical fiber coupling mirror, and the femtosecond pulse realizes the convergence of light beams by the optical fiber coupling mirror and is coupled into the high nonlinear photonic crystal fiber; the beam expander expands the diameter of the femtosecond pulse beam which is transmitted in space to fill the entrance pupil of the fiber coupling mirror, thereby realizing the high-efficiency coupling of the femtosecond pulse focused by the fiber coupling mirror to the high-nonlinear photonic crystal fiber;
the third step: when the femtosecond pulse is transmitted in the high nonlinear photonic crystal fiber, optical solitons with the wavelength moving towards the long wavelength direction are generated, and the wavelength of the optical solitons is increased along with the increase of the power of the femtosecond pulse light incident to the high nonlinear photonic crystal fiber; and the wavelength tuning range of the ultrashort pulse is determined by the wavelength range of the anomalous dispersion region of the high nonlinear photonic crystal fiber;
the fourth step: the output end of the high nonlinear photonic crystal fiber is directly connected with the multimode fiber, and the light beam which is output by the high nonlinear photonic crystal fiber and transmitted by the multimode fiber directly enters the single mode fiber; when light beams are transmitted in the multimode optical fiber, high-order guided modes in the multimode optical fiber are excited, the high-order guided modes interfere with each other, and images of an incident light field can be repeated at specific periodic positions of the multimode optical fiber, namely, a self-imaging phenomenon occurs; the length of the multimode fiber is integral multiple of a self-imaging period corresponding to the central wavelength of the transmission ultra-short pulse, so that the central wavelength of the ultra-short pulse transmitted by the multimode fiber is converged at the emergent end face of the multimode fiber and then coupled into the fiber core of the single mode fiber, and partial light beams with other wavelengths around the central wavelength of the ultra-short pulse are coupled into the cladding of the single mode fiber, so that the ultra-short pulse is rapidly lost; the multi-mode and single-mode fiber cascade structure is utilized to realize selective transmission of the ultra-short pulse spectrum, and the compression of the line width of the ultra-short pulse spectrum is realized;
the fifth step: the electric control optical fiber telescopic platform controls the telescopic quantity of the length of the multimode optical fiber by the electric signal generated by the signal generator, so that the transmission wavelength of the multimode-single mode optical fiber cascade structure is changed, and the electric control tuning of the narrow-linewidth spectral center wavelength output by the multimode-single mode optical fiber cascade structure is realized;
and a sixth step: the signal generator controls the electrically controlled adjustable optical attenuator to change the femtosecond pulse optical power incident to the high nonlinear photonic crystal fiber, so as to change the optical soliton wavelength output by the high nonlinear photonic crystal fiber; the electric control optical fiber telescopic platform realizes the ultra-short pulse wavelength tuning and the spectral line width compression output by the multimode-single mode optical fiber cascade structure by changing the length of the multimode optical fiber; the signal generator simultaneously controls the electric control variable optical attenuator and the electric control optical fiber telescopic platform to enable the electric control variable optical attenuator and the electric control optical fiber telescopic platform to be matched with each other, the attenuation of each electric control variable optical attenuator corresponds to the telescopic quantity of one electric control optical fiber telescopic platform, and tuning of the output optical soliton wavelength of the high nonlinear photonic crystal optical fiber and change of the transmission filtering center wavelength of the multimode-single mode optical fiber structure along with the optical soliton center wavelength are achieved; the multimode-single mode fiber structure performs spectral filtering on the central wavelength of the optical soliton, and simultaneously eliminates the influence of residual pump femtosecond pulses in the output beam of the high nonlinear photonic crystal fiber;
therefore, the large-range continuous tuning of the ultra-short pulse wavelength and the compression of the spectral line width of the ultra-short pulse with the corresponding wavelength are realized.
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