CN113328328A - All-fiber femtosecond seed laser based on large mode field fiber - Google Patents

All-fiber femtosecond seed laser based on large mode field fiber Download PDF

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CN113328328A
CN113328328A CN202110761035.7A CN202110761035A CN113328328A CN 113328328 A CN113328328 A CN 113328328A CN 202110761035 A CN202110761035 A CN 202110761035A CN 113328328 A CN113328328 A CN 113328328A
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fiber
mode
field
double
pump
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尹雨松
刘学松
袁正印
马飞
杜星
叶世锋
葛浩
刘金柱
李广强
周晓杰
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Futongni Technology Suzhou Co ltd
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H01S3/06708—Constructional details of the fibre, e.g. compositions, cross-section, shape or tapering
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H01S3/06754—Fibre amplifiers
    • H01S3/06758—Tandem amplifiers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
    • H01S3/094069—Multi-mode pumping
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
    • H01S3/0941—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light of a laser diode

Abstract

The invention discloses a full-large mode field fiber femtosecond seed laser, which comprises a large mode field fiber femtosecond oscillator and a two-stage large mode field fiber preamplifier sharing a multimode semiconductor laser pump, a large mode field fiber pulse stretcher and a large mode field fiber pulse selector, wherein an oscillator resonant cavity comprises a double-cladding large mode field chirped fiber grating, a double-cladding large mode field gain fiber, a large mode field fiber coupler and a semiconductor saturable absorption reflector; the resonant cavity of the fiber femtosecond seed laser oscillator completely adopts large mode field fibers, so that the oscillator can directly output higher average power than that of a single mode fiber with a core diameter of 6 mu m.

Description

All-fiber femtosecond seed laser based on large mode field fiber
Technical Field
The invention relates to the technical field of fiber laser, in particular to an all-fiber femtosecond seed laser based on a large mode field fiber.
Background
The femtosecond pulse laser has wide application in the fields of material processing, biological medical treatment, scientific research, national defense and the like. At present, a main stream high-energy/high-power femtosecond laser still mainly adopts a Chirped Pulse Amplification (CPA) structure, namely, a low-energy and high-repetition-frequency ultrashort seed laser pulse generated by a laser oscillator through a mode locking principle is stretched to hundreds of picoseconds or longer by a pulse stretcher, and the repetition frequency is reduced by a pulse selector, wherein in the process, a multi-stage preamplifier is generally needed due to the insertion loss and damage threshold limit of related devices; then the amplified signal is sent to a single-stage or multi-stage main amplifier for large energy/high power amplification; and finally, compressing the pulse width to femtosecond magnitude by a pulse compressor. Ultrafast lasers generally involve solid-state laser technology and fiber laser technology, depending on the working substance morphology. Compared with a solid laser, the fiber laser has the advantages of small volume, light weight, easy maintenance, large heat dissipation area and the like, and is a key development direction of the femtosecond laser technology in recent years. With the gradual maturity of the related technologies, the femtosecond seed oscillator, the optical fiber type pulse stretcher, the pulse selector and the multistage preamplifier have low requirements on heat dissipation due to low bearing energy/power and can use a flexible optical fiber with a small core diameter, so that an independently packaged module can be made, which is beneficial to reducing the product volume of the femtosecond laser and is convenient for large-scale generation of the femtosecond laser and maintenance in the using process, and the part is defined as the femtosecond seed laser in the invention.
The existing mainstream femtosecond seed laser technical scheme has the following defects: 1. the oscillator fiber generally uses a single-mode single-clad fiber with the core diameter of 6 mu m, and the pulse energy output by the oscillator is very small due to the limitation of nonlinear effect, so a first-stage preamplifier needs to be used immediately behind the oscillator; 2. the pumping sources of the oscillator and the pre-amplifier generally use a low-power single-mode output semiconductor laser, and the pumping laser belongs to a high-value easily-consumed device, because pumping light and laser are jointly transmitted in a single-mode fiber core, return light of signal light easily damages the single-mode semiconductor laser; 3. because the maximum injectable power of the single-mode semiconductor laser is low, three to four stages of independent optical fiber preamplifiers are required in the processes of pulse stretching and pulse selection, which means that three to four semiconductor pump lasers and corresponding driving power supplies and control loops are required, which undoubtedly increases the cost, the volume and the complexity of a system and also reduces the working reliability.
Although the prior art adopts the double-clad large-mode-area photonic crystal fiber as the gain medium of the femtosecond laser oscillator to improve the output pulse energy of the oscillator, as in patent CN 100437323C, the limitation of this solution is also obvious: 1. the fiber laser is not of an all-fiber structure, the volume is increased due to the use of a spatial light path, and the environmental interference resistance is reduced, so that the great advantages of the fiber laser are lost; 2. photonic crystal fibers are still currently very expensive.
Disclosure of Invention
The invention aims to provide an all-fiber femtosecond seed laser based on a large-mode-field optical fiber, so as to solve the problems in the background technology.
In order to achieve the purpose, the invention provides the following technical scheme:
a full-fiber femtosecond seed laser based on a large mode field fiber comprises a large mode field fiber femtosecond oscillator and a two-stage large mode field fiber preamplifier sharing a multimode semiconductor laser pump, a large mode field fiber pulse stretcher and a large mode field fiber pulse selector;
the large mode field fiber femtosecond oscillator comprises a multimode semiconductor pumping source, a multimode pumping protector, a first multimode pumping beam splitter, a double-cladding large mode field chirped fiber grating, a first double-cladding large mode field gain fiber, a large mode field fiber coupler, a semiconductor saturable absorption reflector and a photoelectric detector.
The input port of the multimode pumping protector is connected with the multimode semiconductor pumping source and used for protecting the multimode semiconductor pumping source;
the input port of the first multimode pump beam splitter is connected with the output port of the multimode pump protector, one of the two output ports of the first multimode pump beam splitter is connected with the input port of the second multimode pump beam splitter and used for pumping a two-stage large mode field fiber preamplifier, and the other output port of the first multimode pump beam splitter is connected with one end of a double-cladding large mode field chirped fiber grating and used for pumping a large mode field fiber femtosecond laser oscillator;
the double-cladding large-mode-field chirped fiber grating and the semiconductor saturable absorption reflector form a fiber oscillator resonant cavity, one end of a first double-cladding large-mode-field gain fiber is connected with one end of the chirped fiber grating and is used as a working substance in the resonant cavity, the resonant cavity inner coupler has four ports, three ports are respectively connected with the semiconductor saturable absorption reflector, the photoelectric detector and the first double-cladding large-mode-field gain fiber, a fourth port outputs femtosecond seed pulses generated by the fiber oscillator resonant cavity, and the femtosecond seed pulses generated by the fiber oscillator resonant cavity correspond to the first large-mode-field fiber isolator;
the two-stage large-mode-field optical fiber preamplifier comprises a first large-mode-field optical fiber preamplifier and a second large-mode-field optical fiber preamplifier, the first large-mode-field optical fiber preamplifier comprises a first signal pumping beam combiner, a second double-cladding large-mode-field gain optical fiber and a first pumping stripper, a signal input port of the first signal pumping beam combiner is connected with an output port of a large-mode-field optical fiber circulator, a pumping input end of the first signal pumping beam combiner is connected with one output port of the second multi-mode pumping beam splitter, and a signal output port of the first signal pumping beam combiner is connected with one port of the second double-cladding large-mode-field gain optical fiber;
the first pump stripper is connected with the other port of the second double-clad large-mode-field gain fiber and is used for stripping residual pump light which is not absorbed by the second double-clad large-mode-field gain fiber;
the output port of the first large-mode-field optical fiber preamplifier is connected with the input port of a second large-mode-field optical fiber isolator, and the second large-mode-field optical fiber preamplifier comprises a second signal pumping beam combiner and a third double-cladding large-mode-field gain optical fiber;
and a signal input port of the second signal pumping beam combiner is connected with an output port of the large mode field optical fiber pulse selector, a pumping input port is connected with the other output port of the multimode second pumping beam splitter, and a signal output port is connected with one end of a third double-cladding large mode field gain optical fiber.
Preferably, the double-maximum mode field optical fiber preamplifier is provided with at least two double-maximum mode field optical fiber preamplifiers, and the double-maximum mode field optical fiber preamplifier and the large mode field optical fiber femtosecond oscillator share one semiconductor pump laser.
Preferably, the other end of the third double-clad large mode field gain fiber is connected with a second pump stripper for stripping the residual pump light which is not absorbed by the third double-clad large mode field gain fiber.
Preferably, the other end of the second pump stripper is connected with the input port of the third large mode field fiber isolator.
Preferably, the output end face of the third double-clad large mode field gain fiber included in the second large mode field fiber preamplifier is a bevel.
Preferably, the large mode field optical fiber circulator is connected with an optical fiber pulse stretcher and a pulse stretching controller.
Preferably, a plano-convex lens, an optical garbage can, a pumping light splitting sheet and a spatial optical isolator are correspondingly arranged on one side of the third double-cladding large-mode-field gain fiber.
A femtosecond seed laser consisting entirely of large mode field fibers, comprising: the fiber femtosecond oscillator, the two-stage large-mode-field fiber preamplifier and the large-mode-field fiber pulse stretcher which are pumped by the multimode semiconductor laser are shared.
Preferably, the large-mode-field fiber preamplifier is a two-stage large-mode-field fiber preamplifier at least, and shares one semiconductor pump laser with the large-mode-field fiber femtosecond oscillator.
Preferably, the double-cladding large-mode-field chirped fiber grating, the double-cladding large-mode-field gain fiber, the large-mode-field fiber coupler, the large-mode-field fiber isolator, and the large-mode-field fiber pulse stretcher, and the tail fibers of the signal light input and output ends of the large-mode-field fiber pulse selector and the pump beam combiner are polarization-maintaining fibers.
Compared with the prior art, the invention has the beneficial effects that:
1. the volume is littleer, and the structure is compacter: the femtosecond laser oscillator resonant cavity adopts large mode field optical fiber, so that the oscillator can directly output higher average power than that of single mode optical fiber with 6 μm core diameter, and no preamplifier is needed before entering the pulse stretcher, thus at least one stage of preamplifier and related devices are saved; the oscillator and the two-stage preamplifier can share one multimode semiconductor pump source, namely the whole seed laser can reach the output index which can be reached by a plurality of semiconductor pump sources only by one semiconductor pump source;
2. the reliability is higher: the multimode pump is used, pump light is mainly transmitted in the optical fiber cladding, laser is transmitted in the fiber core, and the multimode pump source and the fiber core are almost separated, so that the multimode pump source has better damage resistance than a single-mode pump source, and the isolation degree of the multimode pump protector is higher; only one semiconductor pump source and related driving power supply are needed;
3. the cost is lower: although the cost of the large mode field optical fiber is slightly higher than that of the single mode optical fiber, the cost of the whole laser is mainly on an optical fiber device, and the manufacturing principle and the manufacturing process of the large mode field optical fiber device and the single mode optical fiber device are basically the same, so that the cost of a single device is close; in addition, the optical fibers used in the present invention are all conventional step index fibers, without having to employ expensive photonic crystal fibers.
Drawings
Fig. 1 is a schematic structural diagram of an all-fiber femtosecond seed laser oscillator based on a large mode field fiber.
Fig. 2 is a schematic structural diagram of an all-fiber femtosecond seed laser preamplifier based on a large mode field fiber.
FIG. 3 is an output spectrum of an all-fiber femtosecond seed laser oscillator and preamplifier based on large mode field fiber.
FIG. 4 is a broadened single pulse waveform output by an all-fiber femtosecond seed laser amplifier based on a large mode field fiber.
Fig. 5 is an autocorrelation curve of a pulse compressed all-fiber femtosecond seed laser based on a large mode field fiber.
FIG. 6 is a schematic diagram of a femtosecond seed laser preamplifier consisting entirely of large mode field fibers.
In the figure: 1. a multimode semiconductor pump source; 2. a multimode pump protector; 3. a first multimode pump beam splitter; 4. double-cladding large-mode-field chirped fiber grating; 5. a first double-clad large mode field gain fiber; 6. a photodetector; 7. a large mode field fiber coupler; 8. a semiconductor saturable absorption mirror; 9. a first large mode field fiber isolator; 10. a large mode field fiber optic circulator; 11. a large mode field fiber pulse stretcher; 12. a pulse stretching controller; 13. a first signal pump combiner; 14. a second double-clad large mode field gain fiber; 15. a first pump stripper; 16. a second large mode field fiber isolator; 17. a large mode field fiber pulse selector; 18. a second signal pump combiner; 19. a third double-clad large mode field gain fiber; 20. a second multimode pump beam splitter; 21. a plano-convex lens; 22. a light garbage can; 23. a pump beam splitter; 24. a spatial light isolator; 25. a second pump stripper; 26. and a third large mode field fiber isolator.
Detailed Description
The technical solution in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.
Example 1
Referring to fig. 1 to 6, in embodiment 1 of the present invention, a structural schematic diagram of an all-fiber femtosecond seed laser oscillator based on a large mode field fiber includes a multimode semiconductor pump source 1, a multimode pump protector 2, a first multimode pump beam splitter 3, a double-clad large mode field chirped fiber grating 4, a first double-clad large mode field gain fiber 5, a photodetector 6, a large mode field fiber coupler 7, and a semiconductor saturable absorption mirror 8.
The multimode semiconductor pump source 1 provides pump light for both the oscillator and the two-stage preamplifier. The multimode semiconductor pump source 1 output fiber is a multimode fiber with a typical core diameter of 105 μm. Preferably the pump laser 1 is wavelength locked, i.e. the output power and wavelength drift with temperature is small, typically 9W average output power. The input and output fibers of the multimode pump protector 2 are multimode fibers, with a typical core diameter of 105 μm. The input port of the multimode pumping protector 2 is connected with the output end of the multimode semiconductor pumping source 1 and is used for protecting the multimode semiconductor pumping source 1.
The output port of the multimode pump protector 2 is connected with the input port of the first multimode pump beam splitter 3. The first multimode pump beam splitter 3 has two output ports in common. The port a is connected to an input port of the second multimode pump beam splitter 20 in fig. 2, and provides pump light for the two-stage fiber preamplifier; the port b is connected with one end of the double-cladding large-mode-field chirped fiber grating 4 to provide pumping light for the fiber oscillator.
The typical value of the reflection center wavelength of the double-cladding large-mode-field chirped fiber grating 4 is 1030nm, the typical reflection bandwidth is about 10nm, the average reflectivity is 30-40%, and a proper dispersion amount can be provided. In particular, the double-cladding large-mode-field chirped fiber grating 4 is engraved on the fiber core of a double-cladding large-mode-field passive fiber, preferably the fiber core of a single-mode double-cladding large-mode-field passive fiber, preferably the fiber core of a polarization-maintaining single-mode double-cladding large-mode-field passive fiber, the typical value of the diameter of the fiber core is 10 μm, and pumping light can pass through the double-cladding large-mode-field chirped fiber grating 4 without loss.
The other end of the double-cladding large-mode-field chirped fiber grating 4 is connected with one end of a first double-cladding large-mode-field gain fiber 5. The first double-clad large mode field gain fiber 5 is used as a working substance in the resonant cavity, preferably is a single-mode double-clad large mode field ytterbium (Yb) doped fiber, preferably is a polarization-maintaining single-mode double-clad large mode field Yb doped fiber, and the typical value of the fiber core diameter is 10 μm. In particular, the first double-clad large-mode-field-gain fiber 5 may also be an active ion of other elements such as erbium (Er), Er-Yb co-doped, holmium (Ho), thulium (Tm) doped, etc. to generate laser light of other wavelengths.
The large mode field optical fiber coupler 7 has four ports, wherein three ports c, d and e are respectively connected with the first double-cladding large mode field gain optical fiber 5, the photoelectric detector 6 and the semiconductor saturable absorption mirror 8, and the port f outputs femtosecond seed pulses generated by the oscillator. The optical fibers of the four ports of the large-mode-field optical fiber coupler 7 are all large-mode-field passive optical fibers, preferably single-mode large-mode-field passive optical fibers, preferably polarization-maintaining single-mode large-mode-field passive optical fibers, and the typical value of the fiber core diameter is 10 μm. The photoelectric detector 6 is used for converting the femtosecond laser pulse signal into a time sequence electric signal. The double-cladding large-mode-field chirped fiber grating 4 and the semiconductor saturable absorption mirror 8 form a fiber oscillator resonant cavity. The semiconductor saturable absorption reflector 8 has shorter relaxation time parameters and is responsible for the self-starting of the femtosecond mode locking; the oscillator totally adopts large mode field optical fibers, and the total dispersion amount of the optical fibers in the cavity is adjusted, so that the total dispersion amount of the optical fibers is matched with the different sign dispersion amount of the double-cladding large mode field chirped fiber grating 4, and the net dispersion amount in the resonant cavity can be finely adjusted and controlled.
Typically, for femtosecond laser light around 1030nm, dispersion-managed soliton mode-locking is achieved by preferably making the net dispersion in the cavity less negative. The oscillator outputs a smooth gaussian shaped spectrum with a left-right symmetry, with a spectral width of about 6nm, as shown by the solid line in fig. 3. On the other hand, under the condition that the pulse energy density and the pulse repetition frequency of the incident semiconductor saturable absorption mirror 8 are kept unchanged, the resonator can support larger single-pulse energy oscillation, and output higher average power than the existing femtosecond laser oscillator based on the single-mode single-clad fiber, wherein the typical value is about 10 mW; typical values of the repetition frequency are 20-40 MHz.
Referring to fig. 2, in embodiment 1 of the present invention, an all-fiber femtosecond seed laser preamplifier based on a large mode field fiber is shown, in which an input port of a first large mode field fiber isolator 9 is connected to a port f of a large mode field fiber coupler 7 of an oscillator, and an output port is connected to an input port r of a large mode field fiber circulator 10. The port II of the large mode field optical fiber circulator 10 is connected with an adjustable large mode field optical fiber pulse stretcher 11.
The seed pulse output by the oscillator can enter the large-mode-field optical fiber pulse stretcher 11 for pulse width stretching without passing through any pre-amplifier. The pulse stretching controller 12 can adjust parameters such as the second-order dispersion amount, the high-order dispersion amount, the central wavelength and the like provided by the large mode field optical fiber pulse stretcher 11 by changing stress, temperature and the like. After stretching, the pulse width can be broadened to hundreds of picoseconds or even longer, typically around 500 ps.
The optical fibers of the first large-mode-field optical fiber isolator 9, the large-mode-field optical fiber circulator 10 and the large-mode-field optical fiber pulse stretcher 11 are all large-mode-field passive optical fibers, preferably single-mode large-mode-field passive optical fibers, preferably polarization-maintaining single-mode large-mode-field passive optical fibers, and the typical value of the fiber core diameter is 10 μm. The laser pulse with the broadened pulse width enters a first large-mode-field optical fiber preamplifier to pre-amplify the average power.
The first signal pump combiner 13 has two input ports and one output port. The signal input port g is connected with the output port of the large-mode-field optical fiber circulator 10, the signal beam combination output port is connected with one end of the second double-clad large-mode-field gain optical fiber 14, the two optical fibers are all double-clad large-mode-field passive optical fibers, preferably single-mode double-clad large-mode-field passive optical fibers, preferably polarization-maintaining single-mode double-clad large-mode-field passive optical fibers, and the typical value of the fiber core diameter is 10 micrometers;
the pump input port h is connected to the output port i of the second multimode pump splitter 20, which is all multimode fiber with a typical core diameter of 105 μm. The second double-clad large-mode-field gain fiber 14 is used as a working substance of the first large-mode-field fiber preamplifier, is preferably a single-mode double-clad large-mode-field Yb-doped fiber, is preferably a polarization-maintaining single-mode double-clad large-mode-field Yb-doped fiber, and has a typical fiber core diameter of 10 μm; in particular, the second double-clad large-mode-field-gain fiber 14 may also be an active ion of other elements such as erbium (Er), Er-Yb co-doped, holmium (Ho), thulium (Tm) doped, etc. to generate laser light of other wavelengths.
The other end of the second double-clad large mode field gain fiber 14 is connected to one end of a first pump stripper 15. The optical fibers at two ends of the first pump stripper 15 are all double-cladding large-mode-field passive fibers, preferably single-mode double-cladding large-mode-field passive fibers, preferably polarization-maintaining single-mode double-cladding large-mode-field passive fibers, and the typical diameter value of the fiber core is 10 μm; the refractive index of the coating layer of the middle section of the first pump stripper 15 is greater than that of the inner cladding, and is used for stripping the residual pump light which is not absorbed by the second double-cladding large-mode-field gain fiber 14. The average power of the stretching pulse after passing through the first large mode field fiber preamplifier was about 100 mW.
An input port of the second large mode field fiber isolator 16 is connected to an output port of the first pump stripper 15, and an output port is connected to an input port of the large mode field fiber pulse selector 17. The large mode field fiber pulse selector 17 may be an acousto-optic modulator or electro-optic modulator that functions to reduce the pulse repetition frequency of tens of MHz generated by the oscillator to the desired typical value of 100kHz to 2 MHz. The fiber types at the two ends of each of the second large-mode-field fiber isolator 16 and the large-mode-field fiber pulse selector 17 are all large-mode-field passive fibers, preferably single-mode large-mode-field passive fibers, preferably polarization-maintaining single-mode large-mode-field passive fibers, and the typical value of the fiber core diameter is 10 μm. And the stretching laser pulse with the reduced repetition frequency enters a second large mode field optical fiber preamplifier to pre-amplify the pulse energy. The second signal pump combiner 18 has two input ports and one output port. The signal input port l is connected with an output port of the large-mode-field optical fiber pulse selector 17, the signal beam combination output port is connected with one end of a third double-cladding large-mode-field gain optical fiber 19, all the two paths of optical fibers are double-cladding large-mode-field passive optical fibers, preferably single-mode double-cladding large-mode-field passive optical fibers, preferably polarization-maintaining single-mode double-cladding large-mode-field passive optical fibers, and the typical value of the fiber core diameter is 10 micrometers; in particular, the signal beam combining output port may also be a few-mode double-clad large-mode-field passive fiber, preferably a polarization-maintaining few-mode double-clad large-mode-field passive fiber, with a typical core diameter of 15-30 μm, which matches the fiber type of the third double-clad large-mode-field gain fiber 19.
The pump input port k is connected to the output port j of the second multimode pump splitter 20, which is all multimode fiber with a typical core diameter of 105 μm. The third double-clad large-mode-field gain fiber 19 is used as a working substance of the first large-mode-field fiber preamplifier, is preferably a single-mode double-clad large-mode-field Yb-doped fiber, is preferably a polarization-maintaining single-mode double-clad large-mode-field Yb-doped fiber, and has a typical fiber core diameter of 10 μm; or a few-mode double-clad large-mode-field Yb-doped fiber, preferably a polarization-maintaining few-mode double-clad large-mode-field Yb-doped fiber with a typical core diameter of 15-30 μm, and in particular, the second double-clad large-mode-field gain fiber 14 may also be an active ion of other elements such as erbium (Er), Er-Yb co-doping, holmium (Ho), thulium (Tm), and the like, to generate laser light of other wavelengths. The output port of the third double-clad large mode field gain fiber 19 is processed into a beveled end face, the typical value of the bevel angle is 8 degrees, and the purpose is to reduce the length of the fiber through which the amplified pulse passes as much as possible, thereby reducing the adverse effects of the nonlinear effect on the time domain and the spectral characteristics of the amplified large energy pulse. The amplified laser pulse is coupled into a free space from the bevel end face and collimated by the plano-convex lens 21.
The femtosecond seed pulse generated by the oscillator directly enters the input port of the large-mode-field optical fiber circulator 10 without passing through any pre-amplifier after passing through the first large-mode-field optical fiber isolator 9, and the pulse width is widened to a proper width by the adjustable large-mode-field optical fiber pulse stretcher 11 and then is output through the output port of the optical fiber circulator; pre-amplifying the average power by a first large-mode-field optical fiber pre-amplifier;
the collimated light beam is mixed with a considerable amount of residual pump light which is not absorbed by the third double-clad large mode field gain fiber 19, and the residual pump light is reflected by the pump beam splitter 23 and guided to the light-receiving garbage can 22. Then, the energy-amplified stretched laser pulse is finally output after passing through the spatial optical isolator 24.
In embodiment 1 shown in fig. 2, the maximum single pulse energy of the final output may be greater than 5 uJ; a typical output spectrum is shown by the dashed line in fig. 3, the spectral width is broadened to about 7nm due to the nonlinear effect and gain shaping, and the spectral shape is also developed from a gaussian shape generated by the oscillator to a parabolic shape, which illustrates that the linear chirp inside the pulse through pulse stretching and energy amplification dominates.
Example 2
If it is not necessary to reduce the pulse repetition frequency outputted from the fiber femtosecond oscillator, the large mode field fiber pulse selector can be omitted, as shown in fig. 6, which is a schematic diagram of the structure of a femtosecond seed laser preamplifier composed entirely of large mode field fiber in embodiment 2. The difference from example 1 is:
1. the splitting ratios of the output ports i and j of the second multimode pump beam splitter 20 are different;
2. the length of the third double-clad large mode field gain fiber 19 can be longer due to higher repetition frequency, reduced single pulse energy and reduced nonlinear effect;
3. optionally, without using a free space pump light splitting mode, the output end of the third double-clad large mode field gain fiber 19 is connected to a second pump stripper 25 for stripping the residual pump light not absorbed by the third double-clad large mode field gain fiber 19;
4. optionally, a third large mode field fiber isolator 26 is used in place of the spatial optical isolator. The final output average power of the present embodiment can be larger than 5W, and the typical value of the pulse repetition frequency is 20-40 MHz.
The working principle of the invention is as follows: the femtosecond laser pulse after stretching and power pre-amplification enters a large mode field optical fiber pulse selector 17, the pulse repetition frequency is reduced to a required frequency value from tens of MHz, then the femtosecond laser pulse enters a second large mode field optical fiber pre-amplifier to pre-amplify pulse energy, the stretched femtosecond laser pulse after energy pre-amplification is coupled and output from the other bevel end face of a third double-clad large mode field gain optical fiber 19 to a free space and then is collimated by a lens, and the residual pump light which is not absorbed by the third double-clad large mode field gain optical fiber 19 and mixed in a collimated light beam is reflected by a pump beam splitter and guided to a light-receiving garbage can; compared with the technical scheme of the existing single-mode single-cladding fiber femtosecond laser oscillator and multi-stage independent pre-amplifier, the laser has the advantages of more compact structure, smaller volume, lower cost and higher reliability.
It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof.

Claims (9)

1. An all-fiber femtosecond seed laser based on a large mode field fiber is characterized by comprising a large mode field fiber femtosecond oscillator and a two-stage large mode field fiber preamplifier which share a multimode semiconductor laser pump, a large mode field fiber pulse stretcher and a large mode field fiber pulse selector (17);
the large mode field fiber femtosecond oscillator comprises a multimode semiconductor pumping source (1), a multimode pumping protector (2), a first multimode pumping beam splitter (3), a double-cladding large mode field chirped fiber grating (4), a first double-cladding large mode field gain fiber (5), a large mode field fiber coupler (7), a semiconductor saturable absorption reflector (8) and a photoelectric detector (6);
an input port of the multimode pump protector (2) is connected with the multimode semiconductor pump source (1) and used for protecting the multimode semiconductor pump source (1), an input port of the first multimode pump beam splitter (3) is connected with an output port of the multimode pump protector (2), one of two output ports of the first multimode pump beam splitter (3) is connected with an input port of the second multimode pump beam splitter (20) and used for pumping a two-stage large-mode-field fiber preamplifier, and the other output port of the first multimode pump beam splitter is connected with one end of the double-cladding large-mode-field chirped fiber grating (4) and used for pumping a large-mode-field fiber femtosecond laser oscillator;
the double-cladding large-mode-field chirped fiber grating (4) and the semiconductor saturable absorption reflector (8) form a fiber oscillator resonant cavity, one end of the first double-cladding large-mode-field gain fiber (5) is connected with one end of the chirped fiber grating and serves as a working substance in the resonant cavity, the coupler in the resonant cavity has four ports, three ports are respectively connected with the semiconductor saturable absorption reflector (8), the photoelectric detector (6) and the first double-cladding large-mode-field gain fiber (5), the fourth port outputs femtosecond seed pulses generated by the fiber oscillator resonant cavity, and the femtosecond seed pulses generated by the fiber oscillator resonant cavity correspond to the first large-mode-field fiber isolator (9);
the two-stage large mode field fiber preamplifier comprises a first large mode field fiber preamplifier and a second large mode field fiber preamplifier, the first large mode field fiber preamplifier comprises a first signal pump beam combiner (13), a second double-cladding large mode field gain fiber (14) and a first pump stripper (15), a signal input port of the first signal pump beam combiner (13) is connected with an output port of a large mode field fiber circulator (10), a pump input end of the first signal pump beam combiner (13) is connected with one output port of the second multimode pump beam splitter (20), and a signal output port of the first signal pump beam combiner (13) is connected with one port of the second double-cladding large mode field gain fiber (14);
the first pump stripper (15) is connected with the other port of the second double-clad large-mode-field gain fiber (14) and is used for stripping residual pump light which is not absorbed by the second double-clad large-mode-field gain fiber (14);
the output port of the first large mode field optical fiber preamplifier is connected with the input port of a second large mode field optical fiber isolator (16), and the second large mode field optical fiber preamplifier comprises a second signal pumping beam combiner (18) and a third double-cladding large mode field gain optical fiber (19);
and a signal input port of the second signal pumping beam combiner (18) is connected with an output port of the large mode field optical fiber pulse selector (17), a pumping input port is connected with the other output port of the second signal pumping beam combiner (18), and a signal output port is connected with one end of the third double-clad large mode field gain optical fiber (19).
2. The all-fiber femtosecond seed laser based on the large-mode-field fiber as claimed in claim 1, wherein there are at least two dual-stage large-mode-field fiber pre-amplifiers, and the dual-stage large-mode-field fiber pre-amplifier and the large-mode-field fiber femtosecond oscillator share one semiconductor pump laser.
3. The all-fiber femtosecond seed laser based on the large-mode-field fiber as claimed in claim 1, wherein the other end of the third double-clad large-mode-field gain fiber (19) is connected with a second pump stripper (25) for stripping the residual pump light not absorbed by the third double-clad large-mode-field gain fiber (19), and the other end of the second pump stripper (25) is connected with the input port of a third large-mode-field fiber isolator (26).
4. The all-fiber femtosecond seed laser based on the large-mode-field fiber as claimed in claim 1, wherein the output end surface of the third double-clad large-mode-field gain fiber (19) included in the second large-mode-field fiber preamplifier is a bevel.
5. The all-fiber femtosecond seed laser based on the large-mode-area fiber as claimed in claim 1, wherein a large-mode-area fiber pulse stretcher (11) and a pulse stretching controller (12) are connected to the large-mode-area fiber circulator (10).
6. The all-fiber femtosecond seed laser based on the large-mode-field fiber as claimed in claim 1, wherein a plano-convex lens (21), an optical trash can (22), a pump beam splitter (23) and a spatial optical isolator (24) are correspondingly arranged on one side of the third double-clad large-mode-field gain fiber (19).
7. A femtosecond seed laser consisting entirely of large mode field fibers, comprising: the fiber femtosecond oscillator, the two-stage large-mode-field fiber preamplifier and the large-mode-field fiber pulse stretcher which are pumped by the multimode semiconductor laser are shared.
8. A femtosecond seed laser composed entirely of large-mode-area fibers as set forth in claim 7, wherein said large-mode-area fiber preamplifier is a two-stage large-mode-area fiber preamplifier and shares one semiconductor pump laser with said large-mode-area fiber femtosecond oscillator.
9. The femtosecond seed laser comprising the large-mode-field fiber as set forth in any one of claims 1 to 7, wherein the double-cladding large-mode-field chirped fiber grating (4), the double-cladding large-mode-field gain fiber, the large-mode-field fiber coupler (7), the large-mode-field fiber isolator, the large-mode-field fiber pulse stretcher (11), the large-mode-field fiber pulse selector (17), and the signal light input and output end pigtails of the pump combiner are polarization-maintaining fibers.
CN202110761035.7A 2021-07-06 2021-07-06 All-fiber femtosecond seed laser based on large mode field fiber Pending CN113328328A (en)

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