CN113328328B - All-fiber femtosecond seed laser based on large-mode-field optical fiber - Google Patents

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

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CN113328328B
CN113328328B CN202110761035.7A CN202110761035A CN113328328B CN 113328328 B CN113328328 B CN 113328328B CN 202110761035 A CN202110761035 A CN 202110761035A CN 113328328 B CN113328328 B CN 113328328B
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optical fiber
field
fiber
double
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CN113328328A (en
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刘学松
尹雨松
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

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
  • Plasma & Fusion (AREA)
  • Optics & Photonics (AREA)
  • Lasers (AREA)

Abstract

The invention discloses a full large mode field optical fiber femtosecond seed laser, which comprises a large mode field optical fiber femtosecond oscillator and a double-stage large mode field optical fiber preamplifier which share one multimode semiconductor laser pump, and a large mode field optical fiber pulse stretcher and a large mode field optical 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 optical fiber, a large mode field optical fiber coupler and a semiconductor saturable absorption reflector, and the double-stage large mode field optical fiber preamplifier comprises a large mode field optical fiber isolator, a signal pump beam combiner, a double-cladding large mode field gain optical fiber and a pump stripper; the resonant cavity of the optical fiber femtosecond seed laser oscillator adopts large-mode-field optical fibers, so that the oscillator can directly output higher average power than a single-mode optical fiber with a 6 mu m core diameter, and compared with the technical scheme of the existing single-mode Shan Baoceng optical fiber femtosecond laser oscillator and a multistage independent preamplifier, the optical fiber femtosecond seed laser oscillator has the advantages of more compact structure, small volume, lower cost and higher reliability.

Description

All-fiber femtosecond seed laser based on large-mode-field optical fiber
Technical Field
The invention relates to the technical field of fiber lasers, 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, biomedical science, scientific research, national defense and the like. Currently, the main stream high energy/high power femtosecond laser still mainly adopts a Chirped Pulse Amplification (CPA) structure, namely, a laser oscillator generates low energy and high-repetition frequency ultrashort seed laser pulses through a mode locking principle, the pulse width is stretched to hundreds of picoseconds or longer through a pulse stretcher, and then the repetition frequency is reduced through a pulse selector, wherein in the process, a multistage preamplifier is generally required due to the insertion loss and damage threshold limit of related devices; then the high-power/high-energy amplification is carried out by sending the high-power/high-energy amplification to a single-stage or multi-stage main amplifier; finally, the pulse width is compressed to the femto second level by a pulse compressor. Ultrafast lasers generally relate to solid state laser technology and fiber laser technology, depending on the morphology of the working substance. Compared with a solid laser, the optical fiber laser has the advantages of small volume, light weight, easy maintenance, large heat dissipation area and the like, and is an important development direction of the femtosecond laser technology in recent years. With the gradual maturation of the related technology, the femtosecond seed oscillator, the optical fiber type pulse stretcher, the pulse selector and the multistage preamplifier have low heat dissipation requirement due to small bearing energy/power, and can use flexible optical fibers with small core diameters, so that an independently packaged module can be manufactured, the volume of the femtosecond laser product is reduced, and the large-scale generation and maintenance in the use process of the femtosecond laser are facilitated, wherein the part is defined as the femtosecond seed laser in the invention.
The technical scheme of the existing mainstream femtosecond seed laser has the following defects: 1. the oscillator fiber generally uses a single-mode Shan Baoceng fiber with a 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 that a first-stage preamplifier is needed to be used immediately after the oscillator; 2. the pump sources of the oscillator and the pre-amplifier generally use a low-power single-mode output semiconductor laser, and the pump laser belongs to a high-value vulnerable device, because pump light and laser propagate in a single-mode fiber core together, and signal light returns to the single-mode semiconductor laser to be extremely vulnerable; 3. because of the low maximum injectable power of single-mode semiconductor lasers, three to four stages of independent fiber preamplifiers are required in total in the pulse stretching and pulse selecting processes, which means that three to four semiconductor pump lasers and corresponding driving power supplies and control loops are required, which clearly increases the cost, volume and complexity of the system and reduces the working reliability.
Although the prior art scheme adopts a double-cladding large-mode-area photonic crystal fiber as a gain medium of a femtosecond laser oscillator so as to improve the output pulse energy of the oscillator, as disclosed in the invention patent CN 100437323C, the limitation of the scheme is obvious: 1. the optical fiber structure is not all-optical fiber structure, the volume is increased by using a space optical path, the environment interference resistance is reduced, and the great advantage of the optical fiber laser is lost; 2. photonic crystal fibers are still very expensive at present.
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 above purpose, the present invention provides the following technical solutions:
a full-fiber femtosecond seed laser based on a large-mode-field optical fiber comprises a large-mode-field optical fiber femtosecond oscillator and a two-stage large-mode-field optical fiber preamplifier which share one multimode semiconductor laser pump, a large-mode-field optical fiber pulse stretcher and a large-mode-field optical fiber pulse selector;
The large-mode-field optical 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 optical fiber, a large-mode-field optical fiber coupler, a semiconductor saturable absorption reflecting mirror and a photoelectric detector.
The input port of the multimode pump protector is connected with the multimode semiconductor pump source and is used for protecting the multimode semiconductor pump source;
The input port of the first multimode pumping beam splitter is connected with the output port of the multimode pumping protector, one of the two output ports of the first multimode pumping beam splitter is connected with the input port of the second multimode pumping beam splitter and is used for pumping the double-stage large-mode-field optical fiber preamplifier, and the other output port of the first multimode pumping beam splitter is connected with one end of the double-cladding large-mode-field chirped optical fiber grating and is used for pumping the large-mode-field optical fiber femtosecond laser oscillator;
The double-cladding large-mode-field chirped fiber grating and the semiconductor saturable absorption reflecting mirror 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 used as a working substance in the resonant cavity, four ports are shared in the resonant cavity, wherein the three ports are respectively connected with the semiconductor saturable absorption reflecting mirror, the photoelectric detector and the first double-cladding large-mode-field gain fiber, 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;
the two-stage large-mode-field optical fiber pre-amplifier comprises a first large-mode-field optical fiber pre-amplifier and a second large-mode-field optical fiber pre-amplifier, wherein the first large-mode-field optical fiber pre-amplifier comprises a first signal pump beam combiner, a second double-cladding large-mode-field gain optical fiber and a first pump stripper, a signal input port of the first signal pump beam combiner is connected with an output port of a large-mode-field optical fiber circulator, a pump input end of the first signal pump beam combiner is connected with an output port of a second multimode pump beam splitter, and a signal output port of the first signal pump 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-cladding large-mode-field gain fiber and is used for stripping residual pump light which is not absorbed by the second double-cladding large-mode-field gain fiber;
the output port of the first large-mode-field optical fiber pre-amplifier is connected with the input port of the second large-mode-field optical fiber isolator, and the second large-mode-field optical fiber pre-amplifier comprises a second signal pump beam combiner and a third double-cladding large-mode-field gain optical fiber;
The second signal pump beam combiner signal input port is connected with the output port of the large mode field optical fiber pulse selector, the pump input port is connected with the other output port of the multimode second pump beam splitter, and the signal output port is connected with one end of the third double-cladding large mode field gain optical fiber.
Preferably, at least two bipolar large-mode-field optical fiber preamplifiers are arranged, and the bipolar large-mode-field optical fiber preamplifiers and the large-mode-field optical fiber femtosecond oscillator share one semiconductor pump laser.
Preferably, the other end of the third double-cladding large-mode-field gain fiber is connected with a second pump stripper for stripping residual pump light which is not absorbed by the third double-cladding 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 optical fiber isolator.
Preferably, the output end face of the third double-cladding large-mode-field gain optical fiber contained in the second large-mode-field optical 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, one side of the third double-cladding large-mode-field gain optical fiber is correspondingly provided with a planoconvex lens, an optical garbage can, a pumping beam splitter and a space optical isolator.
A femtosecond seed laser composed entirely of large mode field optical fibers, comprising: the optical fiber femtosecond oscillator and the two-stage large-mode-field optical fiber preamplifier and the large-mode-field optical fiber pulse stretcher are used for pumping a multimode semiconductor laser.
Preferably, the large mode field optical fiber pre-amplifier is at least two stages of large mode field optical fiber pre-amplifiers, and shares one semiconductor pump laser with the large mode field optical fiber femtosecond oscillator.
Preferably, the signal light input and output end tail fibers of 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 are polarization-maintaining fibers.
Compared with the prior art, the invention has the beneficial effects that:
1. The volume is smaller, and the structure is more compact: the femtosecond laser oscillator resonant cavity adopts large mode field optical fibers, so that the oscillator can directly output higher average power than a single-mode fiber with a 6 mu m core diameter, and therefore, no preamplifier is needed before the oscillator enters a pulse stretcher, and at least one-stage preamplifier and related devices are omitted; the oscillator and the two-stage pre-amplifier can share one multimode semiconductor pumping source, namely, the whole seed laser can reach the output index which can be reached by a plurality of semiconductor pumping sources only by one semiconductor pumping source;
2. The reliability is higher: the multimode pump is used, the pump light mainly propagates in the fiber cladding, and the laser propagates in the fiber core, and the two are almost separated, so that the multimode pump source has better damage resistance than the 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 a 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 are basically the same as those of the single-mode optical fiber device, so that the cost of the single device is approximate; furthermore, the optical fibers used in the present invention are all conventional step index fibers, and expensive photonic crystal fibers are not necessary.
Drawings
Fig. 1 is a schematic structural diagram of an all-fiber femtosecond seed laser oscillator based on a large-mode-field optical fiber.
Fig. 2 is a schematic structural diagram of an all-fiber femtosecond seed laser preamplifier based on a large-mode-field optical fiber.
Fig. 3 is an output spectrum of an all-fiber femtosecond seed laser oscillator and a preamplifier based on a large mode field fiber.
Fig. 4 is a stretched single pulse waveform of an all-fiber femtosecond seed laser amplifier output 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 composed 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 optical fiber circulator; 11. a large mode field optical 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 optical 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. pumping the beam splitter; 24. a spatial optical isolator; 25. a second pump stripper; 26. and a third large mode field optical fiber isolator.
Detailed Description
The technical solutions 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. 1-6, in embodiment 1 of the present invention, a structure 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 dual 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 has a wavelength locking function, i.e. the output power and wavelength drift with temperature are small, and the average output power is typically 9W. The input and output fibers of the multimode pump protector 2 are multimode fibers, with a typical value of 105 μm core diameter. The input port of the multimode pump protector 2 is connected with the output end of the multimode semiconductor pump source 1 and is used for protecting the multimode semiconductor pump source 1.
The output port of the multimode pump protector 2 is connected with the input port of the first multimode pump splitter 3. The first multimode pump splitter 3 has two output ports in common. Wherein port a is connected to the input port of the second multimode pump splitter 20 in fig. 2, and provides pump light for the two-stage optical fiber preamplifier; the port b is connected with one end of the double-cladding large-mode-field chirped fiber grating 4 to provide pump 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 the proper dispersion can be provided. In particular, the double-cladding large-mode-field chirped fiber grating 4 is inscribed on the fiber core of the double-cladding large-mode-field passive fiber, preferably the fiber core of the single-mode double-cladding large-mode-field passive fiber, preferably the fiber core of the polarization-maintaining single-mode double-cladding large-mode-field passive fiber, the fiber core diameter is typically 10 mu m, and the 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 a resonant cavity, preferably a single-mode double-clad large-mode-field ytterbium (Yb) -doped fiber, preferably a polarization-maintaining single-mode double-clad large-mode-field Yb-doped fiber, and has a typical core diameter of 10 μm. In particular, the first double-clad large-mode-area gain fiber 5 may also be an active ion doped with other elements such as erbium (Er), er-Yb co-doped, holmium doped (Ho), thulium doped (Tm) and the like, so as to generate laser light with other wavelengths.
The large-mode-field optical fiber coupler 7 is provided with 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 reflector 8, and the port f outputs femtosecond seed pulses generated by the oscillator. The fibers of the four ports of the large-mode-field fiber coupler 7 are all large-mode-field passive fibers, preferably single-mode large-mode-field passive fibers, and preferably polarization-maintaining single-mode large-mode-field passive fibers, and the typical value of the fiber core diameter is 10 mu m. The photodetector 6 is used to convert the femtosecond laser pulse signal into a time-series electrical signal. The double-cladding large-mode-field chirped fiber grating 4 and the semiconductor saturable absorption reflecting mirror 8 form a resonant cavity of the fiber oscillator. The semiconductor saturable absorption reflector 8 has shorter relaxation time parameters and is responsible for self-starting of femto-second mode locking; the oscillator is all made of large-mode-field optical fibers, and the total dispersion quantity of the optical fibers in the cavity is adjusted so as to be matched with the different-number dispersion quantity of the double-cladding large-mode-field chirped fiber grating 4, so that the net dispersion quantity in the resonant cavity can be finely regulated and controlled.
Typically, for femtosecond lasers around 1030nm wavelength, it is preferable to make the net dispersion in the resonant cavity take on a small negative value, so that the mode locking of the soliton can be realized. The oscillator outputs a smooth gaussian spectrum of bilateral symmetry with a spectral width of about 6nm, as shown in solid lines in fig. 3. On the other hand, under the condition that the pulse energy density and the pulse repetition frequency of the laser incident to the semiconductor saturable absorption reflector 8 are kept unchanged, larger single-pulse energy oscillation can be supported in the resonant cavity, and the average power is higher than that of the existing single-mode Shan Baoceng-based optical fiber femtosecond laser oscillator, and the typical value is about 10mW; the repetition frequency is typically 20-40 MHz.
Referring to fig. 2, in embodiment 1 of the present invention, a full-fiber femtosecond seed laser pre-amplifier based on a large-mode-field fiber is schematically shown, 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 ① of a large-mode-field fiber circulator 10. The port ② of the large mode field fiber circulator 10 is connected to an adjustable large mode field fiber pulse stretcher 11.
The seed pulse output by the oscillator can enter the large-mode-field optical fiber pulse stretcher 11 to carry out pulse width stretching without passing through any pre-amplifier. The pulse stretching controller 12 can adjust parameters such as second-order dispersion amount, higher-order dispersion amount, center 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 stretched to hundreds of picoseconds or even longer, typically around 500ps.
The fibers of the first large-mode-field fiber isolator 9, the large-mode-field fiber circulator 10 and the large-mode-field fiber pulse stretcher 11 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 core diameter is 10 μm. The laser pulse with the widened pulse width enters a first large-mode-field optical fiber pre-amplifier 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 an output port ③ of the large-mode-field optical fiber circulator 10, the signal beam combination output port is connected with one end of a second double-cladding large-mode-field gain optical fiber 14, 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 mu m;
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-cladding large-mode-field gain fiber 14 is used as a working substance of the first large-mode-field fiber pre-amplifier, and is preferably a single-mode double-cladding large-mode-field Yb-doped fiber, and is preferably a polarization-maintaining single-mode double-cladding large-mode-field Yb-doped fiber, and the typical value of the fiber core diameter is 10 μm; in particular, the second double-clad large-mode-area gain fiber 14 may also be an active ion doped with other elements such as erbium (Er), er-Yb co-doped, holmium doped (Ho), thulium doped (Tm) and the like to generate laser light with other wavelengths.
The other end of the second double-clad large mode field gain fiber 14 is connected to one end of the first pump stripper 15. The fibers at the 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, and preferably polarization-maintaining single-mode double-cladding large-mode-field passive fibers, and the typical value of the fiber core diameter is 10 mu m; the refractive index of the middle fiber coating layer of the first pump stripper 15 is larger than that of the inner cladding layer, and the middle fiber coating layer of the first pump stripper 15 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 optical fiber pre-amplifier is about 100mW.
The input port of the second large mode field fiber isolator 16 is connected to the output port of the first pump stripper 15, and the output port is connected to the 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 an electro-optic modulator, and functions to reduce the pulse repetition frequency of tens of MHz generated by the oscillator to a desired typical value of 100kHz to 2MHz. The fiber types at both ends 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, and 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 the output port of the large-mode-field optical fiber pulse selector 17, the signal beam combining 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 mu m; in particular, the signal beam combining output port may also be a few-mode double-clad large-mode-field passive optical fiber, preferably a polarization-maintaining few-mode double-clad large-mode-field passive optical fiber, with a typical core diameter of 15-30 μm, which is matched with the fiber type of the third double-clad large-mode-field gain optical 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-cladding large-mode-field gain fiber 19 is used as a working substance of the first large-mode-field fiber pre-amplifier, and is preferably a single-mode double-cladding large-mode-field Yb-doped fiber, and is preferably a polarization-maintaining single-mode double-cladding large-mode-field Yb-doped fiber, and the typical value of the fiber core diameter is 10 μm; the second double-clad large-mode-area gain fiber 14 may be an erbium (Er) doped fiber, an Er-Yb co-doped fiber, a holmium (Ho) doped fiber, or a thulium (Tm) doped fiber, which is preferably a polarization-maintaining small-mode double-clad large-mode-area Yb doped fiber, with a typical core diameter of 15 to 30 μm. The output port of the third double-clad large-mode-area gain fiber 19 is processed into a bevel end face, and the typical value of the inclination angle is 8 degrees, so that the length of the fiber through which amplified pulses pass is reduced as much as possible, and the adverse effect of nonlinear effects on the time domain and spectral characteristics of the amplified large-energy pulses is reduced. The amplified laser pulses are collimated by the plano-convex lens 21 after being coupled into free space from the bezel end face.
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 preamplifier 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 also mixed with a considerable amount of residual pump light which is not absorbed by the third double-cladding large-mode-field gain optical fiber 19, and the residual pump light is reflected by the pump beam splitter 23 and is guided to the light receiving garbage can 22. The energy amplified stretched laser pulses then pass through a spatial opto-isolator 24 and are finally output.
In example 1 given in fig. 2, the maximum single pulse energy of the final output may be greater than 5uJ; a typical output spectrum is shown in dashed lines in fig. 3, with the spectral width being broadened to about 7nm due to nonlinear effects and gain shaping, and the spectral shape also evolving from gaussian to parabolic by the oscillator, which illustrates that the internal linear chirp of the pulse after stretching and energy amplification dominates.
Example 2
The large mode field fiber pulse selector can be omitted if it is not necessary to reduce the pulse repetition frequency outputted from the fiber femtosecond oscillator, as in example 2 given in fig. 6, a schematic configuration of a femtosecond seed laser preamplifier entirely composed of a large mode field fiber. The difference from example 1 is that:
1. The output ports i and j of the second multimode pump beam splitter 20 have different splitting ratios;
2. The third double-clad large-mode-field gain fiber 19 may be longer in length due to the lower repetition frequency, lower single pulse energy, and lower nonlinear effects;
3. Alternatively, a free space pumping splitting mode is not required, and the output end of the third double-cladding large-mode-field gain fiber 19 is connected to a second pump stripper 25, so as to strip the residual pump light which is not absorbed by the third double-cladding large-mode-field gain fiber 19;
4. alternatively, a third large mode field fiber isolator 26 is used in place of the spatial optical isolator. The final output average power of this embodiment may be greater than 5W, with a typical value of 20-40 MHz for pulse repetition frequency.
The working principle of the invention is as follows: the femto-second laser pulse after stretching and power pre-amplification enters a large-mode-field optical fiber pulse selector 17, the pulse repetition frequency is reduced from tens of MHz to a required frequency value, then enters a second large-mode-field optical fiber pre-amplifier to pre-amplify pulse energy, the stretched femto-second laser pulse after energy pre-amplification is coupled and output to a free space from the other bevel end face of a third double-cladding large-mode-field gain optical fiber 19 and then is collimated by a lens, and residual pump light which is mixed in a collimated beam and is not absorbed by the third double-cladding large-mode-field gain optical fiber 19 is reflected by a pump beam splitting sheet and is guided to a light receiving garbage can; compared with the technical scheme of adding a multistage independent pre-amplifier into the existing single-mode Shan Baoceng optical fiber femtosecond laser oscillator, 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 characteristics thereof.

Claims (3)

1. The all-fiber femtosecond seed laser based on the large-mode-field optical fiber is characterized by comprising a large-mode-field optical fiber femtosecond oscillator and a two-stage large-mode-field optical fiber preamplifier which share one multimode semiconductor laser pump, a large-mode-field optical fiber pulse stretcher and a large-mode-field optical fiber pulse selector (17);
the large-mode-field optical fiber pulse stretcher is a large-mode-field optical fiber pulse stretcher (11) and a pulse stretching controller (12);
The large-mode-field optical fiber femtosecond oscillator comprises a multimode semiconductor pump source (1), a multimode pump protector (2), a first multimode pump beam splitter (3), a double-cladding large-mode-field chirped fiber grating (4), a first double-cladding large-mode-field gain optical fiber (5), a large-mode-field optical fiber coupler (7), a semiconductor saturable absorption reflector (8) and a photoelectric detector (6);
The 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), the input port of the first multimode pump beam splitter (3) is connected with the output port of the multimode pump protector (2), one of the two output ports of the first multimode pump beam splitter (3) is connected with the input port of the second multimode pump beam splitter (20) and used for pumping the double-stage large-mode-field optical 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 the large-mode-field optical 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, the resonant cavity is used as a working substance in the resonant cavity, the resonator is provided with four ports, wherein the 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 optical fiber pre-amplifier comprises a first large-mode-field optical fiber pre-amplifier and a second large-mode-field optical fiber pre-amplifier, the first large-mode-field optical fiber pre-amplifier comprises a first signal pump beam combiner (13), a second double-cladding large-mode-field gain optical 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 optical fiber circulator (10), a pump input end of the first signal pump beam combiner (13) is connected with one output port of a 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 optical fiber (14);
The first pump stripper (15) is connected with the other port of the second double-cladding large-mode-field gain optical fiber (14) and is used for stripping residual pump light which is not absorbed by the second double-cladding large-mode-field gain optical fiber (14);
The output port of the first large-mode-field optical fiber pre-amplifier is connected with the input port of a second large-mode-field optical fiber isolator (16), and the second large-mode-field optical fiber pre-amplifier comprises a second signal pump beam combiner (18) and a third double-cladding large-mode-field gain optical fiber (19);
the output end of the second large-mode-field optical fiber isolator (16) is connected with a large-mode-field optical fiber pulse selector (17);
The signal input port of the second signal pump beam combiner (18) is connected with the output port of the large-mode-field optical fiber pulse selector (17), the input port of the second signal pump beam combiner (18) is connected with the other output port of the second multimode pump beam splitter (20), and the signal output port is connected with one end of the third double-cladding large-mode-field gain optical fiber (19);
The two-stage large-mode-field optical fiber preamplifiers are at least two, and the two-stage large-mode-field optical fiber preamplifiers and the large-mode-field optical fiber femtosecond oscillator share one semiconductor pumping laser;
The other end of the third double-cladding large-mode-field gain optical fiber (19) is connected with a second pump stripper (25) for stripping residual pump light which is not absorbed by the third double-cladding large-mode-field gain optical fiber (19), and the other end of the second pump stripper (25) is connected with an input port of a third large-mode-field optical fiber isolator (26);
The large-mode-field optical fiber circulator (10) is connected with a large-mode-field optical fiber pulse stretcher (11) and a pulse stretching controller (12).
2. The all-fiber femtosecond seed laser based on the large mode field optical fiber according to claim 1, wherein an output end face of the third double-clad large mode field gain optical fiber (19) included in the second large mode field optical fiber preamplifier is a bezel.
3. The all-fiber femtosecond seed laser based on the large-mode-field optical fiber according to claim 1, wherein a plano-convex lens (21), an optical garbage can (22), a pumping beam splitter (23) and a space optical isolator (24) are correspondingly arranged on one side of the third double-cladding large-mode-field gain optical fiber (19).
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