CN101826696A - High-energy low-repetition-frequency fiber laser - Google Patents
High-energy low-repetition-frequency fiber laser Download PDFInfo
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- CN101826696A CN101826696A CN200910078961A CN200910078961A CN101826696A CN 101826696 A CN101826696 A CN 101826696A CN 200910078961 A CN200910078961 A CN 200910078961A CN 200910078961 A CN200910078961 A CN 200910078961A CN 101826696 A CN101826696 A CN 101826696A
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Abstract
The invention relates to a high-energy low-repetition-frequency fiber laser, which comprises a pump light source used for providing energy for generating laser light, a gain fiber used for providing gains, a mode locking device used for modulating the laser light so as to generate ultrashort pulses, and a single-mode transmission fiber used for laser transmission, wherein the gain fiber, the single-mode transmission fiber and the mode locking device form a resonant cavity of the fiber laser; the generated laser light is oscillated in the resonant cavity; and under the condition that the lengths of the gain fiber and the mode locking device are not increased, the length of the single-mode transmission fiber is increased according to a relation that the length L of the resonant cavity is inversely proportional to the repetition frequency frep of the fiber laser and according to the reduction of the needed repetition frequency frep. By increasing the length of the resonant cavity, the high-energy low-repetition-frequency fiber laser reduces the repetition frequency of pulses in the resonant cavity, correspondingly improves the energy of a single pulse, further simplifies the structure of an amplification system, improves the efficiency of an amplifier and effectively reduces the cost.
Description
Technical field
The present invention relates to mode locked fiber laser and amplifier region, be specifically related to a kind of fiber laser of high-energy low-repetition-frequency.
Background technology
Mode locked fiber laser is meant and utilizes mode-locking technique to produce the experimental provision of femtosecond and psec ultrashort pulse in fiber laser.Mode-locking technique is a kind of method that laser beam is modulated, and makes oscillation longitudinal modes different in the light beam have definite phase relation, thereby makes each mode-coherent stack obtain ultrashort pulse.In general, ultrashort pulse is meant femtosecond and picopulse, wherein 1 femtosecond=1 * 10
-15Second, 1 psec=1 * 10
-12Second.General mode locked fiber laser utilizes semiconductor laser as pump light source, the special optical fiber that utilizes doped with rare-earth elements is as gain fibre, utilize monomode fiber as base components in the laser chamber, utilize the characteristic of saturable absorber to produce femtosecond or picopulse again.For this kind mode locked fiber laser, the repetition rate of the ultra-short pulse laser of generation is generally 20MHz-100MHz.In experiment and using, for obtaining lower repetition rate, the ultrashort pulse that the modelocked fiber laser oscillator is produced is reduced to low-repetition-frequency 1kHz-200kHz by pulse selector (being specially electrooptic modulator or acousto-optic modulator) with high repetition frequency 20MHz-100MHz, and the ultra-short pulse laser of this kind low-repetition-frequency is coupled in fiber amplifier or the Solid State Laser amplifier, after one or more levels amplification, obtain the high energy pulse that pulse energy is 1 μ J-10mJ.
Above-mentioned high energy pulse has very important application in fields such as industrial processes, medical treatment.Characteristics such as the mode locked fiber laser of doped with rare-earth elements ytterbium is high owing to its gain, the pulse of support Broadband Femtosecond come into one's own.Ultra-short pulse laser is applied in microfabrication and the medical treatment, requires that its pulse energy is stabilized in that 10 μ J are above, pulse duration＜500fs, repetition rate be 100kHz-1MHz.And the pulse menu mode of traditional employing electrooptic modulator is difficult to make repetition rate to be reduced to greater than 200kHz, generally 1kHz-100kHz scope (representative value is 10kHz, and the electrooptic modulator of 200kHz is very expensive).And shortcoming clearly: bulky, energy consumption causes thermal effect, can't use air cooling, operating voltage too high (about 4000V) etc.If adopt acousto-optic modulator as the pulse menu mode, efficient is lower, always has the loss more than 20%.
The technical papers title that is published in 2005 is " High energy femtosecond Yb cubicon fiber amplifier, " (L.Shah, Z.Liu, I.Hartl, G.Imeshev, G.C.Cho, and M.E.Fermann, Opt.Lett.30,2754 (2005)).This paper is mentioned, and by 3 grades of amplifications, pulse energy is brought up to 200 μ J (repetition rate is 100kHz).The repetition rate of pulse is by acousto-optic modulator the 45MHz of oscillator to be reduced to 100kHz.The pulsewidth of amplifying after compressing is 500fs.
The technical papers title that is published in 2007 is: " 90W average power 100 μ J energy femtosecond fiber chirped-pulse amplification system, " (F.
D.Schimpf, O.Schmidt, B.
K.Rademaker, J.Limpert, and A.T ü nnermann, Opt.Lett.32,2230-2232 (2007)).Pulse energy after this paper will amplify is brought up to 100 μ J.Remain laser oscillator pulse (repetition rate is 18MHz) is reduced to 900kHz by acousto-optic modulator, and amplification compression back pulsewidth is 500fs.
The technical papers title that is published in 2007 is " Scaling of femtosecond Yb-doped fiber amplifiers to tens of microjoule pulse energy via nonlinear chirped pulse amplification " (L.Kuznetsova and F.W.Wise, Opt.Lett.32,2671-2673 (2007)), this paper is introduced acousto-optic modulator in the Yb optical fiber laser amplifier system.The repetition rate of fibre laser oscillator is from 60MHz (pulse energy 8nJ), and behind acousto-optic modulator and laser amplifier, the repetition rate of pulse reduces to 3MHz-0.15MHz, amplifies pulse energy accordingly and is 1.5 μ J-30 μ J to the maximum.The advantage of this system is that pulsewidth is 240fs, narrow than above-mentioned two bibliographical informations.
Existing high-energy 3MHz-0.15MHz pulse amplifying technique all adopts electric light or acoustooptic modulation technology, and electric light or acoustooptic modulation system cost an arm and a leg, and is unfavorable for promoting.
Existing electric light or the acoustooptic modulation method utilized selected monopulse technology, very low (the reduction multiplying power of apparent weight complex frequency and deciding of selected pulse energy, pulse energy after the menu is the 1/R (wherein R is the reduction multiplying power of repetition rate) of initial pulse energy, generally in tens pJ magnitudes), the pulse energy serious waste needs some grades of preamplifiers just can reach the pulse (generally be to two-stage) of tens nJ magnitudes.
Existing electric light or acoustooptic modulation method select the modulation contrast of monopulse technology lower, in the menu pulse often with faint little original burst signal, and along with amplification variation.
In fiber laser and solid state laser, utilize mode-locking technique, can obtain ultrashort pulse.Specifically in fiber laser, realize that the method for locked mode mainly contains: nonlinear polarization rotary process, saturable absorber method (wherein saturable absorber is meant semiconductor saturable absorbing mirror, carbon nano-tube saturable absorber) etc.The nonlinear polarization rotary process can be regarded a kind of artificial saturable absorber method as, saturable absorber is placed in the laser chamber, absorption to endovenous laser changes with distribution of light intensity, it is very strong to light absorption when light field is more weak, when light field is strong, light absorption is reduced, when reaching a particular value, make the laser pulse of intensity maximum stand minimum loss, thereby obtain strong mode locking pulse.
For Rear Earth Doped Fiber Laser, commonly used has er-doped and mixes two kinds of fiber lasers of ytterbium.Wherein for erbium doped fiber laser, operation wavelength is near the 1550nm, common single mode optical fibres (as the SMF28 optical fiber of Corning Incorporated), and dispersion values is for negative.Because the output wavelength of this kind fiber laser is operated in communication band, therefore important application is arranged in optical communication and obtain extensive concern.Accordingly, at this wave band, also there are dispersion compensating fiber and dispersion shifted optical fiber etc.So the non-general single mode fiber of this kind also has zero chromatic dispersion and positive dispersion type, so can the chromatic dispersion of whole laser be managed and compensate by the combination of these optical fiber.
For ytterbium-doping optical fiber laser, operation wavelength is near the 1030nm, and the general single mode fiber chromatic dispersion is just (as the xp-1060 optical fiber of Nufern company), is different from erbium doped fiber laser, near be difficult to find a job monomode fiber with negative dispersion 1030nm is outside special photonic crystal fiber.But there is very big shortcoming in this kind photonic crystal fiber: price is very expensive and by offshore company's monopolization (such as the Crystal Fiber company of Denmark), and welding is difficulty (common heat sealing machine can't weld, and needs former factory to weld) etc. very.Therefore if positive dispersion in the chamber is compensated, in laser, can only place such as body grating, the large-scale dispersion compensation device of prism equity.The very big like this simplification that reduces fiber laser.
In ytterbium-doping optical fiber laser, if there is not grating equity negative dispersion device to compensate, this laser is called mixes ytterbium Totally positive dispersion fiber laser.Locked mode method commonly used has two kinds, and the one, utilize the nonlinear polarization pivoting mechanism in conjunction with the spectral filtering effect.(repetition rate is more than the MHz) generally speaking, output pulse width arrives a few ps levels, the shortest reaching about 200fs after the compression at hundreds of fs.Another is to utilize saturable absorber (as semiconductor saturable absorbing mirror), and similarly under the condition, output pulse width is near 1ps, and compression afterpulse width also is about 200fs.
For the fibre laser oscillator of routine, utilize the method for nonlinear polarization rotation to obtain mode locking pulse, repetition rate is 20MHz-100MHz.In the up-to-date document of delivering, utilize the fiber laser of this kind technology locked mode also only to be reduced to the 1MHz magnitude, corresponding laser oscillator chamber inner fiber length is 200 meters (ring cavity structures).In other words, if the chamber long (promptly increasing the length of monomode fiber in the chamber) of lengthening laser, the nonlinear polarization rotation technique does not utilize the locked mode of saturable absorption method effective.Thereby obtain the low-repetition-frequency pulse in order to overcome this difficult point, method in common is above-mentioned by the agency of now: obtain the pulse of 20MHz-100MHz repetition rate in laser oscillator, put into pulse selector outside the chamber or in the laser amplifier and make pulse repetition frequency be reduced to the 1kHz-200kHz magnitude.
Summary of the invention
Main purpose of the present invention is to provide a kind of fiber laser of high-energy low-repetition-frequency, and the pulse seed source of the low-repetition-frequency of suitable amplification and application is provided, and amplifies efficient and simplifies amplifier system to improve.
For achieving the above object, the technical solution used in the present invention is as follows:
A kind of fiber laser of high-energy low-repetition-frequency, comprise and be used to provide the pump light source, the gain fibre that is used to provide gain that produce laser institute energy requirement, be used for thereby laser is modulated clamping apparatus that produces ultrashort pulse and the single-mode transmission optical fiber that is used for laser transmission, wherein constitute the resonant cavity of fiber laser by gain fibre, single-mode transmission optical fiber and clamping apparatus, the laser that is produced is at internal oscillation, under the situation that gain fibre, clamping apparatus length all do not increase, the length of single-mode transmission optical fiber is according to long L of resonant cavity and repetition rate f
RepThe relation that is inversely proportional to is according to required repetition rate f
RepReduction and increase.
Preferably, described resonant cavity is an annular chamber, and the length of described single-mode transmission optical fiber increases by following formula:
L
0＝c/nf
rep-L
1
Wherein, L
0Be single-mode transmission optical fiber length, c is a vacuum light speed, and n is the refractive index of single-mode transmission optical fiber, L
1Be length in the chamber of gain fibre and clamping apparatus.
Preferably, described resonant cavity is a linear cavity, and the length of described single-mode transmission optical fiber increases by following formula:
L
0＝c/2nf
rep-L
1
Wherein, L
0Be single-mode transmission optical fiber length, c is a vacuum light speed, and n is the refractive index of single-mode transmission optical fiber, L
1Be length in the chamber of gain fibre and clamping apparatus.
Preferably, described clamping apparatus is a saturable absorber.
Preferably, described saturable absorber is semiconductor saturable absorbing mirror or carbon nano-tube saturable absorber.
Preferably, gain fibre is the gain fibre of doped with rare-earth elements erbium, ytterbium, holmium or thulium.
Preferably, described clamping apparatus is the nonlinear polarization Rotation Controllers, is provided with spectral filter in the described fiber laser.
Preferably, described nonlinear polarization Rotation Controllers is half-wave lengthy motion picture, quarter-wave lengthy motion picture group or the optical fiber type Polarization Controller that is used for Polarization Control.
Preferably, described spectral filter is birefringent filter, interferometric filter or the optical fiber type spectral filter that is used for spectral filtering.
Preferably, gain fibre is the gain fibre of doped with rare-earth elements erbium, ytterbium, holmium or thulium.
Preferably, be provided with optical fiber type isolator or split type spatial separation device in the described resonant cavity, be used to make laser one-way transmission in described resonant cavity of generation to form vibration.
Preferably, described fiber laser uses as the seed source of fiber amplifier.
Preferably, after described fiber laser or fiber amplifier, be connected with reflective gratings, be used for the laser pulse after amplifying is compressed, transmission-type grating grating compression set to, body grating or optical fiber type grating.
Utilize the fiber laser of high-energy low-repetition-frequency provided by the invention, have following beneficial effect:
1) long by improving the chamber, reduced the repetition rate of pulse in the chamber, the corresponding single pulse energy that improved;
2) owing to improved single pulse energy, increase self phase modulation in the fiber laser cavity, helped the broadening of pulse spectrum;
3) before amplification, do not need to continue to select individual pulse, therefore simplified system configuration greatly, effectively reduce cost with electric light or acousto-optic modulator;
4) owing to improved single pulse energy, be equivalent to increase one-level or two-stage optical fiber preamplifier, further simplified the structure of amplification system, improved efficiency of amplitude, effectively reduce cost.
Description of drawings
Fig. 1 is the optical fiber laser structure figure of high-energy low-repetition-frequency in the embodiment of the invention 1;
Fig. 2 obtains spectrogram in the embodiment of the invention 1;
The mode locking pulse sequence of Fig. 3 for obtaining in the embodiment of the invention 1;
Fig. 4 is the optical fiber laser structure figure of high-energy low-repetition-frequency in the embodiment of the invention 2;
Fig. 5 is the optical fiber laser structure figure of high-energy low-repetition-frequency in the embodiment of the invention 3;
Fig. 6 is the optical fiber laser structure figure of high-energy low-repetition-frequency in the embodiment of the invention 4;
Fig. 7 is the optical fiber laser structure figure of high-energy low-repetition-frequency in the embodiment of the invention 5.
Among the figure: 1, pump light source; 2, pump light Transmission Fibers; 3, wavelength division multiplexer; 4, single-mode transmission optical fiber; 5, gain fibre; 6, optical collimator; 7,1/4 wavelength sheet; 8,1/2 wavelength sheet; 9, split type spatial separation device; 10, polarization splitting prism; 11, space formula Faraday rotator; 12, condenser lens; 13, saturable absorber; 14, the single-mode transmission optical fiber of Zeng Jiaing; 15, laser output mouth; 16, optical fiber type one-way isolator; 17, optical circulator; 18, optical fiber type Polarization Controller; 19, optical fiber power beam splitter; 20, optical fiber type speculum; 21, optical fiber type faraday rotation mirror.
Embodiment
The fiber laser of the high-energy low-repetition-frequency that the present invention proposes is described as follows in conjunction with the accompanying drawings and embodiments.
The fiber laser of the high-energy low-repetition-frequency that the present invention proposes, with prior art identical be that it comprises: be used to provide the pump light source that produces laser institute energy requirement; Be used to provide the gain fibre of gain; Be used for thereby laser is modulated the clamping apparatus that produces ultrashort pulse; The single-mode transmission optical fiber that is used for laser transmission, wherein constitute the resonant cavity of fiber laser by gain fibre, single-mode transmission optical fiber and clamping apparatus, the laser that is produced vibrates in resonant cavity, unlike the prior art be, under the situation that gain fibre, clamping apparatus length all do not increase, the length of single-mode transmission optical fiber is according to long L of the resonant cavity of fiber laser and repetition rate f
RepThe relation that is inversely proportional to is according to required repetition rate f
RepReduction and increase.
Experimental principle of the present invention is described below:
After the pump light that is produced by pump light source was injected into the resonant cavity of fiber laser, laser began to carry out light generation in resonant cavity.Because gain medium is the broadband,, the transient pulse of a maximum is always arranged in the noise spectrum owing to the bat between the pattern.The time interval between the pulse is inversely proportional to the long L of resonant cavity of fiber laser, repetition rate f
RepInverse for pulse interval.In the fiber laser of annular cavate, repetition rate f
RepFor:
f
rep＝1/ΔT＝c/nL. (1)
In the fiber laser of linear cavate, repetition rate f
RepFor:
f
rep＝1/ΔT＝c/2nL
Wherein Δ T is a pulse interval, and c is a vacuum light speed, and L is that the resonant cavity of fiber laser is long, is specially length L in the chamber of gain fibre and clamping apparatus
1With the single-mode transmission optical fiber length L
0Sum, n is the refractive index of single-mode transmission optical fiber.
As seen, as long as increase the long L of resonant cavity, the repetition rate of pulse will reduce.The annular chamber of common fiber laser is about and is 2m-10m, and corresponding pulse repetition frequency is 100MHz～20MHz.If pulse repetition frequency is reduced to 1MHz～0.150MHz, chamber length need be increased to 200m～1300m.
The present invention reduces repetition rate by the length that increases single-mode transmission optical fiber.The present invention proposes the fiber laser of high-energy low-repetition-frequency, and resonant cavity both can adopt annular chamber also can adopt linear cavity, and when resonant cavity adopted annular chamber, the length of described single-mode transmission optical fiber increased by following formula:
L
0＝c/nf
rep-L
1
When resonant cavity adopted linear cavity, the length of single-mode transmission optical fiber increased by following formula:
L
0＝c/2nf
rep-L
1
Wherein, L
0Be single-mode transmission optical fiber length, c is a vacuum light speed, and n is the refractive index of single-mode transmission optical fiber, L
1Be length in the chamber of gain fibre and clamping apparatus.
If the length of single-mode transmission optical fiber in the fiber laser cavity is brought up to 200m～4km, then can directly pulse repetition frequency be reduced to 50kHz-1MHz (referring in the annular chamber that if be applied in the linear cavity, repetition rate is half of annular chamber).And theoretical epicoele inner fiber can be longer, and corresponding repetition rate can be lower, to the 10kHz magnitude.In so long fiber laser,, be difficult to the self-starting locked mode if do not carry out dispersion compensation.The present invention preferably utilizes saturable absorber as clamping apparatus, and saturable absorber is semiconductor saturable absorbing mirror or carbon nano-tube saturable absorber.The semiconductor saturable absorbing mirror of the high modulation degree of depth as the self-starting device, can not rely on the nonlinear polarization pivoting mechanism and the independent locked mode of realizing in fiber laser.In placing the fiber laser lumen of semiconductor saturable absorbing mirror, the weak pulse that semiconductor saturable absorbing mirror absorbs in the laser cavity dashes, and hard pulse in the chamber is absorbed seldom, makes to have only a pulse to obtain bigger gain in the chamber and grow up and finally stable.The resonant cavity internal dispersion of this pulse shaping mechanism and laser is irrelevant, still is that the dispersion management type all is suitable for to the Totally positive dispersion type.Therefore, if adopt saturable absorber as clamping apparatus, its gain fibre can be any in the gain fibre of doped with rare-earth elements erbium, ytterbium, holmium, thulium.
The technology that directly produces the high-energy low-repetition-frequency pulse in fiber laser disclosed in this invention is different from above-mentioned disclosed electric light and the acousto-optic modulator of utilizing and reduces existing methods such as repetition rate, and main difference is as follows:
The present invention can directly produce the seed pulse of 0.15MHz-3MHz repetition rate by the length of single-mode transmission optical fiber in the control resonant cavity, has saved expensive electric light or acoustooptic modulation system;
When reducing pulse repetition frequency, average power is but almost constant, thus can directly produce pulse up to the 100nJ magnitude, and do not need preposition amplification;
Contrast utilizes the not high problem of modulation ratio that occurs in electric light or the acousto-optic modulator, and the present invention directly produces the spike train of high-contrast, and pulse is clean, and no leakage pulse is exaggerated.
Fiber laser to annular cavate, be illustrated in figure 1 as a kind of preferred implementation of the fiber laser of high-energy low-repetition-frequency of the present invention, in this fiber laser: be used to provide the pump light source 1 that produces laser institute energy requirement to insert wavelength division multiplexer 3 by pump light Transmission Fibers 2, wavelength division multiplexer 3 is as the resonant cavity of pumping coupler with pump light source 1 incoming fiber optic laser, and providing can source inlet.Be connected to form the resonant cavity of annular by single-mode transmission optical fiber 4, gain fibre 5 and clamping apparatus.Clamping apparatus adopts saturable absorber 13 in the present embodiment, is connected with condenser lens 12 before the saturable absorber 13, and the laser focusing that is used for producing is at saturable absorber 13.Because difference with the prior art of the present invention has been to increase the length of single-mode transmission optical fiber 4, therefore be different from prior art with the single-mode transmission optical fiber 14 that increases herein.Preferably, the fiber end face of single-mode transmission optical fiber can be connected with optical collimator 6 in this fiber laser, resonant cavity partly can also connect 8 groups of 1/4 wavelength sheet 7 and 1/2 wavelength sheets, is used to adjust the polarization direction of the laser pulse that is produced, and the coupling of control laser.Be connected with in resonant cavity under the situation of saturable absorber 13,8 groups of this 1/4 wavelength sheet 7 and 1/2 wavelength sheets are not necessarily.Also be connected with split type one-way isolator 9 in the resonant cavity, form by 4 elements, be respectively polarization splitting prism 10, the 1/2 wavelength sheet 8, space formula Faraday rotator 11 and the polarization spectroscope 10 that connect successively, one-way isolator 9 is used to make laser one-way transmission in resonant cavity of generation to form vibration.With the laser output mouth 15 of one of them polarization splitting prism 10 as fiber laser, with the ultrashort pulse output that produces in the resonant cavity, another polarization splitting prism 10 connects the two ends and the saturable absorber 13 of single-mode transmission optical fiber 4, forms T shape structure.Gain fibre in this device can be any in the gain fibre of doped with rare-earth elements erbium, ytterbium, holmium, thulium.Condenser lens 12 adopts ordinary lens or non-spherical lens.Saturable absorber 13 adopts semiconductor saturable absorbing mirror or carbon nano-tube saturable absorber etc.Above-mentioned single-mode transmission optical fiber 14, for erbium doped fiber laser (being that gain fibre is an erbium-doped fiber), can be that operation wavelength is single-mode transmission optical fiber, dispersion shifted optical fiber of 1.5 μ m etc., for ytterbium-doping optical fiber laser (being that gain fibre is a Yb dosed optical fiber), can be that operation wavelength is 1 μ m single-mode transmission optical fiber, also can be that operation wavelength is 1.5 μ m single-mode transmission optical fiber.Split type one-way isolator 9 can be arranged on the arbitrary end in wavelength division multiplexer 3 two ends.
Utilize the course of work of above-mentioned fiber laser as follows: after opening pumping source 1, it to be adjusted to certain power, to form vibration in the resonant cavity of fiber laser.Increase the luminous power of pumping source 1, after the locked mode threshold value that reaches fiber laser, can obtain mode locking pulse output.At this moment, detect output spectrum, can see locked mode spectrum on spectrometer, typical spectral shape is (ytterbium-doping optical fiber laser utilizes the semiconductor saturable absorbing mirror locked mode) as shown in Figure 2.Simultaneously, survey diode with high speed optoelectronic and survey output pulse signal, can see the mode locking pulse sequence on oscilloscope, typical mode locking pulse sequence is (pulse repetition frequency 381.3kHz this moment) as shown in Figure 3, above-mentioned locked mode modulated process is a prior art, no longer describes in detail here.
Fiber laser for annular cavate, be illustrated in figure 4 as the another kind of preferred implementation of the fiber laser of high-energy low-repetition-frequency of the present invention, most of device and annexation thereof are identical with embodiment 1 in the present embodiment, different is that present embodiment adopts optical fiber type one-way isolator 16 to replace split type unidirectional spatial separation device 9, one-way isolator 16 is used to make laser one-way transmission in described resonant cavity of generation to form vibration equally, can be placed on position shown in Figure 4, also can be placed on the other end of wavelength division multiplexer 3 arbitrarily, shown in Fig. 4 dotted line.Still be connected with two polarization splitting prisms 10 in the resonant cavity, one of them polarization splitting prism 10 is as laser output mouth 15 output ultra-short pulse lasers; Another polarization splitting prism 10 connects the two ends and the saturable absorber 13 of single-mode transmission optical fiber 4, forms T shape structure.
Fiber laser for annular cavate, be illustrated in figure 5 as another preferred implementation of the fiber laser of high-energy low-repetition-frequency of the present invention, in this fiber laser: be used to provide the pump light source 1 that produces laser institute energy requirement to insert wavelength division multiplexer 3 by pump light Transmission Fibers 2, wavelength division multiplexer 3 is as the resonant cavity of pumping coupler with pump light source 1 incoming fiber optic laser, and providing can source inlet.Be connected to form the resonant cavity of annular by single-mode transmission optical fiber 4, gain fibre 5 and clamping apparatus.Clamping apparatus adopts saturable absorber 13 in the present embodiment, the polarization splitting prism 10 that utilizes optical circulator 17 to replace among the embodiment 2 in the present embodiment, and optical circulator 17 connects the two ends and the saturable absorber 13 of single-mode transmission optical fiber 4, forms T shape structure.The single-mode transmission optical fiber 14 that comprises increase in the same present embodiment.Do not adopt 1/4 wavelength sheet 7 and 8 groups of polarization directions of adjusting laser pulse of 1/2 wavelength sheet in the present embodiment, but adopt optical fiber type Polarization Controller 18 to adjust the polarization direction of laser pulse.The difference of present embodiment and embodiment 1,2 also is to utilize the laser output mouth 15 of optical fiber power beam splitter 19 as fiber laser.Identical with embodiment 2, be connected with one-way isolator 16 in the resonant cavity, one-way isolator 16 makes laser one-way transmission in described resonant cavity of generation form vibration, can be placed on position shown in Figure 5, also can be placed on the other end of wavelength division multiplexer 3 arbitrarily, shown in Fig. 5 dotted line.Gain fibre in this device can be any in the gain fibre of doped with rare-earth elements erbium, ytterbium, holmium, thulium.Saturable absorber 13 adopts semiconductor saturable absorbing mirror or carbon nano-tube saturable absorbing mirror etc.
Core concept of the present invention is the length that adds long optical fibers in the resonant cavity of fiber laser, utilizes mode-locking technique to obtain stable high-energy low-repetition-frequency pulse.Based on this core concept, utilize the nonlinear polarization rotation technique to replace utilizing saturable absorber also can obtain mode locking pulse output separately.Above-mentionedly point out, resonant cavity for the fiber laser of length like this, utilize the nonlinear polarization rotation technique not utilize the easier acquisition mode locking pulse of saturable absorber separately, but by meticulous adjusting wave plate group, still can obtain the stable mode-locking pulse.Fiber laser for annular cavate, as shown in Figure 6, in this fiber laser: be used to provide the pump light source 1 that produces laser institute energy requirement to insert wavelength division multiplexer 3 by pump light Transmission Fibers 2, wavelength division multiplexer 3 is as the resonant cavity of pumping coupler with pump light source 1 incoming fiber optic laser, and providing can source inlet.Be connected to form the resonant cavity of annular by single-mode transmission optical fiber 4, gain fibre 5 and clamping apparatus.Same present embodiment comprises the single-mode transmission optical fiber 4 of increase.The fiber end face of single-mode transmission optical fiber 4 can be connected with collimater 6 in this fiber laser, and resonant cavity partly is connected with 8 groups of 1/4 wavelength sheet 7 and 1/2 wavelength sheets, is used to adjust the polarization direction of laser pulse, and the coupling of control laser.Resonant cavity partly is connected with polarization splitting prism 10, and polarization splitting prism 10 is as the 15 output ultrashort pulses of laser output mouth.Gain fibre can be any in the gain fibre of doped with rare-earth elements erbium, ytterbium, holmium, thulium.
For fiber laser to linear cavate, as shown in Figure 7, in this fiber laser of present embodiment: be used to provide the pump light source 1 that produces laser institute energy requirement to insert wavelength division multiplexer 3 by pump light Transmission Fibers 2, wavelength division multiplexer 3 is as the resonant cavity of pumping coupler with pump light source 1 incoming fiber optic laser, and providing can source inlet.Be connected to form linear resonant cavity by single-mode transmission optical fiber 4, gain fibre 5 and clamping apparatus.The single-mode transmission optical fiber 14 that comprises increase in the same present embodiment.One end of linear resonant cavity is a saturable absorber 13, and the other end is optical fiber type speculum 20 or optical fiber type faraday rotation mirror 21, guarantees the laser vibration back and forth in linear cavity that is produced.Utilize the laser output mouth 15 of optical fiber power beam splitter 19 as fiber laser.
The fiber laser of high-energy low-repetition-frequency provided by the present invention, the seed source that can be used as fiber amplifier uses.After described fiber laser or fiber amplifier, be connected with reflective gratings to, transmission-type grating grating compression set, be used for the laser pulse after amplifying is compressed, body grating or optical fiber type grating.
Can sum up by above embodiment, one of key of the present invention is the selection of optical fiber kind and fiber lengths.For ytterbium-doping optical fiber laser, operation wavelength is 1 mu m waveband.And 1 μ m gain fibre price is relatively costly, and the optical fiber price domestic consumer of the above length of 1km relatively is difficult to bear.The present invention proposes, and in ytterbium-doping optical fiber laser, removes and selects for use outside the 1 μ m gain fibre, uses common communication instead and still can reach the locked mode requirement with single-mode transmission optical fiber, the very big like this cost of having saved.For erbium doped fiber laser, operation wavelength is selected the positive dispersion single-mode transmission optical fiber than selecting the negative dispersion single-mode transmission optical fiber to produce and is had more high-energy and nondividing pulse at 1.5 mu m wavebands.
Two of key of the present invention is a pulse enable mechanism.The present invention relies on saturable absorber (semiconductor saturable absorbing mirror and the carbon nano-tube saturable absorber etc. of the high modulation degree of depth, modulation depth is greater than 20%, preferred adopt 30%, make fiber laser relatively easy, and can guarantee the self-starting of locked mode and recover certainly) start locked mode.
Above execution mode only is used to illustrate the present invention; and be not limitation of the present invention; the those of ordinary skill in relevant technologies field; under the situation that does not break away from the spirit and scope of the present invention; can also make various variations and modification; therefore all technical schemes that are equal to also belong to category of the present invention, and scope of patent protection of the present invention should be defined by the claims.
Claims (13)
1. the fiber laser of a high-energy low-repetition-frequency, comprise and be used to provide the pump light source that produces laser institute energy requirement, be used to provide the gain fibre of gain, be used for thereby laser is modulated clamping apparatus that produces ultrashort pulse and the single-mode transmission optical fiber that is used for laser transmission, wherein by gain fibre, single-mode transmission optical fiber and clamping apparatus constitute the resonant cavity of fiber laser, the laser that is produced is at internal oscillation, it is characterized in that: at gain fibre, under the situation that clamping apparatus length does not all increase, the length of single-mode transmission optical fiber is according to long L of resonant cavity and repetition rate f
RepThe relation that is inversely proportional to is according to required repetition rate f
RepReduction and increase.
2. the fiber laser of high-energy low-repetition-frequency according to claim 1 is characterized in that, described resonant cavity is an annular chamber, and the length of single-mode transmission optical fiber increases by following formula:
L
0＝c/nf
rep-L
1
Wherein, L
0Be single-mode transmission optical fiber length, c is a vacuum light speed, and n is the refractive index of single-mode transmission optical fiber, L
1Be length in the chamber of gain fibre and clamping apparatus.
3. the fiber laser of high-energy low-repetition-frequency according to claim 1 is characterized in that, described resonant cavity is a linear cavity, and the length of single-mode transmission optical fiber increases by following formula:
L
0＝c/2nf
rep-L
1
Wherein, L
0Be single-mode transmission optical fiber length, c is a vacuum light speed, and n is the refractive index of single-mode transmission optical fiber, L
1Be length in the chamber of gain fibre and clamping apparatus.
4. according to the fiber laser of claim 2 or 3 described high-energy low-repetition-frequencies, it is characterized in that described clamping apparatus is a saturable absorber.
5. the fiber laser of high-energy low-repetition-frequency according to claim 4 is characterized in that, described saturable absorber is semiconductor saturable absorbing mirror or carbon nano-tube saturable absorber.
6. the fiber laser of high-energy low-repetition-frequency according to claim 4 is characterized in that, described gain fibre is the gain fibre of doped with rare-earth elements erbium, ytterbium, holmium or thulium.
7. according to the fiber laser of claim 2 or 3 described high-energy low-repetition-frequencies, it is characterized in that described clamping apparatus is the nonlinear polarization Rotation Controllers, is provided with spectral filter in the described fiber laser.
8. the fiber laser of high-energy low-repetition-frequency according to claim 7 is characterized in that, described nonlinear polarization Rotation Controllers is half-wave lengthy motion picture, quarter-wave lengthy motion picture group or the optical fiber type Polarization Controller that is used for Polarization Control.
9. the fiber laser of high-energy low-repetition-frequency according to claim 7 is characterized in that, described spectral filter is birefringent filter, interferometric filter or the optical fiber type spectral filter that is used for spectral filtering.
10. the fiber laser of high-energy low-repetition-frequency according to claim 7 is characterized in that, described gain fibre is the gain fibre of doped with rare-earth elements erbium, ytterbium, holmium or thulium.
11. the fiber laser of high-energy low-repetition-frequency according to claim 2 is characterized in that, is provided with optical fiber type isolator or split type spatial separation device in the described resonant cavity, is used to make laser one-way transmission in described resonant cavity of generation to form vibration.
12. the fiber laser of high-energy low-repetition-frequency according to claim 1 is characterized in that, described fiber laser uses as the seed source of fiber amplifier.
13. the fiber laser of high-energy low-repetition-frequency according to claim 12, it is characterized in that, after described fiber laser or fiber amplifier, be connected with reflective gratings to, transmission-type grating grating compression set, be used for the laser pulse after amplifying is compressed, body grating or optical fiber type grating.
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