applied sciences

Article High Compact, High Quality Single Longitudinal Mode Hundred Picoseconds Based on Stimulated Brillouin Scattering Pulse Compression

Zhenxu Bai, Yulei Wang *, Zhiwei Lu *, Hang Yuan †, Zhenxing Zheng †, Sensen Li, Yi Chen, Zhaohong Liu, Can Cui, Hongli Wang and Rui Liu

Received: 1 December 2015; Accepted: 15 January 2016; Published: 20 January 2016 Academic Editor: Totaro Imasaka

National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, Harbin 150001, China; [email protected] (Z.B.); [email protected] (H.Y.); [email protected] (Z.Z.); [email protected] (S.L.); [email protected] (Y.C.); [email protected] (Z.L.); [email protected] (C.C.); [email protected] (H.W.); [email protected] (R.L.) * Correspondence: [email protected] (Y.W.); [email protected] (Z.L.); Tel.: +86-0451-8641-2759 (Y.W. & Z.L.) † These authors contributed equally to this work.

Abstract: A high beam quality hundred picoseconds single-longitudinal-mode (SLM) laser is demonstrated based on stimulated Brillouin scattering (SBS) pulse compression and aberration 3+ compensation. Flash-lamp-pumped Q-switched Nd :Y3Al5O12 (Nd:YAG) SLM laser with 4+ 4+ Cr :Y3Al5O12 (Cr :YAG) as a saturable absorber is used as the seed source. By combining master-oscillator-power-amplifier (MOPA), a compact single-cell with FC-770 as working medium is generated as pulse compressor. The 7.8 ns SLM laser is temporally compressed to about 450 ps, and 200 mJ energy is obtained at 1064 nm without optical damage. The energy stability is better than 3% with beam quality factor M2 less than 1.8, which makes this laser system an attractive source for scientific and industrial applications.

Keywords: picosecond laser; stimulated Brillouin scattering (SBS); pulse compression; high energy

1. Introduction Ultrashort with high energy and high power intensity have wide application prospects, such as laser induced plasma (LIP), space debris detection, laser , nonlinear optics, , and laser processing [1–5]. Generally, the so-called ultrashort pulse refers to the lasers with pulse width between nanosecond (~ns) and femtosecond (~fs). In terms of output characteristics, the picosecond laser has shorter pulse width and high peak power compared with nanosecond laser; on the other hand, compared with the femtosecond laser, the picosecond laser also has higher stability and easily achieves high energy. These characteristics make picosecond lasers more attractive for many applications. At present, mode-locking technique is the main method for obtaining picosecond pulses with pulse energies a few nano-joule and repetition rate up to hundreds MHz [6–8]. It is necessary to achieve significant higher pulse energies while keeping the pulse duration amplifiers. In this case, regenerative amplifiers are commonly used to strongly amplify selected individual optical pulses from a mode-locked train emitted by oscillators and allow multi-pass through the medium placed in an optical resonator. Although the amplification gain is about 105 or 106 of the injected energy, the maximum energy obtained after the regenerative amplifier is just in the order of few milli-joules as reported [9–12]. In addition, the structure of the regenerative amplifier is very complicated with poor stability. Such a low single pulse energy is still unable to meet the needs of a lot of applications.

Appl. Sci. 2016, 6, 29; doi:10.3390/app6010029 www.mdpi.com/journal/applsci Appl. Sci. 2016, 6, 29 2 of 8 Appl. Sci. 2016, 6, 29 2 of 8 (SLM) are often required such as optical sensing, laser radar and optical frequency standard. That means that the mode-locked picosecond laser cannot meet the application requirements in those Moreover, in some specific areas, ultra-short pulse lasers with single longitudinal mode (SLM) are fields. often required such as optical sensing, laser radar and optical frequency standard. That means that the The stimulated Brillouin scattering (SBS) phenomenon was discovered decades ago and has mode-locked picosecond laser cannot meet the application requirements in those fields. been widely used in phase stabilization, pulse compression, beam combination and ultrafast pulse The stimulated Brillouin scattering (SBS) phenomenon was discovered decades ago and has shaping [13]. Among them, in the field of laser pulse compression, SBS is an effective method to been widely used in phase stabilization, pulse compression, beam combination and ultrafast pulse compress a long pulse for its high energy reflectivity and high compression ratio. In particular, SLM shaping [13]. Among them, in the field of laser pulse compression, SBS is an effective method to output can be obtained by SBS pulse compression due to the Stokes spectrum is the result of compress a long pulse for its high energy reflectivity and high compression ratio. In particular, convolution of the pump spectrum and the SBS medium gain spectrum. The temporal schematic SLM output can be obtained by SBS pulse compression due to the Stokes spectrum is the result of diagram of SBS pulse compression process is shown in Figure 1. The medium of SBS cell can be liquid, convolution of the pump spectrum and the SBS medium gain spectrum. The temporal schematic gas or solid. By SBS pulse compression, Stokes light with pulse width of few and sub- nanoseconds diagram of SBS pulse compression process is shown in Figure1. The medium of SBS cell can be liquid, can be obtained [14–16]. In order to achieve pulse compression, different types of SBS compressor gas or solid. By SBS pulse compression, Stokes light with pulse width of few and sub- nanoseconds were designed including taped waveguide, one-cell geometry, oscillator amplifier, compact two cell, can be obtained [14–16]. In order to achieve pulse compression, different types of SBS compressor scalable two cell, and so on. Three kinds of the most widely used structures are schematically were designed including taped waveguide, one-cell geometry, oscillator amplifier, compact two cell, depicted in Figure 2. Figure 2a shows the original single-cell pulse compressor with long focal-length scalable two cell, and so on. Three kinds of the most widely used structures are schematically depicted lens. This structure suffers disadvantages of easily occuring optical breakdown and low compression in Figure2. Figure2a shows the original single-cell pulse compressor with long focal-length lens. This ratio that make it unsuitable for practical application [17,18]. Figure 2b is the basic two-cell structure structure suffers disadvantages of easily occuring optical breakdown and low compression ratio that with two separate cells: one relatively short cell as a generator, and the other as an amplifier. A short make it unsuitable for practical application [17,18]. Figure2b is the basic two-cell structure with two focal lens is adopted to obtain higher reflectivity with less sensitivity to injected energy. In order to separate cells: one relatively short cell as a generator, and the other as an amplifier. A short focal lens ensure the amplification effect, length of the SBS cell in Figure 2a,b should be no less than shortest is adopted to obtain higher reflectivity with less sensitivity to injected energy. In order to ensure the theoretical length L: amplification effect, length of the SBS cell in Figure2a,b should be no less than shortest theoretical length L: τ ⋅c L = τ ¨ c (1) L “ (1) 2nn where τ isis the the pump pump pulse pulse duration duration at at full full width width at athalf half maximum maximum (FWHM), (FWHM), c is ctheis light the light speed, speed, n is ntheis refractive the refractive index index of SBS of medium. SBS medium. Figure Figure 2c is the2c compact is the compact single-cell single-cell structure structure involving involving a concave a concavemirror coated mirror for coated high forreflectivity high reflectivity at pump at pumpwavelength wavelength that redirects that redirects the generated the generated Stokes Stokes light lightthrough through the same the same cell cellfor foramplification. amplification. In this In this structure, structure, the the standing-wave standing-wave field field formed formed by by the forward transmissiontransmission pumppump lightlight and and backward backward transmission transmission Stokes Stokes light light can ca enhancen enhance the the stability stability of SBSof SBS generation, generation, then then improve improve the stability the stability of the pulseof the compression; pulse compression; meanwhile, meanwhile, can also effectively can also compresseffectively the compress cell length the [cell19– length21]. [19–21].

Pump

Distance (a)

Stokes (b)

(c) Pump light

Stokes light

(d) Focus Time SBS cell Figure 1. Temporal schematic diagram of simulated Brillouin scattering (SBS) pulse compression process ( a) pump pump light light incident incident to to the the SBS SBS Cell; Cell; (b ()b backward) backward Stokes Stokes light light produced produced near near the thefocus; focus; (c) (pumpc) pump light light energy energy transfer transfer to tothe the leading leading edge edge of of the the Stokes Stokes light; light; ( (dd)) compressed compressed Stokes light obtained with a sharp rising edge.

Although manymany studies studies on on SBS SBS pulse pulse compression compression technology technology have have been carriedbeen carried out, most out, people most havepeople just have paid just attention paid attention to the pulse to the compression pulse compression ratio. Moreover, ratio. Moreover, most of most the light of the sources light sources used in theirused previousin their previous studies arestudies commercial are commercial lasers, and lasers, there and is there no combination is no combination of the laser of the amplifier. laser amplifier. To the bestTo the of ourbest knowledge,of our knowledge, little research little research on the beam on the quality, beam stability quality, andstability applicability and applicability of the SBS of pulse the compressionSBS pulse compression laser has been laser carried has been out. carried out. Appl. Sci. 2016, 6, 29 3 of 8

Appl.Appl. Sci. 2016,, 6,, 2929 33 of of 8 8

f SBS Cell

Pump (a) f SBS Cell

Pump (a) Length 1

Beam λ/4 SBS Cell Length 1 SBS Cell splitter (Amplifier) f (Generator) Beam λ/4 SBS Cell SBS Cell splitter f (b) (Amplifier) (Generator) (b) Length 2

Stokes Length 2 Stokes SBS Cell f SBS Cell f (c) (c) Length 3 Length 3

Figure 2. Three kinds of pulse compression device based on SBS technology (a) single-cell structure; (FigurebFigure) two-cell 2.2. ThreeThree structure; kindskinds ofof(c) pulsepulse compact compressioncompression single-cell device structure. based Beam on SBS splitter: technology polarizing (a) single-cell plate or structure; polarizing (b) two-cell structure; (c) compact single-cell structure. Beam splitter: polarizing plate or polarizing prism;(b) two-cell λ/4: quarter structure; wave (c) plate. compact single-cell structure. Beam splitter: polarizing plate or polarizing prism;prism; λλ/4:/4: quarter wave plate. In this paper, we present a compact high beam quality hundred picoseconds laser system based on SBSInIn thispulsethis paper,paper, compression we we present present and a a compact compactaberration high high compensa beam beam quality qualitytion hundred techniques.hundred picoseconds picoseconds Three-plan laser laser resonant system system based reflector based on on SBS pulse compression and aberration compensation techniques. Three-plan resonant reflector methodSBS pulse is compressionused to generate and aberrationSLM Q-switched compensation output techniques. combined Three-planwith Cr4+:Y resonant3Al5O12 (Cr reflector4+:YAG) method crystal method is used to generate SLM Q-switched output combined with4+ Cr4+:Y3Al5O124+ (Cr4+:YAG) crystal asis usedsaturable to generate absorber. SLM Double-pass Q-switched amplifier output combined is equipped with with Cr :Ya 3Alcompact5O12 (Cr single-cell:YAG) crystal SBS pulse as as saturable absorber. Double-pass amplifier is equipped with a compact single-cell SBS pulse compressorsaturable absorber. in our design. Double-pass Multi-constituent amplifier is equipped medium with FC-770 a compact is used single-cell in the compressor SBS pulse compressor for its high compressor in our design. Multi-constituent medium FC-770 is used in the compressor for its high reflectivityin our design. and Multi-constituent relatively low price. medium SLM FC-770 seed ispu usedlses inof the 9 mJ compressor energy with for its 7.8 high ns reflectivitypulse width and are reflectivity and relatively low price. SLM seed pulses of 9 mJ energy with 7.8 ns pulse width are producedrelatively from low price. the oscillator. SLM seed After pulses passing of 9 mJ through energy the with SBS 7.8 pulse ns pulse compressor, width are 7.8 produced ns pulses from are the then produced from the oscillator. After passing through the SBS pulse compressor, 7.8 ns pulses are then compressedoscillator. After to less passing than through500 ps by the the SBS SBS pulse pulse compressor, compressor. 7.8 ns The pulses output are thenenergy compressed and compressed to less thancompressed 500 ps by to the less SBS than pulse 500 compressor. ps by the SBS The pulse output compressor. energy and compressedThe output pulseenergy width and ascompressed a function pulse width as a function of operating voltages are given. Maximum energy 200 mJ is obtained with ofpulse operating width voltagesas a function are given. of operating Maximum voltages energy are 200 given. mJ is Maximum obtained with energy beam 200 quality mJ is obtained factor M2 withless beam quality factor M2 less than 1.8 and energy fluctuation measured less than 3%. thanbeam 1.8 quality and energy factor fluctuationM2 less than measured 1.8 and energy less than fluctuation 3%. measured less than 3%. 2. Experimental Section 2.2. Experimental Experimental Section Section TheTheThe experimentalexperimentalexperimental setup setupsetup of theof hundredthe hundred picoseconds picosecondspicoseconds laser system laserlaser based systemsystem on SBS basedbased pulse on on compression SBS SBS pulse pulse compressioniscompression shown in Figure isis shownshown3. The inin laserFigureFigure system 3. The is composedlaserlaser systsystemem of aisis SLM composedcomposed oscillator, of of a anaSL SL opticalMM oscillator, oscillator, isolator, an an a optical beam ,expanderisolator, a a beam andbeam a SBSexpanderexpander pulse compressor.andand a SBS pulse compressor.compressor.

PartPart 1: SLM Oscillator PartPart 2: 2: Optical Optical isolator isolator 4+ M1 AD P1 Cr4+:YAG:YAG EE M2 P2 λ/2 FR M1 AD P1 Nd:YAG M2 P2 λ/2 FR M3 Nd:YAG M3

Part 4: SBS pulse compressorcompressor PartPart 3: 3: Beam expander

L3 λ/4/4 PP 3 L2L 2 L1L 1 MM5 5 SBSSBS CellCell 3 Nd:YAGNd:YAG 3

M4M 4

SBS-PCMSBS-PCM DistortionDistortion IncidentIncident wavefront wavefront DistortedDistorted wavefront wavefront SBS-PCM reflected wavefront SBS-PCM reflected wavefront Output OutputOutput wavefront wavefront Output Illustration of SBS-PCM distortion compensation Illustration of SBS-PCM distortion compensation FigureFigureFigure 3. 3. Schematic SchematicSchematic layoutlayout layout ofof ofthe the hundred hundred picosecondspicoseconds picoseconds laser laser laser system. system. system. SLM: SLM: SLM:single single singlelongitudinal longitudinal longitudinal mode; mode; SBS-PCM:mode;SBS-PCM: SBS-PCM: StimulatedStimulated Stimulated BrillouinBrillouin Brillouin scattering scattering phasephase phaseconjugationconjugation conjugation mirror;mirror; mirror; FR: FR: Faraday Faraday FR: Faraday rotator; rotator; rotator; YAG: YAG: Y3Al5O12. YYAG:3Al5O Y123.Al 5O12. Appl. Sci. 2016, 6, 29 4 of 8

Part 1 is the flash-lamp-pumped SLM oscillator based on three-plan resonant reflector. The 3+ 3+ Nd :Y3Al5O12 (Nd:YAG) rod has a length of 120 mm and a diameter of 3 mm with a 1.0% Nd concentration. The oscillator is passively Q-switched by a Cr4+:YAG crystal with initial transmittance 7.5%. Lower transmittance of the saturable absorber can guarantee the Q-switched output with narrower linewidth and pulse width. The laser cavity is a compact plane-plane configuration with ˝ cavity length of 480 mm. M1 is a 0 concave high reflectivity (HR) mirror at 1064 nm. M2 is a flat mirror with reflectivity of 50% at 1064 nm as an (OC). A Fabry–Perot (F-P) etalon E with the thickness of 2 mm is inserted into the cavity to restrict the SLM output through the interaction with the output mirror M2, which is equivalent to a composite etalon. P1 is a Brewster angle polarizer. An aperture diaphragm (AD) is used to filter high-order mode. Part 2 is the optical isolator, consisting a polarizer P2, a 1/2 wave-plate and a Faraday rotator (FR), used to guarantee the stability of the system by preventing backward Stokes returning to the oscillator. Part 3 is the beam expander system with a factor of 2 expands the diameter of the laser beam from oscillator. Part 4 is the SBS pulse compressor consists of a flash-lamp-pumped Nd:YAG amplifier and a SBS cell. 1.0% doped Nd:YAG crystal with a diameter of 7 mm and a length of 140 mm is used as gain medium. FC-770 is chosen as the SBS medium in the SBS cell, which is a multi-constituent perfluorinated compound with molecular formula of C7F15NO. FC-770 has the characteristics of low phonon lifetime (0.57 ns), small Brillouin frequency shift (1081 MHz) and moderate gain coefficient (3.5 cm/GW), which is a suitable medium for SBS pulse compression [22,23]. To reduce the optical breakdown caused by impurities, we filter the FC-770 medium in a vacuum using two layers Millipore filter with pore size 0.05 µm. Stimulated Brillouin scattering phase conjugation mirror (SBS-PCM) and pulse compressor are two primary functions of the SBS cell in our experiment. As a PCM, it can compensate the phase distortion caused by thermal effect and inhomogeneity of the gain crystal [24]. As the illustration of SBS-PCM distortion compensation shows, optical distortion occurred when the seed light (red line) first passes through the laser medium. Due to phase reversal property of the SBS-PCM, after double passing through the optical component, distorted light can be recovered to diffraction-limited (green line) as the original. Meanwhile, the reflected light path from SBS-PCM is strictly same as the incident path, no matter what angle of the SBS cell placed or changed. These remarkable characteristic is helpful for us to realize high beam quality amplified output with high stability. As a pulse compressor, the SBS cell can realize the high efficiency pulse compression of the high energy pulses. The adopted structure of the SBS compressor in our design is a compact single-cell. The length of the SBS cell is 600 mm. To obtain higher reflectance, high compression ratio and sufficient acting length, we insert a convex lens L3 with focal length 1000 mm in front of the SBS cell. M5 is a concave total reflection mirror at 1064 nm with focal length 500 mm. After passing through M5, pump light is focused on the position close to the near incident end of the cell, producing the backward scattering Stokes light. Based on calculation, the beam size at the focus is about 1.6 mm. Through a 1/4 wave-plate, backward Stokes light is reflected from the polarizer P3 with polarization angle rotated from p- to s-polarized.

3. Results and Discussion The SLM oscillator provides 7.8 ns laser pulses with single pulse energy of 9 mJ at 1064 nm wavelength. The repetition rate of the oscillator is adjustable from 1 to 10 Hz. Figure4 shows the SLM seed pulse and compressed Stokes pulse we obtained in our experiment, respectively. The waveform of the oscillator output was given in Figure4a showing a rather smooth shape. The diameters of the oscillator output is about 2.3 mm with angle of divergence less than 0.5 mrad. The temporal profile of the output was detected by a photodetector (UPD-40-UVIR-P; ALPHALAS GmbH, Gottingen, Germany) with rising time less than 40 ps, and recorded by an oscilloscope (806Zi-A; Teledyne LeCory Inc., New York, NY, USA) at a bandwidth of 6 GHz. The compressed pulse profile with steep rising edge and pulse width around 450 ps is shown in Figure4b. Appl. Sci. 2016, 6, 29 5 of 8 Appl. Sci. 2016, 6, x 5 of 8 Appl. Sci. 2016, 6, 29 5 of 8 FigureFigure5 5represents represents the the experimental experimental curve curve of of output output energy energy and and compressed compressed pulse pulse width width with with thethe change changeFigure of 5of represents amplifier amplifier voltage, thevoltage, experimental as as well well as as curve the the energy energy of output and and pulseenergy pulse width width and compressed stability stability (error (error pulse bar). bar). width From From with the the thecurve,curve, change we we can ofcan amplifier see see that that the voltage,the output output as energy wellenergy as is theis near near energy linear linear and growth growth pulse from fromwidth 650 650 stability to to 850 850 V. (error OneV. Onemajor bar). major From reason reason the is curve,thatis that with we with the can increase thesee increasethat of the the outputof injected the energyinjected energy, is energy thenear SBS linear, reflectivitythe growthSBS reflectivity is from obviously 650 isto improved.obviously850 V. One Theimproved. major reflectivity reason The weisreflectivity that obtained with we isthe higher obtainedincrease than isof 75%higher thewhen injected than the 75% energy voltage when, exceedsthe the SBS voltage 650reflectivity V. exceeds Correspondingly, is 650 obviously V. Correspondingly, the improved. output pulse The the widthreflectivityoutput narrows pulse we width downobtained narrows rapidly is higher whiledown than improving rapidly 75% whilewh theen amplifier improvingthe voltage voltage the exceeds amplifier from 650 350 voltageV. to Correspondingly, 650 V.from This 350 is becauseto 650 the V. outputthat,This at is thispulsebecause stage, width that, the narrows leadingat this stage,down edge ofthe rapidly the leading Stokes while edge pulse improving of is the amplified Stokes the amplifier pulse greatly is dueamplified voltage to the from fastgreatly growing350 dueto 650 to gain V.the Thisoffast the isgrowing compressor.because gain that, of However, at the this compressor. stage, from the 700 leading However, V to higher edge from of voltage, the700 Stokes V to no higher obvious pulse voltage, is pulseamplified no width obvious greatly compression pulse due to width the or fastchangescompression growing were gain observed.or changes of the Thecompressor. were mean observed. reason However, forThe the mean from increasing re 700ason V energy tofor higher the contributes increasing voltage, no littleenergy obvious to thecontributes compressionpulse width little processcompressionto the compression is that or the changes Stokes process were pulse isobserved. widththat the is closeTheStokes mean to pu the lsere FC-770ason width for medium isthe close increasing phononto the FC-770energy lifetime. contributesmedium The measured phonon little torootlifetime. the mean compression The square measured error process (RMSE) root is mean that of output squarethe Stokes energy error pu (RMSE) atlse 850 width V of is output lessis close than energy to 3%. the at FC-770 850 V ismedium less than phonon 3%. lifetime. The measured root mean square error (RMSE) of output energy at 850 V is less than 3%.

(a) (b) (a) (b)

7.8 ns 450 ps 7.8 ns 450 ps

Figure 4. Temporal pulse profiles of the (a) SLM oscillator seed output; (b) SBS pulse width Figure 4. TemporalTemporal pulse pulse profiles profiles of of the the ( aa)) SLM oscillator seed output; ( (b)) SBS pulse width compressed output. compressed output.

Figure 5. The output energy and pulse width as a function of amplifier voltage. Figure 5. TheThe output energy and pulse width as a function of amplifier amplifier voltage.

Figure 6 represents the spatial beam characteristics of the output from polarizer P3 in both near- Figure 6 represents the spatial beam characteristics of the output from polarizer P3 in both near- fieldFigure and far-field,6 represents which the were spatial measured beam characteristics by a Charge ofCoupled the output Device from (CCD) polarizer (Camyu P Co.in bothLtd., field and far-field, which were measured by a Charge Coupled Device (CCD) (Camyu 3Co. Ltd., near-fieldChongqing, and China). far-field, As which shown were in Figure measured 6a,b, bythe a measured Charge Coupled beam diameter Device (CCD) of near-field (Camyu is Co.about Ltd., 6.1 Chongqing, China). As shown in Figure 6a,b, the measured beam diameter of near-field is about 6.1 Chongqing,mm in x-direction China). and As 5.9 shown mm inin Figurey-direction.6a,b, theThe measurednear-field beambeam diameterdivergence of angle near-field is less is than about 0.9 mm in x-direction and 5.9 mm in y-direction. The near-field beam divergence angle is less than 0.9 6.1mrad. mm The in x -direction2D and 3D and far-field 5.9 mm laser in y intensity-direction. distribution The near-field profiles beam are divergence given in Figure angle is6c,d, less which than mrad. The 2D and 3D far-field laser intensity distribution profiles are given in Figure 6c,d, which 0.9obtained mrad. Theby a 2Dlong and focal 3D length far-field lens. laser Both intensity the ne distributionar-field and profilesfar-field areshow given good in beam Figure quality.6c,d, which obtained by a long focal length lens. Both the near-field and far-field show good beam quality. obtainedFigure by a7 long shows focal the length measured lens. Bothbeam the radius near-field unde andr output far-field energy show of good 200 beammJ and quality. pulse width Figure 7 shows the measured beam radius under output energy of 200 mJ and pulse width around 450 ps at various distances from the lens with f = 150 mm. The transverse output beam profile around 450 ps at various distances from the lens with f = 150 mm. The transverse output beam profile was measured by using a 90/10 knife-edge technique. The result indicates a beam quality of was measured by using a 90/10 knife-edge technique. The result indicates a beam quality of M 2 1.80 M 2 1.75 2 x= , 2 y= in both directions perpendicular to the axis of propagation. M x 1.80 , M y 1.75 in both directions perpendicular to the axis of propagation. Appl. Sci. 2016, 6, 29 6 of 8

Figure7 shows the measured beam radius under output energy of 200 mJ and pulse width around 450 ps at various distances from the lens with f = 150 mm. The transverse output beam profile was 2 measured by using a 90/10 knife-edge technique. The result indicates a beam quality of Mx “ 1.80, 2 Appl.MAppl.y “ Sci. Sci.1.75 2016 2016 in, ,6 6,both, 29 29 directions perpendicular to the axis of propagation. 66 of of 8 8

(a)(a) (b)(b)

(c)(c) (d)(d)

FigureFigure 6.6. IntensityIntensityIntensity distribution distributiondistribution of ofof SBS SBSSBS pulse pulsepulse width widthwidth compressed compressedcompressed output outputoutput (a) 2D((aa)) distribution2D2D distributiondistribution of near-field; ofof near-near- field;(field;b) 3D ((b distributionb)) 3D3D distributiondistribution of near-field; ofof near-field;near-field; (c) 2D (( distributioncc)) 2D2D distributiondistribution of far-field; ofof far-field;far-field; (d) 3D ((d distributiond)) 3D3D distributiondistribution of far-field. ofof far-field.far-field.

2 2 Figure 7. Measured beam characteristics of M222 with M2 ==1.80 and M2 2 == 1.75 . FigureFigure 7. 7. MeasuredMeasured beam beam characteristics characteristics of of MM withwith MMxxx “1.801.80 and MMy yy“ 1.75.1.75 .

4.4. ConclusionsConclusions Conclusions InIn summary, summary, wewe developeddevelopeddeveloped aa highhighhigh beambeambeam qualityqualitqualityy andand highhigh compactcompact hundredhundred picosecondspicoseconds SLMSLM laserlaser system system based based on on SBS SBS phase phase conjugateconjugate comp compensationcompensationensation andand pulsepulse compressioncompression technologies. technologies. CompactCompact single-cellsingle-cell structurestructure isis adoptedadopted inin ourour experiment.experiment. ByBy buildingbuilding aa standing-wavestanding-wave fieldfield withwith thethe forwardforward transmissiontransmission pumppump lightlight andand backwardbackward transmissiontransmission StokesStokes light,light, highhigh stabilitystability pulsepulse compressioncompression isis obtained.obtained. OwingOwing toto thethe excellentexcellent prpropertiesoperties ofof FC-770,FC-770, thesethese pulsespulses werewere delivereddelivered withwith aa near-diffractionnear-diffraction limited limited spatialspatial intensityintensity profile. profile. Also, Also, differentdifferent outputoutput energyenergy andand compressedcompressed pulsepulse widthwidth werewere comparedcompared experimentally.experimentally. InIn ththee operationoperation ofof thethe wholewhole system,system, maximummaximum energyenergy 220220 mJmJ andand compressedcompressed pulsepulse aroundaround 400400 psps waswas achievedachieved withwith nono breakdownbreakdown ofof anyany opticaloptical devicedevice oror media.media. TheThe overalloverall energyenergy jitterjitter ofof thethe systemsystem isis lessless thanthan 3%.3%. TheThe excellentexcellent energy,energy, stabilitystability andand Appl. Sci. 2016, 6, 29 7 of 8

Compact single-cell structure is adopted in our experiment. By building a standing-wave field with the forward transmission pump light and backward transmission Stokes light, high stability pulse compression is obtained. Owing to the excellent properties of FC-770, these pulses were delivered with a near-diffraction limited spatial intensity profile. Also, different output energy and compressed pulse width were compared experimentally. In the operation of the whole system, maximum energy 220 mJ and compressed pulse around 400 ps was achieved with no breakdown of any optical device or media. The overall energy jitter of the system is less than 3%. The excellent energy, stability and transverse beam profile performances make this system a promising candidate for scientific and industrial applications.

Acknowledgments: This work is supported by the National Natural Science Foundation of China (grant Nos. 61378007, 61138005 and 61378016). Author Contributions: All the authors have made significant contributions to this study and have approved this submission. Zhenxu Bai built the whole system and drafted the main manuscript; Yulei Wang and Zhiwei Lu supervised the research and provided the experimental platform; Hang Yuan and Zhenxing Zheng designed the pulse compressor; Sensen Li, Yi Chen and Zhaohong Liu contributed to the data analyss and results discussion; Can Cui revised the article and responsed the reviewer’s comment; Hongli Wang and Rui Liu recorded the experimental data. Conflicts of Interest: The authors declare no conflict of interest.

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