Divided-pulse nonlinear amplification and simultaneous compression

Divided-pulse nonlinear amplification and simultaneous compression Qiang Hao, Qingshan Zhang, Tingting Sun, Jie Chen, Zhanhua Guo, Yuqing Wang, Zhengru Guo, Kangwen Yang, and Heping Zeng Citation: Applied Physics Letters 106, 101103 (2015); doi: 10.1063/1.4914882 View online: http://dx.doi.org/10.1063/1.4914882 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/106/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in 120 fs, 4.2 nJ pulses from an all-normal-dispersion, polarization-maintaining, fiber laser Appl. Phys. Lett. 103, 121111 (2013); 10.1063/1.4821776 Waveform shaping of stretched-pulse fiber laser output with a hollow photonic-crystal fiber Appl. Phys. Lett. 102, 171113 (2013); 10.1063/1.4801934 Simultaneous second-harmonic generation, third-harmonic generation, and four-wave mixing microscopy with single sub-8 fs laser pulses Appl. Phys. Lett. 99, 181124 (2011); 10.1063/1.3658456 Compression of 1.8 μ m laser pulses to sub two optical cycles with bulk material Appl. Phys. Lett. 96, 121109 (2010); 10.1063/1.3359458 Efficient optical pulse compression using chalcogenide single-mode fibers Appl. Phys. Lett. 88, 081116 (2006); 10.1063/1.2178772

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APPLIED PHYSICS LETTERS 106, 101103 (2015)

Divided-pulse nonlinear amplification and simultaneous compression Qiang Hao,1 Qingshan Zhang,1 Tingting Sun,1 Jie Chen,1 Zhanhua Guo,2 Yuqing Wang,1 Zhengru Guo,1 Kangwen Yang,1 and Heping Zeng1,3,a)

1 Shanghai Key Laboratory of Modern Optical System, and Engineering Research Center of Optical Instrument and System, Ministry of Education, School of Optical Electrical and Computer Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China 2 Shanghai Langyan Optoelectronic Science and Technology Co., LTD, Shanghai 200433, China 3 State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, China

(Received 15 January 2015; accepted 3 March 2015; published online 11 March 2015) We report on a fiber laser system delivering 122 fs pulse duration and 600 mW average power at 1560 nm by the interplay between divided pulse amplification and nonlinear pulse compression. A small-core double-clad erbium-doped fiber with anomalous dispersion carries out the pulse amplification and simultaneously compresses the laser pulses such that a separate compressor is no longer necessary. A numeric simulation reveals the existence of an optimum fiber length for producing transform-limited pulses. Furthermore, frequency doubling to 780 nm with 240 mW average power and 98 fs pulse duration is achieved by using a periodically poled lithium niobate crystal at room C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4914882] temperature. V

As ultrafast fiber lasers offering crucial advantages over their solid-state counterparts with their easy operation, robust performance, compact size, and cheap pump, much attention has been paid to replace bulk lasers with fiber lasers.1–3 The competition is represented by Ti:sapphire lasers and frequency-doubled erbium-doped fiber lasers. If one wants absolute cutting-edge of electric fields with extremely high pulse energies, Ti:sapphire lasers are still the best choice.4 But if one wants modest-energy pulses with dozens of optical cycles, ultrafast fiber lasers are the worthy choice.5,6 The fiber architecture brings high coupling efficiency between pump laser and doped ions, and produces large surface-tovolume ratio to comfort heat dissipation. Even though current fiber technologies have promoted great improvements in achieving compact and reliable laser systems which can deliver both high average/peak powers and high beam qualities, the art of laser fiber still encounters difficulties in its small fiber core where high-intensity laser is confined and produces several strong nonlinear optical effects which may significantly deteriorate pulse shape, limit pulse peak power, or even damage the gain fiber. Largemode-area double-clad fiber can reduce nonlinear optical effects and provide enough laser gain for high-power amplification, but mode dispersion as well as mode speckle perturbs the frequency chirp of pulses which are hardly compressed to less than 100 fs.7,8 Although photonics crystal fiber (PCF) technology supports ultrashort pulse amplification of superior beam quality, the intrinsic air-hole structure of PCF is quite difficult to be incorporated into an all-fiber architecture, and current PCF amplifiers have to be assisted by bulk optics.9,10 Therefore, high-power ultrashort fiber lasers demand improved designs for compact all-fiber amplification and sub-100-fs pulse compression.

a)

Author to whom correspondence should be addressed. Electronic mail: [email protected]. Telephone: þ86-21-62232108. Fax: þ86-2162237211.

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In order to minimize nonlinear optical effects in highpower fiber amplifier, it is important to obtain long stretched pulses. Chirped-pulse amplification (CPA) as well as divided-pulse amplification (DPA) is the technique to relieve excessive nonlinear phase shifts in fiber amplifiers.11–14 In CPA, strong stretching and compression are introduced by bulk media (such as grating-pairs), which are inconvenient for applications requiring a compact and alignment-free laser source. CPA with fiber-based pulse stretcher has enabled amplifier to generate pulses of milli-joule energy and hundreds watts average power with hundreds of femtoseconds pulse duration, but sub-100-fs pulse compression is still difficult mainly due to practical challenges arisen from spectral gainnarrowing and high-order dispersion. The recently developed DPA technique employing polarized pulse duplicating could divide a pulse into a sequence of replicas with controllable time delays before amplification, and recombine those replicas after amplification. Compared with CPA, DPA does not require impractical large amount of dispersion and delicate alignment on the diffraction gratings, and thus possesses the integration potential in getting a compact laser system with both high energy and femtosecond duration. Lately, DPA and CPA have been implemented together to provide a compact tabletop system delivering 320 fs pulse duration with 430 lJ pulse energy at 96-kHz repetition rate.15 Moreover, divided-pulse spectral broadening was demonstrated and sub-100-fs pulse compression was realized by using chirpedmirror compressor.16 So far, a challenge still exists in the art of ultrafast fiber laser design to generate high-energy sub100-fs pulses. A straightforward solution is to combine nonlinear spectral broadening17,18 and divided-pulse amplification12 by seeding an upchirped pulse into a negativedispersion fiber amplifier. In this letter, we demonstrate a fiber laser system with divided pulse amplification in an anomalous dispersion double-cladding nonlinear fiber amplifier, where pulse amplification and compression were simultaneously carried out so that no separate compressors were required. The laser

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FIG. 1. Schematic of laser setup. LD: 480-mW laser diode. WDM/OI/OC: an integrated component with the function of wavelength division multiplexer, optical isolator, and optical coupler. EDF: erbium-doped singlemode fiber. DCF: dispersion compensation fiber. F-PBS: fiber-coupled polarization beam splitter. SMFA: single-mode-fiber amplifier. FRM: Faraday rotator mirror. Col1 and Col2: high-power collimator. HWP: halfwave plate. PBS: polarization beam splitter cube. DCFA: double-clad fiber amplifier.

system delivered 122-fs pulse duration and 600-mW average power at 1560 nm with a repetition rate of 80 MHz. Frequency doubling to 780 nm with 240-mW average power was further realized by a periodically poled lithium niobate (PPLN) crystal at room temperature. The experimental setup is shown in Fig. 1. Our laser system consisted of a mode-locked fiber oscillator, a fiber stretcher providing a certain amount of frequency up-chirp, a single-mode fiber (SMF) amplifier, a pulse-divider, a doubleclad fiber amplifier, and a PPLN crystal. The oscillator was an Er-doped fiber laser with a repetition rate of 80 MHz at 1560 nm. To simplify the laser setup and improve the modelocking stability, we used an innovative fiber-coupled hybrid device WDM/OI/OC to function as wavelength-division multiplexing (WDM) for pump and laser coupling, optical isolator for unidirectional lasing, and PM-fiber for lasing output. In addition, an electric polarization controller (EPC) replaced the conventional mechanical polarization controller to realize active control of passive mode-locking.19 By presetting the voltage on the EPC, stable mode-locking was obtained via nonlinear polarization rotation evolution. Furthermore, the temporal duration and spectral bandwidth of the output laser pulses could be accurately controlled by the EPC device. A photodiode and an electric loop were

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applied to monitor and feedback control the EPC, ensuring long-term stable mode-locking operation. The fiber oscillator used 0.82-m Er-doped fiber and 1.74-m SMF-28e. The corresponding fiber dispersion parameters are 51 and 19 ps/nm/km at 1560 nm, respectively. The laser operated in the stretched-pulse regime and produced positively chirped pulses due to the short fiber length between the Er-doped fiber and the hybrid device WDM/OI/ OC. An output average power of 5 mW, corresponding to a pulse energy of 63 pJ, was obtained at the output port with 200-mW pump power. Figure 2(a) shows the temporal and spectral curves of the seed laser pulses retrieved from a second-harmonic generation frequency-resolved optical gating spectrogram. The full-width at half-maximum (FWHM) duration of the pulse is 1.5 ps and FWHM spectral bandwidth is 28.0 nm, generating a time-bandwidth product of 5.2. Such a large time-bandwidth product is preferred by fiber-based chirped pulse amplification, since it may shorten the fiber used in the subsequent stretcher. It is well-known that overlong fiber stretcher may induce excessive nonlinear evolution and high-order dispersion, which is quite difficult to be completely compensated in the pulse-compressing stage. We used 1-m long dispersion compensation fiber with group-velocity dispersion of 51 ps2/km at 1560 nm to counteract the negative dispersion of transmission fiber. A double-pass configuration with the combination of a fibercoupled polarization beam splitter and a faraday rotator mirror (FRM) were used to cancel the environmental influence on the non-PM dispersion compensation fiber. In the power preamplifier, a dual-pass configuration of bidirectionally pumped single-mode-fiber pre-amplifier was used to boost the laser average power to 96 mW. Similar to the stretcher, the Faraday-rotation mirror connected to the preamplifier reflected the incident pulse to suppress the amplified spontaneous emission (ASE) noise and rotated the polarization of the pulses by 90 to cancel all birefringence effects in the pre-amplifier. A fiber-based polarization-beam-splitter was used to couple the stretched pulse to the pre-amplifier and reflected the pre-amplified pulses to subsequent components. Due to the limited transmission bandwidth of WDM and reflection bandwidth of Faraday rotator mirror as well as spectral gain-narrowing effect, dramatic decrease of spectral

FIG. 2. The temporal (red curves) and spectral (blue curves) intensity of laser pulses from oscillator (a) and preamplifier (b).


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bandwidth was observed in the pre-amplifier. The output characters of the pre-amplifier are shown in Fig. 2(b). The FWHM duration of the pre-amplified pulses is 1.9 ps, and FWHM spectrum bandwidth is 14.4 nm, generating a timebandwidth product of 3.37. Spectral gain-narrowing is the main limitation to generate well-compressed pulses in femtosecond fiber amplifiers. A well-known technique for generating sub-100-fs pulses is nonlinear pulse amplification, which makes use of nonlinear spectral broadening to overcome the gain bandwidth limitation. For a fiber laser running at wavelengths shorter than 1.3 lm, e.g., Yb-fiber laser, the concept of nonlinear pulse compression has to apply a compressor to dechirp the positive chirp imposed by the interplay between self-phase-modulation and group-velocity dispersion in optical fibers. While for a fiber laser running at wavelengths longer than 1.3 lm, e.g., Er-fiber laser, fibers with anomalous dispersion may carry out the pulse amplification and simultaneously compress the laser pulses without additional requisite pulse compressors. Our objective is to find the equilibrium which can not only produce sufficient optical nonlinearity to broaden the spectrum but also restrict excessive nonlinear effects to ensure highquality temporal integrity around the wavelength of 1.55 lm. To study nonlinear compression in anomalous dispersion fiber, we numerically simulated the pulse evolution in the nonlinear fiber amplifier. An adaptive split step Fourier method was used to solve the generalized nonlinear Schr€ odinger equation20 X inþ1 @ n A @A a b ¼ Aþ n! n @T n @z 2 n2 ð1 þ icA RðT 0 ÞjAðz; T T 0 Þj2 dT 0 ;

(1)

1

where A ¼ A(z, t) is the complex amplitude of the pulse envelope of pulses, a is the laser gain coefficient, bn are the

dispersion coefficients at center frequency x0, and c is the nonlinear coefficient. The right-hand side of Eq. (1) models laser gain, self-phase modulation, dispersion, and intra-pulse Raman scattering. The procedure of this simulation was to find an optimized fiber length in the main fiber amplifier. For simplicity, the laser gain was assumed to be evenly distributed along the fiber length, while other parameters were chosen in accordance with the experimental parameters, where nonlinear parameter c is 3 W1km1, laser gain coefficient is 3.0 dB/m, b2 is 22 ps2/km, and b3 is 0.08 ps3/km at the central wavelength of 1560 nm. A 1.9-ps pulse with 5 mW average power at 80 MHz repetition rate and 14.4 nm spectral width was carried out in the simulation. Simultaneous laser amplification, spectral broadening, and pulse compression could be observed during the amplification process in an anomalous dispersion gain fiber seeded by an upchirped pulse. Figures 3(a) and 3(b) show the temporal and spectral evolution along the propagation fiber. As shown in Fig. 3(c), the pulse evolution could be roughly divided into three operation regimes: near-linear regime (5 m). With lower pulse energy and longer pulse duration, the amplification works in the nearlinear regime where temporal compression could be observed in the initial fiber length. As the pulses are amplified into the moderate-nonlinearity regime, self-phase modulation as well as anomalous dispersion jointly dominates, resulting in obvious spectral broadening and pulse shortening, as shown in Figs. 3(c) and 3(d). With a further increase in pulse energy, soliton-effect compression which leads to the formation of pulse pedestals and eventual break-up is observed in the over-nonlinearity regime. The numerical simulation reveals the existence of an optimum fiber length for nonlinear pulse amplification and compression. According to our simulation, a duration of 60 fs could be

FIG. 3. Simulation results of pulse evolution in nonlinear fiber amplifier. Temporal (a) and spectral (b) evolutions along the fiber amplifier. (c) The variance of pulse duration vs propagation fiber length. (d) The spectra at different propagation distances, 3 m (green curve), 4.9 m (red curve), and 5.2 m (blue curve).


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FIG. 4. (a) The measured autocorrelation trace of the divided replicas. Inset shows the schematic of the pulse divider. Three cylinders represent YVO4based dividers with given direction of crystal OA and s-polarized direction shown as green dashed-dotted lines and red dotted lines, respectively.

obtained with a gain fiber of 4.7 m and a total gain of 14 dB. The highest attainable average power of transform-limited sub-100-fs pulses is 130 mW, corresponding to a limited pulse energy of 1.6 nJ. The concept of divided pulse amplification was then carried out to reduce nonlinear optical effects and thus extract higher pulse energy from the fiber amplifier. The pulse division and combination were achieved by employing cascaded YVO4-based dividers with the help of a Faraday rotator mirror to make the reflected pulse replicas pass through the same crystal series with crossly changed polarizations. In the pulse combination port, the phase delays for all the pulse replicas are automatically matched with identical round-trip optical path lengths. As depicted in the inset of Fig. 4, three YVO4 crystals with lengths of 10, 20, and 40 mm divided the initial pulse into eight replicas. The polarization-mode delay between ordinary and extraordinary waves in YVO4 is 0.7 ps/mm at 1550 nm. The first and third YVO4 crystals (from left to right) had their crystal optical axes (OA) oriented at an angle of 45 to the horizontal plane, while the second YVO4 crystal axis was oriented

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in the horizontal plane. Therefore, eight replicas were generated to mitigate excessive nonlinear phase shifts in the main fiber amplifier. Figure 4 shows the measured autocorrelation trace of the pulse string, indicating polarization-mode delay between adjacent replicas matches well with the designed time of 10-mm increment length of YVO4. Experimentally, the divided pulses got amplified in a dual-pass double-cladding fiber amplifier (DCFA), which consisted of 1.5-m single-mode fiber and 0.6-m 12/130 Erdoped double-clad fiber with numerical aperture (NA) of 0.2 for the fiber core and 0.46 for the cladding. With a pump power of 4.3 W from a wavelength-stabilized laser diode at 976 nm, the DCFA delivered 600 mW average power (measured by a integrating sphere photodiode power sensors S145C, Thorlabs) at 1560 nm, corresponding to 7.5 nJ pulse energy. Along the first pass of DCFA, pulse evolution worked in the so-called near-linear regime. Subsequently, as the pulse reflected by the FRM and passed through DCFA again, self-phase modulation and anomalous dispersion enforced the pulse amplification into the moderatenonlinearity regime. Figures 5(a) and 5(b) show the FROG measurement (15–100-USB, Swamp Optics) of the combined pulse. The linear spectral phase indicates that a nearly transform-limited pulse was obtained with the FWHM duration of 122 fs if a Gaussian pulse shape was assumed. As evaluated by our numeric simulation, optical nonlinearity limits the attainable pulse energy less than 1.6 nJ in a singlepass DCFA (without pulse dividing). In principle, 12.8 nJ pulse energy should be possible with 8 pulse dividing, while the experimentally attained pulse energy was much lower than the ideal case. The difference was mainly caused by the splicing loss between different type fibers with mismatched effective mode areas (as mainly determined by the core diameter and NA mismatch), as well as by the insertion loss of Faraday reflection and coupling loss in the pulse divider/combiner. Besides, new spectral components generated in the double-pass nonlinear amplifier also affected the combining efficiency through breaking the spectral symmetry of the pulse division/combination processes.14

FIG. 5. (a) The retrieved temporal intensity (blue curve) and phase (red curve) of laser pulses from main amplifier. Inset shows the measured FROG trace. (b) The retrieved spectral intensity (blue curve) and phase (red curve) of laser pulses from main amplifier. Inset shows the retrieved FROG trace. (c) The pulse autocorrelation trace of 780 nm laser pulses and its fit for a Gaussian profile. (d) The optical power stability of the 780 nm femtosecond laser. Inset shows the measured spectrum of 780 nm laser pulses.


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A PPLN crystal with 20.9 lm poling period and 0.45 mm length was used for frequency-doubling the amplified pulses at room-temperature. A typical autocorrelation (pulseCheck, APE) and the corresponding spectrum (HR4000, Ocean Optics) of the frequency-doubled pulses at 780 nm are shown in Fig. 5(c) and the inset of Fig. 5(d), respectively. The pulses were near-bandwidth limited with 0.64 time-bandwidth product and 98 fs pulse duration (FWHM) if a sech2 pulse shape is assumed. An average power of 240 mW (photodiode power sensors S121C, Thorlabs) was achieved with 600 mW incident power at 1560 nm, corresponding to 40% conversion efficiency. Figure 5(d) shows the 24-h optical power stability of the 780 nm femtosecond laser and reveals power fluctuations (standard deviation) of 1%. In conclusion, we have demonstrated divided-pulse nonlinear amplification in an Er-doped double-cladding fiber amplifier. Nonlinear amplification in the gain fiber was accompanied by simultaneous spectral broadening and pulse compression. The polarization-multiplexing nonlinear amplification and compression may pave a way for high-power and high-energy ultrashort pulse amplification in quasisingle-mode fibers. In addition, the frequency-doubled erbium-doped fiber laser provides competitive sub-100-fs laser pulses to replace standard Ti:sapphire lasers for various ultrafast scientific researches and industrial applications. This work was partly supported by the National Key Scientific Instrument Project (2012YQ150092), National Basic Research Program of China (2011CB808105), National Natural Science Foundation of China (Nos. 11404211, 61127014, and 11374370), and Shanghai Municipal Science and Technology Commission (13ZR1458100). 1

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