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Abstract
Parabolic pulses with linear self-phase-modulation-induced frequency chirp are attractive in ultrafast laser fiber amplification system for the functionality of nonlinearities suppression. In this paper, we present an effective way of parabolic pulse evolution by passive spectral amplitude shaping with a pair of chirped fiber Bragg gratings (CFBG). By this approach, a high-energy high-peak-power Yb-doped fiber chirped pulse amplification (CPA) system is demonstrated. The oscillator is a dispersion-managed passively mode-locked Yb-doped fiber laser with a broadband Gaussian-shaped spectrum which is evolved to parabola in a following preamplifier with pre-chirping management by a CFBG compressor. The pulses are then stretched with a CFBG stretcher, based on frequency-to-time mapping, the temporal profiles of the pulses show an identical parabolic envelope to the spectrum. The shaped pulses are further amplified with three stages of all-fiber amplifiers and compressed by a grating-pair compressor. The pulse duration is compressed to 172 fs with a pulse energy of 27 µJ. The central pulse encompasses 72% of total pulse energy, corresponding to a pulse peak power of 113 MW. No obvious pulse degeneration is noticed at nonlinearity accumulation B-integral as high as 12 rad. This configuration shows a significant potential for nonlinearity tolerance in high-energy operation compared with conventional CPA system.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Recently, high-energy ultrafast femtosecond laser systems are in great demand for various fields such as high precision industrial processing, clinical surgery and high harmonic generation (HHG) [1–4]. Compared with conventional femtosecond Ti:sapphire laser systems, fiber-based laser systems provide good beam quality, robust stability, and compact configuration at repetition rate of kilohertz to megahertz. Several works have been reported to show excellent high average power performance of fiber-based femtosecond laser systems [5,6].
However, reaching both high energy and ultrashort pulse duration in fiber-based femtosecond laser systems is more challenging than solid-state systems due to the interaction of nonlinearity, gain and dispersion in all-fiber amplifiers [7]. The compressed pulse quality will further degrade with uncompensated nonlinear spectral phase and high order dispersion [8]. The unwanted pulse distortions cause the reduction of pulse peak power after compression. The consequence is even worse in all-fiber femtosecond system because of the pigtail fiber linking each stage of fiber amplifiers and the front-pumped configuration which will cause more nonlinearity accumulation [9].
To improve the pulse quality of high-energy ultrafast fiber lasers, several methods are employed to manage the nonlinearity. Linear CPA is the common approach for fiber-based high-energy femtosecond system [10–12]. Usually, the pulses are sufficiently stretched to nanosecond-level duration in a grating-based stretcher and amplified in large mode area (LMA) fibers or Rod-type photonic crystal fibers (rod-PCF). The peak power intensity during amplification is significantly attenuated leading to a mitigation of nonlinear effect. The nonlinear accumulation is qualified by the B-integral which is requested for less than 1 rad in a linear amplification system [8]. Pulse energy of 400 µJ and pulse duration of 330 fs has been obtained by the combination of a 2-ns stretched pulse and a rod-PCF amplifier [13]. These Rod-CPA systems can provide enough energy for many applications, but the complicated free-space structure reduce the stability of the system. In addition, long distance between the two grating pair and the large grating size are demanded to compensate the chirp of nanosecond-level stretched pulse, which make the system less compact and more cost.
Parabolic pulses have been reported as proper candidates in CPA systems for nonlinearity suppression because the parabolic intensity profiles lead to linear self-phase-modulation-induced frequency chirps. The features result in an attractive high-quality pulse after amplification and compression. Several approaches for parabolic pulse generation have been demonstrated in [14–23]. Self-similar amplification can produce parabolic intensity profile in optical fibers with normal group-velocity dispersion (GVD) [14]. A grating-pair compressor is usually used for pre-chirping management to facilitate the evolution in the amplifier. After amplification, the pulses can be decreased to sub-100 femtosecond. The shortest pulse duration of 24 fs with pulse energy of 1 µJ is demonstrated by a hybrid femtosecond fiber laser amplification system [15]. An alternative solution based on parabolic pre-shaping without pre-chirping is also demonstrated [16]. The system starts from sub-nanojoule, 9-ps seeds, pulses are shaped passively in fiber and preamplifier. The system generates nearly transform-limited (TL) pulses shorter than 300 fs at pulse energy exceeding 4 µJ. To ensure a self-similar amplification, the pulse duration after amplification is usually kept at around tens of picosecond, and the pulse energy is limited up to several microjoules due to the stimulated Raman scattering [17,18]. It also has been reported using adaptive learning optimization algorithms and liquid crystal spatial light modulator (LC-SLM) to generate parabolic waveforms in time domain. The pulses can be further compressed to less than 200 fs with a pulse energy of 0.25 µJ [19].
In addition, parabolic pulse has been generated by spectral amplitude shaping, a parabolic temporally stretched pulse is obtained when the spectrum is shaped to a parabolic profile. This method is based on dispersion-induced frequency-to-time mapping combined with spectral shaping. Mostly, the shaping approaches are actively controlled by utilizing LC-SLM [20,21]. By spectral amplitude modulation with the LC-SLM, accumulated B-integral of 16 can be controlled [22]. This approach allows the generation of nanosecond-level parabolic pulse compared to self-similar amplification. It represents an efficient way to realize high energy and ultrafast femtosecond pulse with large B-integral, but the structure of pulse shaper which consists of gratings and LC-SLM adds complexity and instability of the system.
In this paper, we report an Yb-doped all-fiber-integrated CPA system based on passive spectral amplitude shaping, which deliver a pulse duration of 173 fs and a pulse energy of 27 µJ at a repetition rate of 500 kHz, corresponding to a central pulse peak power of 113 MW. The 550-ps parabolic pulse is generated in a preamplifier with a pair of CFBGs and further amplified in three stages of all-fiber amplifiers. The final stage is an LMA-PCF amplifier which is simply spliced to the preamplifier through a piece of transition fiber. After a grating-pair compressor, the pulse can be compressed to as short as 173 fs with an efficiency of 77%. The method of passive spectral amplitude shaping provides major benefit in nonlinearity tolerance as high as 12 rad. In addition, only a few free-space components are utilized at the end-pumped and compression structure, the optimized compression distance between the two gratings is 55 cm. This approach is compact, highly versatile, and can be applied to most of CPA systems.
2. Experimental setup
The experimental schematic diagram of the high energy femtosecond fiber CPA laser is shown in Fig. 1. The whole system consists of four parts: an all-fiber oscillator, a CFBG-based pulse shaper, three stages of all-fiber power amplifiers, and a grating-based pulse compressor. All the components employed in the oscillator and fiber amplifiers are polarization-maintained to ensure a strictly linear polarization mode with high environmental stability.
Fig. 1. Schematic diagram of the high energy femtosecond fiber CPA system. SESAM, semiconductor saturable absorber mirror; LD, laser diode; WDM, wavelength division multiplex; CFBG, chirped fiber Bragg grating; ISO, isolator; PM, polarization maintaining; Yb-SMF, ytterbium-doped single mode fiber; DCF, double-clad fiber; DDPG, digital delay and pulse generator; AOM, acoustic optical modulator; DM, dichromic mirror; F, focusing lens; HR, high reflecting mirror; HWP, half wave plate.
The oscillator is a home-made dispersion-managed SESAM passively mode-locked fiber laser in a linear configuration. A CFBG (Teraxion, DMR) is used as the dispersion-management component which possesses a reflectivity of 25% and provides an anomalous dispersion β2= −0.25 ps2. The reflection spectrum is gaussian-shaped profile centered at 1064 nm with a 3-dB bandwidth of 28.01 nm. The total cavity length is accurately adjusted to 10.8 m with a total net dispersion of 0.02 ps2 to obtain a Gaussian-shaped spectrum, corresponding to a repetition rate of 18.52 MHz. The gain is provided by a 0.7-m long Yb-doped fiber (Nufern, PM-YSF-HI-HP, 250 dB/m absorption at 976 nm) pumped by a single-mode LD with maximum output power of 350 mW. The pulses are extracted from the 20% port of the coupler.
The pulses from the oscillator are firstly amplified in a core-pumped fiber amplifier, then a CFBG compressor (Teraxion, DMR) is employed. The parameter of the CFBG compressor is almost the same as the one used in the oscillator. The CFBG is acted as an all-fiber compressor for dispersion compensating to ensure an effective nonlinear amplification in next amplifier. The Gaussian spectrum is shaped to parabola in the cladding-pumped amplifier which consists of a 4.8-m Yb-doped double-clad fiber (LIEKKI, Yb1200-6/125DC-PM, 2.6 dB/m absorption at 976 nm). After the spectral shaping process, the pulses are temporally stretched by a CFBG stretcher (Teraxion, TPSR) with a GVD of −32.02 ps/nm, and a third order dispersion (TOD) of −0.261 ps/nm2. The reflection spectrum is flat-top profile centered at 1064 nm with a 3-dB bandwidth of 25.2 nm which ensures little effect on the signal spectral profile. The CFBG stretcher provides enough GVD for frequency-to-time mapping. The temporal profiles show nearly identical parabolic shape to the amplitude profiles of the spectrum. Although a temporal asymmetry of the front edge induced by TOD is found inevitably, the consequence can be rectified to some extent by the gain shaping in the further amplifier. The combination of CFBGs and fiber amplifier plays an important role in parabolic evolution as an all-fiber structured pulse shaper, which makes the system compact and stable.
Three stages of Yb-doped fiber amplifiers are used to scale up the average power. The gain media of the first-stage preamplifier is a 4-m Yb-doped double clad fiber (LIEKKI, Yb1200-6/125DC-PM) which is cladding-pumped by a 350-mW LD. The length of the active fiber is accurately selected to adjust the temporal asymmetry by the gain-induced pulse shaping. The isolators used in the amplification are integrated with a 30-nm band-pass filter centered at 1064 nm for stimulated spontaneous emission (ASE) suppression. A fiber-coupled AOM (Gooch & Housego, Fiber-Q) is employed to reduce the repetition rate to 500 kHz drived by a DDPG which is triggered from the 1% tap port of the isolator. A 3-m double-clad 10/125 µm ytterbium-doped fiber (Nufern, PLMA-YDF-10/125-M, 4.9 dB/m at 976 nm) is used in the second-stage preamplifier as the gain medium. The final amplification is implemented in 3-m Yb-doped LMA-PCF with a core diameter of 40 µm and an inner-clad diameter of 200 µm (NKT Photonics, AeroGAIN-BASE-1.1) which is water cooled at 25 °C. The input signal port is a 0.4-m transition fiber (SC-250/14-PM-Ge) which can be simply spliced to the pigtail fiber of the isolator. The splicing loss is measured to be 0.3 dB. The pump laser is assembled from four pieces of 30 W multi-mode LD by a PM combiner and coupled into the PCF by two aspheric focusing lens. A dichromic mirror is used to separate the amplified signal from the pump light. The end-pumped structure is the only free-space part in the fiber amplifiers.
In the last part, the collimated output pulses are compressed by a 1600 lines/mm transmission grating-pair compressor (LightSmyth Technologies) with aperture size of 30.8×11.3 mm and 125×19 mm, respectively. The two gratings are arranged in Littrow angle to ensure a best diffraction efficiency of 95% and a roof mirror is used to create a double-pass configuration. The total compression efficiency is estimated to 77% by adjusting the polarization through a half wave plate (HWP).
In our experiment, the temporal profile of the stretched pulses is measured by a 12.5 GHz free-space coupling photodetector (Newport) followed by a 25 GHz digital oscilloscope (Agilent), and the spectrums are monitored by optical spectrum analyzer (Yokogawa), the compressed pulses are measured by an autocorrelator (APE) and a commercial frequency-resolved optical gating (FROG) based on second harmonic generation (Mesa Photonics). The beam quality factor is measured by a scanning slit beam profiler system (BeamScope-P8).
3. Results and discussion
The dispersion-managed passively mode-locked laser operated with an average output power of 0.5 mW, pulse duration of 1.83 ps, and repetition rate of 18.52 MHz at a pump power of 35 mW. It exhibited an approximate Gaussian-shaped spectral with a bandwidth of 10.4 nm centered at the wavelength of 1064 nm. The pulses could be compressed to 162 fs which was very close to the TL pulse duration of 160 fs. The autocorrelation (AC) trace of the oscillator, pulse train, and the spectral profile of the oscillator are shown in Figs. 2(a), 2(b) and 2(c), respectively.
Fig. 2. (a) The autocorrelation trace of the oscillator. (b) the mode-locked pulse train with the repetition rate of 18.52 MHz. (c) The spectrum of the dispersion-managed mode-locked oscillator.
On the basis of femtosecond self-similar amplification theory, the evolution of spectral and temporal profiles is mainly decided by the pulse duration and pulse energy [14,23]. In our experiment, we use a preamplifier and a CFBG compressor to control the two key parameters. The laser from the oscillator was pre-amplified to 2 mW and the CFBG compressor was used to compress the pulses to 200 fs. The spectrum was broadened to 18 nm and shaped to a parabola by nonlinear amplification in the cladding-pumped amplifier with the increasing of pump power up to 370 mW. To further increase the pump power, the spectrum was deviated from the optimal profile and broadened to more than 30 nm with SPM-induced spectral modulation. The spectral profile versus different pump power and the parabolic fitting of optimal spectral profile are showed in Fig. 3(a). An exponential function included 100 terms in the series is used to fit the parabolic profile in our experiment, which is described in Eq. (3) of [24]. To evaluate the quality of spectral evolution, the pulses could be compressed to 100 fs by a piece of hollow-core photonic bandgap fiber with negative dispersion, indicating that the pulses were spectrally broadened with nearly linear chirp in this stage. The AC trace is shown in Fig. 3(b).
Fig. 3. (a) The spectral intensity monitored after the cladding-pumped amplifier versus different pump power (colored line), parabolic fitting of the spectral profile at 370 mW pump power (dot red). (b) The autocorrelation trace of the compressed pulse after spectral shaping.
After that, the pulses exported from the cladding-pumped amplifier were stretched to 580 ps through the CFBG stretcher. The temporal profile of the dispersed pulses was approximate identical to the shape of the input optical spectrum due to frequency-to-time mapping, but indicated a slightly asymmetrical front end induced by TOD. The temporal profile and the related spectrum are shown in Figs. 4(a) and 4(d). To quantify the temporal profile of pulse in our system, we employ a misfit parameter M between the intensity profiles of actual pulse |ψ|2 and an ideal parabolic fitting |ψP-FIT|2 [25]:
Fig. 4. The temporal profiles (dashed blue) of pulse and parabolic fitting (dot red) at different positions of the fiber amplifiers. (a) after CFBG stretcher, (b) after 6/125 amplifier and (c) after 10/125 amplifier; (d), (e) and (f) the corresponding optical spectrum of the pulse measured with 0.1 nm spectrum resolution.
In the final amplifier, the input average power of 140 mW was scaled up to 20 W with a pulse energy of 40 µJ at the repetition rate of 500 kHz, which corresponded to 40% slope efficiency for 52 W pump power, as shown in Fig. 5(a). The spectra from the main amplifier at different pulse energies are presented in Fig. 5(b), the 3-dB spectral bandwidth after amplification was kept around 15 nm supporting a TL pulse duration of 140 fs. However, there was a certain level of unavoidably ASE around 1030 nm at full power operation, it still showed a relatively high signal to ASE ratio of 37 dB and the ASE background could be estimated to 7.5 mW in the linear scale spectrum. Further power amplifying was limited by the nonlinearity-induced pulse quality degradation and ASE lasing which may cause a catastrophic damage to the PCF. The beam quality M2 values were measured to be 1.18 (parallel) and 1.23 (perpendicular), and the measured power fluctuation (root mean square) in over 2 hours was 0.25% at average power of 20 W, as shown in Fig. 5(c). The polarization extinction ratio (PER) was measured to be 15 dB.
Fig. 5. (a) Average output power of the main amplifier versus the change of pump power, experimental data (blue dot), linear fitting (solid red). (b) The optical spectrum at different pulse energy. (c) M2 measurements, beam diameters as a function of distance from laser beam waists (red, black dot), measured power stability in 2 hours (solid blue).
The amplified pulses were collimated to a grating-pair compressor with an optimized compression distance of 55 cm and the shortest pulse duration of 172 fs was obtained when the average power from the amplifier was 17.5 W. The pulse duration and the temporal phase were characterized by SHG-FROG, as displayed in Fig. 6. Considering the compression efficiency of 77%, the maximum average power after the compressor was measured to be 13.5 W, corresponding to a pulse energy of 27 µJ. The peak power of the central pulse was estimated as 113 MW considering the 28% remained energy in the subpulse which is calculated from the retrieval pulse trace. By simulating the pulse evolution with a commercial software, the B-integral in fiber pre-amplifiers was calculated to be 5.3 rad, and the B-integral in the final amplifier was calculated to be 6.7 rad when the average output power was 17.5 W. Pulse deterioration appeared when the B-integral exceeded 12 rad in whole fiber amplifier chain.
Fig. 6. Retrieval temporal intensity profile of the compressed pulse (solid red) and the temporal phase (dashed blue) obtained from the SHG-FROG measurement.
To prove the effect of nonlinearity tolerance on the parabolic spectral amplitude shaping and to reveal the influence of the profile on the recompressed pulse quality, we generated a Gaussian-shaped spectrum with the same bandwidth to the parabola by directly amplifying the pulse in the preamplifier without shaping process, the M factor was calculated to be 0.07. Then the pulses were amplified with the same condition of the parabolic pulse, the AC traces of the compressed pulses in the two comparative experiments at pulse energy of 5 µJ, 15 µJ and 35 µJ were monitored, respectively. The results are illustrated in Fig. 7.
Fig. 7. (a) (b) and (c) AC traces at pulse energy of 5 µJ, 15 µJ and 35 µJ with Gaussian-shaped spectrum (M = 0.07); (d) (e) and (f) AC traces at pulse energy of 5 µJ, 15 µJ and 35 µJ of a parabolic- shaped spectrum (M = 0.048).
The pulse energy was transferred from the central peak to a pedestal in the AC trace as pulse energy increased from 5 µJ to 35 µJ without parabolic shaping (M = 0.07), indicating that pulse distortions degraded the pulse contrast and decreased the peak power with the increasing of pulse energy. This result clearly revealed that the Gaussian profiles suffered from a strong nonlinear chirp imposed by SPM, which cannot be compensated by simple grating-pair compressors. On the contrary, parabolic pulses (M = 0.048) showed almost identical AC traces with the increase of pulse energy until 35 µJ, indicating that the SPM-induced nonlinear chirp was controlled by parabolic shaping process. Although a slight pedestal was observed from the AC trace, this method still demonstrated a better nonlinearity tolerance when the B-integral reached 12 rad in the amplifier, enabling to generate high energy femtosecond pulses with high quality. The pedestal was probably induced by the imperfect parabolic shaping, mismatched high-order dispersion of the stretcher and compressor, or the gain shaping effect in the fiber amplifiers.
4. Conclusion
In this paper, we present a temporal parabolic pulse amplified in all-fiber configuration based on spectral amplitude shaping by a pair of CFBGs. The pulse from the oscillator is evolved to a parabola with a CBFG-based pulse shaper and further amplified in three-stages of all-fiber-integrated amplifiers. The pulses can be compressed to 172 fs with a pulse energy of 27 µJ by a pair of gratings. The central pulse encompasses 72% of total pulse energy, corresponding to a pulse peak power of 113 MW. Free-space components only exist at the pump coupling and compression part. The nonlinearity tolerance in this configuration is confirmed to as high as 12 rad by comparative experiments. The system is compact with high reliability which is attractive in many applications since the absence of active spectrum modulator and rod-PCF amplifier.
Funding
National Research and Development Program of China (2017YFB0405201); National Natural Science Foundation of China (61527822).
Acknowledgments
The authors acknowledge the funding support from the National Natural Science Foundation of China.
Disclosures
The authors declare no conflicts of interest.
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