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Abstract
A high-average-power, high-pulse-energy picosecond chirped pulse amplification (CPA) laser system based on an extra-large-mode-area (XLMA) triple-clad fiber (TCF) was demonstrated. The ultrashort pulses, generated from all-fiber mode-locked oscillator, stretched and then were pre-amplified to 10 W through a series of fiber power amplifiers. Subsequently, the average output power was amplified to 620 W corresponding to a pulse energy of 0.62 mJ via XLMA TCF. Additionally, the amplified pulses were compressed to a pulse duration of 7.6 ps with an average power of 423 W and a compression efficiency of 68.2%. The ultrashort laser is a promising light source for application of water-guided laser processing, albeit with a beam quality factor of 20 and 21 along two orthogonal axes.
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1. Introduction
Ultrafast fiber lasers with high average power and high pulse energy are in great demand in industrial material processing as it can reduce thermal diffusion of processing area and improve spatial resolution of nanofabrication, for instance, hard material processing, glass cutting, high-precision micromachining [1–9]. Generally, ultrafast fiber oscillator with nJ-scale pulse energy and mW-scale average power is unable to meet application requirements, so further power amplification is required. Fiber amplifier in a master-oscillator-power-amplifier (MOPA) scheme has a simple architecture [10]. However, peak power and pulse energy are limited by fiber nonlinearities, such as self-phase modulation, cross-phase modulation, and so on [11]. Several methods have been proposed to reduce the influence of nonlinear effects, for instance, using CPA technology, employing a large-mode-area (LMA) fiber, shortening fiber length, and employing backward pumping regimes [12–16].
It is well known that fiber nonlinearity is inversely proportional to the mode field area of fiber. Therefore, a promising way to reduce the nonlinear effects is to use large-mode-field (LMF) fiber coinciding with CPA technique [17–27]. In 2010, a double-clad Yb-doped fiber with mode field diameter of 27 µm was used to realize an average power of 830 W at a repetition rate of 78 MHz with a pulse duration of 640 fs [28]. In 2016, a 30/250 Yb-doped double-clad fiber (DCF) was used to achieve an average output power of 107 W with pulse duration of 566 fs at repetition rate of 17.57 MHz [29]. In terms of pulse energy, a 30/250 µm Yb-doped double-clad fiber was used to achieve pulse energy of 112 µJ at a repetition rate of 1 kHz [30].
Photonic crystal fibers (PCFs) have attracted extensive attention due to their large mode area and single fundamental mode operation. In 2011, T. Eidam et al. reported a fiber CPA system with pulse energy of 2.2 mJ, pulse width of 500 fs at repetition rate of 5 kHz by using large-pitch PCF with a mode diameter of 108 µm as main amplifier, but the average power was only 11 W [31]. In the same year, Fabian et al. reported a 294-W, 73.5-µJ ultrafast laser in a Yb-doped large-pitch fiber (LPF) with a mode field diameter of 62 µm [32]. In 2014, a triple-clad LPF with a core diameter of 35 µm was used to achieve an output average power of 100 W [33]. In the same year, a rod-type fiber with a mode field area of ∼4500 µm2 was used to generate a pulse energy of 100 µJ at 1 MHz repetition rate with a pulse duration of 270 fs [34]. In 2019, C.P.K. Manchee et al. demonstrated a pulse energy of 400 µJ at 200 kHz repetition rate and a pulse duration of 330 fs by using a rod-type PCF with a core diameter of 85 µm and cladding diameter of 260 µm. Nevertheless, the average power achieved was only 80 W [35].
The results of ultrafast pulses fiber amplifier presented above are depicted in Fig. 1. As can be seen, fiber nonlinearity presents many challenges in ultrafast pulse amplification even with LMF-PCF, resulting in a dilemma that high average power and high pulse energy cannot be obtained simultaneously. Fiber diameter of more than 80 µm enables high pulse energy than fiber diameter of less than 80 µm. In terms of average power and pulse energy of these fibers, the average power increases with decreasing pulse energy. In order to obtain high average power and high pulse energy, we propose an TCF amplifier which has a higher damage threshold and heat resistance than DCF amplifier because of the third cladding [36]. Furthermore, XLMA-TCF shows obvious advantages over DCF with respect to laser-running stability and service life, as pump light rays were reflected by the second glass-based clad [37]. Although XLMA-TCF supports multimode transmission resulting in beam quality deterioration, for some laser-processing applications, the uniformly distributed laser intensity leads to a high-quality smooth cut that is free from cracking and chipping [38]. For example, in water-guided laser processing, since the total number of modes in the TCF is less than that in water waveguide, the laser beam undergoes total internal reflection between water and air media and is guided into the workpiece [39–41], as shown in Fig. 2.
In this paper, we demonstrate a backward-pumped CPA system based on a Yb-doped XLMA TCF. The system delivers an average power of 620 W at 1-MHz repetition rate, corresponding to a pulse energy of 0.62 mJ. After compression, a pulses duration of 7.6 ps at average power of 423 W was obtained with compression efficiency of 68.2%. The corresponding pulse energy and peak power are 0.42 mJ and 55.7 MW, respectively. To the best of our knowledge, this is the first demonstration of high average power and high pulse energy at 1 MHz repetition rate by using single-channel TCF amplifier.
2. Experimental setup
The experimental setup of the high-average-power, high-pulse-energy CPA system is depicted in Fig. 3. The CPA laser system consists of a passively mode-locked Yb-doped fiber oscillator, preamp I (1-m single clad YDF), acoustic optical modulator (AOM), preamp II(1-m single clad YDF), fiber stretcher, preamp III (2-m double clad YDF), main amplifier (1.2-m TCF) and a pair of transmission gratings.
Fig. 3. Schematic of the CPA laser system; Seed: all PM dispersion-managed mode-locked ytterbium fiber oscillator; M1-M2: Dichroic mirror; M3: Plate mirror.
The seed was a polarization-maintaining (PM) dispersion-managed fiber oscillator by using a semiconductor saturable absorber mirror (SESAM). In the oscillator, the gain medium was a 1-m Yb-doped PM fiber (Nufern. PM-YSF-HI-6/125), pumped by a single-mode diode emitting at a wavelength of 976 nm. A chirped fiber Bragg grating (CFBG) was used as dispersion-management component to improve the spectral and temporal quality.
The pulses from the oscillator were firstly amplified in preamp I. The gain medium was a 1-m Yb-doped PM fiber (Nufern. PM-YSF-HI-6/125). After that, the amplified laser was traveled to an AOM to reduce the repetition rate. The laser was then propagated in a 1300-m long single mode PM fiber to stretch the pulse duration.
After stretching, the pulses were further amplified in a series of Yb-doped power amplifiers. The gain medium of the preamp II was a 1-m PM-YSF. In the preamp III, a 2-m long double-cladding PM Yb-doped fiber (PLMA-YDF-20/130-VIII) was used as the gain medium with a cladding absorption coefficient of 10.2 dB/m at 976 nm. At this stage, the fiber was coiled to a radius of 5 cm to maintain fundamental mode operation and was cut at an angle of 8° at the output facet to avoid Fresnel reflection. In the main amplifier, a 1.2-m-long Yb-doped TCF (Nufern) with 100/400/480 µm core-clad diameter, 0.11/0.22/0.46 NA, and approximately 26.3 dB/m cladding absorption at 976 nm was used. The gain fiber was backward pumped by a wavelength-stabilized 140-W laser diode.
Finally, the pulses were injected into a transmission grating pair (LightSmyth.T-1600-1030s-130X20-94) with a line density of 1600 lines/mm. Considering the introduced group delay dispersion (GDD) was 29.9 ps2, the distance between the two gratings was optimizing to 52 cm.
The temporal and spectral profiles of the output pulses were monitored by an ultrafast photodetector with a 25 GHz digital oscilloscope (Agilent DSO-X92504A) and a spectrum analyzer (Yokogawa, AQ6370D) with a resolution of 0.02 nm. The pulse durations were measured by an autocorrelator (APE, GmbH).
3. Results and discussion
3.1 All-fiber front end
The performance of the dispersion-managed mode-locked laser which consists of PM fibers and components was characterized. As shown in Fig. 4(a), the oscillator spectral bandwidth was 13 nm at the central wavelength of 1032 nm. The pulse energy was on the order of picojoules-level at a pulse repetition rate of 45 MHz. The pulse duration was measured to be 1.5 ps, as shown in Fig. 4(b).
Fig. 4. (a) The spectrum of the mode-locked fiber oscillator; (b) The autocorrelation trace of the mode-locked fiber oscillator; (c) The spectrum after the stretcher and III amplifier; (d) The pulse duration after the stretcher and III amplifier.
The pulses from the oscillator were firstly amplified in preamp I. The seed was amplified to 37 mW at the pump power of 330 mW. After passing through an AOM, the repetition rate was reduced to 1 MHz, corresponding to an average power of 0.8 mW. In order to reduce the influence of nonlinear effects in the amplification stages, the pulses were stretched to 669 ps by propagating through a 1300-m long single-mode PM fiber (Nufern PM 980-XP, β2 = 0.023 ps2/m). The GDD introduced by the stretcher was 29.9 ps2. Due to SPM accumulation, the spectra bandwidth was broadened to 25 nm. The spectrum and pulse duration after stretcher are shown in Fig. 4(c) and Fig. 4(d).
After stretching, the pulses were amplified to 82 mW, corresponding to pulse energy of 82 nJ, at pump power of 360 mW in preamp II. In preamp III, the maximum average power was limited to 10 W for avoiding strong nonlinear effects, although higher average power could be obtained. As shown in Fig. 4(c), the 3-dB spectrum bandwidth was 9.9 nm and the pulse duration was 551 ps (see Fig. 4(d)) due to gain narrowing effect.
3.2 Main amplifier
In the main amplifier, high pump absorption of 26.3 dB/m@976 nm of TCF results in heavy heat accumulation. Considering the influence of nonlinear effects and thermal effects on the stability of the system, the theoretical design and analysis of the fiber length and heat generation were carried out. Firstly, the output power and residual power as a function of fiber length was simulated.
As shown in Fig. 5(a), the average power gradually increase as fiber lengthens from 0.7 m to 1.4 m, while the residual pump power gradually decreases. The amplified signal power increases from 560 W to 720 W, and the residual pump power decreases from 200 W to 10 W. The increasement in fiber length allows efficient absorption of pump light, resulting in higher output power than short fiber length. When fiber length reaches 1.2 m, the amplified power and the residual pump power tend to be flat. In addition, thermal load as a function of gain fiber length at the pump power of 800 W was simulated, as shown in Fig. 5(b). When the output power was 600 W, the heat accumulation at the end face of the fiber reached 150 W/m. Combined with the above simulation results, a 1.2-m active fiber was used to minimize the nonlinear effect and maximize the efficiency. The fiber was spliced with an end cap to improve damage threshold and a water-cooling device was designed according to the spliced end cap, as shown in Fig. 6(a). The fiber was mounted inside a water pipe, and cooled by circulated cooling water at 16 °C. The thermography camera recorded a maximum temperature of 27.2 °C on the end cap surface at the highest pump power, shown in Fig. 6(b).
Fig. 5. (a) The output power and residual power as a function of fiber length at the pump power of 800 W; (b) Thermal load as a function of gain fiber length, inset: zoomed figure at thermal load of 150 W/m.
As shown in Fig. 7(a), the output power increases linearly with a light conversion efficiency of 74.2% and the highest average power of 620 W was obtained at pump power of 836 W. The output average power can be further increased with the increasement of pump power. Although the triple-clad fiber is a non-PM fiber, the polarization extinction ratio is still better than 12.5 dB at the maximum output power. The amplified beam quality was measured, as shown in Fig. 7(b). Due to the large-mode-field area, the fiber allows muti-transverse modes transmission with an output beam quality factor of 20 and 21.5 along two orthogonal axes, making it a promising laser source for laser processing with high efficiency and high standard quality, such as water-jet guided laser source. In the experiment, we have not observed transverse mode instability, but rather slight beam jitter caused by environmental perturbations. The amplified beam profile was recorded by a camera (Pyrocam III) for 10 seconds (see Visualization 1). The amplified pulses were detected by high-speed photodetector (Newport 818-BB-35) and high-speed oscilloscope, as shown in Fig. 7(c). The amplitudes of two adjacent pulses are slightly different, indicating that the laser possesses a relatively good stability. The final output spectrum with a bandwidth of 7.4 nm at a center wavelength of 1032.2 nm was recorded by optical spectrum analyzer (wavelength resolution: 0.02 nm), as shown in Fig. 7(d). This spectrum bandwidth enables pulse compression to 212 fs according to Fourier transform limit. Nevertheless, nonlinearity and third-order dispersion accumulated in the main amplifier and the fiber stretcher cannot be compensated by using a pair of gratings.
Fig. 7. (a) The output power of the main amplifier versus pump power; (b) The beam quality factor (M2) after the main amplifier; (c) Oscilloscope trace of pulse trains at 1 MHz repetition rate; (d) The measured spectrum after the main amplifier.
3.3 Compressor
The output pulses from the main amplifier were compressed by a transmission grating pair. As shown in Fig. 8(a), the autocorrelation trace measured by APE autocorrelator was fitted with sech2 profile and the pulse duration was 7.6 ps by optimizing the distance between the two gratings. The pulses experienced GVD and nonlinearity in 1300-m fiber stretcher without loss and gain, and then were amplified to an average power of 10 W with a Gaussian-shaped spectrum affected by gain-narrowing effect. In the main amplifier, the pulses accumulated nonlinear phase shift and the calculated B-integral was 6.74. Due to the contributions of accumulated nonlinearity and third-order dispersion, the amplified pulses can only be compressed to 7.6 ps with slight pedestal by using a transmission grating pair. In the following work, a chirped fiber Bragg gratings and chirped volume Bragg gratings will be employed to stretch and compress the pulses, which will further reduce the pulse duration and increase the pulse peak power. After compression, an average power of 423 W was achieved at the input power of 620 W, as shown in Fig. 8(b), corresponding to a compression efficiency of 68.23%, and a peak power of 55.7 MW.
Fig. 8. (a) The autocorrelation trance of the pulse after the compression at average power of 423W; (b) The output power of compressor versus the increase of input power.
4. Summary
In conclusion, we demonstrated a high-average-power picosecond CPA system with 0.4-mJ pulse energy at 1-MHz repetition rate based on an XLMA TCF. At the pump power of 836 W, the maximum output power of 620 W was achieved, corresponding to an amplification slope efficiency of 74.2%. After compression, the maximum average power of 423 W with a pulse duration of 7.6 ps was obtained, corresponding to a peak power of 55.7 MW. This paper simultaneously realizes high average power and high pulse energy by using TCF, laying the foundation for further amplification.
Funding
National Natural Science Foundation of China (62105009, 62035002); National Key Research and Development Program of China (2017YFB0405201).
Disclosures
The authors declare no conflicts of interest.
Data Availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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