Monolithic Yb-Doped Femtosecond Fiber Laser With >300 W Average Power

In this submission, we report a maximum 638-W fiberized chirped-pulse-amplification (CPA) laser system with a bulk reflection grating compressor. The system is able to emit 338-W compressed pulses of 624 fs centered at 1 µm, corresponding to a pulse energy of 1.3 µJ at a pulse repetition rate of 258 MHz. The output beam quality factor M2 is measured to be <1.3, without observing transverse-mode instability. The standard deviation of the power fluctuation is quantified to be 0.6 W over a time span of more than 1 hour, at an output power of up to 100 W.


I. INTRODUCTION
I N THE past two decades, high-power, high-repetition-rate (HRR) femtosecond (fs) fiber lasers hold great importance in a variety of scientific and industrial applications [1], [2], [3], [4]. For instance, a HRR fs laser source with µJ-level pulse energy can dramatically improve the signal-to-noise ratio (SNR) of strong-field applications by increasing the data acquisition rate [5], [6]. Frequency comb generation in vacuum and extreme ultraviolet (VUV/XUV) spectral regions, as well as in mid-infrared regions, could also be obtained from high-power fs fiber laser, employing noble gas-filled enhancement cavity and difference-frequency-generation (DFG) technology [7], [8], [9], [10], [11], [12]. Material processing using high-power HRR burst ultrashort pulse has opened a new regime of micromachining, named ablation-cooled regime [13], [14], in which fs pulses produce precise material ablation while avoiding detrimental heat accumulation, leading to minimize thermal damage to adjacent material. Generally, the most common way to build a high-power HRR fs fiber laser is to employ master-oscillatorpower-amplifier (MOPA) architecture; that is, a mode-locked fiber oscillator provides picosecond (ps) seeding pulses, and then a following fiber-based power amplifier is applied to boost both average power and pulse energy of the laser system. In the final step, a grating-based compressor is used to compensate the pulse frequency chirp, leading to fs pulse with high peak power. Directly amplifying fs-level pulse in a power amplifier results in strong nonlinear effects due to the rapidly increased peak power during the amplification, which eventually causes pulse degradation or material damage. To solve this problem, chirped-pulse amplification (CPA) was introduced to circumvent deleterious nonlinearities in power amplifiers [15]. More specifically, the seeding pulses are linearly stretched to nanosecond (ns) in duration prior to the fiber amplifier such that nonlinear effects are significantly alleviated and the pulse is linearly amplified. Finally, the amplified pulses are compressed using a pulse compressor composed of either a Treacy type grating pair or Chirped Volume Bragg Grating (CVBG). Conventional rare-earth-doped fiber amplifiers usually have a core diameter in the range of 4-12 µm, and thus a fiber CPA system usually produces fs pulses with tens to hundreds of nJ energy and several to several tens of watt average power [16], [17], [18]. Such fs fiber laser is suitable for applications in ophthalmology [19], [20], [21]. The emergence of large-mode-area (LMA) Yb-fibers enables power scaling of fs fiber-CPA systems to kilowatt (kW) regime [22], [23], [24], [25].
High-power fs fiber lasers are the perfect candidate for industrial applications in a challenged work environment. To date, the majority of high-power fs fiber lasers employs spatial coupling schemes in the final stage of the fiber power amplifier. Such scheme avoids the difficulty to perform mode-matching and fusion splicing between fibers with different core-sizes. Spatial coupling makes mode-matching much easier between different This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ Fig. 1. Schematic of high-power, high-rep-rate fiber laser system. LD, laser diode, WDM, wavelength-division-multiplexer; YDF, ytterbium-doped fiber; Col, collimator; HWP, half-wave plate; QWP, quarter-wave plate; PBS, polarized beam splitter; ISO, isolator; HR, highly-reflective mirror; PM, polarization-maintaining; CIR, circulator; BPF, band-pass filter; CPS, cladding power stripper; HP, high power; OSA, optical spectrum analyzer; AUTO, auto-correlator. types of fiber through using a combination of a few spherical lenses. On the other hand, once fusion splice is engaged between passive and gain LMA fibers, the splicing quality significantly affects the performance of power amplifier. Especially for the kW-level Yb-fiber laser systems, the accumulated heat may severely impact on spliced interface, resulting in either fiber breakdown or bullet-shaped core damage backward propagating inside the core, known as fiber fuse effect [26], [27], [28].
However, an all-fiber integration format is more favored since the light wave is confined inside the fiber, naturally immune from environmental disturbance. Moreover, a monolithic fs fiber laser without any spatial coupling between cascaded amplifiers could significantly improve the stability of laser running performance.
In this article, we demonstrate a monolithic high-power HRR Yb-fiber laser system, producing a maximum 638 W average power at a pulse repetition-rate of 258 MHz. The laser has a center wavelength of 1050 nm and de-chirped pulse duration of 624 fs. The obtained maximum pulse energy is 1.3 µJ, corresponding to peak power around 2 MW.

II. EXPERIMENTAL SETUP
The experimental setup of the 638-W Yb fiber laser system is illustrated in Fig. 1. The laser system mainly consists of a mode-locked fiber oscillator providing seed pulses, a Chirped Fiber Bragg Grating (CFBG) utilized as pulse stretcher, two stages of polarization-maintaining (PM) fiber pre-amplifiers, a Yb-doped LMA main amplifier, and a reflection type dielectric grating-pair served as pulse compressor.
The home-built HRR fiber ring oscillator employs a 20cm long highly doped Yb-fiber (LIEKKI Yb1200-4/125) as gain medium, a 21-cm single-mode fiber (SMF, HI1060), and 46-cm spatial structure to form a resonant cavity. A wavelengthstabilized 976-nm laser diode with a maximum output power of 750 mW is used to pump the oscillator through a wavelengthdivision-multiplexer (WDM). A pair of reflective gold gratings (600 line/mm groove density) is used to manage the cavity dispersion, giving a net dispersion of around zero. The oscillator is mode-locked based on nonlinear polarization rotation with a repetition rate of 258 MHz, and operated at the stretched-pulse regime, where the oscillator exhibits the broadest mode-locked spectrum with lowest intensity-and phase-noise [29], [30]. In order to isolate the oscillator from environmental perturbations (airflow, vibration, etc.), it is mounted on a 35-mm thick breadboard which is fully-covered with a plexiglass panel optical enclosure, and a vibration absorbing mat is put underneath.
The output pulse from the fiber oscillator is coupled into PM980 fiber for subsequent stretcher and amplifiers. A spatial isolator and a half-wave plate (HWP) are put in between to prevent back reflection and align the pulse polarization along the slow axis of the PM fiber. A piece of PM fiber coupler with 99/1 coupling ratio is utilized to extract 1% of the seed pulse for monitoring the oscillator's power and spectrum, while the rest 99% of the power is fed into following stretcher and amplifiers. Then, a four-port PM fiber circulator and a set of CFBG are combined together to form a pulse stretcher. Double-pass configuration is employed in the stretcher, and the stretched pulse duration is estimated to be ∼2 ns. We use a band-pass filter to discard the spectrum located outside of CFBG reflection band to improve signal pulse SNR.
After the pulse stretcher, three stages of Yb-doped fiber amplifiers are used to scale up the laser power. The first-stage single-mode fiber amplifier is used to compensate the signal loss from the CFBG stretcher. A 60-cm long Yb-doped fiber with a core diameter of 6 µm is used as gain fiber, which is forward core-pumped by a 950-mW wavelength-stabilized 976-nm LD. One percent of power is used for characterization and monitoring. The second-stage amplifier consists of a pump beam combiner, a 500-cm long Yb-doped double-cladding gain fiber (DCF) with a 10-µm core diameter, a high-power fiber isolator, and a cladding power stripper (CPS). A beam combination of two fiber-coupled 18-W 976-nm multimode LDs constitutes the pump. The pump beam combiner also serves as a mode-adaptor between two different core-size fibers (6 µm versus 10 µm). In-band back reflection is avoided by high-power fiber-pigtailed isolators placed between amplifiers. The fiber employed in the first-and second-stage amplifier are all PM ones, which are used to exempt NPR induced nonlinear phase by preserving polarization state of amplified pulses. An extra CPS is employed at the end of 2nd amplifier to remove residual pump power. The core diameter of the CPS is carefully chosen to be 20 µm in order to match the core size of subsequent power amplifier fiber. Careful splicing and mode-matching are performed between HP-ISO-output 10/125 and CPS-input 20/400 fibers. The main amplifier at the last stage is standing on a water-cooled cold plate (60 × 60 cm), including a commercially available 790-cm long 20/400 Yb-doped DCF, a (6+1) ×1 high-power pump beam combiner, 4 fiber coupled 250-W 976-nm multi-mode LDs, and a 300-W power level CPS (HP-CPS). In order to guide the pump light inside the cladding of 20/400 LMA fiber, both of the spliced points are re-coated with UV-cured low-refractive-index material. Accumulated heat resulting from residual pump is dissipated by direct water circulation through HP-CPS mental jacket. An 8-degree end cap made of core-less fiber is spliced to the end facet of gain fiber. The gain fiber is coiled to a 20-cm diameter circle to increase higher-order mode loss, preserving the fundamental mode of the beam. Finally, the output pulse of the main amplifier is de-chirped by a pair of gratings which have a double-pass configuration. Both of the gratings have a groove density of 1740 line/mm, and are vertical polarization sensitive. Several wedges and mirrors are used in the setup to couple a small portion of a high-power laser pulses for the following diagnosis.

III. RESULTS AND DISCUSSION
The HRR mode-locked fiber oscillator has an average power of 200 mW and the optical spectrum is centered at 1035 nm with a Full-Width-Half-Maximum (FWHM) of 6 nm, shown as the blue curve in Fig. 2(a). Fig. 2(b) shows the stretched pulse detected by an InGaAs photo-detector (EOT, ET3000A) and recorded by a digital oscilloscope (Keysight, DSOX6004A, 2.5 GHz bandwidth). The pulse train exhibits a temporal period of 3.88 ns, corresponding to a fundamental repetition rate of 258 MHz. The inset of Fig. 2(b) illustrates the recorded radiofrequency (RF) spectrum with ∼80 dB SNR, confirming a stable fundamental mode-locking. The output pulse from oscillator is measured with an auto-correlator (APE, pulse-check) and the duration is measured to be 1.82 ps, as illustrated in Fig. 2(c). The pulse is subsequently stretched to 2.1 ns by the CFBG stretcher (TeraXion, TPSR, 135 ps 2 ) and then measured by a fast photodetector (Newport, Model 1014) together with a high-speed digital oscilloscope (Tektronix, DPO73304DX). The temporal profile of stretched pulse is depicted in Fig. 2(d).
Due to the 10-nm bandwidth of CFBG and the insertion loss of the 4-port fiber circulator, only 9-mW average power of seed pulse is left for the following core-pumped amplifier. The stretched pulse spectrum is plotted in green curve shown in Fig. 3(a). After the 1st stage amplifier, output power is scaled up to 610 mW. The amplified spectrum is plotted as the light blue curve in Fig. 3(b). Optical spectra before and after the amplifier exhibit similar shapes, confirming the linear pulse amplification process without obvious optical nonlinearities. A double-cladding pump scheme is employed in the 2nd stage amplifier, and output power is further increased to 22.3 W when the 976-nm pump diode delivers 36-W power to the gain fiber. Residual pump power is measured to be 1.8 W, and separated by a dichroic mirror standing at the end of 2nd stage amplifier. The red curve depicted in Fig. 3(b) shows the amplified spectrum of 2nd amplifier. Obviously more amplification takes place on the red part rather than blue one, which is mainly resulted from the signal re-absorption due to the relatively long gain fiber length (500 cm). Compared with the previous 1st amplified signal spectrum, the compensation of intensity on the longer wavelength part provides us more bandwidth for the following power amplifier. While the CPS can be utilized to remove residual pump power from 2nd amplifier, it can also be used to filter out the backward amplified spontaneous emission (ASE) light generated from the main power amplifier, protecting the front end of the laser system.  A maximum output power of 638 W is achieved in the main amplifier. As described in Fig. 4(a) , the red curve shows the final output spectrum in the logarithmic scale. A distinctly narrowed spectrum bandwidth from 10 nm to 3.5 nm indicates a severe gain-narrowing effect. Furthermore, the re-absorption effect also makes a contribution to the narrowed spectrum. Four 250-W LDs are beam-combined to pump the 20/400 LMA gain fiber. We measure the output power as a function of increasing pump power [blue triangles in Fig. 4(b)]. It is challenging to use single line-fitting slope efficiency to evaluate the main amplifier because the pump wavelength is slowly shifted toward a longer wavelength as the pump power increased. This pump wavelength drift is determined by both applied pump current and LD temperature, with a total drift wavelength range of 16 nm (from 962 to 978 nm). This pump drift phenomenon is commonly observed in the 100-W class 976-nm pump LDs, which can be alleviated through wavelength stabilization using Volume Bragg Grating. The pump wavelength as a function of increasing pump power is also marked in red squares in Fig. 4(b). The slope becomes larger as the pump wavelength approaches 976 nm, indicating the amplifier becomes more efficient. This mainly attributes to the fact that Yb-fiber absorption peak locates at around 976 nm. As we launch full pump power, 638 W signal power is obtained, corresponding to optical-to-optical efficiency of ∼64%.
A pair of reflective dielectric gratings and a roof mirror are utilized to form a Treacy-type pulse compressor. Diffraction efficiency of single grating is measured to be ∼97% for vertical polarization. The measured compressed signal power is presented as blue circles in Fig. 4(b). In the experiment, we obtain 338-W compressed pulses at the maximal pump power, corresponding to 53% compression efficiency. Considering the 4-pass configuration efficiency of 88%, the extra 35% compression loss is mainly due to the polarization degradation inside the main fiber amplifier. In order to maximize the compression efficiency, we tune the incident angle slightly away from the Littrow angle. 63 degrees incident angle is used in the experiment. The grating pair has a vertical distance of ∼ 47 cm, providing total amount of −134 ps 2 group delay dispersion (GDD) compensation. We also adjust the GDD of CFBG through the temperature control array to get a optimized compression quality pulse.
The auto-correlation (AC) trace measured by a commercial auto-correlator is illustrated in Fig. 5(a). The shortest pulse is 624 fs with a deconvolution factor of 1.41 (Gaussian assumption). We also numerically calculate the spectrum-supported Fourier-transform-limited (FTL) pulse, and plot it in the blue curve in Fig. 5(a). The compressed pulse duration is nearly two times of FTL pulse duration. This is probably due to the nonlinear chirp induced by gain-narrowing effect in the final stage power amplifier. As shown in Fig. 5(b), the beam quality M 2 is measured to be 1.28 and 1.26 for horizontal and vertical directions, respectively. The inset of Fig. 5(b) shows the beam profile recorded by a CCD camera. Transverse-mode instability is not observed at all power level. We also measure the power stability of the laser system, and the results are plotted in Fig. 5(c). When the system operates at 100 W, the standard deviation of power fluctuation is measured to be 0.6 W over one hour recording.

IV. CONCLUSION
In summary, we demonstrate a monolithic HRR Yb-doped fiber laser system that delivers up to 638 W amplified pulses centered at 1050 nm. These pulses are compressed to 624 fs in duration with a good beam quality through fine adjustment of grating pair and out-coupling lens. Further power scaling to kW-level is reachable since no observation of detrimental nonlinear effect (such as fuse effect) in the main amplifier during the experiment. Such a high power HRR laser system holds great potential for many applications, such as material processing of carbon fiber reinforced polymers and generation of high power mid-infrared pulses.