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Abstract
In this work we presented a compact femtosecond laser system based on Yb doped fiber seed laser and efficient Yb:YAG crystal rod power amplifier. Matched pair of chirped fiber Bragg grating stretcher and chirped volume Bragg grating compressor were used to obtain high fidelity - Strehl ratio 76%, pulses of 764 fs duration, 104 µJ energy at 200 kHz repetition rate at the output of the laser system.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Sub-picosecond high peak power laser systems are used in many fields of science and industry, e.g. material processing, ultrafast spectroscopy, nonlinear imaging, etc [1–3]. Ytterbium (Yb) doped laser crystals (e.g. Yb:YAG, Yb:CaF2, Yb:KYW) possess some highly desirable properties, like low quantum defect, broad amplification bandwidth, low excited state absorption, for efficient generation of ultrashort pulses. However, direct amplification of ultrashort pulses to high peak power is limited by various nonlinear effects (Kerr lensing, self-focusing, self-phase modulation, Raman scattering, etc.). Chirped pulse amplification (CPA) is a common method to significantly reduce these effects. In a CPA scheme, pulses are stretched, amplified and then re-compressed to achieve high peak power transform-limited pulses at the output and to significantly reduce peak power in the amplification medium [4].
Diffraction grating compressor was the most common choice in ultrashort pulse CPA systems for three decades [4–7]. Diffraction grating compressor has many desirable properties, e.g. large and easily variable dispersion, inherently smooth dispersion profile, simple geometry. It is also the method of choice for compressing pulses to the record high peak powers due to relatively straightforward scaling of the gratings size [8]. When pulses are chirped to hundreds of picoseconds or longer pulse duration, diffraction grating based pulse compressors become bulky, which is disadvantageous for industrial applications. The diffraction grating compressors require precise alignment to achieve good spatial and temporal properties of the output pulses [9,10]. Therefore long-term opto-mechanical stability of the compressor becomes critical. Compression instabilities may be especially problematic in industrial environment, where thermal and mechanical perturbations may compromise precise alignment of the system.
During the last decade implementation of chirped volume Bragg gratings (CVBG) in ultrafast laser systems evolved [11–14]. Improvements in holographic recording of the gratings as well as in homogeneity of photorefractive glass allowed to demonstrate high compression efficiency while also preserving good beam quality [11,12]. Moreover, compressor based on CVBG is very compact, simple to align and much less sensitive to misalignment as compared to diffraction grating compressor. These properties are very desirable for industrial ultrafast lasers.
CVBG design allows realization of both stretcher and compressor using the same grating by reflecting laser pulse before and after amplification from the opposite sides of CVBG [14–16]. This configuration is convenient in all-solid-state laser system due to typically low amplifier chain dispersion which does not significantly affect the amplified and compressed pulse duration if it is left uncompensated. It is not the case for fiber lasers which are increasingly common as front-ends of ultrashort pulse solid-state lasers [17,18]. In such configuration fiber dispersion cannot be neglected and additional compensation element is required [11]. Chirped fiber Bragg grating (CFBG) stretcher with CVBG compressor is a very attractive combination to perform this task due to the possibility to precisely compensate dispersion of the system as well as to conveniently perform the tuning of CFBG dispersion. This configuration was demonstrated earlier in fiber laser systems [17–20]. However, pulse energy and therefore peak power from fiber lasers is limited by relatively small mode area of single mode fibers. Higher peak power pulses can be obtained using free-space solid state amplifiers and this stimulated us to adapt combination of fiber-optic stretcher and bulk compressor for hybrid Yb ions amplification system.
In this work, we present the application of CFBG stretcher and CVBG compressor configuration in sub-picosecond laser system based on fiber laser front-end and solid state Yb:YAG final amplifier.
2. Experimental setup and results
Developed CPA laser system layout is shown in Fig. 1. The all-in-fiber seed source was based on the architecture presented in our previous work [19,20]. It consisted of SESAM based fiber oscillator, generating 1 ps duration pulses with spectral bandwidth of 1.55 nm at full width at half maximum (FWHM) which are then linearly up-chirped to 4 ps and spectral bandwidth broadened to 3.6 nm at FWHM. The seed laser pulses were further chirped to 220 ps duration using CFBG stretcher with group velocity dispersion (GVD) of β2 = −33.7 ps2. Additional fiber amplifier was used after fiber-coupled acousto-optic modulator (AOM) to increase the output energy, generating 2.3 μJ, 200 kHz repetition rate laser pulses at 1030 nm central wavelength and with spectral bandwidth of 3.6 nm at FWHM. The output beam from the seed laser was collimated to be of 1 mm diameter (at 1/e2 intensity level). The beam shape was Gaussian with beam quality factor (M2) close to 1.
Fig. 1 Layout of the developed laser system consisted of all-in-fiber seed source, free-space Yb:YAG amplifier and pulse compressor. HWP1, HWP2 – half-wave phase retardation plates, FR – Faraday rotator, M1, M2 – mirrors, SP – pump/signal wavelength separator, LD – laser diode output beam collimator, P1, P2, P3 – polarizers, SM – spherical mirror, CFBG – chirped fiber Bragg grating, CVBG – chirped volume Bragg grating, QWP – quarter-wave phase retardation plate.
The input and output beams of the double-pass free-space amplifier was decoupled via polarization using half-wave plate (HWP1), Faraday rotator (FR) and a pair of polarizers (P1 and P2). In the amplifier Yb:YAG crystal rod (Yb3+ concentration 3.6%, rod length 12 mm) pumped longitudinally by a continuous wave laser diode (LD) was used. LD radiation was coupled to 105 μm core diameter and 0.22 numerical aperture (NA) fiber. Maximum laser diode power was 120 W at 940 nm central wavelength and 4 nm bandwidth (FWHM). Seed and pump beams were focused to respectively 0.36 mm and 0.33 mm diameter overlapped spots in the amplifier crystal. Second pass was realized by back reflecting amplified signal by concave spherical mirror (SM) with 100 mm radius of curvature. SM was placed at 120 mm distance from the amplifier crystal to compensate pump induced thermal lens.
After double-pass through amplifier crystal seed was amplified from 460 mW to average output power of 24 W at 200 kHz pulse repetition rate (120 µJ pulse energy). For 84 W pump power it corresponded to 28% amplifier efficiency. Amplifier output power and total gain dependences on input seed power are plotted in Fig. 2. In tested setup high gain saturation level was attained – 1% change of input power transforms to 0.13% output power change when the amplifier is seeded with average power higher than 0.3 W. Such level of saturation is beneficial for overall power stability of the system as it becomes less sensitive to seed power fluctuations. On the other hand, at low seed power, 40 dB double pass gain was achieved. There is a potential possibility to obtain pulses of multi-mJ energy from this configuration at lower repetition rates when seeded with µJ level pulses from the fiber laser. Realization of this potential is planned in our future work.
Fig. 2 Average output power (black curve, left axis) and total amplifier gain (red curve, right axis) of double-pass Yb:YAG amplifier versus seed power. Pulse repetition rate was 200 kHz. Amplifier was pumped by 84 W power.
At maximum seed power spectral bandwidth after amplification in Yb:YAG crystal was reduced from 3.6 nm to 2.2 nm at FWHM due to gain narrowing effect. Consequently chirped pulse duration was reduced from 220 ps down to 150 ps as measured by 20 GHz bandwidth oscilloscope and photodetector with 35 ps response time. Measured spectrum of chirped pulses before and after amplification at maximum output power are shown in Fig. 3.
Fig. 3 Spectrum of the chirped seed pulses (black line) and spectrum of the chirped pulses after double-pass Yb:YAG amplifier (red line). Inset: the same spectra in logarithmic scale.
The initial spectral modulation as seen from Fig. 3 could arise from group delay ripples of the CFBG stretcher or from the internal reflections in micro-optical components used in all-in-fiber seed source [20]. It was previously shown that small initial spectral modulation leads to the appearance of small amplitude (~1%) satellite pulses in the range of 3-5 ps from the main compressed pulse. The spacing of these pulses is related to the modulation frequency of the spectrum. However, overall temporal quality of the compressed pulse does not degrade significantly in this case [21,22].
At the output of the laser, beside the laser radiation bunched in ultrashort pulses, there unavoidably is a certain level of continuous wave (CW) amplified spontaneous emission (ASE) coming from the pumped amplifier stages. This CW background can be relatively separated into two components: one that comes from the Yb-doped fiber amplifiers and another added by the ASE from pumped Yb:YAG crystal. ASE level from the fiber laser can be estimated from the broad spectral shoulders visible in the logarithmic scale (inset of Fig. 3). By smoothly extrapolating these shoulders one can obtain ~30 dB ASE level at the central 1030 nm wavelength. Because there is no temporal gating in this realization of the laser system, both CW background and pulsed radiation will be similarly amplified in Yb:YAG amplifier, resulting in ~30 mW of amplified background. The CW background that results from Yb:YAG amplifier was not directly measured but the upper bound can be estimated from Fig. 2: when the input seed power is very small (1 μW), total gain reaches ~40 dB, which corresponds to the output power of ~10 mW. The large part of this output power is still the actual amplified signal. Furthermore, at high input seed power, output power after amplifier is 24 W, resulting in considerable Yb3+ upper-state depletion and reducing ASE. Conservatively one can estimate that ASE from Yb:YAG amplifier is <1 mW at full output power, insignificant amount compared to the ASE from the fiber amplifiers.
Beam quality after amplification deteriorated (M2 ~1.3) due to thermal lens aberrations so we used spatial filtering to obtain diffraction limited beam quality. Spatial filtering was performed using an aperture near focal plane of the focusing lens. Aperture size was chosen so that only the central lobe was transmitted. Spatial filter resulted in ~3% power loss of the amplified beam. After spatial filter beam quality parameter M2 was measured using standard z-scan technique focusing the beam by a positive lens of well-defined focal length and tracing the beam radius change along propagation direction. Best fit yielded M2~1.0 indicating a diffraction limited beam quality. Measurement graph is shown in Fig. 4.
Fig. 4 Beam radius measurement along propagation direction of focused spatially filtered amplified beam (red and black dots). Beam quality parameter was estimated to be M2~1.0 from the fit (red and black curves). Images of the beam profile at several positions are shown as insets (beam size was normalized for clarity).
After amplification and spatial filtering chirped pulses were compressed by CVBG compressor. Output beam was separated from the input by using a polarizer (P3) and a quarter-wave plate (QWP), as shown in Fig. 1. The efficiency of CVBG compressor was 87%, yielding 20.8 W of output power and 104 μJ pulse energy.
Dispersion of the CFBG stretcher was matched to the dispersion of the CVBG compressor accounting for the dispersion of ~40 m of fiber in the seed laser. Residual dispersion could be compensated by applying thermal gradient on the CFBG (for more details see [19,20]). Optimally compressed pulses were characterized by second harmonic generation frequency-resolved optical grating (SHG-FROG) autocorrelation method. The pulse duration retrieved by FROG algorithm (Swamp Optics) using a 1024x1024 grid was 764 fs (Fig. 5). Transform-limited pulse duration derived from output spectrum was 644 fs. Mismatch between measured and transform-limited pulses could be attributed to the accumulated nonlinear phase in the single mode fiber seed laser due to small mode area (~95 µm2) of the doped fiber. A residual phase retrieved from FROG was ~2 rad in the time range from −0.7 ps to 2 ps encompassing 94% of total pulse energy and ~6 rad in the time range from −2 ps to 2 ps encompassing 98% of total pulse energy. Strehl ratio of the compressed pulse, defined as the ratio of actual peak power of the pulse to the one of the transform limited pulse, was 76%.
Fig. 5 Envelope of the compressed pulses retrieved from SHG FROG measurement compared to transform-limited pulse shape calculated from the measured spectrum (left axis) and retrieved temporal phase (right axis). Inset: measured and retrieved FROG traces (0.15% retrieval error).
Beam quality after CVBG compressor was characterized by the same z-scan technique of focused beam (Fig. 6). Slight irregularities in the beam profile were observed near focal position, origin of which is not currently clear. Overall beam quality was good with estimated beam quality parameter M2 <1.2.
Fig. 6 Beam radius measurement along propagation direction after CVBG compressor. Z-scan graph of amplified beam after CVBG compressor (red and black dots). Beam quality parameter M2<1.2 was estimated from the fit (red and black curves). Images of the beam profile at a few positions are shown as insets (beam size was normalized for clarity).
3. Summary
In this work a compact ultrafast laser system based on fiber seed laser and Yb:YAG crystal rod power amplifier was developed. The key novelty of the system was application of CFBG stretcher and CVBG compressor with matched dispersion which enabled to obtain nearly bandwidth-limited compressed sub-picosecond pulses. Presented laser system produces pulses of 764 fs duration and 104 μJ energy at 20.8 W average power with perfect nearly diffraction-limited beam profile. Proposed laser architecture enables to construct truly compact and robust high energy ultrashort pulse lasers for wide range of applications.
Funding
Research Council of Lithuania (LAT-10/2016).
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