We report on a picosecond two-stage double-pass chirped pulse amplifier based on a low doping level Yb:YAG rods. After compression, it provides output pulses with a pulsewidth of 1.15 ps and an energy of more than 20 mJ at a repetition rate of 100 Hz with a beam quality of M2 ∼1.05. These pulses were frequency doubled in a two-cascaded second harmonic converter based on LBO and BBO crystals with an output energy of 12 mJ and 5 mJ at 515 nm, suitable for simultaneously pumping OPCPA cascades.
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Lasers with high peak and average powers delivering short pulses of the order of a few picoseconds are in demand for material processing , pumping of optical parametric chirped pulse amplifiers (OPCPA) [2,3], ultrafast spectroscopy , or as a direct pumping source for XUV lasers . Chirped pulse amplification (CPA)  is a common method for significantly reducing the associated nonlinear effects when amplifying ultrashort laser pulses to a high peak power. Among readily accessible laser media with a wide bandwidth, Yb3+-doped materials are the most promising, since a low quantum defect reduces the fraction of the pump power transferred to heat. Since highly ordered Yb:YAG crystals have not only good thermal conductivity, but also a high gain when pumped with high-brightness laser diodes, they have become the most popular material for lasers with high peak and average powers of ps and sub-ps pulsewidth range.
There are several approaches to creating compact high-energy ps and sub-ps laser sources with excellent beam quality, as well as good long-term and short-term stability. Fiber amplification [7,8] has no competitors in terms of stability, reliability and beam quality, but cannot achieve high peak power due to nonlinear effects. The geometry of thin Yb:YAG disks makes it possible to overcome both thermal and nonlinear optical effects and obtain high average and peak power. Indeed, Yb:YAG thin disk regenerative amplifiers provide pulse energies of ∼20–200 mJ with a pulsewidth after compression of less than 2 ps at a repetition rates of ≥ 1 kHz [9–15] and even of 500 mJ with a pulsewidth of 2 ps at 100 Hz  using an additional ring amplifier, although such lasers are expensive due to the complexity of manufacturing and alignment. Yb:YAG Innoslab amplifiers demonstrated a high average power of 1.1 kW  with a pulsewidth of 615 fs, while an increase in pulse energy of up to 54 mJ was achieved [18,19] with an average power of ∼0.5 kW. However, the high manufacturing costs and difficulties in aligning multi-pass slabs, as well as in suppressing ASE and parasitic generation, hinder the wider use of such schemes. An alternative approach is the use of single crystal fibers (SCF), providing an average power of 160 W  or a pulse energy of several mJ [21,22]. In fact, the high surface-to-volume ratio in SCF provide good thermal management, however waveguide propagation in Yb:YAG does not provide a significant advantage . The concept of thin tapered Yb:YAG rods [24,25] looks promising, since it allows one to increase efficiency due to homogenization of the pump intensity, especially if this allows the use of low brightness pump diodes. Nevertheless, the energy of the output pulses in SCF with an aperture of about 1 mm is limited to a few mJ due to the damage of AR coatings. Cryogenically cooled Yb:YAG rods with an aperture of more than 2 mm provide energy scaling up to 70 mJ  and average power up to hundreds of watts, although with a pulsewidth of 6 ps. In addition, the use of cryogenic cooling is rather difficult for industrial lasers. A multi-pass amplifier based on а Yb:YAG rod provides a pulse energy of 200 mJ and a pulsewidth of < 0.9 ps at a repetition rate of 10 Hz . An additional amplifier stage using an active-mirror architecture increases the pulse energy to 0.78 J after compression at a repetition rate of 2 Hz . At repetition rates ≥ 10 kHz, gain modules based on thin Yb:YAG rods provide energies of 2.5 mJ  and 3.5 mJ . Therefore, active research is still underway to develop CPA based on Yb:YAG rods for lasers generating tens of mJ with a pulsewidth of ps and sub-ps range at a repetition rate of 100 Hz.
In this paper, we report on the development of a compact modular picosecond laser, consisting of all-in-fiber seed source, a two-stage double-pass CPA based on conduction-cooled YAG rods with low Yb doping level, a pulse compressor and two-cascaded frequency doubler. To reduce the heat load, CPA stages are pumped by high-brightness laser diodes at a wavelength of 940 nm with a repetition rate of 100 Hz. After compression the laser provides nearly bandwidth-limited pulses of 1.15 ps FWHM pulsewidth with excellent beam quality M2 ∼1.05 and energy fluctuations of 0.75%. The possibilities of scaling the output energy by combining a larger number of pumping laser diodes in the second stage of CPA or more powerful diodes, as well as increasing the repetition rate by reducing the pump pulsewidth, are discussed. This demonstrates progress in achieving a sufficiently high output energy with a short pulsewidth and excellent beam quality derived from a reliable and compact CPA based on widely available Yb:YAG rods. In addition, these pulses were frequency doubled in a two-cascaded second harmonic converter  based on LBO and BBO crystals, providing two beams with output energy of 12 mJ and 5 mJ at 515 nm with an overall conversion efficiency of 85%. A laser source with such an output energy and a pulsewidth is well suited for OPCPA pumping , especially when using white light supercontinuum as its seed, excited by ∼1 ps pulses in YAG crystals.
2. Experimental setup
The seed source for the two-stage double-pass CPA (Fig. 1) was based on an all-in-fiber passively mode-locking laser  generating spectrally broadened and temporally stretched pulses of 3.6 nm FWHM and ∼0.22 ns FWHM at a central wavelength of 1030 nm. The maximum seed pulse energy was limited to ∼6.5 µJ at a repetition rate of 21 kHz by a nonlinear phase distortion in the fiber amplifier. Seed pulses were selected using a BBO Pockels cell and thin film polarizers (TFP) providing attenuation of more than 30 dB. The CPA operated at a repetition rate of 100 Hz to reduce the thermal load on the Yb:YAG rods. The Faraday isolator (ISO) protects the seed source from backward amplified and depolarized radiation coming from the amplifier. A collimated gaussian seed beam ∼0.6 mm in diameter at 1/e2 level was directed to the first stage of the CPA while the amplified beam after the second pass was decoupled by polarization using a quarter-wave retardation plate (QWP), a Faraday rotator (FR) and a thin film polarizer (TFP).
An Yb:YAG rod 2×2×20 mm3 in size with 2% at. doping level was bonded to a water-cooled copper heatsinks set to 14°C. The choice of doping level is determined by the results of a comparative investigation  of Yb:YAG gain modules of various rod dimensions and doping levels: 1%, 2% and 5%. Reducing the doping level from 5% to 2% made it possible to achieve higher gain and output power due to improved heat dissipation with elongation of the rod. However, a further decrease in doping to 1% with a corresponding elongation of the rods leads to a deterioration in the overlap of the pump and seed beams due to the divergence. In this case, even the waveguide propagation of the pump beam in a cylindrical SCF is not sufficient to compensate for the loss of gain . Finally, excessive elongation of the Yb:YAG crystal leads to an undesirable increase in the B-integral, especially for the second stage of a double-pass amplifier. Structurally, the self-made gain module consisted of two copper plates interconnected to form a square hole for the Yb:YAG rod. A water-cooling channel passed through these plates surrounding the crystal. The ends of the crystal 0.5 mm protruded beyond the holder. A layer of indium about 0.1 mm thick filled the gap between the crystal and the holder. For better thermal contact, the crystal in the holder was placed in a furnace, which ensured melting and uniform distribution of indium in the gap. After cooling, the gain module was placed in a test bench to determine depolarization. In case of unsatisfactory results, the melting process was repeated again. This gain module was pumped using a fiber-coupled laser diode (nLIGHT module with a maximum power of 135 W CW at a wavelength of 940 nm with a spectral bandwidth of 4.2 nm FWHM, a fiber core diameter of 105 µm and NA = 0.22) operating at 70% duty cycle (pulsewidth 7 ms FWHM at a repetition rate of 100 Hz). A further decrease in the duty cycle while maintaining the emission wavelength made it necessary to increase the temperature of the used laser diode above 37°C, therefore, we chose pulsewidth of 7 ms. The output beam from a laser diode with an average power of 87 W was imaged by a collimator up to a diameter of 0.56 mm at a depth of ∼5 mm in the laser rod. A flat mirror (M) reflected the amplified beam for the second pass, while the induced thermal lens was compensated by adjusting the separation of the lenses in the relay telescope. A Faraday rotator in combination with a quarter-wave retardation plate (QWP) was used to suppress induced depolarization .
Separation of the CPA stages was carried out using the Faraday rotator, a polarizing cube and a half-wave retardation plate made it possible to suppress self-lasing. To reduce the energy density at the laser crystal of the second stage, the input beam was expanded to ∼1.4 mm at 1/e2 level. The Yb:YAG rod with a larger cross-section: 5×5×20 mm3 and the same doping level of 2% at. was mounted using silicate glue to eliminate residual stresses associated with inhomogeneous solidification of indium. This rod was pumped with а combined output of seven laser diodes, providing pulses of 2 ms FWHM with a total average power of 89 W at a repetition rate of 100 Hz (nLIGHT fiber coupled laser diodes with output power of 70 W CW at a wavelength of 940 nm, a fiber core diameter of 105 µm and NA = 0.22). The corresponding optimal duty cycle of laser diodes of 20% was found experimentally. A Lightcomm beam combiner with an output fiber diameter of 400 µm was mounted on a water-cooled heatsink and provided a power loss of less than 5%. The selection of the emission wavelength and the corresponding arrangement of the laser diodes on a common heatsink made it possible to obtain an integral spectral bandwidth of only ∼5 nm. The output beam from a beam combiner was imaged by a collimator up to a diameter of 1.45 mm at a depth of ∼5 mm in the laser rod. In the second stage, the induced thermal lens was compensated by adjusting the distance between the concave spherical mirror (SM) (R = 800 mm) and the laser rod, although the relay imaging can also be implemented here. Spatial filters were added at the output of amplifier stages to control spherical aberration. Iris apertures (AP) were placed near the focal plane of the focusing lens, and the diameter of the aperture was selected to transmit only the central lobe with an insignificant energy loss of < 5%.
The amplified pulses are directed to a folded 4-pass compressor based on an 1842 grooves/mm transmission grating with a diffraction efficiency of more than 97% from Gitterwerk GmbH (the overall efficiency exceeds 90%). This grating compressor operates near the Littrow configuration, where the angle of incidence and diffraction of ∼72° are the same for the central wavelength. To prevent Kerr lensing and damage to the optical coatings of the elements, the amplified beam in front of the compressor was increased to ∼6 mm. The dispersion of the diffraction grating was precisely matched to the dispersion of chirped fiber Bragg grating (CFBG) stretcher (Teraxion HPSR-1030-18F[1842L-82-(33.75)]-0P2-0R) in the seed laser by adjusting the distance between the roof and the flat mirror in the pulse compressor. The compressor occupies a small area of 15×30 cm2. A small fraction of the compressed pulse (< 0.1 mJ) was deflected using an attenuator formed by a half-wave retardation plate (HWP) and a thin-film polarizer (TFP) to generate a white light supercontinuum in a YAG crystal as an OPCPA seed source . The remaining energy was used for frequency doubling in two successive stages based on LBO 15×15×2 mm3 and BBO 15×15×2 mm3 crystals placed in an oven with thermal controllers set to a temperature of 40°C. The second-harmonic output pulses can be used as OPCPA pumping source . The entire scheme described above is assembled on an optical breadboard measuring 1×1.5m2. Spectral measurements were carried out using an Yokogawa AQ6373 optical spectrum analyser; measurements of the temporal shape of pulses before compression – with 20 GHz Tektronix DPO72004C oscilloscope and a photodetector Picometrix (8 ps temporal resolution), and after compression – by APE PulseCheck-50 autocorrelator, the pulse energy was measured using Coherent J-10MT-10kHz and Ophir PE50-DIF-ER-C meters.
3. Results and discussion
The double-pass small signal gain in the first Yb:YAG CPA stage was 28 dB (Fig. 2(a)). The energy density on the surface of the optical elements was maintained below 2 J/cm2. A thermal lens of ∼55 mm and a depolarization loss of 5.5% were observed at an average pumping power of 87 W. An increase in the pulse repetition rate to 200–300 Hz of the first CPA stage can be achieved by using shorter pump pulses (< 7 ms), which would reduce the thermal load on the crystal. Amplified pulses with an output energy of 2.35 mJ (Fig. 2(a)) at a repetition rate of 100 Hz, a pulsewidth of ∼110 ps FWHM and a spectral width of 1.7 nm FWHM (Fig. 2(b)) were directed to the second CPA stage. Thus, a total signal gain of almost 40 dB was achieved with low input seed energy (Fig. 2(a)). A thermal lens of ∼300 mm and a depolarization loss of 2.5% were observed at an average pumping power of 89 W. The optimal pulsewidth of the pumping pulses (Fig. 2(a) – inset) in the second stage of the amplifier is a compromise between population inversion and gain deterioration due to heating.
Amplified pulses with an output energy of 22.5 mJ (Fig. 2(a)) a pulsewidth of ∼90 ps FWHM and an excellent beam quality of M2 ∼1.02 (Fig. 3(a)) were directed to the compressor. The output energy stability after pulse compression StDev ± 0.75% (Fig. 3(b)) was observed during 30 min of laser operation without a protective housing in the air flows created by the dust filtration system. Although the gain narrowing from the initial spectral width of 3.6 nm FWHM of the seed source to 1.6 nm FWHM (Fig. 2(b)) after amplification, we obtained compressed pulses of 1.15 ps FWHM (Fig. 4(a)) with an overall efficiency of > 91% by optimizing the temperature gradient applied to CFBG installed in a fiber seed source. The amplified pulse spectrum width of ∼1.6 nm FWHM corresponds to a calculated pulsewidth of 1.04 ps FWHM for a transform-limited pulse.
At maximum output pulse energy, the total B-integral accumulated by amplified pulses passing through a two-stage amplifier-compressor reaches 1.8. Moreover, no significant changes in the autocorrelation trace were observed with the reduction in the output energy. Our estimates show that the main sources of nonlinearity are the Yb:YAG rod and the Faraday rotator in the second stage of the amplifier. The initial spectral modulation found in Fig. 2(b) may occur due to group delay ripples of the CFBG stretcher or due to internal reflections in the micro-optical components used in all-in-fiber seed source . The visible wings on the sides of the autocorrelation trace and the mismatch between the measured and transform-limited pulses can be attributed to the accumulated nonlinear phase in a single-mode fiber laser and an uncompensated higher-order dispersion and can be seen in the autocorrelation of a compressed seed (Fig. 4(a)). The beam quality after the compressor remains almost unchanged (M2 = 1.05 on Fig. 4(b)), while the energy decreases to ∼20 mJ.
Frequency conversion in two successive cascades provides conversion efficiency of 62% and 70%, respectively with an overall efficiency of 85%. The intensity of the fundamental pulse was chosen to saturate the conversion at both cascades, which led to lower output energy fluctuations. Two separate second harmonic outputs with an energy of 12 mJ and 5 mJ at a wavelength of 515 nm allow individually pumping OPCPA stages.
Combining more laser diodes for the second CPA stage or more powerful diodes would increase the pump power by several times. Moreover, diodes with a power exceeding 130 W and a fiber core of 105 µm at a wavelength of 940 nm and 969 nm, as well as fiber beam combiners designed for 19 inputs became widely available. The measured laser induced damage thresholds (LIDT) of AR-coatings on Castech crystals are 2.2 times higher than the LIDT of Altechna crystals used, while the LIDT of separators is even higher. Thus, a double increase in the output energy can occur even with a constant diameter of the amplified and pump beams. In this case, the 5 × 5 mm2 Yb:YAG rod aperture used by us seems excessive when the diameter of the amplified beam remains 1.4 mm at 1/e2 level, and reducing the rod aperture will allow more efficient heat dissipation. However, it is much more difficult to achieve the quality of an amplified beam with a significant increase in the pump power, which is difficult to comprehensively evaluate and predict without conducting experiments, since depolarization and spherical aberrations increase significantly. Nevertheless, in earlier experiments  using a pump power of 140 W, i.e. 1.6 times higher than that used in the second CPA stage, beam quality of M2 < 1.2 was maintained. Thus, according to our estimates, energy scaling up to 50 mJ is possible without a significant decrease in the beam quality. However, it is necessary to carefully isolate the amplification stages, for example using a Pockels cell. In addition, together with a decrease in the pulsewidth of the pump pulses, it will become possible to increase the repetition rate to 200–300 Hz, at least without increasing the energy of the output pulses.
We reported on the development of a compact and modular picosecond laser design consisting of all-in-fiber seed source, a two-stage double-pass CPA based on Yb:YAG rods, a pulse compressor, and a two-crystal cascaded frequency doubler. The use of Yb:YAG rods with low doping level in combination with pulsed pumping by high-brightness laser diodes made it possible to obtain nearly transform limited 1.15 ps FWHM pulses with an energy exceeding 20 mJ, an energy stability of 0.75%, and exceptional beam quality of M2 = 1.05 at a repetition rate of 100 Hz. Frequency doubling with an overall efficiency of 85% allows to direct 12 mJ and 5 mJ outputs for pumping of OPCPA stages, while picosecond pulses allowed us to generate a stable and passively synchronized white light supercontinuum for its seed.
Lietuvos Mokslo Taryba (LAT-10/2016).
We thank Augustinas Petrulenas and Vytenis Girdauskas for their great contribution to the experiments.
The authors declare no conflicts of interest.
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