100-mJ, 100-W cryogenically cooled Yb:YLF laser.

We present a diode-pumped Yb:YLF laser system generating 100-mJ sub-ps pulses at a 1-kHz repetition rate (100 W average power) by chirped-pulse amplification. The laser consists of a cryogenically cooled 78 K, regenerative, eight-pass booster amplifier seeded by an all-fiber front end. The output pulses are compressed to 980 fs in a single-grating Treacy compressor with a throughput of 89%. The laser will be applied to multi-cycle THz generation and pumping of high average power parametric amplifiers.

Continuous-wave Tm:YLF laser with ultrabroad tuning (1772-2145 nm).

We report detailed experimental data aiming for rigorous investigation of Tm:YLF laser performance, especially with a focus on tuning behavior. Continuous-wave (cw) lasing performance of Tm:YLF crystals with thulium dopings in the 2-6% range is investigated under diode and Ti:Sapphire pumping at 792 nm and 780 nm, respectively. While employing the c-axis, we have achieved cw lasing thresholds below 20 mW, laser output power up to 1.42 W, and laser slope efficiencies as high as 70% with respect to absorbed pump power. The passive loss of the Tm:YLF crystal is estimated to be as low as 0.05% per cm, corresponding to a crystal figure of merit above 10000. Via employing this low-loss crystal and a 2-mm thick off-surface optical axis birefringent filter (BRF) with strong sideband rejection, a record cw tuning range covering the 1772-2145 nm interval is demonstrated (except a small gap between 1801-1815nm region). Detailed lifetime and emission cross section measurements have been performed to explain the observed performance, and strategies for further performance enhancement are discussed.

Broadly tunable (993-1110 nm) Yb:YLF laser.

We have investigated room-temperature continuous-wave (cw) lasing performance of Yb:YLF oscillators in detail using rod-type crystals with low Yb-doping (2%). The laser is pumped by a low-cost, high brightness, 10 W, 960 nm single-emitter multimode diode. Laser performance is acquired in both E//a and E//c configurations, using 12 different output couplers with transmission ranging from 0.015% to 70%. We have estimated the passive loss of the Yb:YLF crystal as 0.06% per cm, corresponding to an impressive crystal figure of merit above 4000. The low-doping level not only reduces the system losses but also minimizes the thermal load as the low doped crystals enable distribution of heat load in a greater volume. Using the advantages of lower loss and improved thermal behavior, we have achieved cw output power above 4 W, cw slope efficiencies up to 78%, and a record cw tuning range covering the 993-1110 nm region (117 nm). The output power performance achieved in this initial work is limited by the available pump power, and future room-temperature Yb:YLF systems have the potential to produce higher output power levels.

Semiconductor saturable absorber mirror mode-locked Yb:YLF laser with pulses of 40 fs.

We have generated pulses as short as 40 fs with an average power of 265 mW from a semiconductor saturable absorber mirror (SESAM) mode-locked Yb:YLF oscillator employing a 1% transmitting output coupler (OC). The room-temperature laser is pumped by a low-cost 960 nm single-emitter multimode diode and dispersion compensation is provided via double chirped mirrors (DCMs). The 40-fs pulses are centered around 1050 nm with a width of 34 nm at a repetition rate of 87.3 MHz. By increasing the output coupling to 5% and by using Gires-Tournois interferometer (GTI) mirrors for dispersion compensation, we have also demonstrated 380-fs pulses with 1.85 W of average power around 1025 nm at a repetition rate of 190.4 MHz. Using an intracavity off-surface optic axis birefringent filter, the central wavelength of the pulses could be tuned in the 1020-1025 nm and 1019-1047 nm ranges for the 5% and 1% transmitting OCs, respectively. To the best of our knowledge, these are the shortest pulses and highest average and peak powers generated from room-temperature Yb:YLF lasers to date.

Mode-locked Cr:LiSAF laser far off the gain peak: tunable sub-200-fs pulses near 1 µm.

We report, to the best of our knowledge, the first mode-locking results of a Cr:LiSAF laser near the 1 µm region. The system is pumped only by a single 1.1 W high-brightness tapered diode laser at 675 nm. A semiconductor saturable absorber mirror (SESAM) with a modulation depth of 1.5% and non-saturable losses below 0.5% was used for mode-locking. Once mode-locked, the Cr:LiSAF laser produced almost-transform-limited sub-200-fs pulses with up to 12.5 mW of average power at a repetition rate of 150 MHz. Using an intracavity birefringent filter, the central wavelength of the pulses could be smoothly tuned in the 1000-1020 nm range. Via careful dispersion optimization, pulse widths could be reduced down to the 110-fs level. The performance in this initial study was limited by the design parameters of the SESAM used, especially its passive losses and could be improved with an optimized SESAM design.

Highly efficient cryogenic Yb:YLF regenerative amplifier with 250 W average power.

We report an efficient diode-pumped high-power cryogenic regenerative amplifier operating at 1019 nm employing the c axis of Yb:YLF. Compared to the usually selected 1017 nm transition of the a axis, the c-axis 1019 nm line has a three-fold higher emission cross section and still possesses a full-width at half-maximum (FWHM) of 6.5 nm at 125 K. The chirped-pulse amplifier system is seeded by a fiber front-end with energy of 30 nJ and a stretched pulse width of 2 ns. In regenerative amplification studies, using the advantage of higher gain in the c axis, we have achieved record average powers up to 370 W with an extraction efficiency of 78% at a 50 kHz repetition rate. The output pulses were centered on 1019.15 nm with a FWHM bandwidth of 1.25 nm, which supports sub-1.5 ps pulse durations. The output beam maintained a TEM00 beam profile at output power levels below 250 W with an M2 below 1.2. Above this power level, the thermally induced lensing in Yb:YLF created a multimode output beam. The thermal lens was rather dynamic and deteriorated the system stability above a 250 W average power level.

High power (>500W) cryogenically cooled Yb:YLF cw-oscillator operating at 995†nm and 1019†nm using E//c axis for lasing.

We present record continuous wave (cw) output power levels from cryogenically cooled Yb:YLiF4 (Yb:YLF) lasers in rod geometry. The laser system is pumped by a state-of-the-art 960 nm diode module, and vertically polarized lasing was employed using the E//c axis of Yb:YLF. Lasing performance was investigated at different output coupling levels in different cavity configurations and the laser crystal temperature was estimated via monitoring the emission spectrum of the gain media. We have obtained a cw output power up to 400 W at a wavelength of 995 nm. The absorbed pump power was around 720 W, and the laser output had a TEM00 beam profile with an M2 of 1.3 in both axes. At higher absorbed pump power levels with increasing laser crystal temperature, we observed a lasing wavelength shift from 995 nm to 1019 nm. In this regime cw output power levels above 500 W have been achieved at an absorbed pump power of 750 W. Further power scaling was limited by the onset of strong thermal lensing. We discuss underlying physical mechanisms for the wavelength shift and present detailed temperature measurements under lasing conditions.

High-power passively mode-locked cryogenic Yb:YLF laser.

We report, to the best of our knowledge, the first mode-locked operation of Yb:YLF gain media at cryogenic temperatures. A saturable Bragg reflector was used for initiating and sustaining mode locking. Once aligned, the system was self-starting and quite robust. Using output couplers in the 10-40% range, 3-5 ps long pulses with an average power as high as 28 W were achieved. The repetition rate was 46.45 MHz, and the corresponding pulse energy and peak power were as high as 602 nJ and 126.5 kW, respectively. The central wavelength of the mode-locked pulses could be tuned in the 1013.5-1019 nm range using an intracavity birefringent filter. The achieved output power performance is two to three orders of magnitude higher than previous room-temperature Yb:YLF systems.

20-mJ, sub-ps pulses at up to 70 W average power from a cryogenic Yb:YLF regenerative amplifier.

We report, what is to our knowledge, the highest average power obtained directly from a Yb:YLF regenerative amplifier to date. A fiber front-end provided seed pulses with an energy of 10 nJ and stretched pulsewidth of around 1 ns. The bow-tie type Yb:YLF ring amplifier was pulse pumped by a kW power 960 nm fiber coupled diode-module. By employing a pump spot diameter of 2.1 mm, we could generate 20-mJ pulses at repetition rates between 1 Hz and 3.5 kHz, 10 mJ pulses at 5 kHz, 6.5 mJ pulses at 7.5 kHz and 5 mJ pulses at 10 kHz. The highest average power (70 W) was obtained at 3.5 kHz operation, at an absorbed pump power level of 460 W, corresponding to a conversion efficiency of 15.2%. Despite operating in the unsaturated regime, usage of a very stable seed source limited the power fluctuations below 2% rms in a 5 minute time interval. The output pulses were centered around 1018.6 nm with a FWHM bandwidth of 2.1 nm, and could be compressed to below 1-ps pulse duration. The output beam maintained a TEM00 beam profile at all power levels, and possesses a beam quality factor better than 1.05 in both axis. The relatively narrow bandwidth of the current seed source and the moderate gain available from the single Yb:YLF crystal was the main limiting factor in this initial study.

Efficient, diode-pumped, high-power (>300W) cryogenic Yb:YLF laser with broad-tunability (995-1020.5 nm): investigation of E//a-axis for lasing.

We present, what is to our knowledge, the first detailed lasing investigation of cryogenic Yb:YLF gain media in the E//a-axis. Compared to the usually employed E//c-axis, the a-axis of Yb:YLF provides a much broader and smooth gain profile, but this comes at the expense of reduced gain product. We have shown that, despite the lower gain, which (i) increases susceptibility to cavity losses, (ii) raises lasing threshold, and (iii) inflates thermal load, efficient and high-power lasing could be achieved in the E//a axis as well. A record continuous-wave (cw) powers above 300 W, cw slope efficiencies of 73%, and a tuning range covering the 995-1020.5 nm region were demonstrated. In quasi-cw lasing experiments, via minimization of thermal effects, slope efficiencies can be scaled up to 85%. In gain-switched operation, sub-50-µs long pulses with a peak power exceeding 2.5 kW at multi-kHz repetition rate were attained. We measured a beam quality factor below 1.5 for laser average powers up to 100 W and below 3 for laser average powers up to 300 W. Power scaling limits due to thermal effects, laser dynamics in pulsed pumping, and multicolor lasing operation potential were also investigated. The detailed results presented in this manuscript will pave the way towards development of high-power and high-energy Yb:YLF oscillators and amplifiers with sub-500-fs pulse duration.