Cryogenic Yb Research Articles

Pushing the tuning limits of femtosecond Yb-based solid-state lasers to 1 µm: a Yb:YVO4 laser tunable down to 1004 nm.

We have obtained what we believe to be the shortest fs tuning wavelength (1004 nm) from a Yb-based solid-state laser system. As the working horse, we have chosen Yb:YVO4 due to its blueshifted gain spectrum. While using a single 10 W 952 nm diode for pumping and a semiconductor saturable absorber mirror (SESAM) for mode-locking, we have achieved 266 fs long pulses with up to 1.24 W of output power around 1021 nm. Using an intracavity birefringent filter, the central wavelength of the fs pulses could be tuned smoothly between 1004-1038 nm and 1009-1061 nm while employing the E//a and E//c axis of the material, respectively. By using an additional hard aperture to increase the modulation depth, we have also attained 100 fs level pulses in the 1012.5-1019 nm range, which shows the suitability of the system in seeding cryogenic Yb:YLF-based amplifiers.

Wavelength-switching performance and thermal lens effect of cryogenic Yb:YAG lasers

Wavelength-switching performance and thermal lens effect of cryogenic Yb:YAG lasers pumped by a 940 nm laser diode are presented. A maximum continuous wave output power is reached 13.16 W at 80 K with an optic-optic efficiency of 25.6% at an incident pump power of 60 W. The wavelength-switching performance is observed and explained by absorbed pump power threshold behavior as temperature. Moreover, we directly characterize the thermal lens effect by wavefront measurement with a Shack-Hartmann wavefront sensor. The wavefront peak-to-valley value (PV) is 0.338 µm (about 0.33λ) at 80 K, corresponding to a thermal lens dioptric power of 0.15 m-1. The substantial decrease of the thermal lens effect contributes to the significant improvement of laser performance, which is consistent with the brilliant beam quality factor M2 of about 1.10 at both x and y directions.

Externally pumped yellow laser source emitting at 577 nm

We demonstrated a 577 nm yellow laser source with a proper combination of Raman medium and frequency doubling medium, pumped externally. Cryogenic Yb:YAG oscillator acts as an external pump source, and it generates 1.2 mJ energy at 1 KHz emitting around 1029 nm. Using a Raman resonator with Ba(NO3)2 as Raman medium, stimulated Raman scattering around 1153.4 was generated. Finally, 577 nm yellow laser was generated using the frequency-doubling medium LBO. A maximum average output power of 65 mW with a pulse duration of 18 ns at 1 KHz was achieved.

One-joule 500-Hz cryogenic Yb:YAG laser driver of composite thin-disk design.

We present results on the development of a cryogenic Yb:YAG multi-pass laser amplifier based on a composite thin-disk design and demonstrate one-joule, diffraction limited, chirped 234-ps pulses with 50% optical-to-optical efficiency. High beam quality was obtained for repetition rates up to 400 Hz. The hardware was disassembled and thoroughly inspected after accumulating 80 hours of use at repetition rates from 100 to 500 Hz and exhibited no signs of damage. This laser driver is now commissioned to a dedicated laboratory where a grating compressor is producing 5.2-ps pulses used in the development of a compact x ray source based on inverse Compton scattering.

Cavity-dumped nanosecond Cr:LiSAF laser in the 985\u20131030\xa0nm region for versatile seeding of Yb-based amplifiers

We report nanosecond (ns) cavity-dumped operation of a low-cost diode-pumped Cr:LiSAF laser around 1000 nm. The system is pumped with one 1-W single-emitter multimode diode at 665 nm. A Pockell cell (PC) and thin-film-polarizer (TFP) combination placed inside the cavity chops up an adjustable portion of the intracavity power and creates a variable time-dependent output coupler. Via adjusting the length and magnitude of the electrical signal going into the PC, output pulses with pulsewidths in the 2.5–500 ns range and with peak power levels above 10 W are generated at repetition rates up to 100 kHz. The central wavelength of the pulses could be smoothly tuned in the 985–1030 nm region, and is only limited by the anti-reflection coating bandwidth of the current PC and TFP. This versatile nanosecond source with 100 nJ level energies could serve as an attractive low-cost seed source for Yb-based amplifiers, including the cryogenic Yb:YLF systems.

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.

A 61-mJ, 1-kHz cryogenic Yb:YAG laser amplifier* *Project supported by the National National Science Foundation of China (Grant Nos. 12004262 and 62005184) and the Natural Science Foundation of Top Talent of SZTU (Grant No. 202024555101039).

We report a diode-pumped rod-type Yb:YAG laser amplifier operating at 1 kHz.Cryogenic cooling method was adopted to make the Yb:YAG crystal work with four-level behavior. A single-frequency fiber laser acts as the seed in an actively Q-switched Yb:YAG oscillator. The resonator delivers 5.75-mJ pulses at 1 kHz with a pulse duration of approximately 40 ns. The pulses were amplified to 61 mJ in a four-pass rod-type Yb:YAG amplifier with optical-to-optical efficiency of 24% in the main amplifier. The M 2 parameter of the output laser is < 1.4.

Error analysis of contactless optical temperature probing methods for cryogenic Yb:YAG

In this work, we have investigated six different in situ optical contactless temperature probing methods for cryogenic Yb:YAG systems. All the methods are based on variation of fluorescence spectra with temperature, and they either look at the width of the emission line, the ratio of the emission intensity at different wavelengths and to the overall spectral change at selected wavelength intervals. We have shown that, for Yb:YAG crystal with homogeneous temperature distribution, one can perform real-time contactless optical temperature measurements with a ± 1 K accuracy in the 78–300 K range. We have further tested the methods in measuring the average temperature of Yb:YAG rods at up to 500 W absorbed pump power level. We have seen that, a real-time temperature measurement accuracy of ± 5 K is feasible in both lasing and non-lasing situations for estimating the average temperature of crystals under nonhomogeneous thermal load. The techniques are quite valuable in evaluating the bonding quality of Yb:YAG crystals in cryogenic systems. Moreover, the real-time temperature information provides feedback on parameters like cavity alignment status and extraction efficiency to the laser engineers while optimizing the system.

Comparative investigation of lasing and amplification performance in cryogenic Yb:YLF systems

We present detailed experimental results with cryogenic Yb:YLF gain media in rod-geometry. We have comparatively investigated continuous-wave (cw) lasing and regenerative amplification performance under different experimental conditions. In the cw lasing experiments effect of crystal doping, cw laser cavity geometry and pump wavelength on lasing performance were explored. Regenerative amplification behavior was analyzed and the role of depolarization losses on performance was investigated. A recently developed temperature estimation method was also employed for the first time in estimating average crystal temperature under lasing conditions. It is shown that the thermal lens induced by transverse temperature gradients is the main limiting factor and strategies for future improvements are discussed. To the best of our knowledge, the achieved results in this study (375 W in cw, and 90 W in regenerative amplification) are the highest average powers ever obtained from this system via employing the broadband E//a axis.

Detailed investigation of absorption, emission and gain in Yb:YLF in the 78–300 K range

We present spectroscopic measurements focusing on a detailed investigation of temperature dependence of absorption, emission and gain in the uniaxial Yb:YLF laser gain medium. Measurements are carried out in the 78&#x2013;300 K range, but we especially targeted our attention to the 78&#x2013;150 K interval, which is the desired working range of liquid nitrogen cooled cryogenic Yb:YLF lasers/amplifiers. A tunable (770&#x2013;1110 nm) Cr:LiSAF laser with around 100 mW continuous-wave output power and sub-0.2 nm bandwidth is used as an excitation source. The average power of the Cr:LiSAF laser is low enough to prevent heating of the sample, and its spectral flux (W/nm) is high enough to enable large signal-to-noise ratio measurements. Measured absorption data is used to cross-check the validity of the emission measurements, while the measured temperature dependent small-signal gain profile provided a second independent confirmation. The acquired absorption cross section curves match the previous literature quite well, whereas the measured strength of c-axis emission is stronger than some of the earlier reports. Direct measurements of small signal gain confirmed the emission cross section data, where single pass gain values above 50 have been measured for the 995 nm transition of E//c axis at 78 K. We further provide simple analytic formulas for the measured temperature dependence of absorption and emission cross section. We hope the presented results to be useful for the development of next generation of cryogenic Yb:YLF laser and amplifier systems.

Comparison of different in situ optical temperature probing techniques for cryogenic Yb:YLF

We present, what is to our knowledge, the first detailed set of experiments comparing different in situ optical temperature estimation methods for Yb:YLF (Yb:LiYF4) crystals used in cryogenic laser applications. The proposed temperature estimation methods are based on the temperature dependence of emission spectra of Yb:YLF in E//c axis, and looks at either the variation of the spectral intensity ratio of different wavelengths, or to the full-width half-maximum (FWHM) of the emission lines, or to the overall absolute integrated spectral change with respect to a reference temperature (also known as Differential Luminescence Thermometry: DLT). We have shown that by using the DLT method we can estimate the temperature of Yb:YLF crystals in the 78-300 K range with an accuracy better than ±1 K. The other methods work well in the 78-150 K range, and provide a fast temperature estimation with ±2 K accuracy. The benefit of the proposed technique has been demonstrated via evaluation of thermal contact quality of different Yb:YLF crystals, where we have seen that, a temperature estimation accuracy of ±5 K is feasible even for samples under nonhomogeneous thermal load. We hope the findings presented in this work to be useful to laser engineers and scientists working with cryogenic Yb:YLF systems.

Diode pumped cryogenic Yb:Lu3Al5O12 laser in continuous-wave and pulsed regime

We report on diode pumped cryogenic Yb: Lu3Al5O12 (LuAG) laser in the continuous-wave and pulsed regime. Under given experimental condition, we determined an optimum crystal temperature of 140 K for efficient laser operation. A maximum output power of 10.46 W with slope efficiency values of 48.6 and 67% with respect to incident and absorbed power respectively were achieved for an output coupler transmission of 40%. Yb:LuAG laser performed significantly better at 140 K with an output power 16 times and slope efficiency 7 times better than the values at 240 K. Self-starting passively Q-switched Yb:LuAG lasers were also characterized using Cr:YAG saturable absorbers with three different initial transmissions. A maximum pulse energy of 0.215 mJ and a peak power of 1.46 kW were obtained corresponding to the shortest pulse duration of 148 ns.

Eight-pass Yb:YLF cryogenic amplifier generating 305-mJ pulses

We report record output pulse energies from a cryogenic 8-pass Yb:YLF amplifier system operating at 10 Hz repetition rate. When seeded with 20-mJ, 1-ns stretched pulses, the amplifier produced output pulses with 305 mJ of energy at 1018.5 nm with a spectral width supporting sub-ps pulse-duration. The output beam profile was quite symmetric and had a measured beam quality factor (M2) of ∼1.45. To achieve this performance, the diameter of the beam inside the gain element is gradually increased via an adjustable telescope from around 4.6 mm to 6.2 mm. This enables adjustment of the fluence to a moderate value in subsequent passes: high enough for efficient extraction and low enough to prevent laser induced damage. To our knowledge, this is the highest pulse energy reported from cryogenically cooled Yb:YLF amplifiers to date. Further scaling in peak power and repetition rate is anticipated in future work.

Laser induced damage in coatings for cryogenic Yb:YAG active mirror amplifiers.

We report results of a study of the laser induced damage threshold (LIDT) behavior of ion beam sputtered HfO2/SiO2 multilayer coatings on Yb:YAG using 1-on-1 and N-on-1 test protocols. The tests were conducted at ambient, vacuum, and cryogenic conditions using 280 ps pulses at λ=1030nm. The 1-on-1 LIDT of antireflection (AR) stacks is found to be only slightly reduced under vacuum and cryogenic conditions, while that of high reflectivity (HR) stacks is insensitive to environmental conditions within the uncertainty of the measurements. Cryogenic N-on-1 tests show the LIDT of the HR coating is almost the same as in the 1-on-1 tests. Conversely, the cryogenic N-on-1 test of the AR coating shows damage at ∼13J/cm2, a fluence lower than the 20.4J/cm2 of 1-on-1 tests. The AR damage behavior is found to be affected by imperfections at the Yb:YAG surface. These findings show that high surface quality is required to increase energy extraction from active mirror laser amplifiers.

Power and energy scaling of rod-type cryogenic Yb:YLF regenerative amplifiers

We numerically investigate the power and energy scaling potential of cryogenic Yb:YLF regenerative amplifiers in rod geometry. Our approach is based on solving the coupled set of equations describing thermal behavior of the material and its effect on spectroscopic properties, gain, and overall amplification. The approach is first benchmarked with earlier experimental data. By carefully analyzing the sensitivity of the system to operation parameters, we see that the relatively low gain nature of the Yb:YLF and the onset of thermal effects are the main factors that limited the performance in earlier experimental work. We show that usage of dual-rod geometry promises much improved performance. Specifically, we demonstrate that sub-250 fs pulses with an average power of up to 270 W and a peak power above 500 GW can be extracted directly from a single-stage Yb:YLF regenerative amplifier employing dual Yb:YLF rods. We further show that by adjusting the spot size in the regenerative amplifier, one can operate the amplifier in either high-energy mode ( &gt; 100 m J at 1 kHz) or high-average-power mode ( &gt; 25 m J at 10 kHz, with &gt; 250 W ). We also discuss pros and cons of Yb:YLF with respect to Yb:YAG, and underline the need for measurement of population and photo-elastic-effect-induced lensing in Yb:YLF to obtain a better understanding of Yb:YLF systems. The findings presented in this work can be used for the design and development of next-generation high-average and peak-power Yb:YLF amplifier systems.

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.

Core-pumped mode-locked ytterbium-doped fiber laser operating around 980 nm

In this letter, we first demonstrate a core-pumped passively mode-locked all-normal-dispersion ytterbium-doped fiber oscillator based on nonlinear polarization evolution operating around 980 nm. The dissipative soliton fiber laser pulse can be compressed down to 250 fs with 1 nJ pulse energy, and the slope efficiency of the oscillator can be as high as 19%. To improve the dissipative soliton laser output spectrum smoothness, we replace the birefringent plate based intracavity filter with a diffraction-grating based filter. The output pulse duration can then be further compressed down to 180 fs with improved spectral-smoothness. These schemes have potential applications in seeding cryogenic Yb:YLF amplifiers and underwater exploration of marine resources.

High-gain (43 dB), high-power (40 W), highly efficient multipass amplifier at 995 nm in Yb:LiYF4

A simple implementation of a multipass amplifier along with the use of a cryogenic Yb:LiYF4 (YLF) gain medium has enabled the demonstration of a bulk amplifier with an unprecedented combination of large-signal gain (43 dB), efficiency (>50% optical), average output power (40 W) and a near-diffraction-limited output beam.