Cryogenic Yb3+-Doped Solid-State Lasers

Cryogenically cooled solid-state lasers promise a revolution in power scalability while maintaining a good beam quality because of significant improvements in efficiency and thermo-optic properties. This is particularly true for Yb lasers because of their relatively low quantum defect and relatively broadband absorption even at cryogenic temperatures. Thermo-optic properties of host materials, including thermal conductivity, thermal expansion, and refractive index at low temperature, are reviewed and data presented for YAG (ceramic and single crystal), GGG, GdVO₄, and Y₂O₃. Spectroscopic properties of Yb:YAG and Yb:LiYF₄ (YLF) including absorption cross sections, emission cross sections, and fluorescence lifetimes at cryogenic temperatures are characterized. Recent experiments have pushed the power from an end-pumped cryogenically cooled Yb:YAG laser to 455-W continuous-wave output power from 640-W incident pump power at an of M² 1.4.     

Low-heat high-power scaling using InGaAs-diode-pumped Yb:YAG lasers

We report to our knowledge the highest to date quasi-CW output power, 600 W and pulse energy, >1 J, for an InGaAs diode-pumped Yb:YAG laser. In separate preliminary results, we have also obtained 225 W of average output power under true CW diode pumping. This performance was obtained using a laser head designed to be part of a master oscillator power amplifier (MOPA) operating at 3 kW. We summarize why the diode-pumped Yb:YAG crystal laser is ideal for scaling to high average powers and the different approaches being pursued. We also report our latest results for side-pumped rod devices     

Semiconductor saturable absorber mirrors (SESAM's) for femtosecondto nanosecond pulse generation in solid-state lasers

Intracavity semiconductor saturable absorber mirrors (SESAM's) offer unique and exciting possibilities for passively pulsed solid-state laser systems, extending from Q-switched pulses in the nanosecond and picosecond regime to mode-locked pulses from 10's of picoseconds to sub-10 fs. This paper reviews the design requirements of SESAM's for stable pulse generation in both the mode-locked and Q-switched regime. The combination of device structure and material parameters for SESAM's provide sufficient design freedom to choose key parameters such as recovery time, saturation intensity, and saturation fluence, in a compact structure with low insertion loss. We have been able to demonstrate, for example, passive modelocking (with no Q-switching) using an intracavity saturable absorber in solid-state lasers with long upper state lifetimes (e.g., 1-μm neodymium transitions), Kerr lens modelocking assisted with pulsewidths as short as 6.5 fs from a Ti:sapphire laser-the shortest pulses ever produced directly out of a laser without any external pulse compression, and passive Q-switching with pulses as short as 56 ps-the shortest pulses ever produced directly from a Q-switched solid-state laser. Diode-pumping of such lasers is leading to practical, real-world ultrafast sources, and we will review results on diode-pumped Cr:LiSAF, Nd:glass, Yb:YAG, Nd:YAG, Nd:YLF, Nd:LSB, and Nd:YVO₄     

Diode-pumped neodymium lasers repetitively Q-switched by Cr4+:YAG solid-state saturable absorbers

We report on the experimental results of a continuously diode-laser pumped Nd:YAG laser, operating at 1064 nm and repetitively Q-switched by a Cr⁴⁺:YAG solid-state saturable absorber. End-pumping the Nd:YAG with a 10-W fiber-coupled diode-laser we could either optimize the energy or the average output power, depending on the choice of the saturable absorber and the output coupler. The maximum energy was ≈200 μJ in single TEM₀₀, 17 ns pulses at 6 kHz, whereas a maximum average power of ≈2 W with 32-ns pulses at 20 kHz was obtained. We also present preliminary results of a repetitively Q-switched Nd:YVO₄ laser at 1064 nm. The repetitive Q-switching operation is described by an improved model, which accounts for the behavior of both the active medium and the solid-state saturable absorber. The results of the model agree fairly well with the experimental data. Experimental results of second harmonic conversion are also reported and interpreted using a depleted pump model     

Diode-pumped tunable Yb:YAG miniature lasers at room temperature:modeling and experiment

We present a simple design rule for diode-laser pumped quasi-three-level lasers by using the M² factor. The validity of this model was demonstrated by diode-pumped Yb:YAG laser experiments. The maximum output power of 1.33 W and optical slope efficiency of 63% were obtained in a 400-μm Yb:YAG chip miniature laser. Using a 200-μm Yb:YAG chip, a 70% optical slope efficiency was reached. In a coupled-cavity configuration, with a quartz birefringent tuning filter, 8.2 THz (29 nm) of tuning was obtained at room temperature. By changing to a calcite birefringent filter, single-axial-mode oscillation with an output power of 500 mW was observed     

Low threshold laser-diode side-pumped Tm:YAG and Tm:Ho:YAG lasers

We report results from transverse laser-diode pumping of Tm:YAG and Tm:Ho:YAG rods. Using two 60-W quasicontinuous-wave laser-diode bars and a special dielectric coating structure on the barrel surface of the laser rod, laser operation was obtained at room temperature with threshold pump energies below 100 mJ and with output pulse energy above 10 mJ in free-running operation and 2 mJ in Q-switched operation. The Tm:Ho:YAG laser was more susceptible to a temperature increase in the material and performed significantly poorer in the Q-switched mode of operation than the Tm:YAG laser. This was predicted by a model accounting for up conversion and the dynamic equilibrium between the upper levels in thulium and holmium in Tm:Ho:YAG     

Ce3+-activated fluoride crystals as prospective activemedia for widely tunable ultraviolet ultrafast lasers with direct 10-nspumping

New possibilities have been investigated for recently developed solid-state tunable ultraviolet (UV) laser materials such as Ce³⁺ ion-activated LuLiF₄ (LLF) and LiCaAlF₆ (LiCAF). With their broad-gain width, demonstrated reliability, and high efficiency, they are attractive for ultrashort pulse generation and amplification. To prove that, we have demonstrated UV picosecond-pulse amplification using Ce:LLF. For such new laser materials, we proposed a passive self-injection seeding scheme for the direct generation of short-pulse trains, which does not require CW operation capability or an external short-pulse seeding laser, Using this simple scheme, a UV sub-nanosecond pulse train is directly and passively generated from Ce:LLF pumped by a standard 10-ns KrF excimer laser, and Ce:LiCAF pumped by the fourth harmonic of a conventional 10-ns Q-switched Nd:YAG laser     

High-power/high-brightness diode-pumped 1.9-/spl mu/m thulium and resonantly pumped 2.1-/spl mu/m holmium lasers

We report high power (>36 W) with beam propagation factor M/sup 2//spl sim/2 in a diode end-pumped Tm:LiYF/sub 4/ (Tm:YLF) laser generating output near the 1.91-/spl mu/m region. Using the 1.91-/spl mu/m emission and high brightness achieved with the Tm:YLF laser we resonantly end-pump the Holmium /sup 5/I/sub 7/ manifold in Ho:YAG and demonstrate /spl sim/19 W of continuous-wave (CW) output. The diode-to-Holmium optical to-optical conversion efficiency achieved is /spl sim/18%. Using a CW pumped and repetitively Q-switched configuration, the Tm:YLF pumped Ho:YAG laser achieves >16 W of output power with an M/sup 2//spl sim/1.48 at 15 kHz. A Q-switched frequency range of 9 to >50 kHz is also achieved.     

Laser-diode end-pumped passively Q-switched intracavity-frequency-doubling Nd:GdVO/sub 4//KTP Green laser with GaAs saturable absorber

The intracavity photon density is assumed to be Gaussian spatial distributions, and the nonlinear loss that is due to second-harmonic generation (SHG) to the photon-density equation is given under Gaussian spatial distributions in the rate equations for a laser-diode end-pumped passively Q-switched intracavity-frequency-doubling Nd:GdVO/sub 4//KTP laser with a GaAs saturable absorber. These space-dependent rate equations are solved numerically. The dependences of pulsewidth, pulse repetition rate, single-pulse energy, and peak power on incident pump power are obtained for the generated-green-laser pulses. In the experiment, a laser-diode end-pumped passively Q-switched intracavity-frequency-doubling Nd:GdVO/sub 4//KTP laser with a GaAs saturable absorber is realized, and the experimental results are consistent with the numerical solutions.     

A 2.65-kW Yb:YAG single-rod laser

We report a continuous-wave average output power of 2.65 kW from a single Yb:YAG laser rod pumped with 9000 W from 940 nm InGaAs laser diodes. To the best of our knowledge, this is the highest average output power ever reported from a single Yb:YAG gain element. The optical-to-optical efficiency (i.e., output power to raw laser diode optical power) was 28%. We also obtained 860 W with an M/sup 2/ of 2.1 when pumping with 6000 W, obtaining 14% optical-to-optical efficiency.     

Advanced Ti:Er:LiNbO3 waveguide lasers

This paper reviews the latest developments of diode-pumped Ti,Er:LiNbO₃ waveguide lasers emitting at wavelengths around 1.5 μm. In particular, harmonically mode-locked lasers, Q-switched lasers, distributed Bragg reflector (DBR)-lasers, and self-frequency doubling lasers are discussed in detail. Supermode stabilized mode-locked lasers have been realized using a coupled cavity concept; a side mode suppression ratio of 55 dB has been achieved at 10-GHz pulse repetition rate with almost transform limited pulses. Q-switched lasers with a high extinction ratio (>25 dB) intracavity electrooptic switch emitted pulses with a peak power level up to 2.5 kW and a pulsewidth down to 2.1 ns at 1-kHz repetition frequency. Numerical simulations for both lasers are in a good, almost quantitative agreement with experimental results. A DBR-laser of narrow linewidth (≈3 GHz) with a permanent (fixed) photorefractive grating and 5 mW output power has been realized. Self frequency doubling lasers have been fabricated with a periodic microdomain structure inside an Er-doped laser cavity; simultaneous emission at the fundamental wavelength, 1531 nm, and at the second harmonic wavelength, 765 nm, has been obtained     

Ce3+:LiCaAlF6 crystal for high-gain orhigh-peak-power amplification of ultraviolet femtosecond pulses and newpotential ultraviolet gain medium: Ce3+:LiSr0.8Ca0.2AlF6

To develop high-peak-power ultrashort pulse laser systems in the ultraviolet region, a large Ce³⁺:LiCaAlF₆ (Ce:LiCAF) crystal, a tunable ultraviolet laser medium with large saturation fluence and broad gain spectrum width, was grown successfully with a diameter of more than 70 mm. To demonstrate high small signal gain, a four-pass confocal amplifier with 60 dB gain and 54 μJ output energy was constructed. Chirped pulse amplification (CPA) in the ultraviolet region was demonstrated using Ce:LiCAF for higher energy extraction. A modified bow-tie-style four-pass amplifier pumped by 100-mJ 266-nm 10-Hz pulses from a Q-switched Nd:YAG laser had 370-times gain and delivered 6-mJ 290-nm pulses. After dispersion compensation, the output pulses can be compressed down to 115 fs. This is the first ultraviolet, all-solid-state high-peak-power CPA laser system using ultraviolet gain media, and this demonstration shows further scalability of the Ce:LiCAF CPA system. Additionally, a new gain medium, Ce³⁺ :LiSr_0.8Ca_0.2AlF₆, with longer fluorescence lifetime and sufficient gain spectrum width over 18 nm was grown to upgrade this system as a candidate for a final power amplifier gain module     

Fiber-Laser-Pumped Er:YAG Lasers

Hybrid fiber-laser-pumped solid-state lasers exploit high-power cladding-pumped fiber lasers for direct (in-band) pumping of a crystal-based solid-state laser to reduce heating in the laser crystal, and hence allow scaling to higher power in both continuous-wave (CW) and pulsed modes of operation. In this paper, we briefly review the attractions of the hybrid laser approach for generation of output in the ~ 1.6 mum wavelength regime and consider the main design considerations for efficient operation of hybrid lasers based on Er:YAG in both CW and pulsed modes of operation. Examples of hybrid Er:YAG lasers, pumped by Er,Yb codoped fiber lasers at 1532 nm, with CW output powers up to 60 W at 1645 nm and 31 W at 1617 nm and slope efficiencies of 80% and 47% with respect to incident pump power, respectively, are described. In Q-switched mode of operation, pulse energies up to 30.5 mJ were obtained, limited by coating damage. Finally, the prospects for further increase in output power and improvement in overall performance in CW and Q-switched modes of operation will be discussed.     

Modeling of passively Q-switched Er/sup 3+//Yb/sup 3+/-codoped clad-pumped fiber lasers

We present the modeling of passively Q-switched Er/sup 3+//Yb/sup 3+/-codoped double-clad fiber lasers (EYDFL). Both two-level and three-level saturable absorbers (SAs) are used. The model is based on detailed rate equations of SAs, Er/sup 3+/, and Yb/sup 3+/ populations in the fiber, and light propagation equations including spontaneous emissions due to both Er/sup 3+/ and Yb/sup 3+/. Laser output characteristics including pulse energy, width, peak power, repetition frequency (PRF), and average power are computed as functions pump power, fiber length, single-mode/multi-mode fibers, Er/sup 3+//Yb/sup 3+/ concentrations, out-coupling reflectivity, focus area on the SA, and SA doping and length. Several transition metal-doped crystal SAs and semiconductor SA mirrors (SESAM) are considered and compared.     

1.047-μm Yb:Sr5(PO4)3F energystorage optical amplifier

The pumping and gain properties of Yb³⁺-doped Sr₅ (PO₄)₃F (Yb:S-FAP) are reported. Using a tunable, free running 900-nm Cr:LiSAF oscillator as a pump source for a Yb:S-FAP rod, the saturation fluence for pumping was measured to be 2.2 J/cm² based on either the spatial, temporal, or energy transmission properties of the Yb:S-FAP rod. The emission peak of Yb:S-FAP (1047.5 nm in air) is shown to overlap with that of Nd:YLiF₄ (Nd:YLF) to within 0.1 nm, rendering Yb:S-FAP suitable as an effective power amplifier for Nd:YLF oscillators. The small signal gain, under varying pumping conditions, was measured with a cw Nd:YLF probe laser. These measurements implied emission cross sections of 6.0×10^-20 and 1.5×10^-20 cm ² for π and σ polarized light. Respectively, which fall within the error limits of the previously reported values of 7.3×10^-20 and 1.4×10^-20 cm² for π and σ polarized light, obtained from purely spectroscopic techniques. The effects of radiation trapping on the emission lifetime have been quantified and have been shown to lead to emission lifetimes as long as 1.7 ms, for large optically dense crystals. This is substantially larger than the measured intrinsic lifetime of 1.10 ms. Yb:S-FAP crystal boules up to 25×25×175 mm in size, which were grown for the above experiments and were found to have acceptable loss characteristics (<~1%/cm) and adequately large laser damage thresholds at 1064 nm (~20 J/cm² at 3 ns). Overall, diode-pumped Yb:S-FAP amplifiers are anticipated to offer a viable means of amplifying 1.047-μm light, and may be particularly well suited to applications sensitive to overall laser efficiencies, such as inertial confinement fusion energy applications     

230 mW of blue light from a thulium-doped upconversion fiber laser

We demonstrate a powerful diode-pumped blue laser source, consisting of a 7-W diode at 807 nm that pumps a Nd:YAG laser giving 1.6 W with good beam quality at 1123 nm, and a thulium-doped upconversion fiber laser. The maximum output power achieved at 481 nm is 230 mW. We also describe the behavior of a reversible loss which is generated in the fluoride fiber during high power operation     

Diode arrays, crystals, and thermal management for solid-statelasers

We summarize our efforts in the development of solid-state lasers, including the laser diode arrays, pump light delivery, approaches to thermal management, and novel gain media. Our interests are in developing unique solid-state lasers, including those operating at higher powers, offering less common wavelengths, and having other specialized features. In this paper, we discuss high-power Tm:YAG and Yb:YAG lasers. The gas cooled slab laser concept using Yb:S-FAP, and side-pumped Er:YAG and Cr:ZnSe lasers. We address the optical and thermal physics of these systems and also mention several additional gain media that have the potential of offering unique performance characteristics: Ce:LiSAF, APG-2 laser glass, Dy:LaCl₃, and Yb:BCBF     

Waveguide lasers in the C-band fabricated by laser inscription with a compact femtosecond oscillator

Femtosecond laser writing of high-quality optical waveguides by means of a compact diode-pumped cavity-dumped Yb:glass laser oscillator is reported. Waveguides have been written on an erbium-ytterbium co-doped phosphate glass for active device production. A detailed analysis of the optical characteristics of waveguides written with different repetition rates, pulse energies, and translation speeds is presented, and an optimum set of writing parameters is established. Coupling losses as low as 0.1 dB/facet with standard telecom fibers and propagation losses lower than 0.4 dB/cm have been obtained. Characterization of the active properties of the waveguides is also presented, together with the demonstration of waveguide laser action in the whole C-band with peak output power of more than 30 mW.     

Diode-pumped Nd:YLF master oscillator for the 30-kJ (UV), 60-beamOMEGA laser facility

The operational conditions of the OMEGA laser system require an extremely reliable and low-maintenance master oscillator. We have developed a diode-pumped, single-frequency, pulsed Nd:YLF laser for this application. The laser generates Q-switched pulses of ~160-ns duration and ~10-μJ energy content at the 1053-nm wavelength with low amplitude fluctuation, (<0.6% rms) and low temporal (build-up time) jitter (<7-ns rms). Amplitude and frequency feedback stabilization systems have been used for high long-term amplitude and frequency stability     

Active tracker laser (ATLAS)

A high brightness diode-pumped, Nd-YAG solid state laser has been designed, fabricated, and tested. This phase conjugated master oscillator/power amplifier (MOPA) device produces 20-ns Q-switched pulses at 2500 Hz at an average power of 690 W and a beam quality of 1.1×DL when the pump diodes are operated at 27.5% duty cycle. With an external KTP doubler, this device has produced 175 W of green average power at a beam quality of 1.5 × DL and a conversion efficiency of 45% over continuous operating times as long as one hour. This 1.06 μm result is believed to be the highest average power brightness achieved, and the 532-nm performance is both the highest average green power and the highest average brightness ever reported