Upconversion-pumped femtosecond thulium laser at 2309†nm mode-locked by a GaSb-based SESAM.

We report on a femtosecond thulium laser operating on the 3H4 → 3H5 transition with upconversion pumping around 1 µm and passively mode-locked by a GaSb-based SEmiconductor Saturable Absorber Mirror (SESAM). This laser employs a 6 at.% Tm:LiYF4 laser crystal and a polarization maintaining Yb-fiber master oscillator power amplifier at 1043 nm as a pump source addressing the 3F4 → 3F2,3 excited-state absorption transition of Tm3+ ions. In the continuous-wave regime, the Tm-laser generates 616 mW at ∼2313 nm with a slope efficiency of 10.0% (vs. the incident pump power) and a linear polarization (π). By implementing a type-I SESAM with a single ternary strained In0.33Ga0.67Sb quantum well embedded in GaSb for sustaining and stabilizing the soliton pulse shaping, the self-starting mode-locked Tm-laser generated pulses as short as 870 fs at a central wavelength of 2309.4 nm corresponding to an average output power of 208 mW at a pulse repetition rate of 105.08 MHz and excellent mode-locking stability. The output power was scaled to 450 mW at the expense of a longer pulse duration of 1.93 ps. The nonlinear parameters of the SESAM are also reported.

Tm:CALGO lasers at 2.32†µm: cascade lasing and upconversion pumping.

We report on the first laser operation of a disordered Tm:CaGdAlO4 crystal on the 3H4 → 3H5 transition. Under direct pumping at 0.79 µm, it generates 264 mW at 2.32 µm with a slope efficiency of 13.9% and 22.5% vs. incident and absorbed pump power, respectively, and a linear polarization (σ). Two strategies to overcome the bottleneck effect of the metastable 3F4 Tm3+ state leading to the ground-state bleaching are exploited: cascade lasing on the 3H4 → 3H5 and 3F4 → 3H6 transitions and dual-wavelength pumping at 0.79 and 1.05 µm combining the direct and upconversion pumping schemes. The cascade Tm-laser generates a maximum output power of 585 mW at 1.77 µm (3F4 → 3H6) and 2.32 µm (3H4 → 3H5) with a higher slope efficiency of 28.3% and a lower laser threshold of 1.43 W, out of which 332 mW are achieved at 2.32 µm. Under dual-wavelength pumping, further power scaling to 357 mW at at 2.32 µm is observed at the expense of increased laser threshold. To support the upconversion pumping experiment, excited-state absorption spectra of Tm3+ ions for the 3F4 → 3F2,3 and 3F4 → 3H4 transitions are measured for polarized light. Tm3+ ions in CaGdAlO4 exhibit broadband emission at 2.3 - 2.5 µm making this crystal promising for ultrashort pulse generation.

Excited-state absorption and upconversion pumping of Tm3+-doped potassium lutetium double tungstate.

We report on a bulk thulium laser operating on the 3H4 → 3H5 transition with pure upconversion pumping at 1064 nm by an ytterbium fiber laser (addressing the 3F4 → 3F2,3 excited-state absorption (ESA) transition of Tm3+ ions) generating 433 mW at 2291 nm with a slope efficiency of 7.4% / 33.2% vs. the incident / absorbed pump power, respectively, and linear laser polarization representing the highest output power ever extracted from any bulk 2.3 µm thulium laser with upconversion pumping. As a gain material, a Tm3+-doped potassium lutetium double tungstate crystal is employed. The polarized ESA spectra of this material in the near-infrared are measured by the pump-probe method. The possible benefits of dual-wavelength pumping at 0.79 and 1.06 µm are also explored, indicating a positive effect of co-pumping at 0.79 µm on reducing the threshold pump power for upconversion pumping.

Watt-level efficient 2.3 µm thulium fluoride fiber laser.

We report on an efficient mid-infrared thulium (Tm) fiber laser operating on the 3H4→3H5 transition and featuring an upconversion pumping scheme. This laser comprises a heavily Tm3+-doped (2.50 mol. %) zirconium fluoride glass fiber pumped by a tunable Yb fiber laser around 1.05 µm corresponding to the 3F4→3F2,3 excited-state absorption transition. The laser generates 1.24 W at 2269-2282 nm with a slope efficiency of 37% in the quasi-continuous-wave regime. The Tm-glass fiber exhibits a broadband 3H4→3H5 emission with a bandwidth of 173 nm, making it very promising for femtosecond fiber oscillators at ∼2.3µm.

Widely tunable in-band-pumped Tm:CaF2 laser.

This Letter presents the efficient laser operation of a Tm:CaF2 crystal in-band pumped at 1610 nm by an Er-Yb-codoped fiber laser system. A laser slope efficiency of 55% (versus incident pump power) was achieved in a continuous-wave regime, with a maximum output power of 1.25 W at ∼1.88µm in a nearly diffraction-limited beam (M2=1.14). We also demonstrated a continuous tuning range of 180 nm, which extends to short wavelengths down to 1773 nm.

Laser operation of highly-doped Tm:LiYF4 epitaxies: towards thin-disk lasers.

Quasi-continuous-wave laser operation of 20 at.% Tm:LiYF4 thin films (84-240 µm) grown by Liquid Phase Epitaxy (LPE) on undoped LiYF4 substrates is achieved. The 240 µm-thick Tm:LiYF4 active layer pumped at 793 nm with a simple double-pass scheme generated 152 mW (average power) at 1.91 µm with a slope efficiency of 34.4% with respect to the absorbed pump power. A model of highly-doped Tm:LiYF4 lasers accounting for cross-relaxation, energy-transfer upconversion and energy migration is developed showing good agreement with the experiment. The pump quantum efficiency for Tm3+ ions is discussed and the energy-transfer parameters are derived. These results show that LPE-grown Tm:LiYF4 thin films are promising for ~1.9 µm thin-disk lasers.

Watt-level Tm:LiYF4 channel waveguide laser produced by diamond saw dicing.

Low-loss surface channel waveguides with a cross-section of 30 × 30 µm2 are produced by diamond saw dicing of 6.2 at.% Tm3+, 3.5 at.% Gd3+:LiYF4 films grown by liquid phase epitaxy (LPE) on (001)-oriented bulk undoped LiYF4 substrates. Pumped by a Ti:Sapphire laser at 783 nm, a continuous-wave Tm:LiYF4 waveguide laser generated 1.30 W at 1880 nm (for π-polarization) with a slope efficiency of 80% with respect to the absorbed pump power. The laser threshold was at 80 mW. The waveguide morphology was studied revealing low roughness (3 ± 2 µm) as expressed by the propagation losses of <0.3 dB/cm. A combination of LPE and diamond saw dicing is a promising technology for multi-watt single-mode channel waveguide lasers and amplifiers.

Tm,Ho:LiYF4 planar waveguide laser at 2.05 µm.

The first holmium fluoride waveguide laser, to the best of our knowledge, is reported using a 25-µm-thick Gd3+-ion-modified 4.5 at. % Tm3+, 0.5 at. % Ho3+-codoped LiYF4 active layer grown by liquid phase epitaxy on (001)-oriented LiYF4 substrate. Pumped by a Ti:sapphire laser at 797.2 nm, the planar waveguide laser generates 81 mW of continuous-wave (CW) output at ∼2051 nm with a slope efficiency of 24%. Power scaling up to 186 mW at 2051 nm and 2065 nm in quasi-CW regime is demonstrated. The parameters of the Tm3+↔Ho3+ energy transfer are determined. Tm,Ho:LiYF4/LiYF4 epitaxies are promising for waveguide lasers and amplifiers at above 2 µm.

2.3 µm Tm3+:YLF mode-locked laser.

A passively mode-locked Tm:YLF laser emitting at 2.3 µm is reported for the first time, to the best of our knowledge. The continuous-wave stable mode-locking operation is obtained with a semiconductor saturable absorber mirror at a repetition rate of 100 MHz. The average output power is 165 mW for a pulse duration of 94 ps.

Mid-infrared rotated image singly resonant twisted rectangle optical parametric oscillator based on HgGa(2)S(4) pumped at 1064 nm.

We compare linear, planar ring, and rotated image singly resonant twisted rectangle (RISTRA) type nanosecond optical parametric oscillator cavities using HgGa2S4 nonlinear crystal pumped by 8 ns pulses at 1064 nm from a low beam quality pump source. The input-output characteristics and the output idler beam quality at 6300 nm are compared for two values of the pump beam diameter presenting different cavity Fresnel numbers and magnitudes of the spatial walk-off effect due to birefringence. The RISTRA cavity ensures in all cases a circular output beam profile but is advantageous in terms of beam quality with respect to the planar ring only at a large pump beam diameter.

Narrow-bandwidth, ~100 ps seeded optical parametric generation in CdSiP2 pumped by Raman-shifted pulses at 1198 nm.

Low-threshold, efficient optical parametric generation at ~4.64 µm is demonstrated using CdSiP2 nonlinear crystal pumped by 150 ps Raman shifted pump pulses at 1198 nm in noncritical configuration at 1 kHz repetition rate. Maximum single pulse idler energy of 6 µJ and total conversion efficiency of 30% are achieved. Seeding at the signal wavelength with a distributed feedback laser diode enables ~25 fold narrowing of the bandwidths down to ~10 GHz, resulting in a Fourier product of ~1 for the ~100 ps long signal (1615 nm) and idler (4.64 µm) pulses.

Midinfrared optical parametric oscillator based on the wide-bandgap BaGa4S7 nonlinear crystal.

The orthorhombic biaxial crystal BaGa(4)S(7) has been employed in a 1064 nm pumped optical parametric oscillator generating <6 ns long idler pulses with energies as high as 0.5 mJ at 6.217 µm and average power of ~50 mW at 100 Hz. Notwithstanding the relatively low nonlinearity, ~3 times above threshold operation has been achieved at pump intensities more than 5 times below the crystal surface damage limit.

Narrow-bandwidth, mid-infrared, seeded optical parametric generation in 90° phase-matched CdSiP2 crystal pumped by diffraction limited 500 ps pulses at 1064 nm.

Low-threshold, efficient optical parametric generation at ~6100 nm is demonstrated using CdSiP2 nonlinear crystal at 1 to 10 kHz repetition rates with relatively long 500 ps pump pulses at 1064 nm. Maximum single pulse energy of 8.7 µJ and average power of 79 mW are achieved for the idler. Seeding at the signal wavelength is employed using a distributed feedback laser diode, which enables approximately tenfold narrowing of the idler bandwidth down to less than 1 nm.

Optical parametric generation in CdSiP2 at 6.125 µm pumped by 8 ns long pulses at 1064 nm.

A 21.4 mm long noncritically cut CdSiP2 crystal, pumped by 8 ns pulses at 1064 nm in a double-pass configuration for pump, signal, and idler, generated 523 µJ, 5.8 ns idler pulses at 6.125 µm. The average power of 52.3 mW at the repetition rate of 100 Hz is the highest ever achieved at such wavelengths with direct down conversion from the 1 µm spectral range.

Picosecond mid-infrared optical parametric amplifier based on the wide-bandgap GaS(0.4)Se(0.6) pumped by a Nd:YAG laser system at 1064 nm.

Operation of a GaS(0.4)Se(0.6) optical parametric amplifier is demonstrated with a 5-11 µm idler tuning range, maximum energies of ∼10 µJ for sub-30-ps pulse durations, and performance ∼3 times better than with pure GaSe.

Subnanosecond, 1 kHz, temperature-tuned, noncritical mid-infrared optical parametric oscillator based on CdSiP(2) crystal pumped at 1064 nm.

Operation of an optical parametric oscillator based on CdSiP(2) and pumped at 1064 nm is demonstrated at a repetition rate of 1 kHz. The maximum output idler energy of 24 microJ at 6.125 microm corresponds to an average power of 24 mW. Increasing the crystal temperature up to 150 degrees in the noncritical (90 degrees) configuration leads to idler wavelength tuning from 6.117 to 6.554 microm. Subnanosecond pulse durations are obtained for the signal and idler as a result of the 1 ns pulse duration of the pump, made possible by the rather short crystal and cavity lengths (approximately 1 cm).

Nd:YAG pumped nanosecond optical parametric oscillator based on LiInSe2 with tunability extending from 4.7 to 8.7 microm.

LiInSe(2) is one of the few (only 5) non-oxide nonlinear optical crystals whose band-gap (2.86 eV) and transparency allowed in the past nanosecond optical parametric oscillation in the mid-IR without two-photon absorption for a pump wavelength of 1064 nm. However, the first such demonstration was limited to the 3.3-3.78 microm range for the idler and the average idler power did not exceed 2.5 mW. Here we report broadly tunable operation, from 4.7 to 8.7 microm, of an OPO based on LiInSe(2), achieving maximum idler pulse energy of 282 microJ at approximately 6.5 microm, at a repetition rate of 100 Hz (approximately 28 mW of average power).