Knut Stenersen

Modeling of laser-pumped Tm and Ho lasers accounting for upconversion and ground-state depletion

Models for numerical simulations of laser-pumped thulium- and thulium-holmium-doped lasers have been developed. In the models, upconversion losses and ground-state depletion in both thulium and holmium are accounted for, as well as spatial dependencies of the pump and resonator modes. The models apply to CW operation and to the build-up of population inversion prior to lasing in pulsed modes of operation. It is shown that upconversion losses in Tm:Ho:YAG significantly reduce the output from the laser in both CW and Q-switched mode. Simulations of CW lasers show good agreement with experimental results.

Midinfrared laser source with high power and beam quality

A simple scheme for generation of high power in the midinfrared is demonstrated. By using a 15 W thulium-doped fiber laser emitting at 1907 nm to pump a Q-switched Ho:YAG laser, we obtained 9.8 W at 2096 nm at a 20 kHz pulse repetition rate with excellent beam quality. The output of this laser was used to pump a doubly resonant zinc germanium phosphide based optical parametric oscillator, and we obtained 5.1 W average power in the 3-5 microm range with M2 approximately = 1.8.

A 22-watt mid-infrared optical parametric oscillator with V-shaped 3-mirror ring resonator

We report on a ZnGeP(2)-based optical parametric oscillator (OPO) with 22 W of output power in the 3-5 µm range and a beam quality factor M(2) ≈1.4. The OPO uses a novel V-shaped 3-mirror ring resonator that allows two passes of the beams through the same nonlinear crystal. The pump is a 39 W hybrid Tm:fiber laser/Ho:YAG laser.

High power and efficient long wave IR ZnGeP2 parametric oscillator

A high power, efficient, and tunable laser source in the 8-10 microm range, based on a ZnGeP(2) optical parametric oscillator (OPO) pumped by a hybrid 2.1 microm laser is demonstrated. The hybrid laser consists of a Q-switched Ho:YAG laser pumped by a 15 W CW thulium fiber laser. With 8.9 W of 2.1 microm pump power we obtained 0.95 W at 8 microm with an M(2)-value of 2.7 from an OPO with two walk-off compensating crystals.

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.

High-pulse-energy mid-infrared laser source based on optical parametric amplification in ZnGeP2

Nonlinear optical conversion of 1.064 µm pulses from a Q-switched Nd:YAG laser to the mid-infrared is demonstrated. The experimental setup is based on a two-stage master-oscillator/power-amplifier (MOPA) design with a KTiOPO4 based MOPA in the first stage and a KTiOAsO4/ZnGeP2 based MOPA in the second stage. The setup can be tuned to provide output at 8 µm or in the 3–5 µm wavelength region. We obtain more than 8 mJ at 8 µm, and up to 33 mJ at 3–5 µm. The measured beam quality factors are in the range M2=2–4 for both wavelength regions.

Efficient conversion from 1 to 2 µm by a KTP-based ring optical parametric oscillator

Conversion of Q -switched 1.064-microm Nd:YAG laser pulses to the 2-2.2-mu; m region with 46% efficiency is demonstrated with a KTP-based type 2 phase-matched optical parametric oscillator (OPO) with two pairs of walk-off compensating crystals in a ring resonator. With 10 mJ of pump energy, we obtain 2.5 mJ at the 2.06-microm signal and 2.1 mJ at the 2.2-microm idler, with a beam quality of M(2) approximately 1.4 . With a ZnGeP(2) -based OPO pumped by the signal from the KTP OPO we achieved 14% conversion efficiency from 1.064microm to the 3-5-microm range.

Importance of pump-beam group velocity for backconversion in optical parametric oscillators

We present numerical simulations that indicate that backconversion in optical parametric oscillators depends strongly on the group-velocity mismatch (GVM) between the pump and the signal beams. Small GVM tends to suppress modulation of the signal, and the resultant narrow signal spectrum can favor strong backconversion. Large GVM permits a wider spectrum, which in turn can reduce backconversion. Comparison of simulation results and experimental data supports our theory.

Mid-infrared optical parametric oscillator synchronously pumped by an erbium-doped fiber laser

A mid-infrared synchronously pumped optical parametric oscillator pumped by a femtosecond erbium-doped fiber laser is demonstrated and characterised. The idler is tunable from 3.7-4.7 μm, with a maximum average power of 37 mW and a pulse length of ∼ 480 fs at 4 μm. We compare the experimental results with numerical results based on an extensive simulation model.

Fiber laser pumped Mid-IR source

We report on a compact and efficient 3-5 μm laser source, based on a 1.9 μm Tm:fiber laser, pumping a Q-switched 2.1 μm Ho:YAG-laser, which in turn pumps a ZnGeP2 (ZGP) optical parametric oscillator (OPO). Recent advances in Tm:fiber laser technology has resulted in practical high power 1.9 μm sources, which are ideal pump sources for high pulse rate Q-switched Ho:YAG lasers. Using a 15 W commercial Tm:fiber laser we obtained 5.6W average output power from the Ho:YAG laser and 3 W from the ZGP OPO.