Leonard A. Pomeranz

Efficient mid-infrared laser using 1.9-µm-pumped Ho:YAG and ZnGeP 2 optical parametric oscillators

We report >4.2-W mid-infrared (mid-IR) output (3.8 and 4.65 µm) and >2.1 W at 3.5 µm in ZnGeP2 (ZGP) optical parametric oscillators (OPO) pumped by a holmium-doped yttrium aluminum garnet (Ho:YAG) laser, directly pumped by a diode-pumped 1.9-µm thulium-doped yttrium lithium fluoride (Tm:YLF) laser. Optical-to-optical efficiency achieved is >7.2% (laser diode to mid-IR). In addition, a Ho:YAG-pumped ZGP OPO operation is achieved over a Ho:YAG temperature range of 80 °C at the 8-W (Ho:YAG) and 3-W (ZGP OPO) power levels.

Advances in nonlinear optical crystals for mid-infrared coherent sources

Advances in growth of the birefringent crystals ZnGeP2 and CdSiP2, as well as all-epitaxial processing of orientation-patterned semiconductors GaAs (OP-GaAs) and GaP (OP-GaP), are extending solid-state laser output deep into the mid-infrared. These materials exhibit the highest nonlinear coefficients and broadest infrared transparency ranges among all practical nonlinear optical crystals. In this review paper we describe the attractive properties of these materials, along with the unique capabilities and novel crystal growth and processing that continue to provide record-breaking conversion efficiencies and output powers in the mid-infrared.

1-μm-pumped OPO based on orientation-patterned GaP

Orientation patterned gallium phosphide (OP-GaP) is a new nonlinear optical (NLO) crystal which exhibits the highest nonlinear coefficient (d14=70.6 pm/V) and the longest infrared cut-off (12.5 μm) of any quasi-phase-matched (QPM) material that can be pumped at 1-μm without significant two-photon absorption. Here we report the first 1064nm-pumped OPO based on bulk OP-GaP. Multi-grating OP-GaP QPM structures were grown by producing an inverted GaP layer by polar-on-nonpolar molecular beam epitaxy (MBE), lithographically patterning, reactive ion etching, and regrowing by MBE to yield templates for subsequent bulk growth by low-pressure hydride vapor phase epitaxy (LP-HVPE). The pump source was a diode-end-pumped Nd:YVO4 monoblock laser with an RTP high-voltage Q-switch (1064 nm, 1W, 10kHz, 3.3 ns) which was linearly polarized along the <100> orientation of the AR-coated 16.5 x 6.3 x 1.1 mm3 OP-GaP crystal (800-μm thick HVPE layer, 20.8 μm grating period only 150 μm thick) mounted on a copper blocked maintained at 20°C by a thermo-electric cooler. The OPO cavity was a linear resonator with 10-cm ROC mirrors coated for DRO operation (85%R at signal, 55%R at idler). The pump 4σ-diameter at the crystal face was 175 μm. The observed OPO signal (idler) threshold was 533 mW (508 mW) with a slope efficiency of 4% (1%) and maximum output power 15 mW (4 mW). The signal (1342 nm) and idler (4624 nm) output wavelengths agreed well with sellemier predictions. Orange parasitic output at 601.7nm corresponded to 9th order QPM sum frequency mixing of the 1064-nm pump and the 1385-nm signal.

Optical parametric oscillation in quasi-phase-matched GaP

Orientation patterned gallium phosphide (OP-GaP) is a new quasi-phase-matched (QPM) nonlinear optical (NLO) semiconductor for mid-infrared frequency generation. It overcomes several limitations of ZGP, the current NLO crystal of choice for 2-μm-pumped optical parametric oscillators (OPOs): OP-GaP exhibits lower 2-μm absorption loss, higher thermal conductivity, noncritical phase matching via quasi-phase matching (QPM), and a larger band gap that allows for pumping at 1064 nm. Here we report the first OPO based on bulk OP-GaP. Multi-grating OP-GaP QPM structures were grown by polar-on-nonpolar molecular beam epitaxy (MBE), lithographically patterned, reactive ion etched, and regrown by MBE to yield templates for subsequent bulk growth by low-pressure hydride vapor phase epitaxy (LP-HVPE). A Tm-fiber-pumped Ho:YAG pump laser was line narrowed with a volume Bragg grating (2090nm, 20W, 20kHz, 12 ns) and linearly polarized along the <100> orientation of the AR-coated 16.5 x 6.3 x 1.1 mm3 OP-GaP crystal (QPM layer = 800 μm thick, grating period = 92.7 μm) mounted on a copper blocked maintained at 20°C by a thermo-electric cooler. The OPO cavity was a linear resonator with 10-cm ROC mirrors coated for DRO operation (85%R at signal, 55%R at idler). The pump spot size at the crystal face was 250 μm. The observed OPO threshold was 3.1 W (44 MW/cm2) with a slope efficiency of 16% and a maximum output power of 350 mW until surface damage occurred at 1.25 to 1.5 J/cm2. The signal (3.54 μm) and idler (5.1 μm) output wavelengths agreed well with sellmeier predictions.

Recent advances in all-epitaxial growth and properties of orientation-patterned gallium arsenide (OP-GaAs)

We report on all-epitaxial growth of large diameter (3-inch), large aperture (≫1.5mm thick), low-loss (≪0.005cm⁻¹) QPM GaAs. 2-µm-laser-pumped OPO performance was comparable to that of ZnGeP<inf>2</inf>.

Efficient Mid-Infrared Optical Parametric Oscillator Based on CdSiP 2

We report the first optical parametric oscillator based on the new mid-infrared nonlinear optical crystal CdSiP2. Pumping with a 2W, 1.99-micron Tm:YALO laser produced 340 mW average power output (signal + idler) at 27% slope.

First OPO based on orientation-patterned gallium phosphide (OP-GaP)

Optical parametric oscillation was achieved for the first time in OP-GaP. Tm-fiber-pumped Ho:YAG (2090nm, 20W, 20kHz, 12ns) generated 350 mW signal (3.54μm) plus idler (5.1μm) from a 92.7-μm grating period crystal in a linear DRO.

HVPE of Orientation-Patterned Gallium Phosphide (OP-GaP) with Novel Quasi-Phasematched Device Structures

We report the successful growth and fabrication of engineered quasi-phasematched grating structures - parallel gratings, tandem gratings, linear- and curved fan gratings, and chirped gratings - in the new nonlinear optical semiconductor OP-GaP.

High efficiency compact mid-IR sources

We describe mid-IR sources utilizing ZGP and CSP Optical Parametric Oscillators (OPO) directly pumped by high efficiency 1.94 μm Tm:YAP Q-switched lasers. Compact Q-switched Tm:YAP lasers, implemented using Cr:ZnS saturable absorbers, generated 29 kW peak power pulses and an average power of 4W. The OPOs, constructed using the latest generation ZGP and CSP crystals with low 1.94 μm absorption, were operated at near-degeneracy with mid-IR output in the 3.6-4.2 μm range. Various doubly-resonant OPO configurations were evaluated, including single-pass pump pass and double-pass pumping. Maximum mid-IR powers of 2.3 W and 2.5 W and optical conversion efficiencies of 58% and 64% were measured for ZGP and CSP double-pass pump OPOs, respectively.

Advances in growth and power scaling of the nonlinear crystal CdSiP2 (Conference Presentation)

CdSiP2 (CSP) is a nonlinear optical chalcopyrite semiconductor developed as a wider-band-gap analog of ZnGeP2 (ZGP) to enable mid-infrared generation with widely-available 1- and 1.55-micron pump laser sources. CSP has an even higher nonlinear coefficient (d36=84.5 pm/V) than ZGP (d36=79 pm/V), and its lower thermal conductivity (13.6 W/mK vs 35 W/mK for ZGP) is more than offset by nearly 10-fold lower absorption losses in the 1.06- to 2.1-micron wavelength range, making CSP an attractive alternative to ZGP even for power-scaling 2-micron-pumped OPOs. CSP growth presents significant crystal growth challenges compared to ZGP including: a higher melting point and vapor pressure that push the limits of fused silica based growth technology, a higher reactivity with boat materials and fused silica ampoules, an increased incidence of twin formation, and a negative c-axis thermal expansion coefficient (which makes it prone to cracking). Despite these difficulties, recent advances in crystal growth from stoichiometric melts using the horizontal gradient freeze (HGF) technique have resulted in scaling boule diameters from 19 to 28 millimeters. Improved refractive index data was recently measured at AFRL over a wide temperature range from 85K through 450K and fit to a temperature-dependent Sellmeier equation. More accurate thermal conductivity measurements along the major crystallographic axes are also reported. Finally, we report the first multi-watt CSP OPO: over 5 Watts of average power output (near-degenerate signal plus idler) were achieved at a slope efficiency of 60% from tandem, walk-off-compensated CSP crystals in a linear resonator pumped by a q-switched 32-nanosecond 1.94-micron Tm:YAP laser.