Cadmium silicon phosphide, CdSiP2 or CSP, exhibits the highest nonlinear Figure of Merit (d2/n3) among all bulk birefringent crystals that can be pumped at wavelengths ≤ 2 microns - and the highest among all nonlinear optical (NLO) crystals at any wavelength with practically low absorption losses. Although a 2-micron pumped optical parametric oscillator (OPO) and numerous 1-micron-pumped CSP OPOs have been demonstrated previously, here we report the first 1.5-micron-pumped CSP OPO.
The pump laser for this experiment was a non-critically phase-matched (NCPM) KTP singly-resonant OPO generating 6.3 ns, 10 mj pulses at 1.57 microns, which was in turn pumped by a 1.064-m 5-Hz, 7-ns, 20-mJ Nd:YAG laser. The 1.57-m signal from the KTP OPO pumped a 5x5 mm2 x 5.5-mm long CSP crystal cut for type I phase matching at 63°) inside a linear flat-flat cavity. The measured absorption coefficient at 1570 nm was approximately 0.015cm-1. The input mirror was highly transmitting (97%T) at 1.57 microns and highly reflective from 2.5 to 3.8 microns. The best results were achieved at signal and idler wavelengths of 2.67 microns (50%R output coupling) and 3.83 microns (30%R output coupling) respectively, yielding 2.24 mJ of combined signal and idler output energy for a total optical conversion efficiency of 32%. The slope efficiency was 42%. At 66 MW/cm2 pump peak intensity the OPO was running nearly 4 times over threshold with no signs of roll-over. The CSP crystal was angle-tuned to produce output wavelengths ranging from 2.2 microns to 5.05 microns.
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.
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.
Orientation patterned gallium phosphide (OP-GaP) is a new nonlinear optical (NLO) crystal which exhibits the highest nonlinear coefficient (d<sub>14</sub>=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:YVO<sub>4</sub> 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 mm<sup>3</sup> 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 9<sup>th</sup> order QPM sum frequency mixing of the 1064-nm pump and the 1385-nm signal.
Fiber lasers are advancing rapidly due to their ability to generate stable, efficient, and diffraction-limited beams with
significant peak and average powers. This is of particular interest as fibers provide an ideal pump source for driving
parametric processes. Most nonlinear optical crystals which provide phase-matching to the mid-IR at commercially
available fiber pump wavelengths suffer from high absorption above 4μm, resulting in low conversion efficiencies in the
4-5μm spectral region. The nonlinear optical crystals which combine low absorption in this same spectral region with
high nonlinear gain require pumping at longer wavelengths (typically >1.9μm). In this paper, we report a novel mid-IR
OPO pumped by a pulsed thulium-doped fiber laser operating at
2-microns. The eyesafe thulium-fiber pump laser
generates >3W of average power at >30kHz repetition rate with
15-30ns pulses in a near diffraction-limited beam. The
ZnGeP<sub>2</sub> (ZGP) OPO produces tunable mid-IR output power in the
3.4-3.99μm (signal) and the 4.0-4.7μm (idler) spectral
regions in both singly resonant (SRO) and doubly resonant (DRO) formats. The highest mid-IR output power achieved
from this system was 800mW with 20% conversion efficiency at 40kHz. In a separate experiment, the 3W of 2-micron
light was further amplified to the 20W level. This amplified output was also used to pump a ZGP OPO, resulting in 2W
of output power in the mid-IR. To our knowledge, these are the first demonstrations of a fiber-pumped ZGP OPO.
Converting near infrared signals in a nonlinear medium is an attractive way to generate terahertz radiation due to the
availability of near-IR lasers and nonlinear materials. However, these terahertz generation schemes are typically
inefficient and are often cumbersome, which may limit their use in certain applications. We have developed and
demonstrated a compact, fiber pumped optical terahertz source based difference frequency mixing (DFM) of nanosecond
pulses in zinc germanium phosphide (ZGP). With this setup, we have successfully generated 2mW of average power
terahertz radiation at 2.45THz. This has enabled us to perform active, real-time terahertz imaging experiments using an
uncooled microbolometer array. In performing these experiments, we have also developed a theoretical model for
terahertz generation based on DFM of IR pump signals. In this paper, we discuss our compact fiber pumped terahertz
source technology, imaging system, model, and how we intend to overcome some of the common issues associated with
optical terahertz generation.
Design and projected performance of a lightweight, 1064 nm, laser transmitter is described. A 40 mJ, diode side-pumped Nd:YAG zig-zag slab oscillator is configured to operate on either of two Nd:YAG absorption bands (795 nm or 807 nm) to provide stable operation over a 100°C temperature range, using minimal power for diode wavelength stabilization.
Laser radar systems are required for various military applications including obstacle detection, target recognition, and terrain mapping. Each application requires different system parameters such as pulse energy, repetition rate, and field of view. This paper presents a review of a multifunction laser radar system developed and tested for the Cooperative Eyesafe Laser Radar Program (CELRAP) of the U.S. Army CECOM Night Vision and Electronic Sensors Directorate.
Laser radar systems are required for various military applications including obstacle detection, target recognition, and terrain mapping. Each application requires different system parameters such as pulse energy, repetition rate, and field of view. This paper is the second in a series of papers describing the progress toward a multifunction laser radar system under construction for the Cooperative Eyesafe Laser Radar Program (CELRAP) of the U.S. Army CECOM Night Vision and Electronic Sensors Directorate.
Laser radar systems are required for various military applications including obstacle detection, target recognition, and terrain mapping. Each application requires different system parameters such as pulse energy, repetition rate, and field of view. This paper presents a review of a multifunction laser radar system under construction for the Cooperative Eyesafe Laser Radar Program (CELRAP) of the U.S. Army CECOM Night Vision and Electronic Sensors Directorate.