Peter A. Ketteridge

Mid-infrared ZnGeP 2 parametric oscillator directly pumped by a pulsed 2 μm Tm-doped fiber laser

We have demonstrated what we believe to be the first mid-infrared optical parametric oscillator (OPO) pumped directly by a pulsed Tm-doped fiber laser. The Tm-fiber pump laser produces 30 ns pulses with a repetition rate of 30 kHz at a wavelength of 2 μm. The ZnGeP2 (ZGP) OPO produces 20 ns mid-IR pulses in the 3.4-3.9 μm and 4.1-4.7 μm spectral regions simultaneously. More than 658 mW of mid-IR output power has been generated with a total OPO slope efficiency greater than 35%.

Compact, high average power, fiber-pumped terahertz source for active real-time imaging of concealed objects

We have modeled and demonstrated a scalable, compact, fiber-pumped terahertz source based on difference frequency mixing (DFM) in zinc germanium phosphide (ZGP) capable of producing high average and peak-power pulses. Currently, our terahertz source produces 2 mW of average THz power and >40 W of peak power with sub-nanosecond pulses at a repetition rate of 100 kHz in the range of 2-3 THz without cryogenic cooling or ultra-fast optics. This high average power laser-based terahertz output enables the real-time imaging of concealed objects using an off-the-shelf uncooled microbolometer focal-plane array. With this THz system, we have imaged objects obscured inside in a common shipping envelope, demonstrating the potential of compact laser-based terahertz sources for use in security screening applications.

2-μm-pumped 8-12-μm OPO source for remote chemical sensing

A 100 Hz, optical parametric oscillator (OPO) lidar breadboard is designed, built and tested for remote chemical sensing in the 8 - 12 micrometers range. Continuous tuning is achieved by angle tuning a type II, silver gallium selenide (AgGaSe2) OPO crystal pumped in a single step by a 2.088-micrometers pump laser. The pump source for the OPO consists of a temperature stabilized, continuously pulsed, resonantly pumped Ho:YAG (2.088-micrometers ) laser, end-pumped by a diode- end-pumped Tm:YLF (1.9-micrometers ) laser. The 9 mm X 5 mm X 25 mm-long OPO crystal was mounted on a computer-controlled galvanometer scanner for rapid wavelength tuning (1.5 micrometers between shots). Continuous tunability was demonstrated from 7.9 to 12.6 micrometers with energies in the 50 - 400 (mu) J range. Quantum slope conversion efficiencies up to 40% were obtained. Far-field beam divergence measurement showed the output of the OPO to be 2.6 times diffraction limit. The improved OPO beam quality over previous studied tandem OPO systems is attributed to the reduced Fresnel number of the OPO cavity (idler resonating) and the better beam quality of the pump source. A LabWindows based data collection and analysis system is implemented. The effectiveness of the OPO as a source for chemical sensing is demonstrated by the collection of the absorption spectrum of ammonia.

Compact Fiber-Pumped Terahertz Source Based on Difference Frequency Mixing in ZGP

We have demonstrated a novel terahertz source based on dual-wavelength amplification in polarization maintaining Yb- doped fiber and frequency mixing in a zinc germanium phosphide (ZGP) crystal. The system consists of two orthogonally polarized signals, whose difference frequency is in the terahertz region, amplified in a single all-fiber amplifier chain and mixed in a ZGP crystal to generate high-peak-power terahertz radiation. Currently, 2 mW of average terahertz power (20 W peak, 20 nJ/pulse) has been produced at a repetition rate of 100 kHz and pulsewidths of 1 ns with a conversion efficiency of 0.137%. We have also developed a terahertz mixing model, which coincides well with our experimental data.

Pulsed Tm-doped fiber lasers for mid-IR frequency conversion

Fiber lasers are an ideal pump source for nonlinear frequency conversion because they have the capability to generate short pulses with high peak-powers and excellent beam quality. Thulium-doped silica fibers allow for pulse generation and amplification in the 2-micron spectral band. This opens the door to a variety of nonlinear crystals, such as ZnGeP2 (ZGP) and orientation patterned GaAs (OPGaAs), which cannot be pumped by Yb- or Er-doped fiber laser directly due to high losses in the near-IR band. These crystals combine low losses with high nonlinearities and transparency for efficient nonlinear mid-IR converters. Using such nonlinear crystals and a pulsed Tm-doped master oscillator fiber amplifier (MOFA), we have demonstrated efficient mid-IR generation with watts of output power in the 3-5μm region. The Tm-doped MOFA is capable of generating from 10 to 100W of average output power at a variety of repetition rates (10kHz - >500kHz) and pulse widths (10ns - >100ns). Total mid-IR power is only limited by thermal effects in the nonlinear materials. The use of Tm-doped fiber-pumped OPOs shows the path toward compact, efficient, and lightweight mid-IR laser systems.

Real-time terahertz imaging system for the detection of concealed objects

A terahertz imaging system and method of use including a compact Yb-doped fiber laser-pumped ZGP crystal as a THz source and an uncooled microbolometer array as a detector. According to the present invention, semiconductor lasers are also drive current modulated to produce desired laser pulsewidth, repetition rate and wavelengths needed for DFG THz generation in various non-linear crystals. The fiber-coupled semiconductor lasers provide at least two wavelengths that will produce THz radiation by DFG in non-linear converter. These two wavelengths are combined and amplified in a single Yb fiber amplifier chain. Yb amplifier is staged in continually increasing core diameters to provide significant signal amplification while suppressing deleterious non-linear effects.

Multi-watt mid-IR fiber-pumped OPO

We have demonstrated a multi-watt mid-IR ZGP OPO pumped by a pulsed Tm-doped fiber laser operating at 1.995 um. 2 W of total mid-IR output power has been generated in the 3.4-3.9 mum and 4.0-4.7 mum spectral regions simultaneously.

Thulium fiber laser-pumped mid-IR OPO

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 ZnGeP2 (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.

High power pulse amplification in Tm-doped fiber

We report >20 W of average output power at 1.995 mum from a pulsed Tm-doped fiber amplifier system operating at 100 kHz. Pulse energies of >325 muJ have been generated at 50 kHz with 13 ns pulses in the same amplifier.

Compact fiber pumped terahertz source

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.