Peter G. Schunemann

Octave-spanning ultrafast OPO with 2.6-6.1µm instantaneous bandwidth pumped by femtosecond Tm-fiber laser

We report the extension of broadband degenerate OPO operation further into mid-infrared. A femtosecond thulium fiber laser with output centered at 2050 nm synchronously pumps a 500-μm-long crystal of orientation patterned GaAs providing broadband gain centered at 4.1 µm. We observe a pump threshold of 17 mW and output bandwidth extending from 2.6 to 6.1 µm at the -30 dB level. Average output power was 37 mW. Appropriate resonator group dispersion is a key factor for achieving degenerate operation with instantaneously broad bandwidth. The output spectrum is very sensitive to absorption and dispersion introduced by molecular species inside the OPO cavity.

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%.

Ultralow Gradient HGF-Grown ZnGeP 2 and CdGeAs 2 and Their Optical Properties

ZnGeP 2 and CdGeAs 2 have long been recognized as promising crystals for infrared frequency generation. They exhibit the highest nonlinear optical coefficients ( d 36 equals 75 pm/V and 236 pm/V for ZnGeP 2 and CdGeAs 2 , respectively) among all known compounds that possess adequate birefringence for phase matching. ZnGeP 2 s transparency range (0.62−13 μ m) makes it the optimum material for shifting the wavelength of 2- μ m pump lasers into the 3–5- μ m range via optical parametric oscillation (OPO), whereas that of CdGeAs 2 (2.3–18 μ m) is better suited for doubling the frequency of CO 2 lasers (9–11 μ m) into the same range via second-harmonic generation. In both cases however, the application of these materials has been hindered by great difficulty in achieving crack-free single crystals, and by large defect-related absorption losses. The horizontal-gradient-freeze (HGF) growth technique has been instrumental in overcoming these difficulties. “Ultralow” axial gradients (1–3°C/cm) have been used to control stoichiometry by minimizing vapor transport as well as to eliminate cracking due to anisotropic thermal expansion. (The a -axis and c -axis thermal-expansion coefficients of ZnGeP 2 differ by a factor of two, whereas those of CdGeAs 2 differ by a factor of 15.) In addition, oriented seeds were used to ensure monocrystalline nucleation (because even a small degree of polycrystallinity can lead to cracking even in low gradients) and growth along preferred directions to facilitate fabrication of device crystals. Finally growth was performed in a two-zone, transparent furnace in order to monitor and control the seeding-and-growth process.

Mid-IR frequency comb source spanning 4.4–5.4 μm based on subharmonic GaAs optical parametric oscillator

Broadband mid-IR output suitable for producing 1000-nm-wide frequency combs centered at 4.9 μm was achieved in a degenerate subharmonic optical parametric oscillator (OPO) based on 500-μm-long Brewster-angled orientation-patterned GaAs crystal. The OPO was synchronously pumped at 182 MHz repetition rate by 100 fs pulses from a Cr²⁺:ZnSe laser with the central wavelength of 2.45 μm and the average power of 100 mW.

Refractive-index measurements and Sellmeier coefficients for zinc germanium phosphide from 2 to 9 µm with implications for phase matching in optical frequency-conversion devices

Recent experiments in mid-IR frequency conversion reveal that older Sellmeier models for zinc germanium phosphide are inadequate for predicting phase-matching loci in zinc germanium phosphide optical frequency-conversion devices. This results in compromised device performance. We conduct a complete study of the refractive indices of zinc germanium phosphide from 2 to 9 µm and calculate new Sellmeier coefficients. The phase-matching calculations based on our new refractive-index data predict much more closely the results of several different mid-IR frequency-conversion experiments.

Broadly tunable noncritically phase-matched ZnGeP 2 optical parametric oscillator with a 2-µJ pump threshold

We report a high-repetition-rate (1-10-kHz) optical parametric oscillator (OPO) based on noncritically phase-matched ZnGeP2 (ZGP). The pump source was an OPO based on periodically pole lithium niobate that was pumped in turn by a Q-switched diode-pumped 1-microm Nd:YAG laser. The ZGP OPO yielded continuously tunable output from 3.7 to 10.2 microm by tuning of the pump wavelength from 2.3 to 3.7 microm. At the optimal pump focusing, the minimum ZGP OPO threshold achieved was 2 microJ, which is to our knowledge the lowest ever reported for a singly resonant OPO. The output energy in the 6-8-microm range was > 20 microJ, and the quantum efficiency of converting 1-microm radiation to the mid IR exceeded 10%.

Noncritical singly resonant optical parametric oscillator operation near 6.2 μm based on a CdSiP 2 crystal pumped at 1064 nm

CdSiP(2) is employed in a nanosecond, 90 degrees -phase-matched, singly resonant optical parametric oscillator pumped at 1064 nm to produce idler pulses near 6.2 microm with an energy as high as 470 microJ at 10 Hz.

Femtosecond parametric generation in ZnGeP(2).

We report traveling-wave optical parametric generation in short (2-mm) ZnGeP(2) samples with reduced anomalous absorption, using femtosecond pump pulses near 2 mum . The signal and the idler waves generated could be tuned from 2.5 to 10 mum , and they extend the tunability of the beta-barium borate optical parametric generator used as a pump source to the mid-infrared. At a single-pass internal conversion efficiency of 2.5% we estimate pulse durations of 75 fs (signal near 3 mum) and 200 fs (idler near 6 mum).

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.

Direct measurement of ZnGeP2 birefringence from 0.66 to 12.2 μm using polarized light interference

The birefringence (Δn) of ZnGeP2 has been measured directly from polarized light interference spectra obtained in transmittance over the 0.66–12 μm wavelength range from samples of six different thicknesses. The Δn values were determined from the positions of fringe maxima (Δn=kλ/t) and then compared to previously published data which were obtained by a different technique. It was found that the interference fringe method results in values of Δn accurate to ±0.00005. The data are shown to exhibit much less scatter as a function of wavelength than previous results and can lead to more accurate calculations of phase‐matching angles for second‐harmonic generation applications.