Darrell J. Armstrong

3.4-μm ZGP RISTRA nanosecond optical parametric oscillator pumped by a 2.05-μm Ho:YLF MOPA system

We report on the first demonstration of ZGP OPO based on Rotated Image Singly-Resonant Twisted RectAngle (RISTRA) cavity. For the OPO signal wave we achieved a near diffraction-limited beam at 3.4 μm with pulse energy of 10 mJ at repetition rate up to 500 Hz. As a pump source for the ZGP OPO, we utilized a 2-μm, TEM00, Ho:YLF MOPA system producing > 55 mJ energy per pulse at repetition rate range from single shot to 500 Hz.

Measurement of the χ (2) tensors of KTiOPO 4 , KTiOAsO 4 , RbTiOPO 4 , and RbTiOAsO 4 crystals

We use the separated-beams method to measure the second-order nonlinear optical tensors of the crystals KTiOPO4, KTiOAsO4, RbTiOPO4, and RbTiOAsO4 for second-harmonic generation of 1064-nm light. Our results agree well with most previous measurements but have improved precision.

Generation of radially polarized beams with an image-rotating resonator

We show how a passive image-rotating optical resonator can be used to convert a linearly polarised, lowest-order Gaussian beam into a radially polarized beam. The image and polarization rotation of the cavity removes the frequency degeneracy of the modes, making it possible to select the radially polarized mode by cavity tuning. With the addition of gain, the same cavity should operate as a radially polarized laser when injection seeded at the proper wavelength.

Nanosecond optical parametric oscillator with 90° image rotation: design and performance

According to numerical models of nanosecond optical parametric oscillators, cavities with 90° image rotation can produce high-quality beams even if the Fresnel number of the cavity is large. We review the properties of such image-rotating cavities and present a method for designing them. The laboratory performance of one promising design is characterized, demonstrating its ability to produce high-quality beams with good efficiency.

High-power, high-energy ZGP OPA pumped by a 2.05-μm Ho:YLF MOPA system

We report a ZGP OPO/OPA system capable of producing >30 mJ at a signal wavelength of 3.4 μm. Our pump source for the OPO/OPA is a 2.05-μm, diffraction-limited Ho:YLF master-oscillator power-amplifier (MOPA) producing ~150 mJ energy per pulse at repetition rate from single shot to ~500 Hz (>100 mJ at 1 kHz). The Ho-MOPA output was split ~2:1 into two beams: one (~50 mJ per pulse) pumping an image-rotating RISTRA ZGP OPO to generate a 3.4-μm signal wave with >10 mJ of pulse energy and near diffraction-limited beam quality, and the other to pump a ZGP OPA stage to increase the signal wave to ~30 mJ per pulse.

Measurement of the chi(2) tensor of GdCa4O(BO3)3 and YCa4O(BO3)3 crystals

Using the separated-beam method, we have measured the full second-order nonlinear optical tensors of the crystals GdCa4O(BO3)3 and YCa4O(BO3)3. Our d tensors (d = chi(2)/2) differ from those of previous reports but give nearly the same values of d_eff for type-I doubling of 1064-nm light. However, for other wavelengths or for type-II doubling, our tensors give values for d_eff distinctly different from the alternative tensors.

Absolute measurement of the effective nonlinearities of KTP and BBO crystals by optical parametric amplification

Absolute magnitudes of the effective nonlinearity, deff, were measured for seven KTP and six BBO crystals. The d(eff), were derived from the parametric gain of an 800-nm signal wave in the sample crystals when they were pumped by the frequency-doubled, spatially filtered light from an injectionseeded, Q-switched Nd:YAG laser. The KTP crystals, all type II phase matched with propagation in the X-Z plane, had d(eff) values ranging from 1.97 to 3.50 pm/V. Measurements of gain as a function of phase velocity mismatch indicate that two of the KTP crystals clearly contain multiple ferroelectric domains. For five type I phase-matched BBO crystals, d(eff) ranged from 1.76 to 1.83 pm/V, and a single type II phase-matched BBO crystal had a d(eff) of 1.56 pm/V. The uncertainty in our measurements of d(eff) values is ±5% for KTP and ±10% for BBO.

Demonstration of improved beam quality in an image-rotating optical parametric oscillator

We performed laboratory and numerical modeling studies of an optical parametric oscillator with 90 degrees intracavity image rotation. We found that the signal beam was more symmetric than that from comparable cavities without image rotation, and it had low values of the beam quality factor, M(2) . Oscillator performance agreed well with our numerical model.

All Solid-State High-Efficiency Tunable UV Source for Airborne or Satellite-Based Ozone DIAL Systems

We designed, built, and tested two laboratory prototype nanosecond UV sources for airborne or satellite-based ozone differential absorption lidar (DIAL) remote-sensing systems. Our prototypes use a 532-nm second-harmonic pulse from a Q-switched injection-seeded Nd:YAG laser to pump an optical parametric oscillator (OPO) that generates a tunable signal wavelength near 803 nm. The OPO signal is mixed with additional 532 nm light either inside the OPO cavity, or in a subsequent sum-frequency generation (SFG) stage, to generate 10-ns pulses at 320-nm. Our system designs result from an integrated, iterative approach where operating parameters including the pump-beams spatial profile, the second harmonic generation efficiency, the OPOs cavity geometry, output coupling, crystal lengths, and the length of the SFG crystals, are all determined from numerical modeling. By using this approach, we obtained 320 nm pulse energies approaching 200 mJ with overall optical conversion efficiency-from 1064 to 320 nm-exceeding 20%. To optimize efficiency, we incorporate three important design characteristics: a pump beam having a high-quality flat-topped spatial profile, an image-rotating non-planar ring-cavity OPO capable of generating high-quality large-diameter flat-topped beams, and pulse injection seeding of the OPO to achieve near-zero cavity buildup time to enhance the efficiency of sum-frequency mining. We believe additional optimization of our designs may eventually yield UV pulse energies approaching 300 mJ with optical conversion efficiencies comparable to those of our current systems.

Generation of vortex beams by an image-rotating optical parametric oscillator

We generate optical vortex beams in a nanosecond optical parametric oscillator based on an image-rotating resonator. This efficient new method of vortex generation should be adaptable to pulsed or continuous lasers.