Publications

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We have numerically modeled an efficient method of doubling the 1064 nm wavelength of a Q-switched Nd:YAG laser using a lambda-doubling nanosecond optical parametric oscillator (LDOPO). The LDOPO cavity is based on the four-mirror nonplanar RISTRA geometry, denoting rotated-image singly-resonant twisted rectangle, and contains a single type-II KTP crystal. By using the polarization-rotating properties of this cavity, and modifying its geometry to incorporate polarization-selective mirrors with angles of incidence near Brewster's angle, this design obtains stable, singly-resonant oscillation at degeneracy. If the pump laser is injection-seeded, and the LDOPO contains an intra-cavity étalon for single-longitudinal-mode oscillation, the phase of the wavelength-doubled 2128 nm light remains locked to the phase of the pump, independent of cavity length, so active frequency stabilization is not required. Numerical analysis indicates that a pulse-injection-seeded LDOPO can obtain 1064 nm to 2128 nm conversion efficiency exceeding 61%. However, analysis of a complete system incorporating a primary low-energy LDOPO that pulse-injection-seeds a secondary higher-energy LDOPO indicates total 1064 nm to 2128 nm efficiency of approximately 57%. A 2128 nm lambda-doubling system having conversion efficiency > 50% may offer a cost-effective alternative to conventional two micron laser sources such as Tm:Ho:YAG.

We have developed analytical and numerical models to address propagation of light through bent fiber amplifiers, including the mode distortion that results from fiber bending, and changes in beam profile resulting from self-focusing. © 2005 Optical Society of America.

Sandia National Laboratories has developed high-energy all-solid-state UV sources for use in laboratory tests of the feasibility of satellite-based ozone DIAL. These sources generate 320 nm light by sum-frequency mixing the 532 nm second harmonic of an Nd:YAG laser with the 803 nm signal light derived from a self-injection-seeded image-rotating optical parametric oscillator (OPO). The OPO cavity utilizes the RISTRA geometry, denoting rotated-image singly-resonant twisted rectangle. Two configurations were developed, one using extra-cavity sum-frequency mixing, where the sum-frequency-generation (SFG) crystal is outside the OPO cavity, and the other intra-cavity mixing, where the SFG crystal is placed inside the OPO cavity. Our goal was to obtain 200 mJ, 10 ns duration, 320 nm pulses at 10 Hz with near-IR to UV (1064 nm to 320 nm) optical conversion efficiency of 25%. To date we've obtained 190 mJ at 320 nm using extra-cavity SFG with 21% efficiency, and > 140 mJ by intra-cavity SFG with efficiency approaching 24%. While these results are encouraging, we've determined our conversion efficiency can be enhanced by replacing self-seeding at the signal wavelength of 803 nm with pulsed idler seeding at 1576 nm. By switching to idler seeding and increasing the OPO cavity dimensions to accommodate flat-top beams with diameters up to 10 mm, we expect to generate UV energies approaching 300 mJ with optical conversion efficiency approaching 25%. While our technology was originally designed to obtain high pulse energies, it can also be used to generate low-energy UV pulses with high efficiency. Numerical simulations using an idler-seeded intra-cavity SFG RISTRA OPO scaled to half its nominal dimensions yielded 560 μJ of 320 nm light from 2 mJ of 532 nm pump using an idler-seed energy of 100 μJ.

We have built and tested a highly efficient source of pulsed 320 nm light based on intra-cavity sum-frequency-generation in a self-injection-seeded image-rotating nanosecond optical parametric oscillator. The four-mirror nonplanar ring optical cavity uses the RISTRA geometry, denoting rotated-image singly-resonant twisted rectangle. The cavity contains a type-II xz-cut KTP crystal pumped by the 532 nm second harmonic of Nd:YAG to generate an 803 nm signal and 1576 nm idler, and a type-II BBO crystal to sum-frequency mix the 532 nm pump and cavity-resonant 803 nm signal to generate 320 nm light. The cavity is configured so pump light passes first through the BBO crystal and then through the KTP crystal with the 320 nm light exiting through the output coupler following the BBO sum-frequency crystal. The cavity output coupler is designed to be a high reflector at 532 nm, have high transmission at 320 nm, and reflect approximately 85% at 803 nm. With this configuration we've obtained 1064 nm to 320 nm optical-to-optical conversion efficiency of 24% and generated single-frequency λ = 320 nm pulses with energies up to 140 mJ.

During the past several years Sandia National Laboratories has carried out proof-of-concept experiments to demonstrate tunable, efficient, high-energy ultraviolet nanosecond light sources for satellite-based ozone DIAL. We designed our UV sources to generate pulse energies ≳ 200 mJ at 10 Hz in the range of 308-320 nm with optical-to-optical efficiency approaching 25%. We use sum-frequency generation to mix the 532 nm second harmonic of Nd:YAG with near-IR light derived from a self-injection-seeded image-rotating nonplanar-ring optical parametric oscillator. Laboratory configurations using extra- and intra-cavity sum-frequency generation were designed and tested, yielding 1064 nm to 320 nm conversion efficiencies of 21% and 23% respectively, with pulse energies of 190 mJ and 70 mJ. These energies and efficiencies require pump depletion in the parametric oscillator of at least 80% and SFG efficiency approaching 60%. While the results reported here fall slightly short of our original goals, we believe UV pulse energies exceeding 250 mJ are possible with additional refinements to our technology. Although the sources tested to date are laboratory prototypes with extensive diagnostics, the core components are compact and mechanically robust and can easily be packaged for satellite deployment.

We report the experimental realization of a new type of optical parametric oscillator in which oscillation is achieved by polarization rotation in a linear retarder, followed by nonlinear polarization mixing. The mixing is performed by a type II degenerate parametric downconversion in a periodically poled KTP crystal pumped by a 1064 nm pulsed Nd:YAG pump. A single, linearly polarized beam, precisely at the degenerate wavelength is generated. The output spectrum has a narrow linewidth (below the instrumentation bandwidth of 1 nm) and is highly stable with respect to variations in the crystal temperature.

Conventional wisdom contends that high-energy nanosecond UV laser sources operate near the optical damage thresholds of their constituent materials. This notion is particularly true for nonlinear frequency converters like optical parametric oscillators, where poor beam quality combined with high intra-cavity fluence leads to catastrophic failure of crystals and optical coatings. The collective disappointment of many researchers supports this contention. However, we're challenging this frustrating paradigm by developing high-energy nanosecond UV sources that are efficient, mechanically robust, and most important, resistant to optical damage. Based on sound design principles developed through numerical modeling and rigorous laboratory testing, our sources generate 8-10 ns 190 mJ pulses at 320 nm with fluences ≤ 1 J/cm 2. Using the second harmonic of a Q-switched, injection-seeded Nd: YAG laser as the pump source, we convert the near-IR Nd:YAG fundamental to UV with optical-to-optical efficiency exceeding 21%. © 2005 Materials Research Society.

We've generated high-quality flat-top spatial profiles from a modified Continuum Powerlite 9010 Nd:YAG laser using the Gaussian-to-flat-top refractive beam shaper available from Newport Corporation. The Powerlite is a flashlamp-pumped, Q-switched, injection-seeded Nd:YAG laser manufactured in 1993 that delivers ∼ 1.6 J at 10 Hz using an oscillator and two 9 mm diameter amplifier rods. While its pulse energy is impressive, its beam-quality is typically poor, an all too common characteristic of research-grade Nd:YAG lasers manufactured in the late 1980's and early 1990's. Structure in its near-field spatial fluence profile is reminiscent of round-aperture diffraction that is superposed with additional "hot spots." These characteristics are largely due to poor beam quality from the oscillator coupled with over-filled amplifier rods, and reflect a design philosophy from the era of organic dye lasers. When these older laser systems are used for tasks like pumping optical parametric oscillators (OPO's), or for other applications demanding good beam quality, their designs are simply inadequate. To improve the 9010's beam quality we spatially filter the oscillator beam and remove the resulting Airy rings with an iris, then collimate and magnify the remaining central disk so its diameter is appropriate for input to the refractive shaper. The output of the beam shaper is then double-pass amplified through two amplifier rods with thermally induced focusing compensated by a negative lens before the first pass and by a convex mirror before the second pass. Using this approach we've obtained single-pass energy exceeding 250 mJ with little degradation of the flat-top profile and ∼ 950 mJ after double pass amplification. After double-passing the two amplifier rods the beam suffers some degradation in symmetry and uniformity, but is still much improved compared to the beam obtained using the 9010's original factory configuration. We find the modified 9010's fiat-top profile improves conversion efficiency when used for our applications in crystal nonlinear optics.

A UV generation system consisting of a quasi-monolithic nonplanar-ring-oscillator image-rotating OPO, called the RISTRA OPO, is presented. High beam quality and the absence of mirror adjustments due to the monolithic design make this OPO well-suited for demanding applications such as satellite deployment. Initial tests of self seeding using low-quality flattop beams with poor spatial overlap between the OPO's cavity mode and the spatial mode of the injected signal pulsed showed pump depletion of 63%.

The heuristic theory of second harmonic generation with focused beams in walkoff-compensating crystals was developed and presented. Intercrystal phase-shift due to the anti reflective dielectric coatings on the crystals was measured. The conversion efficiency and the far-field second harmonic beam profiles were compared and it was found that theoretical developments were in good agreement with experimental results.

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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.

We present numerical modeling and laboratory studies of degenerate type I nanosecond optical parametric oscillators. Because the signal and idler waves are identical and parametric gain is phase sensitive, their round-trip phase is a critical parameter. We show that signal spectrum, transverse mode, and conversion efficiency are all strongly influenced by this phase. We also examine the influence of signal-wave injection seedine and phase-velocity mismatch. © 2003 Optical Society of America.

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The goal of this project was to increase the power of vertical cavity surface emitting lasers and to convert their wavelength into the blue/ultraviolet and the infrared for sensing applications. We have increased the power to the multi-watt level and have generated several milliwatts of blue light using optical pumping. Electrical pump has been less successful, but we have identified the problems and begun work to overcome them using a bottom emitting design.

It is shown that the combination of pulse-front slant, k-vector tilt, and crystal birefringence often permits exact matching of both phase and group velocities for three-wave mixing in birefringent crystals. This exact match makes possible more-efficient mixing of short light pulses, and it permits efficient mixing of chirped or broadband light. I analyze this process and present examples. © 2001 Optical Society of America.

The wavelength variation of the second-order nonlinear coefficients of KNbO3, KH2PO4 and LiB3O5 crystals was discussed. The second-order nonlinear coefficients were measured using optical parametric amplification and second-harmonic generation over a wide range of wavelengths for the crystals. The results showed that Miller scaling was a useful approximation for the crystals.

We compare frequency doubling of broadband light in a single nonlinear crystal with doubling in five crystals with intercrystal temporal walk-off compensation and with doubling in five crystals adjusted for offset phase-matching frequencies. Using a plane-wave dispersive numerical model of frequency doubling, we study the bandwidth of the second harmonic and the conversion efficiency as functions of crystal length and fundamental irradiance. For low irradiance, the offset phase-matching arrangement has lower efficiency than a single crystal of the same total length but gives a broader second-harmonic bandwidth. The walk-off-compensated arrangement gives both higher conversion efficiency and broader bandwidth than a single crystal. At high irradiance, both multicrystal arrangements improve on the single-crystal efficiency while maintaining a broad bandwidth. © 2001 Optical Society of America.

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