Arlee V. Smith

Mode instability in high power fiber amplifiers.

For powers exceeding a sharp threshold in the vicinity of several hundred watts the beam quality from some narrow bandwidth fiber amplifiers is severely degraded. We show that this can be caused by transverse thermal gradients induced by the amplification process.

Bulk and surface laser damage of silica by picosecond and nanosecond pulses at 1064 nm

We measured bulk and surface dielectric breakdown thresholds of pure silica for 14 ps and 8 ns pulses of 1064 nm light. The thresholds are sharp and reproducible. For the 8 ns pulses the bulk threshold irradiance is 4.75 +/- 0.25 kW/microm2. The threshold is approximately three times higher for 14 ps pulses. For 8 ns pulses the input surface damage threshold can be made equal to the bulk threshold by applying an alumina or silica surface polish.

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.

Peak-power limits on fiber amplifiers imposed by self-focusing

We have numerically investigated the behavior of the fundamental mode of a step-index, multimode (MM) fiber as the optical power approaches the self-focusing limit (P(crit)). The analysis includes the effects of gain and bending (applicable to coiled fiber amplifiers). We find power-dependent, stationary solutions that propagate essentially without change at beam powers approaching P(crit) in straight and bent fibers. We show that in a MM fiber amplifier seeded with its fundamental eigenmode at powers <<P(crit), the transverse spatial profile adiabatically evolves through a continuum of stationary solutions as the beam is amplified toward P(crit).

Optical Damage Limits to Pulse Energy From Fibers

The irradiances or fluences encountered in pulsed fiber lasers and amplifiers are sometimes high enough to destroy the fiber. The ultimate limit is set by the intrinsic damage threshold irradiance of doped silica. Other nonlinear processes such as self-focusing and stimulated Brillouin scattering are also important. We present here the results of our measurements of the damage threshold, and of our analysis of how self-focusing and Brillouin scattering influence the power limit.

Increasing mode instability thresholds of fiber amplifiers by gain saturation

We show by numerical modeling that saturation of the population inversion reduces the stimulated thermal Rayleigh gain relative to the laser gain in large mode area fiber amplifiers. We show how to exploit this effect to raise mode instability thresholds by a substantial factor. We also demonstrate that when suppression of stimulated Brillouin scattering and the population saturation effect are both taken into account, counter-pumped amplifiers have higher mode instability thresholds than co-pumped amplifiers for fully Yb3+ doped cores, and confined doping can further raise the thresholds.

Phase distortions in sum- and difference-frequency mixing in crystals

We show that if two waves are incident on a quadratically nonlinear crystal, with the third wave generated entirely within the crystal, a phase-velocity mismatch (Δk ≠ 0) leads to intensity-dependent phase shifts of the generated wave only if there is walk-off, linear absorption, or significant diffraction of at least one of the waves as well as significant energy exchange among the waves. The result is frequency broadening and wave-front distortion of the generated wave. Although the induced phase distortions are usually quite small, they may be significant in applications that require high spectral resolution or pointing accuracy.

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.

Injection-seeded titanium-doped-sapphire laser

We demonstrate injection seeding of a pulsed, laser-pumped, titanium-doped-sapphire ring laser by both continuous-wave dye and diode lasers. As little as 100 microW of seed light is required to produce 4 mJ of 30-nsec TEM(00) output having a bandwidth of less than 25 MHz FWHM. Using an atomic resonance filter we find that more than 99.9% of the energy is at the 780-nm seed wavelength. We discuss the spatial and longitudinal mode-matching requirements for successful seeding.

Numerical Models of Broad Bandwidth Nanosecond Optical Parametric Oscillators

We present three new methods for modeling broad-bandwidth, nanosecond optical parametric oscillators in the plane-wave approximation. Each accounts for the group-velocity differences that determine the operating linewidth of unseeded optical parametric oscillators, and each allows the signal and the idler waves to develop from quantum noise. The first two methods are based on split-step integration methods in which nonlinear mixing and propagation are calculated separately on alternate steps. One method relies on Fourier transforming the fields between t and ω to handle propagation, with mixing integrated over a Δz step; the other transforms between z and kz in the propagation step, with mixing integrated over Δt. The third method is based on expansion of the three optical fields in terms of their respective longitudinal empty cavity modes, taking into account the cavity boundary conditions. Equations describing the time development of the mode amplitudes are solved to yield the time dependence of the three output fields. These models exclude diffraction and group-velocity dispersion but can be readily extended to include them.