Nonlinear Optical Crystals: A Complete Survey

We present a theory of ultrashort-pulse second-harmonic generation (SHG) in materials with longitudinally nonuniform quasi-phase-matching (QPM) gratings. We derive an expression for the output second-harmonic field generated in an arbitrary QPM grating from an arbitrary fundamental field, valid for arbitrary material dispersion in the undepleted-pump approximation. In the case when group-velocity dispersion can be neglected, a simple transfer-function relationship describes the SHG process. This SHG transfer function depends only on material properties and on the QPM grating design. We use this SHG transfer function to show that nonuniform QPM gratings can be designed to generate nearly arbitrarily shaped second-harmonic output pulses. We analyze in detail a technologically important example of pulse shaping: the generation of compressed second-harmonic pulses from linearly chirped fundamental input pulses. The efficiency of these interactions as well as the limits imposed by higher-order material dispersion are discussed.

Ultrashort-pulse second-harmonic generation with longitudinally nonuniform quasi-phase-matching gratings: pulse compression and shaping

The optical biaxial nature of crystals in the potassium titanyl phosphate (KTP) family result in anisotropic transmission that depends on the polarization direction of the transmitted radiation with respect to the fundamental crystal axes. Knowledge of the polarization-dependent crystal transmission is important for all wavelength-conversion applications and in particular is the only limitation on possible combinations of wavelengths when one uses the quasi-phase-matching techniques recently developed for the KTP family materials. In this study, polarized transmission spectra of KTiOPO4 (KTP), RbTiOPO4, RbTiOAsO4 (RTA), and KTiOAsO4 were measured over the 0.3–6-µm wavelength range with a spectrophotometer and a Fourier-transform infrared spectrometer. Seven crystal samples were studied, including four samples of KTP crystals of different origins. Variations in spectral transmission on the short- and long-wavelength edges, as well as visible-wavelength transmission and OH- absorption properties, are presented and discussed. The transmission of one sample of KTP and of RTA was also measured before and after periodic electric field poling.

Transmission measurements in KTP and isomorphic compounds

We present results from what we believe is the first reported example of an optical parametric oscillator based on periodically poled RbTiOAsO4. The oscillator is pumped by a femtosecond self-mode-locked Ti:sapphire laser and, with a single-grating 2-mm-long crystal and one mirror set, a combination of pump and cavity-length tuning provided wavelength coverage from 1060 to 1225  nm (signal) and 2.67 to 4.5 µm (idler). Average output powers were as much as 120  mW in the signal and 105  mW in the idler and interferometric autocorrelations recorded at signal and idler wavelengths of 1.1 and 3.26 µm, respectively, imply pulse durations of 125 and 115  fs, respectively.

Broadly tunable infrared femtosecond optical parametric oscillator based on periodically poled RbTiOAsO(4).

The availability of continuously tunable narrow-band laser sources emitting in the mid-infrared region of the electromagnetic spectrum between 2.5 μm and 15 μm is important for a large variety of applications. One way to achieve both broad tunability and narrow linewidth is frequency conversion in a nonlinear optical material, in particular difference frequency generation (DFG).

Mid-IR Difference Frequency Generation

Rubidium titanyl arsenate (RTA), a crystallographic isomorph of potassium titanyl phosphate, shows promise for nonlinear-frequency generation throughout the 1–5-µm spectral region. Difference-frequency generation in an RTA crystal produced tunable output in the 3.2–4.2-µm wavelength range. A single 1.064-µm Nd:YAG laser pumped both a LiNbO3 optical parametric oscillator used to generate a tunable signal beam and the RTA crystal used for difference-frequency generation. Conversion efficiencies were limited to 4% primarily by the large beam divergence of the signal beam. Phase-matching measurements were in excellent agreement with new IR-corrected RTA Sellmeier equations based on refractive-index measurements.

D. Small

Papers

Rubidium titanyl arsenate difference-frequency generation and validation of new Sellmeier coefficients