G. D. Boyd

Solid-state low-loss intracavity saturable absorber for Nd:YLF lasers: an antiresonant semiconductor Fabry–Perot saturable absorber

We introduce a new low-loss fast intracavity semiconductor Fabry-Perot saturable absorber operated at anti-resonance both to start and sustain stable mode locking of a cw-pumped Nd:YLF laser. We achieved a 3.3-ps pulse duration at a 220-MHz repetition rate. The average output power was 700 mW with 2 W of cw pump power from a Ti:sapphire laser. At pump powers of less than 1.6 W the laser self-Q switches and produces 4-ps pulses within a 1.4-micros Q-switched pulse at an approximately 150-kHz repetition rate determined by the relaxation oscillation of the Nd:YLF laser. Both modes of operation are stable. In terms of coupled-cavity mode locking, the intra-cavity antiresonant Fabry-Perot saturable absorber corresponds to monolithic resonant passive mode locking.

LINEAR AND NONLINEAR OPTICAL PROPERTIES OF ZnGeP2 AND CdSe

The refractive indices of ZnGeP2, point group 42m, have been determined from 0.64 to 12 μ. ZnGeP2 has positive birefringence with reasonable temperature tunability and a band gap in the visible. Its nonlinear optical coefficient d14 for second harmonic generation (SHG) measured relative to d14 of GaAs is 0.83 ± 15%. The birefringence of ZnGeP2 is not large enough to allow phase‐matched SHG, but nondegenerate phase‐matched three‐frequency mixing is possible and the parametric oscillator threshold power is calculated for several situations. A redetermination of the coefficient d31 in CdSe is made and the materials compared. It is concluded that ZnGeP2 may be a most promising material for parametric generation in the 0.8– to 12‐μ region.

Linear and nonlinear optical properties of AgGaS 2 , CuGaS 2 , and CuInS 2 , and theory of the wedge technique for the measurement of nonlinear coefficients

The refractive indices of the ternary A^{I}B^{III}C_{2}^{VI} semiconductors AgGaS 2 , CuGaS 2 , and CuInS 2 have been measured over the entire range of transparency of these crystals. The optical nonlinear coefficients for second-harmonic generation have also been determined. Three-frequency collinear phase matching is analyzed in detail for AgGaS 2 . The birefringences of CuGaS 2 and CuInS 2 are not large enough to permit three-frequency phase matching within the transparent regions. A parametric oscillator threshold calculation for a pump wavelength 0.89 μ, which is within the range of the GaAs injection laser, indicates that AgGaS 2 is promising for this application. The upconversion efficiency in AgGaS 2 for sum mixing of the CO 2 laser ( \lambda = 10.5 \mu ) with the xenon ion laser ( \lambda = 0.597 \mu ) is also calculated. The result indicates that, depending upon system requirements and the availability of high optical quality material, AgGaS 2 can be comparable to ZnGeP 2 for upconversion. In Appendix II, we present a theory of the wedge technique for the measurement of nonlinear coefficients. This theory takes into account losses and assumes a Gaussian beam geometry. Furthermore, a discussion of units in nonlinear optics is given.

Resonant optical second harmonic generation and mixing

Experimental and theoretical results are described on the enhancement of optical second harmonic generation (SHG) and mixing in KDP by the use of optical resonance. Both resonance of the harmonic and of the fundamental are considered. Large enhancements are possible for resonators with low loss. Using a planoconcave harmonic resonator containing 1.23 cm of KDP, the authors achieved a loss < 4 percent per pass. This resulted in an enhancement of ∼ 500 times the harmonic power internal to the resonator and ∼ 10 times external to the resonator. When resonating, the fundamental enhancements of ∼ 5 were observed. The theory includes the effect of double refraction. This results in a coupling coefficient of the generated harmonic power to the transverse modes of the harmonic resonator. The experimental results are in substantial agreement with the theory.

Linear and nonlinear optical properties of some ternary selenides

The refractive indices of the ternary A^{I}B^{III}C_{2}^{VI} semiconductors CuAlSe 2 , AgGaSe 2 , CuGaSe 2 , and AgInSe 2 have been measured over most of the transparency range of these crystals. The optical nonlinear coefficients for second-harmonic generation of AgGaSe 2 CuGaSe 2 , and AgInSe 2 have also been measured. Three-frequency colinear phase matching is analyzed in detail for AgGaSe 2 . The birefringences of the other three crystals are not sufficient to permit three-frequency colinear phase matching within the range of the measured index. The merits of AgGaSe 2 for nonlinear optical applications are evaluated in comparison with other promising infrared nonlinear materials.

Refractive Index as a Function of Temperature in LiNbO3

The refractive index vs wavelength of LiNbO3 has been measured between 0.42 μ and 4.0 μ at 25°C and 80°C. The measured values have been expanded in a three‐term polynomial by a least‐squares best fit and the temperature dependence of the polynomial coefficients determined. The results are used to calculate the phase‐matching temperatures normal to the optic axis in nonlinear optical second‐harmonic generation and parametric experiments. The calculated results are compared with experimentally measured values.

Optical Nonlinearities in LiIO3

Clear, transparent (0.5–5.0 μ), solution‐grown, single crystals of negative uniaxial LiIO3 show no optically induced damage and remain stable in the temperature range 20°–256°C. The crystals are of point group 6 and are optically active and pyroelectric. We have measured the linear electrooptic coefficients in the visible and the optical activity in the infrared. For λ1=1.0845 μ phase‐matched second harmonic generation (PMSHG) occurred at θm=28.9° and this angle changed by less than 0.3° in the range 20–256°C. PMSHG was employed to determine the single domain quality of the crystals and to measure | d31(LiIO3)/d36(KDP) |=11. At 1.06 μ we have measured the coherence lengths and the ratio of the nonlinear coefficients | d33/d31 |=0.8.

Multiple quantum well reflection modulator

We demonstrated a quantum‐confined Stark effect electroabsorption modulator consisting of quantum wells of AlGaAs and GaAs on an epitaxial multilayer dielectric mirror, all grown by molecular beam epitaxy. The resulting reflection modulator avoids problems of substrate absorption, and has relatively high contrast ratio (up to ∼8:1 with peak reflectivity of 25% at 853 nm) because the light passes twice through the quantum wells. Reflection modulators are of interest for bidirectional communication systems, in parallel arrays of optical switching and processing devices and for optical interconnects. For the latter there exists the possibility of this device grown on the same substrate alongside a GaAs integrated circuit or even on Si substrates.

Linear and nonlinear optical properties of ternary A II B IV C 2 V chalcopyrite semiconductors

The refractive indices of ZnSiAs 2 , CdGeP 2 , and CdGeAs 2 have been determined over a wide range of wavelengths and the optical nonlinear coefficient for second-harmonic generation from the 10.6-μ CO 2 laser have been measured. The absorption coefficient versus wavelength is given for the above materials as well as for ZnGeP 2 , a previously discussed material. Three-frequency phase-matched mixing is described for each material.

Phase‐matched submillimeter wave generation by difference‐frequency mixing in ZnGeP2

Using two step‐tunable CO2 lasers, we have observed phase‐matched generation of frequencies 70 <ν<110 cm−1 by nonlinear mixing in a birefringent ternary semiconductor, ZnGeP2. An observed power of ∼1.7 μW at 83.37 cm−1 gave a signal‐to‐noise ratio of ∼1000 with a Ge:Ga detector. In combination with tunable optical lasers, this technique should yield a tunable source of submillimeter wave radiation for high‐resolution spectroscopy.