Efficient coupling into tapered proton-exchanged LiNbO(3) waveguides fabricated by vertically controlled immersion.

Planar LiNbO(3) waveguides with a 5-mm tapered region and a 10-mm homogeneous slab waveguide region were fabricated by a double proton-exchange process. The 4-microm waveguide entrance gives rise to a coupling efficiency as high as 93%; the precise control of the depth profile of the tapered region results in a low 1-dB power transfer loss from the tapered region to the 0.4-microm-depth working region. The techniques developed provide an alternative to prism coupling into planar waveguides in Cerenkov second-harmonic generation applications.

Generation of ultrashort second-harmonic pulses by compressing the Cerenkov output.

Ultrashort optical second-harmonic pulses based on a traditional phase-matched Cerenkov second-harmonic generation scheme, with an additional prism to equalize the arrival time, are proposed. Proton-exchanged KTP, LiNbO(3) waveguides, and several prism materials are considered in the evaluation. There is no constraint on crystal length, making high efficiency possible as well as preserving an ultrashort pulse width. The proposed approach may be a practical and efficient way to achieve convenient ultrashort harmonic pulses.

External-cavity semiconductor-laser array insensitive to paraxial misalignment.

We report a tilted-mirror external-cavity gain-guided laser array whose output power is insensitive to small misalignments of the cavity elements. This novel configuration delivers a near-diffraction-limited (1.5 times diffraction-limited) single-lobed beam in the entire current range without the need for realignment of the cavity. The light-current characteristic is linear and kink free, with a higher efficiency compared with the free-running array. A compact graded-index lens design that can emit a collimated beam with a circular cross section is also described.

Current sensing using bismuth rare-earth iron garnet films.

Ferrimagnetic iron garnet films are investigated as current-sensing elements. The Faraday effect within the films permits measurement of the magnetic field or current by a simple polarimetric technique. Polarized diffraction patterns from the films have been observed that arise from the presence of magnetic domains in the films. A physical model for the diffraction is discussed, and results from a mathematical analysis are in good agreement with the experimental observations. A method of current sensing that uses this polarized diffraction is demonstrated.

Paint removal using lasers.

Experiments to investigate the potential for practical laser graffiti-removal systems are reported. A universal engineering curve for the time needed for removal of paint from nonconductive substrates that was valid over a range of 10(7) in intensity was measured with a variety of lasers. Comparable times were measured for conductive substrates, when pulses shorter than the thermal conduction times were used. Analysis suggests that Q-switched Nd:YAG lasers may be the most efficient means for removing graffiti and other unwanted paint. An 1-m(2) area of paint 14 µm thick can be removed in approximately 10 min with a 50-Hz laser system of 15-W average power.

High-efficiency generation of ultrashort second-harmonic pulses based on the Cerenkov geometry.

Simultaneous phase- and group-velocity-matched generation of ultrashort optical second-harmonic pulses based on a quasi-phase-matched Cerenkov second-harmonic generation scheme is proposed. We compare our scheme with other schemes with respect to its ability to achieve high conversion efficiency, its enhancement with respect to cw second-harmonic generation, and its sensitivity to waveguide-period fluctuation; ion-exchanged KTP and LiNbO(3) waveguides are considered in the evaluation. We believe our proposed scheme is easy to implement, and there is no constraint on crystal length in achieving high efficiency as well as in preserving an ultrashort pulse width. 100% efficiency is potentially obtainable from sufficiently long crystals.

Balanced detection technique to measure small changes in transmission.

A novel technique to measure changes in optical transmission of better than 10(-3) with time resolution of better than 10 ns is discussed. The photocurrent from two photodiodes is subtracted with a balanced detection circuit to eliminate the common signal; carrier-induced change in absorption for a semiconductor single quantum well produces a large differential signal.

Loss-induced phase-sensitive spatial light modulator with a wide field of view.

We investigate the feasibility of achieving practical phase modulation by changing only the absorption of the device. We present a spatial light modulator that consists of a three-mirror cavity with lossy slabs inserted between the mirrors. When only the absorption of these slabs is changed, this device can have a gradual change in phase, but no intensity modulation. Theoretical analysis by the use of numbers appropriate to multiple quantum wells shows that the total phase-tuning range can be as large as 2π with an absorption coefficient change of less than 9000 cm(-1); the length of each multiple-quantum-well slab is ~ 1.5 µm. This gives an expected 16° field of view with the nanosecond speed typical of semiconductors.

Wavelength shift of the reflectivity peak of Bragg reflector structures.

We discuss the wavelength for maximum reflectivity of a Bragg reflector that has both index and absorption modulation. We show there that the blue shift away from the Bragg wavelength can be no larger than that given by the wavelength of maximum coupling, which we derive analytically. For gratings with fewer layers, the blue shift will be smaller than this maximum value. In typical semiconductor Bragg reflectors, this shift may be as much as 32 nm.

Role of stimulated four-photon mixing and efficient Stokes generation of stimulated Raman scattering in excimer-laser-pumped UV multimode fibers.

Efficient frequency downconversion by stimulated Raman scattering (SRS) was obtained in a XeF excimer-laser-pumped fused-silica multimode fiber at 351 nm. Twenty orders of Stokes generation were observed covering a spectral range of ~8800 cm(-1), a number we were able to predict numerically using a model for SRS only but incuding a finite gain width. Gains from stimulated four-photon mixing were found to be not important in SRS Stokes generation. Only occasionally did we observe new frequency lines in addition to the normal Raman lines, which are attributed to phase-matched wave mixing processes.

Effect of front-facet reflections on the reflectivity of Bragg reflectors.

Typical Bragg mirrors have an air-dielectric interface that can have a reflectivity spectrum different from that calculated by simple coupled-wave theory because of Fresnel reflection at their surface. This additional reflection can add constructively or destructively, depending on the sequence of the Bragg layers. We show a method of analysis for calculating reflectivity of Bragg mirrors in the presence of additional reflections.

Room-temperature 1.06-0.53-microm second-harmonic generation with MgO:LiNbO(3).

Room-temperature 1.06-0.53-microm second-harmonic generation (SHG) achieved with LiNbO(3) doped with 7 mol. % MgO has been studied. Phase matching was achieved with angle tuning. SHG conversion efficiency of 45% was obtained with a 12-mm-long crystal and a fundamental peak-power density of 140 MW/cm (2). SHG performance of MgO:LiNbO(3) is compared with that of KTP and LBO crystals. Various phase-matching parameters of MgO:LiNbO(3) were calculated as functions of the fundamental wavelength, using the experimentally determined Sellmeier equations. It was found that room-temperature, noncritically-phase-matched Type I SHG can be achieved in this crystal at 1.053 microm, where Nd:YLF lasers operate.

Analysis of absorbing and leaky planar waveguides: a novel method.

We present a simple and general approach for analysis of absorbing and leaky waveguides. The real and imaginary parts of the propagation constant beta of a planar optical waveguide are obtained by evaluating a function, defined through the eigenvalue equation, in real beta. The applicability of the method is demonstrated for simple structures. To apply the method for arbitrarily graded inhomogeneous or multilayered structures we use a simple matrix approach to obtain the eigenvalue equation. The method is straightforward, accurate, and requires no iteration in the complex beta plane.

Spatial light modulation by beam coupling in GaAs crystals.

We present the first reported demonstration of spatial light modulation in GaAs using the photorefractive effect. The demonstration was done with a special configuration in which polarization rotation created by beam coupling was utilized.

Research on nonlinear optical materials: an assessment.

The seven papers making up this assessment are based on the Workshop on Nonlinear Optical Materials held in April 1986.