Propagation of ir light in flexible hollow waveguides: further discussion.

A refinement of theoretical optical loss estimates is given for 10.6-microm radiation propagating in hollow metal waveguides.

Calculation of the Optical Effects of a 27.5-m Spherical Mirror.

People standing under a large spherical mirror see the world inverted, hanging above them as a real image. The shape of this image world depends upon where on the floor the observer stands. In this paper formulas for calculating image positions as a function of observer position are derived and depicted in diagrams of typical image worlds. The images formed by light reflecting several times in the mirror are also calculated. There are both an erect real image world and doubled rings of inverted images surrounding the single-reflection image world.

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.

Photorefractive effect in LiNbO3 directional couplers.

Experimental results on the photorefractive effect in Ti-indiffused LiNbO3 directional couplers at GaAs and He-Ne wavelengths are presented. The photorefractive effect has been observed as drift in the switching state of this device at GaAs wavelengths (0.81 and 0.85 microm) for intensities (powers) as low as 32 W/cm2 (3.2 microW). Analysis of this effect indicates refractive-index changes as large as 8 x 10(-4) may be occurring. Measurements also suggest the possibility of two time constants in the relaxation of the photorefractive effect in these devices.

Nonlinear refraction at the absorption edge in InAs.

The results of measurements of nonlinear refraction at the absorption edge in InAs between 68 and 90 K taken with an HF laser are compared with those of a band-gap resonant model in which the contribution of the light-hole band is included and found to account for more than 40% of the observed nonlinear refraction. A generalized expression for the nonlinear index is derived by using the complete Fermi-Dirac distribution function. Good agreement between theory and experiment is obtained, with no free parameters.

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.

Optimum epilayer structures for integrated optics lasers.

This paper is a theoretical evaluation of double heterostructure AlGaAs laser eapilayer structures designed to optimize performance in integrated optical circuits. The thickness and AlAs concentration are varied to maximize coupling efficiency between the active laser region and the passive waveguide region of the integrated optical circuit. This maximization is performed consistent with maintaining a low laser threshold and ease of fabrication. Both an asymmetric large optical cavity and a symmetric large optical cavity with an internal epitaxially grown tapered active layer are considered. It is shown that with suitable design, both efficient coupling and low threshold can be achieved.

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.

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.

Propagation of infrared light in flexible hollow waveguides.

Designs are described for flexible hollow waveguides with low ir loss over distances on the order of 1 m. Transmission losses at 10.6 microm are calculated, and the effect of bends and twists is discussed. A hollow combination metal-dielectric-walled rectangular waveguide is proposed that can have transmission as high as 95% for a 1-m length, while bent with a 1-m radius of curvature.

Losses in multimode hollow metal infrared waveguides.

Experimental measurements of loss in a hollow metal waveguide show quantitative agreement with the multimode theory of losses using a complex refractive index.

Degenerate four-wave mixing in a mercury-argon discharge.

Phase conjugation has been obtained with pump powers as low as 0.5 mW on the 546.1-nm line of atomic mercury. Collisional processes that oppose the effects of optical pumping sustain the signal. Line splittings observed in the Doppler regime are compared with theory. Reflectivities agree with those obtained from a model that includes pump absorption.

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.

Coupling characteristics of LiNbO3 directional couplers.

An analytic model of a Ti:LiNbO3 channel waveguide has been developed to predict conveniently the coupling coefficient of directional couplers based on near-field intensity measurements. This model is compared to experimental results at wavelengths of 0.63 and 0.81 microm and indicates an accuracy of predicting coupling coefficients within a factor of 2 without prior knowledge of the fabrication parameters.

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.