Oleg M. Efimov

Two-dimensional pseudo-random optical phased array based on tandem optical injection locking of vertical cavity surface emitting lasers.

We demonstrate, both theoretically and experimentally, a pseudo-random, two-dimensional optical phased array (OPA) concept based on tandem injection locking of 64-element vertical cavity surface emitting laser (VCSEL) arrays. A low cavity-Q VCSEL design resulted in an injection locking optical power of less than 1 μW per VCSEL, providing large OPA scaling potential. Tandem injection locking of two VCSEL arrays resulted in measured controllable optical phase change of 0-1.6π. A high quality beam formed with suppressed grating lobes due to the pseudo-random array design was demonstrated with performance close to simulated results. A preliminary 2.2° x 1.2° beam steering example using the tandem arrays was also demonstrated.

Compact electro-optic modulator for direct integration into an X-band antenna array front-end

A compact electro-optic modulator comprising a waveguide grating formed on a small piece of lithium niobate can be integrated directly into an X-band antenna array element. A prototype link with this modulator was demonstrated.

High power microwave pulse impact on an all-dielectric lithium niobate modulator

Measurements and multiphysics simulations of all-dielectric LiNbO3 grating modulators exposed to high power microwave pulses were conducted. Thinned substrate devices exposed to 180kV/cm fields show no significant degradation in measured optical spectrum or link gain.

Analog pulse-position modulation for free-space optical transmission of microwave signals

A pulse-position modulated optical link with chirped electro-optic waveguide grating modulator and semiconductor optical amplifier based demodulator has multi-gigahertz bandwidth and spur free dynamic range exceeding 85 dB.Hz4/5.

Enhanced light scattering in coatings templated with interfacially stabilized colloids

Controlling inter-pore distances enables tuning the color or whiteness of microvoid coatings. While pore spacings have been modified in limited area inverse opal films, little work has occurred studying the feasibility of controlling pore spacings and thus the appearance of scalable, spray-on, microvoid inorganic coatings. In this work we investigated using interfacially stabilized colloidal templates to increase pore spacing and thus enhance Mie scattering in porous silica films. Coatings were fabricated by forming monodisperse colloids with or without a polyvinylpyrrolidone (PVP) interfacial stabilizing layer, dispersing them in a silica precursor solution, and spraying this suspension on a substrate. The films were cured and the colloids subsequently solution extracted at mild temperatures to create porous surfaces. Coatings made with PVP coated colloids had thicker pore walls and scattered light approximately 3× more efficiently than coatings made with bare colloids. Furthermore, a viewing angle dependent color shift was observed in the PVP colloid templated coatings. Side illumination of the samples with white light causes an orange appearance under angles of specular reflection, while a light blue appearance is observed out of these angles because of strong Mie scattering of short-wavelength radiation in both situations. Lastly, modeling based on Mie scattering confirms that it is the dominant optical effect in these coatings and explains the appearance of these coatings. The approach of using interfacially stabilized colloids to improve pore separation applies to many porous films and should be considered when increased light scattering is desired.

Damage threshold measurements: Self-focusing or intrinsic damage?

The laser-induced damage (LID) thresholds of pure fused silica (Corning 7980) have been measured with single temporal mode nanosecond pulses at 1064 nm. The laser beam has been focused by spherical and conical lenses into 1.6 μm diameter spots. In the case of pseudo-Bessel beam (conical lens) which inherently was not subjected to self-focusing the threshold has been close to the intrinsic threshold in fused silica. However, the measurement with pseudo-Gaussian beam (spherical lens) has shown about 30% lower value of threshold. Complete identity in the cross-section distributions of beam intensities and considerable difference in measured thresholds indicate that self-focusing influence on the LID of dielectrics even for tight focused laser beams.

Intrinsic laser-induced damage in bulk transparent dielectrics

Our results from experiments on laser-induced damage in transparent dielectrics are incompatible with the well-known avalanche ionization model. The mechanism of damage may involve a collective response of the dielectric, such as “dielectric-metal” phase transition.

Single and multiple pulse laser-induced breakdown in transparent dielectrics in the femto-nanosecond region

This review is devoted to a long-term investigation into the nature of the laser-induced damage of silciate glasses. As an important result, we show that the threshold power density of the intrinsic damage of the boro-silicate glass at ~1 μm wavelength does not depend on pulse duration from 2 x 10-13 to 3 x 10-8s as long as self-focusing is avoided. This result cannot be explained by existing theories and indicates that the damage mechanism involves a collective response of a certain volume in the dielectric as a whole, rather than the accumulation of electrons via individual generation processes like multiphoton, tunneling, or avalanche. Special attention in the research was paid to investigation into the processes of multiple pulse damage and subthreshold modification of boro- and lead-silicate glasses.

Intrinsic and multiple pulse laser-induced damage of transparent dielectrics in the femto-nanosecond region

This review is devoted to a long-term investigation into the nature of the laser-induced damage of silicate glasses. Besides damage threshold measurements, it includes data about linear and nonlinear absorption, multiple pulse damage, and subthreshold modification of media. The experiments were carried out over a wide range of wavelengths, pulse durations, and spot sizes. As an important result, we show that the threshold power density of the intrinsic damage of the boro-silicate glass at ~1 µm wavelength does not depend on pulse duration from 2x10-13 to 3x10-8s as long as self-focusing is avoided. This result cannot be explained by existing theories and indicates that the damage mechanism involves a collective response of a certain volume in the dielectric as a whole, rather than the accumulation of electrons via individual generation processes, like multiphoton, tunneling, or avalanche. Special attention in the research was paid to investigation into the processes of electron excitation in alkali- and lead-silicate glasses.

Collective excitation of transparent dielectrics under femtosecond pulses

The goal of this work is to measure the intrinsic laser-induced damage (LID) of a borosilicate glass for femtosecond pulses and to compare it with data obtained for the nanopicosecond region under conditions when self-focusing is avoided. This study demonstrates that the intrinsic LID irradiance of industrial borosilicate glass does not depend on pulse duration for 2 /spl times/ 10/sup -13/ - 3 /spl times/ 10/sup -8/ s pulses. This finding is in favor of the LID mechanism, which is based on the phase transition scenario, rather than the currently accepted view based on a multi-photon ionization with ensuing avalanche development.