Anthony S. Kewitsch

Self-focusing and self-trapping of optical beams upon photopolymerization

We demonstrate theoretically and experimentally that optical beams are self-focused and self-trapped upon initiating photopolymerization. This unique nonlinear optical phenomenon is dependent on the optical exposure and produces permanent index-of-refraction changes larger than 0.04. The resulting nonlinear wave equation is shown to be nonlocal in time and displays self-trapped solutions only for sufficiently low average optical intensities.

All-fiber zero-insertion-loss add-drop filter for wavelength-division multiplexing

We developed and fabricated an all-fiber add-drop filter by recording a Bragg grating in the waist of an asymmetric mode converter-coupler formed by adiabatic tapering and fusing of two locally dissimilar, single-mode optical fibers. The insertion loss of the device was ~0.1 dB .A narrow spectral bandwidth (<1 nm) and a large add-drop efficiency (>90%) were also demonstrated. In addition, the filter was polarization independent.

Nonlinear optical properties of photoresists for projection lithography

Optical beams are self‐focused and self‐trapped upon initiating crosslinking in photoresists. This nonlinear optical phenomenon is apparent only for low average optical intensities and produces index of refraction changes as large as 0.04. We propose using the self‐focusing and self‐trapping phenomenon in projection photolithography to enhance the resolution and depth of focus.

Coherent combining of the output of two semiconductor lasers using optical phase-lock loops

We have experimentally demonstrated current-injection optical phase-lock loops (OPLLs) based on commercial single-section semiconductor distributed-feedback (DFB) lasers. Using two parallel OPLLs, we have obtained 87% efficient coherent power combining of the two DFB lasers. The rms differential phase error between the two lasers is about 30 degrees .

Electric-field multiplexing/demultiplexing of volume holograms in photorefractive media

We propose a new method of volume hologram multiplexing/demultiplexing in noncentrosymmetric media. Volume holograms may be multiplexed by tuning the material parameters of the recording medium (such as refractive index or lattice parameters) while keeping the external parameters (wavelength and angles) fixed. For example, an external dc electric field alters the index of refraction through the electro-optic effect, effectively changing the recording and reconstruction wavelengths in the storage medium. Then the storage of holograms at different fields, hence different indices of refraction, is closely related to wavelength multiplexing. We demonstrate this concept in a preliminary experiment by electrically multiplexing two volume holograms in a strontium barium niobate crystal.

Tunable quasi‐phase matching using dynamic ferroelectric domain gratings induced by photorefractive space‐charge fields

We demonstrate a method of dynamic, tunable quasi‐phase matched second‐harmonic generation using optically induced polarization gratings with periods equal to twice the coherence length. These gratings increase the peak second‐harmonic conversion efficiency by a factor of 17 above a poled strontium barium niobate crystal, to 0.01% for fundamental beam intensities of 0.8 MW cm−2. We generate quasi‐phase matching spectral response peaks as narrow as 0.175 nm and tailor the response by writing an ensemble of gratings in the same volume, each of which enhances the second‐harmonic generation at a predetermined wavelength.

Selective page-addressable fixing of volume holograms in Sr0.75Ba0.25Nb2O6 crystals

We demonstrate selective fixing of volume holograms in photorefractive media. Each holographic page may be fixed individually and overwritten without destroying the other fixed pages. We present experimental results describing this process in Cr-doped Sr(0.75)Ba(0.25)Nb(2)O(6) at room temperature, with hologram lifetimes exceeding 100 days during continuous readout with an intense beam (1 W/cm(2)).

Coherent beam combining with multilevel optical phase-locked loops

Coherent beam combining (CBC) technology holds the promise of enabling laser systems with very high power and near-ideal beam quality. We propose and demonstrate a novel servo system composed of multilevel optical phase lock loops. This servo system is based on entirely electronic components and consequently can be considerably more compact and less expensive compared to servo systems made of optical phase/frequency shifters. We have also characterized the noise of a 1064 nm Yb-doped fiber amplifier to determine its effect on the CBC and studied theoretically the efficiency of combining a large array of beams with the filled-aperture implementation. In a proof-of-concept experiment we have combined two 100 mW 1064 nm semiconductor lasers with an efficiency of 94%.

Simple methods of measuring the net photorefractive phase shift and coupling constant

We report measurements of the photorefractive phase shift and coupling constant of several photorefractive materials. We solve the problem of beam coupling and diffraction in a material with a dynamically written grating for arbitrary input beams. These solutions are used to determine the beam coupling as a function of the photorefractive phase ø and coupling constant g when one beam is either sinusoidally phase modulated or ramped in phase. Experimental results are obtained for LiNbO(3), BaTiO(3), and for paraelectric potassium lithium tantalate niobate as a function of applied electric field.

Coherent Power Combination of Semiconductor Lasers Using Optical Phase-Lock Loops

Heterodyne optical phase-lock loops (OPLLs) enable the precise electronic control over the frequency and phase of a semiconductor laser (SCL) locked to a ldquomasterrdquo reference laser. One of the more interesting applications of OPLLs is the creation of coherent arrays by locking a number of ldquoslaverdquo SCLs to a common master laser. In this paper, we demonstrate the coherent power combination of various high-power semiconductor lasers using OPLLs in both the filled-aperture and tiled-aperture configurations. We further demonstrate the electronic control over the phase of each individual SCL using a voltage-controlled oscillator. It is feasible to combine a large number of SCLs using this approach, leading to compact, efficient, and cost-effective high-power and high-radiance optical sources.