Joel Falk

Enhanced spontaneous Raman scattering and gas composition analysis using a photonic crystal fiber

Spontaneous gas-phase Raman scattering using a hollow-core photonic bandgap fiber (HC-PBF) for both the gas cell and the Stokes light collector is reported. It was predicted that the HC-PBF configuration would yield several hundred times signal enhancement in Stokes power over a traditional free-space configuration because of increased interaction lengths and large collection angles. Predictions were verified by using nitrogen Stokes signals. The utility of this system was demonstrated by measuring the Raman signals as functions of concentration for major species in natural gas. This allowed photomultiplier-based measurements of natural gas species in relatively short integration times, measurements that were previously difficult with other systems.

Improved sensitivity gas detection by spontaneous Raman scattering

Accurate, real-time measurement of the dilute constituents of a gaseous mixture poses a significant challenge usually relegated to mass spectrometry. Here, spontaneous Raman backscattering is used to detect low pressure molecular gases. Rapid detection of gases in the approximately 100 parts in 10(6) (ppm) range is described. Improved sensitivity is brought about by use of a hollow-core, photonic bandgap fiber gas cell in the backscattering configuration to increase collection efficiency and an image-plane aperture to greatly reduce silica-Raman background noise. Spatial and spectral properties of the silica noise were examined with a two-dimensional CCD detector array.

The effects of focusing in the three‐frequency parametric upconverter

This paper extends the theory of parametric upconversion to describe the mixing of two focused optical beams with unequal confocal parameters. The results of this paper have greatest applicability when focusing of one input beam is limited by physical constraints, e.g., damage. The effects of phase matching, walk‐off, and diffraction are treated herein. The analysis shows that if the beams have fixed confocal parameters b1 and b2, optimum mixing generally occurs with b2≠b1. If both confocal parameters can be arbitrarily chosen, maximum efficiency occurs with b1=b2= (crystal length)/2.84 in the absence of Poynting vector walk‐off, and with b1≠b2 in the presence of walk‐off. For weak focusing upconversion efficiency is written in a closed form which shows explicitly the effect of walk‐off.

Numerical study of transient stimulated Brillouin scattering

Seeded, transient, stimulated Brillouin scattering (SBS) is studied using a new noniterative, numerical algorithm to solve this two‐point boundary value problem. The coupled pump, Stokes, and phonon equations are solved numerically, including the effects of pump depletion and a finite phonon lifetime. The noniterative approach leads to efficient computation (<30 s of cpu time, VAX 8000 series computer for 500 by 5000 points in space and time, respectively). The numerical study predicts transient oscillations in the pump and the Stokes intensities. The frequency of these oscillations depends on the phonon lifetime and the depleted, exponential SBS gain. This work is the first known prediction of these oscillations.

Theory of phase conjugation by stimulated Brillouin scattering

The accuracy of the phase-conjugate signal produced by stimulated Brillouin scattering in a waveguide is theoretically investigated. Contradictions between earlier phase-conjugation theories are resolved and are shown to be due solely to differing treatments of the non-phase-matched Brillouin output. We find that this output dominates the nonconjugate signal from stimulated Brillouin scattering.

Stimulated Raman scattering at kHz pulse repetition rates

Stimulated Raman scattering (SRS) has been achieved at kHz pulse repetition rates. The Raman oscillator consists of a lithium iodate crystal placed inside a Nd:YAlO3 laser, with feedback provided at the Stokes wavelengths by mirrors surrounding the crystal. First and second Stokes outputs at 1.18 and 1.31 μm, respectively, were generated from oblique longitudinal‐optic (LO) phonons in the lithium iodate. These results open the possibility of realizing substantial average output powers from Raman oscillators for the first time.

Measurements of stimulated Brillouin scattering phase-conjugate fidelity

We describe high-accuracy measurements of stimulated Brillouin scattering phase-conjugate fidelity. The dependence of fidelity on pump intensity and on pump-beam divergence is in rough agreement with theoretical predictions for the low-reflectivity limit, even for operation deep into the saturation regime.

Limits to the NEP of an intracavity LiNbO(3) upconverter.

Limits to low noise equivalent power (NEP) operation of a lithium niobate upconverter are investigated. Upconversion is achieved inside the optical cavity of an Ar-ion laser. Limits to NEP are imposed by limits to conversion efficiency and by noise present in the upconversion process. Conversion efficiency is limited y thermal effects in the lithium niobate. Thermally induced wedging, focusing, and aberrations are caused by the lithium niobate absorption at the 514.5-nm argon pump wavelength. The primary component of noise in the upconverter is due to upconversion of thermal radiation from the lithium niobate crystal. The lowest NEP, at lambda = 3.4,microm, achieved in this study was 8.9 x 10(-14) W/Hz(1/2).

Limits to the efficiency of beam combination by stimulated Brillouin scattering

The probability distribution function of the phase difference between two thermal-noise-initiated stimulated Brillouin scattering beams was calculated and measured as a function of the correlation between the noise sources. This correlation is determined by the spatial overlap of the two pump beams. Perfect phase locking of every pair of Brillouin-scattered pulses is shown to be impossible.

Nonlinear optical properties of CdSe quantum dots

The nonlinear optical properties of CdSe quantum dots are investigated using self‐saturation and degenerate four‐wave mixing techniques. The saturation of the room temperature absorption coefficient, for a wavelength in the vicinity of the first exciton peak, is measured. The measured data can be predicted from a description of the quantum dot as a two level, homogeneously broadened, saturating system. The variation of the four‐wave mixing reflectivity with intensity and with wavelength is reported. All four‐wave mixing data are also consistent with the homogeneously‐broadened two‐level model.