Robert J. Kruhlak

Characterization of chromatic dispersion in photonic crystal fibers using scalar modulation instability

A simple and accurate method is proposed for characterizing the chromatic dispersion of high air-filling fraction photonic crystal fibers. The method is based upon scalar modulation instability generated by a strong pump wave propagating near the zero-dispersion wavelength. Measuring the modulation instability sideband frequency shifts as a function of wavelength gives a direct measurement of the fibers chromatic dispersion over a wide wavelength range. To simplify the dispersion calculation we introduce a simple analytical model of the fibers dispersion, and verify its accuracy via a full numerical simulation. Measurements of the chromatic dispersion of two different types of high air-filling fraction photonic crystal fibers are presented.

Solitary pulse propagation in high gain optical fiber amplifiers with normal group velocity dispersion

Pulse propagation in high gain optical fiber amplifiers with normal group velocity dispersion is studied using numerical simulations of a generalized nonlinear Schrodinger equation including a complex Lorentzian gain profile. In particular, when the bandwidth of the input pulse is significantly less than that of the amplifier transition, an initial period of propagation where parabolic pulse characteristics are observed is followed by a solitary wave regime where the pulse temporal and spectral widths are stabilized by the limiting effect of the transition. The intensity and chirp characteristics of the resulting solitary pulse are considered in detail and dimensional analysis yields an empirical expression for the solitary wave peak power in excellent agreement with simulations.

Polarization modulation instability in photonic crystal fibers

Polarization modulation instability (PMI) in birefringent photonic crystal fibers has been observed in the normal dispersion regime with a frequency shift of 64 THz between the generated frequencies and the pump frequency. The generated sidebands are orthogonally polarized to the pump. From the observed PMI frequency shift and the measured dispersion, we determined the phase birefringence to be 5.3 x 10(-5) at a pump wavelength of 647.1 nm. This birefringence was used to estimate the PMI gain as a function of pump wavelength. Four-wave mixing experiments in both the normal and the anomalous dispersion regimes generated PMI frequency shifts that show good agreement with the predicted values over a 70 THz range. These results could lead to amplifiers and oscillators based on PMI.

Cross-phase modulation instability in photonic crystal fibers

We report on the observation of cross phase modulation instability (XPMI) in photonic crystal fiber (PCF). The rapidly varying dispersion of the fiber around its zero dispersion wavelength gives rise to a complicated phasematching curve.

Permanent refractive-index modification in germanium-doped optical fibers by use of red light

We observed photoinduced birefringence in elliptical-core optical fibers by using a continuous-wave krypton-ion laser. We induced the birefringence by injecting 20 mW of 647-nm or 50 mW of 676-nm light into the fiber at 45 degrees to the slow axis. The rate of change of the refractive index was found to be proportional to the square of the average power. Polarization mode couplers written into the fibers have been stable for more than 2 years and can be erased by use of light polarized perpendicularly to the original writing beam.

PCF based optical parametric oscillators and amplifiers

This study demonstrates photonic crystal fiber (PCF) based optical parametric oscillators and amplifiers. In the experiment, a wide range of fibres with varying zero GVD points around the visible red pump wavelength are used. The resulting upshifted sideband is in the yellow region of the spectrum of pumping in the normal dispersion regime (low group index mode) in agreement with simple predictions based on the linear phase matching curve, while the sidebands corresponding to conventional modulation instability are close to the pump wavelength when pumping in the anomalous dispersion regime (the high group index mode of the fibre).

A General Theory of Inhomogeneous Broadening for Nonlinear Susceptibilities: The Polarizability and Hyperpolarizability

While nonlinear optical spectroscopy is becoming more commonly used to study the excited states of nonlinear-optical systems, a general theory of inhomogeneous broadening is rarely applied in lieu of either a simple Lorentzian or Gaussian model. In this paper, we generalize all the important linear and second-order nonlinear susceptibility expressions obtained with sum-over state quantum calculations to include Gaussian and stretched Gaussian distributions of Lorentzians. We show that using the correct model to analyze experiments that probe a limited wavelength range can be critical and that this theory is better able to fit the subtle spectral features - such as the shoulder region of a resonance - when both models produce qualitatively similar responses.

A General Theory of Inhomogeneous Broadening for Nonlinear Susceptibilities: The Second Hyperpolarizability

A general theory of inhomogeneous broadening is rarely applied to nonlinear spectroscopy in lieu of either a simple Lorentzian or a Gaussian model. In this paper, we generalize all the important third-order nonlinear susceptibility expressions obtained with sum-over-state quantum calculations to include Gaussian and stretched Gaussian distributions of Lorentzians. This theory gives a better fit to subtle spectral features-such as the shoulder of the electroabsorption peak, and is a more accurate tool for determining transition moments from spectroscopy experiments.

Optical parametric amplification in the normal dispersion regime using a PCF

Photonic crystal fibres (PCFs) can now be constructed exhibiting a wide range of dispersion profiles and nonlinearities, which have enabled the demonstration of many nonlinear optical phenomena. In our experiments, we have used a range of fibres with varying zero group velocity dispersion (GVD) points around the visible red pump wavelength. Upshifted sidebands can be generated which span the entire visible range from the blue to the red, whilst the corresponding downshifted sidebands range from approximately 750 to 1000 nm. We have demonstrated that newly developed PCFs can be used as efficient parametric frequency generators, and, in principle, a PCF based fibre optic parametric oscillator could generate a tunable all solid state source spanning the entire visible spectrum. Such a source would, however, require the production of PCFs with very uniform fibre diameter and the development of higher power single transverse mode solid state lasers in the red.

Scalar modulation instability near zero GVD using a PCF

Scalar modulation instability has been demonstrated in the normal dispersion regime using a PCF leading to efficient upconversion of a red pump to wavelengths throughout the visible.