A topical review of numerical and experimental studies of supercontinuum generation in photonic crystal fiber is presented over the full range of experimentally reported parameters, from the femtosecond to the continuous-wave regime. Results from numerical simulations are used to discuss the temporal and spectral characteristics of the supercontinuum, and to interpret the physics of the underlying spectral broadening processes. Particular attention is given to the case of supercontinuum generation seeded by femtosecond pulses in the anomalous group velocity dispersion regime of photonic crystal fiber, where the processes of soliton fission, stimulated Raman scattering, and dispersive wave generation are reviewed in detail. The corresponding intensity and phase stability properties of the supercontinuum spectra generated under different conditions are also discussed.

Supercontinuum generation in photonic crystal fiber

Linear optics with photon counting is a prominent candidate for practical quantum computing. The protocol by Knill, Laflamme, and Milburn [2001, Nature (London) 409, 46] explicitly demonstrates that efficient scalable quantum computing with single photons, linear optical elements, and projective measurements is possible. Subsequently, several improvements on this protocol have started to bridge the gap between theoretical scalability and practical implementation. The original theory and its improvements are reviewed, and a few examples of experimental two-qubit gates are given. The use of realistic components, the errors they induce in the computation, and how these errors can be corrected is discussed.

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Linear optical quantum computing with photonic qubits

An overview is presented of the current state-of-the-art in silicon nanophotonic ring resonators. Basic theory of ring resonators is discussed, and applied to the peculiarities of submicron silicon photonic wire waveguides: the small dimensions and tight bend radii, sensitivity to perturbations and the boundary conditions of the fabrication processes. Theory is compared to quantitative measurements. Finally, several of the more promising applications of silicon ring resonators are discussed: filters and optical delay lines, label-free biosensors, and active rings for efficient modulators and even light sources.

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Silicon microring resonators

Optical frequency combs provide equidistant frequency markers in the infrared, visible and ultraviolet, and can be used to link an unknown optical frequency to a radio or microwave frequency reference. Since their inception, frequency combs have triggered substantial advances in optical frequency metrology and precision measurements and in applications such as broadband laser-based gas sensing and molecular fingerprinting. Early work generated frequency combs by intra-cavity phase modulation; subsequently, frequency combs have been generated using the comb-like mode structure of mode-locked lasers, whose repetition rate and carrier envelope phase can be stabilized. Here we report a substantially different approach to comb generation, in which equally spaced frequency markers are produced by the interaction between a continuous-wave pump laser of a known frequency with the modes of a monolithic ultra-high-Q microresonator via the Kerr nonlinearity. The intrinsically broadband nature of parametric gain makes it possible to generate discrete comb modes over a 500-nm-wide span (approximately 70 THz) around 1,550 nm without relying on any external spectral broadening. Optical-heterodyne-based measurements reveal that cascaded parametric interactions give rise to an optical frequency comb, overcoming passive cavity dispersion. The uniformity of the mode spacing has been verified to within a relative experimental precision of 7.3 x 10(-18). In contrast to femtosecond mode-locked lasers, this work represents a step towards a monolithic optical frequency comb generator, allowing considerable reduction in size, complexity and power consumption. Moreover, the approach can operate at previously unattainable repetition rates, exceeding 100 GHz, which are useful in applications where access to individual comb modes is required, such as optical waveform synthesis, high capacity telecommunications or astrophysical spectrometer calibration.

Optical frequency comb generation from a monolithic microresonator

Slow light with a remarkably low group velocity is a promising solution for buffering and time-domain processing of optical signals. It also offers the possibility for spatial compression of optical energy and the enhancement of linear and nonlinear optical effects. Photonic-crystal devices are especially attractive for generating slow light, as they are compatible with on-chip integration and room-temperature operation, and can offer wide-bandwidth and dispersion-free propagation. Here the background theory, recent experimental demonstrations and progress towards tunable slow-light structures based on photonic-band engineering are reviewed. Practical issues related to real devices and their applications are also discussed. The unique properties of wide-bandwidth and dispersion-free propagation in photonic-crystal devices have made them a good candidate for slow-light generation. This article gives the background theory of slow light, as well as an overview of recent experimental demonstrations based on photonic-band engineering.

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Slow light in photonic crystals

We study a Mach-Zehnder nonlinear fiber interferometer for the generation of amplitude-squeezed light. Numerical simulations of experiments with microstructure fiber are performed using linearization of the quantum nonlinear Shrodinger equation. We include in our model the effect of distributed linear losses in the fiber.

Amplitude squeezing in a Mach-Zehnder fiber interferometer: Numerical analysis of experiments with microstructure fiber.

This Letter reports theoretical and experimental studies of spectrum reversal with tunable wavelength offset in the optical-frequency regime—two widely separated spectral sidebands can always behave as mirror images of one another with respect to the center frequency of the controlling pump pulse. We call this interesting physical phenomenon “spectral mirror imaging.”

Demonstration of nondegenerate spectrum reversal in optical-frequency regime

We explore the spectral effects due to cross-phase modulation and walk-off in picosecond fiber optical parametric oscillators. The output spectrum exhibits pump-power-dependent broadening, which can be quite asymmetric associated with a redshift or a blueshift depending on pump synchronization. By slightly increasing the cavity length, one obtains a blueshifted spectrum and a conversion efficiency as high as 15%.

Cross-phase-modulation-induced spectral effects in high-efficiency picosecond fiber optical parametric oscillators.

The Casimir force was predicted in 1948 as a force arising between macroscopic bodies from the zero-point energy. At finite temperatures, it has been shown that a thermal Casimir force exists due to thermal rather than zero-point energy and there are a growing number of experiments that characterize the effect at a range of temperatures and distances. In addition, in the rapidly evolving field of cavity optomechanics, there is an endeavour to manipulate phonons and enhance coherence. We demonstrate a way to realize a Casimir spring and engineer dilution in macroscopic optomechanics, by coupling a metallic SiN membrane to a photonic re-entrant cavity. The attraction of the spatially localized Casimir spring mimics a non-contacting boundary condition, giving rise to increased strain and acoustic coherence through dissipation dilution. This provides a way to manipulate phonons via thermal photons leading to ‘in situ’ reconfigurable mechanical states, to reduce loss mechanisms and to create additional types of acoustic nonlinearity—all at room temperature. An optomechanical cavity comprising a re-entrant cavity and membrane resonators can be tuned in and out of the Casimir regime. At the transition between the two regimes, the mechanical resonators exhibit a change in stiffness—the Casimir spring.

Casimir spring and dilution in macroscopic cavity optomechanics

A technique for generating variable pulse delays uses one or more nonlinear-optical processes such as cross-phase modulation, cross-gain modulation, self-phase modulation, four-wave mixing or parametric mixing, combined with group-velocity dispersion. The delay is controllable by changing the wavelength and/or power of a control laser. The delay is generated by introducing a controllable wavelength shift to a pulse of light, propagating the pulse through a material or an optical component that generates a wavelength dependent time delay, and wavelength shifting again to return the pulse to its original wavelength.

Jay E. Sharping

Papers

Amplitude squeezing in a Mach-Zehnder fiber interferometer: Numerical analysis of experiments with microstructure fiber.

Demonstration of nondegenerate spectrum reversal in optical-frequency regime

Cross-phase-modulation-induced spectral effects in high-efficiency picosecond fiber optical parametric oscillators.

Casimir spring and dilution in macroscopic cavity optomechanics

All-optical, continuously tunable, pulse delay generator using wavelength conversion and dispersion