Andrew M. C. Dawes

Broadband SBS Slow Light in an Optical Fiber

In this paper, we investigate slow light via stimulated Brillouin scattering (SBS) in a room temperature optical fiber that is pumped by a spectrally broadened laser. Broadening the spectrum of the pump field increases the linewidth Deltaomegap of the Stokes amplifying resonance, thereby increasing the slow-light bandwidth. One physical bandwidth limitation occurs when the linewidth becomes several times larger than the Brillouin frequency shift OmegaB so that the anti-Stokes absorbing resonance substantially cancels out the Stokes amplifying resonance and, hence, the slow-light effect. We find that partial overlap of the Stokes and anti-Stokes resonances can actually lead to an enhancement of the slow-light delay-bandwidth product when Deltaomegapsime1.3OmegaB. Using this general approach, we increase the Brillouin slow-light bandwidth to over 12 GHz from its nominal linewidth of ~30 MHz obtained for monochromatic pumping. We controllably delay 75-ps-long pulses by up to 47 ps and study the data-pattern dependence of the broadband SBS slow-light system

Distortion management in slow-light pulse delay

We describe a methodology to maximize slow-light pulse delay subject to a constraint on the allowable pulse distortion. We show that optimizing over a larger number of physical variables can increase the distortion-constrained delay. We demonstrate these concepts by comparing the optimum slow-light pulse delay achievable using a single Lorentzian gain line with that achievable using a pair of closely-spaced gain lines. We predict that distortion management using a gain doublet can provide approximately a factor of 2 increase in slow-light pulse delay as compared with the optimum single-line delay. Experimental results employing Brillouin gain in optical fiber confirm our theoretical predictions.

12-GHz-Bandwidth SBS Slow Light in Optical Fibers

We increased the bandwidth of SBS slow light in an optical fiber to 12.6 GHz. We delayed 75-ps pulses by up to 47 ps and studied the data pattern dependence of the broadband SBS slow-light system.

Transverse patterns for all-optical switching

We demonstrate an all-optical switch that operates at ultra-low-light levels and exhibits several features necessary for use in optical switching networks. An input switching beam, wavelength λ, with an energy density of 10−2 photons per optical cross section [σ = λ/(2π)] changes the orientation of a two-spot pattern generated via parametric instability in warm rubidium vapor. The instability is induced with less than 1 mW of total pump power and generates several μWs of output light. The switch is cascadable: the device output is capable of driving multiple inputs, and exhibits transistor-like signal-level restoration with both saturated and intermediate response regimes. Additionally, the system requires an input power proportional to the inverse of the response time, which suggests thermal dissipation does not necessarily limit the practicality of optical logic devices.

Collimated blue light generation in rubidium vapor

We describe an experiment for generating and characterizing a beam of collimated blue light (CBL) in a rubidium vapor. Two low-power, grating-feedback diode lasers, operating at 780.2 nm (5S1/2→5P3/2) and 776.0 nm (5P3/2→5D5/2), respectively, provide step-wise excitation to the 5D excited state in rubidium. Under the right experimental conditions, cascade decay through the 6P excited state will yield a collimated blue (420-nm) beam of light with high temporal and spatial coherence. We investigate the production of a blue beam under a variety of experimental conditions and characterize the spatial coherence and spectral characteristics. This experiment provides advanced undergraduate students with a unique opportunity to investigate nonlinear optical phenomena in the laboratory and uses equipment that is commonly available in laboratories equipped to investigate diode-laser-based absorption spectroscopy in rubidium.

Absorption-induced trapping in an anisotropic magneto-optical trap

We report on a simple anisotropic magneto-optical trap for neutral atoms that produces a large sample of cold atoms confined in a cylindrically-shaped volume with a high aspect ratio (100:1). Due to the large number of trapped atoms, the laser beams that propagate along the optically thick axis of the trap to cool the atoms are substantially attenuated. We demonstrate that the resulting intensity imbalance produces a net force that spatially localizes the atoms. This limits both the trap length and the total number of trapped atoms. Rotating the cooling beams by a small angle relative to the trap axis avoids the problem of attenuation, and atoms can be trapped throughout the entire available trapping volume. Numerical and experimental results are reported that demonstrate the effects of absorption in an anisotropic trap, and a steady-state, line-center optical path length of 55 is measured for a probe beam propagating along the length of the trap.

Improving the bandwidth of SBS-based slow-light delay

Frequency modulating the pump laser in SBS slow-light delay systems increases the effective Brillouin bandwidth by nearly two orders of magnitude, making the fiber Brillouin amplifier technique applicable to all-optical controllable delay of Gb/s data.

Simultaneous quantum-state measurements using array detection

Here we present the results of the first experiment that uses array detection to measure quantum states of an optical beam [1]. We demonstrate that an array detector can be used to simultaneously measure the quantum states of many different spatial modes of the same beam. Furthermore, we show that array detectors can allow for an improvement in effective detection efficiency over standard detectors when using balanced homodyne detection. This improvement comes from the fact that the local oscillator (LO) and signal fields need not be mode-matched when using array detectors. In our experiment array detection is found to be over forty times more efficient than standard detection for measurements of a particular field mode. The technique we use for determining the state of our field modes is quantum state tomography (QST), and yields the density matrix in the Foch-state basis ρmn [2,3].

Carrier-frequency dependence of a step-modulated pulse propagating through a weakly dispersive single narrow-resonance absorber

We observe interference between the optical precursors and the main signal for small optical depth α0 L ∼ 1, in which the main signal cannot be entirely absorbed. Since the main signal oscillates at the carrier frequency of the input pulse and precursors oscillate at medium resonance frequency, in our case carrier frequency dependence of the total transmitted field is observed as a form of modulation patterns oscillating at the detuning frequency. To distinguish between the Sommerfeld and Brillouin precursors for the case of weakly dispersive off-resonance medium, we utilize asymptotic precursor theory under the assumption of small detuning.

Direct observation of optical precursors in a region of anomalous dispersion

We observe the creation of optical precursors when a step-modulated optical pulse propagates through a linear resonant absorber. The precursors are the dominant part of the transmitted field, displaying 100% transmission at their maximum amplitude.