Measuring laser-speckle statistics in scaled-laboratory experiments.

Generating fully developed speckle in a repeatable way is of interest to ongoing scaled-laboratory experiments. Such experiments often look to validate theoretical and numerical predictions for numerous laser-based applications. Unfortunately, experimental constraints such as camera-pixel sampling, residual-sensor noise, and cover-glass etaloning limit one's ability to match the statistics of fully formed speckle in a straightforward way. In this paper, we develop expressions for the speckle probability density function (PDF) and speckle contrast, which account for the effects of camera-pixel sampling (relative to the size of the speckles), as well as Gaussian-distributed additive noise. We validate these expressions using wave-optics simulations, which also account for the separate effects of cover-glass etaloning. Next, we set up an experiment that limits the effects of the cover-glass etaloning (as much as possible). The results show excellent agreement with the expressions we develop for the speckle PDF and speckle contrast. This agreement will enable future scaled-laboratory experiments to match the statistics of fully developed speckle in a straightforward way.

Parity-time-symmetry-breaking gyroscopes: lasing without gain and subthreshold regimes.

We show that the lasing threshold for two coupled resonators (CRs) corresponds to lasing without gain (LWG), a phenomenon analogous to lasing without inversion in atomic systems, when parity-time (PT) symmetry is broken. The use of LWG for gyroscopes may resolve some of the difficulties associated with PT-symmetric gyroscopes. In particular, we find that PT-symmetric systems suffer from undamped Rabi oscillations, whereas LWG systems are overdamped. In addition, observation of enhanced sensitivity should be more straightforward in LWG gyros because the enhancement remains above unity even at couplings far from the exceptional point (EP). Finally, LWG gyros operate more like conventional laser gyroscopes with one frequency for each output direction, and therefore there is no ambiguity in the direction of rotation. Gain saturation in CR systems is found to dramatically boost the size of the sensitivity enhancement, eliminate the Rabi oscillations, and enlarge the parameter space around the EP over which the enhancement is expected to occur. A second situation with broken symmetry is also examined: CR systems below threshold. Whereas the pole in sensitivity coincides with the EP at threshold, the pole can occur far away from the EP for subthreshold systems. Our analysis also puts previous results on passive and active fast-light cavities using atomic vapor cells into the context of EP-enhanced sensing with non-Hermitian Hamiltonians.

Closed-loop superluminal passive cavity.

We demonstrate the operation of a closed-loop fast-light cavity that allows rapid (~10 ms) measurements of the cavity mode frequency and its uncertainty. We vary the scale factor by temperature tuning the atomic density of an intracavity vapor cell. The cavity remains locked even as the system passes through the critical anomalous dispersion where a pole is observed in the scale factor. Positive and negative scale-factor enhancements as large as |S| ≈70 were obtained. To our knowledge, these are the first experiments that demonstrate a scale-factor enhancement in a closed-loop fast-light device by changing the optical path length, laying the groundwork for the improvement of cavity-based metrology instruments such as optical gyroscopes.

High-precision, accurate optical frequency reference using a Fabry-Perót diode laser.

We show that the optical output of a temperature and current-tuned Fabry-Perót diode laser system, with no external optical feedback and in which the frequency is locked to Doppler-free hyperfine resonances of the 87Rb D2 line, can achieve high frequency stability and accuracy. Experimental results are presented for the spectral linewidth, frequency stability, and frequency accuracy of the source. Although our optical source is limited by a short-term spectral linewidth greater than 2 MHz, beat signal measurements from two such sources demonstrate a frequency stability of 1.1 kHz, or minimum Allan deviation of 4×10-12, at an integration time τ=15 s and with a frequency accuracy of 60 kHz at τ=300 s. We demonstrate the use of the optical source for the precision measurement of hyperfine level frequency spacings in the 5P3∕2 excited state of 87Rb and provide an accurate frequency scale for optical spectroscopy.

Quantum-Noise-Limited Sensitivity-Enhancement of a Passive Optical Cavity by a Fast-Light Medium.

We demonstrate for a passive optical cavity containing a dispersive atomic medium, the increase in scale factor near the critical anomalous dispersion is not cancelled by mode broadening or attenuation, resulting in an overall increase in the predicted quantum-noise-limited sensitivity. Enhancements of over two orders of magnitude are measured in the scale factor, which translates to greater than an order-of-magnitude enhancement in the predicted quantum-noise-limited measurement precision, by temperature tuning a low-pressure vapor of non-interacting atoms in a low-finesse cavity close to the critical anomalous dispersion condition. The predicted enhancement in sensitivity is confirmed through Monte-Carlo numerical simulations.