Theoretical modeling and experimental demonstration of Raman probe induced spectral dip for realizing a superluminal laser

Demonstration of a superluminal laser using electromagnetically induced transparency in Raman gain

We report the realization of a superluminal laser in which the dip in the gain profile necessary for anomalous dispersion is produced via electromagnetically induced transparency caused by the optical pumping laser. This laser also creates the ground state population inversion necessary for generating Raman gain. Compared to a conventional Raman laser with similar operating parameters but without the dip in the gain profile, the spectral sensitivity of this approach is explicitly demonstrated to be enhanced by a factor of ∼12.7. Compared to an empty cavity, the peak value of the sensitivity enhancement factor under optimal operation parameters is inferred to be ∼360.

Observation of a highly superluminal laser employing optically pumped Raman gain and depletion

In this paper, we report a Raman laser which is extremely sensitive to a variation of the cavity length, using a scheme employing two stable isotopes of Rb. One isotope is used for producing a broad gain spectrum via the optically pumped Raman gain process, while the other is used for producing a narrow dip via the optically pumped Raman depletion process. By tuning the frequencies of the two Raman pumps, the center frequencies of the gain and dip can be aligned to the same frequency. This approach allows tuning of the gain and dip parameters independently over a broad range of operating conditions. With such a configuration, we can produce a negative dispersion around the two-photon resonance frequency in the vapor cell, which leads to a group index that is close to zero. By theoretically matching the experimental observations, we can infer that the sensitivity of such laser is enhanced by a factor of more than 2800, which is nearly a factor of three larger than the highest value reported previously using a different approach.

Electromagnetically induced transparency in Raman gain for realizing a superluminal ring laser

We describe an approach for realizing a superluminal ring laser using a single isotope of Rb vapor by producing electromagnetically induced transparency (EIT) in Raman gain. We show that by modifying the detuning and the intensity of the optical pump field used for generating the two-photon population inversion needed for generating Raman gain, it is possible to generate a dip in the center of the gain profile that can be tuned to produce a vanishingly small group index, as needed for making the Raman laser superluminal. We show that two such lasers, employing two different vapor cells, can be realized simultaneously, operating in counter-propagating directions in the same cavity, as needed for realizing a superluminal ring laser gyroscope. This technique, employing only one isotope, is much simpler than the earlier, alternative approach for realizing a superluminal Raman laser based on Raman gain and Raman dip in two isotopes [Zhou et. al, Opt. Express27, 29738 (2019)10.1364/OE.27.029738]. We present both an approximate theoretical model based on four levels as well as the results of a model that takes into account all relevant hyperfine states corresponding to the D1 and D2 transitions in 85Rb atom. We also present experimental results, in good agreement with the theoretical model, to validate the approach.

Phase correlation during two-photon resonance process in an active cavity

In this paper, we experimentally demonstrate a strong correlation between the frequencies of the Raman pump and the Raman probe inside an optically pumped Raman laser. We show that the correlation is due to rapid adjustment of the phase of the dipoles that produce the Raman gain, following a sudden jump in the phase of the Raman pump. A detailed numerical model validates this interpretation of the phase correlation. The width of the spectrum of the beat between the Raman pump and the Raman laser is significantly narrowed due to this correlation. As a result, the minimum measurable change in the cavity length, for a given linewidth of the Raman pump laser, is substantially reduced. Therefore, this finding is expected to enhance the sensitivity of such a laser in various metrological applications (e.g., accelerometry).

Hyperfine-state dependence of highly efficient amplification from diode-pumped cesium vapor

We report optical amplification with an optical-to-optical conversion efficiency of 70 ± 1% from a diode-pumped Cs vapor cell. When pump (852 nm; D₂-line) and signal (895 nm; D₁-line) lasers with a narrow spectral width of ∼2 MHz are resonant on the hyperfine states (F = 3 or 4) of the 6S_1/2 state, we observe that the amplification factors are significantly changed according to the hyperfine-state combination of the pump and signal lasers. We find that the optical frequencies of the pumping and signal lasers need to be controlled near the hyperfine state of 6S_1/2 (F = 4) to obtain an efficient diode-pumped alkali amplifier (DPAA). To realize highly efficient optical gain conditions, both the spatial modes of the pump and signal lasers are made to overlap in the Cs vapor cell with the use of a single-mode optical fiber. An amplification factor of 430 ± 15 is achieved under the following conditions: cell temperature of 90 °C, signal power of 0.1 mW, and pump power of 200 mW. We believe that our results can aid in the development of highly efficient diode-pumped alkali-vapor lasers and amplifiers.

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.

Superluminal Raman laser with enhanced cavity length sensitivity

We demonstrate experimentally a superluminal ring laser based on optically pumped Raman gain, and a self-pumped Raman depletion for producing anomalous dispersion, employing two isotopes of rubidium. By fitting the experiment data with the theoretical model, we infer that the spectral sensitivity of the superluminal Raman laser to cavity length change is enhanced by a factor of more than a thousand, compared to a conventional laser.

White light cavity formation and superluminal lasing near exceptional points

We study theoretically a superluminal laser system comprising active and passive (lossy) coupled micro-resonators with equal gain/loss. It is shown that when the system satisfies the white light cavity (WLC) condition, corresponding to zero group index, it also forms a PT-symmetric system (PTSS) at its exceptional point (EP). Slightly above lasing threshold, in the broken symmetry regime near the EP, the system exhibits "superluminal" lasing - a unique lasing condition which is highly attractive for sensing and precision metrology applications. It is also shown that some of the latest experimental studies involving PTSSs have indirectly demonstrated such superluminal lasing.

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.

Demonstration of a highly subluminal laser with suppression of cavity length sensitivity by nearly three orders of magnitude

We have demonstrated a laser in which the frequency shift due to small cavity fluctuations is far less than what would be expected from a conventional laser. The factor of sensitivity suppression is inferred to be equal to the effective group index experienced by the laser, implying that this laser is subluminal. We have observed a suppression factor as high as 663. Such a laser is highly self-stabilized compared to a conventional laser, and is expected to have a far smaller Schawlow-Townes linewidth. As a result, this laser may have potentially significant applications in the fields of high-precision optical metrology and passive frequency stabilization.

Tailorable dispersion in a four-wave mixing laser

We present experimental results demonstrating controllable dispersion in a ring laser by monitoring the lasing-frequency response to cavity-length variations. Pumping on an N-type level configuration in Rb87, we tailor the intra-cavity dispersion slope by varying experimental parameters, such as pump-laser frequency, atomic density, and pump power. As a result, we can tune the pulling factor, i.e., the ratio of the laser frequency shift to the empty cavity frequency shift, of our laser by more than an order of magnitude.