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.

Elimination of optical phase sensitivity in a shift, scale, and rotation invariant hybrid opto-electronic correlator via off-axis operation.

The hybrid opto-electronic correlator (HOC) uses a combination of optics and electronics to perform target recognition. Achieving a stable output from this architecture has previously presented a significant challenge due to a high sensitivity to optical phase variations, limiting the real-world feasibility of the device. Here we present a modification to the architecture that essentially eliminates the dependence on optical phases, and demonstrate verification of the proposed approach. Experimental results are shown to agree with the theory and simulations, for scale, rotation and shift invariant image recognition. This approach represents a major innovation in making the HOC viable for real-world applications.

Integration of a PQ:PMMA holographic memory device into the hybrid opto-electronic correlator for shift, scale, and rotation invariant target recognition.

The hybrid optoelectronic correlator (HOC) combines optical and electronic signal processing to achieve the same functionality as traditional optical correlators but without the need for dynamic materials. Here we propose and demonstrate the integration of a PQ:PMMA holographic memory device (HMD) into the HOC as a high-speed all-optical database for reference images. Using a PQ:PMMA HMD for one of the inputs eliminates one of the key speed limitations in the HOC. The observed correlation signal agrees with simulations but highlights the need for high quality holographic substrates in this application.

Thick PQ:PMMA transmission holograms for free-space optical communication via wavelength-division multiplexing.

Phenantrenequinone doped poly(methyl-methacrylate) (PQ:PMMA) is a holographic substrate that can be used for angle or wavelength multiplexed Bragg gratings. However, efficient writings can be done only using a high-power, long-coherence volume laser over a limited wavelength range. This constraint makes it difficult to write gratings that would diffract several different read wavelengths into a single direction. We describe the rules for writing such gratings, taking into account the differences in the mean index seen by the write and read wavelengths. We further demonstrate the use of such a transmission hologram for wavelength-division multiplexing in a free-space optical communication system.

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).

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.

Demonstration of shift, scale, and rotation invariant target recognition using the hybrid opto-electronic correlator.

Previously, we had proposed a hybrid opto-electronic correlator (HOC), which can achieve the same functionality as that of a holographic optical correlator but without using any holographic medium. Here, we demonstrate experimentally that the HOC is capable of detecting objects in a scale, rotation, and shift invariant manner. First, the polar Mellin transformed (PMT) versions of two images are produced, using a combination of optical and electronic signal processing. The PMT images are then used as the reference and the query inputs for the HOC. The observed correlation signal is used to infer, with high accuracy, the relative scale and angular orientation of the original images. We also discuss practical constraints in reaching a high-speed implementation of such a system. In addition, we describe how these challenges may be overcome for producing an automated version of such a correlator.

Experimental demonstration of the hybrid opto-electronic correlator for target recognition.

Optical target recognition using correlators is an important technique for fast verification and identification of images. The hybrid opto-electronic correlator (HOC) recently proposed by us bypasses the need for nonlinear materials such as photorefractive polymer films by using detectors instead, and the phase information is yet conserved by the interference of plane waves with the images. In this paper, we demonstrate experimentally the basic working principle of the HOC architecture using currently available technologies. For matched reference and query images, the output signal shows a sharp peak, indicating a match is found. For an unmatched case, a much lower peak value is observed, indicating no match. We also demonstrate the dependence of the output signal on the phases of the interfering plane waves and describe a technique using an interferometer and a servo for optimizing the output signal. As such, the work reported here paves the way for further development of the HOC for practical applications.

Lasing dynamics of super and sub luminal lasers.

We study theoretically the lasing properties and the cavity lifetime of super and sub-luminal lasers. We find that obtaining the necessary conditions for superluminal lasing requires care and that a laser operating under these conditions can under some conditions tend towards bi-frequency lasing. In contrast, conditions for a subluminal laser are less stringent, and in most situations its steady-state properties are well predicted by the self-consistent single-frequency laser equations. We also study the relaxation time of power perturbation in super and sub-luminal lasers using a finite-difference-time-domain tool and present the impact of the lasing power, the group velocity and the dispersion properties of the cavity on the relaxation dynamic of such perturbations. For the subluminal laser, we find that the time constant changes by a factor that is close to the group index. In contrast, for the superluminal laser, we find that the time constant does not change by the factor given by the group index, and remains close to or above the value for an empty cavity. These finding may be interpreted to imply that the quantum noise limited linewidth of the subluminal laser decreases with increasing group index, while the same for the superluminal laser does not increase with decreasing group index. The implications of these findings on the sensitivity of sensors based on these lasers are discussed in details.