Photonics/Optics

Photon Counting Using Edge-Detection Algorithm

Improved optical communications links can be used in building-to-building networks in high-attenuation conditions such as rain or fog. NASA’s Jet Propulsion Laboratory, Pasadena, California New applications such as high-data-rate, photon-starved, free-space optical communications require photon counting at flux rates into gigaphoton-per-second regimes coupled with sub-nanosecond timing accuracy. Current single-photon detectors that are capable of handling such operating conditions are designed in an array format and produce output pulses that span multiple sample times. In order to discern one pulse from another and not to over-count the number of incoming photons, a detection algorithm must be applied to the sampled detector output pulses. As flux rates increase, the ability to implement such a detection algorithm becomes difficult within a digital processor that may reside within a field-programmable gate array (FPGA).

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Optical Structural Health Monitoring Device

This device detects microscopic cracks and surface structural changes in components. Dryden Flight Research Center, Edwards, California This non-destructive, optical fatigue detection and monitoring system relies on a small and unobtrusive light-scattering sensor that is installed on a component at the beginning of its life in order to periodically scan the component in situ. The method involves using a laser beam to scan the surface of the monitored component. The device scans a laser spot over a metal surface to which it is attached. As the laser beam scans the surface, disruptions in the surface cause increases in scattered light intensity. As the disruptions in the surface grow, they will cause the light to scatter more. Over time, the scattering intensities over the scanned line can be compared to detect changes in the metal surface to find cracks, crack precursors, or corrosion. This periodic monitoring of the surface can be used to indicate the degree of fatigue damage on a component and allow one to predict the remaining life and/or incipient mechanical failure of the monitored component.

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Widely Tunable Mode-Hop-Free External-Cavity Quantum Cascade Laser

This technology is suitable for spectroscopic applications, multi-species trace-gas detection, and measurements of broadband absorbers. Lyndon B. Johnson Space Center, Houston, Texas The external-cavity quantum cascade laser (EC-QCL) system is based on an optical configuration of the Littrow type. It is a room-temperature, continuous- wave, widely tunable, mode-hop-free, mid-infrared, EC-QCL spectroscopic source. It has a single-mode tuning range of 155 cm-1 (≈8% of the center wavelength) with a maximum power of 11.1 mW and 182 cm-1 (≈15% of the center wavelength), and a maximum power of 50 mW as demonstrated for 5.3 micron and 8.4 micron ECQCLs, respectively. This technology is particularly suitable for high-resolution spectroscopic applications, multi-species trace-gas detection, and spectroscopic measurements of broadband absorbers.

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Non-Geiger-Mode Single-Photon Avalanche Detector With Low Excess Noise

Applications include quantum key distribution for the financial industry and photon-starved optical communications needs. NASA’s Jet Propulsion Laboratory, Pasadena, California This design constitutes a self-resetting (gain quenching), room-temperature operational semiconductor single-photon-sensitive detector that is sensitive to telecommunications optical wavelengths and is scalable to large areas (millimeter diameter) with high bandwidth and efficiencies.

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Using Whispering-Gallery-Mode Resonators for Refractometry

Refractive and absorptive properties are inferred by correlating predictions with measurements. NASA’s Jet Propulsion Laboratory, Pasadena, California A method of determining the refractive and absorptive properties of optically transparent materials involves a combination of theoretical and experimental analysis of electromagnetic responses of whispering-gallery-mode (WGM) resonator disks made of those materials. The method was conceived especially for use in studying transparent photorefractive materials, for which purpose this method affords unprecedented levels of sensitivity and accuracy. The method is expected to be particularly useful for measuring temporally varying refractive and absorptive properties of photorefractive materials at infrared wavelengths. Still more particularly, the method is expected to be useful for measuring drifts in these properties that are so slow that, heretofore, the properties were assumed to be constant.

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Focusing Light Beams To Improve Atomic-Vapor Optical Buffers

Atomic-vapor optical buffers could be made to perform more nearly optimally. NASA’s Jet Propulsion Laboratory, Pasadena, California Specially designed focusing of light beams has been proposed as a means of improving the performances of optical buffers based on cells containing hot atomic vapors (e.g., rubidium vapor). There is also a companion proposal to improve performance by use of incoherent optical pumping under suitable conditions.

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Digitally Enhanced Heterodyne Interferometry

This design mitigates cyclic error and improves measurement sensitivity. NASA’s Jet Propulsion Laboratory, Pasadena, California Spurious interference limits the performance of many interferometric measurements. Digitally enhanced interferometry (DEI) improves measurement sensitivity by augmenting conventional heterodyne interferometry with pseudo-random noise (PRN) code phase modulation. DEI effectively changes the measurement problem from one of hardware (optics, electronics), which may deteriorate over time, to one of software (modulation, digital signal processing), which does not. DEI isolates interferometric signals based on their delay. Interferometric signals are effectively time-tagged by phase-modulating the laser source with a PRN code. DEI improves measurement sensitivity by exploiting the autocorrelation properties of the PRN to isolate only the signal of interest and reject spurious interference. The properties of the PRN code determine the degree of isolation.

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