Photonics/Optics

Two-Photon-Absorption Scheme for Optical Beam Tracking

This approach reduces cost for free-space optical communication receivers. NASA’s Jet Propulsion Laboratory, Pasadena, California A new optical beam tracking approach for free-space optical communication links using two-photon absorption (TPA) in a high-bandgap detector material was demonstrated. This tracking scheme is part of the canonical architecture described in the preceding article. TPA is used to track a long-wavelength transmit laser while direct absorption on the same sensor simultaneously tracks a shorter-wavelength beacon. The TPA responsivity was measured for silicon using a PIN photodiode at a laser beacon wavelength of 1,550 nm. As expected, the responsivity shows a linear dependence with incident power level. The responsivity slope is 4.5 × 10–7 A/W2. Also, optical beam spots from the 1,550-nm laser beacon were characterized on commercial charge-coupled device (CCD) and complementary metal-oxide semiconductor (CMOS) imagers with as little as 13.7 μW of optical power (see figure). This new tracker technology offers an innovative solution to reduce system complexity, improve transmit/receive isolation, improve optical efficiency, improve signal-to-noise ratio (SNR), and reduce cost for free-space optical communications transceivers.

Posted in: Photonics, Briefs, ptb catchall, Tech Briefs, Photonics

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On Calculating the Zero-Gravity Surface Figure of a Mirror

As well as gravity reversing between two configurations, mount forces must reverse to within the St. Venant scale.An analysis of the classical method of calculating the zero-gravity surface figure of a mirror from surface-figure measurements in the presence of gravity has led to improved understanding of conditions under which the calculations are valid. In this method, one measures the surface figure in two or more gravity-reversed configurations, then calculates the zero-gravity surface figure as the average of the surface figures determined from these measurements. It is now understood that gravity reversal is not, by itself, sufficient to ensure validity of the calculations: It is also necessary to reverse mounting forces, for which purpose one must ensure that mounting-fixture/mirror contacts are located either at the same places or else sufficiently close to the same places in both gravity-reversed configurations. It is usually not practical to locate the contacts at the same places, raising the question of how close is sufficiently close. The criterion for sufficient closeness is embodied in the St. Venant principle, which, in the present context, translates to a requirement that the distance between corresponding gravity-reversed mounting positions be small in comparison to their distances to the optical surface of the mirror.

Posted in: Photonics, Briefs, Photonics

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Fiber-Coupled Planar Light-Wave Circuit for Seed Laser Control in High Spectral Resolution Lidar Systems

The compact, efficient, and reliable design enables use on small aircraft and satellites.Precise laser remote sensing of aerosol extinction and backscatter in the atmosphere requires a high-power, pulsed, frequency doubled Nd:YAG laser that is wavelength-stabilized to a narrow absorption line such as found in iodine vapor. One method for precise wavelength control is to injection seed the Nd:YAG laser with a low-power CW laser that is stabilized by frequency converting a fraction of the beam to 532 nm, and to actively frequency-lock it to an iodine vapor absorption line. While the feasibility of this approach has been demonstrated using bulk optics in NASA Langley’s Airborne High Spectral Resolution Lidar (HSRL) program, an ideal, lower cost solution is to develop an all-waveguide, frequency-locked seed laser in a compact, robust package that will withstand the temperature, shock, and vibration levels associated with airborne and space-based remote sensing platforms.

Posted in: Photonics, Semiconductors & ICs, Briefs, TSP, Photonics

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Calibrating Photovoltaic Cells

The basic function of a photovoltaic cell is to convert input — sunlight energy expressed in irradiance (W/m2) — into output — useable electrical power — with as little loss as possible. To quantify the ability of the system to accomplish this conversion, one can simply compare the output to the input by forming a ratio of the two. This ratio, expressed in percentages, is known as the power conversion efficiency (PCE) of the device and it is a key parameter of electrical performance. Since the PCE is used to compare the performance of photovoltaic devices, it is critical that accurate estimates be made for the PCE. The estimate is dependent on knowing, with a high degree of accuracy, the actual conditions, including irradiance and cell temperature, under which the parameter is measured.

Posted in: Articles, Features, ptb catchall, Photonics

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New Scanning Probe Microscopy Techniques for Analyzing Organic Photovoltaic Materials

Organic photovoltaic (OPV) materials are an emerging alternative technology for converting sunlight into electricity. OPVs are potentially inexpensive to process, highly scalable in terms of manufacturing, and compatible with mechanically flexible substrates. In an OPV device, semiconducting polymers or small organic molecules are used to accomplish the functions of collecting solar photons, converting the photons to electrical charges, and transporting the charges to an external circuit as a useable current.

Posted in: Application Briefs, Applications, ptb catchall, Photonics

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Qualification and Selection of Flight Diode Lasers for Space Applications

NASA’s Jet Propulsion Laboratory, Pasadena, California The reliability and lifetime of laser diodes is critical to space missions. The Nuclear Spectroscopic Telescope Array (NuSTAR) mission includes a metrology system that is based upon laser diodes. An operational test facility has been developed to qualify and select, by mission standards, laser diodes that will survive the intended space environment and mission lifetime. The facility is situated in an electrostatic discharge (ESD) certified cleanroom and consist of an enclosed temperature-controlled stage that can accommodate up to 20 laser diodes. The facility is designed to characterize a single laser diode, in addition to conducting laser lifetime testing on up to 20 laser diodes simultaneously.

Posted in: Photonics, Briefs, ptb catchall, Tech Briefs, Photonics

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High-Speed Operation of Interband Cascade Lasers

NASA’s Jet Propulsion Laboratory, Pasadena, California Optical sources operating in the atmospheric window of 3–5 μm are of particular interest for the development of free-space optical communication link. It is more advantageous to operate the free-space optical communication link in 3–5-μm atmospheric transmission window than at the telecom wavelength of 1.5 μm due to lower optical scattering, scintillation, and background radiation. However, the realization of optical communications at the longer wavelength has encountered significant difficulties due to lack of adequate optical sources and detectors operating in the desirable wavelength regions.

Posted in: Photonics, Briefs, ptb catchall, Tech Briefs, Photonics

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