Photonics/Optics

Range-Gated Metrology With Compact Optical Head

A compact, single-fiber optical head requires minimal internal alignment. NASA’s Jet Propulsion Laboratory, Pasadena, California This work represents a radical simplification in the design of the optical head needed for high-precision laser ranging applications. The optical head is now a single fiber-optic collimator with dimensions of order of 1×1×2 cm, which can be easily integrated into the system being measured with minimal footprint. Previous heads were significantly larger, with multiple optical elements requiring careful alignment. The new design has only one optical fiber per head, rather than four, making it much easier to multiplex between tens or hundreds of heads. It is capable of subnanometer precision, consistent with the demanding requirements of new missions.

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

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Performance of 1mm² Silicon Photomultipliers

A silicon photomultiplier (SPM) is a new type of semiconductor detector that has the potential to replace the photomultiplier tube (PMT) detector in many applications. In common with a PMT detector, the output of an SPM is an easily detectable current pulse for each detected photon and can be used in both photon counting mode and as an analogue (photocurrent) detector. However, the SPM also has a distinct advantage over PMT detectors. The photon-induced current pulse from a PMT varies greatly from photon to photon, due to the statistics of the PMT multiplication process (excess noise). In contrast, the current pulse from an SPM is identical from photon to photon. This gives the SPM a distinct advantage in photon counting applications as it allows the associated electronics to be greatly simplified. Identical pulses also mean that the SPM can resolve the number of photons in weak optical pulses, so-called photon number resolution. This is critical in a number of applications including linear-optics quantum computing.

Posted in: Articles, Features, ptb catchall, Photonics

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Biomedical Imaging Using Ultrashort Laser Pulses

The field of optical microscopy experienced significant gains in resolution and speed following the introduction of lasers. Unfortunately, these gains came at the expense of sample degradation caused by the continuous flux of intense light. Taking advantage of the two-photon absorption process, Webb and Denk implemented a microscope based on the use of near-IR light pulses capable of causing simultaneous multiple fluorophore excitation. Two-photon microscopy is now widely applied in the biomedical imaging field due to the absence of out-of-focus photobleaching and reduced photodamage and fluorescence scattering. These advantages are brought about collectively by the inherent instantaneous peak intensity and narrow focal plane of excitation. Given that peak intensity increases with decreasing laser pulse duration, one would expect extensive use of available ultrashort (sub-10 fs) pulse laser systems in the field of biomedical imaging. However, most two-photon microscopes still use the same pulse duration that Webb and Denk used in 1990 (≈150 fs).

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

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MACOS Version 3.31

NASA’s Jet Propulsion Laboratory, Pasadena, California Version 3.31 of Modeling and Analysis for Controlled Optical Systems (MACOS) has been released. MACOS is an easy-to-use computer program for modeling and analyzing the behaviors of a variety of optical systems, including systems that have large, segmented apertures and are aligned with the technology of wavefront sensing and control. Two previous versions were described in “Improved Software for Modeling Controlled Optical Systems” (NPO-19841) NASA Tech Briefs, Vol. 21, No. 12 (December 1997), page 42 and “Optics Program Modified for Multithreaded Parallel Computing” (NPO-40572) NASA Tech Briefs, Vol. 30, No. 1 (January 2006) page 13a. The present version incorporates the following enhancements over prior versions:

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

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Mitigating Photon Jitter in Optical PPM Communication

Compensation based partly on photon-arrival statistics would yield gain. NASA’s Jet Propulsion Laboratory, Pasadena, California A theoretical analysis of photon-arrival jitter in an optical pulse-position-modulation (PPM) communication channel has been performed, and now constitutes the basis of a methodology for designing receivers to compensate so that errors attributable to photon-arrival jitter would be minimized or nearly minimized. Photon-arrival jitter is an uncertainty in the estimated time of arrival of a photon relative to the boundaries of a PPM time slot. Photon-arrival jitter is attributable to two main causes: (1) receiver synchronization error [error in the receiver operation of partitioning time into PPM slots] and (2) random delay between the time of arrival of a photon at a detector and the generation, by the detector circuitry, of a pulse in response to the photon. For channels with sufficiently long time slots, photon-arrival jitter is negligible. However, as durations of PPM time slots are reduced in efforts to increase throughputs of optical PPM communication channels, photon-arrival jitter becomes a significant source of error, leading to significant degradation of performance if not taken into account in design.

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

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Fast Offset Laser Phase-Locking System

Phases can be locked within a microcycle; known phase noise can be added. NASA’s Jet Propulsion Laboratory, Pasadena, California Figure 1 shows a simplified block diagram of an improved optoelectronic system for locking the phase of one laser to that of another laser with an adjustable offset frequency specified by the user. In comparison with prior systems, this system exhibits higher performance (including higher stability) and is much easier to use. The system is based on a field-programmable gate array (FPGA) and operates almost entirely digitally; hence, it is easily adaptable to many different systems. The system achieves phase stability of less than a microcycle. It was developed to satisfy the phase-stability requirement for a planned spaceborne gravitational-wave-detecting heterodyne laser interferometer (LISA). The system has potential terrestrial utility in communications, lidar, and other applications.

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

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Fiber-Optic Determination of N₂, O₂, and Fuel Vapor in the Ullage of Liquid-Fuel Tanks

A fiber-optic sensor provides feedback control of onboard inert gas generation systems (OBIGGS) and reduces aircraft operational costs. John H. Glenn Research Center, Cleveland, Ohio A fiber-optic sensor system has been developed that can remotely measure the concentration of molecular oxygen (O2), nitrogen (N2), hydrocarbon vapor, and other gases (CO2, CO, H2O, chlorofluorocarbons, etc.) in the ullage of a liquid-fuel tank. The system provides an accurate and quantitative identification of the above gases with an accuracy of better than 1 percent by volume (for O2 or N2) in real-time (5 seconds). In an effort to prevent air-craft fuel tank fires or explosions similar to the tragic TWA Flight 800 explosion in 1996, OBIGGS are currently being developed for large commercial aircraft to prevent dangerous conditions from forming inside fuel tanks by providing an “inerting” gas blanket that is low in oxygen, thus preventing the ignition of the fuel/air mixture in the ullage.

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

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