Photonics

Reducing Temperature Effects in Optical Resonator Sensors

Influences of temperature fluctuations are suppressed by factors of about 105. NASA’s Jet Propulsion Laboratory, Pasadena, California A differential-frequency measurement technique has been devised to reduce the spurious contributions of temperature fluctuations to determinations of physical quantities from readings of optical resonator and interferometric sensors. The technique is applicable mainly to sensors in the form of whispering-gallery mode (WGM) optical resonators. The evanescent electromagnetic fields of such resonators interact with their environments, such that their resonance frequencies change in response to environmental changes (e.g., a change in the index of refraction of the surrounding medium) that one seeks to measure. The resonance frequencies also vary significantly with temperature, with consequent introduction of errors and uncertainties and, hence, effective loss of sensitivity to changes in quantities other than temperature that one seeks to determine.

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Using Laser Vibrometry to Validate Gossamer Space Structures

NASA has been developing large ultra-lightweight structures commonly referred to as Gossamer space structures for many years to reduce launch costs and to exploit the unique capabilities of particular concepts. For instance, dish antennas are currently being pursued because they can be inflated in space to sizes as large as 30 meters and then rigidized to enable high data rate communications.

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Nanophotonics Principles and Applications

The term “nanophotonics” is used to encompass the scientific study of the interaction of matter and light at the nanometer scale. It is possible to design nanometer scale devices to slow down, enhance, produce, or manipulate light by understanding how light behaves as it travels through, or otherwise interacts with, materials at the nanometer scale. Two applications where nanophotonics have had an impact on society are devices used in optical switching for telecommunications and Organic Light Emitting Diodes (OLEDs) used in display technology and lighting.

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Transmissive Diffractive Optical Element Solar Concentrators

These would weigh and cost less than do mirror-type solar concentrators. NASA’s Jet Propulsion Laboratory, Pasadena, California Solar-thermal-radiation concentrators in the form of transmissive diffractive optical elements (DOEs) have been proposed as alternatives to mirror-type solar concentrators now in use. In comparison with functionally equivalent mirror-type solar concentrators, the transmissive, diffractive solar concentrators would weigh and cost less, and would be subject to relaxed mechanical tolerances.

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Nematic Cells for Digital Light Deflection

Smectic A (SmA) prisms can be made in a variety of shapes and are useful for visible spectrum and infrared beam steerage. John H. Glenn Research Center, Cleveland, Ohio Smectic A (SmA) materials can be used in non-mechanical, digital beam deflectors (DBDs) as fillers for passive birefringent prisms based on decoupled pairs of electrically controlled, liquid crystalline polarization rotators, like twisted nematic (TN) cells and passive deflectors. DBDs are used in free-space laser communications, optical fiber communications, optical switches, scanners, and in-situ wavefront correction.

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Improving the Optical Quality Factor of the WGM Resonator

New iterative annealing and polishing increases the resonator’s finesse over the fundamental limit. NASA’s Jet Propulsion Laboratory, Pasadena, California Resonators usually are characterized with two partially dependent values: finesse (F) and quality factor (Q). The finesse of an empty Fabry-Perot (FP) resonator is defined solely by the quality of its mirrors and is calculated as F = πR1/2/(1 – R). The maximum up-to-date value of reflectivity R ≈ 1 – 1.6 × 10–6 is achieved with dielectric mirrors. An FP resonator made with the mirrors has finesse F = 1.9 × 106. Further practical increase of the finesse of FP resonators is problematic because of the absorption and the scattering of light in the mirror material through fundamental limit on the reflection losses given by the internal material losses and by thermodynamic density fluctuations on the order of parts in 109. The quality factor of a resonator depends on both its finesse and its geometrical size. A one-dimensional FP resonator has Q = 2 F L/λ, where L is the distance between the mirrors and λ is the wavelength. It is easy to see that the quality factor of the resonator is unlimited because L is unlimited. F and Q are equally important.

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Ultra-Stable Beacon Source for Laboratory Testing of Optical Tracking

A prototype laser beacon assembly provides reference for testing tracking and pointing systems. NASA’s Jet Propulsion Laboratory, Pasadena, California The ultra-stable beacon source (USBS) provides a laser-beam output with a very low angular jitter and can be used as an absolute angular reference to simulate a beacon in the laboratory. The laser is mounted on the top of a very short (≈1 m) inverted pendulum (IP) with its optical axis parallel to the carbon fiber pendulum leg. The 85-cm, carbon fiber rods making up the leg are very lightweight and rigid, and are supported by a flex-joint at the bottom (see figure). The gimbal-mounted laser is a weight-adjustable load of about 1.5 kg with its center of rotation co-located with the center of percussion of the inverted pendulum. This reduces the coupling of transverse motion at the base of the pendulum to angular motion of the laser at the top.

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