Photonics/Optics

Using Hollow Core Plastic Bragg Fiber to Deliver Ultrashort Pulse Laser Beams

Ultrashort pulse (USP), or “ultrafast,” lasers emit extremely brief pulses of light, generally with duration of a picosecond (10-12 seconds) or less. The pulses are characterized by a high optical intensity that induces nonlinear interactions in various materials, including air.

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

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Scanning Miniature Microscopes Without Lenses

Polarization-sensitive and multicolor versions should also be possible. NASA’s Jet Propulsion Laboratory, Pasadena, California The figure schematically depicts some alternative designs of proposed compact, lightweight optoelectronic microscopes that would contain no lenses and would generate magnified video images of specimens. Microscopes of this type were described previously in “Miniature Microscope Without Lenses” (NPO-20218), NASA Tech Briefs, Vol. 22, No. 8 (August 1998), page 43 and “Reflective Variants of Miniature Microscope Without Lenses” (NPO20610), NASA Tech Briefs, Vol. 26, No. 9 (September 1999), page 6a. To recapitulate: In the design and construction of a microscope of this type, the focusing optics of a conventional microscope are replaced by a combination of a microchannel filter and a charge-coupled-device (CCD) image detector. Elimination of focusing optics reduces the size and weight of the instrument and eliminates the need for the time-consuming focusing operation.

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

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Target-Tracking Camera for a Metrology System

Angular measurements are updated at a rate of hundreds of hertz. NASA’s Jet Propulsion Laboratory, Pasadena, California An analog electronic camera that is part of a metrology system measures the varying direction to a light-emitting diode that serves as a bright point target. In the original application for which the camera was developed, the metrological system is used to determine the varying relative positions of radiating elements of an airborne synthetic-aperture-radar (SAR) antenna as the airplane flexes during flight; precise knowledge of the relative positions as a function of time is needed for processing SAR readings.

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

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Polarimetric Imaging Using Two Photoelastic Modulators

The frame rate is the difference between the resonance frequencies of the modulators. NASA’s Jet Propulsion Laboratory, Pasadena, California A method of polarimetric imaging, now undergoing development, involves the use of two photoelastic modulators in series, driven at equal amplitude but at different frequencies. The net effect on a beam of light is to cause (1) the direction of its polarization to rotate at the average of two excitation frequencies and (2) the amplitude of its polarization to be modulated at the beat frequency (the difference between the two excitation frequencies). The resulting modulated optical light beam is made to pass through a polarizing filter and is detected at the beat frequency, which can be chosen to equal the frame rate of an electronic camera or the rate of sampling the outputs of photodetectors in an array.

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

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Miniature Wide-Angle Lens for Small-Pixel Electronic Camera

The lens design addresses issues peculiar to small-pixel image sensors. NASA’s Jet Propulsion Laboratory, Pasadena, California The figure depicts a proposed wide-angle lens that would be especially well suited for an electronic camera in which the focal plane is occupied by an image sensor that has small pixels. The design of the lens is intended to satisfy requirements for compactness, high image quality, and reasonably low cost, while addressing issues peculiar to the operation of small-pixel image sensors. Hence, this design is expected to enable the development of a new generation of compact, high-performance electronic cameras. The lens example shown has a 60° field of view and a relative aperture (f-number) of 3.2.

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

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Modal Filters for Infrared Interferometry

NASA's Jet Propulsion Laboratory, Pasadena, California Modal filters in the ≈10-μm spectral range have been implemented as planar dielectric waveguides in infrared interfero-metric applications such as searching for Earth-like planets. When looking for a small, dim object (“Earth”) in close proximity to a large, bright object (“Sun”), the interferometric technique uses beams from two telescopes combined with a 180° phase shift in order to cancel the light from a brighter object. The interferometer baseline can be adjusted so that, at the same time, the light from the dimmer object arrives at the combiner in phase. This light can be detected and its infrared (IR) optical spectra can be studied. The cancellation of light from the “Sun” to ≈106 is required; this is not possible without special devices — modal filters — that equalize the wavefronts arriving from the two telescopes.

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

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Taming Residual Bulk Image in CCDs

Residual bulk image (RBI) is a phenomenon observed in certain types of front side-illuminated charge-coupled devices (CCDs). A CCD is an electronic light sensor used in digital cameras. In simplest terms, the sensor exhibits a memory of prior exposures resulting in ghost images appearing in subsequent images. This deferred charge can cause a number of problems in cooled long-exposure scientific applications. At a minimum, the ghost images can create the illusion of a non-existent object (Figure 1, left). Equally serious, they can lead to significant errors in quantitative measurements required for photometric applications.

Posted in: Articles, Features, ptb catchall, Photonics

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