Photonics

Design of a Field-Mapped Beam Homogenizer

Light beam homogenizer designs used for decades are largely based upon lenticular lens arrays that date back to the 1940s and 1950s. A more recent design is from the late 1980s that comprises a pair of crossed-cylindrical lenticular lens structures coupled with a condensing lens. The 1980s design has 10 optical surfaces which represents a significant source of optical loss, long beam path (typically ~ 1.5 meters) and high cost from having so many optical elements. Further, this design requires careful and precise alignment of the light beam in relation to the optics comprising the homogenizer, in particular angular rotation of the lenslets and their positioning in the X-Y plane of the optical axis. These designs are unable to produce a homogenized ring illumination, which is useful for ablating polymer insulation from metal bonding pads in microelectronic fabrication of multichip modules and memory chips.

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

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Optical Displacement Sensor for Sub-Hertz Applications

NASA’s Jet Propulsion Laboratory, Pasadena, California. A document discusses a sensor made from off-the-shelf electro-optical photo-diodes and electronics that achieves 20 nm/(Hz)1/2 displacement sensitivity at 1 mHz. This innovation was created using a fiber-coupled laser diode (or Nd:YAG) through a collimator and an aperture as the illumination source. Together with a germanium quad photodiode, the above-mentioned displacement sensor sensitivities have been achieved. This system was designed to aid the Laser Interferometer Space Antenna (LISA) with microthruster tests and to be a backup sensor for monitoring the relative position between a proof mass and a spacecraft for drag-free navigation. The optical displacement sensor can be used to monitor any small displacement from a remote location with minimal invasion on the system.

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

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Curved Piezoelectric Actuators for Stretching Optical Fibers

Curved actuators produce greater displacements than do flat actuators. Langley Research Center, Hampton, Virginia Assemblies containing curved piezoceramic fiber composite actuators have been invented as means of stretching optical fibers by amounts that depend on applied drive voltages. Piezoceramic fiber composite actuators are conventionally manufactured as sheets or ribbons that are flat and flexible, but can be made curved to obtain load-carrying ability and displacement greater than those obtainable from the flat versions. A curved actuator of this type can be fabricated by bonding a conventional flexible flat actuator to a thin metal backing sheet in a flat configuration at an elevated temperature so that upon cooling to room temperature, differential thermal contraction of the layers causes the laminate to become curved. Alternatively, such a curved actuator can be fabricated by bonding the layers together at room temperature using a curved mold.

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

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Tunable Optical Assembly With Vibration Dampening

Flat actuators are mechanically simple and offer vibration dampening. Langley Research Center, Hampton, Virginia Since their market introduction in 1995, fiber Bragg gratings (FBGs) [wherein “fiber” signifies optical fiber] have emerged as excellent means of measuring such parameters as strain and temperature. Distributed-grating sensing is particularly beneficial for such structural-health monitoring applications such as those of “smart” structures or integrated vehicle health management in aerospace vehicles. Because of the variability of their output wavelengths, tunable lasers have become widely used as means of measuring FBGs.

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

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Cylindrical Piezoelectric Fiber Composite Actuators

Cylindrical actuators offer advantages over flat flexible actuators. Langley Research Center, Hampton, Virginia The use of piezoelectric devices has become widespread since Pierre and Jacques Curie discovered the piezoelectric effect in 1880. Examples of current applications of piezoelectric devices include ultrasonic transducers, micro-positioning devices, buzzers, strain sensors, and clocks. The invention of such lightweight, relatively inexpensive piezo-ceramic-fiber-composite actuators as macro fiber composite (MFC) actuators has made it possible to obtain strains and displacements greater than those that could be generated by prior actuators based on monolithic piezoceramic sheet materials. MFC actuators are flat, flexible actuators designed for bonding to structures to apply or detect strains. Bonding multiple layers of MFC actuators together could increase force capability, but not strain or displacement capability.

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

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Micron-Accurate Laser Fresnel-Diffraction Ranging System

This system would exploit the variation of Fresnel diffraction with distance. Marshall Space Flight Center, Alabama The figure schematically depicts two versions of an opto-electronic system, undergoing development at the time of reporting the information for this article, that is expected to be capable of measuring a distance between 2 and 10 m with an error of no more than 1 μm. The system would be designed to exploit Fresnel diffraction of a laser beam. In particular, it would be designed to take advantage of the fact that a Fresnel diffraction pattern is ultrasensitive to distance.

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

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Spatial Combining of Laser-Diode Beams for Pumping an NPRO

Multiple multimode beams are efficiently combined into one optical fiber. NASA’s Jet Propulsion Laboratory, Pasadena, California A free-space optical beam combiner now undergoing development makes it possible to use the outputs of multiple multimode laser diodes to pump a neodymium-doped yttrium aluminum garnet (Nd:YAG) non-planar ring oscillator (NPRO) laser while ensuring that the laser operates at only a single desired frequency. This optical beam combiner serves the same purpose as does the one described in “Diffractive Combiner of Single-Mode Pump Laser-Diode Beams” (NPO-42411), NASA Tech Briefs, Vol. 31, No. 5 (May 2007), page 16a. Although the principles of design and operation of the present and prior beam combiners are not identical, they are so closely related that it is necessary to devote the next four paragraphs to reiteration of a substantial portion of the cited prior article in order to give meaning to a description of the present beam combiner.

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

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