Detailed reliability studies of high-power, CW, broad-area, GaAs-based laser- diodes were performed. Optical and
electrical transients occurring prior to device failure by catastrophic optical-damage (COD) were observed. These
transients were correlated with COD formation as observed in laser diodes with an optical window in the n-side
electrode. In addition, custom electronics were designed to fault-protect the laser diodes during aging tests, i.e. each time
a transient (fault) was detected, the operating current was temporarily cut off within 4μs of fault detection. The lifetime
of fault-protected 808-nm laser-diode bars operated at a constant current of 120A (~130W) and 35°C exceeded similar
unprotected devices by factors of 2.
The present model of formation and propagation of catastrophic optical-damage (COD), a random failure-mode in laser
diodes, was formulated in 1974 and has remained substantially unchanged. We extend the model of COD phenomena,
based on analytical studies involving EBIC (electron-beam induced current), STEM (scanning transmission-electron
microscopy) and sophisticated optical-measurements. We have determined that a ring-cavity mode, whose presence has
not been previously reported, significantly contributes to COD initiation and propagation in broad-area laser-diodes.
We have been developing a novel thermal-to-visible transducer (TVT), an uncooled thermal-IR imager that is based on a
Fabry-Perot Interferometer (FPI). The FPI-based IR imager can convert a thermal-IR image to a video electronic image.
IR radiation that is emitted by an object in the scene is imaged onto an IR-absorbing material that is located within an
FPI. Temperature variations generated by the spatial variations in the IR image intensity cause variations in optical
thickness, modulating the reflectivity seen by a probe laser beam. The reflected probe is imaged onto a visible array,
producing a visible image of the IR scene. This technology can provide low-cost IR cameras with excellent sensitivity,
low power consumption, and the potential for self-registered fusion of thermal-IR and visible images. We will describe
characteristics of requisite pixelated arrays that we have fabricated.
We describe an uncooled IR camera that is based on a Fabry-Perot Interferometer (FPI) IR-to-visible transducer. The
FPI-based IR camera converts a thermal-IR image to a video electronic image. IR radiation, emitted by an object in the
scene, is imaged onto an IR-absorbing material that is located within an FPI. Spatial variations in temperature of the scene
translate into corresponding temperature variations in the IR-absorbing material, forming a temperature image in the FPI.
Within the FPI, the temperature variations produce variations in optical thickness for any beam of collimated visible light
that is reflected from the FPI. The intensity of visible light reflected by the FPI is a function of optical thickness and thus
forms an image, with thickness variations translating into intensity variations. The reflected light traverses visible optics
that image the IR-absorbing material onto a visible-detector array, where the reflected light is converted into an electronic
image. We will describe in detail the various sources of noise that determine the noise-equivalent temperature difference
(NETD) of an FPI-based infrared camera. The dominant sources of noise are (1) shot noise in the visible-detector array
and (2) temperature fluctuations (thermal noise) in the transducer. For a typical CCD array, we project a total NETD
of approximately 40-50 mK for an FPI-based IR camera that is configured so that shot and thermal noise contribute
approximately equally to the noise.
A new method to detect and localize structural flaws, mode signature analysis, has been developed and demonstrated on a large realistic model bridge. The presence and location of flaws are detected by analyzing the vibrational response of the structure at a single, fixed point to an impulse excitation at a different fixed point. The localization method has successfully pinpointed a defect with a resolution of approximately 4 cm when both the impulse- excitation source and the point at which the vibration is monitored are located several tens of cm away from the defect. The detection method has been demonstrated by 1) contact measurements made by an accelerometer mounted on the structure and 2) remotely, from the time-dependent Doppler shift of a laser beam reflected from the structure. After the accelerometer signal, proportional to acceleration, is integrated to produce a signal proportional to velocity, it is found to be essentially identical to the remotely observed laser-vibrometer signal. Data analysis illustrates the tradeoff between maximizing the probability that an existing flaw is detected and minimizing the probability that a structure that is in good condition is misdiagnosed as faulty.