A Retinal Prosthesis Technology Based on CMOS Microelectronics and Microwire Glass Electrodes.

A very large format neural stimulator device, to be used in future retinal prosthesis experiments, has been designed, fabricated, and tested. The device was designed to be positioned against a human retina for short periods in an operating room environment. Demonstrating a very large format, parallel interface between a 2-D microelectronic stimulator array and neural tissue would be an important step in proving the feasibility of high resolution retinal prosthesis for the blind. The architecture of the test device combines several novel components, including microwire glass, a microelectronic multiplexer, and a microcable connector. The array format is 80 times 40 array pixels with approximately 20 microwire electrodes per pixel. The custom assembly techniques involve indium bump bonding, ribbon bonding, and encapsulation. The design, fabrication, and testing of the device has resolved several important issues regarding the feasibility of high-resolution retinal prosthesis, namely, that the combination of conventional CMOS electronics and microwire glass provides a viable approach for a high resolution retinal prosthesis device. Temperature change from power dissipation within the device and maximum electrical output current levels suggest that the device is acceptable for acute human tests.

Pulse compression using a tapered microstructure optical fiber.

We calculate the pulse compression in a tapered microstructure optical fiber with four layers of holes. We show that the primary limitation on pulse compression is the loss due to mode leakage. As a fiber's diameter decreases due to the tapering, so does the air-hole diameter, and at a sufficiently small diameter the guided mode loss becomes unacceptably high. For the four-layer geometry we considered, a compression factor of 10 can be achieved by a pulse with an initial FWHM duration of 3 ps in a tapered fiber that is 28 m long. We find that there is little difference in the pulse compression between a linear taper profile and a Gaussian taper profile. More layers of air-holes allows the pitch to decrease considerably before losses become unacceptable, but only a moderate increase in the degree of pulse compression is obtained.

Transverse load sensing by use of pi-phase-shifted fiber Bragg gratings.

The birefringence of a pi -phase-shifted fiber Bragg grating can be determined with high accuracy by measurement of the polarization-induced spectral splitting of its narrow central transmission window. The use of this feature for sensing of a load applied in the direction transverse to the optical fiber is demonstrated. A distributed force resolution of 1.4x10(-3) N/mm was obtained, which corresponds to a difference in the principal strains of the fiber core of 0.5mu? . We also show that the transverse load response of the sensor is insensitive to temperature.

Broadband square-pulse operation of a passively mode-locked fiber laser for fiber Bragg grating interrogation.

A long-cavity, passively mode-locked erbium fiber laser operated in the square-pulse regime is demonstrated as a useful light source for interrogating fiber Bragg grating arrays. Output pulses with 4-W peak-power, 10-ns pulse widths, and bandwidths greater than 60nm were used successfully to interrogate 2% fiber Bragg gratings.

Ultrahigh-sensitivity fiber-optic strain and temperature sensor.

We report ultrahigh-sensitivity static strain sensing (noise equivalent strain

Index structure of fiber Bragg gratings in Ge-SiO(2) fibers.

The behavior of the concentration of photoinduced color centers in Ge-SiO(2) optical fibers was compared with that of the index modulation associated with fiber Bragg gratings (FBG's) written in the same fibers. We find that the fluence dependence of the photoinduced Ge E? center, its thermal annealing behavior, and its reaction with H(2) are similar to that of the index modulation generated in both H(2)-loaded and unloaded Ge-SiO(2) fibers. The much higher photosensitivity of H(2)-loaded Ge-SiO(2) fibers is attributed to the much higher formation efficiency of Ge E? centers, with an additional contribution from GeH. A diamagnetic structure, possibly densification, is also found to contribute to the index modulation of FBG's.

Grating sensor array demodulation by use of a passively mode-locked fiber laser.

A fiber Bragg grating sensor array is interrogated by use of a passively mode-locked fiber laser source. A novel demodulation scheme that uses highly dispersive fiber to convert the grating wavelength shift to a temporal shift in the arrival time of the reflected pulses is demonstrated. The source bandwidth of >85 nm permits interrogation of many-grating arrays, and the demodulation technique permits fast sensing of large strains.

Fast-neutron radiation effects in a silica-core optical fiber studied by a CCD-camera spectrometer.

A simple CCD-camera spectrometer was deployed at the Los Alamos Spallation Radiation Effects Facility to characterize fast-neutron irradiation effects in several silica-based optical fibers over the wavelength range ~ 450-1100 nm. The experimental arrangement allowed optical loss spectra to be developed from remotely recovered frame grabs at various times during irradiation without it being necessary to resort to cutback methods. Data recorded for a pure-silica-core/F-doped-silica-clad fiber displayed a peculiar artifact, which is described and mathematically modeled in terms of leaky modes propagating in an optical cladding that is substantially less susceptible to radiation-induced optical attenuation than is the core. Evidence from optical time-domain reflectometry supports the postulate that mode leakage into the cladding may be a result of light scattering from the tracks of ions displaced by the 14-MeV neutrons. These results suggest that fibers with fluorine doping in the core, as well as in the cladding, would be relatively resistant to radiation-induced attenuation in the UV-visible spectral region.

Thermal stability of photoinduced gratings and paramagnetic centers in Ge- and Ge/P-doped silica optical fibers.

The electron-spin-resonance spectra of paramagnetic defects in Ge/P-doped silica fibers prepared with second-harmonic generation (SHG) are compared with those induced in Ge- and Ge/P-doped silica preforms by 5-eV photons. Among the defects observed, the thermal stability of Ge E'-type centers is similar to that of photoinduced Bragg grating efficiency but not of SHG efficiency. The isochronal anneal curves of the Ge(l) and Ge(2) centers in, Ge/P-doped silica correlate well with SHG efficiency, but the agreement is not nearly so good in silica containing only Ge. Only the thermal behavior of photoinduced Ge E'(d1) centers is similar to that of SHG efficiency in both materials, albeit only in the initial stages of annealing.

In-line fiber étalon for strain measurement.

A description and demonstration of a fiber interferometer that uses a short segment of silica hollow-core fiber spliced between two sections of single-mode fiber to form a mechanically robust in-line cavity are presented. The hollow-core fiber is specifically manufactured to have an outer diameter that is equal to the outer diameter of the single-mode lead fibers, thereby combining the best qualities of existing intrinsic and extrinsic Fabry-Perot sensors. A dynamic strain resolution of ~22 nepsilon/ radicalHz at frequencies of >5 Hz with a sensor gauge length of 137 microm is demonstrated.

Permanent photowritten optical gratings in irradiated silicate glasses.

We show that permanent optical gratings can be photowritten into simple silicate glasses by exposure to interfering beams of an Ar-ion laser after the glass has been treated by x rays. Gratings with index modulations as large as Deltan = 10(-5) can be formed in less than a minute by exposure to write beams with intensities of the order of 50 W/cm(2).

Fiber Bragg reflectors prepared by a single excimer pulse.

Narrow-line, permanent Bragg reflection gratings have been created in Ge-doped silica-core optical fibers by interfering beams of a single 20-ns pulse of KrF excimer laser light. Of the fibers studied, the highest reflectance value of ~2% was observed with a linewidth (FWHM) of 0.1 nm, which corresponds to a 2-mm grating length with an index modulation of ~3 x 10(-5).

Modal evolution of induced second-harmonic light in an optical fiber.

We show how the mode of the photoinduced second-harmonic light in an optical fiber changes with preparation time. We also discuss what type of nonlinear interaction can be causing the induced second-harmonic light and give experimental evidence that the initial second-harmonic light is a core-cladding interface effect.

Correlation of single-mode fiber radiation response and fabrication parameters.

Statistically significant correlations have been established between certain fabrication parameters of matched clad, single-mode optical fiber waveguides and their response to an ionizing radiation dose of 2000 rad. The reCOVE:ry data measured at -35 degrees C following exposure have been fit to nth-order kinetic behavior where the adjustable parameters are the initial and permanent incremental losses (A(o) and A(f), respectively), the half-life of attenuation tau, and the order of kinetics n. The set of fibers chosen for analysis had Ge-doped silica cores. In fibers with Ge-F-doped silica clads, A(o) correlates with the concentration of Ge-doped into the fiber core; A(f) correlates with the ratio of oxygen to reagents used during core deposition; and tau and n correlate with a two-way interaction of core oxygen and fiber draw speed. In P-F-doped clad fibers, the P concentration has been found to correlate with the order of the kinetics of recovery.

Correlation of defect centers with second-harmonic generation in Ge-doped and Ge-P-doped silica-core single-mode fibers.

The origin of frequency doubling in Ge-doped silica-core single-mode glass fibers has been investigated with electron-spin-resonance spectrometry. Correlations have been observed between the conversion efficiency and the light-induced Ge E' center concentration.

Radiation-induced attenuation in polarization maintaining fibers: low dose rate response, stress, and materials effects.

The loss induced in polarization-maintaining (PM) fibers by low dose rate [<0.01 Gy/h, where 1 Gy = 100 rads(Si)] radiation exposure has been found to vary from <0.4 to approximately 6 dB/km-10 Gy, depending on the wavelength of measurement and the fiber. Correlations have been established between low dose rate response and the "permanent" induced loss determined by fitting the recovery of the induced loss following high dose rate exposure to nth-order kinetics. Using this technique, both 0.85- and 1.3-microm PM fibers have been found which show virtually no permanent incremental loss and would therefore appear to be resistant to low dose rate radiation environments. The asymmetric stress inherent in PM fibers has been shown to reduce the permanent induced loss, while the recovery of the radiation-induced attenuation was found to be enhanced in fibers with Ge-F-doped silica clads.

Effect of low dose rate irradiation on doped silica core optical fibers.

The optical attenuation induced in multimode doped silica core optical fiber waveguides by a year's exposure to low dose rate (1 rad/day) ionizing radiation was studied, allowing a characterization of fibers deployed in these environments and a determination of the permanent induced loss in the waveguides. Variations in the induced attenuation at 0.85 microm have been observed with changes in the dose rate between 1 rad/day and 9000 rads/min. These dose rate dependences have been found to derive directly from the recovery that occurs during the exposure; the recovery data predict little or no dose rate dependence of the damage at 1.3 microm. The low dose rate exposure has been found to induce significant permanent attenuation in the 0.7-1.7-microm spectral region in all fibers containing P in the core, whether doped uniformly across the diameter or constrained to a narrow spike on the centerline. Whereas permanent loss was induced at 0.85 microm in a P-free binary Ge-doped silica core fiber by the year's exposure, virtually no damage was observed at 1.3 microm.

Radiation damage of optical fiber waveguides at long wavelengths.

Measurements of the radiation-induced optical attenuation at 1.3 microm and the induced absorption spectra (0.4-1.7 microm) of state-of-the-art pure synthetic silica and doped silica core optical fiber waveguides have been undertaken to characterize their radiation response at long wavelengths. It has been observed that radiation-induced absorption bands at long wavelengths can give rise to substantial induced losses at both 1.3 and 1.55 microm in some fibers, especially those doped with P or B; the ratio of the damage at 1.3 and 1.55 microm to that at 0.82 microm in these fibers has been found to be only ~0.29 and 0.71, respectively. In contrast, pure fused silica and binary Ge-doped silica core fibers have shown the greatest hardness at long wavelengths. Suggestions have been made for the optimum wavelengths and preferred fiber compositions to minimize the effects of nuclear radiation in fiber-optic communications systems operating at long wavelengths.