Cryopreservation of auricula shoot tips using the encapsulation/dehydration technique.

Calcium alginate encapsulated, in vitro culture derived shoot tips of the florist's auricula, cultivar Embley, were successfully cryopreserved using the encapsulation/dehydration technique. The use of preculture media containing both sucrose and mannitol was essential for post-thaw growth. After 2 weeks cold acclimatisation at 10 degrees C, shoot tips were encapsulated and incubated in culture medium containing 0.25 M sucrose and 0.25 M mannitol for 48h, followed by 23h dehydration with silica gel (water content of beads approximately 16%) prior to plunging into liquid nitrogen. Post-thaw growth of the shoot tips was without callus formation. The morphology of frozen and non-frozen shoots was comparable.

Guided wave acousto-optic and electro-optic tunable filter designs with relaxed beam-splitter requirements.

New designs for acousto-optic tunable filters and electro-optic tunable filters with the Mach-Zehnder configuration are proposed. The new designs differ from conventional designs in three respects: (1) polarizing beam splitters are not required, (2) the optical path difference for the waveguides between the beam splitters is a half-wavelength, and (3) the relative positions of the polarization coupling regions in the two waveguides are displaced in the propagation direction by half of the spatial period of the perturbation responsible for the coupling. Because the new designs provide an additional degree of freedom in achieving the required beam-splitter performance, they are expected to be much easier to fabricate than conventional filter designs with polarizing beam splitters.

Propagation of light trapped within a set of lowest-order modes of graded-index multimode fiber undergoing bending.

We describe experimental results and a theoretical analysis for propagation in graded-index multimode fiber when diode laser light is launched into the lowest-order propagation modes and the fiber undergoes severe bending perturbations. Experimentally, near-field modal interference images and transmission loss measurements were obtained for different loop diameters. The data indicate that, when the fundamental mode is excited, the light remains in lowest-order modes even for small bend diameters. This is consistent with analysis which predicts that, in a parabolic-index multimode fiber subject to constant diameter bending, the light tends to oscillate between lowest-order modes and remains trapped therein rather than diffusing to high-order modes. Implications of these results for an intrusion-resistant communication system with graded-index multimode fiber are discussed.

Accurate monitoring of an interferometric fiber-optic sensor with a multimode semiconductor laser.

A technique for monitoring interferometric sensors by correlation of the optical signal from the sensor with the signal from a phase-modulated reference interferometer is described. The technique is applied to temperature measurement with a fiber Fabry-Perot sensing head. We extend the 7 degrees C unambiguous measurement range obtained with a 1.3-mum laser diode as the light source to 43 degrees C by adding a second laser diode emitting at 1.55mum . Experimentally, a rms phase-measurement error of +/-0.28rad , corresponding to a temperature error of +/-0.32 degrees C , was achieved with a low-quality multimode Fabry-Perot laser. The phase error was improved to +/-0.14rad , corresponding to a temperature error of +/-0.16 degrees C , by use of a distributed-feedback laser diode.

Railroad bridge instrumentation with fiber-optic sensors.

Fiber-optic sensors were installed on fatigue-critical components in the superstructure of a railroad bridge to monitor dynamic strains induced by trains crossing the bridge as well as to detect the onset of cracks. Each fiber Fabry-Perot interferometer (FFPI) strain gage was adhesively bonded to a stainless-steel strip to facilitate all-weather installation on the steel bridge members by spot welding. FFPI strain sensors were also installed on a rail at an approach to the bridge. Electrical resistive strain gages were colocated with the fiber-optic sensors on the bridge for the purpose of performance verification. In addition to the strain gages, fiber-optic continuity sensors for crack detection were bonded to the structure at critical locations. A telemetry system for transmitting the data over telephone lines was also installed at the bridge site. Dynamic response of the fiber-optic strain sensors is comparable with that of the electrical gages, and their performance has not degraded in the year since the initial installation.

High-performance fiber-optic temperature sensor using low-coherence interferometry.

A temperature-sensor system based on low-coherence interferometry with a fiber Mach-Zehnder interferometer as a phase modulator was implemented. A measurement range of 20 to 800 degrees C with a resolution of 0.025 degrees C (corresponding to 0.0004 fringe) was achieved with a 1-mm-long fiber Fabry-Perot interferometer as the sensing element.

High-sensitivity intrinsic fiber-optic Fabry-Perot pressure sensor.

A new configuration for a pressure sensor in which the motion of a diaphragm produces strain in a fiber Fabry-Perot interferometer (FFPI) is described. The single-mode fiber containing the interferometer is bonded at one end to the stainless-steel diaphragm. The fiber is also attached beyond the interferometer to the sensor housing in such a manner that it is always under tension and experiences a strain in proportion to the deflection of the diaphragm. An analysis relating the expected interferometer phase change to pressure is presented, and the dynamic response of the FFPI sensor to pressure changes produced by an air pump is in good agreement with that measured with a conventional pressure sensor.

Multiplexed interferometric fiber-optic sensors with digital signal processing.

A microcontroller-based digital signal processing system developed for use with fiber-optic sensors for measuring pressure in internal combustion engines is described. A single distributed feedback laser source provides optical power for four interferometric sensors. The laser current is repetitively modulated so that its optical frequency is nearly a linear function of time over most of a cycle. The interferometer phase shift is proportional to the elapsed time from the initiation of a sawtooth until the sensor output signal level crosses a threshold value proportional to the laser output power. This elapsed time, assumed to vary linearly with the combustion chamber pressure, is determined by the use of a digital timer-counter. The system has been used with fiber Fabry-Perot interferometer transducers for in-cylinder pressure measurement on a four-cylinder gasoline-powered engine.

High-performance single-mode fiber-optic switch.

A piezoelectrically actuated four-port Mach-Zehnder switch with 30-micros switching time and 0.25-dB insertion loss is demonstrated at a wavelength of 1.3 microm. The worst-case cross talk is below -21 dB for an arbitrary input polarization. The process of fabricating the switch from commercial fiber couplers with length matching of the interferometer arms is described.

Fiber-optic pressure sensors for internal combustion engines.

Two designs incorporating embedded fiber Fabry-Perot interferometers as strain gauges were used for monitoring gas pressure in internal combustion engines. Measurements on a Diesel engine, a gasoline-fueled engine, and a natural-gas engine are reported.

Metal-embedded fiber-optic Fabry-Perot sensors.

The sensing of temperature and of ultrasonic pressure with fiber-optic Fabry-Perot interferometers embedded in aluminum is demonstrated. The metal parts are cast in air by using graphite molds. Breakage of the fibers at the air-metal interface during the casting process is avoided through the use of stainless-steel stress-relief tubes. The optical phase in an embedded interferometer is found to be 2.9 times more sensitive to temperature change than for the same interferometer in air, in good agreement with model calculations. An embedded interferometer has also been used to detect ultrasonic waves over the frequency range of 0.1-8 MHz.

Embedded fiber-optic Fabry-Perot ultrasound sensor.

A fiber-optic ultrasound sensor is presented. The sensor consists of a continuous length of single-mode optical fiber with a built-in Fabry-Perot interferometer. The acoustic pressure produces changes in the index of refraction along the interferometer cavity through the strain-optic effect, thus modulating the reflected power of the light propagating in the fiber. The dielectric internal mirrors that form the interferometer are fabricated by joining a fiber coating with a TiO(2) film at one end to an uncoated fiber by electric arc fusion splicing. Experimental results have been obtained for sensors embedded in plastic and graphite composite materials, using ultrasound waves in the range from 100 kHz to 5 MHz. Values for the optical phase shift amplitude as large as 0.5 rad were obtained at an acoustic frequency of 200 kHz for a 1.1-cm-long interferometer embedded in plastic.

Optical-fiber Fabry-Perot embedded sensor.

A reflectively monitored optical-fiber Fabry-Perot interferometer was embedded in a graphite-epoxy composite material. Its performance as a temperature sensor was demonstrated from 20 to 200 degrees C. The change in relative phase shift with temperature, Deltaø/øDeltaT, was measured to be 8.0 x 10(-6)/ degrees C for this embedded sensor. This value is 4% lower than for one employing a similar fiber in an air ambient. A thermal expansion coefficient for the composite material in the direction of the fiber axis is estimated from these data to be 2.1 x 10(-7)/ degrees C.

Fiber optic application for thermal switching in vanadium dioxide films.

Large thermally induced changes in the end-interface reflectance and transmittance of silica fibers coated with vanadium dioxide films have been observed.

Performance of a fiber-optic temperature sensor from -200 to 1050 degrees C.

A reflectively monitored Fabry-Perot temperature sensor is characterized over a 1250 degrees C temperature range. Dielectric mirrors for the interferometer are incorporated into a continuous length of single-mode fiber by a fusionsplicing technique. The change in optical phase per unit temperature is nearly constant above room temperature but decreases at lower temperatures. Phase-shift data for the germania-doped silica fiber used in these experiments are compared with calculations based on previously reported information on the temperature dependence of refractive index and length in bulk fused silica.

Optical Kerr effect in fiber gyroscopes: effects of nonmonochromatic sources.

The effects of source bandwidth on the optical Kerr effect are calculated. We show that the nonreciprocal Kerr bias error in fiber gyroscopes may be substantially reduced by using broadband sources.

Intraoperative radiotherapy.

Intraoperative radiotherapy is the term applied to irradiation of unresectable tumors, partially resectable tumors, and regional lymph nodes with external beam radiation at the time of surgical exposure. The procedure is a promising supplement to conventional therapy; it presents no special surgical problems, and it is well tolerated.

Propagation characteristics of single-mode evanescent field couplers.

The propagation of polarized light through a BOTTLE coupler has been investigated experimentally. Relative phase shifts for orthogonally polarized modes and the strength of coupling between these modes have been determined. These results have been related to elements in the transfer matrix for the coupler. The effect of polarization on the sensitivity of an interferometric sensor with a fiber coupler as the output beam splitter has also been investigated.

Contradirectional frequency-selective couplers for guided-wave optics.

Frequency-selective optical couplers in which a periodic perturbation in refractive index induces contradirectional power transfer in parallel single-mode waveguides are analyzed. Expressions for the spectral response are obtained by solving the coupled-mode equations. It is shown that by varying the amplitude of the periodic perturbation along he direction of propagation, the sidelobes present for the case of a uniform perturbation can be substantially reduced. Calculations are made of the mode propagation constants and coupling parameters for two rectangular-core waveguides with a periodic surface corrugation in the region between them. By using these results, design parameters and theoretical performance characteristics for a coupler with high transfer efficiency, low cross talk, and low reflection efficiency in the primary waveguide are determined.