James Sharp

Single-crystal ruby fiber temperature sensor

Single-crystal ruby fibers with nominal diameters in the range from 80 to 500 μm and lengths up to 170 mm have been produced using the laser heated miniature pedestal growth technique. These fibers were tested for use as the basis of fiber optic temperature sensors. Characterization of the fiber fluorescence properties and comparison to bulk samples together with their high melting point indicate their potential for fluorescence-based sensing at extremely elevated temperatures. Good geometrical matching with current fiber optic technology and the use of phase-locked detection techniques also allow high resolution measurements to be obtained.

Er3+ + Yb3+-codoped Al2O3 crystal fibres for high-temperature sensing

Single-crystal sapphire fibres codoped with approximately equal amounts of rare-earth (RE) Er3+ and Yb3+ ions have been fabricated by the laser heated pedestal growth technique. The crystal fibres are then investigated for high-temperature sensing applications. Due to the unique transitions from the 4f electronic configuration in RE ions together with the high dopant concentrations used, strong upconverted emission in the visible has been observed. In particular, emissions in the green and red spectral regions have been exploited in an intensity-based sensing scheme for thermometric measurements up to ~ 1320 and 1420 K respectively.

Graded-index characteristics in single-crystal fibers.

Single-crystal fibers of Ti(3+):Al(2)O (3)have been grown by the laser-heated pedestal growth technique and shown to exhibit radial refractive-index gradients. A refractive-index increase of approximately 12% in the fiber core with respect to the fiber sidewalls has been measured. The index profile can be fitted with a parabolic model. Postgrowth treatment of Cr(3+):Gd(3)Sc (2)Al (3)O(12)fibers has been shown to produce a uniform cladding region with a graded-index core. The core index is some 12.5% larger than the cladding region, with an index profile shape that is approximately parabolic in nature.

Comparative bactericidal activities of lasers operating at seven different wavelengths

Seven laser instruments, delivering radiation at a selection of wavelengths in the range of 0.355 to 118 mm, were investigated for their ability to kill Escherichia coli as a lawn of the bacteria on nutrient agar culture plates. Easily the most effective was a 600-W CO2 laser operating at 10.6 mm, which produced 1.2- cm2 circular zones of sterilization at energy densities of around 8 J cm22 in a 30-msec exposure. Circular zones with an area of 0.7 cm2 were achieved with 200 W from a Nd:YAG laser delivering 8-ms, 10-J pulses of 1.06 mm radiation at 20 Hz. The exposure time, however, was 16 s and the energy density (1940 J cm22) was more than 240 times higher than with the CO2 laser. This difference is believed to be partly due to the much higher absorption of radiation at 10.6 mm than at 1.06 mm, by water in the bacterial cells and the surrounding medium (nutrient agar). Sterilization was observed after exposure to frequency-tripled Nd:YAG laser radiation at 355 nm (3.5 J cm22). Lasers that were totally ineffective in killing Escherichia coli (with their wavelength and maximum energy densities tested) were the far infrared laser (118 mm; 7.96 J cm22), the laser diode array (0.81 mm; 13,750 J cm22), and the argon ion laser (0.488 mm; 2210 J cm22). The speed at which laser sterilization can be achieved is particularly attractive to the medical and food industries.

High-concentration Er:YAG single-crystal fibers grown by laser-heated pedestal growth technique

High-concentration Er:YAG single-crystal fibers have been grown using the laser-heated pedestal growth technique. Instability in the melt and concomitant opacity of fibers were observed at source concentrations higher than 15 mol.%. Spectroscopic examination shows that broadening of the linewidth of the ⁴I(13/2) → ⁴I(15/2) transition is strongly dependent on Er³⁺ concentration.

High-speed, acousto-optically addressed optical memory.

A page-oriented, angle-multiplexed volume holographic optical-memory recording system has been constructed. This memory is addressed by the use of an acousto-optic deflector with a random-access time of 16 µs per page. This enables data transfer rates of 5.28 Gbits/s when pages of binary data are being stored. The reconstruction quality of images stored as memory pages is assessed with the quality achieved with the acousto-optic device compared with that achieved with the original recording optics.

An automated recording system for page oriented volume holographic memories

A system for the automatic acquisition and recording of images from video input scenes to a volume holographic optical memory has been designed and constructed. This memory is intended for use as a template database for reference images for an optical correlator. The system has been tested and commissioned; initial results indicate that system parameters such as hologram reconstruction quality are of a sufficiently high standard for the application.

Experimental Systems Implementation of a Hybrid Optical-Digital Correlator

A high-speed hybrid optical-digital correlator system was designed, constructed, modeled, and demonstrated experimentally. This correlator is capable of operation at approximately 3000 correlations/s. The input scene is digitized at a resolution of 512 x 512 pixels and the phase information of the two-dimensional fast Fourier transform calculated and displayed in the correlator filter plane at normal video frame rates. High-fidelity reference template images are stored in a phase-conjugating optical memory placed at the nominal input plane of the correlator and reconstructed with a high-speed acousto-optic scanner; this allows for cross correlation of the entire reference data set with the input scene within one frame period. A high-speed CCD camera is used to capture the correlation-plane image, and rapid correlation-plane processing is achieved with a parallel processing architecture.

Concentration dependence of upconversion emission from Er:YAG fibers.

Fibers of Er:YAG have been grown using the laser-heated pedestal growth method. Fibers with Er concentrations ranging from 0.5 to 10 mol. % have been produced and excitation at 965 nm has been used to produce intense upconversion emission at green and red wavelengths. The dependence of ground state absorption and upconversion emission on dopant concentration has been studied and shown to exhibit lower levels of self-absorption than previously reported. At Er concentrations of 5 mol. % and above; however, the variation of upconversion emission intensity with pump power deviates from theoretical predictions, exhibiting saturation behavior consistent with concentration quenching.

Comprehensive modeling of multimode fiber sensors for refractive index measurement and experimental validation

We propose and develop a comprehensive model for estimating the refractive index (RI) response over three potential sensing zones in a multimode fiber. The model has been developed based on a combined ray optics, Gaussian beam, and wave optics analysis coupled to the consideration of the injected interrogating lightwave characteristics and validated experimentally through the realization of three sensors with different lengths of stripped cladding sections as the sensing region. The experimental results highly corroborate and validate the simulation output from the model for the three RI sensing zones. The sensors can be employed over a very wide dynamic RI range from 1.316 to over 1.608 at a wavelength of 1550 nm, with the best resolution of 2.2447 × 10−5 RI unit (RIU) obtained in Zone II for a 1-cm sensor length.