The U.S. Navy has unique sensing requirements that require the use of non-traditional sensing techniques. Problems of electromagnetic interference (EMI), reliability, and weight have driven the Navy to pursue fiber optic sensors to meet these requirements. This paper presents an overview of fiber optic sensor technology related to Navy shipboard requirements. The different types of sensors as well as a means of interconnecting these sensors are described. In addition, overviews of two areas relating to future shipboard sensing systems are presented: MEMS Sensors and Hybrid Networks.

Code division multiplexing can be used to increase the number of senors operated on one light emitting diode (LED) and thereby reduce the unit sensor cost of the complete sensor system. We have successfully designed a system to multiplex 32 sensors on one LED with output power of approximately 0.4 mW, at 850 nm wavelength. The LED is modulated with a pseudo-random pulse sequence and distributed through a 2 by 32 coupler to 32 reflective sensors. Sensor response is measured by computing the cross correlation of the transmitted code and the sensor reflection. The response is separated in time by varying the length of the sensor fiber. Sensor noise is reduced by averaging and cross correlation of the sensor response with the transmitted code sequence. All processing is done by microcomputer and a digital oscilloscope. The system was demonstrated with multimode fiber connected to 2 Fabry-Perot sensors and 1 amplitude sensor. Noise values on each sensor imply that more than 32 sensors can be monitored with code division multiplexing technology.

This paper describes the development of a complete fiber optic sensor network demonstrator, installed on a naval warfare simulator system. It incorporates a 64 on/off multiplexed optical sensor network together with single point analog optical sensors. A complete description of the system is given showing the different sensors that have been developed and their specific applications. We also discuss the experimental results and the potential advantages of this technology for naval systems.

The S.E.P. (Societe Europeenne de Propulsion) develop and manufacture cryogenic engines for the 'Ariane' European Space Agency launcher. Since the late 80s the C.N.E.S. (Centre National des Etudes Spatiales), the French Space Agency, has supported the development of fiber optic sensors for the future missions, exploration and diagnosis of spacecraft engines. The environmental conditions in a cryogenic engine are extreme in terms of temperature range, pressure, and vibration levels. We have shown that fiber optics sensors can be ideally suited in these harsh environments. Hence we present in more detail a fiber optic pressure probe that has been developed for engine in-situ measurements.

The paper presents theoretical and experimental investigations on distributed anti-Stokes ratio thermometry (DART) sensing for monitoring temperature in complicated structure. A technique, based on maximizing signal to noise ratio (SNR) of DART, has been proposed. A preliminary prototype DART system based on a microcomputer has been developed and optimized by this technique. It has spatial resolution of 15.6 m (limited by pulse width of the laser), measuring time of 160 s, temperature resolution of 7 degrees Celsius, sensing upper limit of 260 degrees Celsius, and sensing distance of 1.5 km.

The fiber-optic refractometer using visible laser diodes with wavelengths of 650 divided by 670 nm for the liquid refractive-index measurement is presented. The refractive- index measures by fiber-optic sensors of the connected configuration for different liquids with refractive indices from 1.33 to 1.5 have given the accuracy of 5.10^-3. The fiber-optic refractometer was performanced for the distinguish of the salt or sugar content in the mixtures with range of 10^-3 and 5.10^-4, respectively. These refractometers are already to use for the sugar control systems of beverage industry and salt-water environment.

A low-cost fiber-optic pressure sensor is reported designed for use in production automotive engines for combustion monitoring and control. The sensor operates on the principle of changing light intensity due to the reflection oflight from a metal diaphragm moving under the effect of combustion pressure. Two designs are described: one based on the use of a single, multi-mode fiber and the other using two multi-mode fibers. The optoelectronic components used in the sensor include one GaA1As LED operated at 850 am and one or two SI PIN photodiodes. To combat LED intensity and photodiode sensitivity changes over an underhood temperature range of -40°C to 150°C a proprietary auto-referencing technique has been developed that does not require temperature regulation of the optoelectromc package. Telecommunication-grade optical fibers, with core diameters ranging from 50 to 100 microns, are used coated with high temperature coatings. A sensor, as small as 2.8 mm in diameter, can be either directly inserted into an engine head or integrated with a spark plug or a fuel injector. The sensor optoelectromc and electronic components are packaged inside a miniature "smart" connector that connects to an engine controller power supply as well as outputs pressure and sensor health signals. When optimized for high linearity and reduced thermal effects, the spark plug-mounted sensor demonstrates accuracy comparable to that of a laboratory-grade piezoelectric transducer. Under engine operating conditions ranging from idle to full load and maximum RPM, the combined sensor's hysteresis, non-linearity, and thermal shock result in pressure reading accuracy of 1.5% full scale range. With the targeted life-time of 500 Million pressure cycles and price of less than $10, the sensor meets the performance, durability, and cost requirements for production car applications.

We have demonstrated a 15 dB lidar receiver detection sensitivity improvement by employing an Er³⁺-doped fiber preamplifier as the input to an InGaAs P-I-N photodiode. This eye-safe, 1550 nm detection system can significantly improve lidar system performance. It is shown that it is comparatively easier at 1550 nm than at 1064 nm to achieve a higher detection sensitivity for a lidar system using a fiber preamplifier in comparison with that using an avalanche photodiode (APD). The reason is that the Er:fiber preamplifier performs better than a Nd:fiber preamplifier, while the InGaAs APD is less sensitive in comparison with a Si APD. In a lidar receiver using a fiber preamplifier, the beam coupling efficiency from free-space mode to a single- mode fiber is the critical parameter. For moderate target velocities, an automatic front-end alignment system using piezoelectric transducers can be effected to yield a good coupling efficiency. The design and preliminary test results of such a lidar receiver are discussed in terms of optimized optical filter bandwidth, optical preamplifier gain and noise figure, and input saturation level.

A new optical displacement measurement system is presented using a dual interferometric design where a Wollaston prism interferometer is employed in conjunction with a normal Michelson interferometer. This is configured without the use of external polarizers, apart from those associated with the Wollaston prism. It is shown that an optical path difference induced in the Michelson interferometer can be detected using the Wollaston prism in a normal interferometer arrangement, despite the different modes of operation of the two interferometers (the Michelson splits input light by amplitude whereas the Wollaston prism splits light by polarization). Further, the interference pattern produced by the Wollaston prism interferometer changes in a measurable, linear fashion as the optical path difference from the Michelson interferometer alters, and as a result of the shape of the interference pattern, the need for calibration is reduced since a reference point is produced by the zero optical path difference point at the center of the Wollaston prism. A simple theoretical analysis of the system is presented and used to derive a computer model of the optical arrangement. Results from an experimental implementation of the system, using a Wollaston prism with a beam separation of 0.5 degrees and a superluminescent diode, of wavelength 825 nm, as a light source, are included and compared to the results from the computer model.

We developed a laser based inspection system which was used to monitor defect distributions in optoelectronic devices such as diode lasers. Basically the system works as the well-known laser beam induced current (LBIC) technique. Various lasers emitting in the 633-1300 nm wavelength range were employed as excitation source. A number of high power laser diode arrays (LDA) aged under different aging conditions (parameters: injection current, heat sink temperature, time) were inspected with the system. The scans obtained revealed significant differences for different aging levels of arrays. This finding allows us to determine the aging status of LDA and contributes to find methods for giving failure predictions for individual devices.

Frequency-domain measurements of photon migration were conducted in concentrated polystyrene suspensions in order to assess their use for on-line particle sizing in undiluted process streams. Using a numerical inverse algorithm, particle size distributions (PSD) and volume fractions of three latex suspensions with differing size distributions were recovered from phase-shift measurements at fifteen or less visible wavelengths. Comparison to dynamic light scattering measurements show excellent agreement. Since it is the propagation characteristics of multiply scattered light instead of the amount of light detected that is employed to solve the inverse problem, the measurement technique provides a self-calibrating (and therefore especially suitable) method for on-line process monitoring of PSD in the chemical-based industries.

Theoretical and experimental results related to wavelength modulation absorption spectroscopy using single-mode diode lasers are presented. This method uses a phase-sensitive detection technique, and the implications of detection at harmonics greater than the commonly used second-harmonic are discussed. It is shown that several advantages accrue with such higher-harmonic detection. Applications of the method in increasing sensitivity to measurements of density and temperature fluctuations; in increasing wavelength resolution of modulation and pressure broadened congested spectra; and in applications for accurate characterization of absorption lines of molecular spectra, are discussed. It is shown that wing structure of absorption lines can be obtained with much more accuracy using higher harmonic detection than either direct absorption or the commonly used second harmonic detection technique. Experimental results obtained are compared to the predictions of the theory developed.

In recent years there has been renewed interest in using modulation spectroscopy for a variety of scientific and engineering applications. Modulation spectroscopy is often implemented with diode lasers which have the advantages of small size, non-intrusiveness, speed, and ease of use. Several theoretical frameworks have been developed to describe modulation spectroscopy and its limiting cases: frequency modulation spectroscopy (FMS), and wavelength modulation spectroscopy (WMS). In this paper we examine these frameworks and describe a general approach to modulation spectroscopy which takes into account the properties of semiconductor lasers.

As performance levels for sophisticated electronic materials and device processing increase so to does the demand for versatile methods for on-line real time sensing of many process parameters. To that end we have developed a series of tunable semiconductor laser spectrometers for use in detection schemes of gas phase chemical species via infrared absorption spectroscopy. We report here on characterization of dual modulation schemes for enhanced sensitivity, interference fringe free, absorption spectroscopy data.

Compact liquid sensors were constructed by using the output of near-infrared semiconductor diode lasers to measure transmission. Multiple short external cavities were used to force diode lasers to provide a wide spectral coverage (approximately equals 72 nm) for the detection of broad absorption features of liquids. The transmission at discrete laser modes were processed by the principal component regression (PCR) and the partial least squares (PLS) algorithms. A root mean square error of prediction of 0.092% and a correlation coefficient R² of 0.998 were obtained for the detection of water concentration in D₂O. The rms value of 0.056% from 30 independent measurements of 0.5% water in D₂P showed that the method was sensitive to detect 0.1% concentration difference. The application of the PCR and the PLS algorithms in the scheme was studied in a three mixture system of water, acetone and methanol in terms of optimizing factors and detection of 'outliers'. The limiting noise sources were determined by modeling synthetic data.

Virtual displays have tremendous potential in entertainment applications such as video games, head mount displays for personal computers, and mobile World Wide Web viewers. These consumer applications require high quality virtual displays at a cost below $40 per eye. This combination of performance and cost is not realizable with virtual displays based on LCDs or CRTs. However, low cost, high quality virtual displays can be achieved using patented scanned linear array technology and red, green and blue monolithic LED arrays. A 384 by 224 full color virtual display prototype has been built using this approach. The prototype delivers 4-bits of grayscale per color and flicker-free performance at a 60 Hz frame refresh rate. This paper discuses details of our LED based full color virtual display prototype; development of red, green and blue monolithic LED arrays; and work-in- progress to miniaturize the display and scale the resolution to full VGA.

Multiple channel writing devices are important to the graphics arts industry for applications like imagesetting, color proofing, and direct-to-plate writing. Eastman Kodak Company has developed a novel approach to the design of these devices. The laser array consists of 10 groups of 16 laser diodes, which are combined with diffractive optics into 10 individually addressable writing channels. This design gives us the ability to achieve higher printing power levels, as well as a level of redundancy for commercial use. This paper covers aspects of the structure of the array devices, as well as how they are integrated into working printheads. The major part of the discussion is on the characterization of the arrays and the printheads for spot quality and power levels. It touches on the metrics used to predict print quality by looking at spot quality, and on the equipment used to make these measurements in a timely fashion over many focus positions. Finally, there is a brief discussion of the technology transfer that resulted in a successful transition of this project from R&D into manufacturing.

The analysis of many high explosives (HE) involves the use of rotating-mirror cameras and high speed film. Fiducial timing spots are made on the film to provide temporal reference to the experimenter during subsequent evaluation. The writing speed of the 'streak' camera is 10 millimeters per microsecond, thus, the optical fiducial pulse width must be on the order of tens of nanoseconds to generate a useful spot size. For this application, a useful spot size corresponds to a width of 200 to 300 micrometers. Present systems employ light-emitting diodes for this task, mounting them at the focal point of the camera. However, the size and clarity of the current timing spot on the film is less than optimum. Furthermore, experiments involving high explosives require the isolation of the electronic instrumentation from the experimental hardware and passive operation is always preferred if not required. This is due to safety requirements as well as instrumentation ground loop and EMP concerns. Another restriction is the diminished sensitivity of the high-speed film to wavelengths above 600 nanometers, which narrows the field of possible optical sources considerably. A passive, fiber-coupled system based upon a 635 nanometer laser diode has been developed and tested. The development process, final design and test results are presented and the improved signal resolution is compared with current technology.

The utilization of visible laser diodes for laser printing is discussed. First, the characteristics of a multiple- element array of single-mode, individually-addressed red (AlGaInP) laser diodes is described. The benefit of shorter- wavelength blue lasers is then evaluated. Finally, towards the realization of a blue laser diode, we describe results for AlGaInN and its heterostructures, which have been grown by OMVPE and characterized, including electrical injection and optical pumping of InGaN/AlGaN heterostructures.

Law enforcement and military forces are often faced with situations requiring less-than-lethal response options. Low- power, eye-safe laser illuminators have been shown to be effective, non-lethal weapons for a variety of law enforcement and other-than-war military applications. Through the effects of illumination, glare, and psychological impact; lasers can provide unequivocal warning, threat assessment based on reaction to the warning, hesitation, distraction, and reductions in combat and functional effectiveness. This paper discusses ongoing research and development by Science and Engineering Associates into laser illuminator concepts for civilian and military use. Topics include fundamental design and safety issues, laser diode requirements, and laser illuminator concepts, including a grenade shell laser system that converts a standard 40-mm grenade launcher into a laser illuminator.

An optical arrangement is described for the investigation of alignment and optical aberration effects for fast-axis collimation lenses used with laser diode bars. Different lens configurations have been compared, and correction of the collimation errors caused by bowing of the diode bar ('smile') has been achieved by use of a piezo-electric actuator to make a compensating bow in the micro-lens.

Acousto-optic spatial light modulators present new opportunities to create bright video displays with pulsed lasers. Operating characteristics of coherent red, green, and blue light sources to support NTSC, PAL, and HDTV video standards are presented. We describe rear-projection systems under development at COLOR that employ pulsed RGB lasers to produce very bright images suitable for both outdoor and brightly lit indoor large screen display applications.

In an effort to support the advancement of U.S Navy initiatives in improved machinery diagnostics and control, improved information collection for damage control systems, and greater integration of shipboard sensor information the Naval Surface Warfare Center, Carderock Division (NSWCCD) is engaged in an ongoing project to evaluate and develop advanced optical sensors. Initial efforts focused on examining the possibility of replacing some mature conventional sensors with their optical counterparts. Currently, NSWCCD is focusing on specific niche applications where measurements can not be made by the conventional means, or where optical sensors provide a significant performance advantage. Focus areas of innovative fiber optic sensors have included EMI immunity, measurement of absolute strain, smart structure development, and linking advanced sensors to optical communications networks. Sensor technologies being investigated include Fabry-Perot type interferometers, grating type fiber optic sensors, and optically operated MEMS sensors. Commercial, off-the-shelf (COTS) sensors which utilize the preceding technologies are being tested and evaluated in Navy laboratories. In addition, research initiatives are underway to develop new sensors as well as methods for interfacing these sensors with current and future controls, communication, and monitoring equipment.