Employing an optical readout architecture expands the capabilities offered by uncooled thermal imagers, such as
extremely fast frame rates, dual-band imaging, and multi-megapixel resolution. It also affords the ability to incorporate
multiple pixel designs on the same infrared sensor chip, which we have taken advantage of to fabricate an optical
readout photomechanical imager with 12 distinct pixel designs in the sensor chip layout. Using this methodology, we
were able to quickly sort the designs in terms of performance and suitability for manufacturing, and thus, in an expedient
and highly cost-effective manner, determine which pixel designs have merited future consideration for full-scale
prototyping. A fast frame rate MWIR photomechanical imager based on one of the best pixel designs was built and
tested for high-speed imaging of small arms fire.
Nanowire photodetectors of a variety of materials have been attracting increased attention due to their potential for very
high sensitivity detection. Silicon photodetectors are of particular interest for detection in the visible spectrum, having
many benefits including cost of substrate, ease of processing, and ability for integration with conventional fabrication
techniques. Using top-down fabrication techniques results in additional benefits of precise control of number, geometry,
and placement of these wires. To demonstrate the potential of these devices, top-down, vertical silicon nanowire
phototransistor arrays have been fabricated using ebeam lithography and deep reactive ion and inductively coupled
plasma etching. These devices show a much higher phototransistive gain over nanowire photodiodes with similar
geometry under illumination from a 635nm laser. Low temperature measurements also show the dependence of dark
current and sensitivity on temperature. The mechanism responsible for this gain is shown to be dominated by the large
surface-to-volume ratio of nanowires where charge capture and recombination at the surface creates a radial gate bias
which is modulated with light intensity. 3D numerical simulations validate this mechanism and further show the
dependence of device behavior on nanowire doping, geometry, and surface state density. This will allow for the precise
engineering of these devices to achieve the maximum sensitivity obtainable as we strive for the ultimate goal of single
Photonic devices with low insertion loss are important in dense wavelength division multiplexing (DWDM) systems. Currently most of these devices, such as variable optical attenuators (VOA), switches, filters, and dispersion compensators, etc., involve bulk (or micro-optic) components that require conversions between fibers and free-space optical elements leading to high insertion loss. Recently, we have proposed, analyzed, and demonstrated several fiber based devices for DWDM optical communication systems. Here we present an in-line fiber VOA, a 2x2 switchable wavelength add/drop filter, and high performance dispersion compensators. The VOA is built with a side-polished fiber covered with a liquid crystal overlay. By varying the orientation of the liquid crystal molecules using an applied electric field, the loss of the device can be controlled. The 2x2 wavelength switch is designed by recording electrically switchable holographic gratings in a layer of holographic polymer dispersed liquid crystal (H-PDLC) sandwiched between two side-polished fibers. The dispersion compensators are based on high precision fiber Bragg gratings (FBG). A unique method for writing FBGs with arbitrary phase and amplitude distributions is demonstrated. All of these devices are analyzed theoretically and demonstrated experimentally. Both theoretical and experimental results will be presented and discussed. These devices are suitable for DWDM optical information transmission and network management.
A newly developed fiber Bragg grating (FBG) fabrication method is demonstrated. By combining a stable continuous-wave UV laser, a high precision electro-optical UV modulator, a compact and stable holographic setup, and a subnanometer precision air-bearing translation stage, the setup allows both apodization and phase of the FBGs to be continuously changed at each grating line. During the writing process, a fiber is moved constantly by the high-precision air-bearing stage, while the UV laser beam is switched on and off by a customized pulse train with nanosecond timing precision and forms a stable interference pattern on the fiber. The error of this system mainly results from the control signal's timing jitter, which is normally lower than 0.1%. Theoretical analysis and experimental results for gratings with various structures are demonstrated and discussed. They show good agreement, demonstrating that the fabrication method is capable of fabricating gratings with arbitrary apodization and phase design.
Fiber Bragg grating (FBG) is an important element in many applications including filters and dispersion compensators in fiber communication systems. With recently developed inverse scattering algorithm, FBGs with desired reflection spectrum and/or dispersion properties can now be designed. However, most of these designs require arbitrary grating amplitude and phase control. Previously, fabrication of such FBGs relies on the accurate control of the temporal variation of the intensity pattern using a piezo electric translation stage. The precision of this fabrication method is limited by the noise in the control voltage, which is usually larger than 1%. The distortion in piezo response also affects the performance. In this paper, we develop and demonstrate a novel writing technique for arbitrary FBG fabrication. Our technique is based on a translate-and-write configuration. The incorporation of a precisely controlled shutter allows the apodization and phase of the FBG to be continuously changed at each grating line. The shutter error mainly results from the control signal's timing jitter, which is normally lower than 0.1%. Using this writing technique, we demonstrate a Hamming apodized grating with 20mm length, -22 dB minimum transmission, and < -25 dB reflection side lobe suppression. Furthermore, phase-shift in a grating can be fabricated by a simple delay in the control signal. We also demonstrate FBGs with π, π/2, 3π/2 phase-shifts, respectively. Our experimental results are in excellent agreement with theoretical predictions. To show the capability to fabricate a FBG with arbitrary structure, we demonstrate a 35 mm long zero dispersion grating.
The photorefractive effect is a phenomenon in which the local index of refraction is changed by the spatial variation of the light intensity. Although the phrase 'photorefractive effect' has been traditionally used for such effects in electro-optic materials, new materials, including photopolymers and photosensitive glasses, have been developd in recent years and are playing increasingly important roles in optical fiber communication systems. Photopolymers in combination with liquid crystals are ideal materials for wavelength selective tunable devices. The improved optical quality and large dynamic range of photopolymers make them promising materials for holographic recording. Holographic gratings recorded in photopolymers can be employed as distributed Bragg reflectors. The large birefringence of liquid crystals can be used to tune the index of refraction to cover a large wavelength range. In addition, birefringence of liquid crystals can be used to tune the index of refraction to cover a large wavelength range. In addition, the combination of photopolymer and liquid crystal also leads to a new material known as holographic polymer dispersed liquid crystal (H-PDLC) which provides a medium for switchable holograms. Photonic devices made of these materials can be easily incorporated into an optical fiber system because of the low index of refraction of polymers and liquid crystals, and their relatively easy processing techniques. Besides photopolymers, photosensitive glasses are also promising for applications in fiber optic systems. Fiber Bragg gratings (FBGs) have been used as bandpass filters and dispersion compensators. In this paper, we describe the applications of photopolymers, H-PDLCs, and FBGs in fiber optic devices. Specifically, we will describe our recent works on photonic devices such as filters, switches, and dispersion compensators for WDM systems.