An optical time-division multiple-access (OTDMA) network architecture has been proposed which has the potential of avoiding electronic processing of signals at the aggregate network bandwidth. New specifications for the optical components used in this OTDMA network architecture will be required before a practical system is realized. In this paper, we present a model of the OTDMA architecture that relates parameters at the device level such as carrier mobility, physical geometry, charge trapping, and carrier-concentration to system-level performance measured such as bit error rate and noise margin. We present mathematical models of the devices in the system. These models are interconnected into a system-level Monte Carlo simulation model of the OTDMA architecture. The photoconductive AND device, a critical component in the OTDMA receiver, is modeled as a time-varying circuit element (conductance) in a microstrip transmission line. Device-level physics of the photoconductor is incorporated into the microstrip model via a time-varying conductance. We base the simulation model of the AND device on the explicit second order Adams-Bashforth formulation. Alternative simulation modeling approaches, including feed-forward artificial neural networks, are also used with excellent results. Simulation of the OTDMA network is in good agreement with our approximate analysis, in addition to laboratory measurements.
We demonstrate a simple method to modulate a pulse train of arbitrarily narrow optical pulses using off-the-shelf components. The method makes use of the Sagnac loop fiber interferometer only several meters in circumference.
Multiple Quantum Well (MQW) materials and devices have been designed and demonstrated to have large optical nonlinearities which are suitable for use in ultrafast optical TDMA interconnects at 1.3 micrometers . The MQW materials consist of GaAlInAs wells and AlInAs barriers grown lattice-matched to a semi-insulating InP substrate by molecular beam epitaxy. The MQW samples exhibited large absorption changes at 1.3 micrometers due to bandfilling and exchange effects. The carrier saturation densities near the heavy-hole exciton peak were similar to those for GaAs/AlGaAs MQWs. The large optical nonlinearities near 1.3 micrometers were used to demonstrate an all-optical, high-contrast asymmetric reflection modulator suitable for performing all-optical time-division demultiplexing at low pump intensities. The modulator consists of an asymmetric Fabry-Perot etalon which utilizes a nonlinear MQW spacer. The modulator exhibited an on/off contrast ratio of greater than 1000:1 and an insertion loss of 2.2 dB at a pump intensity of 30 kW/cm2. The recovery of the modulator is shown to decay with a time-constant of 725 ps.
A tunable erbium doped fiber ring laser, pumped with a 980 nm InGaAs diode laser was constructed. Output power as a function of pump power and output wavelength for a given fiber length was measured. Spectral and temporal analysis of the signal showed mode-locked pulses of short duration and broad frequency content, as well as a CW component confined to a relatively narrow optical frequency range. The mechanisms for this type of mode-locking are discussed as well as the limitations placed on the lasing line width and spectral tunability.
A semi-conductor TWLA provides the gain medium for an external cavity laser source at 1.32 microns for compatibility with single mode fiber optic systems. The active layer of the waveguide is InGaAsP in an angle stripe geometry. Parameters for CW lasing are established. Pulsed operation is then achieved by two methods: direct RF modulation of the bias current, and regenerative feedback of the converted optical output signal. Both methods yield short pulses of different frequency noise characteristics. The mode-locked rate can be adjusted over a wide range for different applications by varying the cavity length. Wavelength tuning is achieved by replacing the cavity end mirror with a grating the use of an all fiber cavity is examined.
Optical intensity modulators with closely matched frequency responses are required for the realization of an optical adaptive nulling system for microwave/millimeter-wave multi-element antenna systems. Traveling-wave interferometric modulators fabricated in lithium niobate have been demonstrated that track to within 0.03 dB and 0.22 deg in amplitude and phase, respectively, over the 5- to 7-GHz frequency band.
An analog fiber-optic data link has been demonstrated for direct charge readout of wire chambers used in high energy particle detectors. The fiber link consists of a Nd:YAG laser carrier, a Mach-Zehnder external modulator, and a low-noise optical receiver. A charge pulse developed on a sensing wire flows directly into the electro-optic modulator with no preamplification. The substitution of passive modulators and fiber cable for active electronics and copper wire near large collider detectors has clear advantages with regard to radiation damage susceptibility, EMI immunity, physical size, and power consumption. Also, the modulator performance was unaffected for an applied one Tesla magnetic field. Reduction of noise contributions from the laser carrier and optical receiver and exploitation of the 'effective optical gain' properties of external modulators resulted in shot-noise-limited link sensitivity. Modulators were packaged with large termination resistorsenabling increased charge sensitivity with an increased system risetime tradeoff. Energy resolution of the fiber link was comparable to preamps conditional on large terminations and subsequent slow readout. For shorter risetimes required in timing diagnostics, small termination resulted in sensitivity 6.8 times inferior to preamps. Several potential improvements in sensitivity are discussed.
A novel optical control system using nematic liquid crystals and acoustooptic devices is introduced that can provide compact, lightweight, low cost, transmit/receive mode, high performance (greater than 8 bits), truly analog, amplitude and phase control for large phased array antennas. The system provides independent amplitude and phase calibration and control capabilities across the array.
The paper describes a concept of an acoustooptic phased array antenna beam-former with N multiple simultaneous beam-generation capability and wide-band antenna operation feature. The system was experimentally demonstrated using a single-channel acoustooptic device driven by a signal of frequency f(c), and an acoustooptic device with N channels driven by signals with frequencies f(c)+f(1) to f(c)+f(N). It is suggested that the concept can be extended to radar bands by using gigahertz-band acoustooptic devices.
Subcarrier multiplexing (SCM) is a convenient method of implementing multi-access in a lightwave system. Conventionally SCM receivers detect all channels using high speed optoelectronic components, each channel then being selected using heterodyne techniques in the electrical domain. Optical prefiltering is a novel technique where channel selection is accomplished optically. This method has the primary advantage that only a low bandwidth optical detector and receiver is needed to recover the baseband information.
Analytical studies have been made on the effect of diffusion and surface recombination on the frequency dependent characteristics of an ion-implanted GaAs optical field effect transistor. Modulated optical generation and voltage dependent depletion layer width in the active region have been considered whereas photovoltaic effect is ignored in this analysis. Result shows that drain-source current decreases with the increases of modulated signal frequency but diffusion effect increases the modulating frequency range from c.m. to m.m. wavelength. Moreover, I- V changes significantly with the trap center density only when Nr >= 1023/m2 with diffusion effect and >= 1020/m2 without diffusion effect at a particular dimension of the device. This model may be very much useful to measure the sensitivity of the device in terms of trap center density and modulating frequency.
The present paper establishes the photonic superguiding theory in polar crystals with a high nonlinearity. In the microscopic theory it is shown that photons can feel an attractive effective interaction by exchange of virtual phonons. Such an interaction leads to the superguiding state, in which the photons with opposite wave vectors and spins are bound into pairs. The photon pairs travel without scattering attenuations. In the semiclassical theory we find that polar crystals with a high nonlinearity are self-defocusing media with dispersion and the optical fibers made from such crystals can propagate temporal solitons without dispersion. If the photons propagating in such a waveguide enter the superguiding state, the waveguide exhibits both an ultralow energy loss and a high transmission rate.
Using an optical correlator, we experimentally evaluated a binary phase-only filter (BPOF) designed to recognize objects not in the training set used to design the filter. Such a filter is essential for recognizing objects from actual sensors. We used an approach that is as descriptive as a BPOF yet robust to object and background variations of an unknown or nonrepeatable type. We generated our filter by comparing the values of spatial frequencies of a training set. Our filter was easily calculated and offered potentially superior performance to other correlation filters.
A multiple imaging system can produce a two-dimensional array of images from a single input object. The basic components of our implementation are a high-performance Fourier processor and a suitably designed diffraction grating that is placed in the frequency phase. We have made use of a single diffractive element Fourier transform lens, corrected for coma, astigmatism, and field curvature. The lens is designed to have the proper amount of distortion to produce an optical transform. The optical processing system consists of two of these lenses spaced a distance equal to the sum of their focal lengths. We employ an iterative algorithm to produce the necessary phase diffraction grating that generates a multiplicity of equal energy plane waves from a single incident plane wave. The method is based on an iterative Fourier- transform scheme that makes use of known constraints to synthesize the phase structure. The constraints include the known Fourier modulus (array of point sources), the phase-only requirement in the grating plane, and fabrication requirements such as discrete phase levels. The iterative algorithm provides an efficient means to calculate grating profiles that give rise to large arrays of images. System performance over a relatively wide field-of-view is characterized by calculating the modulation transfer function at various points in the image plane.
Using an optical technique, we classified images of natural terrain based on their fractal dimension. We calculated the fractal dimension from an optically generated power spectrum obtained with a magneto-optic spatial light modulator (SLM). By using the fractal dimension to classify images of natural terrain, our post processing was simpler that when a ring-wedge detector was used.
The Artificial Long Delay Optical Processor (ALDOP) generates a large variable delay of a band limited pulse modulated RF signal. The proposed prototype ALDOP has potential use in a Passive Radio Ranging (PRR) system operating in the 82.5 - 97.5 MHz frequency band. The complete system will combine a passive electronic band pass filter with the optically based ALDOP to impart a unique phase shift to each frequency component of an incoming RF signal. The phase shift is generated using optical heterodyne methods that incorporate the transmission and modulation of light through a series of optical components including an acousto-optical cell and a Fresnel Zone Plate.
The switching response of a CdTe-InSb nonlinear etalon subject to an intense light beam is reported. The device is illuminated with a high intensity pulsed pump beam and a low intensity pulsed probe beam. The pump beam has a wavelength that corresponds to a negative slope of the low intensity reflectance spectrum of the etalon and has sufficient power to change the index of refraction in the etalon cavity. This causes a shift of the spectrum, a decrease in the reflectivity, and an increase in the electric field in the etalon cavity which further shifts the spectrum. This process continues until a minimum reflection level is reached. The output yields the convolution of the probe beam with the device response to the pump pulse. It appears that the switching response of the etalon is much faster than could be determined with the 100 ps pulses used.
We designed a programmable delay line using laser diodes to tap an acousto-optic cell. The laser diodes optically tap the Bragg cell at varying distances from the transducer to create delayed versions of the input signal. We describe the optical architecture that uses heterodyne detection to collect delayed versions of the input signal. We present experimental results that verify system performance. Applications for the delay line include radar and signal processing systems.
Optical adaptive processors using photorefractive crystals as time integrating devices have demonstrated greater than 40-dB cancellation of narrowband interference signals. However, we desire to use adaptive optical processors to null broadband radar jamming. We tested the performance of our photorefractive based adaptive processor using wideband noise signals (greater than 100 kHz). The system's operation and experimental results of both narrowband and wideband cancellation are described.
Dynamic arrays of nonlinearly coupled mode-locked laser oscillators offer a means of generating arrays of ultrashort optical pulses where the arrays are both susceptible to rapid reconfiguration in response to small externally introduced electronic signals and yet also stable against random perturbations. We examine one specific example where the pulses in the array can be approximated as lowest order, weakly coupled solitons, propagating on multicore optical fiber.
This paper discusses the synthesis and design procedures for implementation of microwave filters with adjustable transfer functions using fiber optic delay lines along with electro-optical couplers (EOC). It presents two configurations for realizing narrow-band and wide-band filters. It numerically investigates several design configurations in which the desired transfer functions are obtained by adjusting the coupling ratios in EOCs. The paper also discusses the effect of random variations in the length of delay lines and the coupling ratios of EOCs on the transfer function of the filters.
We introduce a novel, time delay-based, transmit-receive mode, remotely located, optical control system for wide instantaneous bandwidth phased arrays. Time delays are achieved using a cascade of free-space and fiber-based optical delay lines using bulk imaging optics for low complexity, two dimensional, free-space optical interconnects. High channel isolation, low insertion loss optical switching between delayed and undelayed paths is based on polarization switching using the low cost nematic liquid crystal arrays and polarizing beam splitters. The system allows for noise reduction via spatial and polarization filtering. A dual- channel time-multiplexed system arrangement is used to implement the fast (e.g., 200 beams/s) radar scan rates using the several millisecond response time nematic liquid crystals.