Future space-based platforms can and will benefit from the implementation of photonics in both analog and digital subsystems. This paper will discuss potential applications and advantages to the platforms through the use of photonics.

A new approach for linearizing electroabsorption modulators (EAM) is presented here. It will be very useful for broadband, analog, radio frequency fiber-optic links. In this approach, two or more EAM will be fabricated in series on the same waveguide. Theoretical analysis shows that cancellation of both the second order and third order harmonic distortions can be obtained simultaneously by using proper electrode length and by biasing appropriately the electrodes, leading to an enhancement of the multi-octave spurious free dynamic range (SFDR). Simulation shows that the SFDR of the optical link for the two-electrode case is 10 dB larger than that for a single electrode case. Experimental confirmation of the theoretical prediction on distortion reduction is also presented.

The best RF link gain and the multi-octave SFDR are obtained when the semiconductor EA modulator is biased at the point where the photocurrent has maximum slope efficiency. Since the maximum slope efficiency corresponds directly to the maximum slope of the photocurrent, the photocurrent output is monitored as the applied bias voltage is swept¹. The voltage at which the modulator photocurrent changes fastest (highest slope) corresponds to the optimum bias point for maximum link gain. During the sweep, the photocurrent output from the modulator is amplified and sent to a microcontroller. The microcontroller records the photocurrent during the sweep and determines the optimum bias. This is preferable to the common way of determining maximum RF gain by tapping off a portion of the optical power because it does not induce optical loss or require additional optical components. This paper will present the design and testing of a module that will stand alone and apply the optimum dc bias for a modulator automatically.

High dynamic range is achieved in the optical domain with our technique thereby providing multi-gigahertz modulation bandwidth. Early measurements show an improvement exceeding 24 dB in third-order intermodulation distortion over that of a single, unlinearized Mach- Zehnder optical modulator. Using this approach, practical measurements at 500 MHz show a spurious-free dynamic range (SFDR) of 105 dB Hz^2/3, where the noise floor spectral density is -141.3 dBm/Hz measured at the optical link output.

Analog-to-digital converters (ADCs) are an essential component of digital receiver systems. Progress at advancing the electronic ADC modules has been very slow due in large part to the difficulties in fabricating the electronic circuitry required for very high resolution and high sampling rate converters. This slow progress has resulted in a bottleneck between the received analog signal and the digital signal processing system. Single or multiple analog signal down conversion stages are required in digital receivers to down convert the received analog signal to an intermediate frequency (IF) that can be processed by the electronic ADC. There has been much recent interest in the use of photonics for direct digitization of the analog signal at the received RF frequency thus eliminating the need for analog down conversion. This paper reviews some of the recent research advancements in photonic ADCs. We will especially focus on the development of a novel photonic ADC module that uses semiconductor saturable absorbers to perform the data quantization. We will also present recent results in the development of a mode-locked fiber laser used as the sampling source in this photonic ADC architecture.

We report on free-space photonic switches that are based on switched volume holographic diffraction grating for RF and telecommunications applications. For example, a cascade of n independently controlled electro-optic (EO) gratings can be configured in a binary tree structure to route a free-space optical carrier to on of 2ⁿ possible output channels. These gratings are comprised of liquid crystal composite materials, which exhibit low loss, high speed, and high switching contrast. Thus, this switching technology enables the construction of compact photonic switching systems with the potential for low insertion loss and low crosstalk. In this paper we describe grating based systems that are fanout capable, due to the inherent analog nature of the switched grating materials. Finally, we present the performance characteristics of the monolithic photonic switching systems constructed from arrays of switched gratings.

When asked to demonstrate the performance of fiber optics engineers often resort to hundreds of thousand dollars worth of test equipment including plotters and displays to show a screen view and to prove the viability of the photonic system. This paper will present a relatively inexpensive and unusual method of allowing visitors to see and hear operation of a microwave link versus that of a coaxial coupled RF link.

A fully automatic semiconductor-laser based Optical Phase Locked Loop (OPLL) subsystem has been developed using a packaged OPLL module and custom electronic circuits. The OPLL module contains a DFB laser and a multi-section tuneable laser operating at 1500 nm. The custom circuits perform the operations required to control the bias currents and temperatures of the lasers. When the subsystem is activated, the emission frequency of one of the lasers is slowly tuned to match the other laser emission frequency, thus beginning the locking process. A fibre coupler taps off a small part of the OPLL module output and detects the laser beat signal in an optical receiver. When this frequency matches that of the input LO signal to the OPLL module, the circuit detects the locked state and ensures that it is maintained. This paper details the design and implementation of the OPLL control and acquisition circuits, together with the performance of the OPLL subsystem.

This paper describes the design and performance of a 1:16 GaAs/A1GaAs photonics integrated circuit (PIC) for implementing the beamforming function in an optically controlled phased-array system. The PIC includes independent phase and amplitude control for all 16 channels and is flip-flop bonded onto a GaAs carrier which contains all the electrical track routing, thus leaving greater space on the PIC for electro-optic interaction and also avoiding crossovers between the optical waveguides and electrical tracks.

The authors present a monolithically integrated photodetector array combiner approach that operates over an extremely wide RF bandwidth. These arrays are suitable for coherent RF signal combining applications such as optically controlled phased array radar. The approach consisted of a monolithically integrated array of high-power, wideband photodetectors distributed along an RF transmission line resulting in a low power consumption, broadband, high power handling optical-to-RF combiner, The authors demonstrated a 4-element photodetector array with a small-signal 3dB bandwidth of 34 GHz. RF models were developed and calibrated to the measured results to predict the performance of larger arrays. The models predict than an 8-, 16-, and 48-element array would have a small-signal 3dB bandwidth of 25 Ghz, 15.7GHz, and 5.8GHz respectively. These arrays showed a reasonable amount of robustness to variations and/or errors in the time delays of the input optical feed network suggesting that implementation outside the laboratory should be practical.

Millimeter wave phased array systems have antenna element sizes and spacings similar to MMIC chip dimensions by virtue of the operating wavelength. Designing modules in traditional planar packaing techniques are therefore difficult to implement. An advantageous way to maintain a small module footprint compatible with Ka-Band and high frequency systems is to take advantage of two leading edge technologies, opto- electronic integrated circuits (OEICs) and multilevel packaging technology. Under a Phase II SBIR these technologies are combined to form photonic modules for optically controlled millimeter wave phased array antennas. The proposed module, consisting of an OEIC integrated with a planar antenna array will operate on the 40GHz region. The OEIC consists of an InP based dual-depletion PIN photodetector and distributed amplifier. The multi-level module will be fabricated using an enhanced circuit processing thick film process. Since the modules are batch fabricated using an enhanced circuit processing thick film process. Since the modules are batch fabricated, using standard commercial processes, it has the potential to be low cost while maintaining high performance, impacting both military and commercial communications systems.

We generated a low phase noise 16 GHz electrical signal of 20 Hz linewidth from a single 1 MHz linewidth diode laser. At 36 Ghz, our frequency angle mm-wave generator produced an electrical signal with an instrument resolution limited linewidth of < 1 kHz. Our approach uses a relatively low frequency (4-8 GHz) phase modulator to generate a multiline sideband spectrum from a single mode laser, which is filtered to pass or select a pair of spectral lines. Interference between the lines on the active area of a photodetector produces an electrical signal that can be set between 4 and 60 Ghz. B y controlling the phase modulator frequency and amplitude we set the sideband frequency spacing between 4 and 8 Ghz, and efficiently couple optical power into sidebands up to the fourth order. Rapid transitioning between sideband spectra best suited for a particular mm-wave frequency could be made in a few milliseconds using a programmable synthesizer and RF amplifier. Two- sideband selection was obtained by splitting the signal between two legs of a filter network and using ultra-narrowband fiber Bragg grating (FBG) fiber filters to select single lines. We controlled the fiber filter strain to tune the filters rapidly, setting the tension using PZT actuators. The generator prototype was packaged as a pair of portable twin rack modules under PC control.

Erbium-doped thin-film waveguides are promising as loss-compensating devices for various photonic integrated circuits. We have addressed major challenges in the development of lossless splitters based on the Er-doped glass films: the requirement of high Er concentrations, the fabrication of quality waveguides, and the development of suitable architectures for beamsplitting and for signal and pump combining and separating, and the limitations of maximum-obtainable gain of Er-doped thin-film amplifiers. Based on these results, we have successfully developed integrated-optic, lossless splitters. The fabricated devices clearly demonstrate lossless transmission of an optical signal at 1.54 (mu) m wavelength through a 1x2 splitter and an Er-doped amplifier that are monolithically integrated on the same substrate.

The demand for higher data capacity and reduced levels of interference in the communications arena are driving dtat links toward high carrier frequencies and wider modulation bandwidths. Circuitry for performing intermediate frequency processing over these more demanding ranges is needed to provide complex signal processing. We have demonstrated photonics technologies utilizing Bragg Grating Signal Processing (BGSP), which can be used to perform a variety of RF filter functions. The desirable benefits of multiple-tap adaptive finite impulse response (FIR) filters, infinite impulse response (IIR) filters, and equalizers are well known; however, they are usually the province of digital signal processing and demand preprocessor sample rates that require high system power consumption. BGSPs provide these functions with discrete optical taps and digital controls while only requiring bandwidths easily provided by conventional RF circuitry. This is because the actual signal processing of the large information bandwidths is performed in the optical regime, while control functions are performed at RF frequencies compatible with integrated circuit technologies. To realize the performance benefits of photonic processing, the Bragg grating reflectors must be stabilized against environmental without unduly taxing the RF control circuitry. We have implemented a orthogonally coded tap modulation technique which stabilizes the transfer function of the signal processor and enables significant adaptive IF signal processing to be obtained with very low size, weight, and power. Our demonstration of a photonic proof-of-concept architecture is a reconfigurable, multiple-tap FIR filter that is dynamically controlled to implement low-pass, high-pass, band-pass, band-stop, and tunable filters operating over bandwidths of 3 Ghz.

As the frequencies and bandwidths in military RF systems escalate, we enter a realm where photonic technologies can play an important role in signal transmission and processing. Since it is very difficult to process large data bandwidths, either tunable or instantaneous, using conventional electronics, conventional receiver electronics becomes a bandwidth bottleneck, thus development is timely for optical processing of wideband signals. A review of critical mission requirements and photonics capabilities reveals a variety of military missions that can benefit immediately or in the near future from the advantages offered by photonics. In this paper we will discuss the implementation of an Integrated Sensor System (ISS) that is greatly simplified through the use of a coherent optical channelizer. In this system the received RF signal, which has been modulated onto an optical carrier, is routed via an all-optical switch to the coherent optical channelizer. A signal channelizer has the capability to process signals at frequencies of up to 100 Ghz and translate all the frequency channels to a convenient I.F. That is compatible with advanced digital receivers currently under development. The channelizer is also used on the transmit side of the system where a signal waveform is electronically synthesized at a convenient I.F. then photonically translated to a desired frequency band and transmitted to the aperture over fiber.

In recent years, many examples have been cited where photonic signal procesors have the potential of enabling the deployment of wideband RF sensor systems whose data rates would otherwise overwhelm the capabilities of traditional digital signal processors. Passive, synthetic aperture millimeter-wave (MMW) imaging arrays are one such class of sensor system that offers a solution to the need for a passive, all-weather look-down sensor for critical DoD and environmental missions. A photonic method of correlating the signals received by the various elements of the array offers a highly efficient, exceptionally wideband capability, exploiting a powerful new technology for aperture- synthesis imaging. Our trade study has helped to establish that a photonically based interferometric imager can significantly reduce the weight and power consumption of the system relative to an all-digital- electronics approach. In addition, the ability of an airborne or space based system to process multi-gigahertz bandwidths offers the first real-time MMW imager operating at video rates. Previous demonstrations of optical correlators have used discrete components, and generate only the real part of the correlation. Using integrated optical waveguide technology developed in part fof the telecommunications industry, we have developed an approach for computing complex correlations over wide bandwidths and in such a way to allow video rate imaging for passive MMW systems. We will describe a general processing architecture, its capability, and our trade study results to provide a motivation for the future development of these types of systems.

Acousto-optic spectrum analyzers (AOSA) are very convenient devices in order to provide the radio air panoramic observation. They can represent the pattern of radio air situation in the way which is the most suitable for direct observation. The parallel data processing which is inherent to AOSA allows to increase the processing rate significantly in comparison with other kinds of panoramic radio receivers. The possibility to use different kinds of AOSA (including analyzers with space, time and composed space/time integration) have been considered, and the information processing rates of each kind have been analyzed from the point of view of the system optical information characteristics providing. The reasons of these parameters increasing limitation have been studied. On a level with calculations, the results of experimental implementation of AOSA-based panoramic receiver have been presented. We have used the space integration AOSA based on Bragg cell designed with tellurium dioxide single crystal with simultaneous analysis of 10MHz sub-band. The results of using of such panoramic receiver in the real system of radio air control have also been discussed.

Acousto-optic tunable filters (AOTF) transmit and process information which modulates wavelength of the light passed through the device. AOTF information possibilities can be described by specific parameters such as information transmission capability. The reasons of this parameter limitations have been analyzed. Among the factors influencing the AOTF transmission capability, such factors as size and form of initial light source as well as features of the medium modulaing the light beam by wavelength, can be considered very important. Theoretical considerations clarifying the sources of information losses in AOTF, have been stated. A set of experiments has been performed in which the dependences of AOTF information characteristics on the system configuration have been measured. The studied devices were designed with tellurium dioxide single crystals in the configuration providing the best recognition of adjacent wavelengths. The possibilities of AOTF information optimization have been discussed.

Analog-to-digital converters operating at unprecedented speeds and resolutions are presently under development using a combination of photonics and electronics techniques. These systems impose stringent performance constraints on the photoreceivers used for photonic - electronic conversion, particularly in regard to linearity and noise. Photodetectors must accommodate optical pulses with very high input powers iwthout saturation, and the pulse input energy must be accurately determined. This paper presents design considerations and simulations of photodetectors and associated preamplifiers to meet these goals.