Recent performance improvements in analog optical links have prompted us to investigate analytically the limits to link performance. In this paper these limits are derived and compared to current state-of-the-art link performance.
We review our progress in the demonstration of photonically controlled phased arrays. In particular, we emphasize the impact made by photonic phased arrays steered via true-time- delay. We describe, in addition, a photonic beamforming architecture that combines RF-heterodyning with a Rotman lines feed for multiple beam formation.
Electro-optic modulators have been widely used in high power, large link budget fiber-optic transmitters for both RF and digital signal transmission applications. The recent development of high performance electro-optic polymers has demonstrated a promising future for these materials in the fabrication of a new generation of electro-optic modulators, switches, switch arrays and other integrated optic devices. The low dielectric constant, flexibility in device processing, compatibility with semiconductor technology, and potentially very large electro-optic coefficient are very attractive features for wide-band, high efficiency waveguide devices. To investigate the feasibility of using the new class of materials in electro-optic modulator and switch applications, we have fabricated both Mach Zehnder and straight channel electro-optic modulators and several types of optical switches using electro-optic polymers. The polymer layers are thermally relating to the system performance, such as half-wave voltage, RF bandwidth, photochemical stability, bias voltage stability, and thermal stability are tested for multiband RF photonic transmission systems and network switching applications. Optical push- pull techniques allow reduction in halfwave voltage, and halfwave voltages of 1V or less appear feasible in very broadband polymer devices.
AlGaN has been in the spotlight lately for its use in blue to UV lasers and LEDs, tolerances to high temperature/harsh environment, high power applications, etc. Little has been examined in the area of electro-optic (EO) modulation and waveguiding. This paper compiles the optical and EO properties of AlGaN. It also models the behavior of AlGaN integrated optic waveguides and switches based on the zero- gap EO directional coupler. Among the advantages of AlGaN integrated optic waveguides and switches is the potential to work at wavelengths for IR to the blue region. Additionally, GaN has a close refractive index match, to glass and polymer optical fibers and waveguides. Preliminary findings suggest that the size of AlGaN EO directional couplers, based on measured nonlinear properties of the material and, assuming a viable PIN structure, could be in the range of approximately 7000 micrometers , operating at TTL voltages, with potentially high modulation switching speeds.
A new scheme for a nonlinear optical (NLO) polymer opto- electronic (OE) modulators/switch is presented that renders the potential to realize a device interaction length that is an order of magnitude shorter than conventional NLO polymer OE devices operating at TTL voltage levels. It utilizes available NLO polymer materials for the corelayer and conductive polymer materials for the cladding layers in order to realize an effective 1 micrometers separation between the electrodes. Using current NLO polymer materials with electro-optic coefficients of 20 pm/V, it is feasible to demonstrate < 2 mm interaction length OE devices operating at 5 volts. These lengths could lead to insertion within multichip modules. A 1 micrometers gap between electrodes also makes it possible to perform in-situ poling.
We have demonstrated slot vee antenna-coupled electro-optic modulators at 24 GHz. The lost vee antenna design allows for a more robust microwave signal feed into the LiNbO3 chip, where the modulator itself acts as a slab waveguide for the microwave signal. The antenna-coupled design overcomes the velocity mismatch problems inherent in LiNbO3 traveling-wave electro-optic modulators.
A novel npin waveguide structure for the dual-function electroabsorption modulator/detector is proposed and fabricated. With the addition of an n-layer to the conventional pin-structure, the device exhibits phototransistor behavior in the detector model. The device has an InGaAsP intrinsic layer with Franz-Keldysh electroabsorption at 1.3 micrometers wavelength. Preliminary results show optical gain in the detector mode and good modulator characteristics.
In this paper, a compact, high efficiency, adjustable fiber optic true time delay (TTD) line for fast steering phase array antenna bean is proposed. The key features of this approach are: (1) sub-ps time resolution so that high accuracy TTD can be achieved for tens of GHz antenna signals: (2) large number of TTD channels can be addressed due to the tunable nature of our unique electro-optic grating material. An analysis on component parameter selection and system performance are provided.
This paper presents a novel architecture for independently steering broadband nulls for phased array antennas. Measurements taken over a ten percent fractional bandwidth on a three-element proof-of-concept system shows a null depth of better than 40 dB uniform across the entire band in this laboratory setup. The architecture presented is best suited for small antenna array applications, for example, self-guided airborne munitions.In a fully integrated, optimized system, null depths of 50-70 dB or greater across a multi-gigahertz bandwidth are anticipated and the critical factors, which influence this performance, are examined.
A cascade of n independently controlled gratings can be used to route an optical carrier through one of 2 inch evenly spaced time delay paths. The resulting optical systems include digital time shifters for phased arrays with the potential for improving attainable performance in terms of insertion loss, crosstalk, and compactness. We describe results from an effort in which these characteristics of free-space optical time delay system based on switched- volume-diffraction gratings were modeled and investigated experimentally. In one experiment, a 1 by 4 router, which constitutes the front end of a 2-bit photonic time delay circuit, was used to validate the low insertion loss and miniaturization capabilities of this technology. We fabricated electrically switched gratings which demonstrated 20 dB contrast and a response time of 15 microseconds. Realistic loss and crosstalk parameters were used in detailed systems modeling to show that practical system can be built using this technology with very low insertion loss and crosstalk. Various configurations are described, including a multi-pass device that may replace many single channel time shifters with a single optical system.
Optical signal distribution for phased array antennas in communication system is advantageous to designers. By distributing the microwave and millimeter wave signal through optical fiber there is the potential for improved performance and lower weight. In addition when applied to communication satellites this weight saving translates into substantially reduced launch costs. The goal of the Phase I Small Business Innovation Research (SBIR) Program is the development of multi-level photonic modules for phased array antennas. The proposed module with ultimately comprise of a monolithic, InGaAs/InP p-i-n photodetector-p-HEMT power amplifier, opto-electronic integrated circuit, that has 44 GHz bandwidth and output power of 50 mW integrated with a planar antenna. The photodetector will have a high quantum efficiency and will be front-illuminated, thereby improved optical performance. Under Phase I a module was developed using standard MIC technology with a high frequency coaxial feed interconnect.
This paper describes potential designs for a miniaturized Acoust-Optic Radio Frequency signal excision system with adaptive frequency control. It is proposed that diffractive micro-optical techniques can be used to reduce the system size to 1 mm height, 10 mm width, and 24 mm length. These dimensions compare in size to an integrated optics waveguide approach, but the micro-optics design uses optimized discrete components. This small dimension comes at the expense of a relatively low time bandwidth product, where a value of N equals 200 was used.
The ability to remotely radiate microwave signals has become an essential feature of modern electronic counter-measures (ECM) systems. The use of fiber optics allows remote microwave links to be constructed which have very low propagation loss and dispersion, are very flexible and light in weight, and have a high degree of immunity from external electromagnetic fields, crosstalk and environmental effects. This combination of desirable characteristics are very beneficial to avionic ECM antenna remoting as well as many other applications. GEC-Marconi have developed high performance fiber components for use in a towed radar decoy. The resulting rugged and compact optical transmitter and receiver modules have been developed and proven to maintain the required performance over the full hostile range of environmental conditions encountered on a fast jet. Packaged fiber optic links have been produced which can achieve a compression dynamic range of greater than 87 dB in 1 MHz bandwidth over a 2 to 18 GHz.
Wideband microwave/millimeter wave photonic link technology suitable for shipboard RF antenna remoting is presented. High performance 1.3 micrometers laser diode based direct modulation links with bandwidths exceeding 3 GHz, indirect modulation links to 20 GHz using Mach-Zehnder and semiconductor electroabsorption modulators, and frequency conversion links with operation beyond 40 GHz using a series modulator configuration are described. The performance currently attained by the broadband analog links makes them strong candidates to replace copper-based shipboard antenna remoting systems operating in the HF to EHF frequency range. Modulation efficiency improvements required for analog photonic links to approach RF transparency are discussed. The photonic links are suitable for insertion into both transmit and receive single- and multi-element navigation, radar, communication, and electronic warfare antenna systems.
Adaptive beamforming is a technique to suppress the interference coupled to the antenna through the antenna sidelobes and minimize the corruption ofthe signal of interest by means of adaptively tracking the source of interference with the nulls of radiation pattern [1-2]. Adaptivity is vital to a host of mission-critical applications, for which jamming suppression in a radar has been studied extensively . As the time-bandwidth product (TBWP) of the adaptive processor increases, the computational burden rapidly exceeds the capabilities of the state-of-the art integrated microelectronics technology. A computation power of 1013 bps may be required. Insufficient parallelism has limited performance of electronic radar processors to approximately 50 dB jam suppression over the bandwidth of 1 MHz.
In this paper, the concept of a hybrid integrated optical sensor system for frequency selective electric field measurement will be presented. The sensor system will be applicable to E-field measurement problems up to frequencies in the microwave regime. It will provide minimum interference with the measured electric field, since the detected signal is transferred into the optical domain within the sensor head which is connected to the read-out via EMI free optical fibers. The key components within the sensor head area a planar antenna connected to a 1.55 micrometers InGaAs/InAlAs waveguide electro-absorption (EA) modulator via a low-power transimpedance amplifier based on GaAs MESFET technology. In order to avoid interference with the measured electric field, the transimpedance amplifier is powered by optical means using an array of photovoltaic cells on GaAs substrate for high-efficient power conversion at 850 nm wavelength. Based on numerical and experimental result the key components will be discussed and evaluated for the application within the sensor system. Furthermore, a novel and flexible technique for fiber-chip coupling will be presented, employing the integration of InP V-grooves with the waveguide EA-modulator.
Two L-band phase-matched fiber optic delay line channels and a broadband fiber optic RF signal processing filter have been designed, fabricated, tested, and evaluated. These two related RF photonic system development efforts are potentially useful in ELINT signal processing of ultrawideband signal. Specifically, two high performance optical delay lines operating at 1 GHz with a 500 MHz bandwidth have been prototype and show prototyped and show improved dynamic range and environmental phase tracking performance over conventional SAW delay lines. In addition, an eight-tap fiber optic transversal filter using wavelet amplitude weighting has been designed, fabricated, and tested in the 50 MHz to 20 GHz frequency range. A high pass wavelet filter useful for ultrawideband signal detection has been optically implemented, and test result presented for sensitivity and dynamic range are promising.
The majority of the signals encountered in nature ar analog while the preferred method of processing these signals is digital. Digital signal processors provide higher resolution, improved flexibility and functionality, and improved noise immunity over its analog counterparts. As a result, the analog-to-digital, (A/D) interface is generally considered to be the most critical part of any signal acquisition and processing systems. Because of the difficulty in achieving high-resolution and high-speed A/D converters, this A/D interface has been and continues to be a barrier to the realization of high-speed, high-throughput systems. Electronic A/D techniques have been investigated but appear to be limited. (Sigma) (Delta) techniques have been successful in providing high-resolution converters but only for audio frequency signal. Optical approaches have also ben investigated to leverage the wide bandwidth and parallelism of optics but most have been limited by the component linearity, device speed, or dynamic range. We previously proposed and demonstrate an optical approach to A/D conversion based on oversampling techniques using a modification of the (Sigma) (Delta) architecture and multiple quantum well modulators as the optical devices. Here we describe other approaches to oversampling A/D conversion incorporating space-time processing of optical signals leading to ultra-high resolution at very high speed.
The demonstration of RF photonic links with bandwidths of 100 GHz or more is expected in the near future, due to rapid and continuing progress in modulator and detector technology. Since it is very difficult to electronically process such a large bandwidth after photodetection, photonic approaches that reduce the burden on the electronics are increasingly relevant.One such approach is an optical channelizer, where an RF modulator optical carrier is optically dispersed onto a detector array. Each element of the array only sees a portion of the original wideband RF spectrum. We analyze the RF performance of optical channelizers in terms of crosstalk and uniformity of response. Both direct detection and heterodyne channelizers will be considered. Analysis of this kind is necessary for our application, since the usual parameters of a dispersive optical system, such as resolution, resolving power or filter linewidth do not provide enough information to determine the RF performance. The analysis has shown that a Fabry-Perot filter based channelizer cannot provide adequate RF performance, while a grating-based channelizer can. For channelizer to 1 GHz channels with a -70 dB crosstalk specification, a total grating length of roughly 80 cm is required, so a multiple bounce geometry is necessary to obtain a reasonably compact system.
We report the generation of high-energy, low-jitter, and short pulses by gain-switching of a tapered stripe gain- guided laser diode via resonant driving. The application in optically clocked sample-and-hold will be discussed.
A packaged diode-laser OPLL sub-system has been constructed for evaluation in a proof-of-concept coherent optical beamforming system. The loop has been implemented with narrow linewidth laser diodes, micro-optics and wide bandwidth electronics to give optimum phase noise performance. The laser diodes are designed for wide bandwidth and high FM-efficiency, while the main challenge in the construction of the packaged OPLL is the realization of a high gain loop, with a small propagation delay. A total phase variance of 0.05 rad2 has been achieved, and that within the 15 MHz system bandwidth is 0.0007 rad2. The OPLL can operate with LO frequencies from 7-15 GHz. This paper details the performance of the completed OPLL module together with key results for the custom FM lasers designed and fabricated for use within it.
Coherent photonic systems promise novel functionality and/or improved performance compared to direct detection photonic system, but have the disadvantage of being sensitive to optical phase noise. The most common approach to this problem is to force one laser to track the phase of the other with a phase locked loop (PLL), so that the phase noise of the lasers cancels out of the RF heterodyne beat note. Although the PLL approach has been implemented for semiconductor lasers, the large linewidth of these lasers and the resulting large PLL loop bandwidth severely constrain the design and limit performance. This disadvantage of the PLL approach is particularly relevant for many applications, since semiconductor lasers are preferred for system insertion.
We present experimental results of coupled opto-electronic oscillators (COEO) constructed with a semiconductor optical amplifier based ring laser, a semiconductor Fabry-Perot laser, and a semiconductor colliding pulse mode-locked laser. Each COEO can simultaneously generate short optical pulses and spectrally pure RF signals. With these devices, we obtained optical pulses as short as 6 picoseconds and RF signals as high in frequency as 18 GHz with a spectral purity comparable with a HP8561B synthesizer. These experiments demonstrate that COEOs are promising compact sources for generating low jitter optical pulses and low phase noise RF/millimeter wave signals.
High-frequency self-pulsing effects in combined index and gain coupled two-section DFB lasers in a single cavity laser are investigated by numerical simulations. The gain coupling increase the modulation index and frequency of the intensity oscillations. Simulation results show that the signals of oscillation frequencies up to 200 GHz may be possible to generate using such devices. Preliminary experimental results agree well with simulation.
Monolithic integration promises to deliver high-speed, mechanically robust, EMI-immune, low cost, light-weight, miniaturized RF modules for Air Force space-based sensors,and UAV and manned air vehicle sensor applications. This paper reviews several new approaches being investigated in the Air Force Research Laboratory's Photonics Center towards the monolithic integration of optical and electronic devices on silicon to form high-speed RF links and antenna elements. These include a completed in-house project to evaluate GaAsN light-emitting material latticed-matched to silicon, and funded programs to develop low-noise ring laser link sources flip-chip bonded onto silicon and also monolithically grown on GaP on silicon, and InGaAs sources and detectors grown on mis-oriented silicon CMOS via selective epitaxy. A new in-house technology application project is discussed.
The primary focus of photonics for antenna systems has, historically, been on the development of link and beam steering techniques. More recent work is focusing on the design of new types of antenna elements or arrays of elements to take advantage of the advantages in photonics. By using photonically controlled devices and materials and materials it is possible to produce revolutionary changes in antenna elements and in the design and properties of arrays, opening the door for a new class of antennas - photonically controlled reconfigurable antennas. In this paper we survey the history and current status of photonically reconfigurable antennas. This will include the evolution of photonically controlled switches for application in antennas. We look at photonic control of reactive devices and the optically variable capacitor and the evolution of this device towards monolithic integration. Finally, we also will look at the state of photonically reconfigurable silicon and its application to antenna design.