An integrated-optic lx2 directional coupler waveguide modulator is described. The device is a modified version of a conventional directional coupler modulator with only one input and two output waveguides. Unlike conventional modulators, this modulator is self-biased. The half-power 3-dB operating point is automatically achieved without the need of a DC bias voltage. In addition, the lx2 modified directional coupler is more efficient than the conventional directional coupler modulator in terms of lower switching voltage and higher linearity.
External modulation of 1.3 μm laser light with LPE grown InGaAsP/InP buried channel waveguides is presented. The waveguide is compatible with single mode fiber and the planar structure has potential for monolithic integration. In this paper, a numerical analysis of electroabsorption (EA) modulation for the waveguide modulator shows interesting results which may help the design of this device.
We report the development of theoretical and experimental study of the transient behaviors of picosecond photoconductivity. A simple photoconductor with conventional neutrality analysis is described and stripline electrodes are used for high speed operation1. The carrier transport mechanism in a photodiode is modeled, which is in a non-neutrality condition2. This leads into the analysis of the high field, high speed transient behaviors of picosecond photoconductivity by solving the transport equation in its original form without neutrality approximation. Indeed, the photogenerated carrier are found to travel in opposite directions. Future analysis will be emphasized on interaction of this behavior with external circuits.
A lossy nonlinear directional coupler is studied numerically. It consists of two lossless planar optical waveguides coupled through a lossy multiple quantum well nonlinear medium. GaAs-based material parameters were used for the analysis, and the input/output optical transfer characteristics are presented here.
Transmission of a 6 GHz, 8-phase phase-shift-keyed (8Φ-PSK) signal over a 12.5 km single-mode fiber in the 1.3 μm wavelength region with a 5 dB power margin was demonstrated for the first time. A digital modem operating at 26 Msymbol/s (78 Mbit/s), a high speed' multimode InGaAsP laser diode, and a high speed PIN diode were used in this fiber-optic transmission system. Floor characteristics and a power penalty observed in the system bit error rate performance were found to be caused by the intensity noise of the laser diode, particularly the reflection-induced intensity noise.
The performance of a commercially available 1.3μm, single mode fiber-optic link is characterized in terms of parameters relevant to microwave and RF engineers. In particular, key link performance parameters are evaluated such as insertion loss, input/output VSWR, bandwidth, noise, compression point, intermodulation distortion (IMD) and dynamic range. A description of performance limitation factors is included for each of these parameters.
This paper describes a fiber-optic RF bandpass filter consisting of a multimode fiber cavity. Performance equations are derived and test results are presented. The filter is capable of high Q, low loss operation up to frequencies beyond 10 GHz. It is expected to be useful in a variety of systems applications to reduce noise bandwidths and remove unwanted out-of-band signals.
This paper discusses the character-istics required of optical lasers if they are to satisfactorily generate microwave R.F. signals. Two applications are considered, an airborne pulse doppler radar and a radar warning receiver. The main feature of each application is the necessity or desirability of generating the master R.F. signal at relatively large distances from the antenna. It is shown that the state of the art of semiconductor lasers is not equal to the requirements imposed by these applications.
Coherent mixing of optical radiation from a tunable CW ring dye laser and a stablized HeNe laser was used to inject broadband microwave signals into GaAs MESFETs (FETs), AlGaAs-GaAs HEMTs and monolithic FET amplifiers up to 55 GHz. Using this technique, amplification of an optically injected signal at 32 GHz, direct injection locking of a 17 GHz GaAs oscillator, and frequency tuning of a 40 GHz oscillator by laser intensity modulation were demonstrated. Comparison of the millimeter wave frequency generation mechanism, noise sources and system performance with other techniques used for signal synchronization and distributed control application are discussed.
Techniques for generating a 35GHz millimeter-wave signal in a microstrip circuit fabricated on GaAs using one or two lasers locked to sidebands of a single master laser will be presented. Ring resonator circuits have been modelled and characterized for achieving efficient output. Varactor diodes were implemented into the circuit for electronic tuning. Stable laser sources have been developed with frequency stability of 20 to 100 MHz. Strong FM sidebands have been produced with a laser diode.
An interdigitated photoconductor (two terminal device) on GaA1As/GaAs heterostructure was fabricated and tested by an electro-optical sampling technique. Further, the photoresponse of GaA1As/GaAs HEMT (three terminal device) was obtained by illuminating the device with an optical signal modulated up to 8 GHz. Gain-bandwidth product, response time and noise properties of photoconductor and HEMT devices were obtained. Monolithic integration of these photodectors with GaAs microwave devices for optically controlled phased array antenna applications is discussed.
A phased array antenna is a multi-element antenna capable of agile electronic beam forming and steering requiring several hundred high frequency, wide bandwidth, interconnections. Conventional distribution methods using waveguide or coax are impractical because they exhibit high attenuation, limited bandwidth, sensitivity to EMI, temperature drifts, and phase instability. Additionally, for large numbers of antenna elements, the size and weight of such systems make them impossible for large scale implementation on communication satellites. RF signal distribution via fiber optic technology is a potential solution to these phased array antenna problems.
This paper reports on the work in progress at Lockheed Electronics Company in the area of acousto-optic processors for adaptive antenna arrays. The work encompasses both theoretical and hardware implementation of such processors. For demonstration of the basic concept, an optical/electronic brassboard has been built and tested. The system, consisting of a 2-element array with one user and one jamming signal, has been shown to achieve almost 25 dB of nulls in less than 3 microseconds for signals in the L-band.
Two technologies which are potential candidates for distributing control and intelligence information in large phased array antennas, are space feeding and fiber optic corporate feeding. A comparison of the two technologies is presented in both qualitative and quantitative form. The comparison is embodied in the hypothetical cases of a V-band and an L-band spaceborne imaging radar. Size, weight, power and performance impacts of the two feeding methods are contrasted.
An acousto-optic processor which forms synthetic aperture radar images in real-time is described. It employs a space and time integrating architecture to perform the required two dimensional matched filtering operation as a sequence of one dimensional processes. The matched filtering in range is performed on each radar return pulse using the acousto-optic device. The azimuthal matched filtering is performed using a fixed reference mask and a charge-coupled device operating in the time delay and integrate mode. This fixed mask architecture has been modified to include a background subtraction capability to reduce the effects of unwanted bias terms on image quality. The effectiveness of this technique will be analyzed for two different time bandwidth product cases. SAR imagery formed using the real-time optical processor will be presented.
This paper reviews the performance enhancement techniques for wideband acousto-optic (AO) Bragg cells. These techniques include transducer apodization, phased array beam-steering, birefringent phase-matching, and anisotropic acoustic beam confinement.
A new technique is developed for improved efficiency with planar phased array Bragg cells. By the preparation of a groved substrate surface prior to the fabrication of a planar phased array acoustic transducer, the symmetry of the planar phased array structure is removed, As a result, one of the two acoustic first order wavefronts can be suppressed. Bragg diffraction efficiency is improved by approximately a factor of two due to the concentration of acoustic power. Several devices have been fabricated successfully on GaP substrate demonstrating the expected efficiency gain.
We describe a 500 MHz, 2 microsecond time aperture, LiNb03 acousto-optic Bragg cell with minimal acoustic beam spreading and high diffraction efficiency. The cell utilizes an apodized phased array transducer. This device is particularly useful in wide bandwidth acousto-optic processing applications, such as optical correlation, where diffracted optical beam quality is important.
Mercurous halide crystals (Hg2C12, Hg2Br2 and Hg2I2) possess unique acousto-optic properties which include very low acoustic velocities and high figures of merit. These properties make mercurous halide Bragg cells very desirable for various EW signal processing applications which are not possible with other acousto-optic materials. This paper discusses and compares the crystals' properties and optimum operating conditions, and identifies the systems' implications for certain EW signal processing applications. The devices' development status is also discussed.
The use of photorefractive crystals as optically addressed time integrating spatial light modulators is discussed. Photorefractive crystals can be used to implement variable filters in acoustooptic systems, and in particular, applica-tions in signal detection and adaptive signal processing are discussed. The bias-free time integrating property of the crystals is demonstrated.
A polarization sensitivity was observed in the bandwidth and interaction efficiency during the investigation of the SAW acousto-optic (AO) interaction in lithium niobate. It was observed that input light linearly polarized along the propagation direction of the acoustic beam allowed an increased interaction bandwidth when compared with input illumination polarized orthogonal to the acoustic propagation direction. The polarization of the optical beam remained unchanged to within one part in 10,000. Experimental findings show that this polarization sensitivity was parameterized by acoustic wavelength. Results of the wavelength parameterization are reported and comparisons drawn to theoretical work performed in the Johns Hopkins University study funded by Harry Diamond Laboratories.
A double balanced optical mixer is proposed for the measurement of optical spectra. It is shown that the in phase and quadrature components of the signal can be extracted. The image rejection ratio for the device and the temperature coefficient of the figure of merit are estimated. Furthermore, it is shown how current clipped correlator designs can be modified to perform the required signal processing.
Monolithic GaAs detector-amplifier channels and arrays have been fabricated for use in the 0.81 - 0.85 pm wavelength range. The circuits were fabricated using a multilayered GaAs/AlGaAs/GaAs structure grown on semi-insulating GaAs. The AlGaAs layer provided electrical isolation between the upper MESFET layer and the lower n- photoconductor layer. Typical rise times of discrete photoconductors in response to 0.84 μm wavelength optical exci-tation were in the range of 1.5 - 4 ns. Rise and fall times of integrated detector-amplifier single channel circuits of 9 ns and 28 ns, respectively, were observed in response to 0.84 pm optical excitation.
It is desirable to have a set of universal requirements for electronic warfare (EW) receivers so that every receiver designer need only fulfill this one set of requirements. It is very difficult, however, to provide such a set of generic requirements for EW receivers. Not only will different missions require receivers with different specifications , but requirements are also driven by available technology. For example, if a Bragg cell has a bandwidth of 1 GHz, the bandwidth of the complete optical processor (receiver) will be limited to 1 GHz or less. Receiver requirements are also dictated by other subsystems in the EW system. For example, if the digital processor following a receiver can process only 100,000 pulses/second, there is no need to design a receiver to receive more than 100,000 pulses/second because the additional pulses received will either be ignored by the processor or, as is more likely, cause a partial or complete processor failure.
Real-time acousto-optic signal processors produce data at rates beyond the handling capability of microprocessors. In order to examine the issues associated with detection and post-processing in AO processors, we consider the AO spectrum analyzer. Spatial and temporal data rate reduction techniques are described, so that new energy arrivals only are reported. These techniques are used to overcome spatial signal aberrations associated with the detection process, and temporal aberrations such as modulation or ringing pixels. False alarm rate vs. hardware complexity tradeoffs are examined. Also, a constant false alarm rate probabilistic filter is described.
This paper reports on a microwave spectrum analyzer demonstration that processes the RF signals with optical techniques. Signals in the 1 to 18 GHz band are channelized to a resolution of .74 GHz. The channelization is performed by modulating the RF signals onto a laser carrier, and then using two Eschelle diffraction gratings to demultiplex the signals according to frequency. The channelized output is monitored with a linear detector array. The report discusses the demonstration design, configuration and results.
The need to process the ever increasing number of exotic signals simultaneously, is a major motivation to develop efficient Bulk Acoustic Wave (BAW) processors. In devices using active input arrays it is important that the phased transducers not only operate over a wideband but also steer the BAWs efficiently over a large angular span. We demonstrate wideband spurious free beam steering of an input transducer array that is fabricated using dielectric well technology. Operation of BAW spectrum analyzers based on active input arrays is modeled, where various issues of importance such as anisotropy and mode conversion are taken into consideration. Acoustic beam profiles are measured via Bragg scattering of focused laser beams. The experiments demonstrate channelization of RF signals in a LiNb03 device and validate our theoretical diffraction model that is based on Huygen's principle.
The requirements of fielding acousto-optic signal processors for electronic warfare applications place stringent demands on mechanical design considerations. The environments that optical processors are required to operate in can drastically limit their functionality. This paper discusses the design constraints, construction, and performance results of two operationally different acousto-optic signal processors.