Search

Future communication systems will require high data rates and flexible modulation. Direct optical phase modulation of two microchip lasers by information-bearing signals allows for high-rate delivery via fiber to a basestation. At the basestation, the coherent optical signals are combined with a reference in a photodetector to produce a microwave/millimeter-wave carrier with arbitrary M-ary quadrature amplitude modulation, which can then be transmitted over a wireless channel. Rapid tuning of the microwave/millimeter-wave carrier, the modulation scheme, and the data rate is achievable through this method with no fixed oscillators at the basestation, thus providing for flexible architectures. Results show a high-quality carrier and, for 4- and 16-QAM, with data rates to 200 Mb/s. Extensions to higher data rates are discussed. Index Terms--Broad-band wireless, coherent fiber-optic link, fiber radio, optical vector modulation (OVM).

Described in this paper are the photoresponse characteristics of microwave transistors, both unipolar [metal-semiconductor FET's (MESFET's) and modulation-doped FET's (MODFET's)] and bipolar [heterojunction bipolar transistors (HBT's)]. Investigation includes time- and frequency-domain measurements. For unipolar devices FET's, the two dominant photodetection mechanisms, photoconductive and photovoltaic, are clearly identified within the same device for the first time. It is shown that even high-speed FET's are limited to a photonic bandwidth of a few megahertz, if photodetection and amplification are to be achieved simultaneously. In contrast, bipolars HBT's can provide optical gain up to the millimeter-wave range. It is shown that their bandwidth to a modulated optical input is closely related to the microwave bandwidth, and that parameters such as base-access resistance and base-emitter capacitance are critical to photoresponse optimization. Index Terms - Heterojunction bipolar transistors, microwave transistors, optoelectronic devices, photodetectors, phototransistors, transient analysis.

This paper concerns a novel hybrid lidar-radar system for underwater surveillance. Simulations and laboratory measurements incorporating the hybrid detection scheme revealed a 17-dB suppression of the water backscatter signal (clutter) and corresponding target contrast enhancement. These results led to the design, implementation and testing of a lidar-radar system for ocean experimentation. Details of this system, in addition to the goals and results of the ocean experiment, are presented.

The use of microwave high-electron mobility transistors (HEMT's) as photodetectors or optically controlled circuit elements have attracted interest. A model of the optical characteristics of HEMT's, which takes into account carrier transport as well as the quantum mechanical nature of the two-dimensional (2-D) electron gas channel, is presented. It is shown that the effect of illumination is equivalent to a shift in the gate to source bias voltage, referred to as the internal photovoltaic effect. The theoretical model is supported by experimental results that demonstrate that the HEMT photoresponse is a nonlinear function of light intensity with very high responsivity at low optical power levels.

Since microwaves do not penetrate water, RADAR, the principal tool for remote sensing of the earth and atmosphere, cannot be used directly for the detection of underwater objects. Currently, aerial light detecting and ranging (LIDAR) systems are therefore preferred for the detection and ranging of objects submerged in the sea. LIDAR provides for large area coverage at high speed, but it lacks coherent detection capability, a shortcoming that severely limits system sensitivity and underwater target contrast. In response to this problem, this paper details the merging of RADAR and LIDAR technologies in the constitution of a hybrid LIDAR-RADAR detection scheme. This new sensor configuration has reduced incoherent backscatter clutter by 17 dB in laboratory experiments and related computer simulations.

A model for the mechanism of negative photoresponse, namely, the decrease of drain current under illumination, in AlGaAs/GaAs MODFET's is presented. Also, a comprehensive experimental study discussing the dependence of this phenomena on gate and drain to source bias voltages and optical power, as well as a comparison with devices that show the usual positive photoresponse, are reported. The negative photoresponse is attributed to trapping of photogenerated carriers in the GaAs buffer layer, causing a change in the potential profile and consequent reduction in the number of carriers in the 2-DEG channel. The above theory is supported by numerical solution of Poisson's and electron continuity equation, using the finite-elements method. Finally, the implications of the negative photoresponse on the high-speed photodetection properties of MODFET's devices are discussed.

In recent years, the usefulness of the MESFET as an optically sensitive microwave element on MMIC's has generated much interest. A theoretical model for the device under steady illumination has been developed previously by the authors. This paper presents an extension of that model to include sinusoidally modulated illumination up to the microwave range. The dependance of the response on the bias conditions, the wavelength, intensity and modulation frequency of the optical input, and the particulars of device structure are incorporated in the model. The importance of the internal photovoltaic effect, not properly accounted for in previous works, is emphasized. The theoretical model is validated by experimental results.

Analysis and systematic design of uniform phased arrays with subarrays of unequal sizes are absent from the literature, in spite of the appeal of such architectures to wide-bandwidth beam steering. Unequally sized subarrays are expected to outperform identical, contiguous subarrays in terms of array grating lobes. A uniform linear array that is divided into contiguous subarrays of random sizes is studied here. Each array element is connected to a phase shifter, and each subarray is implemented with a common time delayer. Closed-form expressions for the average array factor and the variance of the array factor are developed. These expressions enable the formulation of a design procedure that includes the estimation of the peak-grating-lobe level in terms of the probability that the grating lobes will exceed a desired level.

One of the aspects of the merging of microwave and optical technologies is the use of optical signals to switch electronic circuits in general and microwave circuits in particular (including MMIC). During the last decade, the feasibility of optical switching of MMIC's has been demonstrated. This paper presents a novel model for the optical switching of the MESFET, which is the building block of MMIC's. The model predicts the optical switching performance as a function of the optical signal parameters, the bias level, and the device physics and geometry. The results and conclusions from the theory are verified by measurements. The new model can serve as a design tool for designing an optimal MESFET for optical switching purposes.

The MESFET as an optically sensitive microwave element on MMIC's has attracted much attention. The theoretical modelling of the device, however, needs more consideration. This paper proposes an analytical model for the illuminated MESFET, complete in that all major contributions to the optical response are considered. The dependence of the response on bias conditions, the wavelength and intensity of the optical input, and the particulars of device structure, are incorporated in the model. The importance of the internal photovoltaic effect, which has not been properly modelled previously, is emphasized. The novel theoretical model is verified by experimental results.

A study of two methods for decreasing the harmonic and intermodulation distortion in broadband fiber-optic television transmission networks is presented. The analysis covered the enhanced laser modulation linearity approach and the implementation of alternative modulation techniques.It was shown that while the linearity may be improved by 15 dB with an active matching method, the group modulation approach is a better option for multi-channel television transmission.

The photodetection mechanisms in GaAs MESFETs have been investigated by several researchers. Recently the authors have published an in-depth study of the MESFET as an optical detector under constant illumination involving both experimental and theoretical modelling. In this paper we discuss the compressive nature of that photodetection process. Experimental results involving constant illumination, modulated light and pulsed illumination verify the theoretical conclusions. Finally we present a suggested structure of an RF logarithmic amplifier based on the above phenomenon.

This paper reports on the optical generation of a rapidly tunable millimeter-wave subcarrier for lidar--radar. The millimeter-wave signal is generated by beating the output from two Nd: YV[O.sub.4]/MgO : LiNb[O.sub.3] electrooptical monomode microchip laser sections realized monolithically in a single composite crystal. The device has a continuous tuning range up to 45 GHz. The measured chirp rate is 3816 THz/s, the voltage sensitivity is 10.6 MHz/V, and the measured residual phase noise is -106 dBc/Hz at 10-kHz offset. Index Terms--Electrooptic modulation, fiber radio, frequency-chirped lidar--radar, microchip laser, optical heterodyning.

The GaAs metal--semiconductor-metal (MSM) device is a very useful planar and monolithic-microwave integrated-circuit compatible photodetector and electron-detector. As a photodetector, the MSM has been used for many applications in the past, however, in this paper we demonstrate its usefulness as an electron-beam detector as well. We present here a comprehensive analysis of the primary detection mechanism (electric field enhanced collection of generated electrons) as well as a newly identified secondary mechanism. This new mechanism is characterized by a high detection gain, but low speed. Experimental results are presented to verify the analysis, and possible applications are suggested by utilizing each one of the two detection mechanisms. Index Terms--Electron detector, GaAs, MSM device, photo detector, photo tubes.

This paper is concerned with the optical domain generation of high-quality millimeter-wave signals for fiber-radio and other applications. The mode-locked millimeter-wave optical transmitter described here is based on simple electrooptic microchip laser technology. The transmitter can be designed to operate from a few gigahertz to 100 GHz and beyond. The residual phase noise of the laser is below -100 dBc/Hz at 1-kHz offset, which makes it well suited for optically fed millimeter-wave wireless applications. A key feature of the transmitter is its simplicity, the very small number of elements it employs and the high level of integration of the millimeter-wave and photonic components that results in a small, rugged, and reliable package. The paper describes the design, fabrication, and experimental evaluation of the transmitter. Index Terms--Fiber radio, microchip lasers, millimeter-wave generation, mode-locked lasers, optical fiber communication, optical transmitters.

A novel optical frequency down-conversion approach employing the quadratic electrooptic effect is proposed, analyzed, and verified. This down-conversion method can seamlessly integrate with an optical phase-locked-loop linear phase demodulator. It has the potential to achieve a large dynamic range and low conversion loss. Comprehensive theoretical studies are performed to investigate the noise, efficiency, and linearity of this down-conversion approach. Experiments are then conducted. It is found that the unwanted fourth-order electrooptic effect of the InP modulator device limits the dynamic range. [ABSTRACT FROM AUTHOR]

Using the Maxwell-Bloch equations, we have studied dynamics associated with an electro-optically tunable laser modulated by a sinusoidal signal with a frequency near the cavity free spectral range. We find that frequency modulation (FM) modelocking may occur when a higher order FM mode acquires the lowest threshold. For short-cavity lasers, the pulse waveform in modelocking is complicated, and unlike the simple Gaussian shape suggested by previous studies. In order to explain instability observed in laser FM operation, we develop a linear stability analysis for the lasing FM mode. Our analysis shows that this type of instability is attributed to mode competition and to laser relaxation dynamics. We then confirm the analytic results with numerical simulations. [ABSTRACT FROM AUTHOR]