This paper describes a lowloss microstrip matching circuit with large bandwidth for connecting an Ortel SL1010 laser diode of nominal impedance of 2 ohms, together with parasitics, to a 50 ohm microstrip line. This technique utilizes a microstrip Chebychev transformer without very wide line widths, to obtain the match at a center frequency of 10.5 Ghz with bandwidth of 9 Ghz, insertion loss of less than 1.5 dB and reflection coefficient of better than -10 dB.
The structure and performance characteristics of a GaAs surface emitting laser (SEL) diode and a 2-D array of SELs is reported. The SEL has a very short optical cavity with a distributed feedback structure, and emits light perpendicular to the wafer surface. The laser shows low threshold current and has an estimated high frequency response of tens of GHz. A 2-D array of SELs with 6 x 16 elements was designed and demonstrated.
This paper describes the theory and experimentation of the amplification of optical signals by damping rate adjustment of Semiconductor Diode Lasers using the facet pumped optical modulation technique., It was found that the modulation damping rate of the probe laser depends on the relative position of the pump laser wavelength with respect to the Fabry Perot resonant wavelength of the probe laser. This effect is a result of the carrier induced index change. It is shown experimentally and theoretically that the effect on the damping rate can be both positive and negative. The positive effect results in a larger damping rate, therefore a decrease in the resonant peak height. The negative effect results in a smaller overall damping rate, therefore an increase in the resonant peak height. It is this increase in the resonant peak height that is of interest, it allows amplification of the modulation frequency of the optical signal near the resonant frequency of the probe laser.
High dynamic range electro-optic modulator (EOM) for wide-band microwave fiber optic link is a system issue. This paper describes EOM sensitivity improvement, nonlinearity reduction fabrication process optimization, and the associated performance requirements of EOM driver, diode laser, photodetector, post-detector amplifier, and measurement equipment.
A novel modulation technique is presented which facilitates the transmission of high frequency, fractional bandwidth
signals in fibers. This approach uses the advantages of modelocked semiconductor lasers and external modulators. Narrow
band (~ 10 GHz) modulation can be readily achieved at high center frequencies. Experimental realization of this concept is
demonstrated with an acoustooptic modulation of modelocked optical pulses from a GaAs laser diode at a center frequency
of 2.05 GHz and a bandwidth of 68.7 MHz.
Fiber optic links can be used in a phased array antenna system to provide interconnections for a variety of digital control signals as well as analog rf signals. In particular, programmable fiber optic delaylines used as rf phase shifters in a beam forming network are examined in this paper.
Chirping characteristics due to the electroabsorption modulation in bulk InGaAsP, InGaAs/GaAs multiple quantum wells, and InGaAs/GaAs superlattice is accessed in terms of the measured electroabsorption spectra and the corresponding electrorefraction spectra calculated from the Kramers-Kronig transform. The optimal designs for intensity modulation and phase modulation of laser light are discussed.
Planar GaAs and GaAs/A103Ga0.7As Schottky barrier photodiodes with high-speed and high-sensitivity have been fabricated and characterized. In order to improves the sensitivity of photo-diodes, surface passivation and antireflection coatings have been performed using various dielectric films, such as Si02, Si3N4, and polyimide. The internal quantum efficiency of 60% to 77% and the responsivity of 0.47 A/W to 0.6 A/W were measured for the wavelength range of 0.5 μm to 0.84 μm. A rise time of 8.5 ps and a 3-dB cutoff frequency of 42 GHz were measured for the GaAs/A10.3Ga0.7As photodiode, whereas 15 ps and 24 GHz were measured for the GaAs photodiode.
Fiber optic links are being considered to transmit digital and analog signals in phased array antenna feed networks in space communications systems. The radiating elements in these arrays will be GaAs monolithic microwave integrated circuits (MMIC's) in numbers ranging from a few hundred to several thousand. If such optical interconnects are to be practical it appears essential that the associated components, including detectors, be monolithically integrated on the same chip as the microwave circuitry. In this paper the general issue of monolithic integration of microwave and optoelectronic components is addressed from the point of view of fabrication technology and compatibility. Particular attention is given to the fabrication technology of various types of GaAs optical detectors that are designed to operate at a wavelength of 830 nm.
Recent developments in linear Charge-Coupled Device (CCD) arrays have been aimed at extending the performance of Acousto-Optic (AO) processors. The results of these efforts have proven to be very successful. This paper will review the various detector devices and performance parameters for AO signal processing systems. A discussion of three arrays under contract development to Harry Diamond Laboratories will be given along with test results of the array developed by Fairchild Weston Systems.
Fiber optic (FO) links offer the promise of providing the microwave system designer with new flexibility and capabilities for use in radar and avionics systems. Results presented in this paper show a significant reduction in link RF insertion loss from the typical 40 dB at X-band (9 GHz) to 7 dB over a 3 dB bandwidth of 800 MHz, while maintaining a signal-to-noise ratio (SNR) of greater than 125 dBc/Hz. This 33 dB improvement can lead the way to many uses of fiber optics in microwave designs, especially for short distance applications, where currently used waveguide or coax cables have low insertion loss. The improved link insertion loss was obtained in a joint internal research and development (IR&D) project of the Ortel Corporation and Hughes Aircraft Company, in which both optical performance and RF narrow bandwidth passive reactive matching improvements were made on the typical Ortel high frequency laser transmitter and photodiode detector receiver units.
Quantitative criteria for assessing the suitability of a fiber optic link for microwave transmission are discussed. Direct and indirect modulation schemes for fiber optics are compared. Some recent developments in the implementation of 18 GHz fiber optic links using lithium niobate modulators are described.
A unique application of a long fiber optic delay line as a radar repeater to improve radar testing capabilities is described. Using a 31.6 kilometer long experimental externally modulated fiber optic link with a DFB laser, we demonstrated the first known generation of 152 microsecond delayed ideal target at X-band (10 GHz) frequencies having the phase stability and signal-to-noise ratio (SNR) needed for testing modern high resolution Doppler radars.
Optical techniques for broadband microwave signal generation and detection have been developed to characterize monolithic microwave integrated circuits (MMICs). Emphasis is on the enhancement of measurement accuracy and the identification of limitations. De-embedded complex S-parameters are derived from time-domain data obtained from both a GaAs photoconductive switch and electro-optic sampling of a Ka-band MMIC power amplifier. These parameters are directly compared with those measured from a network analyzer of the same circuit.
Techniques for generating microwave/millimeter-wave signal in a monolithic resonant ring circuit are described. The microwave signal can be generated by focusing modulated light into a photoconductive microstrip gap or by mixing two sideband locked lasers at the photodiode. Varactor and PIN diodes can be integrated inside the ring for frequency tuning and power switching.
Direct second harmonic extraction up to 5.8 GHz using an electronic injection-locked oscillator is demonstrated experimentally for timing synchronization in Gbit/s modified-Manchester-coded (MMC) fiber optic transmission system. We show, by theoretical modeling and experimental results, the optimization of the second harmonic power in the received data and present the receiver eye diagram with the regenerated synchronization signal.
Research in the use of guided wave optics for processing of wideband signals is reviewed, and advantages and limitations of the technology are discussed. The signal processors employ electrooptic and fiber optic components to perform such functions as spectral analysis of radio-frequency signals, correlation and matched filtering, code and waveform synthesis, signal delay and storage, and analog-to-digital and digital-to-analog conversion. In most cases the guided-wave approach is distinguished by the ability to perform a particular function at very high analog bandwidths or digital data rates. In concluding remarks, an effort is made to provide some perspective on competing technologies and to indicate some areas where future research might prove fruitful.
We present novel concepts for the formation of broadband beams for phased arrays. These concepts involve the use of multichannel acoustooptic devices which serve as variable delay lines. A photorefractive crystal is used to increase the optical efficiency of the beamforming task.
Large aperture phased array antennas operating at millimeter wave frequencies are designed for space-based communications and imaging platforms. Array elements are comprised of active T/R modules which are linked to the central processing unit through high-speed fiberoptic networks. The system architecture satisfying system requirements at millimeter wave frerquency is T/R level data mixing where data and frequency reference signals are distributed independently before mixing at the T/R modules. This paper demonstrates design procedures of a low loss high-speed fiberoptic link used for transmission of data signals over 600-900MHz bandwidth inside satellite. The FO link is characterized for transmission of analog and digital data. A dynamic range of 79dB/MHz was measured for analog data over the bandwidth. On the other hand, for bursted SMSK satellite traffic at 220Mbps rates BER of 2x10-7 was measured for Eb/No of 14.3dB.
A new architecture for optically controlled phased array antenna, providing true time delays, is described. Heterodyne generation of the microwave signal is analyzed, especially with regard to spectral bandwidth and signal to noise ratio. Some of the requirements of optoelectronic devices implemented in the architecture (laser, spatial light modulators, detectors,...) are discussed. Experimental set up and results are presented.
An acousto-optic multichannel programmable time-delay device using a multichannel Bragg cell is proposed. The key is to illuminate the Bragg cell with a narrow optical beam. Absolute time delays can be changed by varying the distance between the optical beam and the transducer plane, while differential delays across the channels can be obtained by rotating the optical beam about its optical axis. Various ways for embodiment of the concept are given for both discrete and continuous delays. Experimental verification has been carried out with both direct detection and heterodyne detection. Delays ranging from a fraction of a nanosecond to a few microseconds are possible. The delay line device can be used in many broadband applications in communications, radar, and signal processing systems.
This paper reports the progress on the development of an interferometric acousto-optic receiver. The receiver is designed to receive radio frequency (RF) signals as input and generate digital words as output which includes frequency, pulse amplitude, pulse width and time of arrival (TOA). The system consists of the following basic building blocks: the optical bench, the RF reference, the IF portion and the digital encoder. This paper presents alternative designs and corresponding receiver performance. This paper also describes some of the problems and suggests possible solutions to improve the performance.
The large time-bandwidth products of acousto-optic spectrum analyzers require dedicated post-processing hardware for real-time signal parameter estimation with low false alarm rates. Parameters of interest include center frequency, time-of-arrival, time-of-departure, and amplitude. Maximum likelihood parameter estimation can be performed with a spectrogram correlation process, which is a 2-d matched filter operating on the time-frequency history of the spectrum analyzer. The filter is matched to the known covariance of the signal. A data collection board was constructed to allow capture of 256 channels at a readout rate of up to 15 μS. This data was used to examine sub-optimal implementations of the 2-d matched filter. A high-accuracy 2-d peak detection algorithm was developed to assess the filter performance. Experimental results are presented, and design tradeoffs are shown to exist between filter complexity and accuracy of parameter estimation.
This paper describes an acousto-optic (AO) receiver for frequency sorting and direction finding (DF) measurement. A novel concept of the cueing system is described that provides high resolution signal analysis covering a wide instantaneous bandwidth. Two critical components for the cued DF receiver are AO Bragg cells and Bulk Acoustic Wave (BAW) delay lines. Recent experimental results of these two devices are reported.
The first demonstration of acousto-optical excision using a GaAs photodetector array is reported. A maximum isolation of 40 dB was obtained using a six element GaAs interdigitated finger heterojunction photoconductor array. The center frequency was 45 MHz, and the swept bandwidth was 3 MHz. The response of discrete GaAs heterojunction photoconductors was linear with bias voltage potentially making them useful for performing multiplication functions in optical analog filters. Concepts of using these photoconductors for optical adaptive filters are discussed.