We review current 18 gigahertz optic link technology with a spurious free dynamic range of 110 dB/Hz2/3. The dynamic range of an externally modulated optic link increases with increasing optical power on the photodetector. One of the limiting factors for dynamic range is the amount of optical power that the photodiodes can handle. A method to allow greater optical information sideband power throught the link by filtering out some of the optical carrier will be discussed. The I/O modulator is normally run with an otpcial modulation depth of 4%. It should be possible to increase this by reducing the amount of optical carrier sent to the detector. This increase in the depth of modulation will reduce the RF loss and noise figure of the optic link thereby reducing the preamplifier requirements to obtain maximum dynamic range. This should make possible a spurious free dynamic range of 120 (db/Hz)2/3 with commercially available 18 GHz photodetectors. This technique could also be used to reduce problems caused by stimulated Brillouin scattering in long lengths of SM fiber.
Various optical architectures for N-bit digital control of microwave signals are introduced that use the photoconductive (PC) effect in microwave waveguides for variable rf attenuation control. A 2-bit optically controlled microwave attenuator based on a silicon PC transmission line device is experimentally demonstrated at 990 Mhz. At 532 nm, this attenuator provided 0, 5.8, 11.2, and 15.6 of attenuation levels.
Electroabsorption modulators (EA) are attractive components for very high speed digital links (up to 40 Gbit/s). The objective of the present work has been to evaluate the analog performance and use of a Franz Keldysh modulator (FK) monolithically integrated with a DFB laser (DFB/FK-modulator) operating at 1550 nm. This DFB/FK-modulator is a combination of a directly modulated laser diode and an external modulator in one chip. The analog performance is therefore depending on both the modulator bias voltage and DFB-laser bias current. The normal optical output characteristic from an EA-modulator has a strongly nonlinear behavior. The modulator described in this paper shows for low bias voltage and/or high laser current a linear bahavior. This linear behavior is mainly due to the hole pile-up effect at the p/i-interface of the modulator. In digital transmission system this hole pile-up effect is a disadvantage, but in an analog transmission system it can be used to achieve better analog performance. Measurements (and simulations) on the DFB/FK-modulator show that its analog performance competes well with direct modulated FP and DFB lasers, especially if the modulator is biased for optimum analog performance.
We report a novel straightforward theoretical analysis and improved high frequency experimental performance of a new type resonant photodetector. A large detection volume is provided by a synchronous drift of photogenerated carriers with a moving interference pattern in a photoconductor. It appears to be suitable for optical generation of microwave signals at high power levels. Performances as a function of applied electric field, photocarrier lifetime, fringe period, and optical power level are analyzed. Experimental results using a GaAs photoconductive layer are presented at frequency f equals 1.6 GHz.
The Franz-Keldysh effect is utilized for high-linearity, electroabsorptive InGaAsP waveguide modulators. Two-tome RF measurements are performed in which the DC bias is adjusted to maximize the spurious-free linear dynamic range. With the proper choice of bias, the sub- octave link dynamic range increases to 123 dB in a 1 Hz bandwidth for 0.25 mA photocurrent.
The phase and amplitude stability of directly and externally modulated analog fiber optic links are investigated and factors which limit link stability are discussed. Link gain, phase noise, and phase delay fluctuation measurements have been performed on fiber-optic links which are currently being considered for Navy shipboard antenna remoting and phased array antenna applications. Measurements indicate that good amplitude and adequate phase stability can be obtained using either directly or externally modulated analog photonic links provided certain design considerations are met.
In this paper, theoretical and experimental results for wideband photonic downconversion systems operating from microwave frequencies through millimeter-wave frequencies are presented. The system consists of a low phase-noise optical heterodyne local oscillator (LO) generator derived froma two-frequency diode-pumped Nd:YAG laser, a millimeter-wave Mach-Zehnder modulator, and a high-speed photodiode. The sum and difference frequency products between the optical LO and the input RF signal are generated upon photodetection. An analysis of photonic heterodyne downconversion is presented, and preliminary experimental downconversion results at Ka-band are presented that are in good agreement with the theoretical prediction of 6 dB conversion loss. Due to the high degree of correlation between the phase fluctuations of the laser modes, the phase noise is much lower than that of previous heterodyne sources, which were typically too noisy for many applications. The free- running optical LO has measured phase noise better than L(1 kHz) equals -90 dBc/Hz at X-band, limited by the measuring system. Finally, novel microwave and millimeter-wave system architectures with enhanced performance and flexibility are discussed, and compared to conventional downlink systems employing electronic mixers.
As large factories, chemical plants and oil refineries become more and more automated, it becomes necessary to relay information from a large number of sensors to central control sites. In addition, it has also become important to transmit large amoutns of information in the form of data, voice, video, etc., to workers around the plant or factory. A unique technique based on optical microwave cellular system is described which is highly suitable for such an application.
Analog fiber-optic links can be used as a connecting infrastructure for modern wireless communication systems. For existing wireless networks, which operate at frequencies below 3GHz, the possibility of using low-cost, commercially available optoelectronic components is very attractive. Also, the development of practical millimeter-wave optical transmitters may be critical to the deployment of future high frequency (> 20 GHz) radio distribution systems. The linearity performacne of low-cost microwave, and millimeter-wave optical trnamitters will be assessed for the wireless application.
An overview of towed decoy theory of operation is discussed to develop microwave link requirements. Different fiber optic microwave link approaches including band shifting with wavelength division multiplexing, extenal modulation, and directly modulated lasers are discussed. A military airborne environment is described. Electro-mechanical towline cables are discussed. Future trends for fiber optic microwave links in electronic warfare systems are identified.
Microwave radar array antennas generally employ a beamformer which functions as the interface between the array elements and the transmitter-receiver, distributing signals to the array elements on transmit, and coherently combining the elemental signal components on receive. A conventional beamformer typically consists of corporate feed manifolds comprised of waveguide or coaxial cables, stripline combiner circuits, etc. These manifolds grow increasingly large and heavy in proportion to the number of elements in the array. Optical bemaformers, which utilize manifolds made up of optical fibers, promise a lightweight and very compact alternative approach. In an optoelectronic implementation the electrical signals are first converted to a modulation on an optical carrier frequency, and then the distribution and combining manifolds are configured with optical fibers. This paper compared conventional and optical implementations for a large, dual band (S and UHF) radar array. A detailed dual band beamformer design is developed for each technology and the two designs are compared with respect to weight, volume and power consuption. Both designs meet the same set of dynamic range, noise figure, bandwidth, power output, and sidelobe weighting requirements.
We describe the developemnt and antenna range demonstration of a UHF 8-element optically- powered directly modulated fiberoptic manifold. Active laser diode drive circuits are used to increase the laser diode sensitivity and reduce the link noise figure. The manifold operates over the 340-400 MHz band, it consumes low power (55 mA at 4 V per channel), has a two- tone dynamic range of 69 dB/MHz and a noise figure of 2.4 dB.
Multichannel fiber-optic delay line architectures using optical polarization switching are proposed that use nonpolarization maintaining single-mode fibers while still maintaining high extinction polarization properties of the switched optical beams. Critical birefringence compensation and noise reduction techniques are introduced and demonstrated for these delay lines showing high optical polarization extinction (> 39 dB) and electrical signal-to-noise ratio (> 92 dB) results.
A method to generate and control one or a set of time delays using fiber optic cavities is described. The time delays can be finely controlled over a continuum resulting from a change in the voltage of a single control signal. Architectures using these devices for controlling transmitting and receiving RF arrays are described. The controller can scan an RF array beam rapidly over a continuum of angles. Issues addressed in this paper include the relationship between design parameters of the device and maximum delay achievable, signal attenuation versus delay, and uncertainty in delay, and the switching speed of the RF array beam pointing direction.
We report the demonstration of a 4-bit optoelectronic-switched silica-waveguide time-delay network. Targeted for insertion into a 96-element L-band conformal array, the optical time- shifter provides 16 programmable time-delays in steps of 0.248 nsec. By characterizing its RF insertion phase and synthesized pulse response, we verified that the relative time-delays generated by the waveguides were within 15 psec of their designed value. The antenna patterns obtained with the waveguide-module steering the central column of the phased array demonstrated greater than 50% instantaneous bandwidth for scan angles as wide as +/- 60 degrees.
We discuss the development of a fiber-optic approach for low-loss true time delay of wideband RF signals for phased-array-antenna beamsteering and general analog signal-processing applications. The device utilizes narrowband optical Bragg reflection gratings written holographically into the core of a single-mode fiber to reflect light of different optical wavelengths. An optical carrier modulated by the RF signal of interest is launched into this delay-line fiber. The desired RF time delay is realized by runing the optical carrier wavelength for reflection from the appropriate grating. Unlike conventional switching schemes, this time- delay element has an RF insertion loss that is independent of the number of time delays (bit resolution) provided. The wavelength selectable nature of the TTD device can simplify beamsteering control and channel multiplexing in phased-array antennas. Experimental results are reported for optical and RF characterization of the Bragg-grating element. Both time and frequency-domain RF data are shown that demonstrate the wavelength-selectable time delay concept.
This paper presents the experimental demonstration and the far field pattern characterization of an optically controlled phased array antenna. It operates between 2.5 and 3.5 GHz and is made of 16 radiating elements. The optical control uses a 2D architecture based on free space propagation and on polarization switching by N spatial light modulators of pxp pixels. It provides 2N-1 time delays values and an analog control of the 0 to 2(pi) phase for each of the pxp signals feeding the antenna (N equals 5, p equals 4).
The problem of obtaining a true-time-delay photonic beamformer has recently been a topic of great interest. Many interesting and novel approaches to this problem have been studied. This paper examines the design, construction, and testing of a dynamic optical processor for the control of a 20-element phased array antenna operating at L-band (1.2-1.4 GHz). The approach taken here has several distinct advantages. The actual optical control is accomplished with a class of spatial light modulator known as a segmented mirror device (SMD). This allows for the possibility of controlling an extremely large number (tens of thousands) of antenna elements using integrated circuit technology. The SMD technology is driven by the HDTV and laser printer markets so ultimate cost reduction as well as technological improvements are expected. Optical splitting is efficiently accomplished using a diffractive optical element. This again has the potential for use in antenna array systems with a large number of radiating elements. The actual time delay is achieved using a single acousto-optic device for all the array elements. Acousto-optic device technologies offer sufficient delay as needed for a time steered array. The topological configuration is an optical heterodyne system, hence high, potentially millimeter wave center frequencies are possible by mixing two lasers of slightly differing frequencies. Finally, the entire system is spatially integrated into a 3D glass substrate. The integrated system provides the ruggedness needed in most applications and essentially eliminates the drift problems associated with free space optical systems. Though the system is presently being configured as a beamformer, it has the ability to operate as a general photonic signal processing element in an adaptive (reconfigurable) transversal frequency filter configuration. Such systems are widely applicable in jammer/noise canceling systems, broadband ISDN, and for spread spectrum secure communications. This paper also serves as an update of work-in-progress at the Rome Laboratory Photonics Center Optical Beamforming Lab. The multi-faceted aspects of the design and construction of this state-of-the-art beamforming project will be discussed. Experimental results which demonstrate the performance of the system to-date with regard to both maximum delay and resolution over a broad bandwidth are presented.
A multistage optically implemented microwave frequency synthesizer has been designed and fabricated. The concept and implementation are described with emphasis on operational characteristics of a prototype 3-stage unit. The synthesizer is designed as an optical analogue of the mix-and-divide approach to frequency synthesis taking advantage of the single-sideband, supressed-carrier modulation produced by the Bragg acousto-optic interaction and the sum- only or difference-only mixing of optical heterodyne detection without the need for any in- stage passband filtering. The synthesizer exhibits a 500 MHz bandwidth and completely switches and settles stably upon a new frequency within 250 nanoseconds. Phase noise was measured to be less than -130 dBc/Hz at 10 KHz offset from the carrier.
The development of an acousto-optic null steering processor for radar ECCM applicaiton is reviewed. The general problem to be solved by this special purpose signal processor is discussed and the advantages of the acousto-optic approach are given. Processor architecture, AO/EO component selection, and experimental discoveries are described. Measurements of the laboratory breadboard model's performance for a variety of multipath and non-multipath signals are also covered. Limitations of the current breadboard model and future directions are discussed.
Radiometer spectrometers are used in millimeter and submillimeter wavelength radio astonomy for the spectral measurement of molecular transitions. The spectrum of interest spans 10's of GHz and the measurement time is large in order to obtain adequate signal-to- noise ratio. The low power/channel and simplicity of acousto-optic technology has led to the current development of acousto-optic spectrometers (AOS) with 1 GHz bandwidth and 1,000 channels in small, low-power consuming packages. The recent development of multichannel AOS's, utilizing multichannel Bragg cell technology, provides increased spectral coverage, reduction in overall data acquisition time, and accommodation and multibeam antennas. This paper describes the design features utilized in this current generation AOS's that provide them with the stability needed to compete favorably with more conventional filter bank spectrometers. Specific attention is given to the design of the key optoelectronic elements in these AOS' including thermally stabilized single-mode semiconductor laser diode light sources, polarization switching single and multichannel Bragg cells, photodetector arrays, and optics, as well as to the design of the system data acquisition electronics. Suppression of system noise, including laser speckle, is also discussed. The performance of these AOS's for specific radio astronomy spectroscopy applications is also described.
The results of experimental investigations of the regime of acoustic interference in acousto- optic cells for optical beam control are represented. The crystals TeO2, LiNbO3, Al2O3 and KRS-6 are used as the materials of interferometers. The forms of frequency responses of interferometers verify sharp changes of input impedance. The transitory regime of interferometer is investigated and the changing of input impedance of interferometer in transient process is shown. The extension of frequency bandwidth approximately 2 times more is observed. The gain of peak power for constructive interference was reached approximately 4 divided by 30 times for different conditions. The results of experimental investigations of quasi- collinear acousto-optic filter using degenerated shear mode in (001) direction of LiNbO3 crystal is presented.
The principle of quadrature signal processing was implemented in a space-integrating acousto- optic correlator (SIAOC). A new SIAOC structure for quadrature filtering of wideband radio signals was developed. Performance analysis for this architecture is described. Simple experimental results obtained with microwave acousto-optic cell (AOC) are presented.
Statistical aspects of correlative processing of clipped signals obtained as a result of 2-level quantization are considered in this paper. Detection of signals buried in Gaussian noise with uniform power spectral density in limited frequency range are investigated in passive mode. The output signal-to-noise ratio estimation has been compared to that for the case of non- clipped signals processed in linear light intensity modulation mode.
Acousto-optic time integrating receivers were first discussed some 18 years ago. This type of receiver can provide a very high sensitivity without too many attendant problems of stability and interference. The disadvantage with them is the loss of information about the received signal parameters due to the integration time used. Clearly any signal parameter with a time constant less than the receiver integration time will be averaged out by the detector and the information will be lost. An acousto-optic time integrating receiver has been developed that has a parallel path after the Bragg cell that allows the operator to direct any diffracted light at a selected frequency to a high-speed photodiode detector that measures the signal parameters. Output from the reciever is a high sensitivity real time display of signal activity against frequency and a table of the signal parameters of a particular signal chosen from the activity display measured at a slightly lower sensitivity.