In an avionic environment, the critical parameters for any transmission media are weight, size, performance, reliability and layout flexibility. This paper describes the advantages of using RF fiber optic links as replacements for waveguides and coaxial cable, and describes three developed links that illustrate the potential performance obtainable utilizing fiber optics. These links are a 2.85 to 3.15 GHz link, a 10.5 to 11.5 GHz and a wideband 2 to 12 GHz link.

The use of coherent detection in optical fiber communications offers advantages of increased signal transmission capacity, improved receiver sensitivity, and flexibility of system design. This paper describes the practical limits to coherent system performance.

Fiber optics and integrated optic circuits have various applications for radar and electronic warfare systems. Examples such as phased array, radar netting, deceptive jammer, and maximum entropy adaptive filter are presented in this paper. Some of the fiber optic and opto-electronic functional devices and building blocks for signal/data processing are also described.

Conventional electrical interconnect and switching technology is rapidly becoming a critical issue in the realization of systems using high speed silicon and GaAs - based technologies. Optical interconnect technology promises to enhance performance, provide relief from the pinout problem, decrease implementation complexity, and provide improvements to the flexibility of systems by allowing real time reconfiguration of these systems. In recent years, rapid progress has been made in VLSI/VHSIC technology that improves on - chip density and speed while packaging these high speed chips is becoming extremely difficult and in some cases limiting system performance. By releasing the bandwidth contraints on interconnects and packaging, the full processing speed capabilities of silicon and GaAs logic can be exploited to dramatically improve system throughput. A number of university, govern-mental and industrial laboratories have been developing technology for on-chip/on-wafer, chip-to-chip and board-to-board high speed optical communication. Both guided wave and free space communication media are being developed. In this paper, a review of some of the state-of-the-art technological developments will be presented.

A symmetric coplanar transmission line, geometrically suitable for incorporating many devices on the same crystal is considered for high-speed integrated optics in LiNb0₃ . The effect of the line dimensions on the efficient use of the available microwave power for electrooptic applications is discussed. A broadband electrode design is described and a 5 GHz phase modulator operating at 1.5μ m is reported.

A novel modified electro-optic Ti:LiNb01 lx2 directional coupler modulator is demonstrated. The modulator consists of a single-mode input waveguide which branches into a pair of optically coupled single-mode waveguides. Due to the symmetry of the modulator configuration, the half-power 3-dB operating point is automatically achieved without the need for a DC bias voltage. It is compatible with an efficient push-pull traveling wave type electrode structure for ultra high speed operation, utilizing the largest r₃₃ electro-optic coefficient. In addition, the device can be used as a lx2 optical switch, requiring a lower drive voltage than a conventional directional coupler switch.

A Ti:LiNb03 interferometer modulator has been fabricated for instrument applications in the 1.3 μm wavelength region. It requires only 3.5 volts switching voltage, and has a fiber-to-fiber insertion loss of -4 dB. This includes a propagation loss of 2 dB and a fiber/device coupling loss of ~ 1 dB/end. Frequency response measurements indicate an optical 3 dB bandwidth of ~ 8 GHz. The corresponding drive voltage/bandwidth figure of merit of .44 volts/GHz is the lowest reported for a 1.3 um modulator.

The possibility of forming a surface wave electro-optic modulator is considered. Such a device is formed by launching a surface electromagnetic wave on an optical planar guide. A grating can be formed via the electro-optic or Kerr effect. Similar to the surface acoustic wave device, depending on the frequency and strength of the the modulation signal, it can deflect the incoming optical beam. However, different from a surface acoustic wave device, the surface wave electro-optic modulator is supposed to work at multi-gigahertz frequencies. In order to have a large deflection angle and a high efficiency, one needs a substrate which can support an electromagnetic modulation signal that is tightly confined to the surface and has a very slow phase velocity. Various materials, phenomena, and structures are considered. The advantage and disadvantage of each are discussed.

Two 10-GHz direct laser modulation links and an external modulation link were demonstrated. A signal-to-noise ratio of 130 dB/Hz was measured for the 1.3 μm wavelength external modulation link and 115 dB/Hz for the 0.8 pm and 1.3 μm wavelength direct modulation links.

The performance and design of fiber-optic links for analog signal transmission at microwave frequencies are discussed. The effect of impedance matching the fiber-optic transceiver using either lossless or lossy circuit elements is presented. In particular, the following performance parameters are examined: link insertion loss, transceiver input and output VSWR, and link bandwidth. The contributions of RF circuit losses as well as optical losses to the overall RF link insertion loss are described. Commercially available fiber-optic links are shown to be unsuitable for most microwave applications. However, fiber-optic links employing lossless impedance matching to the laser diode and photodiode are shown to minimize the link insertion loss and VSWR in a finite frequency band and are therefore practical for many applications involving microwave signal transmission.

The theoretical gain/bandwidth limitations for directly modulated fiber-optic (FO) links are examined. Design curves are presented which enable one to determine the photodiode RC time constant which yields the minimum link loss for a specified receiver bandwidth. The insertion loss characteristics are examined for FO links subject to broadband impedance matching where purely reactive elements are used.

The implementation of optical fiber transversal filters incorporating single-mode fibers and bidirectional couplers using a cascade synthesis procedure is discussed. The filter structures fabricated by this method exhibit good filtering characteristics in a broad frequency region. Single cascaded filters with first passband center frequencies from few MHz to over 3 GHz and with quality factors over 250 have been demonstrated. Many single cascades can also be cascaded together to form a multicascaded structure for shifting the first passband to higher frequencies or for suppression of the out-of-band attenuation level. Multicascaded filters with more than 30 dB out-off-band attenuation level have been demonstrated. The cascade synthesis procedure is very simple and cost-effective.

As the availability of commercial GaAs integrated circuits increases, gigabit optical fiber transmission systems will soon be a reality. Since commercially available lasers and PIN diodes already operate well over a gigabit, the only limiting factor to commercially feasible high data rate systems are the high speed circuits required in the transmitters and receivers. The circuits that require ultra high speed are shown in Figure 1. Microwave Semiconductor Corp. has recently announced a fiber optic chip set that will perform each function shown in Figure 1, excluding the laser and PIN diode. This paper will describe two of the new GaAs integrated circuits, the laser driver and the transimpedance receiver circuit, that operate above a gigahertz.

A high-speed n-GaAs planar Schottky barrier photodetector capable of detecting an analog/digital modulated microwave optical signals up to 20 GHz has been fabricated. The impulse response measurement shows that the photodiode has a risetime of 27 ps and a FWHM of 94 ps. The spectral response measurement yields a responsivity of 0.40 A/W and quantum efficiency of 62 % at 820 nm. Based on the measured RC time constant (C = 0.15 pF and Rs = 7.6 ohm), the photodiode of 25 pm in diameter has a 3-dB bandwidth of 18.5 GHz.

Monolithically integrable lateral PIN detectors have been developed. The device fabrication procedure has been optimized to be compatible with standard integrated circuit fabrication procedures.

Conventional PIN detectors have bandwidth efficiency products of up to 38 GHz. This product is primarily limited by the hole transit time through the depleted layer. We describe a pInP/nInGaAsP/nInP heterojunction waveguide PIN photodetector that provides for light absorption perpendicular to current collection, a feature that results in high-speed, high-efficiency and relatively bias-insensitive operation. The 30 μm waveguide length corresponds to 6 absorption lengths, assuming an optical confinement of 30% and a material absorption coefficient of 0.7 μm.-1 Our uncoated, packaged device has an impulse response of 40 ps (Tektronix S-4 sampling head) and efficiency of 25% at zero bias.

The irregular response of a self-pulsing semiconductor laser under direct current modulation has been studied experimentally. The complex evolution of the oscillation waveform as the driving parameters are varied is found to follow a simple rule determined by the ratio of the frequencies of the external modulation and the intrinsic resonance. Good agreement with the predictions of a rate equation model is obtained.

An analysis of fiber-optic link loss and bandwidth based on simple equivalent circuit models for the laser diode and photodiode is presented. The method of modeling the link by defining a transfer coefficient between the current flowing into the active resistance of the laser diode and the current generated at the photodiode allows conventional circuit analysis to be performed. A closed form expression is derived for the link transfer function for both an impedance matched link and an unmatched link. The link loss is shown to be reduced by more than 15 dB by impedance matching the transceiver at microwave frequencies for typical device parameters. The analysis also demonstrates that impedance matched links can actually have a net RF gain with realizable device parameters at frequencies up to X-band. The transfer function for a link with realistic two-element matching circuits is derived. The bandwidth is then determined. These results define a link loss versus bandwidth tradeoff. The analytical results are verified by experiment.

The design and performance characteristics of a fiber-coupled microwave laser package suitable for optical transmission of broadband or RF signals are described. The package contains a GTE 15-GHz, VPR-BH diode laser coupled to a single-mode fiber; an InGaAs edge detector for monitoring rear facet power; temperature stabilizing components; and microstrip con-nections to the laser. The laser is placed in series with a 5041 transmission line that is terminated externally using a commercially available precision high-frequency load. Major features of the package include a laser carrier design that simplifies internal component assembly while maintaining stable laser-to-fiber alignment to ambients of 60°C, a flexible internal microstrip configuration that relieves expansion stresses at temperature extremes, a hermetically sealed environment free of organic com-pounds, and a reconfigurable RF port that is externally compatible with both coaxial and microstrip transmission media. Broadband performance of the package is currently dominated by the laser's 15-GHz frequency response. Circuit simula-tions of the internal microstrip and parasitic reactances indicate that inherent package bandwidth exceeds 20 GHz. Though overall bandwidth is dominated by the laser, packaging-related in-band ripple is affected by the internal microstrip, amounting to approximately 0.75 dB p/p. A variation of the basic package design incorporates a narrowband reactive impedance-matching network in close proximity to the laser for transferring maximum RF power while maintaining a VSWR that is less than 1.5:1.

The inherent performance advantages of GaAs have been extensively demonstrated and make it very attractive as a semiconductor material for the fabrication of high-speed digital ICs. Its high electron mobility and high peak electron drift velocity qualify GaAs as the material for developing future high-speed integrated circuits. In addition to its electron transport properties, GaAs has an intrinsic carrier concentration that is low enough to yield semi-insulating substrates, reducing device interconnect capacitance. Recent advances in processing and circuit technology have made possible the fabrication of medium-scale integration ICs with production-oriented repeatability.

Optical communication systems which use heterodyne or homodyne detection are commonly referred to as coherent systems. These systems provide a significant improvement in receiver sensitivity, leading to a substantial increase in repeater spacing. Heterodyning also allows the frequency domain multiplexing of several hundred or more optical carriers, with very narrow separation, leading to a vast improvement in capacity. However, these systems place very stringent requirements upon the coherence of the lasers used. The semiconductor lasers intended for use in these systems possess both phase and intensity noise, which limit the system performance. We give an overview of the manner and extent to which these problems degrade a coherent transmission system. The resulting requirements on laser linewidth are noted. Techniques for the linewidth reduction of semiconductor lasers, such as external grating loading and injection locking, are reviewed.

Coherent lightwave communication systems have been shown to offer a 5-20 db improvement in performance compared to conventional direct detection schemes. This results in increased receiver sensitivity as well as selectivity. Aspects pertaining to receiver modeling for coherent lightwave communication systems are studied using the bit error rate (BER) and receiver sensitivity as figures of merit.

The potential for coherent detection to improve the receiver sensitivity and expand the transmission capacity of optical communications systems has now been demonstrated in a number of laboratories, but much remains to be done to develop systems for use in telecommunications networks. In the light of reported experimental results, this paper reviews the performance required of devices and components for coherent systems, and considers the developments needed to bridge the gap between laboratory demonstration and field application.

In this invited tutorial review paper, I discuss the various options available to match the state of polarization of the signal and local oscillator waves in a coherent fiber optical communication system. The various advantages and disadvantages of each technique are discussed and directions for further work are identified.