Integrated-optical interferometric modulators specifically designed and demonstrated for bandpass analog optical systems are described. Techniques such as nondissipative impedance matching for lumped-element modulators and electrooptic phase reversal for traveling-wave devices can provide substantially higher modulator response than conventional baseband designs. Recent results on the use of the polarization-mixing technique to improve modulator linearity at microwave frequencies are also discussed.

Analog fiber-optic links using an integrated-optical intensity modulator have been demonstrated. These links operate at frequencies from 50 MHz to 22 GHz with electrical gain u to 1 1 dB noise figure as low as 6 dB and intermodulation-free dynamic range of up to 1 13 dBHz2/3. The design factors determining optical link performance are discussed briefly and the effect of adding electronic amplifiers is analyzed. SUMMARY Analog optical links are attractive for many applications ranging from cable television distribution to radar-signal transmission at microwave frequencies. These systems often require a link with a low noise figure low electrical insertion loss and high dynamic range. We have demonstrated several intensitymodulated optical links that use a diode-pumped Nd:YAG laser and an integrated-optical modulator to achieve high performance by all these measures. 13 We will first discuss these experimental links and then show how their performance could be improved further by adding electronic amplifiers. Figure 1 illustrates the optical link used in our experiments with an electronic preamplifier and postamplifier added. (The experimental results reported here refer to the electrical ports of the optical link Figure 1. Block diagram of the optical link with an electronic preamplifier (Al) and postamplifier (A3) added. (Experimental results in the paper refer to the optical link alone without the amplifiers. ) 252 / SPIE Vol 1371 High-Frequency Analog Fiber Optic Systems(1990) OPTICAL LINK 161 884-1 INTEGRATED-OPTICAL PIN

A comparison was conducted of fiber optic links for use in electromagnetic environments. Determining which link is optimum for a particular application is dependent on the environment''s electromagnetic bandwidth and amplitude. the device under test the frequency range of interest the environmental conditions and the link''s characteristics. If electromagnetic parameters tested device and environment cannot be altered the link must be adaptable to the device in its environment and operate over the required frequency range. The comparison determined the bandwidth. magnitude linearity versus frequency and temperature risetime signal-to-harmonic ratio and relative merits of each link. The Nanofast 0P300-2A was judged to be the outstanding low radio frequency (RF) FOL. It had a consistant upper 3 dB point (BW ) of 250 MHz and minimum ripple with changing gain: a near linear temperature versus BWU with minimum ripple: a risetime between 1. 7 is and 2. 1 s and a SHR of 42 dB. For high RF use the EOD Sentinal 1000 was the FOL of choice because of its: consistant 1 GH: BWU and low magnitude ripple coefficient with changing gain and temperature a risetime of 0. 5 its and SHR 47 dB.

A relatively simple electronic predistortion unit to linearize a Mach Zehnder intensity modulator was demonstrated. Limitation of the linearity improvement due to the effect of multi-carriers induced third order distortion was explained. A 13 to 16 dB third order distortion reduction was achieved in a 20 channels system. introduction For analog lightwave distribution external modulation is very attractive because high power diode pump laser can be used to achieve good power budget and also the intermodulation distortion (IM) is independent of frequency. The primary disadvantage of external modulation is the nonlinear distortion associated with the standard external modulator. Linearization schemes such as electronic predistortion [1] feed-forward compensation [2] and polarization mixing [3] have been proposed to reduce the nonlinear distortion. Those techniques achieved relatively the same level of linearity improvement. For the present CATV application the primary frequency range of interest is in the 50 to 600 MHz. While the frequency is relatively low the broadband nature of the system and the high dynamic range requirement make the design of broadband compensation difficult. In this paper we demonstrated a unique predistortion circuit which offered fairly good linearization with simpler electronic circuit. In the next section an experimental set-up to evaluate the linearized modulator was described. Several limitations due to the distribution of composite distortion product arising from multi-carriers were explained. The two-tones third order IM distortion measured was below the noise of the spectrum analyzer. The degradation for wideband multi-channels system was discussed. Experiment In this section the linearized modulator and the experimental evaluation techniques were described. This work was intended for multi-carriers CATV system operating in the frequencies range of 50 to 600 MHz. However the approach could be valuable for other applications as well. The goal of this work was to reduce the third order nonlinearity generated by the modulator. With this higher optical modulation index could be employed which would result in a higher link power budget for a given optical source power. The predistortion unit presented here offered separate independent phase and amplitude control. The functional block diagram of the predistortion unit was shown in Figure 1. The modulating RF input was split by using a 10 dB RF directional coupler. The coupled output RF signal was fed into the third order distortion generator. A matched pair diode biased in the opposite direction was used to generate the third order distortion. The distortion generator output was followed by a variable gain amplifier. At the output end a 3 dB RF power combiner was used to sum both the main modulating signal and the generated distortion signal. The output was 8 / SPIE Vol. 1371 High-Frequency Analog Fiber Optic Systems(1990)

High-frequency response (about 50 MHz) is reported for a fiber-optic phase modulator that is coated with vinylidene fluoride/trifluoroethylene (VDF/TrFE) copolymer, 80 about 90 microns thick, on a single-mode fiber of 80 microns diameter and made piezoactive by radial poling. A flat response of optical phase shifts is observed in the frequency range over 10 KHz to 3 MHz whereas at frequencies higher than 4 MHz it shows multiple peaks dominated by radial resonances of the fiber-jacket composite. An excellent agreement exists between the experimental data and theoretical calculations based on radial vibration analysis.

Ten different electro-optic modulator nonlinearity reduction techniques are presented, and performance parameters of each technique are compared. These parameters include linear dynamic ranges, third-order intermodulation products, relative sensitivities, and operation bandwidths. It is concluded that the feedforward, dual-polarization, and anti-parallel push-pull approaches are more favorable than other techniques in terms of fabrication simplicity, nonlinearity reduction, and sensitivity.

The measurement of the relative intensity noise (RIN) of a ridge waveguide laser and a distributed feedback laser under CW, external cavity, direct modulation, and modelocking conditions is presented. The purpose is to determine the relative noise performance of modelocked laser diodes. The results indicate that the RIN of modelocked lasers are comparable to CW lasers but lower than both external cavity lasers (optimized for modelocking but without the applied RF) and directly modulated lasers; the difference can be as much as 5 optical dB. The microwave carriers produced optically by the direct modulation and modelocking of laser diodes are also compared. The comparison determines that modelocked lasers produce less noisy and more RF power efficient microwave carriers. However, no difference in microwave linewidth is detected within the limit of the resolution bandwidth of the detection system.

Computer aided design (CAD) tools can expedite the development of high speed electrooptic modulators. The simulation program can be used to simulate the device performance, therefore, lead to the optimal design. The layout tool can be used to design mask patterns. Custom features specifically developed for electrooptic modulators make the mask pattern design highly efficient. The OPTOMASK layout program and results of simulations on low-loss Y-branch design and on quasi-linear electrooptic modulator design are reported.

A wide bandwidth Mach-Zehnder modulator with on/off switching powers of 20 dBm, a bandwidth of up to 20 GHz, and insertion losses around 5 dB is presented which is based on HCC-1232 polymer. Attention is given to material properties of HCC-1232, the modulator design, and preliminary test results.

The use of fixed, tapped, and programmable fiber delay lines in signal processing applications at bandwidths to 10 GHz is discussed. Optoelectronic ring resonators, traveling-wave photodetectors, and differential amplifiers for microwave signal processing are also described.

An efficient optical adaptive equalizer has been developed for the optical processing of guided lightwave signal waveforms corrupted by time-varying dispersion and attenuation effects. The optical equalizer is based upon a tapped delay line filter built by laser ablative chemical etching. The least-mean-square algorithm is used to adapt an equalizer''s weights in real-time as the optical channel varies. The equalizer is designed for incoherent lightwave systems carrying binary, on/off keyed, digital data.

Partitioning the transmission line on a LiNbO3 traveling wave modulator into short sections and feeding each section from an antenna integrated to the modulator surface can overcome the frequency limitation from material dispersion. A phase modulator at 5-13 GHz with a peak sensitivity better than 100 deg/(Watt) sq rt is demonstrated using an unoptimized microwave feed system. The technique should be scalable to high millimeter-wave frequencies.

The wideband frequency modulation (FM) of an optical carrier by a radio frequency (RF) or microwave signal can be accomplished independent of laser type when indirect modulation is employed. Indirect modulators exploit the integral relation of phase to frequency so that phase modulators can be used to impress frequency modulation on an optical carrier. The use of integrated optics phase modulators, which are highly linear, enables the generation of optical wideband FM signals with very low intermodulation distortion. This modulator can be used as part of an optical wideband FM link for RF and microwave signals. Experimental results from the test of an indirect frequency modulator for an optical carrier are discussed.

The design, component selection, fabrication, testing, and evaluation of an electronically-switched, binary, fiber optic, programmable delay line (BIFODEL) are discussed. It comprises seven delay stages, has a maximum delay of 5 microsec, a resolution of 39 ns, and operates over the 2-18 GHz band with a 500 MHz bandwidth, a dynamic range of 36 dB, and a power consumption of 19 W. A discussion on the relative merits of an optically-switched BIFODEL is also presented.

High-performance, low-cost microwave microwave RF/IF fiber optic links which are based on a heterodyne laser approach are described. The system combines microwave frequency and intermediate frequency fiber optic links into a single link, which provides certain advantages, including relaxation of the stringent requirement of a laser source, minimization of system noise and nonlinearity, the use of multimode fiber optic cable and connectors, and elimination of down-converter requirement. A 12 GHz link with 70 MHz IF has a signal-to-noise ratio (SNR) of better than 120 dB/Hz and a 26.5 GHz up-converted link has a SNR of better than 100 dB/Hz.

The first microwave-phased array antenna steered by optical delay lines is described. The optical ''time shifters'' utilize the propagation of light waves through a finite length of fiber to generate the time delays that control the beam pointing angle. Delay times specified by the antenna steering angle were implemented by switching bias currents of high speed lasers pigtailed to fiber optic delay lines.

A point-to-point fiber optic link capable of transforming microwave signals up to 18 GHz over distances exceeding 20 km is described. This self-contained true time delay line for X-band radar uses indirect modulation to obtain high frequency operation with minimum distortion and noise generation by the optical components. Many of its features can be incorporated into similar systems for airborne operation.

Issues affecting the design of low noise high dynamic range analog fiber optic links are discussed. Particular attention is given to the implications of high optical power sources and low insertion loss external modulators for the achievable noise figure and dynamic range of such systems focusing on antenna remoting.

Detailed analysis has been performed of modal noise reduction by superposition of high frequency modulation. Data obtained revealed that, for both direct and external modulation, the modal noise reduction factor depends on the modulation frequency and fiber length. The fiber mode coupling, chromatic dispersion, and the mode partition noise have been neglected in the analysis. Experimental results confirmed that, for multimode fiber, the modal noise reduction depends almost only on the modulation depth.

The current status of microwave fiber optic links is discussed. It is noted that to date, high-speed links are limited not only by the available bandwidth of analog lasers but also by the fiber dispersion effect on link parameters. New 15 GHz distributed feedback lasers are capable of significantly reducing dispersion problems while providing enhanced bandwidth. An optical isolator with the microwave circuitry makes it possible to reach coupling efficiencies as high as 60 percent.

Interdigitated photodetectors that are compatible with a standard commercial monolithic microwave integrated circuit (MMIC) process have been designed and characterized. A wide variety of interdigitated metal-semiconductor-metal designs were investigated by varying device geometry and type of contact (Schottky and ohmic). The Schottky photodetector devices exhibited quantum efficiencies between 40 percent and 50 percent with 3 dB bandwidths up to 12 GHz and dark currents less than 5 nA. The fastest device, having 0.5-micron fingers with 1.0-micron spacing, had a useful operating bandwidth up to 40 GHz. The impedance of the photodetectors was measured and equivalent circuit models were derived from the data.

A method for determining the transimpedance gain is presented, in which an optical receiver is characterized as a simple microwave amplifier. A simple formulation is obtained using S and Y parameters. The method is applied to a PIN-GaAs MESFET transimpedance amplifier.

A new planar high-speed dual wavelength InAlAs/InGaAs Schottky barrier photodiode with graded superlattice structure is reported. In the detector structure, the top wide band gap n-In(0.52)Al(0.48)As epilayer absorbs photons in the visible to near infrared spectrum with the peak response occurring around 0.8 micron, and the bottom n-In(0.53)Ga(0.47)As epilayer absorbs photons in the 1.0 to 1.6 micron wavelength regime with the peak response occurring at 1.3 micron. The detector is capable of detecting and demultiplexing at both short and long wavelengths simultaneously without the complication of additional components. The response speed measured by the impulse response method is estimated to be about 3GHz.

An InGaAs/InP PIN photodiode integrated with 50 ohms coplanar waveguides has been fabricated on a semi-insulating substrate for high-speed operation. This photodetector exhibits a system limited impulse response of 16 ps, which is considered to be the fastest measured impulse response reported to date for these devices.

Optical heterodyne techniques can be used to generate millimeter-wave signals. Optical FM sideband injection locking can be used to acheive an extremely narrow spectral width millimeter-wave signal. The phase and amplitude of the millimeter-wave signal can be controlled using electro-optic waveguide components.

A variety of high-speed components for analog applications (lasers, external modulators, photodetectors, and receivers) is described. Particular attention is given to an analog video distribution system based on subcarrier multiplexing techniques, and a phased array antenna control system with all-optical modulation control and phase/amplitude control of the antenna elements.

As the modulation bandwidth of a traveling wave modulator is increased, the velocities of the microwave and the lightwave need to be taken into consideration. Velocity mismatch of 20 percent or greater can occur in a III-V modulator and this will result in a bandwidth of approximately 22 GHz-cm or less for an electrical loss of 1 dB/GHz sq rt/cm. To improve the bandwidth of the modulator to greater than 40 GHz, velocity mismatch would have to be reduced to within 10 percent. A new velocity matched III-V modulator is presented.

Directly- and externally-modulated fiber optic links have been compared in terms of gain, noise, and dynamic range. Data obtained indicate that direct modulation has gain and noise figures relatively insensitive to the optical bias level, provided the increased dynamic range at the higher laser bias level; excess optical noise caused by relaxation oscillation in the semiconductor laser limited the noise figure and dynamic range. For the externally-modulated link, gain and noise figures appeared to significantly depend on the optical input power and RF switching power, but the dynamic range is primarily determined by the optical input power. Photodetector linearity is a significant factor limiting the dynamic range of both of these links, especially at lower microwave frequencies.

A new self-equalizing, RF memory loop based on fiber optic delay lines and MMIC technology is presented. A nested loop configuration is proposed which combines fiber optic delay lines with conventional delay lines, forming an optimal signal storage medium for modern radar deception. Combined with adaptive equalization and RF processing circuity implemented in MMIC, a coherent optical RF memory (CORFM) is constructed. The CORFM provides a much wider instantaneous bandwidth than the digital RF memory and and it is capable of storing time-coincident pulses.