Harold V. Roussell

Signal-to-Noise Performance of Two Analog Photonic Links Using Different Noise Reduction Techniques

We demonstrate two analog photonic links that use different noise reduction techniques to achieve high gain and low noise figure without electronic amplification. Both links use a high-power, low-noise master oscillator power amplifier as the optical source, a balanced-bridge dual-output LiNbO3 Mach-Zehnder modulator with a record low Vpi = 1.33 V at 12 GHz, and either one or two high-power rear-illuminated photodetectors. In the first link, both outputs of the quadrature-biased modulator are used to illuminate two photodetectors configured for laser noise cancellation, yielding record high gain (> 17.0 dB) and low noise figure (< 6.9 dB) across the 6-12 GHz band. The second link uses low biasing to maximize the signal-to-noise ratio in one of the two modulator outputs, and thus requires only one photo-detector. This link has lower gain (> 12.7 dB) but also record low noise figure (< 5.7 dB) across this same frequency band.

RF-Over-Fiber Links With Very Low Noise Figure

Previously published analytical models for the noise figure of an amplifierless fiber-optic link fail to predict the measured performance, with a discrepancy of 1.1 dB at 1 GHz that increases to 2.3 dB by 12 GHz. We use an equivalent circuit to derive the effect of an additional source of noise not accounted for in earlier models: thermal noise arising from loss in the modulators traveling-wave electrodes. The electrode thermal noise has a frequency dependence matching that of the links noise figure, such that predictions using the improved model match the measured 1-12 GHz performance of a link with record low noise figure to within ~ 0.4 dB across this band.

Low Noise Figure, Wide Bandwidth Analog Optical Link

We report achieving a record low noise figure for an amplifierless fiber-optic link — ≤ 15 dB over the frequency range 1.0 – 9.5 GHz — via a combination of a low-VπMach-Zehnder modulator with two antiphase outputs, a high-power laser, and a balanced photodetector pair. We also present a complete model for this link that predicts its measured performance to within 1 dB.

Broadband, directly modulated analog fiber link with positive intrinsic gain and reduced noise figure

We report the concept and first demonstration of a broadband amplifierless, directly modulated analog fiberoptic link with an intrinsic gain of +3.8 dB, based on a series connection of diode lasers and a pin photodiode.

Effect of optical feedback on high-dynamic-range Fabry-Perot laser optical links

The effect of optical backreflection is measured on a 450-MHz optical link using a commercial Fabry-Perot laser without an isolator. With a low backreflection photodetector, the dynamic range of the Fabry-Perot link is comparable to a link with a high-linearity distributed feedback laser. With a standard photodetector, the dynamic range is 13 dB lower.

Optical frequency conversion using a linearized LiNbO/sub 3/ modulator

High dynamic range optical frequency conversion is demonstrated using a third-order linearized modulator. A record dynamic range of 122.5 dB/spl middot/Hz/sup 4/5/ was achieved at an input frequency of 400 MHz.

Input impedance conditions for minimizing the noise figure of an analog optical link

It has been previously shown that 3 dB is the lowest noise figure attainable for an amplifierless optical link with perfect lossless impedance matching to the RF source. In a prior experimental link with near-perfect impedance matching, dissipative loss in our input matching circuit prevented us from achieving a measured noise figure of less than 4 dB. Investigation of the effects of input impedance mismatch indicates that mismatch can actually lower the noise figure to below 3 dB even in the presence of some dissipative loss in the input circuit. We have verified this theory by using the mismatch effect to reduce the measured noise figure of our link to 2.5 dB at 130 MHz. We believe this is the first demonstration of amplifierless link noise figure of less than 3 dB. We confirmed the validity of our measurement technique by also measuring the noise figure of a 2.5 dB RF attenuator to be 2.5 dB.

Interferometric modulators for an adaptive nulling system

Optical intensity modulators with closely matched frequency responses are required for the realization of an optical adaptive nulling system for microwave/millimeter-wave multi-element antenna systems. Traveling-wave interferometric modulators fabricated in lithium niobate have been demonstrated that track to within 0.03 dB and 0.22 deg in amplitude and phase, respectively, over the 5- to 7-GHz frequency band.

Low-noise-figure photonic links without pre-amplification

Using optical fiber to retrieve signals from remote sensors has several advantages compared to remoting by means of metallic waveguides such as coaxial cable. Fiber-optic retrieval of an RF signal can be achieved by down-converting and digitizing the signal for conveyance by a digital fiber-optic link, or it can be achieved by conveying the RF signal over an analog fiber-optic link before digitization. The latter approach can be realized with a minimum of hardware and dc power required at the sensing site, provided that the analog fiber-optic link has a sufficiently low noise figure without a pre-amplifier. Early demonstrations of “amplifierless” analog fiber-optic links typically reported very high noise figures—in excess of 30 dB. In the last decade or so, several techniques have been developed to improve this situation. We describe five such techniques and show that they have resulted in much lower measured noise figures for amplifierless links. One technique, for example, has yielded noise figures ≪ 5 dB for amplifierless links at frequencies of up to 10 GHz. The existence of amplifierless links with such low noise figures may enable remote sensing of signals in situations where the size, weight, and power (SWAP) of the remote hardware is of primary concern.

Low-cost, high performance optoelectronic components for antenna remoting

We describe a demonstration of a system in which we incorporated components developed as part of the DARPA Analog Optoelectronic Module Technology Project (TRP). The goal of the TRP program was to bring to manufacturability fiber optic components whose specifications are adequate for both military and commercial applications but whose cost is substantially less than the presently available components. We demonstrated the functionality of the TRP components by using them in a fiber optic link designed to replace one of the coaxial cables that perform antenna remoting in a phased array. For this antenna remoting demonstration, we used an existing linear array at the Makaha Ridge, MIT Lincoln Laboratory Facility. Ordinarily, each subarray is connected to the programmable array computers in the control trailer with coaxial cable. The performance of the array with all coaxial cables connected to each subarray was compared to the case with one of the coaxial cables replaced by the fiber optic link. The results showed no degradation in the performance of the linear array with the TRP fiber optic link replacing one of the remoting coaxial cables.