Experimental and analytical study of the fiberoptic based recirculating memory systems

Reza A. Saedi

doi:10.17918/00008306

Fiberoptic links are useful in many microwave signal processing applications, one of which is the recirculating RF memory loop. As an integral part of memory loop systems, the fiber optic link offers several clear advantages: small size, light weight, immunity to EMI, and low loss over long delays. The development of several alternative techniques for predicting and realizing the total time delay achievable through the fiberoptic (FO) based memory loop was the objective of this study. On the basis of current literature, the FO link was established as the only practical option for the long delay element. The analytical expressions, predicting the total time delay achievable were developed in terms of gain, flatness, noise figure, intermodulation distortion, and dynamic range characteristics of FO delay element. Among them, nose figure and gain flatness greatly influence the performance of the recirculating memory loop. Two sets of low-loss, low noise figure, optical modules operating at 850 and 1300 nm for long and short delays were designed. These achieved optimum performance over the bandwidth of 2-4 GHz. The basic building blocks of the FO links, reactively matched optical transmitter and actively matched optical receiver modules, were fabricated and characterized, and then used in several experiments. The first set of experiments led to a sub-millisecond recirculation of the RF signal; however, gain flatness curtailed the total recirculation times over the full bandwidth. To overcome this limitation, the concept of the adaptive gain equalization technique was demonstrated via a tunable filter. The phase coherency of each individual pulse was then investigated through a circuit designed to sampled pulses after a tunable delay. Finally, the degradation of the offset carrier noise power spectral density and the effect of recirculation on the FM modulated signals were studied. An alternative approach to overcoming the gain flatness problem-to facilitate full recirculation in the optical domain-was considered, and analytical models for predicting the performance of the optical amplifier-based memory loop were developed. Finally, a system architecture, capable of providing time delays over milliseconds for the full instantaneous bandwidth of 2-4 GHz, was analyzed. To extend this approach over full 2-18 GHz, a special optical coding method with a higher dynamic range was recommended.

Experimental and analytical study of the fiberoptic based recirculating memory systems

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Experimental and analytical study of the fiberoptic based recirculating memory systems

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