Stephen Wanuga

Interfaces for high speed fiber-optic links: analysis and experiment

An analysis of directly and externally modulated fiber-optic links is presented. The theoretical analysis is based on the signal flow diagram of the interface circuits to the laser diode, Mach-Zehnder electro-optic modulator, and p-i-n photodiode. System parameters such as gain, noise figure, two-tone intermodulation distortion, and spurious free and compression dynamic range are expressed as a function of frequency and operating point of the laser and external modulator. Two directly and externally modulated fiber-optic links were designed and fabricated to verify the analytical models. The direct modulation FO link was developed at the Ku-band (11.6-12.4 GHz), whereas the external modulation link was at L-band (875-925 MHz). Spurious-free dynamic ranges of 95.8 dB Hz/sup 2/3/ and 113 dB Hz/sup 2/3/, respectively, were achieved. The predictions based on the analytical models match the measured results. >

Comparison of alternative modulation techniques for microwave optical links

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.

Low-loss L-band microwave fiber optic link for control of a T/R module

An experimental directly modulated fiber-optic link assembled and optimized for L-band operation with near-zero RF insertion loss is presented. This was achieved by optimizing the critical coupling of light from the laser diode to a single-mode fiber and by applying reactive matching circuits to the laser and detector in order to minimize RF reflection losses. Microwave performance measurements are made on the fiber-optic-fed T/R module, in terms of noise figure, intermodulation distortion, and dynamic range, and compared with the module performance without the fiber-optic link. It is shown that careful attention to optical coupling and to the reactive matching of the optical laser and detector transducers can lead to a link gain of -1.8 dB at 1.3 GHz and a 150 MHz bandwidth. The 66.5 dB/MHz exp 2/3 spurious-free dynamic range exhibited by the present link in conjunction with a T/R module demonstrated the feasibility of employing fiber optics in many antenna-remoting optically-controlled phase arrays and optical microwave interconnect applications.