Photonic Links with Enhanced Dynamic Range

Increasingly, fiber optic links are being extended into the K-Band frequency regime and beyond for communications and signal processing applications. Higher operating frequencies put stringent demands upon the RC time-constants of photodetectors and receiving electronics. High bandwidth demands very short RC time-constants and thus small active areas for the photodetectors. Unfortunately, such small photodetectors saturate at only one or two milliwatts of received optical power, severely limiting the available dynamic range. Presently, it is relatively easy to provide optical carrier powers in excess of 100 mW, but the wideband detectors that can benefit from this power level do not exist. High frequency analog modulation systems require small modulation depths to avoid distortion and intermodulation products, also putting severe constraints on the dynamic range. This proposal addresses a technique for enhancing the dynamic range by as much as 20 dB. A tunable optical filter is used to suppress the carrier power to lower the mean optical power at the photodetector without affecting the sidebands. A dynamic range greater than 40 dB is possible for system bandwidths as large as 20 GHz. This technique is applicable to both direct analog intensity modulation of laser diodes and external analog modulation by a Mach-Zehnder intensity modulator.