High signal to noise operation of fiber optic links to 18 GHz

Very high signal-to-noise ratio (SNR) transmission through a fiber-optic link is reported for frequencies up to 18 GHz. The dominant noise sources of fiber-optic links are reviewed, and the experimental setup is described. A high-speed buried-crescent-type 1300-nm laser with a threshold current of 10 mA, operated at a bias current of 70 mA was used. The photodiode has a 3-dB bandwidth of 12 GHz and a response at 18 GHz of -8 dB. For signal to noise measurements, several amplifiers covering 0.18-18 GHz were used. The link length was 100 meters. The link was connected by fusion splices, and the photodiode had an optical return loss >55 dB to eliminate enhanced noise due to optical feedback. The best SNR was achieved in the DC-3-GHz region where laser noise is lowest. The SNR steadily dropped for frequencies out to the laser resonance frequency of approximately 10 GHz. From 10-16 GHz, the link SNR is nearly constant. Beyond 16 GHz, receiver noise dominates. The key factors in obtaining these SNR levels are the use of a laser with good low-frequency noise characteristics and the absence of optical reflections.<>