Optically Linked Receiver

Signal information can be transmitted over an optical fiber by encoding the signal as a frequency or phase modulated lightwave carrier. This approach eliminates the need for any light amplification if an adequately sensitive phase demodulator coupled to an analog-to-digital converter is used at the receiving end. A digital output is thus readily obtained. Phase modulation offers several advantages over intensity modulation: wider bandwideth, higher sensitivity, and wider dynamic range. The phase modulation can be produced by electro-optic materials without the need for any active devices. The signal carrier is mixed down and converted into an electrical signal by a fast PIN diode. The intermediate frequency (IF) is greater than the signal bandwidth and less than the diode cutt-off, typically 5-50 GHz. The signal information exists as the frequency or phase variation of the IF. The IF frequency must be measured and converted to digital data at twice the signal bandwidth, up to 10 GS/s. HYPRES proposes a complete digital system to measure the IF signal content. The system detects the time-of-arrival of the zero-crossings of the IF. A very high sensitivity superconducting flux quantizer, already demonstrated by HYPRES, is used for this measurement. The quantizer is followed by a 100 GHz sampling system consisting of an accumulator and averager. The combined system is capable of 1 ps resolution in the progressive phase acquisition, translating into a 20 HGz IF base interval, and correspoding to a signal bandwidth of 5 GHz with 6.7 bits of resolution. For a 0.5 GHz signal bandwidth, the signal resolution will be 10 bits. In Phase I, HYPRES will demonstrate the feasibility of the approach by generating and detecting the zero-crossing frequency of an optical signal. In Phase II, the complete link will be implemented with a signal bandwidth of 5 GHz.