The performance limitation with respect to the data rate is determined mainly by the bandwidth of
the UTC-PD in the transmitter and that of the SBD detector in the receiver. The 3-dB RF bandwidth
of the UTC-PD is 140 GHz (from 270 to 410 GHz), while the RF bandwidth of the SBD detector also
exceeds 100 GHz. Thus, the IF bandwidth of the receiver currently limits the maximum bit rates.
Fig. 1(b) shows eye diagrams demodulated by the receiver and the bit error rate (BER)
characteristics at 40 Gb/s and 42 Gb/s [6]. Error-free ðBER G 10
11
Þ transmission has been
achieved at 40 Gb/s, which is the highest data rate ever reported for Berror-free[ wireless links
without forward error correction (FEC). When the data rate is 42 Gb/s, a floor has been observed in
the BER characteristics, which is caused by the limitation in IF (baseband) bandwidth of SBD
detector. In Fig. 1(b), 8-mA photocurrent of the UTC-PD corresponds to the transmitter output
power of 90 W.