Costas Loop Homodyne Detection for 20-Gb/s QPSK Signal on the Optical Frequency Synchronous Network

Automatic optical heterodyne phase lock by microcomputer-assisted loop filter

“…The target of the phase noise, under 2°, is the value that causes no symbol error, as was demonstrated in reference 6 (Ref. 6). If an optical PLL circuit is to suppress the SSB phase noise to less than 2°, the loop bandwidth must exceed a few megahertz (MHz) even if the light source has spectral-linewidth of the order of kHz.…”

“…The LD used was an external-cavity-structured LD (E-LD) identical to the one used in Ref. 6. Its FM response is −20 MHz/mA to −2.8 MHz/mA between DC and 1 MHz, as shown in Fig.…”

“…The optical frequency pull-in operation was monitored by utilizing our optical frequency discrimination circuit that had been developed for 20 Gbit/s QPSK homodyne detection, see Ref. 6, as shown in Fig. 2(b).…”

“…We have developed optical phase-lock loop (PLL) technology to realize the optical synchronous network and demonstrated stable homodyne detection for 20 Gbit/s QPSK signals [6]. If PLL technology is applied to not only long haul but also short distance optical transmission systems, we can enhance the light source frequency stability (which suppresses the unnecessary bandwidth usage), narrow its spectral line width, raise the symbol rate up to match the performance of pre-amplifiers and high-speed logic-ICs (which is not limited by analogue-to-digital conversion (ADC) speed), and reduce the DSP requirements.…”

“…The MA-PLL starts to work by running a binary search algorithm (BSA) when the optical carrier frequency is set within AE100 MHz from some grid frequency, and locks the carrier phase to the OFCR within 0.45 milliseconds. The loop filter used in the MA-PLL is the same integral, lag and lead loop filter (I&LL-LF) as developed for 20 Gbit/s QPSK signal homodyne detection [6], with some changes to the circuit parameter values. Once signal carrier phase locking to the OFCR commences, it achieves the single-side band phase noise of 1.8°for offset frequencies from 10 to 5 MHz.…”