Simultaneous Demultiplexing, Electrical Clock Recovery, and Optical Clock Generation Using Photocurrent and Subharmonic Frequency in a Traveling-wave Electroabsorption Modulator

We demonstrate for the first time using a traveling-wave electroabsorption modulator simultaneously as a photodetector, a demultiplexer, and an optical pulse generator for clock-recovery and demultiplexing at linerates of 40 Gb/s and 160 Gb/s. Introduction Compact optoelectronics components are desired to bring down the cost and ease the maintenance in high capacity optical communication systems. One way to realize this is through monolithic integration of different devices. Another possibility is to explore more functions in a single device. In this paper, we demonstrate three co-existing functions in a travelingwave electroabsorption modulator (TW-EAM) [1]: detection, demultiplexing, and pulse generation. This is made possible by utilizing the photocurrent and subharmonic frequency in the device. With the addition of a phase-locked loop (PLL) [2], simultaneous demultiplexing, electrical clock recovery, and optical clock generation is realized at 40 Gb/s. This approach is further extended to a linerate of 160 Gb/s by adding another EAM. Principle of Operation Fig. 1(a) shows the setup of this approach for 40 Gb/s operation. The PLL requires an electrical 40 GHz tone extracted from the input 40 Gb/s RZ signal to recover a synchronized 10 GHz clock. This 40 GHz tone is provided by the photocurrent signal coming out of the upper electrical port of the TW-EAM. The 3-dB bandwidth of the TW-EAM as a photodetector [3] is 12 GHz but the roll-off is not fast. At 5 dBm of optical input the electrical 40 GHz tone from the TW-EAM is –36 dBm as shown in Fig. 2(a), which is 30 dB higher than the minimum power level required for phase-locking. The recovered 10 GHz clock is fed back into the lower electrical port of the TW-EAM with a proper phase to generate a gating window for demultiplexing 40 Gb/s to 10 Gb/s and for generating a synchronized 10 GHz optical clock at another wavelength simultaneously. The applied 10 GHz electrical clock travels through the TW-EAM and finally gets blocked by the 40GHz band-pass filter (BPF) at the input of the PLL. As a result, simultaneous demultiplexing, electrical clock recovery and optical clock generation is achieved by utilizing the photocurrent and subharmonic frequency in a TW-EAM. The generated optical clock is separated from the demultiplexed signal by an optical filter. If a counter-propagating scheme is used instead, the wavelength of the optical clock can be the same as the signal wavelength. This scheme can be extended to line-rates of N×40 Gb/s by adding another stage of demultiplexer in the front to extract a 40 Gb/s signal from the higher line-rate [4]. Fig. 1(b) shows the simplified setup for the 160 Gb/s experiment, where an EAM is used as the front demultiplexer. (a)