Experimental Demonstration of Optical PAM-4 Generation for Short-Reach Optical Communications

The demand for higher bandwidth is increasing exponentially due to high-speed applications and increase in the number of users accessing internet. To meet this demand several modulation techniques are proposed which have higher spectral efficiency exploiting different dimensions of the optical carrier amplitude, phase, and polarisation. Further, by combining wavelength division multiplexing scheme with the dimensions of the optical carrier, capacity of the network can be further enhanced. Optical telecommunication networks can be broadly classified as long and short-haul, where m-Quadrature amplitude modulation (m-QAM) schemes are popular in former, and 4-level pulse amplitude modulation (PAM-4) is a promising economical solution for latter. In this paper, a proof-of-concept technique of generating optical PAM-4 signal is proposed. Experimental demonstration and simulation of PAM signal generation and transmission is done at bit rate 240 Mbps and baud rate 120 Mbps over various fiber lengths. Generation of signal optically circumvents the need for high speed electronics transmitter and also it is possible to exploit the complete dynamic range of the optical source characteristics, which otherwise would be limited to linear region. This concept can be extended to Integrated optics for very high signal rate generation optically.