100Gb/s DPSK-3ASK Modulation Format for Metro Networks: Experimental Results

We assembled an experimental setup to investigate the optical signal characteristics of a multi-level 100Gb/s modulation format that demonstrates the possibility to be a low-cost option for higher speed interfaces in metro networks. The modulation format which we are naming DPSK-3ASK is comprised of phase-shift keying as well as amplitude-shift keying to produce 2.5bits/symbol. This encoding allows the system to be realizable with 40Gb/s components. To generate the three amplitude levels for the ASK modulation, we use a 40Gb/s bit-error ratio tester and a multiple-path RF line to drive a commonly available 40Gb/s modulator which is biased at the quadrature of its transfer function. We place a second 40Gb/s modulator biased at the null of its transfer function for phase modulation. We detect the six symbols in this modulation format by first using an optical splitter to direct the signal to two separated paths: an ASK path comprising of a single photo-detector for direct detection, and a PSK path that utilizes a delay-line interferometer followed by a balanced detectors. We measure the bit error rate for the two ASK eyes and for the PSK eye, as well as show the noise tolerance of the modulation format by coupling amplified spontaneous emission noise together with the signal. We discuss practical challenges with the setup, and list possible solutions.