How to make optical coherent detection measurements using EnvCalc
Coherent detection has been recognised for some time as offering superior performance to direct detection. Until recently coherent receivers were prohibitively expensive, and that is why all commercial optical communications receivers use direct detection. However, a coherent receiver which uses real-time digital signal processing technology can meet both the performance and cost needs of future optical communications networks.
Most researchers have not built an actual digital processor that operates at the relevant data rates, 10Gb/s and higher, because of the large amount of resources that task would consume. Instead they have employed a real-time sampling oscilloscope, and then processed a short measurement burst offline on a convenient computing platform. The references listed below all describe this kind of measurement. Although the approach has been used to date to demonstrate optical communications systems, it can also be used to record the electric field of an optical signal, to study signal distortions, or to characterise active and passive optical components.
The optical signal, typically a phase encoded signal such as binary or quadrature PSK, is mixed with light from a separate local oscillator laser in a phase diverse hybrid. The local oscillator is close in optical frequency to the signal, perhaps 1 GHz away, but it is not phase locked to the signal. The outputs of the phase diverse hybrid are observed by photodetectors and captured by the real-time sampling oscilloscope. The EnvCalc software is able to reconstruct the optical signal from the sampled waveforms stored in the oscilloscope. To make use of the EnvCalc software, the user must assemble the hardware configuration and then perform the measurement.
M.G. Taylor, "Measurement of phase diagrams of optical communication signals using sampled coherent detection," NIST Tech. Dig.: Symp. Optical Fiber Measurement, Boulder, CO, vol. 1024, p. 163-166, Sep. 2004.
D.-S. Ly-Gagnon, K. Katoh, K. Kikuchi, "Unrepeatered optical transmission of 20 Gbit/s quadrature phase-shift keying signals over 210km using homodyne phase-diversity receiver and digital signal processing," IEE Electron. Lett., vol. 41, no. 4, p. 59-60, 2005.
S. Tsukamoto, D.-S. Ly-Gagnon, K. Katoh, K. Kikuchi, "Coherent Demodulation of 40-Gbit/s Polarization-Multiplexed QPSK Signals with 16-GHz Spacing after 200-km Transmission," OFC 2005 conference, Anaheim, US, PDP29, 2005.
D.-S. Ly-Gagnon, S. Tsukamoto, K. Katoh, K. Kikuchi, "Coherent detection of optical quadrature phase-shift keying signals with carrier phase estimation," IEEE J. Lightwave Technol., vol. 24, no. 1, p. 12-21, 2006.
K. Kikuchi, "Phase-Diversity Homodyne Receiver for Coherent Optical Communications," COTA 2006 conference, Whistler, Canada, paper CThB3, 2006.
S.J. Savory, A.D. Stewart, S. Wood, G. Gavioli, M.G. Taylor, R.I. Killey, P. Bayvel, "Digital Equalisation of 40Gbit/s per Wavelength Transmission over 2480km of Standard Fibre without Optical Dispersion Compensation," ECOC 2006 conference, Cannes, France, paper Th2.5.5, Sep. 2006.