The ability to measure both the amplitude and phase of an optical signal is increasingly important in modern telecommunications systems. The stepped-heterodyne technique is a very simple linear technique that provides a self-referenced measurement of the spectral amplitude and phase of periodic optical signals. A typical setup requires a discretely tunable optical local oscillator, an optical coupler, a photodiode and a real-time oscilloscope as shown in Fig. 1.
The periodic nature of the signal under test reduces its spectrum to a series of discrete spectral modes spaced at multiples of the repetition rate. The local oscillator is positioned between two adjacent spectral modes of the signal. A measurement of the resultant beat signals on the real-time scope allows us to recover the amplitude of each mode, as well as the phase difference between the two modes. This measurement is then repeated as the local oscillator is stepped across all the spectral modes of the signal. From here it is simple to reconstruct the full electric field of the input signal in either the time or the frequency domain. The key advantage of this technique over other linear methods is that the measurement does not require an external electronic clock synchronised to the optical signal. As such is it ideal for the measurement of high repetition rate passively modelocked laser diodes. In Fig. 2 we reproduce a stepped-heterodyne measured of a 10GHz 33% return-to-zero signal and a 66% carrier suppressed return-to-zero signal. The stepped heterodyne result is shown in blue, independent measurements made with a high resolution optical spectrum analyser and a high speed sampling oscilloscope are shown in red.
For more details on this work please see:
Ricardo Rosales, S. G. Murdoch, R.T. Watts, K. Merghem, Anthony Martinez, Francois Lelarge, Alain Accard, L. P. Barry, and Abderrahim Ramdane, “High performance mode locking characteristics of single section quantum dash lasers,” Opt. Express 20, 8649 (2012)
S. G. Murdoch, R. T. Watts, Y. Q. Xu, R. Maldonado-Basilio, J. Parra-Cetina, S. Latkowski, P. Landais, and L. P. Barry, “Spectral amplitude and phase measurement of a 40 GHz free-running quantum-dash modelocked laser diode,” Opt. Express 19, 13628 (2011)
D. A. Reid, S. G. Murdoch, and L. P. Barry, “Stepped-heterodyne optical complex spectrum analyzer,” Opt. Express 18, 19724 (2010)