Spectrum Shaping for Heterodyne Detection of Wavelength-Swept WDM Signal: Feasibility Study of Mach–Zehnder Interferometer-Type Periodic Filter

In this paper, we investigate the performance of a spectrum shaping technique for the suppression of image-induced crosstalk during the heterodyne detection of wavelength-swept wavelength-division-multiplexed (WDM) signals. In particular, we focus on a Mach-Zehnder interferometer (MZI) filter as a promising candidate for a spectrum shaping filter. We calculate the relationship between the power penalty, signal-to-crosstalk ratio (SCR) and free spectral range (FSR) of the MZI filter, the bandwidth of an IF filter, and the bit rate per channel, taking into consideration three tandem configurations of plural MZIs with different FSRs. And, to clarify the tolerance to the wavelength drift that occurs in actual systems, we investigate the impact of the wavelength mismatch between a WDM signal and a local light. The calculated results provide the criteria for suppressing the image-induced penalty quantitatively, and the SCR improvements that are realized when the tandem configurations are used. The results also indicate that each type of MZI tandem configuration has merits and demerits, depending on the ratio of the bit rate per channel to the channel spacing and the reception quality requirements. Furthermore, we also describe experiments that we performed with respect to the generation, spectrum shaping, and selective heterodyne detection of a three-channel super-dense WDM signal with a channel spacing of 25 GHz transmitted at 1.0 Gb/s per channel, by using tandem configurations of three MZI filters (FSR = 25,12.5, and 6.25 GHz). The experimental results agreed well with the calculated results, which are based on the consideration discussed in Sections III and IV, and the results confirmed that the image rejection was well performed. Moreover, we confirmed that we can suppress the SCR degradation by using the tandem configurations for a spectrum shaper. The allowable normalized wavelength mismatch (f
_m
/?f) for a degradation of 1.0 dB can be enhanced from 0.012 for 1-MZI to 0.016 and 0.04 for 2-MZI and 3-MZI configurations, respectively.