Abstract
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 (${\rm FSR} = 25$, 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 $(f_{m}/\Delta f)$ 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.
© 2010 IEEE
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