A Wavelength Converter Integrated with a Discretely Tunable Laser ...

10 2. Wavelength converter overview
Zehnder interferometer (MZI). Compared to XGM, XPM allows for lower optical powers,
lower chirp, larger extinction ratio and reduced wavelength dependency.
The XPM based wavelength conversion methods are described below. We focus on integrated
devices that employ SOAs as the non-linear element, because integrated devices are much more
stable and compact than their fiber based counter parts. In addition, fiber based converters
suffer from a difference in arm length, resulting in a reduced wavelength bandwidth [13].
Mach-Zehnder interferometer (MZI)
Wavelength conversion in a MZI with SOAs (Fig. 2.3) works as follows: A probe at wavelength
is injected in the MZI and split in two parts. Each part propagates through a
different arms of the MZI. Next, the two parts are recombined again interferometrically and
the resulting probe output power depends on the phase difference between the two arms. The
output is minimum for destructive interference and maximum for constructive interference.
Consequently, the probe output power can be switched by modulating the phase difference
between the two MZI arms. This is realized by injecting a control signal into the SOA in one
of the arms. The control signal can be injected either co-propagating or counter-propagating
with the probe. In counter-propagating conversion no filters are needed at the converter output
to separate the control signal from the probe, but the bit-rate is limited to ¤ Gb/s [14].
In co-propagating mode, the bit rate is limited to ¤ - Gb/s by the carrier recovery time in
the SOA. The limited carrier recovery causes inter-symbol interference for bit rates beyond
the SOA bandwidth. Converters have been fabricated using active layers of either bulk material
[14–19] or multi-quantum well (MQW) material [20–22]. MQW layers require normally
less injection current than bulk, while a bulk layer is advantageous for the reproducible fabrication
of polarization independent behavior. The lengths of the SOAs that were reported in
literature range between 500i and 1200 , and their injection current typically varies
between 100 mA and 280 mA. Static extinction ratios of the converted output of dB and
extinction ratio improvements of up to ¤ dB have been reported [22, 23]. An extinction ratio
of over 30 dB was reported in [24]. In dynamical operation, the extinction ratio varies between
13 dB at 2.5 Gb/s to around 10 dB at 10 Gb/s [14, 20, 25]. The typical control signal input
λprobe
SOA
SOA
λcontrol
λconverted
P
ISRR OSNR
λcontrol
ASE
λconverted
Figure 2.3: a) Illustration of a MZI wavelength converter. A counter propagation scheme
is shown. b) An illustration of an output spectrum from the converter that explains the Input
Signal Rejection Ratio (ISRR) and the Optical Signal to Noise Ratio (OSNR).