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

2.2 Optical gating 11
powers (in the fiber) that are required are between and dBm at 2.5 Gb/s and between
and dBm at 10 Gb/s for the control power. The probe powers are lower, between
¤ and dBm. The input power dynamic range for the control signal is typically 3 dB
in dynamic operation. However, a 10 dB dynamic range, between dBm and dBm, for
penalty free conversion at 5 Gb/s was reported in [22], using integrated pre-amplifiers. The
wavelength span of a converter should, in principle, be such that the converter can convert to
any of the WDM wavelengths of the network in which it is employed. A wavelength span
of the converted output of 26-30 nm was reported in [16, 17, 26] and a span of even 80 nm
(C-band and L-band) at 10 Gb/s in [19]. Optical signal to noise ratios (OSNR) of the converted
output of 40-45 dB (0.1 nm resolution bandwidth) have been realized [14, 19] Apart from the
noise, some of the control signal power exits from the converted output port of the converter,
even in counter-propagating operation, due to reflections in the chip. The input signal rejection
ratio (ISRR) is defined as the power difference between the converted output and the reflected
control signal. The ISSR is typically 25-35 dB [14, 19].
λprobe
φ 1
φ 2
φ 1
φ 2
φ 1 −
SOA
φ 2
time
λconv
Δτ
delay
λcontrol
Figure 2.4: ((top)) MZI wavelength converter in differential control. (bottom)) Output
phase and of the RZ pulses after the SOAs, and their difference as a function of time.
Bit rates of 40-100 Gb/s can be achieved by employing the MZI in a differential control scheme
(Fig. 2.4), which by-passes the limited recovery time of the SOAs. In differential control, the
signal is not injected into just one MZI arm as before, but in both arms with a time delay .
The control signal consists of short Return-to-Zero (RZ) pulses of £¤ ps. When the control
pulse enters the SOA in the upper arm, the phase of the probe is shifted over radians, virtually
instantaneously, and after the pulse passed the SOA, the SOA recovers ”slowly” (§¨ ps). The
same happens in the lower arm, but later. Since the converted output is determined by the
phase difference of the two arms, the phase shifted probe from the upper arm opens the gate
and that of the lower arm closes it later.