Page 233 - Introduction to Information Optics

21.8 4. Switching with Optics

As a result, a very high speed pulse train can be demultiplexed down to a lower
base rate. In addition, the base rate in this scheme can be as high as 100 GHz,
much higher than the value (a few GHz) limited by the carrier lifetime in SOA,
The reasons for this are as follows [13].
First, it takes only a small amount of carrier density change to create an
additional phase shift of p. For example, for a 500-/mi- long InGaAsP SOA
and a wavelength of 1.5/^m, a n phase shift corresponds to a change in
3
refractive index of 10~~ . The rate of change of refractive index with electron
hole pair density is

3
20
(cm ) = 2 x 1(T . (4.27)
dN
17
Therefore, a carrier population change (AN) of 10 cm 3 is needed to have n
18
3
phase shift, compared with a full population inversion of 10 cm" . In other
words, it is not necessary to have a full decay in the SOA to have a n phase
shift. There would be a phase shift of several n during the complete decaying
of the gain and refractive index in SOA.
Second, the time needed for carrier injection via electric current can be
3 3
accomplished at a typical rate of 4 x 10~ cm per picosecond. Therefore, the
17 3
time needed for replenishment of 10 cm"" is only 25 ps. With a higher
injection current and hence a shorter carrier lifetime, it was demonstrated that
a phase shift of n can be achieved in lOps, implying a base rate as high as
100 GHz,
This scheme has found important applications in high-speed demultiplexing
and digital optical 3R regeneration (reamplirication, reshaping, and retiming).
Digital demultiplexers (Fig. 4.14) with lOOGbit/s clock rate and optical digital
regenerator (Fig. 4.15) operating at 160Gbits/s have been demonstrated [11].

Base clock as
control

Demultiplexed
output
Data
11010001111 1 0 i

Fig. 4.14. Digital optical demultiplexers.