Page 231 - Introduction to Information Optics

216 4. Switching with Optics

Controlled switching can also be realized via soliton collisions (or overlap),
When copolarized solitons with different group velocities collide (or pass
through each other), an offset in arrival time and phase occurs for both solitons
relative to the case when there is no soliton collision. For small-frequency
difference, phase shift greater than it can be induced by the collision [1], This
phase shift can be used in the interference of two solitons, and the output can
be changed from zero to unity, depending on whether or not a collision occurs,

4.2.4.3. Semiconductor Amplifier as a Nonlinear Medium
NOLM was first realized using optical fiber as a nonlinear medium. Very
fast switching processing is possible in a glass fiber interferometer because the
response and relaxation times of the intrinsic nonlinear optical refraction in
glass are a few femtoseconds. The main drawbacks stem from the small optical
nonlinearity. For example, to produce a n nonlinear phase shift at a control
power level of 100 mW, the required length of the fiber at 1.55 /im wavelength
is around 200 km.
For an interferometer with long optical path, the stability is significantly
degraded. It is also difficult to integrate such an interferometer into other
optical devices.
A good nonlinear material is needed in order to implement optical switches.
The essential material requirements are sufficient large nonlinear coefficients
and low optical loss coefficients such that switching operation can achieve at
a practical optical power level. The best nonlinear medium to date for
switching applications appears to be semiconductor optical amplifiers (SOA)
[H]-
A SOA is a laser diode with the reflectivity of the end faces minimized.
Strong nonlinearity of SO As was discovered when associated effects, such as
frequency chirping, were easily detected. In fact, much work has been done to
minimize the nonlinear effects in SOAs. The nonlinearity in SO As is a resonant
effect. If an optical beam with photon energy slightly larger than the bandgap
is incident on an SOA, it is amplified through stimulated emission. The
amplification saturates as the conduction band is depopulated. The change in
gain coefficient due to saturation causes the variations in refractive index, as
described by the Kramer-Kronig relations. The change in refractive index is
proportional to the optical intensity, provided the gain is not fully depleted.
8
The refractive nonlinearity of SOAs is 10 times larger than that of silica fiber,
and optical loss is not an issue since the power of the control and data signals
is amplified.
An interesting configuration using a SOA in a NOLM is an asymmetrical
arrangement as shown in Fig. 4.12 [12]. A data pulse is split into clockwise
and counterclockwise components by the input coupler. Since the SOA is
placed asymmetrically with an offset x in the loop, the clockwise pulse will