As for the discrete high speed switch, the LiNbOs-based electro-optic modulator is the
most commonly used device, offering 20GHz bandwidth operation with lumped circuit
design. Beyond this speed the traveling wave type configuration is required for optical
and microwave velocity matching. Being more compact and integrable with lasers, the
electro-absorption switch became successful in lOGbps system. For advanced optical
network, additional functions are required at the nodes, such as add-drop function,
wavelength conversion, and optical cross connect. For these purposes the devices must
satisfy the requirements of compactness, integrability, low insertion loss, wavelength
bandwidth, in addition to the response speed and sensitivity.
The semiconductor optical ampMier (SOA) is a promising device as high-speed all-optical
switch, because of the compactness, speed and low insertion loss. As an example a cross
phase modulation @I'M) switch is composed of symmetric Mach Zehnder(MZ)
interferometer with a pair of SOA inside. Upon exciting one of the SOA's with control pulse,
both gain saturation and phase modulation are induced, causing the imbalance of the
symmetric interferometer, thus the signal pulse is switched. Within the wavelength
bandwidth of SOA the device can operate with two wavelengths, functioning as the
wavelength converter switch. The push-pull symmetric MZ switch was proven to operate at
100 Gbps and beyond, because the response time is related to the rise time and not the fall
time. SOA also shows high conversion efficiency as the four wave mixing device, thus
suitable for wavelength converter: Though a semiconductor saturable absorber switch has
no gain, the resonance enhancement with asymmetric Fabry-Perot microcavity cont,aining
a quantum well saturable absorber was shown to operate below 1 pJ pump pulses. One of
the problems of semiconductor optical switch is the coupling loss issue. In-line fiber-optic
switches such as NOLM being free from this, a number of 0-TDM system trials utilize such
fiber-optic all optical switches, with the sacrifice of compactness.
The comparison among the switches should be made with the consideration of individual
system requirements.