Ultra-fast Switching
In order to optimize transmission speeds for information encoded in femtosecond light pulses, optical switches must also have switching speeds on When the order of femtoseconds. Below are a few selected optical switches that can accommodate ultra-fast optical time division multiplexed signals.
The Optical Stark Effect
When light interacts with a semi-conductor, electron-hole pairs called "excitons" are formed. When a short pulse of light with a wavelength very close to that which excitons in quantum wells absorb, there is a temporary shift in the wavelength that excitons absorb. This phenomenon is known as the "optical Stark effect."
For an optical signal, having the wavelength corresponding to the shifted absorption for excitons, changes in absorbance could stop the transmission of the signal. If this phenomenon were integrated into an optical switch, with an incident light pulse of a few femtoseconds, it would be conceivable to experience switching speeds of less than 10 fs.
The only problem with this method is that it requires a prohibitively high excitation power.
Symmetric Mach-Zehnder Type All Optical Switches
As mentioned above, optical switches must have a low switching energy in order for the device to be practical in the context of a commercial optical system. Generally, the maximum switching energy allowed in practical considerations is 1 pJ (10-12 Joules).
For a symmetric Mach-Zehnder (SMZ) optical switch, the control pulses can alter the characteristics of the nonlinear waveguides. When a signal passes through these waveguides, the difference in nonlinear phase shifts defines the state of the switch. As shown, this switch can split parts of the signal into two fibers using the priciple of interferometry. The switching speed for this device is determined by little more than the length of the control pulses.
Terahertz Optical Assymetric Demultiplexer (TOAD)
The TOAD switch is essentially a fiber loop joined at the base by an optical coupler. The input signal is then split into two equal parts that propagate around the loop in opposite directions and recombine at the coupler to make the output signal. Also on the loop is another coupler for the addition of a control signal and a semi-conductor optical amplifer (SOA) which is used to cause a large phase shift in the signal.
Without any control signal, the switch is off. The two parts of the input signal travel the same distance through the same medium and, after the loop, arrive back at the coupler exactly in phase and return to the input fiber. The purpose of the control signal is to deplete the gain of the SOA and change the index of refraction. After the control pulse, the SOA gradually recovers to its original state. Since the two parts of the signal pass through the SOA at different time, the control signal can be timed to affect one part of the split signal but not the other. In this way, a phase difference is introduced to the parts of the split signal. With the appropriate timing, the two signals can be shifted out of phase such that the entire signal passes to the output fiber. The filter in the output fiber is to absorb any remnant of the control signal.