OPTICAL SWITCHING-SEED,Bipolar Controller Modulator

 OPTICAL SWITCHING

It is a process by which an optical signal can be routed to a desired communication path without converting them to electrical signal.

There are two types of optical switches

• slow switch-used to route around a fault  in the optical fiber
• Fast Switch-uses Electro optic or magneto optic effect for switching

There are 3 broad classification of  switches

1.Mechanical Method

Mirrors and prisms are used to divert incoming optical signals to the required path. They can be controlled easily as they are simple in structure.Miniaturization and application in large scale communication equipment is difficult.

2. MEMS (Micro electro mechanical systems)

The Mechanical components,sensors and actuators are fabricated on a silicon substrate.They are ideal for large scale switching.Their Power consumption is minimal.They need high processing capabilities for manufacturing.

3.Optical Wave guide Method

These switches utilize the refractive index of substrates to guide optical signals to their preferred destination.They can be easily integrated into small components

Multiple Quantum wells(MQW) and the Electro Absorption Modulator

Figure 1 Multiple Quantum well*

• Larger changes in the absorption of material can be realized by Multiple quantum well material(MQW)
• Quantum wells are very thin layers of two different semiconductor materials with different bandgaps eg:GaAs-AlGaAs Quantum wells
• The layers are so thin that electrons and holes are trapped inside the well.They should gain energy to move out of the barriers surrounding them
• When the photon energy is greater than the bandgap,the absorption coefficient of the material increases.
• When a material absorbs photon greater than the bandgap exciton (electron hole bound state)is formed.
• This change in absorption is explained as quantum confined stark effect(QCSE)
• Quantum wells are sufficiently separated(50Å) to avoid coupling between wells
• Voltage applied across the modulator affects the absorption of light.This affects the amount of photo current generated.

How does an electro absorption Modulator modulates the optical signal?

When electric field is applied to the material,there is a change in absorption of light,which in turn changes the transmission of light through the modulator.

Self electro optic Effect Device(SEED)

• SEED is a particular use of Multiple Quantum well Electro absorption modulator. 
• They can exhibit bi stability and perform optical logic function. 
• Electro absorption modulators modulate an Incident optical beam by absorbing some of its optical power.The amount of absorption is controlled by the voltage applied to the modulator.They are made from semiconductor materials.The amount of optical power absorbed depends on absorption coefficient(𝝰)of the material.It depends of the wavelength of light ,which is further dependent on the electric field applied.
• The basic SEED circuit is shown in figure 2

Figure 2:Schematic diagram of SEED*

• In a Multiple quantum well,optical detection is done by

1.Absorption of Incident Photons

2.Recombination of electron hole pairs,excited by photons

3.tunneling of electrons and holes through the barriers to generate photo current

• When the electric field applied is low,rate of tunneling is small ,recombination process dominates and small photo current is generated
• When the electric field is high,rate of tunneling is more and the photo current generated is based on the absorption spectra of the multiple quantum well.
• The Photo current generated is given by

Iph=qPinc𝞪(V)W 

      —---------------

              hv

V - Applied Bias; Pinc- Incident Optical power; W - Width of MQW region

• When the light intensity changes,the photo current generated changes and voltage across the device also changes
• This further influences the absorption and transmission of the modulator
• The state of the device is changed based on the light intensity and thus photonic switching is performed

BIPOLAR CONTROLLER MODULATOR

• 2 optical beams for transmission and control can be used for switching
• The path and effects of the signal and control beam are indistinguishable. To make SEED compatible with Opto electronic Integrated circuit Technology, Gain is important in the circuit.
• To achieve Gain,we can connect a photo transistor in series with the SEED
• The control beam falls on the Transistor and the signal beam on the SEED modulator
• The transistor current is usually high and so the Modulator current should be high.This is achieved by using p-i(MQW)-n modulator in the form of the waveguide
• Another method to improve gain is to use a hetero junction bipolar transistor (HBT)with a multiple quantum well in the base collector junction. By having a transistor,we can apply a large electric field to collector base to cause Quantum confined stark effect (QCSE).The base terminal Provides control for better opto electronic coupling
• The Transistor amplifies the photo current generated in the MQW and by changing its collector base voltage gives a voltage feedback to the MQW-HBT
• The Modulator and the controller are connected in parallel and the load is in series with the controller and Modulator
• The signals are detected in both controller and Modulator. They give a Feedback through the Load and change their light absorption capabilities
• 
• Figure 3: Integrated Controller Modulator

• In a different Application,if there is a change in the light input of the controller,the light passing through the modulator will also be changed.
• In another Application,an On-off Optical signal is used to change the optical states of the modulators