Optical (photonic) Switching

OPTICAL (PHOTONIC) SWITCHING

The optoelectronic switching devices are very useful for computing and light activated logic gates applications.

Optical or photonic switching refers to a phenomenon in which transmission of an optical field through a device is switched among two or more possible states by optical means.

Types of optical switching

There are two types of optical switching such as linear optical switching and non linear optical switching. The Quantum Controlled Stark Effect (QCSE) in p-i (Multi Quantum Well) -n (p type instrinsic - n - type) diode is used for linear optical switching. optically controlled switching and logic devices are based on the QCSE.

Quantum confined stark effect refers to the bending of potential well due to transverse applied electric field and shifting of the absorption edge of exciton to lower energy side and resulting absorption of photons whose energy is less than the original exciton absorption peak energy.

The quantum wells can be reduced in their size as quantum wires and quantum dots. Quantum dots are the nanometre size box like structure. When the electrons are confined in 10 nm scale 3-D semiconductor heterostructures, the electron motion is fully quantized which creates artificial atomic states in semiconductors.

The quantum dots provide the quantum computers which are faster and provide more memory than conventional computers. Each quantum dot consists of about 20 electrons and acts as an optical switch. Here GaAs is the basic material. The spin direction of electron is mainly taken into account for on- off condition.

The nonlinear optical switching is based on the self phase modulation (SPM).

The transmission of light through the device is intensity dependent so that the optical beam itself induces a switching depending on its intensity. This phenomenon is called self phase modulation and it exhibits in optical fiber.

In a short piece of optical fiber, its ends are made highly reflecting through suitable thin film coating. This device is used to select a particular channel in a multichannel optical communication system.

The phase shift introduced by the above fiber depends on intensity and power of the light beam. The nonlinear optical switching provides a faster switching time 10-15 second.

Self Electro Optic Effect Device (SEED)

In p - i (MQW) - n diode, when the reverse bias voltage increases to a large value, the tunneling current varies remarkably as shown in fig.4.36.

The photocurrent-bias voltage characteristic curve, exihibits negative differential resistance (NDR) region (Fig.4.36). The NDR occurs where the heavy-hole (HH) and the light-hole (LH) absorption peaks cross the photon energy of the input light.

This NDR effect is exploited in SEED. Thus the SEED device exhibits photonic switching, bistability and optically induced oscillations.

Slope at NDR range is negative i.e., photo current decreases with increase of bias voltage.

Fig. 4.37(a) shows the p - i (MQW) – n diode in the reverse bias with a series resistor 'Rs'.

Fig. 4.37(b), shows input and output power response of the switching device, Pin is incident optical power and Pout = I2Rs electric output power. Here I is the photo current flowing through resistance Rs.

Working:

• At low bias voltages and low optical power most of the incident light is transmitted. But the photo current increases due to recombination of electrons and holes.

• When the incident optical power increases, the photo current(I) increases due to tunneling of charge carriers. Thus the voltage drop I2Rs across the series resistor increases.

• Since the supply voltage remains constant, the negative bias across the diode decreases. The heavy hole absorption peak is shifted to higher energies.

• Therefore the transmission of light is decreased. This will result more amount of light absorption and increased value of photo current due to tunneling.

• Thus the increase of the input optical power increases the output electric power as shown in fig.4.38.

• At particular input optical power, the heavy hole and light hole absorption peaks cross the photon energy of the input light. Hence there is no absorption of light by exciton or heavy hole or light hole.

Thus the photo current decreases and correspondingly output electric power is also decreased.

• Thus the negative resistance region (NDR) arises as shown in fig. 4.36 and 4.37(b).

• Further increase of input optical power increases the output electric power due to ordinary photon absorption by the diode.

• Thus the state of the device is altered by optical power. That is photonic or optical switching is obtained by light beams with two different powers one for complete transmission and other one for control (zero transmission of light).

• The hysteresis observed in the curve (Fig. 4.37(b)) is due to the asymmetric shapes of the heavy hole and light hole absorption curves. The feedback due to the series resistor is optoelectronic type.

Electro-Optic Switch based on NLO material

An example of an electro-optic switch based on NLO materials is shown in fig. 4.38. The switch is comprised of two parallel waveguides made of NLO materials.

The waveguide channels have a different refractive index from the surrounding material. The light can be switched back and forth between the channels by applying and removing a voltage across the bottleneck. In the absence of an electric field, the light travelling through the lower waveguide interacts with the upper waveguide in a non-linear manner at the bottleneck, causing the light to switch channels.

Switching does not occur when an electric field is applied. The electric field polarizes the NLO material and alters the refractive indices of the two channels, such that the non-linear interaction at the bottleneck is modified and the light stays in the lower waveguide.

Plasmon

A plasmon is a quantum of plasma oscillation. Just as light (an optical oscillation) consists of photons, the plasma oscillation consists of plasmons.

The plasmon can be considered as a quasiparticle since it arises from the quantization of plasma oscillations.

Thus, plasmons are collective oscillations of the free electron gas density. For example, at optical frequencies, plasmons can couple with a photon to create another quasiparticle called a plasmon polariton.