OPTICAL TRANSMISSION

UNDERSTANDING

OF

Information to be transmitted

(Electrical)

Modulation&

Transmitter(Electrical to

Optical )

Recovered Information

Electronics for InformationRecovery

Receiver(Optical to Electrical )

BASIC OPTICAL COMMUNICATION LINK

Optical Fiber

Coupling Arrangement

TRANSMITTER SOURCES

LED

LASER

COUPLING MECHANISM

APD

PIN-FET

RECEIVER

We will discuss ………..

OPTICAL TRANSMITTER

The transmitter converts an electrical analog or digital

signal into a corresponding optical signal.

The source of the optical signal can be either a light

emitting diode (LED), or solid state laser diode (LD).

The most used wavelengths of operation for optical

transmitters are 850, 1300, or 1550 nanometers.

A key advantage of Optical Communication over electrical communication

The input electrical signal need not to modulated by using locally generated carrier .

The signal can directly be modulated by Laser or LED. This arrangement is known as Directly Modulation or Intensity Modulation.

This will reduce the cost of equipment.

TYPES OF OPTICAL SOURCES

Incoherent Source

LED

Edge-LED

Surface –LED

Coherent Source

Laser

Fabry-Perot

Distributed feedback

VCSEL

TYPICAL FIELD PATTERN OF LED AND LASER

Operating Principle of LASER and LED

LASER Works on the principle of Stimulated emission, while LED works on the principle of spontaneous emission

Absorption

Spontaneous Emission

An electron in the valence band acquires some amount of energy,and there by moves from valence band to the conduction band

The electron cannot remain in the higher energy level for a longer duration and thus returns back from the conduction band to the valence band

When it returns back from conduction band to valence band there will be emission of radiation(Photon),the wavelength of which is proportional to the difference in band gap of the two

Spontaneous Emission

Stimulated Emission

A passing photon interacts with an electron in the higher energy level,stimulating the emission of an identical photon as the electron drops down to the lower energy level

Thus single photon results in the emission of an identical photon

The wavelength of the emitted photon is the same as the incident photon and will be in coherence with it

Stimulated Emission

Operation Of PN-Junction Silicon has 4 electrons in its valence band,if we add an

impurity which has 5 valence electrons there will be extra electrons left over after all the bonds are formed(Stable covalent bond and no free electrons)

Thus resulting in the formation of a N-type material

If we dope silicon with a material having only 3 valence electrons,then there will be holes that are formed

Though holes do not have mobility like electrons,they are charge carriers resulting in the formation of a P-type material

Operation Of PN-Junction

If a junction of P and N type material is formed it results in the formation of a PN-Junction

In a PN-Junction there will be movement of electrons and a recombination of electron- hole pair takes place thus, no charge carriers exist in the depletion layer thus preventing any carrier flow

This barrier prevents further flow of electrons

Operation Of PN-Junction

When sufficient energy is applied,electrons are injected into depletion layer

This forces the electrons which are already present to move across the junction resulting in a forward bias

Thus the electron-hole re-combination takes place

When the electrons re-combine with the holes of LASER/LED photons are emitted,wavelength of which depends on the band-gap of the material

Operation Of PN-Junction

Operation Of PN-Junction

Combining,P and N type materials as indicated above results in the formation of a stable covalent bond with no free electrons

By varying the proportion of this P and N type material a PN junction with desired amount of free electrons can be formed

Semiconductors like Ga-As are used for producing such electrons, thus resulting in the emission of radiation

LED

An LED which is formed from a single type of a semiconductor material, is called homo-junction LED and results in the emission of a light which is omni-directional(or in all directions)

This type of LED is suitable for short distances of fiber(Has lesser coupling efficiency)

An LED can also be formed by the creation of a PN junction with different crystalline properties and refractive indices resulting in a hetero junction LED

This allows the LED perform itself as a wave-guide thus coupling light to the maximum extent possible

LED

LASER

LASER is an acronym for:

L-Light

A-Amplification by

S-Stimulated

E-Emission of

R-Radiation

Laser principle

In a practical Laser device,light generating material is placed between a set of reflectors that has been aligned parallel to each other

The optical cavity formed by the two mirrors is called a Fabry Perot Resonator

As energy is applied to the Laser material photons are emitted spontaneous emission

Some of the photons are lost due to reflections from the sides

However a single photon emitted by this spontaneous emission is sustained within the cavity and results in stimulated emission of 2 photons

FABRY-PEROT LASER DIODE

FABRY-PEROT LASER DIODE

These two photons result in the emission of two more and so on thus resulting in the emission of a number of photons

All the photons that are emitted will be in phase and thus LASER source is said to be coherent

The length of the resonant cavity can be varied,resulting in the emission of a desired wavelength

In a semi conductor LASER diode beam is available from both the ends

A photo detector can be placed at one end to monitor the power level of the emitted radiation allowing drive circuitry to keep the output level constant

FABRY-PEROT LASER DIODE

DISTRIBUTED FEED BACK LASER DIODE

Fabry-Perot output is lots of different wavelengths, DFB gives just one.

DFB is similar to Fabry-Perot cavity, but with grating above active layer

Grating is a zig-zag structure, and its different refractive index causes reflection at particular wavelength.

Exact wavelength reflected depends on refractive index change and zig-zag spacing.

Only desired wavelength reflected back into laser cavity and emitted.

Much more expensive than Fabry-Perot lasers due to increased complexity.

DISTRIBUTED FEED BACK LASER DIODE

OUTPUT SPECTRUM OF

FP AND DFB LASER DIODES

FP laser diode the cavity is formed by two mirror by cleaving the end edge of the laser.

FP laser support multi-longitude mode with in the cavity.

In DFB laser diode the cavity is formed by the grating which can only support one longitudinal mode.

VERTICAL CAVITY SURFACE EMITTING LASER(VCSEL)

VCSEL’s have vertical laser cavity and emit light vertically from their surface

Layers of partially reflective mirrors above and below cavity to confine light

More efficient than edge emitters, cheaper to test, and easier to get light into fibers

Currently only low powers and not mass produced at 1550nm.

DFB lasers have a number of significant advantages over FP types:

They can exhibit very narrow line-widths (of the order of 50 kHz).

They have quite low chirp(Spectral Width).

They typically have a very low Noise

ADVANTAGES

DFB lasers are extremely sensitive to reflections.

They are sensitive to temperature variations in two ways:

The stable (average) temperature of the device has a very strong influence on wavelength.

Varying conditions produce significant fluctuations in laser output power.

They have a relatively high cost.

DISADVANTAGES

3 Level Laser

4 Level Laser

2 Types of collusion

He-Ne Laser

Coupling efficiency

Spectral width

Types of modulation

Linearity requirements

Bandwidth

Cost

Required o/p power

Factors to be taken into consideration

while selecting the optical source

COUPLING EFFICIENCY

COUPLING MECHANISM

COUPLING

Coupling

Optical source must ensure that the optical energy must be efficiently coupled into the core of the fiber

Fiber will transmit light launched into its Numerical Aperture

Pig-tailing

Placing the fiber’s tip in very close proximity to an LED or LD.

There are two popular methods for light coupling into the fiber

COUPLING TO A FIBER

Use of a Graded Index Lens (GRIN lens) is fairly common. A GRIN lens is very similar to just a short length of graded index fiber (albeit with a much larger diameter). The lens collects and focuses the light onto the end of the fiber.

A Ball lens is also often used. This is bonded to the surface of the LED with an epoxy resin that has a specific refractive index.

The Direct Coupling method is becoming increasingly popular. Just mount the fiber end so that it touches the LED directly. A common way to do this is to mount the LED inside a

connector so that when a fiber is plugged in. This has the advantage of low cost and low complexity.

Another common way is to fix a ball lens to the end of the fiber.

OPTICAL RECEIVER

The light emerging from the end of an optical fiber link must be detected and converted into electrical pulses for further processing so that the transmitted information can be received. The two types of detectors normally used are;

The Avalanche Photodiode (APD)

The PIN Photodiode (PIN).

LIGHT DETECTORS

Light Detectors

Photons can be absorbed by the semiconductors,there by releasing electrons from the valence band

These electrons combines with holes resulting in the formation of electron-hole pairs

For a photon to be absorbed,the wavelength of the photon should match with that of the band gap

Thus certain materials can only absorb photons of specific wavelength,hence the wavelength of the source and the detector must match

P-I-N DIODE DETETOR

A photodiode is reverse biased,with resistance being more there will be a large depletion region that is formed thus no current flows

If incident photon strikes depletion region the resulting electron-hole pair move towards their corresponding destinations resulting in a current flow

If the photon strikes outside depletion region, time taken by the electron-hole pair to move towards their respective destinations will be more and thus resulting in a delay

P-I-N DIODE DETECTOR

In a PIN diode there is an Intrinsic region that is formed between P and N junctions(Intrinsic region is a region which doesn’t have any dopants)

Since it doesn’t have any charge carriers,the resistance offered by the intrinsic region will be more

Due to this intrinsic region,the width of the depletion region will increase

As a result,there will be a faster response and efficiency also will increase

Thus a PIN diode will overcome the limitations of a normal PN-diode

PIN diode is basically a one to one device,that is one-electron hole pair creating only one electron-hole pair requiring more amplification

PIN Diode

AVALANCHE PHOTO DIODE

An APD is a photo multiplier device which provides light amplification as a part of its function

It has a heavily doped P and N junctions,along with an intrinsic layer resulting in a strong electric field

When a photon strikes a depletion region an electron hole pair is created

If the electric field is strong, these carriers strike a neutral atom and there will be an increase in its kinetic energy which makes an electron to move to its valence band and thus resulting in the formation of another carrier(Electron-hole pair)

Avalanche Photodiode(APD)

An APD is a photo multiplier device which provides light amplification as a part of its function

It has a heavily doped P and N junctions,along with an intrinsic layer resulting in a strong electric field

When a photon strikes a depletion region an electron hole pair is created

If the electric field is strong, these primary carriers strike a neutral atom and there will be an increase in its kinetic energy which makes an electron to move to its valence band and thus resulting in the formation of another carrier(Electron-hole pair)called the secondary carrier

This carrier results in the formation of one more electron-hole pair and so on resulting in a number of such secondary carriers

The gain depends upon the applied bias voltage, larger the value more is the gain

Typically the voltage that is applied will be hundred volts

The bias voltage should not be very high,collision ionization occurs without incident photon

If the gain becomes very high,signal to noise ratio will increase

Avalanche Photodiode(APD)

Avalanche Photodiode(APD)

Detector Responsivity (Detectivity): This is the ratio of output current to input optical power.

Hence this is the efficiency of the device. Spectral Response Range:

This is the range of wavelengths over which the device will operate.

Response Time:This is a measure of how quickly the detector can

respond to variations in the input light intensity. Noise Characteristics:

The level of noise produced in the device is critical to its operation at low levels of input light.

KEY PARAMETERS OF OPTICAL DETECTOR