UNIT-3

Mohammad Asif IqbalAssistant Professor, Deptt of ECE,JETGI, Barabanki

What is an Optic Source?

• The heart of a fiber optical data system

• A Hybrid Device • Converts electrical signals into optical signals

• Launches these optical signals into an optical fiber for data transmission.

• Device consists of an interface circuit, drive circuit, and components for optical source. (LEDs, ELEDs, SLEDs, LDs, etc)

Both n -type and p -type materials are formed byadding a predetermined number of impurityatoms to a silicon base. An n -type material iscreated by introducing impurity elements thathave five valence electrons ( pentavalent ), suchas antimony , arsenic , and phosphorus.

Semiconductor Basics

The p -type material is formed by doping a pure germanium or silicon crystal with impurityatoms having three valence electrons.

Note that there is now an insufficient number ofelectrons to complete the covalent bonds of thenewly formed lattice. The resulting vacancy iscalled a hole and is represented by a small circle ora plus sign, indicating the absence of a negativecharge. Since the resulting vacancy will readilyaccept a free electron:The diffused impurities with three valenceelectrons are called acceptor atoms.

Note that the four covalent bonds are still present.There is, however, an additional fifth electron due tothe impurity atom, which is unassociated with anyparticular covalent bond. This remaining electron,loosely bound to its parent (antimony) atom, isrelatively free to move within the newly formed n -type material. Since the inserted impurity atom hasdonated a relatively “free” electron to the structure:Diffused impurities with five valence electronsare called donor atoms.

Semiconductor Basics

n-type p-type

As we have discussed earlier In an n -type material,the number of free electrons has changedsignificantly, but the number of holes has not changedsignificantly from this intrinsic level. The net result,therefore, is that the number of electrons faroutweighs the number of holes. For this reason: In ann-type material the electron is called the majoritycarrier and the hole the minority carrier.

For the p -type material the number of holes faroutweighs the number of electrons,Thus In a p-type material the hole is the majoritycarrier and the electron is the minority carrier.

When the fifth electron of adonor atom leaves the parentatom, the atom remainingacquires a net positivecharge: hence the plus sign inthe donor-ion representation.For similar reasons, theminus sign appears in theacceptor ion.

NP

Now let's join both of them

Depletion Region

At the instant the twomaterials are “joined” theelectrons and the holes in theregion of the junction willcombine, resulting in a lack offree carriers in the regionnear the junction, Note theonly particles displayed inthis region are the positiveand the negative ionsremaining after theabsorption of free carriers.This region of uncoveredpositive and negative ions iscalled the depletion regiondue to the “depletion” offree carriers in the region.

LIGHT-EMITTING DIODES

• As the name implies, the light-emitting diode is a diode that gives off visible or invisible(infrared) light when energized.

• In any forward-biased p – n junction there is, a recombination of holes and electrons withinthe structure and primarily close to the junction.

• This recombination requires that the energy possessed by the unbound free electrons betransferred to another state.

• In all semiconductor p – n junctions some of this energy is given off in the form of heat andsome in the form of photons.

• In Si and Ge diodes the greater percentage of the energy converted during recombination atthe junction is dissipated in the form of heat within the structure, and the emitted light isinsignificant.

• For this reason, silicon and germanium are not used in the construction of LED devices. Onthe other hand

• Diodes constructed of GaAs emit light in the infrared (invisible) zone during therecombination process at the p–n junction.

Even though the light is not visible,infrared LEDs have numerousapplications where visible light is nota desirable effect. These includesecurity systems, industrialprocessing, optical coupling, safetycontrols such as on garage dooropeners, and in home entertainmentcenters, where the infrared light ofthe remote control is the controllingelement.

Through other combinations of elements a coherent visible light can be generated as shown in the table below

Process of electroluminescence in the LED

P N

Recombination

Recombination

Recombination

Recombination

Recombination

Emitted visible light

Metallic contact

Characteristic and symbol

SLEDs – Surface Emitting LEDs

• Primary active region is a small circular area located below the surface of the semiconductor substrate, 20-50µm diameter and up to 2.5µm thick.

• Emission is isotropic and in lambertian pattern.• A well is etched in the substrate to allow the direct

coupling of emitted light to the optical fiber• Emission area of substrate is perpendicular to axis

of optical fiber• Coupling efficiency optimized by binding fiber to the

substrate surface by epoxy resin with matching refractive index

Surface Emitting LED

ELEDs – Edge Emitting LEDs

• Primary active region is a narrow strip that lies beneath the semiconductor substrate

• Semiconductor is cut and polished so emission strip region runs between front and back.

• Rear face of semiconductor is polished so it is highly reflective while front face is coated with anti-reflective, light will reflect from rear and emit through front face

• Active Regions are usually 100-150µm long and the strips are 50-70µm wide which are designed to match typical core fibers of 50-100µm.

• Emit light at narrower angle which allows for better coupling and efficiency than SLEDs

Edge Emitting LED

LDs – Laser Diodes

• Emit coherent light through stimulated emission

• Mainly used in Single Mode Systems

• Light Emission range: 5 to 10 degrees

• Require Higher complex driver circuitry than LEDs

• Laser action occurs from three main processes: photon absorption, spontaneous emission, and stimulated emission.

Laser Diode Optical Cavity

• One reflecting mirror is at one end while the other end has a partially reflecting mirror for partial emission

• Remaining power reflects through cavity for amplification of certain wavelengths, a process known as optical feedback.

• Construction very similar to the ELEDs.

Lasing Characteristics

• Lasing threshold is minimum current that must occur for stimulated emission

• Any current produced below threshold will result in spontaneous emission only

• At currents below threshold LDs operate as ELEDs

• LDs need more current to operate and more current means more complex drive circuitry with higher heat dissipation

• Laser diodes are much more temperature sensitive than LEDs

Tunable Laser

• Tunable Laser

• Employed in broad-band interconnections and broadcast networks where the need for high power, narrow line width, and a tunable single-frequency emission is a must.

• Laser that is able to produce controllable multiple wavelengths within single cavity.

• Able to switch transmission of different wavelengths without using multiplexer for transmission to many different channels at by tuning the output frequency to its designated channel.

Tunable Laser Cavity

• Consists of an Active Region, and two passive regions: Phase Control and Grating

• Active region is a double heterostructure of a low bandgap between two high gap low index claddings.

• Two passive regions made from semiconductor with intermediate bandgap between active and cladding.

Tunable Laser Operation

• Current is injected into the Active Region causing the entire optical cavity to oscillate in a single longitudinal mode.

• A current is then injected into the grating control region causing a refractive index decrease which induces a shift of the Bragg wavelength and variation in the mode.

• The phase region with the injected phase current allows for recovery in Bragg wavelength in order to keep the same mode in the center of the filter band.

• This results in an output with variable wavelength.

Summary

• Optical light sources convert electrical signals into optical signals and launch them.

• Commonly used light sources include LEDs, ELEDs, SLEDs, and LDs.• LEDs produce nonlinear incoherent light whereas a Laser Diode

produces linear coherent light.• Incoherent light sources used in multimode systems as where Laser

Diodes/Tunable Lasers in single mode systems• Laser diodes must operate above their threshold region to produce

coherent light, otherwise operating as ELED.• Laser diodes are much faster in switching response than LEDs• Tunable laser is able to produce coherent light output with controlled

variable wavelength • Tunable laser is used in multi wavelength systems by replacing a

system where many sources are coupled into a multiplexing device system

THANK YOU!