![Module 2 syllabus
• Optical Sources and detectors: Light Emitting Diode: LED Structures,
Light source materials, Quantum efficiency and LED power, Laser
Diodes: Modes and threshold conditions, Rate equations, External
quantum efficiency, Resonant frequencies, Photodetectors: The pin
Photodetector, Avalanche Photodiodes.
• WDM Concepts: Overview of WDM, Isolators and Circulators, Fiber
grating filters, Dielectric thin-film filters, Diffraction Gratings. [Text1:
4.2 ,4.3, 6.1, 10.1, 10.3, 10.4, 10.5, 10.7]](https://image.slidesharecdn.com/module2optical-241030093146-b19945ce/85/Module-2-optical-pptx-which-includes-all-contents-of-m2-2-320.jpg)
Module 2 optical.pptx which includes all contents of m2
- 1. Optical Communication Module 2 By Bavana B N Assistant Professor
- 2. Module 2 syllabus • Optical Sources and detectors: Light Emitting Diode: LED Structures, Light source materials, Quantum efficiency and LED power, Laser Diodes: Modes and threshold conditions, Rate equations, External quantum efficiency, Resonant frequencies, Photodetectors: The pin Photodetector, Avalanche Photodiodes. • WDM Concepts: Overview of WDM, Isolators and Circulators, Fiber grating filters, Dielectric thin-film filters, Diffraction Gratings. [Text1: 4.2 ,4.3, 6.1, 10.1, 10.3, 10.4, 10.5, 10.7]
- 3. Optical Sources • Optical transmitter coverts electrical input signal into corresponding optical signal. • optical signal is then launched into the fiber. Optical source is the major component in an optical transmitter .Popularly used optical transmitters are Light Emitting Diode (LED) and Laser.
- 4. Characteristics of Light Source of Communication To be useful in an optical link, a light source needs the following characteristics • It must be possible to modulate the light output over a wide range of modulating frequencies. For fiber links, the wavelength of the output should coincide with one of transmission windows for the fiber type used. • To couple large amount of power into an optical fiber, the emitting area should be small. • To reduce material dispersion in an optical fiber link, the output spectrum should be narrow. • The power requirement for its operation must be low.
- 5. • The light source must be compatible with the modern solid state devices. • The optical output power must be directly modulated by varying the input current to the device • Better linearity of prevent harmonics and intermodulation distortion. • High coupling efficiency. • High optical output power. • High reliability. • Low weight and low cost.
- 6. Light Emitting Diodes(LEDs) p-n Junction: • Conventional p-n junction is called as homojunction as same semiconductor material is sued on both sides junction. The electron-hole recombination occurs in relatively layer = 10 μm. As the carriers are not confined to the immediate vicinity of junction, hence high current densities can not be realized. • The carrier confinement problem can be resolved by sandwiching a thin layer ( = 0.1 μm) between p-type and n-type layers. The middle layer may or may not be doped. The carrier confinement occurs due to bandgap discontinuity of the junction. Such a junction is called heterojunction and the device is called double heterostructure. • In any optical communication system when the requirements is 1. Bit rate f 100- 2—Mb/sec. 2. Optical power in tens of micro watts, LEDs are best suitable optical source.
- 7. LED Structures Hetero juncitons: • A heterojunction is an interface between two adjoining single crystal semiconductors with different bandgap. • Hetero junctions are of two types, Isotype (n-n or p-p) or Antisotype (p-n). Double Heterojunctions (DH): • In order to achieve efficient confinement of emitted radiation double heterojunctions are used in LED structure. A heterojunciton is a junction formed by dissimilar semiconductors. • Double heterojunction (DH) is formed by two different semiconductors on each side of active region. Fig. 3.1.1 shows double heterojunction (DH) light emitter.
- 9. • The crosshatched regions represent the energy levels of free charge. Recombination occurs only in active In GaAsP layer. • The two materials have different band gap energies and different refractive indices. • The changes in band gap energies create potential barrier for both holes and electrons. The free charges can recombine only in narrow, well defined active layer side. • A double heterojunction (DH) structure will confine both hole and electrons to a narrow active layer. • Under forward bias, there will be a large number of carriers injected into active region where they are efficiently confined.
- 10. • Carrier recombination occurs in small active region so leading to an efficient device. • Another advantage DH structure is that the active region has a higher refractive index than the materials on either side, hence light emission occurs in an optical waveguide, which serves to narrow the output beam.
- 11. Light Source Materials • The spontaneous emission due to carrier recombination is called electro luminescence. To encourage electroluminescence it is necessary to select as appropriate semiconductor material. • The semiconductors depending on energy bandgap can be categorized into 1) Direct bandgap semiconductors. 2) Indirect bandgap semiconductors. • In direct bandgap semiconductors the electrons and holes on either side of bandgap have same value of crystal momentum. Hence direct recombination is possible. The recombination occurs within 10-8 to 10-10 sec.
- 12. • In indirect bandgap semiconductors, the maximum and minimum energies occur at different values of crystal momentum. The recombination in these semiconductors is quite slow i.e. 10- 2 and 10- 3 sec. • The active layer semiconductor material must have a direct bandgap. In direct bandgap semiconductor, electrons and holes can recombine directly without need of third particle to conserve momentum. • In these materials the optical radiation is sufficiently high. These materials are compounds of group III elements (Al, Ga, In) and group V element (P, As, Sb). Some tertiary allos Ga1-x Alx As are also used.
- 14. • The peak output power is obtained at 810 nm. The width of emission spectrum at half power (0.5) is referred as full width half maximum (FWHM) spectral width. For the given LED FWHM is 36 nm. • The fundamental quantum mechanical relationship between gap energy E and frequency v is given as • where, energy (E) is in joules and wavelength (λ) is in meters. Expressing the gap energy (Eg) in electron volts and wavelength (λ) in micrometers for this application.
- 15. Different materials and alloys have different band gap energies • The bandgap energy (Eg) can be controlled by two compositional parameters x and y, within direct bandgap region. The quartenary alloy In1-x Gax Asy P1-y is the principal material sued in such LEDs. Two expression relating Eg and x,y are
- 16. Quantum Efficiency and Power • The internal quantum efficiency (ηint) is defined as the ratio of radiative recombination rate to the total recombination rate.
- 20. Laser Diode • The laser is a device which amplifies the light, hence the LASER is an acronym for light amplification by stimulated emission of radiation. • The operation of the device may be described by the formation of an electromagnetic standing wave within a cavity (optical resonator) which provides an output of monochromatic highly coherent radiation.
- 21. Principle : • Material absorb light than emitting. Three different fundamental process occurs between the two energy states of an atom. 1)Absorption 2) Spontaneous emission 3) Stimulated emission. Laser action is the result of three process absorption of energy packets (photons) spontaneous emission, and stimulated emission. (These processes are represented by the simple two-energy-level diagrams). Where E1 is the lower state energy level. E2 is the higher state energy level.
- 22. • Quantum theory states that any atom exists only in certain discrete energy state, absorption or emission of light causes them to make a transition from one state to another. • The frequency of the absorbed or emitted radiation f is related to the difference in energy E between the two states. • If E1 is lower state energy level. and E2 is higher state energy level E = (E2 – E1) = h.f. Where, h = 6.626 x 10-34 J/s (Plank’s constant). • An atom is initially in the lower energy state, when the photon with energy (E2 – E1) is incident on the atom it will be excited into the higher energy state E2 through the absorption of the photon
- 24. • When the atom is initially in the higher energy state E2, it can make a transition to the lower energy state E1 providing the emission of a photon at a frequency corresponding to E = h.f. • The emission process can occur in two ways. 1)By spontaneous emission in which the atom returns to the lower energy state in random manner. 2)By stimulated emission when a photon having equal energy to the difference between the two states (E2 – E1) interacts with the atom causing it to the lower state with the creation of the second photon.
- 26. • Spontaneous emission gives incoherent radiation while stimulated emission gives coherent radiation. Hence the light associated with emitted photon is of same frequency of incident photon, and in same phase with same polarization. • It means that when an atom is stimulated to emit light energy by an incident wave, the liberated energy can add to the wave in constructive manner. The emitted light is bounced back and forth internally between two reflecting surface. • The bouncing back and forth of light wave cause their intensity to reinforce and build-up. The result in a high brilliance, single frequency light beam providing amplification.