1. Optical Sources

2. Optical Source Main function Types
Convert electrical energy into optical energy (with the condition that light output to be effectively launched or coupled into optical fiber)
Types
Wideband continuous spectra (incandescent lamps)
Monochromatic incoherent ( LEDs)
Monochromatic coherent (LDs)

3. Optical Sources Requirements
Size and configuration compatible with launching light into the fiber( light should be highly directional)
Should be linear (accurately track the electrical input signal ,minimize the distortion & noise)
Emit light at wavelengths where fiber has low losses, dispersion and where detectors are efficient
Capable of simple signal modulation (direct modulation) ranging from audio frequencies to GHz range
Should couple sufficient optical power to overcome attenuation in the fiber

4. Optical Sources
Should have narrow spectral bandwidth (line width) in order to minimize dispersion
Must be capable of maintaining stable optical output (that does not change with ambient conditions)
Should be comparatively cheap and highly reliable in order to compete with conventional transmission techniques

5. Basic Concept (LASER) Absorption and emission of radiation
The interaction of light with matter takes place in discrete packets of energy or quanta, called photons. The absorption and emission of light causes them to make a transition from one state of energy to another.
E = E2 – E1 = hf
Two types of emission
Spontaneous (entirely random manner)
Stimulated (photons have identical energy, in phase and polarization)

6. Basic Concept (LASER) Population inversion Pumping
More electrons in higher energy level
Necessary to achieve optical amplification
Pumping
Process to achieve population inversion usually through external energy source

7. Basic Concept (LASER)

8. Basic Concept (LASER) Optical feedback and laser oscillations
Photon striking an excited atom causes emission of a second photon which release two more photons creating an avalanche multiplication.
Amplification &Coherence achieved by (Febry – Perot resonator)
Placing mirrors at either end of the amplifying medium
Providing positive feedback
Amplification in a single go is quite small but after multiple passes the net gain can be large
One mirror is partially transmitting from where useful radiation may escape from the cavity
Stable output occurs when optical gain is exactly matched with losses incurred (Absorption, scattering and diffraction)

9. Basic Concept (LASER) Optical feedback and laser oscillations
Oscillation occur over small range of frequencies
These frequencies achieve sufficient gain to overcome cavity losses and radiate out
Thus source emits over a narrow spectral band and the device is not perfectly monochromatic

10. Semi Conductor Emission
Types of material
Conductor
Insulators
Semi conductors (intrinsic & extrinsic)
N type material
Donor impurity added
Increases thermally excited electrons in the conduction band
P type material
Acceptor impurity added
Increases positive charges (holes) in the valance band
PN junction

11. Semiconductor
The properties of Semiconductor lies in between conductor and insulator. The semiconductor has valance band, conduction band and Fermi level ( the gap between two bands ). The Fermi level is relatively small which means that small amount of energy is sufficient to bring about electric current in semiconductors.
Conduction Band
Valance Band
Fermi Level

12. The electrical resistance of semiconductor lies in between conductors and insulators. The increase in temperature can lower the resistance in semiconductors. Silicon is the most common semiconductor used.
The Fermi level can be increased or decreased by adding dopants into Silicon.
If P-type material such as Aluminum, Gallium or Indium are added, which create holes and shortage of electrons. Fermi level is increased. If N-type material such as Phosphorus, Arsenic and Boron are added the result is excess of electrons and Fermi level is reduced.
P.N Junction
If a voltage is applied to P.N Junction ( Forward biased ). The Fermi Level on both sides of Junction will move, so that a current will flow through the P.N Junction. The electrons will flow into the P layer and holes are formed in N layer. The system tries to reach equilibrium by excited electrons flowing to holes . During this process ,energy is released in the form of Photons. It is the mechanism of Light emitting diodes. If p layer and N layer in the P.N Junction are heavily doped and strong current used a population inversion of electrons occurs in an Optical Cavity ,a Laser can be created.

13. Semiconductor Injection Laser
Stimulated emission by recombination of injected carriers
Optical cavity provided in the crystal structure
Advantages are
High radiance due to amplifying effect
Narrow line width of the order of 1nm
High modulation capabilities presently ranging in G Hz
Good spatial coherence which allows the output to be focused by a lens into a spot to provide high coupling efficiency

14. Semiconductor Injection Laser
Efficiency
External quantum efficiency ηD
Ratio of increase in photon output rate for a given increase in the number of injected electrons
ηD = (d Pe/hf) / d I/e)
Internal quantum efficiency ηi
Ratio of the total number of photons produced the cavity to the number of injected electrons
Total efficiency ηT
Ratio of the total number of output photons to the number of injected electrons

15. Semiconductor Injection Laser
Laser Modes
Does not emit single wavelength but a series of wavelengths at different modes
Spacing of modes depends upon the length of the optical cavity
Single Mode Operation can be achieved by
Reducing the length of the optical cavity
Difficult to handle
Have not been practically successful

16. Injection Laser Structure
Types are
Gain guided lasers
Multimode lasers
Exhibits power kinks
Operate both in short and long wavelength
High threshold current and low quantum efficiency
Index-guided lasers
Operate at various wavelengths with a single mode
Threshold current is low
Quantum-well lasers

17. Injection Laser Diode (ILD)
Distributed feedback lasers
Elegant approach to achieve single frequency operation
Use of distributed resonators which utilizes distributed Bragg diffraction grating
Provides periodic variation in refractive index
Only modes near Bragg wavelength is reflected constructively
Broadly classified as
DFB
DBR (Less well developed)

18. ILD Characteristics Threshold current Vs Temperature Dynamic response
Threshold current in general tends to increase with temperature
Dynamic response
Switch-on delay followed by damped oscillations known as relaxation oscillations
Serious deterioration at data rates above 100 Mbps if td is 0.5 ns and RO of twice this
td may be reduced by biasing the laser near threshold current
ROs damping is less straight forward
Damping is slow in DH Double Heterogeneous as compared to BH (Buried Heterogeneous)

19. ILD Characteristics Frequency Chirp
Direct current modulation of a single mode semiconductor laser causes dynamic shift of the peak resulting in linewidth broadening called Frequency Chirp
Combined with chromatic dispersion causes significant performance degradation
Can be reduced by biasing the laser sufficiently above the threshold current
Low chirp in DFB and quantum well lasers

20. ILD Characteristics Noise Mode Hopping Phase or frequency noise
Instabilities like kinks
Reflection of light into the device
Mode partitioning
Mode Hopping
Mode hopping to a longer wavelength as the current is increased above threshold
Mode partitioning is the phenomena which occur in multimode semiconductor lasers when the modes are not well stabilized. Even when the output power of the laser is maintained temperature changes can cause variations in the relative output power of the modes. This spectral fluctuation along with dispersion can produce random distortion.

21. ILD Characteristics Reliability Major problem in ILDs
Device degradation occur
Two types of degradations
Catastrophic degradation
Mechanical damage to mirror facet
Results in partial or complete failure
Gradual degradation
Defect formation in active region
Degradation of the current confining region