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Optical Sources

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Optical Source

• Main function• 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)

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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

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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

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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)

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Basic Concept (LASER)

• Population inversion• More electrons in higher energy level • Necessary to achieve optical amplification

• Pumping• Process to achieve population inversion usually through external energy

source

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Basic Concept (LASER)

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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)

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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

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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

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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

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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.

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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

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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

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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

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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

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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)

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ILD Characteristics

• Threshold current Vs Temperature• 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

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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

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ILD Characteristics

• Noise• 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

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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