Basic concepts. Optical Sources (b) Optical Sources (a) Requirements for light sources (b) Requirements for light sources (a)

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1 Optical Sources (a) Optical Sources (b) The main light sources used with fibre optic systems are: Light-emitting diodes (LEDs) Semiconductor lasers (diode lasers) Fibre laser and other compact solid-state lasers are used in some systems Semiconductor laser diodes Standard HeNe lasers LED Fibre laser Blue solid-state lasers Requirements for light sources (a) Sufficient output power Overcome component losses Laser (mw range) has much higher power than LED (W range) Narrow spectral linewidth Minimizes fiber dispersion and increases transmission capacity in WDM systems Laser has much narrower linewidth (typically 13 nm) than LED (typically 3050 nm) The spectral linewidth depends on device structure Directional light output Increases coupling efficiency Laser (spreading at an angle of 10-0 ) couples more light into fiber than LED (spreading out at larger angles) Requirements for light sources (b) Useful emission wavelength region For low fiber attenuation and small fiber dispersion. (typical windows: nm, 1300 nm, 1550 nm) Emission wavelength depends on semiconductor material from which the light source is made Requirements for light sources (c) Modulation Easily modulated at high bit rates» greater information capacity Speed. Lasers are faster than LEDs Stable light output Cheap and reliable Basic concepts LASER: Light Amplification by Stimulated Emission of Radiation Absorption: Atom excited to higher energy state (i.e. E 1 E ) when bombarded by photon with energy hf 1

2 Spontaneous emission Photon with energy hf is released when atom moves from higher to lower energy state emits light in random directions and out of phase» Incoherent radiation» Mechanism for light generation in LEDs Stimulated emission Photon with energy hf forces atom to return to lower energy state, and generates second photon also with energy hf. Stimulating and stimulated photons have same energy hf (hence same frequency f) and are in phase (hence same polarization)» Coherent radiation» Optical amplification» Mechanism for light generation in lasers Extrinsic semiconductors Group V impurities (e.g. P) Possess five valence electrons Leaves an excess electron for every impurity atom» Known as n-type semiconductor Group III impurities (e.g. B) Possess three valence electrons Each atom covalently bonds with 3 host atoms Constitute an excess hole for every impurity atom» Known as p-type semiconductor The p-n junction A p-n junction is fabricated from a single slice of semiconductor, with one side doped p-type and the other n-type In trying to neutralize charges Free electrons in n-type diffuse across junction to p-type Free holes in p-type diffuse to n-type Electrons & holes close to junction recombine A depletion region (free of mobile charge carriers) establishes a potential barrier between the p and n type regions which restricts the inter-diffusion of majority carriers from their respective regions in the absence of an externally applied voltage LED - Spontaneous emission (a) When the junction is forward biased, both the depletion region width and the resulting potential barrier are reduced Electrons from the n type region and holes from the p type region can flow more readily across the junction into the opposite type region The increased concentration of minority carriers in the opposite type region in the forward biased p-n diode leads to the recombination of carriers across the bandgap LED - Spontaneous emission (b) In band to band radiative recombination the energy is released with the creation of a photon, the energy of which is given by E g hf

3 LED (Light Emitting Diode) Structures of optical sources To achieve a high radiance and a high quantum efficiency Carrier confinement: achieve a high level of radiative recombination in the active region of the device, which yields a high quantum efficiency Optical confinement: prevent absorption of the emitted radiation by the material surrounding the pn junction Homojunctions and single and double heterojunctions have been widely investigated a semiconductor p-n junction, the materials on either side of which are formed by doping the same basic starting material, such junctions are termed homojunctions A heterojunction consists of two adjoining semiconductor materials with different band-gap energies Homojunction stucture (a) Homojunction structure (b) Single heterojunction structure (a) Single heterojunction structure (b) The electrons injected from the n GaAs layer (1) to the p GaAs active layer () are blocked from diffusing over a large distance by the potential barrier provided by the higher-bandgap p AlGaAs layer (3) The active thickness is therefore determined by the thickness of the p-gaas region. For small d, a smaller drive current can give the same carrier density. Thus power efficiency is higher than the homojunction laser The lower refractive index of the AlGaAs layer provides improved optical confinement (only on one side of the active region) 3

4 Double heterojunction structure (a) Double heterojunction structure (b) A layer of GaAs is sandwiched between two layers of the compound GaAlAs which has a wider energy gap than GaAs and also a lower refractive index The carrier and optical confinement may be achieved simultaneously The bandgap differences from potential barriers in both the conduction and valence bands which prevent electrons and holes injected into the GaAs layer from diffusing away The step change in refractive index provides a very much more efficient waveguide structure than was the case in homojunction lasers and the radiation is confined mainly to the active region LED sources LEDs and Lasers Have similar light generating structures Main differences LED:» current density is low» spontaneous emission Laser:» Large current density» Population inversion» Optical feedback resonance» Stimulated emission Laser Basic Operation LASER: Light Amplification by Stimulated Emission of Radiation LASER = Optical oscillator Need a feedback path Need a gain medium to overcome the losses in the light path Need something to start the oscillation Fabry-Perot cavity (1) Consider two mirrors with given reflectivity creating a cavity filled with a passive material of refractive index n If light is injected, the transmission will depend on the reflectivity of each mirror and the wavelength of the light. L n R 1 R 4

5 Fabry-Perot cavity () L Laser Diode n R 1 R Standing wave (resonance of the cavity) exists when: nl i i The separation between two consecutive resonances is: c f nl This cavity acts as a multi-frequency filter Fabry-Perot cavity (3) Resonant Frequencies The cavity can also be seen as an optical feedback loop (part of the light bounces on the mirrors) What happen if the passive material in the cavity is replaced by an active material? L gain R 1 R n Simplified spectra How can we make the gain material and then start the oscillator? f Wavelength output of a FPC (Fabry-Perot Cavity) laser defined by the combination of the gain curve and the axial modes of the cavity Worst case full range linewidth Laser Basic Operation Worst case linewidth: the difference in wavelength between the worst extreme axial modes; i.e. 5 as shown in the figure below The worst case estimate of the pulse spreading by dispersion Stimulated emission 5

6 The Einstein Relations In thermal equilibrium, the population of the two energy levels of a system described by Boltzmann statistics is: N1 g1 exp( E1 / KT ) g1 exp[( E E1) / KT ] N g exp( E / KT ) g g1 exp( hf / KT ) (11) g where N 1 and N represent the density of atoms in energy levels E 1 and E, respectively, with g 1 and g being the corresponding degeneracies of the levels, K is Boltzmann s constant and T is the absolute temperature Under the condition of thermal equilibrium, the lower energy level E 1 of the two level atomic system contains more atoms than the upper energy level E Population inversion (a) To achieve optical amplification it is necessary to create a nonequilibrium distribution of atoms such that the population of the upper level is greater than that of the lower energy level. The condition is known as population inversion Populations in a two energy level system: (a) Boltzmann distribution for a system in thermal equilibrium; (b) a nonequilibrium distribution showing population inversion Population inversion (b) Optical feedback and laser oscillation (a) Population inversion may be obtained at a p-n junction by heavy doping (degenerative doping) of both the p and n type material by passing a high drive current through the diode, which results in the majority charge carriers to excite to a higher energy level The radiation in the laser diode is generated within a Fabry- Perot resonator cavity, as in most other types of lasers Optical feedback and laser oscillation (b) The mirror facets are constructed by making two parallel clefts along natural cleavage planes of the semiconductor crystal The mirrors provide strong optical feedback in the longitudinal direction, thereby converting the device into an oscillator with a gain mechanism that compensates for optical losses in the cavity The device will oscillate (thereby emitting light) at those resonant frequencies for which the gain is sufficient to overcome the losses The sides of the cavity are simply formed by roughening the edges of the device to reduce unwanted emissions in these directions Comparison of LED and laser diode characteristics Optical output power against drive current Radiant output as a function of frequency for a p-n junction laser: (a) below threshold (spontaneous emission); (b) with the laser modes at threshold; and (c) with the dominant laser mode above threshold 6

7 Experimental demonstration (a) Experimental demonstration (b) Power (µw) LED Current (ma) Power (µw) Laser Diode Current (ma) Example 6 (a) Question 4: A GaAs laser operating at 850 nm has a 500 µm length and a refractive index n = 3.7. What are the frequency and wavelength spacings. Example 6 (b) Solution: 8 c 310 f 81.1GHz -6 Ln Ln nm 3.7 Example 7 (a) A particular semiconductor FPC laser is fabricated using active material with a bandgap of 1.3x10-19 J and a refractive index of The cavity length is 75m and it is possible for 7 axial modes to operate under the gain curve. What is the full width worst case linewidth? Example 7 (b) Solution: Estimate the operating wavelength: 6 hc / Eg m 1. 53m max Use max to calculate the axial mode spacing 6 ( ) / Ln 1nm Hence if 7 modes can operate the worse case linewidth is 6nm. 7

8 Multimode laser diode Laser Fabry-Perot ST package Fabry-Perot laser diode Single mode lasers To obtain single-mode operation it is necessary to eliminate all but one of the longitudinal modes Reduce the length L of the cavity until the frequency separation of the adjacent modes is larger than the laser transition linewidth or gain curve Distributed feedback based lasers: the use of distributed resonators, fabricated into the laser structure to give integrated wavelength selectivity Laser - Distributed Feedback (DFB) Feedback in a DFB laser is provided by a series of periodic perturbations built into the structure along the length of the gain medium and sufficiently close to the active region to interact with the evanescent field 0 is the vacuum wavelength and 0 /n is the wavelength in the material: 0 = nd Butterfly package DFB laser diode Laser Vertical Cavity Surface Emitting Lasers (VCSELs) Laser Distributed Bragg Reflector (DBR) Lasers A Bragg grating is printed into a portion of the semiconductor The Bragg grating will reflect one wavelength depending on the period of the grating. Only one mode at the Bragg wavelength will lase. Bragg section 00µm Phase section 130µm Active section 790µm InP p InP n ion implantation Active section Phase section Bragg section Alcatel-Thales 8

9 Comparison Coupling to a fibre (a) Coupling to a fibre (b) Use of a Graded Index Lens (GRIN lens). The lens collects and focuses the light onto the end of the fibre (a) Use a ball lens. An epoxy resin bonds the lens to the surface of the LED, however the refractive index (RI) of the epoxy cannot match to both the RI of the fibre (~1.45) and the RI of the semiconductor (~3.5) (b) Direct coupling. Mount the LED inside a connector and the fibre is mounted in the other half of the connector. Low cost and low complexity (c) Fix a ball lens to the end of a fibre (d) Laser safety (1) Laser light is very dangerous and should be treated as a significant hazard Lasers are classified by hazard potential based upon their optical emission. Necessary control measures are determined by these classifications. In the U.S., laser classifications are based on American National Standards Institute s (ANSI) Z136.1 Safe Use of Lasers. Laser safety (): laser classes and hazards Lasers are grouped according to the degree of hazard Classes 1, and 3a are safe for viewing because of limited power and irradiance Classes 3b and 4 require appropriate precautions to be taken Most medical devices are Classes 3b or 4 and require eye protection and detailed training Laser safety (3): laser ANSI classification Class 1 denotes laser or laser systems that do not, under normal operating conditions, pose a hazard Class denotes low-power visible lasers or laser system which, because of the normal human aversion response (i.e., blinking, eye movement, etc.), do not normally present a hazard, but may present some potential for hazard if viewed directly for extended periods of time (like many conventional light sources). Class 3a denotes some lasers or laser systems having a CAUTION label that normally would not injure the eye if viewed for only momentary periods (within the aversion response period) with the unaided eye, but may present a greater hazard if viewed using collecting optics. Class 3a lasers have DANGER labels and are capable of exceeding permissible exposure levels. If operated with care Class 3a lasers pose a low risk of injury. 9

10 Laser safety (4): laser ANSI classification Class 3b denotes lasers or laser systems that can produce a hazard it viewed directly. This includes intrabeam viewing of specular reflections. Normally, Class 3b lasers will not produce a hazardous diffuse reflection. Class 4 denotes lasers and laser systems that produce a hazard not only from direct or specular reflections, but may also produce significant skin hazards as well as fire hazards. 10

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