Chapter 4. Photonic Sources. Contents. Review of Semiconductor Physics Light Emitting Diode (LED) - Structure, Material,Quantum efficiency, LED Power, Modulation Laser Diodes - structure, Modes, Rate Equation,Quantum efficiency, Resonant frequencies, Radiation pattern
Chapter 4 Photonic Sources
Contents • Review of Semiconductor Physics • Light Emitting Diode (LED) - Structure, Material,Quantum efficiency, LED Power, Modulation • Laser Diodes - structure, Modes, Rate Equation,Quantum efficiency, Resonant frequencies, Radiation pattern • Single-Mode Lasers - DFB (Distributed-FeedBack) laser, Distributed-BraggReflector, Modulation • Light-source Linearity • Noise in Lasers
Review of Semiconductor Physics a) Energy level diagrams showing the excitation of an electron from the valence band to the conduction band. The resultant free electron can freely move under the application of electric field. b) Equal electron & hole concentrations in an intrinsic semiconductor created by the thermal excitation of electrons across the band gap Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
n-Type Semiconductor • Donor level in an n-type semiconductor. • The ionization of donor impurities creates an increased electron concentration distribution. Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
p-Type Semiconductor • Acceptor level in an p-type semiconductor. • The ionization of acceptor impurities creates an increased hole concentration distribution Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Intrinsic & Extrinsic Materials [4-1] • Intrinsic material: A perfect material with no impurities. • Extrinsic material: donor or acceptor type semiconductors. • Majority carriers: electrons in n-type or holes in p-type. • Minority carriers: holes in n-type or electrons in p-type. • The operation of semiconductor devices is essentially based on the injection and extraction of minority carriers. [4-2]
The pn Junction Electron diffusion across a pn junction creates a barrier potential (electric field) in the depletion region. Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Reverse-biased pn Junction A reverse bias widens the depletion region, but allows minority carriers to move freely with the applied field. Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Forward-biased pn Junction Lowering the barrier potential with a forward bias allows majority carriers to diffuse across the junction. Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Semiconductor Device Fabrication • Device fabrication requires a critical study of the crystal structure of the different materials • Crystal structure is the periodic arrangement of atoms (Si or Ge) or group of atoms (GaAs or NaCl etc.) • The spacing between the atoms or group of atoms is called lattice spacing or lattice constant • To create a semiconductor device we need a crystalline base/ substrate for mechanical strength and electric contacts • Thin layers of semiconductor materials are grown on the substrate which should have the same lattice structure as that of the substrate • Lattice matching is required, means lattice constant should be the same • Lattice matching is important as we want to avoid temperature dependent stresses or strain • This is known as epitaxial growth (we can change the impurity level in the adjacent layers)
LED Structure For use in the fiber transmission applications an LED must have • High radiance output Radiance is a measure , in watts, of the optical power radiated into a unit solid angle per unit area of the emitting surface. High radiances are necessary to couple sufficiently high optical power levels into a fiber. • Fast emission response time The emission response time is the time delay between the application of a current pulse and the onset of optical emission. Time delay is responsible for limiting the band width with which the source is modulated. • High quantum efficiency Quantum efficiency is related to the fraction of injected electron-hole pairs that recombine radiatively
continued To achieve high radiance and high quantum efficiency, the LED structure must provide a means to confine the charge carriers and the stimulated optical emission to the active region of the pn-junction where radiative recombination can take place Carrier Confinement: It is important to achieve the high level of radiative recombination in the active region of the device which yields a high quantum efficiency. Optical Confinement: It is important for preventing absorption of the emitted radiations in the surroundings.
Cross-section drawing of a typical GaAlAs double heterostructure light emitter. In this structure, x>y to provide for both carrier confinement and optical guiding. b) Energy-band diagram showing the active region, the electron & hole barriers which confine the charge carriers to the active layer. c) Variations in the refractive index; the lower refractive index of the material in regions 1 and 5 creates an optical barrier around the waveguide because of the higher band-gap energy of this material. [4-3] Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Surface-Emitting LED • Active region is etched circular well. • The circular active area have diameter 50μm and up to 2.5 μm thick. • The emission pattern is almost isotropic with a 120o half-power beam width • This pattern is called Lambertian Pattern i.e., the source is equally bright when viewed from any direction but the power diminishes as cosθ where θ is the angle between the viewing direction and the normal to the surface • The power is half when θ = 60o so half power beam width is 120o Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Edge-Emitting LED Consists of an active junction region (source of incoherent light and two guiding layers). • To match the core diameter, the contact strip is 50-70 m • Length of active region range from 100 to 150m • The emission pattern is more directional. • In the plane parallel to the junction, emitted beam is Lambertian with half power width 120o • In the perpendicular plane half power width decrease to 25 – 35o Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Light source Materials • Material used in the active region should be a direct band gap material. • In direct band gap material, radiative recombination is very high to maintain adequate level of optical emission. • Single element semiconductors are not direct band gap materials. • III (e.g., Al, Ga, In)-V (e.g., P, As, Sb) compounds are direct band gap materials. Few ternary and quaternary combinations of the binary compounds of III-V materials are also direct band gap materials. • For operation in 800 -900 nm spectrum, the material used is the ternary alloy Ga1-x Alx As • The ratio x will determine the band gap of the material or wavelength of the peak emitted radiation. The value of x for active region is chosen so that the emission spectrum should be 800 – 850nm • For x = 0.8, peak is at 810nm
continued • At longer wavelengths, the quarternary alloy In1-x Gax Asy P1-y is one of the premier candidate. • By varying x and y I the active area, LED’s with peak power at any wavelength between 1.0 – 1.7μm • E = h= hc/ • The peak emission wavelength can be expressed as a function of band gap energy Eg in eV by the equation (μm) = 1.240/ Eg (eV)
Spectral width of LED types Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Rate equations, Quantum Efficiency & Power of LEDs • When there is no external carrier injection, the excess density decays exponentially due to electron-hole recombination. • n is the excess carrier density, • Bulk recombination rate R: • Bulk recombination rate (R)=Radiative recombination rate + nonradiative recombination rate
With an external supplied current density of J the rate equation for the electron-hole recombination is: In equilibrium condition:dn/dt=0 n is the steady state electron density in the active region when a constant Current is flowing through it.
Internal Quantum Efficiency The internal quantum efficiency in the active region is the fraction of electron-hole pairs that recombines radiatively Where Rr and Rnr are the radiative and non radiative recombination rates and nr = n / Rnr is the non radiative life time Where is the bulk recombination life time and given as
Optical Power In general r and nr are comparable for direct band gap materials so for simple homojunction LED quantum efficiency would b ½. However double heterostructures can have 60-80% efficiency If the total current injected is I, then the total number of recombinations per second is Rr + Rnr = I/q, substituting this equation in quantum efficiency eq. we get Rr = int I / q Where Rr is the total number of photons generated per second and has energy h Optical power generated internally in the active region in the LED is:
External Quantum Eficiency • In order to calculate the external quantum efficiency, we need to consider the reflection effects at the surface of the LED. If we consider the LED structure as a simple 2D slab waveguide, only light falling within a cone defined by critical angle will be emitted from an LED. [4-10]
Modulation of LED • The frequency response of an LED depends on: 1- Doping level in the active region 2- Injected carrier lifetime in the recombination region, . 3- Parasitic capacitance of the LED • If the drive current of an LED is modulated at a frequency of the output optical power of the device will vary as: • Electrical current is directly proportional to the optical power, thus we can define electrical bandwidth and optical bandwidth, separately. [4-15] [4-16]
[4-17] Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Some LED and Laser Diode Material Mixtures and their Characteristics
LASER Light Amplification by the Stimulated Emission of Radiation
Advantages: Ideal laser light is single-wavelength only. This is not exactly true for communication lasers. Lasers can be modulated (controlled) very precisely. Lasers can produce relatively high power. Indeed some types of laser can produce kilowatts of power. In communication applications, semiconductor lasers of power up to about 20 mW are available Because laser light is produced in parallel beams, a high percentage (50% to 80%) can be transferred into the fiber.
Disadvantages: Lasers have been quite expensive by comparison with LEDs. Reason: Temperature control and output power control is needed. The wavelength that a laser produces is a characteristic of the material used to build it and of its physical construction. Lasers have to be individually designed for each wavelength they are going to use. Amplitude modulation using an analog signal is difficult with most lasers because laser output signal power is generally non-linear with input signal power
Properties of LASER LASER has three main characteristics: • Monochromatic • Coherent • Collimated
Properties of LASER Monochromatic Radiation which occurs at a single wavelength or color.
Properties of LASER Coherent It means all waves vibrate in step, there by constructively reinforcing each adjacent wave. A. = Coherent light Constructive interference B. = Incoherent light Destructive interference OPTICAL INTERFERENCE
Properties of LASER B Collimated It means rays are travelling in the same direction on parallel paths. Collimating lens Source Source Target area A
LASER (Light Amplification by the Stimulated Emission of Radiation) • Laser is an optical oscillator. It comprises a resonant optical amplifier whose output is fed back into its input with matching phase. Any oscillator contains: 1- An amplifier with a gain-saturated mechanism 2- A feedback system 3- A frequency selection mechanism 4- An output coupling scheme • In laser the amplifier is the pumped active medium, such as biased semiconductor region, feedback can be obtained by placing active medium in an optical resonator, such as Fabry-Perot structure, two mirrors separated by a prescribed distance. Frequency selection is achieved by resonant amplifier and by the resonators, which admits certain modes. Output coupling is accomplished by making one of the resonator mirrors partially transmitting.
Pumped active medium • Three main process for laser action: 1- Photon absorption 2- Spontaneous emission 3- Stimulated emission Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Lasing in a pumped active medium • In thermal equilibrium the stimulated emission is essentially negligible, since the density of electrons in the excited state is very small, and optical emission is mainly because of the spontaneous emission. Stimulated emission will exceed absorption only if the population of the excited states is greater than that of the ground state. This condition is known as Population Inversion. Population inversion is achieved by various pumping techniques. • In a semiconductor laser, population inversion is accomplished by injecting electrons into the material to fill the lower energy states of the conduction band.
Fabry-Perot Resonator [4-18] R:reflectance of the optical intensity, k: optical wavenumber
Laser Diode • Laser diode is an improved LED, in the sense that uses stimulated emission in semiconductor from optical transitions between distribution energy states of the valence and conduction bands with optical resonator structure such as Fabry-Perot resonator with both optical and carrier confinements. Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Laser Diode Characteristics • Nanosecond & even picosecond response time (GHz BW) • Spectral width of the order of nm or less • High output power (tens of mW) • Narrow beam (good coupling to single mode fibers) • Laser diodes have three distinct radiation modes namely, longitudinal, lateral and transverse modes. • In laser diodes, end mirrors provide strong optical feedback in longitudinal direction, so by roughening the edges and cleaving the facets, the radiation can be achieved in longitudinal direction rather than lateral direction.
DFB(Distributed FeedBack) Lasers • In DFB lasers, the optical resonator structure is due to the incorporation of Bragg grating or periodic variations of the refractive index into multilayer structure along the length of the diode. Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Laser Operation & Lasing Condition • To determine the lasing condition and resonant frequencies, we should focus on the optical wave propagation along the longitudinal direction, z-axis. The optical field intensity, I, can be written as: • Lasing is the condition at which light amplification becomes possible by virtue of population inversion. Then, stimulated emission rate into a given EM mode is proportional to the intensity of the optical radiation in that mode. In this case, the loss and gain of the optical field in the optical path determine the lasing condition. The radiation intensity of a photon at energy varies exponentially with a distance z amplified by factor g, and attenuated by factor according to the following relationship: 
Z=0 Z=L  Lasing Conditions:  
Continued Previous eq. gives us the information related to the resonant frequencies of Fabry-Perot cavity. Condition for Threshold: The condition to just reach the lasing threshold is the point at which the optical gain is equal to the total loss αt, in the cavity. Using the condition for threshold and eqs.  and  we will get the relation for threshold gain
Threshold gain & current density [4-23] Where αend is the mirror loss in the lasing cavity. Thus For laser structure with strong carrier confinement, the threshold current Density for stimulated emission can be well approximated by: [4-24]
Optical output vs. drive current Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000