Lecture 5: OPTICAL SOURCES

1. Lecture 5: OPTICAL SOURCES Iksan Bukhori, M.Phil.

2. Semiconductor Source • Expected environment : – Small size – 850, 1300, or 1550 nm – Power – Liniarity – Simple modulation – Modulation frequency response – Low cost – High reliability • Source wavelength : – Short wavelength: » 500→1,000 nm » Binary alloy (e.g., GaP: 600-700 nm) » Ternary alloy (e.g., GaAlAs: 800- 900 nm) – Long wavelength: » 1200→1600 nm » Quaternary alloy (e.g., InGaAsP: 1300-1600nm)

3. Optical Sources • Source – LED – Semiconductor laser • LED – Low cost – Medium power – Short distance communication, low bit rate path • Laser – High cost – Enough power – Long distance communication, high bit rate path

4. LED

5. Uncooled laser

6. Cooled laser

7. SEMICONDUCTOR • Semiconductor materials have conduction properties located between metals and insulators. • Silicon (Si) is located in group IV (has 4 electrons in the outer shell) of the periodic table of elements. • Conduction properties can be interpreted with the help of energy band diagrams. • Pure crystals at low temperatures conduction band have no electrons at all and the valence band is very full. • Both bands are separated by an energy gap or band gap that has no energy level. • If the temperature rises, several electrons cross the energy gap towards the conduction band.

8. Semiconductor band structure.

10. (a) Electron excitation from the valence band to the conduction band. (b) The concentration of electrons and holes is the same in intrinsic semiconductors.

11. The concentration of electrons and holes is known as intrinsic carrier concentration : • where: T: absolute temperature kB: Boltzman's constant = 1.38 x 10-23J/0K m: static mass of electrons = 9.11 x 10-31Kg h: Planck’s constant = 6.626 x 10-34JS me: effective mass of electrons mh: effective mass hole

12. Conduction can be increased by doping, namely the addition of a mixture of ingredients from group V, such as P, As, Sb. • If the atom of the material replaces a Si atom, 4 electrons are used for covalent bonding and the fifth free electron is used for conduction. • A mixture called a donor can give an electron in the conduction band. • In this material currents are generated by electrons (negative) n-type material. • Conduction can also be increased by adding material from group III, which has 3 electrons in the outer shell. • 3 electrons form a covalent bond, thus forming a hole that is the same as the donor electron  the concentration of free holes increases in the valence band. • This material is called acceptor and p-type material because conduction is carried out by holes (positive)

13. (a) Type n material level donor (b) Ionization of donor mixtures results in increased distribution       electron concentration

14. (a) Acceptor level for p-type material (b) Mixed ionization of acceptors increases the distribution of hole concentrations

15. Intrinsic and extrinsic ingredients • There is no mix of ingredients called intrinsic ingredients. • Thermal vibrations of crystal atoms  electrons out into the conduction band • Thermal generation process  electron-hole pairs • Recombination process  free electrons release energy down to free holes in the valence band. • Balanced condition: Generation rate = recombination rate • Intrinsic material : pn = p0n0= ni2 p0: balanced hole concentration n0: balanced electron concentration ni: intrinsic material carrier density

16. Intrinsic and extrinsic ingredients • A small amount of chemical mixture in crystals produces extrinsic semiconductors. • Electrical conductivity is proportional to carrier concentration  there are 2 types of charge carrier material: a. Majority carriers: electrons in n-type material or holes in p-type material. b.  Minority carriers: holes in type-n material or electrons in p-type material. • The operation of semiconductor devices is essentially based on the injection and extraction of minority carriers.

17. pn junction Diffusion of electrons across the PN junction produces a potential barrier in the depletion region

18. Reverse Biased Reverse bias widens the depletion area, but allows minority carriers to move freely.

19. Forward Biased Foward bias reduces the potential barrier allowing the majority carrier to diffuse across the junction

20. Direct dan indirect band gap Rekombinasielektrondanemisi photon yangberkaitanpadasuatubahan direct-band-gap (elektrondan hole memiliki nilai momentum sama)

21. Recombination of electrons in an indirect-band-gap material (electrons and holes have different momentum values) requires energy Ephand momentum kph

23. LED

24. Light Generation • Forward-bias pnjunction – More doping than electronic diodes – Additional features to hold charge carriers and light fields • Light Generation – Electron and hole radiatiom recombination – Radiation and non-radiation recombination » Efficiency increases by flooding the light generation area with •High density load carrier and • High power light

25. Light Generation • Forward-biased pnjunction – Hole is injected into material n – Electrons to material p • Recombination Carrier with the majority - carrier near junction • Release energy ≈ energy materialbandgapEg – if radiactive, f ≈ Eg /h • Radiation Transition –Spontaneous emission: » Non-coherence » Random Polarization » Random Direction » Increase noise at signal – Stimulate Emission: » Coherence (same phase, polarization, frequency and direction) • Silicon and germaniumradiator are not efficient – Used mixed semiconductor

26. Double hetero structure GaAlAs x>y  limit carrier& optical guide Diagram energy band Index bias variation

27. Configuration • 2 basic configuration: – Front / surface emission or Burrus – End emission • Front Emission: – The light emitting active area is oriented perpendicular to the fiber axis. – A well etched on the substrate material of the device, where fiber is planted to receive light. – The active circle area is 50 μm in diameter and thick up to 2,5 μm. – Isotropic emission patterns are essentially (lambertiant) with a power pattern cosθ so HPBW 1200.

28. LED front emission

29. End Emission: – Consisting of an active junction area that are incoherent source and two guide layers. – The guide layer has a lower refractive index than the active area but is larger than the surrounding material. – The structure forms a gel guide that directs optical translation to the fiber core. – 50 to 70 μm wide band connection tape to fit 50 to 100 μm fiber sizes. – Emission patterns are more directed than surface emissions. – In the plane parallel to the junction the lambertiant emission pattern, in the direction of the perpendicular junction having a HPBW 25 to 350 corresponds to the thickness of the waveguide.

30. LED end emission

31. Wavelength and material • Wavelength andbandgapenergy Egofmaterial • Wavelength (and energy bandgap) also temperature function, increase: ~0.6nm/C • λ = hc/Eg • λ[μm] =1,24/Eg [eV] • Typical wavelengths – GaP LED » 665 nm » short distance, low cost system. – Ga1-x AlxAs LED and laser » 800 → 930 nm » Initial fiber system – Ga1-xInxAsyP1-yLEDs and lasers » 1300 nm (late ’80an,early ’90an, FDDI datalinks) » 1550 nm (middle’90 - now)

32. Band-gap energy and length of the output gel as a function of the molecular part of Al pd AlxGa1-xAs at room temperature.

33. The spectrum of the LED AlxGa1-xAs with x = 0,008

34. Souce Material • Wavelength resistance and lattice spacing – Lattice spacing: » Atomic spacing layer » Must be the same as layer    made (tolerance of 0.1%) • Horizontal lines are only on the diagram – Most devices     Long wavelengths are made on the InP substrate » Horizontal lines are drawn to the left from the point of InPInP – Short wavelengh » Ga1-xAlxAs g horizontal line

35. Source Materials • Fundamental quantum-mechanical relationship: or • For a mixture of three AlGaAs ingredients, the size of Eg (eV) is the magnitude: • For a mixture of four ingredients In1-xGaxAsyP1-y, the magnitude of Eg (eV) is the magnitude::

36. Example • Material source AlxGa1-xAs with x = 0,07 • How much the Egand λ? • Material source In1-xGaxAsyP1-y, withx = 0,26 • Calculate Egand λ?

37. Internal Quantum Efficiency • Excess electrons in p-type materials and holes in n-type materials occur in semiconductors of light sources due to carrier injection in the device contacts. • The excess density of Δn electrons is the same as the excess hole Δp, because the carrier is injected to form and recombine in pairs for the need for neutrality of the crystal charge. • If the carrier injection stops  the carrier density returns to the balance value. • Carrier excess density: Δn0: the excess density of electrons is injected early

38. Carrier excesses can be combined radiatively or non-radiation. • PD radiation recombination will produce photon emissions. • If electron-holes combine non-radiation  releases energy in the form of heat (vibration of lattice). • Internal quantum efficiency: a radiation combination of electron-hole pairs. • Internal quantum efficiency: Rr: the rate of radiation recombination per unit volume Rnr: non-radiation recombination rate

39. For exponential decrease in carrier excess, radiation recombination lifetime: • Non-radiation lifetime recombination: • Internal quantum efficiency: • Lifetime bulk recombination

40. If the current injected into the LED is I, then the total recombination amount per second is • Substitution of the previous equation produces, • Note that Rris the amount of photons generated per second and each photon has an energy of hv, so the total internal power generated by the LED is

41. Transient response • The basic assumption of a transient response approach: – The capacitance of CSjunctions varies more slowly     because the current is compared to the diffusion capacitance Cd is seen constant Cd is seen constant. – Price Cs between 350 to 1000 pF for medium to large currents. • Based on these assumptions, rise time to the half-current point (also the half-power point) LED: • Rise time 10 s/d 90 %: IP: the function ladder amplitude for driving LEDs IS: diode saturation current lifetime of minority carriers

42. Wave current form