Session 3 Light Sources and other Components

1. Session 3 Light Sourcesand other Components

2. N- and P- Type Semiconductors The N- has a surplus of negative electrons. The P- has a surplus of holes.

3. P-N Junction One of the crucial keys to solid state electronics is the nature of the P-N junction. When p-type and n-type materials are placed in contact with each other, the junction behaves very differently than either type of material alone. Specifically, current will flow readily in one direction (forward biased) but not in the other (reverse biased), creating the basic diode. This non-reversing behavior arises from the nature of the charge transport process in the two types of materials.

4. PN junction • At the junction, electrons fill holes so that there are no free holes or electrons there. • A barrier is formed at the depletion region with an electrostatic field of 0.6V for Si.

5. Forward Biased P-N Junction the negative terminal pushes negative electrons towards the junction. the positive terminal pushes holes towards the junction. • if the voltage is high enough then the barrier will be overcome and current will flow through the junction.

6. LEDs • When the applied forward voltage on the diode, the LED drives the electrons and holes into the active region between the n-type and p-type material, the energy can be converted into infrared or visible photons. • This implies that the electron-hole pair drops into a more stable bound state, releasing energy on the order of electron volts by emission of a photon. • The red extreme of the visible spectrum, 700 nm, requires an energy release of 1.77 eV to provide the quantum energy of the photon. At the other extreme, 400 nm in the violet, 3.1 eV is required.

7. LED Radiation Patterns An LED is a directional light source, with the maximum emitted power in the direction perpendicular to the emitting surface. The typical radiation pattern shows that most of the energy is emitted within 20° of the direction of maximum light. Some packages for LEDs include plastic lenses to spread the light for a greater angle of visibility.

8. Light-emitting Diode (LED) Ppeak P-3 dB BW • Datacom through air & multimode fiber • Very inexpensive (laptops, airplanes, lans) • Key characteristics • Most common for 780, 850, 1300 nm • Total power up to a few W • Spectral width 30 to 100 nm • Coherence length 0.01 to 0.1 mm • Little or not polarized • Large NA ( poor coupling into fiber)

9. Lasers • Laser is an acronym for light amplification by the stimulated emission of radiation • Laser characteristics: • Nearly monochromatic: the light emitted has a narrow band of wavelengths • Coherent: the light wavelength are in phase, rising and falling thought the sine-wave cycle at he same time • Highly directional: the light is emitted in a a highly directional pattern with little divergence.

10. Three basic elements of a laser • A typical laser consists of three things: • a Pump, a Gain Medium, and a Cavity. • The pump would send energy into the gain medium and this would excite the electrons and holes within it. • This process then gets amplified within the cavity and lasing takes place.

11. A semiconductor laser diode • Pump - by applying a potential difference V • Gain medium - modified pn-junction or MQW • cavity – the cleaved surfaces + coating • A feedback circuit is also implemented in order to control the amount of current sent to the laser diode.

12. Properties of LDs • Here is a list of the most important properties of LDs • three general categories: Electrical, Optical, and Temperature. ElectricalOptical • Laser threshold ● Light output power • Operating current ●Slope efficiency • Operating Voltage ●Beam Divergence ●Peak wavelength Temperature • operating temperature • wavelength shift

13. Fabry-Perot (FP) Laser Ppeak Threshold I • Multiple longitudinal mode (MLM) spectrum • “Classic” semiconductor laser • First fiberoptic links (850 or 1300 nm) • Today: short & medium range links • Key characteristics • Most common for 850 or 1310 nm • Total power up to a few mw • Spectral width 3 to 20 nm • Mode spacing 0.7 to 2 nm • Highly polarized • Coherence length 1 to 100 mm • Small NA ( good coupling into fiber) P

14. Distributed Feedback (DFB) Laser P peak SMSR • Single longitudinal mode (SLM) spectrum • High performance telecommunication laser • Most expensive (difficult to manufacture) • Long-haul links & DWDM systems • Key characteristics • Mostly around 1550 nm • Total power 3 to 50 mw • Spectral width 10 to 100 MHz (0.08 to 0.8 pm) • Sidemode suppression ratio (SMSR): > 50 dB • Coherence length 1 to 100 m • Small NA ( good coupling into fiber)

15. Source Characteristics • Characteristic LED Laser • Output lower higher • Speed slower faster • Output pattern (NA) higher lower • Spectral width wide narrow • Single-mode compatibility no yes • Ease of use easier harder • Lifetime longer long • Cost lower higher

16. Output Power • Output power is the optical power emitted at a specified drive current. Output power (mW) Laser LED Drive Current (mA)

17. Spectral width Relative Output LED : 40 nm Laser : 0.1 to 5 nm Wavelength (nm)

18. How they look like • Semiconductor laser diodes come in many shapes and sizes. • Package: TO cans; fiber pigtail; hermetic seal

19. Fiber Optic Detectors • They convert optical signals back into electrical impulses that are used by the receiving end of the fiber optic data, video, or audio link. • Detectors perform the opposite function of light emitters. • The most common detector is the semiconductor photodiode, which produces current in response to incident light. 1 Photodiode; 2 PIN photodiode; and 3 APD

20. Detectors for optical communications • PN photodiodes • Electron-hole pairs are created in the depletion region in proportion to the optical power • Electrons and holes are swept out by the electric field, leading to a current • PIN photodiodes • Electric field is concentrated in a thin intrinsic (i) layer • Avalanche photodiodes • Like pin photodiodes, but have an additional layer in which an average of M secondary electron-hole pairs are generated through impact ionization for each primary pair

21. Material Aspects Responsivity (A/W) • Silicon (Si) • Least expensive • Germanium (Ge) • “Classic” detector • Indium gallium arsenide (InGaAs) • Highest speed 1.0 Quantum Germanium Efficiency = 1 0.5 InGaAs Silicon 0.1 1500 500 1000 Wavelength nm

22. Detector Materials and Wavelength semiconductor detectors for optical communications Material Bandgap Wavelength Peak Respossivity Si 1.17eV 300-1100nm 800nm 0.5A/W Ge 0.775 500-1800 1550 0.7 InGaAs 0.75-1.24 1000-1700 1700 1.1

23. Characteristics of PN photodiodes • Reverse-biased • The active detection area (depletion area) is small; • many electron-hole pairs recombine before they can create a current in the external circuit. • Unsuitable for most fiber-optic communication • Low gain - fairly high optical power is needed to generate appreciable current • The slow response - limits operations to the kHz range.

24. Simple PN photodiode circuit • How to connect a PN photodiode?

25. PIN photodiode • The name comes from the layering of these materials positive, intrinsic, negative — PIN • Basic idea: • Sandwiching a thin layer of a different semiconductor material (of intrinsic conductivity) between the outer p and n layers • Choosing the outer p and n layers to be transparent to light in the working wavelength range

26. PIN photodiode • In the PIN photodiode, the depleted region is made as large as possible. A lightly doped intrinsic layer separates the more heavily doped p-types and n-types.

27. Avalanche photodiode (APD) • operates as the primary carriers, the free electrons and holes created by absorbed photons, accelerate, gaining several electron Volts of kinetic energy. • A collision of these fast carriers with neutral atoms causes the accelerated carriers to use some of their own energy to help the bound electrons break out of the valence shell.

28. Avalanche photodiode • Electron-hole pairs created by absorption of photons are accelerated to energies at which more pairs are created, then the new pairs are accelerated and create more pairs, in an “avanlanche” • Avalanche multiplication creates excess noise • Much better signal-to-noise ratio than with external amplification • APDs require high-voltage power supplies for their operation. The voltage can range from 30 or 70 Volts for InGaAs APDs to over 300 Volts for Si APDs. This adds circuit complexity. • APDs are very temperature sensitive, further complicating circuit requirements.

29. APD vs PIN • In general, APDs are only useful for digital systems because they possess very poor linearity. • Because of the added circuit complexity and the high voltages that the parts are subjected to, APDs are always less reliable than PIN detectors. • At lower data rates, PIN detector-based receivers can almost match the performance of APD-based receivers, makes PIN detectors the first choice for most deployed low-speed systems. • At multigigabit data rates, however, APDs rule supreme.

30. Comparison of PIN and APD Parameter PIN Photodiodes APDs Materials Si, Ge, InGaAs Si, Ge, InGaAs Bandwidth DC to 40+ GHz DC to 40+ GHz Wavelength 0.6 to 1.8 µm 0.6 to 1.8 µm Efficiency 0.5 to 1.0 A/W 0.5 to 100 A/W Circuitry none HV, Temp Sta Cost (Fiber Ready)$1 to $500 $100 to $2,000

31. Detector Characteristics • Respossivity is defined as the ratio of the photocurrent to the optical power, Pin: R = Ip/Pin (units: A/W)

32. Quantum Efficiency • Quantum Efficiency is the Ratio of primary electron-hole pairs created by incident photons to the photons incident on the detector material. • h = (# of emitted electrons)/(# of incident photons) • A quantum efficiency of 70% means seven out of ten incident photons create a carrier.

33. Dark current • The induced current that exists in a reversed biased photodiode in the absence of incident optical power.

34. Minimum detector power • Determines the lowest level of incident optical power that the detector can handle. • The noise floor of a PIN diode tells the minimum detectable power. Noise floor = dark noise/responsivity • R = 0.5 mA/mW, and a dark current of 2nA. The noise floor = 2nA/(0.5mA/mW) = 4nW

35. Response Time • Response Time is the time needed for the photodiode to respond to optical inputs and produce and external current. The response time relates to its usable bandwidth.

36. Response time • BW = 0.35/tr, • The RC time constant of a detector also limits the bandwidth. • BW = 1/(2pRLCd), RL is the load resistance and Cd is the diode capacitance

37. Bias Voltage • 5V for PIN PD to ~ 100V for APDs. • Affects operation. • dark current, responsivity, response time increase with the bias voltage. • Temperature sensitive.

38. Integrated detector/preamplifier • A detector package containing a PIN photodiode and transimpedance amplifier • The output is voltage (V/W) • Integrated package

39. What are transmitters and receivers? • Transmitter: A device that includes a source and driving electronics. It functions as an electrical-to-optical converter • Receiver: A terminal device that includes a detector and signal processing electronics. It functions as an optical-to-electrical converter

40. Basic transmitter concepts

41. LED based transmitter Input Buffer LED Driver Input Bias • The most common devices used as the light source in optical transmitters are the light emitting diode (LED) and the laser diode (LD). LEDs are widely used for short to moderate transmission distances because they are much more economical, and stable in terms of light output versus ambient operating temperature. LDs are used for long transmission distances. can couple many times more power to the fiber than LEDs but are unstable over wide operating temperature ranges and require more elaborate circuitry to achieve acceptable stability.

42. LD-based transmitter • Not on and off but is simple modulated between high and low levels above the threshold current. • Power monitor to compensate temperature changes Input Buffer Modulator Input Bias Current Signal conditioner Ref Gen Duty Cycle Compensation

43. Basics receiver concepts

44. Basic receiver concepts • Sensitivity: the lowest power that is detectable. Determined by • the noise floor - SNR or BER of the system • Detector used • in dB or mW unit • Dynamic range: the difference between the minimum and maximum acceptable power levels.

45. Transceivers • Transceiver: transmitter + receiver

46. Optical Connectors • Optical connectors are the means by which fiber optic cable is usually connected to peripheral equipment and to other fibers. • These connectors are similar to their electrical counterparts in function and outward appearance but are actually high precision devices to tolerances of a few ten thousandths of an inch.

47. SC connector • Snap-in Single-Fiber Connector • A square cross section allows high packing density on patch panels • Used in premise cabling, ATM, fiber-channel, and low-cost FDDI. • Available in simplex and duplex configurations

48. ST connector • The most widely used type of connector for data communications. • A bayonet-style “twist and lock” coupling mechanism allows for quick connects and disconnects, and a spring-loaded 2.5 mm diameter ferrule for constant contact between mating fibers.

49. LC connector • Small Form Factor Connector • Similar to SC connector but designed to reduce system costs and connector density.

50. FC Connector • Twisted-on Single-Fiber Connector • Similar to the ST connector and used primarily in the telecommunications industry. • A threaded coupling and tunable keying allows ferrule to be rotated to minimize coupling loss.