Optical Sources & Detectors Navneet Singh Aulakh Scientist CSIO, Chandigarh.
Nov 28, 2014
Optical Sources & Detectors
Navneet Singh Aulakh Scientist CSIO, Chandigarh.
Block diagram of Optical Communication SystemTRANSMITTER1 2 3
RECEIVERFiber splice 1 2 3
Optical fiber Coding Laser diode Photodiode + decoder
TRANSMISSION MEDIUM
Types of Optical Sources LED (Light Emitting Diodes) LASER (Light Amplification by Stimulated Emission of Radiation)
LEDs Light Emitting DiodeEmits incoherent light through spontaneous emission. Used for Multimode systems with 100-200 Mb/s rates. Broad spectral width and wide output pattern. 850nm region: GaAs and AlGaAs 13001550nm region: InGaAsP and InP
Pure-crystal energy-band diagram
n-type material
p-type material
Electron diffusion across a pn junction
Reverse bias condition
Forward bias condition
Principle of Light Emitting Diode (LED)
Table 6-1: Light-Emitting Semiconductors Material Wavelength Range Bandgap Energy (m) (eV) AlGaInP GaAs AlGaAs InGaAs InGaAsP 1.82 - 1.94 0.61 - Table 6-1: Light-Emitting Semiconductors 0.68 0.9 Material Wavelength 1.4 Range Bandgap Energy (m) - 1.55 (eV) 1.4 0.8 - 0.9 0.95 1.0 - 1.3 AlGaInP 1.82 - 1.94 0.61 - 0.68 1.24 0.73 1.4 0.9 - 1.7 0.9 - 1.35 GaAs AlGaAs InGaAs InGaAsP 0.8 - 0.9 1.0 - 1.3 0.9 - 1.7 1.4 - 1.55 0.95 - 1.24 0.73 - 1.35
Forward Biased Diodes: holes and electrons recombine in some group III - V compounds, a few II - VI compounds, the electrons and holes recombine and produce radiation.
c Eg ! hv ! hf ! h , or: P hc P ! , and: Eg 1.24 P (Qm) ! Eg (eV)
Types of p-n JunctionsHomojunctions:
a s
a s
N carriers are not confined light is not confined inefficient. Heterojunctions: the carriers are confined the light is also confined Historically, single hets, then double hets.
Heterojunctions:
Double-heterostructure configuration
Types of LEDs Surface Emitting LEDs Edge Emitting LEDs
Surface Emitting LEDs
Coupling lens used to increase efficiency. Short optical Links with Large NA fibers. Data rates less than 20 Mbps.
Edge Emitting LEDs
Higher data rates>100Mbps Multimode and Single Mode fibers
Comparision between SLEDs And ELEDs SLEDs generally radiate more power into air ( 2.5 to 3 times) than ELEDs since the emitted light is less effected by reabsorption. SLEDs couples more optical power into large NA (greater than .3 ) than ELED where as the opposite is true for low NAs. Less coupling efficiency in SLEDs as compare to ELEDs. ELEDs have better modulation bandwidth than SLEDs ELEDs have narrower bandwidth than SLEDs.
LED Characteristics Optical Output Power Output Spectrum Modulation Bandwidth
Optical Output Power
P
P
P
I
I
I
1. Ideal light output against current characteristics for an LED 2. Light output of SLEDs against current 3. Light output of ELEDs against current
Power Versus Current Characteristics
Output power against temperatureOptical Output Power
SLED ELED
Temperature
LED Spectral Profile
Modulation Bandwidth
Current ratio I out/I in .707 .500
Electrical 3db point Optical 3 db point
Frequency
Packaging Microlensed LED
Advantages of LEDs Easier Fabrication Lower Cost Simpler Temperature circuit design Lower temperature dependence
Lasers
LASER Basic Operation
Basic Steps required to form a laser beam : There are generally 3 processes: Absorption Spontaneous emission Stimulated emission
Principle Of LASER action1)Absorption:When a photon with certain energy is incident on an electron in a semiconductor at the ground state (lower energy level ) the electron absorbs the energy and shifts to the higher energy level(E2). The energy now acquired by the electron is (E2E1).
2)Spontaneous EmissionAfter the absorption process the electrons at the higher energy level are in an excited state. Now if there is no incident light ,they fall back to the lower energy level during which they give up the energy acquired during absorption in the form of radiation.The spontaneous emission process is as shown in the figure below:
3)Stimulated EmissionOnce the external photon strikes to this excited atom,this leaves its position from the higher energy level and & it will emit the photon. Thus 2 photons are available at the output,one which is striking on the atom and other coming out because of excited emission of two photons and the light amplification takes place. These two photons are in same phase and traveling in the same direction.
Band Gap Materials Direct Band gapSemiconductors like Si Indirect Band gapIII V compounds like GaAlAs , GaAsP, InGaAsP
Types of Lasers Semiconductor Laser (used in advanced optical fiber communication) Solid State Laser (used in atmospheric and free space links) Gas Lasers(used in atmospheric and free space links)
Semiconductor Laser Diode
Types of semiconductor laser diode DFB(Distributed feedback Laser) DBR (Distributed brag reflector Laser) Tunable Laser Diodes
DFB and DBR Lasers
Tunable Laser Diode Laser structure is divided in three subsequent sections which are optically coupled Gain section is also called the active region and it generates light wave of high power. Passive section serve as a wavelength selective reflector A passive phase shifting section adjusts the effective optical length of gain and phase shifting sections together to the wavelength .
Soild State Lasers
In solid state lasers the active area is created by so called Laser active atoms. These active atoms are stimulated by injection of powerful light which is called optical pumping. By employing optical pumping electrons located in the ground level are pumped to an upper level with high energy. Laser operation is finally achieved by stimulated emission
Gas Lasers In Gas lasers gas atoms or gas molecules are employed to produce the active material. Gas Lasers provides the best results as far as spatial and spectral quality of the laser light beam are concerned.
Emission Spectrum of LaserRelative Intensity
1550
P(nm)
Packaging
Packaging
Packaging
Packaging
Advantages of Laser High Modulation rates Narrower Spectral Width Less dispersion induced signal distortion Higher fiber coupling efficiency Greater transmission distance
Comparison between LED and LASERS
Optical Detectors
Requirements of Optical detector High sensitivity at the operating wavelength High Fidelity Large electrical response to the optical signal Short response time to obtain a suitable bandwidth A minimum noise introduced by the detector Stability of performance characteristics Small Size Low bias voltage High reliability Low cost
Types of Photodiodes PN Photodiode P-I-N Photodiode Avalanche Photodiode Phototransistor Photo Darlington Pair
PN Photodiode To induce material to conduct current , one needs to populate the conduction band with electrons. The bandgap of Si is 1.17ev and the bandgap of Ge is .775ev respectively. For excitation of electron at the conduction band Ep=hf=hc/P>Eg
PN PhotodiodeEp p n
Applying external voltage (Reverse bias) enhances the flow of electrons and holes
Input Output Characteristics of PhotodiodeIp(mA) P(mW) Input to a photodiode is light power P Output is current I.e Ip Ip E P So Ip=RP where R is responsivity and its value is constant Responsivity R ranges from .5A/W and this characteristics shows how effectively a photodiode convert light into an electrical signal
Responsivity versus wavelength in PN photodiode Responsivity =Ip/P Photocurrent Ip is the number of electrons ,Ne, flowing per unit time,i.e Ip=Ne/t Light power is light energy per unit of time, where light energy is equal to the energy of photon(Ep) times the number of photons (Np) So P=NpEp/t where Ep=hc/P So R=Ip/P=Ne P/Np h c Where L=Ne/Np is quantum efficiency of a regular communications photodiode ranges from 50% to almost 100 %.
Disadvantages of PN photodiode Narrow depletion Region So the need is to increase the width of the depletion region without manipulating unnecessarily the value of the reverse bias voltage.
P-I-N photodiode A thick, lightly doped intrinsic layer sandwiched between thin p and n regions.
p
i
n
Types of P-I-N photodiodes Front Illuminated Rear Illuminated
Front Illuminated PhotodiodeEp Metal Contacts Depletion region 5um Metal Contacts
Rear Illuminated Photo diodeMetal Contacts Depletion region Ep
Metal Contacts
Advantages of P-I-N photodiode Intrinsic layer is thick, so more number of incident photons enter into this layer and generate electron hole pair, so results in the high quantum efficiency of the device. Reverse biasing voltage is small (usually 50) because the thickness of the depletion region is controlled by the thickness of the intrinsic layer, not by reverse voltage. High bandwidth ( Efforts to improve the bandwidth of 110 Ghz).
Avalanche Photodiode Drawbacks of P-I-N photodiode need of an amplifier to magnify the photocurrent produced by the photodiode.
P+
i
p
N+
APD The quantum efficiency of the APD is M times larger than that of a P-I-N photo diode. R(APD)=M x R(PIN) M depends upon 1 Accelerating voltage 2 Thickness of the gain region 3 Ratio of electrons to holes participating in the ionization process. M ranges from 10 to 500.
APD
Photo Transistor
BIp
Ip
Photo Darlington pair
Ip
Noise Sources in photodiode Shot Noise Thermal Noise Dark current Noise Excess Noise
Shot Noise: Deviation of the actual number of electrons from the average numbers is known as shot noise. Thermal Noise The deviation of an instantaneous number of electrons from their average value because of temperature change is called Thermal Noise. Thermal Noise is often called Johnson noise .
Dark current Noise The dark current noise arises due to dark current which flows in the circuit when the photodiode is in unilluminated environment under bias condition. The magnitude of this current depends on the Operating temperature. Biased voltage Type of detectors
Excess Noise: Cause- Avalanche Multiplication Process
Facilities Available at Fiber Optics Division of CSIO Fiber Handling and Termination Equipment OTDR Splicer Connector Polisher Termination Inspection Equipment Spectrum Analyser, Logic Analyser, CRO, and other Electronics Test Equipment