Lec 4_Optical Sources and Detectors

8/19/2019 Lec 4_Optical Sources and Detectors

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EEE 529: Optical Sources and

Detectors

Danson Njue


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Recap

Optical Transmitter

• The starting point of the optical communication system is the opticaltransmitter where the electrical signal convert to the optical signal bymodulating the optical source. This is done using an electronic circuitknown as the driving circuit.

• The most common devices used as the light source in opticaltransmitters are the light emitting diode (LED) and the laser diode (LD)

• These devices are mounted in a package that enables an optical fiber to

be placed in very close proximity to the light emitting region in order tocouple as much light as possible into the fiber.

• Depending on the nature of the modulating signal, the resultingmodulated light may be turned on-and-off or may be linearly varied inintensity between two predetermined levels


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Optical sources

• The basic requirement for light sources in optical communication

depends on the intended application (long-haul, short-haul). The main

features of the light source are;-

– Power: Source power must be sufficiently high so that the received

signal is detectable – Speed: The source must be suitable for modulation at the desired

rate

– Line-width: The source must have a narrow spectral line width to

minimize the effects of chromatic dispersion in the fibre

– Noise: The source must be free of random fluctuations

– Others include ruggedness, insensitivity to environmental

conditions, reliability, low cost and long lifetime


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Optical sources

• Wideband continuousspectra sources

(Incandescent Lamps)

• Monochromatic incoherent

sources (Light Emitting

Diodes - LED)

• Monochromatic coherent

sources (Light Amplification

by Stimulated Emission of

Radiation - LASER)

The most commonly used sources are LED and Lasers


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Light Emitting Diodes (LED)

•Produced through spontaneous emission of light

• LEDs are fabricated in two main basic structures;

– Surface emitting

– Edge emitting

• Edge Emitter

– More complex and expensive device but offers high output power

levels and high speed performance

– The output power is high because the emitting spot is very small,

typically 30-50 µm, allowing good coupling efficiency to similarly

sized optical fibers.


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LED Structures

• Surface Emitter

– Comparatively simple structure, relatively inexpensive, offer low-to-moderate output power levels, and are capable of low-to-moderate operating speeds.

– Optical output power is as high or higher than the edge-emittingLED, since the emitting area is large, causing poor couplingefficiency to the optical fiber.

– Surface-emitting LEDs are almost perfect Lambertian emitters.This means that they emit light in all directions.

*Check on Lambert cosine law – The main disadvantage with surface emitting is their relatively

broader linewidth hence high dispersion


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Characteristics of LEDs

• There are five major characteristics of LED

– Peak wavelength

– Spectral Width

– Emission pattern

– Power

– Speed


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• Peak wavelength :The wavelength at which the sourceemits the most power. It should be matched to thewavelengths that are transmitted with the leastattenuation through optical fiber. The most common peakwavelength are 780, 850, and 1310 nm.

• Spectral width: Ideally, all the light emitted from an LEDwould be at the peak wavelength, but in practice the lightis emitted in a range of wavelengths centered at the peakwavelength. This range is called the spectral width of thesource.


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• Emission Pattern: The pattern of emitted light affects theamount of light that can be coupled into the optical fiber.The size of the emitting region should be similar to thediameter of the fiber core.

• Power: Usually achieved by coupling as much of a source’s

power into the fiber as possible. The key requirement isthat the output power of the source be strong enough toprovide sufficient power to the detector at the receivingend, considering fiber attenuation, coupling losses andother system constraints. In general, LEDs are less powerful

than lasers.


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• Speed: A source should turn on and off fast enough tomeet the bandwidth limits of the system. The speed isgiven according to a source’s Rise or fall time, the timerequired to go from 10% to 90% of peak power. LEDs haveslower rise and fall times than lasers.

• Linearity: Linearity represents the degree to which theoptical output is directly proportional to the electricalcurrent input. Most light sources give little or no attentionto linearity, making them usable only for digitalapplications. Analog applications require close attention tolinearity. Nonlinearity in LEDs causes harmonic distortion inthe analog signal that is transmitted over an analog fiberoptic link.


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LED materials

•There are many material inconstruct LED, for exampleGaAlAs (gallium aluminumarsenide) for short-wavelengthdevices.

• Long-wavelength devices

generally incorporate InGaAsP(indium gallium arsenidephosphide).

• These material gives differentenergy gap as shown in tablebelow

• Different material also will givesdifferent wavelength for differentapplication

Material Energy Gap Eg (eV)Wavelength

(nm)

Si 1.17 1067

Ge 0.775 1610

GaAs 1.424 876

InP 1.35 924

InGaAs 0.75-1.24 1664-1006

AlGaAs 1.42-1.92 879-650

InGaAsP 0.75-1.35 1664-924


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Laser Diode (LD)

• Light produced through Stimulated Emission

process

• Lasers emits coherent light

• Laser diode light is monochromatic and thespectral/line width of the light is small.

• Since the laser diode is monochromatic, the

light is easily directed into the fiber.


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Characteristic of laser diode –

Monochromatic : The spectral width of the radiatedlight is very narrow. The line width of a laser diodecan be in tenth or hundred of nanometer

– Well directed: A laser diode radiates narrow , welldirected beam that can be easily launched intooptical fiber

– Highly intense and power efficient: A laser diode canradiate hundreds of milliwatts of output power. LDmaking the current to light conversion 10 times

more efficient than it is in the best LEDs.

– Coherent: Light radiates by a laser diode is coherent;where all oscillation are in phase.


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Characteristic of laser diode

•

Linearity: Laser diode is linear in terms of light output versuselectrical current input, but unlike LEDs, they are not stable over

wide operating temperature ranges and require more elaborate

circuitry to achieve acceptable stability

• Emission Pattern: The pattern of emitted light affects the

amount of light that can be coupled into the optical fiber. Thesize of the emitting region should be similar to the diameter ofthe fiber core.

Fig. Laser emission pattern and fiber core


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Laser Diode Material

•The material inside laser diode will present thewavelength of laser diode

• Different material will present differentwavelengths

• Different material also provide different energygaps

• Material in laser diode will provide how long the

transmitter can support and the wavelengthmode and type


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Laser Diode Material

• InGaAs: Indium Gallium Arsenide. Generally used to makehigh-performance long-wavelength detectors

• InGaAsP: Indium Gallium Arsenide Phosphide. Generallyused for long-wavelength light emitters

• Injection Laser Diode (ILD): A laser employing a forward-biased semiconductor junction as the active medium.Stimulated emission of coherent light occurs at a PIN junction where electrons and holes are driven into the

junction


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LEDs vs Lasers

•LEDs are generally more reliable than lasers, but bothsources will degrade over time.

• This degradation can be caused by heat generated by thesource and uneven current densities. LEDs are easier to usethan lasers.

• The light radiate from LED is incoherent light where thereare no optical cavity.

• The output radiation has a broad spectral width, since theemitted photon energies ranges over the range the energy

• Compared to the LED, laser diode is highly intense andpower efficient. LED need 150mA of current to achievepower radiate at 1mW but laser diode only need 10mAcurrent to achieve same power level.


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Optical Detectors

• A fiber optic receiver is an

electro-optic device thataccepts optical signals from anoptical fiber and converts theminto electrical signals.

• An optical receiver consists of :

-Optical detector

-Low-noise amplifier

-Other circuitry.

• Optical detectors convert

optical power to electricalcurrent at the receiver


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Performance criteria of optical detectors

•

Sensitivity has to be matched to the emission spectra ofthe optical transmitter

• Linearity (Linear relationship between the intensity and

the electrical signal)

• High quantum efficiency / high spectral sensitivity• Fast response time

• Stability of performance (temperature sensitivity)

•

Reliability and Robustness• Low Noise

• Lost Cost


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Classification of optical detectors

•

Diodes – PN-diodes

– PIN diodes

– Avalanche photodiodes

– Schottky diodes

• Photo conductors

•

Photo transistors


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Principle of optical detection• The conversion of an optical into an

electrical signal requires the absorption of

the incident light.

• The absorption leads to an excitation of an

electron from the valence to the conduction

band. The excitation leaves ‘holes ’ in the

valence band.

• As such, the excitation leads to photo-

generation of electron-hole pairs, because

the absorption always leads to the

generation of a hole and an electron.

• If now a photon gets absorbed in the

material the electron-hole pairs have to be

separated by an electric field. The energy of

the photon has to be sufficiently high to

excite an electron from the valence to the

conduction band.


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Absorption

• The absorption of a photon produces an electron hole-pair and thus a

photocurrent. The absorption of the photons depends on the absorptioncoefficient α in the medium, which depends on wavelength of the photon

• The photocurrent caused by the absorption of photons is given by;

• where Po is the optical power, λ is the wavelength of the incident light, h is

the Planck constant, and e is the elementary charge.

• The term R accounts for the reflection at the interface of the detector and air,

(1-R) is the light absorbed in the detector and the exponential term considersthe absorption in the medium.

• d is the thickness of the absorber.


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Quantum efficiency of optical detectors

• The quantum efficiency is defined by:

• Quantum efficiency is determined by the absorptioncoefficient. The quantum efficiency is generally below unity,

but can be for its maximum very close to unity.


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Spectral Responsivity

• Describe the spectral sensitivity of the devices. The spectral

responsivity is given by:

• where Iph is the photocurrent and Po is the incident optical

power. The unit of the spectral responsivity is A/W.

• The spectral sensitivity can also be derived from the quantum

efficiency by:


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Assignment 1

• Write short notes on the different types of

photo detectors, clearly outlining the principle

of operation, and advantages and

disadvantages

Lec 4_Optical Sources and Detectors

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