OPTICAL SOURCES
LED
-Requires less complex drive circuitry
-No thermal and optical stabilization circuits are needed
-Fabricated less expensively
LED STRUCTURES
Must have
-High radiance output:
Is a measure in watts, of the optical power radiated into a unit solid angle per
unit area of the emitting surface.
-Response time:
Is the time delay between the application of a current pulse and the onset of
optical emission.
-Quantum efficiency:
Is related to the fraction of injected electron hole pairs that recombine
radiatively
To achieve high radiance and high quantum efficiency the LED structures must
provide a means of confining the charge carriers and optical confinement.
Carrier Confinement:
Is used to achieve high level radiative recombination in the active region yields
high quantum efficiency.
Optical Confinement:
Preventing absorption of emitted radiation by material surrounding pn junction.
To achieve carrier and optical confinement LED configurations such as
homojunctions and single and double heterojunctions have been widely used.
-The most effective of these structures is double heterostructure device.
-Two different alloys on either side of the active region.
-The band gap differences of adjacent layers confine the charge carriers.
-Differences in indices of refraction of adjoining layers confine the optical field.
DIAGRAM
Other parameters influencing the device performance include
- optical absorption in the active region
-Carrier recombination at the heterostructure interfaces
- Doping concentration of the active layer
- Injection carrier density
- Active layer thickness
Two basic LED configurations being used for fiberoptics are
-Surface Emitters
- The plane of the active light emitting region is oriented perpendicularly to
the axis of the fiber.
- A well is etched through the substrate of the device, into which a fiber is
cemented in order to accept the emitted light.
- The emission pattern is isotropic with 120deg half power beam width.
- This pattern from a surface emitter is called Lambertian pattern.
- In this the source is equally bright when viewed from any direction, but the
power diminishes as cos theta .
DIAGRAM
-Edge Emitter
- Consists of an active junction region and two guiding layers.
- This forms the waveguide channel that directs the optical radiation
toward the fiber core.
- Emission pattern is more directional than surface emitter.
- Plane parallel to the junction – half power width of theta=120deg
- plane perpendicular to the junction- half power width of theta=25-35deg
DIAGRAM
LIGHT SOURCE MATERIALS
-The active layer of an optical source must have direct band gap.
-DB produce adequate level of optical emission, group III-V materials are DB
material.
-For the operation in the 800-900nm, the principle material used is Ga1-xAlxAs
-The ratio x of aluminium arsenide to gallium arsenide determines the band gap of
the alloy and the wavelength of the peak emitted radiation.
-The value of x for the active area material is usually chosen to give emission
wavelength of 800-850nm.
-An example of Ga1-xAlxAs LED with x=0.08 peak output occurs at 810nm.
-The width of the spectral pattern at its half power point is known as Full Width
Half Maximum(FWHM) spectral width, is 36nm. (GRAPH)
-A very close match between the crystal lattice parameters is required to reduce
- interfacial defects
- minimize strain in the devices as the temp varies
-Fundamental quantum mechanical relationship
E=h‫=ע‬hc/λ
-Peak emission wavelength in micrometers can be expressed as a function of the
band gap energy Eg by the equation
λ=1.240/Eg
- A heterojunction with matching lattice parameters is created by choosing two
material compositions that have the same lattice constant but difference band gap
energies.
QUANTUM EFFICIENCY AND LED POWER
-The excess densities of electrons n and holes p are equal, since the injected
carriers are formed and recombine in pairs with the reqiurement of charge
neutrality in the crystal.
-When carrier injection stops the carrier density returns to the equilibrium value.
-The excess carrier density decays exponentially with time according to the relation
n=noe-t/‫ז‬
no= initial injected excess electron density
‫ =ז‬carrier life time depends on material composition and device defects
-The excess carrier can recombine either radiatively and non-radiatively
-When there is a constant current flow into an LED a equilibrium condition is
established.
-The total rate at which carriers are generated is the sum of externally supplied and
thermally generated rates.
Derivation
-
Not all internally generated photons will exit the device.
-
To find the emitted power, need to consider external quantum efficiency.
-
Defined as the ratio of the photons emitted from the LED to the number of
internally generated photons.
-
To find external quantum efficiency , reflection effects at the surface of the LED
have to be taken into account.
EXPRESSION&DIAGRAM
MODULATION OF A LED:
-
The frequency response of an LED is largely determined by 3 factors:
1. Doping level in the active region
2. Injected carrier lifetime
3. Parasitic capacitance of LED
- The modulation bandwidth of an LED can be defined in either electrical or optical
terms.
-Modulation bandwidth is defined as the point where the electrical signal power,
designated by p(w) has dropped to half its constant value resulting from the
modulated portion of the optical signal.
-This is the electrical 3dB point that is the freq at which the output electrical power is
reduced by 3dB wrt input electrical power.
p(w)=I2(w)/R
Ratioelec=10log[p(w)/p(o)]=10log[I2(w)/I2(o)]
I(w)= electric current in the detection circuitry
-The electrical 3dB point occurs at that freq point where the detected electrical power
p(w)=p(o)/2
I2(w)/I2(o)=1/2
I(w)/I(o)=0.707
-The modulation bandwidth of an LED is given in terms of 3dB bandwidth of the
modulated optical power P(w), it is specified at the frequency where P(w)=Po/2
- In this case the 3dB bandwidth is determined from the ratio of the optical power wrt
freq w to the unmodulated value of the optical power.
Ratiooptical=10log[P(w)/P(o)]=10log[I(w)/I(o)]=0.500
GRAPH
LASER DIODES
-Laser come in many forms
-The lasing medium can be gas, a liquid, an insulating crystal or a semiconductor.
-For optical fiber systems the laser sources used are semiconductor laser diodes.
-Spatial and temporal coherence, highly monochromatic ,very directional.
-Laser action is the result of 3 key processes:
-photon absorption
- spontaneous emission
- stimulated emission
-In thermal equilibrium the density of excited electrons is very small.
-Stimulated emission will exceed absorption only if the population of the excited
states is greater than that of ground state. This condition is known as Population
Inversion.
-Population Inversion is achieved by various pumping techniques.
Laser Diode Modes and Threshold Conditions
-Virtually all laser diodes in use are multilayered heterojunction devices.
-Stimulated emission in semiconductor lasers arises from optical transitions
between distributions of energy states in the valence and conduction bands.
-This differs from gas and solid state lasers, in which radiative transitions occur
between discrete isolated atomic and molecular levels.
-The radiation in the laser diode is generated within a Fabry-Perot resonator cavity.
-A pair of flat , partially reflected mirrors are directed toward each other to enclose
the cavity.
-The purpose of these mirrors is to provide strong optical feedback, thereby
converting a device into an oscillator with a gain mechanism that compensates for
optical losses in the cavity.
-The laser cavity can have many resonant frequencies.
-The device will oscillate for those resonant frequencies.
-The sides of the cavity are formed by roughening the edges of the device to
reduce unwanted emission in these directions.
Distributed Feedback laser:
-Cleaved facets are not required for optical feedback.
-Lasing action is obtained from Bragg reflectors or periodic variations of the
refractive index which are incorporated into the multilayer structure along the
length of the diode.
-The optical radiation within the resonance cavity of a laser diode sets up a
pattern of electric and magnetic field lines called modes of the cavity.
-These can be separated into two independent sets of transverse electric and
transverse magnetic modes.
-Each set of modes can be described in terms of lateral, longitudinal and
transverse variations of the electromagnetic fields.
-The longitudinal modes are related to the length of the cavity, and determine
the principle structure of the frequency spectrum of the emitted optical radiation.
-Lateral modes lie on the plane of the pn junction. These mode depend on the
side wall preparation and the width of the cavity and determine the shape of the
lateral profile of the laser beam.
-Transverse modes are associated with the electromagnetic field and the beam
profile in the direction perpendicular to the plane of the pn junction.
-To determine lasing conditions and the resonant frequencies, the
electromagnetic wave propagating in the longitudinal direction in terms of
electric field phasor is given by
EQUATIONS
Laser Diode Rate Equations:
-The relation between the optical power output and diode drive current can be
determined by examining the rate equations.
-The total carrier population is determined by carrier injection, spontaneous
recombination and stimulated emission.
-For a pn junction with a carrier confinement region of depth d, the rate equations
are given by
and
C- is the coefficient describing the strength of the optical absorption and emission
interactions
Source of photons
resulting from
stimulated emission
Increase in the electron conctn in
the conduction band as current
flows into the device
No of photons
produced by
spontaneous emission
Decay in the photons
caused by loss
mechanisms
No. of the electrons lost from
the conduction band owing to
spontaneous and stimulated
emissions
- Solving these two equations will yield expression for output power.
External Quantum Efficiency:
-Is defined as number of photons emitted per radiative electron hole pair
recombination above threshold
Experimentally it is calculated from
Eg = bandgap energy in electron volts
dP = incremental change in emitted output power
dI = Incremental change in drive current
λ=emission wavelength
LASER DIODE STRUCTURES AND RADIATION PATTERNS
-Basic requirement for efficient operation of laser diodes, the current flow must be
restricted laterally to a narrow stripe along the length of the laser.
-Three basic optical confinement methods used for bounding laser light in the
lateral direction.
-Gain guided laser
-Positive index waveguide
-Negative index waveguide
LIGHT SOURCE LINEARITY
- In analog applications any device nonlinearities will create frequency
components in the output signal that were not present in the input signal.
-Two important nonlinear effects are harmonic and intermodulation distortions.
- Harmonic distortion –
- Non-linearities are the result of inhomogeneities in the active region of the device
and arise from power switching between the lateral modes in the laser. These are
referred to as ‘kinks’ .
MODAL,PARTITION AND REFLECTION NOISE
Modal or speckle noise: (speckle pattern – mutual interference of set of wavefronts)
-Noise is generated when speckle pattern changes in time.
-The continuously varying speckle pattern that falls on the photodetector produces a
time varying noise in the received signal.
Mode partition Noise:
- Is associated with intensity fluctuations.
Reflection Noise:
-Is associated with linearity distortion caused by some of the light output being
reflected back into the laser cavity from fiber joints.
-The reflected power couples with the lasing modes thereby causing their phases to
vary.
RELIABILITY CONSIDERATIONS:
-Degradation of light sources can be divided into three basic categories.
Internal damage:
- This effect arises from the migration of crystal defects into the active region of the
light source. These defect decrease the internal quantum efficiency.
Ohmic contact deterioration:
-This effect is a function of the solder used to bond the chip to the heat sink, the
current density through the contact and the contact temperature.
Facet damage:
- This degradation reduces the laser mirror reflectivity and increases the
nonradiative carrier recombination at the laser facets.
PHOTODETECTORS
-Receiving device
-Converts variation of optical power into correspondingly varying electric current
-Should be insensitive to variations in temperature
-Different types of photodetectors are in existence:
- photomultipliers
- pyroelectric detectors
- semiconductor based photodiodes
-Semiconductor photodiodes is used exclusively because of its small size,high
sensitivity and fast response time.
-Two types of photodiodes : pin photodiode and avalanche photodiode
The pin Photodetector:
-The device structure consists of p and n regions separated by very lightly n-doped
intrinsic(i) region.
-Large reverse bias voltage is applied , intrinsic region is fully depleted of carriers.
-When photon is incident , can give up its energy and excite an electron from valence
band to the conduction band. This process is known as Photocarriers.
-The high electric field present in the depletion region causes the carriers to separate.
-This gives rise to a current flow in the external circuit, with one electron flowing for
every carrier pair generated.
-This current flow is known as photocurrent.
-As the charge carriers flow through the material , some electron –hole pairs will
recombine and disappear.
-The charge carriers move a distance, this distance is known as diffusion length.
-The time it takes for an electron or hole to recombine is known as carrier life time.
-The life time and diffusion length is related by the expressions
Ln=(Dn‫ז‬n)1/2 and Lp=(Dp‫ז‬p)1/2
-Optical radiation is absorbed in the semiconductor material according to the
exponential law
P(x)= P0(1-e-αs(λ)x)
αs(λ)= absorption coefficient at a wavelength λ
P0= incident optical power level
P(x)=optical power absorbed in a distance x
- A particular semiconductor material can be used only over a limited wavelength
range.
-The upper wavelength cutoff λc is determined by the band gap energy of the
material.
λc= hc / Eg = 1.24/Eg(ev)
-The photoresponse cuts off as a result of very large values of αs at the shorter
wavelengths.
-In this case photons are absorbed very close to the photodetector surface where
the recombination time of the generated electron hole pairs is very short.
-The generated carriers thus recombine before they can be collected by the
photodetector circuitry.
-Taking into account the reflectivity at the entrance of the photodiode then the
primary current resulting from the power absorption is given by
-Two important characteristics of a photodetector are quantum efficiency and
response speed.
-These parameters depend on material band gap, operating wavelength, and the
doping and thickness of p,i and n regions of the device.
-The quantum efficiency η is the number of electron hole carrier pairs generated per
incident photon energy h‫ ע‬and is given by
η= Ip/q / Po/h ‫ע‬
-To achieve a high quantum efficiency , the depletion layer must be thick to permit
large fraction of the incident light to be absorbed.
-The thicker the depletion layer the longer it takes for the photogenerated carriers to
drift across the reverse biased junction.
-The performance of photodiode is characterized by Responsivity R. This is related
to quantum efficiency by
R= Ip/Po = ηq/h‫ע‬
-In most photodiodes the quantum efficiency is independent of the power level
falling on the detector.
-Responsivity is a linear function of the optical power.
-The responsivity is a function of wavelength and of the photodiode material.
Avalanche photodiode
-Internally multiply the primary signal photocurrent
-In order of carrier multiplication , the photogenerated carriers must traverse a
region where a very high electric field is present.
-In this high field region a photogenerated electron or hole can gain enough energy
so that it ionizes bound electrons in the valence band upon colliding them.
-This carrier multiplication mechanism is known as impact ionization.
-The newly created carriers are also accelerated by high electric field to cause
further impact ionization. This phenomenon is the avalanche effect.
- A commonly used structure for achieving this carrier multiplication is the reach
through construction.
-This configuration is referred to as p+πpn+ reach through structure.
-The average number of electron hole pairs created by a carrier per unit distance
travelled is called the ionization rate.
-The multiplication M for all carriers generated in the photodiode is defined by
M=IM/Ip
-IM= average value of total multiplied output current Ip=primary unmultiplied output
current
pin photodiode
avalanche
photodiode
Detector Response Time
Depletion Layer photocurrent:
Schematic representation of a reverse – biased pin photodiode:
The principle noises associated with the photodetectors are
-Quantum noise
-Dark current noise generated in the bulk material of the photodiode
-Surface leakage current noise
Quantum Noise:
Arises from the statistical nature of the production and collection of photoelectrons
when a optical signal is incident on a photodetector.
Bulk dark current:
Arises from the electrons and or holes which are thermally generated in the pn
junction of the photodiode.
Surface leakage current:
Simply the leakage current , dependent on surface defects, cleanliness, bias
voltage and surface area.
-Effective way of reducing surface dark current is through the use of a guard ring
structure which shunts leakage current away from the load resistor.
RESPONSE TIME
The response time of photodiode depends mainly on 3 factors:
The photodiode parameters responsible for these factors are:
-Absorption coefficient
-The depletion region width
-Photodiode junction
-Package capacitances, amplifier capacitances
-Detector load resistance, amplifier input resistance, and photodiode series
resistance
The transit time of the photocarriers:
-Depends on carrier drift velocity and depletion layer width
- The diffusion process are compared with the drift of carriers in the high field region.
-The response time is described by the rise and fall time of the detector output.
Fully depleted region
partially depleted
region
-Devices with very thin depletion regions tend to show slow and fast response
components.
-The fast component in the rise time is due to carriers generated in the depletion
region.
-Slow component arises from the diffusion of carriers that are created from the
edge of the depletion region.
-The junction capacitance is
Comparison of photodetectors