• Photonic and Electronic Device Laboratory


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    • Abstract: Photonic and Electronic Device LaboratoryLecture for Photonics Lab 3Optical FibersPhotodetectors1 This Week’s LaboratoryWe will couple light into an optical fiber and determine the coupling efficiency

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Photonic and Electronic Device Laboratory
Lecture for Photonics Lab 3
Optical Fibers
Photodetectors
1
This Week’s Laboratory
We will couple light into an optical fiber and determine the coupling efficiency
Laser Screw-in Photodetector
Laser
Mount diode Lens Mount Optical Fiber Coupler Optical Fiber
230 mm 220 mm
approx.
5-6 mm
Laser Diode
Current Power
Source Meter
Lens
TE Cooler
Optical Breadboard
Today’s lecture: optical fibers & photodetectors
2
Semiconductor Photodiodes
Performance Requirements of Optical Detectors:
•High sensitivity at operating wavelengths
•High fidelity
•High quantum efficiency
•Large bandwidth
•Low noise
•Stability of performance characteristics
•Small size
•Low bias voltage
•High reliability
3
Principle of Optical Detection
Photogeneration of electron-hole pair: e-
EC
hυ > Eg Eg
EV
hole
Consider a PN photodiode:
depletion
region •Apply reverse bias: electric field separates
the generated carriers
- +
p n •Holes are accelerated left
•Electrons are accelerated right
EC •Collected by external circuit
EV
hυ > Eg
EF
4
Quantum Efficiency vs Wavelength
•Si p-i-n photodiodes can be
fabricated with nearly unity
quantum efficiency if an
anti-reflection coating is applied
•InSb detectors must be cooled
to minimize thermal excitation
from: Saleh and Teich, Fundamentals of Photonics (1991) 5
Photodetector Quantum Efficiency
Quantum Efficiency
number electrons collected
quantum efficiency =
number of incident photons
re (e' s /second)
η=
rp (photons /second)
η is largely determined by the absorption coefficient of the semiconductor material
η is usually less than 1
6
Photodetector Responsivity
Responsivity
IP A
R= , units of
P0 W
where
IP = output photocurrent (A)
P0 = incident optical power (W)
•We can now derive a simple expression for the responsivity
Now,
P0
rp =

P0
and, by definition, re = η rp = η

7
Photodetector Responsivity - continued
•Therefore, we have:
P0
Ip = η q where q = electronic charge

•Since
IP
R=
P0
ηq
•Then we have: R=

η qλ
R=
hc
from: Saleh and Teich, Fundamentals of Photonics (1991) 8
Current-Voltage Characteristic of Photodiode
•The I-V relation of the photodiode is given by:
⎡ qV ⎤
i = is ⎢exp( ) − 1⎥ − i p
⎣ k BT ⎦
•This is the usual I-V relation of a PN junction, with an added photocurrent –ip proportional
to the photon flux
9
Photodiode: Photovoltaic Mode, Short-circuit Mode
Photovoltaic Mode
•Used in solar cells
•Responsivity measured in V/W
rather than A/W
Short-circuit Mode
•Short-circuit current is simply
the photocurrent ip
10
Photodiode: Reverse-biased (Photoconductive) Mode
Reverse-biased, no load resistor
Reverse-biased, with load resistor
•operating point lies on the dashed line
11
Photodiode: Reverse-biased (Photoconductive) Mode
Why add a reverse bias ?
•Strong reverse bias creates a strong electric field in the junction which increases the
drift velocity of the carriers, thereby reducing transit time
•Strong reverse bias increases width of depletion layer, thereby reducing junction
capacitance and improving response time
•Increased width of depletion layer leads to larger photosensitive area. This makes it
easier to collect more light
12
Optical Fibers
Optical Fiber = Cylindrical Dielectric Waveguide
Fiber Application Material Core Cladding Buffer Loss
Type diameter diameter Diameter (dB/km)
Single telecommunications, glass/glass 10 µm 125 µm 250 µm 0.2
mode cable TV
Multi- local area networks glass/glass 62.5 µm 125 µm 250 µm 0.4
mode
plastic display lighting plastic/plastic 400 µm 1000 µm ?? 150 (at λ
= 600nm)
pure laser beam delivery glass/glass 400 µm 470 µm 650 µm 10
silica in medicine
core 13
Optical Fibers - Applications
•Terrestrial and undersea telecommunications links Undersea Optical Fiber Cables
•Cable TV systems
•Remoting of antennas for microwave satellite communications
•Local area networks
•Fiber gyroscopes in navigation (e.g. planes)
•Instrument panel lighting
•Tethered guided missiles
•Laser beam delivery in medicine and for cutting and drilling metals
14
Optical Fibers: Types
Multimode
Single mode
Graded Index
•Refractive indices of core and cladding differ only slightly
•Fractional refractive index change is small: n1 − n2
∆= ≈ 0.001 − 0.02
n1
•Optical communications fibers made out of high-purity SiO2
•Slight changes in refractive index provided by addition of low concentrations of
dopants (titanium, germanium, boron, et.c.) 15
Optical Fibers: Numerical Aperture
Ray Launched into Optical Fiber:
•What is the maximum angle at which light can be launched into the fiber and still guided?
•Apply Snell’s Law at endface:
no sin α = nco cosν i
•We wish to determine the maximum launch angle αmax that sets υi=υcr
where υcr is the critical angle for total internal reflection
16
Optical Fibers: Numerical Aperture, continued
•Consider core-cladding interface, apply Snell’s Law : cladding
υt ncl
nco sinν i = ncl sinν t
υi υr nco
core
•Critical angle is given by:
ncl
sinν cr =
nco
•Now, we have:
no sin α max = nco 1 − sin 2 ν cr
17
Optical Fibers: Numerical Aperture, continued
•Let us define the numerical aperture by:
NA = no sin α max
•Then, we have:
NA = nco − ncl
2 2
18
Modes of an Optical Fiber: Basic Concepts
Ray in Multimode fiber with Accompanying Wave Fronts
•‘Up’ and ‘down’ waves interfere and set up a standing wave pattern
•The standing wave patterns contain an integral number of half wavelengths
•Each acceptable ray path defines a mode of the optical waveguide, each corresponding
to a unique two-dimensional standing wave pattern
•Multi-mode waveguides support many modes due to the large diameter of their cores
compared to the wavelength of light
19
Modal Dispersion in Multi-mode Optical Fibers
Steepest and Shallowest Trajectories for Modes in Multi-mode Fiber
•Let us assume that each mode can be represented by a plane wave
•The group velocity vg of a mode is then given by:
c
vg = sin vi
nco
•Therefore, the propagation time τ over optical fiber of length L is:
nco 1
τ =L
c sin vi
20
Modal Dispersion in Multi-mode Optical Fibers – c’td.
•Therefore, the maximum difference in transit time ∆τ is given by:
nco ⎛ 1 1 ⎞
∆τ = L ⎜
⎜ sin v − sin π / 2 ⎟

c ⎝ cr ⎠
nco ⎛ nco − ncl ⎞
=L ⎜ ⎟
c ⎜ ncl



L NA2

2c nco
•Example: What is the difference in estimated maximum and minimum transit times for
a multi-mode optical fiber with NA=0.2 ?
∆τ 1 NA2 ( 0. 2) 2
= = = 44 nS/km
L 2c nco 2 × (3 ×10 ) ×1.5
8
21
Modal Dispersion in Multi-mode Optical Fibers – c’td.
•Implications of intermodal dispersion:
What bit rate can this optical fiber system support?
The transit time spread ought to be less than the bit period: ∆τ < 1 / B
•A convenient means for characterising the information carrying capacity is the
(bit-rate)*distance product:
Now, we have: B∆τ < 1
So that: 2cnco
BL <
NA2
•For our example, this gives us: BL = 22.5 Mbits/sec km
•NOTE: our assumptions are a little crude, and the real number is 100-1500 Mbits/sec km
22
Optical Fibers
•SIngle mode optical fiber overcomes the problem of intermodal dispersion
•It still suffers from material dispersion (wavelength dependence of refractive index)
and waveguide dispersion
Ranges of Attenuation Coefficients of Silica Glass Single and Multi- Mode Fibers
23


Use: 0.0332