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Lecture 9
Semiconductor Photodetectors
 There are three material systems used:
 Si for λ  1 μm
 InGaAs for λ  1 μm
 Ge for λ  1 μm
Semiconductor Photodetectors
 There are two types of photodiodes:
 PIN (most used & without internal gain)
 Avalanche Photodiode (with internal gain)
Non-ideal Optical Receivers
 Consider effects of quantum noise, thermal noise, dark
current, and avalanche multiplication noise (if
applied).
I  I s  I d  I th
I  Is  Id
N  N s  N d  average electron in time T
eN s
Is 
T
eN d
Id 
T
Non-ideal Optical Receivers
e2  N s  N d 
e N
I 

2
T
T2
e
2
I N   Is  Id 
T
1
B
 bandwidth
2T
I N2  2e  I s  I d  B
2
2
N
2
Non-ideal Optical Receivers
 We then could find a signal-to-noise ratio as:
Ps  I s2 R  signal power
PN  I R  noise power
2
N
2
s
Ps
I
SNR 

PN 2e  I s  I d  B
Non-ideal Optical Receivers
 Consider effect of avalanche gain M
I M I
2
s
I M
2
N
2
2
s
2 x
I N2
x  excess multiplication noise
 M = 1 for ‘PIN’ and M > 1 for APD.
Non-ideal Optical Receivers
 Consider thermal noise power = 4kTB
 k = Boltmann’s constant = 1.38 x 10-23 J/K
 T = absolute temperature
M 2 I s2 R
SNR 
2eR  I s  I d  M 2  x B  4kTB
M 2 I s2

2e  I s  I d  M 2  x B   4kTB / R 
Non-ideal Optical Receivers
 “RC limited bandwidth”
1
B
; C  photodiode capacitance
2 RC
1
R
2 BC
M 2 I s2
SNR 
2e  I s  I d  M 2  x B  8 CkTB 2
 ‘thermal noise dominates at high bandwidth/data rate’.
Non-ideal Optical Receivers
 “RC limited bandwidth”
1
B
; C  photodiode capacitance
2 RC
1
R
 load resistor
2 BC
M 2 I s2
SNR 
2e  I s  I d  M 2  x B  8 CkTB 2
 ‘thermal noise dominates at high bandwidth/data rate’.
Non-ideal Optical Receivers
 Relate signal current to an optical incident power on
photodetector.
eN s ePopt
Is 

T
h
h  Planck's constant
  optical frequency
  quantum efficiency < 1
Non-ideal Optical Receivers
eN s ePopt
Is 

T
h
h  Planck's constant
  optical frequency
  quantum efficiency < 1
Example
 If λ= 1.55 μm and quantum efficiency η = 0.5. What is
Is in term of Pout?
Example
 If the receiver has following parameters: PIN
photodiode, B = 500 MHz, Is = 1 μA, Id = 10 nA, T =
293K, R = 50Ω. Find the SNR.
Non-ideal Optical Receivers
 How to improve the SNR.
1. Increase load resistor
Non-ideal Optical Receivers
 How to improve the SNR.
2. Use photodetector with gain
Example
 A photodetector with unity internal gain and no thermal noise has a
dark current of 1 x 10-13 A. The photodetector is used for digital
transmission at a rate of 500Mbps so that a bit interval has a duration
of 2ns.
(a)
On the average, how many dark current electrons pass through the load
resistor during a bit interval?
Example
(b) What is the smallest threshold value of electrons per bit interval for which
the error rate will be less than 10-5 when a ‘zero’ is transmitted?
Example
(c) Using the threshold value for number of electrons per bit interval
determined in (b), what is the average number of photoelectrons per bit
interval for which that same error rate is achieved when a ‘one’ is
transmitted? A tolerance of  0.2 electrons per bit interval is allowed for a
‘correct’ answer.
Example
(d) What is the average optical power in W incident upon the photodetector
required to achieve a bit error rate of 10-5, assuming an optical wavelength
of 0.85 μm, equal numbers of ‘ones’ and ‘zeroes’ are transmitted, and
photoelectrons are produced by 23% of the incident photons.
Example
 Consider a non-ideal optical receiver, with unity internal
gain in which thermal noise is the dominant noise source.
The operating temperature is 300K. For a binary data
transmission rate of 1 Gbps, and a photodetector
capacitance of 1 pF, how many photoelectrons per bit are
needed on the average for a bit error rate of 10-10? Assume
that B = 1/(2T), where B is the receiver bandwidth and T is
the bit time interval.
Example
Data Transmission Standards
 Original standard for digital communication is based
on coaxial cable/twisted wire pairs (introduced in early
1960’s by AT&T/Bell telephone system).
 T1 or DS-1 is then introduced in pulse code
communication format.
Data Transmission Standards
 T1 or DS-1 is then introduced in pulse code
communication format.
 DS-0: 64 kbps (one 8 ksamples/s x 8 bits/sample voice
channel)
 DS-1: 24 DS-0 + 8 kbps = 1.544 Mbps.
 DS-2: 6.312 Mbps  96 voice channels.
 DS-3: 44.736 Mbps  672 voice channels.
Data Transmission Standards
Data Transmission Standards
 SONET – Synchronous Optical Network: international
standard developed during 1980’s for high data rate
fiber optic transmission.
Data Transmission Standards
 SONET – Synchronous Optical Network: international
standard developed during 1980’s for high data rate fiber
optic transmission.
 SONET is robust in that redundant path are provided to
maintain transmission after node or cable failure called
“self-healing”.
 SONET combines, consolidates, and segregates traffic from
different locations through same ‘hardware’ machine.
Data Transmission Standards
 ATM – Asynchronous Transport Mode
 Time axis divided into 125 μs frames.
 Each frame contains many 53-octet cells.
Data Transmission Standards
 ATM supports voice, video, and data over one medium
and platform.
 ATM supports LAN, MAN, and WAN with one
platform.
 ATM cells can be combined or multiplexed for
transmission over SONET/SDH.
 Note: SDH (Synchronous Digital Hierarchy) –
compatible with SONET primarily used in Europe.
Long-haul (intercity, transoceanic)
 SMF has been cost-effective medium for:
 1983: new long-distance installations.
 1986: transoceanic systems.
 1988: transoceanic systems.
 Initially, installations were at 1.3 μm. Presently, new
installations are designed for 1.53 μm to take advantage
of lower fiber loss (0.2 – 0.25 dB/km) and Er-doped
fiber amplifiers.
 These systems use dispersion shifted fiber.
Long-haul (intercity, transoceanic)
 Light source: DFB InGaAsP laser.
 Modulation: direct modulation, LiNbO3 external
modulator, electroabsorption external modulator.
 Receiver: InGaAs PIN or APD (M = 20-50)
 Fiber optics used for point-to-point transmission of
time-division-multiplexed binary information.
Long-haul (intercity, transoceanic)
 Time division multiplexing, demultiplexing, switching,
routing, buffering are all done electronically.
 Trends: Within the past years, WDM has been being
employed with channel spacing of 100 GHz (0.78 nm)
or 200 GHz (11.56 nm).
Cable TV
 Use SMF for analog signal distribution (coaxial cable
from distribution modes to homes).
 Trends: Fiber-to-the-home (FTTH) and WDM will
play a key role in future cable.
 HDTV favors digital distributions and optical fibers.
LAN
 Both single-mode and multimode fiber are used for
computer networks, industrial, and university
campuses.
 Optical fibers are mainly used for point-to-point
transmission.
 Trend: More SMF, star buses, and WDM will play
important role.
FTTH
 Services to homes: telephone, picture, data,
entertainment on demand, and etc.
 Passive optical networks (PONs) with cascade stars are
used.
 More Competition between traditional telephone
companies and cable TV companies.
 Installed fibers to the curb will be extended to homes
as demand for broadband services ncrease.
Analyzing Optical Networks
 System requirements: Data rate, SNR (analog), BER
(digital), number of nodes, maximum length between
nodes.
 Component specifications:
 Light source: output power, wavelength, bandwidth.
 Fiber: attenuation and dispersion.
 Receiver: quantum efficiency, capacitance, dark current,
and gain.
 Others such as couplers, connectors, filters, and etc.
 Optical power budget.