Transcript Fig. 6-1: pin photodiode circuit

Optical Receivers
Theory and Operation
Xavier Fernando
Ryerson Communications Lab
http://www.ee.ryerson.ca/~fernando
Photo Detectors
• Optical receivers convert optical signal (light)
to electrical signal (current/voltage)
– Hence referred ‘O/E Converter’
• Photodetector is the fundamental element of
optical receiver, followed by amplifiers and
signal conditioning circuitry
• There are several photodetector types:
– Photodiodes, Phototransistors, Photon multipliers,
Photo-resistors etc.
Requirements
• Compatible physical dimensions (small size)
• Low sensitivity (high responsivity) at the
desired wavelength and low responsivity
elsewhere  wavelength selectivity
• Low noise and high gain
• Fast response time  high bandwidth
• Insensitive to temperature variations
• Long operating life and low cost
Photodiodes
• Photodiodes meet most the requirements, hence
widely used as photo detectors.
• Positive-Intrinsic-Negative (pin) photodiode
– No internal gain, robust detector
• Avalanche Photo Diode (APD)
– Advanced version with internal gain M due to self
multiplication process
• Photodiodes are sufficiently reverse biased during
normal operation  no current flow without
illumination, the intrinsic region is fully depleted of
carriers
Physical Principles of
Photodiodes
• As a photon flux Φ penetrates into a semiconductor, it will be
absorbed as it progresses through the material.
• If αs(λ) is the photon absorption coefficient at a wavelength λ,
the power level at a distance x into the material is
Absorbed photons
trigger photocurrent
Ip in the external
circuitry
Examples of Photon
Absorption
pin energy-band diagram
Cut off wavelength:
c 
hc
Eg

1 . 24
E g ( eV )
μm
Cut off wavelength depends on the
band gap energy
Quantum Efficiency
• The quantum efficiency η is the number of the
electron–hole carrier pairs generated per incident–
absorbed photon of energy hν and is given by
Ip is the photocurrent generated by a steady-state
optical power Pin incident on the photodetector.
Avalanche Photodiode (APD)
• APD has an internal gain M, which is obtained by
having a high electric field that energizes photogenerated electrons.
• These electrons ionize bound electrons in the
valence band upon colliding with them which is
known as impact ionization
• The newly generated electrons and holes are also
accelerated by the high electric field and gain
energy to cause further impact ionization
• This phenomena is the avalanche effect
APD Vs PIN
Responsivity ()
Quantum Efficiency () = number of e-h pairs
generated / number of incident photons
 
Ip /q
 
P0 / h
Ip

P0
q
h
APD’s have an internal gain M, hence
 APD   PIN M
where, M = IM/Ip
IM : Mean multiplied current
M = 1 for PIN diodes
mA/mW
Responsivity
When λ<< λc absorption is low
When λ > λc; no absorption
c 
hc
Eg
Light Absorption Coefficient
• The upper cutoff
wavelength is
determined by the
bandgap energy Eg of
the material.
• At lower-wavelength
end, the photo response
diminishes due to low
absorption (very large
values of αs).
Photodetector Noise
• In fiber optic communication systems, the photodiode is
generally required to detect very weak optical signals.
• Detection of weak optical signals requires that the
photodetector and its amplification circuitry be optimized to
maintain a given signal-to-noise ratio.
• The power signal-to-noise ratio S/N (also designated by SNR)
at the output of an optical receiver is defined by
SNR Can NOT be improved by amplification
Notation: Detector Current
• The direct current value is denoted by, IP (capitol main
entry and capital suffix).
• The time varying (either randomly or periodically) current
with a zero mean is denoted by, ip (small main entry and
small suffix).
• Therefore, the total current Ip is the sum of the DC
component IP and the AC component ip .
IP  I p  ip
i
2
p
 Lim
1
T 
T
T /2
i
T / 2
2
p
( t ) dt
Quantum (Shot Noise)
Quantum noise arises due optical power fluctuation
because light is made up of discrete number of photons
i
2
Q
 2 qI p BM
2
F (M )
F(M): APD Noise Figure F(M) ~= Mx (0 ≤ x ≤ 1)
Ip: Mean Detected Current
B = Bandwidth
q: Charge of an electron
Dark/Leakage Current Noise
There will be some (dark and leakage ) current without any
incident light. This current generates two types of noise
Bulk Dark Current Noise
i
2
DB
 2 qI D BM F ( M )
2
ID: Dark Current
Surface Leakage
Current Noise
(not multiplied by M)
i
2
DS
 2 qI L B
IL: Leakage Current
Thermal Noise
The photodetector load resistor RL contributes to
thermal (Johnson) noise current
iT  4 K B TB / R L
2
KB: Boltzmann’s constant = 1.38054 X 10(-23) J/K
T is the absolute Temperature
• Quantum and Thermal are the significant noise
mechanisms in all optical receivers
• RIN (Relative Intensity Noise) will also appear in
analog links
Signal to Noise Ratio
Detected current = AC (ip) + DC (Ip)
Signal Power = <ip2>M2
2
SN R 
ip M
2
2 q ( I p  I D ) M F ( M ) B  2 qI L B  4 k B T B / R L
2
Typically not all the noise terms will have equal weight.
Often thermal and quantum noise are the most significant.
Noise Calculation Example
Limiting Cases for SNR
• When the optical signal power is relatively high, then the shot
noise power is much greater than the thermal noise power. In
this case the SNR is called shot-noise or quantum noise
limited.
• When the optical signal power is low, then thermal noise
usually dominates over the shot noise. In this case the SNR is
referred to as being thermal-noise limited.
Limiting Cases of SNR
In the shot current limited case the SNR is:
2
SN R 
ip
2q(I p )F (M )B
For analog links, there will be RIN (Relative
Intensity Noise) as well
2
SN R 
ip M
2
 2 q ( I p  I D ) M 2 F ( M )  4 k B T / R L  ( R IN ) I p2  B


Typical SNR vs. Received Power
• Note, APD
has an
advantage
only at low
received
power levels
Noise-Equivalent Power
• The sensitivity of a photodetector is describable in terms of the
minimum detectable optical power to have SNR = 1.
• This optical power is the noise equivalent power or NEP.
• Example: Consider the thermal-noise limited case for a pin
photodiode. Then
To find the NEP, set the SNR = 1 and solve for P:
Response Time in pin photodiode
Transit time, td and carrier drift velocity vd are related by
td  w / vd
For a high speed Si PD, td = 0.1 ns
Rise and fall times
Photodiode has uneven rise and fall times depending on:
1. Absorption coefficient s() and
 o r A
2. Junction Capacitance Cj
Cj 
w
Junction Capacitance
Cj 
 o r A
w
εo = 8.8542 x 10(-12) F/m; free space permittivity
εr = the semiconductor dielectric constant
A = the diffusion layer (photo sensitive) area
w = width of the depletion layer
Large area photo detectors have large junction
capacitance hence small bandwidth (low speed)
 A concern in free space optical receivers
Various pulse responses
Pulse response is a complex function of absorption coefficient
and junction capacitance
Comparisons of pin Photodiodes
NOTE: The values were derived from various vendor data
sheets and from performance numbers reported in the
literature. They are guidelines for comparison purposes.
Comparisons of APDs
NOTE: The values were derived from various vendor data sheets
and from performance numbers reported in the literature. They
are guidelines for comparison purposes only.
Part B
OPTICAL RECEIVER
Signal Path through an Optical Link
Fundamental Receiver Operation
• The first receiver element is a pin or an avalanche photodiode, which
produces an electric current proportional to the received power level.
• Since this electric current typically is very weak, a front-end amplifier
boosts it to a level that can be used by the following electronics.
• After being amplified, the signal passes through a low-pass filter to reduce
the noise that is outside of the signal bandwidth.
• The also filter can reshape (equalize) the pulses that have become distorted
as they traveled through the fiber.
• Together with a clock (timing) recovery circuit, a decision circuit decides
whether a 1 or 0 pulse was received,
Optical receiver schematic
Bandwidth of the front end:
CT: Total Capacitance = Cd+Ca
RT: Total Resistance = Rb // Ra
Try Example 6.7 in Keiser
B  1 2 R T C T
Noise Sources in a Receiver
The term noise describes unwanted components of an electric signal that tend to
disturb the transmission and processing of the signal
• The random arrival rate of signal photons produces quantum (shot) noise
• Dark current comes from thermally generated eh pairs in the pn junction
• Additional shot noise arises from the statistical nature of the APD process
• Thermal noises arise from the random motion of electrons in the detector
load resistor and in the amplifier electronics
Probability of Error (BER)
• BER is the ratio of erroneous bits to correct bits
• A simple way to measure the error rate in a digital data stream
is to divide the number Ne of errors occurring over a certain
time interval t by the number Nt of pulses (ones and zeros)
transmitted during this interval.
• This is the bit-error rate (BER)
• Here B is the bit rate.
• Typical error rates for optical fiber telecom systems range
from 10–9 to 10–12 (compared to 10-6 for wireless systems)
• The error rate depends on the signal-to-noise ratio at the
receiver (the ratio of signal power to noise power).
Logic 0 and 1 probability distributions
Pe 
1
2
 P1 (V th )  P0 (V th ) 
Asymmetric distributions
Select Vth to minimize Pe
P0 (V th ) 
P1 (V th ) 


p ( y / 0) dy
V th

V th

p ( y / 1) dy
Deciding Threshold Voltage
Probability of error assuming P  1 P (V )  P (V )
e
th
0
th 
2  1
Equal ones and zeros
Where,
P0 (V th ) 
P1 (V th ) 


p ( y / 0) dy
V th

V th

p ( y / 1) dy
Depends on the noise variance at on/off levels and the
Threshold voltage Vth that is decided to minimize the Pe
Question: Do you think Vth = ½ [Von + Voff] ?
Derived BER Expression
• A simple estimation of the BER can be calculated by assuming the
equalizer output is a gaussian random variable.
• Let the mean and variance of the gaussian output for a 1 pulse be bon
and σ2on, respectively, and boff and σ2off for a 0 pulse.
• If the probabilities of 0 and 1 pulses are equally likely, the bit error
rate or the error probability Pe becomes
Probability of Error Calculation
• The factor Q is widely used to specify receiver performance, since it
is related to the SNR required to achieve a specific BER.
• There exists a narrow range of SNR above which the error rate is
tolerable and below which a highly unacceptable number of errors
occur. The SNR at which this transition occurs is called the
threshold level.
BER as a Function of SNR
BER as a function of SNR when the standard deviations are equal
(σon = σoff) and when boff = 0
Receiver Sensitivity
• A specific minimum average optical power level must arrive at
the photodetector to achieve a desired BER at a given data rate.
The value of this minimum power level is called the receiver
sensitivity.
• Assuming there is no optical power in a received zero pulse,
then the receiver sensitivity is
Where, including an amplifier noise figure Fn, the
thermal noise current variance is
Receiver Sensitivity Calculation
The receiver sensitivity as a function of bit rate will change for a given
photodiode depending on values of parameters such as wavelength,
APD gain, and noise figure.
The Quantum Limit
• The minimum received optical power required for a specific bit-error rate
performance in a digital system.
• This power level is called the quantum limit, since all system parameters
are assumed ideal and the performance is limited only by the detection
statistics.
Eye Diagrams
• Eye pattern measurements are made in the time domain and
immediately show the effects of waveform distortion on the
display screen of standard BER test equipment.
– The eye opening width defines the time interval over which signals can be
sampled without interference from adjacent pulses (ISI).
– The best sampling time is at the height of the largest eye opening.
– The eye opening height shows the noise margin or immunity to noise.
– The rate at which the eye closes gives the sensitivity to timing errors.
– The rise time is the interval between the 10 and 90% rising-edge points
Stressed Eye Tests
• The IEEE 802.3ae spec for testing 10-Gigabit Ethernet (10-GbE) devices
describes performance measures using a degraded signal.
• This stressed eye test examines the worst-case condition of a poor
extinction ratio plus multiple stresses, ISI or vertical eye closure, sinusoidal
interference, and sinusoidal jitter.
• The test assumes that all different possible signal impairments will close
the eye down to a diamond shaped area (0.10 and 0.25 of the full pattern
height).
• If the eye opening is greater than this area, the receiver being tested is
expected to operate properly in an actual fielded system.
The inclusion of all possible signal
distortion effects results in a
stressed eye with only a small
diamond-shaped opening
46
Architecture of a Typical PON
• A passive optical network (PON) connects switching equipment in a
central office (CO) with N service subscribers
• Digitized voice and data are sent downstream from the CO to customers
over an optical link by using a 1490-nm wavelength.
• The upstream (customer to central office) return path for the data and voice
uses a 1310-nm wavelength.
Burst-Mode Receivers
• The amplitude and phase of packets received in successive time slots from
different user locations can vary widely from packet to packet.
• If the fiber attenuation is 0.5 dB/km, there is a 10-dB difference in the
signal amplitudes from the closest and farthest users.
• If there are additional optical components in one of the transmission paths,
then the signal levels arriving at the OLT could vary up to 20 dB.
• A fast-responding burst-mode receiver with high sensitivity is needed
The guard time
provides a sufficient
delay time to
prevent collisions
between
successive packets
that may come from
different ONTs.