Optical Receivers Theory and Operation

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Optical Receivers Theory and Operation

• Xavier Fernando
• Ryerson Communications Lab
• http//www.ee.ryerson.ca/fernando

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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.

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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

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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

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Physical Principles of Photodiodes

• As a photon flux F penetrates into a
  semiconductor, it will be absorbed as it
  progresses through the material.
• If as(?) is the photon absorption coefficient at
  a wavelength ?, the power level at a distance x
  into the material is

 
Absorbed photons trigger photocurrent Ip in the
external circuitry
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Examples of Photon Absorption
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pin energy-band diagram
Cut off wavelength
Cut off wavelength depends on the band gap energy
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Quantum Efficiency

• The quantum efficiency ? is the number of the
  electronhole carrier pairs generated per
  incidentabsorbed photon of energy h? and is
  given by

Ip is the photocurrent generated by a
steady-state optical power Pin incident on the
photodetector.
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Avalanche Photodiode (APD)

• APD has an internal gain M, which is obtained by
  having a high electric field that energizes
  photo-generated 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

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APD Vs PIN

•  

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Responsivity (?)

• Quantum Efficiency (?) number of e-h pairs
  generated / number of incident photons
• APDs have an internal gain M, hence
• where, M IM/Ip
• IM Mean multiplied current

mA/mW
M 1 for PIN diodes
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Responsivity
 
When ?ltlt ?c absorption is low When ? gt ?c no
absorption
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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 as).

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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
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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 .

 
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Quantum (Shot Noise)
Quantum noise arises due optical power
fluctuation because light is made up of discrete
number of photons
F(M) APD Noise Figure F(M) Mx (0 x 1)
Ip Mean Detected Current B Bandwidth q
Charge of an electron
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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
ID Dark Current
Surface Leakage Current Noise
IL Leakage Current
(not multiplied by M)
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Thermal Noise
The photodetector load resistor RL contributes
to thermal (Johnson) noise current
KB Boltzmanns 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

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Signal to Noise Ratio
Detected current AC (ip) DC (Ip)
Signal Power ltip2gtM2
Typically not all the noise terms will have equal
weight. Often thermal and quantum noise are the
most significant.
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Noise Calculation Example
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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.

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Limiting Cases of SNR

• In the shot current limited case the SNR is
• For analog links, there will be RIN (Relative
  Intensity Noise) as well

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Typical SNR vs. Received Power

• Note, APD has an advantage only at low received
  power levels

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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
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Response Time in pin photodiode
Transit time, td and carrier drift velocity vd
are related by
For a high speed Si PD, td 0.1 ns
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Rise and fall times

• Photodiode has uneven rise and fall times
  depending on
• Absorption coefficient ?s(?) and
• Junction Capacitance Cj

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Junction Capacitance
eo 8.8542 x 10(-12) F/m free space
permittivity er 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
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Various pulse responses
Pulse response is a complex function of
absorption coefficient and junction capacitance
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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.
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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.
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Optical receiver

• Part B

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Signal Path through an Optical Link
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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,

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Optical receiver schematic
Bandwidth of the front end CT Total
Capacitance CdCa RT Total Resistance Rb //
Ra Try Example 6.7 in Keiser
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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

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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 109 to 1012 (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).

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Logic 0 and 1 probability distributions
Asymmetric distributions
Select Vth to minimize Pe
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Deciding Threshold Voltage
Probability of error assuming Equal ones and
zeros
Where,
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 ?
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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 s2on, respectively, and
  boff and s2off 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

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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.

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BER as a Function of SNR

• BER as a function of SNR when the standard
  deviations are equal (son soff) and when boff
  0

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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
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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.

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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.

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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

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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
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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.

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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.