1. Optical Fibre Communication Systems
Lecture 4 - Detectors & Receivers
Professor Z Ghassemlooy
Northumbria Communications Laboratory
Faculty of Engineering and
Environment
The University of Northumbria
U.K.

2. Contents Properties and Characteristics Types of Photodiodes Receivers
PIN
APD
Receivers
Noise Sources
Performance
SNR
BER

3. Optical Transmission - Digital
The design of optical receiver is much more complicated than that of
optical transmitter because the receiver must first detect weak, distorted signals and then make decisions on what type of data was sent.
analogue receiver
But offers much higher quality than analogue receiver.

4. Optical Receiver – Block Diagram
Photo-
detection
Amplification
(Pre/post)
Filtering
Signal Processing
Converting
optical
signal into
an electrical
signal
Limiting the
bandwidth,
thus reducing
the noise
power
To recover the
information signal
Optical signal
(photons – hf)
Information signal

5. Photodetection - Definition
It converts the optical energy into an electrical current that is then processed by electronics to recover the information.
Detection Techniques
Thermal Effects
Wave Interaction Effects
Photon Effects

6. Photodiode - Characteristics
An electronics device, whose vi-characteristics is sensitive to the
intensity of an incident light wave. Po
Dark current V I
Forward-biased
“Photovoltic”
operation
Reverse-biased 
“photoconductive”
Short-circuit
Small dark current due to:
leakage
thermal excitation
Quantum efficiency
(electrons/photons)
Responsivity
Insensitive to temperature
variation

7. Photodetector - Types
The most commonly used photodetectors in optical communications are:
Positive-Intrinsic-Negative (PIN) 
No internal gain
Low bias voltage [10-50 = 850 nm, 5-15 = 1300 –1550 nm]
Highly linear
Low dark current
Most widely used
Avalanche Photo-Detector (APD)
Internal gain (increased sensitivity)
Best for high speed and highly sensitive receivers
Strong temperature dependence
High bias voltage[250  = 850 nm, = 1300 –1550 nm]
Costly

8. Photodiode (PIN) - Structure
electron
hole I
Photons
RL (load
resistor)
Output Io
Bias voltage p n
electron
hole I
Depletion
region
No carriers in the I region
No current flow
Reverse-biased
Photons generated electron-hole pair
Photocurrent flow through the diode and in the external circuitry
The power level at a distance x into
the material is:
Where  is the photon absorption coefficient

9. Photodiode (PIN) - Structure
Depletion region width
The capacitance of the depletion layer Cj (F) is:

10. Photodetector - Reponsivity
PIN: APD:
R = Io/Po A/W RAPD = G R
Io = Photocurrent; Po = Incident (detected) optical power
G = APD gain;  = Quantum efficiency = average number of electron-hole pairs emitted re / average number of incident photons rp
Note: rp = Po/hf and re = Io/q
l is the length of the photoactive region
Io = qPo/hf Thus normally  is very low, therefore = 0. So
 = 99% ~ 1

11. Photodetector - Responsivity
Silicon (Si)
Least expensive
Germanium (Ge)
“Classic” detector
Indium gallium
arsenide (InGaAs)
Highest speed
G Keiser , 2000

12. Photodetector - Equivalent Circuit
Photodiode
Contact leads Rs L RL Rj Cj Io
Rs = Small, (i.e s/c)
L = Large, (i.e o/c)
Amplifier
Ramp
Camp
Output
CT = Cj + Camp
RT = Rj || RL || Ramp
The transfer function is given by:

13. Photodetector - Equivalent Circuit
The detector behaves approximately like a first
order RC low-pas filter with a bandwidth of:

14. Photodiode Pulse Responses
Fast response time  High bandwidth
At low bias levels rise and fall times are different. Since photo collection time becomes significant contributor to the rise time.
G Keiser , 2000

15. Photodiode Pulse Responses
Small area photodiode
Large area photodiode
Due to diffusion of carrier from the edge of w
Due to carrier generated in w
w = depletion layer
s = absorption coefficient
G Keiser , 2000

16. Photodetetor – Typical Characteristics
0.1-2
0.5-1
Rise time (ns)
1-5 -
10-500
50-500
0.1-1
1-10
Dark current (nA)
<30 5
20-35
6-10
220
45-100
Bias voltage (-V)
2-5
55-75
2.5-25
50-55 Ge
PIN APD
0.5-2
1.3-2
1.2-3
Capacitance (pF)
60-70 77
65-90
Quantum
Efficiency (%)
50-120
Responsivity
(A/W)
(1550) (1550)
Wavelength range
Peak
(nm)
InGaAS
PIN APD Si
Parameters
Source: R. J. Hoss

17. Minimum Received Power
Is a measure of receiver sensitivity defined for a specific:
Signal-to-noise ratio (SNR),
Bit error Rate (BER),
Bandwidth (bit rate),
at the receiver output. Po Pr
Detector
Amplifier
Power loss
MRP = Minimum Detected Power (MDP) – Coupling Loss

18. MRP Vs. Bandwidth MRP (-dBm)  =1300 Bandwidth (MHz) SNR (dB) 50 30 10
-70
-60
-50
-40
-30
-20
MRP (-dBm)
Bandwidth (MHz) 50 30 10
SNR (dB)
 =1300

19. Selection Criteria and Task
Optical
Optical Sensitivity for a given BER and SNR
Operating wavelength
Dynamic range
Simplicity
Reliability and stability
Electrical
Data rate
Bit error rate (digital)
Maximum Bandwidth (analogue)
Signal-to-noise ratio (analogue)
Task:
To extract the optical signal (low level) from various
noise disturbances
To reconstruct the original information correctly

20. Receivers: Basics
The most important and complex section of an optical fibre system
It sensitivity is design dependent, particularly the first stage or front-end
Main source of major noise sources:
Shot noise current
Thermal noise: Due to biasing/amplifier input impedance
Amplifier noise:
Current
Voltage
Transimpedance noise

21. Receiver - Bandwidth
A range of frequencies that can be defined in terms of:
Spectral profile of a signal
Response of filter networks
Equivalent bandwidth: Defines the amount of noise in a
system
Types of Bandwidth
Ideal
Baseband
Passband
Intermediate-Channel
Transmission
Noise

22. Ideal, Low-pass and Band-pass Filters-3
Ideal
Bbp
Blp
Band-pass filter
Low-pass filter
Higher order
filter
Frequency

23. Noise Equivalent Bandwidth (NEB) B
-3 dB
B3dB B
Filter response
NEB
Defines as the ideal bandwidth
describing the point where:
Area under the response cure =
Area under the noise curve.

24. Optical System Optical drive circuit Light source m(t) Photodiode
Amplifier
Fibre
ip(t)
P(t)
Photocurrent
Average photocurrent
(DC current) Io
Signal current
io(t) +
Photocurrent =

25. Optical Receiver - Model
The received digital pulse stream incident at the photodetector is given by:

26. Optical Receiver - contd.
For m(t) = sin t
The mean square signal current is
For a digital signal
The mean square signal current is

27. Optical System - Noise
Is a random process, which can’t be described as an explicit function of time
In the time domain – Can be characterized in probabilistic terms as:
Mean - correspond to the signal that we are interested to recover
Variance (standard deviation) - represents the noise power at the detector’s output
Can also be characterized in terms of the Root Mean Square (RMS) value
Time average

28. Optical System - Noise
The electric current in a photodetector circuit is composed of a superposition of the electrical pulses associated with each photoelectron
The variation of this current is called shot noise

29. Optical System - Noise Sources
At the receiver:
Additive
Signal dependent
Modal noise
Due to interaction of (constructive & destructive) multiple coherent modes, resulting in intensity modulation.
Photodetector Noise 
Preamplifier (receiver) Noise 
Distortion due to Non-linearity
Crosstalk and Reflection in the Couplers

30. Noise - Source Noise - contd.
LED: Due to:
In-coherent intensity fluctuation
Beat frequencies between modes
LD: Due to:
Non-linearities
Quantum noise: In the photon generation
Mode hopping: Within the cavity
Reflection from the fibre back into the cavity, which reduces coherence
Difficult to measure, to isolate and to quantify
Most problematic with multimode LD and multimode fibre

31. Noise Currents Let noise current be defined as: Shot-noise:
inoise(t) = i(t) - IDC (Amps)
IDC = Photocurrent Io
Noise current from random current pulses is termed as shot-noise.
Shot-noise:
Quantum
Dark current

32. Quantum Shot Noise
The photons arrive randomly in a packet form, with no two
packets containing the same amount of photons.
Random generation of electron-hole pair, thus current.
Variation of the total current generated, about an average value.
This variation is best known as QUANTUM SHOT NOISE.

33. Quantum Shot Noise
The average number of electron-holes pairs per bits is:
Where  the time period.
The probability of detecting n photons in a time period  is follows the Poisson Distribution:
Incoherent light
Y Semenova, DIT, Ireland
Coherent light

34. Quantum Shot Noise
The rate of electron-hole pairs generated by incident photons is:
With an ideal receiver with no noise we have:
Note that, the minimum pulse energy of the quantum limit is:

35. Shot Noise - PIN The mean square quantum shot noise current on Io
The mean square dark current noise (also classified as shot noise)
Where Id = surface leakage current, and B is the electrical bandwidth of the system
Q is the electron charge.
Total shot noise current ITs = Dark current + Photocurrent
The total mean square
shot noise

36. Noise Power Spectrum I2o ITs2 Power spectrum Shot noise Frequency B
Modulation
bandwidth

37. Shot Noise - APD The mean square photocurrent noise where
F = The noise figure = Gx for 0<x<1
G = The optical gain hf RL Av Vi
Bias voltage Vo

38. Noise Currents - contd. Thermal Noise Total Noise PIN APD
RL = Total load seen at the input of the preamplifier
K = Boltzmann’s constant = 1.38x J/K
T = Temperature in degree Kelvin = Co + 273
Total Noise
PIN
APD

39. Electrical Amplifier Noise
Amplifier type BJT JEFT
- Voltage Noise
- Current Noise
Total amplifier noise

40. Receiver Signal-to-Noise Ratio (SNR)iT iA hf
PIN
APD
Note: SNR cannot be improved be amplification

41. SNR - Quantum Limit Shot noise Poisson
The mean square quantum shot noise current on Io
Shot noise
Poisson

42. Type of Receivers - Low Impedance Voltage Amplifier
Simple
Low sensitivity
Limited dynamic range
It is prone to overload and saturation hf CT RL Av Vi Vo
RC limited bandwidth
RL = Rdetector || Ramp.
Ramp= High

43. Type of Receivers - High Impedance Voltage Amplifier with Equaliser
High sensitivity
Low dynamic range hf CT RL
Equaliser Vi Vo Av
Rdetector is large to reduce the effect of thermal noise
Detector out put is integrated over a long time constant, and is
restored by differentiation

44. Type of Receivers - Transimpedance Feedback Amplifier
The most widely used 
Wide bandwidth
High dynamic range
No equalisation
Greater dynamic range (same gain at all frequencies)
Slightly higher noise figure than HIVA hf CT RF Av RL Vi Vo
Bandwidth

45. Transimpedance Feedback AmplifierVi -A
Where is the noise power spectral density, and RT = RL||RF

46. Optical Receiver - Analogue
Employ an analogue preamplifier stage, followed by either an analogue output stage (depending on the type of receiver).
Comms. Special. Inc.

47. Optical Receiver - Digital
1st stage is a current-to-voltage converter.
2nd stage is a voltage comparator, which produces a clean, fast rise-time digital output signal. The trigger level may be adjusted to produce a symmetrical digital signal.

48. Optical Transmission - ISI
Optical pulse spread after traversing along optical fiber
Thus leading to ISI, where some fraction of energy remaining in appropriate time slot, whereas the rest of energy is spread into adjacent time slots.

49. Receiver Performance
Signal-to-Noise Ratio (SNR)
Bit Error Rate (BER)

50. SNR
In analogue transmissions the performance of the system is mainly determined by SNR at the output of the receiver.
In case of amplitude modulation the transmitted optical power P(t) is in the form of:
where M is modulation index, and m(t) is the analogue signal.
The photocurrent at receiver can be expressed as:

51. And for large signal level
SNR
The S/N can be written as
Note, F is the amplifier noise figure.
For PIN: G = 1
So we have
Low input signal level
And for large signal level

52. SNR Vs Receiver Sensitivity
Po(dBm)
Note: Io =RPo
G Keiser , 2000

53. Bit Error Rate (BER) Variance 2on bon 1 vth boff Variance 2off
Probability of Error = probability that the output voltage is less than the threshold when a 1 is sent + probability that the output voltage is more than the threshold when a 0 has been sent 1
bon
boff
Variance 2on
Variance 2off
vth
where q1 and q0 are the probabilities that the transmitter sends 0 and 1 respectively.
Note, q0 = 1- q1.

54. Bit Error Rate (BER)
BER = No. of error over a given time interval/Total no. of bits transmitted
If we assume that the probabilities of 0 and 1 pulses are equally likely
where

55. Bit Error Rate (BER) - contd.
For
off = on =  RMS noise
bon = V, and boff = 0
Thus vth = V/2 and Q = V/2
Therefore:
In terms of power signal-to-noise ratio (S/N)

56. BER Performance
Minimum input power depends on acceptable bit error rate
Many receivers designed for 1E-12 or better BER
G Keiser , 2000

57. Basic Receiver Design Optimized for one particular
AGC -g
Bias
Clock
Recovery
Decision
Circuit
0110
Remote
Control
Temperature
Monitors
& Alarms
Optimized for one particular
Sensitivity range
Wavelength
Bit rate
Can include circuits for telemetry
Agilent Tech.

58. Optical Receivers - Commercial Devices
28 GHz Monolithic InGaAs PIN Photodetector
100 kHz- 40 Gb/s
DC - 65 Gb/s InGaAs PIN Photodiodes
100 GHz Dual-Depletion InGaAs/InP Photodiode

59. Wide-Band Optical Receiver (40 Gb/s)
Bandwidth: 100 KHz to 35 GHz
Responsivity: 0.6 A/W
Wavelength response: nm
Operating current 75 mA
Power dissipation: 400 mW
Power gain: 8 dB
Linearity response
Sensitivity response
Typical eye diagram

60. Wide-Band Optical Receiver (DC - 65 Gb/s)
InGaAs PIN Photodiodes
Reverse bias voltage: +3V
Responsivity: 0.5 A/W at 1300 and 1550 nm wavelength.
Opto-electronic Integrated Circuits (OEICs) which combine optical, microwave, and digital functions on the same chip
Application:
Ethernet fiber local area networks
Synchronized Optical Network SONET,
ISDN,
Telephony
Digital CATV).

61. Regenerator (3R) Receiver followed by a transmitter
No add or drop of traffic
Designed for one bit rate & wavelength
Signal regeneration
Reshaping & timing of data stream
Inserted every 30 to 80 km before optical amplifiers became commercially available
Today: reshaping necessary after about 600 km (at 2.5 Gb/s), often done by SONET/SDH add/drop multiplexers or digital cross-connects
Fibre

62. Summary Photodiode characteristics Types of photodiode: PIN and APD
Photodiode responsivity & equivalent circuit
Minimum received power
Optical receiver:
Types
Bandwidth
Noise
Signal-to-noise ratio
Bit error rate
Receiver design
Regenerator

63. Next Lecturer
Optical Devices