Optical Fiber Communications
EE 534
Chapter 3: Optical sources & detectors
Part (2): Optical detectors
Abdallah Abu Arabia (a.abuarabia@eng.misuratau.edu.ly)
Headlines
Optical receiver
PIN detector
APD detector
Receiver noise sources
Receiver Signal-Noise Ratio
Tutorial#5
2 Abdallah Abu Arabia (a.abuarabia@eng.misuratau.edu.ly)
Optical Receiver
The first receiver element is the photodiode (a pin or an avalanche), which produces an
electric current proportional to the optical 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,
3 Abdallah Abu Arabia (a.abuarabia@eng.misuratau.edu.ly)
Optical Receiver
Requirements:
Compatible physical dimensions (small size)
High responsivity (high sensitivity) at the desired wavelength
and low responsivity elsewhere
Low noise and high gain
Fast response time
Insensitive to temperature variations
Long operating life and low cost
Reserve bias operating
4 Abdallah Abu Arabia (a.abuarabia@eng.misuratau.edu.ly)
Photodetectors
Photodiodes (or photodetectors) are sufficiently reverse biased during normal
operation no current flow without illumination.
Two type of detectors, namely the positive-intrinsic-negative (PIN) and the
avalanche photo diodes (APD), are mostly used in fiber optic receivers.
APD has a self multiplying mechanism so that it has high gain.
The tradeoff of having the gain is the ‘excess noise’ due to random nature of
the self multiplying process.
Compared to short wavelengths (less 800 nm), at high wavelengths
(between 1310 and 1550 nm), APDs have the same excess noise, but they
have an order of magnitude lower avalanche gain. Therefore, APD’s have
relatively low responsivity at longer wavelengths.
5 Abdallah Abu Arabia (a.abuarabia@eng.misuratau.edu.ly)
Photodetectors
Requirements: high efficiency, low noise,Wideband.
Principle: light absorption.
In the semiconductor material, when the incident photon energy
exceeded the band gap energy, each photon absorpted by the
semiconductor generates an electron-hole pairs. Under the influence of
an external electric field, electrons and holes in semiconductors transit
and form the current flow, called photocurrent (Ip).
I p RPin R—Response of the photodetector
(A/W)
6 Abdallah Abu Arabia (a.abuarabia@eng.misuratau.edu.ly)
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PIN Photo Diode
Positive-intrinsic-negative (PIN) is the most widely used
photodiode. The device consists of a p and n type
semiconductor regions separated widely by intrinsic (pure)
layer called “depletion region” absorbs the incident photons. A
photodiode is normally reverse biased at optical
receivers.
As a photon flux Φ=hv penetrates into a semiconductor, it will
be absorbed as it progresses through the material.
Absorbed photons trigger photocurrent Ip in the external
circuitry proportional to photon energy.
7 Abdallah Abu Arabia (a.abuarabia@eng.misuratau.edu.ly)
PIN Photo Diode
If αs(λ) is the photon absorption coefficient at a
wavelength λ, the power level at a distance x into the
material is:
8 Abdallah Abu Arabia (a.abuarabia@eng.misuratau.edu.ly)
PIN Photo Diode
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Avalanche Photodiode (APD)
APD achieves high responsivity (sensitivity) by having an
internal gain.
This internal gain M is obtained by having a high electric field
that energizes photo-generated electrons and holes.
M is a statistical quantity because of the random nature of
avalanche multiplication process due to the atom structure.
These electrons and holes ionize bound electrons 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.
10 Abdallah Abu Arabia (a.abuarabia@eng.misuratau.edu.ly)
APD - Responsivity
Receiver Quantum Efficiency () = number of e-h pairs
generated / number of incident photons.
Ip / q
P0 / h
Responsivity () is the ratio between photo current (Ip) and
optical power (Po).
Ip q
A/W
P0 h
APDs have an internal gain M, where M = IM/Ip and IM is
the mean multiplied current. Hence:
APD PIN M M = 1 for PIN diodes
11 Abdallah Abu Arabia (a.abuarabia@eng.misuratau.edu.ly)
APD - Selectivity
For a given material, there will be a maximum wavelength (C)
of light can be detected. For the light larger than this limit
wavelength can not be detected.
When the photon energy (h) is larger than the band gap (Eg) of
the semiconductor material, valence band electron can absorb a
photon transition to the conduction band, otherwise no matter
how strong the incident light is, the photoelectric effect will
never occur.
hc 1.24
C ( m)
Eg E g (eV )
12 Abdallah Abu Arabia (a.abuarabia@eng.misuratau.edu.ly)
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APD - Selectivity
In the shor t wavelength
section, the absorption
coefficient of the materials
becomes very large. So each
mater ial has a range of
wavelength can be detected.
The rrelationship between the absorption coefficient of
semiconductor and the wavelength
Si：C=1.06m, range：0.5~1.0 m
Ge： C=1.6m, range：1.1~1.6 m
InGaAs： C=1.6m, range：1.1~1.6 m
13 Abdallah Abu Arabia (a.abuarabia@eng.misuratau.edu.ly)
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APD - Selectivity
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Comparison of the photodetector
Si、Ge、InGaAs pin photodiodes common working parameters
Parameter Symbol Unit Si Ge InGaAs
Wavelength nm 400~1100 800~1650 1100~1700
Responsivity R A/W 0.4~0.6 0.4~0.5 0.75~0.95
Dark ID nA 1~10 50~500 0.5~2.0
Current
Rise time r ns 0.5~1.0 0.1~0.5 0.05~0.5
Bandwidth B GHz 0.3~0.7 0.5~3.0 1.0~2.0
Bias VB V 5 5~10 5
voltage
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Comparison of the photodetector
Si、Ge、InGaAs APD common working parameters
Parameter Symbol Unit Si Ge InGaAs
Wavelength nm 400~1100 800~1650 1100~1700
Avalanche M —— 20~400 50~200 10~40
gain
Dark ID nA 0.1~1 50~500 10~50
Current @M=10
Rise Time r ns 0.1~2 0.5~0.8 0.1~0.5
Gain M•B GHz 100~400 2~10 20~250
Bandwidth
Product
Bias voltage VB V 150~400 20~40 20~30 16
16 Abdallah Abu Arabia (a.abuarabia@eng.misuratau.edu.ly)
Comparison of the photodetector
In the short-distance applications, working in 850nm window, often use Si
photodiodes.
In the short-distance applications, working in 1330nm and 1550nm
window, often use Si photodiodes, often use InGaAs photodiodes.
Compared with PIN photodiodes, APD has a multiplier effect of the
carrier, the detection sensitivity is particularly high, but requires a higher
bias voltage and temperature compensation circuitry.
17 Abdallah Abu Arabia (a.abuarabia@eng.misuratau.edu.ly) 17
Receiver Noise Sources
The term noise describes unwanted components of an electric signal that tend to
disturb the transmission and processing of the signal. There are three main types of
noise:
The random arrival rate of signal photons produces quantum (shot) noise.
Thermal noises arise from the random motion of electrons in the detector load resistor
and in the amplifier electronics.
Dark current noise comes from thermally generated electrons-holes pairs in the pn
junction.
18 Abdallah Abu Arabia (a.abuarabia@eng.misuratau.edu.ly)
Shot - Noise
Light is composed of photons, which are discrete packets
of energy.
Thus, the randomness of the arrival time of each photon
generates a random noise component at the output
current of the photodiode which, is referred to as the
shot noise or quantum noise. The shot noise power for
avalanche photodiodes is given by:
2 2
is 2.q.I p .B.M .F ( M )
19 Abdallah Abu Arabia (a.abuarabia@eng.misuratau.edu.ly)
Shot - Noise
Where:
F(M): APD Noise Figure, F(M) = Mx , (0 ≤ x ≤ 1)
Ip: Mean Detected Current “photocurrent”
B = Receiver Bandwidth
q: Charge of an electron
M=1 for PIN diodes
s is the mean-square current, or the noise power
i
2
associated with this current.
20 Abdallah Abu Arabia (a.abuarabia@eng.misuratau.edu.ly)
Thermal - Noise
Thermal noise is due to the resistive elements in the receiver
amplifier. The thermal noise is independent to the optical signal
level but increase with the temperature. The thermal noise power
is given by:
i2
T 4 K BTB / RL
where, T is the absolute temperature in Kelvin, KB is the Boltzman
constant (1.38054 X 10(-23) J/K ), B is signal bandwidth, and RL is
the receiver load impedance.
21 Abdallah Abu Arabia (a.abuarabia@eng.misuratau.edu.ly)
Dark & Leakage Current - Noise
• There will be some (dark and leakage ) current without any
incident light. There are two of dark current, bulk iDB and surface
iDS dark currents. Typically, the iDS term is negligible (IL~0)
compared to iDB.
The noise power associated with 2 2
the bulk dark current is given by: i DB 2qI D BM F ( M )
Where ID: Dark Current
The noise power associated with the 2
i DS 2qI L B
surface leakage current is given by:
Where IL: Leakage Current
22 Abdallah Abu Arabia (a.abuarabia@eng.misuratau.edu.ly)
Signal to Noise Ratio
i p2 M 2
SNR
2q( I p I D ) M 2 F (M ) B 2qI L B 4k BTB / RL
Signal Power = <ip2>M2 , ip photodiode current.
Typically thermal and quantum noise are the most significant.
Previous eq. can rewritten as:
23 Abdallah Abu Arabia (a.abuarabia@eng.misuratau.edu.ly)
Signal to Noise Ratio
24 Abdallah Abu Arabia (a.abuarabia@eng.misuratau.edu.ly)
Tutorial#5
1) The longitudinal modes of a gallium arsenide injection
laser emitting at a wavelength of 0.87 μm are separated in
frequency by 278 GHz. Determine the length of the optical
cavity and the number of longitudinal modes emitted. The
refractive index of gallium arsenide is 3.6.
2) An injection laser has a GaAs active region with a bandgap
energy of 1.43 eV. Estimate the wavelength of optical
emission from the device and determine its linewidth in
hertz when the measured spectral width is 0.1 nm.
25 Abdallah Abu Arabia (a.abuarabia@eng.misuratau.edu.ly)
Tutorial#5
3) The refractive index of the InGaAsP active region of an
injection laser at a wavelength of 1.5 μm is 3.5 and the
device has an active cavity length of 400 μm. For laser
operation at a wavelength of 1.5 μm determine:
(a) the laser emission mode index;
(b) the eligible number of wavelengths inside the cavity; (c) the
frequency separation of the modes in the active cavity in order
to produce constructive interference.
26 Abdallah Abu Arabia (a.abuarabia@eng.misuratau.edu.ly)
Tutorial#5
4) A p–i–n photodiode on average generates one electron–
hole pair per three incident photons at a wavelength of 0.8
μm. Assuming all the electrons are collected calculate:
(a) the quantum efficiency of the device;
(b) its maximum possible bandgap energy;
(c) the mean output photocurrent when the received optical
power is 10−7W.
27 Abdallah Abu Arabia (a.abuarabia@eng.misuratau.edu.ly)
Tutorial#5
5) A p–n photodiode has a quantum efficiency of 50% at a
wavelength of 0.9 μm.Calculate:
(a) its responsivity at 0.9 μm;
(b) the received optical power if the mean photocurrent is
10−6 A;
(c) the corresponding number of received photons at this
wavelength.
28 Abdallah Abu Arabia (a.abuarabia@eng.misuratau.edu.ly)
Tutorial#5
6) When 800 photons per second are incident on a p–i–n
photodiode operating at a wavelength of 1.3 μm they
generate on average 550 electrons per second which are
collected. Calculate the responsivity of the device.
7) An APD has a quantum efficiency of 45% at 0.85 μm.
When illuminated with radiation of this wavelength it
produces an output photocurrent of 10 μA after avalanche
gain with a multiplication factor of 250. Calculate the
received optical power to the device. How many photons
per second does this correspond to?
29 Abdallah Abu Arabia (a.abuarabia@eng.misuratau.edu.ly)
Tutorial#5
8) When 1011 photons per second each with an energy of
1.28 × 10−19 J are incident on an ideal photodiode,
calculate:
(a) the wavelength of the incident radiation;
(b) the output photocurrent;
(c) the output photocurrent if the device is an APD with a
multiplication factor of 18.
30 Abdallah Abu Arabia (a.abuarabia@eng.misuratau.edu.ly)
Tutorial#5
9) Given that the following measurements were taken for an
APD, calculate the multiplication factor for the device.
Received optical power at 1.35 μm = 0.2 μW
Corresponding output photocurrent = 4.9 μA
(after avalanche gain)
Quantum efficiency at 1.35 μm = 40%
31 Abdallah Abu Arabia (a.abuarabia@eng.misuratau.edu.ly)
