fiber-optics-communications-optical-detectors

1. SENECA COLLEGE
School of Electronics &Computer Engineering
Fiber Optics Communications
CHAPTER-4
OPTICAL DETECTORS
By Harold Kolimbiris

2. INTRODUCTION (1)
 PHOTO DETECTION:
 Photo detection is the process whereby optical power is detected and
converted to electrical power.
 Photo-detector devices or optical detectors perform photo-detection.
Optical detectors perform the exact opposite function of that of the optical
sources; that is, they convert electric power into optical power.
 In any optical fiber communications system, the optical source is part of
the transmitter section, while optical detectors are part of the receiver
section.
CHAPTER-4:
OPTICAL DETECTORS

3. INTRODUCTION (2)
 The performance of an optical detector incorporated into the receiver
section of an optical fiber communications system will be determined by
its ability to detect the smallest optical power possible (detector-
sensitivity) and to generate a maximum electric power at its output with an
absolute minimum degree of distortion (low-noise).
 the optical detector device, which is almost always utilized in an optical
receiver is the semiconductor photodiode.
 The two photodetector devices most commonly used in optical fiber
communications systems are the PIN and APD devices.
CHAPTER-4:
OPTICAL DETECTORS

4. PIN – PHOTODETECTORS (1)
 PIN – is the abbreviation of P-region, I-Intrinsic- N-region semiconductor
diode.
 The principal theory on which a PIN photodetector device is based is
illustrated in Fig-1
 When a photon is incident upon a semiconductor photodetector device with
energy larger than the bandgap energy of that device, the energy of the
photon is absorbed by the bandgap and an electron-hole pair is generated
across the bandgap
CHAPTER-4:
OPTICAL DETECTORS

5. PIN – PHOTODETECTORS (2)
 The energy of incident photon is given by,
 Where:
 =Energy of the photon
 =Planck’s constant 6.62x 10-e34
 =Velocity of light 3x10e8
 =Wavelength
 =Bandgap energy
CHAPTER-4:
OPTICAL DETECTORS
λ
hc
Eph =
phE
h
c
λ
gE
2
Ws
sm/
m

6. PIN – PHOTODETECTORS (3)
 It is evident from the above equation the photon energy is inversely
proportional to the wavelength
 Therefore, there exists a wavelength at which the photon energy becomes
equal to the bandgap energy.
 At this photon energy level electron-hole generation will occur.
 The wavelength at which the photon energy becomes equal to bandgap
energy is called the “cut-off wavelength”
CHAPTER-4:
OPTICAL DETECTORS
phE
λ
cλ

7. PIN – PHOTODETECTORS (4)
 The cut-off wavelength in terms of band gap energy is expressed by,
 Semiconductor materials employed in the fabrication of photodetectors are
the same with the materials employed in the fabrication of optical sources.
 Such materials with their corresponding bandgap energy levels are listed
in the table 4-1.(see text)
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OPTICAL DETECTORS
g
c
E
mµ
λ
24.1
=
eV

8. PIN – PHOTODETECTORS (5)
 The cross-section area of a Silicon PIN-diode is shown in fig-1
 When a photon impedes upon the photo-detector, the low bandgap
absorption layer absorbs the photon and an election-hole is generated.
CHAPTER-4:
OPTICAL DETECTORS
Conduct
Sin −
)( layerAbsorbtionSii −
2SiO
ConductConduct
Photons
Silicon PIN diode. Fig-1

9. PIN – PHOTODETECTORS (6)
 These photo-carriers, under the
influence of a strong electric field
generated by a reverse bias
potential difference across the
device, are separated thus forming
a photo current intensity
proportional to the number of
incident photons.
 The DC biasing of a PIN-diode
photo-detector is shown in fig-2
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OPTICAL DETECTORS
ER L
p nAbsorbtion
Layer
+
-
Diode biasing. Fig-2

10. PIN – PHOTODETECTORS (7)
 The generated photocurrent from the PIN-photodetector device develops a
potential difference across the load resistance RL with a frequency
calculated by,
 Where,
=Photon energy is
= Planck’s constant 6.62 x 10-34W.s2
= Frequency
CHAPTER-4:
OPTICAL DETECTORS
h
E
f
ph
=
phE
h
f
eV

11. PIN – PHOTODETECTORS (8)
 PIN-Photodetector characteristics
 The fundamental PIN photodiode operational characteristics are:
 Quantum efficiency (η),
 Responsivity (R),
 Speed,
 Linearity.
 Quantum efficiency (η) is defined by the number of electron-hole pair
generated per impeding photon, expressed by
CHAPTER-4:
OPTICAL DETECTORS
phN
peN ),( +−
=η

12. PIN – PHOTODETECTORS (9)
 Where:
 N (e−,p+)=Number electron-hole generation
 = Number of photons
=Quantum efficiency
The number of electron-hole pair generation is translated to current by
 Where:
 = Photocurrent (mA)
 q = Electron charge = 1.6x10-19C
 = Number of electrons.
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OPTICAL DETECTORS
phN
η
−×= eP NqIPI
−
e
N

13. PIN – PHOTODETECTORS (10)
 Consequently, the number of incident photons is translated to light power
by,
 Where:
 =Light power
 =Number of photons
 h =Planck’s constant (6.628x10-38J.s)
 v =Velocity of light
CHAPTER-4:
OPTICAL DETECTORS
hvNP pho ×=
phN
OP

14. PIN – PHOTODETECTORS (11)
 The efficiency of a PIN photodetector is proportional to the photon energy
absorbed by the absorption layer of the device.
 Larger photon energy requires a thicker absorption layer, allowing longer
time for electron-hole pair generation to take place.
CHAPTER-4:
OPTICAL DETECTORS

15. PIN – PHOTODETECTORS (12)
 Response-Time (speed)
 Response time or speed of a photodetector is referred to as the time
required by the generated carriers, within the absorption region, to travel
that region under reverse bias conditions.
 The key parameter for determining photodetector device performance is
“Responsivity”.
 Responsivity is defined by the ratio of the current generated in the
absorption region per- unit optical power incident to the region.
CHAPTER-4:
OPTICAL DETECTORS

16. PIN – PHOTODETECTORS (13)
 Responsivity is closely related to quantum efficiency and is expressed by

 Where:
 R = Responsivity
 η = Quantum efficiency
 q = Electron charge
 = Energy of the photon. (hv)
CHAPTER-4:
OPTICAL DETECTORS
phE
q
R η=
phE
C19
1059.1 −
×

17. PIN – PHOTODETECTORS (14)
 The Responsivity of a PIN photo diode is the ratio of the generated photo current
per incident of unit-light power.
 A graphical representation of quantum efficiency (η) and responsivity is shown in
fig-3

CHAPTER-4:
OPTICAL DETECTORS
10%
30%
50%
70%
90%
Responsibility-R(µA/mW)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 700 900 1100 1300 1500 1500 1700
0.9 Quantum efficiency (η)
Responsivity (R)
InGaAs
Ge
Si
Quantum efficiency-Responsivity . Fig-3

18. PIN – PHOTODETECTORS (15)
 Fig-3 illustrates the fundamental difference between responsivity and
quantum efficiency
 For different semiconductor materials, the responsivity is linear up to a
particular wavelength, then, drops quickly
 Beyond this point, the photon energy becomes smaller than the energy
required for electron-hole generation.
CHAPTER-4:
OPTICAL DETECTORS

19. PIN – PHOTODETECTORS (16)
 Dark - current (Id)
 Dark - current is defined as the reverse leakage current of the
photodetector device in the absence of optical power impeding upon the
photodetector device.
 Dark current is an unwanted element caused by such factors as current
recombination within the depletion region and surface leakage current.
 The negative effects of such unwanted currents contribute to thermal shot-
noise.
CHAPTER-4:
OPTICAL DETECTORS

20. PIN – PHOTODETECTORS (17)
 Shot noise
 In semiconductor devices, shot noise is the result of electron-hole
recombination and majority carrier random diffusion.
 The power spectral density of shot noise is proportional to the dark current
and is expressed by
 Where:
 =Shot noise power (W)
 =Dark-current (A)
 q=Electron charge (1.59x 10-19 C).
 =Operating bandwidth
CHAPTER-4:
OPTICAL DETECTORS
Wdn qBIP 2=nP
dI
WB

21. PIN – PHOTODETECTORS (18)
 Shot-noise-voltage is expressed by
 Where:
 =Noise voltage
 =Receiver operating bandwidth.
CHAPTER-4:
OPTICAL DETECTORS
nV
Wdn BIV 2=
nV
WB

22. AVALANCH – PHOTODETECTORS (1)
 AVALANCHE PHOTODIODES (APD)
 Avalanche photodetectors are very similar to PIN - diodes with only one
exception; that is, the addition to the APD device of a high intensity
electric field region.
 In this region, the primary electron-hole pairs generated by the incident
photons are able to absorb enough kinetic energy from the strong electric
field to collide with atoms present in this region, thus generating more
electron-hole pairs.
 This process of generating more than one electron-hole pair from one
incident photon through the ionization process is referred to as the
“avalanche effect”.
CHAPTER-4:
OPTICAL DETECTORS

23. AVALANCH – PHOTODETECTORS (2)
 It is apparent that the photocurrent generated by an APD photodetector
device exceeds the current generated by a PIN device by a factor referred
as the multiplication factor (M).
 Then the generated photo current is expressed by,
 Where,
 = Generated photocurrent.
 q = Electron charge (1.59x10-19C)
 =Carrier number
 =Multiplication factor.
CHAPTER-4:
OPTICAL DETECTORS
MqNI eP )( −=
PI
−
e
N
M

24. AVALANCH – PHOTODETECTORS (3)
 The multiplication factor depends on the physical and operational
characteristics of the photodetector device.
 Such characteristics are the width of the avalanche region, the strength of
the electric field and the type of semiconductor material employed
 The cross section area of a short - wavelength silicon APD device is
shown in fig-5
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OPTICAL DETECTORS

25. AVALANCH – PHOTODETECTORS (4)
 The cross section area of a short - wavelength silicon APD device is
shown in fig-4
CHAPTER-4:
OPTICAL DETECTORS
n n
Photons
+
n
P
2SiO
Metal
Conduct
Metal
Conduct
Absorption
Region
+
P
−
P intrinsic
Avalanche region
(Insulator)
Conduct
Guard
Area
Guard
Area
APD Silicon photodetector device . Fig-4

26. AVALANCH – PHOTODETECTORS (5)
 Gain
 The photocurrent gain in an APD device is a function of several elements
such as:
 (a) The wavelength of the incident photons,
 (b) the electric-field strength as a result of the reverse bias voltage,
 (c) the width of the depletion region and
 (d) the types of semiconductor materials used for the fabrication of the
APD device
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OPTICAL DETECTORS

27. AVALANCH – PHOTODETECTORS (6)
 The relationship of the photocurrent gain to biasing voltage for different
wavelengths is shown in fig-5
CHAPTER-4:
OPTICAL DETECTORS
568.2
799.3
1060
472.2
520.8
Wavelength (nm)
Voltage (V)
Currentgain
++
−−− ppnSilicon π
1
2
5
10
20
50
100
200
500
1000
0 100 150 200 250 300 350 400
Photocurrent gain versus reverse biasing voltage for different wavelengths

28. AVALANCH – PHOTODETECTORS (7)
 The function of the guard rings in an APD structure is to prevent edge
breakdown around the avalanche region.
 When silicon materials are used for the fabrication of APD devices, they
exhibit operating wavelengths of between 400nm-to-900nm.
 When InGaAsP materials are used in the fabrication of APD devices,
these devices exhibit operating wavelengths of between 900nm-to-
1600nm
 Photodetector gain, an important parameter of an APD device, is also
temperature dependent.
CHAPTER-4:
OPTICAL DETECTORS

29. AVALANCH – PHOTODETECTORS (8)
 Photodetector Noise
 Avalanche photodetectors exhibit higher noise levels than PIN devices.
 This is a result of the ionization and photocurrent multiplication process
taken place within the APD device.
 The random nature of the incident photons on the APD device results in a
random photocurrent generation at the output of the device
 This current fluctuation is classified as shot-noise expressed by the
following formula.
CHAPTER-4:
OPTICAL DETECTORS

30. AVALANCH – PHOTODETECTORS (9)
 Photodetector noise equation
 Where:
 = Mean-square-spectral density
 = Frequency (Hz)
 q = Electron charge (1.6x10-19 C)
 * I = Primary Photocurrent
 (M) exp2 = Mean square of the avalanche gain

 * Primary photocurrent (I = Ip+Ibr +Idk)
CHAPTER-4:
OPTICAL DETECTORS
2
2
)(2
)(
MqI
df
id P
=
2
)( Pi
f

31. AVALANCH – PHOTODETECTORS (10)
 Dark - Current
 Dark current is referred to as the current present at the photodetector
output at the absence of incident light.
 For an APD device, the dark current is multiplied by the device
multiplication factor (M), resulting in an overall reduction to device
sensitivity.
 The dark current is a non-linear function of the reverse-biased voltage at
avalanche breakdown levels and is referred to as tunneling current.
CHAPTER-4:
OPTICAL DETECTORS

32. AVALANCH – PHOTODETECTORS (11)
 Dark – Current
 Different semiconductor materials exhibit different levels of tunneling
current resulting from different bandgap sizes.
 For example, devices with small bandgap measure small tunneling
currents in comparison to large bandgap devices measuring larger
tunneling currents
 A practical solution for a substantial reduction of the tunneling current is
the fabrication of structures with a separation between the absorption
(low-bandgap) region and the avalanche (high-bandgap) region.
CHAPTER-4:
OPTICAL DETECTORS

33. AVALANCH – PHOTODETECTORS (12)
 Response-Time
 The response time of a photodetector device is the time a carrier takes to
cross the depletion region.
 For APD devices, the response time is almost double that of PIN-devices
 Response time is directly related to depletion region width.

A typical response time of 0.5ns at 800nm-900nm has been achieved.
CHAPTER-4:
OPTICAL DETECTORS

34. AVALANCH – PHOTODETECTORS (13)
 Capacitance
 In a photodetector device, internal capacitance is a parasitic component
effecting the overall response time of the detector
 As with any other capacitance, junction capacitance of an APD device is
determined by the cross-section area and width of its depletion region and
is expressed by,
CHAPTER-4:
OPTICAL DETECTORS
)(2 jR VV
qAN
C
+
=
ε

35. AVALANCH – PHOTODETECTORS (14)
 Where:
 C =Junction capacitance (F)
 ε = Dielectric constant
 A = Depletion area
 N = Doping density (depletion-region)
 = Reverse bias voltage (V)
 =Junction voltage
 q=Electron charge
CHAPTER-4:
OPTICAL DETECTORS
RV
jV

36. AVALANCH – PHOTODETECTORS (15)
 ADVANCED OPTICAL SEMICONDUCTOR DEVICES
 High demand optical networks require high performance optical devices. One way
to improve the performance of such solid-state devices as optical detectors is
through the Resonant-Cavity-Enhancement (RCE) method (Fabry-Perot).
 The utilization of the resonant micro-cavity principle for the design and
fabrication of such optical devices enhances the wavelength selectivity
and resonant optical field, ultimately leading to improved quantum
efficiency at the operating resonant wavelength
 Read more details from TEXT
CHAPTER-4:
OPTICAL DETECTORS