fiber optics communications: optical detectors
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SENECA COLLEGESchool of Electronics &Computer
Engineering
Fiber Optics CommunicationsCHAPTER-4
OPTICAL DETECTORS
By Harold Kolimbiris
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
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
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
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
hc
Eph
phE
h
c
gE
2Ws
sm /
m
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
c
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)
CHAPTER-4:OPTICAL DETECTORS
gc E
m 24.1
eV
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
Conduct Conduct
P h o to n s
Silicon PIN diode. Fig-1
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
CHAPTER-4:OPTICAL DETECTORS
ER L
p nAbsorbtionLayer
+
-
Diode biasing. Fig-2
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
Ef ph
phE
h
f
eV
PIN – PHOTODETECTORS (8)
PIN-Photodetector characteristics The fundamental PIN photodiode operational characteristics are: Quantum efficiency (), Responsivity (R), Speed, Linearity. Quantum efficiency () is defined by the number of electron-hole pair
generated per impeding photon, expressed by
CHAPTER-4:OPTICAL DETECTORS
phN
peN ),(
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.
CHAPTER-4:OPTICAL DETECTORS
phN
eP NqI
PI
eN
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
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
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
PIN – PHOTODETECTORS (13)
Responsivity is closely related to quantum efficiency and is expressed by
Where: R = Responsivity = Quantum efficiency q = Electron charge = Energy of the photon. (hv)
CHAPTER-4:OPTICAL DETECTORS
phE
qR
phE
C191059.1
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 () and responsivity is shown in fig-3
CHAPTER-4:OPTICAL DETECTORS
10%
30%
50%
70%
90%
Respo
nsibility-R
( A
/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 () Responsivity (R)
InGaAs
Ge
Si
Quantum efficiency-Responsivity . Fig-3
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
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
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 2nP
dI
WB
PIN – PHOTODETECTORS (18)
Shot-noise-voltage is expressed by Where: =Noise voltage =Receiver operating bandwidth.
CHAPTER-4:OPTICAL DETECTORS
nVWdn BIV 2
nV
WB
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
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
MqNIeP )(
PI
eN
M
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
CHAPTER-4:OPTICAL DETECTORS
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
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
CHAPTER-4:OPTICAL DETECTORS
AVALANCH – PHOTODETECTORS (6)
The relationship of the photocurrent gain to biasing voltage for different wavelengths is shown in fig-5
CHAPTER-4:OPTICAL DETECTORS
5 6 8 . 2
7 9 9 . 3
1 0 6 0
4 7 2 . 2 5 2 0 . 8
W a v e l e n g t h ( n m )
V o l t a g e ( V )
Cur
rent
gai
n
ppnSilicon
1
2
5
1 0
2 0
5 0
1 0 0
2 0 0
5 0 0
1 0 0 0
0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0
Photocurrent gain versus reverse biasing voltage for different wavelengths
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
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
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
22
)(2)(
MqIdf
id P
2)( Pi
f
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
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
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
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
qANC
AVALANCH – PHOTODETECTORS (14)
Where: C =Junction capacitance (F) = 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
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
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CHAPTER-4:OPTICAL DETECTORS
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