SENECA COLLEGE School of Electronics &Computer Engineering
Fiber Optics Communications CHAPTER-4 OPTICAL DETECTORS By Harold Kolimbiris
CHAPTER-4: OPTICAL DETECTORS INTRODUCTION (1)
PHOTO DETECTION:
Photo detection is the process whereby optical power po wer is detected and converted to electrical power.
Photo-detector devices or optical detectors perform photo-detection. 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 (1)
PHOTO DETECTION:
Photo detection is the process whereby optical power po wer is detected and converted to electrical power.
Photo-detector devices or optical detectors perform photo-detection. 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 (detectorsensitivity) and to generate a maximum electric power at its output with an absolute minimum degree of distortion (low-noise). (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 communicatio ns 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,
E ph
hc
Where: E ph
=Energy of the photon
h
=Planck’s constant 6.62x 10-e34Ws
c
=Velocity of light 3x10e8 m / s
=Wavelength
E g
m
=Bandgap energy
2
CHAPTER-4: OPTICAL DETECTORS PIN – PHOTODETECTORS (3)
It is evident from the above equation the photon energy 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” c
E ph
is inversely
CHAPTER-4: OPTICAL DETECTORS PIN – PHOTODETECTORS (4)
The cut-off wavelength in terms of band gap energy is expressed by,
c
1.24 m E g
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 eV are listed in the table 4-1.(see text)
CHAPTER-4: OPTICAL DETECTORS PIN – PHOTODETECTORS (5)
The cross-section area of a Silicon PIN-diode is shown in fig-1 Conduct
onduct
Photons
SiO2 i Si( Absorbtion layer )
Silicon PIN diode. 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 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
p
Absorbtion Layer
n
+
RL
-
Diode biasing. Fig-2
E
CHAPTER-4: OPTICAL DETECTORS 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,
f
E ph h
Where, E ph
=Photon energy is
eV
h
= Planck’s constant 6.62 x 10-34W.s2
f
= Frequency
CHAPTER-4: OPTICAL DETECTORS 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
N (e, p ) N ph
CHAPTER-4: OPTICAL DETECTORS PIN – PHOTODETECTORS (9)
Where: N (e ,p )=Number electron-hole generation
N ph =
Number of photons
=Quantum
efficiency
The number of electron-hole pair generation is translated to current by
Where: I P
= Photocurrent (mA)
q = Electron charge = 1.6x10-19C N e
= Number of electrons.
I P
q N e
CHAPTER-4: OPTICAL DETECTORS PIN – PHOTODETECTORS (10)
Consequently, the number of incident photons is translated to light power by,
Po
N ph
hv
Where: PO N ph
=Light power =Number of photons
h =Planck’s constant (6.628x10-38J.s) v =Velocity of light
CHAPTER-4: OPTICAL DETECTORS 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
R
q
E ph
Where: R = Responsivity
= Quantum efficiency
q = Electron charge
E ph =
1.59 10
19
C
Energy of the photon. ( hv)
CHAPTER-4: OPTICAL DETECTORS 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 0.9
Quantum efficiency ( ) Responsivity ( R)
90%
0.8
) W m / A ( R y t i l i b i s n o p s e R
70%
0.7
i
0.6
InGaAs
0.5
50%
e
0.4 0.3
30%
0.2 0.1
10%
0 0
700
900
1100
1300
1500
1500
1700
Quantum efficiency-Responsivity
. Fig-3
CHAPTER-4: OPTICAL DETECTORS 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 shotnoise.
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:
Pn =Shot
I d =Dark-current
q=Electron charge (1.59x 10-19 C ).
noise power (W ) ( A)
BW =Operating bandwidth
Pn
2 I d qBW
CHAPTER-4: OPTICAL DETECTORS PIN – PHOTODETECTORS (18)
Shot-noise-voltage
Where:
V n
BW
V n
is expressed by
=Noise voltage =Receiver operating bandwidth.
V n
2 I d BW
CHAPTER-4: OPTICAL DETECTORS 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,
I P=
q = Electron charge (1.59x10-19C)
N e
Generated photocurrent.
=Carrier number
M =Multiplication
factor.
I P
(qN e ) M
CHAPTER-4: OPTICAL DETECTORS 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 Metal Conduct
Photons
Metal Conduct
SiO2 (Insulator) n Guard Area
P Avalanche region
n Guard Area
Absorption Region
intrinsic
Conduct
APD Silicon photodetector device
. Fig-4
CHAPTER-4: OPTICAL DETECTORS 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 1000
500
Silicon n
p p
200
Wavelength (nm)
100
1060
n i a g 50 t n e r r 20 u C
799.3 568.2
10 5 520.8 2
472.2
1 0
100
150
200
250
300
350
400
Voltage (V)
Photocurrent gain versus reverse biasing voltage for different wavelengths
CHAPTER-4: OPTICAL DETECTORS 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 400 nm-to-900nm.
When InGaAsP materials are used in the fabrication of APD devices, these devices exhibit operating wavelengths of between 900 nm-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 d (i P ) 2
Where:
(i P )
2
f
df
= 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)
2qI ( M ) 2
CHAPTER-4: OPTICAL DETECTORS 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 (lowbandgap) 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 0.5 ns 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, C
qAN
2(V R
V j )
CHAPTER-4: OPTICAL DETECTORS AVALANCH – PHOTODETECTORS (14)
Where: C =Junction capacitance (F)
= Dielectric constant
A = Depletion area
N = Doping density (depletion-region)
V R
= Reverse bias voltage (V)
V j
=Junction voltage
q=Electron charge
