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

Fiber Optics Communications: OPTICAL DETECTORS

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Page 1: Fiber Optics Communications: OPTICAL DETECTORS

SENECA COLLEGESchool of Electronics &Computer

Engineering

Fiber Optics CommunicationsCHAPTER-4

OPTICAL DETECTORS

By Harold Kolimbiris

Page 2: Fiber Optics Communications: OPTICAL DETECTORS

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

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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

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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

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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

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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

Page 7: Fiber Optics Communications: OPTICAL DETECTORS

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

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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

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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

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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

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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 ),(

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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

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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

Page 14: Fiber Optics Communications: 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

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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

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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

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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

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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

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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

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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

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Wdn qBIP 2nP

dI

WB

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PIN – PHOTODETECTORS (18)

Shot-noise-voltage is expressed by Where: =Noise voltage =Receiver operating bandwidth.

CHAPTER-4:OPTICAL DETECTORS

nVWdn BIV 2

nV

WB

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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

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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

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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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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

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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

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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

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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

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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

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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

Page 31: Fiber Optics Communications: 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

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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

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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.

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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

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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

Page 36: Fiber Optics Communications: OPTICAL DETECTORS

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