Part 6: Optical detector آشکارسازهای نوری. Overview -This is an essential component of an optical fibre communication system, dictates the overall system.

Part 6: Optical detector

نوری آشکارسازهای

Overview

- This is an essential component of an optical fibre communication system, dictates the overall system performance.

- Its function is to convert the received optical signal into an electrical signal.

- It must satisfy very significant requirements for performance and compatibility:• High sensitivity at the operating wavelengths (1200-1600nm).• High fidelity. Linearity is important for analogy transmission.• Short response time to obtain a suitable bandwidth.• A minimum noise introduced by the detector.• Stability of performance characteristics.• Small size for efficient coupling to the fibre and easy packaging

with the following electronics.• Low bias voltages.• High reliability and low cost.

• Device types:• Photodiodes• Phototransistors• Photoconductive detectors• Photomultiplier tubes

: آشکارسازنوری

درکل آن کارآیی که است ارتباطی سیستم دریک فیبرنوری ازیک اساسی ایی مولفهاست شده دیکته .سیستم

میکند تبدیل الکتریکی سیگنال رابه دریافتی نوری سیگنال که است .تابعیموارد وهمسازی اجرا وبرای برخورداربوده زیادی ازاهمیت بایست می فوق تابع

نیازباشد ومورد :زیرضروریمیکنند (1 عمل باال های موج درطول که حسگرهایی

مهم (2 مواردآنالوگ ارسال برای که اصل به ومشابه درجه عالیترین صدایا ایجاداست

مناسب (3 باند پهنای آوردن بدست بدای کوتاه پاسخدهی زمانمیشود (4 داده نشان آشکارساز توسط مقدارنویزکه کمترین

ها (5 مشخصه وانجام درکارآیی وثبات پایداریوپیروی (6 درنظرگرفتن با آسان بندی درفیبروبسته موثر اتصال برای کوچک اندازه

ازالکترونهاپایین (7 ولتازپیشقدر

پایین (8 وهزینه باال اطمینان قابلیت

: انواع ها (1 فوتودیود

فتوترانزیستورها (2فتوکانداکتیو (3 آشکارسازهای

پلیر (4 فتومولتی المپهای

نوری آشکارسازی : اصلOptical Detection Principles

نوری آشکارسازی : اصل

یک شدن برانگیخته منجربه میتواند تخلیه درناحیه فوتون یک برخوردیک شدن برانگیخته منجربه میتواند تخلیه درناحیه فوتون برخوردیکایجادحفره ودرنتیجه هدایت باند به ازباندظرفیت ایجادحفره الکترون ودرنتیجه هدایت باند به ازباندظرفیت الکترون

) ). تولیدفوتون شود (درباندظرفیت ). تولیدفوتون شود درباندظرفیت

وتحت اند ازیکدیگرجداشده پیوندگاه درنزدیکی تولیدشده حامل وتحت زوج اند ازیکدیگرجداشده پیوندگاه درنزدیکی تولیدشده حامل زوججریان توسط جابجایی یک ایجاد باعث که الکتریکی جریان تاثیرمیدان توسط جابجایی یک ایجاد باعث که الکتریکی تاثیرمیدان

) میگردد ( بازگشتی نشتی هرجریان درفزونی ( درمدارخارجی میگردد ( بازگشتی نشتی هرجریان درفزونی درمدارخارجیمیشود میشود جاروب ..جاروب

Optical generation of carriers in a PN junction.

W

Pairs generated here recombine before reaching depletion layer

P NLp

Ln

Pairs generated here recombine before reaching depletion layer

P NW

LpLn

Radiation is assumed to be incident on the whole diode.

• photon can give up its energy and excite an electron (photocarriers)

• The high electric field present in the depletion region causes the carriers to separate and be collected across the reverse biased junction

• Diode current is given by,

where the first term is the dark current and the second term is the photocurrent

: pn دریک ازحاملها شده تولید نور پیوندگاه

W

شده تولید زوجقبل ناحیه دراینالیه به رسیدن ازجدا تخلیهازیکدیگرند

P NLp

Ln

شده تولید زوجقبل ناحیه دراینبه رسیدن ازجدا تخلیه الیهازیکدیگرند

P NW

LpLn

( است داده روی دیود سراسر تابشدر که است فرضشده فوق (درشکل

• میتواند فوتون میتواند یک فوتون یکداده دست از داده انرزی دست از انرزیبرانگیخته الکترون برانگیخته ویک الکترون ویک.شودشود

• ( فوتون کننده (حمل• الکتریکی میدان الکتریکی اعمال میدان اعمال

تخلیه درناحیه تخلیه قوی درناحیه قویحاملها جدایی حاملها باعث جدایی باعثدرسراسر آنها درسراسر وتجمع آنها وتجمعپیشقدر اتصال پیشقدر محل اتصال محل

.معکوسمیشود معکوسمیشود • ازرابطه دیود یک ازرابطه جریان دیود یک جریان

میشود میشود زیرمعین ::زیرمعین

جریان ) به مربوط دوم وعبارت تاریک جریان به مربوط اول عبارت فوق دررابطهاست (دیود

pn detector

p+

SiO2Electrode

net

–eNa

eNdx

x

E (x)

R

Emax

e–h+

Iph

h> Eg

W

En

Depletion region

(a)

(b)

(c)

Antireflectioncoating

Vr

(a) A schematic diagram of a reverse biased pn junctionphotodiode. (b) Net space charge across the diode in thedepletion region. Nd and Na are the donor and acceptorconcentrations in the p and n sides. (c). The field in thedepletion region.

Electrode

Vout

© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

:pnآشکارساز

فتودیوداست درپیوندگاه Pn.یک معکوس ازبایاس شماتیک A) نموداری

وپذیرنده دراطراف p و nمیباشند دهنده اتمهای Νa وNd) بارخالص فاصلهاست تخلیه b.درسراسردیوددرناحیه

میدهد رانشان تخلیه درناحیه C).میدان

کل فتوجریانTotal Photocurrent = e?

e–h+

iph(t)

Semiconductor

(a)

V

x

(b)

(a) An EHP is photogenerated at x = l. The electron and the hole drift in oppositedirections with drift velocities vh and ve. (b) The electron arrives at time te = (L l)/ve andthe hole arrives at time th = l/vh. (c) As the electron and hole drift, each generates anexternal photocurrent shown as ie(t) and ih(t). (d) The total photocurrent is the sum of holeand electron photocurrents each lasting a duration th and te respectively.

E

l L l

t

vevhvh

0 Ll

t

e–h+

th

te

t

0

th

te

iph(t)

i(t)

t

0

th

te

evh /L + eve/Levh /L

ie(t)

ih(t)

(c)

(d)Charge = e

evh/L eve/L

© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

فوتون کل :جریان

در x=L.است تولیدی a یک (EHP فوتونی

میشوند رانده وباولتازهای Veو V h .ازهم هم مخالف درجهت وحفره الکترون

برابر Te=)L-1) / Ve.میباشد الکترونها رسیدن b) زمان

برابر L / V h .میباشد ها حفره رسین زمان

) ) داده وحفره الکترون برای وحفره الکترون شدن ازرانده حاصل فوتونها خارجی جریاناست C).شده

وحفره الکترون فوتون جریان جمع با برابراست کل فوتون d)جریان

• Assume only bandgap transitions, the photocurrent IP produced by incident light of optical power P0 is

Absorption • Assume only bandgap transitions, the photocurrent IP produced by incident light of optical power P0 is

• absorption region α0 is strongly dependent on the wavelength• The upper wavelength cutoff λC is determined by the

bandgap energy of the material:

خود • پدیدهجذبی

میگیرد ازنوارشکافصورت تنها که انتقالی اریا گذ داشتن فرض بااز میباشد نوری ازتوان تابعی که شده تولید فوتون جریان

میگیرد زیرصودت : رابطه

خودجذبی خودجذبی / α0/ ضریب ناحیه بازتاب / d عرض ضریبr } است و e{.بارالکترون نوری P0توان

• است وابسته موج طول به αشدیدا ِ خودجذب ناحیه• میشود تعیین ماده شکاف انرزی باال λC توسط قطع موج طول

Photodiode Materials

Quantum Efficiency

• η ≤ 1• η is a function of the photon wavelength• To obtain a high η the width of the depletion layer,

electrons per second

photons per second hfP0

qI P=

فتودیود های ماده

کوانتومی بازده

η ≤ 1است فوتون موج ازطول η . تابعی

کند : پیروی ذیل ازرابطه بایست می تخلیه نوار عرض یا پهنا به ηبزرگآوردن منظوربدست

برثانیه الکترونها تعداد

برثانیه فوتوتها تعداد hfP0

qI P=

توان : به فوتون جریان نسبت با برابراستضریب Rنوری

Responsivityضریب

:پاسخدهی( R )

Long Wavelength Cut-off

بلندقطع موج : طول

Example

p-n Photodiode pnفتودیود

:pnفتودیود

شوند( ١ جذب وپخش تخلیه درهردوناحیه میتوانند فوتونها .

ازهم (۲ جدا صورت به تخلیه درناحیه شده تولید حامل زوج.وشناورند

وپخش (۳ تجمع دارای تخلیه ناحیه طرف به حفره پخش درناحیهاست .شدگی

: Pn فتودیود

Typical p-n Photodiode Output Characteristic

فتودیود خروجی مشخصه

p-i-n Photodiode p+

i-Si n+

SiO2Electrode

n et

–eNa

eNd

x

x

E (x)

R

E o

E

e–h+

Ip h

h> Eg

W

(a)

(b)

(c)

(d)

Vr

The schematic structure of an idealized pin photodiode (b) The netspace charge density across the photodiode. (c) The built-in fieldacross the diode. (d) The pin photodiode in photodetection isreverse biased.

Vout

Electrode

© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

p-i-n Photodiode Structuresp-i-n ديود فتو ساختار

III-V P-i-N photodiodes - InGaAs/InP

• Epitaxial growth of several layers on a n type InP substrate. • Incident light is absorbed in the low doped n type InGaAs layer• lattice matched In0.53Ga0.47As/InP system, C = 1.67m• Drawback - optical absorption in the undepleted p+ region.

A substrate entry P-i-N photodiode with a p+ InGaAsP layer to improve but charges trap at the InGaAsP/InGaAs interface limits the speed.

Dark Current

Comparisons of Common P-i-N Photodiodesديود فتو نقاطمشترك P-i-N مقايسه

Responsivity

pin Photodiode Operation Modes

Phototransistors

Photoconductive Detectors

transit

Iph

Photoconductore–

h+

Iph Iph Iph Iph

A photoconductor with ohmic contacts (contacts not limiting carrier entry) can exhibit gain. Asthe slow hole drifts through the photoconductors, many fast electrons enter and drift through thephotoconductor because, at any instant, the photoconductor must be neutral. Electrons drift fasterwhich means as one leaves, another must enter.

(a) (b) (c) (d) (e)

© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

Photodiodes• Two types of photodiodes commonly used

– PIN (p-type, intrinsic, n-type) diodes, and

– Avalanche photodiodes (APDs).• PIN Photodiode

– the thickness of the depletion region is controlled by i-layer, not by the reverse voltage

– most of the incident photons absorbed in the thick i-layer - high η

– large electric field across the i-layer - efficient separation of the generated electrons & holes

– the p and n layers are extremely thin compare to i-layer - diffusion current is very small

– The increase in the i-width reduces the speed of a photodiode– The speed of response of the photodiode is limited by

• the time it takes to collect the carriers (drift time)• the capacitance of the depletion layer (RC time constant of the

detector circuit)

• Avalanche photodiodes (APDs). بهمني

• It is a photodiodes with internal gain– An additional layer is added in which secondary electron-hole pairs

are generated through impact ionization.

– Internally multiplied the primary photocurrent before it enters the input circuitry of the following amplifier.

– Commonly used structure: Reach-through APD (RAPD)

– The RAPD is composed of a high-resistivity p-type and p+ (heavily doped p-type )

Avalanche Photodiode )APD)

APD

š p+

SiO2Electrode

n et

x

x

E (x)

R

E

h > Eg

p

Ip h

e– h+

Absorptionregion

Avalancheregion

(a)

(b)

(c)

(a) A schematic illustration of the structure of an avalanche photodiode (APD) biasedfor avalanche gain. (b) The net space charge density across the photodiode. (c) Thefield across the diode and the identification of absorption and multiplication regions.

Electrode

© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

n+

h+

E

šn+ p

e–

Avalanche region

e–

h+

Ec

Ev

(a) (b)

E

(a) A pictorial view of impact ionization processes releasing EHPs andthe resulting avalanche multiplication. (b) Impact of an energeticconduction electron with crystal vibrations transfers the electron'skinetic energy to a valence electron and thereby excites it to theconduction band.

© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

Impact Ionization

Electron Electric Field e Electron Impact Hole Impact hv Ionization Ionization h Ec Hole Ev

Electron Ionization Coefficient, 1

e

Hole Ionization Coefficient, 1

h

• In the high field region of an APD, photogenerated electrons and holes can acquire sufficient energy to create new electron-hole pairs through impact ionization process.

• These secondary carriers gain enough energy to ionize other carriers, causing the avalanche process of creating new carriers.

• The average number of e-h pairs created by a carrier per unit distance travelled is called the ionization rate/coefficient

• The ratio k = β/α is a measure of the photodetector performance

• Small k produces low noise and large gain-bandwidth products

• Due to the random nature of scattering collisions, e & h are characterized by separate probability distributions, each with it average value < e > and < h >.

SiO2

Guard ring

ElectrodeAntireflection coating

nn n+

p+

š

p

Substrate

Electrode

n+

p+

š

p

Substrate

Electrode

Avalanche breakdown

(a) (b)

(a) A Si APD structure without a guard ring. (b) A schematic illustration of thestructure of a more practical Si APD© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

• The measured value of M is expressed as an average quantity since the avalanche mechanism is a statistical process; not every carrier pair generated in the diode experiences the same multiplication

Current Gain Against Reverse Bias for APD

Multiplication Factor

APD Bandwidth

parasitic effect and carrier transit

Hole trapping and diffusion tail

avalanche buildup

Gain

Ban

dwid

th

The response time of APD is limited by• the transit time of carriers across the absorption region• the time taken by the carriers to perform the avalanche multiplication process• the RC time constant incurred by the junction capacitance of the diode and its load

The bandwidth in the very low gain regime is usually limited by the diffusion tail due to incomplete depletion of the absorption layer and by hole trapping in heterostructure APD, due to insufficient electric field at the heterointerface.

At moderate gains (5-20), the bandwidth is almost independent of gain. This bandwidth plateau is mainly dominated by parasitic effects (RC) and carrier transit time.

At high enough gain the bandwidth is limited by the avalanche build-up time.

Gain-Bandwidth Product

etBM 21 M = zero-frequency gain

effective transit time, for drifte ktt RCdrift tt

Often an asymmetric pulse shape is obtained from the APD which results from a relatively fast rise time as electrons are collected and a fall time dictated by the transit time of the holes traveling at a slower speed.

APDs constructed of materials in which one type of carrier largely dominates impact ionization (small k) exhibit low noise and large gain-bandwidth products provided the multiplication process is initiated by carrier with larger ionization coefficient.

Since gain is proportional to the avalanche build-up time, the GB-product is a constant as bandwidth is inversely proportional to the build-up time.

Benefits and Drawbacks with the APD

PiN or APDCharacteristics of common P-i-N Photodiodes

Characteristics of common APDs

Noise in Photodiodes : Direct Detection

Noise: Direct Detection Optical Receiver

Noise sources and disturbances in the optical pulse detection mechanism

Noise 1

Noise 2

Noise 3

SNR

The power signal-to-noise ratio at the output of an optical receiver is defined by

N

S Signal power from photocurrentPhotodetector noise power+amplifier noise power

Noise Equivalent Power (NEP)NEP is the minimum optical signal power that produces SNR = 1.

This is the optical power necessary to produce a photocurrent of the same magnitude as total noise current.

NEP determines the weakest optical signal that can be detected in the presence of noise.

Signal-to-Noise Ratio (SNR)

2

2

noise

p

i

I

N

S

For both signal power and noise power are released at the same load resistance,

average photocurrent

noise

p

i

I

root mean square value of the noise induced current

Quantum Noise•The detection of light by a photodiode is a discrete process - an electron-hole pair is generated from the absorption of a photon.•The photocurrent generated is dictated by the statistics of photon arrivals.• When the detector is illuminated by an optical signal P0, the average number of electron-hole pairs generated in a time is

hf

Prz em

0

•The actual number of electron-hole pairs z that are generated fluctuates from the average according to the Poisson distribution, where the probability that z electrons are generated in an interval is

!

exp)(

z

zzzP m

zm

quantum noise - it is not possible to predict exactly how many electron-hole pairs are generated by a known optical power incident on the detector.

Digital Signaling Quantum Noise• For an ideal receiver (Idark= 0, =1 and able to detect an individual photon), the probability of no electron-hole pairs (z = 0) being generated when an optical pulse of energy E falls on the photodetector in the time interval is

mzP exp10•This error probability represents the bit-error-rate of digital system, [ P(0/1)=10 -9, on the average, one error occurs for every billion pulses sent].

•The minimum optical power (or pulse energy) required to maintain a specific bit- error-rate performance in a digital system is known as the quantum limit.

Analog Transmission Quantum Noise•In analog optical receiver quantum limit manifests itself as a shot noise which has Poisson statistics. The shot noise current is on the photocurrent Ip is given by

ps qBIi 22 •Neglecting other sources of noise the SNR at the receiver is

qB

I

i

I

N

S p

s

p

22

2

•The minimum incident optical power necessary to achieve a specific S/N is hfB

P

qBhf

qP

2200

• In term of the absolute optical power requirements analog transmission compares unfavorably with digital signaling.

hfz

P mmin

hfB

N

SP

2min

Dark Current Noise• A small leakage current flows from the device terminals when there is no optical power incident on the photodiode.• This current contribute to the random fluctuations about the average particle flow of the photocurrent and manifests itself as shot noise.• The mean square value of dark current noise is dd qBIi 22 Thermal Noise• Electron motion due to temperature (external thermal energy) occurs in a random way.• The number of electrons flowing through a given circuit at any instance is a random variable.• The mean square value of thermal-noise current in a resistor R,

R

TBki Bt

42 kB = Boltzmann’s constantT = absolute temperature

Shot Noise•The detector average current Ip exhibits a random fluctuation about it mean value as a result of the statistical nature of the quantum detection process.•The number of electrons producing photocurrent will vary because of their random absorption and recombination.• Deviation of an instantaneous number of electrons from their average value is known as shot noise and its current mean square value is

B = post-detection bandwidthps qBIi 22

Noise in a P-I-N Photodiode

• Three sources of noise: Shot noise, Dark current noise, Shot noise due to background radiation

bdPTS IIIqBi 22• The total shot noise, Ib = background radiation induced current

• For photodiode without internal gain, thermal noise from the detector load resistor and from active elements in the amplifier tends to dominate.

Noise in an APD

• Due to avalanche multiplication gain in an APD, the amount of noise is higher than that in a P-I-N photodiode• An excess noise in the output photocurrent due to gain fluctuation

)(2 22 MFMqBIi ps

)(2 22 MFMqBIi dd

)(2 22 MFMqBIi bb

Shot noise

Dark current noise

Background noise

Noise in a P-I-N and APD Photodiode

Receiver Noise

Noise sources within an amplifier can be represented by a series voltage noise source and a shunt current noise source .

The equivalent circuit for the front end of an optical fiber receiver, including the effective input capacitance Ca and resistance Ra.

2av 2

ai

The total noise associated with the amplifier is dfYviiB

aaamp 0

2222

where Y is the shunt admittance and f is frequency. may be reduced with low detector and amplifier capacitance.

2ampi

When the noise associated with the amplifier is referred to the load resistance RL the noise figure Fn of the amplifier may be obtained. This allows to be combined with the thermal noise from the load resistance to give

Then the SNR can be written as

L

nBdp

p

RTBFk

IIqB

I

N

S4

)(2

2

The SNR at the output of the P-i-N photodiode receiver is

2

2

42 amp

L

Bdp

p

iR

TBkIIqB

I

N

S

2ampi

2ampi

2ti

L

nBampt R

TBFkii

422

The thermal noise contribution may be reduced by increasing the value of the load resistor RL, however this will decrease the post detection bandwidth

adL CCRB

2

1

SNR of P-i-N Photodiode Receiver

The SNR at the output of the APD receiver is

2

2

2

22

42

42

MR

TBFkMIIqB

I

R

TBFkMIIqB

MI

N

S

L

nBxdp

p

L

nBxdp

p

SNR of APD Receiver

The total shot noise current multiplied through impact ionization is given by

xdPSA MIIqBi 22 2 where

xMMF )( , x ~0.3 to 0.5 for Si APDs x ~ 0.7 to 1.0 for Ge or III-VAPDs

For low M the combined thermal and amplifier noise term dominates and giving an improved SNR.For large M the SNR decreases with increasing M at the rate of Mx.For the maximum SNR,

xMRTBFk

MIIqB

opLnB

xopdp 2

4

)(22

and

dpL

nBxop IIxqR

TFkM

42