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

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

1 Part 6: Optical detector آشکارسازهای نوری

2 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 ( nm). 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

3 آشکارسازنوری : مولفه ایی اساسی ازیک فیبرنوری دریک سیستم ارتباطی است که کارآیی آن درکل سیستم دیکته شده است. تابعی است که سیگنال نوری دریافتی رابه سیگنال الکتریکی تبدیل میکند. تابع فوق می بایست ازاهمیت زیادی برخورداربوده وبرای اجرا وهمسازی موارد زیرضروری ومورد نیازباشد: 1) حسگرهایی که درطول موج های بالا عمل میکنند 2) ایجاد صدایا عالیترین درجه ومشابه به اصل که برای ارسال مواردآنالوگ مهم است 3) زمان پاسخدهی کوتاه بدای بدست آوردن پهنای باند مناسب 4) کمترین مقدارنویزکه توسط آشکارساز نشان داده میشود 5) پایداری وثبات درکارآیی وانجام مشخصه ها 6) اندازه کوچک برای اتصال موثر درفیبروبسته بندی آسان با درنظرگرفتن وپیروی ازالکترونها 7) ولتازپیشقدر پایین 8) قابلیت اطمینان بالا وهزینه پایین انواع : 1) فوتودیود ها 2) فتوترانزیستورها 3) آشکارسازهای فتوکانداکتیو 4) لامپهای فتومولتی پلیر

4 اصل آشکارسازی نوری : Optical Detection Principles

5 اصل آشکارسازی نوری : برخوردیک فوتون درناحیه تخلیه میتواند منجربه برانگیخته شدن یک الکترون ازباندظرفیت به باند هدایت ودرنتیجه ایجادحفره درباندظرفیت شود.(تولیدفوتون) زوج حامل تولیدشده درنزدیکی پیوندگاه ازیکدیگرجداشده اند وتحت تاثیرمیدان الکتریکی که باعث ایجاد یک جابجایی توسط جریان درمدارخارجی (درفزونی هرجریان نشتی بازگشتی) میگردد جاروب میشود.

6 Optical generation of carriers in a PN junction. W Pairs generated here recombine before reaching depletion layer P N LpLp LnLn PNW LpLp LnLn 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

7 : pnنور تولید شده ازحاملها دریک پیوندگاه W زوج تولید شده دراین ناحیه قبل از رسیدن به لایه تخلیه جدا ازیکدیگرند P N LpLp LnLn PNW LpLp LnLn ( درشکل فوق فرض شده است که تابش در سراسر دیود روی داده است ) یک فوتون میتواند انرزی از دست داده ویک الکترون برانگیخته شودیک فوتون میتواند انرزی از دست داده ویک الکترون برانگیخته شود. (حمل کننده فوتون) اعمال میدان الکتریکی قوی درناحیه تخلیه باعث جدایی حاملها وتجمع آنها درسراسر محل اتصال پیشقدر معکوس میشوداعمال میدان الکتریکی قوی درناحیه تخلیه باعث جدایی حاملها وتجمع آنها درسراسر محل اتصال پیشقدر معکوس میشود. جریان یک دیود ازرابطه زیرمعین میشود:جریان یک دیود ازرابطه زیرمعین میشود: (دررابطه فوق عبارت اول مربوط به جریان تاریک وعبارت دوم مربوط به جریان دیود است)

8 pn detector

9 :pnآشکارساز یک فتودیوداست.Pn نموداری شماتیک ازبایاس معکوس درپیوندگاه (A میباشندn و p دراطراف اتمهای دهنده وپذیرنده Νa وNd) فاصله بارخالص درسراسردیوددرناحیه تخلیه است.b میدان درناحیه تخلیه رانشان میدهد.(C

10 فتوجریان کل Total Photocurrent = e?

11 جریان کل فوتون: است.x=L فوتونی تولیدی در EHP) یک a ازهم رانده میشوند. V h وVe الکترون وحفره درجهت مخالف هم وباولتازهای میباشد.Te=(L-1) / Ve زمان رسیدن الکترونها برابر (b میباشد. L / V h زمان رسین حفره ها برابر جریان خارجی فوتونها(حاصل ازرانده شدن الکترون وحفره)برای الکترون وحفره داده شده است.(C جریان فوتون کل برابراست با جمع جریان فوتون الکترون وحفره(d

12 Assume only bandgap transitions, the photocurrent I P produced by incident light of optical power P 0 is

13 Absorption Assume only bandgap transitions, the photocurrent I P produced by incident light of optical power P 0 is absorption region α 0 is strongly dependent on the wavelength The upper wavelength cutoff λ C is determined by the bandgap energy of the material:

14 پدیده خود جذبی با فرض داشتن گذ اریا انتقالی که تنها ازنوارشکاف صورت میگیرد جریان فوتون تولید شده که تابعی ازتوان نوری میباشد از رابطه زیرصودت میگیرد : ضریب خودجذبی /α0 / عرض ناحیه خودجذبی d / ضریب بازتاب r } بارالکترون است.}e توان نوری وP0 شدیدا به طول موج وابسته است αِناحیه خودجذب توسط انرزی شکاف ماده تعیین میشود λC طول موج قطع بالا

15 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 =

16 ماده های فتودیود بازده کوانتومی η 1 تابعی ازطول موج فوتون است. η : بزرگ پهنا یا عرض نوار تخلیه می بایست ازرابطه ذیل پیروی کند η به منظوربدست آوردن تعداد الکترونها برثانیه تعداد فوتوتها برثانیه =

17 : برابراست با نسبت جریان فوتون به توان نوریR ضریب Responsivity ضریب پاسخدهی: ( R )

18 Long Wavelength Cut-off

19 طول موج بلندقطع :

20 Example

21 p-n Photodiode pnفتودیود

22 :pnفتودیود ١) فوتونها میتوانند درهردوناحیه تخلیه وپخش جذب شوند. ۲) زوج حامل تولید شده درناحیه تخلیه به صورت جدا ازهم وشناورند. ۳) درناحیه پخش حفره به طرف ناحیه تخلیه دارای تجمع وپخش شدگی است.

23 : Pn فتودیود

24 Typical p-n Photodiode Output Characteristic مشخصه خروجی فتودیود

25 p-i-n Photodiode

26 p-i-n Photodiode Structures p-i-n ساختار فتو ديود

27 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 In 0.53 Ga 0.47 As/InP system, C = 1.67 m 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.

28 Dark Current Comparisons of Common P-i-N Photodiodes مقايسه نقاط مشترك فتو ديود P-i-N Responsivity

29 pin Photodiode Operation Modes

30 Phototransistors

31 Photoconductive Detectors

32 transit

33 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)

34 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 )

35 Avalanche Photodiode (APD)

36 APD

37 Impact Ionization 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

38 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 and.

39 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

40 APD Bandwidth 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.

41 Gain-Bandwidth Product M = zero-frequency gain effective transit time, for 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.

42 Benefits and Drawbacks with the APD

43 PiN or APD Characteristics of common P-i-N Photodiodes Characteristics of common APDs

44 Noise in Photodiodes : Direct Detection

45 Noise: Direct Detection Optical Receiver Noise sources and disturbances in the optical pulse detection mechanism

46 Noise 1

47 Noise 2

48 Noise 3

49 SNR The power signal-to-noise ratio at the output of an optical receiver is defined by Signal power from photocurrent Photodetector 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) For both signal power and noise power are released at the same load resistance, average photocurrent root mean square value of the noise induced current

50 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 P 0, the average number of electron-hole pairs generated in a time is 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 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.

51 Digital Signaling Quantum Noise For an ideal receiver (I dark = 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 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 i s on the photocurrent I p is given by Neglecting other sources of noise the SNR at the receiver is The minimum incident optical power necessary to achieve a specific S/N is In term of the absolute optical power requirements analog transmission compares unfavorably with digital signaling.

52 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 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, k B = Boltzmanns constant T = absolute temperature Shot Noise The detector average current I p 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 bandwidth

53 Noise in a P-I-N Photodiode Three sources of noise: Shot noise, Dark current noise, Shot noise due to background radiation The total shot noise, I b = 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 Shot noise Dark current noise Background noise Noise in a P-I-N and APD Photodiode

54 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 C a and resistance R a. The total noise associated with the amplifier is where Y is the shunt admittance and f is frequency. may be reduced with low detector and amplifier capacitance.

55 When the noise associated with the amplifier is referred to the load resistance R L the noise figure F n 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 The SNR at the output of the P-i-N photodiode receiver is The thermal noise contribution may be reduced by increasing the value of the load resistor R L, however this will decrease the post detection bandwidth SNR of P-i-N Photodiode Receiver

56 The SNR at the output of the APD receiver is SNR of APD Receiver The total shot noise current multiplied through impact ionization is given by where, 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 M x. For the maximum SNR, and


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