Lecture 7 Fiber Optical Communication Lecture 7, Slide 1

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1 Lecture 7 Optical receivers p i n ioes Avalanche ioes Receiver esign Receiver noise Shot noise Thermal noise Signal-to-noise ratio Fiber Optical Communication Lecture 7, Slie 1

2 Optical receivers The purpose of a traitional receiver for OOK is: Convert the optical signal into an electrical signal Recover the ata by: Doing clock recovery Performing ecisions on the obtaine signal In state-of-the-art coherent receivers, aitional functionality is performe in igital signal processing (DSP) Electronic ispersion compensation (EDC) Aaptive equalization Phase synchronization This lecture is about OOK systems Necessary to know about......an still common Fiber Optical Communication Lecture 7, Slie

3 Photoetectors The most critical component is the photoetector Converts the optical signal to an electrical current We want these components to have: High sensitivity Fast response time Low noise High reliability Size compatible with fibers This means that semiconuctor materials are exclusively use Photons are absorbe an generate electron hole (e h) pairs This prouces a photo-current. Basic requirement: The etector material bangap energy (E g ) < the photon energy (hν) Fiber Optical Communication Lecture 7, Slie 3

4 The photocurrent is proportional to the optical power The constant R is the responsivity η = the quantum efficiency = the number of e h pairs per incient photon Ieally η = 1 R R increases with λ until hν = E g R 0 when the photon energy becomes too low Si or GaAs can be use for short wavelengths (λ < 900 nm) InGaAs is most common at 1.3 an 1.55 μm Most communication systems use reverse-biase p n junctions (photoioes) of two main types: p i n photoioes Avalanche photoioes (APD) Photoioes (4.1.1) q h 1.4 [A/W] with in μm I p R P in Fiber Optical Communication Lecture 7, Slie 4

5 p i n ioes (4..) absorption of photons e h pair generation carrier rift ue to built-in an applie fiel inuce current in the external circuit Electric fiel Energy levels p i n ioe: p n junction with an intrinsic (un-ope) layer Response time is limite by the transit time through the i-region W tr v s Responsivity increases with W a trae-off between responsivity an spee High spee (~50 GHz) ioes with η close to unity are available Fiber Optical Communication Lecture 7, Slie 5

6 p i n ioes, performance p n ioes are limite by iffusion (absorption outsie the epletion region) In a p i n ioe, the epletion region is wie (intrinsic, unope) p i n banwith limitations: Parasitic capacitance Reuces the spee of voltage changes Transit time Takes time to collect the carriers The ark current shoul be low Current without input signal Due to stray light an thermal generation of carriers Fiber Optical Communication Lecture 7, Slie 6

7 Examples of p i n ioes Schematic picture of a p i n ioe Green is anti-reflection coating p i n ioes without an with pigtail Important parameters are: Banwith Sensitivity Responsivity Polarization epenence No epenence is preferre Fiber Optical Communication Lecture 7, Slie 7

8 Avalanche photoioes (APDs) (4..3) An APD is a p i n ioe with an extra layer next to the i-region Gives gain through impact ionization an amplifies the signal The responsivity can be >> q/hν The responsivity of an APD is M is the multiplication factor R APD M q h MR The increase responsivity comes at the expense of Enhance noise Reuce banwith Fiber Optical Communication Lecture 7, Slie 8

9 APD multiplication factor The multiplication factor M epens on the geometry of the APD, the electric fiel etc The frequency epenence is M (0) M ( ) 1[ e M (0)] τ e is the effective transit time for the multiplication process A trae-off between multiplication an banwith Si-APDs have very goo performance M > 100, high banwith, relatively low noise Very useful for systems operating near 0.8 μm InGaAs-APDs can be use at 1.3 an 1.55 μm Suffer from smaller multiplication an banwith, an higher noise Fiber Optical Communication Lecture 7, Slie 9

10 Receiver esign (4.3) The igital receiver consists of three parts: Front en (photo-etector, trans-impeance amplifier) Linear channel (amplifier, low-pass filter) Data recovery (clock recovery, ecision circuit) front en linear channel ata recovery h photoioe preamplifier amplifier filter ecision circuit ata voltage supply automatic gain control clock recovery Fiber Optical Communication Lecture 7, Slie 10

11 Receiver front-ens (4.3.1) Transimpeance front-en Rf + - P Cp RL - Vout P Cp + Vout Simple Electrically stable Low sensitivity for small R L High banwith High sensitivity Potentially unstable Small banwith for high R L f 1 R C L p f G R f C p Effective input resistance = R f /G Fiber Optical Communication Lecture 7, Slie 11

12 The linear channel consists of: Linear channel (4.3.) A high-gain amplifier with automatic gain control Constant average output voltage irrespective of the input (within limits) A low-pass filter with banwith chosen to: Reject noise outsie signal banwith Avoi introucing inter-symbol-interference (ISI) The best situation is when the filter (an not other components) limits the overall banwith of the receiver The output voltage spectrum is given by H out (ω) = H T (ω)h p (ω) H p (ω) is the photocurrent spectrum H T (ω) is the total transfer function of the front en an the linear channel Normally, H T (ω) is ominate by the filter transfer function H T (ω) H f (ω) Fiber Optical Communication Lecture 7, Slie 1

13 Data recovery (4.3.3) The ata-recovery section consists of A clock-recovery circuit Extracting a sinusoial component at f = B to enable proper synchronization of the ecision circuit Easily one for an OOK RZ signal with a narrow-ban filter The signal contains a elta function at f = B More ifficult for NRZ No sinusoial spectral components are present Can use a full-wave rectifier to convert the NRZ signal to RZ containing a elta function at f = B A ecision circuit comparing the input voltage with a threshol at the time obtaine from the clock recovery input NRZ ata RZ waveform extracte clock Deciing whether a "1" or a "0" was receive Fiber Optical Communication Lecture 7, Slie 13

14 Eye iagrams The eye iagram is a superposition of all bits on top of each other Looks like an eye Gives a visual way to monitor the receiver performance Left: An ieal NRZ eye iagram Right: An eye iagram egrae by noise an timing jitter A measure RZ eye iagram at 640 Gbit/s Fiber Optical Communication Lecture 7, Slie 14

15 Eye iagram interpretation Fiber Optical Communication Lecture 7, Slie 15

16 Receiver noise (4.4) The etecte photo current in the receiver will contain noise There are two funamental sources of noise Shot noise ue to fiel an charge quantization Thermal noise ue to thermal motion of charges The total current, signal + noise, can be written I( t) R Pin ( t) i ( t) i ( t) s T In aition, there can also be optical noise in P in Comes from lasers an optical amplifiers Will be treate later in the course Remember I p ( t) R Pin ( t) Fiber Optical Communication Lecture 7, Slie 16

17 Shot noise Shot noise arises from the particle nature of the photocurrent Current consists of electrons that can only be escribe statistically Current is not constant but fluctuates Compare with cars on a highway or hails on a roof The variance of the shot noise photocurrent is s i s ( t) Δf is the effective noise banwith of the receiver S s (f) is the shot noise two-sie power spectral ensity (PSD) If the etector ark current I cannot be neglecte we have s q( I I ) f Originating from stray light or thermally generate e h pairs f 0 S s p ( f ) f qi p f Fiber Optical Communication Lecture 7, Slie 17

18 Thermal noise Thermal noise originates from the thermal motion of the electrons The two-sie PSD is hf kbt ST ( f ) R exp( hf / k T) 1 R k B is Boltzmann s constant T is the temperature R L is the loa resistance The noise variance is T i T ( t) B L In aition, thermal noise is also generate in electrical amplifiers Introuce the amplifier noise figure F n to obtain L f 0 S T ( f ) f (4k B T / R L ) f T (4k T / R ) F f B L n Fiber Optical Communication Lecture 7, Slie 18

19 Signal-to-noise ratio (SNR) The ifferent noise sources are uncorrelate We obtain the total noise power accoring to ( I) s T q( I p I ) f (4k B T / R L ) F f n The signal-to-noise ratio (SNR) of an electrical signal is efine as average signal power SNR noise power This efinition is for an analog signal This is not the usual meaning of SNR in igital communication theory Instea E b /N 0 or E s /N 0 is use there E b is the energy per bit E s is the energy per symbol N 0 is the noise PSD I p Fiber Optical Communication Lecture 7, Slie 19

20 For a p i n receiver we have Noise in p i n receivers (4.4.) SNR q( R P When thermal noise ominates, we have When shot noise ominates, we have SNR We note: I R ) f Pin 4( k T / R Different scaling with input power in the two limits Thermal noise ominates at low input power Shot noise ominates at high input power in SNR R 4k L B R Pin qf R Pin TF f n Pin h f 0 B P in P in L ) F f n Fiber Optical Communication Lecture 7, Slie 0

21 Noise in APD receivers (4.4.3) Since R APD = M R, the power of the current increases by M But the noise increases too, so the SNR increase is smaller The APD shot noise variance is The excess noise factor is 1 < F A < M since 0 < k A < 1, (k A = α h /α e, see (4..3)) The SNR becomes qm F ( R P I ) f The shot-noise is increase by M F A F A s SNR qm F A ( M) k M (1 k )( 1/ M) A ( R A P in in A ( MR Pin) I ) f 4( k B T / R L ) F f n Fiber Optical Communication Lecture 7, Slie 1

22 In the thermal noise limit we have A factor of M higher than for the p i n In the shot noise limit we have Noise in APD receivers SNR RLR M P 4k TF f A factor of F A lower than for the p i n ioe A B R Pin SNR qf f n in M Pin h F 0 A f P in P F in A The SNR is increase by an APD in the thermal-noise limit The SNR is ecrease by an APD in the shot-noise limit Fiber Optical Communication Lecture 7, Slie

23 APD avantage over PIN The SNR (Δf = 30 GHz) for a p i n receiver an an APD receiver APD is best at low power p i n is best at high power M = 10 is worse than M = 5 The APD vs the p i n APD avantage over p-i-n ioe SNR M M opt SNR p-i-n 1/M There is an optimum value for M M M opt k A 4kBTF qr ( R P L n in I ) 1/3 Optimum value epens on k A = α h /α e Highest M opt 100 for silicon APD Highest M opt 10 for InGaAs APD Fiber Optical Communication Lecture 7, Slie 3