optical communication unit 5

UNIT-5

Mohammad Asif IqbalAssistant Professor, Deptt of ECE,JETGI, Barabanki

Digital Transmission System (DTS)

• The design of optical receiver is much more complicated than that of optical transmitterbecause the receiver must first detect weak, distorted signals and the n make decisions onwhat type of data was sent.

Error Sources in DTS

!)(

)(0

n

eNnP

Eh

dttPh

N

Nn

r

is the average number of electron-hole pairs in photodetector,

is the detector quantum efficiency and E is energy received in a time

interval and is photon energy, where is the probability

that n electrons are emitted in an interval .

N

h )(nPr

[7-1]

[7-2]

InterSymbol Interference (ISI)

Pulse spreading in an optical signal, after traversing along optical fiber,

leads to ISI. Some fraction of energy remaining in appropriate time slot

is designated by , so the rest is the fraction of energy that has spread

Into adjacent time slots.

Receiver Configuration

The binary digital pulse train incident on the photodetector can be written in the

following form:

t.allfor positive is which shape pulse received theis)( and

digit messageth theofparameter amplitudean is period,bit is where

)()(

th

nbT

nTthbtP

p

nb

n

bpn

[7-3]

• In writing down eq. [7-3], we assume the digital pulses with amplitude V

represents bit 1 and 0 represents bit 0. Thus can take two values

corresponding to each binary data. By normalizing the input pulse to

the photodiode to have unit area

represents the energy in the nth pulse.

the mean output current from the photodiode at time t resulting from pulse

train given in eq. [7-3] is (neglecting the DC components arising from dark

current noise):

nb)(th p

1)( dtthp

nb

n

bpno nTthbMtMPh

qti )()()(

[7-4]

Bit Error Rate (BER)

• Probability of Error= probability that the output voltage is

less than the threshold when a 1 is sent + probability that the

output voltage is more than the threshold when a 0 has been

sent.

be

t

e TBBt

N

N

N

t

t

/1

during ed transmittpulses of # total

interval mecertain ti aover error of #

Error ofy ProbabilitBER

[7-5]

Probability distributions for received logical 0 and 1 signal pulses.

the different widths of the two distributions are caused by various signal

distortion effects.

thv

ed transmitt0 if, exceeds tageoutput volequalizer that theprobablity )0|()(

ed transmitt1 if, than less is tageoutput volequalizer that theprobablity )1|()(

0

1

vdyypvP

vdyypvP

v

v

[7-6]

• Where are the probabilities that the transmitter sends 0 and 1

respectively.

• For an unbiased transmitter

th

th

v

v

ththe

dyypqdyypq

vPqvPqP

)1|()1|(

)()(

01

0011

[7-7]

01 and qq

5.010 qq

10 1 qq

Gaussian Distribution

dvbv

dyypvP

dvbv

dyypvP

thth

thth

vv

th

v

on

v

th

2

off

2

off

off

0

2

on

2

on

1

2

)(exp

2

1)0|()(

2

)(exp

2

1)1|()(

mea

n

mea

n

[7-8]

• If we assume that the probabilities of 0 and 1 pulses are equally likely, then

using eq [7-7] and [7-8] , BER becomes:

Q

Q

QdxxQP

Q

e

/2)exp(-

2

1

)2

(erf12

1)exp(

1)(BER

2

2/

2

[7-9]

dyyx

vbbvQ

x

thth

0

2

on

on

off

off

)exp(2

)(erf

[7-9]

[7-10]

Approximation of error function

Variation of BER vs Q,

according to eq [7-9].

Special Case

In special case when:

Vbb onoffonoff ,0 &

From eq [7-29], we have: 2/Vvth

Eq [7-8] becomes:

)

22(erf1

2

1)(

VPe

[7-11]

Study example 7-1 pp. 286 of the textbook.

ratio. noise-rms-to-signalpeak is

V

Quantum Limit

• Minimum received power required for a specific BER assuming that the

photodetector has a 100% quantum efficiency and zero dark current. For

such ideal photo-receiver,

• Where is the average number of electron-hole pairs, when the incident

optical pulse energy is E and given by eq [7-1] with 100% quantum

efficiency .

• Eq [7-12] can be derived from eq [7-2] where n=0.

• Note that, in practice the sensitivity of receivers is around 20 dB higher than

quantum limit because of various nonlinear distortions and noise effects in

the transmission link.

)exp()0(1 NPPe [7-12]

N

)1(

Eye Diagram

• Standard measure for signaling

• Synchronized superposition of all possible realizations of the signal viewed within a particular interval

• Obtained from measurement or transient simulation

TX RX

channel

Eye Diagram (cont’d)

• Timing jitter• Deviation of the zero-crossing from its ideal occurrence time

• Amplitude noise• Set by signal-to-noise ratio (SNR)

• The amount of noise at the sampling time

Existing Work

• Eye diagram analysis• Analytical eye-diagram model [Hashimoto, CICC’07]

• Only consider attenuation and reflection• Assume perfect match at transmitter end

• Jitter and noise analysis• Data-dependent jitter model [Buckwalter, MicrowaveSymp’04][Ou’DTS’04]

• Only consider two taps of channel response • Enumerate all possible input combinations: [00, 01, 10, 11]

• Clock jitter model [Hanumolu’04][Tao’99]• Clock-data recovery (CDR), DLL, PLL

• Amplitude noise model [Hanumolu’05]

• No general framework to model the jitter and noise and find out what is the worst possible scenario

Eye Mask

• Wider eye = more timing margin

• Higher eye = more noise margin

• How to determine if the eye satisfies the mask or not• Find the worst-case jitter and noise

PCI-Express

Contribution

• Formula-based model for jitter and noise• Use differential signaling as an example

• Utilize multi-conductor transmission line equations

• Can be extended to equalized link

• Consider the pre-emphasis filter at the transmitter end

• Worst-case jitter and noise• Directly find the worst-case input pattern

• Use efficient mathematical programming algorithms

• No need for time-consuming simulation

• Runtime is not determined by the pattern length

• Adequate length can be used according to channel response

Motivations • Higher Spectral Efficiency – QPSK / multi-level QAMs• Higher Data Rates – 40Gbit/s, 100Gbit/s, and even higher• Higher Receiving Sensitivity

Recent Coherent Optical Communication • Coherent detection based on DSP

• Local oscillator (LO) laser • Polarization diversity 90° optical hybrid • Balanced detectors• High speed analog to digital convertor (ADC)• High speed digital signal processing (DSP)

Coherent Optical

Coherent Optical Communications

Coherent Optical Receiver – I

• Advantages:• Multi-level constellations• High data rate• Phase managements• Polarization managements

• Dis-advantages:• Electrical circuit complexity• Speed limitations• Cost issues• Power consumptions

Coherent Optical Receiver – II

• Homodyne OPLL based coherent receiver – Costas Loop• Optical carrier recovering technique

Requiring Stable OPLL

Coherent Optical Receiver – II

• Challenges: • Long loop delays (*1ns)• Narrow loop bandwidth (*100MHz)• Transmitting and LO lasers’ linewidth• Sensitive by external variations

• Solutions: • Integrated circuits (photonic IC, electrical IC)• Feed-forward loop filter topology • Minimizing Interconnection delays • Digitally operating feedback system

Phase Locked Coherent BPSK Receiver

Homodyne OPLL + Costas Loop

• Three blocks: photonic IC, electrical IC, and hybrid loop filter • High speed BPSK data demodulations


Photonic IC

• SG-DBR laser – 40nm tunable ranges • 90° optical hybrid• 4 un-balance photodiodes – 30GHz bandwidth


Electrical IC

• Limiting amplifiers • Phase / frequency detector (PFD) – XOR + delay line

Teledyne’s 500nm InP HBT300GHz ft / fmax


Loop Filter

• Main path by integrator – high gain at DC and low frequencies • Feed-forward path – passive capacitor component

Main Path

Feed-Forward Path

Open Loop Responses

* Challenges: 1. OP amp has lots of delays 2. OP amps bandwidth is limited (100MHz)

Fabricated in UCSB (Mingzhi Lu)

Designed by Eli Bloch usingTeledyne 500nm HBT ProcessLoop filter and system

designed by Hyun-chul Park

Integration on a Single Carrier board

• Compact chip size of 10 x 10mm2

• Total delay (120ps)=PIC (40ps)+EIC (50ps)+Interconnection (30ps)

1GHz Loop Bandwidth is feasible

THANK YOU!

optical communication unit 5

Engineering