ECE 455 Lecture 16

1

Coherent Optical Communications

• HMY 455 • Lecture 16 • Fall Semester 2016

Stavros Iezekiel Department of Electrical and

Computer Engineering

University of Cyprus

ECE 455 Lecture 16

COHERENT COMMUNICATIONS

2

ECE 455 Lecture 16

Coherent Optical Communications

• In the optical domain, coherent refers to systems in which mixing of optical signals occurs.

• Consider mixing in an electronic receiver:

m(t) =

Local oscillator

Mixer

B cos (LO t)

A cos (m t + m)

])[(cos2

])[(cos2

)(cos.)(cos

mLOm

mLOm

LOmm

tAB

tAB

tBtA

ECE 455 Lecture 16

• Mixing creates sum (m + LO ) and difference (m - LO )

frequency terms.

• The sum term is then removed by low-pass filtering:

B cos (LO t)

m(t) =

Local oscillator

Mixer

A cos (m t + m)

LPF

])[(cos2

mLOm tAB

• Downconverted signal: at lower frequency than the original message m(t)

• Still “contains” information on amplitude A, frequency m

and phase m of message.

ECE 455 Lecture 16

Load

resistor

RL

Photodiode

IP

Optical combiner

Phase-locked

local oscillator

laser

RRRR tEE cos~

tEE LOLOLO cos~

RLO EE~~

Incoming signal

• The optical equivalent of a receiver using mixing is:

Coherent optical detector

ECE 455 Lecture 16

• We will consider digital modulation, in which a baseband digital signal can modulate the:

– amplitude ER [ASK - amplitude shift keying]

– frequency R [FSK]

– phase R [PSK]

– of a sinusoidal optical signal:

RRRR tEE cos~

ECE 455 Lecture 16

7

ASK

(OOK)

FSK

PSK

Modulated optical carrier waveforms:

ECE 455 Lecture 16

8

Constellation maps for PSK

ECE 455 Lecture 16

Load resistor RL

Photodiode

IP

RRRR tEE cos~

Incoming signal (from an intensity modulated laser):

• Now, consider a direct detection (DD) receiver:

optical phase

optical frequency (of order 100 THz) = c / R

electric field amplitude; optical power is ER2

In other words, this quantity is modulated by the laser drive current if the laser is intensity modulated.

Electric field of incident optical signal

ECE 455 Lecture 16

However, the frequency response of a photodiode is limited. Consider an ASK waveform:

Envelope Optical carrier

The photodiode cannot detect the fast variations of the optical carrier, it can only respond to the modulation envelope, i.e. it acts as an envelope detector.

ECE 455 Lecture 16

Load resistor RL

Photodiode

IP

RRRR tEE cos~

Incoming signal (from an intensity modulated laser):

The incident optical power is proportional to the square of the E-field, i.e.:

RRRRincident tEEP 222 cos~

RRRincident tEP 22cos12

21

ECE 455 Lecture 16

• We saw in earlier lectures that the photocurrent generated in a photodiode is proportional to the incident optical power, i.e. we might expect:

RRRDDP tEI 22cos12

21

,

• However, even state-of-the-art photodiodes have frequency responses that extend no more than 100 GHz, i.e. the photodiode cannot detect the term 2R. Hence for intensity modulation/direct detection schemes (IM/DD),

2

21

, RDDP EI

ECE 455 Lecture 16 Coherent detection theory

• Photodiodes are only sensitive to intensity fluctuations, i.e. they can only detect modulation of ER.

• So the key to implementing optical FSK and PSK detection is to convert the optical frequency and phase fluctuations into optical intensity fluctuations.

• This is achieved using coherent optical detectors:

ECE 455 Lecture 16

Load resistor RL

Photodiode

IP

Optical

combiner

Phase-locked local oscillator

laser

RRRR tEE cos~

tEE LOLOLO cos~

RLO EE~~

Incoming signal

Coherent optical detector

ECE 455 Lecture 16

• Basic theory: the incoming light beam, i.e. the electric field:

• is added to a beam produced by a stable local oscillator laser:

• The “mixing” process, between the information bearing and local oscillator fields is done before photodetection.

RRRR tEtE cos)(~

tEtE LOLOLO cos)(~

ECE 455 Lecture 16

• As before, the photodiode current is directly proportional to the square of the incident electric field, which in this case is given by:

2

2

,

)cos()cos(

)(~

)(~

tEtE

tEtEI

LOLORRR

LORcohP

• Expanding:

)(cos)(cos2

)(cos

)(cos

22

22

,

ttEE

tE

tEI

LORRLOR

LOLO

RRRcohP

ECE 455 Lecture 16

• Using trigonometric identities for cos x cos y and cos2x, we get:

])[(cos

])[(cos

)2cos(1

)22cos(1

2

21

2

21

,

RLORLOR

RLORLOR

LOLO

RRRcohP

tEE

tEE

tE

tEI

ECE 455 Lecture 16

• Notice that we have components at:

DC

2R

2LO

R + LO

R - LO = IF = intermediate frequency

The photodiode cannot respond to the terms in 2R , 2LO and R + LO because they are outside the detection bandwidth.

• If R LO then the frequency IF will be in the microwave range or less, i.e. it can be detected by the photodiode. Hence the actual photocurrent is:

])[(cos2

212

21

, RIFLORLORcohP tEEEEI

ECE 455 Lecture 16 • Since optical power varies with the square of electric

field,

])[(cos2, RIFLORLORcohP tPPPPI

• is a responsivity term.

• The first two terms, i.e. PR and PLO , are DC terms.

• Hence the signal component is:

])[(cos2, RIFLORcohP tPPi

• Usually, PLO >> PR , and:

])[(cos2, RIFLORLOcohP tPPPI

ECE 455 Lecture 16

Detection of amplitude, frequency and phase modulated signals:

Can detect ASK, FSK and PSK

])[(cos2, RIFLORLORcohP tPPPPI

Advantages of Coherent Detection

• In contrast, IM/DD systems can only detect ASK.

ECE 455 Lecture 16

• Since the signal component is:

])[(cos2, RIFLORcohP tPPi

then by increasing PLO we can increase the value of iP , hence the LO laser acts as the equivalent of an optical amplifier, giving greater receiver sensitivity.

Improved sensitivity (compared to IM/DD systems):

])[(cos2, RIFLORLOcohP tPPPI

ECE 455 Lecture 16

IF = R - LO

• In other words, modulation of R is “downconverted” to lower frequencies (microwave range), allowing channels to be filtered using microwave filters instead of optical filters. •. Because microwave filters have sharper selectivity than optical filters, more channels can be accommodated in a given wavelength range.

])[(cos2, RIFLORLORcohP tPPPPI

Better channel selectivity:

ECE 455 Lecture 16 Comparison of typical optical and microwave filter

transfer functions

ECE 455 Lecture 16

1000

channels 1500

channels

Wavelength (nm)

Att

enu

ati

on

(d

B/k

m)

10 GHz channel spacing

ECE 455 Lecture 16

• When the incoming and local oscillator frequencies are different (i.e. R LO ), we refer to the system as being heterodyne.

• If the frequencies are locked to one another (R = LO ), the system is homodyne. The locking is achieved using an optical phase-locked loop. In this case, the IF frequency is zero (IF = 0), and:

)(cos2

212

21

, RLORLORcohP EEEEI

Homodyne detection

Note that homodyne systems can detect ASK & PSK, but not FSK