Laboratorij za Sevanje in Optiko

Fakulteta za Elektrotehniko

Univerza v Ljubljani

Matjaž Vidmar

matjaz.vidmar@fe.uni-lj.si

http://www.s5tech.net/s53mv/

Optical-fiber time-transfer &synchronization systems: advantages, physical limitations and practical

implementations

Optical-technology advantages and drawbacks

Optical-technology

DRAWBACKS:

fiber dispersion

fiber nonlinearities

microphonics andsensitivity to vibration

high fiber thermalcoefficient

difficult optical-signalprocessing/conversion

incompatibility withexsisting equipment

Optical-technology

ADVANTAGES:

very high frequency

very large bandwidth

very low loss

very high peak power

small size optical-fibercables

low cost optical-fiberhardware

electromagnetic immunity

Main types of optical fibers

125μm

n2

gradient-index glass fiber

multi-modestep-index plastic fiber

n1

n2

SiO2+GeO

2

9μm

NA=0.2

SiO2

SiO2+GeO

2

SiO2

1mm

NA=0.1

NA=0.5

125μm NA=0.1

single-mode glass fiber

n2

n1

n1(x)

50μm

Single-mode fiber attenuation & telecom windows

2dB/km

IRreso-nances

Rayleighscattering

impurities (OH-)

1dB/km

0.2dB/km

0.5dB/km

0.1dB/km

0.05dB/km

f [THz] 400 350 300 250 200 150

5dB/km

fiber attenuation

0.02dB/km

λ [μm] 0.8 1.0 1.2 1.4 1.6 2.0II.window 1300nm

III.window 1550nm

I.window 850nm

UVresonances

HEAT

transmitted light

UV

IR

HEAT

OH-

LIGHTHEAT

Rayleigh

LIGHTLIGHT

LIGHT

back-scatter

Single-mode fiber-attenuation side effects

FWD-scat

Multi-mode, chromatic and polarization-mode dispersion

n1

n2

polarization-mode dispersion

single-mode fiber

non-symmetry

n1

n2

multi-mode dispersion

multi-mode fiber

different paths

n1(λ)

n2(λ)

chromatic dispersion

v(HP) > v(VP)

n1(λ),n

2(λ),waveguide

single-mode fiber

v(λ1) > v(λ

2)

Nonlinearities: non-linear n, Brillouin & Raman

f

F(f)

Δf=11GHz

B=10MHz f

F(f)

Δf=13THz

B=30THz

(λ0=1.5μm) f

0=200THz

Ramanscattering

Brillouinscattering

non-linear refraction index:

SiO2:

(self) phase modulation:

P=100mW, S=1.43GW/m2

(λ0=1.5μm) f

0=200THz

signal P=100mWsignal P=1mW

l=50km, λ0=1.55μm

n0=1.46,

=n⋅2/0⋅l=9.3 rd

n2=3.2⋅10−20m2/Wn=n0n2⋅S

n=n2⋅S=4.58⋅10−11

=n⋅k 0⋅l

Comparison of limitations of optical fibers

step-index Δt = 500ns

gradient-index Δt = 5-50ns

G.652 @ λ=1.55μm Δt ≈ 170ps

G.652 @ λ=1.3μm Δt ≈ 20ps

G.652 new Δt ≈ 300fs

pulse broadening @ l = 10km

chromaticdispersion Δλ = 1nm

multi-modedispersion

G.652 old Δt ≈ 10psPMD

PMAX ≈ 100mW (Raman and/or non-linear n)

PMAX ≈ 1mW (Brillouin in narrowband systems)

Thermal and quantum noise as function of frequency

Noisetemperature log(T)

Frequencylog(f)

Thermal noise:

Quantum (shot) noise:

Pn=B⋅k b⋅T

Pn=B⋅h⋅ f

293K

h=6.626⋅10−34 Js

k b=1.38⋅10−23 J /K

T= h⋅ fk b

OPTICS

RADIO/MICROWAVES

Optical timing systems

P(t)

P(t)

P(t)

t

t

t

CW modulation system

Optical CW system

Pulsedsystem

All-optical(coherent) user

High-coherenceoptical clock (laser) f = 194THz

λ = 1.55μmT ≈ 5fs

Optical CW system

DRAWBACKS:

5fs timing ambiguity?

interferometric noise?

Brillouin scattering?

PMD effects?

user-equipment availability?

ADVANTAGES:

highest resolution

highest accuracy

single-mode fiber

Optical and/orelectrical user

Femtosecondpulsed laser

fcarrier

= 194THz

Tpulse

= 100fs-10ps

Pulsed system

DRAWBACKS:

fiber nonlinearity?

fiber chromatic dispersion?

fiber thermal compensation?

electrical SNR?

optical pulse processing?

ADVANTAGES:

high resolution

high accuracy

single-mode fiber

Electricalpulse source

CW modulation system

LASER AMPsingle-mode fiber

fcarrier

= 194THz

fmodulation

= 3-30GHz

DRAWBACKS:

high photodetectorelectrical noise:jitter 1-10ps

low timingresolution?

ADVANTAGES:

simple temperaturecompensation

standard electricalinterfaces

standard hi-reltelecom components

Electrical clock source3-30GHz +15dBm

Electrical clock user3-30GHz +15dBm

EOM

@ C=1pF, T=300K

Pe≈30mW≈15dBm

C

U

electricaloutputG

electrical amplifier

R

Pn=B⋅k b⋅T B= 12⋅R⋅C

U neff= k b⋅T2⋅C=25.7V eff

U neff=Pn⋅R

photodiode

Photodiode electrical noise

Po≈1mW10mW

Pe≈1W≈−30dBm

LASER AMPsingle-mode fiber

fcarrier

= 194THz

fmodulation

= 3-30GHz

Electrical clock source3-30GHz +15dBm

Electrical clock user3-30GHz +15dBm

CW modulation system with flywheel

FLYWHEEL ADVANTAGES:

bandwidth reductionfactor 104 to 106 (Q)

jitter reduction factor 100 to 1000

remaining jitter10fs to 100fs

FLYWHEEL: very narrowband-pass filter:high-Q cavityor VCXO PLL

EOM

Delay-variation compensation techniques

LASER

AMP

compensationfiber

Electrical clock source3-30GHz +15dBm

Electrical clock user3-30GHz +15dBm

COMPENSATION TECHNIQUES:

DFB LASER electrictuning +/-0.5nm

DFB LASER temperature tuning +/-5nm

compensation fibertemperature tuning

FLYWHEEL

EOM

Peltier

transmissionfiber with

chromatic dispersionD ≈ 17ps/nm.km

λ tuning

LASER

AMP

single-modefiber pair l ≈ 300m

EOM

fX

AMP X

Φ

Electrical clock source3-30GHz +15dBm

CW modulation system with temperature compensation

AMP X

Electrical clock user3-30GHz +15dBm

TRANSMITTER

RECEIVER

AMP

λ

LASER

AMP

single-mode fiber l ≈ 3km

EOM

fX

AMP X

Φ

Faradaymirror

CW modulation system with PMD compensation

AMP X

Electrical clock user3-30GHz +15dBm

TRANSMITTER

RECEIVER

AMP

λ

Electrical clock source3-30GHz +15dBm

uncorrectederror ±ΔΦTap RX

AMP

+

Electricalclock user #N3-30GHz +15dBm

AMP

Electrical clock source3-30GHz +15dBm

Multi-point chain clock distribution

Master TX

End-point RX

Electricalclock user #M3-30GHz +15dBm

distance#1 distance#2

+ΔΦ

+ΔΦ

-ΔΦ

-ΔΦ

Vector sum +ΔΦ

-ΔΦ=+ΔΦ-ΔΦ-ΔΦ

Tap

d#1 ≠ d#2

USER REQUIREMENTS:

optical input & output?

pulsed input & output?

integration with otherequipment?

diagnostics?

Future-developement action list

FLYWHEEL TECHNOLOGY:

ceramic materials fordielectric resonators

VCXO characterization

low-noise frequency multipliers anddividers

LASER TECHNOLOGY:

high-speed electronicwavelength tuning

low-coherence sources

direct laser modulation

safety & reliability

OPTICAL FIBER:

long-term, temperature& PMD characterization

connector performance

ferrite components:isolators, circulators& Faraday mirrors