Frontiers in Optics 2009San Jose, CA

11–15 October 2009

All-Fiber Isolator Based on Faraday Rotation

L. Sun,1,2 S. Jiang,3 J. D. Zuegel,1 and J. R. Marciante1,2

University of RochesterLaboratory for Laser Energetics1Laboratory for Laser Energetics 2Institute of Optics, University of Rochester3AdValue Photonics Inc., Tucson, AZ

Linearpolarizedlight

Magnet

Polarizingfiber

Polarizingfiber

Tb fiber

An all-fiber optical Faraday isolator is demonstrated for the first time with 17-dB isolation

E18296

• All-fiberisolatorsarerequiredforhigh-powerapplications

• Theall-fiberisolatorismadeofafiberFaradayrotator and two pieces of polarizing fiber

• ThefiberFaradayrotatorusesa56wt%Terbium-dopedmulticomponentsilica fiber

– effective Verdet constant = –22.3±0.8 rad/(Tm)

• CorningSP1060fiberisusedasthepolarizingfiber – extinction ratio >17 dB at 1053 nm for 1-m-long PZ

• 17-dBisolationismeasuredforallback-injectedpolarizationstates

Summary

Faraday isolators are critical components in many opticalsystemsandarestillbasedonbulkoptics

E17156a

• Faradayisolatorsareusedinopticalcommunicationsystems, high-power laser systems

• Faraday rotation is the result of circular birefringence induced by a magnetic field applied along the axis of light propagation

• Theangleofrotationisgivenby

i = VBL

V: Verdet constant B: magnetic-field flux density L: length of the crystal

• Arotationangleof45ºisrequiredfor most applications

i

Magnet

Crystal

The small Verdet constant in standard silica fibers isthebottlenecktorealizingall-fiberFaradayisolators

E17157a

• Faradayisolatorsbasedonbulkopticsarelimited in high-power applications

– material/air interfaces, surface reflections, epoxy problem

• Nocommercialall-fiberFaradayrotatorsareavailable

– V = 1.1 rad/(Tm) in silica at 1064 nm

– V = –40 rad/(Tm) in TGG (terbium gallium garnet) at 1064 nm

– for a conventional magnet (B = 0.2 T), 4moffiberisrequiredfor a rotation angle of 45º

– such a long fiber length is impractical in terms of magnet size and polarization mixing

56wt%terbiumisdopedintheSilicafiber to increase the Verdet constant

E17163a

• Core: 56wt%terbium 4-nm diameter

• Cladding: 55wt%terbium 130-nm diameter

• N.A.= 0.14

• Loss= 0.11 dB/cm at 1310 nm

• EffectiveVerdetconstant* –22.3±0.8 rad/(Tm) at 1053 nm 20× larger than silica fiber

*L.Sunet al., Opt. Lett. 34, 1699 (2009).

The magnetic-field distribution of a magnet along the axis direction can be accurately derived

E18297

Br = 0.95 T (residual flux density)a = 15 cm, b = 15 cm, L = 25 cm

The prediction matches the measured magnetic field outside the magnet.

l l

l l

tan

tan

BB

z z a b

ab

z z a b

ab

2 2 4 2

2 2 4 2

zr

2 2 2 1 2

2 2 2 1 2

1

1-- -

r=+ + + +

+ +

-

-

^ ^f

^ ^f

h hp

h hp

9>

9

C

C H

–30–0.4

–0.2

0.0

0.2

0.4

–20 –10 0z (cm)

B (

T)

10 20 30

L

b

aL

z

45º rotation in an all-fiber Faraday rotator can be achieved with 20-cm Tb-doped fiber in the magnet

E18298

• 20-cmTb-doped fiber is placed in the magnet

• Theoreticalmaximumrotation angle >50º

• Meets45ºrequirementfor all-fiber Faraday rotator

–20

–20

0

20

40

60

–10 0

z (cm)

i (

°)

10 20

z = distance between the center of the magnet and the center of the Tb fiber

The fiber polarizers are made from Corning SP1060 polarizing fiber (PZ fiber)

E18299

• Core:8(2.6) nm diameter

• Cladding:125nm diameter

• N.A.= 0.14

• Loss= 0.1 dB/m at 1060 nm

Core Airhole

Cladding

Only one linear polarization can propagate.

Transmission spectrum of orthogonal polarizations in PZ fiber shows an extinction ratio >15 dB

E18300

• Extinctionratio>15 dB

• Bandwidth= 25 nm

• Sufficientextinction for all-fiber polarizers

0

–10

–201040 1060 10701050

Wavelength (nm)

Rel

ativ

e p

ow

er (

dB

m)

30-cm section of fiber

Vertical

Horizontal

1080

An all-fiber Faraday isolator is built by combining a fiber Faraday rotator and fiber polarizers

E17937

• TwosectionsofthePZfiberarealignedthroughtheTb-fiberwith a 45º rotation angle and fusion spliced

• EachsectionofthePZfiberis1m(extinction ratio >17 dB at 1053 nm)

• Thepositionofthemagnetisadjustedtoobtain45ºpolarizationrotation

Linearpolarizedlight

Magnet

Polarizingfiber

PolarizingfiberTb fiber

The extinction ratio of the isolator is measured tobe17dBforallback-injectedpolarizationstates

E18301

• Theisolationismeasuredwhilerotatingthepolarizationdirection oftheback-injectedlight

20

16

12

Iso

lati

on

8

4

00 20 40

Rotation angle (°)

Isolation = 17.0±0.4 dB

Powermeter

All-fiberisolator

Polarization-rotatable light

60 80

Therequiredmagnetdimensionscanbereduced to a few centimeters

E17162a

With cm-scale magnets, all-fiber Faraday devices become practical.

10 rad/Tm

15 rad/Tm

a2 (cm)

L (c

m)

00

4

8

12

16

20

2 4 6 8 10

20 rad/Tm

V = 40 rad/Tm

Magnet

L

a2

z

a1

If V = 20 rad/Tm, then a2 = 4.5 cm, L = 7 cm

Assume Br = 1.35 T (N48 NdFeB magnet) a1 = 2.5 mm

All-fiber Faraday isolators have advantages overbulk-optics–basedisolators

E18302

• All-fiberisolatorshavelowcostcomparedwith bulk-opticsisolators

– Automated, pre-programmed fiber splicing instead of manual alignment

• All-fiberisolatorsaresuitableforhigh-powerapplications – no material/air interfaces, no reflection surfaces, no epoxy

• All-fiberisolatorsareidealforlarge-scaleintegration – hundreds of fiber isolators can be packagedinasinglemagnet Magnet

Fiberbundle

An all-fiber optical Faraday isolator is demonstrated for the first time with 17-dB isolation

E18296

• All-fiberisolatorsarerequiredforhigh-powerapplications

• Theall-fiberisolatorismadeofafiberFaradayrotator and two pieces of polarizing fiber

• ThefiberFaradayrotatorusesa56wt%terbium-dopedmulticomponentsilica fiber

– effective Verdet constant = –22.3±0.8 rad/(Tm)

• CorningSP1060fiberisusedasthepolarizingfiber – extinction ratio >17 dB at 1053 nm for 1-m-long PZ

• 17-dBisolationismeasuredforallback-injectedpolarizationstates

Summary/Conclusions