All-Fiber Optical Magnet Field Sensor Based on Faraday Rotation

L. Sun1,2, S. Jiang,3 and J. R. Marciante1,2

University of RochesterLaboratory for Laser Energetics1. Laboratory for Laser Energetics, University of Rochester2. Institute of Optics, University of Rochester3. AdValue Photonics Inc., Tucson, Arizona

OFC 2010 San Diego, CA 21 March 2010

Linearpolarizedlight input

MagnetDetector

PM

PZ

Tb

An all-fiber optical Faraday magnet field sensor is demonstrated using Tb fiber*

E18755

• All-fiber magnet field sensors are required for strong EMI environments

• The all-fiber magnet field sensor is made of a fiber Faraday rotator and a fiber polarizer

• The fiber Faraday rotator uses a 56 wt% terbium-doped multicomponent silicate fiber

• The sensor can measure magnet fields with

– a measurement range from 0.02 to 3.2 T

– resolution of 2 × 10–6 T

– sensitivity of 0.49 rad/T

Summary

*L. Sun, S. Jiang, and J. R. Marciante, Opt. Express 18, 5407 (2010).

All-fiber magnet field sensors are required for strong EMI environments

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• Magnet field sensors are currently electronic based

– superconducting quantum interference devices (SQUID’s),search coils, fluxgates, Hall-effect sensors, etc.

• Electronic devices do not work properly in strong EMI environments

• All-fiber optical magnet field sensors are suitable for strong EMI environments

– no electronics in measurement region

– robust, low weight, small size, remote sensing

– suitable for sensing in nuclear facilities, magnet levitation rail, etc.

Strong EMI environments need all-optical devices.

All-fiber Faraday magnet field sensors have advantages over traditional magnet field sensors

E18757

• Several all-fiber magnet field sensors were demonstrated* using special material coatings, prohibiting low-cost manufacturing

• We developed an all-fiber magnet field sensor basedon Faraday rotation using terbium-doped fiber

PM

SensorTb PZ

PM

Laser

Detector

*H. Okamura, J. Lightwave Technol. 8, 1558 (1990).

The Faraday effect is the rotation of linear polarized light in the presence of a magnetic field

E17156b

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

• The angle of rotation is given by

i = VBL

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

• Faraday rotation is usedfor optical isolation

i

Magnet

Crystal

Example

The small Verdet constant is the bottleneck to realizing Faraday rotation in standard silica fibers

E18758

• Silica fiber has a small verdet constant

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

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

– for 2-cm silica fiber, sensitivity di/dB = 0.022 rad/T

• Silica fiber has been coiled multi-turns to increase the sensitivity1,2

– however, bend-induced linear birefringence affects the state of polarization and quenches the desired Faraday effect

1G. W. Day et al., Opt. Lett. 7, 238 (1982). 2V. Annovazzi-Lodi, S. Merlo, and A. Leona, J. Lightwave Technol. 13, 2349 (1995).

- such a small sensitivity is useless in most applications

56 wt% terbium is doped in the silicate fiber to increase the Verdet constant

E17163b

• Core: 56 wt% terbium, 4-nm diameter

• Cladding: 130-nm diameter

• N. A. = 0.14

• Loss: 0.11 dB/cm at 1310 nm

• Effective Verdet constant* = –24±1 rad/(Tm) at 1053 nm

– 20× larger than silica fiber

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

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

E18299

• Core: 8 (2.6) nm diameter

• Cladding: 125-nm 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

E18300a

• Extinction ratio >15 dB

• Bandwidth = 25 nm

• Sufficient extinction for all-fiber polarizers

0

–10

–201040 1060 10701050

Wavelength (nm)

Rel

ativ

e p

ow

er (

dB

)

30-cm section of fiber

Vertical

Horizontal

1080

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

E18759

BB

a z L

z L

a z L

z L

a z L

z L

a z L

z L

22

2

2

2

2

2

2

2

zr

12 2 1 2

22 2 1 2

12 2 1 2

22 2 1 2

-

--

-

-

-

=+ +

+

+ +

+

++

+

^ ^

^ ^

h h

h h

9 9

9 9

C C

C C

*

4

Br = 1.35 T (residual flux density)

a1 = 2.5 mm, a2 = 30 mm, L = 40 mm

The prediction matches the measured magnetic field outside the tube.

0.8

Magnet

Bz

(T)

z (cm)–10 –5 0 5 10

0.6

0.4

0.2

0.0

–0.2

–0.4

L

a2

z

a1

L

An all-fiber magnet field sensor is built by combining the fiber Faraday rotator and the fiber polarizer

E18766

• 2-cm Tb fiber is spliced with PM fiber and 1-m long PZ fiber

• The magnet field can be measured from the relative light intensity

I/I0 = cos2 (i0 + i) = cos2 (i0 + VBaveL)

• PM and PZ fibers are aligned with 50º rotation angle (i = 50º)

Linearpolarizedlight input

MagnetDetector

PM

PZ

Tb

The data measured by translating the magnet along the fiber axis agrees well with the theoretical curve

E18767

• Sensitivity is 20× larger than silica fiber

• Maximum measureable B

• Resolution

In a self-referenced configuration

0.6

0.5

0.4

0.3

I/I 0

Bav (T)

0.2

0.1–0.2 0.0 0.2 0.4 0.6

maxII

sinB

B2

2

00 r i iD

D=+^ h8 B

d d 0.49 rad/TB VLavi = =

max .3 2 T= =B VL2r^ h

I/I 10 , 2 10 TB6 6#D D= =- -

E18755

Summary/Conclusions

An all-fiber optical Faraday magnet field sensor is demonstrated using Tb fiber*

• All-fiber magnet field sensors are required for strong EMI environments

• The all-fiber magnet field sensor is made of a fiber Faraday rotator and a fiber polarizer

• The fiber Faraday rotator uses a 56 wt% terbium-doped multicomponent silicate fiber

• The sensor can measure magnet fields with

– a measurement range from 0.02 to 3.2 T

– resolution of 2 × 10–6 T

– sensitivity of 0.49 rad/T

*L. Sun, S. Jiang, and J. R. Marciante, Opt. Express 18, 5407 (2010).