All-Fiber Optical Magnet Field Sensor Based on Faraday Rotation

Transcription

1 All-Fiber Optical Magnet Field Sensor Based on Faraday Rotation Linear polarized light input PM Magnet Tb Detector PZ L. Sun 1,2, S. Jiang, 3 and J. R. Marciante 1,2 University of Rochester Laboratory for Laser Energetics 1. Laboratory for Laser Energetics, University of Rochester 2. Institute of Optics, University of Rochester 3. AdValue Photonics Inc., Tucson, Arizona OFC 2010 San Diego, CA 21 March 2010

2 Summary 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 T sensitivity of 0.49 rad/t E18755 *L. Sun, S. Jiang, and J. R. Marciante, Opt. Express 18, 5407 (2010).

3 All-fiber magnet field sensors are required for strong EMI environments 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. E18756

4 All-fiber Faraday magnet field sensors have advantages over traditional magnet field sensors Several all-fiber magnet field sensors were demonstrated * using special material coatings, prohibiting low-cost manufacturing We developed an all-fiber magnet field sensor based on Faraday rotation using terbium-doped fiber Laser PM Sensor Tb PZ Detector PM E18757 * H. Okamura, J. Lightwave Technol. 8, 1558 (1990).

5 The Faraday effect is the rotation of linear polarized light in the presence of a magnetic field 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 used for optical isolation Example Magnet Crystal i E17156b

6 The small Verdet constant is the bottleneck to realizing Faraday rotation in standard silica fibers 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 = rad/t - such a small sensitivity is useless in most applications Silica fiber has been coiled multi-turns to increase the sensitivity 1,2 however, bend-induced linear birefringence affects the state of polarization and quenches the desired Faraday effect E G. W. Day et al., Opt. Lett. 7, 238 (1982). 2 V. Annovazzi-Lodi, S. Merlo, and A. Leona, J. Lightwave Technol. 13, 2349 (1995).

7 56 wt% terbium is doped in the silicate fiber to increase the Verdet constant 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 E17163b *L. Sun et al., Opt. Lett. 34, 1699 (2009).

8 The fiber polarizers are made from Corning SP1060 polarizing fiber (PZ fiber) Only one linear polarization can propagate. Core Airhole Cladding Core: 8 (2.6) nm diameter Cladding: 125-nm diameter N. A. = 0.14 Loss = 0.1 db/m at 1060 nm E18299

9 Transmission spectrum of orthogonal polarizations in PZ fiber shows an extinction ratio >15 db 0 30-cm section of fiber Horizontal Relative power (db) 10 Vertical Extinction ratio >15 db Bandwidth = 25 nm Sufficient extinction for all-fiber polarizers Wavelength (nm) 1080 E18300a

10 The magnet-field distribution of a magnet tube along the axis direction can be accurately derived a 2 Magnet a L L z B z Br = 2 * z + L 2 z + L 2 a1 2 z L ^ + h C 9a2 2 + ^z + L 2h 2 C 1 2 z - L 2 z - L 2 - a1 2 z L ^ - h C 9a2 2 + ^z - L 2h 2 C B z (T) B r = 1.35 T (residual flux density) a 1 = 2.5 mm, a 2 = 30 mm, L = 40 mm E z (cm) The prediction matches the measured magnetic field outside the tube.

11 An all-fiber magnet field sensor is built by combining the fiber Faraday rotator and the fiber polarizer Linear polarized light input PM Magnet Tb Detector PZ 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/I 0 = cos 2 (i 0 + i) = cos 2 (i 0 + VB ave L) PM and PZ fibers are aligned with 50º rotation angle (i = 50º) E18766

12 The data measured by translating the magnet along the fiber axis agrees well with the theoretical curve I/I Sensitivity is 20 larger than silica fiber di db = VL = 0.49 rad/t av Maximum measureable B B max ^r 2h VL. = = 3 2 T 0.2 Resolution 0.1 DI 2Bmax DB = I0 r sin82^i0 + ihb B av (T) In a self-referenced configuration -6-6 DI/I = 10, DB = 2 # 10 T E18767

13 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 T sensitivity of 0.49 rad/t E18755 *L. Sun, S. Jiang, and J. R. Marciante, Opt. Express 18, 5407 (2010).