Beat frequency dependence of the sensitivity for Faraday-rotation
based heterodyning fiber laser magnetic field sensor
Nengpeng Hu, Linghao Cheng
*
, Li Yu, Yizhi Liang, Hao Liang, Bai-Ou Guan
Institute of Photonics Technology, Jinan University, Guangzhou, P. R. China, 510632
ABSTRACT
The beat frequency dependence of the sensitivity for a Faraday-rotation based heterodyning fiber laser magnetic field
sensor is studied, which shows that lower beat frequency results in higher sensitivity. By lowering the beat frequency to
2 MHz, the maximum sensitivity of about 43 Hz/μT to magnetic field has been achieved for a heterodyning fiber laser
inscribed on an Erbium doped fiber. It also shows that the beat frequency is dependent on the polarization of the 980 nm
pump. Therefore, dynamical tuning of the sensitivity optically has also been demonstrated by tuning the polarization of
the 980 nm pump.
Keywords: Fiber laser sensors, magnetic field sensors, Faraday rotation effect
1. INTRODUCTION
Magnetic field sensors are widely used in various fields, such as navigation, current measurements, spatial and
geophysical exploration. All-fiber magnetic field sensors are highly desired due to their advantages in immunity to
electromagnetic interference, light weight, compact size and large bandwidth[1-4]. Various mechanisms can be
employed to implement all-fiber magnetic field sensors, such Faraday rotation effect, magnetostrictive effect[5],
magnetic force, Lorentzian force, and magneto-fluid materials[6], among which Faraday effect can measure magnetic
fields directly and are hence very popular in implementations[7]. We have report a novel all-fiber magnetic field sensor
based on a heterodyning fiber grating laser and Faraday rotation effect in a very compact package[1]. A magnetic field
can introduce a circular birefringence into the fiber laser cavity, which combines with the intrinsic linear birefringence of
the laser cavity to form an elliptical birefringence. This elliptical birefringence then results in two orthogonally polarized
laser outputs of the fiber laser which can be converted to a beat note after photodetection with the beat note frequency
proportional to the elliptical birefringence[8]. By measuring the beat note frequency with and without magnetic field, the
elliptical and the linear birefringence can then be deduced and therefore the strength of the magnetic field can be
calculated.
We have also shown that the sensitivity of the Faraday-rotation based heterodyning fiber laser magnetic field sensor
can be enhanced by lowering the intrinsic linear birefringence[9]. Because the beat note frequency without magnetic
field is proportional to the linear birefringence, the sensitivity of the fiber laser magnetic field sensor is also beat note
frequency dependent. In this paper, we push the sensitivity to the limit by lowering the beat note frequency as low as
possible. For the fiber laser used in our experiment, which is inscribed on an Erbium doped fiber, the maximum
sensitivity of about 43 Hz/μT to magnetic field has been achieved by lowering the beat note frequency without magnetic
field to 2 MHz. We also show that the beat note frequency without magnetic field is pump polarization dependent
because the population distribution among various energy levels of Erbium ions is pump polarization dependent.
Therefore, by tuning the polarization of the 980 nm pump of the fiber laser, we demonstrate that the sensitivity of the
magnetic field sensor can be tuned optically and dynamically.
2. EXPERIMENT RESULTS AND ANALYSIS
The fiber grating laser used for experiments is a dual-polarization fiber laser inscribed by an excimer laser at 193 nm on
an Erbium-doped fiber (EDF, Fibercore M-12) with grating lengths of 7 mm and 6 mm and a spacing of 6 mm. The EDF
shows an absorption coefficient of 11.3 dB / m at 979 nm. Pumped by a 974 nm source, the fabricated fiber laser emits
*
chenglh@ieee.org; phone +86-20-85222046; fax +86-20-85222046;
Proc. of SPIE Vol. 9655 96552X-1