Low-power (mW) nonlinearities of polarization
maintaining fibers
HANIEH AFKHAMIARDAKANI,
1
LUKE HORSTMAN,
1
LADAN ARISSIAN,
1,2
AND JEAN-CLAUDE DIELS
1,
*
1
University of New Mexico, CHTM 1313 Goddard SE, Albuquerque, New Mexico 87106, USA
2
Physics Department, University of Ottawa, Canada
*Corresponding author: jcdiels@unm.edu
Received 17 July 2019; revised 16 September 2019; accepted 23 September 2019; posted 1 October 2019 (Doc. ID 372988);
published 12 November 2019
For space missions, there is a need for fiber lasers of minimum power consumption involving stabilized freq uency
combs. We exploit the extreme sensitivity of the polarization state of circularly polarized light sent through
polarization-maintaining (PM) fibers to power and temperature variations. Low-power nonlinear transmission
is demonstrated by terminating a PM fiber by an appropriately oriented polarizer. The strong correlation between
the power sensitivity of the polarization state and the temperature dependence of the birefringence of the PM
fiber can be exploited for optical length stabilization in fiber lasers and interferometers.
© 2019 Chinese Laser
Press
https://doi.org/10.1364/PRJ.7.001386
1. INTRODUCTION
Daniel Colladon initiated the field of optical waveguiding by
demonstrating the possibility of guiding light through a curved
stream of water in 1841. In 1953, image transmission through
the first fiberscope was demonstrated [1]. Today, optical fiber
has become an integral part of many fields, including telecom-
munications [2], medicine [ 3], and metrology [4].
Fiber sensors can, in principle, provide the same quality fea-
tures as their free-space counterparts while being cheaper, more
compact, and easier to use. Passive fiber sensors are usually
implemented as Sagnac interferometers [5,6], Michelson inter-
ferometers [7], Fabry–Perot interferometers [8], or microfibers
[9] to measure magnetic fields, strain, torsion, and temperature.
These sensors monitor the phase shift or spectral changes of
transmitted broadband light [8,10].
It is shown here that simply monitoring the polarization
of initially circularly polarized light transmitted through a
polarization-maintaining (PM) fiber leads not only to new sens-
ing methods, but also to power control, saturable absorption,
and the possibility of optical path stabilization. Even at peak
power levels not exceeding a few mW, nonlinear transmission
is detected, with time constants in the microsecond range. All
effects related to the Kerr nonlinear index can be neglected in
the range of powers considered here.
Single-mode (SM) fibers exhibit some birefringence, typi-
cally stress-induced, such that the polarization of a beam sent
through the fiber varies with the positioning and bending of
the fiber. This effect has been exploited for generating short
pulses through polarization mode-locking [11]. PM fibers were
introduced to maintain linear polarization along a preferred
axis. As a very high-order waveplate, it is designed with differ-
ent indices of refraction along two orthogonal axes (the “slow
axis” for the higher index, the “fast axis” along the direction of
lower index). One defines the “beat length” (typically a few
millimeters) as the distance over which the retardation between
slow and fast light equals 2π. Any input polarization other than
linear (along a principal axis) will be periodically modified
along the fiber. While the beat length is considered to be a con-
stant, small variations can take place because of environmental
conditions (temperature, stress, magnetic field) or power var-
iations of the propagating light. By accumulating beat length
variations over long distances, we demonstrate extreme sensi-
tivity in the measureme nt of many parameters affecting the
fiber birefringence.
2. POLARIZATION ELLIPSE MEASUREMENTS
To make a comprehensive determination of the polarization
modification, the polarization ellipse is measured for each value
of a given parameter affecting the beat length. The setup to
measure the change in polarization of the transmitted beam
is shown in Fig. 1. A CW laser diode at 1550 nm generates
linearly polarized light that is made circularly polarized by a
quarter-wave plate at 45° to excite both modes of the PM fiber
independently of the orientation of the input end of the
PM fiber. Fibers of the PANDA type were used, where the
principal axis is defined by “stress rods,” as illustrated in
Fig. 2(a). The transmitted light is collimated and sent through
a polarizer mounted on a LabVIEW-controlled rotational
1386
Vol. 7, No. 12 / December 2019 / Photonics Research
Research Article
2327-9125/19/121386-05 Journal © 2019 Chinese Laser Press