Accurate alignment preserves polarization

Accurate alignment preserves polarization
fiberoptic systems
require specialized fiber
Accurate alignment
preserves polarization
and connectors and
careful assembly and
Omur Sezerman and Garland Best
alignment to achieve
optimal performance.
he performance of many fiberoptic components
and systems is affected by the polarization of the
light traveling through the fiber. These include
fiber interferometers and sensors, fiber lasers, and
electro-optic modulators. Many systems also suffer
from polarization-dependent losses that can affect
system performance. Understanding how to control the polarization of light in a fiberoptic system and how to
properly use polarization-maintaining (PM) components is
vital for successful results.
Polarized light can be classified as linearly polarized, elliptically polarized, or circularly polarized (see Fig. 1). The simplest form of polarized light is linearly polarized light, in
which the electric field oscillates in a single plane of vibration.
In theory, one can generate perfectly linearly polarized light,
but in practice this is not the case.
The polarization-extinction ratio gives a measure of the portion of the beam that is linearly polarized along a single axis.
The extinction ratio can be measured by directing a beam
through a polarizer mounted on a rotation stage and onto a
detector. Light polarized parallel to the transmission axis of
the polarizer will pass through to the detector, whereas light
polarized orthogonal to the transmission axis will be blocked.
The output signal registered by the detector will vary in intensity as the polarizer is rotated, and the extinction ratio is
defined as
ER = 10 log (Pmax/Pmin)
where Pmin and Pmax are the measured maximum and minimum signal intensities in milliwatts.
Polarization and fiberoptics
Bending or twisting optical fiber induces stresses in the materOMUR SEZERMAN is CEO and GARLAND BEST is vice president of engineering at Oz Optics Ltd., Unit #219 Westbrook Rd., West Carleton Industrial
Park, Carp, Ontario, Canada K0A 1L0.
Polarization-maintaining connectors feature a positioning key aligned to
the slow axis of the fiber. The key permits the connector to be mated only
with another connector or component at a single angular orientation.
ial that in turn cause phase changes in the polarization state of
the light traveling through the fiber. If the fiber is subjected to
external perturbations such as changes in position or temperature, the polarization of the output beam will vary as a function of time. This is true for even short lengths of fiber and is
undesirable in many applications such as sensing and
telecommunications, which require a constant output polarization from the fiber.
To address variations in the output beam, manufacturers
have developed PM fibers. In PM fiber, light polarized along
one axis of the fiber travels at a different rate than light polarized orthogonal to that axis. This birefringent behavior creates
two principal transmission axes within the fiber, known as the
fast and slow axes. If the light input to a PM fiber is linearly
polarized and oriented along either the fast or the slow axis,
the fiber output will remain linearly polarized and aligned
with that axis, even when the fiber is subjected to external
stresses. A 1-m-long connectorized patchcord constructed
with PM fiber, for example, can typically maintain polarization to at least 30 dB at 1550 nm.
A variety of polarization-preserving core/cladding structures are currently used in the industry (see Fig. 2). The
Reprinted from the December 1997 edition of LASER FOCUS WORLD
Copyright 1997 by PennWell
ment than for the incodashed lines in the drawy
herent source when the
ings show the slow axis
fiber is flexed, even
within each structure.
when the fiber is ideally
More recently, manufacturx
ers have developed polarAn ideal source for use
izing fibers, which only
transmit light polarized
applications would be
along the transmission axis
one that is polarized to at
of the fiber.
The ability of a PM fiber FIGURE 1. In polarized light, the electric-field oscillation remains in a single plane least 40 dB and with a
to maintain polarization (linear polarization, left), rotates azimuthally while changing magnitude (ellipti- linewidth on the order of
tens of nanometers. Such
depends on the conditions cal polarization, middle), or rotates azimuthally while maintaining a constant
a source does not prounder which light is magnitude (circular polarization, right).
duce coherence effects—
launched into the fiber. Perinterference effects between the light on
haps the most important factor is the ratio reaches a maximum value.
The type of source used can affect the slow axis and the fast axis that create
alignment of the polarization axis of the
light with the slow axis of the fiber. For the measurement. Incoherent sources variations in the Pmax and Pmin meaa perfectly polarized input beam, mis- tend to produce a static, averaged mea- surements and can falsely influence
aligned by an angle θ with respect to the surement of the extinction ratio. Such extinction ratio measurements. Most
slow axis of an ideal fiber, the extinction sources are often used by manufactur- lasers, including diode lasers, are polarers in testing assembled fiber patch- ized to only about 25 dB and have a
ratio is
cords, for example, to characterize per- linewidth of about 1 nm, so coherence
formance. If the product is intended for effects come into play during the alignER ≤ 10 log (tan2 θ)
use with a coherent source, however, ment process.
To eliminate measurement inaccuraTo achieve an output extinction ratio such measurements can be misleading.
greater than 20 dB, the angular mis- Measurements taken of the same fiber cies caused by coherence effects, the
alignment must be less than 6°; to aligned to a coherent source often will fiber must be stressed during alignment.
achieve an extinction ratio of approxi- produce a lower worst-case measure- Mechanical stress induces a phase
change in the light along
mately 30 dB, the angular
the slow axis of the fiber,
misalignment must be less
changing the interference
than 1.8°.
between the slow axis and
the fast axis. The point of
Fiber-to-source alignment
the test is to seek the worstMeeting such strict tolercase result. Failure to do so
ances requires system intecan result in wildly optigrators to use careful alignmistic results.
ment techniques. The typical
Elliptical core
D-shaped elliptical core
The fiber should be bent
alignment configuration conslightly, or wrapped
sists of the laser, coupling
around a mandrel with a
optics, the PM fiber, a polarparts
diameter of at least 50 mm,
izer mounted on a rotary
and the minimum output
platform, and a fiber link to
power through the polarizan optical power meter (see
er measured. Ideally there
Fig. 3). The fiber-to-source
should be no change or
alignment should first be
Elliptical stressed
Rectangular stressed
even a slight decrease in the
adjusted to optimize coucladding
minimum output power; it
pling efficiency. Next, the
is common, however, for
maximum and minimum
this value to increase slightfiber output through the
ly as a result of changing
polarizer should be meainterference effects. If the
sured with an optical power
increase is only one or two
meter, and extinction ratio
decibels, then the polarizacalculated. The fiber connecBow tie
tion axis is correctly
tor should be rotated by a
aligned. If the increase is
small increment, and the
larger, the connector should
measurement process repeat- FIGURE 2. Polarization-maintaining fibers feature a variety of core and
be readjusted.
ed. This should be iterated cladding designs. Stress-applying parts within the fiber structure impart
The best extinction ratio
until the measured extinction mechanical stress to the material to differentiate it into fast and slow axes.
that can be achieved depends on how
well the fiber can be positioned with
respect to the laser. Extinction ratios of
20 dB can easily be achieved by manually rotating the connector. If the connector is positioned with a precision rotary
stage, extinction ratios of 25 to 30 dB
are possible.
For product-testing purposes only, an
alternate method for aligning the polarization axis is to place a half-wave plate
on a rotary stage between the source
and the optics that launch light into the
fiber. The connector remains stationary,
and the half-wave plate is rotated to
align the polarization axis of the beam
with the fiber. Extinction ratios of 35 dB
or better can be realized with this technique.
Polarization-maintaining connectors
Input optics
fiber under
Rotary platform
with polarizer
FIGURE 3. To achieve optimal source-to-fiber alignment in a polarization-maintaining system, the
output from the PM fiber is passed through a polarizer and monitored by a power meter. The fiber
is rotated to determine the angular alignment that provides best contrast between the maximum
output power (polarizer oriented to transmit majority of beam) and minimum output power
(polarizer oriented to extinguish majority of beam).
that by industry convention is aligned
to the slow axis of the fiber. The key
restricts the connector to mating with
other components or connectors at a single angular position, preserving the
angular orientation of the fiber.
The tolerances between the key and
keyway on standard FC connectors are
too loose to accurately maintain angular alignment, so manufacturers have
tightened the key dimension tolerances
on PM connectors, based on FC anglepolished connector (APC) standards.
The polarization-extinction ratio can be
degraded by any stresses or microbends
in the connectors, or by external optical
components that do not maintain polarization properly. Special termination
procedures, stress-free glues, and topquality lenses and optics must be used
to minimize these
stresses and thus
Connector-key tolerances
maintain the highest
possible extinction
R (reduced)
Given the importance of the alignment
OZ Optics
of the PM axis across
a connection, the
choice of connector is
critical. The most
common type of PM
connector in use is a
variation of the NTTFC style connector.
Such connectors have
a positioning key—a
narrow raised region
Output optics
Seikoh Gieken
Diamond SA
N (wide)
Seiko Instruments
Diamond SA
Two APC standards currently exist for
components on the market: a narrow,
or reduced-key design, and a wide-key
design (see table). The two dimensions
are incompatible with one another, so
users should know beforehand the
design being used. Most manufacturers
will offer the alternative standard as an
option, so be sure to ask before you
purchase a connector.
To help distinguish polarizationmaintaining connectors from singlemode connectors, most manufacturers
now use a blue strain-relief boot, or
add a blue dot or stripe to a standard
boot. A proposal has been made to also
identify the connector key width by
engraving notches on the key and keyway. A single notch would identify a
narrow key, while a double notch
would identify a wide key.
Although working with polarizationmaintaining fibers and patchcords may
seem difficult, in reality it is not. All that
is needed is a little attention to detail
and some inexpensive equipment. With
the trend toward common standards
and the increasing range of applications,
polarization-maintaining fiberoptics
faces a bright future.
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