Fiber-Optic Sensors - Applied Optics Fundamentals and Device Applications - page 163

Single-mode connectors are utilized for optimum performance and align-
ment tolerances in connectorization of fiber to fiber. Any alternative bulk
optics approach employing beam splitter-lens combinations to split and
recombine optical powers in the interferometer will surely require excessive
space and alignment tolerance ruggedization.
The guided-wave equivalent of the bulk optic beam splitter is the two-
fiber four-port coupler. These couplers are based on the fact that when two
fiber cores are brought sufficiently close to one other, their modes become
coupled through the evanescent fields. This results in a reciprocal power
transfer from one fiber to the other, with the power transfer ratio depending
on the core spacing and interaction length. A wide range of fabrication pro-
cedures have been developed; however, one technique appears to be superior
[15]. This process employs the fused biconical taper technology, in which
two fibers are twisted around one other, flame heated, and fused, while the
two stages of the fusion station are moved apart. The biconical taper brings
the two fiber cores close to one other but simultaneously decreases the core
diameters considerably, which increases the mode spot radius, both of which
contribute to strong coupling through the evanescent tails of the guided
modes. This technique provides a very rugged power divider without fiber
material removal. Several forms of commercial devices are available.
Fiber-to-fiber connections can be either permanent or semipermanent
joints achieved by bonding or thermal fusion, or detachable joints using
demountable connectors. The first type provides higher reliability and sta-
bility under field conditions; but from the viewpoint of component inter-
changeability and testing during system development, it is more reasonable
to use demountable connectors for coupling the optical source pigtail to the
splitter, and the splitter and combiner to the sensing and reference fibers.
Many conventional connector manufacturers offer unique fiber-optic
designs. Because of the small core diameter of single-mode fiber, the position-
ing accuracy of the connector becomes a challenge, in that the attainment of
a coupling loss smaller than 1 dB requires the fiber end-to-end separation be
maintained below about 30 μm, the core-to-core lateral offset below 1-1.5 μm,
and the axis-to-axis tilt angle below 1° (figures are for a 1300 nm operat-
ing wavelength and will be slightly less stringent at 850 nm) [16]. The gen-
eral principle of the field assembly connector is to use precision-machined
ceramic ferrules, with the fiber inserted and epoxied into the central hole,
and the ferrules then assembled into a coupling nut. Additional active
fiber-positioning techniques can be employed to provide further alignment
optimization. A final consideration in the coupling of single-mode polariza-
tion-preserving fibers is the potential polarization coupling between one of
the two fundamental HE mode polarization states due to alignment error
between the main polarization planes of the two joined fibers. This effect is
proportional to sin 2 θ, where θ is the rotation angle [17]. This coupling can be
minimized with proper design.