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Definition: specialty optical fibers with strong built-in birefringence, preserving the properly oriented linear polarization of an input beam
Optical fibers always exhibit some degree of birefringence, even if they have a circularly symmetric design, because in practice there is always some amount of mechanical stress or other effect which breaks the symmetry. As a consequence, the polarization of light propagating in the fiber gradually changes in an uncontrolled (and wavelength-dependent) way, which also depends on any bending of the fiber and on its temperature.
This problem can be fixed by using a polarization-maintaining fiber, which is not a fiber without birefringence, but on the contrary a fiber with a strong built-in birefringence (high-birefringence fiber or HIBI fiber, PM fiber). Provided that the polarization of light launched into the fiber is aligned with one of the birefringent axes, this polarization state will be preserved even if the fiber is bent. The physical principle behind this can be understood in terms of coherent mode coupling. The propagation constants of the two polarization modes are different due to the strong birefringence, so that the relative phase of such copropagating modes rapidly drifts away. Therefore, any disturbance along the fiber can effectively couple both modes only if it has a significant spatial Fourier component with a wavenumber which matches the difference of the propagation constants of the two polarization modes. If this difference is large enough, the usual disturbances in the fiber are too slowly varying to do effective mode coupling.
Figure 1: Polarization-maintaining fiber of PANDA type, with built-in stress rods.
A commonly used method for introducing strong birefringence is to include two stress rods of a modified glass composition (with a different degree of thermal expansion) in the preform on opposite sides of the core (PANDA fibers) (Figure 1). Stress-inducing elements with other shapes are also sometimes used, e.g. in bow-tie fibers. When a fiber is drawn from such a preform, the stress rods cause some mechanical stress with a well-defined orientation. Another technique is to use an elliptical core causing so-called form birefringence [1]. In a photonic crystal fiber (PCF), strong birefringence can be obtained with an asymmetric arrangement of air holes, but stress elements (which may be index-matched) can also be used [6]. In any case, the birefringent beat length can be so small (a few millimeters or even less) that additional stress effects can cause only a low level of mixing of the polarization states.
Polarization-maintaining fibers are applied in devices where the polarization state cannot be allowed to drift, e.g. as a result of temperature changes. Examples are fiber interferometers and certain fiber lasers. A disadvantage of using such fibers is that usually an exact alignment of the polarization direction is required, which makes production more cumbersome. Also, propagation losses are higher than for standard fiber, and not all kinds of fibers are easily obtained in polarization-preserving form.
The polarization extinction ratio of light coming out of a polarization-maintaining fiber may be lower than that at the fiber input. This can occur as a result of imperfect alignment of the polarization direction at the input, but also be due to some residual degree of mode mixing. The latter effect can be strongly increased by mechanical stress (e.g. in a fiber connector). For applications requiring a very high polarization extinction ratio (e.g. in interferometry), it can be necessary to use an additional high-quality polarizer after the fiber.
Polarization-maintaining fibers should not be confused with single-polarization fibers, which can guide only light with a certain linear polarization.
Bibliography
| [1] | K. Sano and Y. Fuji, “Polarization transmission characteristics of optical fibers with elliptical cross section”, Electron. Commun. Jpn. 63, 87 (1980) |
| [2] | A. Kumar et al., “Birefringence calculations in elliptical-core optical fibers”, Electron. Lett. 20, 112 (1984) |
| [3] | J. Noda et al., “Polarization-maintaining fibers and their applications”, J. Lightwave Technol. 4 (8), 1071 (1986) |
| [4] | D. Mogilevtsev et al., “Design of polarization-preserving photonic crystal fibres with elliptical pores”, J. Opt. A: Pure Appl. Opt. 3, S141 (2001) |
| [5] | J. R. Folkenberg et al., “Polarization maintaining large mode area photonic crystal fiber”, Opt. Express 12 (5), 956 (2004) |
| [6] | T. Schreiber et al., “Stress-induced single-polarization single-transverse mode photonic crystal fiber with low nonlinearity”, Opt. Express 13 (19), 7621 (2005) |
See also: fibers, single-polarization fibers, single-mode fibers, birefringence, polarization of laser emission, mode coupling, Spotlight article 2007-05-19
Category: fibers and other waveguides
This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics Consulting GmbH. Contact this distinguished expert in laser technology, nonlinear optics and fiber optics, and find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, or staff training) could become very valuable for your business!
