Spun fibers are optical fibers (usually single-mode silica fibers) where the fiber has been twisted during the fabrication (drawing) process. Such a procedure is applied in order to obtain special properties concerning the effect on the polarization of light sent through the fibers; the results are substantially different from those obtained with standard fibers or with conventional polarization-maintaining fibers.
Techniques for Making Spun Fibers
The originally proposed technique is based on the steady rotation of the fiber preform during the fiber drawing process . Usually, the preform is rotated at a constant rate, which in combination with a constant pulling speed leads to a certain constant spin rate. For example, one may obtain a spin rate of 3 m−1 (i.e., 3 turns per meter) if the pulling speed is 1 m/s and the preform rotates 3 times per second (i.e., with 180 rpm).
The rotating preform method is well suited for realizing unidirectional spinning with high accuracy. However, high spin rates are hard to obtain in combination with high pulling speeds, as are desirable for telecom fibers, because the preform would then have to be rotated very rapidly.
An alternative approach is to leave the preform fixed but twist the fiber with a suitable kind of spin device , which e.g. may be placed in the fiber drawing tower just before the fiber winding mechanism. The latter may have to be optimized such that the fiber is not twisted on the spool, because that could be disturbing for the application.
The twisting can be achieved, for example, by feeding the fiber through a small gap between two rotating wheels. Those wheels are pushed against the fiber and can be moved in opposite directions along their rotation axes, usually in an oscillating fashion, such that they twist the fiber in a time-dependent way. A second pair of such rotating wheels may be used to avoid twisting of the fiber on the spool.
Because the rotating wheels can be relatively small and lightweight, one can realize high spin rates even in conjunction with high pulling speeds. However, a fundamental limitation of this technique is that uniform twisting is not possible; the wheels can be moved only to a limited extent and then need to be moved back. So the technique is suitable for periodic spinning patterns, e.g. of sinusoidal, triangular or trapezoidal shape, but not for unidirectional spinning. One may also use non-periodic patterns, e.g. with a “frequency modulation”, but in any case with a limited excursion of the wheel movement.
It turned out that various kinds of spin patterns which the twisted-fiber method allows are well suited for low-PMD telecom fibers (see below).
Effects of the Spinning
The spinning of the preform during the fiber drawing effectively leads to a modified distribution of the glass material in the fiber. That is the essential effect of the spinning concerning applications.
Only to a quite limited extent, it also leads to mechanical (torsional) stress inside the fiber, since the spinning affects the fiber mostly in a hot state where the viscosity is not yet very high. Therefore, spun fibers do not exhibit substantial polarization effects based on mechanical stress through photoelastic effects, and indeed these are often neglected in calculations of polarization evolution. In that respect, such fibers are substantially different from fibers which are twisted in the cold state, where stress effects are substantial.
Polarization Evolution in Spun Fibers
The calculation of the polarization evolution in spun fibers is far more complicated than for ordinary polarization-maintaining fibers, for example – particularly when random birefringence must be taken into account and statistical properties must be evaluated. This is the case in a telecom context, for example.
For a birefringence axis of the fiber with constant orientation, as in a standard high-birefringence polarization-maintaining fiber, the Stokes vector (describing the state of polarization in the Poincaré sphere) would exhibit a precession around the birefringence axis. With sufficiently fast spinning, however, the birefringence axis rotates so fast that within any length of fiber where it stays approximately constant there is only a very small angle of precession of the polarization vector. As a result of that, the precession is largely suppressed, and the polarization vector exhibits only a fast oscillation of small amplitude, and in addition possibly a slow drift.
Mathematically, one can treat the situation with a simplified model, neglecting circular birefringence of the fiber, and calculating an equivalent birefringence vector (for a given spin profile) which does not reproduce the mentioned low-amplitude fast oscillation. The equivalent birefringence vector can contain stochastic quantities, describing the randomness of the birefringence. (Curiously, it generally has a deterministic circular birefringence component, although no circular real birefringence of the fiber is assumed.) With those mathematical means, one can obtain relatively simple analytical results (although not without non-trivial calculations) for typical cases of interest . One can, for example, calculate to which extent the birefringence is suppressed by the spinning, compared with an unspun fiber.
Other models can also be used, for example models based on mode coupling theory. These are particularly useful for nominally non-birefringent fibers, where a weak coupling approximation is usually well fulfilled.
Numerical simulation models are often used for simulating the effects of polarization evolution in fibers with random birefringence. Analytical results are available only for a limited set of situations, but can be rather useful e.g. for identifying certain general relations.
Different spin profiles can be considered. For example, one may simply have unidirectional spinning with a constant rotation rate, as is typically obtained with the rotating preform method. Other possibilities are a triangular spin profile, where the direction of rotation is regularly reversed, and triangular, sinusoidal or trapezoidal spin profiles. In some cases, non-periodic profiles are applied, e.g. frequency-modulated patterns as a modified form of sinusoidal patterns.
Fibers From Nominally Non-birefringent Preforms
Telecom fibers, for example, are made from an ordinary fiber preform not having any intentionally introduced birefringence, but only some random birefringence at a moderate level. Without spinning, the fibers would also exhibit only some random birefringence, which however is sufficient to cause problems – for example, in optical fiber communications it leads to polarization mode dispersion (PMD) which can be a limiting factor for the achievable transmission capacity, particularly for large transmission distances. By applying an optimized spin pattern in the fabrication, the effective birefringence can be reduced by a large amount, even though the local birefringence is similarly strong as without spinning. Essentially, one obtains a kind of averaging effect.
In mathematical terms, this can be described as a reduction of the birefringence correlation length. Effectively, this brings the fiber substantially closer to an ideal fiber without any birefringence, i.e., an ideal isotropic waveguide. In that limit case, the fiber would be ideally polarization-preserving – not even for a certain linear input polarization state, but for any input polarization state (e.g. with circular polarization).
For telecom fibers, it is not sufficient in practice to only consider the reduction of polarization mode dispersion against a fiber with some constant level of birefringence, or even a fiber with random birefringence. This is because another important aspect is the sensitivity to additional effects such as macro- or microbending and mechanical stress due to temperature changes. Different spin patterns can differ substantially in terms of the resulting sensitivity to such effects. Also, it matters whether the polarization evolution over a short length of fiber (e.g. in some fiber-optic sensor) or only over long lengths (e.g. in telecom applications) is relevant. Overall, the optimization of telecom fibers, for example, is a rather complex issue, and it took years to identify all aspects which are relevant for PMD, not just to optimize the required fiber fabrication machinery.
Fibers From Birefringent Preforms
Some fibers are spun from preforms with an intentionally introduced birefringence, e.g. with a PANDA or bow-tie design – just as used for fabricating standard (unspun) polarization-maintaining fibers. Without spinning the preform during fiber drawing, one obtains a fiber with a substantial amount of birefringence. The polarization of light is then well preserved during propagation under the condition that the input light is linearly polarized along one of the two axes of the fiber. However, other states of polarization – e.g. with circular or elliptical polarization, or linear polarization with a different initial direction – are not preserved in such fibers.
This behavior is substantially changed when strong spinning of the preform is applied. More specifically, the spinning must be so strong that the spin period (corresponding to one full turn of the preform) is much smaller than the polarization beat length. One again obtains an effective birefringence which is very weak, despite substantial local birefringence. Not only a suitable linear polarization state is preserved, but also other states, for example of circularly polarized light.
A substantial advantage of spun high-birefringence fiber (compared with spun low-birefringence fiber) is that one obtains a substantially lower sensitivity to stress-induced birefringence, related to vibrations or thermal drifts. One thus obtains substantially better performance for example in parametric sensors (see below). However, HiBi fibers are significantly more expensive, and also some of them exhibit higher propagation losses.
Spun Photonic Crystal Fibers
It is possible to utilize the spinning of the fiber also in the fabrication of photonic crystal fibers, containing tiny air holes. Those air holes then describe a helical path for unidirectional spinning, for example. One can also use an offset core (i.e., a fiber core not sitting in the center of the preform), which then also follows a helical path. Such designs again have special polarization properties , which substantially differ from those of all-glass fibers.
Applications of Spun Fibers
The special polarization properties of spun optical fibers are of interest in different areas of application:
- In optical fiber communications, low polarization mode dispersion (PMD) is of interest when realizing very high transmission capacity in long fiber-optic links. Therefore, telecom fibers for high data rates are spun fibers; this technology was already introduced in the 1990s.
- Spun fibers are also used for polarimetric fiber-optic sensors. For example, there are fiber-optic current sensors [3, 12], where the Faraday effect is exploited and influences of fiber birefringence would be disturbing. Such sensors can be used, for example, for sensing high currents at high-voltage transmission lines, where high electrical insulation against ground potential is required. Both AC and DC currents can be measured.
Spun telecom fibers are routinely produced by fiber manufacturers which are established in that application area. Spun fibers for special applications like fiber-optic sensors, however, are often made by smaller manufacturers focusing on such niche markets.
Applications of spun photonic crystal fibers appear not yet have to be established at least in the commercial sector.
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