Polarization preserving optical fibre

Details Polarization preserving optical fibre Polarization preserving optical fibre

2001-09-28

2003-04-01

Palmer, Phan T. H. (Department: 2874)

Optical waveguides

Optical fiber waveguide with cladding

C385S124000, C385S125000

Reexamination Certificate

active

06542681

ABSTRACT:

BACKGROUND OF THE INVENTION
Optical fibres of this type (which may generally be referred to as micro-structured fibres) have been described in a number of references, such as WO 99/64903 and WO 99/64904 describing such fibres having claddings defining Photonic Band Gap (PBG) structures, and U.S. Pat. No. 5,802,236, and Monro et al. (see Optics Letters, Vol 25 (4), p.206, February 2000) defining fibres where the light is transmitted using modified Total Internal Reflection (TIR).
Optical fibres and integrated optical wave guides are today applied in a wide range of applications within the areas such as optical communications, sensor technology, spectroscopy, and medicine. The performance of most such applications is affected by or dependent upon the polarization of the light travelling through the fibre. Many systems also suffer from polarization-dependent losses or polarization-dependent propagation properties that can affect system performance. Design of polarization controlling devices in fibre optical systems is therefore vital. The present invention includes design of novel types of polarization controlling optical fibres, for which the polanzation of light coupled to one end of fibre can be preserved at the output end—so-called polarization maintaining fibres.
Light propagating through an optical fibre can be classified as linearly polarised, elliptically polarised and circularly polarised. If the optical fibre has a deviation away from a perfect circular symmetric design (as is the case for optical fibres in practice), this polarization will generally cause the light to split into separate polarization states (two states for the fundamental mode of optical fibres), which will travel at different speeds along the fibre.
For certain applications it is desirable to enhanced such deviations—and create high-birefringent fibres—as it is e.g., the case for polarization-maintaining fibres (see Agrawal, “Non-linear fiber optics”,Academic Press, Second ed., 1995). For other applications, however, even small deviations may be strongly degrading to the system performance. In particular, for high-speed, long-distance optical communication systems (operating at bit-rates of 10 GHz and beyond), an increased attention is now being paid to polarization effects. For such fast systems even very low degrees of birefringence (<10
−7
) occurring in conventional fibres may cause a crucial dispersion differences between the two polarization states, thereby setting upper limits for transmission speed. To explore the potential of PBG-fibres, it is, therefore, important to predict, understand and tailor the polarization effects, which may result in optical fibres.
BRIEF SUMMARY OF THE INVENTION
The present inventors have realised how to provide a significantly higher degree of freedom for tailoring the polarization properties of micro-structured fibres compared to traditional fibres The present inventors have realised that, in fact, the structure of the cladding elements may act in defining the polarization of light transmitted by the fibre. This type of fibre is realised by providing cladding elements, which are not symmetrical around a centre in the core of the fibre. In this manner, the extent of the cladding elements will differ around radial directions seen from the core(s).
In a first aspect, the invention relates to an optical fibre with a waveguide structure having a longitudinal direction, said optical fibre having:
a core region extending along the longitudinal direction,
a cladding region extending along the longitudinal direction, said cladding region comprising at least 3 primary, elongated elements each having a centre axis extending in the longitudinal direction of the waveguide, each primary element having a refractive index being different from a refractive index of any material adjacent to the primary element,
each primary element having a shape which, in a cross-section perpendicular to the longitudinal direction, deviates from a circular shape and having parts extending outside a circle having the same area as that of the primary element and having its centre at a centre of the primary element, at least one extending part of each primary element extending at least substantially in a predetermined direction.
A centre of the cross-section of the primary element may be the centre of gravity or the position of the centre of the circle, where the smallest possible area of the primary element extends outside the circle.
Thus, the shape, such as an oblong shape, deviates from a circle having the same area-and consequently parts thereof extend outside the circle.
By applying such non-circular shapes of primary elements, the modal extent of the guided modes of the fibres may be adjusted, so that specific properties may be obtained. Such properties may be related to polarization preserving properties and/or dispersion properties.
These properties may be obtained even if primary elements having different cross-sectional shapes, such as oval shapes and rectangular shapes—as long as the longitudinal axes thereof point in at least substantially the same direction. This is also independent of the relative sizes of the shapes.
In the cross-section, two primary elements may be positioned in a manner so that two lines, each intersecting a centre of a respective of the two primary elements, and both intersecting a centre of the core region, form an angle of at the most 120°, such as in the interval of 10°-120°, such as 15°-115°, such as 30°-110°, such as 40°-105°, such as 50°-100°, such as 70°-100°.
Another manner of characterizing this structure is one, wherein, in the cross-section, two primary elements are positioned in a manner so that a first distance between a centre of the core area and a centre of one of the two primary elements is at least 2 times a second distance between a centre of the core area and a centre of the other of the two primary elements. This first distance may more preferably be at least 3 times the second distance, such as at least 5 times the second distance. such as at least 10 times the second distance.
Preferably, the cladding region comprises at least 6, such as at least 8, preferably at least 10, such as at least 15, preferably at least 20, such as at least 30 primary elements.
In the present context, the effective refractive index of the cladding is defined or generated by the refractive index/indices of the primary elements and one or more cladding materials positioned between the primary elements.
In one situation, the primary elements may, in the cross-section, cover at least part of lattice points of an at least essentially 2D-lattice. More preferably, to further increase photonic band gap effects all lattice points of a part of the lattice are covered by the primary elements. Naturally, it is preferred that the lattice points at least substantially coincide with centres of the primary elements.
In this situation, the primary elements may, in the cross-section, be rotation symmetric, where a rotational symmetry of the 2D-lattice is different from the rotational symmetry of the primary elements. This may e.g. be a situation where the rotation symmetry of the primary elements is a 120° symmetry (such as of a triangular shape) and the rotation symmetry of the lattice is that of a hexagonal (having a 60° symmetry). It should be noted that an element not being rotational symmetric (like the shape of a drop) will have a rotation symmetry of 360°.
In the present context, the predetermined direction will be a predetermined direction in the plane of the cross-section. This direction is preferably the same for the actual extending part of each primary element. Naturally, each primary element may have a plurality of extending parts extending in different directions. In that situation, preferably also these other extending parts extend in predetermined directions—one direction for each extending part of a primary element.
One relatively simple manner of defining a cladding is one wherein the at least one extending part of each primary element has at least one ax

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