Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
2002-06-18
2004-03-02
Sanghavi, Hemang (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling structure
C385S011000, C385S042000, C385S051000, C065S384000, C065S411000
Reexamination Certificate
active
06701046
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing an optical coupler and/or an optical splitter between a polarization maintaining optical fibre and an optical fibre which is not polarization maintaining, and a corresponding optical coupler and/or splitter.
Polarization maintaining fibres (PMF) are characterized in that they retain the polarization of the input signal within them throughout their length, if this polarization is orientated in one of the two polarization directions of the fibre.
In particular, these fibres have two principal axes of propagation of the optical signal within them (axes of birefringence), called the slow axis and the fast axis. The said axes are essentially perpendicular to each other and have different characteristics. The fast axis has an effective refractive index which is considerably lower than that of the slow axis, and therefore allows the light beam polarized in the same direction to travel along the fibre at a higher phase velocity than that of the optical beam polarized in the direction of the slow axis. In both directions, however, the output signal from the fibre is kept essentially unaltered. A polarization maintaining fibre is characterized in that it has a difference of more than 10
−4
between the refractive indices between the two axes of birefringence.
For the purposes of the present invention, the term “standard fibre” denotes an optical fibre which is not polarization maintaining. Preferably, the said standard fibre is a single-mode fibre, particularly a step-index optical fibre.
In particular, in a coupler according to the present invention, an optical signal which passes along a polarization maintaining fibre (“principal fibre”) is partially extracted from the said fibre and injected into a standard fibre (“extraction fibre”). This coupler can be used in applications which do not require a defined polarization state in the extracted signal.
A typical application of this coupler is the monitoring of the optical power carried by a polarization maintaining fibre in which the power measurement is carried out by means of a polarization-insensitive photodetector which is directly connected to the extraction fibre.
More generally, the monitoring of the optical power is carried out by means of any apparatus which does not require the use of any component characterized by a polarization-dependent behaviour. Conversely, in this application, the output signal from the principal fibre must have a polarization state which is nominally identical to that of the input to the fibre.
An optical signal transmitted by a polarization maintaining fibre is characterized not only by the optical power associated with it, but also by the extinction ratio of the polarization, which quantifies the degradation of the state of polarization with respect to that which should theoretically be propagated. In the case of nominally linear polarization, the extinction ratio is defined thus:
ER
=
P
C
P
N
(
1
)
where:
P
C
is the optical power associated with the axis of birefringence of the fibre along which the optical signal is sent, called the co-polarized component of the optical signal;
P
N
is the optical power associated with the axis of birefringence of the fibre along which the optical signal is not sent, called the cross-polarized component (theoretically zero).
At the input of the polarization maintaining fibre, the said ratio is infinite. since all the optical power is injected along an axis of birefringence. Subsequently, a small part of the optical power is transferred along the other axis of birefringence, as a result of a possible degradation of the fibre, and consequently the extinction ratio assumes a finite value.
A technology which can be used to produce a coupler and/or splitter between a polarization maintaining fibre and a standard fibre is that of micro-optics.
Splitters made by this technology are marketed, for example, by JDS (Nepean. ON, Canada), E-Tek (San José, Calif., USA) and Micro-Optics (Hackettstown, N.J., USA). With this technology, the principal fibre and the extraction fibre are both terminated with suitable lenses to form collimators between which an optical power divider element is inserted on each of the two fibres.
In these splitters, one of the two fibres may be a standard fibre, and therefore by suitably selecting the splitting ratio, or the coupling ratio (CR) of the optical power divider element inserted between the collimators, it is possible to extract a portion of the signal from the polarization maintaining fibre and to send it along the standard fibre.
The Applicant has observed that the coupling efficiency between collimated beams associated with different fibres (such as polarization maintaining fibres and standard fibres) is less than 100%, and the achieving of relatively low losses for a component made by micro-optics technology makes the assembly difficult and expensive.
There are also known optical couplers and/or splitters between polarization maintaining fibres produced by the fusion technology, in other words optical couplers and/or splitters in which both the principal fibre and the extraction fibre are polarization maintaining optical fibres. A component of this type is used in all applications in which polarization maintaining division of optical power is desired (the extraction fibre also maintains the polarization).
U.S. Pat. No. 5,224,977 describes an apparatus for producing a fused optical coupler between polarization maintaining fibres. The apparatus comprises. in particular, a first polarization maintaining fibre and a second polarization maintaining fibre, means for holding stripped portions of both of the said fibres in a position of mutual alignment and in contact with each other. The apparatus also comprises means for heating the said portions of both fibres, placed adjacent to the said means for holding stripped portions of both of the said fibres. The said means for heating the portions of both fibres oscillate along the said stripped portions with an amplitude decreasing in steps until the fusion of the said two portions is complete.
The essential aspect which distinguishes the production of a fused optical coupler between polarization maintaining fibres from the production of a similar splitter with standard fibres is that, in the first case, the polarization is desired to be maintained in both of the maintaining fibres. It is therefore necessary to use a method which ensures that the two fibres in the coupling area are fused while maintaining the parallel orientation of the axes of birefringence. The Applicant has observed that this method applied to a coupler considerably complicates the process of fabricating the coupler by comparison with the process used in the case of standard fibres; this is because these methods require the pre-alignment of the axes of birefringence of the fibres, visually or by monitoring the polarization state at the output of the fibres, before fusion is initiated; this complication is manifested in a considerably longer operating time than that required to produce couplers and/or splitters between standard fibres, and in a considerably lower efficiency of the process. These couplers are therefore expensive.
The Applicant has tackled the problem of producing couplers between a polarization maintaining fibre and a standard fibre with a simpler and relatively inexpensive technology. The Applicant has observed, however, that a fusion process of the standard type does not permit coupling between a polarization maintaining fibre and a standard fibre.
U.S. Pat. No. 5,293,440 describes a coupler for polarization maintaining fibres produced by the fusion technology, including a rigid substrate and two optical fibres placed adjacent to each other and joined together in their portions in which the coupling takes place.
Each optical fibre is fixed separately by an adhesive to the rigid substrate at each of its ends. The coupling portion of the two fibres can be formed by fusion or any other method of joining the two fibres together
Corio Valeria
Pianciola Aurelio
Vavassori Paolo
Corning O.T.I. SpA
Douglas Walter M.
Rojas Omar
Sanghavi Hemang
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