GB2399652A

GB2399652A - Light guide with plural facets at its end

Info

Publication number: GB2399652A
Application number: GB0304131A
Authority: GB
Inventors: Michael Robert Osborne; Laurence Frederick Forrest; Michael Anthony O'key
Original assignee: OpTek Ltd
Current assignee: OpTek Ltd
Priority date: 2003-02-24
Filing date: 2003-02-24
Publication date: 2004-09-22
Also published as: GB0304131D0

Abstract

An optical fibre, waveguide or light guide, has at its end an optical coupling feature comprising plural optical facets e.g. a multi-facetted asymmetric wedge lens having a main facet orthogonal to the optical axis of the fibre, waveguide or light guide.

Description

IMPROVEMENTS RELATING TO OPTICAL WAVEGUIDES

Field of the Invention:

This invention concerns improvements relating to optical waveguides, optical fibres for example.

Background of the Invention:

Optical fibres and waveguides provide a convenient means of transporting light from one place to another. This is used for the transmission of information in optical communications, for the transmission of signals in optical detection systems, for the transmission of optical power in laser materials-processing equipment as well as a host of other uses.

It is generally required to couple light from the initial source (laser or collection optic) into the fibre or waveguide with as much efficiency as possible. How well this can be done depends on the match between the geometry of the light from that source and the range and angles of light which the fibre or waveguide will accept, and several techniques exist which endeavour to transform the geometry of the source into one that is acceptable by the fibre or waveguide. These include bulk-optics lenses or microlenses, gradient index (GRIN) lenses and lenses machined directly onto the ends of optical fibres.

Where the light originates essentially from a point source, for example a distant object, a small source or a single-cavity laser, high coupling efficiencies can be achieved using conventionally profiled optical elements, including spherical and aspheric surfaces in symmetric and asymmetric arrangements. These have been the subject of much prior art, and methods generally exist for the manufacture of the optics and profiles required.

Less well addressed is the issue of coupling light from extended sources into fibres or waveguides. In particular, there is increasing interest in coupling light from "stripe" diode lasers into large-core fibres.

A "stripe" laser consists of a linear array of individual laser cavities set side-by-side. They therefore tend to retain the small size of a single cavity in one dimension, but are extended in the orthogonal dimension. A typical emitting region may be lam by 100pm with divergences of, perhaps, 30 degrees in the fast axis orthogonal to the small dimension of the source, and 12 degrees in the slow axis orthogonal to the large dimension of the source (Figure 1). In this figure the active laser stripe is shown as 10 and the elliptical cross-section of the emerging beam as 11.

It is typical to want to couple the light from such a source into fibres with circularly symmetric core of diameter slightly larger than the large dimension of the diode stripe, say 105-110,um in the present case. Similarly, a 1 80,am wide stripe laser might be coupled to a 200,um core fibre and so on.

Often the slow axis divergence lies within the acceptance angle (NA) of the fibre, and so no tensing is necessary in this dimension. For the fast axis, the divergence exceeds the NA of the fibre, and some form of tensing is required. In this situation a "wedge" type lens, which has a finite curvature in one dimension and essentially infinite radius of curvature in the orthogonal dimension (i.e. a ridge) is required.

The alignment tolerance in the slow axis is dominated by the excess of the fibre core diameter over the diode emitting region length and is therefore of the order of microns. It is desirable to try to keep the tolerance in the orthogonal dimension as large as possible, and at least comparable to the slow axis. For a conventional "wedge" lens, keeping the positional tolerance wide would dictate a large radius lens, as shown in the image of a laser-cut lens at the left of Figure 2. However, with some manufacturing techniques (e.g. laser cutting or grinding and flaming) it is difficult to produce such a large radius without also producing significant rounding of the "corners" of the wedge, as shown in the orthogonal view of the same lens in the right-hand image of Figure 2. The rounding typically displays a similar radius to that produced in the fast dimension.

This rounding can be a significant limitation to the coupling efficiency which can be achieved as some of the light from the ends of emitter either misses fibre entirely, or is refracted at an angle which is outside the acceptance angle of the fibre as shown in Figure 3.

Summary of the Invention:

The present invention overcomes or at least substantially reduces the problem of the rounding in the direction orthogonal to the intended wedge. It does this by machining a plurality of lens facets which give rise to minimal rounding. In particular, and in order to give even better tolerance to misalignment, one facet can be normal to the fibre axis.

The invention will be described hereinafter by reference to the accompanying drawings.

Description of the Drawings:

Figure 1 shows schematically the laser beam distribution from a "stripe" diode; Figure 2 shows orthogonal views of a conventional laser- cut, large-radius "wedge" lens formed on the end of an optical fibre; Figure 3 illustrates how a lens as in Figure 2 works with a "stripe" diode; Figure 4 shows orthogonal views of a laser-cut, three facet, "wedge" lens embodying the present invention; Figure 5 schematically illustrates consideration applicable to the design of a lens as shown in Figure 4; and Figure 6 shows orthogonal views of a modified form of the lens of Figure 4.

Description of the Embodiments:

Orthogonal views of a lens shape embodying the present invention are shown in Figure 4. As can be seen, the rounding in the orthogonal dimension to the intended wedge is almost completely eliminated.

In many cases it will be found that three facets are sufficient. Figure 5 shows a sample design for light emitted from a point source, S. at angles, 0, up to 30 degrees from the normal. Assuming the fibre to have an NA of 0.16, the acceptance half angle is about 9 degrees.

Considering only the two dimensions shown in this figure, Figure 5a shows light emitted from the source being refracted at the normal facet down the fibre. This is effective up to a value of the distance, x, at which the refracted light reaches the acceptance angle, a, of the fibre. If this is 9 degrees, then the maximum value, A, corresponds to a maximum incidence angle, 0, of typically about 13 degrees.

When or before x is reached, the second facet comes into play as shown in Figure 5b at an angle to the fibre axis. The minimum value that can take at the position x is that at which light is refracted more than the fibre acceptance angle, a, beyond the fibre axis, as shown in Figure 5b. For the above case this is typically about 32 degrees.

For the extreme value of 0, assumed to be 30 degrees in this case, there is a maximum value for the angle, ó, which is that at which light from the source, S. is refracted to within the fibre acceptance angle, a, to the axis, as shown in Figure 5c. In the above case this is about 50 degrees.

Therefore, as can be seen, for the above case, a lens form which consists of a plane facet normal to the fibre axis of a size which subtends a half angle of typically up to 13 degrees to the source, followed by symmetrical angled facets at an angle between about 32 and 50 degrees to the fibre axis will effectively launch all of the light incident on the fibre end into the fibre within the acceptance angle.

Translating the above two-dimensional treatment into the full three dimensions changes the numbers somewhat, but not the concept.

It will be appreciated that the present invention has the added benefit of providing an unusually large tolerance to positional alignment errors in the direction which is usually the most demanding, i.e. orthogonal to the fibre axis in the left-hand image of Figure 4.

It will be clear to those skilled in the art that more than the minimum number of separate facets can be used. Moreover, it will be apparent that the transition between the various facets need not be perfectly abrupt, and that a finite degree of rounding will be both tolerable and indeed beneficial in matching the optical surface to the incident radiation, provided, of course, that the profile in the orthogonal dimension is not compromised. Such a lens, again formed by laser cutting, is shown in Figure 6 where the smooth radius between the facets can be seen and where the lack of rounding in the orthogonal dimension is substantially preserved.

It will also be clear to those skilled in the art that the same invention is equally applicable to light-guiding means other than optical fibres, and should be taken to mean all fibres, waveguides and light-guides in the broadest sense.

Multi-angle lenses have been described previously in both conical and wedge geometries (for example in US 5,101,457 and US 5,455,879) but have in all cases had a "point" or "ridge" opposing the optical source, leading to undesirably tight alignment tolerances. In no case has a lens with a substantially flat end been proposed for use in this type of application.

Claims

CLAIMS: 1. An optical fibre, waveguide or lightguide having at its end an

optical coupling feature comprising a plurality of optical facets.
2. An optical fibre, waveguide or lightguide as claimed in claim 1 wherein one of said facets is substantially orthogonal to the optical axis of the fibre, waveguide or lightguide.
3. An optical fibre, waveguide or lightguide as claimed in claim 1 or 2 wherein said substantially orthogonal facet is generally central of said plurality of facets.
4. An optical fibre, waveguide or lightguide as claimed in any preceding claim wherein the facets are symmetrically disposed about the optical axis of the fibre, waveguide or lightguide.
5. An optical fibre, waveguide or lightguide as claimed in any preceding claim wherein there are at least three said facets.
6. An optical fibre, waveguide or lightguide as claimed in any preceding claim wherein the optical coupling feature is substantially asymmetric so as to function in the manner of a "wedge" lens.
7. An optical fibre, waveguide or lightguide as claimed in any preceding claim wherein the transitions between said facets, or at least some of them, are rounded.
8. An optical fibre, waveguide or lightguide as claimed in any preceding claim wherein said facets are laser cut.
9. An optical fibre, waveguide or lightguide substantially as herein described with reference to Figure 4 or 6 of the accompanying 1 0 drawings.
10. An optical waveguide having at its end an optical coupling feature comprising a multi-face/ted asymmetric wedge lens having a main facet orthogonal to the optical axis of the waveguide.