WO2002088814A2

WO2002088814A2 - Improvements relating to optical fibre collimators

Info

Publication number: WO2002088814A2
Application number: PCT/GB2002/002000
Authority: WO
Inventors: Michael Osborne
Original assignee: Optek Limited
Priority date: 2001-05-01
Filing date: 2002-05-01
Publication date: 2002-11-07
Also published as: WO2002088814A3; GB0110691D0; AU2002307906A1; GB2375186A

Abstract

An optical fibre comprising a cladding (4) around a central core (3) has an end portion (5) formed to permit the optical mode size to increase beyond the limits of the core and a microlens (6) is formed on the end face of the fibre. In a described embodiment, the end portion of the fibre is heat treated by a CO2 laser to disperse the core dopant. A 102 increase in collimated region is obtainable by virtue of this technique.

Description

IMPROVEMENTS RELATING TO OPTICAL FIBRE COLLIMATORS

Field of the Invention: This invention concerns improvements relating to optical fibre collimators for collimating a laser beam emerging from an optical fibre. The invention has particular, though not exclusive, application to the field of telecommunications.

Background of the Invention:

There exist several applications where it is desired to collimate the laser beam emerging from an optical fibre, for example where a Faraday rotator is to be used as an optical isolator in telecommunications systems. Often, it is then required to couple the beam back into an optical fibre. This task is generally achieved using free-space optics as shown in Figure 1 of the accompanying drawings. The collimating lenses shown in this figure may be either single or multiple element lenses.

Such collimators are difficult to fabricate, partly due to the low F# of the lenses involved (and hence aberrations) but also due to the difficulty of

accurately aligning the elements of the lens and the fibre. The possibility of producing a lens form on the fibre end (eg as described in US-A-4 932 989) offers the opportunity to reduce the number of components involved, to directly produce superior aspheric lens forms and to increase the robustness and reduce the physical size of the collimator. However, this approach has a fundamental flaw which is related to the size of the fibre core.

The range over which a Gaussian beam can be effectively collimated can be represented by the Rayleigh range, Z_R, which is the distance over

which the beam increases in diameter from a waist ω₀, to a diameter of V2ω₀

as shown in Figure 2 of the accompanying drawings. For a lowest order

Gaussian beam Z_R is equal to πω₀ ²/λ where λ is the wavelength.

The collimated range therefore depends on the spot size. For example, if it is attempted to collimate the beam from a single-mode fibre using a lens formed on the end of the fibre, the spot size at the fibre must be the fibre

mode-field-diameter (MFD) which is typically 9μm. Setting this diameter as

equal to 2 times the minimum diameter gives the geometry shown in Figure

3 of the accompanying drawings, where the collimated range is only about

170μm which is insufficient for most purposes.

In order to obtain a greater collimated range it is necessary to employ a larger diameter beam on the collimating lens surface. As is described in the prior art acknowledgement of EP-A-0 575 993, the conventional approach, shown in Figure 1 of the accompanying drawings, achieves this through the natural expansion of the beam from the fibre end. It is also known to achieve

this by coupling a length of homogeneous glass fibre to the end of the optical fibre and forming a convex microlens on the end face of the homogeneous

fibre end as taught in EP-A- 0 575 993 and US-A-5 669 464 for example, but the provision of the homogeneous fibre involves additional processing and alignment requirements.

Objects and Summary of the Invention: It is accordingly the object of the present invention to overcome or at least substantially reduce the abovementioned problem.

The present invention combines lensing of the fibre tip with processing of an end portion of the fibre to increase the beam size on the lensing element in order to produce an effective monolithic fibre collimator. Hereinafter described is an embodiment of the invention in which the laser beam is allowed to expand beyond the core region of the fibre just prior to the (effective) end of the fibre in such a way that the beam is of increased diameter at the focussing element. As shown in Figure 4 of the accompanying drawings, if the optical mode is allowed to fill the OD of the

fibre (~125μm diameter), the collimated range can be increased to a useful

~17mm. In the embodiment, the dopant which defines the core of the fibre is diffused from the end portion of the fibre. This can be accomplished using a suitably controlled and spatially defined heat source. A laser is one such heat source. It will be appreciated that the steps of fibre modification to allow mode size increase and lens form manufacture can be carried out in any order

to produce essentially the same result. The above and further features of the present invention are set forth in the appended claims and will be well understood from consideration of the following description given with reference to the accompanying drawings.

Description of the Drawings:

Figure 1 shows a conventional optical fibre collimator; Figure 2 illustrates Gaussian beam optics;

Figure 3 illustrates the effect of providing lenses on the ends of optical fibres as taught in the prior art; Figure 4 illustrates an embodiment of the invention.

Detailed Description of the Embodiments:

Figures 1 to 3 illustrate the prior art and explain the problems which the present invention aims to solve, and Figure 4 illustrates an exemplary embodiment of the present invention.

Referring to Figure 4, first and second optical fibres 1 and 2 are shown, each comprising a core 3 and a surrounding cladding 4, the core 3, as is well known, being defined by doping of the glass material of the optical fibre. As shown, each fibre 1 and 2 has an end portion 5 in which the core dopant has been dispersed by diffusion as a result of heat treatment, for example by means of a laser such as a CO laser. By virtue of the dispersion of the core dopant in the fibre end portions 5, a laser beam traversing the core of optical fibre 1 is permitted to expand before it exits the end face of the fibre where a collimating lens formation 6 is provided, for example by a laser micromachining process such as is described in US-A-4 932 989. Similarly, in the optical fibre 2 an end portion 5 is formed which has the reverse effect. For an optical fibre having an outer diameter (OD) of 125μm and a core

diameter of about 9μm, a collimation range of the order of 17mm can be

obtained as compared to the only 170μm collimation range of the prior art Figure 3 arrangement.

The invention thus is advantageous in that any part of a standard optical fibre can be treated to modify the internal composition of the fibre and thermally expand the core so as to increase the optical mode size within the fibre. No necessity arises to couple an additional optical component to the end of the optical fibre as taught by US-A-5 669464, or to physically shape the fibre end portion as taught by EP-A-0 194 842, EP-A-0 220439 and US-A-4 898 450, and the fibre diameter remains constant throughout the length of the fibre.

The invention having been described in the foregoing by reference to an exemplary embodiment, it is to be appreciated that modifications and variations thereto are possible without departure from the spirit and scope of the invention as set forth in the appended claims.

Claims

CLAIMS:

1. A process for producing an optically modified end to an optical fibre which comprises: a. treating an end portion of the fibre in such a way as to allow the optical mode size to increase within the fibre, and b. forming a lens on the end of the thus modified end portion of the fibre.

2. A process as claimed in claim 1, wherein the fibre comprises a core and surrounding cladding and the increase in mode size is accomplished by thermally diffusing the core of the fibre in a region close to the end thereof so as to permit a light beam traversing the fibre to expand before it encounters said lens.

3. A process as claimed in claim 2, where the core diffusion is carried out by heating the fibre end portion with a laser.

4. A process as claimed in claim 3, where the laser is a CO₂ laser.

5. A process as claimed in any of the preceding claims where the lens form is produced by laser machining.

6. A process as claimed in any of the preceding claims where the optically modified fibre end is designed to provide a substantially collimated laser beam in a given region.

7. An optical fibre having an end portion such as to accommodate an increased mode size within the fibre and wherein a lens is formed on the end of said end portion.

8. An optical fibre comprising a central, doped core surrounded by cladding and wherein the core dopant is diffused at an end portion of the fibre so as to increase the optical mode size at the fibre end, a lens being formed on the fibre end.

9. An optical arrangement comprising first and second optical fibres as claimed in claim 7 or 8, the first optical fibre being arranged to emit a laser beam which is subsequently coupled back into the second optical fibre.

10. An optical arrangement as claimed in claim 9 including an optical isolator, a Faraday rotator for example, between the two optical fibres.