GB2158604A

GB2158604A - Diffraction gratings

Info

Publication number: GB2158604A
Application number: GB08412035A
Authority: GB
Inventors: Kevin Christopher Byron
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1984-05-11
Filing date: 1984-05-11
Publication date: 1985-11-13
Also published as: AU4174985A; GB2158604B; GB8412035D0

Abstract

A diffraction grating is comprised of an array (2) of closely packed bare braded index core optical fibres (1) disposed between a pair of glass plates (3, 4). When illuminated transversely the graded index of the core of each fibre diffracts the light in the same way as the etched or ruled lines of conventional gratings. <IMAGE>

Description

SPECIFICATION Diffraction gratings This invention relates to diffraction gratings and to methods of manufacturing them.

Diffraction gratings are commonly made by ruling or etching lines on a substrate. It is an object of the present invention to provide an alternative construction for, and method of manufacturing, diffraction gratings.

According to one aspect of the present invention there is provided a diffraction grating comprising a plurality of optical fibres arranged side-by-side and closely packed in a layer which is one optical fibre thick.

According to another aspect of the present invention there is provided a method of manufacturing a diffraction grating comprising the step of arranging a plurality of optical fibres side-by-side and closely packed to form a layer of optical fibres which is one optical fibre thick.

Embodiments of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows a cross-section through a transmission grating according to one embodiment of the present invention, and Figure 2 shows an alternative optical fibre packing configuration to that employed in the Fig. 1 embodiment.

Referring firstly to Fig. 1, a transmission diffraction grating is manufactured by arranging a plurality of lengths 1 of optical fibre 3 side-by-side to form a layer or array 2 one fibre thick on a glass plate or substrate 3. A second glass plate 4 may be disposed over the optical fibre array 2 to maintain the fibres in the array, and the plates 3 and 4 may be clamped together by means of a suitable framelike structure, for example, (not shown).

The optical fibre employed is an uncoated (bare) graded-index core fibre. The lengths of fibre are closely packed and thus parallel with one another.

When illuminated transversely as indicated in Fig.

1, the graded index of the core of each length of fibre diffracts the light in the same way as the etched lines of a conventional grating.

With a single layer of fibres the grating spacing may be as low as 10 m, giving 100 lines mm-5.

The number of lines per millimetre may be doubled by adding a second layer 5 of closely packed fibre lengths on top of, and in a closely packed relationship with, the first layer 2 as indicated in Fig.

2. Such a line spacing provides a grating suitable for use in the range -800nm - 2im, that is the near infra- red range, in which optical fibre communications systems conventionally operate.

To avoid reflection losses at the fibre surfaces an index-matching liquid may be introduced into the interstitial spaces between the fibres and the glass plates and the grating then sealed.

To make a reflection grating one glass plate or substrate, substrate 3 with light incident as indicated in Fig. 1, is provided with a reflecting surface. This reflecting surface may be on either of the surfaces of the substrate. If it is on the innermost surface of the grating then the possibility of damage thereto in use of the grating is minimised.

The dispersion of gratings so produced is controllable by the fibre core diameter employed and by the gradient of the core refractive index variation. The use of optical fibres results in gratings which are highly efficient and have a very high damage threshold since the diffraction elements (the fibres) are sandwiched between two glass plates.

Normally a laser has two mirrors forming the optical cavity. If one mirror is replaced with a grating then the line width of that laser output would be narrowed. This is desirable in optical communications where effects due to modal noise etc. need to be minimised if not eliminated. This would be particularly useful in a coherent transmission system using YAG lasers or mid-IR lasers. The grating provides spectral narrowing or dispersion and over the wavelength range mentiones is useful in optical systems, particularly for high power lasers in view of its very high damage threshold.

1. A diffraction grating comprising a plurality of optical fibres arranged side-by-side and closely packed in a layer which is one optical fibre thick.

2. A diffraction grating as claimed in claim 1 comprising a further plurality of optical fibres arranged side- by-side and closely packed in a further layer disposed on said layer and in a closely packed relationship therewith.

3. A diffraction grating as claimed in claim 1 or claim 2 wherein said layer of optical fibres is disposed on a glass substrate.

4. A diffraction grating as claimed in claim 3, wherein said layer, or said layer and the further layer, of optical fibres is sandwiched between the glass substrate and a a glass plate.

5. A diffraction grating as claimed in claim 4, wherein interstitial spaces between the fibres and the glass substrate and plate are filled with an index matching liquid.

6. A diffraction grating as claimed in any one of the preceding claims wherein the optical fibres are comprised of bare graded index core fibres.

7. A diffraction grating as claimed in any one of claims 3 to 5, wherein one major surface of the glass substrate or the glass plate is provided with a reflecting surface whereby the grating comprises a reflection grating.

8. A method of manufacturing a diffraction grating comprising the step of arranging a plurality of optical fibres side-by-side and closely packed to form a layer of optical fibres which is one optical fibre thick.

9. A method as claimed in claim 8, further including the step of arranging a further plurality of optical fibres side-by-side and closely packed in a further layer on said layer and in a closely packed relationship therewith.

10. A method as claimed in claim 9 wherein the plurality of optical fibres are disposed on a glass

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Diffraction gratings This invention relates to diffraction gratings and to methods of manufacturing them. Diffraction gratings are commonly made by ruling or etching lines on a substrate. It is an object of the present invention to provide an alternative construction for, and method of manufacturing, diffraction gratings. According to one aspect of the present invention there is provided a diffraction grating comprising a plurality of optical fibres arranged side-by-side and closely packed in a layer which is one optical fibre thick. According to another aspect of the present invention there is provided a method of manufacturing a diffraction grating comprising the step of arranging a plurality of optical fibres side-by-side and closely packed to form a layer of optical fibres which is one optical fibre thick. Embodiments of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows a cross-section through a transmission grating according to one embodiment of the present invention, and Figure 2 shows an alternative optical fibre packing configuration to that employed in the Fig. 1 embodiment. Referring firstly to Fig. 1, a transmission diffraction grating is manufactured by arranging a plurality of lengths 1 of optical fibre 3 side-by-side to form a layer or array 2 one fibre thick on a glass plate or substrate 3. A second glass plate 4 may be disposed over the optical fibre array 2 to maintain the fibres in the array, and the plates 3 and 4 may be clamped together by means of a suitable framelike structure, for example, (not shown). The optical fibre employed is an uncoated (bare) graded-index core fibre. The lengths of fibre are closely packed and thus parallel with one another. When illuminated transversely as indicated in Fig. 1, the graded index of the core of each length of fibre diffracts the light in the same way as the etched lines of a conventional grating. With a single layer of fibres the grating spacing may be as low as 10 m, giving 100 lines mm-5. The number of lines per millimetre may be doubled by adding a second layer 5 of closely packed fibre lengths on top of, and in a closely packed relationship with, the first layer 2 as indicated in Fig. 2. Such a line spacing provides a grating suitable for use in the range -800nm - 2im, that is the near infra- red range, in which optical fibre communications systems conventionally operate. To avoid reflection losses at the fibre surfaces an index-matching liquid may be introduced into the interstitial spaces between the fibres and the glass plates and the grating then sealed. To make a reflection grating one glass plate or substrate, substrate 3 with light incident as indicated in Fig. 1, is provided with a reflecting surface. This reflecting surface may be on either of the surfaces of the substrate. If it is on the innermost surface of the grating then the possibility of damage thereto in use of the grating is minimised. The dispersion of gratings so produced is controllable by the fibre core diameter employed and by the gradient of the core refractive index variation. The use of optical fibres results in gratings which are highly efficient and have a very high damage threshold since the diffraction elements (the fibres) are sandwiched between two glass plates. Normally a laser has two mirrors forming the optical cavity. If one mirror is replaced with a grating then the line width of that laser output would be narrowed. This is desirable in optical communications where effects due to modal noise etc. need to be minimised if not eliminated. This would be particularly useful in a coherent transmission system using YAG lasers or mid-IR lasers. The grating provides spectral narrowing or dispersion and over the wavelength range mentiones is useful in optical systems, particularly for high power lasers in view of its very high damage threshold. CLAIMS

10. A method as claimed in claim 9 wherein the plurality of optical fibres are disposed on a glass substrate.

11. A method as claimed in claim 10 including the step of positioning a glass plate whereby said layer, or said layer and the further layer, of optical fibres is sandwiched between the glass substrate and the glass plate.

12. A method as claimed in claim 11 including the step of filling the interstitial spaces between the fibres and the glass substrate and plate with an index matching liquid.

13. A method as claimed in claim 11 or claim 12, including the step of providing a major surface of the glass substrate or the glass plate with a reflecting surface whereby the grating comprises a reflection grating.

14. A method as claimed in any one of claims 8 to 13, wherein the optical fibres are comprised of bare graded index core fibres.

15. A diffraction grating substantially as herein described with reference to Fig.1 or Fig. 2 of the accompanying drawings.

16. A method of manufacturing a diffraction grating substantially as herein described with reference to Fig. 1 or Fig. 2 of the accompanying drawings.

17. An optical transmission system or device in which spectral narrowing or dispersion of a light source is accomplished using a diffraction grating.