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1. (WO2018138473) APPARATUS AND METHOD FOR OPTICAL ISOLATION

Pub. No.:    WO/2018/138473    International Application No.:    PCT/GB2018/000015
Publication Date: Fri Aug 03 01:59:59 CEST 2018 International Filing Date: Sat Jan 27 00:59:59 CET 2018
IPC: G02B 6/42
G02B 6/14
H01S 3/00
Applicants: SPI LASERS UK LIMITED
Inventors: VARNHAM, Malcom, Paul
Title: APPARATUS AND METHOD FOR OPTICAL ISOLATION
Abstract:
Apparatus for optically isolating a laser (1) from external reflections, which apparatus comprises a mode filter (19) and a first optical fibre (1), wherein: the first optical fibre (1) is a multimode optical fibre that supports a fundamental mode (3) and at least one higher order mode (4); the mode filter (19) is defined by an optical attenuation which is higher for the higher order mode (4) than for the fundamental mode (3); the mode filter (19) is configured to pass the fundamental mode (3) into the first optical fibre (1); and the apparatus being characterized in that: the first optical fibre (1) comprises a long period grating (10); and the long period grating (10) is defined by a period (13) selected to couple the fundamental mode (3) to the higher order mode (4) of the first optical fibre (1); whereby if the fundamental mode (3) and the higher order mode (4) are reflected back into the first optical fibre (1) as back-reflected fundamental and higher order modes (25), (26), then the mode filter (19) optically attenuates the back-reflected higher order mode (26) more than the back-reflected fundamental mode (25) thereby isolating the external reflection.