A novel grating writing interferometer is demonstrated using a phase mask to split the beam, an optical element to collimate the writing beams, and another phase mask to interfere the beams. This interferometer is compact, highly stable, tuneable and improves beam coherence making it amenable for writing with low coherence sources. It allows unwanted orders to be spatially filtered, improving the grating quality by removing the detrimental effect of allowing the zeroth order to impinge on the fibre. Results were achieved for spot gratings and long gratings that were made up of adjacent sub-gratings using a lens as the collimating element, and translated gratings using mirrors to collimate the beams. The fringe contrast evolution was examined, and a tuning range of over 50nm is demonstrated.
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