Recording of high efficiency volume Bragg gratings in a photopolymer using diffraction from very weak pre-recorded gratings

AbstractA two-step method of writing high diffraction efficiency holographic gratings in photopolymer using just one writing beam during the second step is discussed. The technique is based on the use of weak gratings (<1%) recorded during a first step which during the second step are illuminated with a single beam to form strong highly efficient holographic gratings. The weak grating provides a second interfering beam by diffracting a small portion of the illuminating light. Although effects such as beam pumping and twowave mixing arewell known in dynamic re-writable recording media such as photorefractives, the ability to write with one beam, or ‘pump’ gratings is not usually observed in photopolymer materials. Other methods of recording with a single beam have been reported, especially for data storage, but they use compact optical systems, polarization effects or lenticular devices to produce the required second beam. In this method, the second beam is produced within the photopolymer medium. Earlierwork by the authors reported increases in efficiency of weak photopolymer gratings illuminated by a single exposure beam, and more recently evidence was presented that diffraction at the weak pre-recorded grating was responsible for this effect. Here we demonstrate a strong dependence of the diffraction efficiency on grating thickness and show that the method allows the writing of high diffraction efficiency gratings in adverse conditions using a single beam. The possibility of angular multiplexing is also demonstrated. The technique may find application in writing information using pre-recorded weak holographic gratings and, potentially, in other data storage applications. In this paper it is shown that multiple new gratings can be writtenwhich are angularly separated and distinct from the pre-written grating and it is also possible to choose one grating from among a number of pre-written gratings and selectively amplify it without amplifying the others.