Sources for Holographic Writing of Gratings 105 in the first category have been used extensively for grating fabrication but need to be used with care for high-quality grating production. The primary advan- tage is in the high average and peak power available in the UV region. On the other hand, some frequency mixing methods produce UV radiation of high tem- poral and spatial coherence and are ideally suited for grating formation. Typi- cally, these lasers demonstrate high average power capability but have the disadvantage of lower peak power densities. Nevertheless, these have also been shown to be highly successful for inducing large index changes while maintaining excellent grating quality. 3.5.1 Low Coherence Sources The first source reported for use in grating formation was the excimer-laser pumped frequency doubled oscillator­amplifier dye laser operating in the 240-nm window [13]. This source can produce approximately 0.1 W average tunable radiation between 240 and 260 nm using a b-barium borate (BBO) crys- tal by doubling the (CHRYSL 106) dye-laser output. The laser must be line-nar- rowed to increase the coherence length, and longitudinal pumped dye lasers are preferable since they produce a higher quality beam compared to side-pumped dyes which produce a triangular beam profile. An alternative pump laser for