Directly Photoinscribed Thick Bragg Gratings in Ohara WMS-15 Glass-Ceramic Peter A. Krug, Rodica Matei Rogojan and Jacques Albert Department of Electronics, Carleton University, 1125 Colonel By Drive, Ottawa, ON K1S 5B6, Canada pkrug@doe.carleton.ca Abstract: Volume gratings were UV inscribed in WMS-15 glass-ceramic at 193 and 248nm without additional processing. Weak, easily bleached gratings resulted from fluences below 0.3kJ/cm 2 . Stable gratings with Δn ~ 6×10 -5 were formed at higher fluences. ©2009 Optical Society of America OCIS codes: (050.7330) Volume gratings; (160.5335) Photosensitive materials 1. Introduction Glass-ceramics, which combine many useful properties including a wide range of values (negative, zero or positive) of both thermal expansion and thermo-optic coefficients, are obtained by the controlled nucleation and crystallization of glass [1]. Glass-ceramics that are sufficiently transparent to be used in transmissive optical components have recently been reported and have become available commercially [2,3]. Diffraction gratings have previously been photoinduced in glass-ceramics by UV exposure, followed by chemical etching [4], thermal treatment [5] or both [6]. Here we report, for the first time to our knowledge, the direct photoinscription of diffraction gratings in a glass-ceramic without the need for any subsequent processing. 2. UV Inscription of gratings The composition of Ohara WMS-15 transparent glass-ceramic [7] is a trade secret, but the material is known to contain oxides of Si, Al, Zr, P, and possibly Zn and Sb. Polished 3mm thick samples were exposed through zero order nulled phase mask to spatially uniform beam with energy densities of 226mJ/cm 2 /pulse (25 pulses per second at 248nm) or 36mJ/cm 2 /pulse (100 pulses per second at 193nm). The grating depths were calculated from the angular dependence of diffraction efficiency [8] to be between 127 ± 20 and 254 ± 20μm for the 248nm written gratings but only 8.5 ± 2μm for the 193nm written gratings. From the depths and the Bragg diffraction efficiencies measured using a collimated laser beam at 632.8nm, we determined the index modulation depth of gratings written with UV doses between 0.01 and 30kJ/cm 2 , as shown in Fig. 1. 0.01 0.1 1 10 10 -7 10 -6 10 -5 10 -4 High UV Exposure Dose Wavelength 193nm 248nm Index Modulation Depth UV Exposure Dose (kJ/cm 2 ) Low UV Exposure Dose No anneal 100°(5h) 100° (91h) 200° 300° 400° 500° 10 -7 10 -6 10 -5 10 -4 10 -3 10 -2 Dose (kJ/cm 2 ) 30 10 3 1 0.3 0.1 0.03 0.01 Diffraction Efficiency Maximum Annealing Temperature (°C) Fig. 1. Refractive index modulation depth as a function of dose. Fig. 2. Grating diffraction efficiency as a function of annealing. We see that there are two very distinctly different regions of grating formation. For UV doses less than 0.3kJ/cm 2 we observe the rapid formation of weak gratings, having diffraction efficiency less than 2×10 -5 . For 248nm inscription, these gratings reach maximum strength at the relatively low dose of 0.03kJ/cm 2 , after which their strength decreases with increasing dose up to 0.3kJ/cm 2 . For UV doses of 1kJ/cm 2 and above, much stronger grating growth is observed, with the gratings continuing to grow with increasing UV dose up to the maximum applied dose of 30kJ/cm 2 . The strongest grating in the high UV dose regime, written at 30kJ/cm 2 , has an index modulation depth of around 6×10 -5 , which is around 25 times the modulation depth of the strongest grating in the low UV dose