Bragg gratings with reflectivities greater than 99% have been fabricated in phosphate glasssingle mode fibers by irradiating the fibers with 193nm wavelength, high intensity pulses from an ArF excimer laser through a phase mask. Thermal treatment for 1000h at temperatures up to 170°C did not degrade the gratings and resulted in an increase in grating strength. The refractive index modulation amplitudes obtained exceed 10−4.