In order to decide which UV laser to e mploye in fiber Bragg grating fabricating,the contrast of the UV diffraction fie ld is carefully discussed when the UV lasers with different interference are use d.Two methods are discussed.One is putting the phase mask near the fiber and the other is putting two mirrors between the phase mask and the fiber.The former me thod does not require strict interference characteristic while the other one req uires 100 times higher strict characteristic.