Laser Fabricates Waveguides in LiNbO3
The two most common approaches for fabricating waveguides in LiNbO3 -- high-temperature in-diffusion of titanium and proton exchange -- are complex, involving high temperatures or acid treatment and at least one photolithographic step. Recently, however, a team at the University of Southampton in the UK demonstrated a much simpler technique for fabricating channel waveguides in LiNbO3.
The team wrote waveguides directly into the material with ultraviolet radiation from a frequency-doubled 244-nm argon-ion laser. An acousto-optic modulator in front of the laser ensured constant output power from the laser, and the LiNbO3 was mounted on a translation stage beneath the spatially filtered beam. Peak power and energy density at the crystal surface could be varied from 60 to 600 kW/cm2 and from 20 to 9500 J/cm2, respectively.
The waveguides were characterized at both visible (633 nm) and IR (1523 nm) wavelengths by butt-coupling a fiber to the edge of the crystal or by coupling light into the guide with a microscope objective. The researchers used a suitable imag-ing detector to observe the pattern emerging from the other side of the crystal. Good mode patterns, corresponding to both single- and higher-order-mode propagation within the guide, were observed with visible light. With IR radiation, however, confinement within the guide was much poorer. The scientists plan further experiments with different UV exposure parameters to improve their waveguides' IR performance.
The exact mechanism by which the UV radiation alters the refractive index of LiNbO3 will also be a subject of study. Z-cut crystals supported only TM waveguide modes, and Y-cut crystals supported only TE modes, indicating that the UV radiation increased only the extraor dinary index. A likely mechanism is laser-induced local-ion out-diffusion.
Directly writing waveguides with ultraviolet radiation is much simpler than conventional techniques for fabricating them in LiNbO3.
By measuring the divergence of the 633-nm beam emerging from the waveguide, the group calculated the numerical aperture and subsequently the refractive-index change induced by the UV radiation. This change was 6 X 10–4, but high-order-mode transmission in other guides suggested that a higher index change was associated with higher UV exposure levels. Transmission loss in the waveguides was in the range of 0.7 to 2 dB/cm.
The scientists also observed an approximately 2.5-dB increase in the loss of the waveguides during 30 minutes' illumination with visible light. This loss, the result of photorefractive damage, does not occur with IR radiation and is not an impediment to practical devices at telecom wavelengths, according to Sakellaris Mailis, a researcher at the university.
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