Opt Lett. 2019 Jan 1;44(1):37-40. doi: 10.1364/OL.44.000037.
Mid-infrared optical waveguides were inscribed in sapphire with femtosecond pulses at 515 nm. We show that such pulses induce a smooth negative refractive index change allowing for the inscription of a depressed cladding waveguide by closely overlapping the corresponding type I modification traces. The resulting structure consists of a highly symmetrical, uniform, and homogeneous waveguide. The size and numerical aperture of the waveguides were tailored to achieve efficient transmission in the mid-infrared. Single mode operation at a wavelength of 2850 nm and propagation loss of <0.37 dB/cm are reported for a 33 mm long depressed cladding waveguide. Thermal annealing was performed, and the refractive index contrast was still preserved to 50% (i.e., Δn=∼2.5×10) up to 1400°C.
中红外光学波导是用 515nm 的飞秒脉冲在蓝宝石中刻写的。我们表明，这种脉冲会引起平滑的负折射率变化，通过紧密重叠相应的 I 型修正轨迹，允许写入压低包层波导。由此产生的结构由一个高度对称、均匀和同质的波导组成。波导的尺寸和数值孔径被调整为实现中红外的高效传输。报道了对于一个 33mm 长的压低包层波导，在 2850nm 波长下实现单模操作，传播损耗<0.37 dB/cm。进行了热退火，折射率对比度仍然保持在 50%（即 Δn=∼2.5×10），直到 1400°C。
