We have formed simple polarization-controller arrays by inserting liquid crystal (LC) in trenches cut across planar lightwave circuits (PLCs). We fabricated LC layers for use as polarization controllers on PLCs in two ways; in one, the ultra-thin layer of LC is held in a cell that is inserted into a trench on the PLC while in the other, the trench is directly filled with the LC. The ultra-thin LC cell can change the phase of 1.55-µm light from 0 to 3π while the LC filling can change the phase of light at the same wavelength from 0 to 12π below 5Vrms. Two former parallel-aligned ultra-thin LC cells, where the directions of alignment of the liquid crystals are rotated by 45 relative to each other, are capable of converting light with an arbitrary input polarization to TE or TM polarization. Ultra-thin cells of twisted nematic LC can switch the polarization between TE and TM modes with an extinction ratio of -15dB. The array we fabricated had a pitch of 1 mm and 5 elements, but an array with more than 100 elements and a pitch below 125µm will easily be possible by using finely patterned transparent electrodes. We have also applied our techniques to the fabrication of LC-based variable optical attenuators (VOA) on the PLC.

We have exploited a high-power-tolerant variable optical attenuator (VOA) based on the fused fiber coupler in the all-fiber structure. A newly designed VOA employs the external modulation by forcing an axial stress in the tapered region of the fused fiber coupler. In the tapered region, the axial stress changes the refractive index of silica glasses resulting in a change in the coupling coefficient of the coupler. In this paper, we explain the principle of the novel device, VOA, and the optimized fabrication of the fused fiber coupler for the attenuation. The changes of the transmission spectrum for the coupler and the optical power spectrum for pump laser diode (LD), whose center wavelength is 1.47µm, versus the axial displacement were verified by experiment. The possibility of the wavelength uniformity less than 1dB over the range of 1460-1500nm was also obtained by another coupler under a different fabrication condition. The polarization-dependent loss (PDL) at 1.47µm wavelength was 0.65dB for a maximum displacement of 150µm. The designed device has an attractive feature of another output port of the coupler available as a monitoring tap. The device showed a high attenuation above 34dB and an insertion loss below 0.15dB. The all-fiber structure can provide less alignment, which in turn provides a high power tolerance. This novel design, moreover, has a simple and cost-effective structure.