We experimentally demonstrated polarization-mode dispersion (PMD) compensation by distributing polarizers with only 1 degree of freedom (DOF) along the transmission line. The average power penalty was measured to be 0.4 dB by inserting four compensators, where average differential group delay was 47% of bit period.

In a bidirectional wavelength-reused system, polarization control is used to reduce power penalty induced by coherent mixing of the signal with Rayleigh backscattering. The reduction of the effect of Rayleigh backscattering is theoretical study and experimental demonstration. For a 50km, 10Gb/s bidirectional transmission system, an error floor of about 510-10 under the worst polarization state is occurred. Nevertheless, the power penalty is reduced from 3.5dB to only 0.3dB when the signals are under optimum polarization control. The results indicate that the proposed technique may find vast applications in bidirectional ring networks with add/drop as well as cross-connect nodes using wavelength-reused technology.

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.

This paper describes a new free-space optical switch structure based on cascaded beam shifters (each consists of a liquid-crystal polarization controller array and a birefringent plate). This structure comprises 2-input, 2-output switching elements that are locally connected by links. It is applicable to a variety of switching networks, such as a Clos network. The switching network based on this structure is an analog switch that is transparent to signal format, bit rate, and modulation type, so it can handle various types of optical signals. Theoretical feasibility studies indicate that compact large-scale switches (i.e., 100-1000 ports) with relay lens systems can be implemented using beam shifters with a 0.4-dB insertion loss and a 30-dB extinction ratio. Experimental feasibility studies indicate that a 1024-cell beam shifter module with a 0.5-dB insertion loss and a 23-dB extinction ratio is possible at present. An alignment-free assembly technique using precise alignment guides is also confirmed. An experimental 8-stage, 1024-input 256-output concentrator shows low insertion loss characteristics (6.8dB on average) owing to the low-loss beam shifters and the alignment-free assembly technique. Practical switching networks mainly require the improvement of the extinction ratio of the beam shifter module and the development of a fiber pig-tailing technique. This switch structure is applicable to transparent switching networks such as subscriber line concentrators and inter-module connectors.