A review of higher-order mode pass filtering techniques.

Mode Division Multiplexing (MDM) is regarded as a promising technology to overcome the bottleneck of the future demand for high data transmission rates. Multimode fibers are replacing traditional single mode fibers to cope with the increasing bandwidth requirements. MDM works with multiple light modes, and these modes are bound to be coupled with other propagating modes as they pass through guided media. A mode filter removes unwanted modes from the signal received at the receiver after demultiplexing. As a result, designing a highly potential high-order mode pass filter is desired to meet the capacity crunch using the MDM technology. Currently, remarkable research works have been conducted on mode filtering. This paper presents an overview of recent developments in mode filtering techniques along with their designs and fabrication processes. In particular, the mode filter made from different types of materials is reviewed to illustrate the potential of the designs in improving the system performance. Even though commercial success has not been materialized, this work will provide promising prospects in mode filtering techniques for increased flexibility and the choice of different mode filters.

Reconfigurable high-order mode pass filter for mode-division multiplexing.

In mode division multiplexing (MDM) optical system, the mode filter has become an inseparable part to reduce modal crosstalk and transmit the desired modes unabatedly. As filtering out lower-order mode is difficult, here we propose a reconfigurable structure of a higher-order mode pass filter consisting of two tunable mode converters and a directional coupler (DC) in a three-mode planar waveguide platform. By switching the working states of the mode converters, the structure can also be used as a fundamental mode (TE0) pass filter and hence dynamic output signals can be achieved. For the second-order mode (TE2) transmission, the simulated excess loss is ∼0.61 dB at 1.550 µm and the extinction ratio remains ≥24 dB (power ratio of TE0 & TE2 Launch) and ≥25 dB (power ratio of TE1 & TE2 Launch) at the entire C-band (1.530-1.565 µm). The device has negligible polarization dependence and hence the TM polarization exhibits similar results.