In modern optical communication networks, reconfigurable optical add-drop multiplexers (ROADM) are key devices that play an important role in dynamic wavelength-division multiplexing (WDM). A ROADM enables multiple wavelengths carrying data to be added and/or dropped from a transmission optical fibre port without the need to convert the optical signals to electronic signals and again back to optical signals.
The main advantages of using ROADMs in optical networks are the ability to dynamically allocate the available network bandwidth to individual users while not affecting the traffic, and to equalise the power levels of the different wavelength channels processed through every ROADM.
At the Centre of Excellence in Microphotonic Systems, our researchers have developed a new ROADM structure based on 4-f imaging system implemented using an Opto-VLSI processor and a fibre-lens collimator array. By partitioning the Opto-VLSI processor into pixel blocks and driving each pixel block with a “drop” or a “thru (i.e. through)” steering phase hologram, arbitrary wavelength-division multiplexing channel add-drop multiplexing can be achieved.
Experimental results have shown a low insertion loss of 6dB and crosstalk level of no larger than -30.5dB for the drop and through operation modes. Phase hologram optimisation of the Opto-VLSI processor has been investigated, for maximising the number of wavelength channels while keeping adequate insertion loss and crosstalk levels. In addition, wavelength-division multiplexing power equalisation has been achieved by controlling the maximum phase level of the steering holograms.