Frequency translating add/drop filters designed for on-chip light manipulation

Three-dimensional rendering showing the frequency translating add/drop filters. It includes two active microring resonances, interferometric input/output wavesguides, metal wires connecting to thermo-optic and electro-optic phaseshifters, and vertical grating couplers. Credit: Hayk Gevorgyan, Boston University
Researchers have developed frequency-translating add/drop filters using electro-optically modulated photosnic molecules. This new filter class could offer new opportunities for light manipulation on-chip.

Hayk Gevorgyan, Boston University, U.S., will present the research at Frontiers in Optics + Laser Science Conference, (FiO LS), all-virtual meeting, 01-04 Nov 2021.

You can add/drop filters to change the channel's light channels without affecting other channels. These filters are used primarily in optical data communication but also serve a purpose for quantum information processing and optical neural networks. They are linear time-invariant systems and can drop or add light channels, but they never change their wavelength. The new filters have the ability to change the frequency of light signals that are added or dropped. This is a key characteristic.

Gevorgyan stated that "This is a fundamentally different building block in the toolbox for optical chip designers." It can be used by other people to create more complex systems on the chip because it was implemented in a silicon chip findry process. This frequency shifting filter concept could make it easier to manipulate wavelength channels on chip in order to control congestion of wavelengths in data communications. It could also allow for a new type beam splitter to enable quantum computing using photons.

These new filters are based on earlier work by the researchers, which created frequency translating filters that combined microring modators and linear filters to produce a device with a shift drop-port response passband. The design consisted of two microring resonators that were actively coupled and one bus waveguide. The through and drop ports were frequency-shifted from one another, but they also shared the same physical port. This would make it possible to use linear filtering to separate signals spatially for certain applications.

Researchers created a second order frequency translating add/drop filter that maps to distinct waveguide ports. Two microring resonators are used in the device, with two waveguides attached to each ring. Each of the four connections has equal coupling strengths.

Optical microscope image showing the second order frequency-translating add/drop filter, designed by researchers at Boston University. It was implemented using IMEC’s ISIPP50G active silica photonics process. Credit: Hayk Gevorgyan

The researchers tested the device by first measuring passive optical transmission between ports using no electrical signal. The frequency translating response was then measured. They found that the device had filter passbands that were different from their respective frequencies. Thanks to the careful tuning of phase delays using thermo-optic phases shifters, cross talk between channels was kept below 40 dB.

Gevorgyan says, "While these initial results are encouraging, it is not surprising that the insertion loss is quite high. The lowest recorded loss to date is 13dB. The frequency translating filters must have a insertion loss of less than 5 dB, just like regular filters. This is necessary to be useful in data communication. Optimizing the speed, efficiency, and propagation loss of the p-n junction Phase Shifters used in the device can help achieve this. Quantum applications require even stricter requirements, with a target insertion loss of 0.5 dB. We believe that p-n phase shifters cannot achieve such efficiency due to the inherent loss of carrier plasma dispersion effects. These applications may be possible if they are implemented in material platforms like lithium niobate or barium titanate, or a silicon-organic hybrid.

Continue reading Configuring microwave photonic filters with an external device

Further information: Gevorgyan's presentation will be held on Tuesday, 02/11 at 16:30 EDT (UTC 05:00).