Integrated Mode Converter Improves Fiber Coupling

Stuart M. Hutson

Researchers at the University of California at Santa Barbara have developed a simplified technique to monolithically incorporate mode converters into photonic integrated circuits, potentially providing a reliable, cost-effective method to couple the devices into optical communications networks.

The InP-based circuits are complex conglomerations of passive and active waveguides and grating elements that can perform "lightwave functions" that allow them to operate similarly to tunable wavelength converters and routers.

However, coupling the devices to optical fibers poses a host of problems, not the least of which is an inherently high optical-signal loss from the large mismatch of mode size between a circuit's waveguide and a standard, single-mode fiber. The problem could be solved by using special lensed fibers or micro-optical elements, but these would significantly increase device-packaging complexity, cost and alignment sensitivity.

The researchers form the mode converter onto the circuit by tapering a portion of a waveguide by means of a bromine-based, diffusion-limited wet etchant and a single regrowth step. They believe that their method offers a relatively cheap and reliable alternative.

To test the integration, they incorporated the converter into a distributed Bragg reflector laser. They reported a 70 percent fiber-to-waveguide coupling efficiency with cleaved fibers and an 86 percent efficiency when coupled with conical-lensed fibers. Loss sensitivities for cleaved fibers were better than 1 dB for fiber misalignments within ±1.85 µm laterally and ±1.5 µm transversely.