Laser writing fabricates optical delay lines in gallium arsenide
Researchers at Columbia University (New York, NY) and UCLA (Los Angeles, CA) have demonstrated design and fabrication of optical delay lines in an epitaxial GaAs/AlGaAs substrate using laser etching. The micron-scale accuracy possible with laser fabrication of such a monolithic time-delay network offers greater precision than fiber delay lines. A focused 275-nm laser beam photochemically etched parallel grooves into the GaAs layer to form the network. The lower refractive index of the etched grooves confines the 1.3-µm Nd:YAG laser light launched into the GaAs-layer waveguides. Separation of the grooves was set at 10 mm, which resulted in multimode behavior. This distance was chosen as a compromise between efficient coupling to optical fibers and low losses in the bend sections. The delay lines that were tested had the desired delays and output power uniformity within 4%.
The integrated photonic circuits produced by laser etching included smoothly curved 90° bends, asymmetric splitters of various splitting ratios, as well as linear waveguide sections. The splitter sections have the appearance of microscale railroad switches. Splitting ratios vary depending on the length of the overlap between the "mainline" and the "branch." These devices were fabricated for use in an optically steered phased-array radar system, but the researchers are looking to develop prototypes of other large-scale photonic circuits, including switching arrays and wavelength multiplexers.