Waveguide beamsplitters integrated with an erbium-doped thin-film optical amplifier

Abstract

We developed waveguide beamsplitters integrated with an Er- doped thin-film optical amplifier. A 1 to 2-cm-long Er-doped optical amplifier is monolithically cascaded to a beamsplitter in order to compensate for the losses of the beamsplitter. Both devices use a highly Er-doped silicate glass film as a guiding layer. Two different types of structures were investigated for a beamsplitter, i.e., a Y-branch type and a self-imaging multimode interference (MMI) type device. The beam propagation method (BPM) was used for the analysis and design of the beamsplitter structures. The beamsplitter part was designed to be much shorter than the amplifier (i.e., 1 - 2 mm versus 1 - 2 cm) in order to minimize the absorption of a signal beam by Er ions in the potentially underpumped splitter section. Both the amplifier and the beamsplitter parts have a ridge waveguide structure. A novel process technique was developed and used in forming the ridges. The process does not require etching of an Er-doped film in defining the lateral dimension of a waveguide, but involves a lift-off process with a collimated magnetron sputtering. A 1.7-cm-long waveguide thus fabricated shows a 1.55-micrometer signal enhancement of 15.4 dB with a 980 nm pump power of 40 mW. This enhancement fully compensates for both Er absorption and waveguide losses, and results in a gain of 7.2 dB. This demonstrates that 1 - 2-cm-long waveguide amplifiers can provide an optical gain sufficient to compensate for the 1 by 2 or 1 by 4 splitter losses.