Polymer Optical Amplifiers Are on the Way

Richard Gaughan

Optical polymers are attractive candidates for composing future integrated optical devices. The desirable rare-earth ions are insoluble in the polymers, however, precluding their use in integrated optical amplifiers. Now researchers at the FOM Institute for Atomic and Molecular Physics have overcome the problem.

Researchers have discovered methods of incorporating rare-earth ions into optical polymers. The work may lead to the development of integrated optical amplifiers for telecommunications networks.

In rare-earth ions such as erbium and neodymium, the population of electronic levels within one of the inner shells can be inverted to provide optical gain. The inner shell is shielded from the surrounding medium by the outer shell, enabling narrow linewidths, long inverted-state lifetimes and an energy band structure nearly independent of the surrounding medium.

Because erbium's band structure supports emission in the middle of the telecommunications band at 1.54 µm, it is used in fiber amplifiers worldwide. Today there is interest in performing all of the steps in the telecommunications process optically, rather than electronically, using integrated optical devices for modulation, switching and amplifying optical signals. Inserting erbium into a plastic matrix has presented a challenge.

Lenneke Slooff, working with Albert Polman, has developed methods of embedding rare-earth ions in optical polymers. In one, the erbium is enclosed in a polydentate cage, which serves the functions of holding the ions, attaching to the polymer substrate and providing an energy-transfer mechanism from the ligand to the erbium. In another, neodymium is encased in a lissamine sensitizer, which has an absorption cross section that is four orders of magnitude greater than neodymium alone.

The researchers found that both methods introduced rare-earth ions into an optical polymer that can exhibit gain. The polydentate-encased erbium displayed a luminescence lifetime of 0.8 µs and exhibited a large emission bandwidth of about 70 nm. Nevertheless, it supported a gain of 1.7 dB/cm in a polymethylmethacrylate matrix.

In contrast, the lissamine-functionalized neodymium is such an efficient absorber that traditional butt-end coupling introduced so much energy that the structure of the chemical bonds was modified to reduce emission at 1.34 µm. However, when pumped in a parallel-waveguide-coupling structure, the material exhibited gain.

Moving to electrical pumping

Slooff has investigated other methods of introducing rare-earth ions into polymer-based integrated optical structures and has demonstrated electrically excited luminescence, which opens the door to electrically pumped optical amplification.