Fiber Optics

Gain in single-pass photorefractive polymer optical amplifier reaches factor of five

Sept. 1, 1997

An optical amplifier comprising stacked photorefractive polymer-composite layers increases gain coefficient and optical interaction length to provide gains as large as a factor of five. Such composite sandwiches could rival their inorganic counterparts as optical amplifiers. W. E. Moerner and colleagues at the University of California (San Diego, CA) performed two-beam coupling experiments on devices formed of a polymer composite consisting of poly(n-vinyl carbazole) as a charge-transport layer, a nonlinear optical chromophore (4-piperidinobenzyl idene-malononitrile) to produce the electro-optical response, a plasticizer to lower the glass-transition temperature, and fullerene (C60) to provide photoinduced charge generation at red wavelengths. To produce the amplifier, the researchers sandwiched 140-µm-thick layers of the composite between 1.1-mm-thick glass slides coated with indium tin oxide, stacking several modules to form multilayer devices. An applied electric field poled the chromophores to establish second-order optical nonlinearity and assisted in charge generation and transport within the device. Applying electric fields of approximately 65 V/µm to a three-layer structure and using signal and pum¥beams generated by a 676-nm krypton-ion laser, the researchers demonstrated single-pass small signal gains of 5X. The grou¥also placed a two-layer device in an optical cavity and observed spontaneous oscillation as a result of the two-beam coupling--the device acted as a self-pumped phase-conjugating mirror with a 13% reflectivity for an applied electric field of 75 V/µm.

Using a moving-grating method, the researchers have most recently obtained a single-pass gain of 500X in a three-layer device.