OFC '00: Chalcogenide Films Provide High Nonlinearities for Low Power Optical Switching
EDITOR'S TECH BRIEF
BALTIMORE—By engineering the optical properties of chalcogenide glasses films to maximize nonlinearities for wavelengths near 1.5 µm, researchers have developed promising materials for compact switching waveguides. Electronics will become a limiting factor for photonic components operating at speeds beyond 40 Gb/s, causing designers to turn to optical time division multiplexing (OTDM). Achieving sufficiently fast switching for sub-picosecond pulse widths at powers below 1 pJ necessitates materials with extremely high Kerr nonlinearities.
Stefan Spälter and colleagues at <%=company%> Bell Labs (Murray Hill, NJ) deposited chalcogenide thin films onto silica via pulsed laser deposition, fabricating waveguides and demonstrating self-phase modulation characteristics that show potential for high speed all-optical switching (AOS). The group reported their results at the Optical Fiber Communications Conference (OFC 2000; March 5-10).1
Thin films
The Kerr nonlinearity generates an intensity-dependent refractive index change that triggers an intensity-dependent phase modulation. Converting the phase modulation into an amplitude modulation yields an AOS device.
The group fabricated chalcogenide films based on Ge0.25Se0.75-xTex, varying the telluride to tune the material bandgap to achieve compositions with nonlinear values hundreds of times that of pure silica. They deposited the films using pulsed laser deposition in conjunction with bulk chalcogenide glass samples, achieving films with roughnesses of 1 nm rms.
The project resulted in two types of waveguides. In the first, the films were deposited atop ridged silica waveguides; the chalcogenide-air interface provided lateral confinement. In the second technique, the researchers used a photodarkening effect and refractive index change triggered by illumination at wavelengths close to the bandgap energy (532 nm or 647 nm). Using this technique, the group wrote 3-µm wide channel waveguides into the films, with index contrasts on the order of 10-3 to 10-2.
Testing
Using the photodarkened waveguides, the researchers tested nonlinear pulse propagation through the medium by coupling near-transform-limited, 270-fs pulses with energies of up to 1.04 nJ into the waveguides, at a repetition rate of 13.5 MHz. The results indicated a Kerr nonlinearity of n2=1.5 x 10-14 cm2/W, which is roughly 58 times the value for silica. Combined with the two-photon absorption figure of B=5.3 x 10-12 cm/W, chalcogenide waveguides show good potential for AOS device development.
Unlike silica AOS devices, which can require hundreds of meters of material, chalcogenide waveguides offer the potential of switching components mere centimeters long.
References
1. Stefan Spälter et. al., "Highly nonlinear chalcogenide glasses for ultrafast all-optical switching in optical TDM communications systems," Optical Fiber Communications Conference, paper #ThI4, Baltimore, MD (2000).
By: Kristin Lewotsky, Photonics Online