If two optical-fiber cores are brought close enough together, light traveling in one evanescently couples to the other. By making a two-core fiber and adding a Bragg grating to one of the cores, researchers at the Université Laval (Sainte-Foy, Canada) and the University of Sydney (Sydney, Australia) have created a wavelength-selective device functioning as a channel-dropping filter. The dispersive properties of the grating serve to dephase light in the vicinity of the Bragg wavelength, preventing it from being evanescently coupled.
A dissimilar two-core fiber was assembled by machining flats on two different doped single-core preforms and fusing them together. Precise knowledge of the fiber characteristics was essential to determine the coupling parameters, so the researchers measured dopant concentrations with an electron microprobe. In the experiment, light from a broadband light-emitting diode was launched into one of the cores. Scanning the cores with a spot of ultraviolet (UV) light allowed the researchers to tune the coupling until it reached close to 100% efficiency. A Bragg grating was then written onto one of the cores by scanning the UV spot over a phase mask in proximity to a fiber core. Additional fine-tuning allowed the researchers to restore coupling to high efficiency-but with the addition of a single transmission dip of 0.7-nm bandwidth.
Contrary to first expectations, further UV irradiation did not immediately result in overcoupling, but produced a broadening of the coupling peak to an extended flat-top region of about 40 nm; only after continued irradiation did the efficiency drop from 100%. The grating dropped the transmission in the narrow spectral band by 22 dB. The coupling efficiency degraded over time, a condition preventable by using a different UV write wavelength and by redesigning the device to require a smaller intrinsic detuning, say the researchers. Contact Real Vallee at [email protected].