Infrared laser shapes new miniature devices
A laser beam is small and easy to position, making it useful in forming high-resolution structures in fiber on a much smaller scale than is possible with other heat sources, such as flame. The use of lasers in processing, drawing, and splicing fiber has been limited, however, and not widely accepted in the industry. Now, scientists at the University of Bath (Bath, England) physics department have reportedly developed fiber fabrication techniques using carbon dioxide (CO2) laser beams to process optical fibers on a microscopic scale, creating three new miniature devices.
The second device, a long-period grating (LPG), was formed by heating the fiber with the CO2 laser at 4 W with a focused 0.8-mm beam and slowly elongating it. The resulting microtaper structure repeated at intervals of 280 µm along the length of the narrowed fiber to form the LPG (see The first device, a miniature fused coupler, was made by fusing two already tapered fibers (Corning SMF28) to a uniform inner diameter of 15 µm over a length of 10 mm. The two tapered fibers were placed in parallel and fused by exposure to a 5.6-W, 1550-nm CO2 laser beam with a focused spot of 0.8 mm in diameter for 50 s. The length of the fused interaction region was 200 µm, making it the shortest fiber coupler ever, reports the team (see Fig. 1). The final splitting ratio of 96% shows that the two fibers were well matched in diameter. Other low-loss microcouplers with a variety of splitting ratios were also made. Such microcouplers could lead to miniaturized versions of dense wavelength-division-multiplexing interleavers.
Optical transmission spectra recorded from the LPG showed a steep drop in transmission at 1495 nm after 35 periods, possibly due to coupling to different cladding modes (see Fig. 2). After 55 periods, this notch became overcoupled, and another notch appeared at 1619 nm with a FWHM of only 9 nm. Such LPGs will offer better stability even at high temperatures than most ultraviolet-induced gratings, which degrade over time, claim the researchers.
A new microresonator for whispering-gallery modes (WGMs) was reported as the third device. A fiber narrowed to 15 µm in diameter was microtapered by the laser to a 10-µm diameter at two points 300 µm apart. The resulting cavity between the narrowed necks resembled the middle of a prolate spheroid, and had little transmission loss along the fiber.
The team expects prolate microcavities to have many applications similar to WGMs in microspheres, such as novel add/drop filters and microlasers. "We have considered commercialization of the tapering technique, but we have not licensed the technology at present," said George Kakarantzas, research officer.
REFERENCE
- G. Kakarantzas et al., Opt. Lett. 26, 1137 (2001).
Valerie Coffey-Rosich | Contributing Editor
Valerie Coffey-Rosich is a freelance science and technology writer and editor and a contributing editor for Laser Focus World; she previously served as an Associate Technical Editor (2000-2003) and a Senior Technical Editor (2007-2008) for Laser Focus World.
Valerie holds a BS in physics from the University of Nevada, Reno, and an MA in astronomy from Boston University. She specializes in editing and writing about optics, photonics, astronomy, and physics in academic, reference, and business-to-business publications. In addition to Laser Focus World, her work has appeared online and in print for clients such as the American Institute of Physics, American Heritage Dictionary, BioPhotonics, Encyclopedia Britannica, EuroPhotonics, the Optical Society of America, Photonics Focus, Photonics Spectra, Sky & Telescope, and many others. She is based in Palm Springs, California.