Tripled output reduces grating production times

17 Jun 2002

Frequency-tripled output from an Nd:YAG laser has been used by Australian researchers to write Bragg gratings through fiber coatings. Their method speeds up grating production and reduces fiber-handling requirements.
While gratings have been written before using solid-state diode-pumped lasers, researchers at the Optical-Fiber Technology Centre (OFTC), University of Sydney, believe that this is the first time that gratings have been written by tripling the YAG laser output.
Justin Blows from OFTC said: "Until now it was not possible to buy a commercial system that was sufficiently well engineered to do this sort of experiment. High-repetition-rate frequency-tripled Nd:YAG lasers, such as the Coherent AVIA and Lightwave 210UV, have only recently been released onto the market."
Gratings are typically written using 244 nm light because germanium-doped silica fiber is photosensitive at this wavelength.
While 244 nm light is readily available from sources such as frequency-doubled argon-ion lasers, fiber coatings must be stripped before writing can begin. This reduces the strength of the fiber, which must then be recoated.
When using 355 nm light, stripping the coating is unnecessary and the resulting gratings have a reduced birefringence. This is especially important for multimode fibers where the core diameter is much greater than the absorption depth of 244 nm.
However, the change induced in the photorefractive index by 355 nm light decays over time, so the gratings are annealed to make them more stable. Blows said: "It is still too early to say exactly how similar the stability of these gratings are to 244 nm gratings, but there does not seem to be a substantial difference".
Contact Justin Blows, fax +61 2 9351 1911, e-mail "jblows@oftc.usyd.edu.au".

Story courtesy of Opto and Laser Europe magazine.