Australian researchers recently made long-period gratings in regular silica fiber that exhibit relatively low loss and strong wavelength-dependent transmission and that may be useful for a variety of WDM devices.1 Researchers Mark L. von Bibra, Ann Roberts, and John Canning at the Universities of Melbourne and Sydney (Australia) used focused ion-beam irradiation to change the index of the fiber core.
Most methods for fabricating gratings use fiber that is sensitive to ultraviolet (UV) light. The most common method of making a grating is by irradiating the fiber with UV laser light, using either a mask or wave interference. The ion beam method can be used on any normal fiber and does not require a mask.
Ion bombardment mainly affects the refractive index in a material at the depth at which the ions come to rest. This effect is apparently a byproduct of changing the density of the substrate and of chemical changes in the area as well. The depth to which the ions penetrate depends on the ion's mass, initial energy, and the substrate material. By choosing the ion mass and energy with respect to the application, and using a focused beam and controlled ion dose, researchers can control the volume of the substrate affected and control the extent of the refractive-index change. The index change varies linearly with the ion dose, allowing straightforward irradiation schemes (unlike the nonlinear response of index change to UV irradiation).
Last year, M. Fujimaki (Waseda University, Tokyo) and fellow researchers reported creating long-period gratings using masked ion implantation with 5.1 MeV He2+ ions.2 In Fujimaki's work, the stopping distance of the ions was about 24 µm below the surface of the fiber—because the fiber's core is considerably deeper than this distance, the fiber had to be etched before bombardment for the ions to reach the core. In von Bibra's experiment, hydrogen ions with a stopping distance of 62.5 µm were used, allowing the researchers to change the index in the 125-µm-diameter fiber without etching the fiber. The only preparation was removing the plastic coating on the fiber.
The direct-write method using focused ion beams is borrowed from previous work in bulk silica for writing waveguides, undertaken by the Australian researchers. One of the advantages of direct writing is that it allows researchers to write many different grating profiles, unlike mask methods.
Although direct-write methods are extremely valuable for developing new grating designs, they are less likely to be practical for manufacturing large quantities of devices. "They may also be useful for the low-volume production of specialized devices," said Ann Roberts.
One of the advantages of the system is the ability to choose the shape of the beam spot. By choosing a Gaussian beam, the researchers made a grating with a 500-µm period, with a roughly sinusoidal index change (see figure).
The researchers are optimizing the process to improve transmission properties of the gratings, including work on varying the magnitude of the index modulation and the number of grating periods.
Contact Ann Roberts at a.roberts@ physics.unimelb.edu.au.
- M. L. von Bibra, A. Roberts, and J. Canning, Opt. Lett. 26, 765 (June 1, 2001).
- M. Fujimaki et al., Opt. Lett. 25, 88 (2000).