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Fabrication of long-period fiber gratings by use of focused ion-beam irradiation M. L. von Bibra and A. Roberts School of Physics, The University of Melbourne, Melbourne, Victoria 3010, Australia
J. Canning Optical Fibre Technology Centre, The University of Sydney, Sydney, New South Wales 2006, Australia
Received November 13, 2000 Long-period gratings have been made in nonphotosensitive optical fibers by irradiation of the core of a fiber with a focused beam of high-energy protons. The irradiated fibers exhibit relatively low loss, even before thermal annealing, and possess strongly wavelength-dependent transmission. The absence of a mask provides the opportunity to tailor the grating to a desired profile, and a variety of grating profiles were explored. The profile most resembling a sinusoid was found to produce the cleanest transmission spectra. © 2001 Optical Society of America OCIS codes: 060.0060, 060.2370, 350.2770.
There is currently a great deal of interest in the development of integrated optical and in-fiber devices for optical communications as well as optical sensing. In particular, there is a significant amount of ongoing research into grating structures in which the light-guiding region of the f iber or waveguide contains a periodic longitudinal variation in refractive index. The ref lectivity and transmissivity of the grating are strongly dependent on the wavelength of the light and the grating parameters. These gratings are of great interest, therefore, as frequency-selective devices for wavelength-division multiplexing in communications,1 integrated ref lectors for f iber and waveguide lasers,2 and sensors3 for measurements including strain, pressure, acceleration, temperature, and magnetic field in which changes in the spectral transmission characteristics of the device are monitored. Two types of f iber grating have attracted attention as in-f iber wavelength-selective devices. Bragg gratings have a period of several hundred nanometers, whereas long-period gratings4 have periods of several hundred micrometers, and it is this latter type of grating that is the subject of this Letter. Fibers containing long-period gratings exhibit a transmission minimum when there is strong coupling from the propagating fundamental mode into a cladding mode. Most popular grating fabrication methods require photosensitization of the f iber before the grating is produced by irradiation of the f iber with strong UV laser irradiation by use of either an optical phase mask or interferometric methods.5 Other techniques that have been demonstrated for the production of longperiod gratings include the introduction of periodic microbends into the fiber by use of electric arcs6 and physical deformation of the fiber.7 Most recently, masked ion implantation with 5.1-MeV He21 ions was demonstrated as an alternative fabrication method for long-period gratings.8 Ion irra0146-9592/01/110765-03$15.00/0
diation has long been known to produce signif icant (up to 1% when implantation takes place at room temperature) index enhancement in glasses.9 Measurements10 show that the glass is compacted predominantly at the end of range of the ions when they come to rest within the substrate material, and theory suggests that the compaction is caused mainly by nuclear interactions between the ions and the atoms in the material. The end of range, or stopping distance, of the ions is determined by the mass of the ion and its initial energy as well as the physical properties of the substrate. The actual index prof ile as a function of distance from the irradiated surface is well known to be slightly asymmetric9 – 12 and, for the typical energies used in the research described here, has a width of the order of several micrometers. In the work described in Ref. 8, the f iber was etched so that the 5.1-MeV He21 ions were implanted within the core. The stopping distance of He21 with this energy in silica is only ⬃24 mm, whereas the distance from the center of the core to the surface of an unetched optical fiber is significantly greater. H1 ions (protons) with an energy of 2.4 MeV, however, have a stopping distance in silica of approximately 62.5 mm, and so significant index changes can be produced in the core of a standard 125-mm f iber with no preparation of the f iber other than removing the plastic coating. There are several advantages to using ion implantation rather than existing techniques to produce gratings. First, the index of any commercially available silica f iber can be changed by use of ion implantation, and thus the use of specially prepared photosensitive fibers is not required. Second, at the ion doses that are required for fabrication of gratings, the index change is approximately linear with ion dose. This linear index change eliminates the necessity for complex irradiation schemes of the type required for UV © 2001 Optical Society of America
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irradiation of photosensitive glasses,13 which have a nonlinear response, to produce gratings with well-characterized refractive-index prof iles. Finally, the refractive-index change produced in silica by ion irradiation is known to be stable to above 500 ±C,14 and this fact may present significant advantages in certain sensing applications. In this Letter we investigate the use of a focused beam of 2.4-MeV protons to vary the refractive index within the core of a fiber (Fig. 1). A nuclear microprobe consisting of magnetic quadrupole lenses arranged along the beam line focuses the protons to a submicrometer-sized beam spot. We then apply other magnetic fields to scan the beam across the sample, or we can translate the substrate itself to vary the exposure. Focused ion beams were previously used to modify the refractive index of bulk fused silica to produce waveguides.15 The techniques used in direct-writing optical waveguides are used here to fabricate longperiod gratings in optical f ibers. The technique allows many different grating prof iles to be fabricated, since it is a direct-write technique and the index of individual grating elements can be controlled in a straightforward manner. This type of control is not possible with any scheme relying on a mask. In this Letter we demonstrate that long-period gratings can be produced in silica f ibers by direct writing of an index change into the core with a focused megaelectron-volt proton beam and that the resulting f ibers display distinct minima in their transmission spectra. A variety of standard commercial and specialty silica f ibers were investigated. All fibers had a 125-mm cladding diameter. Before exposure, a 25-mm length of fiber was stripped of its jacket. Transport of ions in materials (TRIM)11 simulations showed that, assuming an approximate silica density of 2.32 g cm23 and a fiber radius of 62.5 mm, a proton energy of approximately 2.6 MeV should be used to locate the end of range within the f iber core. Differential interference contrast images16 taken of optical fibers after they had been irradiated with protons at this energy showed that the end of range was slightly farther below the irradiated surface than anticipated. These images and linear extrapolation suggest that a proton energy of 2.4 MeV should be used to locate the maximum index change in the core. This energy was used to fabricate the gratings discussed below. The fibers were then mounted within the microprobe target chamber15 and irradiated with a focused beam of protons produced by a 5U Pelletron accelerator. Average beam doses for each grating element were in the range 0.7 3 1015 3.1 3 10 15 ions cm22 , and total grating exposure times were typically in the range 15–30 min. To fabricate a grating element, we held the fiber stationary while it was exposed to the fixed focused beam. After each exposure, the fiber was translated along its axis by a distance equal to the grating period. Three different beam-spot shapes were investigated. In the f irst, a Gaussian-like beam prof ile with dimensions of approximately 290 mm 3 40 mm was used. The long dimension of the beam was oriented along the axis of the f iber. Note
that for f ibers fabricated with a period of 250 mm there is considerable overlap between the edges of the areas irradiated in each step and that this produces a graded grating prof ile. In the second scheme an approximately square grating prof ile was produced, and in the third a tightly focused beam that produced small isolated periodic index enhancements along the fiber was used. The transmission of the irradiated fibers as a function of wavelength was then measured with an optical spectrum analyzer with unpolarized light. The result was normalized to the transmission through a corresponding length of identical unirradiated fiber. Of the three irradiation schemes used, the first produced the clearest intensity minima in the transmission spectra. Fibers containing gratings fabricated by the other two techniques had many more closely spaced minima in their transmission spectra as a result of coupling from the f iber fundamental mode into cladding modes via harmonics of the grating spatial frequency.13 A differential interference contrast image of a fiber containing a 500-mm-period grating fabricated with the broad approximately Gaussian beam spot and an average dose of 1.2 3 1015 ions cm22 for each spot is shown in Fig. 2(a), and the normalized intensity along a line through the core of this image is shown in Fig. 2(b). The 250-mm-period gratings were produced with the same beam shape, and therefore the irradiated regions overlap in the resulting grating. A typical spectrum of the transmission through a germanosilicate optical f iber (OFTC GF-1) fabricated by the Optical Fibre Technology Centre (OFTC), Sydney, Australia, and containing a graded grating is shown in Fig. 3. This f iber contains a grating fabricated with an average proton-beam dose for each grating period of 0.7 3 1015 ions cm22 . Previous studies12 showed that this dose corresponds to an average index change of approximately 2 3 1024 共⬃0.015%兲. A number of distinct resonances can be seen in the transmission spectrum. There is evidence of mode splitting in the resonances near 1200, 1250, 1300, and 1400 nm which is probably a consequence of the anisotropy introduced into the fiber by the noncylindrically symmetric index change. In particular, there is a transmission minimum of the order of approximately 7 dB at 1420 nm, with a 1-dB width of ⬃15 nm. There is a background propagation loss of 1 to 2 dB, primarily as a result of color-center defects introduced during the irradiation process.9 Previous
Fig. 1. Schematic showing the use of a focused ion beam for direct writing of a grating in the core of a f iber.
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sion spectra can be fabricated by irradiation with a focused beam of mega-electron-volt protons. The fabrication process is fundamentally independent of the precise nature of the f iber and, hence, can be performed with any silica-based f iber. Optimizing the fabrication process to improve the transmission properties of the gratings is the subject of ongoing work. In particular, we are investigating the effect of varying the magnitude of the index modulation and the number of grating periods. The behavior of the gratings after thermal annealing is also being explored, along with their temperature stability and polarization dependence. The authors thank Roland Szymanski and David Hoxley for their assistance with grating fabrication. The assistance of members of the OFTC, particularly Tom Ryan, is also gratefully acknowledged, as are useful discussions with Andrew Stevenson. A. Roberts’ e-mail address is
[email protected]. Fig. 2. (a) Differential interference contrast image of a fiber containing a grating with a period of 500 mm and fabricated with an average dose of 1.2 3 1015 ions cm22 . (b) Intensity along the core of the fiber as a function of position normalized to the background intensity passing through the fiber cladding.
Fig. 3. Transmission spectrum through a fiber with a grating with a period of 250 mm and fabricated with an average proton dose of 0.7 3 1015 ions cm22 at an energy of 2.4 MeV.
studies15 showed that thermal annealing at 500 ±C for 1 h of waveguides produced by ion irradiation reduces these defects, and propagation losses decrease from as much as 3 dB cm21 to well below 1 dB cm21 . It is thus expected that the propagation loss through these fibers will be significantly reduced after thermal annealing. We have demonstrated that long-period gratings in fibers that exhibit distinct minima in their transmis-
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