Surface nanoscale axial photonics: robust fabrication of high-quality-factor microresonators

Abstract

Recently introduced surface nanoscale axial photonics (SNAP) makes it possible to fabricate high-Q-factor microresonators and other photonic microdevices by dramatically small deformation of the optical fiber surface. To become a practical and robust technology, the SNAP platform requires methods enabling reproducible modification of the optical fiber radius at nanoscale. In this Letter, we demonstrate superaccurate fabrication of high-Q-factor microresonators by nanoscale modification of the optical fiber radius and refractive index using CO 2 laser and UV excimer laser beam exposures. The achieved fabrication accuracy is better than 2 Å in variation of the effective fiber radius. © 2011 Optical Society of America OCIS codes: 060.2340, 140.3945, 230.3990. Surface nanoscale axial photonics (SNAP) is concerned with microscopic optical devices created by smooth and dramatically small nanoscale variation of the optical fiber radius and/or equivalent variation of its refractive index [1,2]. SNAP is based on whispering gallery modes (WGMs), which slowly propagate along the fiber axis and circulate around its surface. Unlike slow light in photonic crystals, which is introduced with the periodic modulation of the refractive index [3], the slow axial propagation of light in SNAP is insured automatically by periodic revolution around the fiber surface. The direction of light propagation considered in SNAP is primarily azimuthal so that the axial propagation is naturally slow. There is both fundamental and applied interest in the development of SNAP. The SNAP devices are fabricated of drawn silica and, hence, exhibit very small losses and high Q factors, similar to those of silica WGM microresonators [4]. In addition, the characteristic axial wavelength of these devices is much greater than the wavelength of light, which makes them convenient for investigation of fundamental properties of light, e.g., tunneling, halting by a point source, and formation of dark states [1,2]. For the same reason, a large axial wavelength simplifies accurate fabrication of high-Q-factor microresonators and other microdevices at the surface of a fiber. Typically, the SNAP photonic elements have tens of micrometer dimensions and a record small attenuation of light with a Q factor in excess of 10 6 [1,2]. This suggests SNAP as a potential platform for miniature integrated photonic circuits having attenuation of light that is orders of magnitude smaller than that in microscopic devices fabricated with the existing photonic platforms [5][6][7].This Letter solves a challenging problem of accurate and reproducible modification of the optical fiber effective radius at nanoscale, which is critical for establishing of SNAP as a practical technology. We propose and demonstrate the fabrication of high-Q-factor SNAP microresonators (Fig. 1) based on IR (CO 2 laser) and UV (248 nm excimer laser) beam exposures.In our experiments, illustrated in Fig. 2, the local perturbations of the optical fiber shape and/or refractive index introduced by the IR and ...

Nanophotonics of optical fibers

“…This problem was solved in [32] using the IR (CO 2 laser) and UV (248 nm excimer laser) beam exposures. It has been shown that it is possible to fabricate coupled SNAP microresonators [32][33][34][35][36] and other devices [37] at the surface of an optical fiber with sub-angstrom precision. A general method of precise variation of the effective radius of an optical fiber is based on local annealing of the fiber surface performed with a CO 2 laser beam [ Figure 8(A)].…”

“…In particular, a series of coupled SNAP microresonators can be fabricated along the Ge-doped fiber by exposure to UV radiation through an amplitude mask as illustrated in Figure 8(B). This approach allows the fabrication of very long chains of couple microresonators with sub-angstrom precision [32,34]. A combination of methods for the local modification of photosensitive optical fiber radii exploiting both UV and CO 2 exposures has been also demonstrated [34].…”

“…The speed of axial propagation of these modes v is much smaller than the speed of light in the fiber material v 0 . The central idea of SNAP (Surface Nanoscale Axial Photonics) [30][31][32][33][34][35][36][37] reviewed in this section is to exploit the sensitivity of WGMs to extremely small variations of the effective fiber radius, which is proportional to the variation of the physical fiber radius Δr(z) = r(z)-r 0 and refractive index…”

Fabrication of surface nanoscale axial photonics structures with a femtosecond laser

“…However, the outstanding fabrication precision achieved in modern microphotonics, which is currently as small as a few nanometers [2,5], is still far beyond the precision required for practical applications [3,11]. Surface nanoscale axial photonics (SNAP) is a new fabrication platform which allows to fabricate photonic structures with an unprecedented subangstrom precision and ultralow loss [12][13][14]. SNAP structures are formed at the surface of an optical fiber with nanoscale effective radius variation (ERV).…”

“…1). Recently several high performance micro-devices, such as coupled ring resonators [13] and bottle resonator delay lines [14], have been demonstrated based on the SNAP platform.SNAP structures are usually fabricated by local annealing of the fiber surface using a focused CO2 laser [12][13][14]. Annealing causes the local relaxation of the residual stress introduced during the fiber drawing process and leads to a nanoscale ERV of the fiber.…”