Fabrication of surface nanoscale axial photonics structures with a femtosecond laser

Abstract

Surface nanoscale axial photonics (SNAP) structures are fabricated with a femtosecond laser for the first time. The inscriptions introduced by the laser pressurize the fiber and cause its nanoscale effective radius variation. We demonstrate the subangstrom precise fabrication of individual and coupled SNAP microresonators having the effective radius variation of several nanometers. Our results pave the way to a novel ultraprecise SNAP fabrication technology based on the femtosecond laser inscription.Fabrication of microscopic photonic devices and circuits with ultrahigh precision and ultralow loss is of great interest due to their potential applications in optical processing [1][2][3][4][5], quantum computing [6], microwave photonics [7,8], optical metrology [9], and advanced sensing [10]. 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). The performance of these structures is based on whispering gallery modes which circulate around the fiber surface and slowly propagate along the fiber axis. The axial propagation of these modes is so slow that is it can be fully controlled by the nanoscale ERV of the fiber [12]. Usually, light is coupled into the SNAP fiber with a transverse microfiber attached to the fiber surface (Fig. 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. While a very high subangstrom fabrication precision has been achieved using this method [13,14], the minimum characteristic length of the introduced ERV is relatively large (~50 μm). SNAP structures can be also fabricated using a UV laser beam exposure of a photosensitive fiber [12,15]. However, the ERV introduced is typically a few nm only, limited by the available magnitude of photosensitivity. In addition, the requirement of photosensitivity of the fiber restricts the applications of this method. Fig. 1. Illustration of a nanoscale ERV caused by the stress introduced by a femtosecond laser inscription inside the fiber. A microfiber is attached to the fiber surface to excite a whispering gallery mode. The microfiber is also used to characterize the ERV. Inset illustrates a magnified ERV introduced by the femtosecond laser inscription.Femtosecond laser inscription is a powerful technology that ha...

Optical frequency combs generated mechanically

“…We assume that, due to the smoothness of the bottle resonator radius variation ( ) r z  and the semiclassical nature of the axial spectrum, the deviation from equidistance of the spectral series considered can be made very small, at least within a relatively small bandwidth. Tuning of eigenfrequencies by CO2 and femtosecond laser post-processing is possible as well [28,29]. We suggest that the maximum deviation from the equidistance of the spectral series considered, max | | m  has the order of 1 MHz, similar to [30].…”

Surface nanoscale axial photonics structures introduced by bending of optical fibers

“…The propagation is described by the one-dimensional Schrödinger equation. SNAP devices can be manufactured with unprecedented sub-angstrom precision [2] by local annealing with focused CO2 laser radiation [1] and by the femtosecond laser inscription [3]. The subangstrom fabrication precision of SNAP devices and their ultralow loss allows one to create complex miniature photonic circuits having promising applications in communication technologies, quantum computing, optomechanics, microfluidics, and sensing.…”

Rectangular SNAP microresonator fabricated with a femtosecond laser

“…Multifunctional SNAP devices can be created by modifying a uniform fiber with the introduction of nanoscale effective radius variation (ERV). SNAP resonant structures with nanoscale ERV can be created by using different approaches including annealing with focused CO2 laser beams [1,2], femtosecond laser inscription [3,4], local heating [5], tapering [6], bending [7], and evanescent coupling with droplets in microcapillaries [8]. One of the important applications of the SNAP platform consists in fabrication of miniature processors of optical pulses.…”

“…For CO2 laser annealing, the maximum demonstrated cutoff wavelength shift is ~ 1 nm and the contrast is limited by the characteristic size of the laser beam ~ 50 µm. For femtosecond laser inscription in the fiber core [3,4], the size of the laser beam can be as small as a micron and the maximum cutoff wavelength shift is, similarly, close to 1 nm. However, the laserinduced stresses inside the fiber spread the effect of the laser inscription along the fiber length having the order of the fiber radius.…”