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...
We UV inscribe and characterize a long-period fiber grating with a period of 25 μm. A series of polarization-dependent dual-peak pairs can be seen in the transmission spectrum, even though only the symmetrical refractive index modification is introduced. The fabricated grating exhibits a lower temperature sensitivity compared with standard long-period gratings and an enhanced refractive index sensitivity of ∼312.5 nm/RIU averaged from 1.315 to 1.395, which is more than four-fold higher than standard long-period gratings in this range. The full width at half-maximum of the fabricated grating is only about 0.6 nm, allowing for high-resolution sensing. Moreover, the grating period is so small that the attenuation dip corresponding to a high-order Bragg resonance can also be seen, which can act as a monitor of the unwanted perturbation to realize dual-parameter sensing.
We propose and demonstrate a highly sensitive refractive index (RI) sensor based on a novel fiber-optic multi-mode interferometer (MMI), which is formed with a femtosecond-laser-induced in-core negative refractive index modified line in a standard single mode fiber. The proposed MMI structure is directly written with femtosecond laser in one step, which removes the splicing process needed in conventional MMI fabrication and also significantly improves the robustness. This device exhibits a high sensitivity to surrounding refractive index, with a maximum sensitivity up to 10675.9 nm/RIU at the RI range of 1.4484-1.4513. The distinct advantages of high sensitivity, compact, robust and assembly-free all-fiber structure make it attractive for real physical, chemical and biological sensing.
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