Sensors and Actuators A 243 (2016) 111–116 Contents lists available at ScienceDirect Sensors and Actuators A: Physical j ourna l h o mepage: www.elsevier.com/locate/sna Fabrication and characterization of laser-ablated cladding resonances of two different-diameter photosensitive optical fibers Md. Rajibul Islam a , Dinusha Serandi Gunawardena a , Yen-Sian Lee a , Kok-Sing Lim a,∗ , Hang Zhou Yang b , Harith Ahmad a a Photonics Research Centre, University of Malaya, 50603 Kuala Lumpur, Malaysia b School of Physics, Northwest University, Xi’an, Shaanxi 710069, China a r t i c l e i n f o Article history: Received 22 December 2015 Received in revised form 21 March 2016 Accepted 22 March 2016 Available online 23 March 2016 Keywords: Laser ablation Laser-ablated clad grating Cladding resonance Refractive index sensing a b s t r a c t In this work, we demonstrate the inscription of grating in both cladding and core of photosensitive single-mode optical fibers with diameter 4.4/125 m and 4.2/80 m. Such inscription is made possible through a hydrogenation process for the photosensitive fiber and employ high energy focused pulsed laser from an ArF excimer laser. In the fabrication, a periodic ablation is formed on the fiber surface and the optical damage is extended from the fiber surface towards the core of the fiber, observable under an optical microscope. As a result, cladding modes are excited and cladding resonances are produced in the output spectra. The response of cladding resonances to ambient refractive index (RI) is characterized and investigated through simulation and experiment. It is observed that cladding resonances in the fiber with smaller diameter are more sensitive to the ambient RI variation. © 2016 Elsevier B.V. All rights reserved. 1. Introduction Fiber Bragg grating (FBG) is one of the most commonly used photonic devices in a variety of industrial sensing applications. The advantages of FBG sensors include small size, wavelength encoding and multiplexing capability. The emergence of photosensitive fiber based on germanium-doped silica fibers [1–3] has enhanced the development and popularity of the FBG in numerous applications. In addition, photosensitive fibers offer exciting prospects for the development of other fiber devices as fiber grating assisted mode couplers [4], sensors [5,6], fiber laser [7], etc. In a typical transmission spectrum of fiber Bragg gratings (FBGs), it is common to observe a comb of resonance wavelengths shorter than the Bragg wavelength. These wavelengths are the result of resonant coupling between the forward-propagating core mode and other backward-propagating cladding modes in the fiber [1]. The coupling into higher azimuthal fiber modes can be realized in tilted FBGs [1,2] or in FBG with an asymmetric transverse index pro- file [8]. However, the fabrication of tilted FBGs is complicated and tedius. Chojetzki et al. reported excitation of cladding resonance by optical damage in fiber during grating formation. Such optical damage occurred in the grating inscription during fiber drawing ∗ Corresponding author. E-mail addresses: kslim@um.edu.my, cosine0 84@yahoo.com (K.-S. Lim). process by KrF excimer laser [9]. Recently, FBG inscription was per- formed on optical fibers which were drawn down to the scale of several micrometer. Fang et al. reported FBGs fabricated on these microfibers using an 800 nm femtosecond laser [10] and Xuan et al. demonstrated a LPG in microfibers inscribed by femtosecond laser [11], where the grating was formed through physical damage of the microfibers. Due to the small diameter of microfibers, this physical damage makes them fragile. The issue can be alleviated by inscrib- ing the grating on microfiber using a 248 nm KrF excimer laser as demonstrated by Zhang et al. [12]. More literature on higher order cladding modes excitation in the fiber by physical damage or deformation in the gratings can be found in [7–10]. In this article, we present a Laser-ablated clad grating (LaCG) inscribed using a ArF excimer laser (193 nm) in 4.4/125 m and 4.2/80 m photosensitive fibers through an asymmetric photo- induced change of cladding using a precise level of high intensity UV laser along with high pressure hydrogenation process. In addi- tion, RI sensitivity of LaCG is measured by the magnitude of spectral shift. These gratings are inscribed on two fibers with different cladding diameters and their sensitivities to ambient RI variation are characterized. To the best of our knowledge, there has been no report to date of laser ablated clad grating using ArF excimer laser. Our aim is to investigate the fabrication of these laser-ablated clad gratings in a non-photosensitive cladding and characterize the resultant cladding resonances by RI sensing. This proposed laser- ablated clad grating can be considered as an alternative to tilted http://dx.doi.org/10.1016/j.sna.2016.03.025 0924-4247/© 2016 Elsevier B.V. All rights reserved.