Investigating the resonance spectrum of optical frequency combs in fiber-optic cavities

We report a detailed theoretical and experimental study of fiberoptic cavities under broadband excitation by mode-locked laser combs. We calculate the effects of fiber dispersion on the cavity transmission. For any integer ratio between the comb repetition rate and cavity free spectral range, the theoretical resonant output spectrum exhibits a narrow group of resonant teeth, surrounded by minor, unevenly spaced resonances. Also, the central resonance can be rapidly and precisely tuned over the entire comb span by only acting on its repetition rate. Experimental observations are provided by a single-mode fiber ring and a telecom-wavelength comb laser. The resulting spectral pattern agrees very well with our theoretical prediction, allowing a thorough characterization of the cavity dispersion and opening new perspectives for comb spectroscopy in dielectric resonators. © 2013 Optical Society of America

Localized strain sensing with fiber Bragg-grating ring cavities

We report the theoretical description and the experimental demonstration of an optical resonator formed by inserting a Fiber Bragg Grating (FBG) in a closed fiber loop. The spectral characteristics of such a resonator strongly depend on the reflectivity of the FBG. In the wavelength region where the FBG reflectivity R is negligible, the system behaves like a conventional ring resonator. On the other hand, when R is not vanishing, a split-mode structure can be observed, associated to the degeneracy removal of two counterpropagating resonant modes. The magnitude of the mode splitting can be used to sense small variations of the FBG physical parameters, such as length, temperature or group index. An example of strain sensing with this setup is reported, showing that the mode splitting is sensitive to a mechanical strain applied to the FBG, while it is almost insensitive to a strain applied to any other point of the resonator. This peculiar feature allows to perform cavity-enhanced, local strain measurements with a reduced sensitivity to environmental perturbations, which represents an important improvement in the framework of the fiber-optic sensors. (C)2013 Optical Society of Americ

Liquid Droplet Microresonators

We provide here an overview of passive optical micro-cavities made of droplets in the liquid phase. We focus on resonators that are naturally created and suspended under gravity thanks to interfacial forces, illustrating simple ways to excite whispering-gallery modes in various slow-evaporation liquids using free-space optics. Similar to solid resonators, frequency locking of near-infrared and visible lasers to resonant modes is performed exploiting either phase-sensitive detection of the leakage cavity field or multiple interference between whispering-gallery modes in the scattered light. As opposed to conventional micro-cavity sensors, each droplet acts simultaneously as the sensor and the sample, whereby the internal light can detect dissolved compounds and particles. Optical quality factors up to 107⁻108 are observed in liquid-polymer droplets through photon lifetime measurements. First attempts in using single water droplets are also reported. These achievements point out their huge potential for direct spectroscopy and bio-chemical sensing in liquid environments. Finally, the first experiments of cavity optomechanics with surface acoustic waves in nanolitre droplets are presented. The possibility to perform studies of viscous-elastic properties points to a new paradigm: a droplet device as an opto-fluid-mechanics laboratory on table-top scale under controlled environmental conditions

Detection of chemicals using a novel fiber-optic sensor element built in fiber loop ring-resonators

We developed an evanescent field access block (EAB) sensor with an etched single mode fiber, fabricated with a core diameter of 8 mu m and reduced cladding diameter down to 11 mu m by hydrofluoric acid (HF) etching technique. Here, we describe the manufacturing process of the packaged sensor block and demonstrate that the sensor is able to detect sample absorption and refractive index (RI) changes. The sensor is tested with different chemical solutions such as ethylenediamine (EDA), dimethyl sulfoxide (DMSO) and ethanol (C2H5OH). At the same time, we fabricated (at CSIR-CGCRI) high quality optical fiber couplers having low splitting ratios (99.5:0.5%), which we used to build a 4m long optical evanescent-wave fiber ring cavity with an embedded EAB. After a careful analysis of the fiber loop characteristics, we performed cavity ring-down measurements in order to detect the presence of liquid samples on the EAB. (C) 2014 Elsevier B.V. All rights reserved

Rheology of complex fluids with vibrating fiber-optic sensors

We demonstrate a new optical rheometer where a fiber Bragg grating sensor (FBG) detects the periodical strain caused by the oscillations of a string tightened within a sample fluid. We show that the viscoelastic moduli of complex fluids can be obtained instantaneously from the FBG optical backreflection signal, without need of any current flow in the wire or magnetic field that could affect the sample chemistry. After a validation with known-viscosity solutions, the technique is employed to monitor the phase transition of a sol-gel compound. The results provide an insight in the early-stage gelation dynamics that cannot be obtained with traditional rheometers, and allow to clearly identify the gel point as the intersection of the viscoelastic moduli

Ionizing Radiation Detectors Based on Ge-Doped Optical Fibers Inserted in Resonant Cavities

The measurement of ionizing radiation (IR) is a crucial issue in different areas of interest, from environmental safety and industrial monitoring to aerospace and medicine. Optical fiber sensors have recently proven good candidates as radiation dosimeters. Here we investigate the effect of IR on germanosilicate optical fibers. A piece of Ge-doped fiber enclosed between two fiber Bragg gratings (FBGs) is irradiated with gamma radiation generated by a 6 MV medical linear accelerator. With respect to other FBG-based IR dosimeters, here the sensor is only the bare fiber without any special internal structure. A near infrared laser is frequency locked to the cavity modes for high resolution measurement of radiation induced effects on the fiber optical parameters. In particular, we observe a variation of the fiber thermo-optic response with the radiation dose delivered, as expected from the interaction with Ge defect centers, and demonstrate a detection limit of 360 mGy. This method can have an impact in those contexts where low radiation doses have to be measured both in small volumes or over large areas, such as radiation therapy and radiation protection, while bare optical fibers are cheap and disposable