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- ItemBunimovich Stadium-like Microsphere for Randomized Fiber Laser Operation(Basel : MDPI, 2018) Silveira, Beatriz; Gomes, André D.; Becker, Martin; Schneidewind, Henrik; Frazão, OrlandoA silica resonator was demonstrated for random laser generation. The resonator consisted of a conventional microsphere fabricated in an optical fiber tip through electric arc discharge, and modifications to its geometry were carried out to create asymmetry inside the silica structure. The resulting Bunimovich stadium-like microsphere promotes multiple reflections with the boundaries, following the stochastic properties of dynamic billiards. The interference of the multiple scattered beams generates a random signal whose intensity was increased by sputter-coating the microstadium with a gold thin film. The random signal is amplified using an erbium-doped fiber amplifier (EDFA) in a ring cavity configuration with feedback, and lasing is identified as temporal and spectral random variations of the signal between consecutive measurements.
- ItemOptical Vernier Effect: Recent Advances and Developments(Weinheim : Wiley VCH, 2021) Gomes, André D.; Bartelt, Hartmut; Frazão, OrlandoThe optical analog of the Vernier effect applied to fiber interferometers is a recent tool to enhance the sensitivity and resolution of optical fiber sensors. This effect relies on the overlap between the signals of two interferometers with slightly detuned interference frequencies. The Vernier envelope modulation generated at the output spectrum presents magnified sensing capabilities (i.e., magnified wavelength shift) compared to that of the individual sensing interferometers that constitute the system, leading to a new generation of highly sensitive fiber sensing devices. This review analyses the recent advances and developments of the optical Vernier effect from a fiber sensing point-of-view. Initially, the fundamentals of the effect are introduced, followed by an extensive review on the state-of-the-art, presenting all the different configurations and types of fiber sensing interferometers used to introduce the optical Vernier effect. This paper also includes an overview of the complex case of enhanced Vernier effect and the introduction of harmonics to the effect.
- ItemDirect observation of modal hybridization in nanofluidic fiber [Invited](Washington, DC : OSA, 2021) Gomes, André D.; Zhao, Jiangbo Tim; Tuniz, Alessandro; Schmidt, Markus A.Hybrid-material optical fibers enhance the capabilities of fiber-optics technologies, extending current functionalities to several emerging application areas. Such platforms rely on the integration of novel materials into the fiber core or cladding, thereby supporting hybrid modes with new characteristics. Here we present experiments that reveal hybrid mode interactions within a doped-core silica fiber containing a central high-index nanofluidic channel. Compared with a standard liquid-filled capillary, calculations predict modes with unique properties emerging as a result of the doped core/cladding interface, possessing a high power fraction inside and outside the nanofluidic channel. Our experiments directly reveal the beating pattern in the fluorescent liquid resulting from the excitation of the first two linearly polarized hybrid modes in this system, being in excellent agreement with theoretical predictions. The efficient excitation and beat of such modes in such an off-resonance situation distinguishes our device from regular directional mode couplers and can benefit applications that demand strong coupling between fundamental- and higher-order- modes, e.g. intermodal third-harmonic generation, bidirectional coupling, and nanofluidic sensing.
- ItemMultimode Fabry-Perot Interferometer Probe based on Vernier Effect for Enhanced Temperature Sensing(Basel : MDPI, 2019) Gomes, André D.; Becker, Martin; Dellith, Jan; Zibaii, Mohammad Ismail; Latifi, Hamid; Rothhardt, Manfred; Bartelt, Hartmut; Frazão, OrlandoNew miniaturized sensors for biological and medical applications must be adapted to the measuring environments and they should provide a high measurement resolution to sense small changes. The Vernier effect is an effective way of magnifying the sensitivity of a device, allowing for higher resolution sensing. We applied this concept to the development of a small-size optical fiber Fabry–Perot interferometer probe that presents more than 60-fold higher sensitivity to temperature than the normal Fabry–Perot interferometer without the Vernier effect. This enables the sensor to reach higher temperature resolutions. The silica Fabry–Perot interferometer is created by focused ion beam milling of the end of a tapered multimode fiber. Multiple Fabry–Perot interferometers with shifted frequencies are generated in the cavity due to the presence of multiple modes. The reflection spectrum shows two main components in the Fast Fourier transform that give rise to the Vernier effect. The superposition of these components presents an enhancement of sensitivity to temperature. The same effect is also obtained by monitoring the reflection spectrum node without any filtering. A temperature sensitivity of −654 pm/°C was obtained between 30 °C and 120 °C, with an experimental resolution of 0.14 °C. Stability measurements are also reported.
- ItemOptical Harmonic Vernier Effect: A New Tool for High Performance Interferometric Fiber Sensors(Basel : MDPI AG, 2019) Gomes, André D.; Ferreira, Marta S.; Bierlich, Jörg; Kobelke, Jens; Rothhardt, Manfred; Bartelt, Hartmut; Frazão, OrlandoThe optical Vernier effect magnifies the sensing capabilities of an interferometer, allowing for unprecedented sensitivities and resolutions to be achieved. Just like a caliper uses two different scales to achieve higher resolution measurements, the optical Vernier effect is based on the overlap in the responses of two interferometers with slightly detuned interference signals. Here, we present a novel approach in detail, which introduces optical harmonics to the Vernier effect through Fabry–Perot interferometers, where the two interferometers can have very different frequencies in the interferometric pattern. We demonstrate not only a considerable enhancement compared to current methods, but also better control of the sensitivity magnification factor, which scales up with the order of the harmonics, allowing us to surpass the limits of the conventional Vernier effect as used today. In addition, this novel concept opens also new ways of dimensioning the sensing structures, together with improved fabrication tolerances.
- ItemGiant refractometric sensitivity by combining extreme optical Vernier effect and modal interference([London] : Macmillan Publishers Limited, part of Springer Nature, 2020) Gomes, André D.; Kobelke, Jens; Bierlich, Jörg; Dellith, Jan; Rothhardt, Manfred; Bartelt, Hartmut; Frazão, OrlandoThe optical Vernier effect consists of overlapping responses of a sensing and a reference interferometer with slightly shifted interferometric frequencies. The beating modulation thus generated presents high magnified sensitivity and resolution compared to the sensing interferometer, if the two interferometers are slightly out of tune with each other. However, the outcome of such a condition is a large beating modulation, immeasurable by conventional detection systems due to practical limitations of the usable spectral range. We propose a method to surpass this limitation by using a few-mode sensing interferometer instead of a single-mode one. The overlap response of the different modes produces a measurable envelope, whilst preserving an extremely high magnification factor, an order of magnification higher than current state-of-the-art performances. Furthermore, we demonstrate the application of that method in the development of a giant sensitivity fibre refractometer with a sensitivity of around 500 µm/RIU (refractive index unit) and with a magnification factor over 850.