Abstract:Single-mode waveguide designs frequently support higher order transverse modes, usually as a consequence of process limitations such as lithography. In these systems, it is important to minimize coupling to higher-order modes so that the system nonetheless behaves single mode. We propose a variational approach to design adiabatic waveguide connections with minimal intermodal coupling. An application of this algorithm in designing the "S-bend" of a whispering-gallery spiral waveguide is demonstrated with approximately 0.05 dB insertion loss. Compared to other approaches, our algorithm requires less fabrication resolution and is able to minimize the transition loss over a broadband spectrum. The method can be applied to a wide range of turns and connections and has the advantage of handling connections with arbitrary boundary conditions.
We present thermally regenerated fiber Bragg gratings in air-hole microstructured fibers for high-temperature, hydrostatic pressure measurements. High-temperature stable gratings were regenerated during an 800°C annealing process from hydrogen-loaded Type I seed gratings. The wavelength shifts and separation of grating peaks were studied as functions of external hydrostatic pressure from 15 to 2400 psi, and temperature from 24°C to 800°C. This Letter demonstrates a multiplexible pressure and temperature sensor technology for high-temperature environments using a single optical fiber feedthrough. © 2011 Optical Society of America OCIS codes: 060.2370, 060.4005, 120.5475, 120.6780. Sensors that operate at high temperatures are needed for a wide range of applications in the energy, automobile, and aerospace industries. For example, fast, accurate, and reliable interrogation of gas pressure information ensures safe and efficient operations of gas turbine, coal boilers, and power plants, where the operating temperatures range from 400°C to more than 1000°C. Optical fiber sensors have always been considered good candidates for harsh environment applications. A single Fabry-Perot interferometer implemented on a fiber tip can perform pressure or temperature sensing beyondCompared with fiber interferometer sensors, fiber Bragg gratings (FBGs) offer greater multiplexing capability [2], and continuous efforts have been made to improve the operating temperature of FBGs. These may involve modifying the chemical composition of the fiber core [3], and using an ultrafast laser to form a Type II damaged grating [4]. Previously, we reported a high-temperature pressure fiber sensor in which the grating was inscribed in an air-hole microstructure fiber with an ultrafast laser [5]. The Type II FBG shows stable and reproducible pressure sensing operation over 800°C. But further optimization of grating linewidth and suppression of the strong laser-induced birefringence are needed to allow better pressure sensing range and accuracy. The relatively large in-line loss with Type II permanent damage also limits sensor multiplexing.Recently, a new type of high-temperature FBG was reported, in which grating structures are regenerated after the Type I seed gratings are erased during a hightemperature annealing process [6][7][8][9][10]. Unlike chemical composition gratings, it has been shown that the regeneration process is independent from dopants in fiber cores [8] and photosensitization processes, such as hydrogen loading [10]. This versatility opens up hightemperature stabilization to other material systems and allows postregeneration of unloaded gratings, including arrays of online draw tower gratings [10]. By carefully controlling the strength of the seed grating and annealing procedures, high-temperature gratings with ∼35% reflectivity and narrow linewidth can be regenerated [7]. Stable operating temperature up to 1295°C was reported for regenerative gratings [8]. The ultrahigh temperature stability, good grating qualities, and rela...
We present spatially resolved Rayleigh scattering measurements in different polarization-maintaining (PM) fibers for high-temperature pressure sensing. The pressure-induced birefringence in the fiber cores is interrogated using polarization-resolved frequency-swept interferometry. The pressure responses of a PM photonic crystal fiber and a twin-air-hole PM fiber are investigated for a pressure range of 0 to 13.8 MPa (0-2000 psi) at room temperature and at temperatures as high as 800°C. The proposed sensing system provides, for the first time to our knowledge, a truly distributed pressure-sensing solution for high-temperature applications. © 2012 Optical Society of America OCIS codes: 060.2370, 060.4005, 120.5475, 120.6780. High-temperature pressure sensing is a challenging but indispensible task for a wide spectrum of applications in energy and aerospace industries. It plays a critical role in ensuring safe and efficient operations of fossil-fuel and nuclear power generation systems. For sensing at high temperatures beyond 800°C, fiber sensors are probably the only option due to their excellent thermal resistivity and immunity to electromagnetic noise and corrosion. In the past few years, point sensors such as Fabry-Perot interferometry devices have been successfully developed for high-temperature applications [1][2][3]. A more challenging issue is to develop multiplexing technology so the entire power system can be monitored using a single fiber. This is very important given the challenging of wiring tens or hundreds of sensors for extreme environment applications. Quasi-distributed pressure sensing can be realized with fiber Bragg gratings (FBGs) multiplexed in pressure-sensitive fibers. However, FBG-based distributed sensing capability is fundamentally limited by the multiplexing density of the FBGs and the consequent high manufacturing cost. Moreover, special grating treatments such as femtosecond laser direct writing and grating regeneration are needed to elevate the survival temperature of FBGs [4][5].In this Letter, we reported for the first time truly distributed pressure sensing at room and high temperature using pressure-sensitive fibers. It is based on the optical frequency domain reflectometry measurement of in-fiber Rayleigh scattering. This technology has been applied to provide a distributed sensing solution for temperature, axial strain, and transverse stress measurements [6][7][8]. In this Letter, air-hole microstructural fibers are used to extend its application for pressure measurement at high temperature. By simultaneously measuring the in-fiber Rayleigh scattering of two orthogonal polarizations, hydrostatic gas pressure applied upon the fiber under test (FUT) are spatially interrogated in subcentimeter resolution over several meters length of optical fibers. Pressure distributions of up to 13.8 MPa (2000 psi) are measured in a temperature range from 24°C to 800°C. The technique demonstrated in this Letter completely eliminates the use of high-temperature stable FBG sensors and the chall...
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