We introduce a rigorous theory of third-harmonic generation in optical waveguides and apply it to design a micro-fiber waveguide for efficient generation of third-harmonic radiation from infrared lasers. Phase-matching with efficient mode overlap is achieved in micro-fibers having a diameter roughly equal to half of the fundamental wavelength. Using a typical solid-state or fiber laser for pumping, high conversion efficiency is possible in only a few centimeters of a micro-fiber.
We demonstrate a long-period grating whose resonance varies in strength but remains fixed in wavelength with either temperature or strain. Using this fiber-grating sensor, we resolved a change of 1 muepsilon of strain or 0.04 degrees C in temperature. Such sensors require no spectrometer or other frequency-selective components and can operate in real time.
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