Photonic crystal fibre (PCF) modal interferometers are widely used for sensing applications. Owing to the intrinsic Mach-Zehnder-like dualmode interference there is a trade-off between sharp spectral response and wide sensing range. A three-beam path interferometer induced by mode-conversion in a single dual mode PCF end face is experimentally demonstrated. Doubled free spectral range is obtained owing to the three-beam path interference, which is suitable in sensing large index change, wide temperature variation and strong applied pressures. As an example, a temperature sensor with the sensitivity of 8.17 pm/8C is demonstrated ranging from room temperature to 5208C. The mechanism and further improvements of the multi-beam path interferometers are also discussed.Introduction: Over the past 30 years, fibre-optic technologies have experienced tremendous growth. A big interest in this field is to develop high performance interferometric fibre-optic sensors [1]. Different approaches, in which the guiding mechanisms of an optical fibre can be modulated by external factors, have been exploited. The hollow structure of photonic crystal fibres (PCFs) provides a convenient access to the evanescent waves of the guided light without complicated pretreatment. The first attempt to construct an interferometer with a PCF was reported in 2001 by MacPherson et al. [2]. Owing to small size, flexibility, immunity to electromagnetic interference and the capability for remote measurements, PCF based modal interferometric sensors have shown advantages in detecting changes of pressure, temperature and environmental refractive index [3][4][5][6]. However, owing to the intrinsic Mach-Zehnder like dual-mode interference, the spectrum is just an ordinary sinusoidal curve. The maximum sensing range is determined by the spectrum's period, i.e. the free spectral range (FSR). If the wavelength shift is large closing to a FSR, the spectrum almost overlapps at its original curve. In this case, it is not easy to read the actual spectral shift. However, wide range sensing is desired in many applications, for example, temperature sensors working in some industrial environments or in fire alarm systems.In this Letter, we propose to tailor the reflective PCF interferometer's spectrum by inducing an extra interfered beam path. A doubled maximum sensing range without resolution reduction is obtained. A temperature sensor working up to 5208C is demonstrated with a sensitivity of 8.17 pm/8C. In comparison with previously reported fibre sensors with 2008C measurement range [4,5], our approach shows a remarkable improvement.
Polarization independent sum frequency generation (SFG) is proposed in an electro-optic (EO) tunable periodically poled Lithium Niobate (PPLN). The PPLN consists of four sections. External electric field could be selectively applied to them to induce polarization rotation between the ordinary and extraordinary waves. If the domain structure is well designed, the signal wave with an arbitrary polarization state could realize efficient frequency up-conversion as long as a z-polarized pump wave is selected. The applications in single photon detection and optical communications are discussed.
Optical parametric amplification (OPA) of arbitrarily polarized light is proposed in a multi-section periodically poled Lithium Niobate (PPLN). External electric field is applied on selected sections to induce the polarization rotation of involved lights, thus the quasi-phase matched optical parametric processes exhibit polarization insensitivity under suitable voltage. In addition to the amplified signal wave, an idler wave with the same polarization is generated simultaneously. As an example, a ~10 times OPA showing polarization independency is simulated. Applications of this technology are also discussed.
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