Optical microfibre devices

In the last decade optical fibre tapers with micrometer diameter (often called microfibers) have been investigated for numerous applications ranging from sensing to wavelength convertors, telecom and optical manipulation. This paper reviews the various applications of microfibres. 1. Optical microfibres Although microfibers with diameters comparable to the wavelength have been manufactured for nearly half of a century [1], their application in devices has been limited because of the perceived difficulty to achieve low-loss propagation [2]. Microfibres have been manufactured from a variety of different materials including silica [3-8], silicon [9,10], phosphate [11], tellurite [11], lead-silicate [12], bismuthate [12] and chalcogenide glasses [13] and a variety of polymers [14-19]. Most of the microfibers exhibited an irregular profile along their length which resulted in high propagation losses in devices. In the last decade the use of the flame-brushing technique [3-8] allowed to manufacture microfibers with losses smaller than 0.01dB/mm, sufficiently low for the manufacture of devices for sensing, comms, wavelength conversion and optical manipulation. Optical microfibers (OMs) are optical fibre tapers with diameters comparable to the wavelength of light propagating in them. They generally fabricated by tapering optical fibres and therefore result connected to conventional optical fibres by transition regions (Fig. 1). If the transition region has a suitably small angle, a single mode launched in the optical fibre pigtail core is adiabatically transferred in a single mode guided by the cladding/external medium interface and no power occurs into other modes. Fig.1 Schematic of operation of an adiabatic optical fibre microwire. A beam launched in the optical fibre pigtail (A) enters the transition region downtaper where it is initially focused (B) until diffraction occurs. The beam then expands and it is guided by the cladding/external medium interface (C). After this point the beam keeps focusing until it reaches the minimum waist diameter (D), where it propagates unaffected. In the uptaper the reverse process occurs and the beam leaved the optical fibre pigtail in the core. Oms are usually fabricated using the so-called flame brushing technique [4], where a heat source – typically a flame – is repeatedly brushed under an optical fibre that is being stretched.