The most successful practical application of single-mode biconical tapering technique has been achieved in the design of low-loss fused directional coupler. By heating, stretching and tapering of two fibers a fused structure was developed in which power coupling from one to the other fiber is possible. These processes can be proceeded in many ways and the wavelength dependence of coupling can be intentionally decreased or increased. Directional couplers, WDMs and filters are easily made by this method. These components are widely used in long-distance telecommunication networks, LAN's, fewer common are the applications in optical metrology especially as the sensing devices.
The easiest form to analyse the device is the single fiber or 2x2 coupler where the energy transfer is achieved by the side coupling of the fundamental modes in 2 fibers aligned and bonded together. Properly made tapered devices are characterised by low excess losses, much below 1 dB. It is well known from experiments that it is quite easy to taper thermally a single fiber, locally lower the diameter more than 40 times without significant losses.
