Post-processing of optical fibre provides a powerful means of manipulating the characteristics of the guided light, opening up opportunities in dispersion engineering, enhancement of optical nonlinearities, mode conversion and spectral filtering. Dispersion engineering in particular is a technique key in the optimization of nonlinear optical processes such as four-wave mixing, soliton propagation and supercontinuum generation. We have constructed two advanced computer-controlled post-processing systems using a butane-oxygen flame and a CO2 laser beam as heat sources, and developed various kinds of structure providing novel optical features.
We routinely form down-tapered and up-tapered fibres with low insertion loss, by stretching and compressing the fibres, respectively, while heating them with a butane-oxygen flame [Kakarantzas (2007)]. These techniques are useful for modifying the mode field size, effective refractive index, group velocity dispersion and optical nonlinearity.
In contrast to conventional fibres, we can also change the properties of PCFs by modifying the size of the air holes through applying pressure and heat. The holes either inflate or collapse depending on the pressure. Various kinds of structure can be created by selective air-hole closure before pressurization and heating.
We have successfully fabricated structural rocking filters in highly birefringent PCFs [Kakarantzas (2003)]. The rocking filter couples the two linearly polarized eigenmodes of the fibre close to a resonant wavelength, and can be formed by periodically twisting the principal axes to and fro along the fibre. We have made rocking filters with broad range of resonant wavelength, coupling ratio and bandwidth by adjusting parameters of rocking profile such as rocking period, rocking amplitude and number of periods. Rocking filters with two resonant wavelengths have also been made. Since the transmission spectrum of the rocking filter allows only indirect and rough estimation of the rocking profile, we have developed a side-scattering measurement technique for directly measuring the internal twist profile and structure of fibres [Stefani (2014)]. By theoretical analysis based on a precise transfer matrix approach [Zang (2010)] and experimental confirmation using low coherence interferometry [Wong (2010)], we have shown that rocking filters have group velocity dispersion profiles that differ dramatically from those of unperturbed fibres near the resonant wavelength. Furthermore, the profiles can be widely controlled by adjusting the rocking profile, suggesting a new application for rocking filters in nonlinear fibre optics.