Thermal post-processing of PCF

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.

Tapering

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.

Upper: scanning electron micrograph of an untreated fibre (left: core diameter 1.8 µm) and a tapered fibre (right: core diameter 0.5 µm), both to the same scale. Lower: optical micrograph (side-view) of an conventional single-mode fibre (Corning SMF-28) up-tapered from 125 µm to 243 µm.

Hole inflation and collapse

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.

Micrographs of various PCFs. (a) SEM of endlessly single-mode (ESM) PCF. (b) SEM of hole-inflated ESM PCF. (c) Optical micrograph of a single-hole fiber made from an ESM PCF via the selective hole-collapsing technique. (d) SEM of a nanoweb fiber. (e) The web is made curved via the selective hole-inflation technique. In all the SEMs, the black horizontal bars correspond to 10 µm.

Structural rocking filters

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.

Characterization of rocking filters. (a) SEM of a highly birefringent PCF used for fabrication of rocking filters. (b) Transmission spectrum of a filter. (e) Measured and calculated group delay of the filter. (d) Group velocity dispersion profile obtained from (c). (e) Typical scattering pattern obtained by the side-scattering measurement, which shows the scattered intensity from a filter into almost forward direction (1°) as a function of fibre rotation angle. (f) Twist profile of a filter determined by the side-scattering measurement.