Laser crystallisation of semiconductor core optical fibres

Summary form only given. Semiconductor core optical fibres offer the exciting prospect of performing non-linear optics and signal processing in a geometry that is inherently compatible with today's fibre optical networks. To fully exploit the potential of these fibres it is important that they can be fabricated with core sizes of the order of 1 μm, or less, so as to enhance their non-linear optical parameter and facilitate low loss integration with standard fibres. In this paper we report on the fabrication of such fibres and present a route to improving the material quality, and hence reducing the losses, via laser annealing [1]. Amorphous silicon (a-Si) optical fibres with core sizes ranging in diameter from 0.8 - 1.7 μm were fabricated using the well-established high pressure microfluidic chemical deposition technique [2]. The micro-Raman spectrum for the as-deposited core material is shown in Fig 1(a) and clearly demonstrates the characteristic broad peak of a-Si at 480 cm-1. The losses of this material are typically as high as 50 dB/cm, and it is therefore unsuitable for the fabrication of the majority of optical devices. To improve the transmission properties, a CW argon ion laser operating at 488 nm and a power of - 2 W was focused onto the core to anneal the material to a crystalline phase. The photon energy at this wavelength is - 2.5 eV which is much greater than the indirect band-gap energy of silicon, 1.1 eV. Thus the photons are strongly absorbed and generate a plasma of free electrons that causes the core to heat up via phonon assisted recombination. A set of high precision stages were used to ensure that the core was kept at the laser's focal point whilst being scanned through the beam. The Raman spectra of an annealed fibre and a single crystal reference are juxtaposed in Fig. 1(b). The difference in peak position for the fibre core spectrum is a consequence of the residual stress associated with the mismatch between the thermal expansion coefficients of the silica cladding and the silicon core materials. Voigt fitting of the spectra shows that they both have a Lorentzian component with a linewidth of 2.7 cm-1, indicating a highly crystalline core material. The optical transmission loss of this annealed silicon optical fibre was measured at an operating wavelength of 1550 nm to be 5.6 dB/cm over a 1mm length, representing the lowest loss that has been reported for a crystalline silicon optical fibre with a sub-micron radius core. Subsequent TEM measurements revealed that large millimetre scale single crystal sections of core material were produced using this technique. We will discuss our efforts to increase the annealed core to a length that is suitable for practical device fabrication, as evidenced by the consistent Raman spectral width measured over 13 mm in Fig. 1(c). We anticipate that this work will lead to the development of in-fibre semiconductor devices that can be seamlessly integrated with the standard single mode fibres found in today's optical networks.