Fibre system technology

Optical Fibres

Conventional optical fibres consist of two, generally concentric glass cylinders of differing refractive indices. Light is guided through the core via total internal reflection, which operates only if the core has a higher refractive index than the encircling cladding. However, it has been known since the first multi-fibre systems were built that losses were incurred due to focal ratio degradation (FRD; non-conservation of Etendue). When used in a spectrograph, fibres need to conserve the angular distribution of the incoming beam between the telescope and the slit, or  the spectrograph will suffer a decrease in performance or become larger and more expensive. Modifying the collimator to make it accept the faster beam either increases the beam size at the disperser leading to greater cost or otherwise requires a faster camera or more detector pixels.

It is useful to consider fibre loss mechanisms from a 'modal' point of view. With this model of modes in mind, fibre loss mechanisms can be considered as two types; mode independent which are intrinsic factors, and mode dependent which are extrinsic factors.

Intrinsic factors: 

• Absorption
• Scattering 

Extrinsic: 

• Micro and Macro bending
• Alignment (perpendicularity between the fibre surface and optical axis)
• Stability of the light source and moveable mechanical parts

 

The quest to understand the behaviour of optical fibres is being approached from both a theoretical and experimental angle. Theoretical models such as those proposed by Gloge (1972) are being developed in order to be able to predict the behaviour theoretically. In addition to this experimental set-ups allow us to investigate the various causes of FRD, for example the wavelength dependence of the phenomena with respect to temperature.

Fibres are a vital component of a variety of astronomical instruments but are predominantly utilised in integral field spectroscopy and multi-object spectroscopy to conveniently rearrange light coming from different places into a common slit. However before these instruments can be designed it is extremely important to understand exactly how the fibres will behave and consequently a great deal of the work in this area aims to understand the fundamental properties of fibres.

The fibre work in Durham is mainly concentrated on conventional step-index multimode optical fibres, however more recently, work has focussed on the properties of photonic crystal fibres (PCFs). 

Photonic Crystal Fibres

The possibility of modifying the guidance properties by introducing a microstructure in the refractive index profile of optical fibreswas first suggested by Kaiser and Astle in 1974. As opposed to optical fibres which have a solid core surrounded by a cladding, PCF's have a holey structure within the cladding and when these fibres were first manufactured in 1996, it became evident that depending on the particular PCFgeometry, light guidance can occur in one of two ways - either by ModifiedTotal Internal Reflection (MTIR) or via the Photonic Band Gap (PBG). The most interesting thing about these fibres is that by altering properties such as the size of the air holes or the spacing between the holes, the fibre will act in extremely different ways. One of the first special characteristics of PCF's to be reported was their ability to be endlessly single moded. This property means that the fiber can, in principle, be scaled to arbitrary dimensions and remain single moded, whereas when the core size is increased in a conventional optical fiber a corresponding decrease in index step between the core and the cladding is required in order to remain single moded. Other interesting applications include highly non-linear fibres which are used for applications ranging from spectroscopy through to telecommunications.