Research

Fabrication of photonic crystal fibres

 

The stack and draw process

The fabrication of high quality custom-designed PCF is central to the research of our division. We use the "stack-and-draw" procedure, which relies on manual assembly of glass capillaries and rods into an appropriate preform stack whose structure corresponds approximately to the desired fibre structure. After inserting the preform stack into a glass tube and fusing during the drawing process, one obtains a microstructured preform or "cane". The final step in PCF fabrication involves drawing the cane into fibre with the desired dimensions, such as cladding-lattice pitch and the outer fibre diameter. By tuning process parameters such as temperature, preform feed rate and drawing speed, as well as the pressure inside the preform, the size of the air-holes and their regularity can be controlled. As for standard fibres, the fabricated PCF is coated with a polymer jacket for improved mechanical strength.

Perform construction and fibre drawing

Fig 1: Steps in the fabrication of photonic crystal fibres. (1) 1 mm thick capillaries are drawn to precise dimensions and then (2) stacked to form the desired "preform". (3) The preform is fused together and drawn down in size to a "cane" (~ 1 mm in diameter). (4) In the final drawing step, the cane is drawn down to fibre and encased in a silica outer cladding.

 

Silica PCF

Silica glass has excellent transparency in the visible and near IR, as well as good drawing properties and excellent mechanical strength. As a result it is the material most commonly used to form PCF. We are fabricating a wide range of different PCF structures for use in many different projects throughout the division. The technology permits a wide range of different structures to be fabricated, from periodic lattices of air channels to nanoweb structures and more complicated architectures. 

High quality hollow-core PCF is essential for our projects on gas-laser devices, optical sensors and particle guidance, and new nano-structured fibres are being developed for experiments on light bullets and chemical sensing. A universal requirement in the division is that group members who need special PCFs in their experiments are expected to participate in their fabrication and design. Figure 2 shows examples of some of the structures made so far.

 

Fibre structures

Fig 2: (a) Hollow-core photonic bandgap fibre; (b) endlessly single-mode PCF; (c) Kagome-lattice hollow-core fibre.
Fig 3: Generation of green, yellow, and red supercontinua for different launched states of polarization in the birefringent core (1550 nm fibre laser source, 60 fs pulses).

 

Soft glass fibres

Compound glasses are of interest because of their enhanced transparency in the infrared and ultraviolet, higher optical non-linearities, stress-optical coefficients and Verdet constants. Examples are lead-silicate glasses, but tellurite and chalcogenide compositions are also of great interest [3-6]. Figure 3 shows the colours produced in a PCF made from Schott SF57 glass, fabricated in Erlangen. The core was birefringent, yielding red and green supercontinuum colors for light polarised along the orthogonal fibre axes (1550 nm fibre laser source, 60 fs pulses, 1.2 nJ per pulse). White light was generated when the pulses were launched at 45°.
Figure 4 shows the broadening of the spectrum of a mode-locked Ytterbium laser depending on the launched pulse energy in a PCF made from Schott SF6 glass. Only 20 pJ were necessary to generate an octave spanning supercontinuum suitable for frequency combs around 1 µm [6].
We fabricate PCF made from these compound glasses and investigate them for use as extremely nonlinear fibres or as low loss waveguides in the ultraviolet and infrared. An additional drawing tower has been installed in our clean room and we have now the possibility to draw soft-glass fibre as well as silica. We also use the stack-and-draw technique to realize this fibre.

 

Figure 4: Left: Evolution of the spectrum from SF6 PCF depending on launched pulse energy; right: SEM picture of SF6 PCF.