WP3- PCF design, production and testing

 

The aim of Work Package 3 is to develop monolithic fiber based optical amplifiers for providing the optical power needed for scribing of solar cells.

 

Specifically this target will be achieved by design and fabrication of active single-mode large mode area photonic crystal fibers (PCFs) for use in high peak power Q-switched and MOPA lasers. The challenge is to:

·         Optimize flexible large mode area PCF for highest possible power performance

·         Develop interfacing technologies that allow simple access to PCFs.

·         Produce non-flexible (rod-type) PCFs enabling new laser tools for solar cell materials processing

 

The motivation for using PCFs is that they offer a unique combination of high beam quality and high output powers, whereas conventional fibers with quite small core size and polymer-based cladding materials are limited in performance, due to the onset of optical nonlinearities and by optical/thermal damage. Expanding the core size is an essential feature that reduces power density, thus reducing nonlinear effects and increasing damage threshold levels. 

 

Fig. 1: Double-cladding photonic-crystal large-mode-area fiber.

 

The fiber shown in Fig.1 has a double clad structure, as it has a micro-structured single mode core for the signal, which is embedded in a multimode core for the pump.

The work carried out in this work package will leverage stand alone PCF technology to monolithic amplifiers by improving both fiber design and splicing and interfacing technology.

 

 

In order to improve the state-of-the art on the PCF technology, the ALPINE project will combine the design competences of the University of Parma, the design and fabrication expertise of Crystal Fibre and the laser system integration expertise of EOLITE Systems.

 

              

          Fig. 2: Cross section used in the numerical model.     Fig. 3: Computed magnetic field distribution. 

 

The figures above show the polarizing PCF structure and the computed magnetic field module distribution on the transverse cross-section. 

Furthermore ALPINE enables close collaboration with all parts in the value chain for production of solar cells, which ensures that the development is targeted towards real industrial needs.

 

 

                             

 

 

Fig. 4: LMA fiber with an asymmetric outer shape            Fig. 5: Preferential bending place of asymmetric LMA fiber

 

Going forward NKTPs plans are to improve bending performance and/or beam quality of optical fibers e.g. by developing fibers with preferential bending orientation. This includes fibers, where, for example, the outer shape of the optical fiber is tailored with an asymmetry to provide bending orientation in a preferred plan of the optical fiber. For example, a DC-200-40-PZ-Yb type fiber where the outer shape is D-shaped, see Fig.4. The flat side of the D-shape is oriented such that it is aligned with an axis between the stress-applying parts (SAPs) of the fiber. Thereby, when bending the fiber, it will by itself orient the SAPs in the bending plan, see Fig.5. Such fibers with preferential bending orientation have improved performance in terms of tighter coiling diameters (for single-mode fiber) and more stable beam quality. Various other types of preferential bending orientation features than D-shape outer cladding can be imagined, for example features in the overcladding (such as for example holes, voids, glass features with different thermal expansion coefficient than the background glass material). These ideas are also relevant for conventional polymer-type double-clad fiber. For example, a double-clad fiber, where the outer shape is asymmetric and is orientated in relation to features in the core and/or the cladding that improved bending and/or beam quality. For example, resonant cladding features that can be utilized to strip higher order mode and provide single-mode fibers are sensitive to bending orientation. Hence, it is an advantage to provide asymmetric outer cladding shapes in combination with various type of cladding features that improves beam quality. Example of fiber designs, where preferential bending orientation will be explored are presented by Thomas Tanggaard Alkeskjold, "Large-mode-area ytterbium-doped fiber amplifier with distributed narrow spectral filtering and reduced bend sensitivity," Opt. Express 17, 16394-16405 (2009), and by John Fini, "Design of solid and microstructure fibers for suppression of higher-order modes," Opt. Express 13, 3477-3490 (2005).

This an example of a photonic crystal fiber which is suitable for amplification to high peak power.