Preform Rare-Earth Profiler (PREP)

Over the last decade, high power fibre amplifiers and lasers have been rapidly developed and successfully commercialized for a number of industrial applications such as cutting, welding, and marking. The industrial fibre laser business is currently worth over $800M/year, with compound annual growth rate of about 13% - the highest among the different laser technologies. One of the main contributors to this success has been the significantly improved rare-earth doped fibre fabrication technologies.

High performance fibres rely on controlled incorporation of refractive index modifying dopants, as well as, gain providing rare-earth ions. High efficiency, high average and/or peak power industrial fibre lasers and amplifiers invariably use large-mode area fibres with complex refractive indices and rare-earth distributions. In most cases, a number of different dopants are used simultaneously in order to control the refractive index and gain distribution, and through it the fibre modality and modal differential gain. Additional dopants are also used to reduce nonlinear effects, such as Stimulated Brillouin and Raman Scattering, and other parasitic effects, such as photodarkening. The various dopants have different sizes, mobility, and diffusion rates and, as a consequence, the resulting refractive index profiles can in general be much different to rare earth distributions within the core, and one cannot be inferred from the other. In addition, depending on the fabrication technique, the distribution of dopants is not uniform along the fibre preform, rendering the drawn fibre performance variable and "patchy". In particular, Modified Chemical Vapour Deposition (MCVD) fabrication technique, among the most versatile and widely used fabrication techniques, is known to suffer from poor repeatability and large variability along the preform length. This compromises significantly the fibre yield and increases the fibre cost. In addition, and even more importantly, currently there is no reliable information regarding the "fitness-for-purpose" of fibre in advance. Its suitability can only be tested and quantified after a full fibre laser has been built and thoroughly tested, adding considerably to the fibre laser module turn-around time, yield and cost. So there is a need for unsuitable preforms or parts of preform to be identified early in the fibre drawing process and be discarded.

Another requirement has lately appeared in the fibre telecom area. Over the last few years there has been a strong resurgence in multimode telecom systems research, with spatial-division multiplexing (SDM) promising to solve the predicted forthcoming telecom capacity crunch. Successful development of SDM systems relies exclusively on the development of high performance multimode fibre amplifiers with carefully optimized rare-earth (Erbium or Thulium) profiles for modal gain equalization. Again, detailed and accurate knowledge of the active dopant distribution over the fibre cross-section along the entire preform length is critical for successful demonstrations of this game-changing approach to single-fibre transmission capacity increase.

The main aim of this proposal is to develop widely applicable, non-destructive characterization techniques for the accurate and detailed determination of active-dopant distribution in fibre preforms and provide the required reliable information well before the preforms are drawn into fibres. Such preform characterization techniques are expected to have a big impact on the performance and cost of advanced high-power fibre laser systems, as well as, currently researched SDM telecom systems, and increase the competiveness of the UK manufacturing basis as well as enhance the UK cutting-edge research activities in these areas.

The PREP instrument will reveal fine detail about the preform designs being used by these companies. As such, many of the measurement results will be highly commercially sensitive. Confidential arrangements will therefore need to be put in place to protect 3rd party preform data and equally to protect participating companies' background know-how. However, this is not expected to restrain the delivery of impact as there are many preform designs being developed within the ORC, and the results from these will be published in journals, in conferences and seminars.

Specific activities to deliver impact include:

a) Existing industrial partners and academic collaborators will have direct access to technical data and progress, through the bi-annual technical meetings. Also the active participation of SPI's CTO and ORC Deputy Director in the MST will ensure proper alignment with both organisation needs.

b) Wider circle of potential users will be reached by publication of key results in international conferences (CLEO, Photonics West, OFC, ECOC) and high impact open-access journals (e.g. Optics Express and IEEE Photonics Journal).

c) Strong engagement with the University RIS team (through the ORC dedicated collaboration manager) to promote results and seek more collaborators for further exploitation of successful results.

d) Working closely South-East Photonics Network (SEPNET) manager (Dr. John Lincoln) to reach out to the >200 network members specialising in Photonics.

The most immediate metric for determining the success of the strategy in delivering value to industrial partners is the expected step change in active fibres and fibre laser modules yield, as well as, fibre laser optical and wall-plug efficiency increases.

The ability to pre-select only "fit-for-purpose" sections of preforms is expected to more than double the current fibre yield, and increase the laser efficiency close to the theoretical limit. Such advances are expected to increase fibre laser wall-plug efficiency from the current ~30% to over 45% - a level that cannot be reached by other laser technologies. In addition, the anticipated high power laser system yield is expected to reach >95% levels. Such performance improvements will reduce cost and increase further the fibre laser market penetration.

In the case of academic collaborators, the most appropriate metric of success is the number of high impact factor publications/conferences and contributions to future research grants/contracts that such novel instrument undoubtedly will enable.

However, there is an additional aim of the project in that it is desirable a commercial instrument to become available within five years that is reliable, robust, and which can be properly calibrated. Prerequisites include: research excellence in order to ensure that the best design approach is taken; good market feedback from the collaborators and other service recipients of the measurement technique; selection of the most appropriate commercialization route; and appropriate licensing of technology.