Bismuth-doped fibre laser systems

Lead Research Organisation: Imperial College London
Department Name: Dept of Physics

Abstract

Over the past decade such rapid progress has been made on the research and development of high power fibre lasers that these devices are becoming the laser of choice in numerous new and traditional applications in the laboratory or the industrial work place, from biological imaging to the cutting and welding of steel plates. These advances have been enhanced by several major contributions to the science and technology such as multiclad fibre structures, reliable high power single stripe pump diodes and unique multimode fibre couplers in pumping technology. To date, the most advanced fibre laser technologies have relied upon rare earth ion doping of conventional silica based fibres. The use of silica based fibres allows ease of handling, as well as compatibility and integration with existing fibre technology. Primarily three principal wavelength regions are covered by the rare earth ions of Yb, Yb:Er and Tm, operating broadly around 1030-1120 nm, 1530-1600 nm and 1750-2100 nm, respectively. Consequently, there are very significant gaps in the spectral range 1000-2000 nm where these doped fibre lasers cannot operate. This can be solved by fibre Raman laser technology, however, scalability to ultra high average power levels is problematic, energy storage hence pulse energy is a particular problem because the stimulated Raman laser utilizes a virtual state in the stimulated scattering process, while additional nonlinearity in the long lengths of fibre necessary for efficient operation means that the linewidths of the lasers are broadened impeding frequency doubling, in particular to important wavelengths in the yellow and orange. In addition, despite the impact Raman devices have made, no conventional rare earth doped silica based fibre laser or fibre amplifier is available in the second telecom window in the region of minimum dispersion of silica based fibres around 1300nm.This basic-research project will address these issues. Recently, our collaborators at IRCICA-PHLAM (Lille University) have produced new Bismuth-doped silica fibres. These fibres exhibit an extremely broad luminescence profile, extending from about 1000nm to 1600nm. In this project we plan to analyse and investigate cw lasing and power scaling of fully fibre integrated formats of the Bi-doped laser, conveniently pumped by Yb fibre lasers. In addition, tunable laser schemes will be investigated as well as line narrowing mechanisms and frequency doubling of the output to the yellow-orange spectral region should be achievable, with average powers in the visible at the watt level. Compact laser systems at these wavelengths and power levels should be of relevance to applications in ophthalmic treatments as well as in cosmetic surgery. The broad gain bandwidth in addition to allowing extensive tunability should also permit the support of ultrashort pulses and we will develop both picosecond and femtosecond fibre lasers based upon this unique material. Broadband ASE operation around 1050nm and 1300nm can be also of relevance to optical coherence tomograpfy applications.The peak of the gain in Bi-doped silica fibre is in the range of 1270 nm, the region of minimum dispersion of single mode silica fibre. Consequently, the potential exists for the development of an extremely broad bandwidth telecommunications relevant fibre amplifier in this spectral range. It can be seen that Bi-doped silica fibre presents for the first time the opportunity to expand the role of the doped fibre technology in unattainable spectral regions as described above and should further enhance the scientific and technological development of these indispensable devices.

Publications

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Description Over the past decade such rapid progress has been made on the research and development of high power fibre lasers that these devices are becoming the laser of choice in numerous new and traditional applications in the laboratory or the industrial work place, from biological imaging to the cutting and welding of steel plates. These advances have been enhanced by several major contributions to the science and technology such as multiclad fibre structures, reliable high power single stripe pump diodes and unique multimode fibre couplers in pumping technology. To date, the most advanced fibre laser technologies have relied upon rare earth ion doping of conventional silica based fibres. The use of silica based fibres allows ease of handling, as well as compatibility and integration with existing fibre technology. Primarily three principal wavelength regions are covered by the rare earth ions of Yb, Yb:Er and Tm, operating broadly around 1030-1120 nm, 1530-1600 nm and 1750-2100 nm, respectively. Consequently, there are very significant gaps in the spectral range 1000-2000 nm where these doped fibre lasers cannot operate. This was shown be solved by fibre Raman laser technology, however, scalability to ultra high average power levels is problematic, energy storage, hence pulse energy is a particular problem because the stimulated Raman laser utilizes a virtual state in the stimulated scattering process, while additional nonlinearity in the long lengths of fibre necessary for efficient operation means that the linewidths of the lasers are broadened impeding frequency doubling, in particular to important wavelengths in the yellow and orange. In addition, despite the impact Raman devices have made, no conventional rare earth doped silica based fibre laser or fibre amplifier is available in the second telecom window in the region of minimum dispersion of silica based fibres around 1300nm.

This basic-research project addressed these issues. We investigated the laser characteristics of Bismuth doped silica based single mode optical fibres. The fibres were produced by our collaborators at IRCICA-PHLAM (Lille University) and at the Kotel'nikov Institute of Radio Engineering and Electronics, Moscow. Using various doping techniques, fibres were produced with luminescence extending from ~1000 nm to 1600 nm, however, it is not possible to currently obtain a single fibre sample simultaneously covering this range. Efficiencies of ~15% and average powers of up to 15W were obtained, limited by available pump power in simple cavity configurations. The power scaling of basic Bi-doped fibre lasers is still currently under investigation.

Extra cavity frequency doubling was achieved in the important yellow/orange spectral region using PPLN, leading to an alternative compact source for sodium guide star experiments. High doping of Bi leads to luminescence quenching and hence long active lengths of active fibre are required. This makes single frequency operation difficult. We have achieved sub 4GHz linewidth (the narrowest yet reported in Bi) using integrated all fibre Fabry Perots

Short pulse operation was also demonstrated for the first time using carbon nanotubes as saturable absorbers to obtain 4 ps pulses in dispersion compensated cavities while without compensation giant (linear) chirp pulses of around 500 ps were produced..

The potential of Bi-doped silica fibre as a communications amplifier was also demonstrated for the first time with 1 ps pulses being amplified by 22 db but with temporal broadening to 6 ps in single pass amplifiers. Moderately high bit rate amplification studies up to 20Gbit/s were undertaken to demonstrate the potential of Bi-doped silica fibre amplifiers in upgrade opf telecommunications networks. To enhance this, preliminary studies of dispersion compensation as well as amplification through the use of pre chirping of the input pulse was also undertaken,

Although many world firsts have been achieved in this programme, further materials research is essential to allow increased Bi concentration and reduced device lengths for improved efficiency
Exploitation Route Telecommunications and materials processing. Frequency doubled high power fibre lasers allowing the orange yellow do have application in medical treatment and opthamology. Additional materials research will be necesary before this Bi-doped silica based fibre amplifier can be seriously considered as a potential telecommunications device.

The scalability of the output power albeit with reduced efficiency does allow a route to obtaining wavelength ranges that are unattainable with rare earth doped fibres and so may have possible commercial application.
Sectors Digital/Communication/Information Technologies (including Software),Healthcare