Tuneable, visible, integrated, fibre-based sources with selectable pulsewidth and repetition rate for biophotonics applications

For many applications, particularly in imaging and microscopy, sources of visible laser radiation are required. Despite the remarkable range of commercial laser systems available, this presents something of a challenge! Since the demise of the highly inefficient liquid organic dye laser, one of the most widely deployed schemes to generate visible laser radiation uses parametric generation, usually pumped by a femtosecond Ti:sapphire laser system, which in itself occupies a considerable footprint, is relatively expensive and is effectively unable to provide versatility in pulse duration and pulse repetition rate. As an alternative, the fibre based supercontinnum source has been deployed and although this source has a somewhat smaller footprint, it still exhibits inflexibility in the repetition rate, with most systems simply using a mode-locked Yb fibre laser (hence fixed repetition rate) as the pump scheme for the nonlinear optical generation processes in fibre which leads to the supercontinuum generation. In addition, to obtain the required wavelength, spectral filtering is necessary. This leads to high inefficiency, with effectively all the other wavelengths and power discarded. Typically average powers in the milliwatt range are obtained from such supercontinuum sources, which tends to be inadequate for many investigations. At the same time, the pulses generated by spectral selection from supercontinuum sources are usually bursts of noise, highly structured and temporally irreproducible.

In this work we propose to move away from the use of conventional laser resonant cavities and utilize seeded amplifiers to generate the required pulse durations in efficient and versatile single pass configurations. To permit wavelength diversity, we will use stimulated Raman generation, which is highly efficient in polarization preserving single mode silica fibres. Usually, stimulated Raman generation evolves from noise, with the spectral peak of the gain being about 440 cm-1 from the peak of the pump wavelength. This corresponds to about 60 nm when pumped at 1060 nm and the gain bandwidth is broad with about 40 nm of tuneability possible in each Raman order. The problem of generation from noise is that excessive gain lengths of fibre are required and consequently additional nonlinear processes take place, such as self-phase modulation, which leads to spectral broadening. In some cases this may not be problematic but when sequential second harmonic generation is used in order to shift operation to the required visible region it leads to severely reduced conversion efficiencies. To overcome this we propose to use low level continuous seeding of narrow band radiation from diode lasers along with powerful pumps generated in single pass master oscillator power fibre amplifier schemes. This acts as a seed source for the Raman gain process, with the Raman gain only taking place during the pump pulse leading to rapid build-up of the pulses with narrow spectral bandwidths that will allow frequency doubling with greater than 70% efficiency. Cascading of the Raman process is also possible to extend the spectral coverage and frequency mixing of two lasers sources will also be demonstrated and will allow the complete visible spectrum to be covered. This will all take place in a very low footprint configuration that will be completely fibre integrated leading to high stability and reproducible operation.
The greatest advantage of the single pass technique is that it allows for controlled pulse duration and repetition rate to be achieved, which when coupled with the broad wavelength operation of the system, will provide a unique source that will be deployed in various applications ranging from stimulated emission depletion (STED) microscopy to the pumping of room temperature masers and should be exceedingly commercially attractive.

The unique and relatively broad temporally and spectrally encompassing characteristics of the proposed instrument should have potential economic impact. In the proposed work plan the key objective and clearly identified deliverable throughout the two year study in all work packages is fully operational fibre-integrated, wavelength versatile laser sources with variable and readily selectable pulse duration and repetition rate that can be taken to potential commercial partners as well as placed in several research laboratories for application assessment. A notable and planned characteristic of the device is the versatility. It can be configured to meet single wavelength operation with fixed pulse duration and repetition rate, representing the cheapest commercial option, or can be readily modified to be tuneable, operate in different wavelength ranges simultaneously and with selectable pulse duration and repetition rates, representing the most versatile and most expensive commercial option. Similarly any combination in-between can be configured from the same template, which is attractive in developing a series of commercial devices. Of course, not all the programme is technology and development, there is a broad component of scientific problems to be addressed. The work is being undertaken in collaboration with two major international photonics based companies which could provide a direct route to commercial take up, however, in association with Imperial College Innovations and ensuring full protection of Imperial College's intellectual property right portfolio, the most suitable and beneficial conduit will be sought for the financial development of this instrument. This could and should involve other photonics based companies.

In addition to fostering collaborations with Industry, the group at Imperial College have always placed emphasis on the training of personnel, the open exchange of information and the promotion of public information and education. If successful in application, this project, like previous high impact research projects undertaken by the group, will be presented at the Imperial College Science Fair, which is open to the general public and press and has a record of very high attendance figures. The exchange of people and information between the group and our industrial partners will foster transfer of skills and knowledge. A key consideration of the programme is the recruitment of the most appropriate post-doctoral research assistant. The candidate identified has a very strong record of public engagement in science and technology and in promotion of the topic, having recently been the winner of the Physics division and the overall Science winner of the SET for Britain Award as well as leading numerous public interaction programmes through OSA and SPIE. Public engagement is seen as a vital component to this research programme and will be treated as a deliverable.
The more general impact on society is likely to come via the commercialization of this instrument. Likely applications areas are the medical imaging arena and in the possible replacement of the supercontinuum source-filter assembly in its diverse applications area, where higher power may be required. There is also a real need scientifically for a compact, medium average power, short pulse visible source.