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

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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.