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

对于许多应用，特别是在成像和显微镜，可见激光辐射源是必需的。尽管商用激光系统的范围很广，但这仍然是一个挑战！由于低效的液体有机染料激光器的消亡，产生可见激光辐射的最广泛部署的方案之一是使用参数产生，通常由飞秒Ti：蓝宝石激光系统泵浦，其本身占据了相当大的占地面积，相对昂贵，并且有效地无法提供脉冲持续时间和脉冲重复率的多功能性。作为一种替代方案，基于光纤的超连续源已经被部署，尽管这种源的占地面积较小，但它在重复率方面仍然表现出不灵活性，大多数系统只是使用锁模Yb光纤激光器（因此固定重复率）作为光纤中非线性光产生过程的泵浦方案，从而导致超连续体的产生。此外，为了获得所需的波长，必须进行光谱滤波。这导致效率低下，实际上所有其他波长和功率都被丢弃了。一般来说，毫瓦范围内的平均功率是从这种超连续源获得的，这对于许多研究来说往往是不够的。同时，超连续谱源的谱选择产生的脉冲通常是噪声的爆发，具有高度结构化和时间不可复制性。在这项工作中，我们建议摆脱传统激光谐振腔的使用，并利用种子放大器在高效和通用的单通配置中产生所需的脉冲持续时间。为了允许波长多样性，我们将使用受激拉曼产生，它在保持偏振的单模硅纤维中是高效的。通常，受激拉曼产生是从噪声演化而来的，增益的光谱峰距离泵浦波长峰约440 cm-1。当泵浦在1060nm时，这相当于大约60nm，增益带宽很宽，在每个拉曼阶中可能有大约40nm的可调谐性。噪声产生的问题是光纤需要过多的增益长度，因此会发生额外的非线性过程，如自相位调制，这会导致频谱展宽。在某些情况下，这可能没有问题，但当为了将操作转移到所需的可见区域而使用顺序二次谐波产生时，它会导致转换效率严重降低。为了克服这个问题，我们提出在单通主振荡器功率光纤放大器方案中使用二极管激光器窄带辐射的低电平连续播种以及产生的强大泵浦。这可以作为拉曼增益过程的种子源，拉曼增益仅在泵浦脉冲期间发生，从而导致具有窄频谱带宽的脉冲的快速积累，从而使频率翻倍，效率高于70%。拉曼过程的级联也可以扩展光谱覆盖范围，两个激光源的频率混合也将被演示，并将允许覆盖整个可见光谱。这一切都将在非常低的占用空间配置中进行，将完全光纤集成，从而实现高稳定性和可重复性操作。单通技术的最大优势在于，它允许控制脉冲持续时间和重复频率，当与系统的宽波长操作相结合时，将提供一个独特的源，将部署在各种应用中，从受激发射耗尽（STED）显微镜到室温脉塞泵浦，应该是非常具有商业吸引力的。

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.