Diamond Raman Lasers

激光的波长覆盖范围受到自然材料的限制。外延半导体的工程化放宽了这一限制，但材料系统之间的差距仍然存在。因此，持续不断地需要有效地转换激光的波长，从有好光源的光谱区域转移到它们稀缺的光谱区域。该项目的目标是一种这样的转换过程--拉曼激光器--特别是利用钻石的新用途来实现功率调节。有效的拉曼转换--由于光在介质中的非弹性散射而产生更长的波长--通常被认为是高功率脉冲激光器或基于长光纤的系统的专利。然而，最近的研究表明，事实并非如此。首先在氢气中(蒙大拿州立大学)，然后在晶体中(白俄罗斯国家科学院；麦格理大学)，已经证明可以通过拉曼散射使中等功率的连续波激光波长移动：将拉曼介质放置在激光腔内，以利用其中的高强度。这种方法很重要，因为它扩展了紧凑型二极管泵浦固体激光器的波长调色板。这类激光器通常基于掺杂金属离子的晶体，在这种掺杂晶体系统中，输出波长被限制在有限数量的潜在激光跃迁。例如，基于拉曼的方法允许将众所周知的1064 nm跃迁移到组织透过率高的1200 nm左右的区域。此外，这种拉曼频移激光器的倍频提供了进入应用丰富但目前源匮乏的黄橙色光谱区域的通道。到目前为止，连续波腔内拉曼激光器的输出功率被限制在几瓦特。这一上限是由拉曼介质中的热问题引起的。通常使用的晶体的低热导率严重阻碍了由于非弹性散射过程而沉积在拉曼介质中的热量的去除。这会导致过度的热透镜效应，从而使升级到更高功率的过程复杂化。与大多数拉曼介质相比，钻石具有更高的拉曼增益系数和更大的导热系数。然而，它作为拉曼介质的使用通常是不被考虑的：因为可用的样品数量很小，而且即使是这些小样品的费用也很高。在光子学研究所的初步研究中，我们已经表明，这一判断过于仓促。首先，最近合成单晶钻石的商业供应将降低成本并提高质量。其次，我们的模型表明，钻石的高热导率和损伤阈值意味着紧聚焦可以使用短晶体(&lt；2 mm)。这项计划将建立在这个平台上，特别针对四个演示：1.第一个基于钻石的连续波腔内拉曼激光器(目标：1240 nm处的12W；通过腔内二次谐波产生的橙色中的5W(620 Nm))2.半导体圆盘激光器的第一次拉曼转换(目标：735 nm处的200 mW和1235 nm处的2W)3.自适应光学在连续波拉曼激光器中的自动光束质量优化的首次使用(目标：10W，M平方和lt；首次使用金刚石微光学拉曼转换紧凑型Q开关激光器(目标：40%效率)实现这些结果将为英国在这一重要的新兴固态激光工程领域建立强大的存在。此外，它还将在新的光谱区域开辟一系列紧凑型光源，用于皮下光动力疗法、水下视觉系统和多光谱成像等各种应用。

The wavelength coverage of lasers is limited by the materials nature permits. This constraint is loosened by the engineering that is enabled in epitaxial semiconductors, but gaps remain between materials systems. Thus, there is a continuing requirement to efficiently convert the wavelength of lasers, moving from spectral regions where good sources exist to those where they are scarce. This project targets one such conversion process - the Raman laser - and in particular the novel use of diamond to permit power-scaling. Efficient Raman conversion - the generation of longer wavelengths due to inelastic scatting of light in a medium - is usually considered the preserve of high power pulsed lasers or systems based on long lengths of fibre. Recent work, however, has shown that this need not be so. First in hydrogen gas (Montana State University) and then in crystals (National Academy of Sciences of Belarus; Macquarie University), it has been shown that continuous-wave lasers of modest power can be wavelength-shifted via Raman scattering: the Raman medium is placed inside the laser cavity to exploit the high intensities there in. This approach is important because it expands the wavelength palette available from compact diode-pumped solid-state lasers. Such lasers are typically based on crystals doped with metal ions and the output wavelengths are limited to the finite number of potential laser transitions in such doped-crystal systems. Raman-based approaches allow, for example, the well known 1064 nm transition in Nd:YAG to be shifted into the region around 1200 nm where tissue transmission is high. Furthermore, frequency doubling of this Raman shifted laser gives access to the applications-rich, but currently source-poor, yellow-orange region of the spectrum.So far, the output power from continuous-wave intracavity Raman lasers has been limited to a few Watts. This ceiling arises from thermal problems in the Raman medium. Removal of the heat deposited in the Raman medium due to the inelastic scattering process is seriously inhibited by the low thermal conductivity of the crystals typically used. This leads to excessive thermal lensing effects that complicate scaling to higher powers. Diamond has a higher Raman gain coefficient than most Raman media and much greater thermal conductivity than all of them. However, its use as a Raman medium is usually dismissed: due to the small sample sizes available and the expense of even these small samples. In initial studies at the Institute of Photonics, we have shown that this judgement is too hasty. First, the recent commercial availability of synthetic single crystal diamond will bring down costs and improve quality. Second, our modelling indicates that the high thermal conductivity and damage threshold of diamond means that tight focussing enables the use of short - and therefore available - crystals (<2 mm). This programme will build on this platform, targeting four demonstrations in particular:1. First CW intracavity Raman laser to be based on diamond (target: 12 W at 1240 nm; 5 W in the orange (620 nm) via intracavity second harmonic generation)2. First Raman conversion of a semiconductor disk laser (target: 200 mW at 735 nm and 2 W at 1235 nm)3. First use of adaptive optics for automated beam quality optimisation in a CW Raman laser (target: 10 W, M-squared < 1.1 at 1240 nm)4. First use of diamond micro-optics to Raman convert a compact Q-switched laser (target: 40% efficiency)Achieving these results will establish a strong presence for the UK in this important emerging area of solid-state laser engineering. Furthermore, it will open the way to a range of compact sources in new spectral regions for applications as diverse as subcutaneous photodynamic therapy, underwater vision systems, and multispectral imaging.