Diamond Raman Lasers

激光的波长覆盖范围受到材料性质允许的限制。外延半导体的工程技术使这一限制得以放松，但材料系统之间仍然存在差距。因此，持续需要有效地转换激光的波长，从存在良好光源的光谱区域移动到缺乏光源的光谱区域。这个项目的目标是一个这样的转换过程-拉曼激光-特别是金刚石的新用途，允许功率缩放。高效的拉曼转换——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，情况并非如此。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；麦考瑞大学），研究表明，中等功率的连续波激光器可以通过拉曼散射进行波长偏移：拉曼介质被放置在激光腔内，以利用那里的高强度。这种方法很重要，因为它扩大了紧凑二极管泵浦固体激光器可用的波长范围。这种激光器通常基于掺杂金属离子的晶体，并且在这种掺杂晶体系统中，输出波长受限于有限数量的潜在激光跃迁。例如，基于拉曼的方法允许将Nd:YAG中众所周知的1064nm跃迁到1200nm左右的区域，该区域的组织透射率很高。此外，这种拉曼位移激光器的频率翻倍使其可以进入光谱中应用丰富但目前源贫乏的黄橙色区域。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。这个上限是由拉曼介质中的热问题引起的。由于非弹性散射过程而沉积在拉曼介质中的热量的去除受到通常使用的晶体的低导热性的严重抑制。这将导致过度的热透镜效应，使缩放到更高的功率变得复杂。金刚石具有比大多数拉曼介质更高的拉曼增益系数和比它们都大得多的导热性。然而，它作为拉曼介质的使用通常被驳回：由于可用的小样本量和即使是这些小样本的费用。在光子学研究所的初步研究中，我们已经表明这种判断过于草率。首先，最近商业化的人造单晶金刚石将降低成本并提高质量。其次，我们的模型表明，金刚石的高导热性和损伤阈值意味着紧密聚焦可以使用短的（因此是可用的）晶体（<2毫米）。本项目将以这一平台为基础，特别针对以下四个方面：第一个基于金刚石的连续波腔内拉曼激光器（目标：1240 nm处12w；通过腔内二次谐波产生橙色（620 nm） 5w）。半导体圆盘激光器的第一次拉曼转换（目标：在735 nm处200mw和在1235 nm处2w）3 .首次在连续波拉曼激光器（目标：10w， m平方< 1.1,1240nm）中使用自适应光学进行自动光束质量优化。首次使用金刚石微光学来拉曼转换紧凑型调q激光器（目标：40%效率）实现这些成果将为英国在固态激光工程这一重要的新兴领域建立强大的存在。此外，它将为一系列紧凑光源在新光谱区域的应用开辟道路，如皮下光动力治疗、水下视觉系统和多光谱成像。

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.