Monolithic, mode-locked Titanium-Sapphire lasers with repetition rates in the range of 30 GHz to 300 GHz

本项目的目标是研究一种新型的锁模掺钛蓝宝石(“Ti：Sa”)激光器。这将是第一个单片式钛：Sa激光器，也是第一个重复频率在30 GHz到300 GHz之间的锁模固体激光器。尽管谐振腔必须很短，但钛：Sa非同寻常的大增益带宽支持大量的纵模。这导致了以前所未有的重复频率产生无背景的超短脉冲。例如，一台重复频率为1太赫兹的钛：铝激光器可能仍有100多个模式。该激光器是一种简单、薄的Ti：Sa圆盘，厚度在0.3 mm到3 mm之间，两侧都有色散补偿涂层。计算表明，在这样一个简单的结构中实现克尔透镜锁模是可能的。我们想要研究这种新型的钛宝石激光器的锁模条件和噪声特性，所提出的单片结构对激光运转具有重要的优势。谐振器内部没有减反射膜。这可以防止标准具效应，减少损失。短小的Ti：Sa晶体只需在晶体的两面镀膜即可实现色散补偿，从而实现准孤子锁模。脉冲重复率将非常恒定，即脉冲定时抖动将很低，因为不会有谐振器反射镜的振动或谐振器内的空气湍流。单片激光系统的这种固有稳定性是从单片单频激光器中得知的。具有数十GHz或数百GHz重复频率稳定飞秒脉冲的锁模激光器将在双梳光谱、天文梳、超快脉冲整形、光通信或频率测量方面有应用。高重复率允许使用中等分辨率的色散系统来分离模式。这导致了用于频率梳应用的紧凑和成本效益高的系统。尽管重复率很高，但这样的激光器的峰值功率仍将比其平均功率高出几十倍或数千倍，这意味着可以预期数百瓦甚至一千瓦。这将使这种钛：Sa激光器有别于锁模半导体激光器，锁模半导体激光器的重复频率已经达到100 GHz，尽管峰值功率不到2W。高峰值功率意味着可以驱动非线性光学过程，这对脉冲特性和应用都是有利的。随着蓝光和绿光二极管激光器输出功率和光束质量的未来进步，应该可以用半导体激光器直接泵浦我们的钛：Sa激光器。这将产生非常紧凑和坚固的飞秒激光和频率梳。

The goal of this project is to investigate a new type of mode-locked Titanium-doped Sapphire (“Ti:Sa”) laser. This will be the first monolithic Ti:Sa laser and the first mode-locked solid state laser to have a repetition rate in the range of 30 GHz up to 300 GHz. The unusual large gain bandwidth of Ti:Sa supports a large number of longitudinal modes despite the resonators having to be short. This leads to background-free ultra-short pulses at unprecedented repetition rates. For example, a Ti:Sa laser with 1 THz repetition rate could still have more than 100 modes. The laser is a simple, thin Ti:Sa disk with a thickness between 0.3 mm and 3 mm that bears dispersion-compensating coatings on both sides. Calculations presented in the proposal show that it is possible to achieve Kerr-Lens mode-locking in such a simple structure. We want to investigate the mode-locking conditions and the noise properties of this new type of Ti:Sa laser.The proposed monolithic setup has important advantages for laser operation. There are no antireflective coatings inside the resonator. This prevents etalon effects and reduces losses. The short Ti:Sa crystals allows dispersion compensation using just the coatings on the two surfaces of the crystal, making quasi-soliton mode-locking possible. The pulse repetition rate will be very constant, i. e. pulse timing jitter will be low, because there can be no vibration of resonator mirrors or air turbulence inside the resonator. This inherent stability of monolithic laser systems is known from monolithic single frequency lasers. A mode-locked laser with stable fs-pulses at tens of GHz or hundreds of GHz repetition rate will have applications for dual-comb spectroscopy, astro-combs, ultrafast pulse shaping, optical communication, or frequency metrology. The high repetition rate allows separating the modes with a dispersive system of moderate resolution. This leads to compact and cost-effective systems for frequency comb applications. Despite the high repetition rate, the peak power of such a laser will still be several tens or thousands times higher than its average power, meaning that hundreds of Watts or even a Kilowatt can be expected. This will set this Ti:Sa laser apart from mode-locked semiconductor lasers which have reached 100 GHz repetition rate albeit with a peak power of less than 2 W. A high peak power means that nonlinear optical processes can be driven which is beneficial for pulse characterization and for applications.With future advances in output power and beam quality of blue and green diode lasers it should be possible to pump our Ti:Sa laser directly with a diode laser. This would yield a very compact and rugged femtosecond laser and frequency comb.