Introduction

Abstract

Theodore Maiman allegedly said about his invention, the laser, that it is a “solution looking for a problem” [1]. Since its first demonstration in 1960, countless applications for lasers have been found in diverse fields of science and technology. Due to its outstanding properties, namely coherence, diffraction limited focusability, high brightness, and either high spectral purity or broadband pulsed operation, the laser has become an indispensable tool for imaging and spectroscopy. For example, with coherent illumination, transparent objects can be observed using phase contrast microscopy [2], and using laser scanning confocal microscopy (LSCM) the image quality of biological objects can be greatly enhanced [3]. While the foundations of confocal microscopy have been laid already in the 1950s, it became a standard technique only after the availability of high brightness laser sources [4]. In spectroscopy, the relative accuracy of atomic hydrogen measurements was improved using laser spectroscopy from $10^{-7}$ to $10^{-10}$ [5]. Later, using the so-called laser frequency comb spectroscopy, it was further improved to $10^{-14}$ [5]. Here, the optical spectrum of the laser consists of many equidistant comb modes and can be described with two microwave frequencies $f_0$ and $f_\text {rep}, f_n = f_0 + n \cdot f_\text {rep}$ [6]. Thus, optical frequencies can be mapped to the frequency standard defined by cesium atomic clocks in one step, instead of complicated chains of frequency dividers, greatly reducing noise. Nowadays, optical frequency combs covering the visible, near infrared and mid-infrared range based on rare-earth doped lasers are commercially available.