## 1 Introduction  
Non-destructive thickness measurements [1] play an increasingly important role in industrial quality control, cost management and process monitoring. In automotive manufacturing, the correct structure of paint and coating layers determines not only the overall aesthetics, but also resistance to UV radiation damage and corrosion. In pharmaceuticals, tablet coatings regulate the release of the drug inside the body. In polymer extrusion, precise thickness measurements can reduce costs while ensuring an uncompromised product quality. All of these industrial applications require a measurement technique that is intrinsically safe, accurate to the micrometer level, and capable of resolving multi-layered samples. Above all, the gauging system needs to be inherently fast, providing at least 1000 thickness readings per second, to match the standard set by alternative methods such as eddy current devices [2]. Initial work on terahertz-based thickness measurements dates back to the 1990s [3; 4], when [PERSON] et al. investigated methods for the extraction of optical constants (i.e., the complex refractive index of samples) via thickness assessments. Subsequent works focused on introducing new algorithms for the extraction of material parameters [5; 6; 7], on paint thickness [8; 9; 10] and coating layer [11; 12] gauging, and on the identification of concealed defects [13]. Most of the work accomplished to-date made use of conventional terahertz TDS systems [14]. A standard TDS system employs a femtosecond laser, with the laser beam being split in two paths. The first pulse train illuminates the emitter--usually a photoconductive antenna--and generates terahertz radiation. The terahertz pulses interact with the sample and arrive at the receiver side, where they are superimposed with the second laser pulse train. In order to \"readout\" the terahertz pulses, the second laser pulse train is time-shifted with the help of a mechanical delay stage. For a delay length of several 100 picoseconds, the measurement speed of conventional systems ranges from a few to few 100 terahertz pulse traces per second [15; 16; 17], and thus falls short of the requirements for high-speed monitoring. In order to overcome the bottleneck in terms of measurement speed, two faster terahertz TDS concepts have been proposed: asynchronous optical sampling (ASOPS) [18; 19; 20] and, more recently, ECOPS [21; 22; 23]. Both techniques operate without a mechanical delay, and have proven capable of recording more than 1000 terahertz pulse traces per second. ASOPS, in brief, employs two pulsed lasers with fixed, albeit slightly different repetition rates. The time delay between the two lasers varies periodically between zero and the inverse repetition rate. Consequently, the time needed to acquire a complete terahertz trace depends on the difference between the two repetition rates. If a sufficiently large difference is chosen, the measurement speed can amount to several thousand scans per second. However, there are several limitations for the use of ASPOS in \"real-world\" applications. First, femtosecond fiber lasers in the telecom wavelength regime of 1.5 \(\upmu\)m--the workhorses in most of today's terahertz TDS systems [15; 24]--have typical repetition rates between 250 MHz and 40 MHz. Using these lasers for ASOPS would lead to unnecessarily long scan ranges from 4 ns to 25 ns, i.e., most of the measurement time would be wasted. Lasers with GHz repetition rates can, in principle, reduce the amount of \"dead time\", yet these repetition rates call for more complex Ti:sapphire systems [20; 25]. The use of Ti:sapphire lasers, in turn, changes the optical wavelength to \(\sim\)0.8 \(\upmu\)m, which complicates pulse transmission through optical fibers. Either way, any ASOPS system that measures 1000 (or more) traces per second with nanosecond scan ranges requires transimpedance amplifiers and data acquisition electronics of an extremely high bandwidth (hundreds of MHz to several GHz).