In the evolution of ultrafast spectroscopy and time-resolved measurements, in particular terahertz time-domain systems (THz-TDS), the demand for high-speed delay engines increases. Applications in scientific and industrial environments involve material science, high-resolution spectroscopy and non-destructive testing. Where ultra-fast techniques become crucial to reduce the acquisition time, mechanical or acousto-optic delay lines are limiting factors. Optical sampling methods are able to overcome these restrictions, by eliminating downsides of mechanical delay lines, such as comparably low scan speeds of tens of Hz. Different technical approaches have been developed to obtain two synchronized, temporally delayed femtosecond pulse trains without using a conventional, mechanical delay line. Common optical sampling techniques employ either a single oscillator or come as twin oscillator systems. The asynchronous optical sampling technique (ASOPS) has proven to enable high scan speeds and high-resolution spectroscopy. Two femtosecond fiber lasers are synchronized by locking electronics and operate in a controlled repetition rate offset state. We have established such dual-laser based systems and integrated them into fully fiber-coupled THz-TDS systems for the scientific community already. Optical sampling by cavity tuning (OSCAT) addresses higher costs that come with dual-laser systems using a single oscillator albeit one with a variable pulse repetition rate. We present a new engine based on the electronically controlled optical sampling principle (ECOPS) - but 10 times faster than achievable with conventional piezo-electric (PZT) based systems. We introduce an optical sampling engine (OSE) bringing ultra-fast, time-resolved measurements to 10 kHz - with unprecedented compactness of 19” 3U.
Tomographic photoacoustic imaging (PAT) allows to overcome the anisotropic image resolution of conventional reflection mode imaging. In order to achieve high-resolution, tomographic images, precise information on the position of each detector element is required. PAT systems that acquire signals from rotating linear transducer arrays come with inevitable transducer misalignments. Up to now, transducer orientation (x/y-tilt) and radial distance uncertainty were measured experimentally or have not been considered. Uncalibrated, these systems suffer from characteristic artifacts yielding misinterpretations of anatomic structures. Herein, we derive the artifact mathematically and investigate an analytical calibration method that enables the calculation and compensation of important transducer positioning parameters: the rotational radius and in-plane tilt. We studied the approach theoretically and evaluated the performance of the developed algorithm both on numerical and experimental data. A PAT system based on a 5-MHz linear transducer array, a multichannel electronics platform with channel data access, a NIR-emitting laser system and a rotating samples is used to demonstrate the benefit of the transducer calibration method providing isotropic resolution of 160 μm.
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