In this presentation, we will review recent progress in the development of ultrafast laser sources with high average power. We will present the state-of-the-art of high-power ultrafast laser sources with potential for driving THz sources, current technological challenges in scaling THz average power, and applications areas that could potentially benefit from these novel sources.
Photoconductive emitters and receivers are widely accepted as the best combination for applications requiring broadband and high dynamic range and are nowadays deployed in most commercially available systems. Novel laser sources with higher repetition rate and power levels are a promising route towards further improvements in this area. We present our first steps in this direction by combining state-of-the-art emitters and receivers with an ultra-stable commercial fs laser (MENHIR-1550 SERIES) at 1 GHz repetition rate as the optical source. The output of the laser is amplified and compressed by a commercial fiber amplifier setup. In this experiment, we use 17 mW as the probe beam and 30 mW as the pump beam with a pulse duration of 150 fs, as these are the best operation points for the emitter and receiver available. The emitter is based on iron doped InGaAs in a strip line geometry with an active region of 50 μm x 50 μm while a fiber coupled dipole antenna with a 10 μm gap is used as the receiver. We demonstrate a 1 GHz repetition rate terahertz time-domain spectroscopy (THz-TDS) system with a dynamic range of 73 dB and a bandwidth of 3.5 THz using state-of-the-art THz photoconductive emitter and receiver with a measurement time of 60 s. This result is part of a larger effort to understand the compromises to be realized in terms of repetition rate and average power to take photoconductive emitters and receivers to the next step in dynamic range enhancement.
Ultrafast Terahertz sources with high average power are of increasing interest for various spectroscopic investigations, currently limited by signal-to-noise ratio. A straightforward path to increase the average power of THz sources is to make use of state-of-the-art femtosecond near infrared driving lasers with higher average power than the commonly used Ti:Sa lasers. Diode pumped solid state lasers based on Yb now reliably provide from hundreds of watts up to kilowatts of average power with sub-ps pulses, while THz generation with more than a few tens of watts driving power remains widely unexplored. Among these technologies, modelocked thin-disk oscillators are particularly attractive to drive high power, high repetition rate THz sources, providing hundreds of watts directly from a compact one-box oscillator without the need for any additional amplification stages, thereby reducing the overall system complexity.
Here, we will present our recent results using optical rectification (OR) in GaP and Lithium Niobate (LN), driven by a home-built Yb:YAG femtosecond modelocked thin-disk oscillator with an average power of more than 100 W at 13 MHz repetition rate. Using GaP, we achieve milliwatt average power levels with a bandwidth extending > 6 THz making this an ideal tool for THz-TDS for example of absorptive samples. Using the tilted pulse front scheme in LN, we achieve THz powers exceeding 40 mW at 13 MHz repetition rate, which represents the highest average power of any THz sources with MHz repetition rate. Additionally to these results, we will present our ongoing investigation of thermal effects and further average power scaling of OR in this unusual excitation regime.
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