Ultrafast laser sources and frequency conversion at high repetition rates
Authors:   , Hamrouni Marin 
Published: October 2024

Mode-locked laser sources have become a cornerstone technology for numerous applications and led to the development of groundbreaking optical tools and systems. In the optical frequency domain, mode-locked lasers feature spectra composed of a set of discrete lines that are equally spaced. This spectral structure gave the name to a new tool called optical frequency comb (OFC), spanning applications such as spectroscopy, astronomy, attoscience, telecommunications, and atomic clocks. The main important features of OFCs are the average power and spectral bandwidth, in addition to the repetition rate and the location of the comb-lines within the electromagnetic spectrum. While nearly all applications benefit from high average power and large spectral bandwidths, the most suitable repetition rates and spectral regions are truly application dependent. Mode-locked lasers generating near-infrared pulses with repetition rates spanning from megahertz to hundreds of megahertz are now mature. Nowadays, driven by application needs in spectroscopy and astronomy, many research projects focus on generating OFCs in the mid-infrared spectral region as well as increasing the repetition rates. Due to the lack of commercially available laser crystals featuring resonant transitions in the mid-infrared region, direct generation of OFCs above 3 μm is technologically challenging. While the emergence of quantum cascade lasers paved a promising way for direct mid-infrared generation, the main approach for realizing broadband mid-infrared OFC sources remains nonlinear frequency conversion of near-infrared pulses delivered by mature mode-locked lasers. There, an important challenge is to deliver sufficient power to achieve efficient nonlinear conversion, especially at high repetition rates since the pulse energy and repetition frequency tend to play against each other. As a result, mid-infrared OFC sources based on gigahertz mode-locked lasers are rare and mostly limited to photonic laboratories. Pursuing the objective of developing compact and cost-efficient OFC sources suitable for real-world applications, this thesis is structured into two main parts. The first chapter presents the development of powerful and broadband modelocked laser oscillators based on Ytterbium (Yb) gain materials. It begins with the description of a Kerr-lens mode-locked Yb:CALGO oscillator operating at 1 GHz repetition frequency. The laser demonstrates the shortest pulse duration and highest average and peak power with respect to previous gigahertz-class oscillators based on Yb gain media. In a subsequent experiment, we perform the full stabilization of the laser, turning the source into a functional broadband OFC. Besides achieving low-noise performance, the stabilized source delivers peak power levels suitable for many applications, including nonlinear frequency conversion. We close the first chapter by addressing the challenge of generating few-cycle pulses at high average power using Yb gain media. Here, we introduce a novel pumping scheme that overcomes the limitations due to the small frequency difference between the pump and laser photons. In particular, our technique enables efficient generation and broad spectral bandwidth at the same time. By implementing this approach in a soft-aperture Kerr-lens modelocked laser oscillator, we obtain record-high optical-to-optical efficiency and high average power compared to previous Ytterbium-based sources operating in the few-cycle regime. In the second part of this thesis, we investigate novel approaches for efficient frequency conversion of ultrafast lasers into the terahertz and mid-infrared spectral ranges. This part consists of three main sections. In the first section, we discuss the development of a compact and cost-efficient terahertz source based on driving the nonlinear process directly inside the cavity of a simple ultrafast bulk mode-locked laser. This way, the terahertz emitter benefits from leveraged power levels available inside the cavity, which reduces the requirement on the overall driving source compared to traditional approaches. In the subsequent section, we demonstrate efficient and broadband mid-infrared generation based on parametric down-conversion inside thin-film lithium niobate waveguides. By exploiting the large nonlinearities and dispersion engineering offered by those nanophotonic devices, we revisit an approach to nonlinear down-conversion, namely gain-trapped optical parametric amplification. This technique relies on the interplay between nonlinear gain and group velocity mismatches, which under specific conditions results in the trapping of the down-converted pulses under the driving near-infrared pump. In the unsaturated regime, this approach enables exponential amplification of the midinfrared pulses over distances exceeding conventional methods, which allows to reach saturation with a few picojoules of in-coupled pulse energy. After saturation, up to 50% of near-infrared pump photons are converted into the 3-4 μm band using only a few picojoules of pulse energy coupled inside the waveguide. Given the low energy requirements, this approach is promising for realizing efficient frequency conversion at multi-gigahertz repetition rates. Lastly, we utilize dispersion-engineered thin-film lithium niobate devices to demonstrate broadband supercontinuum generation and carrier-envelope offset frequency detection at low input power. The approach uses dispersion engineering to match the group velocity of fundamental and second harmonic pulses centered around 2 and 1 μm respectively. In this case, an interplay between saturation and small phase-mismatch occurs over long interaction length, which leads to strong spectral broadenings. As a result, we generate a supercontinuum ranging from the ultraviolet to mid-infrared ranges with only tens of picojoules of in-coupled fundamental energy. The resulting octave-spanning spectrum produces carrier-envelope offset frequency beatnotes that we directly detect at the waveguide output. We believe that the combination of the laser sources presented in the first part with the frequency conversion techniques showcased in the second section will enable the realization of compact mid-infrared OFCs operating at high repetition rates. This leaves exciting prospects for follow up studies.