Browsing by Author "Repgen, Paul"

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    Analogy of harmonic modelocked pulses to trapped Brownian particles improves laser performance
    (SpringerLink, 2023-02-28) Laçin, Mesut; Repgen, Paul; Şura, Aladin; Şenel, Çağrı; İlday, Fatih Ömer
    High-repetition-rate ultrafast lasers are needed for diverse applications. Harmonic modelocking, where multiple identical, equidistant pulses circulate in the cavity, reaches beyond the practical limitations of reducing the cavity length. However, it suffers from stochastic deviations that manifest as supermodes in the radio-frequency spectrum and difficulties in maintaining the same harmonic state, often coupled with trade-offs in pulse energy, duration, or noise performance. Here, we first show that deviations in the temporal positions of the pulses contribute disproportionately more to the supermodes than deviations in their amplitudes. Then, we argue that these fluctuations are analogous to those of trapped Brownian particles. This analogy reveals that supermodes are suppressed by stronger spectral filtering, which corresponds to fluid viscosity, and higher pulse energy reduces the noise, akin to lower temperature. Guided by this intuitive picture, we construct a Yb-fibre laser incorporating strong filtering and high intracavity energies by limiting nonlinear polarisation evolution to a short section of ordinary fibre. The rest of the all-fibre cavity comprises polarisation-maintaining fibre, which additionally improves environmental robustness. We report record-high supermode suppression ratios, reaching 80 dB, excellent long-term and environmental stability, and pulse energy, duration, and noise characteristics that are similar to fundamentally modelocked lasers.
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    ItemOpen Access
    Stable harmonic modelocked laser operation by damping the temporal displacement of optical pulses in analogy to Brownian particles
    (IEEE - Institute of Electrical and Electronics Engineers, 2023-09-04) Laçin, Mesut; Repgen, Paul; Şura, Aladin; Şenel, Ç.; İlday, Fatih Ömer
    Since the first report of modelocking in 1964, every parameter of a typical oscillator has been improved by several orders of magnitude. The exception is the repetiton rate, which is still roughly in the order of the initial 56 MHz. Some applications, such as ablation-cooled material processing, require high pulse repetition rates. Harmonic modelocking presents itself as a solution. However, it has had a limited impact outside the laser research laboratory to date, but its utility can be improved significantly if its reliable, repeatable, and stable operation can be ensured.