Nonlinear Polarization Rotation in Bulk Multipass Cells for Mode-Locking Applications

Nonlinear polarization rotation (NPR) is an effect that exploits Kerr nonlinearity and is routinely used in fiber lasers. Due to the fiber's long propagation length, the accumulation of required nonlinearity for NPR occurs gradually and without degrading the pulse's spatial profile. Consequently, the effect is widely utilized in fiber lasers as a controllable mode-locking mechanism [1], [2]. The NPR effect has also been demonstrated in a free-space geometry of a Herriott-type multipass cell, where it has been used as a filter to enhance the temporal contrast of ultrashort pulses [3]. The quasi-waveguide nature of a Herriott cell allows for smooth nonlinearity accumulation, making the effect ideal for mode-locking a free-space thin-disk oscillator. Passively mode-locked thin-disk lasers have recently evolved into capable power-scalable systems, delivering ~100 fs-long pulses [4]. NPR mode-locking has the potential to combine all the advantages of conventional passive mode-locking techniques based on Kerr lens and SESAM, including a smooth and controllable initiation of the pulsed regime, a high modulation depth, and short fs-long pulse durations. Furthermore, the operation at the stability center of a laser cavity ensures insensitivity to misalignment and simplifies the handling of high-power lasers, facilitating the adoption of free-space ultrafast oscillators in real-world applications beyond the laboratory environment.