Multipass amplifiers with self-compensation of the thermal lens

Abstract

We present a novel architecture for a multi-pass amplifier based on a succession of optical Fourier transforms and short propagations that shows a superior stability for variations of the thermal lens compared to state-of-the-art 4fbased amplifiers. We found that the proposed multi-pass amplifier is robust to variations of the active medium dioptric power. The superiority of the proposed architecture is demonstrated by analyzing the variations of the size and divergence of the output beam in form of a Taylor expansion around the design value for variations of the thermal lens in the active medium. The dependence of the output beam divergence and size is investigated also for variations of the number of passes, for aperture effects in the active medium and as a function of the size of the beam on the active medium. This architecture makes efficient use of the transverse beam filtering inherent in the active medium to deliver a beam with excellent quality (TEM00).

Passive alignment stability and auto-alignment of multipass amplifiers based on Fourier transforms

“…The choice of the Fourier-based amplifiers is motivated by the superior stability of these multipass amplifiers to variations of thermal lens and aperture effects at the active medium compared with state-of-the-art amplifiers based on the 4f-imaging propagation. An extensive comparison between the two multipass architectures can be found in [9]. Three variations of the same Fourier-based amplifier were investigated in this study: the basic layout of Fig.…”

“…Furthermore, it demonstrates theoretically and practically, that the Fourier-based design is well apt for a simple auto-alignment system that mitigates the beam excursion from the optical axis (perfect alignment) caused by the tilts of its various optical components. Hence, this study, combined with the investigations of [9], displays the excellent performance of the Fourier-based architecture with respect to thermal lens variations, aperture effects, beam quality, efficiency and alignment stability, qualifying it for high-power and high-energy applications.…”

“…The dependency of the gain (transmission) on tilts constitutes the sensitivity of the multipass amplifier to mirror tilts. In this study, it is assumed that losses in the multipass system occur only at the active medium whose positiondependent gain (and absorption in the unpumped region) can be approximated by an average gain and a positiondependent transmission function (soft aperture) [9]. It is further assumed that the soft aperture of the active medium can be fairly approximated by a Gaussian aperture [5,6,9,14], i.e., an aperture with Gaussian intensity transmission function τ:…”

“…In this study, Fourier-based multipass amplifiers are considered, i.e., multipass amplifiers where the propagation from active medium to active medium is accomplished using an optical Fourier transform. The beam propagation in the Fourier-based amplifiers takes the form [9] AM − Fourier − AM − SP − AM − Fourier −AM − SP − AM − Fourier − AM − SP − AM... , where SP represents a short free propagation and Fourier any propagation that performs an optical Fourier transform. In contrast to the 4f-based amplifiers, Fourier-based amplifiers show output beam characteristics which are independent on variations of the thermal lens ensued by the pumped active medium [9].…”

“…The concept of Fourier-based amplifiers has been introduced in [10] and successively elaborated in [11,12,13,14,9], where a detailed comparison between Fourierbased and state-of-the-art multipass amplifiers based on the 4f-relay imaging is presented. In contrast to these earlier considerations, here another crucial aspect of laser design is considered, namely, the stability of the amplifier performance to misalignments (tilts).…”