Abstract
The conversion of transverse mode-locked (TML) laser beams from a Hermite-Gaussian (HG) to a Laguerre-Gaussian (LG) mode set using a cylindrical lens mode converter was demonstrated experimentally. By changing the spatial symmetry of the beams new spatio-temporal dynamics for TML lasers were enabled. In particular, the fast linear motion of an oscillating laser spot, generated by a TML laser based on HG modes, was translated into the circular motion of a TML laser beam based on LG modes. The mode conversion was demonstrated successfully for different average mode orders. Apart from an average ellipticity of about 6%, the converted beam profiles remained circular over the propagation from the near- into the far-field. The remaining ellipticity seemed to be introduced by astigmatism of the incident HG TML beam, which could be compensated before conversion. Due to their radial symmetry and high scanning speed TML laser beams based on LG modes are well suited for precision applications like STED or Minflux microscopy.
Highlights
While higher order transverse modes are often suppressed in lasers to obtain a pure fundamental output beam, they have become of increasing interest over the past few years, as their unique properties have found various applications, e.g., in the field of microscopy [1,2,3], in the telecommunication sector [4,5,6] or for quantum experiments [7,8,9]
If transverse modes are generated within a laser cavity, e.g., by a modulation of the spatial loss [14], phase [15], or gain [16] distribution, they will experience a frequency shift, which depends on their mode order [17]
To demonstrate the successful mode conversion of transverse mode-locked (TML) beams, HG TML beams with different oscillation amplitudes 0 have been converted from an HG mode set to an LG mode set with the above-described cylindrical lens mode converter
Summary
While higher order transverse modes are often suppressed in lasers to obtain a pure fundamental output beam, they have become of increasing interest over the past few years, as their unique properties have found various applications, e.g., in the field of microscopy [1,2,3], in the telecommunication sector [4,5,6] or for quantum experiments [7,8,9]. Higher order transverse modes can be generated with a high degree of purity outside of a laser cavity, using for example spatial light modulators [10, 11] or diffractive optical elements [12, 13]. If transverse modes are generated within a laser cavity, e.g., by a modulation of the spatial loss [14], phase [15], or gain [16] distribution, they will experience a frequency shift, which depends on their mode order [17]. As a result of this frequency shift, the interference of multiple transverse laser modes will cause fast spatio-temporal dynamics in the intensity distribution of the output beam. If the modes are phase-locked with each other, a transverse mode-locked (TML) laser with a periodically scanning output beam is realized [18, 19]. The linear oscillation of the spot was converted into a circular rotation,
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