Reconfigurable and tunable twisted light laser

Scientific Reports, Jul 2018

Twisted light, having a helical spatial phase structure and carrying orbital angular momentum (OAM), has given rise to many developments ranging from optical manipulation to optical communications. The laser excitation of twisted light in a reconfigurable and tunable way is of great interest. Here, we propose and experimentally demonstrate an OAM reconfigurable and wavelength tunable twisted light laser with achievable high-order OAM modes on a hybrid free-space and fiber platform. The excited twisted light laser is enabled by a ring resonator incorporating spatial light modulators (SLMs) and bandpass filter (BPF). By appropriately switching the phase pattern loaded onto SLMs and adjusting the BPF, twisted light laser with reconfigurable OAM and tunable wavelength is implemented. In the experiment, the OAM value is varied from −10 to +10 and the wavelength is adjusted from 1530 to 1565 nm covering the whole C band. The obtained results indicate successful implementation of a reconfigurable and tunable twisted light laser with favorable operation performance. Reconfigurable and tunable twisted light laser may open up new perspectives to more extensive OAM-enabled applications with improved flexibility and robustness.

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Reconfigurable and tunable twisted light laser

Abstract Twisted light, having a helical spatial phase structure and carrying orbital angular momentum (OAM), has given rise to many developments ranging from optical manipulation to optical communications. The laser excitation of twisted light in a reconfigurable and tunable way is of great interest. Here, we propose and experimentally demonstrate an OAM reconfigurable and wavelength tunable twisted light laser with achievable high-order OAM modes on a hybrid free-space and fiber platform. The excited twisted light laser is enabled by a ring resonator incorporating spatial light modulators (SLMs) and bandpass filter (BPF). By appropriately switching the phase pattern loaded onto SLMs and adjusting the BPF, twisted light laser with reconfigurable OAM and tunable wavelength is implemented. In the experiment, the OAM value is varied from −10 to +10 and the wavelength is adjusted from 1530 to 1565 nm covering the whole C band. The obtained results indicate successful implementation of a reconfigurable and tunable twisted light laser with favorable operation performance. Reconfigurable and tunable twisted light laser may open up new perspectives to more extensive OAM-enabled applications with improved flexibility and robustness. Introduction In recent years, the space domain of lightwaves has gained great interest in diverse applications. Exploiting the spatial structure of lightwaves enables a new kind of light beam called structured light beyond the well-known Gaussian light beam. In general, structured light manifests distinct spatial amplitude/phase/polarization distribution1,2,3,4,5. Among different kinds of structured light, a promising one is called twisted light, featuring a helical phasefront, a phase singularity, and a doughnut intensity profile. Since the early recognition in 1992 by Allen and co-workers that lightwaves comparising a helical phasefront exp(ilφ) have an orbital angular momentum (OAM) of lħ per photon (l: topological charge, φ: azimuthal angle, ħ: reduced Plank’s constant)2, OAM-carrying twisted light has given rise to many developments in optical manipulation, tweezer, microscopy, imaging, metrology, astronomy and quantum information processing6,7,8,9,10,11,12,13,14,15,16. Very recently, twisted light has also seen its potential applications in free-space, fiber-based and underwater optical communications with increased transmission capacity and efficient spectral usage17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36. Remarkably, for all of the above OAM-enabled extensive applications, the generation of OAM-carrying twisted light is of great importance. Various techniques for generating OAM-carrying twisted light beams have been proposed and demonstrated over the past years6,7,8,18,19,20. In general, there are two kinds of techniques, i.e. external beam shaping and internal beam lasing. The external beam shaping technique usually transforms a Gaussian beam into an OAM-carrying twisted light beam, e.g. computer-generated hologram7,37, mode converte38, spiral phase plate39, inhomogeneous anisotropic media40, fiber41, metamaterials42,43,44,45,46,47 and photonic integrated devices48,49. These tehniques relying on external beam shaping, however, suffer from relatively low conversion efficiency, degradation of beam quality, high power handling limitation, and added complexity with extra bulky mode conversion devices. Alternatively, using internal beam lasing technique, OAM-carrying twisted light can be also generated inside a laser resonator (i.e. twisted light laser), which may overcome some drawbacks of external beam shaping technique. To date there have been lots of prior seminal works related to OAM-carrying twisted light laser and an increasing interest has been gained to control light’s helicty at the source (i.e. OAM states from lasers)50. For instance, (1) Laguerre-Gaussian (LG) modes were generated in Nd:YAG lasers51,52; (2) LG modes seletection was demonstrated in diode-pumped solid-state lasers53; (3) a doughnut laser beam was generated as an incoherent superposition of two petal beams54; (4) high-order mode excitation was demonstrated in end-pumped solid-state lasers55; (5) the controlled generation of higher-order Poincaré sphere beams was demonstrated from a laser56; (6) OAM microlaser was fabricated and demonstrated on the InGaAsP/InP platform57. These works showed impressive lasing performance. Beyond the basic realization of OAM-carrying twisted light laser, the functionality and robustness are also worth taking full consideration. For a robust OAM-carrying twisted light laser, one would expect reconfigurable OAM and tunable wavelength. Fortunately, several approaches have been reported to add OAM reconfigurability or wavelength tunability to the twisted light laser. For example, (1) a digital laser for reconfigurable on-demand laser modes was demonstrated by using a phase-only spatial light modulator (SLM) as the back optical element of the cavity58; (2) (...truncated)


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Nan Zhou, Jun Liu, Jian Wang. Reconfigurable and tunable twisted light laser, Scientific Reports, 2018, Issue: 8, DOI: 10.1038/s41598-018-29868-8