On-demand orbital angular momentum comb from a digital laser

Photonic orbital angular momentum (OAM) carried by phase-structured vortex light is an important and promising resource for the ever-increasing demand towards high-capacity data information due to its intrinsic unlimited dimensionality. Large superpositions of OAM are easy to be produced, but on-demand generation of arbitrary OAM spectra such as an OAM comb similar to a frequency comb is still a challenge; especially, the on-demand OAM comb and arbitrary multi-OAM modes have not yet been realized at the source. Here we report a versatile at-source strategy for developing a flexibly and dynamically switchable on-demand digital OAM comb laser for the first time, to our knowledge, by controlling the phase degree of freedom itself rather than any proxy. For this aim, we present a crucial design idea that a nested ring cavity configuration is composed of a degenerate cavity embedded into a stable ring cavity and a pair of conjugate two-fold symmetric multi-spiral-phase digital holographic mirrors loaded onto reflective phase-only spatial light modulators. In the nested ring cavity, the stable ring cavity and the degenerate cavity meet the requirements of high spatial coherence and supporting any transverse mode, respectively. The paired conjugate holographic mirrors located in mutual object and image planes circumvent the competing issue among different OAM modes and control the number and chirality of modes in OAM combs with ease. Our strategy has also universality as it has the ability of encoding OAM spectra with arbitrary distribution. The realization of a dynamic on-demand multi-OAM-mode laser is an important progress in the infancy of multi-OAM-mode sources. Our idea provides a promising solution for development of emerging high-dimensional technologies; in the future, there will be increasing opportunities in the fundamentals and applications of high-dimensional OAM modes, and beyond. Our strategy not only contributes to the development of new laser technology, but also provides a toolbox for both linear and nonlinear generation of the multiple OAM modes at the source.