Requirement for Orbital Angular Momentum Comb Modes in Digital Lasers

Research Background and Significance

With the rapid advancement of information technology, the demand for high-capacity data transmission has become increasingly stringent. Orbital angular momentum (OAM) of light, due to its inherently infinite dimensions, is considered a potentially powerful information carrier. It is crucial to achieve the generation of arbitrary OAM spectra on demand, especially comb-like OAM modes similar to frequency combs. However, under current technologies, realizing a digital OAM comb laser that can switch freely at the source remains challenging, particularly the real-time tunable multi-OAM modes which are in growing demand. This research aims to develop a digital OAM comb laser that can switch dynamically and conveniently at the source while hoping to further advance high-dimensional technology and explore more opportunities in the basic research and applications of high-dimensional OAM modes.

Source of the Paper

This research was jointly completed by Zhi-cheng Ren, Li Fan, Zi-mo Zheng, Zhi-feng Liu, Yan-chao Lou, Shuang-yin Huang, Chao Chen, Yong-nan Li, Cheng-hou Tu, Jian-ping Ding, Xi-lin Wang, and Hui-tian Wang from the National Laboratory of Solid State Microstructures and the School of Physics at Nanjing University. It was received on May 7, 2024, and published in Optica on July 10 of the same year.

Research Methods and Process

The research team proposed a key design concept which involves embedding a degenerate cavity into a stable ring cavity and creating a flexible and dynamically switchable digital OAM comb laser by controlling the phase degree of freedom rather than other proxies. They adopted a nested ring cavity construction method, utilizing a reflective phase-only spatial light modulator (SLM) loaded with a conjugate doubly folded symmetric multi-spiral phase digital hologram mirror. Throughout the research process, a precisely designed cavity configuration and efficient water-cooled SLM module were used, resulting in higher output power and preventing damage to the SLM.

Research Results

In the experiment, the research team first characterized the OAM spectrum generated by multi-spiral phase hologram in a passive configuration. The results showed that the designed OAM comb mode could generate a comb structure with up to 64 teeth. Subsequently, they explored the feasibility of generating a single OAM mode at the source, verifying that the proposed digital ring laser could achieve single OAM modes with high purity and orders up to |128|. Additionally, the research successfully created OAM spectra with arbitrary distributions by designing specific initial phases, which is crucial for high-dimensional applications.

Conclusions and Application Value

The research successfully realized a digital OAM comb laser that can dynamically switch on demand at the source, making a significant progress in the active control of high-capacity OAM resources and their multiplexing. The provided solution not only contributes to the development of new laser technologies but also offers a toolbox for generating multiple OAM modes both linearly and non-linearly.

Research Highlights

The highlights of this research are:

• Designed an innovative parameterized ring cavity structure capable of direct laser output of multiple OAM modes.
• Achieved single OAM modes with orders up to 128 and OAM comb modes with up to 64 teeth.
• Demonstrated that the design strategy allows for the arbitrary encoding of OAM spectra relative intensity.
• Conducted in-depth attempts in creating multi-mode OAM comb structures with digital lasers.
• Provided new development opportunities for future basic research and applications.