Rapid Generation of Light Beams Carrying Orbital Angular Momentum (1311.4811v1)

Abstract: We report a technique for encoding both amplitude and phase variations onto a laser beam using a single digital micro-mirror device (DMD). Using this technique, we generate Laguerre-Gaussian and vortex orbital-angular-momentum (OAM) modes, along with modes in a set that is mutually unbiased with respect to the OAM basis. Additionally, we have demonstrated rapid switching among the generated modes at a speed of 4 kHz, which is much faster than the speed regularly achieved by spatial light modulators (SLMs). The dynamic control of both phase and amplitude of a laser beam is an enabling technology for classical communication and quantum key distribution (QKD) systems that employ spatial mode encoding.

• The paper demonstrates a novel DMD-based technique to encode phase and amplitude, efficiently generating Laguerre-Gaussian, vortex, and ANG modes.
• It achieves rapid mode switching speeds up to 4 kHz using binary holography with pulse position and pulse width modulation, overcoming SLM limitations.
• The approach is cost-effective and offers high-speed control, promising advancements in both classical optical communications and quantum key distribution.

Rapid Generation of Light Beams Carrying Orbital Angular Momentum

The paper entitled "Rapid Generation of Light Beams Carrying Orbital Angular Momentum" by Mirhosseini et al. presents a novel technique for the efficient encoding of both amplitude and phase variations onto laser beams using a single digital micro-mirror device (DMD). This work addresses the limitations in mode generation and switching speeds experienced with traditional spatial light modulators (SLMs) and introduces a system capable of achieving switching rates up to 4 kHz.

The researchers set forth a method for rapidly generating Laguerre-Gaussian (LG) modes, vortex modes, and angular (ANG) modes by employing binary holography with a DMD. Their approach involves locally modifying the position and width of a binary amplitude grating to encode phase and amplitude information, effectively controlling the characteristics of the diffracted light. The process leverages modulated carrier functions with pulse position modulation (PPM) and pulse width modulation (PWM) techniques to achieve precise mode control. The interferograms and intensity patterns for several generated modes verified the accuracy of this technique, demonstrating clear helical phase structures aligned with theoretical expectations.

In practical communication systems, the OAM modes generated through this research could enhance classical optical communication and quantum key distribution (QKD) by leveraging the dynamic control of phase and amplitude. Of particular importance is the demonstration of rapid mode switching, a capability that sets this technique apart from traditional SLM-based approaches. The mode switching efficiency as obtained in this paper promises advancements in free-space optical communications, particularly in environments with high-dynamic requirements. Furthermore, the implementation of ANG modes, which form a mutually unbiased basis (MUB) relative to the OAM basis, provides a critical component for QKD systems, enhancing their robustness against eavesdropping attacks by increasing tolerance to interception.

The experimental setup succinctly demonstrates the ability of the DMD to encode and switch among various modes such as $\ell = 5$, $\ell = -5$, and $\ell = 0$, highlighting the potential for deploying this method in high-speed optical networks. While the paper notes the current generation efficiency of about 1.5% for OAM modes, it emphasizes that this is not a significant constraint in quantum communications, where low-intensity pulses are often beneficial.

The researchers conclude that their DMD-based approach offers a cost-effective solution compared to phase-only SLMs, with additional benefits such as higher operational speeds and broader spectral capabilities. Future extensions of this research may involve further enhancing the mode generation efficiencies and exploring alternative DMD architectures with higher resolutions and faster switching capabilities, aiming to bridge the efficiency gap with conventional phase-only modulators. These developments could ultimately expand the application scope in both classical and quantum communications. The potential to integrate DMD technology with existing optical infrastructure reinforces its viability as a disruptive innovation in optical signal processing and photonic research.