Orbital angular momentum spectrum diagnostic from a powermeter.

Orbital angular momentum (OAM) spectrum is one of the identity characteristics of laser beams and determines the performance of OAM based advanced applications. Therefore, diagnosing the OAM spectrum of a beam is of great significance. In this Letter, we show that a common commercial powermeter, rather than an array detector such as the charge coupled device, can be employed to accurately measure OAM spectra for beams. In our proposal, a series of probe single OAM modes are introduced to interfere with the incident beam to be measured. Then, a power value sequence is measured through a powermeter. Finally, the OAM spectrum is acquired by analyzing the power value sequence. The favorable experimental results illustrate the effectiveness of our proposal, where the OAM spectra of both single-mode and multiplexed OAM beams are well diagnosed. This work provides a new, to the best of our knowledge, insight of the OAM spectrum diagnostics and is expected to reduce the cost of the measuring system, thereby facilitating the widespread and advanced application of OAM.