The function to measure orbital angular momentum (OAM) distribution of vortexlight is essential for OAM applications. Although there are lots of works tomeasure OAM modes, it is difficult to measure the power distribution ofdifferent OAM modes quantitatively and instantaneously, let alone measure thephase distribution among them. In this work, we demonstrate an OAM complexspectrum analyzer, which enables to measure the power and phase distribution ofOAM modes simultaneously by employing rotational Doppler Effect. The originalOAM mode distribution is mapped to electrical spectrum of beating signals witha photodetector. The power distribution and phase distribution of superimposedOAM beams are successfully retrieved by analyzing the electrical spectrum. Wealso extend the measurement to other spatial modes, such as linear polarizationmodes. These results represent a new landmark of spatial mode analysis and showgreat potentials in optical communication and OAM quantum state tomography.
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