Metasurface-enhanced spatial mode decomposition

Acquiring precise information about the mode content of a laser is critical for multiplexed optical communications, optical imaging with active wave-front control, and quantum-limited interferometric measurements. Hologram-based mode decomposition devices, such as spatial light modulators, allow a fast, direct measurement of the mode content, but they have limited precision due to cross coupling between modes. Here, we report a proof-of-principle demonstration of mode decomposition with a metasurface, resulting in significantly enhanced precision. A mode-weight fluctuation of $6\times {10}^{-7}$ was measured with 1 s of averaging at a Fourier frequency of 80 Hz, an improvement of more than three orders of magnitude compared to the state-of-the-art spatial light modulator decomposition. The improvement is attributable to the reduction in cross coupling enabled by the exceptionally small pixel size of the metasurface. We show a systematic study of the limiting sources of noise, and we show that there is a promising path towards complete mode decomposition with similar precision.

Figure 1

Mode decomposition apparatus. The incoming laser strikes the metasurface and is split into three beams A, B, and C. The lenses focus this light to a large waist in the output plane. Preparatory optics are shown in Fig. 3.

Figure 2

Design and fabrication of the $\mathrm{Au}/\mathrm{Si}{\mathrm{O}}_2/\mathrm{Au}$ trilayer metasurface. (a) The central 100 pixels of the phase distribution encoded on the MODAN metasurface. (b) The illustration of the trilayer meta-atom structure. (c) The scanning electron microscope (SEM) image of the metasurface. Scale bar: 720 nm. (d) The simulated Co polarization and cross polarization in the reflected beam. The incident light is LCP.

Figure 3

Calibration and validation. (a) Validation apparatus. Laser light is filtered though a mode cleaner cavity. A small angular modulation is applied at M1. The beam is then incident on the two devices under test: a QPD and the metasurface phase plate. (b) Calibration signal. 150 s of data were gathered for the calibration procedure. These data show excellent coherence between the QPD and MODAN-based sensors at the 318 mHz modulation frequency.

Figure 4

Noise contributions. “MODAN Signal” and “QPD Signal” show the measured signals, while the beam is centered on the MODAN, but out of any alignment control loop. The manufacturer's photodiode noise estimate from channels A and C is shown by “PD Noise Projection.” “Dark Noise” shows the total noise of the MODAN photodiodes, signal processing, and digitizer electronics, with no laser illumination.