26th Australian Institute of Physics Congress and the Australian and New Zealand Conference on Optics and Photonics

Single-Shot Modal State Tomography of Laser Beams Using Multi-Plane Light Converter

Not scheduled

20m

Belinda Hutchinson Building (The University of Sydney )

Contributed Oral ANZOS | Photonics and Optics (ANZCOP)

Speaker

Rene Kalla

Description

Both short- and long-haul communication networks rely on optical fibres to satisfy the continually increasing demand for data capacity and transmission speed. In short-reach links, such as those within and between data-centre racks, vertical-cavity surface-emitting lasers (VCSELs) are commonly paired with multimode fibres (MMFs) to transmit binary data as rapidly modulated optical pulses.

The capacity of these links is limited by modal dispersion. Each optical pulse typically contains multiple spatial modes that propagate through the fibre at different group velocities. As these modes separate in time, the pulse broadens and overlaps neighbouring bits, increasing transmission errors and limiting the achievable data rate. These limitations can be mitigated by designing VCSELs with improved modal emission or fibres with reduced mode-dependent propagation delays. Both approaches require detailed characterisation of the emitted optical field, including its modal composition, temporal dynamics, spectral content, and polarisation.

Here, we present a passive, single-shot measurement technique capable of capturing the complete modal state of an optical signal with temporal resolution limited only by the detector bandwidth. Our method is based on spatial Stokes tomography [1], in which a seven-plane light converter maps the incident field onto spatially separated intensity spots. These spots encode pairwise interference between all spatial modes, revealing their amplitudes and, where mutual coherence exists, their relative phases. Mutually incoherent contributions are instead represented as an incoherent statistical mixture. Simultaneous detection of all output spots forms the Stokes vector, from which the complete modal state can be reconstructed without an external reference beam, sequential modal projections, or prior knowledge of the modal basis. This approach provides a pathway towards real-time, spectrally and polarisation-resolved characterisation of rapidly evolving multimode laser states, supporting the development of next-generation VCSELs, multimode fibres, and higher-capacity optical interconnects.

[1] Plöschner, M. et al. Nat Commun 13, 4294 (2022).