Using All of the Fiber: The Largest Complete Mode Transfer Matrix

Scatterings image

Joel Carpenter and colleagues at the University of Sydney measured the largest complete mode-transfer matrix ever. The technique can deliver an arbitrary eigenmode of the fiber (left) or a desired image (right).

Researchers in Australia have claimed to measure the largest complete mode-transfer matrix of a multimode fiber so far, consisting of 110 spatial and polarization modes (Optics Expr. doi:10.1364/OE.22.000096). As the mode-transfer matrix of the fiber maps any possible input pattern coupled into the fiber with its corresponding output pattern at the other end, it can be used to image through multimode fiber. Knowledge of the matrix enables recovery of the original image at one end of the fiber from the scrambled pattern arriving at the other end. In the reverse direction, it can also be used to deliver a particular image to one end of the fiber by coupling in the ‘pre-scrambled’ input at the other end.

Joel Carpenter and colleagues at the University of Sydney used spatial light modulators (SLMs) at either end of a 2-m length of 50-µm-diameter core graded-index multimode fiber. An SLM on one end excited each supported spatial and polarization mode at 1545.54 nm sequentially and another SLM measured the corresponding modes at the other end of the fiber. In this way the complete linear behavior of the fiber is characterized at the measurement wavelength by the complex 110 x 110 transfer matrix, containing full amplitude and phase information for all spatial and polarization modes.   
The technique could have diverse applications in biomedical imaging, telecommunications and astronomy, or any field where the modal properties of a waveguide must be completely measured and controlled.

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