Fig. 3: Tuneable mode manipulation. Results in a bimodal fibre.

This fibre is 0.4 metre long and supports one even mode M₁ and one odd mode M₂ (see Supplementary Information 1). a–c Theoretical 2D maps of the output probe mode distribution computed from Eq. (4). The maps show the output probe power fraction coupled to mode M₁ versus the BCB total peak power (horizontal axis) and BCB mode distribution (vertical axis, indicating the fraction of BCB power coupled to mode M₁). These maps indicate how to set the BCB in order to manipulate the output probe, ensuring it reaches the desired mode distribution. The maps correspond to 3 examples with different input probe mode states, which are reported at the top of each panel. For example, in panel a the input probe mode state is characterised by 10% power on mode M₁, 90% on mode M₂, and a relative phase \(\Delta {{{\rm{\phi }}}}_{{in},12}\) between the two modes of 0.3 rad. d–f. Experimental (exp) and theoretical (theory) results for the same input probe mode states as panels (a–c), but with a fixed BCB mode distribution (indicated at the top of each panel and corresponding to the red-dashed lines in panels (a–c)). Arbitrary output probe mode distribution can be achieved by tuning the BCB power. Specifically, in panel (d), full conversion to mode M₁ is achieved when the BCB peak power is ~ 8 kW (3.2 W average power). In contrast, the BCB in f is configured such that it results in almost no variation of the output probe mode distribution. The insets in panels (d–f) show the far-field intensities of the output probe for different values of BCB peak power P_BCB. Error bars of ±3% are added to the measured relative power of each mode, which represents the estimated uncertainty of our mode decomposition algorithm.