Fig. 1. Selective mode suppression can be accomplished with a resonant-cladding structure. Desirable modes are off-resonance and well-confined (left), while resonant unwanted modes have enhanced leakage.

Fig. 2. Resonant suppression of higher-order core modes is accomplished by introducing indexmatched cladding defect waveguides. Defect modes are index-matched with HOMs but not the fundamental core mode.

Fig. 3. Calculated modes of two aircore fibers with standard cladding design. Effective index is plotted versus wavelength for small (a) and large (b) core fibers (with fundamental modes in blue and HOMs in red). Losses (c) show that, as the core increases from small (dashed) to large (solid), HOMs become much more well-guided with respect to the fundamental.

Fig. 4. An air-guiding cladding waveguide region can be designed to achieve broadband indexmatching with unwanted core modes by tuning the waveguide size. Mode solutions (left) are repeated from Fig. 3(b) in black, and a defect-guided mode is superimposed schematically in red, targeting the first HOM group. Deect geometry is depicted schematically (right)

Fig. 5. A fiber design (left) with two air-guiding cladding defects and R_core=4.59µm selectively suppresses the dominant HOMs. Calculated losses (right) show that HOM losses (solid red) are higher than the comparable standard fiber (dashed red, assuming no cladding defects). The fundamental instead shows reduced loss (solid blue) relative to the standard fiber (dashed blue).

Fig. 6. A second design (left) has cladding defects re-designed to index-match a different core radius, R_core=5.26µm. Calculated losses (right) for this structure show almost identical fundamental confinement (solid and dashed blue) for the fibers with and without cladding defects, but substantial suppression of the HOMs by the defects (solid red) compared to the structure without defects (dashed red). Rectangular regions illustrate how HOM suppression can increase usable bandwidth.

Fig. 7. Problematic surface modes can be avoided if the core and cladding defects have appropriate shapes. Generalizing the design rules of [10], we form defects by removing thick glass vertices, not cutting through them.

Fig. 8. Fundamental-mode intensity profiles of the standard (left) and HOM-suppressed (right) designs show very circular and nearly identical modes. The dashed line indicated the core radius, R_core=4.59.

Fig. 9. Intensity profiles of the standard (left) and HOM-suppressed (center) designs highlight the coupling to a defect-guided mode. Intensity plotted vs. radius (right) shows corresponding enhanced penetration through the cladding, leading to increased tunneling losses.