Abstract
We have fabricated an air-cladded mode-group selective photonic lantern, which can (de)multiplex the first two mode groups of a standard two-mode step-index fiber. Instead of relying on a low-index capillary tube, our simple solution uses air to form the surrounding "cladding" and thereby enable guiding at the end of the taper. Characterization of a 25-mm long lantern taper results in multiplexing crosstalk values between -20 dB and -12 dB for both modal inputs. The de-multiplexing values were around -12 dB for the fundamental mode, and slightly higher for the first higher-order (LP11) mode. Microscopic imaging of a taper cross section having a width of 30 μm reveals the presence of an uncollapsed airhole in the structure between the three fibers. The impact of such an airhole is numerically investigated using an eigenmode expansion method based on a full-vectorial mode solver, and is found to play an important role in assuring a more adiabatic mode conversion through the taper.
Highlights
Optical communication using single mode fibers is approaching the fundamental limits imposed by signal to noise ratio and non-linearity [1]
Repeating the measurement on a finer wavelength range reveals that the large variation in the single mode fibers (SMFs) loss in Fig. 9(b) is due to the modal interference between the LP01 and LP11 mode [16] .The long period gratings (LPG) has a higher suppression ratio of 25 dB around 1550 nm and the amplitude of oscillation is lower at the center
Even though the air gap was demonstrated to play an important role, the crosstalk values experimentally observed are still 3–6 dB smaller than numerically predicted. While this discrepancy may to some extend be attributed to uncertainties in fiber parameters, non-perfect taper transitions, or small diffusion of core dopants during tapering, another possible explanation may be that the dissimilar splice into the two mode step index fiber (TMF), which was not included in the simulations, influences the crosstalk values
Summary
Optical communication using single mode fibers is approaching the fundamental limits imposed by signal to noise ratio and non-linearity [1]. Photonic lanterns are fabricated by inserting the single mode fibers (SMFs) in a low refractive index flourine-doped capillary tube, and the whole structure is tapered down [7]. If photonic lanterns have to be used without MIMO components, it is required to excite a specific mode by coupling light through a specific single mode fiber. Such mode selectivity can be attained by introducing dissimilarity between the single mode fibers either in core size or refractive indices. The capillary tubes used in the lantern fabrication need to have a lower refractive index than the silica cladding to ensure wave guiding at the tapered multi-mode end.
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