Ultra-low-loss 5-LP mode selective coupler based on fused biconical taper technique

Abstract

Trapped in the stringent adiabatic transmission condition of high-order modes, low-loss fused biconical taper mode selective coupler (FBT-MSC) has long been challenging to achieve. We identify the adiabatic predicament of high-order modes to stem from the rapid variation of the eigenmode field diameter, which is caused by the large core-cladding diameter difference of few-mode fiber (FMF). We demonstrate that introducing a positive-index inner cladding in FMF is an effective approach to address this predicament. The optimized FMF can be used as dedicated fiber for FBT-MSC fabrication, and exhibits good compatibility with the original fibers, which is critical for the wide adoption of MSC. As an example, we add inner cladding in a step-index FMF to achieve excellent adiabatic high-order mode characteristics. The optimized fiber is used to manufacture ultra-low-loss 5-LP MSC. The insertion losses of the fabricated LP01, LP11, LP21, LP02 and LP12 MSCs are 0.13 dB at 1541 nm, 0.02 dB at 1553 nm, 0.08 dB at 1538 nm, 0.20 dB at 1523 nm, and 0.15 dB at 1539 nm, respectively, with smoothly varying insertion loss across the wavelength domain. Additional loss is less than 0.20 dB from 1465.00 nm to 1639.31 nm, and the 90% conversion bandwidth exceeds 68.03 nm, 166.68 nm, 174.31 nm, 132.83 nm, and 84.17 nm, respectively. MSCs are manufactured using commercial equipment and a standardized process that takes just 15 minutes, making them a potential candidate for low-cost batch manufacturing in a space division multiplexing system.

All-in-Fiber Dynamically Reconfigurable Orbital Angular Momentum Mode Sorting

“…For instance, MBSs with 7 ports 46 or, alternatively, arrays of 2-port beam splitters on a photonic- integrated circuit functioning as a general d-port beam splitter 55 as well as lanterns with 15, 56 19, 57 and 61 58 ports can all be used, showing the potential of our scheme. Furthermore, very recent results have demonstrated photonic lanterns with very low loss (<0.2 dB), 54 further showing the feasibility of our proposal for practical applications. Finally, due to the interferometric nature of our configuration, it can also be used in an active/reconfigurable mode, where the input mode can be routed to a different output on demand, or a unitary transformation can be applied by the phase modulator, allowing the measurement in different mutually unbiased bases in quantum information.…”

“…The optical loss from the PL input to the detectors is 8.3 dB, which is mainly given by the PM and the internal losses of the PL (∼5 dB). These losses can be greatly reduced by new lantern designs that have much lower losses (up to 0.2 dB), 54 showing a path to having a mode sorter with very low losses.…”

Ultra-low-loss all-fiber orbital angular momentum mode-division multiplexer based on cascaded fused-biconical mode selective couplers

“…These distinctive profiles are designed to optimize the performance of the MSC while ensuring compatibility with conventional fibers. 34 The precisely characterized refractive index distribution aids in the accurate simulation of the MSC. The model utilized in simulations is a stepwise simplification based on the measured profiles, as depicted by the blue line in Fig.…”

“…The manufactured MSCs exhibit ideal performance. 34 In subsequent discussions, MSCs targeting LP 01 , LP 11 , and LP 21 modes are, respectively, labeled as MSC0, MSC1, and MSC2 for clarity.…”

“…Notably, recent advancements have witnessed the unveiling of ultra-low-loss MSCs rooted in FBT technology, showcasing losses below 0.2 dB. 34 This development provides a promising cornerstone for the realization of low-loss MDMUXs.…”