Low-loss and compact photonic lantern based on a step-index double cladding fiber

Area of Science:

• Photonics
• Optical Fiber Technology
• Waveguide Engineering

Background:

• Achieving low-loss photonic lanterns (PLs) traditionally requires strict adherence to the adiabatic criterion, limiting device compactness due to long taper transition lengths.
• Existing fiber geometries pose challenges in managing mode field diameter (MFD) expansion during tapering, necessitating longer transition regions for adiabatic operation.

Purpose of the Study:

• To theoretically propose and experimentally demonstrate a low-loss and compact PL by relaxing the adiabatic criterion.
• To investigate the use of step-index double cladding fiber (SI-DCF) to reduce MFD expansion and taper length.

Main Methods:

• Evaluated MFD and effective refractive index (RI) variations during fiber tapering for modified standard single-mode fiber (SSMF), graded-index fiber (GIF), and SI-DCF.
• Fabricated two types of SI-DCF to realize a 3-mode PL.
• Characterized the fabricated PLs for tapering length, insertion loss (IL), mode purity (LP11), and mode coupling using transfer matrix measurements.

Main Results:

• SI-DCF reduced the MFD expansion ratio from 77.73% to 38.81% compared to conventional fibers, halving the required taper length for 3-mode and 6-mode PLs.
• The fabricated 3-mode PL exhibited a 1.5 cm taper length and < 0.2 dB IL.
• Post-splicing with few-mode fiber, the PL achieved an average IL of 0.6 dB and > 13 dB LP11 mode purity across the C-band, with mode coupling < -10 dB at 1550 nm.

Conclusions:

• Relaxing the adiabatic criterion in conjunction with SI-DCF enables the development of significantly more compact and efficient photonic lanterns.
• The demonstrated SI-DCF-based PL offers a promising solution for low-loss, high-mode-purity light coupling in few-mode fiber systems.