Design Constraints of Photonic-Lantern Spatial Multiplexer Based on Laser-Inscribed 3-D Waveguide Technology

Laser-inscribed 3-D waveguide (3DW) technology has been applied to realize photonic-lantern spatial multiplexer (SMUX), which potentially enables lossless mode (de)multiplexing. However, the index contrast Δn between the transparent substrate and the spatially-inscribed waveguides by femto-second laser pulses is generally smaller than
$6 \times 10^{-3}$
. It is simulated that a 3DW SMUX with
$\Delta {\rm n} < 6 \times 10^{-3}$
is able to provide a low mode-dependent loss (MDL) and coupler insertion loss (CIL) when coupling to a three-mode few-mode fiber (FMF). However, in six- and 15-mode cases, to guide all supermodes, few-mode region created by coupled or merged waveguides at the FMF side of the 3DW SMUX cannot be very small. This results in mode-profile mismatch between the 3DW SMUX and a low differential-group-delay FMF, which is generally with Δn around
$1 \times 10^{-2}$
. Instead of employing bulky imaging optics, uptapering FMF is proposed to enlarge the modes of the FMF for minimizing the mode-profile mismatch. Simulation shows MDL and CIL enhancement can be achieved by the uptapering solution. It also points out that as mode number increases to 15, although uptapering FMF still improves coupling performance, low MDL < 1dB cannot be acquired with
$\Delta {\rm n} < = 6 \times 10^{-3}$
. In order to achieve a lossless mode (de)multiplexing for 15 spatial modes,
$\Delta {\rm n} = 1 \times 10^{-2} $
is required for a 15-core 3DW SMUX. Moreover, a fully-packaged dual-channel 3DW coupling circuit including two six-mode SMUXes is introduced and experimentally demonstrated.
${\rm MDL = 7\; dB}$
and CIL < 8 dB is achieved in a loop through measurement with the two SMUXes. Mode-profile mismatch due to the limited Δn of the 3DW device is solved by uptapering FMF with a factor of 1.4.