High-efficiency Mid-Infrared InGaAs/InP Arrayed Waveguide Gratings

We report the fabrication of InGaAs/InP based arrayed waveguide gratings featuring low insertion loss and non-uniformity. This device can be integrated monolithically with on-chip III-V lasers to make compact high-power mid-infrared sources.


Introduction
Arrayed waveguide gratings (AWG) are commonly used passive structures for wavelength division de/multiplexing in integrated photonic circuits [1].AWG can perform on-chip wavelength beam combining (WBC) [2] to produce high power sources, an essential component for many applications ranging from spectroscopy to sensing, and nonlinear optics.On-chip WBC often requires heterogeneous integration of quantum cascade lasers (QCLs) with silicon-based beam combining chips [2], which increases fabrication complexity.Monolithic fabrication of QCLs and passive waveguides is a more desirable approach to realizing such integration.This work presents the fabrication of low-loss InGaAs/InP waveguides for operation in the mid-IR regime.The waveguides are 1.7 μm tall and 5.2 μm wide to support the fundamental transverse magnetic (TM00) mode, best suited for integration with QCLs.We determined the propagation losses in these waveguides by testing two different types of samples and using the Fabry-Perot (FP) fringe contrast and ring resonator methods.These experiments led to the demonstration of a record Q-factor of over 170,000 for λ ≈ 5.2 μm.To further establish the suitability of the InGaAs/InP platform for efficient WBC, we fabricated 5 × 1 AWGs with 40 nm channel spacing and a 1.87 mm 2 footprint, as shown in Fig. 1.AWG samples based on high-efficiency designs were fabricated, and insertion losses as low as 0.9 dB were measured.

Design, Fabrication and Measurement
We designed a quasi-symmetrically cladded waveguide structure by sandwiching 1.2 μm In0.53Ga0.47Ascore between 500 nm InP upper cladding layer and InP substrate.AWG was designed according to the principles discussed by M.K. Smit et al. [3] to obtain the required channel wavelengths.We further added adiabatic tapers between the arrayed waveguide and the star coupler to allow for a smooth strip-to-slab modal transition.The structures were fabricated by electron beam lithography and subsequent etching using ICP-RIE with BCl3/SiCl4/Ar chemistry.We measured waveguide losses and AWG performance using a free-space edge coupling setup as shown in Fig. 1(b).The lock-in amplifier and the tunable bench-top QCL were controlled by a computer using a LabVIEW program to obtain the full spectrum of AWG.Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.

Waveguide propagation loss
The waveguide loss was characterized by Fabry-Perot interference (FP) and ring resonator methods.We measured propagation loss of 1.15 ± 0.47 dB/cm in the straight waveguide and 1.48 ± 0.23 dB/cm in the bent waveguide with a radius of 500 μm.The data analysis was performed according to the protocols mentioned in [4].The propagation loss using the ring resonator was calculated to be 1.17 ± 0.06 dB/cm, consistent with the conclusions of our FP measurement.

AWG insertion loss and non-uniformity
Normalizing the measured off-chip AWG transmission with respect to a reference waveguide leads to a significant error due to the uncertainty in edge-coupling efficiency.Hence, we estimated the efficiency of our AWG samples by comparing the contrast of FP fringes present in the off-chip AWG transmission data with that of a reference waveguide fabricated on the same chip.Eq. 1 was used to infer the on-chip AWG transmission. (1) is the total length of the AWG input and output waveguides, and is the fringe contrast in the measured AWG off-chip transmission.L and r correspond to the length and fringe contrast in the reference waveguide, respectively.
is the propagation loss obtained from previous results.The transmission of input channel 1 is lower than that of channel 5 by approximately 0.6 dB.We account for this difference to a fabrication defect in the input waveguide of channel 1. AWG insertion loss and non-uniformity were estimated to be 0.88 ± 0.08 dB and 0.60 ± 0.18 dB, respectively.

Fig. 1 .
Fig. 1.(a) Schematic of a 5 x 1 arrayed waveguide grating (AWG) multiplexer that can be used to wavelength beam combine 5 element MWIR laser array.The transmission of the AWG is centered around λ ≈ 5.2 μm by design.(b) Schematic of the optical measurement setup.The alignment of the PIC under test is first done by replacing the photodetector shown here with a liquid nitrogen cooled IR camera.
Photo Detector

Fig. 2 .
Fig. 2. (a) Red curve: linear fit to determine the propagation loss in straight waveguides; orange curve: linear fit to determine the propagation loss in waveguide bends.(b) A resonant peak of the 650 nm coupling gap ring resonator fitted with a Lorentz function.(c) Extracted AWG transmission vs. wavelength for all input channels where shaded region represents the error in our calculation method.