Mid-infrared silicon-on-insulator waveguides with single-mode propagation over an octave of frequency

Abstract

Increasing the working optical bandwidth of a photonic circuit is important for many applications, in particular chemical sensing at mid-infrared wavelengths. This useful bandwidth is not only limited by the transparency range of waveguide materials, but also the range over which a waveguide is single or multimoded for predictable circuit behaviour. In this work, we show the first experimental demonstration of “endlessly single-mode” waveguiding in silicon photonics. Silicon-on-insulator waveguides were designed, fabricated and characterised at 1.95 µm and 3.80 µm. The waveguides were shown to support low-loss propagation (1.46 ± 0.13 dB/cm loss at 1.95 µm and 1.55 ± 0.35 dB/cm at 3.80 µm) and single-mode propagation was confirmed at 1.95 µm, meaning that only the fundamental mode was present over the wavelength range 1.95 - 3.80 µm. We also present the prospects for the use of these waveguides in sensing applications.

Mid-infrared waveguides for broadband single-moded guidance

“…The operating principle of the ESM waveguides relies on a small effective index contrast between the fundamental mode and the cladding mode. However, this leads to the inherent drawback that the bending loss is significant at even large bend radii; in [2], our simulations gave a 90° bending loss of approximately 0.6 dB for a radius of 300 µm. Evidently, this is a limiting factor in the use of the ESM waveguides in a practical PIC.…”

“…The impact from the compact bend design is that it facilitates the practical use of the ESM waveguides in a PIC and directly addresses one of the noted drawbacks in [2]. For our application in MIR absorption spectroscopy, spectrometers based on Mach-Zehnder interferometers require three main components for broadband operation: single-mode waveguides, broadband splitters, and optical delay lines.…”

“…We recently demonstrated a waveguide on a silicon-on-insulator (SOI) platform that allowed for single-moded guidance over an octave of frequency [2]. The design of this waveguide is illustrated in Figure 1 and was originally inspired by the well-known endlessly single-mode (ESM) guidance found in photonic crystal fibres [3].…”

“…The design and optical characterisation experiments for these waveguides are fully described in [2]; the designed parameters for the waveguide (as labelled in Figure 1) are given in Table 1. The waveguide propagation loss was determined as 1.46 dB/cm and 1.55 dB/cm at wavelengths of 1.95 µm and 3.80 µm respectively, with single-mode propagation demonstrated at 1.95 µm.…”

Design of GaSb-based monolithic passive photonic devices at wavelengths above 2 µm

“…Various optical passive components at S-MWIR [8,9] wavelengths were demonstrated using SOI up to 3.5 µm [10] and in Ge-on-Si up to 8.5 µm [11]. There are also few demonstrations of heterogeneously integrated InP gain materials on silicon PICs reported around 2.3 µm [12].…”