Low-loss Ge-rich Si_02Ge_08 waveguides for mid-infrared photonics

Abstract

We demonstrate low-loss Ge-rich Si_0.2Ge_0.8 waveguides on Si_1-xGe_x (x from 0 to 0.79) graded substrates operating in the mid-infrared wavelength range at λ=4.6 μm. Propagation losses as low as (1.5±0.5)dB/cm and (2±0.5)dB/cm were measured for the quasi-TE and quasi-TM polarizations, respectively. A total coupling loss (input/output) of only 10 dB was found for waveguide widths larger than 7 μm due to a good fiber-waveguide mode matching. Near-field optical mode profiles measured at the output waveguide facet allowed us to inspect the optical mode and precisely measure the modal effective area of each waveguide providing a good correlation between experiments and simulations. These results put forward the potential of low-index-contrast Si_1-xGe_x waveguides with high Ge concentration as fundamental blocks for mid-infrared photonic integrated circuits.

Efficient 52 µm wavelength fiber-to-chip grating couplers for the Ge-on-Si and Ge-on-SOI mid-infrared waveguide platform

“…The mid-infrared (midIR) wavelength range is of particular interest for these applications due to the orders of magnitude higher optical absorption crosssection in this range as compared to the telecom and visible wavelength range [1][2][3][4]. Up to this point multiple platforms have been suggested for the implementation of midIR photonic integrated circuits (PICs), some of which are suspended Si membrane [5], silicon-on-sapphire (SOS) [6] and graded index GeSi platform [7] which also finds its application in non-linear photonics [8]. While bulk Si is considered transparent only up to 8 µm wavelength, Ge has low losses in the 2-14 µm wavelength range [9], which -together with its compatibility with standard CMOS/MEMS processes -makes it suitable for the implementation of midIR gas sensor PICs for this wavelength range.…”

Ge-rich graded-index Si_1-xGex waveguides with broadband tight mode confinement and flat anomalous dispersion for nonlinear mid-infrared photonics

“…As a result, competitive propagation losses as low as 1 dB/cm for λ = 4.5 µm and 2 dB/cm for λ = 7.4 µm were obtained by ramping the Ge concentration in the Si 1-x Ge x alloy up to ≈ 40 % [14]. More recently, low-loss Ge-rich Si 1-x Ge x waveguides with a top Ge concentration up to 80 % were also demonstrated at λ = 4.6 µm [15]. As Ge transparency extends up to 15 µm, the Ge-rich Si 1-x Ge x platform presents strong advantages in terms of maximum wavelength of operation.…”

“…Simulations of mid-IR waveguides were done using a finite-difference method (FDM) mode solver, taking into account the refractive index wavelength dependence of Si and Ge [15,19]. The proposed waveguide consists of a graded-index Si 1-x Ge x waveguide core region (h core ) from Si to Ge on a Si substrate, as shown in figure 1 (a).…”

“…As an illustrative example, we may compare the modal effective area (A eff ) of the fundamental quasi-TM mode of a representative design using an h core = 11 µm (similar to the one used in previous works, see ref. 15) with a design containing a reduced h core = 6 µm. A moderate difference is observed at λ = 3 µm, being A eff TM (h core = 6 µm) ≈ 4.4 µm 2 and A eff TM (h core = 11 µm) ≈ 5.4 µm 2 , which however becomes progressively larger for longer wavelengths up to a value of A eff TM (h core = 6 µm) ≈ 10 µm 2 and A eff TM (h core = 11 µm) ≈ 13 µm 2 at λ = 8 µm.…”

“…Thus, we used an h core = 6 µm as yielded a good balance between tight mode confinement, good mode overlap with the Ge-rich region and an expected low dislocation density upon fabrication by state-of-the-art deposition techniques. In that regard, suitable techniques that have already provided notable results are reduced pressure chemical vapor deposition (RPCVD) or low energy plasma enhanced chemical vapor deposition (LEPECVD), among others [14,15].…”