The availability of silicon photonic integrated circuits (ICs) in the 2–4 μm wavelength range enables miniature optical sensors for trace gas and bio-molecule detection. In this paper, we review our recent work on III–V-on-silicon waveguide circuits for spectroscopic sensing in this wavelength range. We first present results on the heterogeneous integration of 2.3 μm wavelength III–V laser sources and photodetectors on silicon photonic ICs for fully integrated optical sensors. Then a compact 2 μm wavelength widely tunable external cavity laser using a silicon photonic IC for the wavelength selective feedback is shown. High-performance silicon arrayed waveguide grating spectrometers are also presented. Further we show an on-chip photothermal transducer using a suspended silicon-on-insulator microring resonator used for mid-infrared photothermal spectroscopy.

A compact (1.2 mm²) fully integrated mid-IR spectrometer operating in the 3 μm wavelength range is presented. To our knowledge this is the longest wavelength integrated spectrometer operating in the important wavelength window for spectroscopy of organic compounds. The spectrometer is based on a silicon-on-insulator arrayed waveguide grating filter. An array of InAs_0.91Sb_0.09 p-i-n photodiodes is heterogeneously integrated on the spectrometers output grating couplers using adhesive bonding. The spectrometer insertion loss is less than 3 dB and the waveguide-referred responsivity of the integrated photodiodes at room temperature is 0.3 A/W.

Mid-infrared spectroscopy in the 2-4 μm wavelength range is of cardinal value for many sensing applications. Current solutions involve bulky and expensive systems to operate. Silicon-on-Insulator (SOI) waveguide technology offers means to miniaturize the different parts of the spectrometer. However, the development of on-chip detectors for the mid-infrared wavelength range is in its infancy and the characteristics are not on par with their discrete (cooled) counterparts. In this work, a compact and cheap mid-infrared spectrometer is realized by integrating a SOI arrayed waveguide grating (AWG) spectrometer operating in the 2.3 μm wavelength range with a high performance mid-infrared photodiode. The AWG has 12 output channels with a spacing of 225 GHz (4 nm) and a free spectral range of 3150 GHz (56 nm), which are simultaneously collected by a single, transistor outline (TO)-packaged extended InGaAs PIN photodiode. The response of each AWG channel is discerned by time-sequentially modulating the optical power in each output channel using integrated MachZehnder based thermo-optic modulators with a π-phase shift power consumption of ≈50 mW. As an example, the absorption spectrum of a 0.5 mm thick polydimethylsiloxane sheet is sampled and compared to a benchtop spectrometer to good agreement.

We report on 2.3x μm wavelength InP-based type-II distributed feedback (DFB) lasers heterogeneously integrated on a silicon photonics integrated circuit. In the devices, a III–V epitaxial layer stack with a “W”-shaped InGaAs/GaAsSb multi-quantum-well active region is adhesively bonded to the first-order silicon DFB gratings. Single mode laser emission coupled to a single mode silicon waveguide with a side mode suppression ratio of 40 dB is obtained. In continuous-wave regime, the 2.32 μm laser operates close to room temperature (above 15 °C) and emits more than 1 mW output power with a threshold current density of 1.8 kA/cm2 at 5 °C. A tunable diode laser absorption measurement of CO is demonstrated using this source.

Integrating III-V gain material with silicon photonic integrated circuits enables the realization of advanced laser sources and fully integrated photonics systems for optical communication and sensing applications. The availability of III-V/silicon laser sources operating in the 2-2.5 μm short-wave infrared (SWIR) wavelength range is very valuable for spectroscopic sensing since many important industrial gases and blood glucose have absorption bands in this wavelength range. In this paper, first we present our latest results on heterogeneously integrated III-V-on-silicon distributed feedback (DFB) laser arrays. A III-V-on-silicon DFB laser array covering the 2.27-2.39 μm wavelength range with 6 nm wavelength spacing is reported. This DFB laser array is employed as the light source for tunable diode laser absorption spectroscopy of different gases. A four-channel DFB laser array integrated with a beam combiner is used to perform spectroscopic sensing over a 7nm spectral range without mode hopping at room temperature. Finally, we present our recent advances in widely tunable Vernier lasers based on heterogeneous integration and butt-coupling of the gain section. Continuous tuning near the absorption lines by thermally adjusting the laser cavity length enable high-resolution tunable diode laser absorption spectroscopy (TDLAS) measurements together with wide wavelength coverage.