III-V-on-Silicon Photonic Integrated Circuits for Spectroscopic Sensing in the 2-4 μm Wavelength Range

Abstract

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.

Keywords: integrated spectrometer; mid-infrared; optical sensor; silicon photonics; spectroscopy.

Figure 1

(a) Schematic of two silicon photonic configurations to realize an integrated on-chip mid-infrared absorption spectroscopy sensor. Broadband source and spectrometer, best suited for liquid and solid analytes; (b) Tunable single mode laser source for trace gas detection.

Figure 2

Transparent window of silicon and silicon dioxide, and emission wavelength coverage of semiconductor lasers based on different III–V active regions. InP-based type-I, type-II and GaSb-based type-I quantum well (QW) diode lasers, GaSb-based interband cascade lasers (ICLs), and QCLs are included.

Figure 3

(a) Schematic drawing and (b) scanning electron microscopy (SEM) image of the cross section of a heterogeneously integrated III–V-on-silicon type-II active device.

Figure 4

(a) Schematic of an InP-based type-II DFB laser heterogeneously integrated on a silicon waveguide, the simulated mode intensity distribution in different sections is also included; (b) simulated coupling efficiency of a 180 μm long III–V/silicon SSC as a function of the III–V taper tip width. The inset figure shows the fundamental mode intensity evolution of the SSC with 0.5 μm wide III–V taper tip; (c) calculated coupling strength of the DFB grating as a function of the DVS-BCB thickness for three different etch depths (150, 180, 210 nm) in the 400 nm silicon device layer.

Figure 5

(a) Continuous-wave (CW) light-current-voltage (L-I-V) curve of a III–V-on-silicon DFB laser with grating period of 348 nm; (b) emission spectrum of the DFB laser driven with 190 mA bias current at 10 °C.

Figure 6

(a) Evolution of the DFB laser emission spectrum with increasing heat-sink temperature under 190 mA bias current, (b) with increasing bias current at 5 °C. The inset figure shows the dependence of the lasing wavelength on (a) temperature and (b) bias current at 5 °C, 10 °C, 15 °C; (c) direct TDLAS measurement of CO and the corresponding HITRAN spectrum.

Figure 7

(a) Emission spectra of six 1000 μm-long DFB lasers with different silicon grating period in an array; (b) evolution of the emission spectrum with bias current for four 700 μm long DFB lasers with different III–V waveguide widths in an array.

Figure 8

(a) Schematic of an adiabatically-coupled photodetector integrated on a silicon waveguide; (b) mode intensity distribution in a longitudinal cross section of the III–V/silicon taper taking the active region absorption into account; (c) schematic cross-section of a grating-assisted III–V-on-silicon photodetector.

Figure 9

(a) I-V curve of the heterogeneously integrated adiabatic-taper-based photodetector in the dark, the inset shows the dark current of the device from −1 V to 0 V; (b) I-V curve of the photodetector under different waveguide-coupled input powers at a wavelength of 2.35 μm.

Figure 10

Schematic of a GaSb/silicon hybrid external cavity laser.

Figure 11

(a) Amplified spontaneous emission coupled from the GaSb-based SLD to a silicon waveguide; (b) superimposed spectra of the hybrid laser by thermally tuning only one MRR; (c) both MRRs and (d) phase shifter.

Figure 12

(a) Microscope image of a 2.3 μm silicon arrayed waveguide grating (AWG) spectrometer; the measured spectral responses of all the channels in three AWGs operating at different wavelengths: (b) 2.3 μm; (c) 3.3 μm and (d) 3.8 μm.

Figure 13

Transmission of four different SOI AWGs operating in the 3.3 μm wavelength range with different channel spacing. The insertion loss (2–3 dB) and crosstalk levels (20–21 dB) are indicated by the dashed lines. The high-resolution (50 GHz) AWG can be used as a mid-infrared DFB laser array multiplexer.

Figure 14

(a) Microscope image of the 2.3 μm AWG spectrometer integrated with a InP-based type-II quantum well photodetector array; (b) wire bonded III–V-on-silicon AWG spectrometers on a PCB; (c) photo-response of the 2.3 μm AWG and (d) 3.8 μm III–V-on-silicon AWG spectrometer.

Figure 15

Schematic of the photo-thermal sensing principle. A modulated mid-infrared pump beam is absorbed by the analyte which causes a local temperature change of the microring waveguide. The thermo-optic effect changes the effective index of the waveguide mode, hereby changing the resonance wavelength λ_res of the microring. For a given fixed probe wavelength λ_probe, the change in λ_res produces a change in probe power ΔP_probe which is measured using a near-infrared detector. The absorption spectrum of the analyte can be reconstructed by scanning the pump wavelength and recording the maximum probe modulation ΔP_probe,max.

Figure 16

Microscope image of the SOI MRR suspended on BOX membrane with AZ5214 photoresist as mock-up analyte: (a) top and (b) bottom view; (c) The measured photo-thermal signal is scaled to calculate the absorption coefficient of the analyte and is compared to FTIR measurement. The FTIR signal is collected in reflection at the Brewster angle and TM polarization and is used with the formula in the inset to estimate the absorption coefficient; (d) A tilted SEM image with false coloring shows the various regions of the PIC; (e) A schematic cross section of the suspended MRR is given.

Figure 17

Schematic of a possible free-space measurement configuration. The probe and pump sources flood-illuminate the chip from a certain distance, e.g., 30 cm. The SOI-chip is capped with a reflective or absorbing second layer (e.g., gold-coated silicon) with spacers (not shown in schematic). Small apertures (uncoated areas of the capping layer) are aligned on top of the MRR and the input/output ports of the probe. The probe signal is collected by a near-IR camera.