Four-channel silicon nitride wavelength beam combiners for multi-gas absorption spectroscopy

The detection and measurement of multiple trace gases based on the laser absorption spectroscopy technique requires laser sources with wavelength tunability in multiple wave bands. In this Letter, we present an on-chip wavelength beam combiner based on silicon nitride Mach-Zehnder interferometers (MZIs) and a modified multimode interferometer (MMI) structure to multiplex light with different wavelength spacings. An MZI composed of a standard 1 × 2 MMI and 2 × 2 MMI is employed to digitally multiplex two wavelengths within ∼100 nm spacing. The two wave bands with a spacing of a few hundred nanometers are multiplexed by a compact modified MMI structure. The measured insertion losses of all four channels are around 2.5 dB from the 1650 nm wave band to the 2050 nm wave band. A tunable diode laser absorption spectroscopy (TDLAS) measurement of multi-gas is demonstrated using the device as the wavelength beam combiner.