Demonstration of methane spectroscopy using a vertical-cavity surface-emitting laser at 1.68 µm with up to 5 MHz repetition rate

An electrically pumped InP-based vertical-cavity surface-emitting laser (VCSEL) emitting at 1.685 mum (5935 cm(-1)) was used to probe rovibrational transitions of methane in the 2nu(3) band at pressures ranging from 10 mbar to 1.3 bar. For the first time a long-wavelength VCSEL at 1.68 mum was used for the demonstration of high-speed gas sensing at repetition rates as high as 5 MHz. The observed tuning rate at 1 MHz was 5.2 cm(-1) mus(-1). The maximum continuous current tuning range was more than 16 cm(-1) (4.5 nm), which is very useful for measurements in high-pressure environments. At 1 MHz the tuning range was as large as 2.6 cm(-1), at 5 MHz it was still approximately 0.36 cm(-1). The measured tuning rates of the VCSEL were -3.05 cm(-1) mA(-1) (+0.86 nm mA(-1)) and -0.40 cm(-1) K-1 (+0.11 nm K-1). Survey spectra measured with the VCSEL were compared to calculations using the HITRAN2000 spectroscopic database and found to be in good agreement. A VCSEL was directly contacted on a section of its wafer to demonstrate 'on-chip' testing ability as opposed to edge-emitting lasers that have to be singularized and individually mounted prior to characterization for spectroscopic applications. It is expected that VCSELs will considerably expand the range of operation for diode lasers in rapid in situ species concentration measurements because of their ability of fast and far mode-hop free single longitudinal and transverse mode tuning properties.