A Brief Insight into the Technology of Tunable Diode Laser Spectroscopy

Tunable diode laser spectroscopy is becoming an important tool in the detection and measurement of trace gases. The basic features of tunable diode laser spectroscopy find a unique and interesting use in well established laboratories and field applications related to the detection of trace gases and in some instances the sensitive and selective detection of environmentally important atmospheric trace gases in real time. Numerous analytical instruments have been developed in the past but not all have the desired characteristics for high sensitivity, selectivity, multi-component detection capability, room temperature operation, high reliability, ease of use and cost effectiveness which is provided by tunable diode laser spectroscopy.

Measurement techniques of tunable diode laser spectroscopy

Detection limits – are a factor for both path length and path measurement time and the molecule being measured. For tunable diode lasers operating in the spectral band, the detection limits are within the range of 0.1 to 5 ppm-meters.

-        Path length – the longer the path length the higher will be the absorption rate, the better will be the sensitivity and the lower will be the detection limits. It therefore follows that longer path lengths result in better detection of low concentrations.

-        Measurement time – detection limit scale is approximately the square root of measurement time. For example, if time is increased from one second to one minute, it will enhance the detection limit approximately seven fold.

-        Multiplexed optical signal – this is a unique feature for a multi-path array configuration since multiplexing splits the signal by time instead of power. Multiplexed optical signal allows for use in heavy dust laden applications with longer path lengths. Multiplexing sends approximately 90% of signal power to each measurement path while beam splitting sends approximately 25% of signal power to each measurement path.

The advantages of tunable diode laser spectroscopy

The technology of tunable diode laser spectroscopy has become a firmly entrenched method of detection, identification and measurement of molecular and atomic species in the gas phase. The reasons for its wide acceptance are due to its unique combination of tunability, output power, small size and modulation capabilities. Lasers offer better wavelength resolutions (linewidth) and spectral brightness (power emitted per unit linewidth) than conventional grating or prism monochromators. In short, tunable diode lasers are significantly more convenient to use than other sources. Owing to their surface-emitting property and their associated on-wafer testing capabilities, these lasers offer a high potential for low cost mass production. These lasers also have low electrical power consumption and high tunability.

Applications of tunable diode laser spectroscopy

Applications of this technology include remote sensing, laser cooling and trapping of atoms, frequency standards, length standards, trace gas detection and process monitoring. They can also be used to monitor environmentally important species such as methane, carbon dioxide and water. In the semi-conductor industry, the technology is used for closed-loop control of deposition processes including electron beam, sputtering, molecular beam epitaxy and thermal evaporation resulting in a significant increase in yield of existing devices and making it possible to have new and more improved devices.