Laser based ultra precision gas measurement technology

Laser gas analysis can achieve high sensitivity and selectivity in gas detection. The multi-component capability and wide dynamic range of this detection method help analyze gas mixtures with a wide concentration range. Due to the fact that this method does not require sample preparation or pre concentration, it is easy to adopt in the laboratory or industry.

Gas analysis is crucial for determining the concentration of known gases in the atmosphere or environment containing the target gas mixture. It is widely used in research, development, and industry. The gases that can be analyzed using gas analysis techniques are roughly divided into dirty gases and clean gases.

Environmental pollutants and toxins, including industrial chimney emissions, diesel engine exhaust, and biomethane from wastewater treatment plants, are classified as dirty gases. Gas analysis helps to detect the concentration of these gases.

Due to the increasing focus on research on renewable energy gases, gas analysis techniques are widely used to analyze the concentration of hydrogen sulfide or biomethane produced by anaerobic digestion in wastewater and landfill sites.

In contrast, the ultrapure gases emitted by gas supply companies such as liquefied air and Linde can be considered clean. Ultra pure gases have various applications, some of which include being used as calibration references for analyzers and as raw materials for industrial processes.

Laser based gas sensors measure the light absorbed by gases, which is constrained by the Beer Lambert law when its frequency or wavelength overlaps with the molecular resonance of the gas. The main challenge faced by absorption based gas analysis is the low signal level. Therefore, insufficient signal-to-noise ratio levels can hinder the accurate quantification of gas levels.

Therefore, Beer Lambert's law helps to derive a possible method to improve the sensitivity of optical sensing systems. There are two possible ways to achieve this - by increasing the absorption cross-section or increasing the interaction length. The former is achieved by selecting spectral regions with stronger molecular transitions, while the latter is achieved using a multi pass pool.

Although mass spectrometry, conventional optics, and gas chromatography are effective in detecting trace gases in the environment and atmosphere, laser based gas analysis has always had advantages, especially in industrial applications.

Laser gas analysis is an economically efficient alternative to traditional analytical techniques, thanks to the latest diode and fiber laser technologies. Laser based gas analysis is used for air and water quality monitoring, cancer detection, atmospheric chemistry, industry, transportation and rural emissions, explosive detection, medical applications, national security, vegetation remote sensing, and artwork characterization.

Laser based gas spectroscopy is a reliable tool for trace gas detection, applied in the near-infrared and mid infrared spectral regions with high selectivity and sensitivity, and short collection time. In addition, optical systems based on the infrared region integrate thermoelectric cooling detectors and other semiconductor sources, such as interband cascade lasers, laser diodes, or quantum cascade lasers, which can achieve on-site operations with minimal maintenance.

Common laser and laser-based trace gas detection technologies include:
Semiconductor laser: This type of laser has a small volume and high reliability. However, their industrial applications are hindered by the lack of high-quality, high-power diodes for specific wavelengths. Different wavelengths use different semiconductor materials.

For example, lead salts are used for 3-30 μ The spectral region of m, where antimonides are used for over 1.8 μ The wavelength of m, gallium arsenide and indium phosphide are used for visible to near-infrared wavelengths. Although lead salt lasers are effective in detecting trace gases in the air, they are not suitable for conventional industrial applications as they require low-temperature cooling.

Diode laser spectroscopy: It uses Beer's law to determine the gas concentration in an absorption spectrometer device, which consists of a radiation source, detector, and a closed absorption cell. Diode laser spectroscopy is a highly attractive method for detecting trace gases, as its instrument is simple and allows electrons to achieve the required modulation.

Tunable diode laser absorption spectroscopy with a long path absorption pool can achieve highly sensitive local measurements. It is particularly effective in monitoring most atmospheric trace substances with distinguishable infrared spectra under low pressure.

The integrated wavelength modulation spectrum of diode lasers can generate signals based on substance concentration and reduce laser noise. This complex method requires modulation of the laser wavelength and detection of the signal using a computer-controlled signal averaging device.

Near infrared diode lasers operating at ambient temperatures target weaker overtones and combination bands, while mid infrared lasers operating at low temperatures cover the basic absorption band for ultra sensitive gas research.

A study published in Optics and Laser Engineering proposes a highly sensitive dynamic analysis technique to achieve real-time online monitoring of dissolved trace gases in transformer oil. The technology used here is based on low noise differential photoacoustic units.

The characteristic gas dissolved in oil is separated and pumped into DPAC using headspace degassing. Amplify the emitted laser using an erbium-doped fiber amplifier and reflect it in DPAC to form a dual pass excitation enhancement.

The results show that both the muffler and differential detection method can reduce the noise during the headspace degassing process by more than 80%. The detection limit of the system for acetylene dissolved in transformer oil is 0.1 μ L/L. This study provides a technical solution for dissolved gas analysis, which has advantages such as high detection accuracy and fast response time.

Another article published in "Sensors and actuators B: Chemistry" reported the use of mid infrared fiber coupled laser absorption sensors for simultaneous in-situ detection of ammonia and nitric oxide, suitable for smoke monitoring in selective catalytic reduction exhaust gas.

Two quantum cascade lasers detected the optimal absorption spectra of ammonia and nitric oxide at 1103.45 cm-1 and 1929.03 cm-1, respectively. Coupling two QCLs onto a hollow fiber and delivering them to an open circuit single ended optical probe for in-situ gas detection.

The performance of the sensor was evaluated through a series of experiments at different temperatures and a constant pressure of 1 atmospheric pressure. The average time minimum detection limits for nitric oxide and ammonia are 30 ppb and 14 ppb, respectively. In addition, the uncertainty of ammonia and nitric oxide detection is less than 5%.

Source: Laser Net