A new method of gas analysis

Scientists from the Institute of Physics. PN Lebedev Physical Institute have developed a new method of gas analysis, based on the record of the absorption spectrum of the particles by measuring the phase shift of the diode laser radiation. The method is highly sensitive and allows you to record the absorption spectrum of a high-resolution, which is especially important in dealing with weakly absorbing particles.

A comparison of the spectra obtained with the modified method of recording CAPS and ICOS (integral cavity output spectroscopy).

Phase method is actively used in the study of loss of electromagnetic oscillations and waves in different objects. It consists in measuring the phase shift amplitude modulated light output from the object compared with the phase of the radiation incident on the object. In optics, a similar method is used since 1933 to measure the lifetime of the excited states of the particles. In 1980, with his help it was determined the photon lifetime in an optical cavity, composed of mirrors. Over time, this type of methods applied to measure absorption acquired the name PS-CRDS (phase shift cavity ring-down spectroscopy, spectroscopy that the phase shift of the fading light that passes through the cavity), or CAPS (cavity attenuated phase shift — phase shift at attenuation of radiation in the resonator). Great popularity in the optical studies the method has acquired because it can be used for simple installations, but it shows a high sensitivity measurements of the order of 10-11 cm-1. The main use is in CAPS gazoanaliza, including the study of atmospheric and medical research typically in situations where the gas is required to recognize a small concentration. The new technique, developed by a group of scientists from the Lebedev Physical Institute, was a continuation of work, related to improving the properties of the gas analyzer, they suggested in 2010.

Says one of the developers, leading researcher of the Lebedev Physical Institute, Candidate of Physical and Mathematical Sciences Sergei Tskhai: "In general ringdaun spectroscopy based on the measurement of attenuation of radiation. We also refer to this measurement, but is not recorded changes in light intensity, and it is modulated with a certain frequency and phase shift modulation recorded light after passage through the cavity. Thus, we’re going to measure the amplitude, which is largely due to the appearance of noise. "

In his work physicists used semiconductor diode lasers, the most promising in the highly sensitive spectroscopy. One major advantage of this type of laser is that the frequency adjustment in which by means of the injection current. Such control eliminates the need for an external modulator and facilitates the control of the parameters of the laser radiation.

However, when used with a diode laser CAPS significant problem arises. Amplitude modulation intensity is accompanied by a frequency modulation, which limits the sensitivity of the measurements and reduces the resolution of the spectrum recording. But researchers have found a way out.

Commented Sergey Tskhai: "Laser diode current is controlled. We can define certain current form periodically changing amplitude, and modulate the laser beam at a relatively low, compared with the optical frequency. On one hand, it facilitates the suppression of noise and the other — when the laser diode current and changing the amplitude and wavelength of the radiation. Nevertheless, we were able to identify precisely the absorption spectrum. According to changes of the phase modulation, we define how the frequency of the laser and the optical spectrum of the set. "

This was achieved using a special form of the injection current pulses with a variable depth of modulation. This technique not only preserves the sensitivity of the phase method to the absorption at a fixed wavelength, but at the same time allows you to record the spectrum at high resolution. The latter is very important when dealing with multi-track when necessary absorption attributed to a specific substance with a characteristic spectrum.

The most common method today for measuring the absorbance of the attenuation of light in the cavity (ringdaun) is the method of ICOS (the chart above shows a comparison of ICOS and the method developed at FIAN). Apparently this is due to the fact that the recording of the spectrum in ICOS occurs during continuous scanning wavelength. In the methods of measuring the phase shift of the PS-CRDS and "pure" CRDS spectrum is recorded as bypo points, that is, at each wavelength is measured by the phase shift in the PS-CRDS or the decay time in the CRDS, though, really everything is automated in the experiment and record spectrum is also produced continuously. In developed at FIAN upgrade method PS-CRDS spectrum can be recorded as well. in ICOS — per scan wavelength. In addition, this scanning can be carried out faster than ICOS by a special form of a variable pulse modulation.

"To compare the recording of the same spectrum we do not need to change anything in the installation, just change the shape of the current pulse, which is given to the computer, and signal processing. Sensitivity analysis measurements showed that in our scheme the sensitivity of 50-70 times higher dimensions than ICOS, at one and the same apparatus., "- Says Sergey Tskhai.

It is also important that, in contrast to other highly sensitive laser methods developed in this field in the LPI method allows the use of standard electronic enough and optical equipment, shifting the center of gravity on the hardware and software procedures. While the process of mathematical processing of the data is quite complex, one of the main challenges in its further work on ringdaun spectroscopy, physicists call this part of the simplification of the research, and further increase the accuracy and sensitivity of the diode spectroscopy. In particular, the planned optimization of the instrument for the measurement of carbon isotopes 12C and 13C in carbon dioxide. Such gas analyzers are used in geochemistry — to check evaporation depth gases and medicine, wherein the ratio of stable carbon concentration in the human body can be detected a number of diseases of the gastrointestinal tract.

/ 20.06.2012 / According to the materials ANI "LPI-Inform"

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