PRINCIPLES OF TUNABLE LASERS ABSORPTION SPECTROSCOPY (TDLAS)

GAS ABSORPTION SPECTRUM AND LASER

As we know, Molecules in gas state have long distance with each other. They have clear quantization of their vibration and rotational energy. In optical spectrum, quantized energy corresponds to a series of fine absorption lines. Because these lines are so fine and separated, they hardly coincide with lines from any other molecules. They are like the fingerprint of molecules. The purities of these lines are comparable with Lasers’, while much purer than light sources from other analytical instruments. This is why Lasers naturally fits the job for gas molecules absorption spectroscopy determination. 
The first step towards the realization of a powerful laser-based absorption technique was the development of single-mode diode lasers in a variety of wavelength regions. Diode lasers combine bine good intensity stability with rapid tunability. It was later realized that the full power of laser-based absorption techniques could be obtained only when the absorption technique was combined with some kind of modulation technique.

WHY USING THE COMPLICATED MODULATION?

The Laser line and absorption line are both too narrow. It is like aiming two pinpoints. The center wavelength of laser diodes has some error due to the manufacturing process and are inherently affected by temperature. So Laser diodes always need to be tuned.
Using modulation gives following advantages:
1, Laser wavelength sweep through the absorption line rather than fix on it, which allows more tolerance on system control. It is common to have 1% error in current and 0.1-degree error in temperature, which is enough for Lasers to miss the narrow absorption line. Getting the laser sweep over a slightly large range provide much more stability in real systems.
2, Both bottom and summit of absorption line are measured. It is very useful for background correction to acquire the actual height of absorption, without broadband interferes.
3, Signals are modulated to higher frequency, easier for filtering and noise reduction.
Reyin’s integrated laser modules have built-in sweep scan control with digital modulation, demodulation and filtering. Result can be directly viewed on PC.

IS TDLAS SUITABLE FOR LIQUID PHASE DETERMINATION AND OTHERS?

Unlike gas, molecules in liquid are surround by solvent. Their absorption spectrums are much wider due to the solvent effect, sometimes even wider than the tunable range of many diode lasers. The narrow bandwidth and modulation characters of diode lasers are useless here. Some "coarse" light sources are more cost effective in this situation, such as LEDs and lights resolved by gratings or filters. Thus, TDLAS is often less preferred for liquid phase detection. But all depend on the substance you want to measure because analytical method always submits to properties.