Today, TDLAS (Tunable Diode Laser Absorption Spectroscopy) is a scientifically and industrially recognized measurement technology for the determination of molecule concentration. The process is calibrated and allows for both very sensitive detection and detection with a large dynamic range. Because of the small laser line widths, additional information such as temperature and pressure of the measured group can also be determined. The outline of a general test setup with tunable laser diodes in absorption spectroscopy is shown in Figure 1.
Different configurations can be implemented with LASER COMPONENTS’ spectroscopic systems.
The highest sensitivities, such as those needed in the detection of trace elements in the ppb range, are achieved through a method called derivative spectroscopy. Although this method is not new, it is still used in too few laboratories. Why? Perhaps it is still too unknown? For this reason, we would like to make a point of mentioning the derivative principle and recalling it to memory: in addition to the laser diode controller, a lock-in amplifier and a computer are used. A modulation current with amplitudes in the % range is added to the direct current of the laser diode. This is achieved by the modulation module of the LASER COMPONENTS-made controller L5830 (Figure 2). The generation of an additional external frequency is not necessary. Perhaps one reason that the derivative method is not prevalent is the fact that most of the laser diode controllers do not have an integrated frequency generation and the derivative method can therefore not be inferred. Phase sensitive detection is set up according to a reference and the signal is read out on the computer. The profile of the absorption line can be recorded with a series of different currents, preferably with small increments, applied to the laser diode. When using a Taylor series development, a relationship between the detected signal and the mathematical derivatives is demonstrated, which produces absorption lines as exhibited in Figure 3.
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Fig. 2: Directly measured absoption line | Derivatively measured absorption line |
