Читать книгу Continuous Emission Monitoring - James A. Jahnke - Страница 134

Tunable diode lasers (TDLs) are now being used as infrared sources by a number of instrument manufacturers in North America and Europe for both extractive and in‐situ system analyzers in a wide variety of applications (Mettler‐Toledo 2017). Although first introduced for source monitoring applications in the 1970s (Hinkley 1972; Hinkley and Kelley 1971), diode lasers were not incorporated into CEM systems commercially until advances in communications lasers reduced costs for lasers emitting at wavelengths suitable for gas identification (Frisch 1996; Imasaka and Ishibashi 1990; Lerner 1998).

In a diode laser, an electron from the conduction band combines with a hole in the valence band to emit a photon. In the recombination of electron–hole pairs, some of the excess energy is converted into photons. The photon will stimulate further recombination and will reflect back and forth in the resonant cavity of the laser to emit a coherent beam of light. The laser wavelength is dependent upon the recombination energy, which is a function of semiconductor materials (such as InGaAsP/InP) and the laser design. Operating in the near‐infrared region of the spectrum from 0.5 to 2.5 μm, the wavelengths emitted can be tuned over a narrow range by varying the laser temperature or the injection current.