Near-infrared diode lasers, detection

If near-infrared diode lasers have low-noise characteristics similar to those of mid-infrared diode lasers, and thus minimum absorbances of 10 5 or less are possible, then an approximate detection limit can be calculated for an absorption experiment. For a 200-m optical path, the calculated detection limit is 5 x 1010 molecules/cm3, which is well above levels of H02 expected to be found in the atmosphere. An absorption experiment in this spectral region apparently would require extremely long optical path lengths, and, indeed, a calculation with a 5-km path yields a calculated detection limit of 2 x 109 molecules/cm3, still rather high for tropospheric measurements. Other issues associated with the use of diode lasers in absorption spectroscopy are discussed in the next section. 

U. Gustafsson, J. Sandsten, S. Svanberg Simultaneous detection of methane, oxygen and water vapour utilizing near-infrared diode lasers in conjunction with difference-frequency generation. Appl. Phys. B 71, 853 (2000)... 

Recently, development of a more accurate and robust propane sensor by replacing the fixed-wavelength He-Ne laser with a tunable diode laser was initiated. The strongest absorption of propane in the near-infrared region occurs near 1.68 /xm. Figure 10.4 shows the first known high-resolution spectral data of propane near 1.68 /xm, from which one can identify the optimum detection strategy. In this work, the diode laser was scanned across the absorption peak at 1.6837 /xm at 500 Hz to mecisure propane concentration. This sensor was applied to the head-end of the PDE at Stanford University, as shown in Fig. 10.5. The initial results of propane measurements are shown in Fig. 10.6. These results contain a wealth of information... 

Under excitation of a 980 nm laser diode, a dazzling green spot from both samples could be clearly seen by the naked eye. Figure 3 gives the typical upconversion luminescent emission bands. Green and near-infrared emission bands were detected in both samples, which were very similar to fluorescent emissions. Especially, the red emission band was measured around 667 nm associated with the F5—> lg transition. Furthermore, the ultra-violet and blue upconversion emissions can also be measured. The ultra-violet emissions in the ranges of 381-394 nm and 409-428 nm are assigned to the G4—> lg and G5— Ig transitions, respectively. The blue emission between 473 and 500 nm... 

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