Use of diode laser

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. 

Semiconductor lasers are ideal excitation sources, and they have already been demonstrated to be remarkably versatile and useful in a number of spectroscopic applications, as indicated by Ishibashi and cowotkers (22,23). These solid-state devices are inexpensive, small, easy to use, long-lived, and require little maintenance. The problem with semiconductor lasers is that powerful singlemode lasers are only available at NIR wavelengths, although this is likely to change in the near future. Until this work, however, no report had been made of the use of diode lasers for SERS. Recently, we demonstrated NIR SERS with a diode-laser excitation source and investigated the characteristics of the technique (Angel, S.M. Myrick, M.L. unpublished data). 

For such absorption measurements infrared lasers can be used which coincide with vibrational-rotational transitions of the investigated molecules (CO2 laser, CO laser, HF or DF lasers, etc.). Particularly useful are tunable infrared lasers (diode lasers, color-center lasers, or optical parametric oscillators, Vol. 1, Sect. 5.7), which may be tuned to selected transitions. The usefulness of diode lasers has been demonstrated by many examples [1451]. For instance, with a recently-developed automated diode laser spectrometer, which is tuned by computer control through the spectral intervals of interest, up to five atmospheric trace gases can be monitored in unattended operation. Sensitivities down to 50 ppt for NO2 and 300 ppt for NO have been reported [1371]. 

An interesting approach to the use of diode laser atomic absorption lies in the use of discharges under reduced pressure as atom reservoirs. In the case of helium low pressure microwave discharges, for instance, metastable levels of elements such as halogens, hydrogen, oxygen or sulfur are excited, which can be... 

The light source for excitation of Nd YAG lasers may be a pulsed flashlamp for pulsed operation, a continuous-arc lamp for continuous operation, or a semiconductor laser diode, for either pulsed or continuous operation. The use of semiconductor laser diodes as the pump source for sohd-state lasers became common in the early 1990s. A variety of commercial diode-pumped lasers are available. One possible configuration is shown in Figure 8. The output of the diode is adjusted by composition and temperature to be near 810 nm, ie, near the peak of the neodymium absorption. The diode lasers are themselves relatively efficient and the output is absorbed better by the Nd YAG than the light from flashlamps or arc lamps. Thus diode-pumped sohd-state lasers have much higher efficiency than conventionally pumped devices. Correspondingly, there is less heat to remove. Thus diode-pumped sohd-state lasers represent a laser class that is much more compact and efficient than eadier devices. 

A useftil applicadon of time-dependent PL is the assessment of the quality of thin III-V semiconductor alloy layers and interfaces, such as those used in the fabri-cadon of diode lasers. For example, at room temperature, a diode laser made with high-quality materials may show a slow decay of the acdve region PL over several ns, whereas in low-quality materials nonradiative centers (e.g., oxygen) at die cladding interface can rapidly deplete the free-carder population, resulting in much shorter decay times. Measurements of lifetime are significandy less dependent on external condidons than is the PL intensity. 

One application of modem solid-state electronic devices is semiconductor materials that convert electrical energy into light. These light-emitting diodes (LEDs) are used for visual displays and solid-state lasers. Many indicator lights are LEDs, and diode lasers read compact discs in a CD player. The field of diode lasers is expanding particularly rapidly, driven by such applications as fiber optic telephone transmission. 

Sell, J. A. Herz, R. K. Monroe, D. R. "Dynamic Measurement of Carbon Monoxide Concentrations in Automotive Exhaust Using Infrared Diode Laser Spectroscopy" SAE Paper No. 800463, 1980. 

Finally, we note that future instrument for lifetime-based sensing and imaging can be based on laser diode light sources. At present it is desirable to develop specific probes which can be excited from 630 to 780 nm, the usual range of laser diodes. The use of such probes will allow us to avoid the use of complex laser sources, which should result in the expanded use of fluorescence detection in the chemical and biomedical sensors. 

A fascinating category of experiments can be found in Table IV. These are the use of lasers to determine thermodynamic parameters. These include calorimetry (43), enthalpies of vaporization and vaporization rates (44, 45), and heat capacities (46). Other laser experiments that can be found in Table IV include the use of CW laser spectroscopy to determine the iodine binding-energy curve (47), the study of vibrational line profiles to determine intermolecular interactions (48), two photon ionization spectrometry (49), a study of optical activity and optical rotatory dispersion (50) and the development of several experiments using blue diode lasers (57). 

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