Lead salt laser

Figure 28.2 Selection of commercially available TDLAS systems. Clockwise, from top to bottom universal dual-beam system with internal calibration cell, based on visible-near-IR laser diodes (LTG-LaserTech Inc) multispecies gas monitor, based on mid-IR QCLs (Cascade Technologies) miniature single-gas sensor, based on visible-near-IR laser diodes (Physical Sciences Inc.) single-/multi-gas system, based on mid-IR lead-salt lasers (Campbell Scientific Inc.) hand-held remote gas sensor, based on near-IR laser diodes (Physical Sciences Inc.)...

The problem of unwieldy lead-salt lasers is now diminishing, with the rapid advance of room-temperature near-IR lasers (emerging Irom telecommunication laser technology), and even more so with the advent of the the quantum cascade laser (QCL). 

Table 28.1 TDLAS sensitivities for selected molecular trace gas species, measured with commercial instruments based on near-IR semiconductor diode lasers, mid-IR lead-salt laser diodes and mid-IR QCL sources (note that for the latter only a few analytical measurement data are available, because of their novelty)...

Fig. 5.64. (a) Spectral ranges of laser emission for different semiconductor materials [5.115] (b) dependence of the emission wave number on the composition x of Pbj-xSnxTe, Se, or S-lead-salt lasers (courtesy of Spectra-Physics)... 

For the visible and near-ultraviolet portions of the spectmm, tunable dye lasers have commonly been used as the light source, although they are being replaced in many appHcation by tunable soHd-state lasers, eg, titanium-doped sapphire. Optical parametric oscillators are also developing as useful spectroscopic sources. In the infrared, tunable laser semiconductor diodes have been employed. The tunable diode lasers which contain lead salts have been employed for remote monitoring of poUutant species. Needs for infrared spectroscopy provide an impetus for continued development of tunable infrared lasers (see Infrared technology and RAMAN spectroscopy). 

Laser sources that emit in the mid-ir region of the spectmm (2—5 -lm) are useful for detection of trace gases because many molecules have strong absorption bands in that region. Other appHcations include remote sensing and laser radar. Semiconductor lead—salt (IV—VI) lasers that operate CW at a temperature of 200 K and emission wavelength of 4 p.m are commercially available however, they have relatively low output powers (<1 mW) (120). 

FIGURE 11.5 Variation of laser frequency and signal with current for a typical lead salt diode laser (adapted from Werle et al., 1992). 

Time-resolved spectroscopy is performed using a pump-probe method in which a short-pulsed laser is used to initiate a T-jump and a mid-IR probe laser is used to monitor the transient IR absorbance in the sample. A schematic of the entire instrument is shown in Fig. 17.4. For clarity, only key components are shown. In the description that follows, only those components will be described. A continuous-wave (CW) lead-salt (PbSe) diode laser (output power <1 mW) tuned to a specific vibrational mode of the RNA molecule probes the transient absorbance of the sample. The linewidth of the probe laser is quite narrow (<0.5 cm-1) and sets the spectral resolution of the time-resolved experiments. The divergent output of the diode laser is collected and collimated by a gold coated off-axis... 

In order to determine exact quantitative data it is necessary to obtain high resolution spectra of the dimer. Since a resolution of < 0.001 cm i is beyond the scope of FTIR spectrometers, laser spectroscopic techniques were required. This could be aciiieved in an IR study using sensitive absorption measurements by a lead salt diode laser. 

Linewldth Considerations and Lead-Salt Tunable Diode Lasers... 

For semiconductor lasers in general, and lead-salt diode lasers in particular, linewldth calculations based even on the first order Schawlow-Townes formulation of Eq. (2) will yield numerical values that are typically several kHz to tens of kHz wide. The reasons for this are as follows. 

To begin with, the typical single mode output power of lead-salt diode lasers presently available is generally (much) less than 10 mw. Also, Qc, the "cold" cavity Q calculated for a "solitary" diode laser (the cavity of which is defined by two cleaved end facets) is generally much less than 10. In terms of the usually measured semiconductor laser parameters the full Lorentzian linewidth between half-maximum power points (FWHM) can be written as... 

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