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СКАЧАТЬ 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.

       Quantum Cascade Lasers.

      Quantum cascade lasers (QCLs), developed in 1994 (Faist et al. 1994), are another option for gas monitoring systems. Operating at room temperature, they emit light in the mid‐infrared region of the spectrum, from 2.5 to over 20 μm, a region where many pollutant molecules strongly absorb. In contrast to diode lasers, where the light emitted depends upon the bandgap of the semiconductor material constituting the laser, quantum cascade lasers incorporate dozens of alternating semiconductor layers. The electric potential varies over the length of the device, where the semiconductor layers form potential “wells.” In QCLs, the output light wavelength is dependent upon the layer structure constructed by design, whereas in tunable diode lasers, it is a function of the material.

      The QCL relies on transitions between excited states in the conduction band (intersubband transitions) for photon generation. In operation, electrons tunnel through the “quantum wells,” generating photons as they cascade through different energy levels. One electron emits a photon in each intersubband transition within the quantum well and then tunnels into the next quantum well to emit another photon, cascading down the quantum wells of the structure to emit multiple photons.

      The flexibility in manufacturing, their ability to emit light in the mid infrared, their high optical power output, and their ability to operate at room temperature have made QCLs increasingly attractive for gas monitoring applications (Kosterev et al. 2008).

       Interband Cascade Lasers.

      Interband cascade lasers (ICLs) operate in the wavelength range of 3–6 μm and fill the gap in wavelengths emitted between tunable diode lasers and quantum cascade lasers. ICLs are similar to quantum cascade lasers; however, in ICLs, light is emitted from electron–hole recombination in interband transitions rather than in intersubband transitions. In a sense, ICLs are a hybrid between tunable diode lasers and quantum cascade lasers, in that light is emitted by electron–hole recombination from electron injection. Due to the nature of these transitions, light emission can occur at lower electrical input power than in quantum cascade lasers.

      Wavelength Selection

      Lasers, of course, are used in gas analyzers to monitor at specific wavelengths. Before the advent of lasers, it was necessary to restrict or separate the wavelengths at which molecules absorb from the spectra emitted from broadband radiation sources. Optical filters and diffraction gratings were and still are used to do this in spectroscopic CEM system analyzers. To manufacture an analyzer, it is often easier and less expensive to utilize broadband emission sources, coupled with spectral limiters such as gratings and filters.

       Optical Filters.

      An optical filter allows light only in a narrow spectral region to pass through it. Interference filters, constructed by vacuum deposition of metallic films on glass or other materials, are commonly used in the infrared region of the spectrum. Neutral‐density filters are used to attenuate light of all wavelengths equally, being made of quartz or glass with a thin metal coating having a specified optical density.

       Diffraction Gratings.

Diffraction gratings are commonly used in the ultraviolet and visible regions of the spectrum to select specific wavelengths. A grating consists of a reflective surface finely etched with a large number of parallel lines (on the order of 600 lines/mm). Each line will scatter light impinging on it, causing the light to interfere constructively and destructively to spread out a reflected spectrum.

       Prisms.

      Prisms disperse ultraviolet, visible, and infrared radiation due to differences in the wavelength‐dependent index of refraction in the prism material. Prism spectrometers are available in the UV and visible spectral regions; although they have been used in atomic absorption spectrometers for measuring metals, they are not typically used in pollutant gas monitoring applications.

      Detectors

      The type of detector used in an analyzer is very dependent on the energy of the light that it is sensing. Because light in the infrared region is relatively weak in the energy it carries, special stratagems are often devised to detect infrared intensity changes. Pneumatic, microphone‐type detectors (Luft detectors) traditionally have been used in infrared analyzers; however, solid‐state detectors, cooled with Peltier coolers, are common today. Sensitivity is often increased by not overly limiting the spectral region of the analyzer, but using a broader band of radiation to obtain more light for the detector. The special methods of gas filter correlation and Fourier‐transform infrared spectroscopy take advantage of this technique.

      The most familiar detector in the visible region is the human eye, which is, of course, used as the detector in EPA Reference Method 9 for measuring visible emissions. Phototubes, photomultiplier tubes, photovoltaic cells, and photo‐diode arrays are used in instrumented systems. Photo‐diode arrays are being incorporated increasingly into CEM analyzers (Durham et al. 1990; Saltzman 1990). Diode arrays provide a simple way of measuring multiple wavelengths and are useful for monitoring several gases in one analyzer, rather than using a separate analyzer for each gas.

      Multipath Gas Cells

      Infrared and ultraviolet spectrometers, gas filter correlation spectrometer, Fourier‐ transform infrared spectrometers, and systems using tunable lasers make use of the Beer–Lambert law, either in a simple logarithmic relationship, or more complicated expressions. One characteristic in these instruments in which light energy is absorbed by targeted molecules is that when more molecules absorb the light energy, the greater will be the difference between the incident light intensity I_o and the signal intensity, I. For low concentrations or small absorption coefficients, an instrument may not be sensitive enough to provide accurate measurements. In particular, weaker absorption bands in the near infrared make it difficult to apply tunable diode lasers for gas monitoring unless the pathlength is increased.

      By increasing the measurement pathlength, more opportunities are provided for light energy to be absorbed before reaching the detector. There are several ways of doing this; the simplest is to just increase the length of the measurement cell. This was attempted in some early CEM instruments, but led to bulky, problem‐prone installations, most of which have since been replaced. Other approaches include using multipath cells (such as White and Herriott cells), using high sensitivity, ultralong pathlength, absorption techniques such as cavity ring‐down spectroscopy (CRDS) and integrated cavity output spectroscopy (ICOS), or by measuring across the stack in a double‐pass in‐situ system instead СКАЧАТЬ