Tunable diode laser source

Through the availability of tunable diode laser sources the set-up used for atomic absorption (see Section 4.2.2) can be simplified considerably. In this way, several additional advantages can be realized. As through wavelength tuning, measurements in the wings of the absorption profile can also be made, and possible improvements to the linear dynamic range in a number of cases could be expected. 

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). 

Tunable diode-laser sensors offer considerable promise for combustion research and development and also for process sensing and control applications. These devices are rugged and relatively easy to operate and they have been demonstrated to yield simple and quantitative measurements of species, temperature, and velocity, where line-of-sight measurements are useful or preferred. These techniques will grow in use as costs of laser sources and fiber-optic components decrease and access to more wavelength regions improves. 

AAS with flames and furnaces is now a mature analytical approach for elemental determinations. However, its development has not yet come to an end. This applies to primary sources, where tunable diode lasers open new possibilities and even eliminates the requirement of using expensive spectrometers. It also applies to atom reservoirs, where new approaches such as further improved isothermal atomizers for ETAAS or the furnace in flame technique (see e.g. Ref. [326]) have now been introduced, but also to spectrometers where CCD based equipment eventually with smaller dimensions will bring innovation. Furthermore, it is dear that on-line coupling both for trace element-matrix separations and speciation will enable many analytical challenges to be more effectively tackeld. 

Diode laser sources Already in 1980, lasers had been suggested as excitation sources for atomic absorption spectrometry [11]. Tunable dye lasers can provide virtually any atomic hne between 213 and 900 run with a bandwith corresponding to the natural hne width of an atomic hne and with a comparatively high intensity. However, they have not found widespread acceptance for this apphcation so far due to their cost and complex operation compared to hollow cathode or electrodeless discharge lamps. This situation seems to have changed with the advent of inexpensive, mass produced diode lasers (DL) [12, 13]. 

There are two other sources worth noting, although they are currently used in a very small fraction of the instruments employed. Continuum sources can be used if their intensity is sufficient to minimize noise levels and if the spectrometer has sufficient dispersion to make the spectral bandpass comparable with the absorbing line width. While feasibility has been demonstrated in research laboratories, there currently is no commercial instrument available. At the other extreme, using a very bright, stable somce with a narrow line width has produced viable absorbance readings that are two to three orders of magnitude below those available with HCLs and EDLs. The somce that provides this detection enhancement is a tunable diode laser. 

Four different methods used for integrated-path remote gas sensing are discussed here. One of these (tunable diode laser absorption spectroscopy, TOLAS) uses a narrow linewidth source of radiation (usually a laser diode) and the other three methods use broadband sources of radiation. These three analyze the spectrum of the radiation after it has traversed the atmospheric path in different ways both differential optical absorption spectroscopy (DOAS) and Fourier transform infrared (FTIR) spectroscopy analyze the entire spectrum over the spectral region of interest, whilst absorption correlation methods record the spectrum after it has been filtered optically with either an optical filter or a sample of the target gas itself. These four methods use an active source of radiation. It is also possible to carry out integrated-path remote gas sensing using a passive source. 

Horii CV, Zahniser MS, Nelson DD, McManus JB, Wofsy SC. 1999. Nitric acid and nitrogen dioxide flux measurements a new appfication of tunable diode laser absorption spectroscopy . In Application of Tunable Diode and other Infrared Sources for Atmospheric Studies and Industrial Processing Monitoring, II, Fried A (ed.). SPIE vol. 3758. SPIE-The International Society for Optical Engineering Bellingham, WA 152-161. 

U. Simon, S. Waltman, I. Loa, L. Holberg, T.K. Tittel External cavity difference frequency source near 3.2 p.m based on mixing a tunable diode laser with a diode-pumped Nd. YAG-laser in AgGaS2. J. Opt. Soc. Am. B 12, 322 (1995)... 

The measurements have to be repeated at increasing pump powers, in order to draw a curve indicating the pump power where transparency (G = 0 dB/cm) and saturation of the gain are obtained. It can be also necessary to measure the optical gain at several wavelengths, in order to define what is the operational amplification bandwidth in that case, a tunable signal laser source (typically, a laser diode with external cavity) has to be used. 

There are, however, several advantages to using TDLs for measurement of gas-phase flame species. These include high resolution (typically better than lxl0 cm" ), good spatial resolution (200 to 1 mm), reasonable output power ( 1 mW), and the ability to scan over their spectral range on a millisecond or better timescale. Probably the most widely studied molecular flame species by tunable diode laser spectroscopy is CO. In addition to the reasons for study outlined above in the discussion of broadband source methods, CO possesses several fundamental (v = 0-l) and hot-band transitions (v = 1-2, V = 2-3) which occur within several line widths (approximately 0.05 cm" ) of each other. At room temperature, populations of states from which hot-band transitions occur are very low. However, at flame temperatures, populations of vibrational states other than the v = 0 state may become appreciable. When temperatures (and also species concentrations) are calculated from simultaneous measurement of a fundamental and a hot-band transition, the technique is referred to as two-line thermometry. 

The pump source in the dispersive experiment is typically a nanosecond laser the probe source can be broadband IR light from a globar or tunable IR light from a CO laser or a semiconductor diode laser. Although CO and diode lasers can produce... 

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