Intracavity Absorption Measurements

In the absorption measurements described above an external laser beam was used. A great increase in sensitivity can be acliieved if the sample is placed inside the laser cavity [9,61-9.64]. This is due to the mode competition in a multi-mode laser. The principle of intracavity absorption measurements is given in Fig. 9.4. 

Several species which are weak absorbers or which may not fluoresce readily, may be detected using intracavity laser absorption. This is a much more sensitive method of detection than the conventional absorption measurements because the output power of a laser is very sensitive to small variations in its gain which, in term, depends on the concentration of absorbing species within the cavity. Vibrational state populations for the CO produced in the reaction... 

It is possible to measure the quantum yield of the radical process (eq. 22) by the difference in CO yields with and without NO present. Clark and co-workers (54,55) have used this technique to measure the yield of radical process 22. They find a limiting value of at high NO pressures of 0.70, in good agreement with the values of 0.68 of Lewis and Lee (143) and 0.76 of Horowitz and Calvert (115). The obvious explanation for the increase in CO yield is that complete scavenging of HCO by NO allows an extra CO molecule to be generated. Radical reaction kinetics involving HCO and O2 (and NO) have been recently studied by flash photolysis (215) and laser photolysis/intracavity absorption (56). 

An intracavity photoacoustic detector was also described by Leslie and Trusty In this work absorption coefficients of CH4 were measured in the 2500-2800 cm region, applying a DF laser for excitation of CH4. The weakest CH4 absorption studied was about 10 times weaker than the strong CH4 absorption at 3,39 pm. The intracavity cell was not intended to be resonant in this case, however, by using a resonance found at 310 Hz, a substantially better performance was obtained. The smallest absorption measured with this setup was 1 10" cm with about 10 1 S/Nat a 1 sec time constant. 

The discussion in item 3 has assumed that the mode coupling and mode frequencies were time independent. In real laser systems this is not true. Fluctuations of mode frequencies due to density fluctuations of the dye liquid or caused by external perturbations prevent stationary conditions in a multimode laser. We can define a mean mode lifetime which represents the average time a specific mode exists in a multimode laser. If the measuring time for intracavity absorption exceeds the mode lifetime no quantitatively reliable information on the magnitude of the absorption coefficient a (o) of the intracavity sample can be obtained. 

It is therefore better to pump the intracavity laser with a step-function pump laser, which starts pumping at r = 0 and then remains constant (Fig. 1.13). The intracavity absorption is then measured at times t with 0 < t which are... 

Fig. 1.15 Time evolution of the spectral profile of the laser output measured with time-resolved intracavity absorption spectroscopy [13]...

The enhanced sensitivity of intracavity absorption may be utilized either to detect minute concentrations of absorbing components or to measure very weak forbidden transitions in atoms or molecules at sufficiently low pressures to study the unperturbed absorption line profiles. With intracavity absorption cells of less than 1 m, absorbing transitions have been measured that would demand a path length of several kilometers with conventional single-pass absorption at a comparable pressure [15, 19]. 

All the absorption techniques discussed thus far typically involve the measurement of a very small change in the total transmitted intensity of a light source through an absorbing medium this normally leads to a high background condition that limits sensitivity. State-of-the-art continuous wave (CW) absorption techniques, such as frequency modulation and intracavity methods (no further details provided here, but see Demtroder (2002) for example), can meet or exceed this sensitivity for static absorption measurements. However, they are commonly difficult to implement. 

E.N. Antonov, V.G. Koloshnikov, V.R. Mironenko Quantitative measurement of small absorption coefficients in intracavity absorption spectroscopy using a cw-dye laser. Opt. Commun. 15, 99 (1975)... 

Direct spectroscopic measurements of absorptions could provide substantial and much-needed complimentary information on the properties of BLMs. Difficulties of spectroscopic techniques lie in the extreme thinness of the BLM absorbances of relatively few molecules need to be determined. We have overcome this difficulty by Intracavity Laser Absorption Spectroscopic (ICLAS) measurements. Absorbances in ICLAS are determined as intracavity optical losses (2JI). Sensitivity enhancements originate in the multipass, threshold and mode competition effects. Enhancement factor as high as 106 has be en reported for species whose absorbances are narrow compared to spectral profile of the laser ( 10). The enhancement factor for broad-band absorbers, used in our work, is much smaller. Thus, for BLM-incorporated chlorophyll-a, we observed an enhancement factor of 10 and reported sensitivities for absorbances in the order of lO- (24). 

Figure 10 shows the schematics of the experimental setup used for intracavity laser absorption spectroscopy (ICLAS) of bilayer lipid membranes (BLMs). Simultaneous electrical and ICLAS measurements were carried out in a two-compartment container constructed from two 1 cm path lengths quartz cells (Figure 11). 

Figure 10. Schematics of the experimental setup for intracavity laser absorption spectroscopy (ICLAS). CD chopper driver PM power meter Mj, M2, M3, M4 spherical high reflection mirrors Mp = pump mirror MN monochromator PMT photomultiplier SP silicon photocell PC Pockels cell WF wedged filter LIA lock-in amplifier R recorder MS microscope OF optical fiber S sample (solution on BLM) IEM instruments for electrical measurements (see Figure 2).

In the methods discussed in the preceding sections, the attenuation of the transmitted light (or of the laser power in intracavity spectroscopy) is monitored to determine the absorption coefficient a a>) or the number density of absorbing species. For small absorptions this means the measurement of a small difference of two large quantities, which limits, of course, the signal-to-noise ratio. 

New absorption methods, like intracavity spectroscopy, cavity-ring-down and cavity-enhanced spectroscopy, have demonstrated very high sensitivities in laboratory measurements with DLs. An ultrasensitive technique that combines external cavity enhancement and FM spectroscopy has been developed recently. This method, which has been called NICE-OHMS. or noise-immune cavity-enhanced optical heterodyne molecular spectroscopy, is based on frequency modulation of the laser at the cavity free-spectral-range frequency or its multiple. The MDA of 5x 10 1 X 10 cm ) in the detection of narrow... 

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