Intracavity enhancement

The cavity-enhanced evanescent-wave sensing method described in Sect. 5.2 and applied to gases in Sect. 5.3 can also be employed for the detection of chemicals in liquid solution. In addition, the intracavity enhancement provided by the high-g... 

Intracavity enhancement, relative to conventional single pass absorption spectroscopy, is due to mode competition and to threshold effects. A simple calculation of the latter for a single mode laser, starting with... 

T.D. Harris, Laser intracavity-enhanced spectroscopy, in Ultrasensitive Laser Spectroscopy, ed. by D.S. KUger (Academic, New York, 1983)... 

Sensitivity can be improved by factors of 10 using intracavity absorption, placing an absorber inside a laser resonator cavity and detecting dips in the laser emission spectmm. The enhancement results from both the increased effective path length, and selective quenching of laser modes that suffer losses by being in resonance with an absorption feature. 

In Raman measurements [57], the 514-nm line of an Ar+ laser, the 325-nm line of a He-Cd laser, and the 244-nm line of an intracavity frequency-doubled Ar+ laser were employed. The incident laser beam was directed onto the sample surface under the back-scattering geometry, and the samples were kept at room temperature. In the 514-nm excitation, the scattered light was collected and dispersed in a SPEX 1403 double monochromator and detected with a photomultiplier. The laser output power was 300 mW. In the 325- and 244-nm excitations, the scattered light was collected with fused silica optics and was analyzed with a UV-enhanced CCD camera, using a Renishaw micro-Raman system 1000 spectrometer modified for use at 325 and 244 nm, respectively. A laser output of 10 mW was used, which resulted in an incident power at the sample of approximately 1.5 mW. The spectral resolution was approximately 2 cm k That no photoalteration of the samples occurred during the UV laser irradiation was ensured by confirming that the visible Raman spectra were unaltered after the UV Raman measurements. 

Continuous-Wave Intracavity Dye Laser Spectroscopy Dependence of Enhancement on Pumping Power... 

Intracavity absorption by I- vapor has been studied for a cw dye laser. The sensitivity enhancement varies from l(r at pump powers near threshold (550 mW and 790 mW) to about 500 at the highest pump powers (near 5 watts). The results can be interpreted quantitatively in terms of a previously proposed theory. 

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). 

Mode-locked Nd-glass or ruby lasers have also been used to investigate hole-burning and intramolecular dynamics in molecules, as have intracavity dye laser techniques, which operate on the principle of transferring the loss of intensity at the frequency of the hole in the spectrum into the enhanced gain of a dye laser, whose broadband output overlaps that frequency. 

Fig. 22. Dependence of the acoustic signal of an intracavity cell with Brewster windows on the modulation frequency. In a long tube the lowest resonance is the lowest order longitudinal mode, which is excited to enhance the signal...

Lupei V, Pavel N, Taira T (2003) Basic enhancement of the overall optical efficiency of intracavity frequency-doubling devices for the 1 pm continuous-wave Nd Y3AI5O12 laser emission. Appl Phys Lett 83 3653-3655... 

Compared with the single-pass absorption of a sample with the absorption coefficient a and the absorption pathlength L2 outside the laser resonator where AP/P = -aL2 = - Ay, the intracavity absorption represents a sensitivity enhancement by the factor... 

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]. 

Although most experiments have so far been performed with dye lasers, the color-center lasers or the newly developed vibronic solid-state lasers such as the Tiisapphire laser, with broad spectral-gain profiles (Vol. 1, Sect. 5.7.3) are equally well suited for intracavity spectroscopy in the near infrared. An example is the spectroscopy of rovibronic transitions between higher electronic states of the H3 molecule with a color-center laser [24]. The combination of Fourier spectroscopy with ICLAS allows improved spectral resolution, while the sensitivity can also be enhanced [25, 34, 35]. 

Instead of absorption, weak emission lines can also be detected with the intracavity techniques [26]. If this light is injected into specific modes of the multimode laser, the intensity of these modes will increase for observation times t mode-coupling with other modes. Cavity-enhanced spectroscopy in optical fibres have been reported in [32, 33]. 

The sensitivity can be further enhanced by frequency modulation of the laser (Sect. 1.2.2) and by intracavity absorption techniques. With the spectraphone inside the laser cavity, the photoacoustic signal due to nonsaturating transitions is increased by a factor as a result of a -fold increase of the laser intensity inside the resonator (Sect. 1.2.3). The optoacoustic cell can be placed inside a multipath optical cell (Fig. 1.28) where an effective absorption pathlength of about 50 m can be readily realized [74]. 

The sensitivity of this intracavity technique (Sect. 1.2.3) can even be enhanced by modulating the magnetic field, which yields the first derivative of the spectrum (Sect. 1.2.2). When a tunable laser is used it can be tuned to the center vo of a molecular line at zero field = 0. If the magnetic field is now modulated around zero, the phase of the zero-field LMR resonances for AM = +1 transitions is opposite to that for AM = — 1 transitions. The advantages of this zero-field LMR spectroscopy have been proved for the NO molecule by Urban et al. [143] using a spin-flip Raman laser. 

To achieve large electric fields, the separation of the Stark electrodes is made as small as possible (typically about 1 mm). This generally excludes an intracavity arrangement because the diffraction by this narrow aperture would introduce intolerably large losses. The Stark cell is therefore placed outside the resonator, and for enhanced sensitivity the electric field is modulated while the dc field is tuned. This modulation technique is also common in microwave spectroscopy. The accuracy of 10 " for the Stark field measurements allows a precise determination of the absolute value for the electric dipole moment. 

All these techniques may be combined with intracavity absorption when the sample molecules are placed inside the laser resonator to enhance the sensitivity. Cavity ring-down spectroscopy yields absorption spectra with a detection sensitivity that is comparable to the most advanced modulation techniques in multipass absorption spectroscopy. 

This technique allows two-dimensional reduction of noise without using a digital computation technique. The precision in signal measurement is improved by a factor of 16. Other spectrometric techniques have been developed to improve the enhancement factor, e.g., by using a Mach-Zehnder interferometer or an intracavity quenching effect. 

Nevertheless, it is possible to give a formal description of a statistical-limit molecule in the same terms as previously used in the strong-coupling case. It is well known that the emission spectrum of large molecules (studied up to now only in condensed phases) is composed of narrow bands (considered as the resonance Raman scattering) and broad-band fluorescence. The relative intensity of the first component is enhanced in presence of fluorescence quenchers (Friedman and Hochstrasser, 1975), or in laser intracavity experiments (Bobovich and Bortkevich, 1977). The first component may be related to the emission from nonstationary s> states with redistribution time shorter than the exciting-pulse duration. The second component would be due to the rapid vibrational redistribution. In the limiting case of nonfluorescent molecules only the resonance Raman spectrum persists. The nonradiative deactivation of the excited state would be more rapid here than the vibrational redistribution. 

If some of the simultaneously oscillating laser transitions share a common upper or lower level, such as the lines 1, 2, and 3 in Fig. 5.27c and Fig. 5.29a, gain competition diminishes the output of each line. In this case, it is advantageous to use intracavity line selection in order to suppress all but one of the competing transitions. Sometimes, however, the laser may oscillate on cascade transitions (Fig. 5.29b). In such a case, the laser transition 1 -> 2 increases the population of level 2 and therefore enhances the gain for the transition 2- 3... 

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