Laser linewidth

The potential of a tunable dye laser should not be overlooked. A tunable dye laser, employing an organic dye as lasing material allows one to choose any suitable excitation line within a particular region. This is in contrast to the case of a gas ion laser which has a limited number of emission lines at fixed wavelength. Nevertheless, a tunable dye laser has significant drawbacks such as poor resolution imposed by the dye laser linewidth (1.2 cm-1) and a continuous background spectrum which requires the use of a tunable filter 15-18). 

LLNL AVLIS Laser. The first WFS measurements using a Na LGS were performed at LLNL (Max et al., 1994 Avicola et al., 1994). These experiments utilized an 1100 W dye laser, developed for atomic vapor laser isotope separation (AVLIS). The wavefront was better than 0.03 wave rms. The dye laser was pumped by 1500 W copper vapor lasers. They are not well suited as a pump for LGSs because of their 26 kHz pulse rate and 32 ns pulse length. The peak intensity at the Na layer, with an atmospheric transmission of 0.6 and a spot diameter of 2.0 m, is 25 W/cm, 4x the saturation. The laser linewidth and shape were tailored to match the D2 line. The power was varied from 7 to 1100 W on Na layer to study saturation. The spot size was measured to be 7 arcsec FWHM at 1100 W. It reduced to 4.6 arcsec after accounting for satura-... 

The spectral resolution of the most monochromatic laser yet devised, expressed as frequency of the laser emission divided by the laser linewidth, is approximately 5 x 10. The laser in question is a special-purpose He-Ne laser with a nominal wavelength of 632.8 nm (15,308 cm l or 4.74 x 10 Hz in frequency units) and a linewidth of 7 to 10 Hz. It is difficult to grasp the physical significance of this degree of resolution. One illustration is that, if the spectrum of this laser were displayed on chart paper such that the zero of electromagnetic energy were located at the sun and 15,308 cm l were located at the orbit of Earth, the width of the peak representing the output of the laser would be 3 millimeters. 

With normal efforts for frequency stabilization in the optical frequency region, time-averaged laser linewidths of about 1 Mc/sec have been obtained 3 ), compared to 1000 Mc/sec in case of spontaneous fluorescence lines. 

In cw-dye lasers, pumped.by the focussed beam from an argon laser, linewidths of less than 20 Me/sec ( 10" A) have been achieved. 

Since the single-mode laser linewidth is small compared to the absorption linewidth, one can probe the absorption profile by tuning the laser line across it, getting more information than by measuring the absorption coefficient averaged over the whole doppler width 6). 

With a laser linewidth of less than IKHz spectroscopy has been carried out on water vapor lines around 1885 cm ( v- 5,3jum) with a resolution never obtained before. Application of the spin-flip laser to P- and Q-Branch absorption of NO with 0.08 cm resolution has been reported by Wood et al... 

The obtainable small laser linewidth with tunable wavelength (Section II) improves selection of different molecular transitions and in many cases the selective population of a single excited rotational level can be achieved. 

Fig. 6.20 Ionization width vs electric field for the Na (20,19,0,0) level near its crossing with the (21,17,3,0) level from experiment (data points) and from WKB-quantum defect theory (solid line). The levels are specified as (n./q.ni.M) Because the lineshapes are quite asymmetric (except for very narrow lines), the width in this figure is taken to be the FWHM of the dominant feature corresponding to the (20,19,0,0) level in the photoionization cross section. For the narrowest line, experimental widths are limited by the 0.7 GHz laser linewidth. Error limits are asymmetric because of the peculiar fine shapes and because of uncertainties due to the overlapping m = 1 resonance (from ref. 37).

We assume that we can measure directly the width of the 6sn — 6pn transition, i.e. that Te exceeds the laser linewidth. We also assume that we can use a high enough laser power so that both the 6snt — 6pn( and the 6sn t — 6pn t transitions are broadened to a width substantially in excess of the laser linewidth. In this case, if we compare the detunings Aa>nt and Amaximum values, from Eq. (19.16) we can see that... 

Since the detuning must be large compared to the laser linewidth, if we are measuring a width rn. > less than the laser linewidth, Eq. (19.9) reduces to... 

All of these approaches have energy resolution limited by the laser linewidth, 0.3 cm-1 for the pulsed lasers and 2 MHz for the cw laser. The intensities of the observed transitions cannot, however, be used with complete confidence, for it is... 

In Fig. 22.6(a) we show the spectrum of the transition from the Ba 5d6p F3 state to the 5d5/216d3/2 5/2 states which lie just above the Ba+5d 3/2 limit, and in Fig. 22.6(b) we show the spectrum from the same initial state to the 5d5/214d3/2 5/2 states which lie just below the Ba+ 5d3/2 limit. It is apparent that the envelopes of the excitations are identical. We have been discussing these states as if they were bound states, and in fact for our present purpose they might as well be. First, below the 5d3/2 limit the observed linewidths equal the laser linewidth, and second, there is no visible excitation of the 6seH continua below the 5d3/2 limit. It is also useful to note that a three channel quantum defect treatment of the 5d nd / = 4 series reproduces both the Lu-Fano plot of Fig. 22.2, and the spectra shown in Fig. 22.6. The experimental spectra were reproduced by QDT models using both the a and i channel dipole moment parametrizations described in Chapter 21. 

It must be emphasized that such phenomena are to be expected for a statistical system only in the regime of low level densities. Theories like RRKM and phase space theory (PST) (Pechukas and Light 1965) are applicable when such quantum fluctuations are absent for example, due to a large density of states and/or averaging over experimental parameter such as parent rotational levels in the case of incomplete expansion-cooling and/or the laser linewidth in ultrafast experiments. However, in the present case, it is unlikely that such phenomena can be invoked to explain why different rates are obtained when using ultrafast pump-probe methods that differ only in experimental detail. 

The light source is an argon ion laser operated in a single-frequency mode. More than one watt of power can typically be obtained with a laser linewidth of approximately 10 MHz. The incident-beam polarization can be continually adjusted with respect to the scattering plane. 

Table 7.1 are continuous wave (CW), not pulsed. Second, frequency stability to < 1 cm" is important to assure Raman shift precision and avoid line broadening. Although the Raman shift axis is usually calibrated periodically, the laser frequency must remain stable between calibrations. Third, lasers vary significantly in output linewidth, from hundreds of reciprocal centimeters to much less than 1 cm". For the majority of samples of analytical interest, a laser linewidth below 1 cm" is sufficient. Laser linewidths are often quoted in terms of frequency rather than wavenumber, in which case 1 cm" equals 30 GHz. Lasers are available with < 1 MHz linewidths (< 10 em ), but such lasers would be unnecessarily narrow for most analytical Raman applications. Fourth, lasers differ in their output of light at wavelengths other than the laser line itself. Gas lasers (Ar+, Kr+, He-Ne) emit atomic lines (plasma lines), and solid-state lasers luminesce, both of which can interfere with Raman scattering. Essentially all lasers require a bandpass filter or monochromator to reduce these extraneous emissions. 

Other experimental aspects come into play for a correct application of DEWM. These refer to the relative comparison between the laser linewidths and the Doppler and collisional broadenings. As a matter of fact, the two limits of narrowband and broadband laser line have important consequences for the quantitative analysis of chemical species. The effect of absorption, caused by the frequency degeneracy, is also problematic in quantitative DEWM and has to be evaluated in some instances. To sum up, an all-embracing interpretation of DEWM measurements is not available as it is, by contrast, for CARS. Nevertheless, careful choice of the experimental conditions and rigorous analysis of the corresponding theoretical approximations can lead to reasonable results for both thermometry and concentration measurements, as... 

Yuratich, M. A. "Effects of Laser Linewidth on CARS." MoZec Physics 38 (1979) 625. 

Barton, S., and Garneau, J. M. "Effect of Pump-Laser Linewidth on Noise in Single-Pulse Coherent Anti-Stokes Raman Spectroscopy Temperature Measurements." Optics Letters 12, no. 7 (1987) 486. 

In case the effective laser linewidth is less than the hyperfine splitting(s) excitation will prepare a two-level system. The effect of spin-flips on the coherence in this system will then manifest itself as a 7 ,-type process. No beats are expected in the decay of the optical free induction. With broadband excitation that spans some of the hyperfine splittings spin-flips will be monitored as 7 2-type processes and quantum beats are expected in the photon-echo intensity vs probe delay. Burland et al. also demonstrated the feasibility of optical nutation in this system from which in principle, as from the OFID, the transition dipole could be calculated. 

Fig.3.8 Part of a CO-laser spectrum obtained by tuning the grating angle (see Fig.3.9). The appearing linewidth does not indicate the laser linewidth but the angular region where the gain is above threshold.

---