Intracavity Saturation Spectroscopy

When the absorbing sample is placed inside the resonator of a tunable laser, the Lamb dip in the absorption coefficient a(o)) causes a corresponding peak of the laser output power P(o)) (Fig. 7.14). 

The power P co) depends on the spectral gain profile G(co) and on the absorption profile a (jo) of the intracavity sample, which is generally Doppler broadened. The Lamb peaks therefore sit on a broad background (Fig. 7.14a). With the center frequency co of the gain profile and an absorption Lamb dip 

In a small interval around o)o we may approximate the gain profile G(co — a i) and the unsaturated absorption profile o (o)) by a quadratic function of co, yielding for (7.34) the approximation 

These derivatives are exhibited in Fig. 7.15, which illustrates that the broad background disappears for the higher derivatives. If the absorptive medium is the same as the gain medium, the Lamb peak appears at the center of the gain profile (Fig.7.14b,c). 

If the laser frequency co is modulated at the frequency 2, the laser output Pl(co) = Pi (coo- -asinQt), 

These derivatives are exhibited in Fig.7.15, which illustrates that the broad background disappears for the higher derivatives. 

If the laser frequency w is modulated at the frequency ft, the laser output 

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Fig. 2.18 Schematic arrangement for third-derivative intracavity saturation spectroscopy...

Fig.7.41a-c. Comparison of different techniques for measuring the neon transition ls2 — 2p2 at X = 588.2 nm (a) intracavity saturation spectroscopy (Lamb peak of the laser output Il cd) with Doppler-broadened background) (b) laser-induced dichroism and (c) laser-induced birefringence [7.72]... 

If the discharge cell has windows of optical quality, it can be placed inside the laser resonator to take advantage of the -fold laser intensity (Sect. 6.2.2). With such an intracavity arrangement. Doppler-free saturation spectroscopy can also be performed with the optogalvanic technique (Sect. 7.2 and [6.101]). An increased sensitivity can be achieved by optogalvanic spectroscopy in thermionic diodes under space-charge-limited conditions (Sect. 6.4.5). Here... 

Fiq.l0.24a-c. Intracavity saturated absorption spectroscopy, (a) Experimental setup, (b) "Lamb peak" in the laser output, (c) derivative of a Lamb peak obtained by modulation of the laser frequency... 

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