Spectral gain profiles

Tunable coherent light sources can be realized in several ways. One possibility is to make use of lasers that offer a large spectral gain profile. In this case, wavelength-selecting elements inside the laser resonator restrict the laser oscillation to a narrow spectral interval and the laser wavelength may be continuously tuned across the gain profile. Examples of this type of tunable laser are the dye lasers were treated in the previous section. 

In general, two types of tunable solid state lasers have been developed those based on color centers in alkali halide crystals, and those based on transition metal ions (3d) in a crystalhne host. In both cases, the tunabihty rehes on the large spectral gain profile provided by the active center. 

The frequency of a single-mode laser inside the spectral gain profile of its active medium is mainly determined by the eigenfrequency of the active laser cavity mode. Therefore any instability of resonator parameters, such as variation of cavity length, mirror vibrations or thermal drifts of the refractive index will show up as frequency fluctuations and drifts of the laser line. 

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]. 

The power P co) depends on the spectral gain profile G a>) and on the absorption profile a (co) of the intracavity sample, which is generally Doppler broadened. The Lamb peaks therefore sit on a broad background (Fig. 2.16a). With the center frequency a>i of the gain profile and an absorption Lamb dip at coo we obtain. 

Without frequency-selective elements inside the laser resonator, the laser generally oscillates simultaneously on many resonator modes within the spectral gain profile of the active medium (Vol. 1, Sect. 5.3). In this multimode operation no definite phase relations exist between the different oscillating modes, and the laser output equals the sum intensities L of all oscillating modes, which are more... 

In real mode-locked lasers the amplitudes Ak are generally not equal. Their amplitude distribution Ak depends on the form of the spectral gain profile. This modifies (6.9) and gives slightly different time profiles of the mode-locked pulses, but does not change the principle considerations. 

Similar to cw lasers, active media with a broad spectral gain profile can be used for pulsed lasers. With wavelength-selective optical elements inside the laser resonator, the laser wavelength can be tuned across the whole gain profile. However, the drawback is the widening of the pulse length AT with decreasing spectral width Av due to the principal Fourier limit AT > 27t/Av. 

The frequency spectrum of a laser is determined by the spectral range of the active laser medium, i.e., its gain profile, and by the resonator modes falling within this spectral gain profile (Fig. 5.20). All resonator modes for which the gain exceeds the losses can participate in the laser oscillation. The active medium has two effects on the frequency distribution of the laser emission ... 

If only the axial modes TEMqo participate in the laser oscillation, the laser beam transmitted through the output mirrors has a Gaussian intensity profile (5.32), (5.42). It may still consist of many frequencies = qcl 2nd) within the spectral gain profile. The spectral bandwidth of a multimode laser oscillating on an atomic or molecular transition is comparable to that of an incoherent source emitting on this transition ... 

For lasers with a broad continuous spectral gain profile, the preselecting elements inside the laser resonator restrict laser oscillation to a spectral interval, which is a fraction of the gain profile. 

Fig. 5.46a,b. Intensity stabilization of a cw dye laser by control of the argon laser power (a) experimental arrangement (b) stabilized and unstabilized dye laser output P X) when the dye laser is tuned across its spectral gain profile... 

The wavelengths of the laser radiation are determined by the spectral gain profile and by the eigenresonances of the laser resonator (Sect. 5.3). If the polished end faces of the semiconducting medium are used as resonator mirrors (Fig. 5.65a), the free spectral range... 

Fig. 5.66. (a) Axial resonator modes within the spectral gain profile (b) temperature tuning of the gain maximum and (c) mode hops of a quasi-continuously tunable cw PbS Te diode laser in a helium cryostat. The points correspond to the transmission maxima of an external Ge etalon with a free spectral range of 1.955 GHz [5.112]... 

Fig. 5.80a,b. Spectral gain profiles of different laser dyes, illustrated by the output power of pulsed lasers (a) and cw dye lasers (b) (Lambda Physik and Spectra-Physics information sheets)... 

The tunability range depends on the slope of the repulsive potential and on the internuclear distances R and R2 of the classical turning points in the excited vibrational levels. The spectral gain profile is determined by the Franck-Condon factors for bound-free transitions. The corresponding intensity distribution I (jo) of the fluorescence from the upper vibrational levels shows a modulatory structure (see Fig. 2.14) reflecting the R dependence IV vib( ) of the vibrational wave function in these levels [5.197]. 

Single-mode operation could be achieved if the spectral gain profile is... 

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