Lasing media

Photons of energy hcv are generated initially in the cavity through spontaneous emission. Those that strike the cavity mirrors at 90° are retained within the cavity causing the photon flux to reach a level which is sufflciently high to cause stimulated emission to occur, and the active medium is said to lase. 

The lasing medium in the titanium-sapphire laser is crystalline sapphire (AI2O3) with about 0.1 per cent by weight of Ti203. The titanium is present as Ti and it is between energy levels of this ion that lasing occurs. 

In an OFRR, WGMs reside in both core and wall. Therefore, only a fraction of the WGM interacts with the gain medium. The lasing behavior of this partially interacting system has been analyzed by Moon et al.10 For a given dye concentration, the minimum fraction of the excited molecules, y(X), is given by ... 

As in any other laser, the lasing threshold in a semiconductor laser diode is reached when the gain of the active material overcomes the losses of the laser cavity. These losses have two basic origins, namely the finite reflectivity of the mirrors mid distributed losses due to scattering and parasitic absorption in the active medium. In contrast to other lasers, the mirrors in typical semiconductor lasers are simply formed by cleaved or etched crystal facets. Therefore, the reflectivity (Fresnel reflectivity) is rather low, about 20% in the case of the nitrides. 

In a Q-switched laser, the population inversion, usually produced in the same way as CW operation, builds up by making the cavity quality factor Q unfavorable for lasing. Then, when the pump energy stored in the laser medium is at the desired level, the Q is suddenly adjusted (electro- or acousto-optically) to release the pulse. This results in high peak powers. 

Figure 9.21. FIRLMR spectrum of CH [53]. The spectra were recorded using a laser wavelength of 124.4 pm. The lasing medium was CH2DOH, pumped by the 10P(34) infrared line of carbon dioxide. The rotational transition involved is N = 3 2, J = 5/2 - 5/2, — <—h...

A third and important requirement for lasing action is to have sufficient optical gain irrespective of the active medium. The optical gain is achieved by multiple passes of the photons through the population-inverted medium. Multiple passes of photons is achieved by placing the active medium in an optical resonator. The optical resonator consists of two or more mirrors. A typical optical arrangement for a laser system is shown in Fig. 12.13. 

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