Tunable Solid-State Lasers

Besides their applications as tunable light sources, diode lasers are more and more used as pump lasers for tunable solid-state lasers and optical parametric amplifiers. Monolithic diode laser arrays can now deliver up to lOOWcw pump powers [5.124]. 

Vibronic solid-state laser materials are, e.g., alexandrite (BeAl204 with Cr ions) titanium-sapphire (Al203 Ti ) fluoride crystals doped with transition metal ions (e.g., MgF2 Co++ or CsCaF3 V ) [5.109, 5.126-5.129]. 

The tuning range of vibronic solid-state lasers can be widely varied by a proper choice of the implanted ions and by selecting different hosts. This is illustrated in Fig. 5.73a, which shows the spectral ranges of laser-excited fluorescence of the same Cr ion in different host materials [5.128] and different metal ions in a MgF2 crystal. 

A particularly efficient cw vibronic laser is the emerald laser (BeaAliSiaOigtCr ). When pumped by a 3.6-W krypton laser at Xp = 641 nm, it reaches an output power of up to 1.6 W and can be tuned between 720 and 842nm [5.131]. The slope efficiency dPout/dPin reaches 64% The erbium YAG laser, tunable around X = 2.8 p.m, has found a wide application range in medical physics. 

The different vibronic solid-state lasers cover the red and near-infrared spectral range from 0.65 to 2.5 jxm (Fig. 5.75). Most of them can run at room temperature in a pulsed mode, some of them also run in cw operation. 

Laser Composition Tuning range [nm] Operation temperature [K] Pump 

Certain solid-state materials with transition-element or rare-earth ions as impurities in a host matrix have rather broad gain curves and can thus be tuned over a certain range. This is the case for the Nd Glass laser, which can be tuned in the region 1.0—1.1 pm. The alexandrite (Cr BeAl204) and emerald (Cr Be3Al2Si6Ci8) lasers that bear a close resemblance to ruby lasers 

The absorption and emission spectra of crystalline or amorphous solids can be varied within wide spectral ranges by doping them with atomic or molecular ions [5.121]. The strong interaction of these ions with the host lattice causes broadenings and shifts of the ionic energy levels. The absorption spectrum shown in Fig.5.72b for the example of alexandrit, depends on the polarization direction of the pump light. Optical pumping of excited states 

The future importance of these lasers is derived from the fact that many of them may be pumped by diode-laser arrays. This has already been demonstrated for Nd YAG and alexandrite lasers where very high total energy conversion efficiency were achieved. For the diode-laser-pumped Nd YAG laser values of r = 0.2 for the ratio of laser output power to electrical input power have been reported (20% plug-in efficiency ) [5.125]. 

We have previously mentioned some laser systems with which a certain degree of tunability is possible (dyes and semiconductor lasers). However, during the past two decades a new class of tunable lasers has been developed tunable solid state lasers. They have definitively replaced dye lasers for some spectral regions, offering a higher 

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. 

CoiMgFj Alexandrite Emerald Cr GSGG CnSrAIFj Cr ZnWO ViMgFj 

Chapter 5 will show in more detail how the spectral width of optical transitions of active centers (particnlarly for transition metal ions) is affected by lattice vibrations. For the purpose of this section, we will just mention that these transitions are associated with the outer electrons of the active center (the 3d valence electrons), which show strong interactions with the phonons of the matrix in which they are embedded. As a result, the optical transitions, and particularly the emission lines, are strongly modulated by lattice vibrations. 

Intracavity frequency doubling of these lasers (Chap. 6) covers the visible and near-ultraviolet range [429]. Although dye lasers are still the most important tunable lasers in the visible range, these compact and handy solid-state devices present attractive alternatives and have started to replace dye lasers for many applications. 

For more details about tunable solid-state lasers and their pumping by high-power diode lasers, the reader is referred to [405, 430-433]. 

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Historically, the first type of laser to be tunable over an appreciable wavelength range was the dye laser, to be described in Section 9.2.10. The alexandrite laser (Section 9.2.1), a tunable solid state laser, was first demonstrated in 1978 and then, in 1982, the titanium-sapphire laser. This is also a solid state laser but tunable over a larger wavelength range, 670-1100 nm, than the alexandrite laser, which has a range of 720-800 nm. 

Figure 2.18 The spectral range covered by different tunable solid state lasers based on transition metal ions.

Figure 2.18 shows the range covered by different tunable solid state laser systems based on transition metal ions. As observed, a good variety of matrices have shown tunable laser action on the basis of Cr + as an active ion. The fundamental aspects determining the tunability of those Cr + based systems will be the subject of Section 6.4 in Chapter 6. 

Let us now devote our attention to one of the most popular lasers in the field of optical spectroscopy the Ti-sapphire laser. As shown in Figure 2.18, the spectral range covered by this laser is the largest among the various tunable solid state lasers from 675 nm up to 1100 nm. In this laser, the active medium is formed by optically active Ti + ions in the AI2O3 crystal host. 

The incorporation of Cr" + ions in crystals is presently an active research subject, due to the possibility of realizing new broadly tunable solid state lasers in the infrared, which will operate at room temperature. Moreover, the spectroscopic properties of this ion are particularly useful in the development of saturable absorbers for Q-switching passive devices. At the present time, Cr + YAG is the most common material employed as a passive Q-switch in Nd YAG lasers. This is because the ions provide an adequate absorption cross section at the Nd + laser wavelength (1.06 /um), together with the good chemical, thermal, and mechanical properties of YAG crystals, which are required for stable operation. 

Trivalent chromium ions have been inserted into a wide variety of host materials, and play a central role in the development of tunable solid-state lasers [1-3]. 

A. B. Budgor and A. Pinto, Tunable Solid State Lasers , Springer, BerUn, 1985. 

Emerald (Cr3+ Be3Al2(Si03)6, chromium-doped beryllium aluminium silicate or beryl) is a well known gem, and its beautiful green color has been attracted people for a long time. Nowadays, emerald crystal is also known as a tunable solid-state laser material, and its optical properties have been smdied (1-10). 

S. Furutni, N. Tamaoki, Glass-forming cholesteric liquid crystal oligomers for new tunable solid-state laser. Adv. Mater. 22, 886-891 (2010)... 

Table5.3. Characteristic data of some tunable solid-state lasers...

For sub-Doppler spectroscopy, single-mode cw dye lasers represent the most important laser types besides cw tunable solid-state lasers. Great efforts have therefore been undertaken in many laboratories to increase the output power, tuning range, and frequency stability. Various resonator configurations, pump geometries, and designs of the dye flow system have been successfully tried... 

Jorgensen/C.K. In OSA Proceedinqs on Tunable Solid State Lasers Shand/M.L. Jenssen,H.P./Eds. Optical Society of America Washington DC/ 1989/ Vol. 5/Pp 252-257. 

In the last few years sol-gel glasses have been applied in many novel inventions (2, 26) such as tunable solid-state lasers (containing photochemical resistant organic colorants with very high luminescence yields (27, 28)) nonlinear optical effects due to... 

Although tunable solid-state lasers and optical parametric oscillators are more and more competitive, dye lasers in their various modifications in the visible and UV... 

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