The titanium-sapphire laser

Despite the fact that the first laser to be produced (the ruby laser. Section 9.2.1) has the remarkable property of having all its power concentrated into one or two wavelengths, a property possessed by most lasers, it was soon realized that the inability to change these wavelengths appreciably, that is to tune the laser, is a serious drawback which limits the range of possible applications. 

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. 

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. 

A further advantage, compared with the alexandrite laser, apart from a wider tuning range, is that it can operate in the CW as well as in the pulsed mode. In the CW mode the Ti -sapphire laser may be pumped by a CW argon ion laser (see Section 9.2.6) and is capable of producing an output power of 5 W. In the pulsed mode pumping is usually achieved by a pulsed Nd YAG laser (see Section 9.2.3) and a pulse energy of 100 mJ may be achieved. 

In 1991 a remarkable discovery was made, accidentally, with a Tp -sapphire laser pumped with an Ar+ laser. Whereas we would expect this to result in CW laser action, when a sharp jolt was given to the table supporting the laser, mode locking (Section 9.1.5) occurred. This is known as self-locking of modes, and we shall not discuss further the reasons for this and how it can be controlled. One very important property of the resulting pulses is that they are very short. Pulse widths of a few tens of femtoseconds can be produced routinely and with high pulse-to-pulse stability. Further modification to the laser can 

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The development of lasers has continued in the past few years and 1 have included discussions of two more in this edition. These are the alexandrite and titanium-sapphire lasers. Both are solid state and, unusually, tunable over quite wide wavelength ranges. The titanium-sapphire laser is probably the most promising for general use because of its wider range of tunability and the fact that it can be operated in a CW or pulsed mode. 

Fig. 1. Experimental layout of the pump-probe electron diffraction experiment. 2a> = 2 harmonic, and 3oj = 3ri harmonic of the Titanium-Sapphire laser operating at 800 nm.

The information provided by the highly differential cross sections that have been obtained with the help of the reaction-microscope technique [3-5] has largely terminated the debate about the physical mechanism responsible for NDSI For the situation explored in most experiments, that is, high-intensity low-frequency lasers typified by the titanium-sapphire laser (A 800 nm) at 1014 to 1015 Wcm 2, consensus has developed that NSDI is caused by the rescattering mechanism an electron that is freed by tunneling... 

The Ti " " ion (3d ) gives a broad-band emission in the near infrared due to the T2 transition. The titanium-sapphire laser is based on this emission. 

The wavelength region below 720 nm cannot be accessed directly by the titanium-sapphire laser but is covered by another laser system, e.g., the optical parametric oscillator (OPO). The OPO system consists of a birefringent crystal, e.g., BBO (BaB204), placed within an optical cavity irradiated by an intense pump beam such as a Nd YAG laser that is operated at its third harmonic (355 nm). Through nonlinear interaction the pump beam is split into an idler beam and a signal beam, which generate a... 

The titanium-sapphire laser is perhaps the ultimate near-infrared laboratory laser for Raman spectroscopy. It is a continuous wave laser that can deliver thousands of milliwatts of laser light and is continuously tunable from below 700 to above 1000 nm. It provides a narrow bandwidth with a high-quality spatial mode. The spontaneous emission from the titanium-sapphire crystal must be filtered from the laser beam. 

FIG. 6. Normalized response functions [see Eqs. (74) and (75) and associated discussion] obtained using the fast amplifier (O) with the titanium-sapphire laser k = 798 nm fwhm = 100 fs) and using the old amplifier ( ) with the NdYAG laser (k = 1060 nm fwhm = 8 ns). Solid lines are optimized fits generated using Eq. (76) with parameter values given in text. 

Here, the principal features and characteristics of the ultrafast laser systems used are briefly summarized. Besides the titanium sapphire laser which acts as the workhorse in nearly all of the discussed experiments, a synchronously pumped dye laser is employed to study the ultrafast dynamics of Nas on a picosecond timescale (see Sect. 3.2.2). For measurements with femtosecond time resolution and wavelengths located between 600 and 625 nm a synchronously titanium sapphire pumped optical parametric oscillator followed by frequency doubling is used. To investigate the Nas C state, two mode-locked titanium sapphire lasers have been synchronized. In all cases the essential parameter of the generated laser pulses, the pulse width, has to be determined. This problem is solved by an autocorrelation technique. Hence, the principles of an autocorrelator are briefly described at the end of this section. 

Mode-Locked Titanium Sapphire Laser. In the late 1980s a new highly tunable laser material, Ti +-doped sapphire (Ti Al20s), set out to conquer the world of ultrafast pulses. It looks as if the titanium sapphire laser is set to replace ultrafast dye lasers in almost all applications. The extraordinary... 

The mode-locking mechanism for the titanium sapphire laser can be described by the optical Kerr effect (OKE). Owing to the nonresonant bound-electron nonlinearity of the gain medium, the OKE can be exploited in the laser cavity to simulate a saturable absorber with a virtually instantaneous absorber recovery. However, the picture of saturable absorber is not strictly correct since there is no saturation or depletion of a population across the transition. The OKE leads rather to a nonlinear intensity-dependent refractive index in the optical elements of the laser cavity, given by... 

Fig. 2.6. Femtosecond configuration of the titanium sapphire laser cavity. AOM acusto-optic modulator Pi, P2 pump mirrors Mi to Mio cavity mirrors PRi to PR4 prisms, OC output coupler (by courtesy of Spectra Physics)...

The details of the femtosecond RIKES setups used for the studies described in this chapter have been reported elsewhere (Wiewior et al, 2002 Shirota, 2005 Shirota et al., 2009). In short, the light sources for the setups were titanium sapphire lasers pumped by approximately 3.5 W of 532-nm light from a neodymium vanadate laser (Spectra Physics). The center wavelength of the titanium sapphire lasers was 800-810 ran, with a full width at half maximum value of ca. 60 or 75 nm and a repetition frequency of ca. 85 MHz. The output... 

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