Mode quality

The choice of laser system is dictated by the considerations outlined above. High-energy pulses of two different frequencies are needed, and good temporal synchronization between the pulses is essential. A pulse width of 0.3-1.0 ps is adequate to resolve most dephasing processes. Good mode quality is important to minimize self-focusing. 

An alternate approach used by the Miller group is to pulse amplify a CW dye laser. This approach degrades the laser resolution from about 1 MHz to about 200 MHz but results in a high-power pulsed laser beam with excellent mode quality. This high peak power and high spectral resolution results in excellent spectra with an improved S/N ratio. The only serious drawback of this scheme is the increased complexity and cost of the laser systems. 

However, the high peak power at higher pulse energies can make it more difficult to maintain good mode quality as the laser beam propagates. 

A typical experimental set-up for SHG is shown in Fig. 5.2. For excitation nowadays one usually uses either a mode locked Ti Sapphire laser, that delivers 100 fs pulses at a rate of about 80 MHz, or a pico or nanosecond Nd YAG laser, often in combination with an optical parametric oscillator (OPO). Whereas the first type of system gives a superior mode quality and stability but limited tunability, the second system is more convenient for SHG spectroscopy. 

For some applications, it is necessary to produce RGH lasers at a reasonable output power level with low spatial divergence and narrow bandwidth. To fulfill these requirements, use of either a master oscillator/power amplifier system or an injection-locked resonator is the most convenient method. Figure 26 shows a schematic of an injection-locked resonator. In this scheme the slave oscillator is an unstable resonator that can control the output energy and spatial mode quality. Input pulse can be made by a master oscillator typically consisting of a cavity with a line-narrowing function. An input power level on the order... 

Advantages of the SFR laser are its high output power (about IW for CW operation), its relatively large tuning range and its good mode quality at single-mode operation. 

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