8. Longitudinal Modes

Spectral composition (Longitudinal Laser Modes) of light and their temporal shape are interrelated thus playing a significant role in laser-induced damage phenomena. Depending on lasers optical scheme and pumping approach (flash lamps, light emitting diodes or even pump lasers) different temporal shapes of light intensity can be realized. Most common techniques used to generate lasing light are known as:

  • CW – Continuous Wave lasing – light with constant in time intensity;
  • Pulsed, free running lasing – generates light pulses of microsecond (~10-6 s) durations;
  • Q-Switching technique is mostly used to generate pulses of nanosecond (~10-9 s) durations;
  • Mode-locking – most common technique to generate ultra-short pulses of femtosecond (~10-15 s) and short – with picosecond (~10-12 s) durations.

For all pulsed techniques different spectral bandwidths of gain medium (Nd:YAG, Ti:Sapphire, Yb:KGW) and resonator (or cavity) lengths are used. In case of extremely narrow bandwidths and very short cavity lengths single longitudinal mode (SLM) is excited which is typically very smooth in time Fig. 1 (left). In some cases SLM radiation could be generated by using external laser oscillators (CW mode), injected into Q-Switched laser and then amplified. Other wavelengths are then suppressed because of the fact that strong injected light takes all the energy form gain medium. However in longer resonators (without injection seeding) many laser modes are generated from noise and amplified. These are competing (so called mode beating) for stored energy of active medium and thus are not repeatable from shot to shot. Each longitudinal mode has slightly different wavelength and thus overlapped in time interferes with other modes by producing strongly modulated pattern over all generated pulse – Fig. 1 (right). Such pulses are consisting of many periodic spikes. These pulses interact differently than SLM pulses when exposed to material even if the effective pulse withs are the same. By synchronizing all longitudinal modes in time very short single pulses (spikes) can be generated. In this way as a result of temporal interference femtosecond pulses are produced. The rule of thumb – the more broad spectrum – the shorter pulse duration.

modes_pic

In practice, single- and all types of multi-mode pulses are used. Therefore in order to have most accurate characterization of laser-induced damage threshold is necessary to test components of interest by considering similar temporal operation conditions as close to real application as possible. Otherwise conclusions dawn about absolute value of optical resistance could be not accurate.