Passively Laser is the laser with Passive Q switching.

passively mode locked laser

passively q-switched laser

passively mode locked fiber laser

passively mode-locked laser with an ultra-narrow spectral width

Passive Q switching is an alternative technique, where the active modulator is replaced with a saturable absorber (a passive Q switch). In a Nd:YAG laser, for example, this could be a Cr4+:YAG crystal. There are other saturable absorber crystals for other wavelengths and semiconductor saturable absorber mirrors (SESAMs) for various operation wavelengths.

In a passively Q-switched laser, instead of an electrically controlled modulator, a saturable absorber is used. In its unsaturated state, it introduces a high optical loss. The laser gain must overcome that loss before lasing can start. When the laser radiation becomes stronger, it eventually saturates the losses – i.e., it reduces them to a much lower value. From this point on, the laser power rises rapidly until the gain is also saturated.

The saturable absorber initially introduces a high optical loss. Once the gain reaches this loss level and the pulse begins to build up, the absorber is saturated, that is, its loss is reduced, which further accelerates the pulse buildup. This method is simple and cost effective (eliminating the modulator and its electronics) and is suitable for very high pulse repetition rates, but it typically leads to lower pulse energies and does not allow external triggering of the pulses.

In any case, the laser dynamics and the energy balance impose certain restrictions on the pulse parameters. For example, a higher pulse repetition rate will normally reduce the pulse energy and increase the pulse duration.