Q-switch or a variable attenuator in the laser cavity is a technique for obtaining powerful laser pulses. As name suggests, the idea is introducing variable losses in the laser cavity. The changes in losses result in the different quality factor (Q-factor) of the laser cavity, hence the name Q-switch. Any oscillator, including a laser is an amplifier with positive feedback (meaning that part of the output signal is returned to the input of the amp. In the laser, the pumped active medium acts as an amplifier for light, whilst the cavity mirrors provide the positive feedback, returning part of the produced light into the active medium. When cavity losses are on, the light that has left the active medium cannot come back and the positive feedback required for lasing does not occur. Therefore, even hard pumping of active medium does not result in lasing. This allows creating very strong population inversion, i.e. promoting almost all the atoms of the active medium into their metastable excited states. When the losses are switched off, the Q-factor of the cavity suddenly increases (the feedback appears), and the laser starts lasing. Because the active medium has stored a large amount of energy, the gain of the amplifier is very high and the intensity of laser radiation grows extremely fast. It also disappears very fast, and for the same reason: high intensity radiation produced quickly ‘eats away’ all the population inversion. This way, the so-called giant pulses are produced. Typically, Q-switched lasers produced by different companies us Nd:YAG or Nd:YLF as their active media; they produce pulses with the duration around 10 ns, the energies of which can be in excess of 1 J.
Technically, Q-switching is usually performed using one of the three major methods. The variable loss component of the cavity can be
- Saturable absorber
- Electro-optic modulator
- Acousto-optic modulator.
A saturable absorber is an optical medium that absorbs laser radiation. When most of its atoms are in their ground state, the medium absorbs the laser light and the losses in the cavity are high. However, when most of the absorber atoms become excited, the medium becomes transparent and cavity losses suddenly decrease. Obviously, such modulator is passive, i.e. no external control is necessary (Fig. 5A).
One of the most common active Q-switches is an electrooptic Pockels cell. It is a crystal where high voltage (usually several kV) induces birefringence and makes it act as a half-lambda phase plate. In other words, the phase difference between an ordinary and extraordinary waves passing the crystal becomes equal to p. When linearly polarized light impinges on such crystal at with its polarization axis at 45° to the ordinary axis, its polarization plane gets rotated by 90°. Along with a crystal, a polarizer is inserted into the cavity. When the high voltage is off, the polarization of light generated in the cavity is such that the polarizer blocks it – the losses of the cavity are high and no lasing can occur. When the gain medium accumulates enough population inversion, half-lambda voltage is switched on, the light polarization is rotated and the beam can pass through the polarizer without losses. This results in a giant light pulse, which sweeps down the accumulated population inversion. Pockels cells (Fig. 3B) are used in lasers with different repetition rates – from several Hz to 1 MHz. They are called active because they are actively controlled by an external high voltage source.
Another type of an active Q-switch is an acousto-optic modulator. It consists of a piece of transparent material (e.g. glass or quartz), where a standing acoustic (ultrasound) wave is produced using piezoelectric effect. To achieve this, the transparent material is attached to a piezoelectric transducer – a slab of material with piezoelectric properties (e.g. BaTiO3). Standing acoustic wave produces periodic modulation of refractive index in the material – a phase grating. Laser radiation is diffracted by this grating, changes its direction of propagation and leaves the cavity. When RF field is switched off, the grating disappears and all the light is transmitted by the transparent material. The lasing starts and giant laser pulse is produced (Fig. 3C). Typically, acoust-optic Q-switches produce pulses of hundreds of nanoseconds.
Q-switched lasers are widespread, and wider description is available in the literature . They are widely used for the investigation of nano-, microsecond and longer processes, flash-photolysis spectroscopy. They are also useful as pump (energy) sources for other (e.g. femtosecond) lasers.