Q-Switched lasers or Quality Switched lasers are a type of high-intensity pulsed lasers. In these lasers, the Q-factor of the laser cavity is switched from a high value to a low value and vice versa to generate high-intensity pulses. The Q-factor of a laser cavity is a dimensionless quantity that provides frequency selectiveness and measures the net energy stored in it. A higher Q-factor cavity is more wavelength selective because the spectral width is narrower and thus it operates in a specific wavelength, which results in minimal losses. Similarly, a lower Q-factor cavity is less wavelength selective and shows high losses due to broader spectral width. The process of switching the Q-factor of a laser cavity between low Q and high Q values is known as Q-Switching. These lasers usually generate high-intensity short optical pulses with nanosecond pulse duration known as giant pulses. This technique is mainly used by solid-state lasers.
When a laser is turned on, pumping starts, and the electrons in the ground state get accumulated in the upper laser level. Eventually, the population density of the upper laser level grows far above that of the lower laser level. i.e., population inversion is achieved. At the same time, the laser oscillation building up in the optical cavity draws energy from the gain medium through the stimulated emission process increasing the optical gain. This in turn reduces the population density of the upper laser level which forces the population inversion to reach a steady state value, where the optical gain in the cavity equals and the losses in the same.
For most types of gain media, the time required to reach this steady state is higher than the upper laser level lifetime. But for most Solid-State gain media, the upper laser level lifetime is longer than the time required to reach the steady state. A longer lifetime means that more electrons can be accumulated in the upper level and hence a higher population difference is attainable between upper and lower laser levels. This indicates the possibility of a larger gain in the solid-state lasers.
A solution to this problem is Q-switching; i.e., dropping the Q-factor of the cavity to a lower value during the pumping process and suddenly increasing the Q-factor as the upper-level population is sufficiently built up.
A laser system needs a resonant cavity with a high Q factor to effectively produce a laser output. Such a cavity allows its optical gain to reach the threshold gain, the gain required to overcome the losses in the cavity. By temporarily removing one of the mirrors of the cavity, a low Q cavity can be obtained. The laser system now acts as an amplifier with no feedback and hence doesn’t support laser oscillation. This provides an excellent opportunity to pump the upper laser level such that a larger population inversion is achieved. By re-introducing the mirror back to the cavity, the gain steadily increases and overcomes the loss. At the same time, the net cavity loss is reduced and switches the Q-factor to a high value. The low loss and the high gain in the cavity allow the laser pulse to grow rapidly by depleting the upper laser level population followed by a decrease in the gain resulting in the formation of a giant pulse as shown in the figure below.
Q-Switching can be implemented using:
Rotating mirrors were used first to perform Q-Switching. The optical cavity is formed using a fixed output mirror and a hexagonal-shaped mirror assembly that is mounted on a rotor, which functions as the rear mirror. It is mounted such that while rotating, the mirror forms the laser cavity for a small duration.
Initially, the rotating mirror doesn’t face the fixed mirror and hence can’t form a resonant cavity. This causes Q-factor to have a low value. The gain medium is pumped to have a higher population inversion during this period. As the mirror rotates, it aligns parallel to the fixed mirror forming a high Q-factor resonant cavity. The cavity loss is reduced and it supports the rapid growth of the laser beam which results in the generation of a giant pulse.
The electro-optic effect can be utilized to implement Q-Switching. For that, an electro-optic crystal and a polarizing element (polarizer or gain medium at Brewster angle) are introduced into the laser cavity. Let the laser beam coming through the polarizer be polarized at 0º. When an electric field is applied, the electro-optic crystal introduces a 45º rotation to the laser beam and moves towards the mirror. The beam after reflection from the mirror passes through the crystal again and undergoes another 45º rotation. Now it reaches the polarizer with a 90º polarization. This beam will not be allowed to pass through it resulting in a high loss or low Q cavity. Once the gain medium is pumped to a sufficiently higher population inversion state, the applied electric field is turned off. The polarization of the laser beam will remain the same throughout the cavity. This allows the laser beam to pass through the polarizer. So, it becomes a high Q cavity and supports laser oscillation which generates the giant pulse. Both Pockels cell and Kerr cell can be used to implement this technique.
An acousto-optic crystal (like Quartz crystal) with a piezoelectric transducer attached to it is placed in the laser cavity. On applying an RF signal, strong acoustic waves start to oscillate in the crystal. Certain small regions of the crystal will experience compression while the alternating adjacent regions experience rarefactions. These small regions of alternating compression and rarefaction will experience variation in material stress and will show a corresponding change in the refractive index. Hence, the crystal acts as a diffraction grating. The developing laser beam will be deflected out of the laser cavity due to diffraction, introducing a low Q cavity. The gain medium is pumped to attain a higher population inversion. Then, the RF signal is turned off. This allows the laser beam to propagate undeflected as the acousto-optic crystal becomes transparent and hence forming a high Q cavity that allows a giant pulse to form. This enables lasers to utilize Q-Switching.
A saturable absorber is an optical component that has a certain absorption loss for light. Such materials have low absorption for higher light intensities, i.e. for low laser intensity, a saturable absorber act as a highly absorbing material and at higher laser intensity; it becomes less absorbing for the same wavelength. It is placed near the output mirror in the laser cavity. Initially, the intensity of the gain medium emission is very small. So, this emission is absorbed by the saturable absorber due to a very high absorption coefficient resulting in a low Q cavity. The continuous pumping increases the population density of the upper laser level which in turn increases the intensity of the gain medium emission. Eventually, it reaches a high intensity such that the ground state of the saturable absorber is bleached out and hence can’t absorb the incoming photons anymore, i.e., absorption gets saturated. Hence the saturable absorber becomes transparent resulting in a low-loss cavity with a high Q value. The laser beam oscillates back and forth in the cavity performing stimulated emission, which quickly draws the energy from the gain medium and forms a giant pulse. This technique is a passive technique as no external signal is needed to switch between high Q and low Q states.
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