Q-switching
A laser operating regime in which the cavity loss is rapidly switched from high (low Q) to low (high Q), producing a giant pulse of stored energy. The technique that produces nanosecond pulses with high peak power.
Q-switching produces a single high-energy nanosecond pulse from a laser by:
- Loss-on (low Q): With high intracavity loss, lasing is suppressed. The gain medium is pumped continuously, accumulating population inversion that builds far above the steady-state threshold value.
- Loss-off (high Q): The intracavity loss is suddenly removed. The cavity Q jumps high, the built-up inversion produces enormous round-trip gain, and the laser rapidly extracts the stored energy as a single short pulse.
Pulse parameters for typical Q-switched solid-state lasers:
| Parameter | Range |
|---|---|
| Pulse duration | 1 – 100 ns (set by cavity round-trip time × number of round-trips for energy extraction) |
| Peak power | kW – MW |
| Pulse energy | μJ – J |
| Repetition rate | single-shot to 100 kHz |
| Cavity round-trip time | 0.1 – 10 ns |
| Round-trips for full extraction | 5 – 50 |
Q-switching mechanisms.
| Method | Implementation |
|---|---|
| Active — acousto-optic Q-switch | An intracavity AOM produces variable diffraction loss; loss-off when RF drive removed |
| Active — electro-optic Q-switch | A Pockels cell rotates polarization between crossed polarizers; loss-off when field applied/removed depending on geometry |
| Active — rotating prism or mirror | Mechanical alignment dependent on rotation angle; loss-off when prism reaches alignment |
| Passive — saturable absorber | A SAM bleaches at sufficient pump-induced inversion, automatically self-Q-switching |
| Cavity-dumping | A coupling element (EO switch, AOM) suddenly extracts all built-up cavity energy through an output port |
Comparison with mode locking.
| Aspect | Q-switching | Mode locking |
|---|---|---|
| Pulse duration | ns | fs – ps |
| Peak power | kW – MW (single pulse) | kW (per pulse in train) |
| Rep rate | single to kHz | 10 MHz – 100 GHz |
| Pulse energy per pulse | J – J | nJ |
| Mechanism | Energy accumulation + release | Phase locking of modes |
The two techniques are not mutually exclusive — Q-switched mode-locking combines Q-switching with active or passive mode locking to produce a train of mode-locked pulses gated by the Q-switching envelope.
Applications.
| Use case | Why Q-switching |
|---|---|
| Laser material processing | High pulse energy for vaporization and ablation |
| Range-finding and LIDAR | High peak power for long-distance ranging |
| Medical lasers (tattoo removal, dermatology) | High peak power for selective photothermolysis |
| Nonlinear frequency conversion (SHG, OPO) | High peak intensity required for efficient conversion |
| Plasma generation | Sufficient peak intensity for above-threshold ionization |
For a Q-switched Nd:YAG laser (1064 nm, common research/industrial workhorse): 10 ns pulses with 1 mJ pulse energy correspond to 100 kW peak power, easily sufficient to ablate metals or drive SHG conversion to 532 nm with 30% efficiency.