Functional Crystals

Q-switched lasers are laser that uses either active or passive Q-switching technology so that it emits high-energy light pulses. Typical applications of this kind of laser are laser material processing (such as cutting, drilling, laser marking), pumping nonlinear frequency conversion devices, ranging and remote sensing.

Figure 1: Setup diagram of the active Q-switched laser.

Types of Q-switched laser

The most common type is an active Q-switched solid-body laser. Solid gain media are well suited for storing excitation energy for a sufficiently long period of time, and bulk lasers allow for large mode regions (thus allowing higher pulse energy and peak power) and shorter laser resonators (compared to fiber lasers, for example). The laser resonator contains an active Q switch, the light modulator, which in most cases is an acousto-optic Q switch. For emission wavelengths in the 1-μm spectral region, the most common pulsed lasers are based on neodymium-doped laser crystals, such as Nd:YAG, Nd:YVO4, or Nd:YLF, although ytterbium-doped laser gain media can also be used. A small active Q-switched solid-state laser can emit an average power of 100 MW in pulses of 10ns, with a repetition rate of 1kHz and a pulse energy of 100μJ. Then, the peak power is ≈9 kW. The highest pulse energy and shortest pulse duration are achieved at low pulse repetition rates (lower than the inverse upstate lifetime) at the cost of a reduction in the average output power. A slightly larger Nd:YAG laser with a 10W pump source (such as a diode rod) can achieve a pulse energy of several millijoules. Nd:YVO4 is particularly attractive for short pulse durations and high pulse repetition rates, or for operation at low pump power. Q-switched lasers with longer emission wavelengths are typically based on erbium-doped laser gain media, such as Er:YAG at 1.65 or 2.94 microns, or Tulium-doped crystals at ≈2 microns. Significantly greater pulse energy can be obtained from the amplifier system (MOPA). For a combination of high average power and medium pulse energy, fiber optic MOPA, also known as MOFAs, can be used.

Figure 2: Setup diagram of the passive Q-switched laser. The saturable absorber is a crystal inside the laser resonator (e.g. Cr:YAG).

Especially for low pulse repetition rates, lamp pumping is an economically advantageous option, as discharge lamps are much cheaper than laser diodes for a given peak power. However, for higher power, diode pumping becomes more attractive, also because the thermal effect in the laser crystal is greatly reduced.

A passive Q-switched laser contains a saturable absorber (passive Q-switch) instead of a modulator. For continuous pumping, a regular pulse sequence can be obtained in which the pulse repetition rate increases as the pump power increases. The timing of the pulse cannot usually be precisely controlled by external means. The most commonly used saturable absorber for 1-μm lasers is the Cr:YAG crystal.

Figure 3: Microchip laser, passive Q switching with SESAM. The left side of the laser crystal has a dielectric coating that acts as an output coupling mirror.

Passive Q-switched microchip lasers have a particularly compact setup. Such lasers typically emit pulses of energy between nanojoules and a few microjoules, with an average output power of tens of milliwatts and a repetition rate between several thousand Hertz and several megahertz. In general, the average output power of passive Q-switched lasers is more limited than that of active Q-switched lasers, because the saturable absorber dissipates some energy, so there is a limiting thermal effect. Note that saturable absorbers usually have some non-saturable loss, which tends to dissipate much more energy than is unavoidable in principle. In particular, some smaller Q-switched lasers, but there are also some lasers with longer resonators that contain an optical filter, such as a volume Bragg grating, operating on a single axial resonator mode. This results in a clean time shape and a small optical bandwidth, usually limited by the pulse duration. Other lasers oscillate over multiple resonator modes, which results in a mode jitter effect: the output optical power is modulated at a frequency that is an integer multiple of the resonator's round-trip frequency. Fiber lasers can also be Q-switched either actively or passively. However, the performance of all-fiber devices is quite limited, while Q-switched fiber lasers that contain bulk optical components such as acousto-optic Q-switches (see Figure 4) are less robust and still less powerful than bulk lasers. The relatively small mode area (even when using large mode area fibers) presents problems of fiber nonlinearity and laser-induced damage, which sets limits on the pulse energy, especially the peak power that can be achieved. It should also be noted that the usually very high laser gain in fiber lasers has an important effect on laser dynamics; In particular, it may lead to the formation of complex temporal substructures.

Figure 4: Setup of an active Q-switched fiber laser.

On the other hand, high-power fiber amplifiers are suitable for amplifying pulse train with high average power but moderate pulse energy. In such amplifiers, a certain degree of nonlinear pulse distortion is often acceptable for applications.

Q-switched lasers have a wide range of applications:

1. Laser material processing, such as laser cutting, laser drilling, laser marking, laser pattern, etc.

2. Laser rangefinder

3. LiDAR for 3D imaging

4. Laser-induced breakdown spectroscopy

5. Medical applications, such as dermatology and tattoo removal

6. Pump equipment for nonlinear frequency conversion, such as pulsed optical parametric oscillator

7. Fluorescence spectroscopy