What are Q-switched Lasers?
Q-Switched lasers are pulse lasers that use Q-switching techniques to generate short duration (usually in nanoseconds) pulses. When compared to the mode-locking technique (another technique for generating laser pulses), the Q-switching technique produces higher pulse energy with a lower pulse duration, and lower pulse repetition rates. Q-switched lasers usually use neodymium-doped materials (Nd: YAG, Nd: YLF, or Nd: YVO4) as the active gain medium. They can also use an erbium-doped gain media Er: YAG that enables lasers to operates at a wavelength 1.65 or 2.94 μm wavelength or thulium-doped crystals that operate at a wavelength of 2 μm. The Q-switched laser can be pumped by either using continuous or pulse form and are ideal for applications that demand high laser intensities in nanosecond pulses, such as metal cutting, and pulsed holography.
How does a Q-Switched Laser Operate?
The Q-factor of an optical resonator is defined as the ratio of energy stored in the inside standing wave to the wasted energy from the wave during a round trip between the laser mirrors. High Q-factor means that high levels of energy can be stored in a cavity, whereas a low Q-factor means that energy is emitted rapidly from the cavity.
Schematic set up of a Q-switched laser
Schematic set up of an active Q-switched fiber laser
The Q-switching technique is realized by putting some type of variable attenuator inside the optical resonator of the laser. The attenuator can absorb or lower the power level without disturbing signals. Here, this variable attenuator acts as a Q-switch.
When the Q-switched laser is ON, initially, the attenuator functioning as the light absorber (i.e with a low Q) and absorbs the light coming out from the gain medium (i.e., the light does not hit the opposite mirror). Hence, there is no light returned back to gain medium, and lasing cannot begin. This reduces the Q-factor of the optical cavity.
Now, s population inversion increases due to the pumping process of the laser, energy inside the cavity increases, and after some time, it attains a saturation point. Some energy inside the optical cavity is lost due to spontaneous emission and other processes. At this point, the Q-switch device is quickly changed from the low Q to High Q, allowing the laser light to reflect between two optical mirrors and ensuring the lasing action to begin (stimulated emission). Because the active medium already has high energy (high population inversion), it causes the high-intensity laser light to build up quickly and reduce storage energy quickly. This whole process results in a short duration. high peak power laser pulse as the output.
Two main types of Q-switching are used. They are active Q-switching and passive Q-switching. In the case of active Q-switching, the Q switching device (variable attenuator) is a mechanical device, such as a shutter, chopper wheel, or spinning mirror/prism placed inside the cavity. In passive switching, the Q-switch is a saturable absorber.
Specifications of Q-switched Lasers
Technology: Q-switched laser
Wavelength: Represents the wavelength of laser light emitted from a Q-switched laser. These lasers are available in deep UV range (300 nm to 200 nm) to the IR range. The wavelength is represented in a nanometer (nm).
Tunable: Represents the wavelength tune-ability of the Q-switched laser. Both the tuneable and non-tuneable Q-switched lasers are available.
Fiber-coupled: The Q-switched lasers are available both in fiber couple or non-fiber coupled choices.
Operation mode: Pulsed laser
Laser color: Represents the laser colour. The Q-swiched lasers are available from deep UV to the IR spectrum.
Mode: Represents the mode of operation of laser light. Both the single-mode and multi-mode lasers are available.
Gain medium type: Represents the host material of the Q-switched laser. It can be a Crystal/glass/fiber.
Laser gain medium: Represents the type of material used as the laser gain medium. Usually, Nd: YAG/ Nd: YLF/ Nd: YVO4/ Er: YAG/fiber/thulium-doped crystal is used as the laser gain medium. Mostly, the gain medium Nd: YAG is used.
Power: Represents the power output of the Q-switched laser. It can have a range of milliwatts to kilowatts range.
Pulse width: Represents the pulse duration of the Q-switched laser working in a pulsed mode of operation. Mostly, the duration of pulse range is available in a nanosecond (10-9 s) range.
Pulse energy: Represents the pulse energy of the Q-switched lasers. Usually, it is in the range from µJ to J (joule) range.
Ultrafast laser: The Q-switched laser with pulse width is in a nanosecond (10-9 s) range is called an ultrafast laser.
Repetition rate: Represents the number of emitted pulses per second from a Q-switched laser. It is represented in Hz. For example, the repetition rate of 200 Hz means that 200 pulses per second emitted from the pulse laser.
Beam divergence: Represents the divergence of the laser. Usually, it is represented in milliradian (mrad).