What are Ytterbium-doped Lasers?

A ytterbium-doped laser is a type of solid-state laser that uses crystal/glass doped with ytterbium (0.2 % to 25 %) as a laser gain medium and the external laser light or flashlamp as an optical pumping source to increase the gain of the laser beam. Diode pumping is usually used. Ytterbium is a type of rare-earth ion and is suitable for high power laser applications and wide wavelength tuning. The ytterbium-doped lasers are ideal for precision cutting and scribing, micro-drilling, blind hole machining, welding, sintering/ 3D printing, heat treating, and scientific applications.

Commonly used materials in ytterbium-doped lasers and their output wavelength range are given below.

Table 1: Commonly used materials in ytterbium-doped lasers and their output wavelength range.

Construction of Ytterbium-doped Lasers

Like other lasers that use a solid state gain medium, Ytterbium-doped Lasers use a gain medium that is doped with Ytterbium. The output light from a laser diode is focused on the gain medium through the cylindrical lens (collimator). This cylindrical lens transforms the strong divergence light into more aligned in a specific direction with a uniform beam diameter.

Yb: YAG laser Scheme

The gain medium (lasing material) can be used in the form of a rod or thin disk shape. The gain medium is placed in between two optical coated mirrors. The lower wavelength range can be obtained by frequency doubling (near to green), or tripled (near to UV), or quadrupled (to deep-UV) by using the harmonic generators. It can operate in both CW and pulsed mode operation. In pulsed mode operation, it can emit laser pulse which has duration from femtosecond to nanosecond range (ultrafast laser).

Absorption wavelength of ytterbium-doped lasers

Absorption spectrum of Yb: YAG/glass lasers

The ytterbium-doped crystal/glass materials absorb maximum optical power at a particular wavelength. For example, the peak absorption wavelength range for Yb: YAG is 960 nm.

Hence, a simple GaAs/InGaAs laser diode or diode array with a wavelength range of about 950-980 nm can resonate with the Yb: YAG/Yb: glass absorption wavelength. Therefore a high optical efficiency (50%) is possible when compared to a flashlamp-pumped ytterbium-doped laser. This is because a flashlamp emits light with a broad wavelength and hence most of the light cannot be absorbed by the ytterbium-doped laser.

Operation of ytterbium-doped lasers

Energy levels of Yb³⁺ ions

Quasi-three-level systems

The ytterbium-doped laser is a quasi-three-level laser system, which means that the lower laser level is near the ground state. The pumping and amplification occur because of transitions between the sublevels of the ground state and the excited state. This quasi-three-level system has the characteristics between the three-level and four-level laser system. At room temperature, the ytterbium-doped laser (Yb: YAG) with output wavelength 1030 nm range exhibits the three-level behavior, whereas, at wavelengths beyond 1080 nm, it exhibits four-level behavior.

In three-level behavior, the thermal population on the lower laser level at room temperature leads to the reabsorption effect. Hence during three-level behavior, high pump intensity needs to achieve laser operation. In four-level behavior, there is no reabsorption problem, the more power efficiency is possible.

Advantages of ytterbium-doped lasers

1. Very low frictional heating as compared to neodymium-doped gain mediums (Nd: YAG).
2. High mechanical strength and high thermal conductivity.
3. Small Quantum defect (energy difference between the input photons and output photons) compared to neodymium-doped lasers. This reduces the thermal effects of high power lasers.
4. Gain bandwidth of the laser transition is fairly large when compared to the neodymium-doped lasers. It allows for wide wavelength tuning ranges.  
5. High upper-state lifetime (1-2 ms), which is suitable for Q switching.

Specification of Ytterbium-doped lasers

Technology: Solid-state laser. Mostly, DPSS technology.

Wavelength: Represents the wavelength of laser light emitted from a ytterbium-doped laser. These lasers are available in deep UV range to the IR range. The wavelength is represented in a nanometer (nm).

Tunable: Represents the wavelength tune-ability of the ytterbium-doped laser. Both the tuneable and non-tuneable ytterbium-doped lasers are available.

Fiber-coupled: The ytterbium-doped lasers are available both in fiber couple or non-fiber coupled choices.

Operation mode: Represents the operation mode. These lasers are available both in continuous wave (CW) and pulsed mode operation. The pulse laser is also called a Q-switched laser.

Laser color: Represents the laser colour. The ytterbium-doped lasers are available from deep UV to the IR spectrum. The tuneable ytterbium-doped lasers produce more than one laser colour by tuning.

Mode: Represents the mode of operation of laser light. Both the single-mode and multi-mode ytterbi