This article mainly introduces the characteristics, composition, main elements of laser crystals, as well as common crystal materials and their applications and advantages.

Laser crystal, which can convert the energy provided by the outside world into a highly parallel and monochromatic laser that is spatially and temporally coherent through an optical resonator, is the working substance of a crystal laser.

The laser crystal consists of two parts: the luminescent center and the host crystal. Most of the luminescent centers of laser crystals are composed of activated ions, which partially replace cations in the host crystal to form doped laser crystals. A self-activating laser crystal is formed when the active ions become part of the host crystal composition.

The active ions used in laser crystals are mainly transition metal ions and trivalent rare earths ion. The optical electrons of transition group metal ions are 3d electrons in the outer layer. In the crystal, this optical electron is easily affected by the surrounding crystal field, so its spectral characteristics are very different in different structural types of crystals. The 4f electrons of trivalent rare earth ions are shielded by the 5s and 5p outer electrons, which weakens the effect of the crystal field on them, but the perturbation effect of the crystal field makes the originally forbidden 4f electron transition possible, resulting in narrow-band absorption and fluorescence spectral lines. Therefore, the spectrum of trivalent rare earth ions in different crystals does not vary as much as transition metal ions. The host crystals used in laser crystals are mainly oxides and fluorides. As a host crystal, it requires stable physical and chemical properties and easy to grow optically uniform. A good large-size crystal, and the price is cheap, but the adaptability between it and the activated ion should be considered, such as the radius, electronegativity and valence state of the matrix cation and the activated ion should be as close as possible. In addition, the influence of the host crystal field on the activated ion spectrum should also be considered. For some host crystals with special functions, activated ions can directly generate lasers with certain characteristics after doping. The most used ones are: Nd:YAG, Nd:YVO4.

The general laser crystals are YAG materials, called yttrium aluminum garnet.

As an industrial laser, highly doped yttrium aluminum garnet is used, that is to say, different elements are added to achieve different effects; as an industrial laser, the following properties are concerned:

1. Under the same input power, the heat generation is low
2. The rod body is polished, especially the end face has high precision
3. High doping concentration (such as YD, up to 30 at.% or more), with longer fluorescence lifetime
4. Simple crystal spectrum, no excited state absorption and up-conversion, and no fluorescence concentration quenching

Nd:YAG crystal is a solid-state laser material with the best comprehensive performance. It has the characteristics of high gain, low threshold, high efficiency, low loss, good thermal conductivity and thermal shock resistance, and is suitable for various laser working modes (continuous, pulsed, Q switch, mode locking, frequency doubling, etc.), which are widely used in industrial, medical, military and scientific research fields.

Properties of Nd:YAG

• High gain
• Low threshold
• High efficiency
• Low loss
• High thermal conductivity
• Good thermal shock resistance

Nd,Ce:YAG doped neodymium and cerium (yttrium aluminum garnet) laser rods are the most ideal working materials for repetitive frequency air-cooled lasers, and are widely used in small laser rangefinders and laser medical instruments.

Advantages of Nd,Ce:YAG

• High efficiency, low threshold
• High optical quality
• Good anti-ultraviolet radiation characteristics
• Good thermal stability
• Good repetition frequency characteristics

YCr4+:YAG crystal is an excellent laser crystal. It is suitable for Q-switched, diode-pumped, Nd:YAG, Nd:YLF, Nd:YVO4 pump lamps and other Nd or Yb doped lasers with a wavelength of 0.8-1.2um. It has extremely high stability and reliability, long life and damage resistance, and it will be an ideal passive Q-switch material.

Advantages of YCr4+:YAG

• Long life
• Anti-damage
• High stability
• High reliability

Er:YAG is an excellent 2.94μm laser crystal, which is widely used in laser medical systems and other fields. Er:YAG crystal is the most important working substance of 3mm laser, and has the characteristics of high slope efficiency, laser operation at room temperature, and laser wavelength in the safe band of human eyes. The 2.94mm Er:YAG laser has been widely used in surgery, dermatology, and dentistry in the medical field.

Advantages of Er:YAG

• High slope efficiency
• Work at room temperature
• The working wavelength of the laser is relatively safe for the human eye

Yb:YAG is a promising laser material, which is more suitable for diode pumping than conventional Nd-doped laser materials. Compared with the commonly used Nd:YAG crystal, the diode-pumped absorption band bandwidth of Yb:YAG can effectively reduce the thermal management requirements of laser diodes. It has a long lifetime at high laser levels and a 3-4 times lower thermal load per unit pump power. Ytterbium-doped YAG crystals are expected to replace neodymium-doped YAG crystals for high-power diode lasers, and other related applications.

Advantages of Yb:YAG

• High slope efficiency
• High optical quality
• High thermal conductivity and high mechanical strength
• No excited state absorption and upconversion
• The heat load generated by unit pump power is lower than that of Nd:YAG crystal
• The absorption bandwidth of diode pump is about 8nm@940nm
• Suitable for pumping of commonly used high-power InGaAs laser diodes (wavelength 940nm or 970nm)

CTH: YAG (Cr, Tm, Ho: YAG) is one of the hotspots of solid-state laser research in recent years. It produces excellent crystals of 2.1 μm wavelength lasers, and the 2.1 μm lasers used as working substances are used in medicine, optical communication, remote sensing, etc. And laser radar, laser chemistry, laser spectroscopy, material processing and other aspects show important application prospects.

The main advantages of CTH:YAG

• High pulse output energy
• Suitable for repetitive frequency work
• Can work efficiently at room temperature
• Suitable for lamp pumping and diode pumping
• The wavelength of laser work is relatively safe for human eyes

Nd:YLF crystal is an ideal laser crystal material for continuous laser and mode-locked laser. Its thermal lens effect is very small, the fluorescence line width is wide, and the output linearly polarized light. Nd:YLF can generate 1047nm and 1053nm wavelength lasers, and has gained important applications in inertial confinement laser fusion research projects. The main advantages of Nd:YLF crystals

• Relatively small stimulated emission cross section
• Ultra-large fluorescent linewidth, low excitation light threshold for continuous laser applications, etc.
• Effective single-mode operation, output high power and low beam divergence angle
• The output of linearly polarized laser is beneficial to obtain high-efficiency Q-switching and frequency-doubling output
• Large-diameter round rods or large-sized slats can also obtain uniform mode laser output
• Suitable as oscillator and preamplifier for high power neodymium glass laser system.

Compared with Nd:YAG, Nd:YVO4 has larger stimulated emission cross section and higher absorption coefficient for pump light. It is an excellent laser crystal, suitable for the manufacture of laser diode pumping, especially for low and medium power lasers. It can be made into all-solid-state lasers that output near-infrared, green, blue to ultraviolet and other types. Nd:YVO4 lasers have been widely used in material processing, machinery, wafer inspection, medical inspection and other fields, and Nd:YVO4 diode-pumped solid-state lasers are rapidly replacing traditional water-cooled ion lasers and lamp-pumped lasers in the market.

Main advantages of Nd:YVO4

• Low light damage threshold and high slope efficiency
• The output of biaxial crystal is linearly polarized
• Low frequency pump wavelength, easy to single-mode output
• The stimulated emission cross section is large, and the absorption of the pump wavelength linewidth is high
• The pump bandwidth at 808nm is 5 times that of Nd:YAG
• The stimulated emission cross section at 1064nm is 3 times that of Nd:YAG