Solid-state lasers are a type of laser where the lasing medium is a solid material, typically a crystal or glass, rather than a liquid or gas, doped with rare earth or transition metal ions. The concept of solid-state lasers was first proposed in the 1960s. In 1960, Theodore H. Maiman showed the world the first laser, which was constructed using a ruby crystal as the special material that produced the laser light. They emit laser light across a broad range of wavelengths, from ultraviolet (UV) to infrared (IR), depending on the choice of dopant and the crystal or glass composition. The output power can range from milliwatts (mW) to several watts (W) or even higher depending on the specific laser design, the gain medium, and the pumping mechanisms.

Solid-state lasers are composed of two main components: a solid host material and an active ion that is doped into the host material. The active ion needs to possess specific characteristics, such as sharp fluorescent lines, broad absorption bands, and a high quantum efficiency at the desired wavelength. The host material, on the other hand, should have properties like strength, fracture resistance, high thermal conductivity and optical quality.

Both glasses and crystalline materials have demonstrated these desired characteristics when doped with rare earth ions. Examples of suitable host materials include silicate glasses, phosphate glasses, and various crystalline materials like garnets, aluminates, metal oxides, fluorides, molybdates, tungstates, and more. The active ions commonly used are rare earth ions like neodymium, erbium, holmium, as well as transition metals like chromium, titanium, nickel, and others.

Some notable types of solid-state lasers include the Ruby laser, Nd:YAG laser, Nd:Glass laser, Nd:Cr:GSGG laser, Er:Glass laser, Alexandrite laser, and Titanium:sapphire laser, among others.

The operation of solid-state lasers can be continuous wave (CW), where a continuous output of laser light is produced, or pulsed, where short-duration pulses of high-power laser light are generated.

Construction of Solid-State Laser

To construct a solid-state laser, a laser rod is mounted near an arc or flash lamp. The lamp is connected to a power supply. Both the laser rod and lamp are arranged parallel to each other and are surrounded by a reflector. At the ends of the laser cavity, a highly reflective mirror and an output coupler are placed. To eliminate excess heat, a circulating system is used to cool the laser, typically cooled water or a mixture of ethylene glycol.

Energy Diagram of Solid-State Laser

The active medium used for solid-state lasers is solid materials. Usually, all solid-state materials are optically pumped, where a light source is used as an energy source, which is applied to the gain medium. After absorbing the pump energy, the electrons in the gain medium are excited to a higher energy level. Within the excited state, some electrons undergo a transition from higher energy levels to a specific metastable energy level. 

The metastable state has a longer lifetime compared to the other excited states, allowing the energy to be stored and accumulated. When an electron in the metastable state undergoes a transition back to the ground state, it releases a photon with a specific energy and wavelength. This process is called stimulated emission, and it generates coherent light. 

The generated photons undergo multiple reflections between the mirrors or other reflective elements within the laser cavity. This feedback mechanism amplifies the stimulated emission, creating an intense beam of laser light. A fraction of the amplified light escapes through one of the partially reflective mirrors, constituting the laser output. 

The output beam typically has a narrow linewidth and is characterized by the specific wavelength associated with the energy difference between the metastable and ground states.

Advantages of Solid-State Laser

• In contrast to gas lasers, solid-state lasers typically do not experience material wastage since the laser medium is in a solid state. The active medium in solid-state lasers, such as crystals or glasses, retains its composition and does not consume or deplete during operation.
• Solid-state lasers are capable of producing both continuous and pulsed outputs. 
• Their construction is relatively simple.

Disadvantages of Solid-State Laser

• Solid-state lasers exhibit low efficiency in converting input energy to laser output.
• The divergence of the laser beam is not constant and can vary between 1 milliradian and 20 milliradians. 
• Power loss can occur when the laser rod becomes excessively heated.

Applications of Solid-State Laser

Solid-state lasers have a wide range of applications across various fields. Besides spectroscopy, telecommunications, the applications of solid-state lasers include:

• Materials Processing: Solid-state lasers are widely used for cutting, drilling, welding, and engraving various materials such as metals, plastics, ceramics, and composites. They provide high precision and can handle both macro and micro scale processing tasks.
• Medical and Biomedical: These lasers find applications in medical procedures such as laser surgery, dermatology (e.g., tattoo removal), ophthalmology (e.g., vision correction), dentistry, and cosmetic treatments. They can precisely target and ablate tissues with minimal damage to the surrounding areas.
• Scientific Research: Essential tools in scientific research, including spectroscopy, fluorescence imaging, particle acceleration, and ultrafast phenomena studies. They enable precise and controlled light sources for investigating materials and studying fundamental physical and chemical processes.
• Defense and Security: These lasers are used in defense and security applications, including laser target designators, range finding, directed energy weapons, and laser-based countermeasures. They provide accurate and powerful sources of light for military, aerospace, and security purposes.
• Telecommunications: Solid-state lasers play a crucial role in fiber-optic communications systems, where they are used as optical amplifiers and as sources of light for transmitting signals over long distances with high data rates.