Working Principle And Application Of Solid-state Laser

Solid Laser is a laser that uses solid laser material as the working substance. The ruby laser invented by T.H. Maiman in 1960 was a solid-state laser and the world's first laser. Solid lasers generally consist of laser working material, excitation source, focusing cavity, resonant cavity reflector and power supply.

The solid working material used in this type of laser is made by doping metal ions that can produce stimulated emission into the crystal. There are three main types of metal ions that can produce stimulated emission in solids: (1) transition metal ions (such as Cr3+); (2) most lanthanide metal ions (such as Nd3+, Sm2+, Dy2+, etc.); (3) actinium It is a metal ion (such as U3+). The main characteristics of these metal ions doped into the solid matrix are: relatively wide effective absorption spectral band, relatively high fluorescence efficiency, relatively long fluorescence lifetime and relatively narrow fluorescence spectral lines, so they are prone to particle number inversion and stimulated emission. The artificial crystals used as crystal matrix mainly include: corundum (NaAlSi2O6), yttrium aluminum garnet (Y3Al5, O12), calcium tungstate (CaWO4), calcium fluoride (CaF2), etc., as well as yttrium aluminate (YAlO3), beryllium Lanthanum acid (La2Be2O5), etc. The glass substrate used is mainly high-quality silicate optical glass, such as commonly used barium crown glass and calcium crown glass. Compared with crystalline matrices, the main features of glass matrices are ease of preparation and easy availability of high-quality materials in large sizes. The main requirements for crystals and glass substrates are: easy incorporation of luminescent metal ions for activation; good spectral characteristics, optical transmittance characteristics and a high degree of optical (refractive index) uniformity; physical properties suitable for long-term laser operation and chemical properties (such as thermal properties, anti-degradation properties, chemical stability, etc.). Crystal lasers are typically represented by ruby (Al2O3: Cr3+) and neodymium-doped yttrium aluminum garnet (abbreviated as YAG: Nd3+). Glass lasers are typically represented by neodymium glass lasers.

Solid laser working material

The working material of a solid-state laser is composed of optically transparent crystal or glass as a matrix material, doped with activating ions or other activating substances. This working substance should generally have good physical-chemical properties, narrow fluorescence spectral lines, strong and broad absorption bands and high fluorescence quantum efficiency.

Glass laser working materials are easily made into uniform large-size materials and can be used in high-energy or high-peak-power lasers. However, its fluorescence spectrum line is wider and its thermal performance is poor, making it unsuitable for working under high average power. Common neodymium glasses include silicate, phosphate and fluorophosphate glasses. In the early 1980s, neodymium glass with a negative refractive index temperature coefficient was successfully developed, which can be used in medium and small energy lasers with high repetition rates.

Crystal laser working materials generally have good thermal and mechanical properties and narrow fluorescence spectral lines, but the crystal growth technology to obtain high-quality large-size materials is complicated. Since the 1960s, more than 300 types of oxide and fluoride crystals doped with various rare earth metals or transition metal ions have achieved laser oscillation. Commonly used laser crystals include ruby (Cr:Al2O3, wavelength 6943 Angstroms), neodymium-doped yttrium aluminum garnet (Nd:Y3Al5O12, referred to as Nd:YAG, wavelength 1.064 microns), lithium yttrium fluoride (LiYF4, referred to as YLF; Nd:YLF , wavelength 1.047 or 1.05 microns; Ho:Er:Tm:YLF, wavelength 2.06 microns), etc.

Since 1973, there has been another type of self-activating laser crystal. Its activated ions are a chemical component of the crystal, so the concentration of activated ions is high and fluorescence quenching will not occur. This crystal has high laser gain and low extraction threshold. The main varieties include neodymium pentaphosphate (NdP5O14), lithium neodymium tetraphosphate (NdLiP4O12), and neodymium aluminum borate (NdAl3(BO4)3). They are mostly grown by the molten salt method and have small crystal sizes, so they can be used in small solid-state lasers.

A variety of tunable laser crystals with broadband fluorescence characteristics have been developed, such as chrysoberyl with terminal phonon transition (Cr:BeAl2O4, wavelength 0.701-0.815 microns, operating at room temperature), nickel-doped magnesium fluoride (Ni: MgF2, wavelength 1.6~1.8 microns, working at low temperature), cerium-doped lithium yttrium fluoride with 5d→4f transition (Ce:YLF, wavelength 0.306~0.315 microns, excited by excimer laser, working at room temperature) and the color center of alkali halide Laser crystal (undoped or doped potassium chloride, lithium fluoride, etc., wavelength 0.8~3.9 microns, mostly working at low temperature).

Solid laser excitation source

Solid lasers use light as the excitation source. Commonly used pulse excitation sources include xenon-charged flash lamps; continuous excitation sources include krypton arc lamps, iodine tungsten lamps, potassium rubidium lamps, etc. In small long-life lasers, semiconductor light-emitting diodes or sunlight can be used as excitation sources. Some new solid-state lasers also use laser excitation.

Since only part of the emission spectrum of the light source is absorbed by the working material, plus other losses, the energy conversion efficiency of solid-state lasers is not high, generally between a few thousandths and a few percent.

Solid laser characteristics

Solid lasers can be used as high-energy and high-power coherent light sources. The output energy of ruby pulse laser can reach kilojoule level. The Q-switched and multi-stage amplified neodymium glass laser system has a maximum pulse power of 10 watts. The output power of the yttrium aluminum garnet continuous laser can reach hundreds of watts, and multi-stage series connection can reach kilowatts.

Solid-state lasers use Q-switching technology (visible light modulation) to obtain short pulses ranging from nanoseconds to hundreds of nanoseconds, and use mode-locking technology to obtain ultra-short pulses ranging from picoseconds to hundreds of picoseconds.

Due to optical inhomogeneity of the working material and other reasons, the output of general solid-state lasers is multi-mode. If the working material with good optical uniformity is selected and the resonant cavity is carefully designed and other technical measures are taken, the fundamental transverse mode (TEM00) laser with a beam divergence angle close to the diffraction limit can be obtained, and a single longitudinal mode laser can also be obtained.

Solid-state laser applications and trends

Solid-state lasers have a wide range of uses in military, processing, medical and scientific research fields. It is commonly used in ranging, tracking, guidance, drilling, cutting and welding, annealing of semiconductor materials, micro-processing of electronic devices, atmospheric detection, spectroscopic research, surgery and ophthalmic surgery, plasma diagnosis, pulse holography and laser fusion, etc. . Solid-state lasers are also used as excitation sources for tunable dye lasers.

The development trend of solid-state lasers is the diversification of materials and devices, including the search for new wavelengths and new working materials with tunable operating wavelengths, improving the conversion efficiency of the laser, increasing the output power, improving the beam quality, compressing the pulse width, improving reliability and Extended working life.

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