This article deals with lasers emitting in the blue and violet spectral region, i.e., with a wavelength roughly around 400–500 nm. The choice of laser gain media for such wavelengths is limited, and the achievable performance is typically not as good as in, e.g., the infrared spectral region.
Types of Blue Lasers
The following types of blue lasers are the most common:
- Blue laser diodes , based on gallium nitride (GaN) or related materials (e.g. InGaN) and emitting around 400–480 nm, have been developed quite successfully, now offering substantially better output powers and device lifetimes than green diode lasers. Output powers can now be up to the order of 10 W for a blue broad-area laser diode, for example, and by combining many of such laser diodes, fiber-coupled diode lasers with hundreds of watts or more out of one multimode fiber have become commercially available. One may also generate of the order of 100 W with a diode bar. Another development is that of blue-emitting VCSELs .
- Thulium-doped or praseodymium-doped upconversion lasers based on fibers or bulk crystals can emit around 480 nm, typically with some tens of milliwatts of output power and with good beam quality. Further development for powers of hundreds of milliwatts or even multiple watts appears to be feasible.
- Helium–cadmium lasers (which are gas lasers) can emit hundreds of milliwatts in the blue region at 441.6 nm, with high beam quality.
- Blue or violet light can also be generated by frequency doubling (external to the laser resonator or intracavity) the output of lasers emitting around 800–1000 nm. Most frequently used are neodymium-doped lasers, e.g. Nd:YAG emitting at 946 nm (for 473 nm), Nd:YVO4 at 914 nm (for 457 nm), and Nd:YAlO3 at 930 nm (for 465 nm). Common nonlinear crystal materials for frequency doubling with such lasers are LBO, BiB3O6 (BIBO), KNbO3, as well as periodically poled KTP and LiTaO3. Output powers of multiple watts can be obtained, even with single-frequency operation and high beam quality, although less easily than with 1-μm lasers. Instead of a laser, an optical parametric oscillator may be used.
- High-power optically pumped VECSELs are also very attractive laser sources for frequency doubling with several watts or even tens of watts of output power. Note that other kinds of semiconductor lasers, such as broad area laser diodes, are available with suitable wavelengths, but are less suitable for frequency doubling due to a typically broader linewidth and poor beam quality. There are some diode lasers, however, which deliver some tens of milliwatts of frequency-doubled light.
- Argon ion lasers, based on laser amplification in an argon plasma (made with an electrical discharge), are fairly powerful light sources for various wavelengths. While the highest power can be achieved in green light at 514 nm, significant power levels of several watts are also available at 488 nm, apart from some weaker lines e.g. at 458, 477 and 497 nm. In any case, the power efficiency of such lasers is very poor, so that tens of kilowatts of electric power are required for multi-watt blue output, and the cooling system has corresponding dimensions. There are smaller tubes for air-cooled argon lasers, requiring hundreds of watts for generating some tens of milliwatts.
For wavelengths below ≈ 400 nm, the eye's sensitivity (i.e. its ability to detect small light levels) sharply declines, and one enters the region of ultraviolet light. (See also the article on ultraviolet lasers.) Note that even for wavelengths around or slightly above 400 nm, the retina can be damaged via photochemical effects even for intensity levels which are not perceived as very bright.
Applications of Blue and Violet Lasers
Blue and violet lasers are used e.g. in interferometers, for laser printing (e.g. exposure of printing plates) and digital photofinishing, data recording (Blu-ray Disc, holographic memory), in laser microscopy, in laser projection displays (as part of RGB sources), in flow cytometry, and for spectroscopic measurements. Direct diode laser applications also become more and more feasible due to the performance enhancement of blue laser diodes.
Data recording appears to be the major driver for the development of blue laser diodes.
In most cases, the use of blue and violet lasers is motivated by the relatively short wavelengths, which allows for strong absorption in many materials, for tight focusing, or for resolving very fine structures in imaging applications.
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