Blue laser

History

Before the 1960s and into the late 1990s, gas and argon-ion lasers were common but suffered from poor efficiencies of 0.01% and large sizes. In the 1960s, researchers deposited gallium nitride on a sapphire base to create blue lasers, but a lattice mismatch caused many defects, leading to lifespans under 10 hours and efficiency below 1%. Gallium nitride crystal layer construction required high nitrogen gas pressures and temperatures similar to those for creating synthetic diamonds.

In 1992, Shuji Nakamura at Nichia Chemicals invented the first blue semiconductor LED using an InGaN active region, GaN optical guide, and AlGaN cladding; four years later he produced the first low-power blue laser, though defects remained high at 10⁶–10¹⁰ defects/cm², resulting in a lifespan under 300 hours with pulsed excitation. In the late 1990s, Sylwester Porowski at the Institute of High Pressure Physics in Warsaw developed technology to create gallium nitride mono-crystals with fewer than 100 defects/cm² using magnesium doping.

In 1999, Nakamura used these Polish crystals to produce lasers with twice the yield and ten times the lifespan, reaching 3,000 hours at 30 mW. In the 2000s, Japanese manufacturers mastered production of a 60 mW blue laser with long lifetimes, enabling Blu-ray, BD-R, and BD-RE devices.

Types

Blue direct diode semiconductor lasers are built using inorganic gallium nitride or InGaN gain medium, with many atomic layers forming quantum wells that generate photons. AlGaN cladding contains the photons, and doping materials such as magnesium help build a substrate free of dislocations with uniform carrier distribution. Blue direct diode lasers can also be fabricated with InGaN semiconductors at 445 nm through 465 nm, which appear significantly brighter than GaN 405 nm diodes because the longer wavelengths are closer to the peak sensitivity of the human eye.

Use of phosphorescent direct diode blue organic light emitting diodes for lasers is impractical due to poor lifetimes under 200 hours. Zener diodes can be incorporated into circuitry to minimize ESD failures. Semiconductor lasers can be driven by pulses or continuous wave operation and may emit photons either perpendicular or horizontal to the lasing medium layers. Direct diode infrared semiconductor lasers, available since the 1960s, can be frequency-doubled to the blue range by nonlinear crystals such as BBO or KTP; greater than 1 W power is possible when resonator-enhanced, yielding watt-class sources including a 400 nm blue laser with 2.6 W output.

Violet DPSS laser pointers at 120 mW and 405 nm use a direct diode infrared gallium arsenide laser at 1 W and 808 nm directly doubled without an interposed longer-wave diode-pumped solid state laser.

Blue Visual Appearance

The violet 405 nm laser appears violet to the eye, a color for which human sensitivity is very limited. When pointed at many white objects, such as paper or clothes washed with certain brightening dyes, the dot appears blue due to fluorescence. For display applications requiring true blue, a wavelength of 445–450 nm is used; advances in volume production have made 445 nm InGaN laser diodes an optimal solution for laser phosphor projectors.