LASER is a word created by the inventor from the initials of "Light Amplification by Stimulated Emission of Radiation." LASER is used as the general term for laser oscillators and amplifiers used to emit laser light.
Laser light is artificial light emitted by stimulated emission, and does not exist in the natural world. The characteristics of laser light include high color purity (single wavelength) and phase alignment.
The world's first semiconductor laser began oscillating over 40 years ago with a wavelength of 840 nm and produced infrared light. Sony introduced the world's first commercial semiconductor laser in 1984, using the "MOCVD" method of growing crystals, and was awarded the Okochi Memorial Production Prize in 1990 for its development and mass-production achievements.
Lasers are classified into various types according to the medium used to emit laser light. Broadly speaking, these types include gas lasers, solid-state lasers, semiconductor lasers, and liquid lasers. Lasers have characteristic wavelengths, outputs, and sizes, and play an active role in the medical, industrial, and consumer sectors. Of the various types, semiconductor lasers are compact and have low power consumption, so they are used for familiar applications such as writing and reading signals for optical discs such as Blu-ray Disc and DVD, projector light sources, and printers.
These semiconductor devices emitlaser lightwhen an electric current is applied to the P-N junction of a compound semiconductor.Semiconductor lasers are a type of diode that combines the electrical properties of a diode with properties that produce laser light, and are also called "laser diodes."
Semiconductor lasers are smaller and lighter in weight than gas and other lasers, and are known to be widely used for applications such as recording and playback of optical discs (Blue-ray Disc, DVD, etc.) and light sources for optical fiber communications.
Recently, three primaries of the light can be realized using semiconductor lasers, which enables use for projector applications. In addition, use in 3D sensor applications is also increasing due to the fact that the human eye is unable to perceive infrared light.
Edison's incandescent light bulb produced light by converting electricity into thermal energy. In contrast, semiconductor lasers and light-emitting diodes generate light directly from electricity.
Incandescent light bulb
Electrical energy → Thermal energy → Light
Semiconductor laser (light-emitting diode)
Electrical energy → Light
The materials used in semiconductor lasers are aggregate of many atoms, and large numbers of electrons revolve around these atomic nuclei. When electrons with high energy return to a low-energy state, they emit energy as light. In addition, other electrons are stimulated by this light and also return to a low-energy state and emit light (stimulated emission).
The generated light is repeatedly reflected within the light-emitting layer by the cleavage plane mirrors to produce further stimulated emission and efficiently amplify the light.
Strong phase-aligned light (laser light) is emitted by repeating (amplifying) stimulated emission in this manner.
The stimulated and emitted electrons are continuously replenished by the current applied to the P-N junction, enabling continuous laser light emission.
The application fields for semiconductor lasers are determined by the laser light wavelength and output characteristics.
The wavelength (color) can be changed by the laser active layer material (energy gap). The first semiconductor laser began oscillating over 40 years ago with a wavelength of 840 nm and produced infrared light.
Short-wavelength lasers with an oscillation wavelength of 400 to 800 nm have good light-focusing characteristics, so they are used to read and write optical disc signals. Lasers with wavelengths of the three primaries of light (450 nm, 530 nm, 635 nm) are used for projector applications.
Near-infrared-wavelength lasers with an oscillation wavelength in excess of 800 nm are used for 3D sensors and short-range optical communications.
Long-wavelength lasers with an oscillation wavelength of 1300 to 1600 nm have low transmission loss inside optical fibers, so they are used as light sources for optical fiber communications in long-range communication networks.
|Laser material||Wavelengths||Colors||Example of application|
|GaInN||400〜530nm||Blue-violet to Green||Blu-Ray Disc/Projector|
|AlGaAs||780〜850nm||Infrared||CD/Printer/Optical communications/3D sensor|
An English scholar said that "Light is radio waves that are visible to the eye," and light can indeed be considered a type of electromagnetic radiation like radio waves. Light is a wave, so it has a wavelength (frequency) and a wave height. In order of decreasing wavelength, electromagnetic radiation includes radio waves → microwaves → infrared rays → visible light (red, orange, yellow, green, blue, violet) → ultraviolet rays → X-rays. The visibility is determined by the wavelength.
Semiconductor lasers are used in a variety of fields.
It is expected that new applications will continue to be created as laser technology progresses in the future.
Semiconductors are positioned between conductors, which conduct electricity well, and non-conductors (insulators), which do not conduct electricity. These substances have properties that enable the electric conductivity to be controlled according to various conditions.
"Semiconductor" is generally used as the collective term for electronic components that use semiconductor properties, such as transistors, diodes, and LSI.
Most current semiconductors such as transistors and LSI use silicon (Si) crystals. Semiconductors consisting of two or more elements, like gallium (Ga) arsenide (As) crystals that are used for applications such as semiconductor lasers, are called "compound semiconductors" to differentiate them from single-element semiconductors such as silicon.
Gallium arsenide (GaAs) crystals are a typical Ⅲ-Ⅴgroup compound semiconductor material. The electron movement within the crystal is faster than that of silicon, so GaAs crystals are also used for applications such as ultra-high-frequency devices in addition to semiconductor lasers.
The semiconductor laser manufacturing process includes a process in which a multi-layered crystal is formed on a single-crystal substrate.
MOCVD is a crystal growth method along with MBE and LPE, and grows crystals from gas. The materials used include organic metal, and crystals are grown by a chemical reaction that uses thermal decomposition of molecules.
MOCVD makes it possible to grow good-quality crystals, and has characteristics such as good uniformity of the crystal growth film and the ability to simultaneously process large quantities.
Sony introduced the world's first commercial semiconductor laser using the MOCVD method in 1984, and this technology has since become the main method of growing semiconductor laser crystals.
Optical pickups are optical heads consisting of an optical system and a semiconductor laser, and are used to read signals from the pits (grooves) on discs and to write signals to discs in optical disc systems such as CD, DVD, and Blu-ray Disc.
The roles of optical pickups are as follows:
Optical pickups comprise components such as a semiconductor laser, a laser detection IC, a beam splitter, and a focusing lens.
|Optical pickups||Wavelength used|
In contrast to infrared lasers and long-wavelength lasers used for communications, lasers that emit light visible to the human eye, such as red and blue lasers in the laser light wavelength band of approximately 400 to 700 nm, are called visible lasers.
Red lasers use AlGaInP materials and emit red light in the 630 to 680 nm wavelength band. Red lasers are used for applications such as DVD optical pickups, barcode readers in supermarkets and convenience stores, laser printers, and laser pointers.
Blue lasers use GaN materials and emit blue (blue-violet) light in the 400 to 500 nm wavelength band. Blue-violet lasers with a wavelength of 405 nm are used as Blue-ray Disc lasers.
The Blue-ray Disc principle is largely the same as that of CD and DVD. However, in order to achieve a larger capacity, a blue-violet laser with a wavelength of 405 nm, which is shorter than that used for DVD, is required to record and play back the pits (grooves) on the disc.
The optical pickups of CD/DVD drives require lasers with the wavelengths used to record and play back CD and DVD. Lasers that can emit laser light in two wavelengths with a single package are called two-wavelength lasers.
Two-wavelength lasers make it possible to reduce the number of components of two-wavelength optical pickups, and also help to greatly enhance reliability. This enables realization of simple two-wavelength optical pickups with good mass productivity.
Two-wavelength lasers include monolithic types and hybrid types. Sony developed and introduced the world's first commercial monolithic two-wavelength laser diode, and used it in its PlayStation 2 home video game console and DVD players. Compared to hybrid types that house two LD chips in a single package, monolithic types have fewer components, are easier to assemble, can achieve an emission point interval accuracy of ±1 µm or less, and help to enhance optical performance. These are features unique to monolithic two-wavelength lasers.