Light amplification by stimulated emission of radiation, or laser in short, is a device that creates and amplifies electromagnetic radiation of specific frequency through process of stimulated emission. In laser, all the light rays have the same wavelength and they are coherent; they can travel long distances without diffusing.
To understand how lasers work, we must understand how an atom gives out light. An atom is the smallest particle in the world, and it contains electrons. By introducing extra photon into the atom, the electrons are forced to move into a higher energy level, and now the atom is at an excited state. However, the excited atom is unstable and the electrons always tries to get back to its ground state, therefore releasing the excess energy it originally gained, as a photon of light radiation. This process is called spontaneous emission, as shown below in Figure. 1.
The laser contains a chamber in which atoms of a medium are excited, bringing their electrons into higher orbits with higher energy states. When one of these electrons jumps down to a lower energy state, it gives off its extra energy as a photon with a specific frequency. By introducing more photons into the system, the photons will eventually encounter another atom with an excited electron, which will stimulate that electron to jump back to its original state, emitting two or more photons with the same frequency as the first and in phase with it. This effect cascades through the chamber, constantly stimulating other atoms to emit yet more coherent photons, and this process is called stimulated emissions. In other words, the light has been amplified, as shown below in Figure 2
Furthermore, mirrors at both ends of the chamber cause the light to bounce back and forth across the medium. One of the mirrors is partially transparent, allowing the laser beam to exit from that end of the chamber. By maintaining a sufficient number of atoms in the medium by external energy source in the higher energy state, thee emissions are continuously stimulated, and this process is called population inversion. Ultimately, it creates a stream of coherent photons which is a very concentrated beam of powerful laser light. Lasers have many industrial, military, and scientific uses, including welding, target detection, microscopic photography, fiber optics, surgery, and etc.
Types of Laser:
There are many different types of lasers and below are the five major types.
1. Gas lasers – ex. HeNe gas laser, and CO2 lasers which emit hundreds of watts of power. They are usually used for cutting and welding in industries.
2. Chemical lasers – powered by chemical reaction which permits large amount of energy, mainly for military use and of very high wavelength. Ex. Hydrogen fluoride laser 2700nm.
3. Solid-state lasers – optically pumped through use of solid medium that is doped , such as ion doped crystalline or glass. An example would be a laser pointer.
4. Fiber lasers – light is guided due to internal reflection in optical fiber. They are widely known nowadays for their high output power and high optical quality as well as long lifespan. The reason is due to the properties of fibers that give high surface area to volume ratio, which allows for efficient cooling when supporting kilowatts of continuous output power. Fiber's wave-guiding properties help maintain signal strength and minimize distortion. Fiber lasers are widely used nowadays for telecommunication that spread across regions several kilometers long.
5. Semiconductor lasers – electrically pumped
a) Light-emitting diodes (LEDs) - In a diode formed from a direct bandgap semiconductor, such as gallium arsenide, carriers that cross the junction emit photons when they recombine with the majority carrier on the other side. Depending on the material, wavelengths (or colors) from the infrared to the near ultraviolet may be produced. All LEDs produce incoherent, narrow-spectrum light. LEDs can also be used as low-efficiency photodiodes in signal applications. An LED may be paired with a photodiode or phototransistor in the same package, to form an opto-isolator.
b) Laser diodes - When an LED-like structure is contained in a resonant cavity formed by polishing the parallel end faces, a laser can be formed. Laser diodes are commonly used in optical storage devices and for high speed optical communication.
Laser diode is a laser where the medium is a semiconductor, formed by a p-n junction, as shown in Fig. 3, and powered by electric current. For different types of laser diode structures, please refer to Appendix 3. Basically, a laser diode is a combination of semiconductor chip that emits coherent light and a monitor photodiode chip for feedback control of power output, in a hermetically packaged and sealed case.
The semiconductor materials that are used to create p-n junction diodes that emit light today are: Gallium arsenide, indium phosphide, gallium antimonide, and gallium nitride. The reason that these are being used is because of the three-five compound properties on chemical periodic table. The materials have to be heavily doped to create P – N regions, which rules out others, leaving groups three-five the ideal options.
Their wavelengths can be adjusted by changing the ratio of composition. For instance, the wavelength of the laser beam produced by InP substrate can be increased by increasing the Indium content or lowering the Phosphate content percentage. Longer wavelength usually indicates a longer travel distance.
According to Wikipedia, Laser diodes are numerically the most common type of laser, with 2004 sales of approximately 733 million diode lasers, as compared to 131,000 of other types of lasers. Laser diodes find wide use in telecommunication as easily modulated and easily coupled light sources for fiber optics communication.
Brandnew can offer high-power diode lasers and systems in a wide range of output powers and wavelengths including laser chip, fiber coupled laser diode, single bar and high power diode laser array. BrandNews strengths are in talent employees, quality engineering, process control, product development and volume manufacturing. Our range of products gives this feeling to our customers who rapidly understand that our solutions help them save time in their RD photonic system and integration work.
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