A semiconductor laser diode, commonly known as a laser diode, is a compact semiconductor device that emits coherent light when an electric current passes through it. It works on the principle of stimulated emission and operates as a junction diode.
How it works:
P-N Junction: The core of a laser diode consists of a P-N junction formed by doping different semiconductor materials. The P-type region has an excess of positively charged holes, while the N-type region has an excess of negatively charged electrons.
Injection and Recombination: When a forward-biased current is applied to the P-N junction, electrons from the N-region and holes from the P-region are injected into the active region (also called the "quantum well" or "active layer"). This active region is typically very thin (on the order of nanometers) and is where the light emission occurs.
Population Inversion: The active region's unique design ensures that the injected electrons and holes are confined to a small space. Due to the confined space and high carrier concentration, a phenomenon called "population inversion" is achieved. In population inversion, more electrons are in higher energy states (conduction band) than in lower energy states (valence band), which is essential for stimulated emission.
Stimulated Emission: When an electron in the conduction band recombines with a hole in the valence band, it can release energy in the form of a photon. This is spontaneous emission. In a laser diode, however, the released photon can stimulate other excited electrons to emit photons with the same energy, phase, and direction. This process creates a chain reaction, resulting in the emission of coherent light.
Optical Cavity: To enhance stimulated emission, the active region is sandwiched between two mirrors (one highly reflective and the other partially reflective), forming an optical cavity. The coherent light generated bounces back and forth between the mirrors, further stimulating emission and allowing a portion of the light to pass through the partially reflective mirror as the laser output.
Semiconductor laser diodes have found a wide range of applications due to their compact size, efficiency, and coherent light emission. Some of the most common applications include:
Telecommunications: Laser diodes are extensively used in fiber-optic communication systems to transmit data over long distances with minimal signal loss.
Printing and Imaging: Laser diodes are used in laser printers, barcode scanners, and optical data storage devices (CDs, DVDs, Blu-ray) for high-resolution printing and reading capabilities.
Sensing and Metrology: Laser diodes are employed in various sensing applications, including distance measurement, alignment, and laser rangefinders.
Medical and Biotechnology: Laser diodes are used in medical applications such as laser surgery, dermatology, ophthalmology, and DNA sequencing.
Industrial Cutting and Welding: High-power laser diodes are used in industrial cutting and welding applications due to their ability to generate intense, focused beams.
Laser Pointers: Small semiconductor laser diodes are commonly used in laser pointers for presentations, astronomy, and educational purposes.
Data Storage: Laser diodes play a crucial role in read/write operations for optical data storage devices like DVDs and Blu-ray discs.
These are just a few examples, but the versatility of laser diodes continues to drive innovation in various fields, ranging from consumer electronics to scientific research and beyond.