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Semiconductor laser light-emitting principle and working principle
A semiconductor laser, also known as a laser diode, is a type of laser that uses a semiconductor material as its active medium. It stands out for its compact size, long operational life, and the ability to be easily integrated with electronic circuits through simple current injection. These advantages have made semiconductor lasers widely applicable in areas such as laser communication, optical storage, optical gyroscopes, laser printing, ranging systems, and radar technology.
**Principle of Laser Illumination**
To generate laser light, three key conditions must be met:
1. **Population inversion**: This means that more electrons are in a higher energy state than in a lower one, creating an imbalance necessary for stimulated emission.
2. **Resonant cavity**: The cavity acts as an optical feedback system, allowing light to oscillate and build up in intensity. The simplest form of this is the Fabry-Perot cavity, which uses the natural cleavage planes of the semiconductor crystal as mirrors.
3. **Threshold condition**: The gain produced by the active medium must exceed all losses within the system, including those from the cavity walls and output coupling.
For stable laser operation, a sufficient current must be injected into the semiconductor to achieve the required population inversion. As the current increases, the gain rises until it surpasses the loss, enabling the formation of coherent laser light at a specific wavelength.
The resonant cavity plays a crucial role in amplifying the light through multiple reflections. In most semiconductor lasers, the ends of the crystal are coated with high-reflectivity and anti-reflective films to enhance the feedback mechanism. This setup allows for repeated stimulation of photon emission, leading to sustained laser oscillation.
In terms of gain, the semiconductor's energy bands allow for efficient carrier injection. By applying forward bias to a homojunction or heterojunction, electrons are excited from the valence band to the conduction band, creating a population inversion. When these excited electrons recombine with holes, they emit photons through stimulated emission, which is then amplified within the cavity.
**Semiconductor Laser Characteristics**
Semiconductor lasers, first developed in 1962, offer several unique benefits compared to traditional lasers. They are small, lightweight, and require low driving power and current. Their high efficiency and long service life make them ideal for various applications. Additionally, they can be directly modulated electrically and are easily integrated with other optoelectronic components.
They are compatible with standard semiconductor manufacturing processes, enabling mass production. These features have led to widespread research and commercialization, making semiconductor lasers one of the fastest-growing and most widely used laser technologies today.
**Working Principle of Semiconductor Lasers**
The working principle of a semiconductor laser involves exciting carriers (electrons) within the semiconductor material. These carriers are injected into the active region, where they transition between energy bands. The natural cleavage planes of the semiconductor act as mirrors, forming a resonant cavity that allows light to resonate and amplify.
Three essential conditions must be met for laser emission:
1. A sufficient population inversion must be created.
2. A suitable resonant cavity must exist to provide optical feedback and sustain oscillation.
3. The system must meet the threshold condition, ensuring that the gain exceeds the losses.
This process results in the generation of coherent, monochromatic laser light that can be used in a wide range of applications.
