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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 is characterized by its compact size, long operational life, and the ability to be easily integrated with electronic circuits through simple current injection. These features have made it a popular choice in various applications such as optical communication, data storage, laser printing, distance measurement, and radar systems.
**Principle of Laser Illumination**
To generate a laser, three essential conditions must be met:
1. **Population inversion**: This means that more electrons are in an excited state than in a lower energy state, which is necessary for stimulated emission.
2. **Resonant cavity**: A cavity is needed to provide optical feedback, allowing photons to oscillate and amplify within the system. The most common structure is the Fabry-Perot cavity.
3. **Threshold condition**: The gain from the medium must exceed the total losses in the system, including those from the cavity walls and output coupling.
In order to achieve stable laser oscillation, the semiconductor must provide enough gain to overcome all optical losses. This is achieved by injecting a sufficient current into the device, which creates a population inversion. When the threshold is reached, light at a specific wavelength can resonate within the cavity, leading to amplification and the generation of a coherent laser beam.
The resonant cavity plays a crucial role in forming a laser by providing optical feedback. In semiconductor lasers, this is typically done using the natural cleavage planes of the crystal as mirrors. One end is coated with a high-reflectivity layer, while the other is treated with an anti-reflective coating to allow the laser output.
For FP (Fabry-Perot) cavity lasers, the cavity is formed by cleaving the semiconductor crystal perpendicular to the PN junction plane. This setup allows for multiple reflections of light, increasing the chances of stimulated emission and ultimately producing a laser output.
**Gain Conditions**
To achieve lasing, there must be a population inversion in the active region of the semiconductor. Electrons are excited from the valence band to the conduction band under forward bias. When these excited electrons recombine with holes, they emit photons through stimulated emission. This process continues as long as the population inversion is maintained, resulting in a continuous laser output.
**Semiconductor Laser Characteristics**
Since its invention in 1962, the semiconductor laser has become one of the most widely used laser types due to its unique advantages:
- Small size and lightweight
- Low driving power and current requirements
- High efficiency and long service life
- Direct electrical modulation capability
- Easy integration with optoelectronic components
- Compatibility with standard semiconductor fabrication processes, enabling mass production
These characteristics have led to extensive research and development worldwide, making semiconductor lasers the fastest-growing and most commercially successful laser technology.
**Semiconductor Laser Working Principle**
The working mechanism of a semiconductor laser involves exciting electrons between energy bands using an external current. The natural cleavage planes of the semiconductor act as mirrors, forming a resonant cavity that allows light to be reflected and amplified. As photons travel back and forth within the cavity, they stimulate more emissions, creating a coherent laser beam.
There are three key conditions required for a semiconductor laser to operate:
1. Sufficient population inversion must be achieved.
2. A suitable resonant cavity must be present to provide optical feedback.
3. The gain must meet or exceed the loss, ensuring that the laser can sustain oscillation.
