**Operating Characteristics of Semiconductor Lasers**
Semiconductor lasers exhibit several key operating characteristics that define their performance and application. Understanding these properties is essential for optimizing their use in various technologies.
**1. Threshold Current**
The threshold current is the minimum current required to initiate lasing action in a semiconductor laser. Below this current, only spontaneous emission occurs, and the gain increases with current. Once the threshold is reached, stimulated emission dominates, and coherent laser light is generated. Several factors influence the threshold current:
- **Doping Concentration**: Higher doping concentrations typically result in lower threshold currents because they enhance carrier density and reduce recombination losses.
- **Resonator Losses**: A lower loss in the optical cavity, such as by increasing mirror reflectivity, leads to a lower threshold current.
- **Heterojunction vs. Homojunction**: Heterojunctions, where different semiconductor materials are used, significantly reduce the threshold current compared to homojunctions. For example, at room temperature, the threshold current for a homojunction is over 30,000 A/cm², while a single heterojunction is around 8,000 A/cm², and a double heterojunction can be as low as 1,600 A/cm². This improvement has enabled continuous-wave operation of high-power semiconductor lasers at room temperature.
- **Temperature**: The threshold current increases with temperature. Above 100 K, it rises approximately with the cube of the temperature. Therefore, semiconductor lasers are most efficient when operated at low or room temperatures.
**2. Directionality**
Due to the short cavity length in semiconductor lasers, their directionality is relatively poor. The divergence angle is larger in the vertical plane (perpendicular to the junction) than in the horizontal plane (parallel to the junction). Typically, the vertical divergence is between 20° and 30°, while the horizontal divergence is about 10°. This makes beam shaping and collimation important for practical applications.
**3. Quantum Efficiency**
Quantum efficiency (η) is defined as the ratio of photons emitted per second to the number of electron-hole pairs injected into the active region. At 77 K, GaAs lasers can achieve quantum efficiencies of 70%–80%, but this drops to around 30% at 300 K due to increased non-radiative recombination and thermal effects.
**4. Power Efficiency**
Power efficiency (ηâ‚) is calculated as the ratio of the optical output power to the electrical input power. Due to various losses—such as heat dissipation and internal absorption—the power efficiency is generally low. However, double-heterostructure devices can achieve up to 10% efficiency at room temperature, with higher values (up to 30%–40%) possible at cryogenic temperatures.
**5. Spectral Characteristics**
Semiconductor lasers emit light due to radiative recombination between the conduction and valence bands. This results in a relatively wide spectral linewidth. For example, a GaAs laser has a linewidth of several nanometers at room temperature, making its light less monochromatic. The peak wavelength shifts with temperature: it is around 840 nm at 77 K and 902 nm at 300 K. This temperature dependence must be considered in applications requiring stable wavelengths.
In summary, semiconductor lasers are versatile light sources with unique operating characteristics. Their performance is influenced by factors such as threshold current, directionality, quantum and power efficiency, and spectral properties. Understanding these aspects allows for better design and optimization in real-world applications.
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