**Operating Characteristics of Semiconductor Lasers**
Semiconductor lasers exhibit several key operating characteristics that define their performance and applications. One of the most important is the **threshold current**, which marks the point at which lasing begins. Below this threshold, only spontaneous emission occurs, and the gain increases with rising current. Once the threshold current is reached, stimulated emission dominates, and laser light is generated. Several factors influence the threshold current:
1. **Doping concentration**: Higher doping levels in the crystal lead to a lower threshold current.
2. **Resonator loss**: A higher reflectivity of the cavity mirrors reduces losses, resulting in a lower threshold.
3. **Junction type**: Heterojunctions have significantly lower threshold currents compared to homojunctions. For example, room temperature homojunction lasers have thresholds above 30,000 A/cm², while single heterojunctions are around 8,000 A/cm², and double heterojunctions can be as low as 1,600 A/cm². This improvement has enabled continuous-wave operation at room temperature with output powers reaching tens of milliwatts.
4. **Temperature**: As temperature increases, so does the threshold current. Above 100K, the threshold rises nearly cubically with temperature. Therefore, semiconductor lasers typically perform best at low or room temperatures.
Another important characteristic is **directionality**. Due to the short cavity length, semiconductor lasers have relatively poor directivity. In the plane perpendicular to the junction, the divergence angle can be as large as 20°–30°, while in the plane parallel to the junction, it is about 10°.
The **quantum efficiency** (η) is defined as the number of photons emitted per second divided by the number of electron-hole pairs injected into the active region. At 77K, GaAs lasers can achieve quantum efficiencies of 70%–80%, but this drops to around 30% at 300K. The **power efficiency** (ηâ‚), which measures the ratio of optical power output to electrical power input, is also affected by various losses. Double-heterostructure devices can reach up to 10% efficiency at room temperature, but this improves significantly at lower temperatures, sometimes reaching 30%–40%.
Finally, **spectral characteristics** are influenced by the electronic structure of the semiconductor material. Lasing occurs between the conduction and valence bands, leading to a relatively wide spectral linewidth. For example, GaAs lasers emit light with a linewidth of several nanometers at room temperature, and the emitted light is not highly monochromatic. The peak wavelength shifts with temperature: it is 840 nm at 77K and 902 nm at 300K. These properties make semiconductor lasers suitable for a variety of applications, from telecommunications to medical and industrial uses.
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