Laser diodes are a type of semiconductor light source, first developed in the 1960s. They are commonly referred to as LD (Laser Diode) and stand for "Light Amplification by Stimulated Emission of Radiation." One of their most significant characteristics is high coherence, meaning the emitted light has a consistent wavelength and phase. Initially, they could only emit low-power red light, which was used as an indicator in early devices like those from HP.
The working principle of a laser diode involves a PN junction formed between two doped gallium arsenide layers. The structure includes two reflective surfaces—one highly reflective and one partially reflective—allowing photons to bounce back and forth. When a forward voltage is applied, electrons and holes recombine, releasing photons. These photons stimulate more emissions, leading to a chain reaction that amplifies the light into a coherent laser beam. This process requires the current to exceed a certain threshold, otherwise, it behaves like an LED, emitting incoherent light.
Inside, a typical laser diode consists of several components: the laser emitting region (LD), a photodiode (PD) for monitoring output, a glass cover for protection and resonance, and a metal casing for heat dissipation and shielding. Laser diodes can be classified based on their junction structure, such as homojunction, single heterojunction, double heterojunction, and quantum well. Among these, quantum well lasers are widely used due to their low threshold current and high power output.
Key technical parameters include the operating wavelength (ranging from 635 nm to 980 nm), threshold current (Ith), operating current (Iop), optical output power (Po), dark current (Id), differential efficiency (η), and divergence angles (θ⊥ and θ∥). These parameters define the performance and application range of the device.
To measure the optical output, an optical power meter is used, ensuring all laser light is captured without reflection. The IL (current-light) curve illustrates how the output power increases with current, reaching a maximum before saturation occurs.
Driving a laser diode requires careful control to avoid overcurrent or overvoltage. Common methods include using a constant current source, adding a current-limiting resistor, or incorporating an APC (Auto Power Control) circuit for stable output. Proper heat management is essential, as temperature affects performance and lifespan.
Compared to LEDs, laser diodes offer better coherence, directionality, and monochromaticity, making them ideal for applications like optical communications, medical treatments, cutting, and scanning. However, they are sensitive to electrostatic discharge (ESD) and require careful handling.
Safety is crucial when working with laser diodes. Direct exposure to the laser beam can cause eye damage, so protective measures must always be followed. Additionally, storing and operating the device in a clean environment helps maintain its performance.
In summary, laser diodes are versatile and powerful light sources with a wide range of applications. Their design, operation, and usage require attention to detail to ensure optimal performance and safety.
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