Laser diode resistance

Laser diodes exhibit a low forward resistance that varies with current and temperature, and their dynamic resistance is critical for stable operation.

Forward and Dynamic Resistance

A laser diode is electrically similar to a PIN diode, and its resistance is not constant like a resistor. The forward resistance is the ratio of the applied forward voltage to the current, typically around 1–3 ohms for small laser diodes, but it decreases as the current increases above the threshold . More importantly, the dynamic (differential) resistance is defined as the slope of the voltage-current (V–I) curve above the threshold current, representing how the voltage changes with small variations in current . This dynamic resistance is usually very low, often in the range of a few tenths of an ohm, which makes the diode highly sensitive to current surges.

Threshold Current and L–I–V Characteristics

The threshold current (I_th) is the minimum current at which the diode begins to lase. Below this current, the diode behaves like a normal LED, and above it, the voltage increases only slightly with increasing current due to the low dynamic resistance . The L–I curve (light output vs. current) and the V–I curve (voltage vs. current) are used to determine the diode's slope efficiency and dynamic resistance. The slope of the V–I curve above threshold gives the differential resistance, which is crucial for designing current-limiting circuits to prevent damage .

Practical Considerations

Because laser diodes have very low resistance, even small current surges can cause catastrophic optical damage (COD) or permanent degradation . Therefore, current-limiting resistors, precision laser drivers, and ESD protection are essential. The forward voltage is typically around 1.5 volts, but it varies with temperature and diode material . Temperature increases can reduce resistance slightly but also raise the threshold current, so thermal management is critical for stable operation .

Summary

• Forward resistance: Low, decreases with increasing current.
• Dynamic resistance: Very low above threshold, critical for current stability.
• Threshold current: Defines onset of lasing; resistance behavior changes above this point.
• Circuit design: Requires careful current control and thermal management to prevent damage. Understanding both the static and dynamic resistance of a laser diode is essential for safe and efficient operation in optical and electronic systems.