Driving diode lasers loses much of its mystery if you observe a few simple rules based on their properties. Below its threshold current, a diode laser emits LED light with spontaneous emission only. At the threshold current and above, it begins to generate laser light, and the optical output power rises steeply with increasing diode operating current. The correlation between the optical output power and the diode current is linear up to the maximum power, called differential efficiency (Figure 1). One crucial figure often overlooked in handling diode lasers is the influence of temperature on the relationship between the optical output power and the operating current. The threshold current rises with temperature, but the optical output power and differential efficiency decrease. The driver circuit thus should have a safety feature o that a significant temperature increase will not destroy the laser.