The laser diode threshold is the point at which the optical gain equals the cavity losses, allowing lasing to begin.

Definition and Physical Meaning

The threshold of a laser diode is the condition where the small-signal gain of the diode just compensates for the losses in the optical cavity, enabling coherent laser emission to start . Below this threshold, the diode emits only spontaneous emission, which is incoherent and weak. Above the threshold, stimulated emission dominates, producing a sharp increase in optical output power .

Population Inversion and Gain

For lasing to occur, the diode material must achieve population inversion, meaning more electrons occupy the conduction band than the valence band . This is achieved by injecting a current that separates the quasi-Fermi levels of electrons and holes, creating a state where stimulated emission exceeds absorption . The optical gain is directly proportional to the injected current once population inversion is established.

Threshold Current and Optical Output

The threshold current (I_th) is the electrical current at which the diode reaches the threshold condition. At this point, the junction voltage and carrier density stabilize, and any additional current primarily contributes to stimulated emission, increasing the optical output . The optical power versus current (L/I) curve shows a distinct "knee" at the threshold, marking the transition from spontaneous to stimulated emission .

Factors Affecting Threshold

Several factors influence the laser diode threshold:

• Cavity losses: Higher mirror or internal losses increase the threshold current .
• Gain efficiency: Materials with higher gain per unit current reduce the threshold .
• Temperature: Increasing junction temperature raises the threshold current due to reduced carrier density and gain .
• Mode area and emission bandwidth: Smaller mode areas and narrow emission spectra improve gain efficiency, lowering the threshold .

Practical Operation

In practice, laser diodes are operated well above threshold, typically 3–10 times the threshold current, to achieve stable, high-power, and low-noise output . Understanding the threshold is crucial for designing drive circuits, optimizing efficiency, and preventing damage from excessive currents . In summary, the laser diode threshold is a fundamental parameter that defines the onset of lasing, determined by the balance between optical gain and cavity losses, and is influenced by material properties, device design, and operating conditions .