9.3.2 Semiconductor Diode Laser Concepts

It was a surprisingly small step from the first efficient LEDs to the first semiconductor lasers. Both were demonstrated in 1962 in the same material, gallium arsenide, and the LED was almost overlooked in the race to make the first semiconductor laser.

The simplest diode lasers are structurally similar to LEDs. Both generate light from recombination of electron hole pairs at a forward-biased junction. Below the laser threshold, both generate spontaneous emission with an intensity that depends on the drive current. However, diode lasers have reflective surfaces that create optical feedback. The feedback has little impact when drive current is below the point needed to produce a population inversion, but is critical in crossing the threshold for laser operation.

At low drive current, electron hole pairs (excitons) release their energy by spontaneous emission, as in an LED. As the drive current increases, it produces more electron hole pairs that emit light spontaneously, increasing the likelihood that a spontaneously emitted photon will encounter and stimulate emission from an exciton that has yet to release its extra energy. Once the drive current reaches a high enough level, it produces a population inversion between the exciton state (the upper laser level) and the atoms with the extra electron bound in the valence band (the lower laser level). That leads to a cascade of stimulated emission as the laser crosses the threshold.

Because the excitons are in the thin layer of the junction plane, stimulated emission is most likely to build up along the junction. For this reason, the reflective cavity is aligned along the junction plane, with reflective surfaces perpendicular to the junction, as shown in Figure 9-7.

Semiconductors have a high refractive index, so an uncoated solid air interface reflects much of the stimulated emission back into the semiconductor, as shown in Equation 5-4, providing feedback for the laser resonator. The large population inversion at high drive current makes gain high in semiconductor lasers, so cavities only a few hundred micrometers long can sustain oscillation. One facet often is coated to reflect all the incident light, so all the stimulated emission emerges from the other end, as shown in Figure 9-7.

Diode lasers have a well-defined threshold at which their output shifts from low-power spontaneous emission to higher-power laser operation, as shown in Figure 9-8. Below the threshold, the diode operates as a relatively inefficient LED. Above the threshold, the diode operates as a laser, converting a much higher portion of the input electrical power into light energy, as shown by the steeper slope.

Threshold current is an important factor in semiconductor laser performance. Electrical power needed to reach the threshold current winds up as heat that must be dissipated in the laser. So does the fraction of above-threshold current that is not converted into light. The extra heat is not just wasted power; it also degrades

laser performance and tends to shorten its lifetime, so lowerthreshold lasers tend to have longer lifetimes. High currents also stress the laser by putting highly concentrated power through the junction; this is measured as threshold current density (threshold current divided by the junction area operating as a laser) rather than the total threshold current.

Diodes built to operate as lasers are inherently less efficient as LEDs than those built to operate as LEDs. A main reason is packaging. As you saw earlier, spontaneous emission is emitted in all directions, and LEDs are designed to collect as much as possible of this light. However, edge-emitting diode lasers like the one shown in Figure 9-7 collect light only from the junction layer at the edge of the chip, a much smaller area. When a diode built to operate as a laser is below threshold, only a small fraction of the spontaneous emission it generates emerges through the edge of the junction plane; the rest is trapped within the packaged device.