Laser Diodes: Operating Principles

Emitting Principles

Each electron in an atom or molecule has a specific discrete energy level, as shown in figure 1. The transition of electrons between different energy levels is sometimes accompanied by light absorption or emission with the wavelength expressed as:

C: Light velocity
E1: Energy level before transition
E2: Energy level after transition
h: Planck constant (6.625 × 10–34 J · s)

There are three types of electron transitions, as shown in figure 2.

The first type of transition, shown in figure 2 (a), is known as resonant absorption. An electron transits from the stable low energy level, E1, to the higher energy level, E2, by absorbing light.

Figure 2 (b) shows spontaneous emission. An electron transits from the high energy level, E2, to a more stable low energy level, E1. Simultaneously, the energy balance of |E2 – E1| is released in the form of light. Since each electron at level, E2, transits independently, light is emitted at random and out of phase. Such light is referred to as incoherent light and is one of the typical characteristics of spontaneous emission.

Under thermal equilibrium, the probability of an electron to exist in the lower level, E1, is higher than that in the higher energy level, E2. Therefore, electron transition to a higher energy level (E1 – E2) by light absorption is more likely to occur than light emission as shown in figure 2 (a). To emit light, electrons must exist at E2 with high probability, which is referred to as inversed population.

The third type of transition, shown in figure 2 (c) is stimulated emission. The electrons in the higher energy level, E2, are forcibly transferred to the lower energy level, E1, by incident light. The light generated this time is referred to as stimulated emission light. Its phase is the same as that of incident light, because stimulated emission light is emitted resonant to the incident light. Such stimulated emission light is referred to as coherent light.

Similarly to an electric circuit, laser oscillation requires a feedback function in addition to a gain which exceeds its loss. A laser beam is oscillated by amplification of stimulated emission and positive feedback with mirrors.

Figure 3 shows a Fabry-Pérot resonator which is the most fundamental optical resonator.

The structure of an LD, in principle, is the same as shown in figure 3, which uses cleaved facets to make the reflection mirrors of both surfaces. Incident spontaneous emission light heading to the reflection mirror is amplified by stimulated emission and comes back to the initial position after reflection. This process is subject to losses resulting from light passing through or diffracting at the reflection mirrors and scattering or absorption within the cavity.

When the loss is higher than the amplification gain, the light attenuates. Injected current strengthens amplification gain in an LD and when the gain and the loss are balanced, initial light intensity becomes equal to returned light intensity. This condition is referred to as threshold. A laser oscillates above the threshold when the gain is high enough.

Injection pumping mainly takes place at the p-n junction in a semiconductor laser diode. A semiconductor crystal can obtain higher gain than a gas laser (HeNe for example) due to the higher density of atoms available within the cavity. Therefore, a laser can oscillate with a short resonant cavity of 300 μm and low reflectivity of 30%.