Light sources enable the conversion from electrical signals to optical signals and are core components of optical transmitters and fiber optic communication systems. Their performance directly affects the performance and quality indicators of the fiber optic communication system. This section mainly introduces the structure, working principle, and related characteristics of two types of light sources: laser diodes (LDs, also known as lasers) and light-emitting diodes (LEDs), and provides their technical specifications.
Several physical concepts related to lasers
The concept of photons
Einstein's quantum theory of light states that light is composed of photons with energy hf, where h = 6.628 × 10⁻13 J·s, known as Planck's constant, and f is the frequency of the light wave. These photons are called photons.
When light interacts with matter, the energy of the photon is absorbed or emitted as a whole, establishing the wave-particle duality theory of light.
Atomic energy level
In semiconductor crystals, the orbits of electrons outside the atomic nuclei overlap to varying degrees due to the shared motion of adjacent atoms. As shown in Figure 3-1, the energy levels in the crystal no longer belong to any single atom; they can move over a wider area, even throughout the entire crystal. In other words, the original energy levels have been transformed into energy bands. The energy band formed by the outermost energy levels is called the conduction band, and the inner energy bands are called the valence band. No electrons exist in the intervals between them; this interval is called the band gap.
Figure 3-1 Energy levels in a crystal
Three modes of interaction between light and matter
The interaction between light and matter can be reduced to the interaction between light and atoms, including three physical processes: stimulated absorption, spontaneous emission, and stimulated emission. The energy levels and electronic transitions of these three interaction modes are shown in Figure 3-2.
Figure 3-2 Energy levels and electronic transitions in three modes of interaction between light and matter.
1) Under normal conditions, electrons are usually in a low energy level Ea. Under the influence of incident light, electrons absorb the energy of the photon and transition to a high energy level E2, generating a photocurrent. This transition is called stimulated absorption. This is the working principle of a photodetector.
2) Electrons in the high energy level E2 are unstable. Even without external force, they will spontaneously transition to the low energy level Ea, recombine with holes, and release energy converted into photons that are radiated outwards. This transition is called spontaneous emission. This is the working principle of a light-emitting diode (LED). Spontaneously emitted light is incoherent light.
3) When an electron in the high energy level Ea is excited by an external photon with energy hf, it is forced to transition to the low energy level Ea, recombine with holes, and simultaneously releases a photon with the same frequency, phase, and direction as the excitation light (called an identical photon).
Since this process is generated under the excitation of an external photon, this transition is called stimulated emission. This is the working principle of a laser. Stimulated emission light is coherent light.
Population inversion and light amplification
Stimulated emission is key to laser generation. Let the particle density at the lower energy level be N, and the particle density at the higher energy level be N². Under normal conditions, N > N², meaning stimulated absorption always exceeds stimulated emission; that is, under thermal equilibrium, matter cannot amplify light.
For matter to amplify light, stimulated emission must exceed stimulated absorption, even if N² > N (the number of electrons at higher energy levels is greater than the number at lower energy levels). This abnormal distribution of particle numbers is called population inversion.
Population inversion is the primary condition for a substance to produce light amplification and emit light.
Direct bandgap and indirect bandgap semiconductors
In stimulated emission of light, energy and momentum must be conserved. The band gap shape is related to momentum; based on the band gap shape, semiconductors can be divided into direct band gap and indirect band gap types, as shown in Figure 3-3. In direct band gap semiconductors, the minimum energy level of the conduction band and the maximum energy level of the valence band have the same momentum, and electrons transition vertically, resulting in high luminous efficiency, as shown in Figure 3-3a. In indirect band gap semiconductors, other particles must participate to maintain momentum conservation for electron transitions, as shown in Figure 3-3b. Only direct band gap semiconductor materials can be used to fabricate light-emitting devices; these materials include GaAs, AlGaAs, InP, and InGaAsP.
Figure 3-3 Direct bandgap and indirect bandgap semiconductors
Laser principle
A semiconductor laser is a laser that uses semiconductor materials as its active medium; it is also called a semiconductor laser self-oscillator.
For a laser to emit laser light, the following three conditions must be met: there must be a working substance (also called an activating substance) capable of generating laser light; there must be an excitation source (also called a pump source) capable of putting the working substance into a population inversion state; and there must be an optical resonator capable of performing frequency selection and feedback.
(1) The working substance capable of generating laser light is the substance that can achieve a population inversion distribution. Once activated, the working substance is called the activating substance or gain substance, and it is a necessary condition for laser generation.
(2) The pump source is an external excitation source that causes the working substance to achieve a population inversion distribution. Under the action of the pump source, Ni > Ni, resulting in stimulated emission greater than stimulated absorption, thus amplifying the light.
(3) The optical resonator: The activating substance can only amplify the light. Only by placing the activating substance in an optical resonator to provide necessary feedback and select the frequency and direction of the light can continuous light amplification and laser oscillation output be obtained. The activating substance and the optical resonator are necessary conditions for generating laser oscillation.
1) Structure of an optical resonant cavity. The structure of an optical resonant cavity is shown in Figure 3-4. By placing two parallel mirrors, M1 and M2, with reflection coefficients r1 and r2 respectively, at appropriate positions at both ends of the activating material, the simplest optical resonant cavity is formed, also called a Fabry-Perot cavity or F-P cavity.
If the mirrors are plane mirrors, it is called a plane cavity; if the mirrors are spherical mirrors, it is called a spherical cavity. Of the two mirrors, one must be able to reflect the light completely, and the other must be able to reflect partially.
Figure 3-4 Structure of an optical resonant cavity
2) The oscillation process of laser generation in a resonant cavity. A schematic diagram of a laser is shown in Figure 3-5. When the working medium achieves population inversion under the action of the pump source, spontaneous emission is generated. If the direction of spontaneous emission is not parallel to the axis of the optical resonant cavity, it is reflected out of the resonant cavity. Only spontaneous emission parallel to the axis of the resonant cavity can exist and continue forward. When it encounters a particle at a higher energy level, it induces a stimulated transition, emitting an identical photon in the transition from the higher energy level to the lower energy level-this is stimulated emission. When the stimulated emission light reflects back and forth once within the resonant cavity, and the phase change is exactly an integer multiple of 2π, several stimulated emission lights propagating in the same direction reinforce each other, producing resonance. After reaching a certain intensity, it is transmitted through partial mirror M2, forming a straight laser beam. When equilibrium is reached, the energy amplified by the stimulated emission light during each round trip within the resonant cavity exactly cancels out the energy consumed, at which point the laser maintains a stable output.
Figure 3-5 Schematic diagram of laser
3) Resonance condition and resonant frequency of an optical resonant cavity. Let the length of the resonant cavity be L, then the resonance condition of the resonant cavity is:
In the formula, c is the speed of light in a vacuum; λ is the laser wavelength; n is the refractive index of the activating material; L is the cavity length of the optical resonant cavity; and γ is the longitudinal mode number, γ = 1, 2, 3.
The resonant cavity provides positive feedback only to the wavelength of the light wave satisfying equation (3-1) or the frequency of the light wave satisfying equation (3-2), causing them to reinforce each other within the cavity and resonate to form laser light.
Since stimulated emission light only forms standing waves along the cavity axis (longitudinal direction), these are called longitudinal modes (different modes correspond to different field distributions).
4) Threshold condition for oscillation. The minimum gain limit at which a laser can produce laser oscillation is called the laser's threshold condition (F-P cavity has losses, and light reflection and refraction from mirrors also continuously consume photons). If Gu represents the threshold gain coefficient, then the threshold condition for oscillation is：
In the formula, α is the loss coefficient of the active material in the optical resonant cavity; L is the cavity length of the optical resonant cavity; and and are the reflection coefficients of the two mirrors of the optical resonant cavity.
