Working principle of light-emitting diode
Light-emitting diodes (LEDs) used in fiber optic communication emit invisible infrared light, while LEDs used in displays emit visible light, such as red and green light. However, their light-emitting mechanisms are essentially the same. The emission process of an LED mainly corresponds to the spontaneous emission process of light. When a forward current is injected, the injected non-equilibrium carriers recombine during diffusion, emitting light. Therefore, LEDs are incoherent light sources and are not threshold devices; their output power is basically proportional to the injected current.
LEDs have a wide spectral width (30–60 nm) and a large radiation angle. In low-speed digital communication and narrow-bandwidth analog communication systems, LEDs are the optimal light source. Compared to lasers, LED driving circuits are simpler, and they offer higher production volume and lower cost.
The difference between LEDs and lasers is that LEDs do not have an optical resonant cavity and cannot generate laser light. They are limited to spontaneous emission, emitting incoherent light. Lasers, on the other hand, are stimulated emission, emitting coherent light.
LED Structure
LEDs also mostly use double heterojunction chips. The difference is that LEDs lack cleavage surfaces, meaning they lack optical resonant cavities, and because they do not oscillate like lasers, they have no optical resonance. LEDs are divided into two main categories: surface-emitting LEDs and edge-emitting LEDs. The structure of a surface-emitting LED is shown in Figure 3-11, and the structure of an edge-emitting LED is shown in Figure 3-12.
Figure 3-11 Structure of a surface-emitting LED
Edge-emitting LEDs also employ a double heterojunction structure. Utilizing SiO2 mask technology, a strip-shaped contact electrode (40-50mm) perpendicular to the end face is formed on the strip-shaped contact surface, thus defining the width of the active layer. Simultaneously, an optical waveguide layer is added to further enhance light confinement, guiding the light radiation generated in the active region to the emitting surface, thereby improving the combining efficiency with the optical fiber. One end of the active layer is coated with a high-reflectance film, and the other end with an anti-reflection film to achieve unidirectional light emission. In the direction perpendicular to the junction plane, the divergence angle is approximately 30°, exhibiting higher output coupling efficiency than surface-emitting LEDs.
Figure 3-12 shows the structure of an edge-emitting LED
LED operating characteristics
(1) Spectral characteristics: The spectral linewidth ΔA of LEDs is much wider than that of lasers. The emission spectrum of InGaAsP LEDs is shown in Figure 3-13.
Figure 3-13 Emission spectrum of InGaAsP LED
Since LEDs lack an optical resonant cavity to select wavelengths, their spectrum is primarily based on spontaneous emission, resulting in a broad spectral linewidth. The wavelength corresponding to the maximum luminous intensity on the spectral curve is called the emission peak wavelength λp, and the wavelength difference Δλ between the two half-intensity points on the spectral curve is called the LED spectral linewidth (or simply spectral width), which is a quantity related to temperature T and wavelength λ.
In the formula, c is the speed of light in a vacuum; h is Planck's constant, h = 6.625 × 10⁻³⁴ J·s; and k is Boltzmann's constant, k = 1.38 × 10⁻ J/K.
As can be seen from equation (3-10), the spectral width increases with the increase of the radiation wavelength λ according to λ². Generally, the spectral width of short-wavelength (GaAlAs-GaAs) LEDs is 10~50nm, and the spectral width of long-wavelength (InGaAsP-InP) LEDs is 50~120nm.
The spectral width increases with increasing active layer doping concentration. Surface-emitting LEDs are generally heavily doped, while edge-emitting LEDs are lightly doped; therefore, surface-emitting LEDs have a wider spectral width. Furthermore, heavy doping shifts the emission wavelength towards longer wavelengths. Additionally, temperature changes and variations in carrier energy distribution also cause spectral width changes.
(2) Output Optical Power Characteristics The P-I characteristic of an LED refers to the relationship between the output optical power and the injection current, as shown in Figure 3-14. As can be seen from Figure 3-14, surface-emitting devices have higher power, but are prone to saturation at high injection currents; while edge-emitting devices have relatively lower power. Generally speaking, under the same injection current, the output optical power of a surface-emitting LED is 2.5 to 3 times greater than that of an edge-emitting LED. This is because edge-emitting LEDs are subject to more absorption and interface recombination.
Figure 3-14 P-I characteristics of LED
(3) Temperature characteristics Since LEDs are thresholdless devices, they have good temperature characteristics and do not require temperature control circuits.
(4) Coupling efficiency Under normal application conditions, the operating current of LED is 50-150mA and the output power is a few milliwatts. Because the divergence angle of the beam emitted by LED is large, the coupling efficiency with optical fiber is low, and the power of the fiber is much smaller. It is generally only suitable for short-distance transmission.
(5) Modulation characteristics: LEDs have low modulation frequencies. Under normal operating conditions, the cutoff frequency of surface-emitting LEDs is 20-30MHz, and the cutoff frequency of edge-emitting LEDs is 100-150MHz, mainly due to the limitation of carrier lifetime.
Comparison of Lasers (LDs) and LEDs
Compared to optical diodes (LDs), LEDs have lower output power, wider spectral linewidth, and lower modulation frequency. However, LEDs offer stable performance, long lifespan, ease of use, a wide linear range of output power, and are simpler to manufacture and less expensive.
LEDs are typically coupled with multimode optical fibers for low-capacity, short-distance optical communication systems with wavelengths of 1.31μm or 0.85μm.
Laser diodes (LDs) are typically coupled with single-mode fiber for high-capacity, long-distance optical communication systems at wavelengths of 1.31 μm or 1.55 μm.
Distributed feedback lasers (DFB-LDs) are also primarily coupled with single-mode fiber or specially designed single-mode fiber for new high-capacity optical fiber systems at a wavelength of 1.55 μm, which is currently the main trend in optical fiber communication development.
