In modern technology, light-emitting diodes (LEDs) and

The difference in the principle of light emission: LED uses spontaneous emission recombination of carriers injected into the active area to emit light, while LD uses stimulated emission recombination to emit light. The direction and phase of the photons emitted by the light-emitting diode are random, while the photons emitted by the laser diode are in the same direction and phase.

LED is the abbreviation of Light Emitting Diode. It is widely seen in daily life, such as indicator lights of household appliances, rear anti-fog lights of cars, etc. The most notable features of LEDs are their long service life and high photoelectric conversion efficiency. Basically, in the PN junction of some semiconductor materials, when the injected minority carriers recombine with the majority carriers, excess energy will be released in the form of light, thereby directly converting electrical energy into light energy. When a reverse voltage is applied to the PN junction, it is difficult for minority carriers to be injected, so it does not emit light. This type of diode made using the principle of injection electroluminescence is called a light-emitting diode, commonly known as LED.

LD is the English abbreviation of laser diode. The physical structure of the laser diode is to place a layer of photoactive semiconductor between the junctions of the light-emitting diode. Its end surface is partially reflective after being polished, thus forming an optical resonant cavity. In the case of forward bias, the LED junction emits light and interacts with the optical resonant cavity, thereby further stimulating the emission of a single wavelength of light from the junction. The physical properties of this light are material-dependent. The working principle of semiconductor laser diodes is theoretically the same as that of gas lasers. Laser diodes are widely used in low-power optoelectronic devices such as CD drives in computers and print heads in laser printers.

(1) Difference in working principle: LED uses spontaneous emission recombination of carriers injected into the active area to emit light, while LD uses stimulated emission recombination to emit light.
(2) Difference in architecture: LD has an optical resonant cavity, which allows the generated photons to oscillate and amplify in the cavity, while LED does not have a resonant cavity.
(3) Difference in performance: LED does not have critical value characteristics, and its spectral density is several orders of magnitude higher than that of LD. The light output power of LED is small and the divergence angle is large.

A light-emitting diode is a semiconductor device that generates light by injecting electrons and holes. When electrons and holes recombine, energy is released in the form of photons, producing visible light or other wavelengths of light. In contrast, a laser diode is a special type of light-emitting diode that produces light through stimulated emission of radiation. In a laser diode, when electrons transition from a high energy level to a low energy level, they release photons corresponding to a specific frequency, thereby achieving coherent amplification of light.

The light beams generated by light-emitting diodes are usually incoherent, that is, the phase and frequency of the light waves have no fixed relationship. This makes the light beam of the light-emitting diode spread widely and cannot be highly focused. In contrast, the beams produced by laser diodes are coherent, meaning that the phase and frequency of the light waves have a fixed relationship. This allows the laser diode's beam to be highly focused, allowing for more precise applications.

The spectrum produced by light-emitting diodes is generally broad, containing a variety of wavelengths of light. This makes light-emitting diodes widely used in lighting, display and backlight fields. In contrast, laser diodes produce a narrow spectrum that contains only specific wavelengths of light. This makes laser diodes have higher application value in fields such as communications, measurement and medical treatment.

Light-emitting diodes are generally less efficient because some of the energy is lost as heat. In addition, the power of light-emitting diodes is usually small, limiting their use in high-power applications. In contrast, laser diodes are more efficient because the light waves they produce can be highly focused, thereby reducing energy loss. In addition, laser diodes can be larger in power, making them suitable for high-power applications.

Light-emitting diodes are widely used in lighting, display, backlight, signal transmission and other fields. Due to their lower cost and higher reliability, the market share of light-emitting diodes in these fields is gradually increasing. In contrast, laser diodes are mainly used in communications, measurement, medical, manufacturing and other fields. Due to their high power, high focus and high coherence characteristics, laser diodes have unique advantages in applications in these fields.

(1) Wavelength: that is, the working wavelength of the laser tube. Currently, the wavelengths of laser tubes that can be used as photoelectric switches include 635nm, 650nm, 670nm, 690nm, 780nm, 810nm, 860nm, 980nm, etc.
(2) Threshold current Ith: that is, the current at which the laser tube starts to generate laser oscillation. For general low-power laser tubes, its value is about tens of milliamperes. The threshold current of laser tubes with a strained multiple quantum well structure can be as low as 10mA. the following.
(3) Operating current Iop: That is, the driving current when the laser tube reaches the rated output power. This value is important for designing and debugging the laser driving circuit.
(4) Vertical divergence angle θ⊥: The angle at which the luminous strip of the laser diode opens in the direction perpendicular to the PN junction, generally around 15˚~40˚.
(5) Horizontal divergence angle θ∥: The angle at which the light-emitting band of the laser diode opens in the direction parallel to the PN junction, generally around 6˚~10˚.
(6) Monitoring current Im: that is, the current flowing through the PIN tube when the laser tube is at rated output power.

(1) Resistance measurement method: Remove the laser diode and measure its forward and reverse resistance values with a multimeter in the R×1k or R×10k range. Normally, the forward resistance value is between 20 and 40kΩ, and the reverse resistance value is ∞ (infinity). If the measured forward resistance value exceeds 50kΩ, it means that the performance of the laser diode has declined. If the measured forward resistance value is greater than 90kΩ, it means that the diode has been seriously aged and can no longer be used.
(2) Current measurement method: Use a multimeter to measure the voltage drop across the load resistor in the laser diode drive circuit, and then estimate the current value flowing through the tube according to Ohm's law. When the current exceeds 100mA, if the laser power potentiometer is adjusted (see Figure 5), and there is no obvious change in the current, it can be judged that the laser diode is seriously aging. If the current increases sharply and gets out of control, it means that the optical resonant cavity of the laser diode is damaged.

There are significant differences between light-emitting diodes and laser diodes in terms of working principles, beam characteristics, spectral characteristics, efficiency and power, and application fields. Light-emitting diodes are suitable for applications with low-power, incoherent light sources, such as lighting and displays, while laser diodes are suitable for applications with high-power, highly focused, and highly coherent light sources, such as communications and medical. Understanding these differences helps us better select and apply these two light source technologies to meet the needs of different fields.