Laser diodes increase the intensity of narrow beam illumination

发表于2021-05-19

  1. LEDs and semiconductor lasers (or laser LEDs) work in the same way, that is, when electrons and holes are polymerized, they emit light, and the emission wavelength depends on the material used. The difference is that the spectral range of LED light is relatively narrow, while the light emitted by a semiconductor laser is basically a single wavelength. The emission wavelength of semiconductor lasers can range from infrared to ultraviolet, and has been widely used in fiber optic communications, barcode readers, optical disc readers, and laser printing. But so far, the application of semiconductor lasers in conventional lighting has proven to be unrealistic.


    Like traditional lasers, semiconductor lasers also require resonant cavities for amplification. The resonant cavity is composed of two parallel planes separated by a few hundred microns. These two planes act as mirrors and bounce the emitted photons back into the cavity. At low power levels, the function of a semiconductor laser is similar to that of a traditional LED. When the power is large enough (approximately 4 kW/cm2), the photons ejected between the two "mirrors" begin to stimulate the semiconductor material to emit more photons. When the generation of lasing particles offsets and exceeds the internal loss, the device starts to "laser", that is, to emit coherent light of a single wavelength.


    There are also some similarities between traditional LEDs and semiconductor lasers: both are powered by AC-DC drivers, and when the temperature rises, the light output will drop. But unlike traditional LEDs, semiconductor lasers do not seem to be affected by the Droop effect. The Droop effect increases the drive current, which leads to a decrease in the effect. Regarding lighting product applications, conventional blue LEDs have higher effects than semiconductor lasers, but only at lower input currents. Therefore, considering the required substrate area, it is not theoretical to produce the same level of light from a conventional blue LED.


    Although laser diodes have been presented in the 1960s, they have only recently been energy efficient enough to be used in lighting applications, especially in high-end automotive lights. BMW provided laser headlamps and claimed that it is 10 times brighter than LED headlamps and has 30% higher efficiency. It uses a precisely placed reflector to reflect the blue semiconductor laser inside the headlamp housing to produce a white light beam, and then focuses it through a lens filled with phosphors to produce high-intensity white light.


    Can semiconductor lasers be used for general lighting in the future? The theoretical energy efficiency limit of phosphor-converted white light LED is about 350 lumens/W, while commercial lighting products are close to 200 lumens/W. The energy efficiency of semiconductor lasers is 100 times higher than that of traditional LEDs, so it can provide very high light output with a smaller die size. For application scenarios with limited physical size (such as car headlights), the absorption power of semiconductor lasers is obvious, but the disadvantage of using them for general lighting is that their emitting cavity is very narrow (about 1-2 degrees).


    At present, how many companies are trying to use semiconductor lasers for general lighting, we still don't know, but at least one company has provided related products. SLD Laser launched the LaserLight surface mount device (SMD) as early as 2016. The device uses a blue semiconductor laser, phosphors and a high-lumen package. It can emit about 500 lumens of white light in a 7×7 mm package. The human eye does not hurt. Its precision optics completes a beam angle that does not exceed 2 degrees. The LaserLight SMD device is the world's first semiconductor laser light source to obtain UL 8750 safety certification.


    The most likely situation is that laser semiconductors are first applied to lighting products for special buildings, and these application scenarios require narrow and high-intensity light beams.


    For example, in museums, galleries, wholesale spaces and some other special places, only one corner of the space needs to be illuminated instead of the entire space. This is not only a requirement for space aesthetics, but also simplifies control and maintenance. However, due to the narrow beam of semiconductor lasers, to develop economically feasible conventional lighting products, it may be necessary to separate optical fibers or waveguides to guide and transmit the emitted light.