Laser diodes increase the intensity of narrow beam illumination

- 2021-07-14-

LED and semiconductor laser (or laser LED) work in the same way, that is, when electrons and holes polymerize, light is emitted, and its 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 semiconductor laser is basically a single wavelength. The emission wavelength range of semiconductor laser can be from infrared to ultraviolet, and it has been widely used in optical fiber communication, bar code reader, CD reader and laser printing. But so far, the application of semiconductor laser in conventional lighting has been proved impractical.

Like traditional lasers, semiconductor lasers also need a resonator to amplify. The cavity consists of two parallel planes separated by several hundred microns, which act as mirrors to bounce the emitted photons back into the cavity. At low power levels, semiconductor lasers function like traditional LEDs. When the power is large enough (about 4 kW / cm2), the photons ejected between the two "mirrors" begin to stimulate the semiconductor material to emit more photons. When the generation of laser offsets and exceeds the internal loss, the device starts to "emit laser", that is, to emit coherent light of a single wavelength.

There are some similarities between traditional LED and semiconductor laser: both are powered by AC-DC driver, and the light output will decrease when the temperature rises. However, unlike traditional LEDs, semiconductor lasers do not seem to be affected by the droop effect, which increases the driving current, resulting in a decrease in efficiency (output lumen / input Watt). For lighting applications, conventional blue LEDs have higher efficiency than semiconductor lasers, but only at lower input current. Therefore, considering the required substrate area, it is not practical to generate the same level of light from conventional blue LEDs.

Although laser diodes have appeared since the 1960s, they have only recently been considered for lighting applications, especially in high-end automotive lamps, because of their high energy efficiency. BMW offers laser headlamps, claiming that they are 10 times brighter and 30% more efficient than led ones. It uses a precisely placed mirror to reflect the blue semiconductor laser inside the headlamp shell to generate a white light beam, which is then focused by a lens filled with phosphor to generate high-intensity white light.

Can semiconductor lasers be used for general lighting in the future? The theoretical energy efficiency limit of phosphor converted white LEDs is about 350 lumen / W, while the commercial lighting products are close to 200 lumen / W. The energy efficiency of semiconductor laser is 100 times higher than that of traditional LED, so it can provide very high light output with smaller die size. For applications with limited physical size (such as automobile headlamp), the attraction of semiconductor laser is obvious, but the disadvantage of using it for general lighting is that its emission cavity is very narrow (only about 1-2 degrees).

At present, it is not clear how many companies are committed to using semiconductor lasers for general lighting, but at least one company has provided relevant products. SLD laser company launched the laserlight surface mount device (SMD) as early as 2016. The device adopts blue light semiconductor laser, phosphor and high lumen package, and is packaged in 7 × The 7 mm package can emit about 500 lumens of white light without harm to human eyes. Its precise optical devices achieve a beam angle of no more than 2 degrees. Laserlight SMD device is the world's first UL 8750 certified semiconductor laser source.

Most likely, laser semiconductors are first used in lighting products for special buildings, which require narrow and high-intensity beams. For example, in museums, galleries, retail spaces and other special places, only one corner of the space needs to be illuminated, rather than the whole space. This is not only the need of space aesthetics, but also simplifies control and maintenance. However, due to the narrow beam spread by semiconductor laser, it may be necessary to combine fiber or waveguide to guide and transmit the emitted light in order to develop economic and feasible conventional lighting products.