Analysis of advantages and disadvantages of laser white light illumination and automotive lighting
2019-04-23

Currently, laser sources are based on semi-polar GaN laser diodes combined with advanced phosphor technology. Since the laser concentrates on a small spot on the phosphor to emit light and convert it into white light, the light source can output a safe and highly collimated white light. Laser-based methods, whose properties include a rise in temperature is much lower than that of LEDs. Laser pumping is also practical because devices that emit in near-ultraviolet light are readily available and can effectively perform this function. Solid-state lighting systems based on laser diodes have found applications in high-brightness lighting applications and are now available in several manufacturers.


One of the driving forces behind the development of laser diode pumping lamps is to avoid LED efficiency degradation. Currently, in addition to Soraa and several others, chips used by other LED source manufacturers do not contain wavelengths shorter than about 450 nm. Therefore, when these inexpensive LEDs with a peak wavelength of 450-460 nm are used for phosphor pumping, the resulting light source will be insufficient in the purple portion of the spectrum. This can be avoided by using a white light with a 405 nm UV-emitting laser diode. With short-wavelength phosphor pumping, the spectral output is richer, the spectral coverage is greater, and the color rendering index is higher than typical LED-pumped white light sources (see Figure 1).


激光白光照明及车用照明优点与缺陷解析

Figure 1. The chromaticity point of light from a laser diode-pumped white light is almost neutral (x = 0.3305, y = 0.3309), demonstrating the spectral richness of the source. The black line in the figure is the Planckian trajectory.


In many cases, the quantity that is more important than the quality of white light is the quantity. In this way, laser diodes pump light, so they are ideal for high-intensity light sources for architectural, searchlights and automotive headlights. Another property of the laser source is that the emitted light beam is extremely divergent and nearly parallel. The advantages of laser-pumped light sources have caught the attention of automakers who are actively installing laser headlights for high-end models. Just like the penetration of LED lighting in the automotive industry, high-end models are leading the way, including some of BMW's models (see Figure 2).


激光白光照明及车用照明优点与缺陷解析

Figure 2. BMW's laser headlamps have separate laser diodes, beam combiners, phosphor targets, and reflector assemblies


Laser pump structure


Unlike LEDs, when a laser diode is used to pump the phosphor, a different structure is required. Due to the directional nature of the laser radiation and its high intensity, it is not possible to simply deposit the phosphor on top of the pump unit. Instead, more optical designs are needed, such as a combination of a phosphor plate and a reflector, or a phosphor coated integrating sphere (see Figure 3). For both methods, one or more diodes can be used for pumping by combining multiple laser beams using a suitable technique. This means that there is no upper limit to the optical power of the laser pumping optical module. Another advantage of using a remote pumping structure is that the phosphor is not on the hot component, which prevents the phosphor from heating during operation, greatly extending its useful life. A relatively simple method of laser pumping is to direct the laser to the phosphor plate and collimate the resulting radiation with a reflector (see Figure 3(a)). However, in terms of optical power conversion, it is more effective to use a phosphor coated integrating sphere (see Fig. 3(b)).


激光白光照明及车用照明优点与缺陷解析

Figure 3. Typical optical setup for laser diode pumping


Although the white light source produced by laser pumping has many advantages, its limitations must also be considered. Just like an LED-based lamp, the emitted light contains two distinct components: downconverted light from the phosphor, and the remaining unconverted laser. The difference is that the biggest difference in the use of lasers is the inherent consistency, which leads to spots, that is, visible and dark spots appear on bright surfaces. These spots can not be ignored, the spots are not only distracting, but also have a negative impact on visual perception, hindering the detection of spatial details of the illuminated objects. Tests have shown that spots reduce visual acuity by 40%, reducing the ability to sense high and low spatial frequencies. These problems are part of the reason why direct laser lighting has not become popular. Please note that when using a laser projector, the speckle can be reduced to an acceptable level by some means to reduce the coherence of the laser. But with the laser illuminating the phosphor, the spots can even drop to a lower level. In this case, the spots are small and can be completely ignored. The best optical design ensures that most lasers are converted to longer wavelengths that do not emit spots, while the remaining lasers exhibit very small spots due to the multiple scattering that occurs as it passes through the phosphor layer. reduce. The phosphor-pumped laser system produces light, is rich in color, and has no spots, which is superior to the light produced by LED lamps.


Discussion on laser pumped white light source


The biggest drawback of laser-pumped white light sources is related to diodes. First of all, the price of this chip is much higher than LED. Although widely used in data storage applications, laser diodes are much more expensive than LEDs for equivalent levels of illumination. Due to the high cost, laser-pumped phosphor systems are expensive and can only be deployed in specialized, less cost-conscious lighting applications.


One advantage of a laser pumped illumination system is that it can cover a wide range of output power. Commercial laser diodes have power ratings ranging from a few milliwatts to a few watts, and a more powerful source can be formed by combining the outputs of several lasers. However, this is not conducive to the life of the system. This is because the diode life is lower than the lifetime of the LED, especially when they are operating at high drive currents. The reason for short lifetimes can be traced back to tiny, pre-existing crystal dislocations in the laser structure. These defects increase as the device is driven at high power. Running in this way will gradually form a wide network of dislocations, called dark line defects. They act as non-radiative composite sites, reducing the light output of the device. The brightness of the diode will decrease over time, and as the pump's source output drops, the brightness is too low until the light is off. However, as the maturity of laser diodes increases, the severity of these problems will diminish until they are eliminated.


Although LED lighting is undoubtedly the leader today, in the next few years, the laser system will develop rapidly.


Source: High-tech LED