FAIZ RAHMAN of the University of Ohio in the United States believes that for phosphor-pumped white light sources, switching from LEDs to lasers can increase efficiency, extend service life and produce highly collimated beams.
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.
The process of switching from LED to laser does not happen overnight. On the contrary, LED-based luminaires will remain the primary means of generating white light for the foreseeable future. However, laser diode-based solid-state lighting systems have found application in high-brightness lighting applications and are now available in several manufacturers.
The driving force behind the development of laser diode pumping lamps is to avoid LED efficiency degradation. However, this is not the only benefit. In addition to Soraa and several others, the 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. These devices are cheap, efficient, and available on the market. Using short-wavelength phosphor pumping, the spectral output is richer, the spectral coverage is greater, and the color rendering index is higher than the typical LED-pumped white light source (see Figure 1).
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 high-intensity light sources for architectural, searchlights and car 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).
Figure 2. BMW's laser headlamps have separate laser diodes, beam combiners, phosphor targets, and reflector assemblies.
Laser pump structure
Unlike LEDs, different arrangements are required when using laser diodes to pump phosphors. 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 arrangements 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)).
Figure 3. Typical optical setup for laser diode pumping
Other solutions may be employed depending on whether a low power or high power source is required. For example, a small luminaire can use a beam expander lens to pump the entire phosphor plate. This method, shown in Figure 4, depicts a ray tracing simulation of a single through light.
Figure 4. A high efficiency pumping unit for small laser pump lamps. The pump light (red beam) enters from the left side, expands the lens through the concave beam, extends to the phosphor coated glass surface on the right side, and is incident on the silicone-bonded phosphor layer. The down converted light is shown as blue light. In the figure, the fluorescent plate is shown as opaque and separated from the green mirror in order to see the trajectory of the light within the reflector housing. In an actual lamp, a fluorescent plate (a transparent glass sheet that allows light to escape outward) is sealed to the side of the reflector. Only a small amount of light is reflected back to the laser module on the left (not shown).
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. However, with lasers, the biggest difference is the inherent consistency, which causes spots - visible and dark spots that appear on bright surfaces.
The shortcomings of the spot cannot be ignored. In addition to being distracting, it has a negative impact on visual perception and hinders the detection of spatial details of the illuminated object. Tests have shown that spots reduce visual acuity by 40%, reducing the ability to perceive 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 too small to be completely ignored. This is mainly because the good lamp design ensures that most of the laser is converted to longer wavelengths that do not emit spots. An additional benefit is that the remaining laser exhibits very small spots because its coherence is reduced as it undergoes multiple scattering as it passes through the phosphor layer. 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.
Pros and cons
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. Why is the diode life low? This can be traced back to tiny, pre-existing crystal dislocations in the laser structure. These defects are greatly increased when the device is driven at high power levels. Operating in this manner 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 decreases over time, and as the pump's source output drops, the brightness is too low until the light is off.
Encouragingly, as the maturity of laser diodes increases, the severity of this problem has diminished, but it has not been eliminated. Helping to solve this problem is to transform into a native GaN substrate for epitaxial growth because it reduces the number of interface threading dislocations and increases the lifetime of the laser diode.
Another breakthrough in laser-based white lighting is the reduction of high costs through economies of scale. This will open up new markets for UV and near-ultraviolet laser diodes. Although LED lighting is undoubtedly the leader today, in the next few years, please pay attention to the laser system.
Source: LED network
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