Phosphor pumping: Lasers outshine LEDs
For phosphor-pumped white-light sources, switching from LEDs to lasers increases efficiency, lengthens lifetime and yields highly collimated beams
BY FAIZ RAHMAN FROM OHIO UNIVERSITY
One solution to both these impediments is to use a laser, rather than a short-wavelength-emitting LED, to pump the phosphor and generate white light via colour mixing. Attributes of the laser-based approach include immunity from droop and reduced thermal heating "“ increases in temperature are far lower than those of LEDs. Laser pumping is also practical, as devices emitting in the near UV are easily available, and can effectively serve in this role.
The evolution from LED-pumping to laser-pumping will not take place overnight. Instead, LED-based luminaires are destined to remain the principal means of generating white light for the foreseeable future. However, laser-diode-based solid-state lighting systems are already gaining ground in high brightness lighting applications, and are now commercially available from several manufacturers.
A driving force behind the development of laser-diode-pumped luminaires has been the promise of avoiding the LED efficiency droop. That's not the only benefit, however. With the exception Soraa and possibly a few others, the chips used by the makers of LED sources do not contain wavelengths shorter than about 450 nm. So, when these low-cost LEDs with a peak wavelength of 450-460 nm are adopted for phosphor pumping, the sources that result are deficient in the violet part of the spectrum.
This limitation can be avoided with white lamps that employ 405 nm violet-emitting laser diodes. These devices are cheap, highly efficient, and already mass-produced for the optical data storage market. Armed with short wavelength phosphor pumping, the spectral output is richer, the spectral coverage greater, and the colour-rendering index higher than it is with a typical LED-pumped white-light source (see Figure 1 for a chromaticity diagram, which contains a chromaticity point for a laser diode-pumped white light lamp).
Figure 1. The chromaticity point of light from a laser diode-pumped, white-light lamp is almost neutral (x = 0.3305, y = 0.3309), attesting to the spectral richness of the light source. The black line inside the diagram is the Planckian locus.
In many cases, even more important than the quality of the white light is the quantity. And judged in this manner, the laser diode-pumped source literally shines. Intense light from the laser produces very strong phosphor pumping and equally intense light emission. This allows the pairing of laser diodes and appropriate phosphors to form ideal high-intensity light sources that are suitable for architectural lighting, searchlights and automobile headlights. Another attribute of laser-based sources is their directional nature, as they emit a concentrated low-divergence light beam.
The merits of the laser-pumped light source have not escaped the notice of automobile manufacturers "“ several of them are actively working on laser-powered headlamps for their high-end models. Just as it was with the penetration of LED lighting in the automobile sector, it is premium-quality vehicles that are leading the way, including some models by BMW (see Figure 2).
Figure 2. A BMW laser-based headlamp features individual laser diodes, a beam combiner, phosphor target and reflector assembly.
Laser-pumping architectures
A different arrangement is required when a laser diode, rather than an LED, is used to pump the phosphor. It is no longer possible to simply deposit a phosphor on top of the pump device, due to the directional nature of laser radiation and its high intensity. Instead, more involved optical arrangements are needed, such as the combination of phosphor plates and reflectors, or the use of a phosphor-coated integrating sphere (see Figure 3).
For both these approaches, more than one diode may be used for pumping, so long as a suitable technique is employed for combining the multiple laser beams. This means that there is no upper limit on the optical power available from a laser-pumped light module. An additional advantage of any remote pumping architecture is that the phosphor does not sit on a hot component. This prevents the phosphor from heating up substantially during operation, greatly prolonging its life.
A relatively simple approach for laser pumping is to direct the laser at a phosphor plate and collimate the resultant radiation with a reflector (see Figure 3 (a)). However, it is more efficient, in terms of optical power conversion, to use a phosphor-coated integrating sphere (see Figure 3 (b)).
Figure 3. Typical optical setups for laser diode pumping of wavelength down-conversion phosphors.
Other arrangements may be adopted, depending on whether a low-power or high-power light source is required. For example, small luminaires can use a beam expansion lens to pump the entirety of the phosphor plate. Such an approach is illustrated in Figure 4: it depicts a ray trace simulation for a single pass-through lamp.
Figure 4. An efficient pumping arrangement for a small laser-pumped lamp. Pump light (red beam) enters from the left, passes through a concave beam expansion lens, expands to the diameter of the phosphor-coated glass plate on the right and is incident on the silicone-bound phosphor layer. The down-converted light is shown as a collection of blue rays. The phosphor plate, deliberately shown as opaque, is detached from the green reflector. This allows the trajectory of light rays inside the reflector envelope to be seen. In an actual lamp the phosphor plate, a piece of transparent glass that allows light to escape outwards, is sealed to the reflector rim. Very few rays are back-reflected towards the laser module to the left (not shown).
All of the strengths of white-light sources produced by laser pumping must be weighed against their limitations. Just like lamps based on LEDs, the emission contains two distinct components: down-converted light from the phosphor, and the residual un-converted laser light. However, with a laser, the big difference is intrinsic coherence, which results in speckle "“ a visible pattern of light and dark spots that appear on any illuminated surface.
The downsides of speckle should not be overestimated. As well as being a distraction, it has a negative effect on visual perception, hampering the detection of fine spatial detail in illuminated objects. Tests have shown that speckle decreases visual acuity by up to 40 percent, and reduces the ability to perceive high and low spatial frequencies. These issues are part of the reason why direct laser illumination has never caught on.
Note that with laser projectors, the coherence of the laser light is reduced by elaborate means "“ this tames speckle to acceptable levels. But it reaches even lower levels with laser-illuminated phosphors. In this case, the speckle is so small that it can be completely neglected. That's primarily because good lamp design ensures that the majority of the laser light is converted to longer wavelengths that do not show speckle. However, an additional boon is that the residual laser light shows very little speckle, thanks to a reduction in its coherence when it undergoes multiple scattering as it passes through the phosphor layer. The upshot of the phosphor-pumped laser system is intense, colour-rich, speckle-free light that is superior to that produced by an LED luminaire.
Pros and cons
The greatest downsides of the laser-pumped, white-light source are associated with the diode. First and foremost, this chip is much pricier than the LED. Despite the maturity that stems from its widespread adoption in data storage applications, for an equivalent level of light emission, the laser diode is far more expensive than the LED. Due to this high cost, laser-pumped phosphor systems are pricy, restricting their deployment to specialised, non-cost-sensitive lighting applications.
One strength of laser-pumped lighting systems is that they can span a wide range of output powers. Commercial laser diodes have power ratings that range from a few milliwatts to several watts, and even more powerful sources can be formed by combining the output of several lasers. However, this is detrimental to the longevity of the system. That's because diode lifetimes are inferior to those of LEDs, especially when they are operated at high drive currents. The shorter lifetimes can be traced back to the tiny, pre-existing crystal dislocations in the laser structures. These imperfections multiply greatly when the device is driven at high power levels. Running in this manner leads to the gradual formation of extensive dislocation networks, called dark line defects. They act as sites of non-radiative recombination, reducing the light output from the device. The diode's brightness diminishes over time, and the output of light source pumped by it falls, until it is so low that it is not of any use.
Encouragingly, as the maturity of the laser diode has increased, the severity of this problem has diminished "“ but it is yet to be eliminated. Helping to address this issue has been a shift to native GaN substrates for epitaxial growth, because this trims the number of interfacial threading dislocations and increases the lifetime of the laser diodes.
The other sticking point to adoption of laser-based white lighting, the high cost, may come down through economies of scale. This will open up new markets for violet and near-UV laser diodes; opportunities that were unforeseen only a few years ago. While LED lighting is undoubtedly in the ascendancy today, watch out for laser-based systems in the years that follow.
