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15 Sep 2006
Novalux believes that its Necsel laser platform meets all of the requirements for use in high-volume applications, such as laser-based rear-projection televisions. Greg Niven compares Novalux's extended-cavity surface-emitting lasers with other light sources and explains why we can expect to see laser-based products by the end of 2007.
The display industry is evolving rapidly thanks to consumer demand for high-quality, affordable displays everywhere from home theatres to palm tops. Recent announcements from major consumer electronics companies have touted laser technology, particularly for projection applications currently using ultrahigh-pressure (UHP) lamps. These include rear-projection television (RPTV) and front projectors. Other potential applications include personal projection, digital cinema and speciality lighting.
Lasers offer significant advantages for projection display because of their large colour gamut, high brightness, high-power conversion efficiency and long lifetime. But while a number of solid-state lasers have been developed for projection applications, none has been able to reach the required price points for integration into mass-produced consumer electronics devices. Novalux aims to change this with Necsel, a low-cost laser platform based on high-power surface-emitting diode lasers. Necsel allows efficient frequency doubling into the red, green and blue with the potential to achieve the necessary price and performance requirements for high-volume applications.Laser TV
Home theatre is the first high-volume display application to benefit from laser technology. Several consumer electronics giants are now vying to be the first to market with large-screen, high-definition (HD) RPTVs to cater for the ever-increasing demand for the technology. In fact, while many people today want 50-inch screens, analysts predict that by 2009 the demand will be for 60 inch displays.
These large screen sizes use a miniature version of the image, called a microdisplay, which is typically less than half a square inch. This is then illuminated by a light source, magnified by optics, and projected onto the back surface of a screen.
Current RPTVs rely on white-light UHP arc lamps as their illumination source for the microdisplay. The most common system is based on digital light processing (DLP) micromirror arrays and is being pioneered by Texas Instruments. In a DLP system, a filter wheel transmits red, green and blue light onto the microdisplay in rapid sequence. Another leading technology, 3LCD, splits the white light using dichroic filters and then projects images from three transmissive high-temperature polysilicon (LCD) panels, one each for the red, green and blue beams.
UHP lamps, however, have come under fire for their relatively short lifetime, limited brightness and poor colour coverage. Lamp lifetime is around 8000 h (defined as the time at which 50% of the population has failed) whereas a television's life expectancy is 30,000 h. Replacing the lamp typically costs in excess of $300 (€240) and changing the bulb requires a qualified technician.
Because UHP lamps cannot provide enough light for large screen sizes, sacrifices are made by using high-gain screens, which decrease the viewing angle, and desaturating the colours so that only 75% of the NTSC colour standard is achieved. It is worth noting that this is worse colour coverage than the legacy CRT that it is replacing.
Both LEDs and lasers are attractive replacement light sources. In particular, LEDs are proving an acceptable alternative for use with screen sizes of up to 55 inches. They offer a lifetime of approximately 20,000 h (defined as the time in which they decrease in power by 50%, however this is undesirable for TV viewing) and they produce around 130% of the NTSC colour standard. The drawbacks of LEDs include limited brightness, colour shifts as the device ages, colour balance variation with changing ambient temperature conditions and cost.
Lasers offer significant performance advantages over competitive light sources. Necsel lasers emit saturated primary colours at fixed wavelengths that reach over 200% of the current NTSC broadcast standard resulting in a superior, more lifelike image. Necsel technology is also wavelength selectable, so integrators can create multiprimary laser displays that include colours such as cyan. The brightness of a Necsel array is six orders of magnitude greater than lamps or LEDs. And a Necsel laser's bright, speckle-free output results in clear, vibrant, high-resolution images consistent with true HD content.
Lasers also reduce system cost, which is crucial if laser-based products are to compete in this aggressive marketplace. Source lifetime is over 30,000 h at 100% power output – lasting as long as the TV itself. The unique properties of laser light also enable manufacturers to produce lower-cost light engine architectures than lamp-based systems. So much so that the cost of the lasers is lower than the cost of the components that can be removed from the engines by converting to laser light.
Specifically in a laser TV, the light engine is simply a combination of red, green and blue lasers that project high-power light directly onto the microdisplay. In a DLP system, this eliminates the need for a colour wheel, colour wheel motor, light tunnel and relay optics. In 3LCD light engines, it eliminates the polarizers, colour filters, turning mirrors and fly's eye lenses. What's more, a Necsel laser's low étendue enables smaller, less costly microdisplay panels and more affordable projection optics.
By 2008, laser TV manufacturers aim to mass-produce 50-inch RPTVs exhibiting 500 nits, full high-definition 1920 × 1080 resolution and over 200% NTSC colour coverage. The systems will be lightweight and thin and all for target prices of under $1000. This would catapult RPTV in price and performance ahead of plasma, the leading large-screen-display technology in today's market.
Lasers for high-volume production
Necsel technology is based on high-power, extended-cavity, surface-emitting semiconductor lasers. These lasers can produce high optical power from a single emitter and can be scaled to any required power by using one- or two-dimensional arrays.
The Necsel platform uses a p-doped Bragg mirror with nearly 100% reflectivity that is soldered to the heatsink for efficient heat removal (see figure above). An n-doped Bragg mirror, with reflectivity low enough to prevent lasing, sandwiches the gain region containing several GaInAs quantum wells. These quantum wells are placed at the anti-node of the laser field.
The cavity is stabilized by a thermal lens, or by an intracavity lens, to form a simple semi-confocal cavity with a standard periodically poled bulk lithium niobate (PPLN) crystal inside the resonator. The GaAs substrate is inside the resonator and a standard glass volume Bragg grating is used as an output coupler that also controls the operating wavelength. This allows the use of all flat optics for simplified, low-cost manufacturing.
Unlike edge-emitting diode lasers, Necsel lasers do not suffer catastrophic damage when used in pulsed mode and can be operated at repetition rates of up to 1 MHz. The resulting high peak powers allow efficient nonlinear conversion into the visible. To date, Novalux has demonstrated lasers at 465, 532 and 625 nm – the key wavelengths for an optimized projection system.
Constructed with standard InGaAs semiconductor material, Necsel lasers can be manufactured using wafer-scale methods and known good-die techniques. The technology also utilizes standard components including PPLN (currently mass produced for surface acoustic wave applications), and standard flat glass output couplers.
These characteristics make Necsel the first solid-state laser technology to approach the performance and price demands of high-volume display applications. Moreover, high-volume production will further reduce unit costs. At present, the cost of manufacturing an RGB Necsel light source with more than 3 W per colour is less than $70 for all three colours with production at about one million units per year. This drops to less than $40 for 10 million units per year.
Other emerging applications
High-power diode lasers could also enable devices as varied as pocket projectors and digital cinema systems. This is attractive to next-generation content producers because uniform colour standards could be applied from one device to the next. So from embedded displays in mobile phones to home cinema systems, consumers could expect similar image quality and uniformity. Future laser devices include:
High-power lasers in every home
Today, only low-power lasers are standard in high-volume consumer devices (low-power diode sources are found in every CD and DVD player). Necsel marks the first high-power laser source poised to enable high-volume consumer applications. Strategic partnerships with consumer electronics companies have been announced and the infrastructure for production is being put in place. So when will consumers see laser display products on their store shelves? Better start writing your Christmas list – the target date for laser-based RPTV products is December 2007.
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