The Science Of The Laser Projector

Would it be a surprise if I told you about a display system that was coming so far advanced in color reproduction it would make a fool out of LCD’s and an OLED would blush? Throughout the last few decades we have gone from CRT, LCD, and are moving closer to OLED displays. There is something far more advanced coming in the next five to ten years – a laser projector. In 2008, Sony succeeded in developing the world’s brightest red laser diode array. Featuring 7.2W optical output power and a 635nm wavelength, this red laser diode array is ideal as a light source for projectors and so forth.

Generally speaking, displays combine red, green and blue light to create full-color images. Displays based on laser technology, therefore, use a tri-laser combination (one laser for each of these three colors). Laser displays in general offer key advantages in that they ensure advanced definition and a wide color gamut of 150% – far beyond that of LCD (70-90%) and OLED (100%). Of these three lasers, red laser diode arrays must be able to deliver high brightness along with efficiency and the ability to operate at room temperature in order to be used in projection equipment.

Lasers offer the following advantages in serving as display light sources.

• Excellent monochromaticity of laser light ensures excellent color reproduction. Imagine the most brilliant colors you’ve ever seen; a red so bright and crisp its comparable to a street light.
• Definition can be increased thanks to the excellent light-focusing characteristics of lasers.
• Because the light is polarized, excellent optical efficiency can be achieved without using optical elements to align polarity when lasers are used in conjunction with liquid crystal panels.
• Laser light sources last longer than lamps – about ten times as long, approaching 15,000 to 20,000 hours.

Creating a laser light source requires the combining of red, green and blue lasers (capable of generating optical output as high as several watts). Sony has remained a leader in the development of laser displays, including the development of compact laser light sources capable of high optical output power.

At Expo 2005 in Aichi, audiences were spellbound by the beautiful images displayed by Sony’s Laser Dream Theater, which was the biggest laser display system in the world – an 164 foot laser projection system using an 11-channel sound system, to screen short called “Voyage Around the Earth”. Sony has been very quiet about this technology until very recently, leading us to believe that it is still actively investing great resources into this method.

A laser “array” is composed of multiple light-emitting laser elements arranged side-by-side to form a single element and the array approach increases the optical output capacity achievable with a single device.

The diagram above shows the structure of the red laser diode array developed by Sony. To achieve a high optical output power from a single light-emitting unit, Sony increased the width of the laser stripes through which current flows to produce up to 60µm of light. By broadening the area of the laser stripes in this way, it is possible to reduce the light density at the emission end face. This reduces the possibility of catastrophic optical damage (COD), which occurs when the emission end face is destroyed by the intensity of its own light. Sony was able to raise output to several watts by arranging the laser elements in the array and broadening the laser stripes.

Within the high-output red laser diode array, layers of aluminum indium phosphide (AlInP) cladding are used to confine light inside the light-emitting layer. Previously this material tended to oxidize and was difficult to process, but Sony was able to develop new manufacturing and processing methods to prevent oxidation. A feature of AlInP is that its refractive index is significantly smaller than that of the gallium indium phosphide (GaInP) light-emitting layer (which emits red light). By using AlInP in the cladding layer, it is possible to confine the light produced by the GaInP light-emitting layer in the vicinity of that layer, which has a high refractive index. This means that even if the thickness of the cladding layer is reduced, it will still be possible to minimize the amount of light absorbed by the electrode and the substrate of the lasers. In general, cladding layers exhibit large thermal resistance when heat produced in the light-emitting layer escapes. Cladding layers also have their own electrical resistance, which means that any increase in the thickness of the layer will result in increased electrical resistance. The use of AlInP makes it possible to reduce the thickness of the cladding layers, resulting in lower thermal and electrical resistance overall.

These original Sony technologies have enabled engineers to increase laser oscillation to a level which achieves an optical output of 7.2W at a temperature of 45°C. The new device is also extremely reliable. Tests under extremely demanding conditions (35°C, 6.6W) showed that it could operate over 10,000 hours before optical output was halved.

One way to increase the brightness of a laser display is to raise the optical output power. Another method is to use a wavelength of light to which the human eye is extremely sensitive. In the red wavelength band, luminosity increases as the wavelength is shortened (Figure 5), resulting in higher brightness.

Relationship between wavelength and luminosity

The wavelength of the emitted light can be controlled by changing the percentages of gallium (Ga) and Indium (In) in the GaInP light-emitting layer. Sony’s challenge was to reduce the wavelength of its 645nm laser by 10nm to 635mm. However, when the wavelength is reduced below 640nm, heat output increases causing a rapid decline in the luminance efficiency of the GaInP light-emitting layer. This frustrated efforts to achieve laser oscillation at a high output level. Sony began developing new technologies to limit reductions in luminance efficiency. Now Sony has succeeded in oscillating a laser to produce an optical output of 7.2W at a temperature of 25°C and a wavelength of 635nm.

Before semiconductor lasers can be used as light sources for display devices, they must meet extremely demanding requirements, including not only high output, but also high brightness, operating temperatures, power conversion efficiency and reliability. However, these requirements are being tackled aggressively by several R&D departments in Sony and other companies. We are not very far off from the mass adoption of laser displays, especially laser projectors.