Red Diode Lasers in Laser Display Applications

Laser display can truly reproduce the rich and bright colors of the objective world, and has shocking expressive power, which is called the fourth generation display technology. Compared with the natural light color gamut seen by the human eye, traditional display devices can only reproduce 30% of the color gamut, while laser display can cover 90% of the color gamut, and the color saturation is more than 100 times that of traditional display devices. In addition, laser display can also realize dual high-definition and true three-dimensional display of image geometry and color, which is the best way to achieve high-fidelity images. Therefore, laser display is also known as the "revolution in the history of human vision". In 1966, Korpel et al. first proposed the idea of using laser as a display light source, and then researchers from all over the world invested in the research tide of laser display. The emergence of laser display technology also provides a new opportunity for my country's development in the display field. In order to further promote the development of my country's laser display industry, in the 1980s, my country proposed the national 863 plan for laser full-color display, and established an industry alliance around laser display technology. The light source of laser display has ushered in the era of semiconductor laser after gas laser and solid-state laser. After entering the 21st century, semiconductor laser technology has developed in an all-round way, the power and performance of the device have been greatly improved, and it is more competitive as a light source for laser display. Semiconductor lasers can be directly excited by current, which is more efficient than solid-state lasers; the working material decays slowly and has a longer service life; the light source system is smaller in size and suitable for high integration; the use of semiconductor technology for large-scale production can make the device cost more expensive. Low.

Laser display requirements for red light source

The wavelength selection of the laser display system for the red light source mainly considers two factors: 1) According to the human eye's responsivity to the wavelength, the wavelength that the human eye is sensitive to is selected to obtain higher optical efficiency; 2) The selected wavelength It can expand the coverage of the color gamut, so as to obtain a better color experience. For red lasers larger than 600nm, the shorter the wavelength, the higher the optical efficiency; the longer the wavelength, the larger the color gamut coverage. According to the standard of the National Television Standards Committee (NTSC), when the 620nm red light is selected, the luminous efficacy is 0.33lm/W, and the color gamut at this time can reach 161%; when the 650nm red light is selected, the color gamut is as high as 211. %, the luminous efficacy is reduced to 0.141lm/W. Therefore, in practical applications, it is necessary to comprehensively consider the application scene of the laser display and the performance of the light source system to select the appropriate laser wavelength. At present, the wavelength of red light used for laser display in the world is usually concentrated in 630-650nm, of which the 638nm red semiconductor laser has the best comprehensive performance.

The light source power required for laser display is equal to the screen brightness divided by the optical efficiency of the laser light source, and the screen brightness is equal to the ambient brightness multiplied by the screen area and divided by the contrast of the screen. In simple terms, the output power of the red semiconductor laser is about 50mW for an A4 paper size screen to ensure normal projection needs; for a 40inch (101.6cm) screen, the output power is at least 500mW; and for a large size screen, When the luminous flux is above 1000lm, the output power needs to be above 25W. With the development of red semiconductor lasers, the output power of the device has been greatly improved. At present, the power level of commercial 638nm red semiconductor lasers has reached the watt level. Through photosynthetic beam processing, the power level can meet the requirements of most laser displays. application requirements. The requirements of laser display for the beam quality of the light source mainly depend on the laser display technology used. At present, the mainstream laser display technology is divided into three categories: laser line scanning, laser point scanning and laser projection. The volume and efficiency of laser line scanning are between those of laser projection and point scanning. This technology is mainly used in the field of micro-projection; laser point scanning has high efficiency, small size, and low cost of the whole system, but it does not affect the beam quality and modulation of the light source. The system has high requirements and low brightness, and is only suitable for display applications of small size (less than A4 paper). Laser projection technology does not have high requirements on the beam quality of the light source, and the allowable luminous flux within the safety range of the human eye is large, which is suitable for most display fields.

The basic principle and structure of red semiconductor laser

The wide strip structure is a common design for high-power lasers, as shown in Figure b, which is a common chip structure with index-guided structures. The structure guided by the material refractive index difference can not only limit the injection current and the lateral diffusion of carriers, but also limit the lateral penetration of the optical field. Therefore, the refractive index guiding mechanism can effectively reduce the threshold current of the device, and the heat generated in the active region can be dissipated to the surrounding passive region to maintain the thermal stability of the device.

Technical Difficulties of Red Lasers

1. Shorten the wavelength

The main material of the red light active region is AlGaInP and the substrate GaAs. The theoretical wavelength is 580-680nm. Most of the early wavelengths are around 680nm. To shorten the wavelength, it is necessary to increase the band gap width and increase the Al content. When the Al composition is increased, the band gap width of the active region becomes larger, which shortens the lasing wavelength of the device, but at the same time reduces the energy difference between the active region and the P region, and aggravates the carrier charge in the active region. Leakage increases the threshold current of the device.  In terms of shortening the wavelength of AlGaInP, it is mainly realized by increasing the content of Al in the active region, using a quantum well structure, and mixing quantum wells. The shorter the wavelength of the red semiconductor laser, the more difficult it is to manufacture and the worse the performance. These are the main reasons for restricting the development of short-wavelength red semiconductor lasers, and they are also problems that researchers urgently need to solve.

2 Increase the output power of the device

The main factor affecting the increase of laser power is cavity surface catastrophic optical damage (COMD). COMD mainly occurs on the cavity surface of the laser. When the output power is large, the optical power density of the cavity surface increases. When the cavity surface power density of the AlGaInP laser reaches 1-5 MW/cm2, the number of defects at the laser cavity surface increases. It will continue to increase and migrate to the interior, resulting in COMD of the laser, and the output power drops rapidly. After a lot of theoretical analysis and practical exploration, researchers found that making a non-absorbing window structure on the laser cavity surface can effectively suppress the COMD phenomenon. Zn was diffused into the active region as an impurity by means of rapid annealing, the Zn diffusion enhanced the disorder of the AlGaInP natural superlattice, and also increased the energy band width of the quantum well in the diffusion region. The region outside the active region with a smaller band gap cannot absorb the oscillating laser, which is called the window region. The appearance of the non-absorbing window greatly reduces the temperature of the entire light-emitting region and effectively suppresses the COMD phenomenon. The figure below shows a semiconductor laser with a window structure.