Trends in laser light sources for projection displays - Embedded.com

Trends in laser light sources for projection displays

ABSTRACT
New laser-based light sources are highly desired for projection displays because of the need for longer lifetime, lower etendue, and higher color gamut. High power, frequency doubled red, green and blue (RGB) surface emitting diode laser arrays have been developed for use in low cost projection microdisplay televisions.

INTRODUCTION
Novalux [1] has been developing low-cost projection display light sources using its patented semiconductor laser technology based on high-power, cavity-controlled surface emitting diode lasers that emit circular Gaussian beams (NECSEL: Novalux Extended Cavity Surface Emitting Laser). The Necsel provides an efficient, frequency-doubled source of laser radiation in the red, blue and green using InGaAs-based infrared semiconductor laser material together with periodically poled lithium niobate (PPLN). These devices have the potential for achieving the price and performance requirements of consumer projection display applications. Necsel arrays have operated in the visible with more than 3.5 W of average power in the blue (465-nm) and green (532-nm) and more than 1.2W in the red (621-nm).

ADVANTAGES FOR DISPLAYS
Lasers offer significant advantages as light sources for displays due to their large color gamut, high brightness, ability to be current modulated for gray scale, long lifetime and low manufacturing cost. The high brightness of lasers (~ 5×106 times that of LEDs) allows the use of the smallest possible micro-displays, low screen gain and power levels that outperform lamps or light emitting diodes. Necsel technology will also offer constant lumen output over the life of the system without wavelength shift over time. Necsels can be operated as small single emitters that can be directly modulated for use in scanning displays as well as very high power arrays for cinema, home TV and signage applications. The output from Necsels are polarized which makes them ideally suited for use with liquid crystal micro-displays or back-light units (BLU) for LCD TVs.

The narrow composite beam angle from such arrays will allow use of optical systems that will provide > 50-inch screens with display thickness comparable to plasma TVs but much lighter and 1/3 the power consumption. Figure 1 shows such an example for a rear projection display recently demonstrated by Mitsubishi. This will bring RPTV to a superior price/performance with plasma displays.

Single-beam or small array formats will allow personal (pocket) displays with output of a few hundred lumens. The high-speed modulation capability of Necsels can provide gray scales suitable for video images and head-up displays. A single beam Necsel has already been used in a 100cc RGB single-beam projection display (see Figure 2). Future applications include use with cell phones, automobile head-up and dashboard displays, as well as being the ultimate display for high definition gaming boxes.

The additional advantages of Necsels include wider color gamut, use of the smallest possible (and therefore cheaper) micro-displays, the elimination of many optical components, polarized output and the use of a high f (>f/9) number projection lens for reduced cost optics.

Figure 1. Photo of 52-inch HD laser RPTV announced by Mitsubishi having 500 nits, 4000:1 contrast and thin form factor using a DLP .

Figure 2. Photograph of a 20-lumen “pico-projector” using a MEMS scanner made by Microvision, Inc. The green laser is a single-element Necsel.

Necsel arrays emit circularly symmetric, low divergent beams that all travel parallel to one another. This remarkable property will allow the use of micro-optical or diffractive optical elements to efficiently convert these beams to a rectangular top hat distribution with an image on the micro-display, eliminating several optical components in the system. Variations in the emission intensity among the emitters in the array have little effect as the total lumen output on the micro-display is monitored to maintain constant light output over the life of the display. This is possible as the Necsel arrays do not have to be driven at their maximum power output because of their high brightness (etendue).

Figures 3a and 3b show a schematic of an optical system for an incoherent source such as a lamp or LED. The system using Necsel arrays shows the significant reduction in the number of optical elements, which further increases the optical transmission efficiency. Note that the RGB Necsel lasers can be operated color sequentially, eliminating the need for the color wheel. The cost of manufacture for an RGB Necsel arrays operating at 3-Watts per color is expected to be under $100 for one million units (all three colors).

Figure 3a. Schematic of a color sequential lamp projection system using a color filter wheel and several optical elements.

Figure 3b. Schematic of Necsel based color sequential projection system.

Figures 4a and 4b show a schematic of an optical system for the 3-panel high temperature polysilicon engine. Once again, the system using Necsel arrays shows significant reductions in the number of optical elements, increasing the optical transmission efficiency and lowering the cost.

Figure 4a. Schematic of a 3-panel HTPS projection system using a lamp.

Figure 4b. Schematic of Necsel based HTPS projection engine showing a dramatically reduced size and number of optical elements.

TECHNOLOGY
Necsel devices can produce high average optical power from a single emitter (> 150-mW in visible) and scaled using one or two-dimensional arrays. High power from a single emitter is achieved by use of a large aperture and an external mirror to provide a stable TEM00 mode that can be efficiently frequency doubled using a highly manufacturable, low-cost non-linear material such as periodically poled lithium niobate (PPLN). diode lasers is that they do not suffer catastrophic damage when operated at high optical power levels as well as operating in a fundamental spatial mode (TEM00). The high peak power levels can produce very efficient nonlinear conversion into the visible, especially when the nonlinear material is inside the laser resonator.

Figure 5. A schematic diagram of the Necsel structure.

All the semiconductor materials are based on well-developed GaInAs quantum well devices to provide wavelength coverage over the entire visible region, if required. At present, wavelengths at 465, 532 and 620-nm are required for an optimized projection system. These wavelengths have already been demonstrated at Novalux. Currently, Novalux is starting to scale the manufacture of these light sources for rear and front projection display applications. Surface emitting diode lasers can be tested at the wafer level to provide “know-good-die” ” without the need to process and mount die to determine if the wafer is good like what must be done with edge-emitting diode lasers.

Figure 5 shows a schematic diagram of the device structure. A p-doped Bragg mirror with nearly 100% reflectivity is soldered to the heat sink for efficient heat removal. An n-Bragg-mirror, with reflectivity low enough to prevent lasing, sandwiches the gain region containing several quantum wells of GaInAs. These quantum wells are placed at the anti-node of the laser field. The cavity is stabilized by a thermal lens or by an intra-cavity lens to form a simple semi-confocal cavity with the nonlinear material inside the resonator. The GaAs substrate is inside the resonator and a glass volume Bragg grating (VBG) is used to control the operating wavelength. This allows the use of all flat optics for simplified manufacturing. The nonlinear material is periodically poled bulk lithium niobate (PPLN) and is used for all three wavelengths with only a slight change in the poling period. No wave-guides are used, simplifying the alignment and manufacturing processes.

Figure 6 shows an array device in a developmental package. The array of second harmonic beams all emerge all parallel to each other as can be seen when a lens is placed in the near field in Figure 7. The divergence angle of a single beam is about 10 milli-radians which is about 1.5 time the diffraction limit. Without such a lens, these beams diverge and overlap to form a single circular beam a few tens of cm away. Each beam is incoherent with respect to one another and speckle has been measured in these arrays to decrease as the inverse of the square root of the number of emitters for approximately equal amplitude of each emitter in the array. This is particularly useful in eliminating speckle at the laser source. It was determined that about 35 emitters in the array would provide the necessary speckle reduction, depending on the characteristics of the screen. In addition, the pulsed operation provides some degree of spectral chirp which further reduces the speckle.

These devices are operated pulsed at repetition rates around one MHz. Pulsed operation allows the higher peak power to provide efficient nonlinear conversion to achieve an advantage in a broad temperature range consistent with manufacturer requirements. Broader spectral and temperature phase-matching bandwidths are achieved and the peak power further reduces the length requirement of the PPLN. The high repetition rate also avoids issues that may occur with video content.

Figure 6. Photo of a single-wavelength frequency doubled array in its uncovered package. The Necsel array was 1 by 5 mm in size with 15 emitters.

The power conversion efficiency of Necsel arrays is presently about 5-6% measured as visible output to electrical power into the laser. It is expected that power conversion efficiencies will reach over 10% in production. Reliability of the infrared devices has been demonstrated to meet TELCORDIA standards with mean time to failure of one million hours where the current must be increased by 20% to keep the same power output. This will mean that arrays can, in principle, be designed to keep the wavelength fixed as well as the visible output power to more than 50,000 hours. Note that Necsel arrays do not have to be driven to their maximum output ” which is in contrast to lamps or LEDs. Total lifetime will include the nonlinear crystal and the entire package.

Figure 7. Array of beams operating at 532-nm seen using a lens.

Figure 8. Photo of a 465-nm Necsel array. The volume Bragg grating shows the scattered light from 24 emitters in the 8-mm wide linear array.

The high overall light efficiency as measured by the electrical power into the lasers to the light to the observer is expected to be significantly higher than large screen plasma and LCD displays. Significant power savings worldwide will result in tens of billions of dollars in energy savings. Present efficiencies observed for the arrays exceed 5% while single emitters have exceeded 10%. An efficiency of 10% is expected in production for power levels in excess of 4-Watts. At present, power levels achieved are 3.9-W at 532-nm, 3.7-W at 465-nm and 1.2-W at 621-nm.

Figure 9 shows an example of the average output power from a 15 element linear array at 465-nm. This device was operating at about 5% overall power conversion efficiency in an un-optimized design. The linear rise in the second harmonic output seen in the Figure demonstrates the device is in the linear conversion region where the conversion from the fundamental wavelength to the second harmonic is quite high.

Figure 9. 465-nm average output power as a function of peak drive current into a 15-element linear array. The power efficiency was about 5%. Green arrays produced similar performance.

Power scaling capabilities of Necsel infrared arrays has been shown by operating water-cooled 5 by 5 mm arrays with 225 emitters (15 by 15) at cw power levels exceeding 80 Watts. All emitters operated in parallel, which demonstrated the uniformity of the arrays. The wavelength uniformity showed that the temperature did not vary by more than three degrees C over the entire array at full power. Such high power arrays would be suitable for very large screen format for cinema, lighting and commercial display for advertising. Brightness from these arrays is almost six orders of magnitude greater than for lamps or LEDs.

Figure 10. 8-mm 2D Necsel array on left together with an array mounted on its heat sink.

The maximum specific power levels reached thus far for a one dimensional array are > 4.5-W/cm for both green and blue and are expected to climb to 9 W/cm for a linear array and about 12 W/cm for a 2-row array for all three colors as efficiency is improved.

Figure 10 shows a single array chip together with a mounted chip on a heat sink. Manufacture of the array chips starts with epitaxial growth on 4-inch GaAs wafers with as many as one thousand, 24-element arrays per wafer that can be tested on the wafer prior to dicing and mounting. Power scaling options include more elements in a single chip array limited primarily by the size of the nonlinear material or more arrays on a single mount. All three colors can be mounted in a single package or separately. The GaInAs material system will allow more colors to be manufactures at the same price point for very high-end color gamut displays.

ACKNOWLEDGEMENTS
Our thanks to our co-workers at Novalux, Inc., B. Cantos, G. Carey, M. Jansen, G. Giaretta, S. Hallstein, J. Hofler, W. Hitchens, N. MacKinnon, J.-M. Pelaprat, A. Tandon and A. Umbrasas.

REFERENCES
[1] US Patents 6,243,407, 6,243,407, 6,448,805, 6,614,827, 6,636,539, 6,775,000 and 6,778,582

About the authors
Greg Niven is Vice President of Marketing at Novalux, Inc. He has spent the last 13 years involved with laser technology for the graphic arts and display markets, including focussing the last four on the consumer electronics marketplace. Before joining Novalux at the start of 2005, he spent six years at Coherent, Inc., where he directed their business development efforts in laser display. His prior five years were at Creo, Inc., where he specialized in high-power lasers and optical imaging systems. Greg received his B.S. degree in Engineering Physics from the University of Alberta. He can be reached at gniven@novalux.com

Aram Mooradian, PH.D. is the founder of Novalux, the inventor of Necsel technology, and the company's chief scientist. Previously, he founded Micracor (Acton, Mass.), a company that commercialized the laser technology that he developed while leading the Quantum Electronics Group at the Massachusetts Institute of Technology (MIT) Lincoln Laboratory. Aram is widely published and has been granted more than 40 patents. He has lectured worldwide, including an invitation under the Fulbright program. He has served as advisor to the United Nations, U.S. Department of Defense, NASA, NATO, the Industrial Science Board of Japan, and to France, the United Kingdom, Kuwait, and Canada. He was associate editor of the Journal of Quantum Electronics and Journal of the Optical Society of America. Dr. Mooradian is a fellow of the American Physical Society and the Optical Society of America. He holds a B.S. degree in physics from the Worcester Polytechnic Institute and a Ph.D. in physics from Purdue University. He can be reached at aram@novalux.com

This paper was originally presented at:
13th International Display Workshops (IDW '06)
Otsu, Japan
December 6-8, 2006

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