Electron Beam Pumped VCSEL Light Source For Projection Display a
Michael D. Tiberi*a, Dr. Vladimir I. Kozlovskyb Principia Lightworks, Inc., 6455 Independence Avenue, Woodland Hills, CA 91367 b P.N. Lebedev Physical Institute, 53 Leninsky Prospect, 119991 Moscow, Russia
ABSTRACT An electron beam pumped vertical cavity laser, or an “eVCSEL”, has been developed as a low-cost light source for LCOS and DLP based consumer television. 1000 lumens directed towards the spatial light modulator requires a total power of 144 watts for lasers in the three primary colors. This power surplus allows for high screen brightness for rear projection televisions of diagonals greater than 50 inches and eliminates the need for high gain screens with the benefit of larger viewing angles. Because of the high saturation of laser light, a color gamut approaching that of the human visual system is possible, creating superior image reproduction. Keywords: projection television, laser light source, eVCSEL, electron beam pumped laser, laser CRT
1. INTRODUCTION Laser light sources for rear projection television (“RPTV”) have several distinct advantages over conventional UHP lamps.
Monochromaticity of laser light allows for a color gamut approaching the range of the human visual system.
There is no need for color filters increasing the efficiency of the light modulation system.
Reduction of size of light modulators because of the small etendue of lasers.
No associated ultra-violet or infrared emission.
Significantly longer operating lifetimes from 10,000 to 20,000 hours.
Directionality and small divergence angle of the light cone allow for simple optical coupling.
These advantages, however, have yet to be realized. Intracavity-doubled diode or diode-pumped solid-state (“DPSS”) lasers have been demonstrated with enough power; however, their high costs, low power efficiency as well as integrating large devices or a large number of devices in consumer products make them costly solutions. Furthermore, these types of lasers may require an external scanning system in order to illuminate the light modulator (“SLM”). A practical alternative with none of the technical or cost constraints found in laser diodes or DPSS lasers is Principia’s electron beam pumped vertical cavity laser - an “eVCSEL”.
2. eVCSEL STRUCTURE The laser cavity for an eVCSEL comprises a gain layer of either a bulk single crystal semiconductor or a multiple quantum well structure (MQW), deposited by standard epitaxial deposition techniques such as MOCVD. The gain layer, varying from 5 microns to 10 microns thick by 2 cm2, is sandwiched by a highly reflective mirror and partially
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Projection Displays XI, edited by Ming H. Wu, Proceedings of SPIE Vol. 5740 (SPIE, Bellingham, WA, 2005) · 0277-786X/05/$15 · doi: 10.1117/12.605922
reflective output coupler. Carriers are injected into the cavity by a scanning electron beam between 10 microns and 60 microns in diameter at an accelerating voltage of 35 kV with an e-beam current up to 2mA. The gain layer for eVCSELs consists of quantum wells of InGaP/AlGaInP for red (640nm), ZnCdSe/ZnSSe for green (540nm) and for ZnSSe/ZnMgSSe (460nm) for blue. Semiconductors with bulk materials use single crystals wafers of CdSSe, CdS and ZnSSe for red, green and blue respectively [1].
Figure 1: eVCSEL resonator structure
The divergence angle of an eVCSEL as well as the coherence length is a function of cavity length. The light cone is confined to a 15° angle and the relatively low coherence of the laser eliminates speckle. Since the optical power required for an RPTV is small, RGB eVCSELs can be placed in a single CRT and scanned either sequentially or simultaneously in a three beam system.
3. OPTICAL POWER The eVCSEL is capable of an output power of up to five watts in each primary color. High output is achieved by raising the accelerating voltage which increases the excitation volume and minimizes losses associated with e-beam absorption by the mirrors and non-uniform excitation along the semiconductor depth. For consumer use, however, the high voltage should be 35kV or less and overall power consumption of fewer than 150 watts for all three primary colors. Optical power can be understood by a simple variation of the formula used to calculate the output power of vertical cavity lasers:
P (out ) = kUA(1 −
jth ) j
Where k is the differential efficiency, U is the anode voltage, A is the current, jth is the threshold current density and j is the maximum current density. (It should be noted that for the purposes of calculated estimations the range of k is from 0.02 to 0.12.) Threshold current Ith for multiple quantum well eVCSELs range for 20 µA to 80µA with jth from 8A/cm2 to 32A/cm2 at 35kV. In eVCSELs where the gain layer is a bulk semiconductor Ith is generally ten times higher with a 10% to 15% decrease in output power. 4.00
Red eVCSEL
3.50
Green eVCSEL
Output Power (W)
3.00
Blue eVCSEL 2.50
2.00
1.50
1.00
0.50
0.00 0.00
0.50
1.00
1.50
2.00
2.50
E-Beam Current (mA)
Figure 2: L.I. curve for eVCSEL primaries
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4. SCREEN BRIGHTNESS RPTVs typically have an on-screen brightness of 350 nits for a 52 inch 16:9 display and less for larger diagonals with a screen gain of 3x to 4x. A 1000 lumen eVCSEL white light source allows for larger diagonals with lower gain screens which produce higher on-screen brightness and an increased viewing angle [5]. 1000 Lumen eVCSEL Light Source 2500
Gain 1.0X Gain 1.5X Gain 2X Gain 3X
Screen Nits
2000
1500
1000
500
0 40
45
50
55
60
65
70
75
80
Screen Diagonal (inches)
Figure 3: Screen brightness as a function of size and gain
5. COLOR BALANCE AND GAMUT In order to provide 1000 lumens of light to the SLM, it is necessary to balance the power of the RGB lasers to the desirable white point as follows. Wavelength(nm) 640 530 460
Lumens for D65 247.8 703.1 48.9 TOTAL
Laser Output Power (W) 2.07 1.20 1.20 4.47
Laser Input Power (W) 35 35 77 144
The resulting color gamut exceeds that of Rec. 709 color space (CRT phosphors) and LEDs without the need for additional color primaries. Furthermore, the FWHM of the spectral line is approximately 2 nm which positions the CIE coordinates to nearly the edge of the color space.
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Figure 4: CIE 1976 Chromaticity Diagram gamut comparison
6. CRT A cathode ray tube is used to inject electrons into the laser cavity to produce laser light. There are two important factors for eVCSELs in relation to the CRT; accelerating voltage and electron beam spot size. The accelerating voltage determines the penetration depth of the electron beam into the semiconductor and therefore the excited cavity volume. The depth of electron beam penetration is proportional to U2 and is approximately 2.5 µm for 35kV. For the electron beam spot several factors are of consequence. It is important for optimum eVCSEL performance that laser generation occur perpendicular to the layers in the structure which limits the maximum spot size to approximately 60 microns in diameter. At the lower limit, in an electron beam of less than 10 microns in diameter, diffraction losses begin to dominate leading to an increase in jth. The optimum range for the electron beam is from 10-60 microns in diameter. In Principia’s eVCSEL, the electron beam spot size is 25 microns in diameter using magnetic focus with a pre-focus electrode, or G3 set at 5kV. This parameter allows for a stable spot diameter of up to 2 mA of beam current. Previous electron beam pumped lasers were pumped in a longitudinal mode which requires a transparent epoxy as well as a transparent substrate, usually sapphire, to attach the laser. This has the disadvantage of poor heat conduction as well as limiting the service life of the device. As a light source, the eVCSEL does not require optical transparency in both the epoxy and mounting substrate. Rather, a metal substrate is used with high thermal conductivity and adequate cooling is achieved by forced air convection. The use of a metal substrate to mount the laser requires that the laser be at some angle in relation to the electron beam or parallel and deflected at a right angle by an external magnet. If the laser is at an angle then the electron beam excitation volume will decrease because of increased reflection of the electron beam, leading to lower power efficiency. However, by using a simple magnet to bend the beam by 90º, lower voltage can be used for maximum excitation volume.
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The electron beam is scanned using a sinusoidal deflection scheme [2]. This approach has many advantages: lower power consumption, 50% less heating in the deflection yoke and circuitry and lower RFI and EMI fields.
7. THERMAL MANAGEMENT As in all light emitting devices, effectively managing localized heating has a significant impact on device performance. In an eVCSEL, both instantaneous heating of the impinging electron beam as well as average heating of the scan area needs to be balanced with the output power of the laser. To estimate the minimum instantaneous or pulse heating it is useful to understand the criteria for achieving laser threshold. Let N equal the number of electron hole pairs of carriers per volume unit created during and electron beam pulse. The energy gap of the gain layer must be exceeded by a factor of three to create one pair [3]. Approximately 2Eg will be converted to heat and therefore the following formulas can be used:
∆T =
2Eg N Cρ
where Eg is the energy gap of the gain material, C is the specific heat of the gain layer, ρ is the density of the gain layer. N can then be calculated by the following:
N=
k1 jVt p 3E g z 0
where V is the CRT high voltage, j=4I/πde2 is the current density, tp is pulse duration, z0 is effective penetration depth and k1 is about 0.75 taking into account that some electron energy is not absorbed by the gain layer. During scanning each point is heated in pulses. The dwell time, tp, is determined by the e-beam spot and the scan velocity. The dwell time must first exceed the carrier lifetime of 2 ns but not too long as to quench laser generation by overheating. The optimum range for highest output power and maximum efficiency is a dwell time of 5 ns to 8 ns corresponding to scan velocities between 5·105cm/s and 3·105cm/s at 25 µm in e-beam spot diameter. For ZnSe, I = 1 mA and vsc = 5·105cm/s, ∆T = 38 Kº. In actual operation, a temperature gradient occurs along the excitation area diameter in which the average temperature is Tav+∆T/2, where Tav is background temperature of a given “pixel” before pulse excitation. At I =1 mA with a scanning area of 1×1.4 cm it is necessary to remove about 20 W of heat.
8. DEVICE SERVICE LIFE The most significant issue relating to the service life of an eVCSEL is the output mirror bombarded by the electron beam. In order to achieve a lifetime in excess of 10,000 hours it is necessary that this mirror be as robust as possible. Optical pumping of laser mirrors typically have a damage threshold of >20 J/cm2. In annealed coatings the damage threshold is as high as 2.8 kJ/cm2 [4]. The primary failure mode of laser mirrors is absorption due to defects in the coating. This produces heat which causes potentially catastrophic thermal stresses within the coating. To compare a laser mirror with electron beam pumping of an eVCSEL it is necessary to examine the absorption of energy. In a typical laser mirror this value is about 0.1%, so the actual damage threshold as given in the earlier example is 20 mJ/cm2. In an eVCSEL, absorption is much higher and is close to 10%. For example, in an eVCSEL with an input power of 50 watts in a 25 um spot in 10 ns, the energy absorbed into the optical coupler is only 10 mJ/cm2. For annealed SiO2/TiO2 mirrors the damage threshold is three orders of magnitude higher [4]. Furthermore, these mirrors have a high degradation temperature and have been successfully tested to 400Cº. This is significant: eVCSEL processing of the CRT is at 350Cº which insures the integrity of the laser structure throughout the manufacturing process.
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9. OPTICAL COUPLING One of the aims in designing an optical system for the eVCSEL light source is to first decouple the scanning of the electron beam to that of the light modulator. It is also important that such an optical system be simple and inexpensive. As mentioned earlier, the RGB light sources can be fashioned either in a single CRT or one CRT per color. The total light emitting area of the eVCSEL is viewed as an object or field by the optical system and is the equivalent of a focal plane. The light then forms an external exit pupil and is re-imaged to the first conjugate focal plane after which the first conjugate pupil is formed on the SLM. A second conjugate image is then formed at the pupil of the projection lens. Each point from the focal plane will fully illuminate the pupil of the system. Thus, the SLM is fully illuminated whenever the eVCSEL is being scanned.
Figure 6: Optical arrangement for uniform illumination of an SLM Several optical arrangements can be employed for eVCSEL illumination. For example, an “x” cube assembly can be used with individual RGB light sources. With the addition of external optics, the light can be shaped to conform to the modulator. Some SLMs require illumination by polarized light. For lasers made using bulk semiconductors the highly reflective mirror can be used in a polarization recovery scheme by proper placement of a quarter wave plate and a grid polarizer collecting almost all of the emitted laser radiation. eVCSELs using MQW structures can be designed to emit highly polarized light which can be directly utilized by the SLM.
CONCLUSION Utilizing the superior color reproduction of lasers in RPTV has been a goal of OEMs for some time. Initially, lasers were evaluated as both light and image sources, but image speckle, high costs and packaging problems, made this an impractical solution. Lasers as a light source, however, have become a promising alternative. With improvements in conventional white light sources such as the UHP lamp and the successful introduction and market penetration of SLMs for RPTV, the baseline for performance for light sources has been clearly established. With lasers, lower cost, reliability, and longer device lifetimes, provide the critical factors in moving into the light source market for RPTV. Principia's eVCSEL has addressed these issues and offers a compelling solution. Our lasers allow for 500 nits with screen diagonals up to 70 inches with power requirements comparable to conventional white light sources. Additionally, the eVCSEL offers device lifetime in excess of 10,000 hours, and can be packaged to fit within the footprint of today’s thinner RPTVs utilizing DLP or LCOS technology. For both the consumer, who benefits from improved picture quality, and manufacturers, who can offer brighter images in larger screen sizes with lower gain screens, reducing costs and increasing margins, this is an exciting new window of opportunity.
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ACKNOWLEDGMENTS Principia wishes to thank Radiant Imaging, eBeam Inc. and Exotic-Electro Optics for their outstanding work and dedication to our efforts. We further wish to thank Atul Batra of Mitsubishi Electric for his guidance in the RPTV consumer markets. Our special thanks and gratitude to Professor Yuri M. Popov of the P.N. Lebedev Physical Institute.
REFERENCES [1] Yu. M. Popov, P.G. Eliseev, “Semiconductor Lasers”, Kvantovaya Elektronika 27 (12) 1035-1047 (1997). [2]James R Webb, “Sinusoidal Deflection on a CRT/CPT: Finally a Reality”, SID 04 Digest, 1036-1039 (2004). [3] V.I. Kozlovsky, Kh.Kh. Kumykov, I.V. Malyshev, Yu.M. Popov. Quantum Electronics 32 (2002) 297. [4] Shiuh Chao, Wen-Hsiang Wang, and Cheng-Chung Lee, “Low-loss dielectric mirror with ion-beam-sputtered TiO2–SiO2 mixed films”, Applied Optics, 1 May 2001, Vol. 40, No. 13; pg 2177-2182 [5] Charles W. McLAughlin,, “Progress in Projection and Large-Area Displays”, Proceedings of the IEEE, Vol. 90, No. 4, April 2002; Pg 521-532
*
[email protected]; phone 1 (818) 340 4898; www.principia-lightworks.com
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