640-nm laser diode for small laser display

640-nm laser diode for small laser display Naoyuki Shimada, Makoto Yukawa, Kimitaka Shibata, Kenichi Ono, Tetsuya Yagi, Akihiro Shima Optical Information Processing Device Dept., High Frequency & Optical Device Works, Mitsubishi Electric Corp., 4-1 Mizuhara, Itami, Hyogo 664-8641, Japan ABSTRACT Short wavelength and highly efficient AlGaInP quantum-well laser diode is promising as a red light source for small laser display application. Two kinds of the laser diodes are presented in this paper. A narrow ridge laser diode was designed for single lateral mode. In addition, a broad area laser diode was optimized for the higher power operation. To suppress a carrier leakage from an active layer, AlInP cladding layers were adopted to both of the lasers. Evaluation tests of the fabricated lasers were performed under CW operation. The wavelength of the narrow ridge laser was 636.0 nm under the condition of 25°C and 100 mW. Single lateral mode oscillation and the high wall plug efficiency of 29% were obtained. The beam divergences were 16° and 8° in fast and slow axes, respectively. The broad area laser showed lasing wavelength of 636.9 nm at 25°C for 200 mW output. The wall plug efficiency was 30% under this condition. Both of the lasers showed both high luminance and high wall plug efficiency. These lasers are suitable for small laser display applications. Keywords: laser display, laser diode, semiconductor laser, AlGaInP

1. INTRODUCTION Laser displays, such as a laser TV and a laser projector are very attractive products and expected to expand an application field of laser diodes (LDs). Especially, small laser projectors have a large potential to be innovative products. Investigations of a small laser projector embedded in a mobile phone and a projector for a car instrument panel are in the spotlight recently. Many other interesting applications are also proposed. To compose those small laser projectors, very compact lasers are necessary as light sources for each three primary colors of light, red, green and blue. For red and blue, utilizing laser diodes is a very simple and compact way. For green, a second harmonic wave of an infrared laser output is generally used. A diode pumped solid state laser or a wavelength stabilized laser diode are candidates of the infrared emitter. Luminance of those compact lasers is much higher than that of traditional light sources, lumps or light emitting diodes. That high luminance enables to minimize a size of projector optics without a large expense of luminous flux of the projector and brightness of a projected image. Furthermore, wall-plug efficiency (WPE) of the lasers is much higher than that of the traditional light sources. Battery lifetime is one of the most important specifications for mobile products. The high luminance and the high WPE are not only advantage of the lasers but also very important requirements to the lasers. In a wavelength range of visible red light, short wavelength is desired to achieve high luminance, because sensitivity of human eyes increases as wavelength of a light source become shorter. A 660 nm laser diode is well developed and commercialized for a DVD disc pickup application.1 However, the shorter wavelength of ~ 640 nm is necessary for display applications because the eye sensitivity at 640 nm is almost three times lager than that at 660 nm. The required output power of the 640 nm LD is at least more than 50 mW. But it depends on an implementation method of a projector. For the lasing wavelength of ~ 640 nm, an AlGaInP material system is generally used. It is thought that WPE of a AlGaInP LD is mainly limited by thermal saturation due to carrier leakage from the active layer because conduction band offset between the active layer and the cladding layer is small.2,3 Furthermore, the smaller offset results from the shorter wavelength because of the higher quasi-Fermi level in the active layer. Therefore, suppression of the leakage is strongly required to realize short wavelength AlGaInP LD with high WPE. The major limitation of maximum output of a narrow stripe AlGaInP LD is catastrophic optical damage (COD) at a facet mirror.4,5 For high density of optical power at the facet to achieve high luminance, facet passivation is important to overcome the COD degradation. In this work, we investigated two high power 640 nm LDs with high WPE for small laser displays. A narrow ridge LD was fabricated for high beam quality. We applied AlInP cladding layer and window mirror structure to overcome the

High-Power Diode Laser Technology and Applications VII, edited by Mark S. Zediker, Proc. of SPIE Vol. 7198, 719806 · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.808710

Proc. of SPIE Vol. 7198 719806-1

limitations of the carrier leakage and the COD. Furthermore, the concentration of impurities, the thickness of cladding layers and the cavity length were optimized. In addition, a broad stripe laser diode was fabricated for the high output power.

2. NARROW RIDGE LD 2.1 Mirror scanning method Several methods are known to implement a small laser display. One of them is that we call mirror scanning method. This method uses MEMS mirror to scan a collimated laser beam. The laser beam spot on the screen is moved fast by the scanning mirror and form an image. During the scanning, laser power of each color is tuned to express a pixel with a proper color. This method requires a small beam spot on a screen. Therefore, single lateral mode and diffraction limited laser output are preferred. In this work we adopted narrow ridge stripe structure for a red LD to control the lateral mode. High productivity is expected for future mass production because of its simple structure.

Ridge stripe

Etched channel

p-Electrode SiN Insulator Cladding Layer Contact Layer Active Layer GaAs Substrate Window Region n-Electrode

Laser Emission

Fig. 1. Schematic view of the narrow ridge LD

2.2 Design and fabrication A schematic view of the narrow ridge LD is depicted as shown in the Figure 1. Epitaxial layers were grown on an ntype misoriented GaAs to prevent AlGaInP from spontaneous ordering. The AlInP cladding layers with low refractive index6,7 was employed to confine the guided optical wave strongly. Large optical confinement factor of the active layer reduces the threshold carrier density. Hence a carrier leakage is suppressed effectively by using AlInP cladding.8 To prevent the facets from COD, window mirror structure was adopted.5 Zinc was selectively induced at facet mirror region as impurity and disordering of the active layer was performed by annealing. Two channels were formed by dry etching to make the narrow ridge stripe followed by deposition of a SiN insulator. The ridge shape was designed to achieve high kink power of the L-I curve.1 In the case of a single lateral mode LD with window mirror structure, the kink can be another limitation of the maximum output power, because the lasing mode is not single mode above the kink. The concentration of impurities, the thickness of the cladding layers and the cavity length were optimized to increase the kink power. After forming p-side and n-side electrodes, the wafer was cleaved. Front and rear facets were covered by low reflective and high reflective coatings, respectively. The LD chip width is 200 μm and the cavity length is 1500 μm. The chip was assembled into 5.6φ can package with junction side down configuration, using AuSn solder and submount.


180 CW 15 ∼ 75°C 10°C steps

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Fig. 2. Dependence of the light output on the injection current of the narrow ridge LD at 15 ∼ 75°C

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Fig. 4. Lasing spectra of the narrow ridge LD

2.3 Evaluation result

Figure 2 shows the dependence of the light output power on the injection current (L-I) of the fabricated narrow ridge LD. Threshold currents of 15°C and 75°C were 51 mA and 106 mA, respectively. Characteristic temperature of the threshold currents, T0, for this temperature range is calculated as 84K. At 25°C, the operation current for 100 mW and the threshold were 131 mA and 54 mA, respectively. High output power up to 180 mW were obtained at lower temperatures without observation of a kink and a COD. Even at the high temperature of 75°C, high output power of 99 mW was achieved at injection current of 250 mA. Thermal roll off of the L-I curves are observed. This is due to thermally activated carrier leakage from the active layer to cladding layer. Because an injection current concentrates into narrow ridge stripe, series resistance of the p-cladding layer is high compared to a broad stripe LD discussed in section 3. Generation of Joule heat and voltage drop in the p-cladding occur due to the resistance. The heat generation and voltage drop can be reason why the thermal roll off is enhanced.3 The operating voltage and the series resistance were measured as 2.67 V and 4.5 Ω at 25°C for 100 mW, respectively. Even though the operating voltage is higher than that of the broad stripe laser, the highest WPE at 25°C of 30% was obtained for 138 mW due to the optimization of the thickness of the pcladding layer. For 100 mW, the WPE was 29%. The full angle at half maximum (FAHM) of the divergence angle in the direction which is perpendicular to the substrate surface plane (⊥) was 16° at 25°C for 100 mW under CW operation. Figure 3 shows the evolution of the far field pattern (FFP) in parallel (//) direction of the narrow ridge LD. The FAHM was 8° for 100 mW. The evolution keeping same shape was observed up to 160 mW. The aspect ratio of the beam divergence for 100 mW is small as 2 : 1. These narrow divergence and small aspect ratio is preferable for mirror scanning type displays. The lasing spectra are shown in Figure 4. Short lasing wavelengths of 636.0 nm and 643.0 nm were observed at 25°C and 55°C, respectively. The full width at half maximum (FWHM) of those spectra are narrow as 0.2 nm. The longitudinal mode spacing of the LD is calculated as ∼ 0.04 nm. Thus several longitudinal mode can be included the peak.


3. BROAD STRIPE LD 3.1 Illumination method

Another method to compose a small laser display uses 2D micro-display device (MDD), projection lenses and the lasers. The lasers illuminate MDD very brightly and an image on the MDD is projected to a screen. This method uses the lasers just as a very bright illuminator. We call this method as “illumination method” in this paper. For this method, output power is more important than the beam quality. Therefore we fabricated and evaluated a broad stripe LD to study a possibility of high power operation. Though the beam quality of a broad stripe LD is lower than that of a narrow stripe LD, we think it is acceptable for the illumination method. 3.2 Design and fabrication

The fabrication process of the broad stripe LD was almost same as the narrow stripe LD. The ridge formation process was not included, of course. AlInP cladding was also applied to this LD, expecting same effect as the narrow ridge LD. But the confinement factor of the guided optical wave in perpendicular direction was designed larger than the narrow ridge LD, because illumination method does not require narrow divergence angle of an output beam. Thus the stronger suppression of the leakage was expected. After the epitaxial growth, a GaAs contact layer was etched off and SiN insulator was deposited at outside of a broad stripe active area for selective current injection into the stripe. The stripe width was designed as 40 μm. The LD chip width is 200 μm and the cavity length is 1000 μm.

500 CW 15 ∼ 75°C 10°C steps

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Fig. 5. Dependence of the light output on the injection current of the broad stripe LD at 15 ∼ 75°C


3.3 Evaluation result

Figure 5 represents L-I curves for temperature range from 15°C to 75°C. The low threshold current of 107 mA was obtained at 15°C and the threshold current density is calculated to be 268 A/cm2. At 75°C, threshold current increased to 328 mA and T0 is calculated as 54K. At 25°C, the operation current for 200 mW and the threshold were 298 mA and 120 mA, respectively. The slope efficiency was 1.11 W/A at 25°C and almost linear L-I curves were obtained from threshold to 500 mW output. This very high output indicates enough capability of the output power for small laser display application. Though thermal roll-off was observed for high injection at high temperature of more than 45°C, output power of 200 mW was achieved up to 65°C. Even at 75°C, laser oscillation over 150 mW output at 700 mA was observed. High power operation was achieved for wide temperature range. The operating voltage and the series resistance were 2.2 V and 0.8 Ω at 25°C for 200 mW, respectively. These values are remarkably lower than that of the narrow ridge LD. Low voltage and small resistance due to the broad stripe structure were confirmed. The highest WPE at 25°C was 35% for 467 mW output. Even for 200 mW, the WPE was still high and the value of 30% was obtained. Figure 6 shows the FFPs of the LD in perpendicular and parallel directions. The FAHM of the FFP in perpendicular direction was 34°. The wide divergence is attributed to the stronger confinement of the guided mode by AlInP low refractive index cladding layer. The truncated shape of the FFP edge in this direction was caused by the limited window diameter of the hermetic cap. In parallel direction, the FAHM was ~ 6°. In this direction, the FFP includes multiple peaks and suffers fulmination from the broad stripe structure. The lasing spectra are shown in Figure 7. Short peak wavelength of 636.9 nm and 643.0 nm was observed at 25°C and 55°C, respectively. The FWHM of those spectra are 0.6 nm and 0.7 nm. Multiple lateral modes caused relatively wide spectral line width compared to the narrow ridge LD.

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Fig. 6. Far field patterns of the broad stripe LD


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Fig. 7. Lasing spectra of the broad stripe LD

4. CONCLUSION We designed and fabricated two 640 nm laser diodes. The narrow ridge laser diode showed 636.0 nm lasing wavelength at 25°C for 100 mW output. The wall plug efficiency was 29% under the same condition. In addition, kink free operation up to 160 mW at 25°C was confirmed. The beam divergences were 16° and 8° in vertical and parallel direction, respectively. The narrow divergence and low aspect ratio was accomplished with short wavelength and high wall plug efficiency. Those characteristics are indispensable for laser display implemented by mirror scanning method. The broad area laser diode showed 636.9 nm lasing wavelength at 25°C for 200 mW output. The wall plug efficiency was 30% under this condition. The maximum output power up to 500 mW was obtained. The high output power was accomplished with short wavelength and high wall plug efficiency. Those characteristics can be applicable for laser displays implemented by illumination method. Both of the lasers demonstrated both high luminance and high wall plug efficiency. The remarkable characteristics show that these lasers are suitable for small laser display applications.

ACKNOWLEDGEMENT The authors express their appreciation to T. Ogawa, K. Horie, T. Shirahama and M. Miyashita for fruitful discussions.


REFERENCES 1 2 3 4 5

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T. Yagi, H. Nishiguchi, Y. Yoshida, M. Miyashita, M. Sasaki, Y. Sakamoto, K. Ono and Y. Mitsui, “High-Power high-efficiency 660-nm laser diodes for DVD-R/RW,” IEEE J. Sel. Top. Quantum Electron. 9 (5), 1260-1264, 2003. S. A. Wood, P. M. Smowton, C. H. Molloy, P. Blood and D. J. Somerford, “Direct monitoring of thermally activated leakage current in AlGaInP laser diodes,” Appl. Phys. Lett. 74, 2540-2542, 1999. D. P. Bour, K. J. Beernink and D. W. Treat, “AlGaInP single quantum well laser diodes,” SPIE Visible and UV Lasers, 2115, 269-280, 1994. A. Moser, E. E. Latta and D. J. Webb, “Thermodynamics approach to catastrophic optical mirror damage of AlGaAs single quantum well lasers,” Appl. Phys. Lett., 55, 1152-1154, 1989. H. Tada, A. Shima, T. Utakoji, T. Motoda, M. Tsugami, K. Nagahama and M. Aiga, “Uniform fabrication of highly reliable, 50-60 mW-class, 685 nm, window-mirror lasers for optical data storage,” Jpn. J. Appl. Phys., 36 (5A), 26662670, 1997. Y. Kaneko and K. Kishino, “Refractive Indices measurement of (GaInP)m/(AlInP)n quasi-quaternaries and GaInP/AlInP multiple quantum wells,” J. Appl. Phys. 76, 1809-1818, 1994. M. Moser, R. Winterhoff, C. Geng, I. Queisser, F. Scholz and A. Dörnen, “Refractive index of (AlxGa1-x)0.5In0.5P grown by metalorganic vapor phase epitaxy,” Appl. Phys. Lett. 64 (2), 235-237, 1994. N. Shimada, K. Shibata, Y. Hanamaki, T. Hamaguchi and T. Yagi, “12W CW operation of 640nm-band laser diode array,” Proc. of SPIE, 6876, 68760L, 2008.
