Vertical-cavity surface-emitting lasers (VCSELs) have recently become very attractive as high-power light sources.1 Their high output power makes them suitable for use in a wide range of optical applications, including materials processing, optical pumping, medical treatment, and sensing. VCSELs are not subject to catastrophic optical damage, which occurs when a semiconductor junction is overloaded by exceeding its power density and is a major limiting factor in the maximum achievable output power for edge-emitting lasers. The output power of VCSELs can also be increased using two-dimensional arrays. The reliability of VCSELs is increasingly important for high-power operation. While the selective-oxidation technique2 that is generally employed to form current apertures does provide optical confinement, reducing the energy lost by diffraction, it also introduces defects into the crystal.
A proton-implantation technique enables the formation of small current apertures that are free from crystal defects. Previously, the detailed characteristics of proton-implanted VCSELs have only been researched using output powers in the 10mW class3 because the implantation technique is unable to provide optical confinement, which makes high-efficiency operation of VCSELs with small current apertures challenging. However, the importance of optical confinement decreases as the size of the current aperture increases. Hence, proton-implanted VCSELs with large current apertures provide a possible route toward high-power operation.
Figure 1. (a) A schematic diagram and (b) a cross-sectional scanning electron microscopy image of a proton-implanted VCSEL with 100μm current aperture. Au: Gold. GaAs: Gallium arsenide. AlGaAs: Aluminum gallium arsenide. DBRs: Distributed Bragg reflectors. λ: Wavelength. InGaAs: Indium gallium arsenide. MQWs: Multiple quantum wells. n: n-Type. p: p-Type.
To explore the possibility of efficient operation of proton-implanted VCSELs with large current apertures, we prepared a proton-implanted VCSEL with a current aperture of 100μm. Figure 1 shows a schematic diagram and a cross-sectional scanning electron microscopy image of the bottom-emitting-type VCSEL with a proton-implanted current aperture. The proton-implanted region can be clearly observed due to the strong contrast in the image: see Figure 1(b). We fabricated two devices: one with a single emitter, and another with seven hexagonally arrayed emitters with a spacing of 150μm.
Figure 2. Injection current-output power characteristics of the single emitter device under continuous wave operation at 15°C.
Figure 3. Injection pulsed current-output peak power characteristics of the arrayed device at 20°C.
Figure 2 shows the injection current and output power characteristics of the single-emitter device under continuous wave operation at 15°C. We achieved a maximum output power and a maximum slope efficiency of over 380mW and 0.96W/A, respectively.4 The output power density of the 100μm single emitter device was therefore estimated to be 4.9kW/cm2. To the best of our knowledge, this value is higher than all previous reports on high-power single VCSELs, and confirms the efficient operation of proton-implanted VCSELs with large current apertures. Moreover, the output characteristics are superior compared to all other proton-implanted VCSELs.
Figure 3 shows current injection pulse and peak output power characteristics of the arrayed device at 20°C. The width and repetition frequency of the current injection pulse were 100ns and 10kHz, respectively, and the peak output powers were estimated from time-dependent and time-averaged output powers. We achieved a peak output power of 40.6W at the pulsed current of 50A.5 This is the first demonstration of 10W-class output power from a proton-implanted VCSEL. The previously reported maximum power density of oxide-confined VCSELs was 24.6kW/cm2 with a pulse width of 60ns and a repetition frequency of 100Hz.6 Our fabricated device has an output power density three times greater, in spite of harder drive conditions.
In summary, we have developed high-power VCSELs using proton-implantation techniques and a large current aperture (100μm). Using an arrayed device under 100ns pulse operation at 20°C, a peak output power of 40.6W was achieved. The corresponding power density is estimated to be 73.8kW/cm2, a value three times greater than the reported record for oxide-confined VCSELs. In the future, we hope to improve the thermal management and electrical properties of the device to further increase power and reliability.
Hideyuki Naito, Masahiro Miyamoto, Yuta Aoki, Akira Higuchi, Kousuke Torii, Takehito Nagakura, Takenori Morita, Junya Maeda, Hirofumi Miyajima, Harumasa Yoshida
Hamamatsu Photonics K.K.
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