AlGaAs based semiconductor laser diodes (LDs) in the 800~900-nm band are widely used as illumination light sources for augmented reality (AR), virtual reality (VR), and 3D depth sensors, and applied to various industries such as automobile, security, and games, as shown in Fig. 1 [1, 2].

Generally a laser diode of the 800-nm band is used as an illuminating light source for indoor 3D depth sensors, but the recent popularity of facial recognition in the fields of mobile phones and security, a high-power laser diode of the 940-nm band is required for an outdoor environment. Compared to the 800-nm band, the 940-nm band has the advantage of less interference from sunlight, but the output must be increased, because the sensitivity of commercial Si detectors is low [1-4].

To develop a high-power LD of 940 nm, it is necessary to increase the near-field (NF) emission beam size to reduce the heat generated by the antireflection (AR) facet; this is mainly determined by the epitaxial (EPI) design and growth technology. Normally, the narrow emission of optical power density in the NF of an LD would contribute to a low COD, and would degrade reliability. Therefore, NF beam size must be enlarged and optical density must be reduced, to improve the COD level and reliability [5]. In addition, the high-power LD must have reduced internal resistance, to suppress the heat dissipation that occurs when the output and temperature rise [6].

For high-power LDs, introducing a large optical cavity (LOC) into the separate confinement heterostructure (SCH) epitaxial layers was adopted as the simplest method to increase power and COD level. However, LOC LDs exhibit deteriorated performance, such as increase of I_th and decrease of SE, because of the large cavity [7]. On the other hand, the double-barrier separate confinement heterostructures (DBSCH), which insert a barrier with a low refractive index between the waveguide and cladding layers, possess superior characteristics in both COD level and reliability. The DBSCH also show deterioration of LD performance, though, in slope efficiency and resistance [8, 9]. Therefore, the LOC LDs and DBSCH have limitations in obtaining output of several hundred mW, due to higher threshold and resistance values caused by the deterioration of optical confinement [5].

To solve the conventional problems, a 940-nm high-power LD has been developed as follows. First, we adopted the GRIN-SCH structures of an AlGaAs/InGaAs single quantum well (SQW) with a grading layer between cladding and waveguide layers, to improve carrier confinement and decrease optical confinement, with low far-field vertical (FFV) angle. This is one of the structures that effectively confines high power in the lasing mode, and limits the heating effects that occur at high optical intensities, such as catastrophic optical damage or spatial hole burning [10].

After that, based on the GRIN-SCH structure, an asymmetric structure was applied to broaden the optical-mode distribution to the n-side region, by varying the Al composition and thickness of the p,n-clad layers. This structure dramatically reduces the FFV and increases the COD level. Although I_th can increase due to opticalconfinement reduction, I_op will be similar to that of the symmetric structure, due to the increase of SE [6, 11, 12]. In addition, the doing of p,n-clad layers is optimized to reduce series resistance and minimize free-carrier absorption.

In this study, we report graded-index separate confinement heterostructures (GRIN-SCH) that provide good carrier confinement, and an asymmetric structure that reduces FFV angle to increase COD level. These structural concepts and advantages under high-power high-reliability operation are demonstrated for 940-nm 350-mW AlGaAs/InGaAs transverse single-mode laser diodes.

The grown epitaxial laser structure is a hetero-p-i-n junction. The epitaxial layer was grown by Aixtron AIX-2400 G2 metal-organic chemical vapor deposition (MOCVD) reactor at 100 mbar and 700°C. TMGa and TMAl are used as group-III sources and AsH₃ as the group-V hydride. Cabon and silicon are used as p-type and n-type dopants respectively. Table 1 and Fig. 2 present the layers and transmission electron microscopy (TEM) image of the 940-nm 350-mW transverse single-mode LD.

3-inch n-type GaAs was used as a substrate, and 1.0-µm n-GaAs buffer, 4.0-µm n-AlGaAs lower cladding layer with Si as the n-type dopant, undoped quantum well active region, 0.2-µm p- AlGaAs 1st upper cladding layer, 100-Å p-AlGaAs etch-stop layer, 1.3-µm p-AlGaAs 2nd upper cladding layer with C as the p-type dopant, and 0.3-µm p-GaAs cap layer with C doped at a high level were successively grown in MOCVD. The undoped quantum well active region consists of 2000 Å of AlGaAs of the upper and lower waveguide layers as graded, and a 75-Å InGaAs single quantum well. The target EL wavelength was 940 nm.

The fabrication process was made with a regrowth type of selective buried ridge (SBR), growing a 0.8-µm n-AlGaAs current-blocking layer (CBL) and 3-µm p-GaAs 3rd growth after the ridge etch. For high-power single-mode oscillation, the stripe width was set to be 4 µm and the cavity length was chosen as 1 500 µm. For the metallization, Ti/Pt/Au was used for the p-side and Au-Ge/Ni/Au for the n-side contact respectively.

The facets were antireflection (AR) / high-reflection (HR), coated by sputter. AR reflection was 5% with Al₂O₃/SiO₂ layers, and HR reflection was 95% with Al₂O₃/Si/SiO₂/Si₃N₄ multilayers.

In this paper, we calculated and compared the mode profiles of GRIN-SCH symmetric and asymmetric vertical waveguide structures using the transfer-matrix method (TMM) [13]. Based on the TMM simulation, the 940-nm 350-mW transverse single-mode LD was implemented and evaluated.

First of all, the GRIN-SCH structures, by inserting a grading layer between the waveguide and the cladding layers based on SCH design, enhance carrier confinement while keeping the carriers in the lower grading region of the active layer [5, 10]. At the same time, the optical confinement is reduced by broadening the optical-mode distribution, due to the difference in effective refractive index in the waveguide. To obtain suitable GRIN-SCH LD performance, we adjusted the Al composition and thickness of the grading region. As a result of the TMM simulation, an FFV angle of 27° and confinement of 0.0298 were obtained, as shown in Fig. 3.

After that, to achieve the desired power level and reliability, we adopted asymmetric structures that increase the thickness and refractive index of the n-clad layers, to expand the optical mode more into the n-clad. This method can reduce the far-field divergence angle, by enlarging the optical-near-field distribution [6, 11, 12]. As a result, the asymmetric waveguide structure has a higher COD level by reducing optical intensity at the front facet, and lower internal loss because the absorption coefficient of the p-doped layer is lower than that of the n-doped layer [11]. In the TMM simulation, by changing the Al composition and thickness of the p,n-clad layers, the FFV can be reduced to 16° and the optical mode can be expanded into the n-side, as shown in Fig 4.

The doping concentration of the two structures was optimized to increase the high-power laser’s efficiency and reliability, by reducing internal resistance and free-carrier absorption loss.

After the fabrication process was complete, the wafer was cleaved into chips. The AR/HR facets were coated by sputter with Al₂O₃, Si, SiO₂, and Si₃N₄ multilayers for high-power operation. The coated chips were assembled with TO-56 package (PKG) and evaluated for electricaloptical properties and high-temperature reliability

The 940-nm 350-mW transverse single-mode laser diode was evaluated for electrical-optical (E-O) characteristics at room temperature and reliability at high temperature in the TO-56 PKG. Output power-current-voltage (L-I-V), far-field horizontal (FFH), FFV, and wavelength characteristics were measured by the CW driving method using DENKEN’s DKLDC330 equipment in Japan. The high-temperature (60°C) reliability was measured under auto-power-control (APC) driving conditions using BLUE'ENG’s AIO9220 system in Korea. As shown in Fig. 5, near-field (NF) fundamental-mode beam profiles were imaged by a source meter (Keithley 2400) and CCD camera (WinCamD-UCD23, Datalay).

Figure 6 shows the evaluation results for a GRIN-SCH structure that minimizes confinement loss while reducing the FFV, by grading Al composition and thickness between waveguide and cladding layers. For output power of 350 mW, we obtained I_th of 20 mA, I_op of 465 mA, and FFV angle of 27°, under continuous-wave operation at room temperature.

The GRIN-SCH structure enhances carrier confinement by keeping the carriers well under the active layer, and the optical confinement of the facet surface where the beam is emitted can be reduced by decreasing the effective refractive-index difference. As a result, the output power is stable up to 330 mW and kink-free at room temperature, but there is a problem in that a kink is generated at output higher than 330 mW.

For the GRIN-SCH structure, the characteristics of 940 nm and 330 mW class were obtained. However, there is a need to increase the power level such as kink and COD for stable illumination sources applied in outdoor optical 3D depth sensors. For this purpose, a high-power, high-reliability single-mode device must essentially reduce the FFV. Therefore an asymmetric structure based on the GRIN-SCH was adopted, by adjusting the Al composition and thickness of the p,n-clad layers.

As shown in Fig. 7, for an output power of 350 mW the asymmetric structure was able to obtain an FFV angle of 18°, which is about 9° less than the 27° of the GRIN-SCH structure. I_th increased to 65 mA due to the decrease of optical confinement with the features of the asymmetric structure, the SE increased to 0.98 mW/mA, and I_op was 400 mA, superior to the GRIN-SCH structure because of being kink-free.

Figure 8 shows the diode laser’s NF fundamental mode beam profile. Intensity profiles are shown for GRIN-SCH (symmetric) and asymmetric structures with similar waveguide thicknesses. It can be seen that by asymmetric design the near-field distribution in the laser cavity is different from that of the GRIN-SCH design. In the asymmetric design, the beam intensity penetrates into the n-cladding layer, and the beam size was enlarged. Therefore, it can be concluded that the asymmetric structure has been successfully designed.

In addition, as the doping of the p,n-clad layers increased, the internal resistance decreased, and the heat dissipation characteristics were improved under high-temperature and high-power driving [12, 14].

We were able to develop a 940-nm 350-mW high-reliability laser diode. Table 2 shows the electrical and optical properties of the final laser diode. Figure 9 shows the average COD levels for the GRIN-SCH (for 10 samples) and for the asymmetric structure based on the GRIN-SCH (also for 10 samples). The asymmetric structure enhanced the slope efficiency by reducing internal loss, and by reducing the electrical and thermal resistivities of the diode laser [11]. Figure 10 shows the high-temperature reliability of the asymmetric structure based on the GRIN-SCH (for 10 samples).

In this paper, the results of the development of a 940-nm high-power high-reliability device, used for an outdoor 3D depth sensor, were discussed and proved. The GRIN-SCH structure has excellent carrier confinement characteristics while preventing current leakage, and the asymmetric structure based on the GRIN-SCH design has excellent COD characteristics while reducing the far-field vertical angle. By applying these two structures, the laser efficiency was maximized, because the internal loss was minimized.

We finally succeeded in developing a 940-nm 350-mW transverse single-mode LD with high reliability that can guarantee operation in a 60°C environment and a COD level of 850 mW.