Objective High-brightness semiconductor lasers are preferable light sources for various applications, such as high-efficiency pumping sources, material processing, medicine and biology, and laser display. The critical design objective for high brightness semiconductor lasers is to maximize the output power while maintaining good beam quality. Conventional broad-area high-power laser diodes suffer from poor beam quality due to the excitation of the higher modes and beam filamentation. The tapered structure is introduced to enhance the output power while maintaining the high beam quality. However, the tapered lasers with common contact usually suffer from spatial hole burning and self-focusing at high continuous-wave (CW) output power. The use of separated electrode contacts for the tapered lasers can effectively suppress detrimental effects in the beam quality and increase the possibility for the improvement of the brightness. In present study, we fabricate a high brightness tapered lasers with separated contacts and analysis experimental data suitable for a better understanding of the impact of the current injected into ridge waveguide (RW) section and tapered amplifier section on output power and beam quality.

Methods The tapered laser with separated contacts is fabricated in present work. The epitaxial layer structure grown by metal organic chemical vapor deposition based on a GaAs substrate. After material growth, the ridge waveguide section, the tapered section, and the grooves are defined by standard i-line lithography and formed by inductively coupled plasma dry etching. The etched grooves are down through the active region, designed to restrict the unwanted mode travelling to the ridge waveguide section. The ohmic contact layer in taper section is etched to reduce the lateral current spreading. Then, a silica layer is deposited on the wafer by plasma enhanced chemical vapor deposition serve as electrical insulation, and insulation layer is removed from current injection windows. Next, metal p-electrode is deposited on the whole wafer by electron-beam evaporation, and then metal layer in the electrode separation area is removed using lift-off process. A metal n-electrode is formed by evaporation and metallization after substrate thinning. The wafer is cleaved and the front/rear facet is antireflection/high-reflection films coated, respectively (R_f≈0.1%, R_r≈96%). Finally, single emitters are mounted p-side down on AlN sub-mounts with AuSn solder. Golden wire bonding is used to contact the n-side with the sub-mounts for the measurements.

In this paper, we investigate tapered lasers with a ridge waveguide width of 5 μm, the RW length of 1500 μm, the taper section length 4000 μm, and taper angle of 5°. The output power, the beam propagation ratio, the intensity profile in the beam waist, and the lateral near-field profile are measured in dependence on the currents through ridge waveguide section and taper section. Device testing is performed under CW conditions.

Results and Discussions The CW output power of 7 W is achieved with the current through the ridge section I_RW>300 mA. With the increase of I_ridge, the threshold current of taper section decreases first, and then stays nearly constant (Fig. 3). At I_RW=0 mA, no gain is provided to the optical mode in the RW section. The required threshold gain has to be provided by the taper section, resulting in a large threshold current due to large active volume of taper section. As the I_RW increases, the gain in the RW also increases. The required threshold gain is provided by both the RW section and the taper section, so that the threshold current of taper section decreases gradually. With the increase of I_RW, the beam propagation factor decreases first down to a minimum value and then increases at the current through taper section I_taper=7 A (Fig.4). As the gain in the RW increases, the power inside the RW will increase until gain saturation, the pumping outside the ridge waveguide decreases, and the intensity of the side lobes in the RW section is reduced correspondingly, thus the beam quailty is improved with the I_RW increase from 0 mA to 300 mA. When the I_RW increases to 500 mA, there will be obvious side lobes appear in the lateral near-field, which may be due to the unwanted higher-order modes are excited in the RW section (Fig. 6). Finally, the brightness of the tapered laser with separated contacts is 369 MW·cm^-2·sr^-1 at 6.4 W of the output power.

Conclusions In present study, the impact of driving current on the characteristics of tapered lasers with electrically separated contacts has been investigated. Tapered lasers emitting at 975 nm providing 7 W of output power in CW operation is fabricated. The increasing of I_RW induces the increase of optical gain and the reduction of the threshold current of taper section. In a certain I_RW range, the improvement of beam quality on account of the decrease of pumping outside the ridge waveguide and reduction of the side lobes intensity in the RW section. At high current I_RW, the unwanted higher-order modes are excited in the RW section and are amplified in the taper section, resulting in the deterioration of beam quality. At 6.4 W of the output power, the beam propagation ratio of M²=1.66 (1/e²) is obtained. The brightness of the tapered laser with separated contacts is 369 MW·cm^-2·sr^-1.