905 nm multiple-active-region lasers are mainly employed as the signal source of vehicle lidars. By alternately growing lasers and tunnel junctions on the same epitaxial wafer, lasers will be cascaded in the same structure by the quantum tunneling effect of tunnel junctions. However, the heavily doped tunnel junction (about 1×10²⁰ cm^-3) leads to rather low resistance, and then the lateral expansion of current will increase here, causing more unnecessary heat loss. At the same time, the current injected into different active regions decreases successively, and it is more difficult to emit the next active region. This increases the threshold current of the device and even leads to the impossibility to emit the lasers. In the conventional laser process, the method of isolation channel is often adopted to suppress the lateral expansion of current. However, the conventional process is mainly aimed at lasers with single active regions, and it employs wet etching with shallow etching depth and low precision. We introduce the isolation channel into the structure of multiple-active-region lasers, and simulate and calculate the lateral current density at different positions. Based on the calculated results, the isolation channel with different depths and spacing is designed, and the device is made and tested to study the suppression effect of different structures on the lateral current and optimize the chip structures.

The layer stack grown on a GaAs substrate by metalorganic vapor phase epitaxy consists of n-Al_0.4Ga_0.6As cladding (donator density N_D=1×10¹⁸ cm^-3), p-Al_0.45Ga_0.55As cladding (acceptor density N_A=2×10¹⁸ cm^-3), n- and p-Al_0.3Ga_0.7As optical confinement layers without doping around the active regions and tunnel junctions. The two In_0.08Ga_0.92As quantum wells sandwiched between Al_0.3Ga_0.7As spacer layers are the active regions. The two GaAs TJs are placed between diodes for cascading. An asymmetric large cavity structure with thick n side is designed. By increasing the thickness of the waveguide layer, the effective spot size can be expanded and the COD level of the device can be improved. The asymmetric waveguide can also limit the high order mode and reduce the loss of optical field and carrier. To suppress the lateral expansion effect of the current, in terms of chip structure, we introduce isolation channels on both sides of the active region along the longitudinal direction of the laser (Fig. 3). This structure can be leveraged to isolate the lateral expansion current, reduce the threshold current of the device, and improve the slope efficiency. Additionally, an ICP device is utilized to etch the designed isolated channel structure.

Fig. 5(a) shows the optical power-current (P-I) curve of the device under different etching depths. The threshold current and slope efficiency of wet-etched sample 1 are 1.52 A and 3.07 W/A respectively, and the channel depth of sample 2 is 4.0 μm. After etching through the first tunnel junction, the threshold current is reduced to 1.27 A and the slope efficiency is improved to 3.20 W/A. When the channel depth of sample 3 is 7.0 μm and the etched tunnel passes through the second junction, the influence of the current expansion effect is further reduced and the threshold current is reduced to 1.20 A, with the slope efficiency rising to 3.27 W/A. As the etching depth increases, the ability to isolate channels to suppress the current expansion effect becomes stronger. Fig. 5(b) shows the P-I curves corresponding to different channel spacing. For the device with cavity length of 1 mm, when the channel spacing of sample 5 is reduced from 180 μm to 125 μm of sample 4, the threshold current can be decreased from 0.91 A to 0.64 A, and the slope efficiency can be increased from 3.38 W/A to 3.58 W/A. This indicates that the current can be concentrated in the center of active regions and the lateral expansion effect of the current can be reduced by decreasing the distance between the two channels. Finally, sample 4 with the best initial performance is packaged and tested, driven by a special pulse drive. The P-I curve of the laser diode is measured under the current pulse width of 100 ns and repetition frequency of 10 kHz (0.1% duty ratio), as shown in Fig. 6(a). The peak power and working current are finally measured at 134 W and 38 A. The measured far-field divergence angle is shown in Fig. 6(b), where the vertical divergence angle is 33.3° and the lateral divergence angle is 5.1°.

The isolation channel structure of the 905 nm tunnel cascade semiconductor laser is optimized. To reduce the lateral expansion effect of the current, we introduce isolation channels with different etching depths and spacing on both sides of each laser to study the influence of different structures on the device performance. The epitaxial structure of double quantum wells with InGaAs/AlGaAs asymmetrical large optical cavity is selected, and the cascade is realized through heavily doped GaAs tunnel junctions. The devices with different depths and spacing channels are fabricated by wet and dry etching methods. The photoelectric performance of the devices is tested and compared, and proven by current expansion theory. The experimental results show that the introduction of the isolation channel can suppress the lateral expansion current effect of multiple-active-region lasers in the tunnel cascade. The deeper isolation channel leads to a shorter distance of the double channel and a better limiting effect. The final three-active-region laser with a channel etching depth of 7.0 μm and a spacing of 125 μm can reduce the threshold current to 0.64 A and slope efficiency to 3.58 W/A. The peak power is finally measured at 134 W under a 0.1% duty ratio.