Objective The high power and high beam quality of tapered semiconductor laser output have led to a recent increase in research conducted in this field. Two guided wave modes are mainly used in semiconductor lasers: refractive index and gain waveguides. Little attention is paid to the gain waveguide owing to its unstable mode; instead, refractive index waveguide structures are often used in tapered semiconductor lasers. Although a tapered semiconductor laser with a refractive index waveguide structure can output high power and high beam quality, the product is similar to that of a high-power wide-contact semiconductor laser. The beam is unstable at high power output and is prone to twisting and causing filamentation. This phenomenon occurs for two reasons. The first is that mode filtering is not ideal in the ridge waveguide part, and the beam injected into the tapered area is not the fundamental mode. The second is that the refractive index changes in the tapered amplification area owing to thermal induction or spatial hole burning, which causes the beam to self-focus. In present studies, the difference in the output characteristics of the gain and refractive index waveguide structures for a tapered semiconductor laser is analyzed. Although the output power of a laser with a gain waveguide structure shows a slight decrease, its beam quality is significantly improved. This study provides a reference for the design of tapered semiconductor lasers with high power and high beam quality.
Methods In this study, the professional optical waveguide simulation software, RSoft, was used to compare and analyze the influences of the gain and refractive index waveguide structures on the output characteristics of a tapered semiconductor laser. First, the structural parameters of a tapered semiconductor laser including the length and width of the single-mode region as well as the length and angle of the tapered region were determined through the relevant theoretical analysis. Then, RSoft was applied to the model for simulation. The near- and far-field distributions, beam quality factor and power-current-voltage characteristics under different guided wave modes were finally determined. In addition, to verify the accuracy of the simulation results, tapered semiconductor lasers with gain and refractive index waveguide structures were fabricated separately, and the beam quality factor was measured using the knife-edge method.
Results and Discussions In the analysis of the near-field distribution, the optical field distribution on the back cavity surface of the gain waveguide structure laser was relatively smooth with no high spikes. In contrast, that of the refractive index waveguide structure laser was relatively rough, with numerous small spikes appearing in the single-mode region (Fig. 2). Furthermore, the optical field distribution on the light-emitting surface of the gain waveguide structure laser was relatively uniform with no high-intensity spikes; that of the laser with a refractive index waveguide structure, however, showed two high-intensity spikes (Fig. 3). The far-field characteristic analysis showed a far-field divergence angle of about 2°×40° (slow axis × fast axis) in the gain waveguide structure, and for the refractive index waveguide structure, the angle was about 8°×40°. The far-field divergence angle of the refractive index waveguide structure laser in the direction parallel to the PN junction was larger than that of the gain waveguide structure laser, and the angle was relatively small. The far-field of the gain waveguide structure laser showed only one spot, whereas that of the refractive index waveguide structure laser exhibited two nearly identical spots (Fig. 4). In addition, the beam quality factor of our fabricated device was measured (Fig. 5). In the range of 0--1.5 W, the beam quality factor of the tapered laser with a gain waveguide structure was smaller than that with a refractive index waveguide structure when the output power was constant. Furthermore, the power-current-voltage analysis result indicated that under a voltage of 1.55 V, the output optical power of the tapered laser with a gain waveguide structure was 820 mW, whereas that with a refractive index waveguide structure was 890 mW. Therefore, the output optical power difference between these two lasers was 70 mW (Fig. 6). The slope efficiencies of the gain and refractive index waveguide structures were calculated to be 0.932 W/A and 1.07 W/A, respectively.
Conclusions In this study, the influences of the gain and refractive index waveguide structures on the output characteristics of a tapered semiconductor laser are studied by simulation and experimentation. The results show that under the same voltage condition, the output power of the tapered laser with a gain waveguide structure is relatively lower than that with a refractive index waveguide structure. However, the light field distribution on the output facet is more uniform. The lower output power can effectively reduce the spatial hole burning effects and result in a better far-field distribution. The light confinement effect is stronger in the refractive index waveguide structure than that in the gain waveguide structure, which causes light reflected from the front cavity surface of the tapered laser to be limited to both sides of the single-mode region and prevents dissipation. The light reflected back again enters the tapered area for amplification, resulting in optical power with relatively high output. In the gain waveguide structure, however, the weak light confinement effect causes a large part of the light reflected from the front cavity surface to be lost. Because the light does not re-enter the tapered area for optical amplification, its output optical power is relatively low. Moreover, the strong confinement effect of the refractive index waveguide structure on the light causes most of the light reflected from the front cavity surface to propagate along the back cavity surface through scattering. This in turn causes a relatively messy distribution of the optical field on the back cavity surface. The light reaching the back cavity surface, which is a high-order transverse mode, is reflected from the back cavity surface and propagates along the front cavity surface outside the single-mode area. If its propagation angle is smaller than the tapered angle of the tapered laser, part of the light will likely re-enter the tapered part to strongly affect the beam quality of the device.
