A silicon-based optoelectronic chip provides a good solution for integrated space laser communication and LiDAR applications owing to its small size, low power consumption, low cost, high integration, high modulation bandwidth, and CMOS preparation process compatibility. During hybrid integration of Ⅲ-Ⅴ lasers with silicon-based optoelectronic chips, the coupling loss between the light source and the silicon-based optoelectronic chip is large. Therefore, the power requirement of the laser light source is higher than that of discrete devices, and the output power of the laser which was realized with single-chip integration or hybrid integration to achieve high side mode suppression ratio and narrow linewidth is generally a few tens of milliwatts. Erbium-doped fiber amplifiers are typically required to amplify the optical power of on-chip light sources, but this in turn limits the integration of optoelectronic chips; therefore, high-power on-chip light sources are a pressing problem for current on-chip laser applications. Coherent beam combining can be used to break the output power limit of a single laser. Coherent beam combining typically consists of two methods: one using a master oscillator and power amplifier (MOPA), and the other using an injection-locking technique. However, the spontaneous radiation of the optical amplifier increases the beam noise and makes the laser application system less effective; therefore, the coherent beam combining technique using injection-locking technology is more suitable for coherent detection systems that are more sensitive to laser noise. In 2001, Musha et al. used two injection-locked Nd∶YAG lasers to achieve coherent beam combining of spatial light with a beam combining efficiency of 94%; however, the scheme used discrete devices, and the arm length difference between the coupler arms had to be compensated in real time by an electronic feedback loop. In this study, to overcome the large coupling loss in hybrid integration and meet the requirements of laser light source power for space laser communication, an array of lasers containing four DFB lasers was injected and locked using a seed source, followed by coherent beam combining using a planar optical waveguide coupler to achieve on-chip optical power multiplication.

To achieve on-chip optical power amplification, a laser array containing four DFB lasers was injection-locked to a seed light source, and a planar optical waveguide coupler was used for coherent beam combining. To study the coherent beam combining effect of the injection-locked laser array, an experimental setup was built to measure the optical power, phase noise, and relative intensity noise of different numbers of injection-locked lasers before and after coherent beam combining, and the beam combining efficiency was calculated. First, the beam combining efficiency of the laser array was measured. The number of injected-locked DFB lasers in the laser array was varied by adjusting the magnitude of the operating current of the DFB lasers. Second, we measured the phase noise and intrinsic linewidth of different numbers of lasers after coherent beam combining. The relationship between phase noise and the number of lasers was investigated by theoretical analysis. Finally, the relative intensity noise after the coherent beam combining was measured.

The beam combining efficiencies for the two, three, and four DFB lasers were calculated to reach 91.6%, 87.8%, and 78.3%, respectively. According to the theoretical analysis in this study, the phase noise after the coherent beam combining contains the noise introduced by the master laser and that introduced by the slave laser. The phase noise introduced by the master laser is proportional to the square of the number of DFB lasers, whereas that introduced by the slave laser is proportional to the number of DFB lasers. The intrinsic linewidth after the coherent beam combining is dependent on the power of the master laser and the number of slave lasers; this tends to increase as the number of slave lasers increases, and decreases as the injected optical power increases. The relative intensity noise is demonstrated to increase as the number of slave lasers increase.

The coherent beam combining laser source based on injection-locking technology using a planar waveguide and a DFB laser array has the advantage of miniaturization, and the branches of the coupler do not introduce significant phase differences during the beam combining process owing to thermal noise. Therefore, compared with spatial light and fiber coherent beam combining, the planar waveguide coherent beam combining does not require the feedback control loop of electronics, greatly simplifying the system complexity. In this study, the effect of coherent beam combining on the phase noise and relative intensity noise is investigated through theoretical analysis and experiments. However, both the phase noise and relative intensity noise of the combined beam increase with an increase in the number of DFB lasers contained in the laser array. The present results provide a simple and effective technical means for coherent beam combining of DFB laser arrays; this is expected to be applied to laser applications such as chip-based space laser communication and LiDAR.