# Recent Progress in On-Chip Erbium-Based Light Sources
## 4 Discussion  
Table 1 summarized the experimental results of representative on-chip Er-based waveguide amplifiers. The propagation loss of Al\({}_{2}\)O\({}_{3}\):Er is relatively low and the gain performance is affected by the preparation method. Prepared by magnetron sputtering, the process is simple and the uniformity is good, but the optical gain is relatively low. On the contrary, the optical gain is much higher when prepared by ALD, but the process is complicated and the growth rate is low; Benefit from the single-crystalline structure and the high Er concentration, a net material gain over 100 dB\(\cdot\)cm\({}^{-1}\) have been achieved by Er chloride silicate nanowire. However, it is too hard to fabricate and integrate with other on-chip devices; As a new host material, LN has the characteristics of high nonlinear coefficient and high refractive index, and has been widely used in on-chip modulators. The optical  
Figure 7: **(a)** The structure of the Si\({}_{3}\)N\({}_{4}\):Er waveguide amplifier. **(b)** The damascene process and the optical mode in the waveguide. **(c)** The ion implantation process. **(d)** The optical image of the 0.5 m long waveguide. **(e)** The waveguide amplifier butt-end coupled with fibers **(f)** The calculated Er concentration (green), simulated optical mode (red) and measured concentration (blue) by RBS. **(g)** Measured propagation losses before ion implantation, as-implanted, and after annealing [41]. Reprinted/adapted with permission from Ref. [41]. 2022, The American Association for the Advancement of Science. gain of LNOI:Er is high, but currently Er3+ are difficult to dope and the propagation loss is still large; Si3N4:Er has ultra-low propagation loss, hence a 0.5 m long on-chip waveguide amplifier could be fabricated and over 145 mW on-chip output power was realized, which achieved giant progress in the field of silicon-based on-chip light sources. In summary, the key issues for achieving the on-chip Er-based light sources are compact size, high gain and low loss. In order to meet the above requirements and satisfy the application requirements of on-chip integration, the following aspects can be considered, including the choice of gain mediums, the optimization of waveguide amplifier structures, and the design for resonator structures of the waveguide laser. The choice of gain medium is crucial. The selection of host materials determines the upper limit of the erbium ion concentration in the gain medium, as well as the maximum material gain that can be achieved. Although too high erbium ion concentration will lead to concentration quenching effect, it can be optimized by introducing Yb and Y [53]. The preparation method of gain medium is also important. The film transfer method is difficult to achieve large-scale integration with other on-chip optoelectronic devices. ALD process can achieve high-quality gain mediums, but the steps are complicated and the growth rate is slow, so that the cost is too high to meet the needs for large-scale integration. Magnetron sputtering can achieve low-cost, large-scale fabrication, but the propagation loss in the gain medium is large. How to further improve the quality of the gain medium prepared by magnetron sputtering and reduce the transmission loss is an important direction for the future development. Chemical vapor deposition (CVD) seems a compromise method, combing the advantages of low preparation cost and high film quality, which is widely used in various aspects. The structure of the on-chip waveguide amplifier should be optimized. For the waveguide amplifiers with strip or ridge waveguide structures, the propagation loss is mainly determined by the sidewall roughness, which can be effectively reduced by adopting wide waveguide structures and improving the etching process. For the hybrid waveguide structures which combine with other materials, such Si3N4, the etching of the gain medium can be avoided and the transmission loss can be effectively reduced.