# Recent Progress in On-Chip Erbium-Based Light Sources
## 3 Gain Medium Based on Different Host Materials
### Other Er-Doped Host Material  
Lithium niobate on insulator (LNOI) is a promising material platform for the photonic integrated circuit, due to its broad optical transparency window, high refractive index, high nonlinear coefficient, and large electro-optical effect [66; 67]. So far, numerous LNOI-based  
Figure 4: Scanning electron microscope (SEM) of (**a**) strip loaded waveguide amplifier, (**b**) hybrid waveguide amplifier, (**c**) slot waveguide amplifier [42].  
Figure 5: The 3D scheme diagram (**a**) and the cross section (**b**) of the silicon-based on-chip waveguide laser [63].  
devices have been achieved and commercialized, such as electro-optic modulators and nonlinear optical devices [68; 69; 70; 71; 72; 73]. Recently, LNOI:Er has attracted great attention due to the high Er doping concentration and low propagation loss [74].  
In 2021, [PERSON] et al., achieved a single-frequency laser based on LNOI:Er by introducing mode-dependent loss and gain competition, as shown in Figure 6a [75]. The radius of the microring is 105 \(\upmu\)m, which corresponds to the free spectrum range (FSR) of 192 GHz. The gap between the microring and the bus waveguide is 700 nm, as shown in Figure 6b and the cross section of the waveguide is shown in Figure 6c. There are four modes supported in this structure, containing TE\({}_{00}\), TE\({}_{10}\), TM\({}_{00}\), and TM\({}_{10}\), as shown in Figure 6d. The overlap of TE\({}_{10}\), TM\({}_{00}\), and TM\({}_{10}\) with the rough sidewall is lager than that of TE\({}_{00}\), thus the scattering loss is higher. Considering the gain competition of the microring laser, only the TE\({}_{00}\) mode could exit and lase. Finally, an output power of 2.1 \(\upmu\)W was achieved, with a side-mode suppression of 35.5 dB, and a linewidth of 0.9 MHz. One year later, the research group demonstrated a LNOI:Er thin film waveguide amplifier [76]. The 600 nm-thick LNOI:Er thin film is transferred to the substrate and etched off 410 nm to form the ridge waveguide structure with chromium (Cr) as hard mask. The PL lifetime at 1530 nm is 2.3 ms. Up to 27.94 dB signal enhancement, 16.0 dB internal net gain (6.20 dB/cm), 8.84 dBm saturation power, 4.59 dB/mW power conversion efficiency has been achieved with 4.49 dB noise figure when the waveguide amplifier is 2.58-cm long.  
Compared to other host materials, Si\({}_{3}\)N\({}_{4}\) has several advantages including a wider transparency window [77], ultra-low two-photon absorption effect at telecom wavelengths, and low propagation losses of \(<\)3 dB/m [78]. These properties have made Si\({}_{3}\)N\({}_{4}\) a widely used platform in silicon integrated photonic applications [79; 80]. In 2022, [PERSON] et al. demonstrated an ultra-high gain Er-doped Si\({}_{3}\)N\({}_{4}\) amplifier, as shown in Figure 7a [41].  
Figure 6: The SEM image of (**a**) the microring, (**b**) the pulley coupling region and (**c**) the end facet. (**d**) The four modes supported in the microring waveguide [75].  
The main fabrication processes are shown in Figure 7b,c, containing photonic damascene process and ion implantation. The length of the waveguide amplifier is 0.5 m and the propagation loss is less than 5 dB/m, as shown in Figure 7d. The waveguide amplifier was butt-end coupled and green luminescence from cooperative upconversion was observed, as shown in Figure 7e. The calculated Er concentration, calculated optical mode intensity and measured concentration by Rutherford backscattering spectrometry (RBS) are shown in Figure 7f. The propagation loss of the waveguide increased sharply after ion implantation and decreased after annealing at 1000 \({}^{\circ}\)C in oxygen for 1 h, as shown in Figure 7g. Over 145 mW on-chip output power and over 30 dB of small-signal gain have been observed, which achieved giant progress in the field of silicon-based on-chip light sources.