## 3 Results and Discussion
### Analysis of Light-Current (L-I), Voltage-Current (V-I), and Spectral Test Results  
After completing the accuracy tests and verification for the coating, the PIV characteristics of the fabricated RSOA were first tested to observe whether it operates as a spontaneous emission chip. The test conditions for the RSOA were set to room temperature (25 \({}^{\circ}\)C), with an injection current of 180 mA. The test results are shown in Figure 5. From Figure 5, it can be seen that under the test conditions of 25 \({}^{\circ}\)C and 180 mA, the output optical power of the chip is 0.67 mW, the operating voltage is 1.35 V, and the differential resistance of the chip is 2.4 \(\Omega\). The light-current (L-I) curve in Figure 5 shows that the RSOA has not yet reached lasing and is still in the spontaneous emission state. This is attributed to the use of the bent waveguide structure and the low-reflectance coating on the facet, which effectively suppresses optical feedback from the chip's intrinsic FP cavity.  
Next, the spectrum of the RSOA was measured to study the effects of the reflectance coating on the RSOA's spontaneous emission spectrum, including the 3 dB spectral width and ripple. Firstly, the RSOA chip was connected to a TEC (thermoelectric cooler). Following this, the RSOA chip was powered on, and its operating temperature was controlled. Then, a collimating lens was employed to carry out the coupling process for the RSOA chip. Lastly, a polarization-maintaining fiber was used to couple the output light from the chip into the fiber, which was then connected to a spectrometer for spectral analysis. The test results are shown in Figure 6.  
Figure 4: Comparison of the test and the simulation.  
Figure 5: Light-current (L-I) and voltage-current (V-I) characteristics of the RSOA.  
Figure 6a presents the spectral output of the RSOA with the four-layer Nb\({}_{2}\)O\({}_{5}\)/SiO\({}_{2}\) film system on the output facet. Under the test conditions of 25 \({}^{\circ}\)C and 180 mA, the 3 dB spectral width of the RSOA's spontaneous emission spectrum is approximately 79 nm, covering a wavelength range from 1497.2 to 1576.6 nm. The simulated results in Figure 1 indicate that within the wavelength range of 1498-1582 nm, the reflectance of the RSOA is below \(1\times 10^{-5}\). The test results in Figure 6a closely match the simulated values from Figure 1, confirming that within the wavelength range of 1497.2-1576.6 nm, the RSOA is operating in a low-reflectance state. Furthermore, within this 3 dB spectral-width range, the ripple is less than 1 dB, as shown in Figure 6a. Figure 6b presents the amplified spontaneous emission (ASE) spectrum within the 1548-1552 nm wavelength range. Within this narrow range, the ripple of the RSOA is approximately 0.5 dB. Extensive research has been conducted on RSOAs [20; 21; 22; 23; 24; 25], with the lowest previously reported ripple being 1 dB, while C-band RSOAs typically exhibit a minimum ripple of around 2 dB. The ripple observed in this RSOA represents the best performance reported to date. This performance is primarily attributed to the implementation of the four-layer Nb\({}_{2}\)O\({}_{5}\)/SiO\({}_{2}\) film system in conjunction with a bent waveguide structure. This design suppressed the optical feedback from the FP cavity of the RSOA, effectively lowering the ripple associated with the chip's intrinsic resonance to a new level, and achieved a very low output facet reflectivity. Furthermore, as observed in Figure 6, the ripple intensity varies slightly at different wavelengths. This is attributed to the fact that the reflection of the output facet of the RSOA cannot be entirely suppressed, and the RSOA's intrinsic FP cavity exhibits certain resonance effects. Therefore, achieving extremely low reflectivity at the light-emitting facet is crucial for optimizing the performance of the RSOA.