## 3 Experimental Design and Analysis of Results
### Optical Characteristics Analysis of F-P Etalon External Cavity Diode Laser
The spectrum of the SOA chip is shown in Figure 4. This demonstrates the frequency-selective characteristics of the F-P cavity, where wavelengths corresponding to 1548.860 nm, 1549.660 nm, and 1550.460 nm exhibit minimal loss in longitudinal modes. After undergoing frequency selection with the etalon, final single longitudinal mode selection is achieved by introducing a filter into the FP-ECDL external cavity. Based on the F-P etalon parameters, a 200 GHz filter matching its free spectral range (FSR) was chosen to suppress longitudinal modes outside the target wavelength with greater loss, ensuring that only the longitudinal mode corresponding to the center wavelength of the filter can lase. Considering both reflected and transmitted output light, the reflectance of the external cavity plane mirror is chosen to be 60%. Under the conditions of an injection current of 150 mA and a chip temperature of 25 \({}^{\circ}\)C, the measured lasing spectrum from the spectrometer is illustrated in Figure 5. The central wavelength is 1549.844 nm, and the optical power meter (selecting the 1550 nm wavelength band) measures a laser power of 10.55 mW. The single longitudinal mode characteristics of a single-frequency laser can be measured by the side-mode suppression ratio (SMSR), and a single longitudinal mode can generally be considered when the SMSR is above 50 dB. A higher SMSR indicates better single longitudinal mode characteristics. In Figure 5, the spectrometer measured an SMSR greater than 70 dB for the FP-ECDL laser spectrum operating in continuous-wave mode selection. Figure 3: SOA gain spectrum. wave lasing mode, specifically, the difference between the blue dashed lines, with an exact value of 77.28 dB. It can be observed that the external cavity frequency-selective structure, incorporating the F-P etalon and filter, exhibits excellent single-mode selection characteristics. Furthermore, there is a nearly identical asymmetric shape near the peak of the laser spectrum (peaks at approximately 12 dB on the left and 20 dB on the right). This is attributed to the influence of various intricate factors on the oscillation mode and performance of the FP-ECDL, such as material characteristics, imperfections in the manufacturing of optical components, thermal effects, etc. These factors contribute to a small segment of optical gain near the peak of the laser spectrum that is not as \"smooth\", resulting in these asymmetric spectra. However, due to the extremely subtle nature of this asymmetry, it does not impact the actual performance of the FP-ECDL. Figure 4: Spectrum of the gain chip after F-P etalon frequency selection (2 nm). Figure 5: FP-ECDL laser spectrum. Taking into account the thermal effects and temperature sensitivity of semiconductor lasers, the practical fabrication of FP-ECDL involves the use of a semiconductor thermal electric cooler (TEC) to control the chip temperature. The laser is enclosed with a metal tube along with the external cavity structure. Detailed fabrication processes, such as laser coupling and packaging, are not discussed in this paper. Figure 6 shows an FP-ECDL narrow linewidth single-frequency laser packaged in a butterfly 14-pin metal tube. By linearly varying the injection current of the FP-ECDL, the emitted optical power was tested at different carrier densities. Figure 7 depicts the FP-ECDL laser power characteristic curve with a changing injection current at 25 \({}^{\circ}\)C and a 90% coated reflectance. Figure 6: Butterfly FP-ECDL assembly. Figure 7: Curve for the FP-ECDL laser power variation with injection current.