## 3 Experimental Design and Analysis of Results  
### Optical Characteristics Analysis of F-P Etalon External Cavity Diode Laser  
The complete external cavity optical path structure employed by the F-P etalon external cavity diode laser (referred to as FP-ECDL) is shown in Figure 2, with arrows indicating the direction of the optical path. The gain chip's left facet, in conjunction with the external cavity mirror, forms the complete laser resonant cavity. The collimation lens, etalon, filter, and mirror together constitute the external cavity structure of the FP-ECDL. The unidirectional isolator for forward transmission prevents backscattered light from re-entering the laser cavity, avoiding interference that could affect the laser's normal operation. Eventually, the laser output is coupled and focused into an optical fiber through a fiber collimator, with the optical fiber's end being equipped with a fiber connector as the laser emission port. The active intracavity portion of the FP-ECDL is composed of a semiconductor gain chip, specifically using Thorlabs's Semiconductor Optical Amplifier (SOA) gain chip. The SOA chip has high reflectivity and anti-reflection coatings on its two ends. One output-coupling facet (corresponding to Figure 2, the right facet of the gain chip) has a reflectance less than 0.01%, meaning nearly no reflection capability. The other reflecting cavity facet (corresponding to Figure 2, the left facet of the gain chip) has a reflectance greater than 90%, and the ridge waveguide on this facet has a width of 5 \(\upmu\)m, providing a wide tunable range. Therefore, while the gain chip has a high gain coefficient and can amplify light signals, it does not include a complete resonant cavity and cannot independently generate laser oscillation. However, to effectively achieve strong external feedback and minimize interference from internal cavity longitudinal modes, this type of gain chip is well suited to the basic requirements of FP-ECDL. The gain chip selected for the experimental preparation of FP-ECDL, under conditions of a 150 mA injection current and 25 \({}^{\circ}\)C temperature, yields a complete gain spectrum, as shown in Figure 3, after collimation through a lens and testing with an optical spectrum analyzer. The output power was measured using an optical power meter (selected for the 1550 nm wavelength range), and the recorded power at this moment is \(-6.043\) dBm, approximately 248.713 \(\upmu\)W. This is a typical gain spectrum of a semiconductor optical amplifier. Unlike laser emitters with a narrow linewidth, this gain chip exhibits an exceptionally wide gain spectrum covering a wavelength range from 1440 nm to 1600 nm, spanning 160 nm. The spectrometer measures a peak bandwidth of approximately 70 nm,  
Figure 2: Optical path structure of the FP-ECDL  
with the center peak wavelength near 1537 nm. The gain spectrum line shape in the peak frequency range is remarkably flat. When the gain spectrum peak has a broader bandwidth, it implies that the gains obtained by all longitudinal modes in that range are generally consistent and evenly distributed. Although there might be intense competition among longitudinal modes, single-mode selection can be achieved through an external cavity frequency-selective optical feedback structure. Ultimately, the overall cavity length of the FP-ECDL is approximately 55 mm. Additionally, considering the potential need for lasers in different wavelength bands for inter-satellite laser communication, adjusting the external cavity frequency-selective structure, such as cavity length, frequency-selective component parameters, etc., can allow the production of FP-ECDLs in different wavelength bands to better meet practical requirements. Afterward, longitudinal mode selection was performed using a fused silica standard with a free spectral range (FSR) of 100 GHz.