## 3 Results and Discussion  
The optical spectra of the ECDL system with different configurations were measured by an optical spectrum analyzer (OSA, YOKOGAWA, AQ6370D, Tokyo, Japanese), the resolution of which was 0.02 nm. The resonance wavelengths were obtained by rotating the grating along the vertical direction. Figure 3 shows the measured spectra of different configurations with an injection current of 300 mA. For configuration A, the tunable range was 161.2 nm from 1435.3 nm to 1596.5 nm, as shown in Figure 3a. The SMSR achieved 47 dB. For configuration B, the SMSR achieved 54 dB, as shown in Figure 3b. The tunable range was 130.9 nm from 1455.4 nm to 1586.3 nm. Compared with configuration A, the tunable range was reduced, but the SMSR was improved. In Figure 3c, the ruling density of grating was increased and a highest SMSR of 65 dB was realized for configuration C. And the tunable range was significantly increased. The tunable range achieved 209.9 nm from 1425.4 nm to 1635.3 nm. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Grating** & **Configuration** & **Ruling Density (Grooves/mm)** & **Diffraction Efficiency in the First Order (\%)** \\ \hline \multirow{2}{*}{Grating I} & A & 90 \\  & B & 17 \\ \hline \multirow{2}{*}{Grating II} & C & 1200 lines/mm & 91 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: List of the parameters for configuration A, B, and C.  
Figure 2: (**a**) Configuration A and C: the polarization direction of the incident beam was perpendicular to the direction of grating groove. (**b**) Configuration B: the polarization direction of the incident beam was non-orthogonal to the direction of grating groove. As can be seen in the above experimental results, the grating parameters have a great effect on the characteristics of the external-cavity laser. The SMSRs for three configurations are investigated in Figure 4. Compared with configuration A, the SMSR of configuration B was improved. And the SMSR of configuration C was the maximum. For the configuration A, there were gradual mounts in the optical spectra, as seen in Figure 3a, which greatly reduced the SMSR. For configuration B, the SMSR was improved by thoroughly eliminating the gradual mount, which may be due to the polarization mismatch between the polarization direction of the incident light and the direction of grating groove. For configuration C, the SMSR was significantly improved by increasing the grating groove density and the tunable range was also improved. Figure 3: Optical spectra of the output beam for the ECDLs with (**a**) configuration A, (**b**) configuration B, and (**c**) configuration C. The injection current was 300 mA. The grating groove density has a great influence on the laser performance. For the external-cavity laser, the diffraction angle is related to the grating equation: \(2d\text{sin}\theta_{L}=m\lambda\), where the d is the grating period, the \(\theta_{L}\) is the diffraction angle. According to the equation, grating diffractive angle is related to the number of grating grooves. The higher the number of grating grooves, the larger the diffraction angle. Due to \(\text{d}\theta/\text{d}\lambda=1/(2d\text{cos}\theta)\), a large grating diffraction angle is helpful to improve grating resolution. Meanwhile, a larger diffractive angle produced a larger spot size on the grating, which means more groove periods are effective to provide optical feedback with the function of wavelength selection. And the noise is better suppressed with a large diffractive angle. Compared with configuration A and B, the grating used in configuration C had a higher groove number and a larger diffraction angle according to the grating equation. For the configuration A, B, and C, when the wavelength of output beam was 1550 nm, the diffraction angle was \(27.7^{\circ}\), \(27.7^{\circ}\), and \(68.4^{\circ}\), respectively. Therefore, configuration C, with a better ability to suppress the noise, can achieve the higher SMSR.