# Impact of Intracavity Power Variations toward Ultrashort Pulse Generation
## 5 Pulse Laser Performance
Under this situation, signal re-absorption occurs as a result of lesser energy in the wavelength range of 1.53-1.56 \(\upmu\)m. In this scenario, the EDF is operated in deep saturation level (low population inversion level). In comparison to X, the signal re-absorption process is lower since the pump depletion region is shorter. The EDF saturation level is also lower (high population inversion level) and results in larger output powers for the 1.53-1.56 \(\upmu\)m light. The population inversion factor plays a critical role in determining the lasing wavelength. It is subjected to the gain profile of the material used in the laser cavity. To illustrate its impact, Figure 8c depicts the normalized EDF gain using different population inversion values. The emission and absorption coefficients are obtained from the optical fiber manufacturer. From Equation (2), the value of the population inversion is varied. These values are not accurate and are purposely chosen to elaborate the lasing wavelength phenomenon. As shown in Figure 8c, as the intracavity power increases, the population inversion level decreases. Based on Equation (2), the estimated value of the population inversion level is in the range of 58-65%. When the population inversion is around 65%, the lasing wavelength occurs at about 1530 nm (blue arrow), the highest peak of the EDF gain. When the population inversion level reduces to 58%, the lasing wavelength shifts to longer wavelengths. The lasing wavelength takes place in the range between 1556 and 1560 nm, represented by the green and red arrows. In relation to the experimental findings (Figure 7), a lasing wavelength longer than 1556 nm is recorded for all output coupling ratios smaller than 50%. Figure 7: (**a**) Optical spectrum of EDFL generated by GNP-SA using a 30% output coupling ratio and (**b**) central wavelength and spectral bandwidth for different output coupling ratios. There is an increment trend of the lasing wavelength when the output coupling ratio is decreased; the population inversion level is also decreased. The longest lasing wavelength of 1558.9 nm is recorded using a 10% output coupling ratio. The lasing wavelength of 1531.9 nm is attained using a 50% output coupling ratio (high population inversion level). These results indicate that by changing the feedback reflectivity (intracavity power), the population inversion can be adjusted accordingly. The measured pulse train in the time domain using the 30% output coupling ratio is shown in Figure 9a. For this measurement, the pump power of 975 nm LD is set at 99 mW. The oscilloscope trace indicates a clean pulse train with 89.37 ns time separation between subsequent pulses. In fact, there is no indication of the presence of other soliton phenomena, such as harmonic and multiple pulses. The round-trip time of 89.37 ns represents a pulse repetition rate of 11.19 MHz which is in agreement with the total cavity length of 14.4 m. Figure 9b portrays the value of the repetition rate for all output coupling ratios from 10% to 50%. The repetition rate value fluctuates between 11.08 and 11.19 MHz, which is considered almost constant. This small variation is caused by small differences in the fiber length by changing the optical coupler. Figure 8: (**a**) Schematic diagram of light propagation in the EDF with variation of intracavity powers, (**b**) pump and signal light propagation profile in the EDF and (**c**) the normalized EDF gain with different population inversion levels. X and Y represent the condition of high and low output coupling ratios, respectively. Figure 10a illustrates the frequency domain trace obtained from a radio frequency (RF) spectrum analyzer using the 30% output coupling ratio. The RF spectrum indicates a stable pulse train with a power extinction ratio (PER) of 58.3 dB. This value occurs at 11.19 MHz fundamental frequency, which is matched with the value deduced from the oscilloscope trace from Figure 9a. For other output coupling ratios, the value of PER at the fundamental frequency is more than 53 dB, as shown in Figure 10b.