# Impact of Intracavity Power Variations toward Ultrashort Pulse Generation
## 3 Saturable Absorber Characteristics  
To fabricate GNP-SA, the mechanical exfoliation method was chosen, as illustrated in Figure 2. This technique has been elaborated on in our previous work [45]. In this technique, a clean fiber channel/physical contact (FC/PC) connector is first coated with an index-matching gel. Then it is dipped in a bulk amount of graphene powder and after pulling out the FC/PC connector, a small amount of graphene nanoplatelet is attached on a fiber ferrule with the help of an adhesive provided by the index matching gel, as shown in Figure 2. To minimize its thickness, Scotch tape is utilized to remove the crumpled graphene layer by layer. This process is repeated a few times until a very thin layer of graphene is seen on the fiber ferrule. Then, it is connected to another FC/PC connector using a fiber adapter to assemble a GNP-SA.  
Figure 1: (a) UV–VIS spectrum and (b) Raman spectrum of GNP powder.  
First, it is important to understand the optical characteristics of the fabricated GNP-SA. For optical transmission measurements, a broadband amplified spontaneous emission (ASE) source (Amonics model ALS-CL-17-B-SC) is applied. For the measurement of the reference spectrum, the ASE source is connected to a Yokogawa AQ6370B optical spectrum analyzer (OSA) with a resolution bandwidth set at 0.02 nm. The fabricated GNP-SA is then positioned between the ASE source and the OSA. The transmission spectrum is measured and then compared with the reference. Figure 3 depicts the transmission of the GNP-SA from 1525 to 1575 nm. This transmission loss includes the insertion loss that occurs between the fiber ferrules and the GNP material. For this work, the transmission was more than 60%, which translates to a maximum loss of 40% only in the entire wavelength range.  
For nonlinear absorption measurement, a balanced twin detector approach was employed, as depicted in Figure 4. The experimental setup consists of a pulsed laser source (PLS) with a center wavelength of 1560 nm, a pulse repetition rate of 250 MHz, and pulse duration of 117 fs. A variable optical attenuator (VOA) is used to vary the PLS power as the input variable. A 50/50 optical coupler divides the optical route evenly between path A and path B. Between the VOA and the 50/50 optical coupler, an isolator is inserted to prohibit any back-reflected signal from propagating to the PLS. Path A is utilized to measure the reference power, while path B is meant to measure the power-dependent transmission. The GNP-SA is placed in path B as the device is sensitive to the input power variation. The optical power of both signals is measured using two identical optical power meters (OPMs), namely OPM1 and OPM2.  
Figure 3: Transmission loss of GNP-SAs across conventional band (C-band) wavelength region.  
Figure 2: Fabrication of GNP-SA, A represents a FC/PC pigtail which the deposited GNP and B is another FC/PC pigtail for the mating process.  
The nonlinear absorption curve of GNP-SA is depicted in Figure 5. In accordance with the progression of the curve, the absorbance declines rapidly at first, then slowly towards the saturation regime. To determine the characteristics of a SA, the nonlinear saturable absorption equation [49] was chosen as follows:  
\[\alpha(I)=\frac{\alpha_{s}}{1+\frac{I}{I_{sat}^{\prime}}}+\alpha_{ns}, \tag{1}\]  
where \(\alpha(I)\) is the intensity-dependent absorption, \(\alpha_{s}\) is the saturable absorbance (modulation depth, MD), \(I\) is the laser intensity, \(I_{sat}\) is the saturation intensity, and \(\alpha_{ns}\) is the non-saturable absorbance. Figure 5 depicts the measured nonlinear absorption of the GNP-SA with its curve fitting from Eqn. 1. From the analysis, the GNP-SA produces a modulation depth of 0.8%, non-saturable absorbance of 36.8%, and saturation intensity of 8.7 MW/cm\({}^{2}\).