# Impact of Intracavity Power Variations toward Ultrashort Pulse Generation
## 1 Introduction  
Fiber lasers have been rigorously investigated since the invention of erbium-doped fibers [1]. Rare-earth doped fibers are a highly efficient gain medium that provides excellent beam quality. Another advantage of fiber lasers is the scalability of their output power with an innovative design of double-clad fibers for multimode pumping schemes [2, 3]. Owing to these advantages, fiber lasers have been demonstrated in various applications, such as high power [4, 5], wide tunability [6, 7], multi-wavelength [8, 9], and ultrashort pulse applications. The most sought-after trait of mode-locked ultrashort pulse laser systems in material processing applications is their capability to generate pulses at very high intensity with an extremely short interaction time. Not only would this allow materials such as metals [10] and ceramics [11] to be machined with exceptional precision, transparent and thermally sensitive media such as biopolymers [12] and glass [13] can be processed, too. To generate ultrashort optical pulses, there are two common techniques: active and passive mode-locking. For the active mode-locking method, an optical modulator is normally used to control the generated pulse durations [14, 15]. Its operation relies on the response time of the optical modulator to modulate the oscillating light in the cavity as a result of longer pulse durations. In addition, the electronic driver must be synchronized with the round-trip time of the pulse, which creates another challenge for this technique. The passive mode-locking technique is a popular choice as compared with active mode-locking techniques as they do not require external electronic components. In passivemode-locking, a nonlinear optical device called a saturable absorber (SA) is deployed in the cavity to generate ultrashort pulses. Optical pulses originated from noise fluctuations in the laser cavity. A noise spike of sufficient intensity will begin to saturate the absorber, reducing its losses. The noise spike will be further amplified in subsequent round trips, and the increased intensity will saturate the losses until a stable train of pulses eventually forms. Within these pulses, the central peak intensity saturates the SA more strongly than the pulse wings, leading to a further reduction in the pulse duration. The absorption of the SA decreases with the increment of input optical intensity. Material-based fiber SAs have attracted lots of attention due to their versatility of full integration with other optical components in fiber laser designs. The early concept of this SA was proposed by [PERSON] et al. [16] utilizing carbon nanotubes (CNTs). The proposed device incorporated a CNT layer between two substrates; however, a few bulk optics were still required to focus the light beam for effective interaction. To rectify the requirement of optical alignment using bulk optics, CNTs can be embedded with other polymers to function as a SA in the form of a thin film [17]. For this setup, the CNT thin film can be sandwiched between two fiber ferrules eliminating the requirement of optical alignments. This approach can also be applied to graphene as the nonlinear material in polyvinyl acetate [18] and polyvinyl alcohol [19] as the host polymer. Placing a graphene/polymer thin film on the fiber ferrule is proven to be a simple approach, as demonstrated in the 1.56 \(\upmu\)m [20; 21; 22] and the 2.1 \(\upmu\)m [23]. In addition to graphene, various materials have been demonstrated to generate ultrashort pulses using this technique: zinc phthalocyanine [24], titanium carbide MXene [25], gallium antimonide [26], zirconium disulfide [21], alpha ferric oxide [27], and lanthanum hexaboride [28]. In addition, another technique of optical deposition is also explored to attach graphene directly to the fiber ferrule surface [29]. This approach was first reported based on the CNT material [30; 31]. The optical deposition relies on the interaction between the injected light and material-dispersed solution.