**1. Introduction**

Optical fiber lasers gained by rare‐earth‐ions‐doped fibers with broad gain spectrum of tens of nanometers make them very attractive for ultrashort‐pulse generation via the mode‐locking mechanisms [1, 2]. They are under a growing interest because of their unique features of high efficiency and low consumption, good beam quality, high stability, naturally fiber coupled

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and providing a powerful tool for high‐speed optical communications, precise micromachin‐ ing, biomedical imaging, and other applications [3].

Mode locking refers to phase locking of many different frequency modes in a laser cavity, which induces a laser to produce a continuous train of extremely short pulses. Unlike the Q‐ switched pulses, the mode‐locked pulses are phase coherent with each other. Active and passive mode locking are two different methods of mode locking. Active mode‐locking methods typically involve using external modulators, for example, electro‐optical modulator, which induce a phase or amplitude modulation of the intra‐cavity light including lots of longitudinal modes with a periodic duration according to the cavity length to generate the mode‐locked pulses, whereas in passive mode locking, the generation of the mode‐locked pulse is controlled by the saturable absorber (SA), that is a nonlinear optical element whose loss depends on the laser pulse intensity and causes self‐modulation of the light.

Optical pulses generated from passive mode locking have the phase locking being carried out automatically in the cavity without external electrical components required, which results in extremely short and high stability. A number of potential operating and applying character‐ istics of passively mode‐locked fiber lasers have been attractively demonstrated that pro‐ duce laser pulses with durations from nanosecond to femtosecond, ultrawide bandwidth (∼100 nm) at repetition rates ranging from several kHz to hundreds GHz. The research field in passive mode‐locking fiber laser goes beyond the generation mechanisms and pulse behaviors that can be found in their operation. A lot of attempts and explores have been made to optimize the operation of the laser to suit for the particular application.

**Figure 1.** Basic configuration of fiber‐based nonlinear polarization rotation. Top, basic configuration; bottom, principle model. PC, polarization controller; PD‐ISO, polarization‐dependent isolator.

The first passively mode‐locked fiber laser was demonstrated in 1983 [4]. An all‐fiber, unidirectional, mode‐locked ring laser was first constructed using a type of artificial satura‐ ble absorber, which uses the effect of intensity‐dependent polarization mode coupling in the fiber, that is, nonlinear polarization rotation effect [5]. Since self‐phase modulation and other

nonlinearity effects contribute to changing the refractivity index by field intensity, as well the change of the state of polarization as shown in **Figure 1**. Another type of common saturable absorber is based on the nonlinear interference between two polarization modes, that is, the so‐called nonlinear optical loop mirror [6] and nonlinear amplifying loop mirror [7]. Nonlinear optical loop mirror relies on the nonlinear interference of the fields that counter‐propagate, so the intensity ofthe pulse is determined by the product ofloop length, peak power, and splitting ratio. The longer the loop length, the smaller the peak power required to reach the first transmission maximum. For passively mode‐locked fiber lasers operating at large normal dispersion, a short loop length is preferred for ultrashort‐pulse generation, while nonlinear amplifying loop mirror is designed with a gain medium placed asymmetrically in the Sagnac loop that is a ring‐cavity interferometer. To date, this artificial saturable absorber continues to be an effective approach to generate ultrashort pulses from passively mode‐locked fiber lasers [8].

and providing a powerful tool for high‐speed optical communications, precise micromachin‐

Mode locking refers to phase locking of many different frequency modes in a laser cavity, which induces a laser to produce a continuous train of extremely short pulses. Unlike the Q‐ switched pulses, the mode‐locked pulses are phase coherent with each other. Active and passive mode locking are two different methods of mode locking. Active mode‐locking methods typically involve using external modulators, for example, electro‐optical modulator, which induce a phase or amplitude modulation of the intra‐cavity light including lots of longitudinal modes with a periodic duration according to the cavity length to generate the mode‐locked pulses, whereas in passive mode locking, the generation of the mode‐locked pulse is controlled by the saturable absorber (SA), that is a nonlinear optical element whose

Optical pulses generated from passive mode locking have the phase locking being carried out automatically in the cavity without external electrical components required, which results in extremely short and high stability. A number of potential operating and applying character‐ istics of passively mode‐locked fiber lasers have been attractively demonstrated that pro‐ duce laser pulses with durations from nanosecond to femtosecond, ultrawide bandwidth (∼100 nm) at repetition rates ranging from several kHz to hundreds GHz. The research field in passive mode‐locking fiber laser goes beyond the generation mechanisms and pulse behaviors that can be found in their operation. A lot of attempts and explores have been made

**Figure 1.** Basic configuration of fiber‐based nonlinear polarization rotation. Top, basic configuration; bottom, principle

The first passively mode‐locked fiber laser was demonstrated in 1983 [4]. An all‐fiber, unidirectional, mode‐locked ring laser was first constructed using a type of artificial satura‐ ble absorber, which uses the effect of intensity‐dependent polarization mode coupling in the fiber, that is, nonlinear polarization rotation effect [5]. Since self‐phase modulation and other

model. PC, polarization controller; PD‐ISO, polarization‐dependent isolator.

loss depends on the laser pulse intensity and causes self‐modulation of the light.

to optimize the operation of the laser to suit for the particular application.

ing, biomedical imaging, and other applications [3].

280 282High Energy and Short Pulse Lasers

Other methods of the use of new materials have been extensively investigated. Most com‐ monly passive mode‐locking devices used in research laboratories and commercial fiber or solid‐state lasers are semiconductor saturable absorber mirror (SESAM) [9]. A semiconduc‐ tor absorber mirror consists of semiconductor heterostructures embedded by a multiple‐ quantum‐well structure like GaInNAs/GaAs. Such a semiconductor absorber mirror can achieve a recovery time of less than 1ns, but has some restrictions on the relatively narrow bandwidth operation, the lower antidamage threshold. Even so, semiconductor absorber mirrors have become widely commercially available and popularly utilization for environ‐ mentally robust and stable mode locking.

Various kinds of low‐dimensional materials exhibiting the advantages of ultrafast recovery time and broadband saturable absorption have been presented for mode‐locked fiber lasers, including carbon‐based nanomaterials, such as carbon nanotubes [10], graphenes [11]. Recently, many two‐dimensional (2D) layered materials have been investigated as broad‐ band saturable absorbers for the mode locking [12–14]. Almost at the same time, another kind of nanomaterials, that is, the so‐called topological insulators (TI), are characterized by a linear dispersion band structure with the Dirac point similar to graphene, which possess inherent features of broad response with a flat broadband wavelength absorption as well as high flexibility. This type of material includes bismuth telluride (Bi2Se3) and antimony telluride (Sb2Te3) [15, 16]. The manufacturing technique of these low‐dimensional materials may have more simple process, easy integration, higher modulation depth, higher damage threshold, a broadband wavelength operation.

Most of passively mode‐locked fiber lasers are based on a ring‐cavity configuration, which conventionally contains a wavelength division multiplexerforthe delivery ofthe pump energy to the cavity, a segment of gain fiber with the core doped by rare‐earth ions, an isolator that provides the unidirectional travelling wave inside the cavity, a beam splitter for the output, a polarization controller, and a saturable absorber for the mode locking. In addition, a spectral filter or other optical elements may be inset into the cavity for the lasing stabilization. In an all‐fiber scheme, the cavity contains the active medium and few fiber elements, which offer lower loss for laser pulses. For mode‐locked ring‐cavity fiber lasers, the fundamental repeti‐ tion rate is determined by its cavity length *L*, the relation expression is as follows: *repetition rate* =*c*/*nL*, where *c* and *n* represents the speed of light, and refractive index respectively. On the state of the so‐called harmonic mode locking (HML), the repetition rate can be two or more integer times of the fundamental repetition rate.

To improve the performance of the interaction of nanomaterials and light in fibers, various types of nanomaterial‐based saturable absorbers have been demonstrated. For instance, a transmission‐mode film‐like saturable absorber is fabricated by nanomaterial‐polymer composites sandwiched between two fiber ferrules in a standard fiber connector [10, 11]. **Figure 2(a)** shows the typical configuration of a transmission‐type saturable absorber. This composite can be constituted by several easy fabrication and integration methods of sputter‐ ing, direct synthesis or deposition on the end surfaces of optical fibers, whereas there is a direct physical contact, that is, the laser light is directly transmitted through the nanomaterial film. It is noticed that there may be thermal and mechanical damages within the limited interac‐ tion length for high‐energy pulsed fiber lasers. A promising alternative is lateral interaction with the evanescent waves of the fiber. The saturable absorbers with the evanescent wave interaction have been demonstrated by several fiber structures including tapered fiber [11], D‐ shaped fiber [13], etc. The structures and lateral interaction process of the tapered and D‐ shaped fibers are shown in **Figure 2(b)** and **(c)**. The evanescent wave is generated by total internal reflection of rays at the boundary of the fiber core with a lower index of clad medi‐ um. The nanomaterials like carbon nanotubes that contained in the lower‐index region can raise the nonlinear reflection coefficient due to evanescent wave absorption along the fiber in a longer nonlinear interaction length in a centimeter scale. This configuration is compatible with the fiber format and easy to add the saturable absorber in the cavity by using simple fiber fusing splicing technique.

**Figure 2.** Typical configuration of transmission‐type saturable absorber (a), evanescent field interaction of the D‐shap‐ ed fiber (b), and tapered fiber (c).

Mode‐locked silicate‐based fibers lasers doped by rare‐earth ions have been demonstrated directly operating around 1 μm, 1.5 μm, and 2 μm with very high optical efficiencies. There have also been reported that mode‐locked fiber lasers based on Raman cascading, frequency conversion, and other nonlinear processes. The group velocity dispersion of silica fiber is normally positive at 1 μm. Many studies have demonstrated that all‐normal dispersion passively mode‐locked fiber lasers can generate various kinds of pulses [17], like dissipative solitons, similaritons, noise‐like solitons, and soliton rains.

tion rate is determined by its cavity length *L*, the relation expression is as follows: *repetition rate* =*c*/*nL*, where *c* and *n* represents the speed of light, and refractive index respectively. On the state of the so‐called harmonic mode locking (HML), the repetition rate can be two or more

To improve the performance of the interaction of nanomaterials and light in fibers, various types of nanomaterial‐based saturable absorbers have been demonstrated. For instance, a transmission‐mode film‐like saturable absorber is fabricated by nanomaterial‐polymer composites sandwiched between two fiber ferrules in a standard fiber connector [10, 11]. **Figure 2(a)** shows the typical configuration of a transmission‐type saturable absorber. This composite can be constituted by several easy fabrication and integration methods of sputter‐ ing, direct synthesis or deposition on the end surfaces of optical fibers, whereas there is a direct physical contact, that is, the laser light is directly transmitted through the nanomaterial film. It is noticed that there may be thermal and mechanical damages within the limited interac‐ tion length for high‐energy pulsed fiber lasers. A promising alternative is lateral interaction with the evanescent waves of the fiber. The saturable absorbers with the evanescent wave interaction have been demonstrated by several fiber structures including tapered fiber [11], D‐ shaped fiber [13], etc. The structures and lateral interaction process of the tapered and D‐ shaped fibers are shown in **Figure 2(b)** and **(c)**. The evanescent wave is generated by total internal reflection of rays at the boundary of the fiber core with a lower index of clad medi‐ um. The nanomaterials like carbon nanotubes that contained in the lower‐index region can raise the nonlinear reflection coefficient due to evanescent wave absorption along the fiber in a longer nonlinear interaction length in a centimeter scale. This configuration is compatible with the fiber format and easy to add the saturable absorber in the cavity by using simple fiber

**Figure 2.** Typical configuration of transmission‐type saturable absorber (a), evanescent field interaction of the D‐shap‐

integer times of the fundamental repetition rate.

282 284High Energy and Short Pulse Lasers

fusing splicing technique.

ed fiber (b), and tapered fiber (c).

Dissipative soliton pulses refer to those confined wave packets of light in nonlinear optical systems with the balance of nonlinear gain, loss mechanisms. Dissipative solitons generally show large linear chirp with high pulse energies due to the large dispersion experienced in the fiber cavity and possible realization of larger compression ratio by means of simple chirp pulse compression techniques. Dissipative solitons offer highly desirable properties for some direct application and as the seed laser for pulse amplifier system, such as short light pulses of high pulse energies and the improved output stability with compactness, efficiency, and reliability.

An effective method of reducing the repetition rate of passively mode‐locked fiber laser is to elongate the cavity by simply adding fiber lengths, and at this time, high‐energy ultrashort pulses can be obtained by this effective approach. This method is well suited for fiber lasers where the resonator cavity may reach length in excess of one mile and generate higher energy pulse while maintaining a compact structure [18]. All‐normal dispersion passively mode‐ locked ytterbium‐doped fiber laser in a sense offers an ideal laser source of low repetition rate, long duration, and high‐energy pulses suitable for a range of applications.

Passively mode‐locked fiber laser yields a relatively lower pulse energy in the ultrashort duration because of mode confinement of conventional single‐mode fiber. Also, the longer fiber enhances nonlinearities like stimulated Raman scattering and self‐phase modulation, which lead to the distortions of the pulse and instability of the mode locking. Today, for the sake of the high-energy laser pulses with available pump power in many application fields like micromachining, many researchers pay their attentions to the use of double‐clad fiber for high‐power fiber amplifiers and lasers. The double‐clad fiber can be pumped by high power laser diodes to get the higher gain [19–21], where the pump light is coupled into the larger inner cladding with a higher numerical aperture.
