**5. Typical applications of short‐pulse mode‐locked fiber lasers**

Fiber laser sources with nanosecond pulses have great potential in a variety of applications requiring low temporal coherence, such as optical metrology, or sensorinterrogation based on low‐coherence spectral interferometry technology. Furthermore, the applications of fiber‐ based master oscillator power amplifier sources can also extend to industrial fields, such as laser marking, engraving, and other micromachining in various materials, particularly suitable for cutting high reflectivity materials like titanium, copper [39], silver [40]. Other successful‐ ly commercial examples include the high‐resolution 3D imaging lidar system [41], nonlinear frequency conversion [42], etc. The pulse duration of ultralong cavity mode‐locked fiber laser may be up to several or even hundreds of nanoseconds with higher energy. In contrast to the Q‐switched lasers, obviously, there would be the limited range of direct applications. For this reason, the interests would focus on the high‐chirp solitons or dissipative solitons resonance with the duration as long as nanoseconds that can further be compressed into a picosecond level.

Double‐clad fibers have been extensively used to build fiber amplification systems, exhibit‐ ing desirable characteristics such as high gain, good efficiency, and excellent beam quality. A diode‐pumped mode‐locked ytterbium‐doped fiber seed laser followed by two fiber amplifi‐ ers has been demonstrated [36], where both fiber amplifiers with the design of large‐mode area were cladding pumped. The oscillator produced 30 pJ, 1.8 ps pulses. After two‐scales ampli‐ fying, the output pulses compressed by 830 grooves/mm gratings produced high‐quality 400 nJ pulses with a pulse duration of 110 fs at average power levels in excess of 25 W. A carbon‐ nanotube‐based master oscillator power amplifier has also been reported that a compact picosecond‐level pulse fiber laser with high average power [37]. The seed laser that is the nanotube‐based mode‐locked Nd:YVO4 laser is further amplified with a single‐stage fiber amplifier. An amplified pulse with a pulse width of 15.7 ps, pulse energy of 244 nJ, has been achieved with an average power of 20 W at a repetition rate of 82 MHz. A fiber amplifier contains a seed source and two‐scale amplifiers in that the gain fibers are different size double‐ clad fibers to suppress the nonlinear effects, and single pulse energy of 4.56 μJ with a pulse width of 0.62 ns at 26.3 MHz has been realized [38]. These systems present simple and practical

fiber‐based solutions for high‐average‐power ultrashort‐pulse laser applications.

Fiber nonlinearity is proportional to the length of fiber and inversely proportional to the fiber core size. The developed large‐mode area double‐clad photonic crystal fiber can be consid‐ ered as a possible approach to overcome many of the difficulties as mentioned above. These amplifiers enabled by advancements in photonic crystal fiber manufacturing technology can generate kilowatts to multigigawatts of peak power using direct amplification showing

Fiber laser sources with nanosecond pulses have great potential in a variety of applications requiring low temporal coherence, such as optical metrology, or sensorinterrogation based on low‐coherence spectral interferometry technology. Furthermore, the applications of fiber‐ based master oscillator power amplifier sources can also extend to industrial fields, such as laser marking, engraving, and other micromachining in various materials, particularly suitable for cutting high reflectivity materials like titanium, copper [39], silver [40]. Other successful‐ ly commercial examples include the high‐resolution 3D imaging lidar system [41], nonlinear

**Figure 12.** Schematic diagram of a master oscillator power amplifier system.

294 296High Energy and Short Pulse Lasers

excellent conversion efficiency, diffraction‐limited beam quality.

**5. Typical applications of short‐pulse mode‐locked fiber lasers**

All‐fiber subnanosecond lasers have great potential application in the generation of wide‐ band supercontinuum (SC) source [38]. The supercontinuum laser source is the widely broadened spectrum generated by strong nonlinear effects using highly nonlinear fibers. The output power of this novel supercontinuum light source is high by using the small core fibers, so that the supercontinuum source is of interest to many kinds of applications. For example, an ultrahigh‐resolution optical coherence tomography (OCT) has been investigated by using the ultrashort‐pulse fiber supercontinuum source as fiber‐based, high‐power, wideband sources [43].

Laser beam at the output of the fiber can be easily focused in a spot with a radius of ∼μm, a few nanojoules of energy in a tens femtoseconds pulse resulting in intensities on the order of ∼GW/cm<sup>2</sup> . So, ultrashort laser pulses would not induce heat diffusion during the fast interaction with objects such as various materials and living structures, that is, free from cracks and melting and other thermal effects [44]. For ultrashort laser pulses with the duration below several picoseconds, the pulses interacting period is generally shorter than the lattice heat‐ ing time, which is necessary for energy diffusion processes for most materials. Ultrashort‐pulse fiber lasers presently have provided a stable and reliable platform for many applications.

Material micro‐ and nano‐machining has been for a long time identified as an important and the largest market for high‐power/energy ultrashort‐pulse fiber lasers [45]. High pulse intensities are widely used for permanent transparent material modification. In addition, these extremely high power densities of the pulses resulting in a highly localized disruption of the material matrix with very little energy deposited and few heat transferred to the surround‐ ing material. This is a so‐called laser cold process. A clear processing edge formed by laser ablation is of key importance for medical, photovoltaic, and semiconductor industry, espe‐ cially for thermally sensitive material, like nitinol shape memory alloys, bio‐absorbable polymers like polylactic acids, glass, etc. [46]. In comparison, nanosecond laser pulses micromachining in glass or other materials would leave an undesirable heat‐affected zone, numerous stress fractures, and micro‐cracks around the processing edge.

Although known as a cold ablation process, by precisely controlling localized heat accumu‐ lation to melt material accompanying with the inhibition of shrinkage stress by producing embedded molten pool by nonlinear absorption process, ultrashort laser pulses at high repetition rates (hundreds of kHz and above) have been applied into micro‐welding of materials including glass and plastic, silicon and glass, and medical piece part [47].

Ultrashort‐pulse laser make them ideal sources for time resolved measurement of the faster physical andchemical phenomenon. By controlling the optical carrierfrequency andthe carrier phase of ultrafast lasers, optical frequency combs, spectroscopy and precision metrology of

optical frequency transitions and natural constants have been realized. Pump‐probe measure‐ ments can use ultrashort laser pulses to measure and determine the evolution of a series of ultrafast processes in many kinds of materials, molecules or even in internal states of atoms, with the advantage of shortening the transient behavior resulting from the optical excitation [48]. Femtosecond laser pulses with modest energies generating the intensities above 10<sup>15</sup> W/cm2 are used to determine the elements of the sample in a technique called femtosecond laser‐induced breakdown spectroscopy (fs‐LIBS) [49].

Ultrashort fiber lasers are developing rapidly in the medical and biology applications. These laser pulses can be used as a laser scalpel directly to medical treatment on the one hand. The advantages of accuracy, absence of thermal interaction, and safety have been accepted by a wide customer. On the other hand, the indirect applications refer to high precision and high‐ quality medical devices, such as stents, implants, and catheters, requiring sophisticated manufacturing techniques, which are available from medical industrial manufacturing processes. An example has been reported in 2014, the multifunctional biochips for realizing high‐performance biochemical analysis and cell engineering [50].

In conclusion, passively mode‐locked ytterbium‐doped fiber lasers operating in the normal dispersion regime have been firmly established in the field of various short‐duration pulses, and attracted increasing attention due to their compactness, low cost, and widespread applications. Various technologies have been developed with the aim to realize short‐pulse all‐fiberlaser sources with the desirable energies, durations, average powers and beam quality, as well the environmental stabilization and reliability. This chapter is intended to smoothly understand each topic on this field and has the interest to furtherread and explore high‐energy short‐pulse generation and their applications.
