**4. Amplification of short‐pulse fiber lasers**

Because laser pulses that extracted from the master oscillator are generally of relatively low energies, an additional external amplifier is required for the enhancement of the pulse energy, which is of key importance of power scaling offiberlasers forthe wide applications. Ytterbium‐ doped fiberlaser systems are excellently suited to generate and amplify ultrashort laser pulses due to their large amplification bandwidth supporting pulse durations of few hundred femtoseconds. This approach is benefited from the simple fiber connection between the oscillator and the amplifier. There are mainly two methods, that is, the chirped pulse ampli‐ fication (CPA) and the master oscillator power amplification (MOPA).

Forthe CPA technique, ultrashort pulses are amplified by time stretching of the original pulses and later recompress them back into a short duration after the fiber amplifier [32]. The chirped pulse amplifier system, as schematically shown in **Figure 11**, is composed of a seed laser, a pulse stretcher, amplifier chains, and a pulse compressor. The duration of laser pulses is firstly increased temporally to a much longer duration of the order of 1ns, that is chirped by using a pulse stretcher, for example a grating pair, fiber chirped Bragg grating, etc., which reduces the peak power to a level so that the nonlinear effects in the gain medium can be avoided. The stretched pulse was amplified in next amplifier system, which typically consisted of large‐ mode area single‐mode fiber or photonic crystal fiber (PCF) gained by multimode laser diodes, allowing more peak power generation below the limit of nonlinear optical intensity effects. Finally, a low‐loss compressor is used to temporally compress the pulses to a duration similar to the input pulse duration. The pulse stretcher is necessary for the amplification of ultra‐ short pulses. Or otherwise, high nonlinearities in the fiberinduced by high optical peak power in ultrashort pulses would affect the recompression to an ideal short pulse in the final compression part. Of course, the compressor also needs to tolerate high peak powers with‐ out introducing nonlinear distortions.

**Figure 11.** Schematic diagram of the chirped pulse amplifier for ultrashort‐pulse fiber laser.

power of 1.81 W, which changing into a state of multipulses. The pulse sequence and single pulse, which is a typical noise‐like pulse, have been measured and shown in **Figure 9(a)** and **(b)**, respectively. The repetition period of the pulse is approximately 5 μs. The pulse dura‐ tion is approximately 292.6 ns. When the pump power was 1.81 W, another stable state, that is, soliton rains, could be obtained with carefully adjusting the polarization controller. **Figure 10** shows the pulse train, single pulse, and the spectrum ofthe fiberlaser at the pumping power of 1.93 W. Within each pulse period, the pulse contains background noise, drifted pulse, and phase‐condensed soliton. The intensity of the drifted pulse is about 10% of the phase‐ condensed soliton. The pulse width of phase condensation soliton is about 102.5 ns at 3 dB, as shown in **Figure 10(b)**. The steady soliton rains cannot be maintained once the pump power is above 1.93 W. The maximum output poweris ∼40.3 mW with single pulse energy of ∼201.5  nJ. Output spectra of the noise‐like pulses and soliton rains have been measured and shown in **Figures 9(c)** and **10(c)**. It can be seen that both spectra have several central wavelengths indicating that the presence of filtering effect in the cavity could be used as a multiwave‐

Recently, it has been reported that the generated pulses of an ultralong cavity fiber laser can deliver microjoule‐level energy in the nanosecond range [30]. In the all‐normal dispersion fiber laser systems, the stable mode‐locking pulses exhibits that the formation of pulse shaping is the product of complicated processes of energy conversion. Various nonlinear effects such as self‐phase modulation, dispersion wave, peak clamping, which have strong influence on the stability of mode locking, and combining with high cavity dispersion can lead to complex pulsing phenomena, like wave‐breaking of the soliton pulse as noise‐like pulses in the results above. On the other hand, the Raman‐induced noise‐like pulses can be realized by the Raman

Because laser pulses that extracted from the master oscillator are generally of relatively low energies, an additional external amplifier is required for the enhancement of the pulse energy, which is of key importance of power scaling offiberlasers forthe wide applications. Ytterbium‐ doped fiberlaser systems are excellently suited to generate and amplify ultrashort laser pulses due to their large amplification bandwidth supporting pulse durations of few hundred femtoseconds. This approach is benefited from the simple fiber connection between the oscillator and the amplifier. There are mainly two methods, that is, the chirped pulse ampli‐

Forthe CPA technique, ultrashort pulses are amplified by time stretching of the original pulses and later recompress them back into a short duration after the fiber amplifier [32]. The chirped pulse amplifier system, as schematically shown in **Figure 11**, is composed of a seed laser, a pulse stretcher, amplifier chains, and a pulse compressor. The duration of laser pulses is firstly increased temporally to a much longer duration of the order of 1ns, that is chirped by using a pulse stretcher, for example a grating pair, fiber chirped Bragg grating, etc., which reduces the

effect in a fiber laser with high nonlinearity and dispersion [31].

fication (CPA) and the master oscillator power amplification (MOPA).

**4. Amplification of short‐pulse fiber lasers**

length short‐pulse fiber laser.

292 294High Energy and Short Pulse Lasers

Femtosecond fiber amplifier systems have the potential for millijoule pulse energies at megahertz repetition rate. Fox example, a ytterbium‐doped fiber amplifier system has delivered millijoule level pulse energy at repetition rates above 100 kHz corresponding to an average power of more than 100 W, the compressed pulse is as short as 800 fs [33], where a short‐length PCF with 80‐μm core diameter is employed, which allows the pulse energies up to 1.45 mJ with a stretched pulse duration of 2 ns. Scaling up of pulse energy in an ultrafast fiber laser has been demonstrated that the simultaneous generation of 60 W of compressed average power at 100 kHz, together with 320 fs and 600 μJ pulses [34].

Highly chirped pulse fiber oscillators may create powerful all‐fiber generator‐amplifier systems, that is, the so‐called master oscillator power amplifier (MOPA), which is an attrac‐ tive technology to achieve picoseconds‐level laser pulses with higher average output power and peak power [35–38]. The master oscillator power amplifier, as shown in **Figure 12**, generally consists of a laser oscillatorthat produces the weak seed pulse andseries of amplifiers that increase the laser power to the required level. Chirped pulses from the generator are directly fed to an amplifier without the use of a stretcher or a modulator and compressed after one or more amplifying stages. In addition, the chirped pulses coming out of the master oscillator in the normal dispersion regime, like dissipative solitons, whose energy exceeds the energy of classical solitons by tens or hundreds of times because of longer duration at the same peak power, can be further increased to the required level in one power amplifier, as well compressed by an external compressor.

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.

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

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 excellent conversion efficiency, diffraction‐limited beam quality.
