**1. Introduction**

Ultrafast laser sources and their applications such as high-power supercontinuum and frequency comb have gained much attention in recent decades [1–7]. High-power fiber lasers

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spur a rapid growth of industrial applications including laser cutting, laser marking, and so on [8]. Moreover, supercontinuum and frequency comb are considered as the breakthrough of laser field for their applications covering precision spectroscopy, astronomical observa‐ tions, and optical frequency metrology [9, 10]. This chapter is intended to describe, from experimental point of view, the ultrashort pulse laser oscillators, high-power nonlinear fiber amplifiers, supercontinuum, and frequency combs. Section 2 shows the performance of two types of mode-locked lasers. The first one consisting of bulk and fiber optical components is mode-locked via nonlinear polarization rotation (NPR) mechanism at 1.03 μm. The other one, operating at 1.55 μm, is mode-locked by nonlinear amplified loop mirror (NALM) with polarization-maintaining (PM) fiber components in order to overcome environmental perturbation and thus maintain long-term operation. Section 3 introduces a practical method (spectral tailoring), which facilitates supercontinuum generation in single-mode fiber amplifier at 1.03 μm with a few picosecond laser pulses. The second part in this section introduced broadband supercontinuum generation (from 950 to 2200 nm) by injecting pulses with 72-fs temporal duration, 150-mW average power, and 60-MHz repetition rate at 1560 nm into 20-cm-long PM-HNLF. Section 4 gives a brief introduction of divided-pulse amplifica‐ tion (DPA). To generate transform-limited pulse at 1.55 μm, DPA with polarized pulse duplicating was employed to overcome the gain narrowing effect and control the nonlinear spectral broadening in anomalous dispersion Er-fiber amplifier. As high as 500-mW aver‐ age power at 1560 nm is achieved by ×8 replicas. Moreover, the highest frequency-doubling conversion efficiency reached 56.3% by using a periodically poled lithium niobate (PPLN) crystal at room temperature. Section 5 discusses an all-optical control method via resonant‐ ly enhanced optical nonlinearity (or pump-induced refractive index change, RIC) for highprecision repetition rate stabilization. The standard deviation (SD) of repetition rate can be reduced to a record level of <100 μHz by using the RIC method in a PM figure-eight laser cavity.

### **2. Fiber laser**

Fiber lasers offer several practical advantages, such as excellent spatial-mode quality, effective heat dissipation, and flexible optical path and, recently, are becoming attractive laser sources in both scientific researches and industry applications. Especially, mode-locked fiber lasers with ultrashort pulse duration and high-repetition rate have attracted a lot of attention for their applications in optical sensing, optical communication, optical metrology, and biomed‐ ical imaging and processing [11, 12]. Therefore, various femtosecond/picosecond mode-locked lasers have been constructed and developed. As mode-locked lasers are often affected by environmental perturbations (mechanical vibration and temperature fluctuation), robust and stable oscillator with compact design are urgently needed. In this section, we present a compact femtosecond fiber laser at 1.03 μm by using integrated fiber optical components. The shortest dechirped pulse duration reaches 81 fs for a net cavity dispersion value close to −0.001 ps2 . Another part of this section described a self-started Er-doped laser oscillator, which is modelocked by NALM with PM-fiber configuration. By optimizing the net dispersion, the buildup time can be reduced from 8 min to ~ms order of magnitude.

#### **2.1. Operation regime of mode-locked lasers**

252

cavity.

**2. Fiber laser**

spur a rapid growth of industrial applications including laser cutting, laser marking, and so on [8]. Moreover, supercontinuum and frequency comb are considered as the breakthrough of laser field for their applications covering precision spectroscopy, astronomical observa‐ tions, and optical frequency metrology [9, 10]. This chapter is intended to describe, from experimental point of view, the ultrashort pulse laser oscillators, high-power nonlinear fiber amplifiers, supercontinuum, and frequency combs. Section 2 shows the performance of two types of mode-locked lasers. The first one consisting of bulk and fiber optical components is mode-locked via nonlinear polarization rotation (NPR) mechanism at 1.03 μm. The other one, operating at 1.55 μm, is mode-locked by nonlinear amplified loop mirror (NALM) with polarization-maintaining (PM) fiber components in order to overcome environmental perturbation and thus maintain long-term operation. Section 3 introduces a practical method (spectral tailoring), which facilitates supercontinuum generation in single-mode fiber amplifier at 1.03 μm with a few picosecond laser pulses. The second part in this section introduced broadband supercontinuum generation (from 950 to 2200 nm) by injecting pulses with 72-fs temporal duration, 150-mW average power, and 60-MHz repetition rate at 1560 nm into 20-cm-long PM-HNLF. Section 4 gives a brief introduction of divided-pulse amplifica‐ tion (DPA). To generate transform-limited pulse at 1.55 μm, DPA with polarized pulse duplicating was employed to overcome the gain narrowing effect and control the nonlinear spectral broadening in anomalous dispersion Er-fiber amplifier. As high as 500-mW aver‐ age power at 1560 nm is achieved by ×8 replicas. Moreover, the highest frequency-doubling conversion efficiency reached 56.3% by using a periodically poled lithium niobate (PPLN) crystal at room temperature. Section 5 discusses an all-optical control method via resonant‐ ly enhanced optical nonlinearity (or pump-induced refractive index change, RIC) for highprecision repetition rate stabilization. The standard deviation (SD) of repetition rate can be reduced to a record level of <100 μHz by using the RIC method in a PM figure-eight laser

Fiber lasers offer several practical advantages, such as excellent spatial-mode quality, effective heat dissipation, and flexible optical path and, recently, are becoming attractive laser sources in both scientific researches and industry applications. Especially, mode-locked fiber lasers with ultrashort pulse duration and high-repetition rate have attracted a lot of attention for their applications in optical sensing, optical communication, optical metrology, and biomed‐ ical imaging and processing [11, 12]. Therefore, various femtosecond/picosecond mode-locked lasers have been constructed and developed. As mode-locked lasers are often affected by environmental perturbations (mechanical vibration and temperature fluctuation), robust and stable oscillator with compact design are urgently needed. In this section, we present a compact femtosecond fiber laser at 1.03 μm by using integrated fiber optical components. The shortest dechirped pulse duration reaches 81 fs for a net cavity dispersion value close to −0.001 ps2

Another part of this section described a self-started Er-doped laser oscillator, which is mode-

.

As well known, the main features of mode-locked fiber laser depend on the pulse evolution process, which is relevant to the group-velocity dispersion (GVD) and the nonlinearity in optical fibers. According to the net intra-cavity dispersion, the pulse-shaping process can be roughly distinguished into the four different regimes, such as soliton regime, stretch-pulse regime, parabolic-pulse regime, and giant-chirp pulse regime, corresponding to all-anomalous dispersion, normal-anomalous dispersion, all-normal dispersion, and large-normal disper‐ sion, respectively. Due to the equilibrium between Kerr nonlinearity and GVD, pulses that propagate in all-anomalous dispersion laser cavity keep unchanged in the form of fundamental soliton [13, 14]. While in dispersion-managed laser cavities, the negative dispersion is com‐ pensated by positive dispersion and thus stretch pulse forms. When the net cavity dispersion is optimized to zero, significant variations on pulse duration could be observed [15, 16]. As pulse operates in the all-normal dispersion regime, where laser gain, self-phase modulation, and dispersion co-effect, spectral/temporal filtering effects force linear chirping in the pulse, so that similariton forms [17–19]. While in ultra-long laser cavity, giant-chirped oscillating can be realized with ultralow repetition rate but at high-pulse energy [20–23].

#### **2.2. NPR mode locking at 1.03 μm**

In this section, we designed a compact ultra-fast Yb-doped fiber laser with integrated optical components. By integrating wavelength division multiplexer and optical isolator with

**Figure 1.** Schematic diagram of a passively mode-locked fiber laser via nonlinear polarization rotation mechanism.

collimators, the fiber loop was simplified. Self-started mode-locking could be realized by setting appropriate polarization angle of four intra-cavity wave plates. Due to the normal dispersion of fiber at 1.0 μm, transmission grating pair with 1250 l/mm was used to provide adjustable anomalous dispersion. As a result, 81-fs temporal duration with 65-MHz repetition rate and 0.5-nJ pulse energy was produced.

The mode-locking procedure can be explained in **Figure 1**. Two polarization controllers and a polarization-sensitive isolator (PSI) are used as the key elements for mode-locking. This combination acts as a virtual saturable absorber, which can absorb the low-intensity tail of pulse and transmit high-intensity part such that the pulse could be shortened. The pulse with linear polarization changes to elliptical polarization by twisting the polarization controller. As mentioned, self-phase modulation (SPM) or cross-phase modulation (XPM) can arouse energy coupling between two orthogonal polarizations. Moreover, serious nonlinear polarization rotation is produced by the high gain in the active fiber. Finally, another polarization controller is used to modify the polarization state to facilitate the central part of the pulse getting through the PSI [24].

In our experiment, a Yb-doped fiber laser shown in **Figure 2(a)** was firstly constructed without dispersion compensation elements. Three intra-cavity wave plates including two quarterwave plates, QWP1 and QWP2, and one half-wave plate, HWP, were set with appropriate polarization angles to realize self-started mode-locking. The pigtails of fiber components are Hi1060 fiber with GVD of ~26 fs2 /mm and TOD of ~41 fs3 /mm, while the GVD for the active fiber is 39 fs2 /mm. The repetition rate and pulse duration were measured to be 70 MHz and 13 ps, respectively. By external-cavity dechirping, the pulse can be compressed to 170-fs duration, but with obvious pedestal. The autocorrelation of pulse before and after dechirping is shown in **Figure 3(a)** and **(b)**, respectively.

**Figure 2.** Structure of ultrafast Yb-doped fiber lasers without (a) and with (b) dispersion compensation. Pump diode: 400-mW laser diode at 976 nm; Yb-SMF: Yb-doped single-mode fiber; WDM/Col: the device combines WDM and colli‐ mator; Col/ISO: the device combines collimator and isolator; QWP1, QWP2, and QWP3: quarter-wave plate; HWP: half-wave plate; PBS: polarization beam splitter; GM: a gold-coated mirror.

254

the PSI [24].

fiber is 39 fs2

Hi1060 fiber with GVD of ~26 fs2

is shown in **Figure 3(a)** and **(b)**, respectively.

half-wave plate; PBS: polarization beam splitter; GM: a gold-coated mirror.

collimators, the fiber loop was simplified. Self-started mode-locking could be realized by setting appropriate polarization angle of four intra-cavity wave plates. Due to the normal dispersion of fiber at 1.0 μm, transmission grating pair with 1250 l/mm was used to provide adjustable anomalous dispersion. As a result, 81-fs temporal duration with 65-MHz repetition

The mode-locking procedure can be explained in **Figure 1**. Two polarization controllers and a polarization-sensitive isolator (PSI) are used as the key elements for mode-locking. This combination acts as a virtual saturable absorber, which can absorb the low-intensity tail of pulse and transmit high-intensity part such that the pulse could be shortened. The pulse with linear polarization changes to elliptical polarization by twisting the polarization controller. As mentioned, self-phase modulation (SPM) or cross-phase modulation (XPM) can arouse energy coupling between two orthogonal polarizations. Moreover, serious nonlinear polarization rotation is produced by the high gain in the active fiber. Finally, another polarization controller is used to modify the polarization state to facilitate the central part of the pulse getting through

In our experiment, a Yb-doped fiber laser shown in **Figure 2(a)** was firstly constructed without dispersion compensation elements. Three intra-cavity wave plates including two quarterwave plates, QWP1 and QWP2, and one half-wave plate, HWP, were set with appropriate polarization angles to realize self-started mode-locking. The pigtails of fiber components are

/mm and TOD of ~41 fs3

13 ps, respectively. By external-cavity dechirping, the pulse can be compressed to 170-fs duration, but with obvious pedestal. The autocorrelation of pulse before and after dechirping

**Figure 2.** Structure of ultrafast Yb-doped fiber lasers without (a) and with (b) dispersion compensation. Pump diode: 400-mW laser diode at 976 nm; Yb-SMF: Yb-doped single-mode fiber; WDM/Col: the device combines WDM and colli‐ mator; Col/ISO: the device combines collimator and isolator; QWP1, QWP2, and QWP3: quarter-wave plate; HWP:

/mm. The repetition rate and pulse duration were measured to be 70 MHz and

/mm, while the GVD for the active

rate and 0.5-nJ pulse energy was produced.

**Figure 3.** Autocorrelation trace of chirped pulses generated by picosecond fiber laser (a) and femtosecond fiber laser (c), (b) dechirped pulse of (a); (d) spectra generated by femtosecond fiber laser with different distances between gra‐ tings.

Secondly, a transmission grating pairs was used to manage the intra-cavity dispersion of the Yb-fiber laser, as shown in **Figure 2(b)**. The quarter-wave plate, QWP3, was used to impose 90° polarization rotation on laser pulses by double-passing the grating pairs. Soliton, stretchpulse, and all-normal dispersion regime can be achieved by optimizing the distance between gratings. **Figure 3(d)** compares the various spectral shapes with different grating separations. As shown in **Figure 3(c)**, the shortest pulse duration was measured to be 81 fs. The black curve in **Figure 3(d)** represents the broadest spectra with a 10-dB bandwidth of 100 nm. The uncompensated phase was mainly caused by the accumulated high-order dispersion in fibers as well as intra- and extra-cavity grating pairs.

#### **2.3. Polarization-maintaining figure-eight fiber laser at 1.55 μm**

Compact size, low cost, and free maintenance fiber laser at 1.55 μm are desirable in many applications, such as eye surgery, Terahertz generation, and precision spectroscopy [25–28]. The standard repetition rate of commercial available fiber laser is typically 80 MHz with an optional design from 20 to 250 MHz. In order to combine both high pulse energy and high average power, a 10-MHz repetition rate is the best choice for applications. When the repetition rate is lower than 10 MHz, a pulse picker has to be used between the laser oscillator and the succeeding amplifier.

In this section, we introduce a PM figure-eight laser cavity which is the best option for oscillator operated at 10-MHz repetition rate. **Figure 4(a)** shows the experimental setup of the figureeight laser cavity. The linear loop comprises of a 980/1550 nm wavelength division multiplexer, a segment of Er-doped fiber (PM-ESF-7/125, Nufern), an isolator, a 2-nm bandpass filter at 1550 nm, and an output coupler CP2 with a splitting ratio of 20:80. The active gain applied in the linear loop is to compensate the cavity loses and facilitate self-started mode-locking. The band-pass filter is used to block longer wavelength (Raman self-frequency shift) and reduce the temporal width of the pulse to be self-consistent. Pulses from the linear loop are coupled into NALM via CP1 with a splitting ratio of 45:55.

Over-pump with three LDs was applied to provide enough power for self-started modelocking. Interestingly, the buildup time of mode-locking was found to be closely related to the net cavity dispersion. When the net dispersion was set to −0.115 ps<sup>2</sup> , as long as 8-min buildup time was observed. After optimizing the net dispersion to about −0.062 ps<sup>2</sup> , the time dramat‐ ically decreased to ~ms of magnitude.

Furthermore, we recorded the mode-locked pulse trains triggered by a square wave with 5- Hz modulation frequency which is simultaneously used to drive LD3. **Figure 4(c)** shows two

**Figure 4.** (a) Schematic of a polarization-maintaining figure-eight erbium-doped fiber laser. WDM1, WDM2, and WDM3: 980/1550 nm wavelength division multiplexers; ISO: isolator; EDF1 and EDF2: erbium-doped fiber; BP: 2-nm bandpass filter at 1550 nm; DCF: dispersion compensation fiber; CP1 and CP2: 1550 nm coupler with splitting ratio of 45:55 and 20:80. (b) The initial pulse that polarization-maintaining figure-eight erbium-doped fiber laser generated. (c) The buildup time measured by an oscilloscope. (d) The measurement of power stabilization once mode-locked.

adjacent periods with 50-ms pump duration, and the corresponding buildup time was measured to be 53 and 6 ms, respectively, exhibiting certain randomness. From experimental results point of view, the mode-locking buildup time is a random value in a certain range, which is related to the net cavity dispersion.

Interestingly, multiple-pulse operation was observed as the mode-locking is established, as shown in **Figure 4(b)**. Peak power clamping effect originating from sagnac mechanism resulted in the formation of pulse bunching [29]. Stable single pulse could be obtained by decreasing the pump power. In single-pulse regime, the 5-min power stability was measured to be 0.26%, as shown in the inset of **Figure 4(d)**.
