**5. Dissipative soliton 2 μm fiber lasers mode-locked with 2D materials**

Although 2 μm Tm3+−doped fiber lasers (TDFLs) have valuable applications in sensing, medical surgery, industrial machining, and scientific experiments [68, 69], applications require high-peak-power and/or high-energy laser pulses, which are generally produced by using Q-switching [45, 70] or mode-locking [43, 44] methods. Compared with Q-switching, modelocking can provide much narrower pulse duration and higher peak power.

Passive mode-locking is usually the preferred choice to get short pulses from 2 μm TDFLs, especially with the maturely developed semiconductor SAs as modulators [47]. However, semiconductor SA has some drawbacks such as complex design and growth procedure [71] and narrow working wavelength range. Recently, graphene (a monolayer of two-dimensional (2D) carbon atoms in a honeycomb structure) has attracted great attention for mode-locking of 2 μm TDFLs [72, 73] due to its advantages of large absorption [74], wide operation spectral range [75], and ultrafast recovery time [76]. Another kind of 2D material MoS2 has also been extensively explored to mode-lock fiber lasers [57, 77–80]. Although monolayer MoS2 is a direct band semiconductor (the bandgap determines the energy of photons to be absorbed), studies have proven that layered MoS2 , through introducing stoichiometric defects (non-ideal atomic ratio), also possesses wideband absorption and saturable absorption features [81]. Thereafter, extensive researches have been dedicated to exploring of mode-locking operation and related characteristics of fiber lasers in the 1 μm [57, 77] and 1.5 μm [78–80] wavelength regions. Owing to large anomalous dispersion of the gain fiber and lower absorption of layered MoS2 at 2 μm, mode-locking operation with this kind of 2D material and corresponding behavior in the 2 μm region still need further verification.

Here, through combining the CGFML and multilayer MoS2 , we show that mode-locking capability of layered MoS2 sheets can be definitely extended to the 2 μm wavelength region. With a linear cavity incorporated with the multilayer MoS2 modulator, fundamental mode-locking in the DS regime for 2 μm Tm3+ fiber lasers is achieved. At the same time, through elongating the total fiber length, thus decreasing the mode-locking repetition rate, the pulse energy can be scaled to over 15 nJ.

The multilayer MoS2 was synthesized with the liquid-phase exfoliation method (LPE) [6], and the MoS2 nanosheet was transferred onto a gold mirror acting as SA. Raman spectrum of the MoS2 on the mirror was detected with a spectrometer, and the results are shown in **Figure 17(a)**. The spectral position of the E2g 1 and A1g modes (~383 cm−1 for E2g 1 and ~408 cm − 1 for A1g) shows that the MoS2 sample has a thickness of approximately four layers [82]. With a self-constructed 1940 nm ~800 ps fiber laser as the probe source, the reflection method was used to measure the saturable absorption of the sample, and the transmittance of the multilayer MoS2 on the gold mirror is shown in **Figure 17(b)** [6]. The nonlinear optical parameters were obtained by using a simple saturable absorption model of [57]

are 72 and 95 mW, respectively, corresponding to soliton molecule energies of 3.32 and 4.38

Although 2 μm Tm3+−doped fiber lasers (TDFLs) have valuable applications in sensing, medical surgery, industrial machining, and scientific experiments [68, 69], applications require high-peak-power and/or high-energy laser pulses, which are generally produced by using Q-switching [45, 70] or mode-locking [43, 44] methods. Compared with Q-switching, mode-

Passive mode-locking is usually the preferred choice to get short pulses from 2 μm TDFLs, especially with the maturely developed semiconductor SAs as modulators [47]. However, semiconductor SA has some drawbacks such as complex design and growth procedure [71] and narrow working wavelength range. Recently, graphene (a monolayer of two-dimensional (2D) carbon atoms in a honeycomb structure) has attracted great attention for mode-locking of 2 μm TDFLs [72, 73] due to its advantages of large absorption [74], wide operation spectral

has also been

is a

nJ. While their single-soliton energies are 1.66 and 1.46 nJ, respectively.

**Figure 16.** Experimentally measured doublet (a) and triplet (b) soliton molecule pulse trains [63].

**materials**

126 High Power Laser Systems

ered MoS2

**5. Dissipative soliton 2 μm fiber lasers mode-locked with 2D**

locking can provide much narrower pulse duration and higher peak power.

range [75], and ultrafast recovery time [76]. Another kind of 2D material MoS2

studies have proven that layered MoS2

behavior in the 2 μm region still need further verification.

extensively explored to mode-lock fiber lasers [57, 77–80]. Although monolayer MoS2

direct band semiconductor (the bandgap determines the energy of photons to be absorbed),

atomic ratio), also possesses wideband absorption and saturable absorption features [81]. Thereafter, extensive researches have been dedicated to exploring of mode-locking operation and related characteristics of fiber lasers in the 1 μm [57, 77] and 1.5 μm [78–80] wavelength regions. Owing to large anomalous dispersion of the gain fiber and lower absorption of lay-

at 2 μm, mode-locking operation with this kind of 2D material and corresponding

, through introducing stoichiometric defects (non-ideal

$$\text{T(I)} = \text{1} - \alpha\_o \times \exp\left(-\text{I/I}\_{\text{sat}}\right) - \alpha\_{\text{ns}} \tag{4}$$

here, T(I) is the transmission, α0 is the modulation depth, I is the input intensity, Isat is the saturation intensity, and αns is the non-saturable absorbance. The measured modulation depth α0 , nonsaturable loss αns, and saturation intensity Isat were 13.6%, 16.7%, and 23.1 MW cm−2, respectively. The modulation depth is comparable to that measured in the 1 μm region [57, 77] but larger than that in the 1.5 μm region [79, 80]. This large modulation depth of the MoS2 SA at the 2 μm wavelength region is efficient for suppressing wave breaking in mode-locking operation [83].

**Figure 17.** (a) Raman spectrum of the adopted multilayer MoS2 sheets and (b) nonlinear absorption of the multilayer MoS2 sheets coated on a gold mirror [6].

The schematic diagram of the experimental setup for the MoS2 mode-locked TDFL is shown in **Figure 18**. A 1550-nm-CW Er/Yb-codoped fiber laser with maximum output of ~1 W was used as the pump source, and a WDM coupler was used to launch the pump light (with an efficiency of ~95%). The Tm3+-doped silica gain fiber (5/125 μm, 0.24 NA) has core absorption of ~350 dBm−1 at ~1550 nm, and 12 cm length of gain fiber was adopted. The dispersion of the gain fiber at 1.9 μm is −12 ps <sup>2</sup> km−1. A 4 m length of SMF-28 fiber was spliced at the output end. To provide normal dispersion, 4.6 m dispersion-compensating fiber (DCF) (2.2 μm, 0.35 NA core) was spliced to the gain fiber. The dispersions of the DCF fiber and the SMF-28 fiber at 1.9 μm are 93 and − 67 ps <sup>2</sup> km−1, respectively [52]. The total net cavity dispersion is ~0.05 ps2 . The DCF fiber was butt coupled to the MoS2 sheet, which was transferred onto a high-reflection gold mirror. High reflection of the gold mirror and the ~3.5% Fresnel reflection of the perpendicularly cleaved output fiber facet completed the laser cavity.

Under pumping, the 2 μm laser first went to CW operation when pump power was over 430 mW. When the pump power was increased to over 630 mW, the laser came to the stable Q-switching regime. Then, further raising the pump power to over 700 mW and careful adjusting the MoS2 position, stable mode-locking operation of the TDFL occurred, which could be sustained up to the available maximum pump power. The output power is linearly dependent on the pump power, and the maximum output power is 150 mW, as shown in **Figure 19** [6]. The slope efficiency is 43.6% with respect to pump power. The laser spectrum of the mode-locked TDFL is centered at ~1905 nm with a FWHM bandwidth of 17.3 nm. This spectral width is much larger than that of the 1 and 1.5 μm counterparts [57, 77–79], showing potential much narrower transform-limited pulse duration of this mode-locked TDFL.

product of ~1200, indicating that the mode-locked laser pulse is highly chirped. Chirping

**Figure 19.** Output (left) and spectrum (right) of the mode-locked Tm3+ fiber laser. Square dots are measured data and

layer) has ultrafast recovery times of tens of femtoseconds [84] and ~100 ps [85, 86], corresponding, respectively, to the intraband transition and interband transition of excited free

time-scale pulse durations [79, 80, 87]. Based on the pulse spectral width (17.3 nm) in our experiment, a Fourier transform-limited pulse width of ~247 fs is expected provided that the

(with resolution of 0.1 MHz) is shown in **Figure 21** (left panel). The fundamental pulsing frequency is 9.67 MHz, which is correspondent to the total cavity length. Over the 100 MHz range, no other supermode oscillations are present. The right panel of the figure shows the ninth-order harmonics in a smaller frequency window (20 MHz), and the signal-to-noise ratio

**Figure 20.** Laser pulse train (a) and single pulse (b) of the MoS2 mode-locked Tm3+ fiber laser measured at the maximum

(monolayer or few-

SAs has achieved femtosecond

mode-locked fiber laser, and the RF spectrum

mode-locked fiber laser is comparatively stable.

Developing High-Energy Dissipative Soliton 2 μm Tm3+-Doped Fiber Lasers

http://dx.doi.org/10.5772/intechopen.75037

129

pulse also somehow contributes to high pulse energy. In fact, 2D MoS2

carriers. Recently, mode-locking with similar multilayer MoS2

entire pulse chirp can be compensated.

the solid line is linear fitting [6].

is also >40 dB, showing that the MoS2

output level. Insets show the oscilloscope traces [6].

We also measured the RF spectrum of this MoS2

The laser pulse trains obtained at the maximum output level are shown in **Figure 20(a)** [6]. The 103.4 ns period time corresponds well to the cavity round trip time (the total fiber length is ~10 m), showing that the mode-locking operates at the fundamental frequency of 9.67 MHz. The intensity stability between different pulses is >95%. Considering the 150 mW output power, single pulse energy reaches 15.5 nJ. This is the highest pulse energy ever achieved in mode-locked 2 μm fiber lasers with MoS2 modulators, and this also demonstrates that 2D material MoS2 has a great potential in high-power photoelectronics and integrated photonics.

**Figure 20(b)** displays the single pulse at the maximum power level, which has a Gaussian shape and a FWHM width of 716 ps. This 2 μm DS pulse width is comparable to the 1 μm counterparts [57, 77]. Combined with its spectral width, the 2 μm DS pulse has a time-bandwidth

**Figure 18.** Experimental setup of the mode-locked Tm3+ fiber laser. EYFL, erbium/ytterbium-codoped fiber laser; SMF, single-mode fiber; DCF, dispersion-compensating fiber [6].

The schematic diagram of the experimental setup for the MoS2

. The DCF fiber was butt coupled to the MoS2

achieved in mode-locked 2 μm fiber lasers with MoS2

single-mode fiber; DCF, dispersion-compensating fiber [6].

of the perpendicularly cleaved output fiber facet completed the laser cavity.

~0.05 ps2

128 High Power Laser Systems

ful adjusting the MoS2

integrated photonics.

in **Figure 18**. A 1550-nm-CW Er/Yb-codoped fiber laser with maximum output of ~1 W was used as the pump source, and a WDM coupler was used to launch the pump light (with an efficiency of ~95%). The Tm3+-doped silica gain fiber (5/125 μm, 0.24 NA) has core absorption of ~350 dBm−1 at ~1550 nm, and 12 cm length of gain fiber was adopted. The dispersion of the gain fiber at 1.9 μm is −12 ps <sup>2</sup> km−1. A 4 m length of SMF-28 fiber was spliced at the output end. To provide normal dispersion, 4.6 m dispersion-compensating fiber (DCF) (2.2 μm, 0.35 NA core) was spliced to the gain fiber. The dispersions of the DCF fiber and the SMF-28 fiber at 1.9 μm are 93 and − 67 ps <sup>2</sup> km−1, respectively [52]. The total net cavity dispersion is

high-reflection gold mirror. High reflection of the gold mirror and the ~3.5% Fresnel reflection

Under pumping, the 2 μm laser first went to CW operation when pump power was over 430 mW. When the pump power was increased to over 630 mW, the laser came to the stable Q-switching regime. Then, further raising the pump power to over 700 mW and care-

could be sustained up to the available maximum pump power. The output power is linearly dependent on the pump power, and the maximum output power is 150 mW, as shown in **Figure 19** [6]. The slope efficiency is 43.6% with respect to pump power. The laser spectrum of the mode-locked TDFL is centered at ~1905 nm with a FWHM bandwidth of 17.3 nm. This spectral width is much larger than that of the 1 and 1.5 μm counterparts [57, 77–79], showing potential much narrower transform-limited pulse duration of this mode-locked TDFL.

The laser pulse trains obtained at the maximum output level are shown in **Figure 20(a)** [6]. The 103.4 ns period time corresponds well to the cavity round trip time (the total fiber length is ~10 m), showing that the mode-locking operates at the fundamental frequency of 9.67 MHz. The intensity stability between different pulses is >95%. Considering the 150 mW output power, single pulse energy reaches 15.5 nJ. This is the highest pulse energy ever

strates that 2D material MoS2 has a great potential in high-power photoelectronics and

**Figure 20(b)** displays the single pulse at the maximum power level, which has a Gaussian shape and a FWHM width of 716 ps. This 2 μm DS pulse width is comparable to the 1 μm counterparts [57, 77]. Combined with its spectral width, the 2 μm DS pulse has a time-bandwidth

**Figure 18.** Experimental setup of the mode-locked Tm3+ fiber laser. EYFL, erbium/ytterbium-codoped fiber laser; SMF,

position, stable mode-locking operation of the TDFL occurred, which

mode-locked TDFL is shown

sheet, which was transferred onto a

modulators, and this also demon-

**Figure 19.** Output (left) and spectrum (right) of the mode-locked Tm3+ fiber laser. Square dots are measured data and the solid line is linear fitting [6].

product of ~1200, indicating that the mode-locked laser pulse is highly chirped. Chirping pulse also somehow contributes to high pulse energy. In fact, 2D MoS2 (monolayer or fewlayer) has ultrafast recovery times of tens of femtoseconds [84] and ~100 ps [85, 86], corresponding, respectively, to the intraband transition and interband transition of excited free carriers. Recently, mode-locking with similar multilayer MoS2 SAs has achieved femtosecond time-scale pulse durations [79, 80, 87]. Based on the pulse spectral width (17.3 nm) in our experiment, a Fourier transform-limited pulse width of ~247 fs is expected provided that the entire pulse chirp can be compensated.

We also measured the RF spectrum of this MoS2 mode-locked fiber laser, and the RF spectrum (with resolution of 0.1 MHz) is shown in **Figure 21** (left panel). The fundamental pulsing frequency is 9.67 MHz, which is correspondent to the total cavity length. Over the 100 MHz range, no other supermode oscillations are present. The right panel of the figure shows the ninth-order harmonics in a smaller frequency window (20 MHz), and the signal-to-noise ratio is also >40 dB, showing that the MoS2 mode-locked fiber laser is comparatively stable.

**Figure 20.** Laser pulse train (a) and single pulse (b) of the MoS2 mode-locked Tm3+ fiber laser measured at the maximum output level. Insets show the oscilloscope traces [6].

In the primary experimental operation based on this model, the 2 μm DS mode-locked Tm-doped fiber laser with a linear cavity delivers 4.9 nJ DSs with pulse duration of 579 fs after being dechirped. Then, through increasing pump power or managing the cavity dispersion map, the pulse energy of this DS fiber is improved to ~12 nJ. We also observe that highpulse-energy harmonic mode-locked DSs from 2 μm Tm-doped fiber lasers, with single pulse energy of 6.27, 4.32, and 3.29 nJ for the second- to the fourth-order harmonics. Thereafter, DS

Developing High-Energy Dissipative Soliton 2 μm Tm3+-Doped Fiber Lasers

decreasing the pulsing frequency, the pulse energy is scaled to 15.5 nJ. This improves the pulse energy of 2 μm mode-locked single-mode fiber lasers to approaching the 1 and 1.5 μm counterparts. All these results show that CGFML DS can be an efficient way to produce high-

To further scale the pulse energy of the CGFML DS in 2 μm TDFLs, more condensed GFs (which has been available currently) should be adopted, and the total cavity dispersion map should be optimized. Therefore, with higher pump power, more condensed GFs, and further optimized parameters, ultrafast 2 μm pulses with even higher energy are readily feasible.

This CGFML model can be readily extended to beyond 2 μm, e.g., mid-infrared fiber lasers (usually with anomalously dispersive gain media) to scale DS energy and thus is an efficient

Key Laboratory for Laser Plasmas (Ministry of Education), Department of Physics and Astronomy, Collaborative Innovation Center of IFSA, Shanghai Jiao Tong University,

[1] Jackson SD. Towards high-power mid-infrared emission from a fibre laser. Nature

[2] Geng J, Jiang S. The 2 μm market heats up. Optics and Photonics News. 2014;**25**:36-41

[3] Tang Y, Yu X, Li X, Yan Z, Wang QJ. High-power thulium fiber laser Q switched with

[4] Wang Y et al. High power tandem-pumped thulium-doped fiber laser. Optics Express.

) is investigated, and through

http://dx.doi.org/10.5772/intechopen.75037

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mode-locking of 2 μm TDFL with 2D material (multilayer MoS2

pulse energy scaling route for anomalous dispersive fiber lasers.

single-layer graphene. Optics Letters. 2014;**39**:614-617

[5] Ferman M. E and Hartl I. Nature Photonics. 2013;**7**:868

Yulong Tang\*, Chongyuan Huang and Jianqiu Xu

\*Address all correspondence to: yulong@sjtu.edu.cn

energy ultrafast pulses from 2 μm TDFLs.

**Author details**

Shanghai, China

**References**

Photonics. 2012;**6**:423-431

2015;**23**:2991-2998

**Figure 21.** Radiofrequency spectral profile of the mode-locked Tm3+ fiber laser [6].
