*3.5.2. Ytterbium-doped fiber lasers*

with the use of 32 layers of graphene is depicted in **Figure 13(e)**. It is centered at 1561 nm and has an FWHM of 20 nm, whereas the repetition rate was 100 MHz. The Tm-doped fiber laser was designed analogously to the Er-doped laser, but in this case the cavity was not all-PM, so the laser needs polarization alignment to initiate the mode locking. The oscillator is pumped at 1566 nm wavelength using a laser diode, beforehand amplified in an Erbium-doped fiber amplifier (EDFA). In this case, 12 layers of graphene were sufficient to support stable mode locking at 1968 nm with 10 nm of bandwidth (**Figure 13f**) and 100.25 MHz repetition frequency.

**Figure 13.** Fiber lasers operating at 1 μm (a), 1.55 μm (b), and 1.97 μm (c), and the corresponding optical spectra gener‐

Efficient mode locking of solid-state lasers (SSLs) with the use of real saturable absorbers is quite challenging. The gain of an active medium (bulk crystal) is not as large as in fiber lasers, and in addition, the free-space resonator needs to be carefully aligned. Also the losses introduced by the SA should be possibly small. This is why most of the graphene-based SSLs utilize monolayer or bi-layer graphene. Up till now, mode locking of SSLs ranging from 532

As an example, Baek et al. [20] demonstrated a Ti:Sapphire laser mode-locked with monolayer graphene. The laser generated 63 fs pulses at 800 nm central wavelength. There were also several reports on lasers operating around 1 μm wavelength [27–29]. The most prominent result was obtained by Ma et al. [29]. The authors have demonstrated stable 30 fs pulses centered at 1068 nm from diode pumped Yb:CaYAlO4 laser by using high-quality CVD monolayer graphene as saturable absorber. These are the shortest pulses ever reported from

Broadband saturable absorption of graphene enables to achieve ultrashort pulse generation also in the mid infrared region. For example, Ma et al. [77] demonstrated a SSL based on a Tmdoped calcium lithium niobium gallium garnet (Tm:CLNGG) crystal, generating 729 fs pulses

**3.5. Graphene-based ultrafast lasers – literature examples**

136 Two-dimensional Materials - Synthesis, Characterization and Potential Applications

to 2500 nm has been demonstrated [75, 76].

graphene mode-locked lasers.

ated by those lasers (d, e, f).

*3.5.1. Solid-state lasers*

Mode locking of Yb-doped fiber lasers using real saturable absorbers might be challenging, mostly due to the fact that such oscillators are operated in the all-normal dispersion regime. The dispersion of standard single-mode fibers is normal for wavelengths shorter than ~1300 nm. Thus, a typical Yb-doped laser built from standard components will be characterized by a positive net group delay dispersion (GDD). This implies dissipative soliton operation of such laser. In order to generate a dissipative soliton, several conditions must be fulfilled [78]. The saturable absorber needs to have sufficient modulation depth in order to imitate and maintain the mode locking. In the case of graphene, it implies the usage of multilayer composites. Moreover, dissipative solitons are characterized by a quite large pulse energy, significantly larger than those achieved in conventional soliton lasers (e.g., Er- or Tm-doped). Optical damage of the SA might be a serious issue which precludes mode locking in Yb-doped fiber lasers.

Up to date, there were only few graphene-based, dissipative soliton Yb-doped fiber lasers reported [70, 79–82]. In all cases, the generated optical spectra were narrower than 2 nm. The broadest spectrum of 1.3 nm was achieved by Zhao et al. [79]. The obtained pulse duration was 580 ps. Such long pulse durations originates from the giant chirp, which is a consequence of the large normal dispersion of the cavity. Usually, dissipative soliton pulses from all-normal dispersion (ANDi) lasers are compressible almost to the transform-limited value [83, 84]. Nevertheless, the performance of the graphene-based YDFLs is far worse than of YDFLs utilizing other mode-locking techniques, such as NOLM/NALM, SESAM, or NPR, where broadband spectra with large pulse energies are achieved [85–87].

#### *3.5.3. Erbium-doped fiber lasers*

Erbium-doped fiber lasers are obviously the most popular constructions among all fiber lasers, thanks to the wide availability of cost-effective components for the telecom industry (couplers, isolators, multiplexers, photodiodes, etc.). The dispersion of standard optical fibers is anom‐ alous at 1.55 μm, which implies soliton-type operation of a typical laser (without any disper‐ sion compensation). Such lasers are quite easy to build in all-fiber configuration, without the necessity of using any free-space bulk components.

The first reported graphene-based fiber lasers back in 2009 were Er-doped fiber lasers [16, 17]. Shortly after those reports, a number of papers appeared, demonstrating novel concepts and ultrafast laser setups utilizing various forms of graphene. The shortest ever reported pulse generated from an Er-doped graphene-based fiber laser was 88 fs reported by Sotor et al. [88] in 2015. For a quite long time (over 4 years) the "world record" was held by Popa et al. (174 fs) [63]. In 2014, Tarka et al. [89] reported generation of 168 fs pulses. Both lasers (from [89] and [63]) were characterized by similar cavity design and similar saturable absorber (graphene obtained via LPE), with quite low modulation depth (2.6 and 2.0%). In both cases the pulse duration was also comparable (168 and 174 fs). The significant improvement in terms of pulse duration was possible not only by proper dispersion management, but mainly by increasing of the SA modulation depth to 11% [88]. The parameters of the three lasers with shortest reported pulses are summarized in **Table 2**.


**Table 2.** Summarized parameters of three graphene-based lasers emitting the shortest pulses.

#### *3.5.4. Thulium-doped fiber lasers*

Thulium-doped fiber lasers operating in the 1.9–2.0 μm are currently considered as one of the most important branches of laser technology [90, 91]. The number of applications of such lasers rapidly grows.

Pulsed Tm-doped fiber lasers are suitable for use in many surgical and dermatological procedures. Due to strong absorption of the 1.9–2.0 μm radiation in water, heating of only small areas of human tissues is achieved. The light penetration into the tissue is at the level of microns, which allows precise cutting. In addition, bleeding is suppressed by coagulation [92].

The second application, where Tm-doped fiber lasers might be used is laser spectroscopy, e.g., remote detection of air pollutants. The 1.9–2.0 μm spectral region contains multiple absorption lines of several molecules, especially two harmful greenhouse gases: carbon dioxide (CO2) and nitrous oxide (N2O). Carbon dioxide is the primary greenhouse gas that is contributing to recent climate change. It is absorbed and emitted naturally as part of the carbon cycle (e.g., animal and plant respiration, volcanic eruptions, ocean-atmosphere exchange), but also human activities strongly contribute to the global emission of CO2 (e.g., burning of fossil fuels) [93]. Nitrous oxide is also a major greenhouse gas and air pollutant. The N2O molecules stay in the atmosphere for an average of 120 years before being removed by a sink or destroyed through chemical reactions. Globally, about 40% of total N2O emissions come from human activities [94]. Nitrous oxide is emitted from agriculture (when nitrogen is added to the soil through the use of synthetic fertilizers), transportation (from burning of fuel), and industry activities (e.g., production of synthetic materials). It is predicted that N2O emissions are going to increase by 5% between 2005 and 2020, driven largely by increases in emissions from agricultural activities [93–95].

Up to date there were only few reports on graphene-based Tm-doped fiber lasers. The first mode-locked TDFL was reported by Zhang et al. [96] in 2012. The authors have achieved 3.6 ps pulses at 1.94 μm. The laser was using chemically exfoliated graphene (via LPE method) dispersed in PVA host. Later, Wang et al. [97] demonstrated a TDFL emitting 2.1 ps pulses, based also on graphene exfoliated by ultrasonic method (dispersed in dimethyloformamide). In 2013, Sobon et al. [98] reported an all-fiber Tm-doped laser which generated 1.2 ps pulses at 1884 nm, using a CVD-graphene/PMMA composite. Improvements in the graphene technology and careful cavity optimization allowed the same authors to further shorten the pulse almost twice (654 fs at 1940 nm [73]). An interesting concept of a Tm/Ho-doped fiber laser was presented by Jung et al. [49]. The oscillator was mode-locked by a side-polished (Dshaped) fiber with deposited graphene oxide. Unfortunately, the pulse duration directly from the oscillator was unknown, due to insufficient output power to perform an autocorrelation measurement [49]. Later, the first polarization maintaining laser was demonstrated. The oscillator was capable of generating 603 fs pulses at 1876 nm [99]. The same group also reported chirped pulse amplification (CPA) of a Tm-doped oscillator in a fully fiberized design, achieving 260 fs pulses with more than 1 nJ energy at 1970 nm [100].
