Ultrafast Fiber Lasers with Low-Dimensional Saturable Absorbers: Status and Prospects

Abstract

Wide-spectral saturable absorption (SA) in low-dimensional (LD) nanomaterials such as zero-, one-, and two-dimensional materials has been proven experimentally with outstanding results, including low saturation intensity, deep modulation depth, and fast carrier recovery time. LD nanomaterials can therefore be used as SAs for mode-locking or Q-switching to generate ultrafast fiber laser pulses with a high repetition rate and short duration in the visible, near-infrared, and mid-infrared wavelength regions. Here, we review the recent development of emerging LD nanomaterials as SAs for ultrafast mode-locked fiber laser applications in different dispersion regimes such as anomalous and normal dispersion regimes of the laser cavity operating in the near-infrared region, especially at ~1550 nm. The preparation methods, nonlinear optical properties of LD SAs, and various integration schemes for incorporating LD SAs into fiber laser systems are introduced. In addition to these, externally (electrically or optically) controlled pulsed fiber laser behavior and other characteristics of various LD SAs are summarized. Finally, the perspectives and challenges facing LD SA-based mode-locked ultrafast fiber lasers are highlighted.

Keywords: low-dimensional materials; optically/electrically controlled fiber lasers; saturable absorber; ultrafast fiber laser.

Figure 2

The atomic structure and spectral region of 2D materials. The atomic structure and spectral region of 2D materials such as graphene (a), TMDs (b), phosphorene (c), arsenene (d), antimonene (e), bismuthine (f), MXene (g), and BN (h). Reproduced with permission [82]. Copyright 2019, Wiley-VCH.

Figure 4

Various schemes for incorporating LD SAs in optical fiber. (a) SA sandwiched between two fiber connectors. Reproduced with permission [126]. Copyright 2015, The Optical Society of America. (b) LD SA injected inside a hollow photonic crystal fiber (PCFs). Reproduced with permission [125]. Copyright 2013, The Optical Society of America. (c) LD SAs transferred onto SPF surface. (d) depositing SAs around tapered micro-fiber. Reproduced with permission [127]. Copyright 2016, Springer Nature.

Figure 1

Low-dimensional SAs classification featured as 0D, 1D and 2D structure varieties. [Images are publicly available online].

Figure 3

Graphic illustration of the preparation of the WSe₂-based SA by the CVD method. (a) The transfer scheme of WSe₂ films. (b) optical image of WSe₂ films transferred onto the fiber ferrule end facet. (c) schematic presentation of the few-layer WSe₂ and light interaction. Reproduced with permission [116]. Copyright 2018, RSC.

Figure 5

Schematic illustration Er-doped fiber laser ring-cavity comprising a single layer graphene SA on the SPF. LD: laser diode; EDF: erbium-doped fiber; PC: polarization controller; hybrid component: an integrated wavelength-division multiplexer and isolator. Reproduced with permission [128]. Copyright 2015, The Optical Society of America.

Figure 6

Mode-locked ultrafast fiber laser based on graphene SA (a) soliton pulse spectrum and (b) autocorrelation trace of the mode-locked pulse. (c) RF spectrum of the laser output pulse train (inset: RF spectrum viewed over a wide frequency range) and (d) 3-dB bandwidth and pulse width of the implemented laser as functions of the over-cladding index of the SA. Reproduced with permission [128]. Copyright 2015, The Optical Society of America.

Figure 7

1D SWCNT SA-based mode-locked fiber laser: (a–c) Conventional soliton: (a) Schematic of the fs fiber laser using the SWCNT-filled HOF. The inset figure shows the spliced image between the normal SMF and HOF, where adiabatic mode transition occurs. (b) Measured optical spectrum and pulse duration (inset) of the mode-locked fiber laser. (a–c) Reproduced with permission [188]. Copyright 2009, The Optical Society of America. (c) The output pulse train of the laser shows a repetition rate of 18.5 MHz. (d–f) Dissipative soliton: (d) Configuration of the fiber ring laser including the SWCNT-SA and the DCF. (e) The optical spectrum of the mode-locked laser at net cavity dispersion of 0.087 ps² and (f) Measured pulsed duration fitted with Gaussian pulse. The inset shows the pulse compressed by additional SMF at extra-cavity. (d–f) Reproduced with permission [60]. Copyright 2014, The Optical Society of America.

Figure 8

Mode-locked ultrafast fiber laser in the NIR region with various 2D SAs such as BP (a–c), Bismuthine (d–f), and Ti₃C₂T_x MXene (g–i). (a) AFM micrograph of selected exfoliated BP fakes; (b) Optical spectrum with a bandwidth of 40 nm acquired after 80 h (blue curve), 160 h (red dot line), and 240 h (green dot line), respectively; (c) Autocorrelation trace with a Gaussian fit. (a–c) Reproduced with permission. Copyright 2018, The Optical Society of America [135]. (d) AFM image of few-layer bismuthine. (e) mode-locked soliton pulse spectrum with 3 dB bandwidth of 4.64 nm, and (f) autocorrelation trace of mode-locked ultrafast laser showing the pulse width of 652 fs. (d–f) Reproduced with permission. Copyright 2018, Wiley-VCH [191]. (g) SEM image of Ti₃C₂T_x, (h) mode-locked soliton pulse spectrum with 3 dB bandwidth of 22.2 nm, and (i) autocorrelation trace of mode-locked fiber laser showing the pulse width of 159 fs. (g–i) Reproduced with permission. Copyright 2017, Wiley-VCH [180].

Figure 9

Mode-locked ultrafast fiber laser in the NIR region with various 2D SAs such as one of the TMDs named MoS₂ SA (a–f) and one of the TIs named Bi₂Te₃ (g–i). (a–c) CS with MoS₂ SA: (a) A schematic diagram of the fiber ring laser in anomalous dispersion with MoS₂ SA sandwiched in between two SMF (inset: Photograph of a fiber connector coated with multilayer MoS₂); (b) Optical spectrum of conventional soliton mode-locked pulse with a bandwidth of 2.6 nm; (c) Autocorrelation trace with a Sech² fit showing pulse duration of 1.28 ps. (a–c) Reproduced with permission. Copyright 2014, The Optical Society [200]. (d–f) DS with MoS₂ SA (d) A schematic diagram of the fiber ring laser in normal dispersion with MoS₂ SA deposited on SPF. (e) The optical spectrum of the mode-locked laser at net cavity dispersion of +0.095 ps² and (f) Measured pulsed duration fitted with Gaussian pulse. (d–f) Reproduced with permission. Copyright 2014, The Optical Society [162]. (g–i) CS with Bi₂Te₃ SA: (g) mode-locked fiber laser cavity comprising Bi₂Te₃ SA, (h) mode-locked optical spectrum with 3 dB bandwidth of 2.69 nm, and (i) autocorrelation trace of mode-locked soliton pulse with the FWHM width of 1.86 ps. (g–i) Reproduced with permission. Copyright 2012, AIP [201].

Figure 10

Electrically controlled fiber laser using an all-fiber graphene device and gate-variable properties of fiber laser operation. (a) Schematic diagram of gate-variable all-fiber graphene device. (b) Gate-controlled Optical transition properties of the device. (c) Gate-controlled Electrical transport properties of the device. (d) Fiber laser configuration, including fabricated all-fiber device with bilayer graphene. LD: laser diode; EDF: erbium-doped fiber; PC: polarization controller; hybrid component: an integrated wavelength-division multiplexer and isolator. (e–g) Characteristics of a passively mode-locked fiber laser at an applied V_G of −1.05 V; (e) Measured pulse duration of 423 fs at a repetition rate of 30.9 MHz (inset). (f) Laser output spectrum with a spectral bandwidth of 8 nm at 3 dB. (g) The measured radio frequency spectrum of the laser output (h,i) Q-switched characteristics of a fiber laser at an applied V_G of −0.18 V; (h) Measured output pulse duration of 3.5 ms at a repetition rate of 25.4 kHz (inset) and (i) its optical spectrum. Reproduced with permission. Copyright 2015, Springer Nature [25].

Figure 11

Graphene electro-optic modulator (GEOM) controlled mode-locked ultrafast fiber laser. (a) Laser cavity setup. EDF: erbium doped fiber; LD: laser diode; WDM: wavelength-division multiplexer; ISO: isolator; OC: output coupler; PC: polarization controller; CIR: circulator; L1: collimating lens; L2: focusing lens; AFG: arbitrary function generator. (b) Drive signal at the modulation frequency of 4.35 MHz and synchronized output optical pulse train for fundamental mode-locking operation at the repetition rate of 4.35 MHz. (c) Drive signal at modulation frequency of 8.7 MHz and synchronized output optical pulse train for second harmonic mode-locking operation at the repetition rate of 8.7 MHz. Reproduced with permission. Copyright 2018, Wiley-VCH [26].

Figure 12

Electrically controlled fiber laser using SWCNT SA device and gate-variable properties of fiber laser operation. Soliton ML fiber laser characteristics at V_G = 0 V: (a) optical soliton pulse spectrum shows the 3 dB bandwidth of 7.6 nm, (b) autocorrelation trace shows the pulse duration of 600 fs with oscillation trace (inset showing repetition rate of 50 MHz) of the mode-locked pulse. QS fiber laser characteristics at V_G > 0.7 V: (c) oscillation trace of QS pulse with repetition rate of 27.5 kHz, (d) variation of QS pulse frequency as a function of applied gate voltage showing that, repetition rate is controlled in the range from 23.6 kHz to 28.8 kHz with applied gate voltage V_G varying from 0.8 V to 1.9 V. Reproduced with permission. Copyright 2019, American Chemical Society [67].

Figure 13

Optically controlled in-line graphene SA-based pulsed fiber laser (a) Schematic representation of optically tunable graphene SA. (b) Nonlinear transmission test result of the CW signal beam (1550 nm) TE mode variable with CW control beam powers at 980 nm. (c) Illustration of Er-doped fiber ring laser incorporated with optically controllable in-line monolayer graphene SA device. LD: laser diode; EDF: erbium-doped fiber; PC: polarization controller; WDM: wavelength-division multiplexer and OC: optical coupler. (d) Schematic explanation of fiber laser operating regime as a function of modulation depth in graphene SA, (e) Q-switching operated pulse train with no control beam (P_c = 0 mW) applied, (f) Q-switched mode-locked operated pulse train with a control beam power of 34 mW (inset: an extended view of the pulse train in time scale) and (g) pulse train of CW mode-locked operation with a control beam power of 42 mW. Reproduced with permission. Copyright 2016, OSA [68].