Novel Mode-Locked Fiber Lasers with Broadband Saturable Absorbers

Abstract

At present, there are two main techniques to realize the mode-locking operation in a fiber laser system, which are active and passively mode-locking technologies. The passive mode-locking method with saturable absorber (SA) is the most widely used technology. To realize passive mode-locking, the key research field is to explore high-performance broadband saturable absorber materials with long term stability, easy fabrication, low cost, and high damage threshold in passive mode-locked fiber lasers.This chapter introduces the optical properties of these new type broadband saturable absorbers materials, such as carbon nanotubes (CNTs), graphene, topological insulators, two-dimensional transition metal dichalcogenides (TMDs) and black phosphorus (BP). The main topics include increasing the damage threshold, improving the fabrication technology, and reducing the nonsaturable losses, as well as fabricating different types of new saturable absorbers devices and the ones used for mode-locked fiber lasers, that is:Constructing a fiber laser with single-walled carbon nanotube (SWCNT) heavy water solution as saturable absorber. The saturable intensity and the modulation depth of the proposed saturable absorber device are evaluated to be 20 MW/cm2 and 5.7%, respectively. The nonsaturable loss is evaluated to be 3%. By incorporating the saturable absorber into Yb-doped fiber laser cavities with 2 nm and 8 nm bandpass filters, respectively, two mode-locked fiber lasers are achieved with central wavelengths of 1053.45 nm and 1054.16 nm. The pulse duration is measured to be 512 ps and 194 ps, respectively. The mode-locked fiber laser with 2 nm bandpass filter generates the pulse with signal-to-noise ratio (SNR) of 50 dB and the mode-locked fiber laser with 8 nm bandpass filter generates pulse with SNR of 60 dB.Fabricating the Bi2Te3/polyvinylalcohol (PVA) and tapered fiber Bi2Te3 saturable absorbers, respectively. The Bi2Te3/PVA saturable absorber is fabricated by liquid phase exfoliation method and the tapered fiber Bi2Te3 saturable absorber is fabricated by pulsed laser deposition technology. The modulation depth and tapered fiber Bi2Te3 saturable absorbers are measured to be 8.4% and 10%, respectively. By incorporating the two kinds of saturable absorbers into Yb-doped fiber laser cavities, two mode-locked fiber lasers with pulse durations of 417 ps and 317 psare achieved.Fabricating the WS2/fluorine mica (FM) saturable absorber and DF-WS2 solution saturable absorber (SSA). The WS2/FM SA is fabricated with thermal decomposition method by depositing WS2 layers onto 20 μm thickness of one-layer FM. The WS2/FM SA has a high damage threshold of 406 MW/cm2, two times higher than that of WS2/PVA. The modulation depth and nonsaturable losses of WS2/FM are measured to be 5.8% and 14.8%, respectively, at 1 μm region. By incorporating the WS2/FM into Yb-doped fiber laser cavity, a mode-locked fiber laser is achieved with central wavelength of 1052.45 nm. The maximum average output power is 30 mW. The modulation depth and nonsaturable loss of WS2/FM SA are measured to be 3.1% and 15%, respectively, at 1.55 μm region. By employing the WS2/FM SA, a stable conventional soliton Er-doped mode-locked fiber laser is achieved. The repetition rate is 8.2 MHz and the pulse duration is 830 fs. The DF-WS2 SSA is fabricated based on a D-shaped fiber embedded in WS2nanosheet solution. This kind of SA shows a modulation depth of 11%, a saturable intensity of 5 MW/cm2, and a nonsaturable loss of 18% at 1.55 μm region. By incorporating this kind of saturable absorber into EDF laser cavity, stable fundamental mode-locking operation is achieved with pulse duration of 670 fs at low pump power. 460.7 MHz harmonic mode-locking operation is also achieved at high pump power, and the pulse duration and SNR are measured to be 710 fs and 66 dB, respectively.Fabricating the black phosphorus solution saturable absorber with D-shaped fiber (DF-BP SSA). The saturable absorber is fabricated based on a DF embedded in BP nanosheets N-Methylpyrrolidone (NMP) solution. Such a BP solution method has virtues of good anti-oxidant capacity, high heat dissipation, and high damage threshold. This kind of saturable absorber shows a modulation depth of 7.75% and nonsaturable loss of 10% at 1.55 μm region. By employing DF-BP SSA, a repetition rate of 15.2 MHz mode-locked fiber laser is achieved. The pulse duration is 580 fs and the SNR is 65 dB.