Switchable thulium-doped fiber laser from polarization rotation vector to scalar soliton

Spectrally ultra-narrow soliton bunches based on a figure-9 mode-locked thulium-doped fiber laser

We experimentally demonstrate the formation of soliton bunches in a mode-locked thulium-doped fiber laser with an ultra-narrow spectral output. By adjusting the pump power from 2.4 to 6 W, soliton bunches are maintained at a central wavelength of 1951.2 nm, with a 3-dB spectral bandwidth of ∼50 pm and a pulse repetition rate of 1.64 MHz. Within this pump power range, the number of soliton pulses within the bunch increases from 4 to 19 while maintaining consistent intensity profiles. To the best of the authors' knowledge, this study presents the first experimental demonstration of long-picosecond soliton bunches with stable ultra-narrow spectral linewidths at a wavelength near 2 µm in a figure-9 all-fiber laser. These findings highlight the potential of the proposed laser source for applications in mid-infrared spectroscopy, nonlinear microspectroscopic imaging, and polymer processing.

Anomalous soliton trapping in net-normal dispersion lasers

In a nonlinear optical system with birefringence such as fiber lasers, soliton trapping can be achieved when the fast (slow) component experiences blueshift (redshift) at normal dispersion to compensate for polarization-mode dispersion (PMD). In this Letter, we demonstrate an anomalous vector soliton (VS) whose fast (slow) component shifts to the red (blue) sides, which is opposite to traditional soliton trapping. It is found that the repulsion between the two components is induced by net-normal dispersion and PMD, while the attraction is ascribed to linear mode coupling and saturable absorption. The equilibrium of attraction and repulsion permits the self-consistent evolution of VSs circulating in the cavity. Our results indicate that the stability and dynamics of VSs are worth revisiting and studying in-depth, especially in lasers with complex configurations, despite it being a well-known object in nonlinear optics.

All-fiber ultrafast amplifier at 1.9 μm based on thulium-doped normal dispersion fiber and LMA fiber compressor

The duration reduction and the peak power increase of ultrashort pulses generated by all-fiber sources at a wavelength of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$1.9\,\upmu \hbox {m}$$\end{document}1.9μm are urgent tasks. Finding an effective and easy way to improve these characteristics of ultrafast lasers can allow a broad implementation of wideband coherent supercontinuum sources in the mid-IR range required for various applications. As an alternative approach to sub-100 fs pulse generation, we present an ultrafast all-fiber amplifier based on a normal-dispersion germanosilicate thulium-doped active fiber and a large-mode-area silica-fiber compressor. The output pulses have the following characteristics: the central wavelength of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$1.9\,\upmu \hbox {m}$$\end{document}1.9μm, the repetition rate of 23.8 MHz, the energy per pulse period of 25 nJ, the average power of 600 mW, and a random output polarization. The pulse intensity and phase profiles were measured via the second-harmonic-generation frequency-resolved optical gating technique for a linearly polarized pulse. The linearly polarized pulse has a duration of 71 fs and a peak power of 128.7 kW. The maximum estimated peak power for all polarizations is 220 kW. The dynamics of ultrashort-pulse propagation in the amplifier were analyzed using numerical simulations.

Period multiplication in mode-locked figure-of-9 fiber lasers

Period multiplication is a bifurcation phenomenon, which depicts a typical roadmap from periodic steady-state to chaos. There has been particular interest in studying period multiplication properties in mode-locked lasers, but the mechanism behind is still not well-studied. Here, we experimentally demonstrate a figure-of-9 fiber laser which can generate period-multiplied pulses with long-term stability. Further theoretical insight is obtained by numerical simulations, which clearly shows that the ratio between the distances of each bifurcation point is approaching the Feigenbaum constant. We believe that the laser design presented here could be a stable and reliable ultrafast optical platform to study bifurcation and chaos, which also has potential applications in optical frequency comb and all-optical modulation.

Promotion of pulse peak power by halving the repetition rate based on a vector soliton

We report on an all-fiber mode-locked repetition-rate-switch pulse operation in a Yb-doped fiber laser based on a polarization rotation vector soliton. The polarization controller (PC) in a fiber loop and a polarization-dependent isolator at the output port are incorporated into the laser resonator at the switch of the repetition rate. By adjusting the PC in the cavity, the mode locking can be switched between the fundamental repetition rate and half of it with a tiny pulse width change. Also, the halved pulse exhibits unique properties: a huge promotion in energy and peak power. To the best of our knowledge, this is the first all-fiber seed source with a passive switch of the repetition rate based on a vector soliton.

Single-axis soliton molecule and multiple solitons generation from a vector fiber laser

We investigate various patterns of vector solitons arising in a passively mode-locked fiber laser based on semiconductor saturable absorber mirror (SESAM). By properly adjusting the cavity parameters including the pump power and intra-cavity birefringence, the fundamental vector solitons, vector soliton molecules, and macroscopic vector solitons can be separately observed. In particular, both vector soliton molecule and macroscopic vector solitons exhibit multi-pulse structure along one polarization axis while there occurs single pulse profile at its orthogonal polarization component. Thus, they can be treated as "1 + 2" and "1+n" vector solitons. Moreover, the size of the macroscopic solitons can be manipulated from half of the cavity to even the whole cavity. The generation mechanisms of these vector soliton patterns are also investigated.

1700 nm and 1800 nm band tunable thulium doped mode-locked fiber lasers

This paper presents short wavelength operation of tunable thulium-doped mode-locked lasers with sweep ranges of 1702 to 1764 nm and 1788 to 1831 nm. This operation is realized by a combination of the partial amplified spontaneous emission suppression method, the bidirectional pumping mechanism and the nonlinear polarization rotation (NPR) technique. Lasing at emission bands lower than the 1800 nm wavelength in thulium-doped fiber lasers is achieved using mode confinement loss in a specially designed photonic crystal fiber (PCF). The enlargement of the first outer ring air holes around the core region of the PCF attenuates emissions above the cut-off wavelength and dominates the active region. This amplified spontaneous emission (ASE) suppression using our presented PCF is applied to a mode-locked laser cavity and is demonstrated to be a simple and compact solution to widely tunable all-fiber lasers.

Dual comb generation from a mode-locked fiber laser with orthogonally polarized interlaced pulses

Ultra-high precision dual-comb spectroscopy traditionally requires two mode-locked, fully stabilized lasers with complex feedback electronics. We present a novel mode-locked operation regime in a thulium-holmium co-doped fiber laser, a frequency-halved state with orthogonally polarized interlaced pulses, for dual comb generation from a single source. In a linear fiber laser cavity, an ultrafast pulse train composed of co-generated, equal intensity and orthogonally polarized consecutive pulses at half of the fundamental repetition rate is demonstrated based on vector solitons. Upon optical interference of the orthogonally polarized pulse trains, two stable microwave RF beat combs are formed, effectively down-converting the optical properties into the microwave regime. These co-generated, dual polarization interlaced pulse trains, from one all-fiber laser configuration with common mode suppression, thus provide an attractive compact source for dual-comb spectroscopy, optical metrology and polarization entanglement measurements.