Self-starting Soliton Research Articles

Self-Starting Soliton–Comb Regimes in χ(2) Microresonators

The discovery of stable and broad frequency combs in monochromatically pumped high-Q optical Kerr microresonators caused by the generation of temporal solitons can be regarded as one of the major breakthroughs in nonlinear optics during the last two decades. The transfer of the soliton–comb concept to χ(2) microresonators promises lowering of the pump power, new operation regimes, and entering of new spectral ranges; scientifically, it is a big challenge. Here we represent an overview of stable and accessible soliton–comb regimes in monochromatically pumped χ(2) microresonators discovered during the last several years. The main stress is made on lithium niobate-based resonators. This overview pretends to be rather simple, complete, and comprehensive: it incorporates the main factors affecting the soliton–comb generation, such as the choice of the pumping scheme (pumping to the first or second harmonic), the choice of the phase matching scheme (natural or artificial), the effects of the temporal walk off and dispersion coefficients, and also the influence of frequency detunings and Q-factors. Most of the discovered nonlinear regimes are self-starting—they can be accessed from noise upon a not very abrupt increase in the pump power. The soliton–comb generation scenarios are not universal—they can be realized only under proper combinations of the above-mentioned factors. We indicate what kind of restrictions on the experimental conditions have to be imposed to obtain the soliton–comb generation.

Self-starting bi-chromatic LiNbO3 soliton microcomb

For its many useful properties, including second and third-order optical nonlinearity as well as electro-optic control, lithium niobate is considered an important potential microcomb material. Here, a soliton microcomb is demonstrated in a monolithic high-Q lithium niobate resonator. Besides the demonstration of soliton mode locking, the photorefractive effect enables mode locking to self-start and soliton switching to occur bi-directionally. Second-harmonic generation of the soliton spectrum is also observed, an essential step for comb self-referencing. The Raman shock time constant of lithium niobate is also determined by measurement of soliton self-frequency-shift. Besides the considerable technical simplification provided by a self-starting soliton system, these demonstrations, together with the electro-optic and piezoelectric properties of lithium niobate, open the door to a multi-functional microcomb providing f-2f generation and fast electrical control of optical frequency and repetition rate, all of which are critical in applications including time keeping, frequency synthesis/division, spectroscopy and signal generation.

All-fiber 2 μm thulium-doped mode-locked fiber laser based on MoSe2-saturable absorber

Abstract We demonstrate an all-fiber passive mode-locking laser operating in the 2 μm region that is implemented using a MoSe2-based saturable absorber (SA). A MoSe2-polyvinyl alcohol (PVA) SA is prepared with a modulation depth of 4.98%, a saturation intensity of 168.6 MW/cm2 and a nonsaturable loss of 29.6%. When the MoSe2-PVA SA was inserted into the fiber laser cavity, highly stable self-starting soliton pulse trains were generated with a center wavelength of 1943.35 nm and a 3 dB bandwidth of 4.38 nm. The pulse waveform has a sech2 profile with pulse duration of 980 fs. The fundamental repetition rate is 23.53 MHz and the signal-to-background ratio of the fundamental frequency ranges up to 65 dB. The production of stable and robust pulses using this laser indicates that the MoSe2-PVA SA is suitable for generation of 2 μm femtosecond ultrashort pulses.

Ultrafast erbium-doped fiber laser mode-locked with a black phosphorus saturable absorber

We experimentally demonstrate a passive mode-locked erbium-doped fiber laser (EDFL) using a multi-layer black phosphorus saturable absorber (BPSA). The BPSA is fabricated by mechanically exfoliating a BP crystal and sticking the acquired BP flakes onto scotch tape. A small piece of the tape is then placed between two ferrules and integrated into an EDFL cavity to achieve a self-started soliton mode-locked pulse operation at 1560.7 nm wavelength. The 3 dB bandwidth, pulse width, and repetition rate of the laser are 6.4 nm, 570 fs, and 6.88 MHz, respectively. The average output power is 5.1 mW at pump power of 140 mW and thus, the pulse energy and peak power are estimated at 0.74 nJ and 1.22 kW, respectively. The BPSA was constructed in a simple fabrication process and has a modulation depth of 7% to successfully produce the stable mode-locked fiber laser.

Mode-Locked All-Fiber Lasers at Both Anomalous and Normal Dispersion Regimes Based on Spin-Coated <named-content content-type="math" xlink:type="simple"> <inline-formula> <tex-math notation="TeX">$\hbox{MoS}_{2}$</tex-math></inline-formula></named-content> Nano-Sheets on a Side-Polished Fiber

We demonstrate an all-fiberized mode-locked laser by employing Molybdenum Disulfide (MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) spin-coated onto a side-polished fiber (SPF) as an in-line saturable absorber. Single crystal MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> has been exfoliated via the Li intercalation method and then dispersed with large population of few-layer MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nano-sheets in ethanol. Subsequently, a saturable absorber has been prepared using a relatively simple method: spin-coating the uniform MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> solution on the fabricated SPF without using any polymer or toxic procedure. Power-dependent transmission property of the prepared MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nano-sheets on the SPF was experimentally analyzed, providing the feasibility to apply it as an efficient in-line saturable absorption behavior. By deploying the MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> SPF into a fiber laser cavity with finely tuned laser pump power and cavity dispersion, mode-locked pulses were obtained at anomalous and normal dispersion of the laser cavity. The self-started soliton pulses at the anomalous dispersion regime were generated with a spectral bandwidth of 9.96 nm at 1584 nm and a pulse duration of 521 fs. Moreover, by managing the dispersion to the normal regime, stable pulses at net-normal intra-cavity dispersion with 11.9 ps pulse duration were obtained with a spectral bandwidth of 18.2 nm at 1570 nm. The results prove the effectiveness of developing MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> saturable absorbers and applications for pulsed laser operation.

Self-mode-locking action in a dual-core ring fiber laser

With the help of numerical experiments we demonstrate the self-starting soliton generation in a dual-core ring fiber laser without any external mode-locking mechanism. This result makes evident that the nonlinear energy switching between the active and the passive cores acts as a self-mode-locking mechanism.

Self-starting sliding-frequency fibre soliton laser

The authors demonstrate self-starting soliton generation by inserting a travelling wave acousto-optic modulator and a bandpass filter in a diode-pumped erbium doped fibre ring laser. The continuous frequency sliding of the circulating light inhibits CW oscillation, and favours the emission of solitons that are stably trapped by the filter.