Compact noise-like pulse fiber laser and its application for supercontinuum generation in highly nonlinear fiber

High energy and low noise soliton fiber laser comb based on nonlinear merging of Kelly sidebands

Optical solitons in mode-locked laser cavities with dispersion-nonlinearity interaction, delivers pulses of light that retain their shape. Due to the nature of discretely distributed dispersion and nonlinearity, optical solitons can emit Kelly-sidebands via the frequency coupling of soliton and dispersive waves. In this paper, we generate a high-energy femtosecond laser comb, by using the intracavity Kelly radiations and 3^rd order nonlinearities. By increasing the intracavity power, the soliton envelop and the Kelly-sidebands merge together via four-wave-mixing, forming a super-continuum spectrum, obtaining 3.18 nJ pulse energy. A supercontinuum span covering from 1100 nm to 2300 nm for further self-referenced f-2f stabilization can be directly achieved by using an amplification-free external supercontinuum technique. Our finding not only demonstrates a non-trivial frequency-time evolution based on 'erbium + χ⁽³⁾' nonlinear gains, but also offers a new opportunity to develop practically compact fiber frequency combs for frequency metrology or spectroscopy.

Generation of cylindrical vector beams in a linear cavity mode-locked fiber laser based on nonlinear multimode interference

In this paper, a linear cavity mode-locked pulsed fiber laser generating cylindrical vector beams (CVBs) is proposed and demonstrated based on a nonlinear multimode interference. A homemade long-period fiber grating with a broad bandwidth of 121 nm is used as a mode converter inside the cavity. The saturable absorber was formed by single-mode fiber-graded index multimode fiber-single mode fiber (SMF-GIMF-SMF) structure. By controlling the pump power, the operation states are switchable among continuous-wave, Q-switched mode-locked (QML), and mode-locked regimes. The repetition rate of the QML CVB pulse envelope varies from 57.4 kHz to 102.7 kHz at the pump range of 118 to 285 mW. When increasing pump power to 380 mW, mode-locked CVB pulse repetition rate of 3.592 MHz, and pulse duration of 4.62 ns are achieved. In addition, the maximum single-pulse envelope energy can reach 510 nJ, and 142 mW average-power CVBs with a slope efficiency of as high as 20.2% can be obtained. Moreover, azimuthally and radially polarized beams can be obtained with mode purity over 95% in different operating regimes. The proposed fiber laser has a simple structure, and the operation is controllable in both temporal and spatial domains, which presents a flexible pulsed CVB source for application of laser processing, time or mode division multiplexing system, and spatiotemporal nonlinear optics.

Symbiotic coexistence of noise-like pulses

Noise-like pulse (NLP) can split and then self-assemble into dynamic bound states, named NLP polymer. Here, we reported the first observation, to the best of our knowledge, of the buildup process of bound NLPs in all-normal-dispersion Yb-doped fiber lasers. By designing two NLP fiber lasers, the distinct autocorrelation trace property for the bound NLPs with a short time interval (around 30 ps), and the high-speed oscilloscope trace characterization for the bound NLPs with a relatively broad time interval (∼500 ps) have all been exhibited. Also, we have demonstrated that it was the Raman effect that mediated the NLP bound states. The experiment results showed that though the inter-interval between the NLPs and the NLP width in the bound states are constantly changing, the envelope of each NLP remained localized and the bound NLPs could maintain within a wide pump range. The dynamics of the experimentally observed bound NLPs have also been discussed with fitting models and numerical simulations. In addition, the experimental test results for the coherence of the NLPs and their bound states further indicated that the NLPs had low temporal coherence characteristics.

Ultra-broadband supercontinuum covering a spectrum from visible to mid-infrared generated by high-power and ultrashort noise-like pulses

To facilitate a fiber-based supercontinuum generation system, single-mode fibers with different cutoff wavelengths are introduced to serve as shortpass filters to replace conventional reflective or transmissive filters. Meanwhile, an ytterbium-doped fiber amplifier is adopted to amplify the filtrated pulses, scaling their average power to the watt level up to 4.33 W. Through this approach, ultrashort high-power laser pulses of 1.56 µm and 1.06 µm wavelengths, which are commonly used in optical communications and industrial applications, can be generated by this single system. Furthermore, it is found that the noise-like pulses still maintain their temporal features, even after they undergo multiple optical processes including amplification, supercontinuum generation, and filtration. After that, the generated pulses at 1.06 µm were launched into a photonic crystal fiber to generate a supercontinuum of 1.85 W covering a spectral range from 560 nm in the visible region to 3.5 µm in the mid-infrared region. This is one of the widest records of spectrum in broadband supercontinuum generation.

Conventional Soliton and Noise-Like Pulse Generated in an Er-Doped Fiber Laser with Carbon Nanotube Saturable Absorbers

Conventional soliton (CS) and noise-like pulse (NLP) are two different kinds of pulse regimes in ultrafast fiber lasers, which have many intense applications. In this article, we experimentally demonstrate that the pulse regime of an Er-doped fiber laser could be converted between conventional soliton and noise-like pulse by using fast response saturable absorbers (SA) made from different layers of single-wall carbon nanotubes (CNT). For the monolayer (ML) single-wall CNT-SA, CS with pulse duration of 439 fs at 1560 nm is achieved while for the bilayer (BL) single-wall CNT, NLP at 1560 nm with a 1.75 ps spike and a 98 ps pedestal is obtained. The transition mechanism from CS to NLP is investigated by analyzing the optical characteristics of ML and BL single-wall CNT. The further theoretical simulation illustrates that CNT-SA enables the switching between CS and NLP in anomalous dispersion regime in Er-doped fiber lasers.

High-power supercontinuum generation by noise-like pulse amplification in Yb-doped fiber amplifier operating in a nonlinear regime

In this paper, we report on a supercontinuum generation by amplifying noise-like pulses (NLPs) in a nonlinear Yb-doped fiber amplifier. The NLP source is a homemade Yb-doped all-fiber ring oscillator based on nonlinear polarization rotation mode-locking method. NLPs possess a repetition rate of 11.57 MHz, energy of 16.5 nJ, and 3 dB spectral bandwidth of 42 nm. The intensity autocorrelation function of NLPs has a broad pedestal and a narrow central spike. The pedestal and the spike have temporal widths of 63 ps and 92 fs, respectively. The NLPs are amplified by a Yb-doped fiber nonlinear power amplifier to achieve output power of 7.5 W. Nonlinear effects in the amplifier drastically broaden the spectrum of NLPs and generate high-power supercontinuum light with 10 dB spectral bandwidth of 1130 nm (from 1037 to 2167 nm). The presented supercontinuum generation system is an all-fiber compact structure and has the advantage of not requiring long single-mode, nonlinear, or photonic crystal fibers.

Pump-controlled flexible generation between dissipative soliton and noise-like pulses from a mode-locked Er-doped fiber laser

We experimentally investigate the alternative generation of dissipative solitons (DSs) or noise-like pulses (NLPs) from a passively mode-locked Er-doped all-fiber laser with a 45° tilted fiber grating (45° TFG). The 45° TFG serves as an in-fiber polarizer for mode locking in the all-fiber laser cavity. Under the fixed orientations of the polarization controllers, flexible generation between DSs and NLPs can be precisely controlled by adjusting the pump power only. To the best of our knowledge, the total cavity length 4.92 m of our fiber laser is the shortest one among all NLP fiber lasers. We obtain the DS with 3 dB bandwidth of 20.4 nm centered at 1577 nm and the NLP with 3 dB bandwidth of 25.2 nm centered at 1574 nm. The fundamental repetition rate of the mode-locked pulses is 42.3 MHz. This DS-NLP switchable Er-doped fiber laser can be regarded as a multifunctional optical source for diverse practical and potential applications.

Raman-converted high-energy double-scale pulses at 1270 nm in P2O5-doped silica fiber

This work presents implementation of a new approach to single-cascade Raman conversion of laser pulses from the spectral range around 1.1 µm into the 1.3-µm wavelength region. The proposed conversion technique relies on double-scale pico-femtosecond pulses for synchronous pumping of an external cavity made of phosphosilicate fiber with high-precision adjustment of pulse repetition rate to the inter-mode frequency of the external cavity. This enabled generation of double-scale pulses centered at 1270 nm featuring a record energy of 63 nJ and the pulse envelope duration of 88-180 ps with the sub-pulse duration of 200 fs. The fraction of the radiation that was converted into the 1270 nm range amounted to 47 percent of the total Raman-converted radiation power. The generated results offer promising possibilities for new spectral ranges to be developed in the field of high-energy pulsed sources with unique double-scale temporal structure.

Mechanism of dissipative-soliton-resonance generation in fiber laser mode-locked by real saturable absorber

Generation of dissipative soliton resonance (DSR) is numerically investigated in an all-normal-dispersion Yb-doped fiber laser mode-locked by a real saturable absorber (SA). In the simulation model, the SA includes both the saturable absorption and reverse saturable absorption (RSA) effects. It is found that the RSA effect induced by the SA material itself plays a dominant role in generating the DSR pulses. We also systematically analyze the influence of key SA parameters on the evolution of DSR pulses in the cavity. Our simulation results not only offer insight into the underlying mechanism of DSR generation in mode-locked fiber lasers by means of real SAs, but also provide a guideline for engineering SA parameters to generate optical pulses with the highest possible energy.

Supercontinuum generation in an Er-doped figure-eight passively mode-locked fiber laser

A supercontinuum source based on a figure-eight Er-doped fiber ring cavity has been experimentally demonstrated with low repetition rate. The proposed configuration of the experiment is a figure-eight fiber laser grounded in Nonlinear Optical Loop Mirror (NOLM) technique. A broad spectrum of approximately 410 nm spanning the range 1315-1725 nm at the level of 30 dB can be obtained at a given average power of 2.6 mW and without any amplifier. Such wide spectrum can be directly achieved in the fiber resonator, which makes the structure compact and robust. By changing the pump power, the temporal pulse width can be adjusted accordingly. The pulse width is about 4 ns at the pump power of around 240 mW. The broadband spectrum was generated directly from the fiber resonator, which can be easily applied in numerous areas, such as the optical frequency metrology, optical spectroscopy, optical coherence tomography, optical communications, and medical sciences.

Generation of spatio-temporal extreme events in noise-like pulses NPE mode-locked fibre laser

We experimentally study spatio-temporal generation of extreme events in the radiation of NPE mode-locked fibre laser generating noise-like pulses. We show that new pulses starts from high-intensity spatio-temporal structure which consist of mainly 3 subsequent pulses which are both separated over fast and slow evolution time. Statistical analysis of the noise-like pulse evolution over round-trips shows that the pulse width and intensity varies with a period of around 85 round-trips which does not change from pulse to pulse. The intensity probability density function has a heavy tail originated only from events of pulse formation.