High-energy mode-locked holmium-doped fiber laser operating in noise-like pulse regime

Dumbbell-Shaped Ho-Doped Fiber Laser Mode-Locked by Polymer-Free Single-Walled Carbon Nanotubes Saturable Absorber

We propose a simple dumbbell-shaped scheme of a Holmium-doped fiber laser incorporating a minimum number of optical elements. Mode-locking regimes were realized with the help of polymer-free single-walled carbon nanotubes (SWCNTs) synthesized using an aerosol (floating catalyst) CVD method. We show that such a laser scheme is structurally simple and more efficient than a conventional one using a ring cavity and a similar set of optical elements. In addition, we investigated the effect of SWCNT film transmittance, defined by the number of 40 nm SWCNT layers on the laser's performance: operating regimes, stability, and self-starting. We found that three SWCNT layers with an initial transmittance of about 40% allow stable self-starting soliton mode-locking at a wavelength of 2076 nm with a single pulse energy of 0.6 nJ and a signal-to-noise ratio of more than 60 dB to be achieved.

Frequency-domain-resolved investigation of unitary h-shaped pulses

We experimentally investigate the generation of h-shaped pulse in an all-polarization-maintaining (PM) and all-normal-dispersion (ANDi) mode-locked fiber laser. The generated pulse is demonstrated to be a unitary pulse, instead of a noise-like pulse (NLP). Furthermore, by employing an external filtering system, the obtained h-shaped pulse can be resolved into rectangular-shaped pulses, chair-like pulses, and Gaussian pulses. The authentic AC traces with a double-scale structure of unitary h-shaped pulses and chair-like pulses are observed on the autocorrelator. The chirp of h-shaped pulses is also proved similar to that of DSR pulses. To the best of our knowledge, this is the first time that the existence of unitary h-shaped pulse generation has been confirmed. Moreover, our experimental results reveal the close relationship of formation mechanisms of dissipative soliton resonance (DSR) pulses, h-shaped pulses, and chair-like pulses, which helps to unify the essences of such "DSR-like" pulses.

Spectrally flat mid-infrared supercontinuum pumped by a high power 2 µm noise-like pulse

We demonstrate spectrally flat high-power mid-infrared supercontinuum (MIR SC) generation with record-breaking power of 33.1 W and power conversion efficiency of 75.06%. It is pumped by a 2 µm master oscillator power amplifier system consisting of a figure-8 mode-locked noise-like pulse seed laser and dual-stage Tm-doped fiber amplifiers with repetition rate of 4.08 MHz. Through cascading a piece of ZBLAN fiber with 13.5 µm large core diameter by direct-low-loss fusion splicing, SCs with spectral ranges of 1.9-3.68 µm, 1.9-3.84 µm, 1.9-4.02 µm and average powers of 33.1 W, 29.8 W, 25.9 W are generated. To the best of our knowledge, all of them have achieved the highest output power under the same condition of MIR spectrum range. This high-power all-fiber MIR SC laser system has relatively simple architecture, high efficiency and flat spectrum, demonstrating the advantages of 2 µm noise-like pulse pump in high-power MIR SC generation.

Tunable mode-locked Tm-doped fiber laser based upon cross-phase modulation

We demonstrate the generation of soliton and dissipative soliton in an ultrafast thulium (Tm) doped fiber laser based upon cross-phase modulation (XPM) induced mode-locking. The mode-locking is realized by periodically modulating the 2-µm signal through XPM that is activated by an injected 1.5-µm pulsed laser. Such a mechanism enables the laser to be mode-locked in various operation regimes without any real or artificial saturable absorbers. Thanks to the XPM pulling effect, the wavelength of the Tm-doped fiber laser can be tuned by adjusting the repetition frequency of the 1.5-µm pulsed laser. The maximum tuning ranges achieved in this work for the soliton and dissipative soliton regimes are respectively 11 nm and 15 nm. The outcomes of this work not only provide a continuously and controllably wavelength-tunable ultrafast laser but also offer a passively synchronized dual-color fiber laser system, which is promised for many important applications such as Raman spectroscopy, nonlinear frequency conversion systems, and multi-color pump-probe systems.

Long-term stable, synchronizable, low-noise picosecond Ho:fiber NALM oscillator for Ho:YLF amplifier seeding

We demonstrate a 41.6 MHz, 1.3 ps, 140 pJ Ho:fiber oscillator using a nonlinear amplifying loop mirror (NALM) as saturable absorber. The oscillator is constructed entirely with polarization-maintaining (PM) fibers, is tunable with a center wavelength between 2035 nm and 2075 nm, and can be synchronized to an external RF reference. For our application of Ho:YLF amplifier seeding for dielectric electron acceleration, the laser is tuned to 2050 nm and synchronized to a stable RF reference with 45 fs rms timing jitter in the integration interval [10 Hz, 1 MHz]. We show long term synchronized operation and characterize the relative intensity noise (RIN) and timing jitter of the oscillator for two different Tm-fiber pump lasers.

2.1 μm, 1.52 μJ square-wave noise-like pulse from an all-fiber figure-of-9 Ho-doped oscillator and single-stage amplifier

A 2.1 μm, high energy square-wave noise-like pulse (NLP) in an all-fiber Ho-doped fiber laser is proposed, which consists of an oscillator and a single-stage amplifier. In the figure-of-9 oscillator, mode-locking is achieved based on the nonlinear amplifying loop mirror, employing a long gain fiber to provide sufficient gain in 2.1 μm band and optimizing the cavity length to obtain maximum pulse energy output. With appropriate pump power and polarization state, the oscillator emits a 175.1 nJ square-wave NLP with center wavelength of 2102.2 nm and spike width of 540 fs. The 3-dB spectral width and pulse envelope width are 11.2 nm and 6.95 ns, respectively. The single-stage amplifier employs a bi-directional pump scheme. After amplification, 5.8 W NLP with a slope efficiency of 56.8% is obtained. The pulse energy of NLP is scaled to 1.52 μJ, which is the highest pulse energy of NLP at 2.1 μm to the best of our knowledge. The obtained high-energy square-wave NLP-fiber laser has great potential in mid-infrared laser generation.

570 MHz harmonic mode-locking in an all polarization-maintaining Ho-doped fiber laser

In this paper, we demonstrate a 570.0 MHz harmonically mode-locked all-polarization-maintaining Ho-doped fiber laser based on semiconductor saturable absorbed mirror. Firstly, the laser operates in the 15.4 MHz fundamental mode-locked soliton regime, emitting 2051.5 nm, 1.62 ps soliton pulse without Kelly sidebands. And then, the stable 37th-order harmonic mode-locked soliton with maximum repetition rate up to 570.0 MHz at 2053nm is generated. Moreover, colorful soliton rain behaviors are also discussed.

Switchable generation of a sub-200 fs dissipative soliton and a noise-like pulse in a normal-dispersion Tm-doped mode-locked fiber laser

We report on the switchable generation of a dissipative soliton (DS) pulse and a noise-like pulse (NLP) in an all-fiberized Tm-doped fiber laser in the normal-dispersion region. Mode-locking operation is achieved through a nonlinear polarization rotation component, and the cavity dispersion is compensated using ultra-high numerical aperture (UHNA4) fiber that is easy to integrate and low in cost. At a pump threshold of 510 mW, DS operation can first be achieved without additional filter. The 3 dB spectrum bandwidth of the DS pulse is greater than 50 nm, and the duration of the de-chirped pulse is 193 fs. By increasing the pump power to 880 mW, the mode-locking state can evolve into NLP operation with proper cavity polarization state. The 3 dB spectrum bandwidth and duration of de-chirped coherence spike are 105.6 nm and 121 fs, respectively. Meanwhile, ultra-broadband NLP (over 150 nm considering 3 dB spectrum width) can also be observed with the appropriate cavity parameters. All the proposed pulse patterns present good capacity for achieving narrow pulse width and withstanding high pulse energy.