Experimental and numerical studies of mode-locked fiber laser with large normal and anomalous dispersion

Generation of 99.8 fs, 25 kW Peak-Power, Dispersion-Managed Pulses Directly from an Yb-Doped Figure-of-9 Fiber Laser

We reported on the generation of 99.8 fs, 25 kW peak-power, dispersion-managed pulses directly from a passively mode-locked Yb-fiber laser oscillator with a figure-of-9 configuration. The introduction of strongly injected pump power and optical components with a high damage threshold enables high-power operation, while the polarization-maintaining (PM) fiber supports environmentally stable self-started mode-locking. Mode-locking in the soliton-like and negative-dispersion regime is characterized by the dispersion management via tuning the separation distances between a pair of gratings inside the cavity. The oscillator generates stable pulses with up to 40.10 mW average power at a 16.03 MHz repetition rate, corresponding to a pulse energy of 2.5 nJ. To the best of our knowledge, it is the highest peak-power directly obtained by a laser oscillator with a figure-of-9 configuration.

Generation of mode-locked pulses based on D-shaped fiber with CdTe as a saturable absorber in the C-band region

This study demonstrates the potential of cadmium telluride (CdTe), a part of the quantum dot (QD) family, as a saturable absorber (SA) to generate ultrashort pulses at the C-band region. The SA was fabricated by drop-casting the CdTe material onto the exposed core of the D-shaped fiber. The nonlinear property of the fabricated SA has a modulation depth of 1.87% and saturation intensity of 6.0 kW cm⁻². The mode-locked laser was generated after the SA was integrated into the erbium-doped fiber laser (EDFL) cavity at a threshold pump power of 192.1 mW giving a center wavelength of 1559 nm and a pulse duration of 770 fs. The maximum average output and peak power were measured to be 2.8 mW and 0.208 kW, respectively. The mode-locked fiber laser generated a signal-to-noise ratio (SNR) of 67.7 dB, proving that the generated mode-locked pulses were very stable. The current work indicates that the novel CdTe device can provide stable mode-locked lasers in the C-band region.

This study demonstrates the potential of cadmium telluride (CdTe), a part of the quantum dot (QD) family, as a saturable absorber (SA) to generate ultrashort pulses at the C-band region.

2 μm passively mode-locked thulium-doped fiber lasers with Ta2AlC-deposited tapered and side-polished fibers

In this work, mode-locked thulium-doped fiber lasers operating in the 2 µm wavelength region were demonstrated using tantalum aluminum carbide (Ta2AlC)-based saturable absorbers (SAs) utilizing the evanescent wave interaction. The Ta2AlC MAX Phase was prepared by dissolving the Ta2AlC powder in isopropyl alcohol and then deposited onto three different evanescent field-based devices, which were the tapered fiber, side-polished fiber, and arc-shaped fiber. Flame-brushing and wheel-polishing techniques were used to fabricate the tapered and arc-shaped fibers, respectively, while the side-polished fiber was purchased commercially. All three SA devices generated stable mode-locked pulses at center wavelengths of 1937, 1931, and 1929 nm for the tapered, side-polished, and arc-shaped fibers. The frequency of the mode-locked pulses was 10.73 MHz for the tapered fiber, 9.58 MHz for the side-polished fiber, and 10.16 MHz for the arc-shaped fiber. The measured pulse widths were 1.678, 1.734, and 1.817 ps for each of the three SA devices. The long-term stability of the mode-locked lasers was tested for each configuration over a 2-h duration. The lasers also showed little to no fluctuations in the center wavelengths and the peak optical intensities, demonstrating a reliable, ultrafast laser system.

Route to stable dispersion-managed mode-locked Yb-doped fiber lasers with near-zero net cavity dispersion

We numerically investigate the stability of a dispersion-managed mode-locked Yb-doped fiber laser of near-zero net cavity dispersion. The instability is primarily due to the filtering effect of the chirped fiber Bragg grating. The size of the unstable region is dependent on the modulation depth of the saturable absorbers. At modulation depth higher than 30%, stable mode-locking can operate throughout the dispersion region. Based on the simulation results, stable mode-locking around zero cavity dispersion is experimentally viable by a SESAM with a 34% modulation depth. The fiber laser can generate laser pulses with a 17-nm spectral bandwidth and a 139-fs dechirped pulse duration.

All-fiber passively mode-locked ultrafast laser based on a femtosecond-laser-inscribed in-fiber Brewster device

We report on an in-fiber Brewster device with a 45° tilted fiber grating (TFG) directly written by a plane-by-plane femtosecond laser inscription method. Up to 10 dB polarization-dependent loss was achieved, proving effective polarizing functionality. Furthermore, we employ it as an in-line polarizer to successfully mode lock a fiber laser through the nonlinear polarization rotation technique. A stable soliton pulse train has been generated at 1563.64 nm with a pulse width of 624 fs and pulse energy of 0.42 nJ. With proper polarization adjustment, the laser also can operate in a noise-like regime. The parameters of this kind of 45°-TFG can be flexibly customized owing to the high flexibility and controllability of the femtosecond laser-inscription approach. In particular, such in-fiber polarizing devices inscribed by femtosecond laser inscription without removing the fiber coating are extremely robust for fiber lasers working at a broad wavelength region including the mid-infrared.

Compact all-fiber 2.1-2.7 μm tunable Raman soliton source based on germania-core fiber

Although ultrafast rare-earth-doped fiber lasers mode-locked at near-infrared and ∼3 μm wavelengths have been well developed, it is relatively difficult to achieve ultrafast fiber laser emitting in the 2.1-2.7 μm spectral gap between ∼2 μm (Tm fiber) and ∼2.8 μm (Er or Ho fluoride fiber). In this paper, we report the generation of 2.1-2.7 μm tunable femtosecond Raman solitons from a compact fusion-spliced all-fiber system using a home-made 1.96 μm ultrafast pump source and a MIR-available germania-core fiber. At first, a Tm-doped double-clad fiber amplifier is used to not only boost up the power of 1957 nm femtosecond seed laser, but also to generate the first-order soliton self-frequency shift (SSFS). The first-order Raman solitons can be tuned from 2.036 to 2.152 μm, have a pulse duration of ∼480 fs and can reach a pulse energy of 1.07 nJ. The first-order Raman solitons are further injected into a 94 mol.% germania-core fiber to excite the second-order SSFS. The second-order solitons can be tuned to longer wavelengths, i.e. from 2.157 μm up to 2.690 μm. Our work could provide an effective way to develop compact, all-fiber ultrafast MIR laser sources with the continuous wavelength tuning of 2.1-2.7 μm.

Dissipative-soliton generation with nonlinear-polarization-evolution in a polarization maintaining fiber

A long-term stable picosecond dissipative soliton (DS) is achieved for the first time using nonlinear polarization evolution. The environmental stabilization is performed by a Faraday mirror, which can cancel environmentally induced changes in the birefringence of the fiber. The laser cavity with all-polarization-maintaining fiber components generates DS pulses with 2.9 nJ single pulse energy and 5.9 ps pulse width. The output power test over 2 hours shows the excellent mode-locking stability of this design.

14.5 GHz passive harmonic mode-locking in a dispersion compensated Tm-doped fiber laser

We demonstrate a high repetition rate passive harmonic mode-locking (HML) based on nonlinear polarization evolution (NPE) technique in a Tm-doped ring fiber laser cavity. Small net anomalous cavity dispersion based on dispersion compensation benefited the generation of high repetition rate HML due to the low soliton splitting threshold. Stable HML with a repetition rate of up to 14.5 GHz and a super-mode suppression (SSR) of 19 dB was obtained at the center wavelength of 1982.3 nm, which is about ten times of state of the art at 2 μm band mode-locking fiber laser to our best knowledge. The repetition rate was selectable between 1 GHz to 14.5 GHz through changing the pump power and intra-cavity polarization state, and the SSR better than 25 dB was obtained as the repetition rate less than 5 GHz.

Coexistence of noise-like pulse and high repetition rate harmonic mode-locking in a dual-wavelength mode-locked Tm-doped fiber laser

Coexistence of harmonic mode-locking (HML) and noise-like pulse (NLP) were experimentally observed in a dual-wavelength mode-locked Tm-doped fiber laser for the first time. The coexistence patterns were self-started and maintained within a wide pump range by appropriately setting the intra-cavity polarization state. The HML was obtained at 1955.3 nm with a varied repetition rate range from 324 MHz to 1.138 GHz which benefits from the dispersion compensation, while the NLP observing at 1983.2 nm can operate at either fundamental repetition rate of 4.765 MHz or second harmonic state. Experimental investigations show that the coexistence patterns are caused by the wavelength-dependent phase delay of the mode-locked fiber cavity. Moreover, dual-wavelength NLP was also observed for the first time at 2 μm spectral region by changing the intra-cavity polarization state.

State distributions in two-dimensional parameter spaces of a nonlinear optical loop mirror-based, mode-locked, all-normal-dispersion fiber laser

We present the results of numerical simulations of dissipative soliton generation using nonlinear Schrödinger equations in an all-normal-dispersion (ANDi) mode-locked fiber laser based on a nonlinear optical loop mirror (NOLM). Firstly, systematic and computationally intensive analysis of the pulse state distributions in two-dimensional parameter spaces of an ANDi fiber laser was conducted. In addition, we determined that unstable non-vanishing regions including pulsation and noise-like pulses are directly related to the saturable absorptions of NOLMs and that two critical filter bandwidths separate those regions from stable ones. Finally, we found that the multi-pulsing power threshold can be maximized by using an optimal optical filter bandwidth.

Thulium-doped all-fiber mode-locked laser based on NPR and 45°-tilted fiber grating

A nonlinear polarization rotation based all-fiber passively mode-locked Tm³⁺-doped fiber laser is demonstrated by using a 45° tilted fiber grating (TFG) as an in-line polarizer. The 45° TFG centered at 2000 nm with polarization dependent loss (PDL) of >12 dB at 1850 nm~2150 nm range was UV inscribed for the first time in SM28 fiber using a 244 nm Ar⁺ continuous wave laser and a phase mask with 25 mm long uniform pitch and titled period pattern of 33.7° with respect to the fiber axis. Stable soliton pulses centered at 1992.7 nm with 2.02 nm FWHM bandwidth were produced at a repetition rate of 1.902 MHz with pulse duration of 2.2 ps and pulse energy of 74.6 pJ. As increased pump power, the laser also can operate at noise-like regime with 18.1 nm FWHM bandwidth and pulse energy of up to 250.1 nJ. Using the same 45° TFG, both stable soliton and noise-like mode-locking centered at ~1970 nm and ~2050 nm, were also achieved by shortening and extending the length of Tm³⁺-doped fiber, respectively, exhibiting advantages of broadband and low insertion loss at 2 µm band.

All-fiber passively mode-locked Tm-doped NOLM-based oscillator operating at 2-μm in both soliton and noisy-pulse regimes

A self-starting all-fiber passively mode-locked Tm(3+)-doped fiber laser based on nonlinear loop mirror (NOLM) is demonstrated. Stable soliton pulses centered at 2017.33 nm with 1.56 nm FWHM were produced at a repetition rate of 1.514 MHz with pulse duration of 2.8 ps and pulse energy of 83.8 pJ. As increased pump power, the oscillator can also operate at noise-like (NL) regime. Stable NL pulses with coherence spike width of 341 fs and pulse energy of up to 249.32 nJ was achieved at a center wavelength of 2017.24 nm with 21.33 nm FWHM. To the best of our knowledge, this is the first 2 µm region NOLM-based mode-locked fiber laser operating at two regimes with the highest single pulse energy for NL pulses.

Ultrashort pulses from an all-fiber ring laser incorporating a pair of chirped fiber Bragg gratings

By incorporating two linearly chirped ultrabroadband fiber Bragg gratings of opposite dispersion in an all-fiber ring laser, we demonstrate a mode-locking regime in which a femtosecond pulse evolving in the normal dispersion gain segment is locally transformed into a highly chirped picosecond pulse that propagates in the remaining section of the cavity. By minimizing nonlinear effects and avoiding soliton pulse shaping in this anomalous-dispersion section, low repetition rate fiber lasers can be made to produce high-energy ultrashort pulses. Using this approach, 98 fs pulses with 0.96 nJ of energy are obtained from an erbium-doped fiber laser operated in the highly anomalous dispersion regime at a repetition rate of 9.4 MHz.