4.3 GHz fundamental repetition rate passively mode-locked fiber laser using a silicate-clad heavily Tm3+-doped germanate core multimaterial fiber

High-power, gigahertz repetition frequency self-mode-locked Ho:GdVO4 laser resonantly pumped by a Tm-doped fiber laser

A self-mode-locked Ho:GdVO4 laser with the GHz pulse repetition frequency oscillation near 2.06 µm was demonstrated for the first time to our knowledge. The output performances of the self-mode-locked Ho:GdVO4 laser were investigated for a few output coupler transmittances at the pulse repetition frequency of 1.89 GHz. At the incident pump power of 8.12 W, the maximum average output power was as high as 2.28 W, corresponding to the slope efficiency and optical-to-optical efficiency of 36.3% and 28.1%, respectively. This is the maximum average output for the 2 µm self-mode-locked solid-state laser with a GHz pulse repetition frequency. This work provides a new way for generating an efficient and a high-power ultrafast pulse laser with a GHz repetition frequency in the 2 µm wave band.

Broadband near-infrared amplified spontaneous emission of Er3+-doped germanate glass fiber

Er3+-doped glass and fiber are very attractive for near-infrared (NIR) lasers and photonic applications. In this work, the full width at half maximum (FWHM) of NIR fluorescence emission of the Er3+-doped germanate glass can be broadened from 72 to 99 nm when Al2O3 was added. In addition, the spectroscopic properties, including absorption and emission spectra, Judd-Ofelt intensity parameters, absorption and emission cross sections, gain coefficient, and fluorescence lifetime, of the Al2O3-modified germanate glass were systematically investigated. What is more, silicate-clad heavily Er3+-doped germanate core multimaterial fibers were successfully drawn by a rod-in-tube method. Notably, broadband NIR amplified spontaneous emission (ASE) with an FWHM of 120 nm was achieved in this new fiber. To the best of our knowledge, this is the largest FWHM reported for Er3+-doped germanate glass fibers. These results suggest that the as-drawn Er3+-doped germanate glass fiber with superior performances is a promising candidate for broadband optical amplification.

Efficient three-level continuous-wave and GHz passively mode-locked laser by a Nd3+-doped silicate glass single mode fiber

Nd³⁺-doped three-level (⁴F_3/2-⁴I_9/2) fiber lasers with wavelengths in the range of 850-950 nm are of considerable interest in applications such as bio-medical imaging and blue and ultraviolet laser generation. Although the design of a suitable fiber geometry has enhanced the laser performance by suppressing the competitive four-level (⁴F_3/2-⁴I_11/2) transition at ∼1 µm, efficient operation of Nd³⁺-doped three-level fiber lasers still remains a challenge. In this study, taking a developed Nd³⁺-doped silicate glass single-mode fiber as gain medium, we demonstrate efficient three-level continuous-wave lasers and passively mode-locked lasers with a gigahertz (GHz) fundamental repetition rate. The fiber is designed using the rod-in-tube method and has a core diameter of 4 µm with a numerical aperture of 0.14. In a short 4.5-cm-long Nd³⁺-doped silicate fiber, all-fiber CW lasing in the range of 890 to 915 nm with a signal-to-noise ratio (SNR) greater than 49 dB is achieved. Especially, the laser slope efficiency reaches 31.7% at 910 nm. Furthermore, a centimeter-scale ultrashort passively mode-locked laser cavity is constructed and ultrashort pulse at 920 nm with a highest GHz fundamental repetition is successfully demonstrated. Our results confirm that Nd³⁺-doped silicate fiber could be an alternative gain medium for efficient three-level laser operation.

Enhanced 2-µm and upconversion luminescence properties regulated by network structure in Ho3+/Yb3+ co-doped germanate laser glasses

Rare-earth (RE) ions doped laser glass has attracted the interest of many researchers because of its numerous potential applications in planar waveguides and fiber lasers. In this work, the 2-µm and upconversion luminescence properties of Ho³⁺ are simultaneously enhanced through the design of components used to regulate the network structure of the germanate glass. Furthermore, the thermal, structural, and spectroscopic properties of the Ho³⁺/Yb³⁺ co-doped germanate laser glass are systematically investigated. It is noted that the calculated gain coefficient of the Nb₂O₅ modified germanate laser glass can reach as high as 3.05 cm^-1 at 2047 nm. These results suggest that the prepared germanate laser glass with superior performances is a promising candidate for 2-µm mid-infrared laser materials applications.

GHz-repetition-rate fundamentally mode-locked, isolator-free ring cavity Yb-doped fiber lasers with SESAM mode-locking

A novel fundamentally mode-locked, GHz-repetition-rate ring cavity Yb-doped femtosecond fiber laser is demonstrated, which utilizes polarization-maintaining gain fiber and is enable by SESAM mode-locking. Thanks to the isolator-free structure, the ring cavity laser is operated bidirectionally and the two polarization-multiplexed output pulse trains are demonstrated synchronous. As a result, tunable waveforms one of which is with reduced pedestal and shorter pulse width in comparison with each individual, are generated by combination of the two orthogonal-polarized output pulses. Furthermore, a similar ring cavity structure that generates GHz picosecond pulses is demonstrated. We believe such high-repetition-rate polarization-multiplexed mode-locked fiber lasers could find further uses in various applications in need of gigahertz repetition rate and tunable waveforms.

Broadband high-gain Tm3+/Ho3+ co-doped germanate glass multimaterial fiber for fiber lasers above 2 µm

High-gain Tm³⁺/Ho³⁺ co-doped optical fibers are urgently desired for high-repetition-rate mode-locked fiber lasers at >2 µm. Here, Tm³⁺/Ho³⁺ co-doped germanate glass with low hydroxyl (OH^-) content was prepared by the conventional melt-quenching method combined with the reaction atmosphere procedure (RAP) dehydration technique. The doping concentrations of Tm₂O₃ and Ho₂O₃ are 2.5 mol.% (7.1 wt.%) and 0.25 mol.% (0.7 wt.%), respectively. Thanks to the high Tm³⁺ doping (7.1 wt.%) and low energy transfer efficiency (19.8%) between Tm³⁺ and Ho³⁺ ions, it enables achieving broadband and high-gain performance in the 2 µm region. Then a silicate-clad Tm³⁺/Ho³⁺ co-doped germanate core multimaterial fiber was successfully drawn by using the rod-in-tube method, which has a broadband amplified spontaneous emission (ASE) with a full width at half-maximum (FWHM) of 247.8 nm at 2 µm. What is more, this new fiber has a high gain per unit length of 4.52 dB/cm at 1.95 µm. Finally, an all-fiber-integrated passively mode-locked fiber laser was built by using this broadband high-gain fiber. The mode-locked pulses operate at 2068.05 nm, and the fundamental repetition rate is up to 4.329 GHz. To the best of our knowledge, this is the highest fundamental repetition rate for the all-fiber passively mode-locked fiber laser above 2 µm. These results suggest that the as-drawn multimaterial fibers with broadband high-gain characteristics are promising for high-repetition-rate ultrafast fiber lasers.

Manipulating the polarization dynamics in a >10-GHz Er3+/Yb3+ fiber Fabry-Pérot laser

In this work, we report on the vector and scalar soliton dynamics that result from inevitable fiber birefringence in an 8-mm Er³⁺/Yb³⁺ fiber based Fabry-Férot (FP) laser that has a free spectral range of up to 12.5 GHz. The generation of polarization-evolving vector solitons can largely degrade the performance of application systems, and the underlying mechanisms and manipulation technologies are yet to be explored. To realize the transition from vector to scalar (linearly polarized) state, we here incorporate the polarization selection effect (PSE) in the simulation model and the numerical results verify that only a small amount of PSE is sufficient for manipulating the soliton dynamics. It also reveals that, prominent polarization-dependent intensity discrimination can be acquired via geometry-induced oblique incidence to the Bragg mirror of the semiconductor saturable absorber mirror (SESAM), and we obtain switchable operating states by tilting the SESAM in the experiments. These efforts create a feasible method to manipulate high-repetition-rate pulse and may shed light on understanding the dissipative soliton dynamics in ultrafast fiber FP lasers.