Jianqiu Xu

Development and applications of gain-switched fiber lasers [Invited]

We briefly review the development of gain-switched rare-earth-doped fiber lasers and their applications in wavelength conversion to mid-IR, supercontinuum generation, and medicine in recent years. We illustrate the similarities between gain-switching and Q-switching techniques that will provide tools for the design and optimization of the gain-switched fiber lasers. From the nature of the gain-switched fiber lasers, benefits of this kind of lasers to 2-μm region and in-band-pumped (two-level system) laser systems are obvious. Advantages of in-band-pumped 2-μm lasers are discussed and analyzed with a simple numerical simulation in terms of Tm-doped fiber lasers. We also propose the key factors in the development of the gain-switched fiber lasers and predict the future tendency.

Mode-locked thulium fiber laser with MoS2

Liquid-phase exfoliated 2D material multilayer MoS2 is transferred onto a gold mirror and its saturable absorption at the 2 µm wavelength region is experimentally observed. This transferred MoS2 saturable absorber has a modulation depth of 13.6% and a saturation intensity of 23.1 MW cm−2. This saturable absorber is integrated into a linear Tm3+ fiber laser cavity, and stable fundamental-frequency mode-locking operation is realized at 2 µm with pulse energy of 15.5 nJ, pulse width of ~843 ps, and a repetition rate of 9.67 MHz. The laser spectral width is ~17.3 nm with a center wavelength of 1905 nm. This first presence of mode-locking with multilayer MoS2 sheets in the 2 µm wavelength region verifies that multilayer MoS2 is a good candidate for broadband mode-locking comparable to graphene, as well as a good mode-locker for achieving high pulse energy.

High-power narrow-bandwidth thulium fiber laser with an all-fiber cavity

We report diode pumped high power 2-µm Tm(3+) fiber lasers with an all-fiber configuration. The all-fiber configuration is completed by specially designed fiber Bragg gratings with similar structure parameters matched to the gain fiber. The maximum output power is 137 W with an optical-to-optical slope efficiency of 62% with respect to absorbed 793-nm pump power. The laser wavelength is stabilized at ~2019 nm with a spectral linewidth less than 3 nm across all output levels. To the best of our knowledge, this is the highest 2-µm laser output from a single narrow bandwidth all-fiber laser system.

Efficient Q-switched Tm:YAG ceramic slab laser

Characteristics of Tm:YAG ceramic for high efficient 2-μm lasers are analyzed. Efficient diode end-pumped continuous-wave and Q-switched Tm:YAG ceramic lasers are demonstrated. At the absorbed pump power of 53.2W, the maximum continuous wave (cw) output power of 17.2 W around 2016 nm was obtained with the output transmission of 5%. The optical conversion efficiency is 32.3%, corresponding to a slope efficiency of 36.5%. For Q-switched operation, the shortest width of 69 ns was achieved with the pulse repetition frequency of 500 Hz and single pulse energy of 20.4 mJ, which indicates excellent energy storage capability of the Tm:YAG ceramic.

High Peak-Power Gain-Switched Tm

Resonantly pumped by a 1.914-μm Q-switched Tm:YLF laser, high-peak-power stable laser pulse from gain-switched ~1.94-μm Tm3+-doped silica fiber lasers are reported for the first time. The slope efficiency was over 80% with respect to absorbed pump power, and chaotic spiking was effectively eliminated by using fast gain switching. With a 4-m Tm3+ fiber, pulse energy of 1.3 mJ and pulsewidth of 61 ns were obtained, corresponding to peak power of 21.3 kW.

^{3+}

A high optical signal-to-noise ratio (OSNR) single-frequency 2 μm Brillouin fiber laser (BFL) with watt-level output and high transfer efficiency is demonstrated for the first time to the best of our knowledge. The Brillouin pump is constructed with a two-stage thulium-doped fiber amplifier (TDFA) seeded by a 2 μm laser diode, providing 4.02 W average power with 1 MHz linewidth. Using an optimized length of 14 m for the Brillouin ring cavity, the BFL works stably in single-mode region with 8 kHz linewidth because of the linewidth narrowing effect. The transfer efficiency is 51% with 1.08 W output power and 62 dB OSNR for 3.22 W pump power.

-Doped Fiber Laser

We propose a cascaded tandem pumping technique and show its high power and high efficient operation in the 2-μm wavelength region, opening up a new way to scale the output power of the 2-μm fiber laser to new levels (e.g. 10 kW). Using a 1942 nm Tm(3+) fiber laser as the pump source with the co- (counter-) propagating configuration, the 2020 nm Tm(3+) fiber laser generates 34.68 W (35.15W) of output power with 84.4% (86.3%) optical-to-optical efficiency and 91.7% (92.4%) slope efficiency, with respect to launched pump power. It provides the highest slope efficiency reported for 2-μm Tm(3+)-doped fiber lasers, and the highest output power for all-fiber tandem-pumped 2-μm fiber oscillators. This system fulfills the complete structure of the proposed cascaded tandem pumping technique in the 2-μm wavelength region (~1900 nm → ~1940 nm → ~2020 nm). Numerical analysis is also carried out to show the power scaling capability and efficiency of the cascaded tandem pumping technique.

High signal-to-noise ratio, single-frequency 2 μm Brillouin fiber laser

Efficient continuous-wave laser emission at 1856 nm from a Tm,Mg:LiNbO(3) crystal slab with high Tm(3+) doping concentration is reported. A maximum output power of 2.62 W is realized with a slope efficiency of 19.6% and the beam quality factor M(2) of 1.7 at room temperature. We believe that this is the first demonstration of watt-level laser operation in Tm,Mg:LiNbO(3) crystal and the output power is four orders of magnitude higher than that reported previously in Tm-doped LiNbO(3) crystal. Performance degradation due to the photorefractive effect under high intensity 1856 nm laser is not observed thanks to the co-doping of magnesium ions. Quantitative analysis about the long-term photorefractive effect is also provided. Multi-wavelength laser operation is realized by using different narrow-band output couplers. This demonstration opens up a viable pathway towards 2-μm integrated optic devices for achieving laser oscillation, electro-optic and nonlinear optical effects within just one sample simultaneously.

High power tandem-pumped thulium-doped fiber laser

Compact LD pumped, CW, and passively Q-switched Nd,Mg:LiTaO3 lasers with a stable, synchronous dual-wavelength operation near 1082 and 1092 nm were demonstrated for the first time. Under an incident pump power of 7.69 W, a maximum CW output power of 464 mW was obtained, corresponding to a slope efficiency of 7.6%. By using a monolayer graphene as saturable absorber, the steadily Q-switched Nd,Mg:LiTaO3 laser with dual-wavelength oscillation was achieved. The maximum output power of the Q-switched laser was 365 mW with a repetition rate of 133 kHz. The minimum pulse duration was measured to be 176 ns. The corresponding maximum pulse energy and peak power were 2.75 μJ and 15.7 W, respectively.

Efficient 1856 nm emission from Tm,Mg:LiNbO3 laser.

While the recent discovered new mode-locking mechanism - dissipative soliton - has successfully improved the pulse energy of 1 μm and 1.5 μm fiber lasers to tens of nanojoules, it is still hard to scale the pulse energy at 2 μm due to the anomalous dispersion of the gain fiber. After analyzing the intracavity pulse dynamics, we propose that the gain fiber should be condensed to short lengths in order to generate high energy pulse at 2 μm. Numerical simulation predicts the existence of stable 2 μm dissipative soliton solutions with pulse energy over 10 nJ, comparable to that achieved in the 1 μm and 1.5 μm regimes. Experimental operation confirms the validity of the proposal. These results will advance our understanding of mode-locked fiber lasers at different wavelengths and lay an important step in achieving high energy ultrafast laser pulses from anomalous dispersion gain media.