Lu Zhang

A broadly wavelength-tunable Er-doped fluoride fiber laser with high power flatness was demonstrated in this work based on a 1650 nm pump scheme. Cascaded transitions of ground- and excited-state absorption processes at 1650 nm were used to motivate the population recycling among energy levels and suppress the reabsorption-induced wavelength tunability range shrinkage, as well as a red-shifted and steeper high-gain band. A wavelength-tunable range up to 157 nm from 2702 nm to 2859 nm was achieved at the watt level for the first time, to the best of our knowledge. High power flatness with a power difference less than 5% was realized in a wavelength-tunable range of 117 nm.

Mid-infrared lasers operating in the 3–5 μm region are of interest for spectroscopy and atmosphere sensing applications. Although the 4F9/2→4I9/2 transition in Er-doped fluoride fiber enables efficient 3.5 μm laser emission, power scaling remains challenging due to significant laser quenching effects and limited power handling capability of fluoride fibers. In this work, we demonstrate a high-power 3.5 μm Er-doped fluoride fiber laser with a spatial cavity configuration. Numerical modeling is performed to analyze the power and thermal behavior of the laser to determine an optimized routine for efficient power amplification with low thermal load, based on which a maximum output power reaching 16.4 W at 3.46 μm was experimentally demonstrated. Significant wavelength redshift with the increase of 2 μm pump power is observed under insufficient 0.98 μm pump power due to reduced population inversion, which offers a straightforward method for wavelength tuning in a broadband-feedback cavity.

Single‐frequency lasers in 3–5 µm region are of interest for coherent detection and precision spectroscopy applications. Fiber lasers offer rich frequency selection schemes, but the single‐frequency laser power has been limited to hundred‐milliwatt‐level in the mid‐infrared region due to the immature fiber devices. Here, we directly address this problem by developing a master‐oscillator power‐amplifier (MOPA) tailored for a high‐power single‐frequency laser in the 3.5 µm waveband. Numerical modeling is performed to analyze the power behavior of the MOPA and determine the routine for efficient power amplification with a low‐power single‐frequency laser seed. By optimizing the gain fiber length, 976 nm pump power, and pumping scheme to balance laser efficiency and thermal effects, the laser produces 8.6 W single‐frequency output at 3470 nm, with a slope efficiency of 23.1% relative to absorbed 1990 nm pump power. To our knowledge, this represents the first demonstration of fiber MOPA at 3.5 µm, and also delivers the highest single‐frequency output power ever achieved from mid‐infrared fiber lasers. Furthermore, the modeling also reveals the potential of this MOPA to generate ten‐watt‐level output at >3.7 µm long wavelength region.

We report the first demonstration of Er-doped ZBLAN fiber laser operating above 3800 nm. Numerical modeling is carried out to optimize the cavity arrangement for laser gain improvement at long wavelength, based on which 0.74 W laser output at 3803 nm is experimentally obtained, with a slope efficiency (with respect to the incident 1990 nm pump power) of 9.5%. Experimental observation shows that the gain competition at the 3.5-μm emission peak of the gain fiber is the main obstacle for long-wavelength operation. By reducing the cavity loss to suppress the gain competition, the emission wavelength is further extended to 3810 nm. The corresponding laser wavelength behavior is well explained through the analysis of laser threshold under different cavity loss. Further power scaling and wavelength extension are also discussed based on the modeling, which reveals the potential to generate ten-watt-level output power at 3800-3810 nm by using an optimized all-fiber scheme.

We demonstrate a pulsed Er-doped ZBLAN fiber laser operating at 3.75 μm based on the gain-switching scheme. A diffraction grating is introduced as a wavelength selection component to enable stable lasing in this long-wavelength region that deviates from the emission peak of ⁴F_9/2→⁴I_9/2 transition in Er³⁺. Different from the conventional gain-switching behavior where the pulse repetition frequency of the output laser is same as the that of the pump, the gain-switched laser demonstrated here shows a variable pulse repetition frequency, which accounts for 1/n (n = 4, 3, 2) of the pump pulse repetition frequency, in response to the 1950 nm pump power. The output pulse characteristics, including average output power, repetition frequency, pulse duration, and peak power, are investigated in detail. Over 200 mW average output power at 3.75 μm was obtained at 12 W of 1950 nm pump power. This work demonstrates that the Er-doped ZBLAN fiber laser, in combination with gain-switched scheme, is a feasible and promising approach to generate powerful pulsed emission > 3.7 μm.

A novel 0.98 μm and 1.15 μm dual-wavelength pumping scheme for cascaded mid-infrared Er-doped ZBLAN fiber laser at 3.44 μm and 2.78 μm is proposed and analyzed by a detailed theoretical model. In our pumping scheme, the ⁴ I _13/2 level (the lower level of 2.78 μm laser), which is taken as a virtual ground state, is first populated by 976 nm pump and 2.78 μm transition. The 1150 nm virtual ground state absorption (VGSA), corresponding to ⁴ I _13/2 → ⁴ F _9/2 transition, excites the Er ions accumulated in ⁴ I _13/2 level to ⁴ F _9/2 level (the upper level of 3.44 μm laser), emptying the ⁴ I _13/2 level as well as providing positive gain for 3.44 μm laser. Followed by the 3.44 μm laser transition, the Er ions would rapidly decay to ⁴ I _11/2 level (the upper level of 2.78 μm laser) via nonradiative transition, realizing the re-population of ⁴ I _11/2 level and enhancing the 2.78 μm laser gain. Hence, a cycling of Er ions between ⁴ I _13/2 level and ⁴ F _9/2 level, which includes both 2.78 μm and 3.44 μm laser transitions, is realized via the ⁴ I _13/2 → ⁴ F _9/2 VGSA. An interesting point in our proposed pumping scheme is that both the 2.78 μm and 3.44 μm laser power are dominantly contributed by the VGSA pump, while the main role of 976 nm pump is to provide initial population to sustain the ions cycling. By optimizing the wavelength of VGSA pump, dual-wavelength pump power ratio and laser cavity parameters, the overall optical efficiency of the two laser output power with respect to pump power could be improved to over 55%. This work provides a new insight on the cascaded mid-infrared fiber laser and also a promising pumping scheme for efficient 2.78 μm and 3.44 μm laser generation simultaneously from single Er-doped ZBLAN fiber.

Objectives: This systematic review and meta‐analysis aimed to evaluate the effectiveness of using maxillary protraction during different stages of the dentition by assessing changes in the jaws and inclination of incisors.Materials and methods: MEDLINE (PubMed), Embase, Cochrane, Web Of Science, China National Knowledge Infrastructure and Wanfang Databases were searched without time limitations up to 15 January 2022. Google Scholar was used to search grey literature. We included cohort studies that compared the effect of maxillary protraction by analysing primary outcomes and were grouped in age‐related conditions. Mean differences and 95% confidence intervals were used for statistical analysis, followed by Grading of Recommendations Assessment, Development and Evaluation analysis.Results: Six studies were finally included. The heterogeneity test showed P ≥ .1 and I² ≤ 50%, and a fixed‐effect model was applied. Patients in the early treatment group (ETG) were mainly in the early‐mixed dentition stage, while patients in the late treatment group (LTG) were in the late‐mixed and early‐permanent dentition stage. Meta‐analysis showed that there were no statistical differences (P > .05) between the ETG and LTG groups in terms of SNA (the angle composed by point Sella‐Nasion‐Subspinale), SNB (the angle composed by point Sella‐Nasion‐Supramentale), ANB (the angle composed by point Subspinale‐Nasion‐Supramentale), Wits, U1/SN (the angle composed by the axis of upper incisors and Sella‐Nasion plane) and L1/MP (the angle composed by the axis of lower incisors and the mandibular plane).Conclusion: Our analysis showed that maxillary protraction applied in the late‐mixed or early‐permanent dentition stage did not cause different effects on the maxillary growth, the correction of the intermaxillary relationship, the inhibition of mandibular growth and dental tipping of skeletal class III patients when compared to that in the early‐mixed dentition stage. Collectively, these data provide a theoretical basis for widening the applicable age period of maxillary protraction and choosing the best treatment opportunity for children patients after a comprehensive assessment.

We propose and demonstrate a novel method to achieve pulsed operation of thulium-doped fiber lasers at 1.7 μm. In our design, the thulium-doped fiber laser cavity was embedded within the cavity of an erbium/ytterbium-codoped fiber laser operating at 1.5 μm. The thulium-doped fiber in the erbium/ytterbium-codoped fiber laser cavity served as a saturable absorber, forcing the 1.5 μm laser to operate in a passively Q-switched scheme. The 1.7 μm thulium-doped fiber laser was in-turn pumped by the Q-switched 1.5 μm laser and was gain-switched. This made the pulse duration of 1.7 μm emission much shorter than that of 1.5 μm emission. 1.02 W average output power at 1720 nm was obtained for 10 W of pump power at 976 nm. The pulse repetition frequency, pulse energy and pulse duration were 134 kHz, 7.6 μJ and 102 ns, respectively. A rate equation model was developed to examine the power- and temporal-behavior of the laser system. The calculated pulse duration and pulse repetition frequency matched well with the experimental results. Methods of further improving the laser performance are discussed based on insights gleaned from our modeling.

A watt-level pulsed Tm-doped fiber laser operating at 1720 nm based on the gain-switch effect and intracavity tandemly pump scheme has been proposed and experimentally demonstrated for the first time.

We have demonstrated an efficient 1720-nm all-fiber laser with ring-cavity configuration based on commercial Tmdoped silica fiber and 1570-nm in-band pump source. The rate equation model was built up to analyze the laser performance of Tm-doped fiber, which exhibits strong absorption in 1.7-μm region. The results show that efficient laser operation can be achieved through the optimization of output coupling and the length of Tm-doped fiber. By using homemade couplers, we experimentally achieved 2.36-W laser output power under 6-W launched pump power. The slope efficiency with respect to the absorbed pump power and optical efficiency were 50.2% and 39.3%, respectively. Due to the employment of ring resonator, a narrow laser linewidth of ~4 GHz at maximum output power was observed.