In this paper, we report a high-power single-frequency all-fiber amplifier at 1064 nm based on cascaded hybrid active fibers. The cascaded gain fiber design, where a piece of active fiber with lower dopant concentration is spliced before the highly doped gain fiber, is proposed to effectively alleviate the thermal loads of high-power amplifier. By virtue of hybrid pump at 976 nm and 915 nm to increase the threshold of transverse mode instability (TMI) effect, an output power of 435 W is achieved from the Yb3+-doped fiber with large core diameter of 30 μm. The beam quality is well maintained with a M2 of 1.4 even at the maximum laser power.
Saturable absorption effect of unpumped active fiber can be used as a method of achieving single-frequency operation from an all-fiber oscillator. Here, the effect was studied when the signal was highly absorbed by unpumped thulium-doped fiber and the laser threshold was beyond the available pump power. We proved that the oscillator could be manipulated to the “oscillating state” by injecting auxiliary pump light to the saturable absorber fiber. This allowed using unpumped thulium-doped fiber with higher absorption to enhance the frequency selecting capability. A record single-frequency output power of 2.56 W at 1720 nm was obtained, with a slope efficiency of 44% and a linewidth of 3.3 kHz, the power scaling of which was only limited by the available power of the single-mode 1570 nm pump source. This work offers new insights into how high-power, single-frequency fiber oscillators can be produced.
A polarization-maintaining (PM) solid-core anti-resonant fiber (SC-ARF) based on the anti-resonant waveguide mechanism was proposed in this work. Numerical simulation results indicated that fundamental mode (FM) birefringence could be obtained by filling the core and cladding of designed SC-ARF with As2S3 and As2Se3 materials respectively. With the cladding tubes thickness of 0.82 μm, our fiber presented the birefringence coefficient (Δn) about 1.56x10-5 and the confined loss (CL) less than 0.1dB/m. What’s more, the optimized CL values and band-pass transmission properties were observed by increasing the tubes number and adjusting the incident laser wavelength, which are of pivotal importance in the PM laser systems and show great potential to fabricate the filters in the mid-infrared regime (MIR).
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.
A narrow-linewidth single-frequency Yb-doped nanosecond pulsed fiber laser with 27-kW peak power was demonstrated in this work. By employing the novel triangle-shape pulse with a 1-ns rise time and 13-ns fall time, the spectral linewidth of 88 MHz was achieved at the maximum output power. The signal to noise ratio was about 33 dB and no obvious residual pump power within the output laser. To the best of our knowledge, it is the first time that the performance of triangle-pulse fiber amplifier was demonstrated.
We observed a large optical bistability in a single-frequency thulium fiber laser with ring cavity configuration. A piece of unpumped Tm-doped fiber served as nonlinear saturable absorber (SA), which also acted as a narrow-bandpass filter by forming self-induced gratings with counter-propagate lasers and enabled single-frequency laser operation at 1720 nm. Due to the large absorption cross section of thulium ions at 1720 nm, the unpumped Tm-doped fiber has large variable losses, hence resulting in strong optical bistability. With 0.75-m SA fiber, a 4.8-W wide bistable region was achieved. The evolution of bistable region with different lengths of SA fibers was investigated. The bistable region became narrower with decreasing SA fiber length, and totally disappeared at a SA fiber length of 0.15 m. To the best of our knowledge, this is the first observation of optical bistability in thulium fiber lasers.
For anti-resonant fibers (ARF), when the diameter of cladding tube approaches the effective diameter of fiber core, the fundamental mode (FM) in core appears an index-induced mode coupling with the FM in cladding tubes. The FMs mode coupling between core and cladding forms a super FM in ARF, and simultaneously introduces an obvious suppression on the FM in core, which provides a novel theory for polarization-maintaining ARF. In this paper, we proposed a polarization-maintaining ARF (PM-ARF) with four asymmetrical cladding tubes for 2.5 THz wave. In PM-ARF, the horizontal cladding tubes are designed to generate the suppression on modes, and the vertical tubes are designed to generate the guidance for modes. Profiting from the mode coupling between core and cladding, a birefringence index of 9×10-6 is obtained by the compact PM-ARF. The mode coupling was confirmed can be combined with methods of bi-thickening the core boundary and elliptical core boundary respectively to improve the polarization-maintaining ability. Optimizing the shells of horizontal and vertical cladding tubes into different thickness respectively, and simultaneously working with the mode coupling between core and cladding, the birefringence index of ARF was improved to larger than 1×10-4. Besides, the mode coupling worked with the method of elliptical core boundary, the birefringence index was further improved to higher than 2×10-4.
In this work, the linewidth performance of triangle-shaped pulsed fiber laser (TPL) with 6.5-ns pulse width was investigated numerically and experimentally. The spectral linewidth of TPL changed with different rising times and fixed pulse width. The minimum spectral linewidth could be obtained when the rising time or falling time is equal to 0. Besides, the self-phase modulation (SPM) could be suppressed when the rising time of TPL is less than 3.25 ns, and the tunable linewidth from 77.73 MHz to 86.69 MHz was obtained at 2.1-kW peak power of pulse. It was found that experimental results are consistent with the theoretical analysis.
Single-mode fiber plays an important role in the applications of THz technology. Serving as one of the optimal choices of THz fiber, hollow-core anti-resonant fiber (ARF) with single-layer anti-resonant elements has advantages of low transmission loss, high damage threshold, low dispersion but disadvantage of single mode which can by realized by the high-order-mode (HOM) suppression effect. In this paper, the key factor affecting the HOM suppression effect in THz ARF is investigated and confirmed to be the cross-sectional area ratio of cladding tubes inner side with fiber core, besides, the ratio will slightly change with the variation of the anti-resonant period of THz ARF where the cladding tubes have different shell thickness. By optimizing the anti-resonant elements of THz ARF from round tubes to semielliptical tubes, a high-performance single-mode THz fiber is proposed and the confinement loss of high-order-mode is confirmed larger than 24 dB/m at the same time that of fundamental-mode is controlled less than 0.4 dB/m.
Stimulated Brillouin scattering (SBS) is a well established method to narrow the laser linewidth to kilohertz level, which however suffers high threshold due to the low SBS gain at the region of 2 μm. The hybrid Brillouin/thulium fiber laser (BTFL) is such an approach which could suppress the laser linewidth with low threshold and high efficiency. In this paper, an ultra-narrow linewidth hybrid Brillouin/thulium fiber laser (BTFL) was demonstrated. Through experimentally optimizing the output coupling, pump scheme, Brillouin pump power and cavity length of the laser, 344-mW laser output with a narrow linewidth of 0.93 kHz was obtained, in which the linewidth of Stokes light was suppressed more than 43 times compared with the 40 kHz linewidth of the Brillouin pump. Besides, the influences of output coupling and pump scheme on the power and linewidth behavior of a single-frequency BTFL were also experimentally investigated, and there exists a performance balance among linewidth narrowing, output power and SBS threshold. The BTFL output power was further boosted to 5.5 W by a one-stage cladding-pumped fiber amplifier, and the corresponding spectral linewidth was broadened to 1.93 kHz. The output coupling exerted a significant influence on the BTFL performance.
We designed and analyzed a novel optically pumped gas THz fiber laser (OPTFL) based on hollow-core anti-resonant fiber (HC-ARF). The OPTFL filled with methanol vapor and pumped by a continuous-wave tunable CO2 laser. Combining the rate equations and the THz wave transmission theory in HC-ARF, factors that impacting THz output are analyzed. By investigating the inner structure of HC-ARF, a HC-ARF with an intrinsic single-mode transmission and a low confinement loss lower than 0.2 dB/m in OPTFL is proposed for efficiently transmitting THz waves. Theoretical analysis indicate that by appropriately increasing the cavity length, the output power of OPTFL is expected to reach the magnitude of 100 milliwatts with optimal operating conditions. The results provide an available method for highperformance THz laser source.
A 1720-nm thulium-doped all-fiber laser based on a ring-cavity configuration is demonstrated. The long-wavelength lasing near the 1.9-μm thulium emission peak was suppressed using a wavelength division multiplexer and single-mode– multimode–single-mode (SMS) fiber device, which together served as a short-pass filter instead of the grating devices usually used in 1.7-μm thulium fiber lasers. A stable hundred-milliwatt-level 1720-nm laser output with a narrow spectral linewidth on the order of gigahertz was obtained after optimizing the output coupling, the active fiber length and the SMS device.
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