Zefeng Wang

Fabrication of chirped and tilted fiber Bragg gratings and suppression of stimulated Raman scattering in fiber amplifiers

Stimulated Raman scattering (SRS) is one of the main limits for fiber lasers further power scaling. We report on the suppression of the stimulated Raman scattering in fiber laser amplifier using chirped and tilted fiber Bragg gratings (CTFBGs) for the first time. In this paper, we design and fabricate a CTFBG used to suppress the SRS in 1090 nm fiber laser output, and establish a system to test the effect of suppression. A maximum suppression ratio nearly 25 dB is achieved. Experimental results demonstrate that CTFBGs can increase the Raman threshold and promote the slope efficiency of the whole system, which is significant for further power scaling in high power oscillators and amplifiers.

Watt-level tunable 1.5 μm narrow linewidth fiber ring laser based on a temperature tuning π-phase-shifted fiber Bragg grating

A watt-level tunable 1.5xa0μm narrow linewidth fiber ring laser using a temperature tuning π-phase-shifted fiber Bragg grating (π-PSFBG) is demonstrated here, to the best of our knowledge, for the first time. The π-PSFBG is employed as both a narrow band filter and a wavelength tuning component, and its central wavelength is thermally tuned by a thermo-electric cooler. The maximum laser power is about 1.1xa0W with a linewidth of ∼318u2009u2009MHz (∼2.57u2009u2009pm) and a power fluctuation of less than 3%. The wavelength tuning range of the laser is about 1.29xa0nm with a sensitivity of ∼14.33u2009u2009pm/°C, and the wavelength fluctuation is about 0.2xa0pm. This work provides important reference for tunable fiber lasers with both high power and narrow linewidth.

Ultra-efficient Raman amplifier in methane-filled hollow-core fiber operating at 1.5 μm

We report on what is, to the best of our knowledge, the first ultra-efficient 1.5 μm Raman amplifier in a methane-filled anti-resonance hollow-core fiber. A 1.5 μm single frequency seed laser is coupled into the hollow-core fiber together with a 1064 nm pulsed pump laser using a shortpass dichromic mirror, and then amplified by stimulated Raman scattering of methane. A maximum optical-to-optical conversion efficiency of 66.4% has been obtained, resulting in a record near quantum-limit efficiency of 96.3% in a 2 m long hollow-core fiber filled with only 2 bar methane gas. This kind of gas filled hollow-core Raman amplifier provides a potential method to obtain high efficiency mid-infrared laser sources with low threshold and narrow linewidth in various applications.

Tunable mid-infrared emission from acetylene-filled hollow-core fiber

We report here a step tunable mid-infrared laser emission from acetylene-filled hollow-core fiber. Two kinds of anti-resonant hollow-core fibers are filled with mbar level of acetylene gas, and pumped with a modulated, amplified, narrow linewidth, fine tunable, 1.5 μm diode laser, then 3 μm laser emissions are generated by the intrinsic absorption of acetylene molecules. The laser wavelength is step-tunable in the range of 3.1~3.2 μm when the pump laser is precisely tuned to different absorption lines of P-branch of acetylene. By properly designing the fibers transmission bands, and carefully selecting active gases and pump lasers, this paper provides a novel method for efficient, compact and tunable mid-infrared fiber lasers over a broad spectrum range.

Efficient mid-infrared cascade Raman source in methane-filled hollow-core fibers operating at 2.8 μm

We report here for the first time, to the best of our knowledge, a novel and efficient cascade Raman laser source operating at 2.8xa0μm by two stages of methane-filled hollow-core fibers (HCFs). In the first stage, a commercial 1064.6xa0nm laser is used as the pump source, and an efficient first-order Stokes wave of 1543.9xa0nm is obtained with a quantum conversion efficiency of ∼87% in 2xa0m ice-cream HCF filled with 2xa0bar methane gas. In the second stage, efficient 2.8xa0μm laser emission is also generated by the first-order stimulated Raman scattering of methane, while the pump source is the Stokes wave at 1543.9xa0nm. A maximum quantum conversion efficiency of ∼75% is obtained with 2.2xa0m node-less HCF filled with 11xa0bar methane gas, resulting in a record total quantum efficiency of ∼65%, which is 1.6 times the previous similar result. This work provides a significant efficient method to obtain a wide wavelength range of mid-infrared, even far-infrared fiber laser sources from conveniently available 1xa0μm band lasers with proper HCFs and different active gases.

High-power tunable mid-infrared fiber gas laser source by acetylene-filled hollow-core fibers

High-power tunable pulsed and CW mid-infrared fiber gas laser sources in acetylene-filled hollow-core fibers, to the best of our knowledge, are demonstrated for the first time. By precisely tuning the wavelength of the pump source, an amplified tunable 1.5 μm diode laser, to match different absorption lines of acetylene, the laser output is step-tunable in the range of 3.09~3.21 μm with a maximum pulse average power of ~0.3 W (~0.6 μJ pulse energy) and a maximum CW power of ~0.77 W, making this system the first watt-level tunable fiber gas laser operating at mid-infrared range. The output spectral and power characteristics are systemically studied, and the explanations about the change of the ratio of the P over R branch emission lines with the pump power and the gas pressure are given, which is useful for the investigations of mid-infrared fiber gas lasers.

Efficient 1.5 μm Raman generation in methane-filled negative curvature hollow-core fiber

Efficient 1.5 μm fiber gas Raman source has been demonstrated for the first time in methane-filled negative curvature hollow-core fiber (NC-HCF) with ice-cream-cone shaped cladding. With 4.5 bar methane gas filled, an 8.8 m long NC-HCF was pumped with a 1064 nm microchip laser, generating Stokes spectral line at 1543.7 nm. A maximum Raman conversion efficiency of about 57% has been achieved with 24 mW coupled pump power, and the corresponding quantum conversion efficiency was 82.7%, a record level of such experiment.

Raman laser amplifier in methane-filled hollow-core fiber

We report on an ultra-efficient 1.5 μm Raman amplifier in methane-filled negative curvature hollow-core fiber. A 1.5 μm tunable CW DFB seed laser is coupled into the fiber together with a 1064 nm pump laser using a shortpass dichromic mirror, and then stimulated amplified by Raman scattering of methane. The maximum Raman conversion efficiency of 66.4 % was obtained in the 2 bar methane gas filled, 2 m long hollow core fiber with 50 mW coupled pump power and 22.6 mW coupled seed laser power, and the corresponding quantum efficiency is as high as to 96.3 %, which almost approaches the quantum limit. The introduction of the single frequency seed laser not only reduced the Raman threshold from 17.5 mW to 9.5 mW, but also narrowed the Stokes linewidth from 3.4 GHz to 2.1 GHz with a factor of 60%. This kind of gas filled hollow core Raman amplifier can be a potential method to obtain low threshold, narrow linewidth and high efficiency mid infrared laser source in various application.

Characteristics of 1.9 μm laser emission from hydrogen-filled hollow-core fiber by stimulated Raman scattering

We report here the detailed characteristics of 1.9 μm laser emission from hydrogen-filled hollow-core fiber by stimulated Raman scattering. A 6.5 m hydrogen-filled Ice-cream negative curvature hollow-core fiber is pumped with a high peak power, narrow linewidth, liner polarized subnanosecond pulsed 1064 nm microchip laser, generating pulsed 1908.5 nm vibrational Stokes wave. The linewidth of the pump laser and the vibrational Stokes wave is about 1 GHz and 2 GHz respectively. And the maximum Raman conversion quantum efficiency is about 48%. We also studied the pulse shapes of the pump laser and the vibrational Stokes wave. The polarization dependence of the vibrational and the rotational stimulated Raman scattering is also investigated. In addition, the beam profile of vibrational Stokes wave shows good quality, which may be taken advantage of in many applications.