We report 3.1-3.2 μm mid-infrared emission from acetylene-filled low loss antiresonant hollow-core fiber pumped with an amplified, modulated, narrowband, tunable 1.5 μm diode laser. The maximum power conversion efficiency of ~30%, with respect to the absorbed pump power, is obtained with a 10.5 m length of fiber at 0.7 mbar. The maximum efficiency with respect to the total incident pump power (~20%) and the minimum pump laser energy required (<50 nJ) are both improved compared to similar work reported previously using an optical parametric oscillator as a pump source. This paper provides an effective route to obtain compact mid-infrared fiber lasers.

The high-power narrow-linewidth fiber laser has become the most widely used high-power laser source nowadays. Further breakthroughs of the output power depend on comprehensive optimization of stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS) and transverse mode instability (TMI). In this work, we aim to further surpass the power record of all-fiberized and narrow-linewidth fiber amplifiers with near-diffraction-limited (NDL) beam quality. SBS is suppressed by white-noise-signal modulation of a single-frequency seed. In particular, the refractive index of the large-mode-area active fiber in the main amplifier is controlled and fabricated, which could simultaneously increase the effective mode field area of the fundamental mode and the loss coefficient of higher-order modes for balancing SRS and TMI. Subsequent experimental measurements demonstrate a 7.03 kW narrow-linewidth fiber laser with a signal-to-noise ratio of 31.4 dB and beam quality factors of M_x² = 1.26, M_y² = 1.25. To the best of our knowledge, this is the highest reported power with NDL beam quality based on a directly laser-diode-pumped and all-fiberized format, especially with narrow-linewidth spectral emission.

Rare-earth-doped fiber lasers at 3–5 μm can provide a variety of applications in defense, security, medicine, and so on. Limited by the maximum phonon energy of silicate glass host at room temperature, these lasers are normally based on soft-glass materials, e.g., fluoride or chalcogenide glass. However, due to the limited transparency of these fibers, until now these systems have only achieved coverage up to 3.92 μm. Here, we report a CW fiber laser operating well beyond 4 μm with significant power based on ${\mathrm{CO}}_2$-filled silica hollow-core fibers. By pumping via a homemade 2 μm laser diode, $\sim 82\mathrm{mW}$optical power at 4.3 μm was achieved at room temperature with a maximum laser efficiency $\sim 19.3\%$. Our demonstration represents the longest-wavelength CW fiber laser to date, paving the way towards compact and high-power mid-infrared fiber lasers beyond the 4 μm wavelength.

The average power of fiber lasers has been scaled deeply into the kW regime in the past years. However, stimulated Raman scattering (SRS) is still a major factor limiting further power scaling. Here, we have demonstrated for the first time, to the best of our knowledge, the suppression of SRS in a half 10 kW tandem pumping fiber amplifier using chirped and tilted fiber Bragg gratings (CTFBGs). With specially self-designed and manufactured CTFBGs inserted between the seed laser and the amplifier stage, a maximum SRS suppression ratio of $>15\mathrm{dB}$ in spectrum is observed with no reduction in laser efficiency. With one CTFBG, the effective output power is improved to 3.9 kW with a beam quality $M^2$ factor of $\sim 1.7$ from $<3.5\mathrm{kW}$ with an $M^2$ factor of $>2$; with two CTFBGs, the effective laser power reaches 4.2 kW with an increasing ratio of 20% and an $M^2$ factor of $\sim 1.8$, and further power improvement is limited by the power and performance of the 1018 nm pump sources. This work provides an effective SRS suppression method for high-power all-fiber lasers, which is useful for further power scaling of these systems.

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.

We have demonstrated for the first time, to the best of our knowledge, a novel and effective method to produce a 1.5 μm fiber source by means of Raman wavelength conversion in a gas-filled hollow core fiber. An ethane-filled, anti-resonance, hollow core fiber is pumped with a high peak power pulsed 1064 nm laser, generating a 1552.7 nm Stokes wave by pure vibrational stimulated Raman scattering of ethane. A maximum peak power of about 400 kW is achieved with a 6 m fiber length at 2 bars of pressure. The maximum Raman conversion efficiency is about 38%, and the corresponding laser slope efficiency is about 61.5%. The linewidth of the Stokes wave is 6.3 GHz. If a tunable pump laser is used, this kind of fiber source can easily achieve a broad tuning range near 1.5 μm.