In this paper, we have experimentally demonstrated a 4.45 kW master oscillator power amplification (MOPA) narrow-linewidth fiber laser based on fiber Bragg grating (FBG) with near-diffraction limited beam quality. By optimizing the structure of narrow linewidth fiber oscillator seed, the temporal characteristics of injected seed laser is improved. Combined with optimizing pumping ratio of amplifier stage, multiple nonlinear effects are mitigated. Finally, a 4.45 kW narrow linewidth laser output with near-diffraction limited beam quality is achieved with a slope efficiency of 80.2%. The signal to noise ratio is 24.5 dB at the maximum power. The 3 dB and 20 dB bandwidth are 0.5 nm and 3.63 nm, respectively.
With the development of high power fiber oscillators, it is urgent to fabricate fiber Bragg gratings (FBGs) on large core fibers. Here, a pair of FBGs in fibers with core diameter of 30 μm are fabricated based on femtosecond laser phase mask scanning method. The reflectivity of high-reflectivity FBG (HR-FBG) is more than 99% with the central wavelength of 1080 nm and the 3dB bandwidth of 3.6 nm. The reflectivity of output coupler FBG (OC-FBG) is about 10% with the central wavelength of 1079.8 nm and the 3dB bandwidth of 2 nm. Furthermore, an all-fiber oscillator is built based on the fs-written FBGs, and the maximum output power of 6.4 kW is realized with the optical-optical conversion efficiency of 74%. The temperatures of HR-FBG and LR-FBG are 90 and 49℃, respectively. This work demonstrates that the large core diameter FBG written by femtosecond laser has excellent performance, which is of great significance for the development of high power fiber oscillators.
KEYWORDS: Fiber lasers, Optical amplifiers, Signal to noise ratio, Fiber amplifiers, Oscillators, Laser systems engineering, High power fiber amplifiers, Cladding, Composites, Reflectivity
One-stage master oscillator power amplifier (MOPA) is an efficient way to acquire high-power narrow-linewidth fiber lasers (NLFLs), which have wide usage in beam combination and detection. In this paper, we set up a narrow-linewidth one-stage MOPA fiber laser system which utilize a compact fiber oscillator as the seed laser. By optimizing the temporal characteristics of seed laser with a backward pumping structure of amplification stage, we finally acquire a 3.5 kW near single mode laser output with 3 dB and 20 dB linewidth of 0.26 nm and 1.1 nm, respectively. The slope efficiency reaches to 83.4%. At the maximum power, the signal to noise ratio ratio is ~60 dB on the spectrum and the M2 factor is measured about 1.3.
The average output power of fiber laser oscillators can stabilize in very high levels reaching kW within the recent years. However, a further scaling is limited due to some nonlinear effects, one of which is simulated Raman scattering (SRS). Here we propose a CO2 laser pulse inscribed transmission long period fiber grating (LPFG) in large mode area (LMA) fiber and employ it within a fiber laser oscillator to mitigate SRS. The wavelength of LPFG is designed to match the peak of SRS gain spectrum. By inserting the LPFG before the output coupling FBG (OC-FBG) of the oscillator, an efficient mitigation of SRS is observed. LPFGs represent a highly cost-effective fiber structure with potential for SRS suppession in high power fiber laser systems.
The one-stage master oscillator power amplifier (MOPA) is an efficient way to acquire high-power narrow-linewidth fiber lasers (NLFLs), which have wide usage in beam combination and detection. However, stimulated Raman scattering (SRS) is still one of the main obstacles for further power scaling. In this paper, we set up a counter-pumping kW-level narrowlinewidth one-stage MOPA fiber laser system and tried to use a chirped and tilted fiber Bragg grating (CTFBG) as a broadband rejection filter to suppress the Raman stokes light in the output laser. We finally acquired a 2.5 kW output with 3 dB linewidth of about 0.87 nm, and the SRS isolation ratio is ~42 dB on the spectrum, which is 12.7 dB higher compared with the situation without CTFBG.
Chirped and tilted fiber Bragg gratings (CTFBGs) have attracted a lot of attention for stimulated Raman scattering (SRS) suppression in high-power fiber laser systems in past years. Here, we fabricate CTFBGs in large-mode-area double cladding fibers and demonstrate its application in a 2-kW fiber laser for SRS filtering at the system’s output. A maximum Raman suppression of about 17 dB is obtained at the central wavelength, and the insertion loss is smaller than 0.2 dB for signal laser. In addition, no obvious degradation of laser beam quality is observed. Better suppression effect could be achieved by broadening the rejection bandwidth of the CTFBGs in the future.
We demonstrate here a method for the online temperature measurement of the output-coupling fiber Bragg grating (FBG) in a high power fiber oscillator by a superimposed FBG, which is used as the output-coupling FBG and the temperature monitor simultaneously. The experimental results verify the feasibility of this method.
High power fiber laser is of importance for a wide range of scientific and industrial processes but the transmission distance is till restricted because of stimulated Raman scattering (SRS). We research here on the mitigation of the SRS in highpower fiber laser systems by long period fiber gratings (LPFGs) for longer laser delivery distance. A broadband and high attenuation LPFG is carefully designed and fabricated by a CO2 based inscription system. It has been proven effective in extending delivery distance due to its filtering effect of Raman signal.
Stimulated Brillouin scattering (SBS) is one of the mainly factors those limit the output power in narrow-linewidth highpower fiber laser systems. Here, we propose and demonstrate a novel method for the suppression of SBS in optical fibers using a tilted fiber Bragg grating (TFBG). With a TFBG being inserted between a single-mode fiber (SMF) amplifier and a 150-meter-length single-mode energy-transmitting fiber, not only the backward Stokes wave is rejected, but also an obvious increasing of the SBS threshold is observed with a value of 1.2 times that without the TFBG, which increases the effective laser output power by about 18 %. This work provides a new idea for SBS suppression in fiber. It is very useful for the further power scaling of high-power narrow bandwidth all-fiber lasers.
In this paper, we propose and demonstrate a novel method for the suppression of SBS in optical fibers using a tilted fiber Bragg grating (TFBG). We designed and fabricated a matched TFBG by using an excimer laser and phase mask in single mode fibers according to the operating wavelength of the fiber laser and the tiny frequency shift of SBS. Experimental results demonstrate that TFBGs can suppress the backward-propagating Stokes light caused by SBS to protect the whole system and improve the stable output power, which is very useful for power scaling of high-power narrow-bandwidth all-fiber laser in the future.
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