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.
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.
In this paper, we propose a novel method of weak reflectivity measurement of FBGs by grating scale. We design and manufacture a series of scale gratings in single-mode fiber using excimer laser and phase mask. The weak reflectivity of measured grating could be achieved by comparing the peak reflection resonance with that of scale gratings. Experimental results show that the reflectivity measurement method based on grating scale is simple and quick, and it does not depend on the transmission spectrum of grating and could effectively avoids the influence of higher-order modes. In the future, by improving grating manufacture technology, it is expected that the reflectivity can be measured more accurately.
We measured the temperature of a single point using a uniform fiber Bragg grating (FBG) for the first time. A quasi- Gaussian temperature distribution pattern was achieved by heating at the midpoint of FBG, then the spectral characteristics, mainly including the central wavelength and amplitude of the resonance peaks were investigated in detail. Results show that the attenuation of each peaks accords well with the theoretical predictions, so the temperature of midpoint can be demodulated by the analysis of transmission spectra.
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.
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.
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.
We have demonstrated here, to the best of our knowledge, for the first time the suppression of stimulated Raman scattering (SRS) in a monolithic fiber laser oscillator using chirped and tilted fiber Bragg gratings (CTFBGs). We designed and inscribed CTFBGs in large-mode-area (LMA) fibers according to the operating wavelength of the fiber laser oscillator. A maximum suppression ratio nearly 19 dB or 23 dB is achieved CTFBG insert before the OC grating or after the HR grating. By reducing the insertion loss and improving the transmission spectrum of the CTFBG, a promotion in laser efficiency could be achieved. This work provides a novel idea for SRS suppression in a high-power all-fiber oscillator system, which is very useful for the output power increasing of fiber oscillators.
We have demonstrated here, to the best of our knowledge, for the first time the suppression of stimulated Raman scattering (SRS) in a monolithic fiber laser oscillator using chirped and tilted fiber Bragg gratings (CTFBGs). We designed and inscribed CTFBGs in large-mode-area (LMA) fibers according to the operating wavelength of the fiber laser oscillator. A maximum suppression ratio nearly 19 dB or 23 dB is achieved CTFBG insert before the OC grating or after the HR grating. By reducing the insertion loss and improving the transmission spectrum of the CTFBG, a promotion in laser efficiency could be achieved. This work provides a novel idea for SRS suppression in a high-power all-fiber oscillator system, which is very useful for the output power increasing of fiber oscillators.
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