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.
We report here a monolithic all-fiber laser oscillator with femtosecond-written FBGs and side-pumping coupler. The splice-free resonant cavity is realized by directly writing a pair of fiber Bragg gratings (FGBs) into an ytterbium-doped fiber (YDF) based on the femtosecond laser phase mask scan method. Furthermore, a (2+1)×1 side-pumping coupler is fabricated in the same YDF by the taper-fused method. When pumping with two 976 nm laser diodes (LD), a maximum output power of 1218 W is realized at the 1070 nm, corresponding to the optical-optical conversion efficiency of about 71%. This work displays an extremely compact and stable fiber laser oscillator, which is of great significance to the development of high power fiber lasers.
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.
In this paper, the quantitative relationship between the seed Raman suppression and output of Yb-doped fiber amplifier is studied theoretically. The analytical formula showing the relationship between the Raman suppression (in dB) in the seed light and output of the fiber amplifier is derived. It is revealed that Raman suppression of the fiber amplifier increases linearly with that of the seed light. It is found that such a relationship can be well kept regardless of the pumping scheme. By the comparison of analytic results and numerical calculation results, it is revealed that there is indeed a linear relationship between two Raman suppressions with the slope agreeing well with the theoretical prediction. However, such a relationship only occurs when the Raman noise power of the seed light is greater than the microwatts level. Once it is less than the microwatts level, the output Raman suppression of the fiber amplifier will not raise with a further increase of seed Raman suppression. It is also found that the law is independent of the output signal power of the seed light as well as the pumping scheme of the fiber amplifier, which is also coincident with the theoretical result.
In this paper, we design and fabricate a series of single-mode scale gratings based Fabry Perot (FP) by using excimer laser and chirped phase mask, then use it to measure the reflectivity of weak reflection gratings on large-mode-area doubleclad (LMA-DC) fibers. Experimental results show that the Bragg resonance reflectivity of the LMA-DC weak reflection grating is between 1.19% and 1.49%. The method of measuring weak reflection grating reflectivity based on scale grating is convenient, efficient, and the accuracy is greatly improved.
In this paper, we used a femtosecond laser direct writing system to fabricate all-fiber structure Fabry-Perot (FP) cavities based on fiber Bragg gratings (FBGs) on single-mode fibers, which was used as a scale for weak reflectivity measurement of output-coupling FBGs (OC-FBGs). By this method, the intensity of the Bragg resonance reflectivity peak of OC-FBG can be measured to be as low as ~1%. Compared with the traditional method based on the transmission spectrum, this method is much higher measurement accuracy.
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.
In this paper, we fabricate FBGs-based FPCs using femtosecond laser and the temperature characteristics is studied. The FBGs are inscribed by line-by-line scanning technique, by which the FBGs can be limited to a specific area in the fiber core region. Besides, the grating length, the grating position and the distance between two successive FBGs can be precisely controlled to adjust bandwidth and free spectral range. FP interference is obviously seen in reflection and transmission spectrum. Line-by-line inscribed FPC is a good candidate for sensing application.
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.
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.
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