Directly inscribing fiber Bragg gratings (FBGs) in ytterbium-doped fiber (YDF) using fs-laser can reduce the melting point in fiber lasers, which is significant for developing efficient fiber laser systems. In this paper, the photoluminescence at 1 µm band excited by the fs-laser in YDF was first reported to assist the FBG inscription, to the best of our knowledge. Moreover, we demonstrate that when the fs-laser scans across the fiber, the unique inner-cladding structure of YDF has a non-negligible effect on the focus position and intensity, thereby presenting a challenge to achieve high reproducibility when inscribing FBGs. The refractive index modulation characteristics of fs-laser incident from different angles are also analyzed.
The short-wave infrared band correlated light source, especially near the 2.3 μm band, has a wide application prospect in the fields of optical metrology of combustion process and non-invasive blood glucose measurement. Especially, the 2.3 μm narrow linewidth source has unique advantages in the monitoring of polluted gas and as the pumping source of gas laser. Tm doped fiber laser is an effective method to realize narrow linewidth output in 2.3 μm band. At present, however, the output power of the 2.3 μm band narrow linewidth Tm fiber laser is only in the order of one hundred milliwatts. Here, we demonstrated the first watts-level single-frequency Tm3+-doped fiber laser operating at 2.3 μm.
The mid-infrared fiber lasers operating in the 3-5 μm have attracted great interest due to wide applications. However, the soft glass fiber-based rare-earth-doped laser in the mid-infrared band has limitations in power enhancement and wavelength expansion. Gas-filled hollow-core fiber lasers provide a new way to realize mid-infrared output. Here, we demonstrate an optically pumped CO-filled hollow-core fiber laser. The pump source was a homemade Tm fiber laser operating at 2.33 μm. By tuning the wavelength of the pump source, mid-IR emission from 4.64 to 4.82 μm has been achieved.
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.
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.
Mid-infrared fiber lasers have shown out standing prospects in many fields, so mid-infrared fiber Bragg grating (FBG) as the most essential component in mid-infrared fiber laser oscillators has attracted much attention. Here, we fabricated mid-infrared FBGs on ZBLAN fibers operating nearby 2.8-2.9 μm by femtosecond laser direct inscription technique, and measured their transmission spectra by all-fiber measurement system, which lays the foundation of high-power all-fiber mid-infrared laser oscillators near 2.8 μm.
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.
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.
The average power of diode-pumped fiber lasers have developed deep into the kW regime in the past years. However, stimulated Raman scattering (SRS) is still a major factor limiting the further power scaling. To date, many methods for SRS suppression have been proposed in fiber systems, such as the application of large-mode-area (LMA) fibers or enlarging the fiber mode area, spectrally selective fibers, or lumped spectral filters like long-period gratings (LPGs). The enlarging of fiber mode area must be combined with controlling numerical aperture (NA) for the operation of fundamental mode. Otherwise it will leads to a decreased transverse modal instability (TMI) threshold in fiber lasers, which also limits further power scaling. It is quite difficult to realize by today’s material and manufacturing technologies of fibers. The designing of spectrally selective fibers is usually very complex. Besides, it is also not easy to manufacture such fibers and it is still limited by the maximum fiber core size that can be employed. The working principle of lumped filters is similar to that of spectrally selective fibers, but it is much easier to design and fabricate such filters. LPGs have good filtering properties, but the filtering characters of LPGs is instable for their high sensitivities to the environment variable such as temperature, strain or humidity. Here, we have demonstrated the mitigation of SRS in kilowatt-level diode-pumped fiber amplifiers using a chirped and tilted fiber Bragg grating (CTFBG) for the first time. The CTFBG is designed and inscribed in LMA fibers, matching with the operating wavelength of the fiber amplifier. With the CTFBG being inserted between the seed laser and the amplifier stage, a SRS suppression ratio of ~10 dB is achieved in spectrum at the maximum output laser power of 2.35 kW, and there are no reduce in laser slope efficiency and degradation in beam quality. This work proves the feasibility and practicability of CTFBGs for SRS suppression in high-power fiber lasers, which is very useful for the further power scaling.
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