A Yb/Ce/P (ytterbium/cerium/phosphorus) co-doped 20/400 µm fluoroaluminosilicate double clad fiber is fabricated by conventional modified chemical vapor deposition (MCVD) technology and solution doping process. The measurement shows the fiber core is doped with ∼0.21 mol% Yb2O3, ∼0.05 mol% Ce2O3, ∼0.83 mol% P2O5, ∼0.37 mol% SiF4 and ∼1.61 mol% Al2O3, respectively. Through co-doping a certain concentration of Ce2O3, P2O5, SiF4 and Al2O3 in the fiber core, it is found that the Yb/Ce/P co-doped 20/400 µm fluoroaluminosilicate fiber shows excellent photodarkening (PD) suppression, and PD loss is about 0 dB/m at 633 nm. Based on an all-fiber laser oscillator system and pumped directly by 915 nm laser diodes, a 1.9 kW laser at 1080 nm is achieved with slope efficiency is about 77.1% (pumped at 915 nm), and the beam factor of M2 is 1.32, which clearly reveals that the Yb/Ce/P co-doped fluoroaluminosilicate fiber is a promising candidate for laser applications with enhanced PD resistivity.
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Yb-doped silica-based glass is one of the important lasing materials and has significant applications in many fields, in particular for fiber lasers. With the development of active fiber technology and semiconductor pump source technology [1,2], the Yb-doped fiber lasers are widely used in material processing [3,4], such as laser marking, welding, cutting and cleaning. While they have become the choice of laser sources for many industrial applications, the photodarkening effect of Yb-doped fiber remains a major concern for long term reliability [2,5]. Specifically, PD will lead to the permanent reduction of fiber laser output power and heat generation associated with it may also decrease the mode instability threshold [6,7].
Mechanisms behind PD effects have been widely investigated for many years, and the PD-induced excess losses in the ultraviolet and visible wavelength regions are believed to be caused by formation of color centers [8,9]. However, the root causes of color center formation are still under debate [3,10]. A variety of techniques have been considered to mitigate the effect of PD, such as fiber composition design [3,5,8,11–16], structure design , gas loading , thermal bleaching [18,19] and photobleaching [10,20]. Among those methods above, fiber composition design is the most efficient approach to realize, which can be done easily during the preform fabrication process.
PD has been greatly reduced with the addition of co-dopants (Al [3,11,12], P [11,13–15], Ce [5,16], etc.) in the core composition. Al doping can increase the solubility of rare earth ions and effectively suppress PD , but excessive Al ions will form Al-OHC related color centers , which will lead to PD. Rare earth materials have good solubility in phosphate glass , the dissolution efficiency of P to Yb3+ clusters is higher than that of Al , which mitigates the light-induced darkening effect. At the same time, P can combine with Al to form AlPO4 and decrease the refractive index of optical fiber . Ce ions doping can effectively suppress the PD effect, because Ce ions have two kinds of ionic states: positive tetravalent (Ce4+) and trivalent (Ce3+), so they have functions of trapping color center related holes [5,16]. In the gain fiber, fluorine is widely used as dopant to reduce the numerical aperture of the fiber core. However, most of the researches focus on the single doping of Al, P and Ce, or the co-doping of two materials in the silica glass with Yb, to modify the PD performance. Few papers have reported that Ce, P and Al co-doping are combined to prepare Yb/Ce/P co-doped fluoroaluminosilicate fiber, study its PD and laser performance.
In this paper, Yb/Ce/P co-doped 20/400 µm fluoroaluminosilicate double clad fiber is prepared by MCVD technology in combination with solution doping process. As comparison, the Yb-doped and Yb/Ce co-doped 20/400 µm fluoroaluminosilicate double clad fibers are also prepared by the same method. In addition, the slope efficiency and power stability of Yb/Ce/P co-doped 20/400 µm fluoroaluminosilicate are tested by a 1.9 kW all-fiber oscillator. This work may provide a new composition design into the preparation of Yb-doped double cladding fiber with excellent PD suppression.
2. Experimental details
2.1 Preform and fiber fabrication
The preparation process of Yb/Ce/P co-doped 20/400 µm fluoroaluminosilicate fiber is almost in agreement with that reported in our previous work . Firstly, the core preform is fabricated by MCVD technique in combination with conventional solution doping technique, which includes the following steps: highly silica soot layer deposition, in order to reduce the refractive index, SiF4 is also added in the deposition process, then soaking in a Ce/Yb/Al-containing alcoholic solution, drying, sintering, and collapsing into a solid rod. Subsequently, the core preform is jacketed with suitable tube and then processed to an octagonal shape with a 20/400 core-to-clad ratio. Finally, the shaped preform is drawn into a Yb/Ce/P co-doped 20/400 µm fluoroaluminosilicate fiber by a drawing tower and coated with a low index polymer which provides a pump NA of 0.46. As comparison, the Yb-doped and Yb/Ce co-doped 20/400 µm fluoroaluminosilicate double clad fibers are also prepared by the same process.
2.2 Photodarkening measurement setup
The PD measurement configuration is shown in Fig. 1, which provides a worst-case test of PD in Yb-doped double-clad fibers (YDF) as proposed by J. J. Koponen et al [23–25]. In order to detect the loss caused by the PD more easily, 633 nm is selected as a probe since the PD induced loss can be easily detected, and the report indicates that the optical loss at this wavelength is 71 times  than that of the signal light and it can reflect the results of PD more quickly. The PD measurement uses a 976 nm laser diode (LD) as the pump source. Both 976 nm and 633 nm LD’s are launched into the fiber core through a custom wavelength division multiplexer (WDM). The output pigtail of WDM is a single mode fiber (Corning HI1060), which is fused with the sample Yb-doped fiber through a mode field adapter (MFA). The other side of the sample YDF is also fused with the output pigtail of WDM through a mode field adapter. A bandpass filter is inserted before the power meter to suppress unwanted pump wavelength, and avoid the test result error. In this article, to accelerate the PD, the inversion of fiber is saturated at approximately 50%. Through the simulation of inversion of Yb-doped fiber, the 976 nm pump power of 20/400 µm fiber is 1.1 W and the length of sample fiber is about 50 mm. To avoid the effect of temperature on the PD test, the PD measurement system and pump lasers diodes are cooled by a water chiller and the temperature is kept at 23°C.
2.3 Laser performance testing
Laser performances of the Yb/Ce/P co-doped 20/400 µm fluoroaluminosilicate fiber are tested by an all-fiber oscillator configuration, as shown in Fig. 2. In brief, the laser oscillator configuration consists of the fiber Bragg gratings (FBGs) and large-mode-area (LMA) gain fiber. The high-reflection (HR) FBG and an output coupler (OC) FBG are made of 20/400 µm passive fibers with the central wavelength of 1080 nm. The reflectivity of HR FBG is about 99.5% and about 10% for the OC FBG. Yb/Ce/P co-doped 20/400 µm fluoroaluminosilicate fiber is fused between HR and OC FBGs. The gain fiber length of Yb/Ce/P co-doped 20/400 µm fluoroaluminosilicate fiber is 36 m and reaches the total cladding pumping absorption 18 dB at 915 nm. Via a (6 + 1) x1 fiber coupler, the fiber laser cavity is pumped by ten 250 W pump lasers diodes (5 pumps at each end) with central wavelength of 915 nm. The pump delivery fibers of semiconductor lasers are 200/220 µm with NA 0.22 which are fused with the signal fiber of a (6 + 1) x1 fiber coupler directly. A home-made cladding power stripper (CPS) is spliced to remove leaked signal light and unabsorbed pump light, ensuring the accuracy of slope efficiency calculation. The fiber laser beam is collimated and output by a quartz block holder (QBH), and detected by a power meter at the end. In addition, it is split by beam splitters and attenuated to mW-lever for spectrum and beam quality analysis. The splice spots of the fiber components take advantage of low fuse-loss as 0.15 dB and are coated with a low index polymer which provides a pump NA of 0.46 in the all-fiber oscillator system. The laser slope efficiency, signal laser spectrum, beam quality and output power stability are also tested for further study.
3. Experimental results and discussion
3.1 Fiber characteristics
The cross-section of the Yb/Ce/P co-doped 20/400 µm fluoroaluminosilicate fiber is shown in Fig. 3(a). It has a core of 20.3 µm and a cladding of 400.8 µm (flat to flat) in diameter. For further study and analysis, the measured refractive index profile of the Yb/Ce/P co-doped 20/400 µm fluoroaluminosilicate fiber is given in Fig. 3(b) and is measured by optical interferometry. The effective NA of the Yb-doped fiber core relative to the silica glass cladding is calculated to be ∼0.064, corresponding to a refractive index difference between the core and silica glass cladding ∼0.0014. The apparent central dip of the refractive index is due to dopant evaporation during the high temperature collapsing process [1,26].
To further investigate the elemental distribution, the concerntration distribution of Yb2O3, Ce2O3, P2O5, Al2O3 and SiF4 across the core region of Yb/Ce/P co-doped 20/400 µm fluoroaluminosilicate fiber is characterized by electron probe microanalysis (EPMA) and presented in Fig. 4. The core dopant concentration of Yb2O3, Ce2O3, P2O5, Al2O3 and SiF4 is estimated to be 0.21 mol%, 0.05 mol%, 0.83 mol%, 1.61 mol% and 0.37 mol%, respectively. In addition, the molar ratio of Ce2O3/Yb2O3, Yb2O3/Al2O3, and P2O5/Al2O3 is calculated as 1:4, 1:7.7 and 1:2, respectively. When P2O5/Al2O3 is less than 1, we use the follow formula (1) [1,27] to calculate the refractive index difference Δn of Yb/Ce/P co-doped 20/400 fluoroaluminosilicate fiber:
The pump absorption spectrum of Yb/Ce/P co-doped 20/400 µm fluoroaluminosilicate fiber is tested with the cut-back method by a broadband source light injection, as shown in Fig. 5(a). The cladding pump absorption of 915 and 976 nm is 0.50 and 1.92 dB/m, respectively. Compared with the conventional commercial 20/400 µm fiber (Fig. 5(a)), the cladding pump absorption is increased by nearly 25% at 915 nm, which is mainly due to the increase of Yb-doped concentration. However, the increase of Yb-doped concentration will cause a significant increase in the risk of PD [12,28] of Yb-doped 20/400 µm fiber, which limits the industrial use of the Yb-doped fiber. The core background attenuation spectrum of Yb/Ce/P co-doped 20/400 fluoroaluminosilicate fiber is tested by model 2500 fiber analysis system (Photon Kinetics, Inc.) with the cut-off method, as shown in Fig. 5(b). The core background loss of Yb/Ce/P co-doped 20/400 µm fluoroaluminosilicate fiber at 1200 and 1300 nm is 11.8 and 4.8 dB/km, respectively. From the test results of 1300 nm, the prepared fiber shows low core background attenuation. In addition, the background loss of the fiber core at 1380 nm is 5.8 dB/km, which indicates good drying control during the fiber preparation process. The quite low values of core background attenuation shows that co-doping process does not bring negative loss to Yb-doped double clad fiber and which is beneficial in high power fiber lasers for the low thermal burden and high efficiency.
3.2 PD results and discussion
The PD measurement of Yb-doped, Yb/Ce co-doped and Yb/Ce/P co-doped 20/400 µm fluoroaluminosilicate fiber is tested by the PD measurement configuration as shown in Fig. 1, and carried out under the same test conditions. After 7200 s of PD test, the results are shown in Fig. 6, Yb-doped 20/400 µm fluoroaluminosilicate fiber and Yb/Ce co-doped 20/400 µm fluoroaluminosilicate fiber reflects PD loss attenuation coefficients α1 and α2 of 71.3 dB/m and 18.5 dB/m, respectively. It is interesting to note that Yb/Ce/P co-doped 20/400 µm fluoroaluminosilicate fiber shows excellent PD suppression results. There is essentially no detectable PD indued loss at 633 nm under the current test conditions. The results show Yb/Ce/P co-doped fiber composition is more suitable for the suppression of PD.
Compared with the Yb-doped 20/400 µm fluoroaluminosilicate fiber (α1=71.3 dB/m), the Yb/Ce co-doped 20/400 µm fluoroaluminosilicate fiber has an obvious PD suppression performance (α2=18.5 dB/m), it is because there are two valence states in Ce ion which can effectively suppress the hole related color center, so Ce co-doping improves the PD suppression effect. Compared with Yb/Ce co-doped 20/400 µm fluoroaluminosilicate fiber (α2=18.5 dB/m), the Yb/Ce/P co-doped 20/400 µm fluoroaluminosilicate fiber has a further PD suppression performance (α3=0 dB/m), phosphorus has more excellent cluster solubility, which improves the performance of PD effectively. Yb/Ce/P co-doped 20/400 µm fiber shows the result of excellent suppression of PD (α3=0 dB/m), reflecting the combination of Al, P and Ce to prepare Yb/Ce/P co-doped 20/400 µm fluoroaluminosilicate fiber, the composite system gives full play to its own PD mitigation effect.
The very low solubility of rare earth element in pure silica glass has been widely reported, most researchers agree that the PD is dependent on the occurrence and dimensions of Yb3+ clusters [13,15,19]. Therefore, in order to increase the solubility of rare earth in the optical fiber as much as possible, aluminum added to silica glass has been proved to be able to improve the dissolution of Yb3+ clusters. Compared with aluminum, phosphorus doping shows better solubility of Yb3+ clusters. P2O5 belongs to the oxide of glass forming body, and [PO4] is the basic unit of phosphate glass . The doping of P2O5 can reduce the melting temperature and Rayleigh scattering of the glass, allowing good transmission in the UV region . P2O5 doping is helpful to the nucleation and growth of nanocrystals during annealing process . In addition to promoting the dissolution of Yb3+ clusters, phosphorus combines with aluminum to form AlPO4 , reducing the refractive index of fiber core. What's more, the state of Al-O-P chemical bond increases the distance between ytterbium ions, which effectively prevents the cluster effect and greatly improves the luminous efficiency of ytterbium ions . At present, the research shows that Ce-doped can obviously improve the radiation resistance of glass [30,33], a generally accepted explanation is that there are Ce3+/Ce4+ redox pairs in Ce-doped glass, which can capture the color centers [16,31]. This conclusion can also be used to explain the photodarkening suppress of Ce-doped gain fiber. A small amount of Ce co-doping in the fiber can effectively suppress the hole related color center, and the prepared Yb/Ce/P co-doped 20/400 µm fluoroaluminosilicate fiber has excellent PD suppression performance through the coordination of each component and mutual compensation.
3.3 Laser performance
During the all laser performance experiments, the fiber laser system and pump lasers diodes are cooled by a water chiller and the temperature is kept at 23°C. The laser performance of Yb/Ce/P co-doped 20/400 µm fluoroaluminosilicate fiber is tested by the all-fiber laser oscillator system as shown in Fig. 2. An output power of 1.9 kW is obtained when the pump power of 2.5 kW is launched into the oscillator system, which corresponded to linearly-fitted slope efficiency is 77.1%, as shown in Fig. 7(a). The high laser efficiencies also indicate that the higher optical-optical conversion efficiency, the lower fiber background loss and fusion loss. The laser output spectrum is analyzed by an optical spectrum analyzer (OSA) at full power, the characteristic peak of output laser is centered on 1080 nm, as shown in Fig. 7(b). It is obvious that characteristic peak of stray light and parasitic lasing is invisible, which shows a good nonlinear control. The beam quality (M2) of the Yb/Ce/P co-doped 20/400 µm fluoroaluminosilicate fiber is evaluated at maximum laser output power by an Ophir’s BeamSquared system, which is depicted in Fig. 7(c), the results show M2 values along X and Y directions are 1.32 and 1.32, respectively. The tested results show that a near single mode laser output is obtained with the Yb/Ce/P co-doped 20/400 µm fluoroaluminosilicate fiber. The excellent beam quality is attributed to the low numerical aperture control of the Yb/Ce/P co-doped 20/400 µm fluoroaluminosilicate fiber, and design of the bending filter mode in the laser structure.
PD will lead serious performance degradation and lifetime limitation for rare earth co-doped silica fiber devices and its nature has been extensively studied in literature . Effective mitigate and measurement of PD effect have also been widely reported. In addition to the PD accelerated aging test method used earlier in this paper, a commonly used method in industry is to test rare earth co-doped silica fiber for a long-time continuous working, and observe the long-time working stability of the signal light [4,34]. In order to further verify the PD performance, the Yb/Ce/P co-doped 20/400 µm fluoroaluminosilicate fiber prepared in this paper has been tested with the laser setup as shown in Fig. 2, the laser setup is kept for more than 200 hours and the output power presents a relative small power fluctuation from 1893 W to 1910 W (Power variation = 0.9%), the power fluctuation during laser operation is caused by the minimal variation of working conditions such as water cycle refrigeration, and the results are as shown in Fig. 8. After 200 hours of full power test, the laser power of the optical fiber signal has not been significantly reduced, which shows that the Yb/Ce/P co-doped 20/400 µm fluoroaluminosilicate fiber prepared in this paper has an excellent PD suppression effect, and can meet the long-term use of industrial fiber laser.
In this work, we prepare a Yb/Ce/P co-doped 20/400 µm fluoroaluminosilicate fiber by conventional MCVD method combined with solution doping process. It is found that co-doped with a certain amount of Ce, P and Al has no evidently negative effects on the core background attenuation of the Yb doped double clad fiber. Compared with Yb-doped and Yb/Ce co-doped 20/400 µm fluoroaluminosilicate fiber, the Yb/Ce/P co-doped 20/400 µm fluoroaluminosilicate fiber shows excellent PD suppression performance. After 7200 s PD accelerated aging test, there is no obvious loss caused by PD at 633 nm in Yb/Ce/P co-doped 20/400 µm fluoroaluminosilicate fiber. The reason for good suppression of PD is the coordination and complementarily of Al, P and Ce. With an all-fiber oscillator configuration and pumped at 915 nm, 1.9 kW laser output power is achieved with a slope efficiency of 77.1% at 1080 nm, and the beam factor of M2 is 1.32. The Yb/Ce/P co-doped 20/400 µm fluoroaluminosilicate fiber-based oscillator system is kept at 1.9 kW output over 200 hours, and no sign of power degradation is obtained. It implies excellent laser stability and low PD effect. The results demonstrate that Yb/Ce/P co-doped core composition design can meet the requirements of low PD fiber for commercial high power fiber lasers.
National Key R&D Program of China (2016YFB0402200, 2017YFB1104400).
The authors declare that there are no conflicts of interest related to this article.
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