Absrtact: in the application of high-power fiber lasers and optical amplifiers, in order to effectively couple the pump energy into the active fiber, the cladding size and numerical aperture of the fiber must be large. Moreover, in order to reduce the nonlinear effect, it is also required that the concentration of rare earth ions should be high and the numerical aperture should be small in a relatively large core. In military and industrial laser applications, in order to obtain power output of more than 100 kW, people also perform coherent combination on the output of multiple lasers / amplifiers. At this time, polarization maintaining double clad fiber (pm-dcf) is needed. This paper will focus on the progress made in the development of pm-dcf. We also report a pm-dcf with panda polarization distribution, whose ytterbium doped core has a value aperture of 0.06 and a diameter of 30um. We will also discuss several key factors that must be considered when designing pm-dcf.

Key words: ytterbium doped fiber, polarization maintaining, polarization maintaining double clad fiber, large mode field area, laser fiber, amplifier fiber

1. general

Ytterbium doped fiber has very high output power and excellent conversion efficiency in a wide wavelength range (from 975 nm to 1200 nm). Different from erbium-doped fiber, excited state absorption and concentration quenching can be avoided in ytterbium doped fiber laser and optical amplifier. These characteristics of ytterbium doped fiber, coupled with the emergence of double cladding technology, make the industry begin to have a strong interest in high-power fiber lasers and optical amplifiers to meet different applications. Ytterbium doped double clad fiber is attracting more and more attention. Its existing and potential applications include military, aviation, material processing, printing and marking, spectral analysis and telecommunications.

For many applications of high power fiber lasers and optical amplifiers, it is necessary to work in a stable linear polarization state. The structure of high power optical amplifier (or laser) is based on coherent beam combining of the outputs of several fiber amplifiers. With the increasing demand for military and industrial applications with output power exceeding 100kW (continuous), the demand for polarization maintaining double clad fiber is also on the rise. It has been reported that non polarization maintaining fiber can be used to realize polarization maintaining. However, these schemes have limitations. The preferred scheme is the polarization maintaining double clad fiber. Although passive polarization maintaining fiber has been commercialized for many years, active polarization maintaining fiber has not appeared until recently. Kliner et al. Reported that a polarization maintaining Yb doped double clad fiber amplifier was fabricated by using a bow tie polarization maintaining fiber. Although the bow tie polarization maintaining double clad fiber has its value for theoretical research or experimental application, there are still a lot of insurmountable problems in the producibility, consistency and future performance upgrading of the optical fiber preform.

Single mode ytterbium doped double clad fiber is very suitable for manufacturing compact lasers with diffraction limited output beam quality. However, the further improvement of output power is limited by amplified spontaneous emission and nonlinear effects, such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS). These limitations can be overcome by using single-mode fibers with large mode field area (LMA) and low numerical aperture. The low numerical aperture of the core can suppress spontaneous emission, and the large mode field area can effectively improve the threshold of SRS and SBS. Another method is that researchers use multi-mode fiber doped with rare earth elements and combine the following technical means to suppress the generation of high-order modes: the fiber adopts a specific winding mode; Secondly, the injection conditions of seed beam are optimized; Thirdly, the optical fiber is designed with specific refractive index distribution and doping distribution; Fourth, a specific cavity design is adopted. By using multi-mode fiber, combined with specific technical means, the single-mode operation of the fiber can be realized, and the performance similar to that of large mode field area fiber can be achieved.

In the long run, in order to meet the application of high-power lasers and optical amplifiers with continuous output up to 100kW, it is necessary to develop polarization maintaining, low numerical aperture, large-size fiber core. In addition, the structural design of optical amplifiers may involve the coherent combination of dozens or hundreds of optical amplifiers. Therefore, in the process of selecting prefabricated parts and optical fiber manufacturing technology, we must fully consider the scalability of mass production capacity of prefabricated parts and optical fibers and the production consistency of these products. This article will discuss how to determine the appropriate technology. We will further discuss the design considerations and the technical progress in developing ytterbium doped panda pm-dcf with large core size and low numerical aperture. We also report a panda pm-dcf with a numerical aperture of 0.06, a ytterbium doped core of 30um and an inner cladding diameter of 0.37400um.

2. design, fabrication and analysis

2.1 bow tie polarization maintaining double clad fiber

As a part of this research work, we have evaluated the process of making bow tie polarization maintaining double clad fiber using MCVD. Figure 1 (a) shows the steps of making bow tie polarization maintaining fiber. The substrate is a synthetic quartz tube of high quality. During its rotation, several layers of borosilicate glass are deposited on its inner wall. Next, the substrate stops rotating. When a ribbon burner is used to sinter a certain part of the substrate, the boron in the glass will volatilize from the selected part of the deposited layer. Next, the substrate is rotated 180 degrees and the same operation is performed. In this process, it must be ensured that the two parts producing boron volatilization are symmetrical in center and the same in size. Before deposition of ytterbium doped fiber core, several layers of glass need to be deposited on the inner wall. These glass layers are used as a buffer layer between the borosilicate stress components and the fiber core to ensure that the evanescent field will not propagate too long in the stress components. Ytterbium doped cores are deposited into the substrate by solution doping., The substrate (tube) with several deposition layers collapses to form a rod. The collapsed preform is further processed to obtain the required inner cladding. Then, in the drawing process, a fluoroacrylic coating with low refractive index is added as the second cladding of the entire optical fiber, and the pump light is conducted in the second cladding. Using this method, the bow tie ytterbium doped polarization maintaining double clad fiber can be fabricated. 

2.2 panda polarization maintaining double clad fiber

The fabrication of this optical fiber is divided into two stages. The fabrication of stress components and rare earth doped prefabricated parts are carried out separately. We will discuss the outstanding advantages of this method later. Rare earth element doped preform is realized by proprietary solution doping technology, so as to obtain very consistent distribution of rare earth elements and Co doped components. Figure 1 (b) lists the main steps of making panda PM optical fiber. A high quality synthetic quartz tube is used as the substrate. Rare earth doped glass is deposited on its inner wall. The tube then collapses into a glass rod. Next, we draw it according to the expected core and inner cladding specifications. In another process, we use MOCVD process to make a circular stress element with the expected composition. In the preform doped with rare earth elements, two holes of specific size are dug symmetrically on both sides of the fiber core. Then, two circular stress elements are inserted into the two holes to form a whole with the prefabricated parts. Next, the prefabricated parts with stress components are drawn according to our expected specifications, and fluoroacrylic materials with low refractive index are added as cladding. According to this step, we have fabricated two panda type polarization maintaining double clad fibers. The fibers have a 10um ytterbium doped core diameter, a numerical aperture of 0.08, an inner cladding diameter of 400um, and a numerical aperture of 0.45. The second fiber has a core diameter of 30um, a numerical aperture of 0.06, an inner cladding diameter of 400um, and a numerical aperture of 0.37.

2.3 design of polarization maintaining fiber

The polarization maintaining characteristics of polarization maintaining fiber depend on the distribution of residual stress components on the fiber core. The residual stress component comes from the difference between the stress component and the thermal expansion coefficient of the core and cladding. The composition, position and shape of the stress component determine the birefringence effect in the fiber. The component design of stress components and the geometric structure design of polarization maintaining double clad fiber are realized by using the internally developed method. The multi-stepped linked model based on this method can predict the refractive index and thermal expansion coefficient of the deposited glass according to the composition of the glass. With these values as input parameters, we can predict the birefringence effect of optical fiber based on the consideration of geometric structure. We have used these mathematical models to design passive 125um and 80um diameter polarization maintaining fibers for Telecom and fog applications.

2.4 analysis

We analyze the optical properties of polarization maintaining ytterbium doped double clad fiber, including crosstalk, beat length, absorption, fluorescence lifetime and slope efficiency.

Polarization crosstalk is tested according to tia/eia-455-193 (fotp-193) polarization maintaining crosstalk measurement method for polarization maintaining fibers and devices. A test system composed of high-quality crystal polarizer, low birefringence optical devices and computer-controlled precision alignment system can measure the repeatability of crosstalk as low as -45db. The tested sample is a number of optical fiber rings with a diameter of 10 inches. The total length of the optical fiber of each optical fiber ring is 10 meters. We peel off the outer cladding of most of the optical fibers in the entire optical fiber ring and immerse the optical fiber ring in oil with high refractive index. The purpose is to eliminate the light transmission in the cladding and ensure that the light only propagates in the fiber core.

The beat length of the optical fiber is measured by the S18 dispersion measurement system of GN nettest according to the wavelength scanning technology (also known as the fixed analysis method). The fully polarized light is injected into the optical fiber through a polarizer (installed at the end). The output power is recorded as a function of wavelength. Then carry out the reference scan (no polarizer is added at this time), which can eliminate the impact of any non PMD power fluctuation on the test results. In weakly mode coupled fibers, such as polarization maintaining single-mode fibers, the relationship between power and wavelength has a periodic peak and trough distribution pattern, and the beat length corresponding to each wavelength can pass through the spacing between adjacent peaks

The 915nmsdl-6380-l2 laser diode of JDSU is used as the light source for the measurement of optical absorption characteristics. Its driving source is ILX 39800. The 8163a optical multimeter with optical probe of Agilent company is used at the power measurement end. The probe adopts integrating sphere technology, which can ensure that the power measurement is insensitive to the numerical aperture. There is also a 915nm band-pass filter (spectrogon company) to block any fluorescence generated due to the 915nm output signal. We use the standard cut back method to analyze the optical absorption characteristics.

We also tested the fluorescence lifetime of some optical fiber samples. The laser mentioned above is used as the pump light source. A small section of optical fiber in the fiber ring is placed next to an InGaAs detector and a 1110nm band-pass filter (spectrogon, 70nm wide FWHM) after stripping the outer layer, so the fluorescence can be detected somewhere in the radial radiation range of the optical fiber. The detector, band-pass filter and a fluke sw90w line analyzer are used to measure the fluorescence attenuation. The lifetime is given by three e-folding time parameters that can describe the attenuation characteristics (E1, E2, E3). When the noise in the signal is large, we can also use the logarithmic fitting of fluorescence attenuation to better predict the components in the life time (E2, E3).

When measuring the slope efficiency, we still use the 915nm pump laser as the pump light source. The light from the pump laser is collimated and focused through the objective lens. When selecting the objective lens, the matching problem between the numerical aperture of the laser tail fiber and the Yb doped fiber must be carefully considered. We put a laser mirror (99.8% reflectivity to the excitation wavelength and 95% transmittance to the pump wavelength) in front of the focusing objective. The bandpass filter shown in Figure 3 and the optical probe using integrating sphere technology are used to eliminate the influence of pump light on the power measurement reading.

3. results and discussion

3.1 fabrication of bow tie and panda polarization maintaining fiber

The fabrication techniques of two completely different polarization maintaining fibers are discussed above. This helps us to understand the advantages and disadvantages of each method. This evaluation is conducted according to two standards: 1) is this technology applicable when making double clad fiber? 2) Can this technology ensure the upgradability, repeatability and consistency of mass production of prefabricated parts?

The advantage of bow tie manufacturing technology lies in that it can simultaneously realize the fabrication of stress components and rare earth doped fiber cores in one process step. Secondly, the distance between the stress component and the core can be controlled by controlling the buffer layer deposited between the stress layer and the core. The distance between the stress component and the fiber core is smaller, which means that for the stress component with certain size and composition, greater birefringence effect can be obtained. However, this technology also has very obvious disadvantages. Firstly, stress components and rare earth doped cores need to be deposited on the same substrate tube, which makes it impossible for us to independently control the polarization and lasing characteristics of the fiber. Secondly, although the stress component can be placed closer to the fiber core, the size of the stress component (which can be deposited) is limited, which also limits the size of the preform with certain birefringence. In other words, this technology limits mass production., Most double clad fibers require the inner cladding to have a non-circular shape. In order to design a desired inner cladding shape, some process steps such as grinding or heat treatment are required. In the bow tie optical fiber preform, grinding (or heat treatment) needs to be carried out when the stress component already exists. Because there are already stress components in the prefabricated parts, the prefabricated parts are quite fragile and are very easy to be broken by mechanical impact (or thermal shock) during grinding (heat treatment). Therefore, the bow tie polarization maintaining fiber is not suitable for mass production.

Panda type polarization maintaining fiber not only has its advantages, but also overcomes the defects of bow tie type polarization maintaining fiber technology. In the production process of panda type polarization maintaining fiber, the rare earth element incorporation and stress component fabrication are actually carried out separately, so we can independently and effectively control the polarization characteristics and the components of rare earth element doped glass. Secondly, we can make a large size of stress-induced components, so we can effectively increase the size of prefabricated parts. This is conducive to the upgrading of prefabricated parts., In order to obtain a non-circular dimension, all processes can be completed before adding stress components, which is equivalent to indirectly improving the production capacity. Panda polarization maintaining technology is very suitable for manufacturing polarization maintaining double clad fiber, and it is also a technology suitable for mass production.

Table 1 lists the size and polarization characteristics (beat length and crosstalk) of various polarization maintaining double clad fibers with bow tie design and stress component design. Fiber 1 is a bow tie ytterbium doped pm-dcf. Because the size of the stress region that can be deposited is limited, we fabricated an optical fiber with an inner cladding diameter of 200um to obtain the birefringence characteristics. Fiber 2 is a stress component type ytterbium doped pm-dcf. Because it is relatively easy to make large-scale stress components, we can make optical fibers with 400um inner cladding. We measured the beat length of the above two fibers by using the wavelength scanning method and the calculated birefringence value. It can be seen from table 1 that the size of optical fiber 1 is minimized to make the birefringence effect reach its value (the beat length at 633nm is only 4mm). The beat length of fiber 2 at 633nm is 2.7mm. Results show that panda polarization maintaining double clad fiber is easier to obtain high birefringence than bow tie polarization maintaining double clad fiber.

3.2 optical properties of ytterbium doped polarization maintaining double clad fiber

In the overview of this paper, we mentioned that the application of high-power fiber lasers and amplifiers requires the fiber to have a low numerical aperture and a large core, so as to obtain high pulse power and improve the threshold of nonlinearity. In addition, in order to achieve output power up to 10000 watts or 100000 watts, the output of multiple fiber lasers also requires coherent beam combining, which means that we also need to develop polarization maintaining ytterbium doped double clad fibers. Kliner et al. Showed a polarization maintaining amplifier made of a bow tie polarization maintaining double clad fiber with low numerical aperture. However, its core diameter is only 10um. Recently, it has been reported that multimode rare earth element doped fiber can obtain single-mode output by using different configurations. This technology is expected to make fiber lasers with an output of more than 100 kW a reality. However, the premise is the polarization maintaining double clad fiber with multi-mode low numerical aperture and rare earth doped core.

We have made two panda type polarization maintaining fibers and one bow tie type polarization maintaining fiber sample. All fiber samples have low numerical aperture, ranging from 0.06 to 0.08. Table 1 also shows other parameters, such as core size, numerical aperture, cladding size, absorption, etc. All fiber cores are ytterbium doped and suitable for CO doping to enhance the same type dispersion of ytterbium ions. However, this kind of CO doping will often improve the refractive index of the core. Therefore, in order to obtain a low core numerical aperture, we need to limit the degree of CO doping. Considering the above factors, we must also ensure that there is enough co doping to prevent the phenomenon of quenching of fluorescence.

In order to understand their efficiency, we measured the fluorescence lifetime of all fiber samples. Figure 4a) shows typical values of fluorescence lifetime of these fibers. All these fluorescence lifetime values are about 0.9 MS, which is equivalent to the lifetime of ytterbium doped silicate glasses reported in some literatures. In addition, e2/e3 time and E1 time are very close, which indicates that ytterbium ions decay at the same decay rate. For example, ytterbium ions dissipate at the same time. Having the same order of e-folding time does not necessarily mean that the fluorescence quenching characteristics of optical fibers are low. Paschotta et al. Reported that under specific excitation conditions, even if the ytterbium ion concentration is lower than 1200ppm (by weight), and there is no quenching phenomenon within the measured fluorescence lifetime, there is still ytterbium ion fluorescence quenching phenomenon in silicate glass fiber. The emission quenching phenomenon mainly comes from the non radiative decay within a few microseconds. In most cases, this phenomenon is difficult to be detected by the measurement system. Paschotta et al. Produced several optical fiber samples (with 2300ppm ytterbium ion concentration by weight) without fluorescence quenching. Therefore, they explained the non radiation effect as processing induced. In another article, burshtein et al reported a similar ytterbium ion fluorescence quenching phenomenon with a rate of 6-300 milliseconds. Considering that the response rate of our measurement system is in the order of 10 milliseconds, we are not sure that Yb doped double clad fiber is still effective if the non radiative effect is in the order of 1-10 milliseconds. However, quenching rates of 100-300 milliseconds have never been observed.

Another conclusive fiber characteristic parameter is slope efficiency, as shown in figure 4-b). The measured slope efficiency is 77%, the excitation wavelength is 1090nm, and the threshold is 250MW. The measured slope efficiency is very close to 84% of the quantum limit at this pump condition and input signal wavelength. The results clearly show that we can fabricate a doped fiber with low numerical aperture, and the fiber efficiency and rare earth ion concentration can reach a certain level.

Using this glass composition, we have fabricated a panda polarization maintaining double clad fiber. The ytterbium doped fiber core diameter is 30um, the numerical aperture is 0.06, and the inner cladding diameter is 400um. The coating is a polymer with low refractive index, making the numerical aperture of the inner cladding 0.37. On top of the coating with low refractive index, we added a protective layer of electric grade acrylic material. The polarization maintaining double clad fiber with multimode core exhibits absorption characteristics of 0.67db/m (915nm) and 2.2db/m (975nm). The measured beat length of the fiber is 4.4mm (633nm), corresponding to the birefringence effect of 1.44x10e-4. Here is a polarization maintaining double clad fiber with 30um core diameter. Our next step is to increase the birefringence effect in the fiber. In the following sections, we will discuss the design considerations for making polarization maintaining double clad fibers. The results show that the birefringence can be greatly improved. Therefore, a polarization maintaining double clad fiber with a multimode core with low numerical aperture is feasible, and it can be predicted that it will play an important role in the development and production of high-power fiber lasers and optical amplifiers.

3.3 polarization characteristics and design standards

Figure 5 shows several key structural parameters affecting birefringence effect in polarization maintaining double clad fiber, including the size of stress component (DS), and the position (DP) of stress component relative to inner clad diameter (DF) and linear diameter (DC). In addition to the geometric parameters, the composition of the stress bar also determines the possible birefringence effect in the fiber. Figure 6 shows the effect of the size and position of the stress bar on the birefringence (beat length) effect of the optical fiber. From figure 6-A), we can see that by increasing the size of another component (DS), we can increase the birefringence (or reduce the beat length) while other parameters remain unchanged. Similarly, figure 6-b) shows that the birefringence effect can also be increased if the stress bar is moved to the core. 

Theoretically speaking, we can obtain large birefringence effect by reasonably designing these two parameters. However, due to the distance between the stress component and the core, DS and DP are actually limited. This limiting distance can be obtained from the distance between the inner edges of the two stress components (DI). If Di is too small, the probability of overlap between mode field and stress components will increase, resulting in an increase in fiber attenuation and bending loss at the lasing wavelength and the amplifier signal wavelength. In order to avoid the overlap between the mode power distribution and the stress components in the optical fiber, we define the standard di/mfd>5. For small core size single-mode fiber used in medium and small power applications, we can obtain sufficient birefringence effect by using conventional stress components and working within the safety margin. However, for large core fibers used in high-power applications, it is still a big challenge to obtain enough birefringence even if they work within the safety margin.

Fiber 2 is a polarization maintaining double clad fiber used in medium power applications. Its core is small and its diameter is 10 microns. The measured result of optical fiber 2 is that the beat length is 2.7mm (633nm), corresponding to the birefringence of 2.31x10e-4. Figures 7-a) and b) show the relationship between the expected beat length and the size of the stress component. The measured experimental data (beat length) of optical fiber 2 is shown in figure 7-a). In addition, we have drawn a vertical line indicating the safety margin standard. For the size of the stress bar on the right of this vertical line, it cannot be realized because the limiting distance Di is too small and di/mfd<5. Obviously, optical fiber 2 is within the limit ratio, and a very small beat length can be obtained. From figure 7-a), we can also see that for an optical fiber with a small core size, we can get a beat length less than 2mm, which does not exceed the limit ratio range.

Another vertical line is also given, which specifies the limiting ratio of a polarization maintaining double clad fiber with a 30um diameter core (fiber 3). The size of the stress component to the left of the limit ratio line is feasible. Therefore, we can expect that the stress bar size of pm-dcf with single-mode core and pm-dcf with multi-mode core can be smaller. In order to obtain a large birefringence effect, it is necessary to move the stress bar near the center of the optical fiber. The expected beat length in this case is shown in figure 7-B). Comparing figures 7-a) and 7-B), we can see that even with the same stress bar size, the beat length of the optical fiber at position 2 of the stress bar is larger than that of the optical fiber at position 1. Fiber 3 is a pm-dcf with a large core (30um), which is suitable for high-power applications. When the stress component of this fiber is at position 2, we can obtain the following parameters: beat length 4.4mm (633nm), corresponding to birefringence of 1.44x10e-4 (see Fig. 7-B). In order to meet the limit ratio, the size of the stress component must be kept at a relatively small value. Therefore, it is difficult to obtain a considerable birefringence effect in a relatively small core fiber. Figure 7 clearly illustrates this point: for large core fibers, the limiting condition (di/mfd=5) is easy to meet the requirements for a specific birefringence effect. Therefore, for large core fibers, we must change the composition of stress components. In this way, we can still obtain large birefringence effect in the case of small size stress components. Figure 7 shows the relationship between the expected beat length and the size of the stress component in the case of another composition. At present, the stress component with this composition is often used to make PM optical fiber used in fiber optic gyroscope, which requires a very small beat length. Through this composition, we can also obtain a very large difference in the coefficient of thermal expansion, corresponding to high birefringence. From Fig. 7-B), we can observe that with this stress component composition, we can obtain birefringence effect comparable to that of small core fiber under the condition of small stress component and meeting the limiting ratio conditions. In the large core fiber, we get the birefringence effect of 3.5x10e-4, which is comparable to the birefringence effect of the standard stress rod small core fiber.

4. conclusion

The demand of high-power fiber lasers and optical amplifiers for low numerical aperture, large-size (multimode) core pm-dcf is growing. In a low numerical aperture (0.06) ytterbium doped pm-dcf, a slope efficiency of 77% is obtained. After comparing the fabrication technologies of the two PM fibers, we found that the birefringence effect of panda pm-dcf fiber can be significantly improved. Considering the need of mass production, panda pm-dcf fiber is more suitable than bow tie pm-dcf fiber. We also demonstrated a pm-dcf fiber with panda shape. Its ytterbium doped core has a numerical aperture of 0.06, a diameter of 30um, an absorption coefficient of 0.67db/m (915nm), 2.2db/m (975nm), an inner cladding diameter of 400 microns and a numerical aperture of 0.37. The experimental and simulation results show that there are some factors to be taken into account in the fabrication of pm-dcf fiber with multi-mode ytterbium doped core. The analysis shows that pm-dcf with large birefringence core is feasible. Therefore, pm-dcf fiber with low numerical aperture and multi-mode core is feasible. It can be expected that it will play an important role in the R & D and production of high-power fiber lasers and optical amplifiers.

Shenzhen PUSMAI technology Co.,LtdIt is a manufacturer of passive optical fiber components in China integrating R&D, production, sales and after-sales. Currently, it mainly focuses on polarization-maintaining optical fiber components and high-power optical fiber components in the 405nm~2050mm band, PUSMAI product PM/High power optic circulator , PM/High power optic isolator , PM/High power optic patch cord  , PM/High power optic optic switch and also provides customers with Pump laser EDFA ,CATV EDFA , High power EDFA Solutions and customized services.