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Abstract
With the research of hollow-core fiber with large core diameter, the coupling efficiency from hollow-core fiber with large core diameter to single-mode fiber is difficult to increase through the traditional technology, we proposed a novel coupling method to improve the coupling efficiency by attaching a pure silica small ball at the front end of single-mode fiber, the coupling efficiency of 50% from hollow-core fiber with a large core diameter of 110 µm to single-mode fiber can be achieved.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Hollow-core fibers (HCFs) have attracted increasing interest due to their ultra-low nonlinearity, and low group velocity dispersion, which are excellent candidates for high-power beam delivery [1], in particular of ultrashort pulses [2,3] and gas-based nonlinear optics applications [4–7]. In most applications of HCFs, the coupling of HCF with traditional single-mode fibers (SMFs) is unavoidable, and the transmission efficiency of the coupling is very important. In order to minimize the coupling loss, fusion splicing is one of the commonly methods for HCF/SMF coupling. In 2007, Limin Xiao et al. proposed that the splicing loss from SMF-28 to HC-1550-02 was 1.87 dB, while that from HC-1550-02 to SMF-28 was 2.48 dB, by using the technology of controlling micro-hole collapse [8]. In 2010, Kiarash Zamani Aghaie et al. used the SMF with optimized parameters to reduce the splicing loss to 0.79 dB [9]. In 2014, Shoufei Gao et al. reported that a high V value transition fiber was added between HC-1550-02 and SMF-28, by optimizing the transition fiber, the splicing loss was reduced to 0.73 dB [10].
In recent years, the insertion coupling method has been proposed. In 2016, S Xie et al. proposed a coupling method from SMF to HCF with a core diameter 12.1 µm by using tapered nanospike to couple into HC-PCF [11,12], in 2017, Danyun Fan et al. proposed the coupling from SMF to HCF that inserts the corroded SMF into the core of HC19-1550-01 with a core diameter of 20 µm [13], Naiqian Zhang et al. proposed low loss coupling from SMF to LC-HCF by insert a tapered fiber into the core of PCF with the mode diameter of 35 µm [14].
All the above studies are aimed at the coupling between HCF and SMF with similar core diameters, and mode field matching can be achieved by the above schemes. However, it is difficult to achieve the matching of mode field with a difference of 5-10 times through the traditional technology when splicing the large core hollow-core fiber (LC-HCF) with SMF. In especial the anti-resonance HCF [15–17], the core diameter of which is generally within the range of 50∼100 µm in order to achieve low confinement loss.
In 2016, Ximeng Zheng et al. reported the LC-HCF with the core diameter 65 µm spliced to standard SMF with a total insertion loss of 3.5 dB by tapering the LC-HCF with a down-ratio 2.4 [18]. But the extra controllable vacuum pressure in the range of few mbar was needed to maintain the delicate core-contour negative curvature during tapering, which increases the difficulty of preparation. At present, no other literature on improving the coupling efficiency from LC-HCF to SMF was reported.
In this paper, we proposed a novel coupling method to improve the coupling efficiency from LC-HCF to SMF, theoretical calculations show that the coupling efficiency get significant increasement as the numerical aperture (NA) of the SMF increases, and the preparation process of coupling structure was investigated in detail. The coupling efficiency of 50% from LC-HCF with a core diameter of 110 µm to the SMF was achieved experimentally when the NA of SMF is 0.22.
2. Coupling structure design
The coupling structure from LC-HCF to SMF is as shown in Fig. 1. The LC-HCF is at left side, the SMF is at right side. And the SMF with processed outer diameter is inserted into the core of the LC-HCF. At the front end of the SMF, a pure silica ball is attached. The function of the ball is to convergence the light from the LC-HCF. When the convergence angle is smaller than the NA angle of the SMF, the light from the LC-HCF can be coupled into the core of the SMF. The convergence angle and the NA angle are determined by the ball and the SMF respectively, which means the coupling efficiency is not directly related to the core diameter difference between LC-HCF and SMF. Higher coupling efficiency can be achieved through this method on the condition of large difference in core diameter, which is superior to traditional technology.
The diameter of the ball is d1, and the distance between the center of the ball and the front end of the SMF is Z, the diameter of the thinner fiber connected between the ball and the SMF is d2. Since the ball needs to be inserted into the core of LC-HCF, d1 is set to be less than the core diameter of LC-HCF.
Figure 2 is the cross section of the LC-HCF homemade in our laboratory. The cladding consists of 8 tubes that are in contact with each other. The curvature radius of the core boundary are about 31.5 µm to 34.5 µm, and the wall thickness of tubes are about 1.75 µm to 1.9 µm. The outer diameter of LC-HCF is about 400 µm, and the core diameter of LC-HCF is about 110 µm.
One of the anti-resonant transmission windows of the LC-HCF is shown in the Fig. 3(a). The simulated confinement loss of fundamental mode is lower than 10 dB/km at the range of 1550 nm ∼1700nm. The mode content of macro-bending status of the LC-HCF is also analyzed, when the bending radius is larger than 1400 mm, the high-order mode loss is higher than 1000 dB/km, and the fundamental mode loss is less than 10 dB/km, in addition, the distortion of the fiber mode caused by bending can be neglected. It can be assumed that only fundamental mode transmission exists in the core when the bending radius is larger than 1400 mm.
Fig. 3. (a) Anti-resonant transmission window of the LC-HCF, (b) the bending loss and mode field distribution of fundamental mode and high-order mode at 1600 nm.
The power coupling coefficient transmitted to the core of SMF was simulated and calculated by the ray tracing software TracePro. TracePro normally used to calculate the split ratio in the coupler and the pump coupling efficiency in fiber laser, etc., which exhibits certain reference significance.
The fundamental mode distribution fits the zero-order Bessel function. Due to the complexity of the processing of the Bessel function and the Gaussian function is close to the Bessel function, the Gaussian distribution is used to approximate the Bessel function. The key is to find the appropriate mode field radius, so that the error caused by the alternative solution is as small as possible. A grid source is defined with a radius of 55 µm and the waist radius of Gaussian beams (1/e^2) is about 44.7 µm. The field distribution of grid source and the electric field distribution of the LC-HCF core mode were shown in Fig. 4, which are basically consistent.
In order to improve the coupling efficiency, it is necessary to increase the diameter of the ball, i.e., increase the receiving area while reduce the convergence angle. And d1 is set to be 110 µm for the calculation process. The coupling efficiency with different distance Z from the center of the ball to the front end of SMF were calculated, as shown in Fig. 5. The distance Z has great influence on the coupling efficiency, and it should be kept within an appropriate range, otherwise the convergent light will couple into the core of SMF incompletely.
In addition, the influence of NA and core diameter dcore of the SMF on coupling efficiency are also analyzed. It can be seen from Fig. 5 that the NA of SMF has great influence on the maximum coupling efficiency. The maximum coupling efficiency of 19%, 25%, 30%, 36%, and 46% can be achieved when the NA of SMF are 0.12, 0.14, 0.16, 0.18, and 0.22 respectively. The core diameter dcore is coupled with the distance Z, the distance Z can regulate in a large range with freedom when dcore is large.
As a comparison, the coupling efficiency of direct coupling from LC-HCF to SMF is also calculated, which is 3.2%. The main factor affecting the coupling efficiency is the mismatch of core diameter between LC-HCF and SMF. By the use of proposed coupling method, the coupling efficiency from LC-HCF with a core diameter of 110 µm to SMF can be increased from 3.2% of direct coupling to 46% when the NA of SMF is 0.22. It is worth noting that the maximum coupling efficiency determined by the NA of SMF, and the NA of traditional SMF is rarely higher than 0.22. In order to further improve the coupling efficiency, a single-mode photonic crystal fiber can be used, which can achieve a higher NA than traditional SMF.
3. Preparation process and result
The preparation of the structure based on Fujikura LZM100 CO2 laser splicing and glass processing system, which have excellent heating system and high precision movement control system, and could be used for fusion various optical fibers and shaping process.
Before preparing the structure, the diameter of the thinner fiber d2 need to be solved firstly. The coupling efficiency is influenced by the curvature radius of the light receiving sphere, the larger the curvature radius, the more light can be concentrated into a smaller convergence angle, and the limit of the curvature radius is a standard sphere. The curvature radius is directly related to the non-circularity of the final ball and determined by the diameter of the thinner fiber d2 during the melting process. Therefore, the non-circularity of the ball needs to be minimized as much as possible. When the thinner fibers with same diameter d2 are used to fuse into spheres with different expected diameters d1, the results are as follows in Fig. 6.
The relationship between the non-circularity of prepared spheres and the ratio of expected diameters and the thinner fiber diameters d1/d2 are as follows in Fig. 7. In order to minimize the non-circularity of the prepared ball, it suggested that the ratio of d1/d2 is set to be above 3. In this experiment, the expected diameter of the ball d1 is 110 µm, and the diameter of connected fiber d2 is 30 µm, and the ratio of d1/d2 is 3.67.
LZM 100 could be used to control the diameter of the ball and the distance between the center of the ball and the front end of the SMF with high precision movement control system. The pure silica thinner fiber with a diameter of 30 µm (drawn from a pure silica fiber with outer diameter of 125 µm by LZM 100) and the SMF with a diameter of 110 µm (etched by 40% HF solutions for 7 minutes from 125 µm) were placed into the LZM100, the preparation process is shown in Fig. 8.
Firstly, the thinner fiber and the SMF were spliced together. The center of hot zone was shifted to SMF direction during splicing due to the diameter difference between the thinner fiber and the SMF. Secondly, the thinner fiber and the SMF were synchronously moved. The moving distance L directly influenced the final distance Z, which need to be precisely set. According to the conservation of volume, the distance L can be roughly estimated by the following formula,
A long heating time was required in the final step, in order to precisely control the heating time, a two-step heating of rough heating and finish heating can be used. The ball was heating to near the expected diameter at a time through the rough heating, and then the diameter of the ball was approached to the expected diameter in multiple times through the finish heating. The diameter of the ball was measured and judged after each finish heating. If the ball diameter is less than 109 µm, the program returns to finish heating, else stops.
Two structures were prepared with different type of SMFs. One of the SMFs is common SMF with NA 0.14, the diameter of the core is 9 µm, the other one is CS 980_125-22/250 with NA 0.22, the diameter of the core is 3.5 µm. The repeatability was pretty good and the diameter of the balls were 110 µm, the non-circularity of the balls were less than 3%, the distances between the center of the balls and the front end of the SMF were 110 µm, ensured that the highest coupling efficiency was achieved. After aligning the XY-direction on the LZM 100, the SMF and the ball was inserted into the LC-HCF by driving the Z-direction motor, as shown in Fig. 9.
The coupling efficiency was measured by a supercontinuum source from 1250 nm to 1650 nm and Yokogawa optical spectrum analyzer AQ6370C. The bending radius of the LC-HCF remained greater than 1400 mm to ensure that single mode transmission in the LC-HCF. The output power from LC-HCF as the coupling input power, the output power from the SMFs through the ball were measured, as shown in Fig. 10. It can be seen that 1550∼1650 nm is located in the transmission window of the LC-HCF, the coupling efficiency was increased as the NA of the SMF increased. The insert loss with the NA of 0.14 is about 8 dB around 1600 nm, and the insert loss with the NA of 0.22 is about 3 dB around 1600 nm, 16% and 50% of coupling efficiency were achieved with the NA of 0.14 and 0.22 respectively, which are consistent with the theoretical simulation results.
According to the measured results of the coupling efficiency, it correlated with the wavelength, and was very similar to the transmission band. In a negative curvature anti-resonant HCF, the fundamental mode power is more concentrated in the core in the middle of the transmission band, but the same grid source distribution was used for the simulation calculations in different wavelength. Therefore, the coupling efficiency in the middle of the transmission band is higher than the calculation. The core mode is easier to couple with cladding tube mode at the edge of the transmission band [19,20], it is speculated that when the light in LC-HCF propagates to the ball, some power of fundamental mode is coupled into the cladding tubes at the edge of the transmission band, and resulting in reduced efficiency of coupling to the SMF at the edge of the transmission band.
As a comparison, the output power of direct coupling from LC-HCF to SMF without the ball is also measured as shown in Fig. 10. The insert loss through the SMF without the ball was about 15 dB around 1600 nm, the coupling efficiency was less than 5%. By using the proposed coupling method, the coupling efficiency from LC-HCF with a core diameter of 110 µm to SMF was increased by an order of magnitude compared to direct coupling.
4. Conclusion
In order to response to the need of coupling application between LC-HCF and SMF, in this paper, we proposed a novel coupling method to improve the coupling efficiency from LC-HCF to SMF by attaching a pure silica small ball at the front end of SMF. Theoretical calculations showed that the coupling efficiency increased as the fiber NA increased. The coupling efficiency is not limited by the mismatch of core diameter, which means the proposed coupling method can achieve higher coupling efficiency than traditional technology on the condition of large difference in core diameter. And the preparation process of coupling structure was investigated in detail. Experimentally, by using the proposed coupling method, the coupling efficiency from the LC-HCF with a core diameter of 110 µm to the SMF was increased by an order of magnitude compared to direct coupling. 50% of coupling efficiency from LC-HCF to SMF was achieved with the SMF NA of 0.22.
Funding
National Natural Science Foundation of China (61535009).
Disclosures
The authors declare no conflicts of interest.
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