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Abstract
We propose and demonstrate a new method of direct writing large-area fiber Bragg grating by femtosecond laser through the coating. By adding an adjustable diaphragm before the focusing objective, we can precisely control the length of the refractive index modulation line along the femtosecond laser incident direction up to 29.1 µm. In combination with femtosecond laser scanning fabrication technology, a uniform refractive index modulation plane can be inscribed in the fiber in a single scanning. Based on the plane-by-plane inscription method, we have fabricated a high-quality high-reflectivity fiber Bragg grating and a chirped fiber Bragg grating on 20/400 double-clad fiber core. The reflectivity of both gratings is greater than 99%, and the insertion loss is as low as 0.165 dB and 0.162 dB, respectively. The thermal slope of chirped fiber Bragg grating without any refrigeration is 0.088 °C/W and there is no obvious temperature increase when using the water cooling. Therefore, the fabrication method of large-area fiber Bragg grating based on diaphragm shaping can efficiently fabricate high-quality fiber Bragg grating in the large core diameter fiber, which has an important application prospect in high-power all-fiber oscillators, especially all-fiber oscillators in special wavebands.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The fiber Bragg grating (FBG) is a crucial component of fiber application and has been widely used in various fields such as fiber communication, fiber sensing, and fiber lasers. The ultraviolet laser exposure method has been commonly used to fabricate FBGs in high-power fiber lasers [1,2]. However, the process of hydrogen loading, coating-stripping, recoating, and thermal annealing makes it difficult to fabricate FBGs and limits further development [3,4]. The femtosecond laser inscription technology has emerged as a solution to these challenges [5]. This method can fabricate FBG on the fiber of non-photosensitive material faster and more reliably without stripping the coating. There are two primary methods to fabricate FBGs based on the femtosecond laser inscription technology, which are phase mask technology [6–10] and direct written technology [11–14]. Compared with the phase mask method, the direct written method is more flexible in inscription factors such as focus shape, period, and position of FBG [15,16].
Femtosecond laser direct writing technology has three methods of inscription: point-by-point [14,17], line-by-line [16,18], and plane-by-plane [11,19]. The small-size FBG consistent with the diffraction limit has been fabricated by the aberration corrections technology [14,20,21]. Due to the small refractive index (RI) modulation area relative to the fiber core size, FBGs fabricated by the point-by-point inscription method and line-by-line inscription method have strong cladding mode loss and birefringence characteristics [22–25]. FBGs fabricated based on the plane-by-plane inscription method effectively improve the above situation and are more advantageous for use in fiber lasers [26,27]. Currently, various plane-by-plane inscription methods have been developed. Based on the beam-shaping technology, such as adding a cylindrical lens or a slit before the focusing objective, the shape of the femtosecond laser focus in fiber is adjusted to a plane. This results in the RI modulation plane in fiber can be increased to a size slightly larger than the core of single-mode fiber (SMF) [28–31]. Moreover, the liquid concave lens made by high RI oil can increase the size of the focus to a plane that is slightly smaller than the core of SMF [32]. Using the femtosecond laser scanning technology, the RI modulation planes can be fabricated by the focal point of line shape along the femtosecond laser incident direction. The moderate numerical aperture (NA) objective has been demonstrated to elongate the focus, which has fabricated the RI modulation planes similar in size to the SMF core [33]. The spherical aberration benefits extending the RI modulation line with a negative region and positive region, and the positive region is demonstrated that have a length of longer than the core of the SMF [34]. By stacking multiple-layer RI modulation lines fabricated by the line-by-line inscription method, the RI modulation plane of a size of 24 × 24 µm is demonstrated, which covers the entire core of 20/400 passive fiber [35]. However, the method is time-consuming since each RI modulation plane needs multiple scans [36]. Table 1 displays the relative reports of the size of the RI modulation plane fabricated by the plane-by-plane inscription method in the silica-based fiber core. Due to the slow fabrication process and small size of the RI modulation plane, these plane-by-plane inscription methods have limitations when used in high-power fiber lasers. Therefore, when fabricating FBGs in the large core diameter fiber for use in high-power fiber lasers, a faster, larger, more uniform, and adjustable RI modulation size plane-by-plane inscription method is needed.
Table 1. Comparison of the RI modulation size between our work with other related reports
In this paper, we propose a new method of direct writing for fabricating FBG on large core diameter fibers. The method involves inserting an adjustable diaphragm before the objective, which benefits to elongate the length of the RI modulation line. Assisted by a single scanning strategy, the uniform RI modulation plane can be realized, with a maximum size of 30 × 29.1 µm. Using this method, we have fabricated high-quality high-reflectivity fiber Bragg grating (HR-FBG) and a chirped fiber Bragg grating (CFBG) in the core of the large mode area (LMA) Ge-doped 20/400 double-clad passive fiber. The HR-FBG exhibits a reflectivity greater than 99.6% at 1079.916 nm, an insertion loss of 0.165 dB, and a thermal slope of 0.05 °C/W. The CFBG has a 3-dB bandwidth of 2.704 nm in the transmission spectrum and a reflectivity greater than 99.1%, as well as an insertion loss of less than 0.162 dB and a thermal slope of 0.088 °C/W. It is worth mentioning that the FBGs fabricated by this method have retained the fiber coating. We study the bending broken diameter of the 20/400 fiber states of recoated, unstripped, and stripped. The minimum bending broken diameter of the unstripped fiber is less than 6 mm, which is significantly smaller than the stripped fiber (22.6 mm) and the recoated fiber (16.6 mm). To the best of our knowledge, this is the largest size of FBG fabricated in silica-based fiber using femtosecond laser plane-by-plane inscription, and each RI modulation plane is fabricated in a single scanning, but not multiple scanning.
2. Experimental setup and theoretical analysis
During the fabrication process, the femtosecond laser with a wavelength of 515 nm, a pulse duration of 290 fs, and a repetition rate of 200 kHz is used. The power of the femtosecond laser can be adjusted using a motorized power attenuator. Figure 1 shows the experimental setup of the femtosecond laser plane-by-plane inscription system and the real-time spectrum-measured system. As Fig. 1 displays, a coaxial adjustable diaphragm is added before the long working distance microscope objective (50×, NA = 0.42). The diameter of the beam incident into the objective is changed by the femtosecond laser through the adjustable diaphragm. The LMA Ge-doped 20/400 double-clad passive fiber, which has a core diameter of 20 µm and an inner cladding diameter of 400 µm, is placed in an oil groove fixed on a 3D high-precision air-bearing motion stage. The femtosecond laser is focused by the objective into the fiber core. A real-time spectrum-measured system is used to measure the spectrum of FBG during fabrication.
Fig. 1. Experimental setup of plane-by-plane inscription system and real-time spectrum-measured system. (CCD: Charge-coupled Device; ASE: Amplified Spontaneous Emission; OSA: Optical Spectrum Analyzer; MFA: Mode Field Adapter)
By narrowing the adjustable diaphragm, the radius of the beam incident into the objective is reduced, thereby increasing the Rayleigh length Z0 of the laser focus in the fiber core [11]. The waist radius w0 of the focus after the objective and Z0 can be expressed as
where f is the focal length of the objective, λ is the wavelength of the femtosecond laser and w is the spot radius of the femtosecond laser beam incident into the objective. According to the inscription system shown in Fig. 1(a), the relationship between the aperture of diaphragm d and w is d = 2w. Equation (2) indicates that a smaller diaphragm aperture leads to a larger Z0. Therefore, reducing the d allows for the fabrication of a longer RI modulation line along the femtosecond laser direction in the fiber core.Figure 2 shows the FBG fabrication process using the femtosecond laser plane-by-plane inscription method. The femtosecond laser first passes through the adjustable diaphragm and is then focused into the fiber by the objective, thereby inducing the RI modulation line in the fiber along the femtosecond laser incidence direction. By conducting a single scanning in the plane perpendicular to the femtosecond laser incidence direction, a uniform RI modulation plane can be obtained. A series of periodically aligned RI modulation planes can be obtained by scanning according to the inscribed path as shown in the inset (a) of Fig. 2, thus inscribing the large-area FBG in the fiber core. The inset (b) of Fig. 2 shows the relative position details between the coverslip, index-matching oil, fiber, and oil groove. The fiber is placed in a square oil groove which is filled with index-matching oil (RI = 1.464). The fiber is entirely immersed in the index-matching oil. The cylindrical shape of the fiber surface, which affects laser focusing, is counteracted by immersing the fiber in the index-matching oil. A coverslip is then placed on the oil groove to eliminate the curved surface of the index-matching oil caused by its surface tension. It is important to note that the fiber should not touch any part of the oil groove or coverslip.
Fig. 2. Schematic diagram of the FBG fabrication by the femtosecond laser plane-by-plane inscription method. (Inset: (a) schematic diagram of the scanning method and (b) schematic diagram of the relative position details between the coverslip, index-matching oil, fiber, and oil groove.)
3. Fabrication of RI modulation plane
A series of RI modulation planes is fabricated near the core of LMA Ge-doped 20/400 double-clad passive fiber by adjusting the power before the objective P and the different d. Each RI modulation plane has a scanning length of 30 µm with a scanning velocity of 60 µm/s. In the inscription process, the repetition rate of the femtosecond laser is set to 200 kHz. Figure 3(a) displays the length of the RI modulation line L with the different P when the d = 0.8 mm, showing a linear fitting relationship between the L and the P. Due to the limitation of the femtosecond laser output power, the maximum P is 8.5 mW when the d is 0.8 mm. Therefore, we use the P = 8 mW to compare the L in four different d, as Fig. 3(b) shows. According to Fig. 3(b), the L increases as the d decreases, which is consistent with theoretical analysis.
Fig. 3. Micrographs of (a) the L in different P when the d = 0.8 mm and (b) the L in different d when P = 8 mW, as well as (c) the linear fitting relationship between the L and the P in different d, and (d) the polynomial fitting relationship between the L and the d when P = 8 mW.
Figure 3(c) illustrates the relationship between the L and the P in different d is obtained by linear fitting. Figure 3(d) displays the excellent polynomial fitting relationship between the L and the d when the P = 8 mW. The results indicate that our femtosecond laser plane-by-plane inscription method can precisely control the size of the RI modulation plane in the fiber according to different application requirements.
To address deviations in fiber core alignment during the inscription process, the maximum L is used to fabricate a series of RI modulation planes into the fiber core, with the P = 8.5 mW and the d = 0.8 mm. The fabrication process similarly involves a scanning length of 30 µm and a scanning velocity of 60 µm/s. Figures 4(a) and (b) display the RI distribution near the fiber core before and after the fabrication of the RI modulation plane, respectively. It is worth noting that both figures use the same colorbar to exhibit the fiber core and cladding RI modulation intensity Δn induced by the femtosecond laser. The measurement result demonstrates that the uniform RI modulation plane near the fiber core can be fabricated, and cover the entire fiber core. The refractive index reconstruction algorithm produces errors at the edges of the RI modulation plane, which results in the RI modulation plane shown in Fig. 4(b) being slightly smaller than the actual area [37]. Besides, wherever the fiber core and the fiber inner cladding, the maximum Δn is up to 1.5 × 10−3. Figure 4(c) displays the microscope images of the cross-section of the RI modulation plane, which measures the size of 30 × 29.1 µm. This demonstrates the precise control of the size of the RI modulation plane with a uniform RI modulation intensity.
Fig. 4. The RI distribution of the cross-section of (a) the fiber core before the inscription and (b) the fiber core after the inscription, as well as (c) the microscope images of the cross-section of the RI modulation plane.
4. Fabrication and spectra of FBGs
Based on the above research, a third-order HR-FBG is fabricated by the femtosecond laser plane-by-plane inscription method on the LMA Ge-doped 20/400 double-clad passive fiber core through the coating. The RI modulation planes of 30 × 29.1 µm are fabricated using inscription parameters of the d = 0.8 mm and the P = 8.5 mW, with a scanning length of 30 µm and a scanning velocity of 60 µm/s for each period. The top view and side view micrographs of the HR-FBG are as Figs. 5(a) and (b) shows. The HR-FBG has a period of 1.115 µm and a total length of 11.1 mm. The Eq. (3) shows the relative to the order m and the period $\Lambda $, and the effective RI ${n_{eff}}$ of the fiber core mode [38].
where the ${\lambda _{Bragg}}$ is the resonance wavelength of the FBG. The transmission spectrum of HR-FBG is measured in real-time during the fabrication process.Fig. 5. The micrographs of (a) the top view and (b) the side view of the HR-FBG, as well as (c) the measured transmission and reflection spectra of the HR-FBG (R: reflectivity; CW: center wavelength; 3-dB BW: 3-dB bandwidth; IL: insertion loss).
Figure 5(c) shows the measured transmission and reflection spectra of the HR-FBG. The center wavelength is 1079.916 nm, and the reflectivity is greater than 99.6%. The 3-dB bandwidth is 0.86 nm, and the bandwidth of greater than -20 dB in the transmission spectrum is reduced to 60 pm. The narrow bandwidth can offer strong feedback on specific narrow wavelength ranges. Additionally, the average insertion loss of the HR-FBG in the transmission spectrum is less than 0.20 dB.
Then, we use the method to fabricate the CFBG. For increasing the bandwidth, the length of the CFBG should be much longer than the HR-FBG. However, this significantly increases the insertion loss of CFBG. To reduce insertion loss, the size of the RI modulation plane is controlled to more closely match the size of the fiber core. Therefore, the size of the RI modulation plane is decreased to 24 × 23.6 µm, corresponding to d = 0.8 mm and P = 7 mW. The top view and side view micrographs of the CFBG are as shown in Figs. 6(a) and (b). Additionally, the period of the third-order CFBG is similar to the width of the RI modulation plane, which can create an approximate waveguide structure in the fiber core, resulting in increased insertion loss. Therefore, the CFBG is fabricated as a fourth-order to reduce insertion loss and decrease period errors caused by slight vibrations of the high-precision air-bearing motion stage. In the CFBG writing process, the first period is 1.486 µm, and each subsequent period increases linearly by 5 × 10−5 nm to introduce a chirp rate of about 0.74 nm/cm in the total length of CFBG is 38 mm.
Fig. 6. The micrographs of (a) the top view and (b) the side view of the CFBG, as well as (c) the spectra of transmission and reflection of the CFBG.
The transmission and reflection spectra of CFBG are displayed in Fig. 6(c). The reflectivity of the CFBG is extracted by the transmission spectrum, which is greater than 99.1% near the center wavelength of 1079.3 nm. Due to the linear increase of periods, the 3-dB bandwidth is broadened to 2.704 nm in the transmission spectrum of Fig. 6(c). By increasing the chirp rate or the length of the CFBG, the 3-dB bandwidth can be further broadened. In addition, the average insertion loss of the CFBG in the transmission spectrum is less than 0.31 dB, indicating the potential for use in high-power all-fiber oscillators.
5. Characterization measurement of FBGs
The insertion loss and temperature characteristics of FBGs in fiber lasers are important performance indicators that directly affect the output power and slope effect of the laser. The experimental setup shown in Fig. 7 is used to measure the insertion loss and temperature characteristics for the HR-FBG and the CFBG. The fiber in the experimental setup is all LMA Ge-doped 20/400 double-clad passive fiber, and all fiber components are fabricated on the same fiber type. A 1070 nm laser outputs the signal laser to the FBG through a cladding light stripper (CLS) to remove the light in the inner cladding. Another CLS, located behind the FBG, is used to filter the light scattered to the inner cladding by the FBG. Finally, the quartz block head (QBH) outputs the signal laser. Both the HR-FBG and the CFBG are fabricated on the 1 m length of the 20/400 fiber.
Fig. 7. Experimental setup for measuring the insertion loss and the temperature characteristics of FBGs.
Before the measurement process, the 1 m length of the 20/400 fiber without any FBG is utilized to measure the output power of the experimental setup at a series of currents as the calibration power. The calibration power can indicate the power through the FBGs. In the calibration process, the experimental setup has a maximum output power of 846 W. However, during stable operation, the experimental setup outputs 425.6 W at 24 A. Subsequently, the FBG under test is used in place of the fiber without FBG, and the output power is measured at a series of corresponding currents according to the above calibration process. The insertion loss is then calculated by the measured output power and the calibration power. During the measurement process, both sides of the FBG under test are fixed on a fiber support and the FBG is hanging in the air without using any other refrigeration to measure its real temperature T using a thermal camera.
Figure 8(a) displays the measurement results of the insertion loss of HR-FBG, with an average insertion loss of 0.165 dB. The measured value experiences instability due to slight fluctuations in the output power of the 1070 nm laser source. Figure 8(b) displays the temperature characteristics of HR-FBG. There is no observable temperature increase when the calibration power is less than 200 W. The thermal slope fitted is 0.05 °C/W, and the maximum temperature is only 37.7 °C at 425.6 W. In addition, the HR-FBG shows no noticeable temperature rise when using the water cooling at the highest calibration power of 846 W.
Fig. 8. (a) The measured and the average insertion loss of the HR-FBG, and (b) the measured temperature and the linear fitting thermal slope of the HR-FBG.
In high-power all-fiber oscillators, the CFBG can be subjected to high-power lasers to achieve self-annealing without stripping the coating [39]. Due to the experimental limitations, the CFBG can only be heating annealed with the coating stripped at present to study the insertion loss and temperature characteristics after annealing. The heating annealing process involves heating the CFBG to 400 °C in 10 minutes, keeping it at that temperature for 5 minutes, and then cooling it to room temperature using an air fan for 20 minutes. The insertion loss and temperature characteristics of the CFBG are measured before and after the annealing in the same way as shown in Fig. 7. The measurement results are shown in Figs. 9(a) and (b). Before the annealing, the average insertion loss and the thermal slope are up to 0.304 dB and 0.327 °C/W, respectively. Because the temperature of CFBG is up to 70.1 °C when the calibration power is 167.9 W, the further increase of calibration power is stopped. Following the annealing process, the average insertion loss and the thermal slope are significantly reduced to 0.162 dB and 0.088 °C/W, respectively. This demonstrates the effectiveness of the annealing process in optimizing the insertion loss and temperature characteristics of CFBG. Furthermore, the CFBG does not exhibit any obvious temperature increase when using the water cooling at the maximum 846 W calibration power with or without annealing.
Fig. 9. (a) The measured and the average insertion loss of CFBG, and (b) the measured and the linear fitting thermal slope of CFBG.
In high-power all-fiber oscillators, the fiber is always bent and fixed to the water-cooled plate. Therefore, it is important to consider the bending strength of the fiber for actual application. We have designed an experiment to demonstrate the bending strength of the fiber in three states: recoated, unstripped, and stripped, with stripping lengths of 50 mm and 100 mm. The micrograph and the corresponding diameter of these three fiber states are displayed in Figs. 10(a), (b), and (c). The process of stripping and recoating optical fibers involves the use of a coating stripper and a fiber recoater, respectively. The boundaries between the stripped and recoated sections, as well as the stripped and unstripped sections, are shown in Figs. 10(d) and (e). The experiment involved bending the fiber into a circular shape and gradually reducing its diameter until it broke. Therefore, A smaller bending broken diameter indicates higher bending strength. The experimental result is displayed in Table 2. The recoated fiber has a bending broken diameter that is 26.6% and 20% smaller than the stripped fiber with stripping lengths of 50 mm and 100 mm, respectively. This indicates that the recoated fiber has greater bending strength than the stripped fiber. Additionally, the bending broken diameter of the 100 mm stripped length is smaller than that of the 50 mm length for both stripped and recoated fibers. The reason for this is that the fiber ring diameter of the 100 mm stripped length is larger than that of the fiber ring 50 mm stripped length when there is only the stripped region on the fiber ring. As a result, when the diameter is continually reduced, the fiber ring of 100 mm stripped length has more stable mechanical properties to show the smaller bending broken diameter. The bending broken diameter of the unstripped fiber is less than 6 mm, which is smaller than in other situations. The results indicate that fabricating FBG without stripping the coating is important for maintaining its mechanical strength.
Fig. 10. The micrograph of fiber states of (a) recoated, (b) unstripped, and (c) stripped, as well as the boundaries micrograph of (d) unstripped and recoated and (e) unstripped and stripped.
Table 2. The bending breaking diameter of three optical fiber states
6. Conclusion
In this paper, a diaphragm-based femtosecond laser direct writing method is proposed, which can be used for large-area FBG fabrication. We demonstrate the good fitting relationship of the length of the RI modulation line to the diaphragm aperture and the power before the objective. Therefore, we can fabricate a uniform RI modulation plane in a single scanning, and precisely control its size, in which the maximum size of 30 × 29.1 µm. To the best of our knowledge, this is the largest size of FBG, which is fabricated using the femtosecond laser direct writing method in silica-based fiber. Using this method, we fabricate the high-quality HR-FBG and CFBG in the LMA Ge-doped 20/400 double-cladding passive fiber core. The HR-FBG has a reflectivity of greater than 99.6% at 1079.916 nm, an insertion loss of 0.165 dB, and a thermal slope of 0.05 °C/W. The CFBG has a reflectivity of greater than 99.1% near 1079.3 nm and a 3-dB bandwidth of 2.704 nm. By annealing the CFBG, the insertion loss and the thermal slope are reduced from 0.304 dB to 0.162 dB and from 0.327 °C/W to 0.088 °C/W, respectively. Both FBGs have no obvious temperature increase at the calibration power of 846 W when using the water cooling. In addition, we demonstrate that the fiber without stripping has greater bending strength than the stripped fiber and the recoated fiber, which shows the necessity of fabricating FBG through the coating. In summary, this method for FBG inscription offers several advantages, including reducing the insertion loss and the thermal slope, the fast fabrication, the large and uniform RI modulation plane, the adjustable size, and the fabrication through the coating, making it highly advantageous for high-power all-fiber oscillators in special wavelengths.
Funding
National Natural Science Foundation of China (11974427, 12004431); Science and Technology Innovation Program of Hunan Province (2021RC4027).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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