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Abstract
Chirped and tilted fiber Bragg gratings (CTFBGs) are important all-fiber filtering components in high-power fiber lasers for stimulated Raman scattering (SRS) suppression. The fabrication of CTFBGs in large-mode-area double-cladding fibers (LMA-DCFs) by femtosecond (fs) laser is reported for the first time to the best of our knowledge. The chirped and tilted grating structure is obtained by scanning the fiber obliquely and moving the fs-laser beam relative to the chirped phase mask at the same time. By this method, the CTFBGs with different chirp rates, grating lengths, and tilted angles are fabricated, and the maximum rejection depth and bandwidth are ∼25 dB and ∼12 nm, respectively. To test the performance of the fabricated CTFBGs, one is inserted between the seed laser and the amplifier stage of a 2.7 kW fiber amplifier, and an SRS suppression ratio of ∼4 dB is achieved with no reduction in laser efficiency and degradation in beam quality. This work provides a highly fast and flexible method to fabricate large-core CTFBGs, which is of great significance to the development of high-power fiber laser systems.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
High-power fiber lasers have been extensively researched and applied in many fields due to the advantages of good beam quality, high efficiency, compactness, and stability in the past decade [1,2]. With the development of high-brightness pump sources, LMA-DCFs components, and new pumping schemes, the output power of fiber laser has been rapidly improved [3–6]. However, the fiber nonlinear effects are main limiting facts for power scaling, especially SRS. Many methods have been proposed to suppress SRS in high-power fiber lasers, such as LMA fibers [7], spectrally selective fibers [8,9], and lumped spectral filters like the long-period grating (LPG) [10–12] and CTFBG [13–20]. With the LMA fibers, the enlarged mode area could lead to degradation of the beam quality and reduce the threshold of transverse mode instability (TMI) [21]. The fabrication of spectrally selective fibers is complex and immature. The filtering characteristics of the LPG are not stable due to the high sensitivity to temperature, strain, and bending. In contrast, the CTFBG with good stability may be more suitable for SRS suppression in high-power fiber lasers. Up to now, the CTFBGs have been utilized to suppress SRS in multi-kW fiber lasers [15–20] and successfully commercialized [22–24], which highlights its significant application value.
The traditional method of fabricating CTFBGs is using nanosecond ultraviolet (UV) lasers and a chirped phase mask [25,26]. However, the UV laser exposure method requires the photosensitization of the fiber, so the fiber should be hydrogen-loading [27] and thermal annealing [28] before and after the FBG inscription, and the time of hydrogen-loading and thermal annealing increases with the growth of fiber diameter. For example, when the CTFBGs are inscribed in the LMA-DCFs with core/inner cladding diameter of 20/400 µm [14,26] or 25/400 µm [19] which are usually used in the high-power fiber laser system, the time of hydrogen-loading generally exceeds 30 days. Moreover, when the CTFBGs are used to handle high-power laser, a thermal annealing time of at least 15 days is required to remove residual hydrogen and hydroxyl in the CTFBGs [14,19]. With the development of fs-lasers inscription technology, it provides a promising method to replace the UV-laser exposure method for FBG inscription [29,30]. The fs-lasers do not require the photosensitivity of fiber and can realize through-the-coating inscription, indicating that the hydrogen-loading, thermal annealing, stripping coating and recoating are no longer necessary, which extremely shortens the FBG fabrication time.
There are two main types of fs-laser inscription technology, namely fs-laser direct inscription technology [31] and fs-laser phase mask technology [32]. The former is often used to inscribe FBGs in the fiber sensing field due to the flexibility of inscription and relatively high insertion loss [33]. While the latter has the advantages of low insertion loss and excellent repetition of inscription, which is more suitable for high-power FBGs inscription. Recently, the chirped FBGs (CFBGs) fabricated by the fs-laser phase mask technology have been used as cavity mirrors in high-power fiber laser oscillators [34–38], and the maximum output power of 5 kW has been achieved [35,38], indicating that the fs-laser phase mask technology has the potential to fabricate high-power FBGs. So far, two schemes have been proposed for tilted FBGs (TFBGs) inscription based on the fs-laser phase mask technology. One is to rotate the phase mask around the fs-laser beam axis, but the fabricated fs-TFBGs have a relatively high insertion loss and small refractive index modulation region [39]. The other is to make the focal spot of the fs-laser scan obliquely in the fiber core [40–42], and the fs-TFBGs inscribed by this scheme have low insertion loss. However, the scanning scheme has only been used for TFBGs fabrication in single-mode fibers. Moreover, to date, though the fs-CFBGs and fs-TFBGs have been fabricated, there are no reports on the CTFBG fabricated by fs-lasers.
In this paper, the CTFBGs are fabricated in LMA-DCFs based on the fs-laser phase mask technology. To the best of our knowledge, this is the first time to fabricate CTFBGs by fs-lasers. The tilted grating planes are obtained through oblique-scanning of the fiber, meanwhile a larger chirp is introduced into the grating by moving the fs-laser beam relative to the chirped phase mask. Furthermore, the method principle of CTFBGs fabrication is numerically analyzed, and the CTFBGs with different chirp rates, grating lengths, and tilted angles are fabricated to demonstrate the flexibility and stability of the inscription setup. The fabrication time of each CTFBG is less than 1 hour, and the insertion loss is less than 0.1 dB. The maximum rejection depth and bandwidth of the fabricated CTFBGs are ∼25 dB and ∼12 nm, respectively. Moreover, a high-power fiber amplifier is established to test the CTFBGs. By introducing one CTFBG between the seed and the amplifier stage, an SRS suppression ratio of ∼4 dB is realized without a decrease of laser efficiency and degradation in beam quality.
2. Fabrication and measurement
2.1 Inscription method and principle
Based on the fs-laser phase mask technology, the CTFBGs were inscribed in LAM-DCFs with core/inner cladding diameter of 20/400 µm, and the inscription setup is shown in Fig. 1. We used a fs-laser source system to generate collimated 515 nm fs-laser beam with 290 fs pulse duration, 210 µJ pulse energy and 1 kHz repetition rate, and the fs-laser beam was reflected by a set of reflecting mirrors and then was incident on the cylindrical lens. The fs-laser beam (beam diameter ωo≈3 mm) was focused by the cylindrical lens (focal length f = 25 mm) through the linear chirped phase mask into the fiber core. Two phase masks were used in the inscription setup with the same pitch of 1577.4 nm and chirp rates of 0.4 or 2 nm/cm, respectively. The focal line width ω≈5.4 µm can be obtained according to the ω≈4λf/πωo, but the actual focal line width in the fiber core was smaller than 5.4 µm due to the cylindrical lens effect of the fiber’s circular surface. The actual focal line width ωac≈3.7 µm can be evaluated as ωac≈ω/ncl, where ncl is the refractive index of fiber cladding [29], which is much smaller than the fiber core diameter. Thus, the tilted grating plane can be realized when the fiber was scanned obliquely via the piezoelectric platform. Meanwhile, because the cylindrical lens and reflecting mirrors were both placed on a one-dimensional translation stage that can move along the X-axis, the chirped grating structure can be elongated by moving the fs-laser beam relative to the linear chirped phase mask and fiber.
To further explain the oblique-scanning strategy for inscribing CTFBG, a schematic illustration is presented in Fig. 2. The green patterns are interference fringes in the fiber core. The black and red dotted lines are the scanning paths of the interference fringes with the routine and maximum scanning angles, respectively, which can be seen as the formed tilted grating planes. Thus, the tilted angle of the grating plane with the routine scanning angle can be expressed as follows [41]:
According to Eq. (2), in the case of a certain focal line width ωac, the maximum tilted angle θmax is determined by the minimum grating period, so the θmax = arctan(Λg/2ωac).
Moreover, the modes coupling characteristics of the CTFBG are also shown in Fig. 2. Obviously, the CTFBG has the characteristics of not only TFBG but also CFBG. When the grating planes are tilted in a TFBG, the forward core modes would be coupled to numerous backward cladding modes, forming a series of discrete cladding mode resonances at the short wavelength range in the TFBG transmission spectrum. Moreover, the forward core modes also would be coupled to backward core modes at the Bragg resonance wavelength, and the Bragg reflection can be observed in the TFBG reflection spectrum. When the chirped grating period is introduced in a TFBG to form a CTFBG, the backward cladding mode resonances can be broadened, causing the fact that the adjacent cladding mode resonances can overlap and form a wide rejection band in the CTFBG transmission spectrum. And the bandwidth of core mode Bragg resonances also increases in the CTFBG reflection spectrum. It is worth noting that when using the oblique-scanning strategy to inscribe the CTFBG, the grating period Λg + Cx along the X-axis does not change at different tilted angles. Thus, the core mode Bragg resonance wavelength λBragg = 2ncore(Λg + Cx) is also not changed with the tilted angle, where ncore is the effective refractive index of core mode, and the bandwidth of core mode Bragg resonances can be expressed as ΔλBragg = 2ncoreCL, where L is the grating length.
2.2 Measurement results
As a band rejection filter, the rejection band characteristics of CTFBG can be designed via the tilted angle, grating length and chip rate. In our inscription setup, different tilted angles and grating lengths of CTFBG can be implemented flexibly and conveniently by the vibration length of the piezoelectric platform and the moving length of the translation stage, respectively. The chirp rate of CTFBG can be changed by replacing the phase masks with different chirp rates. Thus, some CTFBGs with different parameters were fabricated to demonstrate the excellent performance of the inscription setup. The fabrication time of each CTFBG is less than 1 hour, which is greatly shortened compared with the that of UV-laser exposure method. Figure 3 presents the microscope images of the side of CTFBGs with different tilted angles. The tilted grating structure almost coves the entire fiber core by setting the scanning length of the piezoelectric platform along the Z-axis to an appropriate value. At the same time, the vibration length along the X-axis is set to different values to realize different tilted angles.
Fig. 3. The microscope images of the side of CTFBGs with tilted angles of (a) 6.4° and (b) 8.6°, which are viewed in the X-Z plane.
The spectra of the CTFBGs inscribed in LMA-DCFs with different parameters were measured, as shown in Fig. 4. By comparing the transmission spectra of the CTFBG I, CTFBG II, and CTFBG III, the rejection depth and bandwidth of the CTFBG are positively correlated with the chirp rate and grating length, that is, the total chirp of the grating. From the changes in the transmission spectra of CTFBG III and CTFBG IV, we can see that when the tilted angle of the CTFBG increases, the rejection bandwidth increases and depth decreases, and the cladding mode resonance wavelength moves to a shorter wavelength. As a result, the CTFBG III has a maximum rejection depth of ∼24 dB, and the CTFBG IV has a maximum rejection bandwidth of ∼12 nm. As can be seen from the reflection spectra of the CTFBGs, the Bragg resonance wavelength is not correlated with the tilted angle, and the bandwidth of Bragg reflection increases with the increase of the chirp rate and grating length, which is consistent with our analysis in Section 2.1. Furthermore, by using a commercial 1070 nm fiber laser and a power meter, the insertion losses of the four CTFBGs at 1070 nm are measured via the cutoff method, and all the insertion losses are less than 0.1 dB.
Fig. 4. The measured spectra of CTFBGs with different tilted angles, chirp rates and grating lengths. (a) CTFBG I, 6.4°, 0.4 nm/ cm, 10 mm; (b) CTFBG II, 6.4°, 2 nm/ cm, 10 mm; (c) CTFBG III, 6.4°, 2 nm/ cm, 20 mm; (d) CTFBG IV, 8.6°, 2 nm/ cm, 20 mm.
3. Application for SRS suppression
A high-power fiber amplifier system was established to test the performance of the fabricated CTFBGs, as shown in Fig. 5. The seed source is a commercial 1070 nm continuous-wave fiber oscillator whose output fiber is a 20/400 LMA-DCF. A CLS is inserted between the seed source and the amplifier stage to prevent the unabsorbed backward pump light from the seed source. The output and input signal fiber of the combiner is also the 20/400 LMA-DCFs, and the pump source is wavelength-stabilized fiber-coupled laser diodes (LDs) at 976 nm. The length of ytterbium-doped fiber (YDF) is 13 m, and its core/inner cladding diameter is 20/400 µm. The CLS and quartz block head (QBH) at the output end are used to remove the residual pump light and eliminate the feedback of the output facet, respectively. The CTFBG is inserted between the seed source and the amplifier stage without any other change to the system.
Figure 6(a) presents the measured output spectra at different pump power of the amplifier stage without CTFBGs. The output power of the seed source is 50 W, and the SRS occurs when the pump power increases to 2455 W. The center wavelength of Raman light is near 1123 nm, corresponding to the Raman shit of 1070 nm signal wavelength. Thus, the CTFBG IV with the rejection band center wavelength of ∼1123 nm is selected to suppress the SRS. To avoid the effects of the insertion loss of CTFBG IV and the loss of two splice points, the seed power remains unchanged at 50 W. Moreover, the CTFBG IV is neither recoated nor cooling-packaged, and its temperature does not increase obviously after handling 50 W laser power. The measured output spectra with CTFBG IV are shown in Fig. 6(b). We can see that the intensity level of the Raman light is lower than that without CTFBG IV at the same pump power. Because CTFBG IV filters out the Raman light component in the output power from the seed source, the Raman light intensity injected in the amplifier stage is reduced, suppressing SRS in the amplifier stage. Figure 6(c) displays the SRS suppression spectrum, which is obtained by subtracting two output spectra with and without the CTFBG IV at the maximum pump power. The real SRS suppression ratio of ∼4 dB is less than the maximum rejection depth of CTFBG IV of 14 dB. This is because the Raman light component in the output power from the seed source is very small, and the Raman light component filtered by CTFBG IV is limited, which was also observed in the previous reports on SRS suppression by CTFBG written by UV laser [15,16,20]. Thus, the performance of fs-written CTFBG is normal, and the low SRS suppression is due to the fiber laser system. Figure 6(d) shows the output power evolution and the beam quality without and with CTFBG IV. Because the Raman light ratio of total output power is low, the slope efficiency remains ∼80% and the maximum output power is still ∼2.7 kW with CTFBG IV. The M2 factor at the maximum output power increases slightly to 1.33 with CTFBG IV. Considering the influence of two splice points on the beam quality, the CTFBG IV does not cause the degeneration in beam quality.
Fig. 6. The measured output spectra at different pump power (a) without CTFBG and (b) with CTFBG; (c) the SRS suppression spectrum at the maximum output power; (d) the output power evolution without and with CTFBG. Insert is output beam profile at the maximum output power
4. Conclusion
We have fabricated CTFBGs in LAM-DCFs by fs-lasers for the first time. The chirped and tilted grating structure is implemented by scanning fiber obliquely and moving the fs-laser beam relative to the chirped phase mask at the same time. The principle of inscribing CTFBGs by this method is numerically analyzed, and CTFBGs with different parameters are fabricated. The fabrication time of each CTFBG is less than 1 hour, and the insertion loss is less than 0.1 dB with the maximum rejection depth and bandwidth of ∼25 dB and ∼12 nm, respectively. Furthermore, a 2.7 kW fiber amplifier is used to investigate the performance of CTFBGs, and the SRS suppression ratio of ∼4 dB is achieved when a CTFBG is introduced between the seed laser and the amplifier stage. The introduction of the CTFBG has no effect on the laser efficiency and beam quality.
Using fs-laser to inscribe the CTFBG not only greatly shortens the fabrication period, but also promotes the development and application of the CTFBG. The further work that can be carried out is to achieve through-the-coating inscription of the CTFBG by using a cylindrical lens with high NA and short focal length. The power handling capability of the fs-written CTFBG would be studied by introducing it at the output end of high-power fiber lasers. Also, since the fs-laser has no requirement on fiber photosensitivity, fs-CTFBG can be directly inscribed in the YDF to suppress SRS.
Funding
National Natural Science Foundation of China (11974427, 12004431); Science and Technology Innovation Program of Hunan Province (2021RC4027); State Key Laboratory of Pulsed Power Laser Technology (SKL2020ZR05, SKL-2021-ZR01).
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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