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Abstract
We fabricated a fiber-optic directional coupler based on evanescent field coupling between side-polished large mode area (LMA) double clad fibers (DCFs) for a high power fiber laser. The tapping ratio of the fabricated coupler was measured to be - 32 dB. The fundamental mode coupled in a core of the lower side-polished fiber (SPF) was transferred to the upper SPF without clad-mode coupling. Two SPFs were directly faced to increase an optical handling power up to 740 W. The tapping ratio of the coupler was constantly maintained at the applied laser output. The beam quality of the laser including the fabricated coupler was maintained to be 1.22, without mode distortion by the coupler.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Recently, high power Ytterbium-doped-fiber(YDF) lasers have been intensively studied because of their advantages, such as high beam quality, high efficiency, high stability, and small footprint. The lasers can be potentially applicable for science, industry, and defense [1–3]. For the defense application, the high power single mode fiber lasers are required because the laser should be delivered to far targets. Since the 1 kW fiber laser was reported, the kW-levels single mode fiber lasers based on the oscillator or amplifier have been demonstrated [4–9]. To increase the laser output power, beam-combining methods were reported such as spectral beam- combing (SBC) and coherent beam-combining (CBC) [10–13]. The output power of beam-combined lasers was increased up to dozens of kW [14–16]. High power fiber laser channels with narrow linewidth were included in the laser systems. In the narrow linewidth high power fiber laser channels, the range of the used LMA DCF core diameter in a power amplifier is from 20 μm to 30 μm and the numerical aperture (NA) of the core is around 0.065. The clad diameter of LMA DCF is 400 μm. The fundamental mode and high-order-mode (HOM) such as linear polarization (LP) 11 and 02 mode could be coupled in the core of the fiber. The LP 02 mode is totally removed by fiber coiling in the high power fiber laser [9]. The polarization extinction ratio (PER) and beam quality of the laser channel are degraded by the coupled and induced LP 11 mode.
In the beam-combined laser systems, the indirect methods of the laser output power measurement have been studied to monitor the output power of the laser channel. The kW-levels fiber laser output power is indirectly monitored by a photo-detector(PD). It is placed near the delivery fiber. The accuracy of the indirect methods was relatively low compared to the measurement of output power using a power meter because the measured value was related with the position of PD. The usage of fiber-optic coupler could be an alternative to increase the accuracy of the measurement, which is a direct measurement of the tapped laser output power. To use the coupler in the high power fiber laser, the performance of the coupler was satisfied with several requirements such as high optical handling power, low insertion loss, suppression of mode distortion, robust structure, compatibility with delivery fiber and mitigations of thermal increase. Also, the coupler can be potentially used for polarization state monitor of the high power fiber laser because the polarization state of tapped laser is correlated with characteristics of the laser. In the previous work [13], the bulk optics were used for the polarization state monitor of the laser, which was burden to decrease the size and weight of the laser channel.
The fused, thin-film-based, and directional couplers have been used for the fiber lasers, optical communications and sensor applications. These fiber couplers cannot be used for the high power fiber laser tap monitoring because the optical handling power is limited by structure of the coupler and properties of inserted materials in the couplers. Among the fiber couplers, the directional fiber coupler is consisted of two side-polished fibers (SPFs). The SPF can be applied for optical sensors, saturable absorbers, and modulators [17–20]. The coupler based on SPF has several advantages such as robust structure, compatibility with delivery fiber, and high optical damage threshold. The distance between cores and interaction length of SPFs are related with a tapping ratio. The coupler with more than - 30 dB tapping ratio is suitable for the kW-levels high power fiber laser because the tapped laser output power is related with the propagation loss of the laser. The tapped laser output power by the coupler should be minimized. In the previous works [21], the refractive index matching oil is applied on a surface of SPF, which supports the mode coupling and reduces a fabrication difficulty of the coupler. However, the high power laser beam can be absorbed at the refractive index matching material, which causes the thermally induced damage in the coupler.
In this study, we proposed an improved fiber-optic coupler for high power fiber laser. The core size and clad size of side-polished LMA DCF were 25 μm and 400 μm, respectively. The core-coupled mode in the lower SPF was transferred to the upper SPF via evanescent field coupling. It should be noted that refractive index matching oil was not used during this work. Although the fiber-optic directional couplers were already demonstrated, the LMA DCF-based fiber optic coupler was not reported yet, to the best our knowledge. The clad mode coupling in the upper SPF was suppressed by alignment of the fiber. The fiber coupler with - 32 dB tapping ratio was fabricated for the monitor of high power laser output power. The coupler was inserted in a 740 W single-mode fiber laser to evaluate the performance of the coupler. The tapping ratio was constantly maintained in the 740 W laser. Thermal increase by absorption and scattering of the laser was not observed at the transferred region. The beam quality of the laser with the coupler was measured to be 1.22.
2. Experimental results of the fiber coupler
Fig. 1. (a) The schematic illustration of the directional coupler (inset: photography), (b) cross-section of the transferred region in the coupler and (c) transmission spectrum in the coupler ports
Figure 1 shows a schematic illustration of the directional coupler which is made of two side-polished LMA DCFs. The size of the quartz block was 40 (W) x 10 (D) x 5 (H) mm. The core and clad size of LMA DCF(LMA-GDF-25/400-M+, Nufern, fiber core numerical aperture(NA): 0.065) were 25 μm and 400 μm, respectively. The LP01, LP11, LP02, and LP21 mode could be guided in the fiber at 1064 nm. The fiber radius of curvature was 25 cm, which is an optimized condition for the fabrication of the SPF [20]. In the preparation of the SPF, the fiber was mounted on the V-grooved quartz block using UV epoxy. Then, the fiber was grinded to remove an upper clad of the fiber. After grinding, it was polished to reduce the insertion loss of the SPF. The refractive index matching oil was not applied on the surface of the SPF to increase the optical and thermal damage threshold value of the coupler. The SPFs were faced as shown in Fig. 1(b). It was estimated that the distance between the centers of two fiber cores was around 45 μm. The two SPFs were intentionally aligned to reduce the tapping ratio of the coupler. Although the oil was not applied on between two SPFs, the evanescent field in the lower SPF could be coupled with the core of the upper SPF. The UV epoxy was used to fix the aligned two SPFs. Guide lines were carved on the surface of the quartz block to prevent the UV epoxy moving from edge to center of the SPF. As a result, no absorption materials were in the transferred region of the coupler. Figure 1(c) shows the wavelength dependence of the coupler. An 1 μm broadband source was used as an input source. The wavelength dependence was not observed at both ports, which shows that the fabricated coupler could be used for the broadband operation.
Fig. 2. The mode profile of (a) core-coupled single mode in the upper SPF and (b) clad-coupled high order mode in the coupler
The upper SPF was delicately aligned to suppress the clad-mode coupling in the upper SPF. The fundamental mode fiber laser was prepared for the fabrication of the coupler. For removal of the HOM in the core, the bare LMA DCF was coiled to induce the propagation loss of HOMs. The diameter of coiling was less than 10 cm. The calculated fiber bending losses for LP01 and LP11 mode at coiling diameter of 10 cm were 0.03 dB/m and 4.5 dB/m, respectively. The fiber bending loss of LP01 mode was measured to be less than 0.2 dB including a splicing loss of LMA DCF. Then, high refractive index oil was applied on the surface of clad, which was similar to clad light stripper (CLS). At the transferred region, the position and shape of the fundamental mode in the SPF was changed by the radius curvature of the fiber [22]. After the coupler, the tapped laser became the well-defined single mode in the fiber as shown in Fig. 2(a), where HOMs was not observed. Also, the mode distortion by the structure of the coupler was not observed. When the upper SPF was misaligned, the transferred fundamental mode was coupled with the clad of the upper SPF. Then, the mode became HOMs as shown in Fig. 2(b). An alignment of SPF was optimized to fabricate the coupler with more than - 30 dB tapping ratio. The tapping ratio of the fiber coupler was measured to be - 32 dB at the applied laser output power. Although the coupler was suitable for the high power fundamental mode tapping, the LP11 mode could be theoretically transferred from the lower SPF to the upper SPF because the same fiber was used for SPFs and the effective refractive index of two modes were similar. The calculated effective refractive index of the fundamental mode and LP11 mode was 1.4512 and 1.4508, respectively. For those reason, the suppression of transferring HOMs was limited in the coupler. Also, the tapping ratio depending on HOMs would be changed by difference of evanescent field.
The fabricated coupler was inserted in the high power fiber laser system to evaluate the characteristics of the coupler. The high power fiber laser was a master oscillator power amplifier (MOPA) as shown in Fig. 3(a). The seed laser was modulated by the filtered pseudo-random bit sequence (PRBS) signals of 10 GHz linewidth. The center wavelength of seed laser was 1064 nm. The 10/125 μm and 25/400 μm Ytterbium doped DCF were used for pre-amplifier and power amplifier, respectively. The delivery fiber was 25/400 μm Germanium doped fiber (GDF). The maximum output power of the laser was 740 W levels, which was limited by the applied pump power. The coiled 25/400 μm YDF was enough to remove the HOM in the laser. The mode instability (MI) was not observed during the operation of the laser. Before the coupler, the CLS was placed in the laser to avoid an optical damage of the coupler by the residual pump power and clad-guided mode of the laser. The end-cap and collimator were included in the laser. The beam quality of the laser without the coupler was lower than 1.3. Figure 3(b) shows the performance of the coupler in the high power fiber laser. The measured average tapping ratio of the coupler was - 32 dB as a function of the applied laser output power. The tapping ratio of the coupler was maintained when the high power laser was propagated in the coupler. The insertion loss was measured to be - 0.20 dB at the applied laser output power of 740 W. The beam quality of the laser with the coupler was measured to be 1.22 as shown in Fig. 4. The measured beam quality of the laser with the coupler was similar to that of the laser without the coupler. Also, the mode distortion was not observed by the coupler. Figure 4 inset shows the focused single mode profile of the laser. The coupler was compatible with delivery fiber because the fiber of the coupler was the same fiber.
Fig. 3. (a) the schematic diagram of the 740 W MOPA fiber laser and (b) the evaluated performance of the fabricated coupler as a function of applied laser output power
Fig. 4. Measured beam quality of the fiber laser with the coupler at laser output power of 740 W (Inset: the focused mode profile of the laser)
In case of high power fiber laser, the thermal increase was induced by laser scattering at the defected surface of the fiber. When the 740 W laser was propagated into the coupler as shown in Fig. 5(a), the thermal increase was not observed at the transferred region as shown in Fig. 5(b). However, the thermal increase was observed at the edge of the coupler, which resulted from the clad-coupled light scattering at the defected surface of the fiber. Although the clad-coupled mode was suppressed by the alignment of the SPF, the tapped laser could be partially coupled in the clad. The fiber could be mechanically damaged in the fabrication process. The fabrication process should be optimized to mitigate the mechanical damage of the fiber. Figure 5(c) shows the thermal increase of the coupler depending on the laser output power. When the initial temperature of the coupler was 21.5 ℃, the observed temperature was increased up to 39.3 ℃ as a function of laser output power. Though the fabricated coupler was evaluated at 740 W laser output power due to the limitation on the pump power, the coupler could be used for the kW-level high power fiber laser. When the kW-level high power fiber laser was propagated into the coupler, the estimated temperature of the coupler was 46 ℃. It is lower than the thermally damaged temperature of the fiber [23].
Fig. 5. (a) The schematic diagram of the coupler, (b) the thermal image of the coupler, and (c) the thermal increase of the coupler as a function of laser output power
3. Conclusion
The LMA DCF-based optical fiber coupler for the high power fiber laser was fabricated using two SPFs. The SPFs were mechanically coupled to increase the optical handling power up to 740 W. The insertion loss and coupling ratio of the coupler were measured to be - 0.20 dB and - 32 dB, respectively. The coupler was inserted in the high power fiber laser to evaluate the thermal increase and the performance degradation of the coupler. The thermal increase was observed at the edge of the coupler. The temperature of the fiber was increased as a function of the laser output power. The maximum temperature was 39.3 ℃ at the applied laser output power of 740 W. However, the thermal increase was not observed at transferred region of the coupler. The beam quality of the laser including the coupler was measured to be 1.22. The coupling ratio and beam quality were constantly maintained during the operation of the laser, respectively. As the experimental results, the fabricated coupler was suitable for the kW-level high power fiber laser to monitor the laser output power. Also, the size and the weight of the narrow linewidth high power fiber laser for beam combining could be decreased by usage of the coupler.
Acknowledgments
We wish to thank KS Photonics that have contributed device fabrication.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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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