1. Introduction

Different applications including sensor technology, material processing, or medical usage require short pulse or continuous wave laser sources with high output power and high beam quality [1–3]. A prime example is the development of single-frequency laser sources for the next generation of interferometric gravitational wave detectors, which require very high output power of up to $500\,$W with linear polarization in the TEM$_{00}$-mode [1]. For the development of such laser sources, current research is focused on the potential of fiber-based amplifier systems. One important advantage of these over crystal-based laser sources is the simplified thermal management due to the favorable surface-to-volume ratio as a fundamental characteristic of optical fibers. The consideration of thermal lensing thoroughly affects the beam quality of crystal-based amplifiers [4], which limits the opportunities for power scaling without redesigning integral parts of the laser system. Fiber-based systems, meanwhile, are less affected and power scaling is enabled in a modular approach. This allows realization of higher output power levels without degradation of beam quality, when the system is upgraded throughout its lifespan. Furthermore, fiber-based laser systems are distinct because of their high efficiency and excellent beam quality on a small footprint. In particular, single-frequency amplifiers in master oscillator power amplifier (MOPA) configurations have been investigated to deliver highest beam quality with output powers of $200\,$W and above [5–7]. Based on these fiber-based amplifiers, coherent beam combining has been investigated as an additional power scaling approach [8].

Power scaling in fiber-based laser and amplifier systems with narrow laser linewidth is limited by non linear effects such as stimulated Brillouin scattering (SBS) [9] or transverse mode instabilities [10]. Fibers with larger core diameters are used to reduce the occurring optical intensity and shift the onset of non linear effects to higher optical power. Standard step-index fibers are limited to core sizes of around $25\,\mathrm {\mu }$m ($\text {NA}=0.07$) when near single-mode beam quality is required. Thus, other fiber designs such as the photonic crystal fiber (PCF) or the chirally coupled core (CCC) fiber have been investigated. The design of these fibers allows for larger core diameters of $34\,\mathrm {\mu }$m and above while maintaining a high $\text {LP}_{01}$ mode content. While the PCF achieves this through introduction of air-filled capillaries [11], the CCC-fiber is based on an all-solid design; it uses one or more additional side cores that are helically wound around the signal core in an otherwise standard step index structure. The side core helix is composed to fulfill a quasi phase matching condition between higher-order modes propagating in the fiber’s signal and side cores, which allows for coupling of these modes [12]. Moreover, the side core is designed for single-mode guidance only, so that higher order modes experience high loss into the fiber’s cladding. This effectively enables operation with high fundamental mode content at core diameters beyond the single-mode condition of standard step-index fibers. Additionally, the all-solid design is advantageous because fiber integration can be carried out via existing methods for standard fibers (e.g., fiber splicing, end-cap fusing, and cladding-light-stripper manufacturing). As demonstrated by Hochheim et al. [6], by introducing this fiber concept, further power scaling in a single-frequency MOPA configuration can be achieved.

To realize high output power in conjunction with alignment-free and low-maintenance operation, fiber-based lasers or amplifiers require highly efficient fiber components such as mode field adapters, cladding light strippers, fiber end caps and, most importantly, signal-pump combiners (SPCs). An SPC couples the required pump light of multiple high-power laser diodes into the active gain fiber of the laser system. Theeg et al. [13] introduced an efficient SPC design concept (pump coupling efficiency >90%) with side-fused intermediate fibers (IFs) and an uninterrupted (splice-less) signal fiber (SF). A schematic representation of this SPC structure is presented in Fig. 1. Because of the uninterrupted signal core, and thus low signal loss, the SPC is usable in counter-pumped configurations, which are beneficial for SBS suppression [14], without degrading the beam quality. In a previous publication [15], we presented the first SPC with an integrated signal feed-through $34/250$-$\mathrm {\mu }$m CCC-fiber with four multi-mode pump input fibers and pump coupling efficiency of 78%. The lower coupling efficiency is caused by discontinuous fusing between IFs and the SF as a result of the uneven cladding structure of the CCC-fiber [16], created by the rotation of the octagonal preform during fiber drawing. In this previous work, it was concluded that the cladding structure must be altered or the design and manufacturing parameters of the SPC must be adjusted to improve the pump coupling efficiency.

 

Fig. 1. Schematic representation of the SPC design using coreless intermediate fibers (IFs) with the advantage of an uninterrupted core of the CCC signal fiber (CCC-SF). The coupling efficiency of power within the pump fibers (PFs) to the CCC-SF can be controlled by adjusting the taper length (TL) and taper waist (TW).

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In this work, we present the development of an SPC based on a $34/250$-$\mathrm {\mu }$m CCC-fiber that is partially ablated into a circular shape to allow for even and continuous side-fusing along the fiber. First, ray tracing simulations of an SPC model were performed to optimize different combinations of production parameter values toward high pump light coupling efficiency. The simulation clearly indicates that the cladding structure of the CCC-fiber affects the pump coupling efficiency. Although the simulations show that a higher degree of fusion between the IFs and SF increases the pump coupling efficiency, this approach was neglected because of manufacturing parameter restrictions of the SPC processing rig.

Second, an additional production step in the SPC manufacturing process based on $\text {CO}_2$-laser ablation was applied that selectively removes the wavy cladding structure before the IFs were side fused. During laser machining, the CCC-fiber was rotated around its axis while the laser beam was scanned over the lateral fiber edge to selectively ablate parts of the cladding to form an even and round cladding along the CCC-fiber. We confirmed the successful removal and high surface quality of the cladding with scanning electron microscopy (SEM) imaging and a power loss measurement, which shows no indication of power losses beyond the measurement error of the experimental setup.

Third, the preprocessed CCC-fiber was integrated into an efficient and low-loss SPC in the presented side-fused design. A coupling efficiency of over 90% was achieved with four $106.5/125$-$\mathrm {\mu }$m input pump fibers having the NA of $0.22$. The signal-to-pump isolation of $30\,$dB was measured, which guarantees safe operation of the pump diodes in a counter-pumped amplifier configuration even at very high signal power. Additionally, the SPC was stress tested for $5\,$h at the input power of >$500\,$W and no indication of degradation is observed. The power loss of 3% within the housing was measured and 6% of the input power was transmitted within the IFs. This SPC represents a necessary development step toward monolithic CCC-fiber-based laser and amplifier systems, and thereby enables further power scaling of all-fiber-based systems that provide a high-power laser beam in conjunction with exceptional beam quality.

2. Simulation of a signal-pump combiner with an integrated feed-through CCC-fiber

In this section, ray tracing simulations of the SPC model with an integrated CCC-fiber are presented. These simulations agree well with the experimentally obtained pump power coupling efficiencies reported in our previous work on CCC-fiber integration into the SPC design [15]. This confirms that the cladding structure of the CCC-fiber distinctly impacts the SPC coupling efficiency. Furthermore, approaches for increasing the coupling efficiency, particularly regarding modification of the described cladding structure, are investigated.

The theoretically achievable coupling efficiency of an SPC with an integrated feed-through CCC-fiber was determined by modeling the SPC structure (c.f. Figure 1) using the commercial ray tracing software Zemax OpticStudio in the non-sequential mode. The three-dimensional SPC model was derived from the representation shown in Fig. 1 with one pump input fiber (PF). As introduced by Theeg et al. [13], this SPC design uses tapered IFs to achieve high coupling efficiency of light from the PF to the target SF. The coupling efficiency is affected by three parameters: the taper length (TL), the taper ratio (TR) of the IF, and the degree of glass fusion between the IF and SF. The TR is defined as the ratio between the initial diameter of the IF and its diameter at the taper waist. For all cladding structures, the refractive index of glass at $976\,$nm was used. The PF core was modeled to have the NA of $0.22$, while the CCC-SF coating was modeled to result in a cladding NA of $0.48$. The SPC-production rig allows a degree of glass fusion overlap of approximately $1\,\mathrm {\mu }$m between the IF and SF. Therefore, all simulations were performed with the overlap of $1\,\mathrm {\mu }$m between the fibers. It should be noted that the fiber diameter of the CCC-fiber varies along the fiber because of its distinctive cladding structure. Here, for the general overlap along the fusion zone, the average fiber diameter was considered. The pump source was simulated as multiple rays each carrying the same amount of power, which were launched into the PF with NAs of up to 0.22. The rays were collectively detected at the SF and IF outputs, respectively, and the coupling efficiency was calculated.

The simulated coupling efficiency of an SPC with a $34/250$-$\mathrm {\mu }$m CCC-SF dependent on the TL and TR is presented in Fig. 2. As expected, higher values of the TL and TR increase the coupling efficiency. As a comparison, the simulations were additionally conducted using a standard step-index fiber of equal cladding diameter, the results of which are presented alongside the experimental results with the fixed TR of five in Fig. 3(a). From this, the CCC-SPC exhibits lower coupling efficiency of pump power to the SF and a higher amount of power transmitted in the IF. As a general trend, it is deduced that the difference in coupling efficiency reduces with larger TLs. The noticeable dips on the coupling efficiency curve of the CCC-fiber-based SPC is attributed to the IF positioning on the CCC-fiber cladding. It should be noted, that this effect could not be experimentally replicated because of variations of IF positioning relative to the SF during manufacturing. The experimental results of our previous work for a standard step index and CCC-SF with TLs of $14$ and $19\,$mm and TR of $5$ agree with the results gained by this simulation. The coupling efficiency reached in practice is likely lower because of imperfections that are not accounted for in the simulation or because the assumed fusion overlap varies. As the simulation shows, even slight changes around the assumed $1$-$\mathrm {\mu }$m overlap, lead to a significant change in coupling efficiency, particularly when using the CCC-SF. Figure 3(b) displays simulation results where the fixed TL of $13\,$mm, TR of five, and a varied fusion overlap were used. As previously described, an averaged fusion overlap was assumed for the simulation with the inhomogeneous CCC-fiber cladding diameter. As shown, the relative difference in coupling efficiency is lower for larger fusion overlaps.

 

Fig. 2. Simulated coupling efficiency (CE) of an SPC model with integrated CCC-fiber, which was simulated with different TLs and TRs at a fixed degree of glass fusion between the IF and CCC-SF. A map of the CE is presented over the feasible TR and TL ranges.

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Fig. 3. Simulation of the CE of the SPC model with integrated CCC-fiber compared with a similar SPC using a standard step-index feed-through fiber with the cladding diameter of $250\,\mathrm {\mu }$m dependent on (a) TL and (b) fiber’s fusion overlap. Both simulations used the TR of five. Fig. (a) also highlights the experimentally obtained coupling efficiency presented in [15].

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Another approach to allow improved fusion and thereby improved coupling efficiency is removal of the wavy cladding shape of the CCC-fiber. This was investigated by implementing a CCC-fiber model modified around the fusion zone with the IF. At this position, the wavy cladding structure was removed reducing the cladding diameter by $10\,\mathrm {\mu }$m. A transition zone on the fiber was modelled to emulate gradual removal of the cladding, as would be feasible during manufacturing. The result is presented in Fig. 3 and is nearly identical to the result obtained with the SPC using the step-index LMA-SF and the same TR, TL, and fusion overlap.

In conclusion, the simulations show the impact of TR, TL, degree of glass fusion, and cladding geometry on the coupling efficiency of an SPC. In particular, the cladding structure of the CCC-fiber strongly affects the coupling efficiency. Higher values for either production parameter presented can theoretically be used to reduce this difference in coupling efficiency toward SPCs with standard step-index fibers. While increasing the TL and TR during manufacturing is challenging, the degree of glass fusion cannot be increased further during manufacturing altogether. Hence, geometrical restructuring of the CCC-fiber’s cladding is the most practical approach to manufacture SPCs with high pump coupling efficiency.

3. Rotationally symmetric $\text {CO}_2$-laser ablation of the CCC-fiber cladding

As demonstrated through the simulations, the most promising and applicable approach to achieve a coupling efficiency comparable to a standard step-index-fiber-based SPC (90% pump power to target fiber) is selective ablation of the wavy CCC-fiber cladding structure to enable continuous fusing of the IF along the CCC-fiber. It is evident that any technique used to remove cladding material from the fiber should result in highest surface quality of the fiber cladding to maintain its low-loss waveguiding properties. $\text {CO}_2$-laser-based ablation of the fiber material has the potential of precise and selective glass removal, allowing for minimal contamination of the fiber and enabling arbitrary structuring and optimisation of the fiber’s cladding surface. Wellmann et al. demonstrated a $\text {CO}_2$-laser-based approach in which a flat surface was machined on one side of a fiber to access its core light [17]. In this work, this approach was transferred to perform symmetrical ablation of the CCC-fiber, as described in the following section.

The following setup was used to perform the cladding removal: the CCC-fiber was positioned under a $\text {CO}_2$-laser beam. The fiber was rotated to achieve symmetrical ablation. For this process, a Synrad 48-2 $\text {CO}_2$-laser was used. The laser beam could be scanned in 2D by a galvanometer scanner over an area of $50\,\text {mm}^2$. The beam waist diameter was calculated as $65\,\mathrm {\mu }$m with the Rayleigh length of $0.3\,$mm. The fiber was fixed to two rotatable motor stages, which could be positioned in three-dimensions within the laser-processing zone. An extraction system was installed to safely remove the ablated glass material. The positioning and ablation processes were monitored using several cameras.

Before the ablation process was performed, the CCC-fiber was prepared by removing the polymer coating, after which it was thoroughly cleaned. As shown in Fig. 4, the fiber was positioned so that the edge of the fiber cladding reached into the beam waist during the ablation process. With the fiber in place, the laser beam was scanned laterally along the fiber (cf. Figure 4). During the ablation process, the fiber was rotated around its axis to enable symmetric removal around the cladding surface. A smooth transition zone at the beginning and the end of the machined section was achieved via a flat entry angle, as shown in Fig. 4. The laser power was manually controlled using a PWM signal to control the amount of removed glass material from the fiber. All process parameters are presented in Fig. 4. The scan frequency $f_\text {scan}$, describing how often a scan pattern is driven; the scan velocity $v_\text {scan}$, describing how fast the laser beam is scanned over the processing zone; and the angular velocity of the fiber $\omega$ must be matched to achieve symmetrical ablation of the fiber cladding. It was found that the scan frequency of $1\,$Hz in conjunction with the scan pattern velocity of $0.1\text {m}\,\text {s}^{-1}$ and angular velocity of the rotating fiber of $0.6\,\text {min}^{-1}$ produces results with symmetrical ablation and low loss of pump light. From the microscopy images, variation in the cladding diameter of the used CCC-fiber batch was measured as up to $8\,\mathrm {\mu }$m because of its uneven cladding shape resulting from the octagonal shape spun during fiber drawing. Therefore, to remove the undesired cladding structure completely, it was determined that the ablation should reduce the fiber diameter by at least $10\,\mathrm {\mu }$m to ensure complete removal. The machining process was controlled by incrementally increasing the laser power up to the maximum of $3.75\,$W until the desired ablation depth was realized.

 

Fig. 4. Schematic representation of the CO$_2$-laser ablation process including the most important process parameters: laser power $P$, scan frequency $f_\text {scan}$, angular velocity of the fiber $\omega$, length of the scan pattern $l_\text {p}$, length of the ablation zone $l_\text {z}$, scan velocity $v_\text {scan}$, and the angle $\alpha$ at which the beam is directed into the fiber. The fiber was positioned in the focus plane (y-dimension) and moved into the beam so that only the outer regions of the fiber’s were ablated. The fiber was rotated while the laser beam was scanned along the fiber to achieve rotationally symmetric restructuring.

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A power transmission experiment was conducted using light at the wavelength of $976\,$nm from a nLIGHT e12 laser diode that was coupled into the cladding of the machined fiber piece while the output was recorded. Measurements were performed at multiple power levels between $11$ and $91\,$W. The average output-to-input ratio is $0.99$. The detected loss of 1% of the input power is within the measurement error of the experimental setup (relative error $\Delta _{P}=0.05$). Additionally, visual inspections were performed using a microscope and by concurrently injecting light at the wavelength of $633\,$nm into the fiber. No scattering along the machined sections were found, indicating low optical transmission loss. Therefore, it can be determined that CO$_2$-laser ablation is capable of producing the required optical surface quality to allow low-loss optical waveguiding. Furthermore, the machined CCC-fiber samples were analyzed via SEM, capable of an image resolution of $1\,$nm, to evaluate the ablation process and potential surface defects. Figure 5 compares the center of the ablated CCC-fiber piece to a pristine fiber. While the cladding structure is slightly visible on the pristine fiber piece (cf. 5 b)), the image clearly shows full removal of the wavy cladding structure on the laser-ablated section (cf. 5 a)). No impurities were detectable via the SEM imaging. The diameter of the ablated portion of the fiber was $232.5\,\mathrm {\mu }$m at its center. The exceedance of the required ablation of $10\,\mathrm {\mu }$m in diameter is related to uncertainties of the CO$_2$-laser machining process. Nevertheless, neither the SPC manufacturing nor the fiber stability is influenced by the slightly decreased fiber diameter and it is concluded that the CCC-fiber cladding structure was successfully optimized for the SPC production.

 

Fig. 5. Comparison of SEM images of a CCC-fiber section with (a) CO$_2$-laser-ablated cladding structure and (b) pristine CCC-fiber.

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4. SPC with an integrated feed-through CCC-fiber

In this section, the SPC production process is briefly introduced, before the manufactured component using the $\text {CO}_2$-laser machined CCC-fiber is characterized and evaluated.

The production process of the SPC can be divided into two steps: In the first step, two coreless pure silica IFs with the cladding diameter of $108\,\mathrm {\mu }$m were spliced to $106.5$/$125$-$\mathrm {\mu }$m PFs with the NA of $0.22$. The fibers were then thoroughly cleaned and tapered to the desired TR and TL. Tapering was performed by heating the fibers in an aluminum oxide crucible with a hydrogen gas flame to adjust their viscosity while controlling the longitudinal fiber tension using two motorized stages. In the second step, the prepared fibers were wrapped around the $\text {CO}_2$-laser machined CCC-fiber so that the entire converging taper was in contact with the modified portion of the CCC-fiber cladding. Afterward, the resulting fiber bundle was fused together. The fusion progress can be observed during the process by coupling pump light into one of the input fibers while monitoring the coupled power propagating within the CCC-fiber. Once sufficient fusing was achieved, the fiber bundle was fixed within a metal casing and hermetically sealed.

 

Fig. 6. Setup for measuring the coupling efficiency. Four high-power nLIGHT element e12 laser diodes with the maximum output power of $125\,$W at the wavelength of $976\,$nm were used as pump sources for the PFs.

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As stated, an SPC with a modified CCC-fiber using four $106.5/125$-$\mathrm {\mu }$m pump input fibers was manufactured. For comparison with the standard step-index LMA fibers, the TL of $19\,$mm and TR of five was used. The key parameter for the SPC is the efficiency of pump light coupling to the target signal fiber. Therefore, the PFs were connected to four high-power laser diodes, which were operated at several power levels, while the output of the SPC feed-through fiber and the power remaining in the IF were monitored. The setup is presented in Fig. 6. Based on this measurement, the coupling efficiency was accurately determined at several power levels. The resulting pump to CCC-fiber cladding coupling efficiency of the manufactured SPC is presented in Fig. 7. For input powers of $16$ - $503\,$W, an average coupling efficiency of $90\!\pm \!4\%$ was measured. Seven percent of the remaining power was transmitted within the IFs. The remaining three percent were assumed to be lost and absorbed in the housing, and should be dissipated during operation. As presented, each pump power input was also characterized individually. The measurements show that the different pump inputs vary slightly in coupling efficiency between 88% and 91%. This can be attributed to slight imperfections or asymmetry in the manufacturing process. At this efficiency level, in combination with the low loss, this component is comparable to similar SPCs using standard step index SFs (found in [18]). In particular, the low loss indicates that the SPC can be operated long term even at high input power levels. To test the component’s performance, a five-hour stress test at input power of >$500\,$W was performed. For this, the SPC was placed on a heat sink, which was stabilized at a temperature of approximately $18\,^{\circ }$C. The coupled power to the CCC-SF, the power remaining within the IF, and the housing temperature were recorded. The results are shown in Figs. 8(a) and(b), and confirm that the heat load to the SPC is controllable with a simple cooling strategy. While the temperature of the SPC housing rises to approximately $21^{\circ }$C, it remains at this level for the entire duration of the high-power test. The periodic variations of the temperatures (cf. Fig. 8(b)) are related to temperature variations of the cooling system. The coupled pump power also varies with the temperature fluctuation, but this is likely because of temperature fluctuations in the pump diodes, which are cooled by the same cooling system. The coupled power remains at the same level within the five hours at the calculated coupling efficiency of $91\%$.

 

Fig. 7. Coupling efficiency characterization of the SPC with a feed-through CCC-fiber and four pump input fibers. This includes several input powers of which the average coupling efficiency and the proportion of power loss into the housing was calculated (a). Each PF was also characterized individually (b).

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Fig. 8. Five-hour stress test of the SPC with the prepared CCC-fiber and four input PFs. The pump diodes were driven at maximum capacity to the cumulative input power of $503\,$W. The output of the CCC-SF and the IFs is presented in a). At maximum input power, the average of $460.7\,$W was measured at the SF output and that of $32.8\,$W at the IFs. Thus, in this experiment, the coupling efficiency of 91% and loss of $11.5\,$W into the housing was calculated, which relates to $2.2$% of the input power. The difference in coupling efficiency calculated before (see Fig. 7) is within the error margin. The resulting heat load was dissipated using a water-cooled heat sink shown in b).

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4.1 Signal-to-pump isolation

When the SPC is used in a counter-pumping configuration, the signal propagates in direction of the pump diodes, hence the signal should be sufficiently isolated towards the PFs. The setup depicted in Fig. 9 was used to measure the signal-to-pump isolation. A signal from an amplified single-mode diode was coupled into the core of the CCC-fiber on the IF-output side. The amplifier was based on a standard $10/125$-$\mathrm {\mu }$m Ytterbium-doped single-mode fiber with the core NA of $0.075$. Approximately $2\,$W of signal light power was coupled to the CCC-fiber. By detecting the output of the PFs and the CCC-fiber, the signal-to-pump isolation was calculated. The signal-to-pump isolation was measured to be around $30\,$dB at input powers of $0.5$ - $1.7\,$W. The signal-to-pump isolation over several input powers is presented in Fig. 10(a). Although it varies between the different PFs, this is comparable to the SPC using the standard step-index fiber [13] and sufficient for high power operation.

 

Fig. 9. Setup for measuring the signal-to-pump isolation of the SPC. Light of a MOPA-setup was coupled into the core of the CCC-SF. The MOPA was seeded using a $1,064$-nm single-mode diode and used an Ytterbium-doped standard PM fiber with a $10$-$\mathrm {\mu }$m core diameter and the core NA of $0.075$, which was cladding pumped using a multi-mode diode at the wavelength of $976\,$nm. Signal light coupled into the cladding of the SF was stripped by application of high index glue on the cladding (CLS). The power coupled from the cladding of the signal fiber to the pump input fibers was then tracked separately and compared with the power remaining in the signal fiber.

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Fig. 10. (a) Signal-to-pump isolation of the SPC with a CO$_2$-laser machined CCC-fiber. Highlighted are the highest and lowest average isolation for the individual PFs. (b) Excerpt of an S$^2$-measurement taken from a relevant detector position. No peaks beyond the measurement noise were detected, indicating that no higher order modes were generated by the SPC.

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4.2 Signal feed-through properties

Although, the side-fused design of the SPC allows for pump-coupling with minimal influence on the signal light guidance, it is necessary to investigate the SPC regarding signal transmission loss and transversal modal composition after propagating through the component. Because a counter-pumping configuration is required to suppress the early onset of SBS, the SPC is placed at the output side of the laser or amplifier setup. Therefore, the low signal loss and no degradation of the beam quality must be proven.

This is done by evaluating the signal attenuation and by investigating the modal content after propagation through the SPC. The modal composition was evaluated by applying the S$^2$-method. This allows accurate quantification of the number of modes and the relative power within each mode compared with the fundamental mode. A detailed description of the S$^2$-method can be found in [19]. Figure 10(b) shows a representative result of the performed S$^2$-measurement. No higher order modes were detected and the power within higher order modes is >$23\,$dB below the LP$_{01}$-mode. It should be noted that some amount of power within higher order modes is not detected by the S$^2$-method because of the modal filtering effect of the CCC-fiber and is, therefore, present as power loss. Nevertheless, assuming a higher order mode suppression of $22\,$dB/m [20], even a slight onset of higher order modes is detectable using the S$^2$-method because approximately $50\,$cm of CCC-fiber is used behind the SPC. As discussed, the excitation of higher order modes can occur as signal transmission loss because of the CCC-fiber properties. The signal transmission was measured at the signal input power of $1.7\,$W and the attenuation of $0.23\,$dB was recorded. Considering the additional power loss caused by the higher order mode filtering of the CCC-fiber, this is comparable to values found for SPCs using a step-index LMA fiber in an otherwise identical design (see results in [13]).

4.3 Structural investigation using computer tomography

In addition to the optical characterization, the SPC was structurally investigated via computer tomography (CT) imaging with a resolution of up to $0.5\,\mathrm {\mu }$m. This allows assessment of the positioning of the tapered IF, the constant fusing along the fiber, and allows measurement of the degree of glass fusion. An SPC with two input ports was prepared for the CT and was fixed onto a glass substrate to avoid interference with the measurement instrument. As presented, the coupling efficiency of the SPC is higher compared with the component without cladding modifications (coupling efficiency of 78%, as shown in [15]). Therefore, the cladding modification of the CCC-fiber performed in this work successfully improves the device. Nevertheless, the CT-imaging results of the SPCs provide further evidence for this assumption. Two representative excerpts of the tomography recordings are presented in Fig. 11. Displayed are the cross-sections of the SPC in the fusion zone of PF and CCC-SF. Figure 11(a) shows the SPC with a pristine CCC-fiber, where the octagonal cladding structure and a resulting gap between one PF and the CCC-SF is visible. Figure 11(b) shows the SPC based on the machined CCC-fiber, revealing no gaps between the IF and the CCC-SF, and thus sufficient glass fusion between the CCC-SF and the IFs.

 

Fig. 11. CT images of two SPCs with a CCC-SF in the transverse plane at comparable longitudinal positions of the SPCs: (a) with a pristine cladding structure and (b) with a restructured cladding using the CO$_2$-laser. Side fusing to a pristine CCC-fiber results in gaps between SF and IF, which can be identified in the images and lead to a lower coupling efficiency of approximately 80%. In contrary, such gaps cannot be identified for the SPC that uses the restructured CCC-fiber cladding, leading to constant fusing along the fiber and improved coupling efficiency.

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5. Conclusion

In this work, we developed an SPC with an integrated feed-through $34/250$-$\mathrm {\mu }$m CCC-fiber with the pump-to-signal coupling efficiency of 90% and low loss into the housing. The SPC relies on a side-fused design concept and enables a high signal-to-pump isolation of $30\,$dB, low signal loss of $0.23\,$dB, and unaffected beam quality, which was confirmed using the S$^2$-method. Furthermore, the SPC is high-power tested with an input power of >$500\,$W. No degradation was observed during high power operation. Enabling fiber components based on the CCC-fiber potentially allows for increased output power in an all-fiber configuration by raising the limiting threshold currently set by non linear effects.

The developed SPC is based on precise and rotational symmetric ablation of the CCC-fiber’s cladding using a CO$_2$-laser beam. For the first time, the machining process enabled the restructuring of a fiber’s cladding to improve the surface quality, by removing the wavy structure of the CCC-fiber’s cladding. This wavy surface structure was identified to lower the pump coupling efficiency in previous work.

Further optimization on this work lies primarily in automation of the various production processes to eliminate errors introduced by manual work and reduce manufacturing time. An advanced automation allows achievement of larger TRs and TLs to further improve the coupling efficiency of the component as theoretically proposed. Based on the developed fiber components, further research should focus on the realization of high power amplifiers with diffraction-limited beam quality. To enable even further power scaling in laser or amplifier systems, transferring the production to even larger fiber diameters (CCC-fiber and PFs) is conceivable.