Cladding Light Research Articles

Experimental study of chemical-etched high-power cladding mode stripping device toward high attenuation

Fiber cladding mode stripping device with high-power high-attenuation stripping capability and negligible perturbation to signal light has been developed by employing Teflon-capillary segmental-etching fabrication method. By using different etchant solutions, surface roughening and cladding reducing structures can be obtained. The influence of the roughened length on the attenuation of cladding power has been studied experimentally, showing that roughened surface strips cladding light well at length below 5 cm, but becomes less effective beyond that. The impact of the cladding reduction has also been investigated, indicating that reduced cladding can help deplete the cladding light effectively, and the attenuation is able to be improved from 23.0 dB to 43.2 dB by etching the cladding diameter from 400 μm to 50 μm. By combining the roughened surface and reduced cladding structures, cladding mode stripper with 7 °C/kW of temperature rising rate on the etched area has been made, and stripped power up to 2.1 kW at 39 dB of attenuation has been demonstrated, with the device length merely 16 cm. The maximum temperature on the device housing at full operation power for the case water flux being 3.1 L/min and 5.6 L/min is 67 °C and 50 °C, respectively. To the best of our knowledge, this is the first report of >2-kW power cladding mode stripping device with 39-dB attenuation. All results indicate the effectiveness of the proposed fabrication technique to make cladding mode stripping device for high power and superior beam quality fiber laser systems.

Efficient Suppression of Backward Light Scattering for Optical Fiber Cladding Light Stripper

This study investigates a method for the effective suppression of backscattering within optical fiber cladding light strippers (CLSs) with a periodically grooved structure. First, the relationship between the backscattering of cladding light and the surface roughness of the cladding for a periodically grooved CLS is theoretically investigated using the ray-tracing method. It is found that the backscattering of cladding light depends on the surface roughness of the grooves and that undesired backscattering significantly affects the stripping capability of a CLS. Based on the theoretical results, we propose a periodically grooved CLS structure with multiple unetched sections of a suitable length and verify its effectiveness by comparing its lightwave stripping performance with that of a conventional CLS design in both theoretical and experimental ways. Unlike the conventional structure, our proposed structure demonstrates almost uniform lightwave stripping without localized overheating.

Fiber cladding dual channel surface plasmon resonance sensor based on S-type fiber

Fiber cladding surface plasmon resonance (SPR) sensors have few structures, and a clad SPR sensor based on S-type fiber is proposed in this paper. This new type of fiber cladding SPR sensor was formed by electrofusing an S-shaped structure on the fiber to couple the light in the fiber core to the cladding. In this paper, the effects of fiber parameters on the performance of the sensor were studied by simulation and experiment. Based on the conclusion that the smaller the core diameter is, the closer the working band of the SPR resonance is to long wavelengths, and that the geometric characteristics mean that a multimode fiber can receive the fiber cladding light from a small core diameter few-mode fiber, a dual channel SPR sensor with a double S-type fiber cascade was proposed. In the refractive index detection range of 1.333–1.385 refractive index units (RIU), the resonant working band of channel I is 627.66 nm–759.78 nm, with an average sensitivity of 2540.77 nm/RIU, and the resonant working band of channel II is 518.24 nm–658.2 nm, with an average sensitivity of 2691.54 nm/RIU. The processing method for the S-type fiber cladding SPR sensor is simple, effectively solving the problem of this type of SPR sensor structure and the difficult realization of a dual channel. The sensor is expected to be used in the fields of medical treatment and biological analysis.

Experimental Research on High-Power Cladding Light Stripper Based on Domestic Power Delivery Fiber

Experimental Research on High-Power Cladding Light Stripper Based on Domestic Power Delivery Fiber

＞500 W passively-cooled fiber cladding light stripper

To realize of passive-cooling high power fiber cladding light stripper, it is important to optimize the thermal management of both the fiber and the package. By using Teflon capillaries to make segmental etching configuration on fiber, using copper as the package material, and optimizing the package structure through finite element thermal simulations, cladding light stripper capable of handling 500 W power was designed and fabricated. It was experimentally verified that the stripping efficiency reached 23.7 dB and the temperature increase rate on the bare fiber of cladding light stripper was as low as 0.007 ℃/W. In addition, at 540 W of power injection, cladding light stripper could work continuously if mounted on water-cooled cold plate, and could work for 50 s each time if mounted on cold plate filled with phase-change material, with the maximum temperature of package being 58.7 ℃ and 80 ℃ respectively. The researches and results could provide valuable information to the design and development of high power fiber lasers.

Study on tens-of-Watt all-fiber oscillator operating near 980 nm with commercially-available double-cladding ytterbium-doped fiber

In this paper, a more-than ten-Watt all-fiber oscillator operating near 980nm is demonstrated with a commercially-available 20/125-μm double-cladding Yb-doped fiber (DCYF) firstly, to the best of our knowledge. The 15-W output power is eventually obtained with the slope efficiency of 13 % and central wavelength around 978 nm. Moreover, the power scalability of the 980-nm fiber oscillator is also investigated numerically. It is revealed that more-than 20 % slope efficiency can be obtained with this kind of DCYF. It is also found that besides pump power, the cladding light stripper (CLS) is another limitation in the 980-nm fiber oscillator, and more-than 50-W output power can be obtained if more-than 80-W residual pump light can be filter out with each CLS.

>2 kW High Stability Robust Fiber Cladding Mode Stripper With Moderate Package Temperature Rising

High power cladding mode stripper device has been developed in robust configurations, which demonstrated high attenuation and low temperature rise, allowing to remove 2.16 kW of power with efficiency being 21.7 dB. The maximum temperature in packaging did not exceed 55 °C at a room temperature of 20 °C. The signal insertion loss was measured to be below 0.05 dB, indicating the ability to strip the cladding light efficiently but keep the core light almost intact. A 40-min stress test has been carried out, which confirmed the working stability and reliability of the proposed cladding mode stripper. Moreover, the device presented is in a compact and closed pack, more than 2 kW of power could be completely dissipated inside a housing with merely 200 mm of length, which could not only be benefit for suppressing nonlinear effects during power delivering but also be easily integrated into high power monolithic fiber laser systems.

Suppressing the amplified spontaneous emission in the high-power 1018-nm monolithic fiber laser by decreasing the feedback from the inner reflections

We showed that decreasing the feedback that comes from the inner reflections of counter-propagating light could suppress the amplified spontaneous emission (ASE) in the output of 1018-nm fiber lasers. With a simulation model to solve the power spectrum of multiple signals, we calculated the values of the feedback required for aimed power scaling. We realized low feedback by modifying the counter-propagating side of the setup, which included a home-made cladding light stripper on a no-core fiber that was angle-cleaved. With that, we demonstrated a 1018-nm monolithic fiber laser producing 506.4 W with 1 .58 - M 2 beam quality, 84.9% optical–optical efficiency, and 148-pm linewidth. The maximum intensity of the ASE noises was > 50 d B lower than that of the 1018-nm signal, which could allow further power scaling with safety and stability.

Highly Efficient Fiber Cladding Light Stripper Fabricated by Chemical Mask Etching Method

We presented the chemical mask etching method to fabricate an all-fiber cladding light stripper (CLS). The morphology and density of the micron-sized microstructures generated on the fiber surface could be flexibly controlled in fabrication process, which was beneficial to the temperature performance and high-power operation capacity of the device. Based on the method, a theoretical model was developed to optimize the structure of the CLS. Initial experimental processing parameters were also obtained to realize uniform scattering and high attenuation coefficient. An all-fiber cladding light stripper with a high attenuation coefficient of 42 dB was achieved when the stripped cladding light was 1.117 kW. The temperature rising rate was only 0.011 °C/ input-Watt under the condition of air cooling, thanks to all-fiber structure and the uniform scattering design. The average stripped power density reached 9.19 × 106 W/m2 within a 135 mm long CLS. The CLS fabricated by this technique has great application in the high power fiber laser system.

Practical, rapid, and process-on-place polymer-free cladding power stripping for high power fiber lasers.

We have studied polymer-free cladding power strippers for high-power fiber laser applications. A practical, rapid, and process-on-place chemical etching technique is presented to form surface roughness for efficient removal of the cladding light. The technique is methodically studied and performance contours are determined for frequently used 130, 250, and 400 µm double-clad fibers. The stripping efficiency and the thermal performance of the fabricated strippers are investigated with respect to such process parameters as etching time and etched fiber length. At least 15 dB attenuation with $ \lt {0.045}(^\circ {\rm C}/{\rm W})$<0.045(∘C/W) thermal slope is demonstrated for all fiber types. To show the power scalability of the proposed technique, ${\sim}{600}\;{\rm W}$∼600W power is stripped with an ultra-low thermal slope of ${\sim}{0.021}(^\circ {\rm C}/{\rm W})$∼0.021(∘C/W) over a 400 µm fiber.

Atomic layer deposition of zirconium oxide thin film on an optical fiber for cladding light strippers

Cladding light strippers are essential components in high-power fiber lasers used for removal of unwanted cladding light that can distort the beam quality or even damage the whole fiber laser system. In this study, an Atomic Layer Deposition system was used for the first time to prepare the cladding light stripper devices using a 40 nm thick zirconia layer grown on optical fiber. The thickness of the zirconia coating was confirmed using the Scanning Electron Microscopy (SEM) and the Ellipsometry techniques. The elemental analysis was also performed using the wavelength dispersive X-ray spectroscopy technique. The Raman spectroscopy and XRD data confirm the structure of the atomic layer deposition-grown zirconia thin films to be predominantly amorphous. The cladding light stripper devices formed using the zirconia thin films with the lengths of 8.5 and 15.5 cm were able to strip approximately 30% (~1.5 dB) and 40% (~2.3 dB) of the unwanted cladding light.

Combined method for the fabrication of high-power cladding light stripper using a buffered oxide etchant.

Cladding light strippers (CLSs) are vital and one of the critical components for high-power fiber laser applications. In this study, we show the first studies of the formation mechanisms and optimum conditions of a CLS device using a buffered oxide etchant by a combined method of stain (wet) etching and vapor-phase etching. This high-power CLS was shown to result in a stripping performance of ∼17.2 dB at the launched power of 333W (pump limited). The thermal imaging demonstrates that the maximum temperature reached when operating the device at maximum launched power was ∼75°C. The atomic force microscopy (AFM) results show that the combined method yields crystal-like structures with the height in microscales, whereas other conventional methods give only nanoscale roughness. The method also preserves the diameter of the CLS device close to the bare fiber with about 10μm tapering leads to a high surface area to strip unwanted light, which is good for heat dissipation. The combined method possesses the outcome of two methods, including both the crystal-like structures and nanosized hillocks, resulting in high-power stripping performance and robustness.

Asymmetric taper scrambler for improved pump homogeneity and absorption in side-pumped double-clad fibre lasers

A multimode optical fiber scrambler (MOFS) based on an asymmetric taper is demonstrated. The MOFS contains an adiabatic down-taper transition and an abrupt up-taper transition made into a double cladding fiber. This device improves the pump power absorption by two principal effects which adapt the side-coupled cladding light. The first one is to homogenize the ring-shaped light distribution from a side pump combiner before being launched into an active double-clad fiber. Secondly, provided the conservation of etendue, the low NA of that multimode beam is transformed during its propagation through the longer taper transition of the MOFS device. Then, a net NA increment occurs, so that it fills the full input NA of this fiber at the taper waist. The up-taper transition is non-adiabatic for the multimode beam so that it exits from the taper with the transformed NA. The MOFS was inserted into a >200 W fiber laser cavity and significantly improved the effective pump absorption from 0.6 dB m−1 to 1.8 dB m−1.

Microstructured fiber cladding light stripper for kilowatt-class laser systems.

An all-glass microstructured high-power cladding mode stripper capable of handling cladding light of up to a power of approximately 350W with stripping efficiencies >22 dB is presented. An optimized graded structure pattern increased the device's reliability and its power-handling capabilities. Subjected to a 500h stress test, the device shows no degradation.

Cladding Light Stripper of High Average Stripped Power Density With High Attenuation of 39 dB and Low Temperature Rise

We demonstrate a 150-mm long cladding light stripper for kW-level operation with high attenuation and low temperature rising. The first 100 mm of the device was designed based on theoretical simulation to achieve uniform extraction of the high-power scattered cladding light and it was constructed by self-developed experimental device for continuous gradient chemical etching. While in the last 50 mm, we reduced the cladding diameter from 400 to 65 μm and then roughed the surface to extract the residual low NA cladding light more efficiently. The power handling capability of this design was tested up to 1.01 kW for an hour stably without causing any damage. The cladding light stripper achieved a high attenuation of 39 dB and a low temperature elevation of 0.05 °C/ input-Watt under the condition of air cooling. The maximum temperature on the surface of the device was only 78.5 °C with an average stripped power density of 7.156 × 10 6 W/m 2 . To the best of our knowledge, the cladding light stripper reported here has the highest average stripped power density and attenuation coefficient without compromising temperature performance. This cladding light stripper is well suitable for application in multikilowatts class fiber lasers.

Heat suppression of the fiber coating on a cladding light stripper in high-power fiber laser.

We present a theoretical model for the thermal effect of the fiber coating on a high-power cladding light stripper, which is fabricated by chemical etching. For the input and output of the fiber coating, a novel segmented corrosion method and increasing attenuation method are proposed for heat suppression, respectively. The relationship between the attenuation and temperature rise of the fiber coating at the output is experimentally demonstrated. The temperature distribution of the fiber coating at the input as well as the return light power caused by scattering are measured for the etched fiber with different surface roughness values. The results suggest that the rise in temperature is primarily caused by the scattering light propagating into the coating. Finally, an attenuation of 27dB is achieved. At a room temperature of 23°C and input pump power of 438W, the highest temperature of the input fiber coating decreases from 39.5°C to 27.9°C by segmented corrosion, and the temperature rise of the output fiber coating is close to 0.

2 kW single-mode fiber laser employing bidirectional-pump scheme

A 2 kW single-mode fiber laser with two cascade home-made cladding light strippers (CLSs) has been demonstrated by employing bidirectional-pump scheme. 2.009 kW signal power is obtained when pump power is 2.63 kW and the slope efficiency is 76.6%. Raman Stokes light is less than -47 dB at 2.009 kW even with a 10-m delivery fiber with core/inner cladding diameter of 20 μm/400 μm. The beam quality M2≤1.2 and the spectral FWHM bandwidth is 4.34 nm. There is no transverse mode instability and the output power stability of ±0.14% is achieved by special thermal management for a more uniform temperature distribution on the Yb-doped gain fiber.

The thermal effect of cladding power stripper for high power fiber lasers

Lasers have been widely used in many fields such as machinery, medicine, and military. With the development of fiber lasers, high power lasers become more and more desirable. Although lasers with power level of kilowatt or even million watt have been fabricated, there could be many problems associated with the increased power. The most important problem needed to be solved is how to strip unnecessary pump light before laser emit out from the devices. With the progresses in the past years, people have developed some methods to overcome this problem. However, these methods also have some disadvantages, such as too small stripping area to achieve high power light emission, and unsuitability of the technology for mass production. Glue method is the earliest proposed way for stripping the unnecessary light. Its biggest advantage is the simple process. But it also has a big disadvantage that glue cannot endure underhigh temperature in the range of 60°C to 70°C. To solve this problem, people have made attempts for years by either forced cooling or rasing the bearing temperature of glue, however, there is still no good solution. In fiber lasers, the superfluous pump light is dense, which has contributions from the pump light that is not fully absorbed by gain fiber and from high order laser due to misalignment elements. To efficiently strip the inner cladding light, and to get higher stripping power and more uniform heat distribution, we have used an optimized high index glue to fabricate the cladding power stripper. The basic idea is to improve the heat distribution in cladding power stripper by handle the structure of double clad fiber. The specific method is to rotate the cutting area at outsourcing, so the pump light can emit out uniformly, and the glue will have a low working temperature. Experiments and FE simulation are conducted to study the thermal effect of cladding power stripper. The results show that the highest stable operating temperature is 50.0°C when the cladding power is 132 W, and the temperature of cladding power stripper increases linearly with the increase of the stripping power. There is no overheating point in the stripper in the test indicating that the proposed method is applicable for kilowatt level fiber lasers. The experimental results are consistent with FE simulations, which verifies the correctness of simulation model and parameters. As the working temperature is much lower than 60°C, the results demonstrate that such stripper structure is reliable and its properties can be well described by simulations. Such a cladding power stripper only changes the structure of the double clad fiber to optimization the heat distribution, and the result shows this method are useful. The cooling effect of heat source was not perfect, which is the reason why the stripper temperature is 50.0°C when stripping power are only 132 W. So there is an opportunity to change the structure of heat source or use water-cooling or air-cooling to auxiliary heat dissipation for heat source. If both the structures of double clad fiber and heat source are optomized, this cladding power stripper can be used for million watts level lasers without problem. In addition, the simple structure of glue method cladding power stripper, enables its mass production, therefore, this stripper has a great application potential.

A diffraction-limited laser of 25/400 Yb3+/Al3+/P5+/F− silica fiber with a zigzag refractive index profile

In this work, a Yb3+/Al3+/P5+/F− co-doped low-NA preform with larger than 0.4 wt.% Yb3+ doping concentration and zigzag refractive index profile was fabricated based on solution doping modified chemical vapor deposition technology. The modality of the preform refractive index profile was simulated based on a circle-symmetry-distribution fiber model. According to the simulation results a step-index 25/400 double cladding fiber with core NA of ~0.04 was drawn. Pumped with a 970 nm laser diode, a diffraction-limited oscillator-laser output with ~71% slope efficiency and M2 of ~1.06 were achieved at bending diameter ⩾50 cm after stripping cladding light. For bending diameters less than 50 cm, the laser efficiency measured at stripping cladding light decreases with decrease in bending diameter.

High power cladding light stripper using segmented corrosion method: theoretical and experimental studies.

We present the segmented corrosion method that uses hydrofluoric acid to etch the fiber of a fiber laser for removing high-power cladding light to improve stripping uniformity and power handling capability. For theoretical guidelines, we propose a simulation model of etched-fiber stripping to evaluate the relationship between the etched-fiber parameters and cladding light attenuation and to analyze the stripping uniformity achieved with segmented corrosion. A two-segment etched fiber is fabricated with cladding light attenuation of 19.8 dB and power handling capability up to 670 W. We find that the cladding light is stripped uniformly and the temperature distribution is uniform without the formation of hot spots.