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High-brightness fiber-coupled diode module using dense wavelength beam combining technology based on single emitter for material processing and fiber amplifier pumping

High-brightness fiber-coupled diode module using dense wavelength beam combining technology based on single emitter for material processing and fiber amplifier pumping

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(Summary description)Fiber-coupled diode modules have various applications in material processing and fiber laser pumping because of their high efficiency and high reliability. Commercial fiber-coupled diode modules using spatial beam combining and polarization beam combining cannot be employed in high-brightness applications, for example metal cutting, which demands a laser power exceeding 1 kW with a BPP of a few mm*mrad. Dense wavelength beam combining (DWBC) technology showed the possibility of further scaling-up the output power of fiber-coupled diode modules while maintaining the same beam quality that allows for fiber-coupled diode modules to be used in high-brightness applications. The efficiency, reliability, and brightness of fiber-coupled diode modules can be improved by using single emitters instead of laser diode bars as power sources in DWBC. Two types of high-brightness 100 µm/0.22 NA 2 kW fiber-coupled diode modules employing single-emitter-based DWBC technology, which have a wavelength range from 953 to 991 nm with 50% efficiency and a narrower wavelength range with 48% efficiency respectively, were developed for material processing and Raman fiber amplifier pumping. Furthermore, we combined 15 high-brightness 100 µm/0.22 NA 1.4 kW fiber-coupled diode modules into a 600 µm/0.22 NA fiber, achieving more than 22 kW at the output.

High-brightness fiber-coupled diode module using dense wavelength beam combining technology based on single emitter for material processing and fiber amplifier pumping

(Summary description)Fiber-coupled diode modules have various applications in material processing and fiber laser pumping because of their high efficiency and high reliability. Commercial fiber-coupled diode modules using spatial beam combining and polarization beam combining cannot be employed in high-brightness applications, for example metal cutting, which demands a laser power exceeding 1 kW with a BPP of a few mm*mrad. Dense wavelength beam combining (DWBC) technology showed the possibility of further scaling-up the output power of fiber-coupled diode modules while maintaining the same beam quality that allows for fiber-coupled diode modules to be used in high-brightness applications. The efficiency, reliability, and brightness of fiber-coupled diode modules can be improved by using single emitters instead of laser diode bars as power sources in DWBC. Two types of high-brightness 100 µm/0.22 NA 2 kW fiber-coupled diode modules employing single-emitter-based DWBC technology, which have a wavelength range from 953 to 991 nm with 50% efficiency and a narrower wavelength range with 48% efficiency respectively, were developed for material processing and Raman fiber amplifier pumping. Furthermore, we combined 15 high-brightness 100 µm/0.22 NA 1.4 kW fiber-coupled diode modules into a 600 µm/0.22 NA fiber, achieving more than 22 kW at the output.

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Information

Hao Yua, Shaoyang Tana, Huadong Pana,b, Shujuan Suna, Pengyuan Lina, Huan Hua, and Jun Wanga,b

aSuzhou Everbright Photonics Co., Ltd, Suzhou 215163, P.R. China

bSichuan University, Chengdu 610065, P.R.China

 

 


ABSTRACT

 

Fiber-coupled diode modules have various applications in material processing and fiber laser pumping because of their high efficiency and high reliability. Commercial fiber-coupled diode modules using spatial beam combining and polarization beam combining cannot be employed in high-brightness applications, for example metal cutting, which demands a laser power exceeding 1 kW with a BPP of a few mm*mrad. Dense wavelength beam combining (DWBC) technology showed the possibility of further scaling-up the output power of fiber-coupled diode modules while maintaining the same beam quality that allows for fiber-coupled diode modules to be used in high-brightness applications. The efficiency, reliability, and brightness of fiber-coupled diode modules can be improved by using single emitters instead of laser diode bars as power sources in DWBC. Two  types of high-brightness    100 µm/0.22 NA 2 kW fiber-coupled diode modules employing single-emitter-based DWBC technology, which have a wavelength range from 953 to 991 nm with 50% efficiency and a narrower wavelength range with 48% efficiency respectively, were developed for material processing and Raman fiber amplifier pumping. Furthermore, we combined 15 high-brightness 100 µm/0.22 NA 1.4 kW fiber-coupled diode modules into a 600 µm/0.22 NA fiber, achieving more than 22 kW at the output.

 

Keywords: Dense wavelength beam combining, single emitter, high brightness, fiber-coupled module

 

1.INTRODUCTION

 

Direct diode lasers (DDLs) have many advantages such as high efficiency, high reliability, compact size, and light weight. They have been widely used in material processing. Conventional DDLs use combining techniques such as spatial beam, polarization beam, and coarse wavelength beam to scale up the output power. However, the improvement in brightness is still far from the requirements of thick metal cutting, not to mention efficient pumping of Raman fiber amplifiers, which require brightness greater than 160 MW/cm2-sr[ 1]. Both dense wavelength beam combining (DWBC) and coherent beam combining (CBC) are suitable techniques to achieve further brightness improvement. However, the complexity and difficulty of high-power laser diode CBC, partic- ularly high-power broad area laser diode CBC, limit the maximal output power, which is below 50 W[ 2- 3]. The DWBC approach eliminates the need for fine phase control at the expense of spectral brightness[ 4].

 

A laser diode bar, which is effective for assembling optical components, is typically used as the power source in DWBC. However, the smile effect and low polarization ratio of the laser diode bar reduce the brightness of the combined beams and the efficiency of high-brightness DDLs. Furthermore, owing to the compactness of the laser diode bar, thermal cross-talk among the emitters is inevitable. To achieve high brightness, high reliability, and high efficiency, we developed a single-emitter-based DWBC technology. Despite the time required for assembling optical components of single emitters, which is greater than that of laser diode bars because the beam of each emitter has to be individually collimated and aligned, the separated assembly provides five degrees of freedom to amend pointings of beams.


We designed a 1.5 kW free-space output engine using single-emitter-based DWBC technology to lay a solid foundation for high-power fiber-coupled models. All high-brightness fiber-coupled modules constructed by the 1.5 kW free-space output engine benefit not only from the simplicity of manufacturing but also from the conve- nience of maintenance provided by modular designs.

 

2. 1.5KW FREE-SPACE OUTPUT ENGINE


The 1.5 kW engine included 9xx nm 120 µm ridge width laser diode chips mounted on an expansion-matched AlN submount for better heat dissipation. Our 350 W/50 µm[ 5] and 600 W/100 µm[ 6] fiber-coupled modules incorporated commercial M12 modules without fibers as power sources. In the 1.5 kW free-space output en- gine, all chips on submounts (COSs) were soldered on free-space output version M12 modules called M12F. These modules were designed for laser wireless power transmission systems [ 7]. All optical components in the M12F module such as FACs, SACs, and mirrors, were assembled using active-alignment machines developed by Everbright Photonics to optimize the near field (NF) and far field (FF) of the M12F module.

 

In theory, the free running wavelength of the chips should be near the stabilized wavelength to achieve high efficiency and high side-mode suppression ratio. By implementing a sophisticated epitaxial layer design and optimizing the facet anti-reflection (AR) coating, the wavelength stabilizing range of the new 9XX nm 120 µm ridge width single emitter can be greater than 20 nm at 14 A and 25 °C, as shown in Figure 1, while keeping the same output power. All chips were grown and fabricated using high-power 6-inch wafer production line of Suzhou Everbright Photonics Co., Ltd.

Figure 1. Wavelength stabilizing range of 969 nm M12F module.

 

A schematic diagram of the 1.5 kW engine is shown in Figure 2. There are 16 modules in the engine,  whose wavelengths range from 953 to 991 nm. All COSs were combined along the FA direction to minimize the degradation of the beam quality. The beams of 16 M12F modules were recollimated by SA and FA recollimating lenses with an effective focal length of 750 mm and 1000 mm, respectively. To accommodate as many channels as possible in a 40 nm bandwidth, that is, to maximize the brightness, we chose a transmission grating with a line density of 1851 lines/mm as the dispersive element. Different feedbacks ranging from 3% to 15% were tested, and we found that 3% feedback was sufficient to stabilize the wavelength from the threshold to the operating current.  Therefore,  an output coupling (OC) mirror with only one surface coated with AR coating was used  to provide approximately 3% Fresnel reflection. An optical filter between the grating and the OC mirror was inserted to prevent crosstalk between adjacent chips. All wavelength-stabilized beams were expanded by a 3X Galileo telescope at the output of the OC mirror.

 

Figure 3 shows the DWBC output power and the spectrum of the 1.5 kW engine. Note that this engine achieves a peak power conversion efficiency (PCE) of 52.1% at 9 A. The maximal output power, 2.1 kW, was reached at 14 A. The peak PCE would exceed 53% if 5 dead chips were replaced in this engine. The AR coating of optics in the 1.5 kW engine was not optimized for the wavelength interval ranging from 953 to 991 nm. Instead, the AR coating was designed for approximately 99.5% transmission over a broad wavelength range in which a few percent of power is lost because of reflection. The design central wavelength of the first M12F module was.

 

Figure 2. 1.5 kW  free-space output engine layout.

 

953.4 nm whereas the last was 991.2 nm. In comparison, the actual locking wavelength of the first M12F module was 953.7 nm whereas the last was 991.5 nm. This proves the correctness of the design and the accuracy of the assembly.

 


Figure 3. Left: LI and PCE curves for 1.5 kW free-space output engine. Right: Spectrum measured at 14 A.

 

3.2KW FIBER-COUPLED MODULES

 

The 1.5 kW engine exhibits s-polarization with a purity greater than 20 dB, which indicates that the brightness can be further improved by using polarization beam combining technology with negligible loss. The 2 kW fiber- coupled module consists of two 1.5 kW engines. A half-wave plate was installed in the module to rotate the plane of polarization from one engine; a polarizing beam combiner superposes two beams from different engines in both NF and FF. Figure 4 shows the testing results of the module developed for material processing with   an output power of 2.1 kW and a PCE of 50.5% at 8 A. Each engine output more than 1.8 kW of power from  a 100 µm/0.22 NA quartz block head (QBH) fiber during the test when they were independently switched on. However, the output end cap of the fiber was damaged when the current reached 9 A with both engines powered on. The number of peaks in the spectrum diagram exceeded 16 because there was a 0.5 nm difference between the central wavelengths of both engines.

 

After fine-tuning the central wavelength in the 1.5 kW engine assembly, the spectra from both engines fully overlapped. This can be clearly seen in Figure 5. To reduce the spectral width, 12 M12F modules were removed. The operating current was set to 13 A to achieve a 2 kW output. Figure 6 shows the test results for the narrow wavelength range 2 kW/100 µm fiber-coupled module. This module was measured at 25 °C and 70% relative humidity. The PCE was 48.2% at 13 A, but it should be noted that the output of this module was bare fiber without an AR coating. The output power dropped below 2% after a 400-hour aging test.

 

Fused fiber combiners are essential components for the realization of alignment-free and robust all-fiber sys- tems for direct diode lasers to scale power. In addition to input beam quality, the cladding power of the input

 


Figure 4. Left: LI and PCE curves of broad wavelength range 2 kW/100 µm fiber-coupled modules. Right: Spectrum measured at 8 A.

 


Figure 5. Output spectrum of narrow-wavelength-range 2 kW/100 µm fiber-coupled modules measured at 13 A.

 

fibers of the fused fiber combiner must be controlled because both affect the coupling efficiency[ 8]. In a commer- cial multi-emitter fiber-coupled module[ 9], which adopts spatial beam stacking and polarization multiplexing, the FF of the emitter has a significant impact on the NA of the output fiber. However, in the 2 kW module, the NA is determined by the NF of the chip, and the NA further improves at high current as the NF along the slow axis shrinks[ 10]. In the proposed optical design of the 2 kW/100 µm fiber-coupled module, the beam beyond
0.17 NA is truncated by apertures, which means that almost 100% of power is contained within 0.17 NA. 2.1 kW power from a 100 µm fiber, with 0.17 NA corresponding to brightness exceeding 300 MW/cm2-sr. This value allows for DDLs to be employed for thick metal cutting and Raman fiber amplifier pumping. Additionally, a cladding power stripper (CPS) was spliced in the middle of the 100 µm/0.22 NA fiber to strip the cladding light. The temperature of the CPS undergoing 2 kW power delivery is 40.5 °C, as shown in Figure 7.

 

4. 21KW DDL

 

The target was to achieve 21 kW power output from an 600 µm/0.22 NA QD fiber. Two distinct optical designs, namely free-space coupling and fiber combining, can scale the output power to 21 kW, both with specific ad- vantages and disadvantages. Free-space coupling provides a higher brightness but increases the complexity of.

 

Figure 6. Left: LI and PCE curves for narrow wavelength range 2 kW/100 µm fiber-coupled module. Right: Power fluctuation over 400 hours.

 


Figure 7. Thermal image of 100 µm/0.22 NA fiber CPS without active cooling with 2 kW input.

 

the DDL design and assembly. In contrast, fiber combining significantly improves the reliability with approxi- mately 20% BPP degradation. For example, the BPP of the 1.5 kW free-space output engine is approximately  7 mm*mrad and it is evident that 29 engines can be coupled into a 600 µm/0.22 NA fiber by using the BPP optimizing formula[ 11]. If the coupling source were a 100 µm/0.22 NA fiber-coupled 1.5 kW engine, the maximal coupling number would reduce to 19. However, 15 100 µm/0.22 NA fiber-coupled 1.5 kW engines are sufficient to enable a 21 kW output. Some of the 1.5 kW engines were installed without short-wavelength nor long-wavelength M12F modules, and the average output power of each engine exceeded 1.4 kW at 12 A. Figure 8 shows the con- figuration of the 21 kW fiber-coupled module. Fifteen modules were spliced into a 19:1 fiber combiner with an output of 600 µm, i.e., 0.22 NA.

 

Figure 8. Schematic diagram of 21 kW  DDL.

 

Figures 9 and 10 show the power and spectrum measurements of 21 kW DDL. The 21 kW laser achieved 22.9 kW CW power at 13 A. The rollover started at 10 A due to insufficient cooling capacity. The full spectrum superposition of the 15 modules is evident that there are only 16 peaks in the spectrum diagram. The power loss introduced by fiber combining was also analyzed by switching on each fiber-coupled module individually. The difference between the output power from the 600 µm/0.22 NA QD fiber and from the 100 µm/0.22 NA bare fiber varied from -60 to +30 W, which corresponded to the ratio of output power over input power from -4% to+2%. We believe that both differences in coupling efficiency among input ports of the fiber combiner and the AR coating on the 600 µm/0.22 NA fiber end cap contribute to the output power gain/loss difference.

 

5.CONCLUSIONS

 

Single-emitter-based DWBC technology exhibits high efficiency and high reliability in brightness enhancement. We realized a DWBC technology based on a single emitter and implemented it into 1.5 kW free-space output engines with 52.1% peak PCE and 2.1 kW DWBC output. The wavelength stabilizing range of single emitters can exceed 20 nm at high operating current by optimizing the epitaxial design and facet AR coating. Two types of 2 kW fiber-coupled modules based on 1.5 kW free-space output engine for various applications were demonstrated with an ex-fiber PCE of 50.5% and a brightness of 300 MW/cm2-sr. The output power could be scaled to 3.6 kW corresponding to a brightness level of 500 MW/cm2-sr if the end cap of 100 µm/0.22 NA QBH fiber withstands multi-kilowatt power. A total of 23 kW from a 600 µm/0.22 NA QD fiber was also achieved by.

 


Figure 9. Left: LI curve for a 21 kW DDL. Right: Spectrum measured at 13 A.

 


Figure 10. Left: 21 kW DDL prototype. Right: Power measurement at 13 A.

 

combining 15 fiber-coupled 1.4 kW modules. By splicing 2 kW fiber-coupled modules into the 19:1 combiner, we expect to increase the power from 600 µm/0.22 NA fiber to 40 kW with a PCE of approximately 50%. Single- emitter-based DWBC technology can be effectively applied for brightness improvement and cost reduction of diode lasers in other wavelength ranges.

 

ACKNOWLEDGMENTS

 

This study was funded by the National Key Research and Development Program of China (2018YFB1107300).

 

REFERENCES

 

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R. K., Chann, B., Fan, T. Y., Turner, G. W., et al., “Active coherent beam combining of diode lasers,”
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[8]Liu, Y., Yu, H., Califano, A., Braglia, A., and Perrone, G., “Modeling and optimization of multimode fused fiber combiners,” in [Components and Packaging for Laser Systems II ], 9730, 97300N, International Society for Optics and Photonics (2016).
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