Project works towards goal of next-generation laser processing system with high-power blue laser diodes
KOJI TOJO, SHINICHIRO MASUNO, RITSUKO HIGASHINO, and MASAHIRO TSUKAMOTO
Since 1960, the latest light source technology has always been used for laser processing. The progress of laser processing technology is closely related to national projects. In Japan, the development of laser processing technology has been carried out from 1977, achieving the world’s top position in carbon dioxide (CO2) laser processing. However, since 2001, activity has not progressed strongly—as a result, in current global laser technology, Japan lags behind Germany and the United States.
In Germany, government investment based on national strategy is still active. Under these circumstances, a number of research and development projects in Japan are resurging—among them is the “Development of advanced laser processing with intelligence based on high-brightness and high-efficiency laser technologies (2016-2020)” project. Within the project, development of processing technology with blue laser diodes, together with short-wavelength and short-pulse lasers, has been adopted, with the ultimate goal of a next-generation processing system with kilowatt-class blue laser diodes.1
In addition to the advantages of laser diodes, such as excellent light intensity and waveform remote controllability, as well as suitability for automation and unmanned machine tools utilizing the Internet of Things (IoT) and artificial intelligence (AI), which will be common in the field of manufacturing in the future, if the processing performance of an excellent power blue laser processing system can be quickly accomplished, there is a possibility that Japan will assume a higher position again by changing the present situation. In Germany, since 2016, the national project dubbed EffiLAS has been undertaking a national policy on the development of kilowatt-class blue laser diodes for direct processing.2 Therefore, it could be said that Japan’s position is unpredictable and not guaranteed as it was in the past.
A paradigm shift inevitably occurs even in the light source world. With the recent rise of environmental awareness and the Nobel Prize in Physics award in 2014, gallium nitride (GaN) light-emitting devices have gained a lot of attention, especially in the field of lighting. Blue laser diodes have reached the mass production era, mainly for projector light sources, by ever-increasing high brightness and high output of the blue diode elements. In addition, it can be said that the power blue laser has many merits compared to current light sources, with the potential to rewrite the industrial map of laser processing.
Blue laser diode element
The performance improvement of blue laser diodes, which have been used as a small-output light source for applications such as pickups, has been remarkable. Recently, output of 3 W or more per element have become commercially available.
FIGURE 1 shows the transition of the output and wall-plug efficiency (WPE) obtained from NICHIA Corporation’s blue laser diode per element (drawn by the author based on the company’s data). With drastic development in the past 10 years, we can see clearly that the output has improved about 10X and the efficiency has about doubled. The main application is the replacement of a lamp in a projector, and it is used together with a phosphor that generates green or red light. Since blue laser diodes have a longer life and a smaller size compared to lamps, they have been rapidly spreading in recent years in such lighting and display applications.3
FIGURE 1. Power and wall-plug efficiency performance trends of the blue laser diode are shown.
On the other hand, the current application of blue laser diodes to various processing purposes requires a little more time. With regards to the brightness of the light source, an important parameter for processing applications, the blue laser diode already outperforms the infrared (IR) laser diode. When comparing the power density of the element’s end face on the basis of commercial products, while about 6 × 106 W/cm2 at the high output product (11 W/emitter) for the near-IR laser diode, it is about 12 × 106 W/cm2 for the blue laser diode (3.5 W/emitter).
In laser processing, the light absorption efficiency of the material is also an important factor. In general, the reflectance decreases in the shorter wavelength region around less than blue,4 so if a blue laser diode is used, it is expected that equivalent processing can be performed with remarkably lower power than a near-IR laser diode. As a result, considerably lower power consumption can be expected and greater efficiency.
The TABLE shows the superiority of blue laser diodes compared with near-IR laser diodes and second-harmonic generation (SHG) lasers (another type of short-wavelength laser). Besides the high-brightness characteristics and the low-reflection characteristics mentioned above, blue laser diodes have advantages such as back-reflection resistance, low failure rate, and direct modulation characteristics, among others.
In general, the elements of visible-light laser diodes are individually mounted on the TO-package, so the output level per element is limited.5, 6 Along with the increase of output in the future, it is necessary to pay close attention to the development trend of the laser bar in addition to multi-emitter packages, in which multiple diode elements are mounted on one large package.
In blue laser diodes, which have more than 2X the heat generation per unit area compared to near-IR laser diodes, it is necessary to cool effectively. Also, it is essential to create a reliable packaging technology capable of strictly managing the sealing to prevent the dust collection effect of suspended particles because of the high photon energy particular to blue laser diodes.
To develop a power blue laser to satisfy higher power requirements, it is necessary to form a laser module using a multitude of packages. As a module form, there are various ways to accomplish this, such as space-beam output, single-fiber output, and bundle-fiber output. By adopting a single-fiber output-type module, which does not impede the high-brightness characteristics of the blue laser diode and enables high-freedom handling, a general connector that is also used in the near-IR is possible, and high versatility can be obtained. In addition, the fiber-coupled modules have various other advantages, such as easy thermal design because of the separation of the light source section and the light-emitting section, an optimum placement of parts, and an easy scaling-up of outputs.
We are developing a fiber-coupled high-brightness blue diode laser module (Blue Direct Diode Laser, or B-DDL) with a high output by combining the blue laser diodes, considering laser processing applications. It is a versatile laser module that allows a blue laser beam to be freely delivered by an optical fiber with a 100 μm core diameter without impairing the high-brightness characteristics of the laser diode. In the following sections, we introduce the basic technology, the combining technology, and some of the characteristics of the B-DDL.
Beam combining
The practical power per emitter of the visible-light laser diode device has so far been about 3.5 W at a single wavelength in commercially available products, and a beam combiner that bundles outputs of a multitude of elements is indispensable for obtaining a higher output. The method of beam combining is divided into a coherent method and an incoherent method (FIGURE 2). Among them, the incoherent method can be described as practical without requiring a delicate phase control between the lasers.
FIGURE 2. Laser beam combining methods are shown.
The incoherent method includes a spatial-combining method that spatially combines a multitude of beams, a polarization-combining method that combines orthogonal polarized lights by a polarization beamsplitter, and a wavelength-combining method that combines different wavelengths coaxially.7FIGURE 3 shows a conceptual diagram of each method with their advantages and disadvantages, and it is also possible to use each method in combination.
FIGURE 3. The schematic shows spatial beam combining (a), polarization beam combining (b), and wavelength beam combining (c).
Among them, spatial combining is suitable for combining a multitude of laser diode elements of the same wavelength to obtain a high output. The brightness B of the laser is expressed by (1) using the laser output P, the area S of the irradiation spot surface, and the solid angle Ω of the laser beam:
where λ is the wavelength, M2 is a parameter indicating the beam quality described by (2) using the beam spot radius w0 and beam divergence half angle θ0, which is the minimum value 1 with the ideal Gaussian beam.8
As shown in FIGURE 4, when a multitude of laser diodes having a mode field diameter w1 and a divergence angle θ1 are arranged in a line and combined to a multimode fiber (mode diameter w2, divergence angle θ2) with lenses of focal lengths f1 and f2, the maximum number N that can be combined is approximated by the following equation (3):
where Ff is the space filling factor of the beam and M12 and M22 are the beam quality of the laser diode and the fiber, respectively. When the space-filling factor Ff = 1 and the optimum coupling condition f2w1/f1 w2 = 1, the number of beams that can be combined takes the maximum value of NMAX = M22 / M12. If a 450 nm laser diode device with Mx2 = 11.7 and My2 = 5.0 is used, and if a multimode fiber (NA = 0.2) with a core diameter of 100 μm is used as the fiber, the maximum number of beams that can be spatially combined is about 80.
FIGURE 4. An example of combining to a multimode fiber is shown.
FIGURE 5 shows the appearance of the 20 W type of BLUE IMPACT series fiber-coupled power blue laser module using spatial combining. We reached high brightness that has not existed in the past, and its use has already spread to copper laser processing and laser soldering, among other applications.9
FIGURE 5. The 20 W BLUE IMPACT blue direct-diode laser (DDL) module is shown.
High output power
The output scalability of spatial combining is limited by the core diameter and numerical aperture (NA) of the fiber. Using both wavelength and polarization methods is necessary to combine a higher power to a narrow-core-diameter fiber. Fortunately, GaN-based diodes cover a relatively wide-band-gap energy range, and in principle it is a material capable of generating visible light from violet to red. It also covers the short-wavelength visible light region from 375 to 532 nm, even at the commercial product level of laser diodes, and wavelength combining can be performed by selecting elements of various wavelengths.
FIGURE 6 shows a conceptual diagram of a high-power module that combines space-, wavelength-, and polarization-combining methods. Submodules obtained by spatially combining laser diodes of plural wavelengths, such as λ1 to λ3,are integrated into one beam by a dichroic filter and then further combined by polarization-combining. Since the beams combined by polarization- and wavelength-combining do not involve an increase in the cross-sectional area and the spread angle, a high output can be achieved while maintaining the same quality as the submodules.
FIGURE 6. This schematic shows spatial beam combining, wavelength beam combining, and polarization beam combining technics for power scaling of the DDL module.
FIGURE 7 shows the appearance of the 100 W BLUE IMPACT series, which produces a high output. It achieves a high output and a high brightness shown by a beam parameter product (BPP) value of 10 with a light output of 100 W from a 100-μm-diameter fiber with 1 MW/cm2 or more of the power density.
FIGURE 7. The 100 W fiber-coupled BLUE IMPACT blue-DDL module is shown.
Laser characteristics
Here, we introduce the characteristics of the fiber-coupled BLUE IMPACT high-brightness blue laser diode module. The lineup is available with up to 100 W output and fiber core diameter from 50 μm, according to the laser outputs required. FIGURE 8 shows an example of the spectrum and output characteristics of the 20 W version. A 450 nm blue output of 20 W is obtained from a fiber having a core diameter of 100 μm, and the electro-optical conversion efficiency is about 20%. Although it is lower than the WPE of the blue laser diode element alone, this is because of a loss of ambient light that is not able to fit in the NA of the fiber and the influence of Fresnel reflection on the fiber end face. In this example, the spectrum is expanded about 4 nm around 448 nm—depending on the laser diode device to be mounted, it is possible to make it narrower or wider. When narrowband is used, it is also expected to be used as a solid-state laser or an excitation light source for phosphors.
FIGURE 8. Output characteristics of the 20 W blue-DDL module include the spectrum of output beam (a) and output power vs. laser diode current (b).
FIGURE 9 shows how the beam diameter changes in the optical axis direction in the vicinity of the focal point when concentrating the fiber output to 1:1 with the lens, and an example of the beam profile at the focal position. It can be condensed to 100 μm equivalent to the fiber core diameter and 70 of M2 as the theoretical value is obtained, as the NA of fiber output light is 0.2.
FIGURE 9. The beam profile of the 20 W blue-DDL shows the near-field pattern of the output beam (a) and the beam propagation characteristic (b).
FIGURE 10 shows the near-field pattern (NFP) and output characteristics of a 100-W-type beam profile. The fiber is a step-index with a core diameter of 100 μm, and the NFP at the output end has a top-hat shape reflecting it. The M2 value of 70 was attained—the same as the theoretical value.
FIGURE 10. Output characteristics for 100 W blue-DDL module show the near-field pattern of the output beam (a) and output power vs. laser diode current (b).
Examples of processing with power blue laser
In this section, we show examples of laser processing using a power blue laser, mainly on the results obtained using the BLUE IMPACT 100 W type. With the advancement of the electric vehicle (EV) shift in the automotive industry and lithium ion batteries accompanying the advancement of mobile devices, expectations for laser welding of pure copper are increasing. Since a blue laser shows a light absorption rate of 40% or more to pure copper and it can be melted with a smaller output than a conventional IR laser, it is possible to suppress sputtering caused by excessive energy. By doing this, mild processing can be accomplished.
FIGURE 11 shows the results of lap welding copper foil with a 100 W power blue laser. The copper foil is scanned with a laser at a speed of about 10 mm/s from the top surface in a state where three sheets of copper foil are stacked at a thickness of 30 μm. Since the fiber output with a core diameter of 100 μm is concentrated with a projection ratio of 1:1, the laser spot diameter on the sample surface is also 100 μm. A good weld with suppressed heat influence on debris and surroundings is obtained.
FIGURE 11. Lap welding results for pure-copper sheets of 30 µm by the 100 W power blue laser show the top view (a) and cross-section (b) of the welding bead.
FIGURE 12 shows an example of a 3D printer capable of forming pure copper with a blue laser diode developed by Osaka University as a result of the New Energy and Industrial Technology Development Organization (NEDO) project “Development of advanced laser processing with intelligence based on high-brightness and high-efficiency laser technologies” and its layered samples.10 A 100 W power blue laser is mounted on a 3D printer based on the selective laser melting (SLM) method, and the laser focusing head on the XY stage is scanned to melt and solidify the copper powder only in required portions on the powder bed. A laser-focused spot diameter of 100 μm on the powder bed is realized. This makes it possible to laminate pure copper having high electrical conductivity and high thermal conductivity, which has previously been difficult to melt with a laser. This technology is expected to be applied to industrial fields such as aerospace, space, and EVs.
FIGURE 12. 3D printers utilizing a 100 W power blue laser are shown, which enables pure-copper direct prototyping; (a) shows a SLM machine with the power blue laser and (b) shows 3D prototyping samples made from pure copper powder.
New paradigm shifts have started in the industrial field because of IoT, AI, etc. Laser processing technology with advantages such as conformity to numerical control and remote processing availability with no need for tool change, will play a leading role in this new generation field. While laser processing by a power blue laser has just begun, it has the potential to be a core tool for cutting-edge manufacturing in the next generation with the progress of the future development.
ACKNOWLEDGEMENTS
A part of this report was conducted by the NEDO Project “Development of advanced laser processing with intelligence based on high-brightness and high-efficiency laser technologies.”
BLUE IMPACT is a trademark of Shimadzu Corporation.
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KOJI TOJO is with Shimadzu Corporation, Kanagawa, Japan; www.shimadzu.com, while SHINICHIRO MASUNO, RITSUKO HIGASHINO, and MASAHIRO TSUKAMOTO ([email protected]) are with the Joining and Welding Research Institute at Osaka University, Osaka, Japan; www.jwri.osaka-u.ac.jp/en.