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A blue diode laser has a higher absorption rate than a traditional laser, while the maximum power is limited. We report the structure and laser beam profile of a 250 W high-power blue laser (445 nm) for material processing. The absorption rate of the blue laser system for the steel was 2.75 times that of a single-mode fiber laser system (1070 nm). The characteristics of the steel after laser irradiation were determined, validating the potential of this high-power blue laser for material processing, such as heat treatment and cladding. The cost of the developed laser system was lower than that of the existing one. To the best of our knowledge, this is the first blue laser with a power as high as 250 W.
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Development of a high-power blue laser
(445 nm) for material processing
HONGZE WANG,1,*YOSUKE KAWAHITO,1,4 RYOHEI YOSHIDA,2YUYA NAKASHIMA,3AND KUNIO SHIOKAWA3
1Joining and Welding Research Institute (JWRI), Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
2Graduate School of Engineering, Osaka University, Suita, Osaka 567-0871, Japan
3Japan Fuji Electric Corporation, Tokyo 191-8502, Japan
4e-mail: kawahito@jwri.osakau.ac.jp
*Corresponding author: wanghz@jwri.osakau.ac.jp
Received 20 April 2017; revised 12 May 2017; accepted 12 May 2017; posted 15 May 2017 (Doc. ID 293204); published 6 June 2017
A blue diode laser has a higher absorption rate than a tradi-
tional laser, while the maximum power is limited. We re-
port the structure and laser beam profile of a 250 W
high-power blue laser (445 nm) for material processing.
The absorption rate of the blue laser system for the steel
was 2.75 times that of a single-mode fiber laser system
(1070 nm). The characteristics of the steel after laser irra-
diation were determined, validating the potential of this
high-power blue laser for material processing, such as heat
treatment and cladding. The cost of the developed laser sys-
tem was lower than that of the existing one. To the best of
our knowledge, this is the first blue laser with a power as
high as 250 W. © 2017 Optical Society of America
OCIS codes: (140.2020) Diode lasers; (350.3390) Laser materials
processing; (300.1030) Absorption.
https://doi.org/10.1364/OL.42.002251
Lasers have been widely used in material processing (e.g., weld-
ing, cutting, cladding, and heat treatment) because of their high
stability, quality, and processing speed [14]. Various kinds of
laser systems for processing have been developed since 1960
[511], including CO2lasers (wavelength = 10640 nm),
Nd: YAG rod lasers (1064 nm), Yb: fiber lasers (1070 nm),
Nd:YAG disk lasers (1030 nm), and infrared diode lasers
(808980 nm). However, the high cost and low energy conser-
vation efficiency of these laser systems have restricted their
development scale and speed.
The laser absorption of a material determines the energy
conversion efficiency from light and has been investigated ex-
tensively [1215]. When a material is irradiated with a laser,
part of the laser energy is absorbed by the electrons. The ab-
sorption rate of the material is determined by the interaction
between the laser and the material, where the wavelength of the
laser, the surface status and properties of the material, and the
processing parameters can affect the absorption rate. For laser
processing at a high power density (generally higher than
106Wcm2), a keyhole is formed, the laser beam penetrates
the keyhole, the keyhole walls produce multiple reflections,
and a high absorption rate is achieved independently of the
laser wavelength [16]. For laser processing at a low power den-
sity, the absorption rate is strongly dependent on the laser wave-
length [14]. Thus, a laser system with a wavelength in a
particular range could improve the absorption rate considerably
for a material at a low power density.
Fresnels equation is used to calculate the absorption rate of
materials [17,18]. The absorption rate of the ideal flat surface is
calculated from the incidence angle and the optical parameters
related to the material and wavelength of the laser. The calcu-
lated result showed that the absorption rate of iron decreased as
the laser wavelength increased [14,19]. Though there was a
difference between the calculated absorption rate and the actual
value because the effect of surface roughness was ignored, the
obtained variation tendency would be acceptable. Thus, a laser
system with a shorter wavelength (e.g., a diode laser) would
have a higher absorption rate for iron.
Diode lasers are increasingly used because of their relatively
low cost and high energy conservation efficiency [20,21].
Various materials have been used for diode lasers, and the wave-
length of the laser depends on the material, for example,
InGaAs (940990 nm), AlGaAs (720880 nm), AlGaInP
(630690 nm), and GaN (445 nm)[20,22,23]. In his
Nobel lecture, Nakamura [24] predicted that GaN-based laser
diodes may be used for the next generation of solid-state light-
ing because of their high electrical-to-optical energy conversion
efficiency and low cost. Since GaN-based laser diodes were first
reported in 1994 [22,25], they have been widely used in ap-
plications, including lighting and optical storage. GaN laser
diodes with a peak wavelength in the range of blue light
(400480 nm) are called blue lasers [22]. Generally, the maxi-
mum power of a single GaN laser diode is small (e.g., 4 W), and
is not suitable for applications in welding and heat treatment.
Diode arrays containing many single laser diodes are used to
achieve high output powers [26,27]. Although high-power laser
systems with wavelengths longer than 630 nm have been widely
used for production, high-power blue laser systems (445 nm),
which would have a higher light absorption rate, have never
been produced.
We developed a high-power blue laser system with a maxi-
mum output power of 250 W. The absorption rate of the blue
Letter Vol. 42, No. 12 / June 15 2017 / Optics Letters 2251
0146-9592/17/122251-04 Journal © 2017 Optical Society of America
laser system was measured experimentally. The characteristics
of the steel after laser irradiation were determined, and we dem-
onstrated the potential of our high-power blue laser for material
processing.
Figure 1shows our blue laser system. Figure 1(a) shows a
schematic of the light path. Two laser diode clusters were
placed vertically, and the light from these two clusters was com-
bined by a mirror and focused by a convex lens. The mirror
could be divided into eight sub-areas; four areas were covered
with anti-reflection coatings, and the other four areas were
covered with high-reflection coatings. A blue laser from the
LD cluster on the upper side rayed on the areas covered with
a high-reflection coating; then it was reflected to the convex
lens. While a blue laser from the LD cluster on the left side
rayed on the areas covered with anti-reflection coatings, then
penetrated the mirror, raying to the convex lens. Finally, light
rayed to the convex lens focused at the focus point. A schematic
of the laser diode cluster is shown in Fig. 1(b). Each cluster was
made up of four groups (G1, G2, G3, and G4), and each group
was made up of eight laser diodes. The power of the single laser
diode was 4 W, and the wavelength was 445 nm. Thus, the
blue laser system had a maximum designed power of
256 W. Figure 1(c) shows a photograph of the blue laser equip-
ment. Cooling water was used to remove the large amount of
heat produced during the laser operation. Figure 1(d) shows a
photograph of the blue laser passing through water. Sixty-four
laser beams, each from a laser diode, were clearly observed in
the water, and the focusing of these laser beams could be clearly
viewed. The diameter of laser beam decreased to the minimum
value at the focus position, and then increased. A high power
density was achieved at the focus position, or at positions with a
small defocus distance.
Figure 2shows the designed power density distribution of
the blue laser system at the focus position. Figure 2(a) shows
the overall distribution of the power density. The blue laser
focused on an approximately rectangular area, with a high
power density that decreased quickly to zero outside the area.
Figures 2(b) and 2(c) show the power density distributions
along the Xand Yaxes, respectively. According to the defini-
tion of the diameter of a circular laser beam [28], the dimension
of the blue laser focused zone is the zone where the laser power
density is higher than 1e2of the peak. The length of the
focused area along the Xaxis was 0.50 mm, and that along
the Yaxis was 1.33 mm.
The maximum power of the developed blue laser system was
measured by L300W-LP1-50 laser measurement sensor (Ophir
Optronics Solutions Ltd), and the value was 250 W. As the
total power of 64 laser diodes was 256 W, it meant that the
part of light with the power of 6 W was lost in the system,
and the output efficiency of this system was 97.7%. The profile
of the actual laser beam was measured by SP620U CCD cam-
era (Ophir Optronics Solutions Ltd). Figure 3shows the mea-
sured profile of a laser beam at the focus position. The color in
the figure represents the laser intensity, and the color bar is
shown on the right. The actual profile in Xdirection was wider
than the designed one, and two laser intensity peaks appeared
in the profile, which were caused by the position offset of two
LD clusters. In the defocus situation, the dimension of the laser
beam profile would be larger, and these two laser intensity
peaks would be more obvious.
The water calorimetric method [28] was used to measure the
absorption rate of the blue laser. The upper surface of the steel
was irradiated with the laser, and the lower surface of the
steel was cooled with a flow of water. The temperatures of
the water at the entrance and the exit were measured, and
the mass of the water flow was recorded. The absorption rate
of the single-mode fiber laser with the maximum power of
500 W was also measured as a control. The wavelength of this
fiber laser was 1070 nm, and the beam parameter product was
1.04mm mrad. The defocus distance of the fiber laser was set
to be 5.0 mm, ensuring that the heat conduction mode was
the same for the fiber laser as for the blue laser. The power of
Fig. 1. Blue laser system: (a) schematic of the light path, (b) sche-
matic of the laser diode cluster, (c) photograph of the blue laser equip-
ment, and (d) photograph of the blue laser passing through water. LD:
laser diode.
Fig. 2. Designed power density distribution of the blue laser system at the focus position: (a) overall view, (b) along the Xaxis, and (c) along the
Yaxis.
2252 Vol. 42, No. 12 / June 15 2017 / Optics Letters Letter
the fiber laser was set to be 250 W. The intersection angle be-
tween two kinds of laser beams and the normal line of the plane
was 5°, the moving speed of the specimen was 5 mm/s, and the
laser irradiation time was 6 s. For each laser, the experiments
were repeated three times, and the average value was used as the
absorption rate. Figure 4(a) shows the variation in water tem-
perature during the experiment. The variation of water temper-
ature in the blue laser experiment was larger than that in the
fiber laser experiment. In addition, it took a longer time to cool
the specimen in the blue laser experiment than in the fiber laser
experiment. The reason for these phenomena was that more
heat was absorbed by the steel in the blue laser experiment.
The measured flow speeds of water in the blue laser experiment
and the fiber laser experiment were 7.8 and 4.8 g/s, respectively.
Then the absorption rate could be calculated based on the mea-
sured water temperature variation and the flow speed of water.
Figure 4(b) shows that the absorption rate of the blue laser was
2.75 times that of the fiber laser, which confirmed that the
blue laser system had a considerably higher absorption rate
for the steel, compared with the single-mode fiber laser system
(1070 nm).
The suitability of the blue laser for heat treatment was va-
lidated by using stainless steel SUS420J1, which initially con-
sists of the austenite phase. Laser heating converts the austenite
phase to the martensite phase, and increases the hardness.
During the laser heat treatment, the power density of the laser
was reduced by setting the defocus distance to 15 mm to
avoid melting the surface. The power was set as 250 W; the
speed was set as 1 mm/s; and the intersection angle between
the laser beam and the normal line of the plane was set as
5°. The profile of the laser beam at the defocus status was mea-
sured with a test paper. The test paper was irradiated with the
blue laser at the power of 30 W for 1 s; the area of the paper
irradiated with the laser was burned away, recording the laser
profile. Figure 5(a) shows that the blue laser system at this de-
focus distance had two laser intensity peaks, and that the laser
intensity between the two peaks was low. Thus, the direction of
the heat treatment should affect the properties of the treated
zone substantially. Two typical laser treatment directions were
examined. The first direction was parallel to the line passing
through the centers of two peaks [(i) in Fig. 5(a)], and the char-
acteristics of the treated section are shown in Fig. 5(b). The
second direction was perpendicular to the line passing through
the centers of the two peaks [(ii) in Fig. 5(a)], and the character-
istics of the treated section are shown in Fig. 5(c).
In the first treatment, the metal was treated with two peaks
sequentially, and heat accumulated in the zone corresponding to
the center of the peak. A treated depth of 0.76 mm was obtained
because of the heat accumulation, which was larger than that of
the second treatment. In the second treatment, the two laser in-
tensity peaks were moved in parallel, and two treated regions were
Fig. 3. Measured laser beam profile of the developed blue laser sys-
tem at the focus position.
Fig. 4. Absorption rate experiment: (a) variation in water temper-
ature and (b) measured absorption rate.
Fig. 5. Laser hardening experiment: (a) profile of the defocused laser beam and laser hardening directions; (b) section showing the characteristics
of the specimen hardened in the (i) direction; (c) section showing the characteristics of the specimen hardened in the (ii) direction.
Letter Vol. 42, No. 12 / June 15 2017 / Optics Letters 2253
obtained with a greater overall width of 5.06 mm, compared with
the first treatment. Thus, the first heat treatment direction pro-
duced a deeper heat-treated region, whereas the second heat treat-
ment direction produced a wider heat-treated region.
Our blue laser system can be used in laser cladding, heat
treatment, and other applications that require no keyhole.
The high absorption rate makes this blue laser promising for
saving energy in material processing. After mass production
is achieved, the cost of the developed blue laser system may
be as low as $10,000 per kilowatt, which is significantly cheaper
than the existing laser system. In the future, a blue laser system
with higher power will be produced with the structure intro-
duced in this Letter, which can be used in laser welding, and
heat treatment with a greater depth requirement. However, the
difficulty to control the position offset of LD clusters will
increase with the laser diode number.
Funding. Advanced Research Program for Energy and
Environmental Technologies, New Energy and Industrial
Technology Development Organization (NEDO).
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2254 Vol. 42, No. 12 / June 15 2017 / Optics Letters Letter
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The influence of the nonradiative recombination in a multiple quantum well of GaN-based blue laser diodes (LDs) has been are studied experimentally and theoretically by analyzing the optical and electrical properties of LDs with various thickness and indium content of quantum wells (QWs). It is found that when keeping the LD emission wavelength nearly unchanged, the LD device performance with thinner QW and higher indium content of InGaN QWs is much better than the LD with thicker QW and lower indium content, having smaller threshold current density, higher output optical power and larger slope efficiency. Typically, the threshold current density is as low as 0.69 kA/cm ² , and the corresponding threshold current is only 250 mA. The lifetime is more than 10,000 hours at a fixed injection current of 1.2 A under a room-temperature continuous-wave operation. Characteristics of photoluminescence (PL) microscopy images, temperature dependent PL spectra, time-resolved PL and electroluminescence spectra demonstrate that a reduction of the nonradiative recombination centers and an improvement of homogeneity in QWs are the main reason for the performance improvement of GaN-based LD using thinner QW layers with a higher indium content in a certain range. Moreover, theoretical calculation results demonstrate that using a thinner quantum well is also helpful for improving the device performance if the change of alloy material quality is considered during the calculation.
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A high-power, wavelength-tunable, all-fiber integrated thulium-doped fiber laser (TDFL) at 2 μm is presented. The TDFL has a compact configuration which only consists of a low power seed oscillator and a stage of fiber power amplifier. The seed oscillator adopts a tunable band-pass filter as the wavelength selective element, matching the gain spectrum of thulium-doped fiber. It can provide ∼5 W single-mode seed laser with superb spectral characteristics, and the lasing wavelength is adjustable from 1890 to 2050 nm. The fiber power amplifier provides a total gain of ∼17 dB at 2 μm which boosts the signal power to the 300 W-level. The maximum average power reaches 327.5 W at 1930 nm with the highest slope efficiency of 57.4%. This TDFL can afford >270 W lasing operation over the whole tuning range of 140 nm spanning from 1910 to 2050 nm, together with high spectral quality and power stability. This is the first demonstration, to the best of our knowledge, on an all-fiber integrated wavelength-widely-tunable TDFL at 2 μm with output power at the 300 W-level. The results are of great interest for many applications.
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Mode-locked pulses in the dissipative soliton resonance (DSR) regime enable extremely high pulse energy, but typically have the limited peak power of <100 W and a nanosecond-long pulse duration. In this Letter, we demonstrate high-peak-power, ultrashort DSR pulses in a compact Er:Yb co-doped double-clad fiber laser. The linear cavity is simply formed by two fiber loop mirrors (FLMs) using a 50/50 optical coupler (OC) and a 5/95 OC. The 5/95 FLM with a short loop length of 3 m is not only used as the output mirror, but also acts as a nonlinear optical loop mirror for initiating high-peak-power DSR. In particular, the mode-locked laser can deliver ∼100 ps DSR pulses with a maximum average power of 1.2 W and a peak power as high as ∼700 W. This is, to the best of our knowledge, the highest peak power of DSR pulses obtained in mode-locked fiber lasers.
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We present the first demonstration of a spectrally beam combined diode laser array with subsequent sum-frequency generation (SFG). The combined beam of the diode laser array with 19 emitters has the same beam quality as a single emitter, and the wavelength of each emitter is different. The blue light is generated by sum-frequency mixing of pairs of emitters in the diode laser array. About 93 mW of blue light power is produced using a PPLN crystal. Compared with the SFG of two emitters, this approach can increase the number of lasers participating in nonlinear frequency conversion. Thus, it can enhance the available power.
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Estimation of the temperature-dependent absorption coefficient in laser welding process is known as high non-linear inverse problem. Thus, it is necessary to propose a robust and efficiency method to estimate this coefficient. In this paper, a modified Newton–Raphson method combined with the concept of future time is presented to estimate the absorption coefficient in the laser welding process. The special characteristics of this method are that no preselected functional form for the unknown absorption coefficient is necessary and non-linear least-squares is not needed in the algorithm. Two examples have been fulfilled to demonstrate the proposed method. The results show that the proposed method is an accurate, stable, and efficient method to estimate the absorption coefficient in the laser welding process. In addition, two kinds of future time description are considered in this work. One is a linear type and the other is a constant type. From the estimated results, it is evidenced that the accuracy of the estimated results for the linear type of future time description is higher than that for the constant one of future time description.
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Shuji Nakamura discovered p-type doping in Gallium Nitride (GaN) and developed blue, green, and white InGaN based light emitting diodes (LEDs) and blue laser diodes (LDs). His inventions made possible energy efficient, solid-state lighting systems and enabled the next generation of optical storage. Together with Isamu Akasaki and Hiroshi Amano, he is one of the three recipients of the 2014 Nobel Prize in Physics. In his Nobel lecture, Shuji Nakamura gives an overview of this research and the story of his inventions.
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This chapter surveys the fundamentals of semiconductor lasers using a Fabry-Perot LD as an example. Current versus light output characteristics are described using rate equations. Transverse modes and longitudinal modes of semiconductor lasers are explained, and control of transverse modes are described according to the structures of the waveguides of the semiconductor lasers. For semiconductor lasers with antiguiding structures, which are added in the second edition, a brief analysis is given using the multiple-layer model in Chap. 3, and several journal papers are reviewed. Modulation characteristics, lifetime, and noises of semiconductor lasers are also explained. This chapter will make it possible for readers to understand key words that are used in journal papers for semiconductor lasers.
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We have obtained cw laser action on a number of rotational transitions of the Sigmau+-Sigmag+ vibrational band of CO2 around 10.4 and 9.4mu. The laser wavelengths are identified as the P-branch rotational transitions from P(12) to P(38) for the 00°1-10°0 band and from P(22) to P(34) for the 00°1-02°0 band. Strongest laser transition occurs at 10.6324mu (vacuum). A cw power output of about 1 mW has been measured. All these laser transitions can also be made to oscillate under pulsed discharge conditions with a small increase in the peak laser power output. No R-branch transitions have been seen to oscillate either under cw or pulsed discharge conditions. The wavelength measurements are in reasonable agreement with earlier measurement of the bands in absorption, but there are slight differences. These are ascribed to possible pressure-dependent frequency shift effects. A study has been made of the time dependence of the laser output under pulsed excitation, and some conclusions about possible excitation processes are given. Theoretical interpretation given earlier for laser action on vibrational-rotational transitions is discussed in a generalized form. The theory is applicable to both the linear polyatomic molecules and the diatomic molecules.
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This study was undertaken in order to assess absorption of a 10kW fiber laser beam with tightly focused spot diameter of 200μm in Type 304 austenitic stainless steel or A5052 aluminum alloy in bead-on-plate welding with a wide range of welding speeds from 17 to 300mm/s, by using water-calorimetric measurement. The maximum absorptions of the steel and aluminum were 89 and 93% at the lowest speed, respectively. Their absorptions were high over 80% at less than 100mm/s speeds and were reduced greatly by more than 20% at the highest speed. X-ray transmission in-site observation demonstrated that the center part of an incident beam with a bell-shape profile was delivered directly to a tip of a keyhole below 100mm/s speed. High-speed video camera observation indicated that an incident beam was more partially exposed out of a keyhole inlet over 100mm/s, which led to great decrease in laser absorption.
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Numerous publications demonstrate the great potential of lasers for surface treatment (hardening, cladding, alloying). Nevertheless, substitution of classical techniques has turned out to be difficult mainly due to high costs. The paper discusses ways to reduce laser-related costs by increasing the energy coupling efficiency. Following theoretical considerations, experimental results are presented. A calculation of energy coupling requires reliable values of optical constants at process temperature. For materials of technical interest like steels those data are very rare if not lacking completely. An attempt was made, therefore, to calculate optical constants by an extrapolation based on well-known room temperature values of iron and on the electrical resistivity of the alloy. The results fit the existing experimental data satisfactorily in the wavelength range 0.5 to 15 mum. A method is presented allowing to measure the coupling efficiency under process conditions. Results show additional contributions to the coupling rate from oxidation and additive materials. As expected from the theoretical results, a strong influence of the laser parameters wavelength and polarization is observed. e.g., reducing the wavelength from 10 to 1 mum increases the coupling rate by a factor two to three.