Download Current status of the development of the Kumgang

Current status of the development of the
Kumgang laser
Hong Jin Kong,1,* Sangwoo Park,1 Seongwoo Cha,1 Heekyung Ahn,1 Hwihyeong Lee,1
Jungsuk Oh,1 Bong Ju Lee,2 Soungwoong Choi,2 and Jom Sool Kim3
1
Department of Physics, Korean Advanced Institute of Science and Technology(KAIST), 291 Daehak-ro, Yuseong-gu,
Daejeon, 305-701 South Korea
2
Department of Advanced Green Energy and Environment, Handong Global University, 558 Handong-ro, Buk-gu,
Pohang, Gyeongsangbuk-do, 791-708 South Korea
3
Laser Spectronix, 219 Gasan digital 1-ro, Geumcheon-gu, Seoul, 153-704 South Korea
*
[email protected]
Abstract: The Kumgang laser, a 4 kW (4 x 0.1 J @ 10 kHz / 8.5 ns)
coherent beam combination laser using self-controlled stimulated Brillouin
scattering phase conjugation mirrors (SBS-PCMs), is being developed. The
front-end (FE) and the pre-amplifier (PA) are completed. The FE produces
pulse energy of 0.51mJ and a pulse width of 8.5 ns with a 10 kHz repetition
rate (0.51mJ @ 10 kHz / 8.5 ns) and a 95 MHz linewidth. The PA amplifies
up to 200 W (20 mJ @ 10 kHz / 8.5 ns) with an input power 5.1 W.
©2014 Optical Society of America
OCIS codes: (140.3480) Lasers, diode-pumped; (140.3538) Lasers, pulsed; (140.3570) Lasers,
single-mode.
References and links
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laser systems using stimulated Brillouin scattering phase conjugate mirrors for a fusion driver,” JKPS 56(11),
177–183 (2010).
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2(1), 4–6 (1978).
S. K. Lee, H. J. Kong, and M. Nakatsuka, “Great improvement of phase controlling of the entirely independent
stimulated Brillouin scattering phase conjugate mirrors by balancing the pump energies,” Appl. Phys. Lett.
87(16), 161109 (2005).
H. J. Kong, J. W. Yoon, J. S. Shin, and D. H. Beak, “Long-term stabilized two-beam combination laser amplifier
with stimulated Brillouin scattering mirrors,” Appl. Phys. Lett. 92(2), 021120 (2008).
H. J. Kong, J. S. Shin, J. W. Yoon, and D. H. Beak, “Wave-front dividing beam combined laser fusion driver
using stimulated Brillouin scattering phase conjugation mirrors,” Nucl. Fusion 49(12), 125002 (2009).
H. J. Kong, J. S. Shin, J. W. Yoon, and D. H. Beak, “Phase stabilization of the amplitude dividing four-beam
combined laser system using stimulated Brillouin scattering phase conjugate mirrors,” Laser Part Beams 27(01),
179–184 (2009).
J. S. Shin, S. Park, H. J. Kong, and J. W. Yoon, “Phase stabilization of a wave-front dividing four-beam
combined amplifier with stimulated Brillouin scattering phase conjugate mirrors,” Appl. Phys. Lett. 96(13),
131116 (2010).
H. J. Kong, S. Park, S. Cha, and J. S. Kim, “0.4 J/10 ns/10 kHz-4 kW coherent beam combined laser using
stimulated Brillouin scattering phase conjugation mirrors for industrial applications,” Phys. Status Solidi 10(6 c),
962–966 (2013).
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Electron. 11(3), 567–577 (2005).
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2(1), 4–6 (1978).
H. J. Kong, S. Park, S. Cha, and M. Kalal, “Coherent beam combination laser system using SBS-PCM for high
repetition rate solid-state lasers,” Opt. Mater. 35(4), 807–811 (2013).
M. J. Damzen, V. Vlad, A. Mocofanescu, and V. Babin, Stimulated Brillouin Scattering: Fundamentals and
Applications (CRC press, 2003)
J. S. Shin and H. J. Kong, “Phase fluctuation of self-phase-controlled stimulated Brillouin scattering waves via
K8 glass,” Opt. Commun. 285(13–14), 2977–2979 (2012).
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for a phase-conjugated stimulated Brillouin scattering mirror,” Appl. Opt. 36(16), 3739–3744 (1997).
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Received 14 Aug 2014; revised 28 Oct 2014; accepted 29 Oct 2014; published 17 Nov 2014
(C) 2014 OSA 1 December 2014 | Vol. 4, No. 12 | DOI:10.1364/OME.4.002551 | OPTICAL MATERIALS EXPRESS 2551
17. H. Park, C. Lim, H. Yoshida, and M. Nakatsuka, “Measurement of stimulated Brillouin scattering characteristics
in heavy fluorocarbon liquids and perfluoropolyether liquids,” Japanese Journal of Applied Optics 45(6A),
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scattering via phase-conjugation mirror for repetitive laser pulse,” Review of Laser Engineering 29(2), 109–114
(2001).
1. Introduction
A coherent beam combining with stimulated Brillouin scattering phase conjugate mirrors
(SBS-PCMs) is one of the most promising practical techniques for building a laser with high
energy and a high repetition rate [1]. The coherent beam combination technique utilizes a
small-gain medium that can be cooled rapidly, allowing it to operate at a high repetition rate
with a high output energy level. Furthermore, the phase-conjugation property of the SBSPCM prevents the degradation of the amplified beam quality [2]. Therefore, coherent beam
combination lasers using SBS-PCMs have been studied for more than a decade, with the
feasibility of these systems experimentally proved by Kong et al. [3–7].
At KAIST, the development of a 4 kW beam combination laser system using self-phasecontrolled SBS-PCMs, called the “Kumgang laser,” started in 2012 and is scheduled to be
completed in 2015 [8]. This paper presents the recent progress on the development of the
Kumgang laser.
2. Beam Combination and Stimulated Brillouin Scattering Phase Conjugate Mirrors
(SBS-PCMs)
To build a high-power laser, the size of the gain medium of the laser should be large so as not
to undergo optical damage and to enlarge the stored energy. The large gain medium can have
a large thermal load. To reduce the thermal load, the repetition rate of the high-power laser
must be limited. Therefore, it is difficult to achieve both high power and a high repetition rate
in this type of laser system.
One of the most promising techniques to resolve this thermal problem is known as
coherent beam combining [1]. However, to combine laser beams coherently, the wave-front
of the laser beams needs to be a plane wave-front or TEM00 mode [9]. Furthermore, the
piston error between the beams should be matched [9]. To the best of our knowledge,
effective methods to create a plain wave-front include the use of an adaptive optics system
[10], degenerated four-wave mixing (DFWM) [11], and a phase conjugate mirror (PCM) [12].
Among numerous media, liquid media are utilized for a high-energy and high-power laser,
because it is insensitive to optical damage [13].
SBS can occur in gases, liquids, and in solid materials [14, 15]. Among numerous media,
liquid media are utilized for a high-energy and high-power laser, is insensitive to because it is
insensitive to optical damage [13]. Specifically, fluorocarbon and perfluorinated carbon
liquids have good phase-conjugation properties for a high-energy and highly repetitive laser
[16, 17]. The Kumgang laser will utilize the Galden HT-70 heat transfer fluid as the SBS
media for the SC-SBS-PCMs. Table 1 shows the properties of the Galden HT-70 fluid [17].
Table 1. Physical and Optical Properties of the HT-70 Fluid
Parameters
Average molecular weight
Kinematic viscosity
Density
Boiling point
Refractive index
Absorption coefficient
Optical breakdown threshold
Phonon lifetime
SBS gain coefficient
Brillouin linewidth
Brillouin shift
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Unit
u
centiStroke
g/cm3
°C
cm−1
GW/cm2
ns
cm/GW
MHz
MHz
Value
410
0.5
1.68
70
1.28
<103
>100
0.9
5.7
359
1,166
Received 14 Aug 2014; revised 28 Oct 2014; accepted 29 Oct 2014; published 17 Nov 2014
(C) 2014 OSA 1 December 2014 | Vol. 4, No. 12 | DOI:10.1364/OME.4.002551 | OPTICAL MATERIALS EXPRESS 2552
Fig. 1. The experimental setup of the reflectivity measurements of the SBS-PCM using the
Galden HT-70 fluid.
Figure 1 shows the experimental setup of the reflectivity measurements of the SBS-PCM
using the Galden HT-70 fluid. The laser beam from the oscillator passes through a half-wave
plate (HWP), a polarizing beam splitter (PBS) and a quarter-wave plate (QWP). The beam is
split by a wedged window (W), allowing the input energy to the SBS-PCM to be measured.
The laser beam is focused onto a SBS cell by a focusing lens (FL). The focal length of the FL
is 150 mm in this case. The beam reflected by the SBS-PCM is split by W once again, and the
output beam energy is measured. The output energy of the reflected beam is calibrated with a
conventional mirror.
Figure 2 shows the reflectivity of the SBS-PCM using the Galden HT-70 fluid due to the
input energy of the laser beam. This figure shows that the reflectivity of the HT-70 exceeds
50% when the input energy is greater than 10 mJ. This result implies that the input energy of
the main amplifiers in the beam combination laser system should be greater than 10 mJ for an
efficient beam combination.
To apply this SBS-PCM to the Kumgang laser, the input power limitation of the SBSPCMs should be considered. According to Yoshida et al. [18], a high-power laser beam is
absorbed by SBS media, with thermal lensing and convection occurring in the SBS media.
These effects reduce the focal spot intensity of the beam. As a result, the reflectivity of the
SBS-PCM is decreased. The SBS-PCMs for the Kumgang laser has been specially designed
to reduce the thermal effect caused by the high input power.
#220031 - $15.00 USD
Received 14 Aug 2014; revised 28 Oct 2014; accepted 29 Oct 2014; published 17 Nov 2014
(C) 2014 OSA 1 December 2014 | Vol. 4, No. 12 | DOI:10.1364/OME.4.002551 | OPTICAL MATERIALS EXPRESS 2553
100
90
Reflectivity (%)
80
70
60
50
40
30
20
10
0
-10
0
10 mJ
50
100
150
200
250
Input Energy (mJ)
Fig. 2. The reflectivity of the Galden HT-70 fluid given the input energy.
3. A 4 kW Coherent beam combination laser system using self-controlled SBS-PCMs
Fig. 3. A schematic diagram of the Kumgang laser system.
Figure 3 shows a schematic diagram of the Kumgang laser. The front-end (FE) produces a
single-frequency seed beam of 1064 nm at a repetition rate of 10 kHz. The average output
power of the FE is 5.1 W (0.51 mJ @ 10 kHz / 8.5 ns). The seed laser beam from the FE is
amplified by a pre-amplifier (PA) from 5.1 W (0.51 mJ @ 10 kHz / 8.5 ns) to 200 W (20 mJ
@ 10 kHz / 8.5 ns). The gain medium of the PA is an Nd:YAG rod. After the amplification
process by the PA, the laser beam will be divided into four sub-beams by a coherent beam
divider/combiner for its main amplification. This will be operational within one year. The
wave-front distortion of each sub-beam can be compensated for by SBS-PCMs [2]. After the
reflection at the SBS-PCMs and after passing through the main amplifier module (MA) again,
the sub-beams will be combined by the coherent beam divider/combiner. Each sub-beam will
be amplified by the MA to obtain a laser output of 1 kW (0.1 J @ 10 kHz / 8.5 ns). As a
#220031 - $15.00 USD
Received 14 Aug 2014; revised 28 Oct 2014; accepted 29 Oct 2014; published 17 Nov 2014
(C) 2014 OSA 1 December 2014 | Vol. 4, No. 12 | DOI:10.1364/OME.4.002551 | OPTICAL MATERIALS EXPRESS 2554
result, the average output power of each sub-beam will be 1 kW (0.1 J @ 10 kHz / 8.5 ns).
The total coherently combined output power is expected to be 4 kW (0.4 J @ 10 kHz / 8.5
ns). Table 2 presents a summary of the primary design parameters, criteria, and current status
of each part of the Kumgang laser.
Table 2. A Summary of the Design Parameters, Criteria, and Current Status of the
Kumgang Laser System
Part
Front-end
Pre-amplifier
Main amplifier module
Design parameters
Rep. rate (kHz)
Linewidth (MHz)
Pulse width (ns)
Output power (W)
Wavelength tuning range (nm)
Output power (W)
Output power (W)
Criteria
10
<359
<10
>3
1
>200
>1,000
Current status
10
95
8.5
5.1
1
200
-
4. Single-frequency hybrid-Master-Oscillator Power Amplifier (MOPA) front-end
Fig. 4. A schematic diagram of the FE: LD, the single-frequency diode laser oscillator;
FC/APC, the fiber connector/angled physical contact; AOM, the acousto-optic modulator;
WDM, the wavelength-division multiplexor; Yb-fiber, the Yb-doped fiber amplifier; BPF, the
band-pass filter; FC, the fiber collimator; HWP1 and HWP2, half-wave plates; PBS, the
polarizing beam splitter; FR1 and FR2, Faraday rotators; TFP1 and TFP2, thin-film polarizers;
PC, the Pockels cell; QWP, the quarter-wave plate; HR1 and HR2; high reflectors; A1 and A2,
apertures; and GM, the Nd:YAG rod gain module.
Figure 4 shows a schematic diagram of the FE of the Kumgang laser. The FE is the singlefrequency hybrid master-oscillator power amplifier (MOPA) system. The master oscillator
[MO] consists of a diode laser oscillator (LD), an acousto-optic modulator (AOM), a pump
LD, a wavelength-division multiplexor (WDM), a Yb-doped fiber amplifier (Yb fiber), and a
band-pass filter (BPF). The LD produces a CW single-frequency seed beam. The CW laser
beam from this LD is shaped to have a pulse width of 10 ns and a 10 kHz repetition rate by
the AOM. The laser beam is then amplified by the Yb fiber and passes through the BPF. The
output power of the FE is 100 μW (10 nJ @ 10 kHz / 9.5 ns).
Nd:YAG rod-regenerative amplifier (RA) amplifies the output beam of the MO to 5.1 W
(0.51 mJ @ 10 kHz / 8.5 ns). The output beam of the MO is collimated by a fiber collimator
(FC) and travels from the Faraday rotator (FR1) to the half-wave plate (HWP2). After passing
#220031 - $15.00 USD
Received 14 Aug 2014; revised 28 Oct 2014; accepted 29 Oct 2014; published 17 Nov 2014
(C) 2014 OSA 1 December 2014 | Vol. 4, No. 12 | DOI:10.1364/OME.4.002551 | OPTICAL MATERIALS EXPRESS 2555
through HWP2, the beam is injected into the RA cavity, as indicated by the dotted line. The
RA cavity consists of thin-film polarizers (TFP1 and TFP2), a Pockels cell (PC), a quarterwave plate (QWP), an Nd:YAG gain module (GM), apertures (A1 and A2), and high
reflectors (HR1 and HR2). A PC high-voltage driver operates at a quarter-wave rotation
voltage level when the PC is activated. When the seed beam is injected into the RA cavity,
the beam is trapped inside the RA cavity by the activated PC. After 30 round trips, the beam
is dumped out of the RA cavity by the deactivated PC. The output beam from the RA cavity
passes through HWP2 and FR2 and is reflected out by the PBS.
FSR
1500MHz
FWHM
95MHz
FSR=7.5GHz
Single-frequency
Fabry-Perot fringe
(A)
Single frequency line-width
MHz
Scan voltage
(B)
Fig. 5. (a) A Fabry-Perot interferogram of the output beam of the RA, and (b) the interference
spectrum of the output beam of the RA measured by the scanning Fabry-Perot interferometer.
With a Fabry-Perot interferometer, the spectral line shape of the RA laser beam is
obtained, as shown in Fig. 5(a), which shows the single-frequency operation. The measured
linewidth of the output beam of the RA was 95 MHz, as shown in Fig. 5(b). Figure 6(a) and
6(b) show the pulse shape of the leak pulse of the RA and the output pulse of the RA,
respectively. The pulse width is tunable from 5 ns to 10 ns by controlling the activation
timing of the PC with a delay/pulse generator; it is typically 8.5 ns in this experiment. Figure
6(c) shows the output beam profile of the RA.
Fig. 6. (a) The pulse shape of the RA cavity leak pulse, (b) the output pulse shape of the RA,
and (c) the output beam profile of the RA.
To optimize the amplification with Nd:YAG rod amplifiers, it is necessary to fit the center
wavelength of the FE to the gain curve of the Nd:YAG rod amplifiers. By tuning the
temperature and the pumping current of the LD, the wavelength of the FE can be tuned to a
wavelength range of 1 nm. Figure 7 shows the output wavelength of the LD as a function of
the temperature of the LD with 100.0 mA of pumping.
#220031 - $15.00 USD
Received 14 Aug 2014; revised 28 Oct 2014; accepted 29 Oct 2014; published 17 Nov 2014
(C) 2014 OSA 1 December 2014 | Vol. 4, No. 12 | DOI:10.1364/OME.4.002551 | OPTICAL MATERIALS EXPRESS 2556
Center Wavelength (nm)
@ LD Current=100.0mA
FSR=1500MHz
1065.2
1065
1064.8
LD Spectrum
1064.6
1064.4
Linewidth <7.5MHz
(resolution limit)
Temp scan
~200MHz/0.01C
1064.2
1064
16
18
20
22
24
26
LD Temperature (C)
28
30
Fig. 7. The center wavelength of the output beam of LD as a function of the LD temperature.
Output power (W)
Figure 8 shows the output power of the RA. The output power is 5.1 W (0.51 mJ @ 10
kHz / 8.5ns), and the power fluctuation is 0.49% during a time of one hour.
Fig. 8. The output power of the RA during a time of one hour.
5. Nd:YAG Rod Pre-Amplifier
Fig. 9. A schematic diagram of the PA: FE, the front-end; FI, the Faraday isolator; PBS, the
polarizing beam splitter; RL1~4, image relaying lenses; GM1~3, Nd:YAG gain modules; PR,
the polarization rotator; FR, the Faraday rotator; HR, the high reflector; and A and B,
measurement points.
The Nd:YAG rod PA has been built and tested. Figure 9 shows the setup of the Nd:YAG rod
PA. The beam from the FE passes through the Faraday isolator (FI) and is reflected at the
polarizing beam splitter (PBS). After reflection at the PBS, the beam travels back and forth
inside the PA and is amplified twice. After the amplification process by the PA, the output
beam passes out through the PBS.
Figure 10(a) shows the output power of the PA as measured at point B in Fig. 7 as a
function of the input power measured at point A in Fig. 7. The measured output power was
200 W (20 mJ @ 10 kHz / 8.5 ns) with an input power of 5.1 W (0.51 mJ @ 10 kHz / 8.5 ns).
#220031 - $15.00 USD
Received 14 Aug 2014; revised 28 Oct 2014; accepted 29 Oct 2014; published 17 Nov 2014
(C) 2014 OSA 1 December 2014 | Vol. 4, No. 12 | DOI:10.1364/OME.4.002551 | OPTICAL MATERIALS EXPRESS 2557
Figure 10(b) shows the output beam profile of the Nd:YAG rod PA. This good output beam
profile was obtained with the help of an image relaying system.
Fig. 10. (a) The output power of the PA at point B as a function of the input power measured at
point A, and (b) the output beam profile of the PA.
6. Conclusion
In this paper, the authors successfully demonstrated the single-frequency hybrid MOPA FE
and Nd:YAG rod PA of the Kumgang laser. The FE produces a 10 kHz pulse laser beam
which has a pulse width of 8.5 ns and a linewidth of 95 MHz. The PA can amplify laser
beams to 200 W (20 mJ @ 10 kHz / 8.5 ns) with an input power 5.1 W (0.51 mJ @ 10 kHz /
8.5 ns). With this FE and PA, the Kumgang laser will be completed by the middle of 2015.
The Kumgang laser will be applied to cut a micro-SD RAM card using a hologram, which
will be a new form of machining technology in the future. This technology will be more
reproducible, more fast and efficient than the conventional scanning method, because it does
not need scanning of the focused laser beam.
Acknowledgment
This work was supported by the “Dual-Use Technology Program” at the Agency for Defense
Development (ADD) of the Republic of Korea (UM12012RD1) and by the National R&D
Program through the National Research Foundation of Korea (NRF) funded by the Ministry
of Education, Science and Technology (2013-036083).
#220031 - $15.00 USD
Received 14 Aug 2014; revised 28 Oct 2014; accepted 29 Oct 2014; published 17 Nov 2014
(C) 2014 OSA 1 December 2014 | Vol. 4, No. 12 | DOI:10.1364/OME.4.002551 | OPTICAL MATERIALS EXPRESS 2558