Front-end system used in PM-Yb fiber laser for high by yrf69717

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       Front-end system used in PM-Yb fiber laser for high-peak power laser system




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       2008 J. Phys.: Conf. Ser. 112 032014

       (http://iopscience.iop.org/1742-6596/112/3/032014)

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The fifth International Conference on Inertial Fusion Sciences and Applications (IFSA2007)     IOP Publishing
Journal of Physics: Conference Series 112 (2008) 032014                   doi:10.1088/1742-6596/112/3/032014




Front-end system used in PM-Yb fiber laser for high-peak
power laser system

                YOSHIDA Hidetsugu, TSUBAKIMOTO Koji, FUJITA Hisanori,
                NAKATSUKA Masahiro, MIYANAGA Noriaki, and IZAWA Yasukazu
                Institute of Laser Engineering, Osaka University, Suita, Osaka, Japan

                hideyo@ile.osaka-u.ac.jp

                Abstract. We have demonstrated the generation of optional pulse shape by Yb doped fiber
                laser system for EUV lithography laser and IFE front-end system. Yb fiber laser system
                operated the polarization-maintained pulses for single-transverse and -longitudinal mode using
                fiber Bragg grating (FBG). The several output waveforms obtained including rectangular
                pulses from 1ns to 12.5 ns with a 500-ps rise time, and Gaussian pulses from 1 ns to 25 ns. The
                increase in output power has been achieved to 30W (300µJ, 100kHz) with a 30-µm core Yb
                double-clad LMA fiber.



1. Introduction
   The output power of cw Yb-doped fibers (YDF) dramatically increases to achieve diffraction
limited beam during the last two years. Yb doped silica fiber has several advantages, such as broadly
gain bandwidth, high output power efficiency and large saturation fluence. Recently, the most high
power works, such as few hundred watts, are focused on the Yb-doped fiber operation around 1 µm
region because the output efficiency could be achieved extremely higher than 60-80 %[1, 2]
   We have demonstrated the generation of optional pulse shape by single-mode YDF laser system.
Yb fiber laser system operated the polarization-maintained pulsed for single-transverse and -
longitudinal mode using fiber Bragg grating (FBG). The laser oscillator can be tuned at four
wavelengths of 1030, 1053, 1064 and 1080 nm.

2. Arbitrary pulse-shape generation using a polarization-maintained mode Yb fiber laser
system
           The schematic layout of YDF laser system is shown in Fig. 1. The YDF pulse laser system
consists of a master oscillator, a LiNbO3 (LN) modulator for temporal pulse shape, and three YDF
amplifier stages. The master oscillator is a combination of a YDF oscillator using FBG’s and a 30-cm
YDF amplifier. We have succeeded in imprinting all FBG cavity oscillators of different reflectivity. A
part of oscillator is made by writing over 50-mm linear Bragg gratings in YDF using a 248-nm light
source. The refractive index modulation of fiber grating was about 1 x 10-5. The active cavity made by
FBG’s is a 40 mm long, and length of a rear high-reflection mirror and output coupler made by FBG
are 20 mm and 10 mm, respectively. The active length of the side hole polarization maintaining YDF




c 2008 IOP Publishing Ltd                             1
The fifth International Conference on Inertial Fusion Sciences and Applications (IFSA2007)     IOP Publishing
Journal of Physics: Conference Series 112 (2008) 032014                   doi:10.1088/1742-6596/112/3/032014

                                                           Yb:fiber




                1064nm            LN
                FBG-FL            Modulator
                                        Pulse shape            20m
                                        control

                                         13 GHz                       977 nm LD                  Power
                                         100-ps step                  325 mW                     monitor

              Temp. control.



               !"#      !"#
                                              Yb:fiber                    Yb:fiber
                          980 nm LD
                          200 mW
                                                         20m                         20m




             Power                            977 nm LD          977 nm LD           977 nm LD
             monitor                          325 mW             325 mW              325 mW



                                      Fig. 1 Optical layout of YDF laser front-end system

also uses a 20-% reflective FBG. The reflectivity of rear and output end of the FBG are 99% and 85%,
respectively. The FWHM reflection bandwidth for 99.5-% FBG is about 0.15 nm, and the Bragg
wavelength is 1064.1 nm at room temperature.
   The YDF is pumped by a 977 nm laser diode through the fiber coupler. The YDF has Yb3+
concentration of 10000 wtppm, N.A. of 0.21, and a cutoff wavelength of 800 nm. The core and clad
diameters are 6.5 µm and 125 µm, respectively. When the YDF is pumped with a 200-mW laser diode,
the oscillator produces about 40 mW CW in a single longitudinal mode. The oscillation threshold was
about 39 mW of pumping power, and the slope efficiency was about 20%. The master oscillator is
fixed at 1064.10-nm wavelength, and is precisely tuned within 0.1 pm of the desired value by
temperature-controlling within 0.01 degrees.
   The Bragg wavelength changes by strain and the thermo-optic effects. The FBG define the cavity
end, active region and the output coupler, which are narrow band reflectors that allow only one cavity
mode to lase. The laser bandwidth was measured with a 20-km long self-heterodyne delay line. In
laser operation the linewidth of the YDF oscillator was about 100 kHz, whereas the linewidth of the
phase-shifted YDF was 150 MHz. The oscillation linewidth was independent of the output power.
   An electronic-pulse-shape generator supplies an arbitrary-voltage pulse to an integrated optic LN
amplifier modulator which modulates the laser pulse. The electronic waveform generator works by
adding rectangular pulse voltages from 1 ns to 12.5 ns with rise time of 500 ps, and Gaussian pulse
voltages from 8 ns to 25 ns. Figure 2(a) shows several rectangular pulses and (b) Gaussian pulses
when second amplifier is pumped with a 325-mW LD. The LN modulator has bandwidth of 12.5-GHz
bandwidth, and the temporal resolution of pulse shape is limited to about 80 psec.
   The amplified pulse energy as a function of a pulse repetition rate is shown in Fig.3. The output
energy of about 620 nJ with 12.5-ns Gaussian pulse was obtained when the pumping power was 975
mW at repetition rate below 10 kHz. At a repetition rate of 50 kHz, the output energy increases to
about 770 nJ at 975-mW pumping power because no SBS (stimulated Brillouin scattering) generated.
At lower repetition rate below 10 kHz, the output energy decreased to about 20% because the shade of
band-pass filter due to SBS generation with high stored energy. After passing through three amplifiers
and undergoing several optical component losses, the 20-ns pulse has a peak power of about 20W at
650-mW pumping power.




                                                                      2
The fifth International Conference on Inertial Fusion Sciences and Applications (IFSA2007)     IOP Publishing
Journal of Physics: Conference Series 112 (2008) 032014                   doi:10.1088/1742-6596/112/3/032014


                                              FWHM
                                               1ns
                                                                       2.2 ns                       3 ns




                                                          1ns/div.                      1ns/div.                 1ns/div.


                                              6.5 ns                    10.5 ns                     12.5 ns




                                                          2ns/div.                (a) 2ns/div.                   2ns/div.



                                              9ns                       16ns                        25ns




                                                         2ns/div.                       5ns/div.                 5ns/div.
                                                                                  (b)


    Fig.2 (a) several rectangular pulses from 1 ns to 12.5 ns in rise time by 500 ps, and (b) Gaussian
                                      pulses from 8 ns to 25 ns.

                                                            )
                                                            !
                                                            *
                                                                            !"#$%&'(
                                              800
                 Output energy / pulse (nJ)




                                              700
                                                       LD power 975mW

                                              600

                                              500
                                                       LD power 650mW

                                              400

                                              300

                                              200
                                                       LD power 325mW
                                              100

                                                0
                                                 101            102            103            104          105          106
                                                                      Repetition rate (Hz)

     Fig. 3 The amplified pulse energies as a function of a pulse repetition rate for YDF fiber system.




                                                                                3
The fifth International Conference on Inertial Fusion Sciences and Applications (IFSA2007)     IOP Publishing
Journal of Physics: Conference Series 112 (2008) 032014                   doi:10.1088/1742-6596/112/3/032014

                                          30

                                                   1054 nm
                                          25       1064 nm
                                                   1080 nm
                       Output power (W)   20


                                          15


                                          10


                                           5


                                           0
                                               0   20        40       60     80   100

                                                        LD input power (W)

                          Fig.4 The output power as a function of LD input power.

3. Power amplifier system used in LMA Yb fiber.
             Two optical isolators were inserted to prevent the back-reflection into nano-second Yb
fiber laser oscillator, which could disturb the laser operation. Large-mode area (LMA) Yb double-clad
fiber was pumped with a fiber coupled laser diode (LD) of 980 nm. The maximum pump power was ~
90 W. A 3-m long polarization-maintained LMA fiber with core diameter 30 µm (NA: 0.07), and clad
diameter 400 µm (NA: 0.46) was used. Figure 4 shows the output power of LMA main amplifier. The
output power has been achieved to 30W (300µJ, 100kHz) at a wavelength of 1053 nm.
The beam quality was observed with near- and far-field patterns of the amplified laser pulse. The focal
spot size was about1.3 times the diffraction-limited value with a bending radius of 75 mm.


4. Conclusion.
           We have demonstrated the generation of optional pulse shape by Yb doped fiber laser
system for EUV lithography laser and IFE front-end system. Yb fiber laser system operated the
polarization-maintained pulses for single-transverse and -longitudinal mode using fiber Bragg grating
(FBG). The laser oscillator can be tuned at four wavelengths of 1030, 1053, 1064 and 1080 nm. The
several output waveforms obtained including rectangular pulses from 1ns to 12.5 ns with a 500-ps rise
time, and Gaussian pulses from 1 ns to 25 ns. The increase in output power has been achieved to 30W
(300µJ, 100kHz) with a 30-µm core Yb double-clad LMA fiber.
           For applications, this system can be used as the fiber source for interferometer, nonlinear
frequency converter and seed light source for IFE (Inertial Fusion Energy) laser system.
          A part of this work was performed under the auspices of the Ministry of Education, Culture,
Science, and Technology, Japan (MEXT) under the contact subject “Leading Project for EUV
lithography source development”.

References
[1] Y. Jeong, J. K. Sahn, D. N. Payne and J. Nilsson, Opt. Express.12, 6088 (2004).Another
        reference
[2] A. Tunnermann, Workshop on Fiber laser 2006, (2006).



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