THPMY041

Proceedings of IPAC2016, Busan, Korea

NEODYMIUM AND YTTERBIUM HYBRID SOLID LASER OF RF GUNFOR SUPERKEKB
X. Zhou†, T. Natsui, M. Yoshida, R. Zhang, Y. Ogawa, KEK/SOKENDAI, Tsukuba, Ibaraki 3050801, Japan
T. Shibuya, TITECH, Tokyo 152-8550, Japan
Abstract
By development of the Yb-doped laser system, morethan 1.0 nC electron with double-bunch has been obtainedin 25 Hz. The Yb-doped laser system is already forcommissioning for the linac. Next, a new laser system isdevelopment to improve the stability and reliability.

and Yb-doped fiber amplifier also can be used with both
Yb:YAG and Nd:YAG amplification. The two parts of thesignal pulse was separated after fiber part andamplification respectively.

LASER SYSTEM

Copyright © 2016 CC-BY-3.0 and by the respective authors

INTRODUCTION
The electrum beam of the repetition rate of 50 Hz
double-bunch is requested for injector linac ofSuperKEKB. By development of the Yb-doped laser
system, more than 5.0 nC and 3.0 nC electron beam withsingle-bunch has been generated in the 5 Hz and 25 Hz
respectively [1, 2]. Also more than 1.0 nC electron withdouble-bunch has been obtained in 25 Hz [2]. The Ybdoped laser system is already for commissioning for thelinac.
When the pump source repetition rate increases from 25
Hz, the heat in the Yb:YAG crystal cannot be dissipatedeffectively, which cause the thermal increase. Although thecrystal thickness and soldering components was improved,
the thermal conductivity is insufficient with 50 Hz pumpcondition. The thermal lens effect is changed lead to theserious decline of the amplify efficiency and beam quality.
Therefore, for 50 Hz repetition rate, several ten mJ pulse
amplification, a new laser system is development.
The most improvement is two kinds of a synthetic
crystalline material that used as a laser crystals for solidstate lasers based on neodymium-doped YAG (Nd:YAG)
and Ytterbium-doped YAG (Yb:YAG).
Compared with the commonly used Nd:YAG crystal,
Yb:YAG crystal has a much larger absorption bandwidth
to reduce thermal management requirements for diodelasers, a longer upper-state lifetime, three to four times
lower thermal loading per unit pump power. Yb-doped
crystal is more suitable for diode-pumping for high power
diode-pumped lasers. On the other hand, Nd:YAG rods
with high optical homogeneity, high damage threshold,consistentperformanceandhighprocessing
accuracy. Although the pulse shaping of Nd:YAG is
difficult to be adjusted because of the narrow spectrum
gain, the structure of the Nd:YAG is simple, thatmeans reliable convenience and less maintenance.
Therefore, both the Yb:YAG and Nd:YAG are appropriate
candidate for 50 Hz double bunch amplification.
The Emission wavelength of Yb:YAG and Nd:YAG arearound 1030 nm and 1064 nm with the bandwidth of ~2.0
ns and ~0.2 ns, which contained in the spectrum of Ybdoped fiber oscillator. Therefore, the Yb-doped oscillator____________________________________________† email address: xiangyu.zhou@kek.jp

ISBN 978-3-95450-147-2
3748

Figure 1: Layout of Laser system.
As the Fig. 1, the laser system starts with a 114 MHz
Yb-doped fiber oscillator with the all normal dispersion(ANDi) structure. A transmission grating pair stretcher
was employed to expend pulse to ~30 ps and separate thepulse to two parts with the center wavelength of 1030 nm
and 1064 nm. Then the two kinds of pulses can beamplified by Yb:YAG and Nd:YAG crystals respectively.
The weak pulses reduced repetition rate by asemiconductor optical amplifier (SOA) pulse picker andwere amplified by the Yb-doped single mode fiber
amplifier. Then the 1064 signal was amplified by Ybdoped double clad fiber amplifier. To obtain the mJ-classpulse energy, a Yb:YAG thin-disk regenerative solid-state
amplifier and a Nd:YAG rod regenerative solid-state
amplifier were employed. The fiber part structure isfinished, and several ten nJ pulse energy low repetition ratepulses were generated after the Yb-doped fiber
amplification stage.

ANDi Structure Yb-doped Fiber Oscillator

Figure 2: ANDi type Oscillator.

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Proceedings of IPAC2016, Busan, Korea

the effective distance between the second grating and theimage of the first grating. For matching the gain area ofYb:YAG (2 nm bandwidth at 1030nm center wavelength)
and Nd:YAG (0.2 nm bandwidth at 1064nm centerwavelength) crystal, two spatial slit were performed forspectral shaping between the grating pair and reflector,
where the spectrum is spatially separated with a parallelbeam(as Fig. 4). The pair of transmission gratings with agroove density of 1740 grooves/mm is used to stretch thepulse to several tens ps.

Figure 4: Grating stretcher

For 1030 and 1064 nm pulses, the spectrum width isdifferent, which result the distance of two gratings is notmatch. Since the 1030 nm and 1064 nm seed pulses withthe different diffraction angle at the first grating, the twoseed pulses can be separated and stretched with two secondgratings respectively.

SOA Pulse Picker and Yb Single Mode Fiber Amp

Figure 3: The spectrum of the oscillators.
The spectral filter of variable center wavelength wasincorporated in the cavity. The spectrum of the output
signal pulses is show in Fig. 3. Then two ANDi typeoscillators was employed to generate the seed pulses withthe center wavelength of 1030 and 1064 nm respectively.
And a commercial fiber oscillator that produce fromMenlosystems was also import. The spectral range is coverfrom 1025 to 1050 nm.
The three oscillators are set with different polarizations
that combine by two PBS. The polarization of the each
oscillators were controlled by wave plates.

Grating Stretcher
The grating-pair is the cleanest method to stretch a pulse
by introducing dispersion. The dispersion is controlled by

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Figure 5: SOA pulse picker

SOA is amplifiers which use a semiconductor to provide
the gain medium, is very promising in all-optical signal
processing because they are compact, easy to manufacture
and power efficient. The structure is similar to laser diodes
but with anti-reflection design elements. The salientfeature of the SOA pulse picker is that the selected pulses
were amplified to compensate for the loss arising from thefiber coupling.
As the Fig. 5, the SOA inside after the stretcher, wasused as the pulse picker and was pulse biased to reduce therepetition rate from 114 MHz down to 10.38 MHz. Thepulse picker was driven by a ~5 ns 100 mV electrical pulse

ISBN 978-3-95450-147-23749

Copyright © 2016 CC-BY-3.0 and by the respective authors

The Yb-doped fiber oscillator was stable with goodpulse quality and stability. But the noise signal wasgenerated easily for unwanted modifications in signal
processing. Then the oscillator was restructured. The needto compensate group-delay dispersion (GDD) is ubiquitousin femtosecond pulse oscillator. Also, a pair of diffraction
gratings have been used to compensate the GDD in our oldoscillator structure. The oscillator cavity have segments ofnormal and anomalous GDD. Pulse formation and pulse
evolution become more complex as the cavity GDD
approaches zero.
An Yb-doped fiber oscillator with a cavity consisting
only of elements with normal GDD was described. By
increasing the nonlinear phase shift accumulated by thepulse and inserting a spectral filter in the cavity, selfamplitude modulation via spectral filtering is enhanced.
The pulse duration and chirp decrease in the spectral filter.
The resulting spectral broadending is balanced by gain
narrowing. By cutting off the wings of the spectrum, gain
dispersion shapes the temporal profile of the chirped pulse.
Although, the gain spectrum is limited, the stable pulse
trains can even be produced without dispersion
compensation.
As the Fig. 2, a spectral bandpass filter is inset into thecavity. The beam reflects or transmits the polarizer beam
splitter (PBS) controlled by wave plates. Two piezoelectrictransducer (PZT) is used to control the cavity length
locking the repletion rate with the accelerator trigger by asynchronization system. The repetition rate of oscillators
are 114.24 MHz (10.38*11 MHz) with the pulse energy of0.2 nJ.

THPMY041THPMY041

Proceedings of IPAC2016, Busan, Korea

at 10.38MHz amplified through a high power RF amplifier.
By SOA stage, the average power of the 1030 nm and 1064
nm signals were increased from 330 μW to 400 μW andfrom 250 μW to 710 μW respectively.

Yb-doped Double Clad Fiber Amplifier (1064 nm)

Figure 6:Yb single mode fiber amplifier
In the low power amplification, single-mode Yb-doped
fiber with the core diameter 4 μm was coupled with the 976
nm laser diode (LD) pump by a wavelength-divisionmultiplexing (WDM) coupler same as the oscillator, which
compact the construction. Because the amplified
spontaneous emission (ASE) easy to be generated. TheASE noise will be amplified with the weak seed pulse
during each amplifier. For increase the seed pulse amplify
effectively, one LD was separated to 3 parts by fiber
coupler with the pump power of 15%, 35%, 50%,respectively. Seed pulse was amplified step by step (AsFig;. 6). All the 3 amplifiers were coupled each other withno free space.

Grating Pair for ASE Remove at 1064 nm

Figure 9: Double clad fiber amplifier by 1064 nm

To compensate the energy lose in the grating pair, theoutput of grating pair is seeded into the fiber amplifier byan Yb-doped polarization-maintain (PM) double clad fiberof 3.0 m. The core diameter and inner clad diameter of fiberare 10 μm and 125 μm, cladding backwardly pumped by a
4 W fiber-coupled laser diode emitting at 976 nm. The
output power of 1064 nm signal was up to 1.0 W (Figure9).
A pulse picker with an EO modulator was employed toreduce the pulse repetition rate.

IN THE FURTURE

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Figure 7: The pulse shape after the SOA

After the grating stretcher and the SOA pulse picker, theoptical pulse with time duration of ~30ps is separate twoparts with center wavelength of 1030 and 1064 nm. As theFig. 7, the ASE noise added to the signal during the SOA
amplification by 1064 nm. Fortunately, the wavelength of
the noise signal is around 1030 nm. Therefore an additionaltransmission grating was used to separate the noise. Thenoise signal was cute by a slit after grating pair (Fig. 8).

For low-energy sub-μJ pulse amplification, regenerative
amplifier system is a good selection. In regenerative
amplifiers with low input seed energy, a large numberof round trips are required to reach energy
saturation. Especially, for 1030 nm Yb:YAG thin-disk
amplification, because of low gain per reflection at the thindisk, the regenerative amplifier is the most suitable
amplifier concepts.
Next, 1030 nm and 1064 nm parts inject into Yb:YAG
thin-disk regenerative amplifier and a Nd:YAG rod
regenerative amplifier respectively.

CONCULUTION
For SuperKEKB project, to obtain the 50 Hz, mJ-classpulse energy, a hybrid solid laser is developed. After anANDi type Yb-doped fiber oscillator and fiber amplifier,the signal pulses were separated to two parts with thecenter wavelength of 1030nm and 1064nm. An Yb:YAG
thin-disk regenerative solid-state amplifier and a Nd:YAG
rod regenerative solid-state amplifier were employed. Now,
the fiber part structure is finished.

REFERENCES

Figure 8: Grating pair for ASE remove

ISBN 978-3-95450-147-2
3750[1] X. Zhou, et al., WEPME018, IPAC14, Dresden,Germany, 2014.
[2] X. Zhou, et al., WEPMA044, IPAC15, Richmond,
USA, 2015.

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