DEVELOPING AN YB/ND DOPED HYBRID SOLID LASER OF RF GUNFOR SUPERKEKB PHASE II COMMISSIONING
X.Zhou†, T. Natsui, M. Yoshida, R. Zhang, Y. Ogawa, KEK/SOKENDAI, Tsukuba, Ibaraki 3050801, 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 Nd:YAG laser
system is development to improve the stability andreliability.

INTRODUCTION
The electron beams with a charge of several nC and anormalized emittance of less than 10 µm are expected tobe generated in the photocathode RF gun for injector
linac of SuperKEKB accelerator project. By developmentof the Yb-doped laser system, more than 1.0 nC electron
has been obtained in 25 Hz. The laser system is alreadyfor commissioning phase I. But, the 30 ps pulse width
stretch limit the pulse energy of the amplifier laser system.
And the nonlinear effect was occurred.
As well-established laser material, Nd:YAG rods withhigh optical homogeneity and high damage threshold,simplify the design of high pulse energy amplifier.
Therefore, a new Yb-doped fiber and Nd:YAG hybrid
laser system is development to increase the pulse energy
and stability of the laser source. The emission wavelengthof the Nd:YAG is narrowband around 1064 nm. Also, anew laser room was built to increase the stability of thetemperature humidity condition. For phase IIcommissioning, more than 2 nC low emittance electron
beam is expected. For laser source, ~20 ps, 500µm
Ultraviolet (UV) pulse source is required.
Also, a chirped pulse amplification (CPA) Yb-doped
fiber and Yb;YAG thin-disk laser system is prepared forthe phase III commissioning, both pulse energy and pulse
shaping controller are expected. The emission wavelengthof the Yb:YAG is broadband around 1030 nm.

Figure 1: Layout of Laser system
To obtain the mJ-class pulse energy, 1064 nm signal
was send to an Nd:YAG amplification system. After theamplification, the UV pulse was generated by 2 second
harmonic generation (SHG) stages. This laser source wasdeveloped for the Phase II commissioning in this year.
On the other hand, 1030 nm signal was send to a Ybdoped fiber polarizing double-clad photonic crystal fiber(PCF) amplifier and Yb:YAG thin-disk amplifier with theCPA system. This laser source was developed for thePhase III commissioning in next year.

FIBER PART FOR 1030NM AND 1064NM

LASER SYSTEMAs the fig. 1, the laser system starts with Yb-doped
fiber oscillator. The 1064 nm seed pulses was generated
with an all normal dispersion (ANDi) structure fibercavity. The 1030 nm seed pulses was generated with acommercial fiber oscillator (Menlo Systems GmbH).
Both oscillator were synchronized with 2856 MHz and10.38 MHz triggers from accelerator.
Then, the 1030 and 1064 nm seed pulses inject into thesame fiber amplifier systems reducing the repetition rate
and amplifying respectively. The fiber amplifier system
include a grating pair stretcher, semiconductor opticalamplifier (SOA) pulse picker, Yb-doped fiber amplifiers
E.O. pulse picker and ASE noise reducer.
____________________________________________*xiangyu.zhou@kek.jp

Figure 2: Fiber part structure
A stable 114.24 MHz (10.38*11 MHz) Yb-doped fiberANDi type oscillators was developed. The center
wavelength was chosen by a bandpass filter in the cavity.
Here, the oscillator was employed to generate the seedpulses at center wavelength of 1064 nm. And acommercial fiber oscillator that produce was also import.
The spectral range is cover from 1025 to 1050 nm. This
commercial oscillator was used for 1030 nm laser
amplifier.
For the low power amplification, single-mode Ybdoped fiber with the core diameter 4 µm was coupledwith the 976 nm laser diode (LD) pump by a wavelengthdivision multiplexing (WDM) coupler same as theoscillator. Because the amplified spontaneous emission

(ASE) easy to be amplified with the weak seed pulse bystrong pump power, the 700mW pump LD was separated
to 3 parts with the pump power of the 15%, 35%, and50%, respectively. All the 3 amplifiers were coupled eachother with no free space.
1740 grooves/mm transmission grating pair stretcher
was employed to expend pulse to ~20 ps and reshape thecenter of the spectrum shaping at 1064 nm (0.3 nmbandwidth), which fit for the gain area of the Nd:YAGcrystal.
The SOA pulse picker inside after the stretcher toreduce the repetition rate from 114 MHz down to 10.38
MHz. The pulse picker was driven by a ~5 ns 100 mVelectrical pulse at 10.38 MHz amplified through a highpower RF amplifier. Another 3 steps Yb-doped fiber
amplifier was employed to compensate the energy lose.
For 1064 nm amplification, the ASE noise of 1030 nm
added to the signal during the SOA amplification.
Therefore an additional transmission grating was used toseparate and cut the noise by a slit.
After fiber amplifier, an Electro-optic module (E.O.)
worked as a pulse picker is used to pick out the 1-25 Hz
signal from a sequence of pulses of 10.38 MHz. The E.O.
was synchronized to the pulse repetition frequency of
10.38 MHz, with the signal to noise ratio of more than100.
The detail of fiber part that shows in fig.2 wasdescribed at ref [1].

ND:YAG PART WITH REGENERATIVEAMPLIFIER
Diode pumped solid state (DPSS) Nd:YAG laser issolid state lasers made by pumping a solid Nd:YAG gain
medium with a laser diode. The main advantages of diodelasers are overall laser efficiency and extended pumpsource lifetime. 808 nm pump light was generated by thediode pump module with the pulse width of 200µs at 25Hz. The Nd:YAG rod crystal of 1.1% dope-rate was usedwith the AR coating at 1064 nm by both surface.

factor by one round-trip. The pulse energy of 2.0 mJ wasobtained.
After regenerative amplifier, a 2-pass amplifier wasused by a 4!93 mm size Nd:YAG crystal. UV pulses
were generated by using two frequency-doubling stages
with 5 mm thickness beta barium borate (BBO) crystal.
More than 400 µJ UV pulses was generated to RF gun.
The temperature of the laser room (measured at thebase of the regenerative amplifier) had a significant effecton the system performance, as shown in Fig. 4. The
regenerative amplifier (green curve) and 2-pass amplifier(blue curve) maximum output pulse energy decreased by
90% while the room temperature range (pink curve)change of 0.3 °C. By contrast, the fiber output power (redcurve) was stable.

Figure 4: Output log date for regenerative amplifier
Although, a compact efficient regenerative amplifiersystem was constructed, the stability was an issue forapplication. Then, we developed multi-pass amplifier
instead of the regenerative amplifier to increase the pulse
energy after fiber amplifier.

ND:YAG PART WITH MULTI-PASSAMPLIFIERS

Figure 5: Nd:YAG Rod multi-pass amplifier

Figure 3: Nd:YAG Rod regenerative amplifier
For low-energy nJ pulse amplification, regenerative
amplifier system with large number of round trips is agood selection for compact the laser structures. Firstly aregenerative amplifier was employed, consists of a ringcavity having a Nd:YAG rod crystal. Consider theresponse speed of the pockels cell (~20 ns), the total
cavity length of the one round trip is about 7 m (Fig. 3).
The crystal size is 2!79 mm, with 2-3 amplification

The Multi-pass amplifier is simple and stable than theregenerative amplifier, because of no cavity structure.
Since multi-pass amplifier work well when only few
passes are required, several stages are needed for highenergy amplification.
As the fig. 5, the amplified pulses from fiber amplifier
first pass through a 2-pass high-gain amplifier with 2!79
mm size Nd:YAG crystal. The beam is then reflected off
the Faraday isolator and sent to a 8-pass low-gain
amplifierwith4!93
mmsizeNd:YAG
crystal. Successive beam paths are closer to the opticalaxis so that the passes has the maximum overlap in thegain
crystaltomaximize
thegain.
The
amplification factor of the high-gain and low-gain
amplifier are about 50 and 5 respectively. After the twostages multi-pass amplifier, the pulse energy was add toseveral µJ. Although the amplification has not saturated,
the ASE noise was occurred when the pump power wasincreased.
After the second multi-pass amplifier stage, a Pockels
cell is employed to rid of the noise that remained fromE.O. pulse picker. Then, the signal pass through a 4-pass
low-gain amplifier and 2-pass low-gain final amplifier.
Between the 2 stage amplifiers, a telescope lens system
was used to adjust the beam size to compensate thethermal lens effect that generated at each amplifiers. The
optical isolators were employed between the each
Nd:YAG amplifier stages to isolation the feedback. The
total output energy of the multi-pass amplifier was 4.5 mJ.
4 mm and 5 mm thickness beta barium borate (BBO)
crystal are used to two frequency-doubling stages. For thefirst SHG stage, the 2.1 mJ green pulse was generated at
center wavelength of 532 nm. The conversion efficiencyof the first SHG stage was more than 45%, that means
low amplitude and phase noise remained in the signal
pulses.
Because the transport energy loss of the shorter
wavelength is larger than the longer wavelength, thesecond SHG stage was set up near the RF gun. The
distance of laser beam transmits from the new laser room
to the RF gun is about 15 m by several reflect mirrors and2 pair of telescope lens systems. 80% 532 nm pulse
energy was remained in the transport line. The conversion
efficiency of the second SHG stage was more than 25%,
with the UV pulse energy of 510 µm was obtained. Thetotal ω-4ω efficiency was up to 10%.

LASER PROPERTY

From experience, the pulse energy of laser source isstrong enough for the 2 nC electron beam generation. Fig.
7 shows the beam profile of the 2ω pulses. The profile
need be optimized to increase the beam charge.

Figure 7: Beam profile of the 2ω
The laser was stable working more than one monthwithout maintenance. Since, some velocity and position
drift were occurred at pulse picker and Pockels cell, there-adjustment should be performed.

IN THE FURTURE: YB:YAG AMPLIFIERPARTFor the Yb-doped fiber and Yb:YAG thin-disk laser for
the Phase I commissioning, the main issuers are nonlinear
effect since the high pick energy of the ps pulse, andthermal effect since the bad thermal conduction of thethin-disk cooling system. Firstly, the CPA system shouldbe performed, that stretched pulse width to ns level.
Secondly, the cooling condition of the Yb:YAG thin-disk
crystal should be improved with the new soldering
technology and vacuum chamber [2].

CONCULUTION

Figure 6: pulse time domain profile
The pulse shape in the time domain was measured bystreak camera that shows in Fig 6. The pulse width is near
22 ps, better than the pulse source of the Phase I laser
source.
The 266 nm UV laser pulses are injected into thecathode in RF gun with the angle of 60°. For beamcommissioning, 1.57+/-0.032 nC beam generation fromthe gun was achieved with the bunch length of 22 ps in 25
Hz by single or double bunch. The emittance of εNx:
22.955+/-0.879, εNy: 11.376+/-7.876 was measured.

For SuperKEKB project, to obtain the 20 ps, 25 Hz,
mJ-class pulse energy, a hybrid solid laser is developed.
After an Yb-doped fiber oscillator and fiber amplifier, the1064 nm and 1030 nm signal pulses inject into an
Nd:YAG amplifier system and Yb:YAG amplifier system.
4 stages of Nd:YAG rod multi-pass solid-state amplifier
were employed to instead of the Nd:YAG rod
regenerative solid-state amplifier. 22 ps, 510 µJ UVpulses source was generated by laser system. The laser
source shows the good beam quality and stability for
Phase II commissioning.

REFERENCES[1] X. Zhou, et al., “Neodymium and Ytterbium hybridsolid laser of RF gun for SuperKEKB”, THPMY041,
IPAC’16, Busan, Korea, (2016).

[2] R. Zhang, et al., “New Achievements of the Laser
System for RF-Gun at SuperKEKB Injector”,
TUBAB056, IPAC17, Copenhagen, Denmark,proceedings of this conference.