1. The High Power Light Source for LCGT
My name is Noriaki Ohmae from Japan.
My talk is about the high power laser used in LCGT.
Prof. Norikatsu Mio is the chief of LCGT laser group, so if you contact to our group officially, please contact him.
Noriaki OHMAE
Department of Applied Physics, University of Tokyo
ERATO KATORI Innovative Space-Time Project, Japan Science and Technology Agency (JST)
Norikatsu MIO (Chief of LCGT Laser Group)
Photon Science Center, University of Tokyo

2. Requirement of LCGT Laser Power
Laser power = 180 W
Requirement was changed from 150 W to 180 W.
Laser power incident to PRM = 80 W + sideband power
Old plan was 75 W (total).
Main Interferometer
Input Optics MC
This is the simple optical configuration of LCGT.
Laser power in arm cavities is four hundred kilowatts, and that in the power recycling cavity is eight hundred and twenty-five watts.
We designed the power recycling gain of eleven, so the laser power incident to power recycling mirror should be eighty watts.
However, this power stands for the only carrier power.
Including the sideband power, about eighty five watts of incident laser power is required.
Assuming the transmittance of input optics, here, input optics means mode cleaner, pre mode cleaner, modulators and so on, required power of the laser is about one hundred and eighty watts.
This value was changed from one hundred fifty watts.
FP-cavity
PMC
Mod.
Mod.
PRM
FP-cavity BS
Laser
Transmittance = 50%
180 W
SEM
80 W (Carrier)
400 kW
825 W (Carrier)

3. Master: Nd:YAG NPRO (2 W)
Original Design
Solid-state-laser-based design
Injection Locking
Master: Nd:YAG NPRO (2 W)
Slave: Nd:YAG rods (100 W)
Power Amplifier
PZT
QWP
Nd:YAG rod
Intensity stabilization
（Current Control）
quartz rotator
This is the original design of the laser used in LCGT.
Originally, laser used in LCGT is design by solid-state lasers.
Using a injection locking, one hundred watts single-frequency output.
Here, NPRO is used as the master laser, and a ring laser constructed by the Nd:YAG rods is used as the slave laser.
After that, laser power is further increased by the power amplifier.
Using current control, laser power is stabilized.
Using PZT, crystal temperature, and wideband EOM, laser frequency is controlled.
Based on this design, we have researched for several years.
WB-EOM
Nd:YAG NPRO BW
180W
PBS FI
EOM FI 2W FI
110W
10 kHz < f < 1 MHz
Wideband EOM PD
f < 10 kHz
PZT, Crystal Temperature
Frequency Stabilization

4. Prototype Injection-Locked Laser
Single-stage injection-locked laser
Output power > 100 W
Single frequency (l = 1064 nm）
Single transverse mode (M2 ~ 1.0)
Linear polarization (horizontal)
EOM FI PD
PZT
Nd:YAG rod
Nd:YAG NPRO
quartz rotator
mixer
oscillator 2W
100W
Brewster window
Slave Laser
Master Laser
We have developed the prototype injection-locked laser.
This is the setup.
Master laser is Nd:YAG NPRO.
This is the slave laser.
This photo shows the slave laser.
Using injection locking we achieved 100-W, single-frequency, single-transverse mode, linear polarization output.
Laser modules
Slave laser

5. Slave Cavity Design Laser module Developed by MITSUBISHI Nd:YAG rod
Side-pumped by LDs
Linear cavity design by MITSUBISHI
Convex mirrors
To realize TEM00 output
Ring cavity design
Folded design
To reduce astigmatism
quartz rotator
Nd:YAG rod
This photo shows the high power laser module.
This laser module consists of two Nd:YAG rods and quartz rotator.
These Nd:YAG rods are side-pumped by the laser diodes.
This configuration enables to compensate for the thermal birefringences in Nd:YAG rods.
This laser module was developed by the Mitsubishi Electric corporation for industrial applications.
This is the linear cavity designed by the Mitsubishi.
Using convex mirrors, TEM00 output was realized when the huge thermal lensing is made in Nd:YAG rod.
Expanding this design, we made the ring laser.
To reduce the astigmatism caused by the oblique incidence of curved mirrors, we adopted the folded design.
In addition, inserting the Brewster windows, the polarization characteristics was improved.
PZT
Brewster window
Two rods and a rotator to compensate
for the thermal birefringences
quartz rotator
Nd:YAG rod

6. Output Characteristics
Single frequency operation
Measured with a scanning Fabry-Perot intereferometer
Output power > 110 W
Gradual power reduction
Caused by drift of alignment
Free running (without injection locking)
Injection locking
Left figure shows the laser frequency of the slave laser.
This spectrum is measured with a scanning Fabry-Perot interferometer.
Upper shows the frequency of the free running ring laser.
Lower shows that of injection-locked laser.
As you see, single-frequency operation was realized by the injection locking.
Right figure shows the output power.
Output power was more than one hundred and ten watts.
However, you can see the gradual reduction of the output power.
This was caused by the drift of the mirror alignment of the ring cavity.
After realignment, laser power was recovered.

7. Output Characteristics
Near TEM00
TEM00 power has not measured.
Polarization Extinction Ratio > 20 dB
Limited by residual thermal birefringences
This is the intensity distribution of the one hundred watts laser.
M-squared values was almost one point zero.
However, we have not measured TEM00 power.
Right figure shows the polarization characteristics.
Polarization extinction ratio of the one hundred laser was greater than one by one hundred.
This is the intensity distribution of this point.
This polarization extinction ratio was limited by the residual thermal birefringences.

8. Output Characteristics
Relative intensity noise
Free running (no stabilization)
RF-RIN is small.
Cavity pole of slave laser
Frequency noise
Free running (no stabilization)
Almost the same level as NPRO
Left graph shows the relative intensity noise of the one hundred watts laser and that of NPRO.
Both results was obtained with no stabilization.
In RF region, laser noise was reduced.
This was caused by the slave laser cavity.
Right figure shows the frequency noise of the one hundred watts laser.
The frequency noise level was almost the same level as that of NPRO.

9. Wideband Frequency Control Experiment
Wide frequency control bandwidth (near 1 MHz) is required in LCGT.
Large cavity induces the phase delay in the frequency control loop.
Slave cavity length = 2.5 m
PZT
Transfer function
WB-EOM input => Error Signal
Nd:YAG rod
Reference cavity
Finesse ~ 2900
FSR = 714MHz
quartz rotator
After slave cavity
Before slave cavity
Then, we researched the frequency controllability.
Wide frequency control bandwidth of near one Mega hertz is required in LCGT.
However, large laser cavity induces the phase delay in the frequency control loop.
The cavity length of our slave laser is about two point five meters.
This is the setup to stabilize the frequency relative to this reference cavity.
As a fast actuator, wideband EOM is generally used.
This graph shows the transfer functions from the input signal of the wideband EOM to the frequency error signal.
We observed the phase delay when the wideband EOM was placed in the master laser output.
Therefore, we chose this setup.
Nd:YAG NPRO
Brewster window FI
EOM FI 2W
Wideband EOM
EOM
100W PD PD
piezo
oscillator
oscillator
mixer
mixer
>10kHz
<10kHz
Frequency Stabilization

10. Wideband & High-Gain Frequency Control
Wideband control: UGF = 800 kHz
High-gain control: 300 dB at 100 Hz, 180 dB at 1 kHz
Initial state (for lock acquisition)
final state (for high-gain control)
UGF = 800 kHz
Control gain = 300 dB
Then, we tried the wideband and high-gain control.
Left graph shows the open loop transfer function of the frequency control loop for lock acquisition.
Unity gain frequency of eight hundred kilo hertz was achieved.
Right figure shows the open loop transfer function after boosting the control gain in low frequencies.
We achieved high-gain control of three hundred dB at one hundred hertz and one hundred and righty dB at one kilo hertz.
These results shows the frequency of the one hundred laser can be controlled in the same way as that of an NPRO.
Frequency of the 100-W injection-locked laser can be controlled in the same way as that of an NPRO.

11. Optical Components for High-Power Lasers
Electro-optic (EO) crystal
MgO:SLN (1-mol% MgO-doped stoichiometric LiNbO3)
Made by OXIDE corp.
MgO:SLN crystal (2x2x20mm3) was used in the frequency control experiment.
No damage (P = 100 W, beam diameter = 0.5 mm).
If an appropriate EO crystal is not chosen, a crystal will be destroyed by thermal stress.
Next, I show you the optical components I broke for several years.
Optical components for high-power laser should be selected carefully.
We researched MgO-doped stoichiometric lithium niobate.
This is a special lithium niobate.
Thermal property of this crystal was very good, and we used this EO crystal in the frequency control experiment.
There was no damaged in the condition of laser power of one hundred watts and the beam diameter of zero point five millimeter.
If an appropriate crystal is not chosen, the crystal will be destroyed by the thermal stress.
This figure shows the destroyed RTA crystal.
However, this RTA crystal was made more than ten years ago, therefore, I think the crystal growth technique was not adequate.
Destroyed RTA (RbTiOAsO4) crystal
This crystal was old (made more than 10 years ago)

12. Optical Components for High-Power Lasers
Mirror coating damage
Damage threshold of mirror coating is written as about 1MW/cm2.
Since we believed that it depends on the condition (dusts, defects, environment), we tried to destroy mirror coatings.
Setting: Power density on the beam center = 43.5 MW/cm2
Laser power = 95 W, Beam radius = 11.7 mm, Incident angle = 10 deg.
Sample: New mirror x2, used (old) mirror x1
Result: Only one spot with huge scattering on the used mirror was destroyed by 43.5 MW/cm2.
=> There seems to be a margin from 1 MW/cm2.
Then, mirror coating.
The spec of the damage threshold of mirror coating is often written as about 1MW/cm2.
However, we believed that it depends on the condition, for example, dusts, defects, and environments.
So, we tried to destroy the mirror coating.
Detailed setup is this.
Laser power is 95W.
Beam radius was 11.7um.
Incident angle was 10 degrees.
This setup realized the power density on the beam center was 43.5 MW/cm2.
We prepared three samples: two new mirrors, and one used old mirror.
As a result, only one spot with huge scattering on the used mirror was destroyed by 43.5MW/cm2.
Therefore, there seems to be a margin from 1MW/cm2 for new mirror and clean environment.

13. Optical Components for High-Power Lasers
During 100-W laser research for several years, a few mirror coatings were destroyed.
100-W injection-locked laser
PZT
100-W injection-locked slave laser
Frequency doubling experiment
Brewster window
External cavity
During 100-W laser research, for several years, a few mirror coating were destroyed.
One was a mirror used in the slave cavity.
Laser power was almost 1kW, and beam diameter was about 300um.
Right setup shows the frequency doubling experiment.
In this experiment, this mirror was destroyed.
The condition was almost the same as this condition.
And these destructions suddenly happened.
Through these experiments, we studied that optical components are weak for dusts.
Therefore, management of cleanliness is very important.
LBO(LiB3O5)
Dichroic mirror
quartz rotator
Nd:YAG rod
l= 532 nm (Green)
Optical component damage is sensitive to dust. => Management of cleanliness is important.

14. Application: Second Harmonic Generation
Single-pass SHG in PPMgSLT.
SLT has a relatively large thermal conductivity (8 W/m/K).
Green power = 19 W
Cavity-enhanced SHG in LBO
LBO has a high damage threshold.
Green power = 88 W
100-W injection-locked laser
Dichroic mirror
Dichroic mirror
LBO
l= 532 nm (Green)
100-W injection-locked laser
These are the applications of high power lasers developed for gravitational-wave detectors.
We also did the frequency doubling experiment with two configurations.
Left shows the single-pass SHG using PPMgSLT.
Only using single-pass configuration, we obtained 19W green second harmonic output.
Right shows the cavity enhanced configuration.
In this experiment, we used LBO crystal for its high damage threshold.
We obtained 88W green output with about 80% conversion efficiency.
A half of the residual laser power was lost and the other was reflected from the cavity.
Therefore, coupling of the laser to the cavity was about 90%.
We used 100W laser in such experiments.
Therefore, we have experiences to handle high-power lasers.
PPMgSLT
l= 532 nm (Green)
External cavity
We used the 100-W laser in many experiments.
=> We have experiences to handle high-power lasers.

15. Design Change of LCGT Laser
Which is better, injection locking or MOPA ?
Single-frequency, CW laser output with over 100 W can be realized by using injection locking and MOPA.
Injection locking has many advantages compared with MOPA.
However, an injection locking system is more complex than a MOPA system; this is quite critical for easy operation.
We have a 10-year history of developing a prototype laser based on the injection-locking scheme.
We recognized that the injection-locking system is quite sensitive to the alignment.
It seemed to be difficult to keep the best performance without well-trained operators.
MOPA is easier for assembly and maintenance than injection locking.
Fiber laser technology has progressed rapidly.
There is commercially available fiber amplifier with an output of over 10 W.
Here, I explain the design change of the LCGT laser.
Using both injection locking and MOPA can realize single-frequency, CW laser output with more than 100W.
We believe that injection locking has many advantages compared with MOPA, but injection locking system is more complex than a MOPA system.
And we recognized that the injection locking system is quite sensitive to the alignment.
So, it seems to be difficult to keep the best performance without well-trained operators.
In addition, fiber laser technology has progressed rapidly.
In fact, there is commercially available fiber laser amplifier with an output power more than 10W.
Therefore, we changed the plan for LCGT laser.
=> We changed the plan of LCGT laser.

16. Current Design of LCGT Laser
Commercially available components will be used.
Nd:YAG NPRO
40-W fiber laser amplifiers
80-W output by adding outputs of two amplifiers coherently
MITSUBISHI Nd:YAG laser modules
Assembly of the whole system will be asked to a company.
MITSUBISHI Nd:YAG laser modules
Fiber laser amplifiers
200 W
40 W
80 W
Nd:YAG NPRO
Coherent addition
40 W
0.5 W
This is the current design of LCGT laser system.
There is commercially available fiber laser amplifier with 40W output.
By using coherent addition, near 80W laser can be realized.
After that, laser power is amplified by the solid-state laser amplifier.
In iLCGT, we will use a 40-W laser system for easy and fast start.
In bLCGT, we will install 180-W laser system.
And assembly of these laser system will be asked to a company.
iLCGT
bLCGT

17. Future Possibilities of LCGT Laser
Now, there is no commercially available fiber laser amplifier with an output power of more than 100 W and low noise.
However, fiber laser technology has progressed rapidly.
Therefore, the design might be changed as time goes by.
Nd:YAG NPRO
MITSUBISHI Nd:YAG laser modules
Fiber laser amplifier
200 W
However, the progress of laser technology is very fast.
Now, there is no commercially available fiber laser amplifier with an output power of more than 100W and low noise.
However, in the research level, high-power fiber amplifier was realized.
Therefore, the design might be changed by time goes by.
As a first step, near 100-W laser amplifier can be available without coherent addition.
Then, we will not need to use solid-state laser amplifier.
Further, seed laser also can be replaced by the fiber laser, then all fiber laser system will be used.
Therefore, we continue to research these possibilities.
< 2 W
~ 100 W
Nd:YAG NPRO
Fiber laser amplifier
200 W
< 2 W
Yb-doped fiber laser
Fiber laser amplifier
200 W
~ 100 mW

18. Laser & Input Optics Room
We will prepare two laser systems on separate tables.
for backup, maintenance, and upgrade.
LASER 1
INPUT OPTICS TABLE
PMC, Modulators,
Isolators, MMT, …
This is the rough sketch of the laser and input optics room in LCGT.
This is the mode cleaner.
We will prepare two laser system on separate tables for backup, maintenance, and upgrade.
LASER 2 MC

19. Summary
We have developed a prototype solid-state-laser-based 100-W injection-locked laser for LCGT.
And we have tested a lot using the 100-W laser.
Fiber laser technology has progressed rapidly.
LCGT laser design was partially changed from solid state laser to fiber laser.
In iLCGT, 40-W laser will be used.
``Nd:YAG NPRO and Yb-doped fiber laser amplifier’’ will be used.
In bLCGT, 180-W laser is required.
``NPRO + Fiber amplifier + Solid state laser amplifier’’ will be used.
This is the summary of my talk.
We have developed prototype 100-W laser using 100-W laser.
And we have tested a lot using this laser.
LCGT laser design was partially changed from solid-state laser to fiber laser.
In iLCGT, we will use 40-W laser.
This laser consists of NPRO and fiber laser amplifier.
In bLCGT, 180-W laser will be used.
In current plan, we will use NPRO and fiber amplifier and solid-state laser amplier.
That’s all. Thank you.