1. Lasers for Advanced Interferometers
Benno Willke
ILIAS WG3
Hannover, October 2004
opening remarks: Laser not limiting noise source beside quantum noise, laser not limited by technology or fundamental problems, however R&D and developement is required to meet demanding stability and reliability requirements

2. Requirements - Topology
Sagnac:
broadband source to reduce scattered light noise
power control
recycled Michelson:
coherence control
spatial control
squeezed light IFOs
different wavelength
Prestabilized Laser System (PSL)
Laser design depends strongly on topology which is not driven by available lasers as laser is not one of the limiting noise sources.

3. Requirement – High Power
Laser Limits
IFO Limits
stress fracture
birefringence / depolarization
spatial distortions
cavity stability / thermal lens
spurious oscillations in high gain material
thermal problems due to heating of surface/bulk (cavity stability, limit in cooling for cryogenic detectors)
scattered light noise
radiation pressure fluctuations in IFO and in frequency reference (low f performance)
trade off:
high input power
high power recycling gain
easier control problem
easier lock acquisition (radiation pressure dynamics)
spatial and temporal filtering
good frequency reference
easier lock acquisition: smaller dynamic effects due to rad pressure of build up light

4. Requirement – Noise Frequency Noise Coupling Limits via arm asymmetry
options: change IFO readout (DC readout / external modulation)
sensing noise
shot noise in PDH signal
discriminator slope
Intensity Noise
Coupling
Limits
via radiation pressure noise
technical RIN sees common mode rejection (same pendulum TFs) in IFO
quantum noise can be reduced by squeezing techniques and QND
via RIN on photo detectors
non perfect dark fringe contrast
requires passive filtering for rf readout
via rad. press. in frequency reference
sensing noise
shot noise
pointing combined with detector inhomogeneity

5. Requirement – Noise / Design
Spatial Fluctuations
Coupling
Limits
via RIN caused by cavities
via higher order mode “waste light” on IFO photodiodes
additional shot noise on PDH detector
fluctuations in spurious interferometers
finesse of modecleaners
thermal problems in modecleaner
noise introduced by modecleaner
Design Requirements
stability / reliability
soft failure mode
easy to maintain / rare maintenance interval
good efficiency
good stationarity / low glitch rate
high bandwidth / large range actuators
Laser design depends strongly on topology which is not driven by available lasers as laser is not one of the limiting noise sources.

6. Laser Design common concept: different power stage concepts:
laser diode pumped solid state lasers
transfer frequency stability of low power master laser to high power stage
Maser Laser Power Amplifier (MOPA)
injection locked oscillator
different power stage concepts:
rods
zig-zag slabs
fibers
thin disc lasers / active mirror laser

7. Nd:YAG Master-Laser NPRO (non-planar ring oscillator)
output power: 800mW
frequency noise: [ 10kHz/f ] Hz/sqrt(Hz)
power noise: /sqrt(Hz)

8. High Power Stage main problem: thermal design solutions
stress fracture
thermal lensing – spatial profile
birefringence with tangential and radial principle axis
solutions
reduce deposited heat – Yb:YAG, high efficiency
propagate beam perpendicular to temperature gradient – zig-zag, thin disc lasers
increase interaction length – fiber lasers
compensate birefringence

9. Face-pumping - Edge-pumping
zig-zag
plane
Face-
pumping
zig-zag
slab
Cooling
Edge-
pumping
zig-zag
plane
Pumping
Cooling
Stanford High Power Laser Lab Adelaide University

10. End pumped slab geometry
808nm Pump
undoped end
signal OUT
3.33cm
1.51cm
1.51cm
0.6% Nd:YAG
signal IN
undoped end
Motivation -> Higher efficiency
Near total absorption of pump light.
Confinement of pump radiation leads to better mode overlap
808nm Pump
1.1mm X 0.9mm
Stanford High Power Laser Lab

11. Stanford High Power Concept
Pump Power = 130
Output TEM00Power = 50 W
Mode-matching
optics
10W LIGO
MOPA
System
ISOLATOR
20 W
Amplifier
Lightwave Electronics
Mode-matching
optics
2-pass End Pumped
Slab #1
2-pass End Pumped Slab #2
Pump Power = 430 W
Expected TEM00
Output Power = 160W
TO PRE MODE
CLEANER

12. Nd:YVO4 - Virgo Laser (20W)
End Pumped Rods
Nd:YAG - GEO600 Laser (14W)
Nd:YVO4 - Virgo Laser (20W)
Laser Zentrum Hannover

13. LZH High Power Concept f 2f QR BP
output
from Master

14. Fiber Lasers
courtesy H. Zellmer

15. Fiber Laser Result of Jena Group
Backscattered
signal
NPRO
9.4 m Yb-doped
LMA-fiber
Dichr. mirror
Fiber coupled
laser diode
Isolator
To experiment
Input-output diagram
Yb-doped LMA-Fiber
Core:  = 28.5 µm, NA = 0.06
MFD 23 µm
Doping. 700 ppm (mol) Yb2O3
Pumpc.:  = 400 µm, NA = 0.38, D-Form
Seed: 800 mW
Diffraction limited (M2 = 1.1)
Polarization 82% (10:1)

16. Advanced LIGO Laser – Requirements
Power / Beamprofile:
165W in gausian TEM00 mode
less than 5W in non- TEM00 modes
Drift:
1% power drift over 24hr.
2% pointing drift
Control:
tidal frequency acuator +/- 50 MHz, time constant < 30min
power actuator 10kHz BW, +/-1% range
frequency actuatot BW:<20o lag at 100kHz, range: DC-1Hz: 1MHz, 1Hz-100kHz: 10kHz

17. Injection Locked Oscillators - Hannoverf 2f QR
YAG / Nd:YAG / YAG
3x 7x40x7 FI
EOM
NPRO
20 W Master BP
High Power Slave
modemaching
optics
YAG / Nd:YAG
3x2x6
output
key elements:
undoped bonded end-caps
birefringence compensation
pumplight homogenization

18. Prestabilized Laser PSL
frequency stability:
stabilize master laser to rigid or suspended-mirror cavity
power stability:
feed-back to pump source of high power stage
passive filtering at rf
spatial profile
passiver modecleaning
active mode compesation
loops with highest bandwidth up to 1MHz UGF

19. frequency noise requirement
PSL same as LIGOI

20. intensity noise requirement

21. Adv LIGO - PSL optical layout
NPRO 1W
GEO typ ring laser 15W
high power
ring laser
200W
spatial filter resonator (PMC)
AOM
frequency reference resonator
possible solution, conceptual design not finished and not reviewed

22. PSL – stabilization scheme
intensity stabilization outer loop
injection locking
intensity stabilization inner loop
PMC loop
frequency stabilization inner loop
what is the current status?
frequency stabilization outer loop

23. Power Noise Reduction

24. faster relock possible depending on piezo ramp
Relock Time
relock time < 500 ms
faster relock possible depending on piezo ramp

25. Birefringence Compensation
The figure shows some experiments on the optimized double had design. The green curve shows the non
polarized output power of the system without QR in place. It can be seen that the bifocusing acts at a pump
power level of nearly 350 W and the resonator gets unstable in one of the polarization directions. With QR the
bifocusing can be compensated and the output power can further increased ( blue curve). To achieve linear
polarisation the third experiment was done with BP inside the resonator. The output power was limited below
60W (red curve). With QR in place the linear polarized output power reached the same level as in non polarized
operation ( black curve ). * The BFC works very well *

26. End-Pumped Rods f 2f QR BP
output
from Master

27. high power stage status Feb 2004
linear polarized with birefringence compensation

28. Summary different high power stages: end-pumped slabs end-pumped rods
fiber amplifier
different topologies:
MOPA
injection locking
Advanced LIGO pre-stabilized laser system
status of laser development
possible stabilization schemes