High-Power Fundamental Mode Single-Frequency Laser - LIGO

High-Power Fundamental Mode Single-Frequency Laser
Maik Frede, Ralf Wilhelm, Dietmar Kracht, Carsten Fallnich
Laser Zentrum Hannover, Hollerithallee 8, 30419 Hannover, Germany
Phone:+49 511-2788-235, Fax:-100, Email:mf@lzh.de
Frank Seifert, Benno Willke
Albert-Einstein-Institut, Callinstr.38, 30167 Hannover, Germany
Abstract: The first results on an injection-locked high-power Nd:YAG ring laser with
195W single frequency output power in a stable linearly polarized fundamental mode
operation for the next generation of gravitational wave detectors will be presented.
©2004 Optical Society of America
OCIS codes: (140.3520) Lasers, injection-locked; (140.3570) Lasers, single-mode
Introduction
For the fundamental research of gravitational wave detection a specific laser source is needed to achieve
a detector sensitivity of 10 -21 . Therefore, a stable and low-noise fundamental mode, single frequency
laser with an output power of nearly 200W has to be realized [1,2]. Beside high amplitude- and
frequency-stability the long-term stability and an easy maintenance has to be taken into account. We
present the first injection-locked high-power ring laser with a single frequency output power of 195W in
a stable linearly polarized fundamental mode operation.
Pound-Drever-Hall
locking-electronic
LIGO-P040053-00-R
Fig.1: Setup of the injection-locked high-power laser. HP-FI: High power isolator
with thermal lens and birefringence compensation.

Laser concept
To achieve a linearly polarized fundamental mode output power in the range of 200W a high power laser
with four end-pumped Nd:YAG laser rods was developed. To compensate the thermally induced
birefringence each two of the laser rods are combined with a birefringence compensation [3]. To achieve
high long-term stability and an easy maintenance each laser rod is pumped with a fiber bundle of 10
fiber-coupled diodes each delivering up to 30W of pump power. The diodes were operated by nearly
75% of their nominal power to increase the diode lifetime. In case of long-term stability and
maintenance, the derated operation of the diodes enables the compensation of a diode failure or diode
degradation. A pump-light homogenizer was connected to the fiber ends to achieve a homogenous
pump-light distribution [4]. To reduce the maximum temperature and surface stresses at the 40mm long
0.1 at.% doped Nd:YAG rod, two 7mm long undoped end caps were attached [5,6]. Furthermore, a
double pass of the pump light was realized for smoothing the longitudinal temperature distribution. In
order to perform injection-locking for single frequency operation the laser system was used in a ring
resonator configuration. The setup of the whole system is sketched in Figure 1.
To achieve a stable single frequency operation with a locking-range of a few MHz the frequency
stability of a 0.8W single frequency master laser (NPRO) [7] is transferred via an intermediate 12W ring
oscillator [8] to the high-power slave laser. The locking of the high-power laser via the intermediate
stage increases the locking range by a factor of 4 because the locking range is proportional to the square
root of the master to the slave laser power. An active length control system based on the Pound-Drever-
Hall scheme [9] was used to keep the difference between master and slave laser cavity frequencies to a
value within the injection-locking range. A first electronic system stabilized the 12W laser cavity to the
frequency of the NPRO. If these lasers are in a non-locked condition a sawtooth voltage is applied to a
piezo mirror of the 12W laser system. Therefore, the laser cavity length is changed and the cavity is
scanned through her eigenfrequencies, until the cavity length fits to the frequency of the NPRO. If the
frequencies coincide, the injection-locking starts and the laser operates in a unidirectional instead of a
bidirectional mode. In this case a photo-diode at the output port detects twice the output power and the
Pound–Drever–Hall stabilization locks the cavity length. To lock the high-power laser a similar
procedure starts immediately as soon as the first two stages are stably locked. With these locking
electronics a stable single frequency operation and a fast re-lock time was achieved.
Experimental results
With a pump power of nearly 800W the free running high-power laser operates with a linearly polarized
output power of 180W (90W for each direction). The laser runs in a stable fundamental mode operation
which is monitored by a photo diode and an electrical spectrum analyzer. In free running operation and
only one circulating transversal mode, this mode beats with the circulating longitudinal modes and
produces beat signals with a distance of the free spectral range (FSR) of the laser cavity. In our
measurements no higher transversal modes could be detected between the two beat signals of the
fundamental mode. This measurement can also be used to verify the single frequency operation, because
if there is only one circulating longitudinal mode, no beat signals can be detected.
Starting the locking of the system as described above, the laser starts circulating in only one direction
instead of the bidirectional operation. Therefore, the laser output power increased by nearly a factor of
two, see Figure 2. After a few minutes the laser achieved the thermal equilibrium and operates in a
stable single frequency operation. Only external disturbances can disrupt the operation, as seen at the

elock peak. In this operation a linearly polarized fundamental mode output power of 195W was
achieved. This is to the best of our knowledge the highest injection-locked single-frequency output
power with an optical to optical efficiency > 24%.
Output Power [W]
200
180
160
140
120
100
injection-locked slave
unlocked high power laser
+ injected 12 W laser
re-lock
80
01:12 01:13 01:15 01:16 01:17 01:19 01:20 01:22
Time [mm:ss]
Fig.2: Locking process of the high power ring laser.
To analyse the locking range the Pound-Drever-Hall error signals have been evaluated, see Figure 3. The
measured locking ranges of 4.4 MHz and 2.9 MHz are below the calculated results of 4.8 and 6.3 MHz
for the intermediate and the high power slave laser, respectively.
PD DC [V]
3,0
2,5
2,0
1,5
1,0
0,5
0,0
-0,5
-1,0
-1,5
intermediate 12 W
high power slave
-0,0005 0,0000 0,0005
Fig.3: Pound-Drever-Hall error signals measured from the intermediate and the high power slave laser
by changing the cavity length near the resonance frequency.
t [s]
1
0
-1
-2
-3
-4

Especially the locking range of the high-power laser has to be improved to guaranty a long and stable
locking. Independent from the locking range it was demonstrated for the first time that a slave laser with
an output power above 100W can be stably coupled to single frequency operation.
The first long term test showed stable locking over 8 hours with only a few re-locks caused by external
disturbances. For these external disturbances like temperature or pressure changes which changed the
cavity length above the piezo range or for some strong mechanical shock to the optical table an autolock
feature was implemented. The dropping output power is detected by the electronic and starts
immediately the re-lock of the system as described above. The re-locking process of the complete
system takes less than 400 ms as shown in Figure 4.
PD DC [V]
0,0
-0,4
-0,8
-1,2
-1,6
-2,0
high power laser
intermediate 12 W
-2,4
-0,4 -0,2 0,0 0,2 0,4
Fig.4: Relock-time of the complete laser system. Measured is the output of the intermediate
and the high-power laser.
t [s]
Conclusion
We presented a high-power ring resonator with four end-pumped laser heads injection-locked via an
intermediate 12W laser to a single frequency NPRO. With this setup 195W linearly polarized, singlefrequency,
fundamental mode output power was demonstrated. This is the first time that injectionlocking
for a high power laser was applied and from our best knowledge the highest injection-locked
single-frequency output power.

Outlook
After optimization of the locking range, in order to achieve less sensitivity to external disturbances, the
system has to be further characterized in respect of beam quality as well as amplitude- and frequencynoise.
Therefore, a mode-analyzer-cavity (MAC) will be used. By scanning the MAC through its free
spectral range, a complete mode spectrum of the laser can be measured and all higher-order modes can
be detected. By locking the MAC to the laser also the frequency stability of the laser can be measured, if
thermal and acoustic coupling to the MAC is avoided. To achieve the amplitude- and frequencystabilization
needed for gravitational wave detection active control loops will be implemented.
References:
[1] LIGO II Conceptual Project Book, LIGO M990288-A-M (1999)
[2] Advanced LIGO homepage: http://www.ligo.caltech.edu/advLIGO/
[3] M.Frede at al., “High power fundamental mode Nd:YAG laser with efficient birefringence compensation”,
Optics Express 12, 3581-3589 (2004)
[4] M. Frede et al., "High-power single-frequency Nd:YAG laser for gravitational wave detection",
Class.Quantum Grav. 21, 895-901 (2004)
[5] Jan Sulc et al., “Comparison of different composite Nd:YAG rods thermal properties under diode pumping ”
SPIE-Int. Soc. Opt. Eng, 4630, 128-134 (2002)
[6] M. Tsunekane et al., “Analytical and Experimental Studies on the Characteristics of Composite Solid-State
Laser Rods in Diode-End-Pumped Geometry ” IEEE, J. of Selected Topics in Quantum Electronics, 3,
9-18 (1997)
[7] T.J Kane et al., “Monolithic, unidirectional single-mode Nd:YAG ring laser,” Opt. Lett. 10, 65-67 (1985)
[8] I. Zawischa et al., “The GEO 600 laser system”, Class. Quantum Grav. 19, 1775-1781 (2002)
[9] Eric D. Black, “An introduction to Pound-Drever-Hall laser frequency stabilization”, Am. J. Phys. 69,
79-87 (2000)