We report on the use of a fixed-spacer Fabry–Perot ring cavity to filter spatially and temporally a 10-W laser-diode-pumped Nd:YAG master-oscillator power amplifier. The spatial filtering leads to a 7.6-W TEMinfinity beam with 0.1% higher-order transverse mode content. The temporal filtering reduces the relative power fluctuations at 10 MHz to 2.8 x 10^-/sqrtHz, which is 1 dB above the shot-noise limit for 50 mA of detected photocurrent

Byer, R. L.

Gustafson, E. K.

King, P. J.

Savage, R. L., Jr.

Seel, S. U.

Uehara, N.

Willke, B.

English

Caltech Authors - Main

1704 OPTICS LETTERS / Vol. 23, No. 21 / November 1, 1998Spatial and temporal filtering of a 10-W Nd:YAG laser with a
Fabry–Perot ring-cavity premode cleaner
B. Willke, N. Uehara, E. K. Gustafson, and R. L. Byer
Edward L. Ginzton Laboratory, Stanford University, Stanford, California 94305-4085
P. J. King, S. U. Seel, and R. L. Savage, Jr.
LIGO Project, MS 18-43, California Institute of Technology, Pasadena, California 91125Received June 29, 1998
We report on the use of a fixed-spacer Fabry–Perot ring cavity to filter spatially and temporally a 10-W
laser-diode-pumped Nd:YAG master-oscillator power amplif ier. The spatial f iltering leads to a 7.6-W TEM00
beam with 0.1% higher-order transverse mode content. The temporal f iltering reduces the relative power
f luctuations at 10 MHz to 2.8 3 1029y
p
Hz, which is 1 dB above the shot-noise limit for 50 mA of detected
photocurrent.  1998 Optical Society of America
OCIS codes: 120.2230, 330.6610, 330.6790, 140.3530.In recent years the need for high-power lasers
with good spatial and temporal characteristics has
increased, with applications in laser machining,
medicine, quantum optics, and gravitational-wave in-
terferometry. Good pointing stability as well as pure
Gaussian profiles are the key features for precision
machining, whereas pure spatial profiles and power
f luctuations near the shot-noise limit are crucial in
gravitational-wave interferometry. Spatially and
temporally filtered laser-diode-pumped solid-state
lasers are promising sources for these applications.
The most challenging application that requires high
power (10 W) with an extraordinary degree of pu-
rity for both the spatial and the temporal modes
is gravitational-wave interferometry.1,2 The master-
oscillator power-amplif ier (MOPA) concept was pro-
posed in 1995 by the Stanford group3 for the use
in the American Laser Interferometer Gravitational-
Wave Observatory (LIGO) because of its ability to
scale the output power while maintaining the master-
oscillator coherence and operational reliability.4 To
reach the demanding spatial mode and temporal noise
requirements these experiments require a combination
of active stabilization and passive filtering of the laser
source.5 Fabry–Perot interferometers have been pro-
posed as mode cleaners6,7 to filter the laser beam spa-
tially and temporally before its introduction into the
laser interferometer for gravitational-wave detection.
We report the use of a fixed-spacer triangular-ring
cavity, called the premode cleaner, to filter the out-
put of a 10-W Nd:YAG MOPA8 built by Lightwave
Electronics Corporation under contract with the LIGO
project. The master oscillator is a laser-diode-pumped
monolithic Nd:YAG nonplanar ring oscillator (NPRO;
Lightwave Electronics Model 126-1064-700). The
master-oscillator output (320 mW) is transmitted
through an electro-optic modulator, a pair of cylindri-
cal lenses to circularize the beam, and isolation optics
before the output enters the power amplifier. The
power amplifier consists of four Nd:YAG laser rods,
each pumped by two diode bars operating at a power of
16 W each. The double-passed amplifier has a power
gain of 33 and generates 10 W of output power.0146-9592/98/211704-03$15.00/0The 10-W laser beam is mode matched into the
premode-cleaner cavity as shown in Fig. 1. The pre-
mode cleaner consists of three mirrors in a ring con-
figuration with an opening angle of 87.13 deg and a
round-trip length of 42 cm, similar to the design of
Uehara et al.9 Two plane mirrors are attached (with
epoxy) to a fused-silica prism, which is itself attached
to a 17.5-cm-long rigid fused-silica spacer. Both the
spacer and the prism have holes with sufficient clear-
ance to avoid clipping the beam. A curved mirror with
a 1-m radius of curvature is attached to a piezoelec-
tric transducer, which is then attached to the other end
of the fused-silica spacer. A finesse of 220 was mea-
sured for the p polarization, whereas the finesse for the
s polarization was 4440. For this experiment we used
the p polarization, for which we obtained 94% power
throughput for the TEM00 Gaussian mode.
We used several techniques to compare the spatial
profile of the 10-W laser beam before and after fil-
tering. First a small fraction of the light was picked
Fig. 1. Schematic of the experimental setup and the
premode-cleaner Fabry–Perot ring cavity design. A de-
tailed description of the length-control scheme is given
in the text. EO, electro-optic modulator; PZT’s, piezoelec-
tric transducers; L1, L2, mode-matching lenses; M1, M2,
25.4-mm-diameter plane mirrors; M3, 7.75-mm-diameter
mirror with 1-m radius of curvature. Cavity round-trip
length, 42 cm. 1998 Optical Society of America
November 1, 1998 / Vol. 23, No. 21 / OPTICS LETTERS 1705off before and after the premode cleaner and sent into
a beam analyzer (BeamScan, Photon, Inc.). Figure 2
shows the measured beam profiles in the horizontal
and vertical directions. The data of Fig. 2(a) were
taken before the premode cleaner. The faint curves
are calculated best-fit Gaussian profiles, the parame-
ters of which were obtained with a nonlinear fit. The
beam profiles have a good overall Gaussian character,
but deviations between measurement and fit are evi-
dent. Similar distortions as well as the notable dif-
ference of the horizontal and vertical beam diameters
were measured in the NPRO master-oscillator spatial
profile, although most of the NPRO ellipticity was cor-
rected by the pair of cylindrical lenses. Figure 2(b)
shows the measured beam profiles of the 7.6-W laser
beam following the premode cleaner. The measure-
ments and best-fit curves are indistinguishable within
the resolution of this plot. The residual between mea-
surement and fit is less than 1% for each individual
point, and the ellipticity is reduced to less than 1% dif-
ference in horizontal and vertical beam diameter.
We determined the residual power in the higher-
order spatial modes of the 10-W laser by scanning
the premode cleaner over one free spectral range and
measuring the transmitted and ref lected light. This
procedure corresponds to measurement of the laser
beam profile expanded into the normal modes of the
cavity. Figure 3 shows a measurement of the relative
power transmitted through the premode cleaner as a
function of the premode cleaner’s cavity length. The
TEM00 mode is the dominant mode, and the distance
between the two TEM00 peaks corresponds to one free
spectral range of 713 MHz. Pictures taken with a
CCD camera of the TEM00 mode and the largest non-
TEM00 mode are shown as insets. By scanning the
premode cleaner length slowly over a free spectral
range of the cavity we measured the transmitted
power and the relative frequency position of the five
most significant non-TEM00 modes with respect to
the TEM00 mode. Adding all these contributions, we
found 9.3% of the MOPA output power in higher-order
modes. We obtained the same result by measuring
the ref lected power instead of the transmitted power
while scanning the premode-cleaner length over a free
spectral range.
The cavity must be kept in resonance with the
laser when the cavity is used as a premode cleaner.
We locked the cavity resonance frequency to the
laser frequency, using the Pound–Drever–Hall tech-
nique.10 The light was phase modulated with the
electro-optic modulator following the master oscillator
where the light power was less than 350 mW. At
this lower power we were able to use a small-aperture
modulator (2 mm) and still operate at low intensity
in the crystal. We used a modulation frequency of
20 MHz and a modulation index of 0.06. The phase
modulation sidebands as well as the carrier were
amplified and then mode matched to the premode
cleaner. The signal of the fast photodiode measuring
the ref lected light power was then demodulated and
filtered to provide an error signal for the length-control
servo system. The piezoelectric transducer was used
as the actuator of this servo system to lock the cavityto the laser. The bandwidth of this loop was limited
by a mechanical resonance of the fused-silica spacer
at 9 kHz.
With this servo system in place we measured 7.6 W
of transmitted light power for an incident power of
9.1 W. After we account for the power in non-TEM00
modes and the phase modulation sidebands, this trans-
mitted light power corresponds to 94% throughput of
the light power in the TEM00 mode. To get an accu-
rate measurement of the spatially filtered laser pro-
file we mode matched the light transmitted by the
premode cleaner into a second ring cavity. By scan-
ning the length of the second cavity we determined that
0.1% of the light power after the premode cleaner was
in non-TEM00 modes. This measurement confirmed
the calculated residual power in higher-order modes.
In addition to performing spatial mode filtering,
the premode cleaner reduces the technical power
noise of the 10-W laser, which is important for ex-
periments that require a shot-noise-limited laser
source. The gravitational-wave interferometers, for
example, use heterodyne techniques that transform
Fig. 2. Measured horizontal and vertical beam profile of
the 10-W laser and best-fit Gaussian profiles: (a) before
the premode cleaner and (b) following the premode cleaner,
where measurement and fit agree to within the pen width.
Fig. 3. Transmitted power of the 10-W laser through the
premode cleaner as a function of the cavity length. Insets,
pictures (taken with CCD camera) of the dominant TEM00
mode and the largest non-TEM00 mode. The higher-order
mode content before the premode cleaner was ø10%,
whereas following the premode cleaner the relative power
in non-TEM00 modes was reduced to 1023. FSR, free
spectral range.
1706 OPTICS LETTERS / Vol. 23, No. 21 / November 1, 1998Fig. 4. Power noise of the 10-W laser relative to the shot
noise at 50 mA of detected photocurrent. Upper curve,
measurement before the premode cleaner; lower curve,
measurement following the premode cleaner. The power
noise following the premode cleaner is shot-noise dominated
above 7 MHz and less than 1 dB above the shot-noise limit
for frequencies higher than 10 MHz.
the gravitational-wave signal into the megahertz
frequency band to achieve sensitivity limited by the
shot noise of the detected light. Thus it is crucial to
reduce the technical noise at the modulation frequency
to the shot-noise limit. The LIGO gravitational-wave
interferometer requires the power noise of the laser to
be no more than 0.5% above the shot noise of 600 mW
of detected light at 25 MHz. As an active feedback
control loop cannot reduce the technical noise to this
level,11 we used the premode cleaner to achieve this
goal. The power noise filtering of a Fabry–Perot
cavity can be described by HT sf d ­ 1yf1 1 sfyfcd2g2,
where HT sf d is the power f luctuation transfer function
for light transmitted through the cavity at Fourier
frequency f and where fc is the cavity half-width at
half-maximum.
Figure 4 shows the relative power noise of the high-
power MOPA laser relative to the shot-noise level for
50 mA of detected photocurrent before and after the
premode cleaner. The power noise above shot noise
is a measure of the fraction of technical noise that
is present in the laser beam. Two noise sources are
responsible for the technical noise in the MOPA be-
fore the premode cleaner (upper curve): the technical
noise of the NPRO, which is amplified by the power
amplifier, and noise introduced by the amplification
process.12 The lower curve shows the filtered power
noise following the premode cleaner. Using the cal-
culated power noise transfer function of the premode
cleaner, we expect 8.2 dB of power noise filtering at
4 MHz, which is in good agreement with the measure-
ment. At 6 MHz the power noise amplitude spectral
density following the premode cleaner is 3 dB above
shot noise, which indicates that the technical noise
equals the shot noise of 50-mA detected photocurrent.
The peak at 10 MHz is the filtered power f luctuation
impressed by the electro-optic modulator as the result
of a spurious 10-MHz signal present in the rf-oscillator
signal. This modulator was turned off while we mea-
sured the amplitude noise before the premode cleaner.In conclusion, a 10-W laser-diode-pumped Nd:YAG
MOPA was spatially and temporally filtered with a
fixed-spacer Fabry–Perot ring cavity. This premode
cleaner provides a light source with an excellent spatial
profile and low power noise at radio frequencies. Both
the MOPA design and the passive filtering offer the
possibility to increase the laser power to 100 W and
beyond. Although this light source was developed to
meet the needs of high-sensitivity interferometry for
the detection of gravitational waves, the demonstrated
concept is versatile, and the premode cleaner can easily
be implemented in different types of laser systems to
yield spatial and temporal filtering.
We thank David Shoemaker and Stan Whitcomb for
proposing the use of a premode cleaner as a passive
power noise filter and for suggesting this collaborative
research. B. Willke thanks the German Alexan-
der von Humboldt-Stiftung for his Feodor-Lynen
fellowship. This research is supported in part by
the National Science Foundation (NSF) under co-
operative agreement PHY-9210038 and NSF grant
PHY-9630172.
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Spatial and temporal filtering of a 10-W Nd:YAG laser with a Fabry-Perot ring-cavity premode cleaner

A high-power, diode-laser-pumped solid-state laser for precision interferometry,’’ Ph.D. dissertation

Measurements of quantum noise in a cw laser-diodepumped Nd:YAG linear optical amplifier,’’ submitted to Opt.

The MOPA concept for the LIGO gravitational-wave detector was proposed in the GALILEO proposal,

https://authors.library.caltech.edu/2896/1/WILol98.pdf

Spatial and temporal filtering of a 10-W Nd:YAG laser with a Fabry-Perot ring-cavity premode cleaner

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