A high-power, low-noise photodetector, in conjunction with a current shunt actuator, is used in an ac-coupled servo to stabilize the intensity of a 10^-W cw Nd:YAG laser. A relative intensity noise of 1×10^-8 Hz^-1/2 at 10 Hz is achieved

Abbott, Richard

Ottaway, David

Rollins, Jameson

Weiss, Rainer

Zucker, Michael

English

Caltech Authors - Main

1876 OPTICS LETTERS / Vol. 29, No. 16 / August 15, 2004Solid-state laser intensity stabilization at the 1028 level
Jameson Rollins, David Ottaway, Michael Zucker, and Rainer Weiss
LIGO Laboratory, Massachusetts Institute of Technology, MIT NW17-161, 175 Albany Street, Cambridge, Massachusetts 02139
Richard Abbott
LIGO Laboratory, California Institute of Technology, Mail Stop #18-34, Pasadena, California 91125Received February 5, 2004
A high-power, low-noise photodetector, in conjunction with a current shunt actuator, is used in an ac-coupled
servo to stabilize the intensity of a 10-W cw Nd:YAG laser. A relative intensity noise of 1 3 1028 Hz212 at
10 Hz is achieved. © 2004 Optical Society of America
OCIS codes: 140.3850, 040.5150.Laser interferometry is a powerful measurement tool
that has allowed phenomenal improvements in metrol-
ogy for industrial and scientific applications. Laser
intensity f luctuations can limit the sensitivity of in-
terferometry,1,2 as well as other high-precision optical
measurements.3,4
Laser intensity f luctuations limit the sensitivity of
interferometers through several mechanisms. One
obvious coupling is masking the signal at the detector;
often this effect is minimized by holding the output
of the interferometer to a minimum intensity and
using a modulation–demodulation technique to shift
the measurement frequency up to where the inten-
sity f luctuations are smaller. Another coupling is
through radiation pressure on the sensing mirrors,
which in the case of gravitational-wave detectors are
suspended test masses in the range from 1 to 40 kg.
The impact of the photons causes displacement noise
at the interferometer output.
Intensity f luctuations come in two basic forms.
The first, caused by the Poisson distribution of pho-
tons in the light beam, is known as shot noise and
leads to a standard deviation equal to the square root
of the intensity. The second is known as technical in-
tensity noise and is caused by f luctuations in excess of
the Poisson f luctuations, which are typically linearly
proportional to the light power. We deal with the
latter f luctuations here and address the challenge of
approaching the Poisson-limited intensity f luctuations
in a measurement band from 10 to 100 Hz.
The required intensity noise performance for the
Advanced Laser Interferometer Gravitational Wave
Observatory (LIGO) can be seen in Fig. 1. The most
demanding part of this requirement is a relative
intensity noise of 2 3 1029 Hz212 at 10 Hz. In this
Letter we report on the progress toward meeting this
tight requirement in a single-frequency solid-state
master oscillator power amplifier (MOPA) laser5 by
feedback control of the current to the amplif ier pump
diodes.
Current plans for the Advanced LIGO call for the
use of a 180-W injection-locked laser system.6 This
higher-power system may be accompanied by higher
relative intensity noise levels, which may require
more servo gain to reach the same relative intensity
noise level. Other than issues of servo gain, the main0146-9592/04/161876-03$15.00/0hurdles to achieving excellent intensity noise perfor-
mance are sensor noise, nonlinearities in the servo
electronics, and beam geometry f luctuations. These
other limitations are common to both injection-locked
and MOPA configurations; therefore the following
system will be applicable to the Advanced LIGO.
The laser that was used to test and develop this in-
tensity stabilization servo is a prestabilized laser (PSL)
system that is similar to the PSL system used in the
initial LIGO.2 It consists of a 10-W MOPA that can be
locked to a high-finesse reference cavity for frequency
stabilization.7
More important, in the context of this work the PSL
also has a monolithic triangular Fabry–Perot cavity
known as the premode cleaner (PMC).8 This cavity
acts as a high-precision spatial mode filter that re-
duces laser beam geometry f luctuations by a factor
Fig. 1. Relative intensity noise (RIN) as measured by
in-loop and out-of-loop photodetectors. The expected shot
noise and measured electronic noise levels are shown in
the light blue and green traces, respectively. The black
curve shows the Advanced LIGO requirement. (Data
taken June 29, 2003.)© 2004 Optical Society of America
August 15, 2004 / Vol. 29, No. 16 / OPTICS LETTERS 1877of 130. This is important because the responsivity of
the photodiodes varies across their surfaces. Hence,
variations in the position of the beam on the photo-
diode lead to variations in the photocurrent, resulting
in increased noise. The frequency cutoff of the filter
cavity is 1.75 MHz, which, being well outside the band-
width of the servo control system (30 kHz), has no ef-
fect on the design of the intensity control servo.
The MOPA has two main diode current adjust-
ment actuators useful for intensity stabilization: a
high-range, but relatively slow, low-frequency current
adjust (LFCA) and a lower-range, but faster, current
shunt (CS). Both actuators modulate the current to
the main power amplif ier diodes of the laser.
The LFCA sums directly to the power amplif ier
pump-diode current supply at the MOPA control box.
It has a range of approximately 10% of the total
drive current, with a response of up to 20 kHz, at
which point it falls off steeply at a rate of roughly
80 dBdecade.
The CS is a circuit in parallel with the power
amplifier pump diodes that is able to modulate the
current to the diode at approximately 1% of the total
current.9 The response is f lat up to 3 kHz, where
there is only a single pole, making this actuator useful
to much-higher frequencies.
The design of the intensity stabilization servo
was guided primarily by the decision to make the
photodetector, and therefore the entire control loop, ac
coupled. Although this makes the design of the servo
slightly more complicated, it bypasses the need for a
highly stable external dc reference. The basic block
diagram for the servo can be seen in Fig. 2. The
photodetector is placed after the PMC to take advan-
tage of its beam-stabilizing properties.
The CS alone does not have enough range below
1 Hz to control the free-running laser intensity noise.
To circumvent this problem, the control signal to the
CS is picked off, put through a low-pass f ilter, and
then fed into the LFCA. This servo provides a gain
of more than 80 dB at 10 Hz, with a unity gain fre-
quency greater than 30 kHz.
The design of the photodetector is crucial to the
success of this experiment. To achieve a relative
intensity noise level of 1029 Hz212, it is necessary to
make a shot-noise-limited detection of a dc photocur-
rent of 300 mA with a DI of 300 pAHz212. The
photodetector utilizes a photodiode in an ac-coupled
topology instead of the standard transimpedance con-
figuration usually used with photodiodes. Utilizing a
dc transimpedance configuration would have required
the use of operational amplif iers with both low-noise
and high-current capabilities. Instead, it was decided
to split the problem into two parts.
The bias voltage and high-current supply for the
photodiode are handled by a voltage regulator. The
voltage regulator is controlled by a feedback loop such
that a constant reverse bias is maintained across the
photodiode (see Fig. 3). This is done so that appreci-
able ohmic heating does not occur when the diode is
run at reduced light levels.
The photocurrent is sunk across a resistor to con-
vert the photocurrent into a voltage. The critical in-tensity noise stabilization signal is then ac coupled at
the photodiode output with a 1-Hz high-pass f ilter and
processed by a low-noise LT1128 operational amplif ier
with an input-referred voltage noise of 1 nVHz212.
Careful consideration is also given to the effects of
noise at frequencies outside the servo bandwidth. It
is well known that high-frequency noise can encounter
a slew of rate limits in any of the amplif ication stages,
causing broadband noise that infects the sensitive fre-
quency band. Although this is true throughout the
servo chain, it is especially important at the front end,
i.e., within the photodetector. Low-pass f iltering is
used extensively, at every amplification stage, provid-
ing attenuation of many orders of magnitude for sig-
nals above 100 kHz.
The photodiodes used are Hamamatsu G5832-02
2-mm diodes. These are used because of their demon-
strated ability to handle fairly large power levels (with
a linear response above 250 mW), as well as their high
surface uniformity (with a 2% loss of sensitivity at
half the distance from the diode center10). They were
also measured to have a quantum efficiency of 93%.
The current best relative intensity noise level
achieved by the servo is approximately 1 3
1028 Hz212 at 10 Hz and approximately 5 3
1029 Hz212 at 100 Hz (Fig. 1). This level is mea-
sured with the out-of-loop detector (see Fig. 2). The
Fig. 2. Block diagram of the intensity stabilization servo.
The fast path uses the fast CS actuator, whereas the slow
path uses the low-bandwidth, high-range, LCFA. The
control signal comes from the in-loop detector, and the
out-of-loop detector is used for reference. RFPD, radio-
frequency photodetector; PD, photodetector.
Fig. 3. Schematic of the photodetector circuit. The bias
feedback control loop can be seen on the left and the pream-
plification on the right. 1, unity gain amplifier; G, invert-
ing gain amplif ier.
1878 OPTICS LETTERS / Vol. 29, No. 16 / August 15, 2004level given by the in-loop detector is artif icially low
since the servo loop attempts to nullify the detector’s
electronic noise by adding its inverse to the light.
The data in Fig. 1 were taken with a dc photocur-
rent of 140 mA on the out-of-loop photodetector.
This corresponds to an incident light power of
175 mW. However, this relative intensity noise level
can be achieved with powers as low as 100 mW.
The achieved intensity noise level is higher than
the calculated shot noise level, which at 140 mA is
23 1029 Hz212 (see Fig. 1). At this time the limiting
factor in these measurements is not known, although
a candidate is the residual relative motion between the
light beam and the photodiode.
Figure 1 shows that the noise f loor measured by the
in-loop photodiode is well below that measured by the
out-of-loop diode, indicating that the achieved intensity
noise is not limited by a lack of loop gain in the servo
loop between 10 and 150 Hz.
Various electronic noise sources were also carefully
characterized. Figure 1 shows that the measured
input-referred photodetector dark noise is approxi-
mately a factor of 10 less than the measured intensity
noise.
Electronic grounding noise was discovered to be a
critical issue. This was not due to electromagnetic
pickup but instead to problems with noise on the
ground reference that were subsequently adding
noise to the signal in the detector. Improvements
were made over the initial measurements by sending
the signals from the photodetector to the readout
electronics differentially.
The light leaving the PMC is highly polarized, but
further polarization filters were used to ensure that
polarization jitter at the beam splitter would not cause
differential intensity variations at the in-loop and
out-of-loop photodiodes. To eliminate the possibility
that frequency noise would be converted to intensity
noise in the PMC, a frequency stabilization servo was
used to reduce the frequency noise of the laser with no
improvements in the intensity noise.
Finally, extensive measures were also taken to
reduce the effects of any possible environmental noise
sources. For instance, neutral-density filters were
placed in front of the photodetectors to minimize the
effects of scattered light, and the entire experiment
was placed in an enclosure that significantly reduced
the amount of acoustic noise and the effects of air
currents, once again with minimal effect.
Further work is required to achieve the extremely
demanding intensity stabilization required for the Ad-vanced LIGO. Planned improvements include the de-
velopment of in-vacuum photodetectors, moving the
detectors after the LIGO suspended mode cleaner, and
the use of multiple in-loop detectors.
Other researchers have observed similar perfor-
mance at 100 Hz by use of a different photodetector
topology.11 At present it is not known what is limit-
ing the noise performance of either experiment at this
level.
In conclusion, we have achieved intensity stabiliza-
tion of a solid-state laser at the 1 3 1028 Hz212 level
at 10 Hz. To our knowledge this is the f irst time that
such performance has been reported.
This research was supported by the National
Science Foundation under Cooperative Agreement
PHY-0107417 and LIGO Internal DCC P030072-00-R.
The authors gratefully acknowledge the insightful
comments on the manuscript given by David H.
Shoemaker. D. Ottaway’s e-mail address is ottaway@
ligo.mit.edu.
References
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Solid-state laser intensity stabilization at the 10^-8 level

Entwicklung einer quantenrauschbegrenzten Leistungsstabilisierung für ein

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Solid-state laser intensity stabilization at the 10^-8 level

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