Influence of radio frequency harmonics in demodulation on a laser interferometer

1
Poster No.12 / Refer. No.154
Influence of radio frequency harmonics
in demodulation on a laser interferometer
Noriyasu NakagawaA, Koji AraiB, Kazuaki KurodaA,
and TAMA collaborationC ICRR, Univ. of TokyoA,
National Astronomically Observatory of JapanB,
and TAMA collaborationC
Abstract
Laser
Radio frequency harmonics present in both the
local oscillator and the photocurrent of the
interferometer. They may effect to the
sensitivity of the interferometer. In this
poster, the contribution of the harmonics to the
sensitivity is experimentally investigated.
Modulation and demodulation techniques are
essential to control the interferometer . The
laser is modulated at 15.235MHz (1f) in TAMA300.
EOM
LO is required for a mixer to down-convert RF
signal to audio frequency. The table shows actual
power of radio frequency harmonics. LO is often
assumed to be a monochromatic wave.
Gravitational waves (GW) reach an interferometer,
then the output light from the interferometer is
detected by a photo detector (RF port). The power
of the RF port is shown as the table below. In
the RF port, GW signals appear at 1f, while 2f
signals are also generated from the beating of
modulation sidebands.
Harmonic distortion of LO port dB
Harmonic distortion of RF port dB
This reveals that radio frequency harmonics
exist. Even-order ones are due to the circuit,
odd-order ones are due to an intrinsic character
of the mixer. In the mixer, LO works as a switch,
hence it behaves as square waves.
This table reveals that the second-order
harmonics actually exists and has larger power
than 1f in the RF port. In the measurement,
higher order harmonics are also identified
Ideal
Actual
in the demodulated signal. Once they are
down-converted to the audio frequency band, it is
impossible to distinguish the harmonic components
from gravitational wave signals.
The demodulation multiplies LO and RF, and
down-convert the result of LORF to audio
frequency band (AF). GW signal is included
GW
NOISE
Actually we measured the sensitivity and the
influence by harmonics, and compared them. This
measurement of influence was noticed on these
ways. Both measurement of the sensitivity
and the influence by harmonics used same RF
signal source. In the measurement of
the influence by harmonics, 1f or 2f were
reduced to see directly the influence by
harmonics. We measured an upper limit the
influence by harmonics. As a result of
this measurement (right figure), over 70Hz was
limited by an electrical noise of circuit,
nevertheless the influence by the harmonics lies
under the sensitivity of TAMA300.
It is found that the influence of harmonics is
not limiting current noise. This is the upper
limit of the influence by harmonics.