The primary objective of the Decihertz Interferometer Gravitational Wave Observatory (DECIGO) mission is to detect and observe black holes' and galactic binaries' gravitational waves (GWs), at frequencies ranging from 10−2 to 101 Hz (from 0.1 to 100 s in averaging time). This low-frequency range is inaccessible to ground-based interferometers, due to unshieldable background noise and to the fact that ground-based interferometers are limited to a few kilometers in length. Our research is focused on efforts to stabilize semiconductor, Nd:YAG, and fiber lasers, for use as GW detectors' optical sources. In present-day and future detectors, frequency and phase noise may place certain limitations on sensitivity and stability. Our goal is primarily to design robust experiments. In this report, we compare existing methods: Faraday, Faraday peak, and saturated absorption spectroscopy. In these, the laser frequency is stabilized to Rb as an atomic frequency reference by a feedback-loop control system. From the frequency stability of these models, we can predict the characteristics of the three systems through dynamic stability analysis, by analyzing the dynamic Allan variance. We find the optical frequency stability, expressed as the Allan deviation (the square root of the Allan variance), to be 3.3×10−11, 2.9×10−12, and 1.2×10−12 in the respective methods.
Scientists throughout the world are seeking to enhance the capabilities of satellite-to-satellite tracking laser
interferometer-based optical systems used to measure the alterations in earth's gravitational field that indicate critical
changes in the environment. These systems must be able to measure infinitesimal fluctuations in the relative velocities of
two satellites, using a light source that oscillates at a level of frequency stability rated better than 10-13 in the square root
of the Allan variance. In our experiments, semiconductor laser frequency stabilization that typically requires a brief
direct modulation of the laser injection current to obtain an error signal, was accomplished using the Faraday effect of Rb
absorption lines. This effectively modulates the reference frequency of the stabilization system, i.e., the Rb absorption
line, by modulating the magnetic field applied to the Rb absorption cell, instead of the oscillation frequency of the laser
diode. Most recently, we used the Faraday method, in conjunction with a precision temperature controller. For present
purposes, we also use the PEAK method, to obtain the most accurate signal possible, comparing it with saturated
absorption spectroscopic readings, to determine the noise-source.
The precise interferometric systems employed in today's artificial satellites require semiconductor lasers of the
highest caliber. To this end, efforts to stabilize their oscillation frequencies and narrow spectrum line-widths
continue relentlessly. While a number of different approaches have been tested, none have provided overall,
long-term stability. Most recently, we employed a Doppler-free absorption line of Rb atoms, with a precision
temperature controller and an improved laser mount. In this instance, relative optical frequency stability rated
9.07×10-13≤σ(2,τ)≤7.54×10-10, in averaging time for 0.01s≤τ23s. By introducing an optical feedback, which
narrows the laser's linewidth, we obtained improved frequency stability.
The precise interferometric systems employed in today's artificial satellites require semiconductor lasers of the highest callibur. But,
one particularly large obstacle has stood in the way of their broad application; the stabilization of their oscillation frequencies. While a number of different approaches have been tested, none have provided overall, long-term stability. Most recently, we used a Doppler-free absorption line of Rb atoms with a precision temperature controller and an improved laser mount; in this instance, relative optical frequency stability rated 9.07×10-13 ≤ σ(2,τ) ≤ 7.54×10-10, in averaging time for 0.01s ≤ τ ≤ 23s. Furthermore, we heated the Rb cell to up to 313K, in order to enhance the control signal and improve oscillation frequency stability.
The precise interferometric systems employed in today's artificial satellites require semiconductor lasers of the highest callibur. But,
one particularly large obstacle has stood in the way of their broad application; the stabilization of their oscillation frequencies. While a number of different approaches have been tested, none have provided overall, long-term stability. Most recently, we used a Doppler-free absorption line of Rb atoms with a precision temperature controller and an improved laser mount; in this instance, relative optical frequency stability rated 9.07x10-13≤&sgr;(2,&tgr;)≤7.54x10-10, in averaging time for 0.01s≤&tgr;≤23s. By introducing optical feedback, which narrows the laser's linewidth, we obtained improved frequency stability.
The primary objective of the Deci-hertz Interferometer Gravitational Wave Observatory (DECIGO) mission is to detect
and observe black holes' and galactic binaries' gravitational waves (GWs), at frequencies ranging from 10-2 to 101 Hz.
This low-frequency range is inaccessible to ground-based interferometers, due to unshieldable background noise, and
the fact that ground-based interferometers are limited to a few kilometers in length. Our research is focused on efforts to
stabilize semiconductor-, Nd:YAG- and fiber- lasers, for use as GW detectors' optical sources. In present-day- and future
detectors' frequency- and phase-noise may place certain limitations on sensitivity and stability. Our goals (shared with
scientists around the world) are; first, to design robust experiments that will measure a variety of noises (random-walk
FM, flicker-FM, white FM, flicker PM and white PM), in order to verify existing models, and second, to find ways to reduce sensitivity to spurious noise. Current models predict a variety of frequency- and phase-dependent noise slopes, but, a conclusive distinction between noise-models can only be made when the exact points at which the noises occurred are known. In order to increase the sensitivity of the experiment, the laser frequency is stabilized to an atomic-frequency-reference by a feedback-loop control system.
A method of detecting gravitational-field variations using laser diodes is described. While the GRACE project is
currently using the Doppler microwave system to measure the velocities of satellites flying in tandem, in the future, more
advanced laser interferometry will be employed. It is hoped that we will be able to measure infinitesimal changes in their
velocities, by using frequency-stabilized lasers rated at better than 10-13 in the square root of the Allan variance (&sgr;) for 1s
< τ < 100s. As laser light sources, these devices will be notable for their compactness, energy efficiency, lightweight and
high frequency-stability. This thesis describes the improved frequency stabilization obtained through the use of the
magneto-optical effect of the Rb-D2 absorption line, and the adaptation of the PEAK method, in order to obtain a precise
control signal. The method allows us to modulate the reference frequency of the stabilization system (the absorption
spectrum of the Rb-D2 absorption line) by modulating the magnetic field applied to the Rb absorption cell, instead of the
oscillation frequency of the laser diode. In so doing we are able to achieve a frequency stabilized laser diode (&sgr; = 9 x
10-12), while maintaining its linewidth, at an averaging time of 40s. In the next stage, we will test frequency-stabilized
laser optical sources that are to be used in detecting and observing gravitational waves.
Laser interferometers detect gravitational waves with a degree of accuracy and efficiency unimaginable even a few years
ago. The semiconductor lasers that are the primary light source for these devices are small, lightweight, durable and
energy-efficient. On the downside, the devices currently available are still marked by broad oscillation spectra, and
heightened sensitivity to fluctuations in injection current and /or ambient temperature. By applying a small sine wave to
the injection current, we modulate the oscillation frequency. This frequency-modulated beam is introduced to the
Avalanche photo diode through the Rb cell in the saturated absorption optical setup. The resulting signal and a reference
signal are detected simultaneously and combined, to produce an error signal, which, when fed back to the injection current,
stabilizes the diode's oscillation frequency at 2.12x10-12 ⩽ &sgr;(2,τ) ⩽ 5.88x10-11 in the averaging time between 0.4s to 65s.
An optical feedback method, which introduces the laser beam reflected by a mirror or a grating to the semiconductor laser
itself, is reported to narrow oscillation linewidth and improve frequency stability. We are now combining these two
techniques to further improve frequency stability.
The present work discusses methods of stabilizing the frequencies of commercially-available laser diodes. Laser diodes are generally compact and long-lived. The frequency stability, which makes them ideal for onboard laser interferometer light-sources, in applications such as the satellite-to-satellite tracking systems used to verify fluctuations in earth's gravity field, which, in turn, indicate other critical changes in the environment, is the key characteristic of this work. We used the devices typically operating at 780nm, and their frequencies can be stabilized using either of two systems; one, employing the Doppler-free absorption line of Rb atoms and another, sing the Faraday effect of the Rb absorption line. In both cases, the use of the proper modulation frequency and amplitude improved frequency stability, overall, attaining 2.05×10-12 and 2.73×10-11, respectively, in the square root of the Allan variance, by measuring the beat-note between two independently-stabilized laser diodes.
Currently in the initial stages of development, the endeavor aims to use satellite-to-satellite tracking laser interferometer-based optical technique, to document fluctuations in earth’s gravitational field indicating other critical changes in the environment. This system must be able to measure infinitesimal changes in the relative velocity of the two satellites, using a laser light source, which oscillates at frequency stability better than 10-13 in the square root of the Allan variance. We have stabilized the laser’s oscillation frequency using the Faraday effect of Rb absorption lines. This method modulates the reference frequency of the stabilization system by modulating the magnetic field applied to the Rb absorption cell, instead of the oscillation frequency of the laser diode. Furthermore, we have adapted the “double optical feedback” to the laser diode for narrowing its oscillation spectrum and improving its frequency stability. In recent years, a “femtosecond optical comb generator” has been developed as a new reference frequency source for absolute frequency measurement. This optical comb generator is controlled by the microwave frequency standards systems and provides stability of 4x10-13 at an averaging time of 1s and at the order of 10-15 at 1000s averaging time. We have measured the frequency stability of our system using the optical comb. We obtained the best spectrum narrowing effect using two gratings as external reflectors in the double optical feedback setup. The obtained results were 6.269x10-11 ≤ σ ≤ 1.516x10-10 (24.11kHz ≤ f ≤ 58.31kHz) from 1s to 39s in the averaging time.
Currently in the initial stages of development, the endeavor aims to use satellite-to-satellite tracking laser interferometer-based optical technique, to document fluctuations in earth's gravitational field indicating other critical changes in the environment. This system must be able to measure infinitesimal changes in the relative velocity of the two satellites, using a laser light source, which oscillates at frequency stability better than 10-13 in the square root of the Allan variance. The constraints placed upon the system will ultimately require that the interferometer's light-source be small, lightweight, extremely efficient, durable, and possess high frequency-stability. Present-day technology of a laser diode possesses all of these characteristics, except the last and most critical one; frequency stability. So, our ongoing efforts are all focused on the search for a method of stabilizing the oscillation frequency of the satellite-to-satellite laser-diode light source. We used a Rubidium absorption line as a frequency reference of the frequency stabilization system and the "double optical feedback" method to narrow the laser diode's oscillation linewidth, in order to improve its frequency stability. We have measured the frequency, to determine its degree of stability, by comparing the "femtosecond optical comb generator"- and laser diode-frequencies.
Osamu Miyakawa, Seiji Kawamura, Benjamin Abbott, Rolf Bork, Peter Fritschel, Lisa Goggin, Jay Heefner, Alexander Ivanov, Fumiko Kawazoe, Conor Mow-Lowry, Alexei Ourjoumtsev, Sihori Sakata, Michael Smith, Kenneth Strain, Robert Taylor, Dennis Ugolini, Stephen Vass, Robert Ward, Alan Weinstein
KEYWORDS: Demodulation, Interferometers, LIGO, Prototyping, Surface plasmons, Modulation, Mirrors, Scanning electron microscopy, Sensors, Control systems
The LIGO Laboratory 40m prototype interferometer at Caltech is being commissioned to prototype an optical configuration for Advanced LIGO. This optical configuration has to control five length degrees of freedom, and its control topology will be significantly more complicated than any other present interferometers. This paper explains the method of sensing, controls and lock acquisition.
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