Projects at Caltech

The Gravitational data stream from multiple interferometers in the LIGO (and other gravitational waves detector) projects is expected to contain mostly noise, with occasional signals from astrophysical sources of gravitational wave (GW) bursts.

The LIGO Data Analysis System (LDAS) searches through individual detector data streams for such bursts and populates a database with these “event triggers.”

In preparation for real data, I am developing a Monte Carlo simulation of the results of such searches, populating a test database and then performing a study of the statistics of single interferometer bursts, as well as coincidence events from multiple interferometers. All these are due to signals from simulated GW bursts.
 

The low thermal noise motion of the mirrors in interferometric gravitational wave detectors can be significantly increased by their dielectric mirror coatings. The goal of this project was to set up an experiment which would allow us to measure the mechanical losses introduced by the mirror coatings and if possible reduce these mechanical losses.

In order to achieve our goal, our experimental set-up had to be set up so that we could measure the stress-strain dependence of the coatings. Translated: How does the strain on the coating vary as you apply a varying force to it? A vise was used to apply a force to the optics by just rotating the vise handle. After a lot of R&D, it turned out that the best set up consisted of just a vise, the mirror and a strain gauge. Using this set up, we saw a signal that was reproducible within 10% from the mirror coating which varied linearly. This was achieved using a He-Ne laser, but by using a light source in the IR region we think that we should be seeing a much bigger signal. Therefore, that is our aim now.

The Laser Interferometer Gravitational-wave Observatory (LIGO) will directly measure the distortion (10-18 to 10-22m) of spacetime due to an impinging gravitational-wave.  The biggest challenge facing LIGO is reducing the background noises and increasing the sensitivity.  In order to assist in tackling this issue, an end-to-end time domain simulation program for LIGO (E2E) has been developed, which can calculate the LIGO detector response to the signal caused by the GW under a realistic operational condition.

Many important noise sources have already been built into the program.  A current endeavor is to write a simulation program of GW sources so that E2E can simulate a state with both the GW signal and noises. This program presently calculates the time-series variation of the strain (the ratio of change in arms’ lengths to the sum of their lengths) for coalescing compact binaries and spin-down supernovae, then graphs the results.

The program is designed to be flexible in choice of source location and detector location and orientation.  The correlation of signals observed by two detectors is a very important factor to ensure that those observed signals are from a real signal. This program generates signals with proper correlations due to spatial separations and detector orientations. 

The program can also visualize the time series of the result. The trend of the signal frequency change is a key for the signal detection, and this tool makes it intuitively understandable. By over-plotting the results from two different detectors, one can easily understand the effect of the time delay and difference of detector orientations
 

The purpose of this work is the characterization of the control loop Pre-Stabilized laser (PSL). It’s the source of light of the LIGO 40m interferometer at Caltech. The 40m interferometer is a small copy of the real LIGO interferometer used to detect Gravitational waves.

In order to minimize the noise of the PSL, there are several Stability Control Systems. In this work, I test these Stability Controls and measure the noise of the laser when these controls are engaged.

I measure the frequency noise of the laser, the position and angle fluctuations of the beam, and test the frequency servo system and the Pre-mode cleaner servo. I study the frequency spectrum of the noise up to 1 kHz and the long term-fluctuations.
 

The position of mirrors in the Advanced LIGO interferometer must be controlled to sub-nanometer precision in order to achieve full sensitivity to gravity waves. This sensing of the mirror positions via ‘Pound Drever Demodulation’ requires correct demodulation phases to distinguish the different signals present in the output light.

I compare two simulations of Advanced LIGO that operate in the frequency and time domains respectively, finding the error signals and demodulation phases that they predict, and examining the consistency of these two different approaches.
 

The pre-stabilized laser (PSL) is an essential part of LIGO setup, where precision levels of 10-21 have to be achieved. The laser beam has to precisely follow the desired path on the optical table to ensure that elements such as the reference cavity and the pre-mode cleaner perform their tasks properly. My project consists of understanding, characterizing, and testing the optical layout of the PSL.

During the course of my work, I have learned the basics of Gaussian optics, which governs laser beam propagation, paying special attention to the concepts of mode matching and mode mismatch. I have implemented a set of Matlab routines for calculating the beam path, creating a complete graphical representation, and manipulating the layout as to ensure the optimal placement of optical elements.

The results of my calculations have uncovered at least one mistake in the layout, which has been promptly corrected, greatly improving the performance of the pre-mode cleaner. The rest of the layout needs verification, as more mistakes may be present. Once the layout is verified and corrected as necessary, short and long-term stability in frequency and intensity has to be studied in detail in order to characterize the performance of the PSL.
 

Gravitational wave detectors have very stringent precision requirements the position measurement of suspended mirrors. The suspension mechanics, under stress, must not introduce additional noise, which could come from metal creep. Creep rate in thermally treated maraging steel springs used in certain mirror suspensions is being measured with LVDTs at constant temperature. The deformation, which is expected to be of a few micron, will be measured over a time period of several days using a PC controlled data acquisition system. The experimental setup will be described together with the read out and analysis programs and first results will be presented.

When Einstein formulated General Relativity, he made numerous predictions including the existence of gravitational waves.  Until now, though, they have been impossible to detect.  LIGO, the Laser Interferometer Gravitational-Wave Observatory, has been built to overcome this.  Major difficulties arise as a result of the fact that gravitational waves are inherently weak; LIGO is expected to detect stretching on the order of 10-18 meters.

With the need for such precise measurements, a very large number of unwanted effects have to be minimized.  Thus, physical environmental effects must be monitored with care and analyzed.  Among the tools needed are a weather monitor, an accelerometer, seismometers, and vacuum monitors.  Each of these devices must be connected to the network and queried by the database, and the data coming from them must be analyzed.  In order to accomplish all this, we must setup the hardware; write code to query each device and format the data; create GUIs to display the data; and design data analysis programs.

Such systems have been designed and built for the two LIGO observatory sites.  In this project I implement a Physical Environmental Monitoring system for the Caltech 40-meter Interferometer Prototype Laboratory, and analyze the data obtained.

As a consequence of the fluctuation-dissipation theorem, energy dissipation in optics suspensions increases the thermal noise in interferometric gravitational wave detectors. The dissipation of kinetic energy is proportional to the internal friction of the material and inversely proportional to the dilution factor. For certain fiber thicknesses, the change of violin mode frequencies of the fiber with temperature is inversely proportional to the dilution factor. The dilution factor of a double-pendulum fused silica suspension is determined by measuring violin modes as a function of temperature. Results will be applied to deviations of measured energy dissipations from predictions.
 

There are gravitational wave sources (such as supernovae and binary black hole mergers), which emit waveforms for which no good model exists. The filtering of such burst signals should be as general as possible with minimal a priori assumptions on the waveforms. Those filters are very sensitive to non-stationary noise (producing fake signals) as well as to gravitational wave bursts. Such fake signals can be dramatically reduced when working in coincidence with other detectors. This work focuses on optimizing burst search algorithms in the time-frequency domain.

Projects at the LIGO Hanford Observatory

The interferometers at the LIGO Hanford Observatory utilize phase modulation to produce rf sidebands that are used in the detection of gravitational waves. The sidebands and carrier resonate at different levels in the various optical cavities of the interferometer. In order to fully understand the operation of the interferometer, it is necessary to measure sideband power relative to carrier power in each optical cavity. Because of alignment drifts, it is desirable to perform these measurements in parallel. A system consisting of optical spectrum analyzers and remote controlled digital oscilloscopes and ramp generators has been implemented to automate the measurement process in the 2K interferometer. Data has been obtained with the interferometer in several configurations, and is compared against theoretical predictions. Similar systems will also be installed for the Hanford 4K interferometer and the Livingston 4K interferometer.
 
 

The Laser Interferometer Gravitational-wave Observatory (LIGO) is currently building three long baseline interferometers at two sites with the goal of detecting gravitational waves from astrophysical objects.  Electromagnetic radiation caused by lighting may generate coincidence events which mimic real burst events.  The goal of this project is to determine both how lightning strikes of differing peak currents and distances are detected by magnetometers installed at both sites and how this information can be used to veto coincidence events.  The magnetometer data was scanned for spikes associated with known events, and the relationships between signal strength, distance, and peak current examined.   A “glitch” finder was then developed to search for impulses in the 60 Hz contaminated magnetometer signal.  Events flagged as possible lightning strikes during LIGO’s fourth engineering run are compared to a data set obtained from Global Atmospherics Inc. cataloging all cloud-to-ground lightning strikes for the run.

We explore how dissipation in the viscous boundary layer (VBL) at the crust-core interface of a neutron star governs the damping of r-mode perturbations in the fluid interior. Two models are considered: one assuming a normal fluid interior, the other taking the core to consist of superfluid neutrons, superfluid protons and normal fluid electrons. Surprisingly, there is much more dissipation in the superfluid model. This is due to the pinning of superfluid vortices at the crust, which creates a large electron shear.  We also find that magnetic fields much larger than (T/108K)2. 109 Gauss greatly affect the VBL, tending to increase the critical angular velocity at which r-modes become unstable. In the superfluid case, the effects of mutual friction actually counteract this, reducing r-mode dissipation. We evolve these two models in time, accounting for accretion, and explore the possibility of a stable equilibrium state. Neutron stars in such a state are then investigated as possible sources for the current and enhanced versions of LIGO.

The LIGO interferometer fine-alignment process is lengthy and tedious, on occasion requiring hours of manual manipulation. To automate this process, we have written Align, a sequencing program in Tcl/Expect that interacts with existing processes to find the optimal alignment of the LIGO interferometer, one cavity at a time. After it is initiated, Align contacts the Experimental Physics and Industrial Control System (EPICS) to find the initial locked positions of the mirrors. Align uses EPICS to repeatedly reposition the mirrors one at a time around this initial position, adjusting in yaw and pitch. In each new position, Align interacts with Diagnostic Test Tools (DTT) to run a sine response test on the cavity currently being aligned, which applies a sine wave of frequency f to dither the mirror, measures the power output of the cavity, and runs a fast Fourier transform on the results. Align extracts the fundamental frequency and first harmonic coefficients and combines them in a ratio, p(f)/p(2f). The intent of Align is to reposition the mirrors until this ratio is minimized, the minimal ratio corresponding to the optimal alignment for that cavity. When the first cavity has reached optimal alignment, Align iterates through the remaining coupled cavities of the interferometer.
 

Projects at the LIGO Livingston Observatory

Several environmental noise sources can have significant effect on the extremely sensitive LIGO detector. To veto signals due to environmental origin, we have to monitor, identify and record noteworthy local events, such as earthquakes, lightning, thunder, rain, planes, etc. In this project, we used sensitive outside microphones to detect the acoustic signatures (thunder) of local lightning events, estimated their relative location and determined their strength at the detector. We recorded and identified thunders in the vicinity (R~O(20Km)) of the LIGO detector. In order to do this, we set up low frequency, pre-amplified microphones at the end stations and the LVEA. We found that using a simple threshold on the band-limited RMS data, we can construct an on-line trigger for each individual microphone signal. In case of double or triple coincidence within the allowed time window (~20s), we will cross-correlate the time series pairs and determine the exact time delay between the sites. Based on this information, we provide real time data of individual triggers, signal strength at the buildings. If the strength and thunder rate allows, we will locate the origin of the thunder.
 

This project consists primarily of the design and implementation of a least-squares algorithm which uses data from an array of seismometers to approximate the speed and direction of incoming seismic waves at a given frequency. The algorithm was tested on a narrow 5 Hz peak in the displacement power spectrum, which is caused by an oil pipeline to the west of the observatory. The ultimate goal is to find the source of a broad peak (around 1-2 Hz) in the spectrum, which is causing the interferometer to lose lock, and seems to be caused by human activity.
 

The high mechanical Q, thermal conductivity, and density of Al2O3 (sapphire) are all beneficial for use in high precision optics.  However, sapphire’s optical absorption has not been adequately characterized or optimized.  This experiment focuses on spatial (two-dimensional) measurement of the optical absorption of synthetic sapphire at 1064 nm.  Photothermal deflection absorption spectroscopy is used to measure absorption along a line, and the sample is translated to provide spatial resolution.  These measurements will be correlated with chemical analysis measurements of the same samples, and the results will be used to refine synthetic sapphire production and annealing methods for reduction of optical absorption at 1064 nm.
 

The purpose of this project is to rank and determine the best methods to eliminate acoustic noise around and within the Pre-Stabilized Laser (PSL) table. In order to do this efficiently without interfering with present activities at the PSL, we set up a test area which consist of a fairly accurate copy of the optical table and its cage geometry. Different noise reducing/reflecting materials will be examined to see which material, arrangement, etc. lowers the amount of noise the best.

One of the major goals of LIGO is to develop and exploit gravitational wave detection in conjunction with other observations. The need for close collaboration with other gravity wave (GW) detectors and with other experiments capable to detect supernovae (neutrinos, GBR and optical) is particularly important for burst sources such as supernovae. Putting several detectors in coincidence can provide the astronomical community with a very high confidence early warning of the supernova's occurrence. LIGO, as part of GCN network, is currently receiving GRB alerts, which need to be filed into the database.  

Our code is developed to receive the incoming triggers from the LIGO collaborators via socket communication and replaces the e-mail parser in place. The received information is then appropriately parsed and formatted to generate a LIGO_LW format file, which in turn is parsed again and the data is inserted in the LIGO database table.