Seismic isolation system

2. Seismic isolation system

4. Transmitted light in Y-arm Recombined lock in Louisiana Saturday Sunday Friday Transmitted light in X-arm

5. 4 KM 4 KM BASIC SYMMETRIC CHAMBER The LIGO beam splitter is suspended from a vibration isolated platform within this chamber. The air pressure in the chamber is approximately 10-9 torr, or about one trillionth of an atmosphere. Light entering the chamber from the left is split into two beams. One beam continues straight on through the chamber, passing through the input test mass, the similar chamber visible behind this chamber to the right, and then continuing on for 4 km through the beam tube to the end mirror, also located within a similar chamber. LASER The beam splitter must be carefully isolated from external sources of vibration. Otherwise, swinging of the beam splitter can cause a differential change in the optical path length which mimics the effect of a gravitation wave (see figure 1 below). While the chamber rests directly on the floor, the seismic isolation system is separately supported by the blue pillars around the chamber. Air bearings, within the black rubber booties, are used to “float” the internal components of the seismic system so that they can be precisely aligned. Support tubes connect to the external cross beams through bellows in the sides of the chamber. A support table and four layers of steel cylinders and damped springs are used to isolate the optical table from ground motion. Figures 2 and 3 illustrate the degree to which ground vibration is minimized by the seismic isolation system. Figure 2 shows the “vertical transfer function”, the ratio of the vertical vibration amplitude of the optical table to the motion of the concrete floor, as a function of frequency. Below around 10 Hz, the seismic isolation system amplifies the ground motion because of the resonances in the structure. Above around 100 Hz, the optical table’s motion is reduced by about one million relative to the motion of the floor. Figure 3 shows the actual measured motion of the optical table due to the vibration of the earth (the product of the ground motion spectrum multiplied by the transfer function shown in Figure 2). The low frequency cut-off in LIGO’s sensitivity results from the amplitude of the ground vibration and the degree to which it is isolated from the interferometer by the seismic isolation system. Figure 2 Figure 1 Figure 3 OUTPUT TO Y ARM Beam splitter motion changes relative lengths of X and Y arms Displaced position Initial beam splitter position Measurement noise floor TO X ARM Measurement noise floor Predicted performance FROM LASER The vacuum chambers for LIGO were fabricated by Process Systems International, Westboro, Massachusetts. The seismic isolation system was designed by Hytec Systems, Los Alamos, New Mexico.

6. HORIZONTAL ACCESS MODULE This horizontal access module or “HAM” chambers house smaller suspended optic elements which require less vibration isolation than the main interferometer. Here, the mode cleaner, which is composed of optics suspended in this HAM chamber and the one 10 meters to the left, acts as a spatial filter to reduce the pointing jitter and frequency noise of the incident light by about a factor of 100. The optics are supported on a pendulum (see figure 1) which rests on a vibration isolated table having three layers of steel plates and damped springs. The entire spring and steel plate assembly rests on support tubes which penetrate the vacuum shell to the left and right where they are connected to cross beams (see figure 2). The cross beams rest on air bearings (the rubber booted components above the blue vertical piers) that are used to float the entire assembly for initial alignment. The scissors tables beneath the air bearings are used for coarse alignment. The effectiveness of the HAM seismic isolation system is illustrated in the figures below. The top two graphs show the transfer functions for vertical and horizontal motion (the ratio of the amplitude of the internal optical table to the amplitude of the motion of the concrete floor as a function of frequency). The bottom two figures show the measured displacement of the table when the displacement spectrum of the floor is multiplied by the seismic isolation transfer function. The optic’s pendulum suspension (with a natural frequency of about 1 Hz) provides some additional horizontal vibration isolation. 4 KM 4 KM You are here Figure 1 Figure 2

7. HORIZONTAL ACCESS MODULE Laser beam path Optical table Seismic isolation Damped springs Air bearing Scissors table Seismic pier Damped springs Seismic isolation stacks Scissors table Air bearing Laser beam direction Seismic pier

8. DAMPED SPRINGS FOR SEISMIC ISOLATION LIGO's seismic isolation system uses passively damped stages (cascaded mechanical filters) to isolate the optics tables from the earth's seismic motion. In order to push the isolation frequency down to a first resonance of about 1 Hz, HYTEC Inc., as a contractor to LIGO, has developed a helicoil spring with high damping (~2%). The spring is a Constrained Layer Damped (CLD) spring. (HYTEC has applied for a patent on the CLD spring.) When the helicoil spring compresses, a torsional shear strain develops across a damping layer between the outer phoshor-bronze tube and an inner segmented aluminum tube. The helical springs are internally damped so that Q < ~5. The CLD springs are beingproduced by Pegasus Manufacturing Inc. of Hamden, CT. The damped spring provides passive isolation of the suspended optics from ground motion. It also maintains the total motion of suspended test masses within the control range of the suspension actuators. Test masses and beam splitter suspensions are mounted on 4 layer sandwiches of springs and steel. Support optics are mounted on 3 layer sandwiches. Shown below are photographs of the seismic isolation system being assembled. On the left, a three layer system is being built to isolate the small suspended support optics, while on the right the four layer stack comprising the seismic isolation for the large optics, such as the input and end test masses, is visible. The brown ribbons that are visible are kapton electrical cables which carry the sensing and control signals to the suspension system. These cables are carefully attached to intermediate points on the seismic isolation system so that mechanical vibration will not propagate from the outside world onto the vibration isolated suspensions.

9. LIQUID NITROGEN CRYO PUMP Spring hangars isolate the mechanical vibration due to boiling of the liquid nitrogen so that it does not degrade the performance of the interferometer. LIQUID NITROGEN - 80K CRYOPUMP LIQUID NITROGEN - 80K Most molecules, whose paths are indicated by the arrows, stick to the cold inner surface of the cryo pump. Very few have the proper trajectory to pass on thorugh without hitting the pump. GATE VALVES The operating vacuum of the LIGO interferometer is maintained during at a pressure of around 10-9 torr, or about one trillionthof the normal atmospheric pressure. In order to achieve this very high vacuum level, liquid nitrogen cryo-pumps are used to remove water and other volatile materials from the vacuum chamber. The cryo-pump contains an annular vessel which is filled with liquid nitrogen, at a temperature of 80K, which provides a cold surface on which molecules of condensable gases stick, much like the frost build up in a refrigerator. The liquid nitrogen which keeps the cryo-pumps cold is supplied by the four approximately 13,000 gallon large white tanks outside this building and at both end stations. The pump can be “defrosted” by closing the gate valves on either side of the pump and then warming up the cold surface to evaporate the contaminants, which are then vented outside. To prevent the boiling liquid nitrogen from inducing mechanical vibration and noise into the interferometer, the annular cold reservoir is suspended from specially designed vibration isolating spring hangers within the pump. There are only two pumps along each arm of the interferometer, separated by 2.5 miles. The LIGO beam tubes must be fabricated and kept extremely clean in order to maintain such a high level of vacuum within the interferometer. Typically, the total outgassing rate of the beam tube, for all sources other than hydrogen, is less than 10-17 torr-liters/sec/cm2. Hydrogen is the major component of residual gas which remains in the interferometer after it is cleaned and baked out, due to desorption from the stainless steel. It is pumped from the system by the large ion pumps visible on top of the beam tube manifolds in the corner station and at either end.

10. Photodetector senses edge of magnet and outputs a voltage Solenoidal Electromagnet Solenoidal coil driver Voltage is converted to a position correction SUSPENSION WIRE GUIDE ROD MAGNET PHOTO DETECTOR Position LED Magnet Voltage Mirror OPTICS SUSPENSION SYSTEM This is a prototype of the large optic suspensions used in the LIGO interferometer for the mode matching telescope and the recycling mirror. The optics are suspended like a pendulum to isolate them from vibration. The roughly 10 kg mass of the optic is supported by a wire with a diameter of only 12 one-thousandths of an inch. The optic (a plexi-glass dummy for display purposes) is suspended by a single wire loop. The wire contacts the optic just above its mid-point. Since the suspension point is above the center of mass, the mirror is stable. The guide rods visible on the side of the optic are used to precisely establish the contact point between the wire and the optic. The orientation of the mirror must be very carefully controlled. Small magnets visible on the face and sides of the mirror react to solenoidal magnetic fields from actuators in the located in the suspension tower to control the pitch and yaw of the mirror, as well as the linear displacement. The screws on the front and back faces are safety stops used for installation of the tower assembly. The entire assembly must be carefully cleaned and baked in a vacuum oven to remove any residual contamination prior to its installation within the interferometer. Detail of wire stand off and magnet for position control ACTUATOR DETAIL

11. LARGE LIGO OPTICS The optic in the glass case is one of the actual glass optics used in the LIGO interferometer. Made of ultra-pure fused silica glass, it has an optical absorption of less than 10 ppm at the laser wavelength of 1.06 microns. The mirror’s surface is “super polished”, which means that the surface is shaped to conform to the design tolerance within very high precision, typically with an rms variation over the entire surface of about 1-2 nanometers. One side of the surface is polished to be flat, while the other side has a radius of curvature of 10-14 km. The face of the optic is coated has a special low-loss reflective dielectric coating. This technology, originally developed for devices such as ring laser gyroscopes, make possible the high precision control of light necessary for LIGO. The coating is faintly visible as a reddish tinge on the front surface of the mirror. It doesn’t look like a mirror because it is highly reflective only to light very near a wavelength of 1.06 microns (infrared) which is invisible to the human eye. Once polished, the mirror is measured to confirm that the surface has the necessary smoothness. Large optic being suspended and prepared for installation in the interferometer. Measurement of the surfacesmoothness in the optical metrology laboratory at Caltech +4.0 +2.0 0.0 -2.0 -4.0 Results of the measurement of surface smoothness. The colors indicate the degree of departure from the design requirement, typically around one one-thousandth of a wavelength of the laser beam.

12. TCL/TK Command Layer CustomAPI.tcl CustomAPI.rsc genericAPI.tcl genericAPI.rsc TCL Main Master Interpreter TCL Master Interpreter Normal Priority: Commands & Messages Operator Socket C/C++ Package Layer srcindepparam CustomAPI.so LDAS API Structure binaryinspiral ringdown directedperiodic srcindepkey TCL Master Interpreter Exceptional Priority: Errors & Warnings Emergency Socket program ppgeneric genericAPI.so TCL/TK Layer frameset program statistics spectrum frame C++ Socket Class Objects trigger filter Binary Data: Streamed & C++ Objects Data Socket datasource fpfft fpgeneric C/C++ Layer LIGODataAnalysisSystem The initial LDAS hardware and software (right) was integrated into the Hanford Observatory in mid June of 1999. The hardware includes ATM and fast ethernet networks for LDAS as well as server CPUs for storing and retrieving LIGO data. The installed software allows user requests for LDAS data to be made both local to the site and at any remote home sites on the internet. Future versions of LDAS will support requests for analysis of LIGO data. The first elements of LDAS hardware & software (left) at the Livingston Observ-atory was installed in late October of 1999. The second LDAS system is virtually identical to Hanford’s LDAS system. Both sites have their LDAS software synchronized with the development software at Caltech on a daily basis using the T1 links to each site. The LIGO Data Analysis System or LDAS (above top) is designed to analyze LIGO data streams, searching for gravitational wave signatures. To achieve this the LDAS system consists of its own private network of distributed computers working collectively to analysis the data. At the LIGO Observatories, the most challenging LDAS task will be to search for the gravitational waveforms characterizing the final inspiral of a compact binary system made up of combinations of neutron stars and black holes. These searches will utilize a technique known as optimal or Wiener filtering, using a theoretical template to match against the collected data. The number of interesting astrophysical templates is large requiring large amounts of parallel computing performance for detection (see figures above). The LDAS software design is highly modular and extensible (block diagram above). It contains components for data input/output, database connectivity, parallel processing, client services, management and user interfaces. All components are being developed on top of the Unix operating system. Users of the LDAS system interact by generating simple text based requests of the form outlined in the web page shown above. These requests are sent via sockets to a port on the LDAS managerAPI where the requests are interpreted and processed. The LDAS system will use IBM’s DB2 database server to manage the metadata (data about data) such as instrumental behavior, data quality, statistics, data analysis results and data products. The metadataAPI acts as the sole client to this database server, making requests for data inserts and queries of table contents using ODBC (open database connectivity) standard calls. Each of the LDAS software component which provide a system level service are called a LDAS APIs (shown to the right). LDAS APIs integrate two technologies into one unified module, combining the Tcl/Tk script language with its graphical widgets for easy interpreter interfacing along with the compiler efficiency of object oriented C++. Central to the LDAS system is the managerAPI (see figure above). It acts as a broker, receiving the requests for a particular analysis from users on its operator socket. These request commands are then assigned to individual assistant managers which first parse the commands, checking for proper content, then construct a schedule of API level commands based on the parsed contents, and finally sequence the commands through each LDAS API involved in the analysis. The manager can initiate multiple assistant managers, each of carrying out unique user requests. This allows the LDAS system to multitask user requests. The results of the requested analysis are then made available to users in several different formats depending on the type of data products requested. These data products can be delivered using email, anonymous ftp, web pages or directly to remote LDAS APIs. Because of the complexity and computational demands on the LDAS, distributed computing techniques are required to deliver the needed performance. This is achieved using Unix sockets. These sockets send and receive commands, messages and data (see above) between all LDAS APIs. Since no single API carries out the full sequence of analyses on the data, these transmission rates for LDAS are critical to the systems ability to operate at pick performance. Numerous measured data transmission rates are given for the ILWD (Internal Light Weight Data) data types in the plot to the right. The database server manages hundreds of units of information organized into tables (shown above). These tables can have references between various members which assist in the determination of significant relationships between the instrument’s behavior and the astrophysical results from the data analysis. Building these tables with their characteristic relationships is a critical function of the metadataAPI & relational database. Performance measurements for carrying out these task are shown in the plot to the left.

13. Pre-Stabilized Laser • 10W at 1.06 mm by a Master-Oscillator-Power-Amplifier (MOPA) • The master oscillator is a 700mW model 126-1064-700 NPRO (Non-Planar-Ring-Oscillator) diode pumped, narrow linewidth, single frequency laser manufactured by Lightwave Electronics. • This beam is double passed through a power amplifier which consists of 4 Nd:YAG rods which are each pumped by a pair of 20 W diodes. • Frequency Stabilization • Part of the laser output is sent to a reference cavity which is a 13 cm long quartz spacer tube with mirrors optically contacted to each end. The cavity hangs by 2 wire loops in a a vacuum chamber which is thermally insulated and temperature stabilized. • The laser frequency is matched to the reference cavity length by a reflection locking signal from the cavity. This signal is applied in 3 ways: • Slow control (<0.1Hz) to a heater on the master oscillator laser crystal. • Fast control (0.1 Hz-12 kHz) to a PZT on the master oscillator laser crystal. • High frequency control (12kHz-1 MHz) to a Pockels cell on the output of the master oscillator laser • Wideband control signals needed to keep hanging parts of the interferometer locked are fed into the input of the frequency stabilization. In order for the reference cavity to remain locked, the light going into the reference cavity is frequency shifted by an acousto-optic modulator. • Tidal changes to the interferometer armlength over the period of the day are compensated for by changing the reference cavity length using the temperature stabilization circuit. • Intensity Stabilization • Part of the laser output is measured by a photodiode and an error signal is then generated which is fed to a current shunt on the power supply of the pump diodes in the power amplifier. • Pre-Mode Cleaner • The output from the laser is passed through a 3 mirror ring cavity in order to filter the laser frequency noise, intensity noise and beam jitter. • The light reflected from the input to the cavity is used to create reflection locking signal. This is fed into a PZT glued to one of the mirrors in the ring cavity.