Minutes of nanoBPM meeting on 4 December, 2003

"Laser Seismometer", T.Tauchi

(transparencies, 10 pages, pdf, 1MB )

Tauchi and Matsuda visited the Earthquake Research Institute at University of Tokyo on 2 December, 2003. They discussed with A.Araya and Y.Ohtake. The laser seismometer is a sensor using a laser interferometer, which has been developed by A. Araya since 1992 for ground motion(GM) measurements at the gravitational wave observatory. So, the original sensor has aimed to measure horizontal GM. Major characteristics is as follows; laser diode with wavelength of 850nm, wideband from 0.1Hz to 1kHz, and a long-period reference pendulum with fo=0.2Hz. The low frequency is realized by two orthogonal springs supporting the pendulum. Variations due to GM can be detected by difference of two interferometer lights in order to cancel an effect of the laser intensity fluctuation.@@The difference signal shall be fed back accurately to the pendulum in circuit of servo filter. The feedback force is generated by a coil. Current in the coil is proportional to the ground acceleration. First prototype is a tabletop sensor.

Performance has been measured by two sensors at the Black Forest Observatory(BFO) in Germany, whose geology is granite, as well as in Stuttgart city. The BFO spectra were measured to be about 10 times larger than those of the Low Noise Model (LNM) which is the smallest one at Earth. Difference between two sensors were below the LNM at frequency of greater than 0.1Hz, which can be representative of noise level of the sensor. Amplifier and interferometer noises were also measured, which may compose the majority of the sensor noise.

At present, Araya develops the borehole type sensor, which has outer diameter of 140mm and a reference pendulum (tungsten alloy) of 88mm length with fo=3Hz. Laser is provided by fiber, i.e. fiber link and no power supply at the acceleration sensor. The designed noise level is 1-2 x 10-10m/s2/sqrt(Hz) at 0.01 - 100Hz.

For our purpose, following improvements must be necessary;

  1. further downsizing ( < 140mm diameter x 100mm height )
    Long vertical spring must be replaced with a shorter one as well as downsizing the other components.
  2. vacuum vessel
  3. non magnetization
    Relevant objects shall be spring, pendulum and a coil. The coil control must be replace with a capacitive control.

We also need more detailed consideration on the frequency range of the inertial reference. Since the inertial system would correct microseism at 0.2Hz and 3Hz which should be coherent GM, it may contradict with the optical anchor system.

"Final design of the reference system", H. Yamaoka

(transparencies, 9 pages, pdf, 1.1MB , 9 pages, ppt, 2.5MB )

Major changes are removal of the channel frame and longer reference bar for the optical anchor. So, the granite table of 1m(width) x 1.5m(length) x 0.5m(thick) is supported four level jacks. The reference system is assembled from the bar, side plates and a base plate on the granite table. Horizontal position of the reference system can be adjusted by sliding the base plate.

Constructing the FEM model by ANSYS, thermal effects (+/- 1 deg.) were calculated on deformation at the reference bar. The deformation can be caused by different thermal expansion coefficients of materials which would be used in the reference system, and their own weights. The deformation was calculated with many combinations of SUS, CFRP, GFRP and Alumina for the reference bar, side plates and base plate with the granite table, while no SUS option is applied for the bar because of its heavy weight. The best choice would be all CFRP for the least deformation, i.e. +3 (1.39) um and -11.5 (0.52) um at the bar center (difference between the center and edge of the bar) for +1 and -1 degree-C change, respectively. However, the side and base plates can be made of SUS for better machining performance. We should remind that temperature must be controlled as possible as we can.

Resonant frequencies were also calculated with the FEM model. The first resonant frequency is 88Hz, and the 2nd, 3rd and 4th are 110, 143 and 239Hz, respectively. They are high enough for small enhancements of less than 1nm/Hz1/2 as results of spectrum analysis.

The design should be finalized with miner modifications such as shortening the granite table for space of laser beams etc. in next week. After reviewing in details, the reference system must be ordered as soon as possible.

After the meeting, Yamaoka updated the design, i.e. final one(pdf,89KB).

"A proposal of the measurement method for reference bar device and ground motion utilizing interferometer and linear transition of light in vacuum", Y. Higashi and Y. Honda

(transparencies, 6 pages, pdf, 729KB )

Laser interferometer system was proposed to measure variations of position at the reference bar from the floor (ground). Also, an alignment system with a Gaussian laser beam was proposed to measure the ground motion at the floor, where the laser profiles are measured by QPD's. The laser beams and optical elements must be set in vacuum in order to avoid air effects (index change, convection etc. ). A single light source must be used for compensating the laser frequency change.

The interferometer system consists of two arms. One arm is rigidly fixed at the ground, while the other is vertical with a mirror on the reference bar. There must be 2 mirrors at both ends of the reference bar. The alignment system measured three vertical GMs just beneath the three nano-BPMs. The vacuum system was shown in sketch too. Since the floor can not be sealed, a vacuum vessel should be isolated from the floor. Another major issue is attraction due to vacuum at the reference bar. The attraction must be stabilized by using small diameter vacuum pipe with bellows.

At the next meeting, these systems will be updated with more engineering studies.

Next meeting will be held in 25 December, at 1:30pm-.