Search for a Stochastic Background of 100-MHz Gravitational Waves with Laser Interferometers

This Letter reports the results of a search for a stochastic background of gravitational waves (GW) at 100 MHz by laser interferometry. We have developed a GW detector, which is a pair of 75-cm baseline synchronous recycling (resonant recycling) interferometers. Each interferometer has a strain sensitivity of $\sim {10}^{-16}{\mathrm{Hz}}^{-1/2}$ at 100 MHz. By cross-correlating the outputs of the two interferometers within 1000 seconds, we found $h_{100}^2{\Omega }_{\mathrm{gw}}<6\times {10}^{25}$ to be an upper limit on the energy density spectrum of the GW background in a 2-kHz bandwidth around 100 MHz, where a flat spectrum is assumed.

Figure 1

Schematic view of a synchronous recycling interferometer. GW signals are enhanced in the recycling cavity and detected with the photodetector.

Figure 2

Schematic view of the experimental setup of one of the interferometers. EO: electro-optic phase modulator; FI: Faraday isolator; DAQ: data acquisition system. The laser frequency is stabilized to the recycling cavity by the Pound-Drever-Hall technique. GW signal sidebands are once converted to intermediate-frequency (IF) signals at the PD2. Then, the IF signals are mixed with a local oscillator (LO), and converted to audio-frequency (AF) signals. The AF signals are recorded with the DAQ. The EO2 is used to simulate GW signal sidebands for calibration.

Figure 3

Strain sensitivities of the interferometers estimated from the IF signals. The solid red and dashed green lines represent the sensitivities of IFO-1 and IFO-2, respectively.

Figure 4

Variation of ${\widehat{\mu }}_Z$ with respect to the observation time period is shown as filled red circles. The area enclosed by black curves is its two-sided 90% confidence interval.