Search for High Frequency Signals at the Hanford 4k Interferometer

It is well known that the sensitivity of the LIGO interferometers decreases at higher frequencies and indeed, for an “optimally” oriented g.w. (i.e. with the propagation vector normal to the plane of the arms) vanishes at the free spectral range (fsr) frequency ν = ν0 = c/2L. However “optimal” orientation is a special case and when averaged over directions of incidence and polarization of the g.w. the interferometer response remains finite at ν = ν0 [1]. Furthermore the response of the Fabry-Perot cavity at the fsr is greatly enhanced [1,2]. This can be understood as parametric conversion from the carrier frequency to the next longitudinal mode of the interferometer. Such parametric conversion has been recently demonstrated at the Hanford 4K interferometer (IFO) for single arm operation as well as with the full IFO locked in power recycled mode [3]. The data were obtained by imposing a (longitudinal) sinusoidal drive on ITMX and sweeping the frequency around ν0. Fig.1 shows the response of the AS I signal for only the X-arm locked. The curve is the theoretical prediction using the mirror reflectivities and only the amplitude has been normalized for best fit. The response with the full IFO is shown in Fig.2. We observe a broad enhancement at ν = ν0 of the same width as found in Fig.1, (FWHM∼300 Hz) and superimposed on it a narrow peak with FWHM ∼8 Hz. This latter peak reflects the, so called, “double cavity pole”. Namely resonance in both the arm cavity and the power recycling cavity. To express the signal quantitatively we use the following notation