Photon-Noise-Limited Laser Transducer for Gravitational Antenna

We have constructed and tested a laser interferometer transducer for a long, wideband, laser-linked gravitational radiation antenna. Photon-noise-limited performance was achieved using 80 μW from a single mode Spectra-Physics 119 laser in a modified Michelson interferometer on a vibration isolation table in a quiet room. A piezoelectric driver on one of the interferometer mirrors was used to generate subangstrom (3 × 10⁻¹⁴-m) vibrations of known amplitude. The measured displacement sensitivity of the system in the kilohertz region was $1.3\times {10}^{-14}\hspace{0.17em}\text{m}/{\text{Hz}}^{\frac{1}{2}}$, which compares well with the calculated photon noise limit of $1.06\times {10}^{-14}\hspace{0.17em}\text{m}/{\text{Hz}}^{\frac{1}{2}}$. This is the smallest vibrational displacement measured directly with a laser to date.