Abstract
We have developed a method to equip homodyne interferometers with the capability to operate with constant high sensitivity over many fringes for continuous real-time tracking. The method can be considered as an extension of the "J1 . . .J4" methods, and its enhancement to deliver very sensitive angular measurements through Differential Wavefront Sensing is straightforward. Beam generation requires a sinusoidal phase modulation of several radians in one interferometer arm. On a stable optical bench, we have demonstrated a long-term sensitivity over thousands of seconds of 0.1 mrad/√Hz that correspond to 20 pm/ √Hz in length, and 10 nrad/ √Hz in angle at millihertz frequencies.
Highlights
Optical interferometers with sub-wavelength resolution are useful in many optical metrology applications, such as, for example, length measurements, gravitational wave detection, wavefront sensing, and surface profiling, among others
Our technique was developed in the context of continuously measuring the position and orientation of a free-floating test mass for space-based gravitational wave detection [1], the method is useful for other applications as well
Other techniques for the optical readout of free-floating test masses at millihertz frequencies are currently under investigation, such as a polarizing heterodyne√interferometer reaching a sensitivity of about 300 pm/ Hz [2], a compact √homodyne interferometer with a sensitivity of 100 pm/ Hz [3], and a robust implementation of √an optical lever with a readout noise level of 100 pm/ Hz [4]
Summary
Optical interferometers with sub-wavelength resolution are useful in many optical metrology applications, such as, for example, length measurements, gravitational wave detection, wavefront sensing, and surface profiling, among others. Other techniques for the optical readout of free-floating test masses at millihertz frequencies are currently under investigation, such as a polarizing heterodyne√interferometer reaching a sensitivity of about 300 pm/ Hz [2], a compact √homodyne interferometer with a sensitivity of 100 pm/ Hz [3], and a robust implementation of √an optical lever with a readout noise level of 100 pm/ Hz [4] Another method to do this is heterodyne interferometry as developed for LISA √Pathfinder [5] with a sensitivity of better than 5 pm/ Hz [6]. With the availability of inexpensive processing power, this computational complexity is often preferable to additional optics and electronics hardware needed for the optical heterodyning
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