Meaning
Dual-frequency laser metrology systems generate beat frequencies to measure displacement with sub-nanometer resolution. Optical configurations utilizing two slightly offset laser wavelengths form a heterodyne interferometer that isolates displacement phase from ambient intensity fluctuations. Polarizing beam splitters separate orthogonal linear polarizations, directing one frequency to a reference receiver and the other to a movable target mirror.
Phase detection electronics compare the resulting intermediate frequency beat signal against a reference signal to determine position changes.
Carrier Generation
Zeeman-split helium-neon lasers or acousto-optic modulators produce two distinct optical frequencies with a stable frequency difference. Zeeman splitting relies on an axial magnetic field to split the laser transition into two circularly polarized components of opposite handedness. Acousto-optic modulators shift laser light frequency through interaction with acoustic waves in a crystal substrate.
Frequency stability determines absolute displacement measurement accuracy.
Phase Demodulation
Photodetectors convert optical beat signals into electronic intermediate frequency signals. Digital phase meters measure phase differences between reference and measurement signals using high-speed sampling clocks.
Assembly Calibration
Integration testing verifies optical alignment and beam orthogonality before mounting units into precision positioning equipment. Test technicians adjust polarizers to minimize periodic non-linearity errors caused by polarization crosstalk. Verification protocols establish positioning uncertainty budgets prior to final system sign-off.
