Two-wavelength Heterodyne Research Articles

Fiber-based two-wavelength heterodyne laser interferometer.

Displacement measuring interferometry is a crucial component in metrology applications. In this paper, we propose a fiber-based two-wavelength heterodyne interferometer as a compact and highly sensitive displacement sensor that can be used in inertial sensing applications. In the proposed design, two individual heterodyne interferometers are constructed using two different wavelengths, 1064 nm and 1055 nm; one of which measures the target displacement and the other monitors the common-mode noise in the fiber system. A narrow-bandwidth spectral filter separates the beam paths of the two interferometers, which are highly common and provide a high rejection ratio to the environmental noise. The preliminary test shows a sensitivity floor of 7.5pm/Hz at 1 Hz when tested in an enclosed chamber. We also investigated the effects of periodic errors due to imperfect spectral separation on the displacement measurement and propose algorithms to mitigate these effects.

Resonant, heterodyne laser interferometer for state density measurements in atoms and ions

A resonant, two-wavelength heterodyne interferometer has been developed to measure state densities of specific atomic populations. Using a tunable diode laser, one of the wavelengths is set near an absorption line and is sensitive to resonant enhancement of the refractive index. The other wavelength is at least several linewidths away and is used to measure nonresonant effects. The subtraction of the phase shifts of the two interferometers allows one to measure the population density in the lower state of the line being investigated. The interferometer system has been tested using a plasma, created by an inverse pinch plasma source, to measure the state density of Hn=2 by making resonant observations near the Hα (6563Å) transition. Average line-of-sight densities of 5.4×1011cm−3 were measured with a minimum resolvable density of 7.3×109cm−3. The transverse spatial resolution was 1–2mm. The technique described is applicable for measuring lower state densities in any atom or ion where the oscillator strengths are known for a given transition within the tuning range of the laser.

Time-multiplex two-wavelength heterodyne interferometer with frequency-ramped laser diodes

A two-wavelength heterodyne interferometer that is based on a time-multiplexing of interference signals has been constructed by using two frequency-ramped laser diodes. The on/off ramp-current modulation of laser diodes produces a time-multiplexed beat signal. One can measure the tested phase at a synthetic wavelength from the demultiplexed heterodyne signals.

Overcoming coherence length limitation in two wavelength interferometry — an experimental verification

An experimental verification of the possibility of interferometric measurements at distances at which the optical path difference exceeds the coherence length of the laser source will be presented. A two-wavelength heterodyne interferometer setup using one diode laser source and an acousto-optic modulator for the generation of the second wavelength is described and measurements at a distance of 110 m are shown.

Two-wavelength heterodyne interferometer with an expanded measurement range

Two-wavelength heterodyne interferometer with an expanded measurement range

Rough surface interferometry with a two-wavelength heterodyne speckle interferometer

If a rough surface is illuminated by a coherent lightwave of wavelength λ1, it is not possible to determine the surface profile from the phases of the speckle field formed by the scattered light. If the rough surface is illuminated, however, by an additional coherent wave of wavelength λ1, the phase differences between the two speckle fields do contain information about the macroscopic surface profile even if subject to a statistical error. It is shown that (1) the macroscopic surface profile may be determined from the phase differences if the effective wavelength Λ = λ1λ2/|λ1−λ2| is sufficiently larger than the standard deviation of the microscopic profile of the illuminated surface, and (2) the statistical error is reasonably small if the phase measurements are obtained from speckles of sufficient intensity. Using a heterodyne interferometer we demonstrate the feasibility of this technique. In the first experiment we determine the radius of curvature of a rough spherical surface. In the second experiment the macroscopic surface contour on two ophthalmic lenses of the same power variation, one with a grounded surface and the other with a polished surface, was determined.