Measurement of piezoelectric transformer vibrations by digital holography

Dual wavelength digital holographic imaging of layered structures

We present a dual wavelength digital holographic technique for three-dimensional microscopic imaging of layered structures, where layers are separated from one another by the axial distances exceeding the wavelength of imaging light. Our methodology not only provides the three-dimensional structure of each layer, but also allows the height differentiation of distinct layers. We have also implemented a technique suppressing low intensity signal when no reliable phase information can be extracted, based on the quality of the interference fringe pattern. We utilize a dual wavelength setup, where the combination of two overlapping interferometers enables simultaneous acquisition of two phase profiles. We demonstrate that this imaging modality is particularly well-suited for imaging of multilayered electrode structures embedded in glass.

Heterodyne holography with full control of both the signal and reference arms

Heterodyne holography is a variant of phase-shifting holography in which the reference and signal arms are controlled by acousto-optic modulators. In this review paper, we will briefly describe the method and its properties, and we will illustrate its advantages in experimental applications.

Vibration of low amplitude imaged in amplitude and phase by sideband versus carrier correlation digital holography

Sideband holography can be used to get field images (E0 and E1) of a vibrating object for both the carrier (E0) and the sideband (E1) frequency with respect to vibration. Here we propose to record E0 and E1 sequentially and to image the product E1E0* or the correlation 〈E1E0*〉. We show that these quantities are insensitive to the phase related to the object roughness and directly reflect the phase of the mechanical motion. The signal to noise can be improved by averaging E1E0* over a neighbor pixel, yielding 〈E1E0*〉. Experimental validation is made with a vibrating cube of wood and a clarinet reed. At 2 kHz, vibrations of amplitude down to 0.01 nm are detected.

Video-rate computational heterodyne holography

We present a versatile computational image rendering software of optically acquired holograms. The reported software can process 4 Megapixel 8-bit raw frames from a sensor array acquired at a sustained rate of 80 Hz. Video-rate image rendering is achieved by streamline image processing with commercial computer graphics hardware. For time-averaged holograms acquired in off-axis optical configuration with a frequency-shifted reference beam, wide-field imaging of one tunable spectral component is permitted. This software is validated by phase-stepped hologram rendering, and noncontact monitoring of surface acoustic waves by single and dual sideband hologram rendering. It demonstrates the suitability of holography for video-rate computational laser Doppler imaging in a heterodyne optical configuration.

Phase-sensitive narrowband heterodyne holography

We report on amplitude and phase imaging of out-of-plane sinusoidal surface vibration at nanometer scales with a heterodyne holographic interferometer. The originality of the proposed method is to make use of a multiplexed local oscillator to address several optical sidebands into the temporal bandwidth of a sensor array. This process is called coherent frequency-division multiplexing. It enables simultaneous recording and pixel-to-pixel division of sideband holograms, which permits quantitative wide-field mapping of optical phase-modulation depths. Additionally, a linear frequency chirp ensures the retrieval of the local mechanical phase shift of the vibration with respect to the excitation signal. The proposed approach is validated by quantitative motion characterization of the lamellophone of a musical box, behaving as a group of harmonic oscillators, under weak sinusoidal excitation. Images of the vibration amplitude versus excitation frequency show the resonance of the nanometric flexural response of one individual cantilever, at which a phase hop is measured.

Absolute measurement of small-amplitude vibrations by time-averaged heterodyne holography with a dual local oscillator

We report a demonstration of the measurement of the ratio between an optical modulation sideband component and the nonshifted light component by time-averaged heterodyne holography in off-axis and frequency-shifting configuration, through coherent frequency-division multiplexing with a dual optical local oscillator. Experimental results of sinusoidal vibration sensing are reported. This technique enables absolute measurements of subnanometric out-of-plane vibration amplitudes.