Zeeman laser interferometer errors for high-precision measurements

High-Bandwidth Heterodyne Laser Interferometer for the Measurement of High-Intensity Focused Ultrasound Pressure

As a high-end medical technology, high-intensity focused ultrasound (HIFU) is widely used in cancer treatment and ultrasonic lithotripsy technology. The acoustic output level and safety of ultrasound treatments are closely related to the accuracy of sound pressure measurements. Heterodyne laser interferometry is applied to the measurement of ultrasonic pressure owing to its characteristics of non-contact, high precision, and traceability. However, the upper limit of sound pressure measurement is limited by the bandwidth of the interferometer. In this paper, a high-bandwidth heterodyne laser interferometer for the measurement of high-intensity focused ultrasound pressure is developed and tested. The optical carrier with a frequency shift of 358 MHz is realized by means of an acousto-optic modulator. The selected electrical devices ensure that the electrical bandwidth can reach 1.5 GHz. The laser source adopts an iodine frequency-stabilized semiconductor laser with high-frequency spectral purity, which can reduce the influence of spectral purity on the bandwidth to a negligible level. The interference light path is integrated and encapsulated to improve the stability in use. An HIFU sound pressure measurement experiment is carried out, and the upper limit of the sound pressure measurement is obviously improved.

Spectrally Selective Time-Resolved Emission through Fourier-Filtering (STEF)

We demonstrate a method for separating and resolving the dynamics of multiple emitters without the use of conventional filters. By directing the photon emission through a fixed path-length imbalanced Mach-Zehnder interferometer, we interferometrically cancel (or enhance) certain spectral signatures corresponding to one emissive species. Our approach, Spectrally selective Time-resolved Emission through Fourier-filtering (STEF), leverages the detection and subtraction of both outputs of a tuned Mach-Zehnder interferometer, which can be combined with time-correlated single photon counting (TCSPC) or confocal imaging to demix multiple emitter signatures. We develop a procedure to calibrate out imperfections in Mach-Zehnder interferometry schemes. Additionally, we demonstrate the range and utility of STEF by performing the following procedures with one measurement: (1) filtering out laser scatter from a sample, (2) separating and measuring a fluorescence lifetime from a binary chromophore mixture with overlapped emission spectra, (3) confocally imaging and separately resolving the standard fluorescent stains in bovine pulmonary endothelial cells and nearly overlapping fluorescent stains on RAW 264.7 cells. This form of spectral balancing can allow for robust and tunable signal sorting.

A compact high-precision periodic-error-free heterodyne interferometer

We present the design, bench-top setup, and experimental results of a compact heterodyne interferometer that achieves picometer-level displacement sensitivities in air over frequencies above 100 MHz. The optical configuration with spatially separated beams prevents frequency and polarization mixing, and therefore eliminates periodic errors. The interferometer is designed to maximize common-mode optical laser beam paths to obtain high rejection of environmental disturbances, such as temperature fluctuations and acoustics. The results of our experiments demonstrate the short- and long-term stabilities of the system during stationary and dynamic measurements. In addition, we provide measurements that compare our interferometer prototype with a commercial system, verifying our higher sensitivity of 3 pm, higher thermal stability by a factor of two, and periodic-error-free performance.

Resolution verification sensing device for a stereo digital image correlation system based on dual-frequency laser interference

Digital image correlation (DIC) is widely used in materials mechanics, nondestructive testing, and other fields due to its advantages of noncontact, full-field measurement, and simple experimental setup. As an optical measurement method to measure the three-dimensional shape and deformation of an object, measurement accuracy is one of the most important technical indicators of DIC. If DIC is made into a measuring instrument, the resolution is an important parameter that must be provided. In general, the higher the measurement accuracy of the instrument, the higher the resolution of the instrument. At present, the research on DIC focuses on the analysis of factors affecting measurement accuracy, the noise reduction of measurement results, and the improvement of correlation algorithms. There are few reports on the verification of the resolution of DIC measurement instruments. However, accuracy analysis and resolution verification of the measurement instrument is a vital technical task to ensure the credibility of the measurement data. In this paper, a high-precision dual-frequency laser interferometer principle is used to design a sensing device to verify the measurement resolution of the DIC instrument. The accuracy and resolution of the self-made stereo DIC instrument were tested and evaluated using this sensing device.

Synthetic model of nonlinearity errors in laser heterodyne interferometry

The development of laser heterodyne interferometry raises the requirements of measurement resolution and accuracy. However, periodic nonlinearity errors mainly suppress the accuracy of laser heterodyne interferometry. Based on the generation mechanism of nonlinearity errors, the sources of nonlinearity errors in laser heterodyne interferometry are first analyzed in this paper. Then, a synthetic model is established to analyze the influences of various nonlinearity error sources on the first- and second-harmonic nonlinearity errors. The first-harmonic nonlinearity errors can be reduced and suppressed by adjusting the orientation error of optical elements in a heterodyne interferometer. Furthermore, the azimuthal misalignment of the polarization beam splitter (PBS) is the main source of the second-harmonic nonlinearity errors. Therefore, when in heterodyne interferometer, the azimuthal misalignment of the PBS should be avoided if possible. This study provides theoretical basis for reducing and compensating nonlinearity errors in a laser heterodyne interferometer.

Optical activity measurement by use of a balanced detector optical heterodyne interferometer

What we believe to be a novel amplitude sensitive optical heterodyne polarimeter in which a Zeeman laser is associated with balanced detector detection was set up. The aim was to measure the optical activity of a quartz crystal with a Cornu depolarizer at high accuracy. The features of this novel polarimeter, which include the use of a two-frequency laser that ensures the accuracy of the measurement, are discussed. Furthermore, the detection sensitivity of the optical activity of a quartz crystal was measured as 8.5x10(-10). To our knowledge, this is the highest sensitivity obtained for optical activity measurement of a quartz crystal when the error of the measurement is also analyzed.

Simultaneous absolute measurements of principal angle and phase retardation with a new common-path heterodyne interferometer

This study demonstrates a new method for simultaneously measuring both the angle of the principal axis and the phase retardation of the linear birefringence in optical materials. We used a circular common-path interferometer (polariscope) as the basic structure modulated by an electro-optic (EO) modulator. An algorithm was developed to simultaneously measure the principal axis and the phase retardation of a lambda/4 or lambda/8 plate as a sample. In the case of a lambda/4 plate, the average absolute error of the principal axis is approximately 3.77 degrees, and that of the phase retardation is approximately 1.03 degrees (1.09%). The retardation error is within the 5% uncertainty range of a commercial wave plate. Fortunately, the nonlinear error caused by the reflection phase retardation of the beam splitter dose not appear in the new system. Therefore the error could be attributed to misalignment and defects in the EO modulator or the other optical components. As for the repeatability of this new common-path heterodyne interferometer, the average deviation for the principal axis is 0.186 degrees and the phase retardation is 0.356 degrees. For the stability, the average deviation for the principal axis is 0.405 degrees and the phase retardation is 0.635 degrees. The resolution of this new system is estimated to be approximately 0.5 degrees, and the principal axis and phase retardation could be measured up to pi and 2pi, respectively, without ambiguity.

A review of recent work in sub-nanometre displacement measurement using optical and X-ray interferometry

This paper reviews recent work in the field of displacement measurement using optical and X-ray interferometry at the sub-nanometre level of accuracy. The major sources of uncertainty in optical interferometry are discussed and a selection of recent designs of ultra-precise, optical-interferometer-based, displacement measuring transducers presented. The use of X-ray interferometry and its combination with optical interferometry is discussed.

Analytical modeling of the periodic nonlinearity in heterodyne interferometry

The periodic nonlinearity that arises from nonideal laser sources and imperfections of optical components limits the accuracy of displacement measurements in heterodyne interferometry at the nanometer level. An analytical approach to investigating the nonlinearity is presented. Frequency mixing, polarization mixing, polarization-frequency mixing, and ghost reflections are all included in this investigation. A general form for the measurement signal, including that of the distortions, is given. The analytical approach is also applicable to homodyne interferometry.

Common-path optical heterodyne profilometer: a configuration

A novel configuration that combines a linearly polarized He-Ne laser and a birefringent lens to produce a common-path polarized optical heterodyne profilometer with respect to the heterodyned P and S waves has been set up. In this profilometer a linear polarized frequency-stabilized He-Ne laser was used with an acousto-optical modulator to replace the Zeeman laser as the light source that had two polarization eigenstates in different temporal frequencies. The proposed interferometer shows a more symmetric and ideal common-path structure than the conventional optical heterodyne profilometers with the Zeeman laser. The phase error aroused by the elliptical polarization and the nonorthogonality of the two eigenpolarization modes of the Zeeman laser can be reduced. The system's resolution in the vertical direction reaches 2 A, and in a 27-mum scanning range the repeatability of the surface profile measurements is shown to be 5 A.