Integrated longwave infrared (LWIR) photonics hold promise for enhancing on-chip molecular sensing due to the strong light-matter interaction in the LWIR spectrum, which is orders of magnitude more intense than in the near-infrared. However, conventional photonic materials suffer from high optical losses in this range. Specifically, silicon and III-V materials exhibit absorption losses due to multiphonon processes, which limit their applicability for LWIR systems. To address this issue, our work introduces a hybrid germanium-on-zinc selenide (GOZ) platform. This platform leverages the lower multiphonon absorption onset frequencies of germanium and the suitable cladding properties of zinc selenide to reduce optical losses. By employing a direct wafer bonding technique, our study achieves a waveguide system that is transparent from 2 μm to 14 μm, with measured optical losses as low as 1 cm−1 at 7.8 μm, indicating a significant improvement over traditional materials. Our findings demonstrate that the GOZ platform effectively reduces the intrinsic optical losses typical of epitaxiallygrown materials in LWIR devices, thereby paving the way for advancements in quantum and nonlinear photonic applications.
Free-standing thin-film waveguides and slot waveguides offer excellent properties for gas sensing: high air confinement factors resulting in strong light-analyte interaction, reduced material absorption loss, and negligible Fabry-Perot fringes. We demonstrate that such waveguides combined with the sensitive and selective technique of mid-IR laser absorption spectroscopy can detect methane and carbon dioxide down to 300 ppb and 30 ppb levels, respectively. Isotope specific detection of CO2 with excellent 13C isotope ratio accuracy of 0.3‰ has also been shown. The unprecedented sensitivity together with miniature sensor footprint and microlitre sample volumes open new application areas in biology, environmental sensing and industral process monitoring.
Mid-infrared tuneable diode laser absorption spectroscopy (TDLAS) realized with photonic integrated circuits (PICs) has the potential to create small and portable sensing devices with exceptional sensitivity. Here we present a photonic crystal (PhC) W5 waveguide-based sensor, which provides stronger interaction between the light and the analyte than a free space beam, as well as low scattering and material absorption losses. We demonstrate its performance by analysing the transmission spectra of two CO2 isotopes at 4345 nm.
Publisher’s Note: This paper, originally published on 19 July 2019, was replaced with a corrected/revised version on 4 February 2022. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.
Conventional parabolic trough solar concentrators have the benefit of only requiring 1-axis tracking and having a line-focus suitable for heat extraction using long receiver pipes. However, by being 1-axis concentrators, their fundamental limit of concentration is 212x, compared to the 45 000x limit for 2-axis concentrators. We propose to use two recent developments from nonimaging optics to develop practical high-concentration line-focus concentrators. The first is the use of beam-steering lens arrays to redirect sunlight, allowing a concentrator to benefit from 2-axis tracking without being aimed directly towards the sun. The second is the use of étendue squeezing to increase concentration across one axis, at the cost of reduced concentration across the second axis. We show how these two developments may be used to create line-focus concentrators not limited by the 212x concentration limit, and present our work towards designing a practical system implementing these concepts.
Sensors in mid-infrared spectroscopy based on attenuated total reflection (ATR) sensing with internal reflection elements (IREs) facilitate easier measurements of aqueous solutions or other opaque analytes. Micromachined silicon (Si) elements are an attractive alternative to conventional IREs, as they can be produced cheaply with silicon processing. Techniques for surface modifications are also easily integrated into the wafer process, and surface structures such as micropillars or nanoparticles can thereby be used for signal enhancement. Replacing the classic Fourier transform infrared (FTIR) spectrometers with tuneable quantum cascade lasers (QCLs) also opens up new avenues for sensing. In this study, the performance of basic and signal-enhanced Si IREs has been compared for measurements in a spectroscopy setup with a fibre-coupled tuneable QCL source. These IREs had V-shaped microgrooves etched on the underside for more efficient in-coupling of light, while the signal enhanced IREs also had micropillars on the top surface. The results are also contrasted with measurements done in a standard ATR-FTIR spectrometer, using an Alpha II spectrometer with a single-reflection diamond ATR crystal. Various concentrations of glucose (0-5000 mg/dl) in aqueous solutions were used to characterise the system performance. The quality of the signal enhancement was evaluated with regard to sensitivity and noise level in the acquired spectra. The microstructured Si IREs gave a signal enhancement of up to a factor of 3.8 compared to a basic Si element, with some concomitant increase in noise. The absorbance was higher for both types of Si IREs as compared to the diamond ATR crystal. The effective enhancement and the limit of quantification improved by a factor up to 3.1 in the signal-enhanced IREs compared to the basic Si element.
A fiber-coupled transmission spectroscopy setup using a pulsed external-cavity quantum cascade laser (EC-QCL, 1200-900 cm−1 ) has been developed and demonstrated for measurements of aqueous solutions. The system has been characterised with regard to the laser noise and optimal optical pathlength. Solutions with glucose were used to further test the setup, and glucose concentrations down to physiologically relevant levels (0-600 mg/dl) were investigated. Albumin, lactate, urea, and fructose in various concentrations were added as interfering substances as their absorption bands overlap with those of glucose, and because they may be of interest in a clinical setting. Analyte concentrations were predicted using partial least-squares (PLS) regression, and the root-mean-square error of cross-validation for glucose was 10.7 mg/dl. The advantages of using a convolutional neural network (CNN) for regression analysis in comparison to the PLS regression were also shown. The application of a CNN gave an improved prediction error (8.3 mg/dl), and was used to identify important spectral regions. These results are comparable to state-of-the-art enzymatic glucose sensors, and are encouraging for further research on optics-based glucose sensors.
An essential part of a concentrated solar power system is the solar tracker. Tracking is usually implemented by rotating the entire optical system to follow the sun, adding to the bulk and complexity of the system. Beam-steering lens arrays, on the other hand, enable solar tracking using millimeter-scale relative translation between a set of lens arrays stacked in an afocal configuration. We present an approach for designing and comparing beam-steering lens arrays based on multi-objective optimization, where the objective is to maximize efficiency, minimize divergence, and minimize cost/complexity. We then use this approach to develop new configurations with improved performance compared to previously reported results. As an example of a design suitable for high-concentration applications, we present a system consisting of four single-sided lens arrays that can track the sun with a yearly average efficiency of 74.4% into an exit-cone with divergence half-angle less than ±1◦. We also present a simplified system consisting of three single-sided lens arrays, which can be implemented with less mechanical complexity and potentially lower cost. This simplified system achieves 74.6% efficiency and a divergence half-angle of less than ±2.2◦, and might be relevant for low or medium concentration applications. We believe that these results demonstrate the previously untapped potential of beam-steering lens arrays. If such designs are successfully manufactured, they may become an attractive alternative to conventional external solar trackers for a range of solar energy applications.
Diabetes mellitus 1 requires tight control of the blood glucose levels to avoid harmful effects of either too high (hyperglycemia) or too low (hypoglycemia) blood sugar. Due to the availability of low-cost components, fiber- coupled near infrared (NIR) absorption spectroscopy could be a feasible measurement method. From the molar absorptivity of glucose, it is shown that to achieve high accuracy using near infrared spectroscopy for glucose sensing, relative noise levels should not exceed 0:003 %. Two supercontinuum (SC) sources and one broadband lamp were investigated with a low-cost portable spectrometer. The SNR of the two SC sources was limited by amplitude fluctuations and could be improved by averaging. The SNR of the broadband source was found to be largely limited by the detector noise due to the weak intensity. 16 aqueous glucose samples ranging from 0 to 500mm were measured with the broadband source and an SC laser. A partial least squares regression (PLSR) model was built for both measurement sets, yielding root mean square errors of 49 and 54mm, illustrating how the limit of detection is restrained by the high relative intensity noise. A reference arm setup was built and could account for much of the variability of the SC source. A glucose measurement series using this setup and five samples (100 to 500mm) yielded a root mean square error of 10:6mm. The results indicate that an SC source can be feasible for absorption spectroscopy in a reference arm setup.
We present a sensor design based on a Mach-Zehnder interferometer utilizing sub-wavelength gratings (SWGs) that were included in the waveguide to compensate for the short optical path length and to provide phase modulation. According to 2D finite element method simulations, it is possible to achieve 3-fold enhancement in sensitivity and 50% increase in modulation frequency with the inclusion of SWGs in the sensing arm as well as in the reference arm.
Investigating photonic components as refractive index sensors requires reliable numerical models. Performing 3D simulations of large or complex components requires powerful hardware. Alternatives to time-consuming 3D simulations are approximations such as the effective refractive index method. We propose our improved method that more closely follows 3D results, named the inverse effective index method. The accuracy of our approximation method is verified by comparing experimental results with simulations. Ring resonators with 40μm radius are simulated and fabricated. Their qualities as refractive index sensors are assessed by probing their response in DI-water and in various saline solutions. The fabricated resonators are fitted with a microfluidic channel for sample delivery. A tunable laser is used to induce resonance in the ring resonators and the resonance frequencies are measured for all solutions. The accuracy between simulation and experimental data is found to rely heavily on the approximation technique being used. The differences vary from 300% to 6% depending on the approximation method. The fabrication and characterization of the ring resonator-based refractive index sensor took approximately two days, whereas the 2D simulations took under an hour to perform. This demonstrates the usefulness of a proper simulation tool to conduct accurate performance estimations in a short amount of time.
Conventional tracking solar concentrators track sunlight by rotating the concentrator optics to face the sun, which adds to the cost and bulk of the system. Beam-steering lens arrays, in contrast, allow solar tracking without bulk rotation of the optics. It consists of lens arrays stacked in an afocal configuration, and tracking is implemented by relative translation between these lens arrays. In this work, we present a phase-space methodology for analyzing and optimizing the performance of the beam-steering, and for revealing optical aberrations in the system. Using this methodology, we develop a beam-steering lens array with simulated ≈70% efficiency across a two-axis
±40° tracking range, and a divergence of the outgoing beam of less than ±0.65°. We also present a functional small-scale prototype and demonstrate the feasibility of the concept for solar tracking. Beam-steering lens arrays can be placed in front of conventional concentrator optics and operated with little or no external tracking. This may enable low-cost robust concentrated solar power systems, and could also find other applications such as solar lighting and steerable illumination.
The ability to conduct diagnostic functions on a single chip has long been of interest to the medical community. Decentralization of laboratories combined with reduced costs, increased speed and a higher throughput of potential assays are all driving forces for lab-on-a-chip technology. The small chip sizes facilitate low sample volumes, which in turn allow better control of the molecular interactions close to the sample surface. The design and quality of transducers, microfluidics and functionalization processes have all improved over recent years. Despite the growing interest for lab-on-a-chip components, several challenges remain. Combining all three disciplines into a high-quality well-functioning chip that is cheap to fabricate while providing reproducible results is challenging. A project attempting to address these challenges is presented. The main goal is to design and fabricate a labon-a-chip silicon photonic biosensor with multiple channels for detection of antigens with improved sensitivity and selectivity compared to state-of-the-art. As a proof-of-concept, the sensor is designed for simultaneous detection of three distinct antigens: C-reactive protein (CRP), lipocalin and tumor necrosis factor (TNF). The main challenge lies within their respective concentrations as well as the specificity for each analyte, where concentrations vary from the mg/ml to pg/ml regime. Multiplexing is achieved by using photonic crystal resonators, which function as drop-filters, allowing for single input/output while simultaneously probing select transducers that are functionalized for different chemistries. The individual resonator designs facilitate different limit-of-detections (LODs) and dynamic ranges for each analyte. Preliminary results from the first single channel prototype are presented, while work on the multiplexed sensor continues.
We present a numerical study of the sensitivity of guided modes due to the presence of small mechanical vibrations. Analysis is performed utilising finite elements method (FEM) with help of both 2D and 3D models, where changes to the field distributions associated with the guided modes are computed in order to read the position of a conducting plate suspended over a waveguide. The sensitivity related to the guided modes is analysed and a possibility to improve the sensitivity by combining the sensing effect of several modes is investigated. By utilizing FEM, the optical response to the system is studied in terms of both indiviudal response to the perturbation and the collective response by using the concept of multimode interference (MMI). The preliminary models predict a theoretical sensitivity limit of 2:4 %/nm in terms of normalised power transmission for a MMI of only 4:1 μm length.
If the arc lamp in standard display projectors is replaced by lasers, the projector can gain several advantages such as improved color gamut, higher contrast and brightness. Coherent by nature, the laser also adds one significant disadvantage to the projector: the presence of speckle, which manifests itself as noise-like intensity fluctuations over the image. In this paper, we present an overview of our work concerning correlated speckle patterns. We give general formulas for the speckle contrast of the sum of N partially correlated speckle patterns. It is shown how the speckle contrast depends on the correlations between the individual speckle patterns, and for the case of a fully developed speckle pattern we present a general expression for the correlations.
As an example, we look at speckle suppression by discrete rotation of a diffraction pattern on a diffuser. We use a sinusoidal phase grating which has the advantage that the zeroth order can be extinguished without loss of power. Hence, no part of the diffraction pattern remains stationary when rotated. Expressions for correlations are found, with different degrees of generality, which enable us to find expressions for the contrast. Furthermore, the influence of the diffraction pattern's complex amplitude distribution on the correlation and speckle contrast is investigated.
The opto-mechanical properties of silicon together with its fabrication flexibility make it an excellent candidate for
many integrated photonic applications. The recent trend in miniaturizing optical components, while maintaining
stringent demands for their performance opens a way for various coupling mechanisms to be used for sensing
and switching. However, as the system grows in complexity, the number of possible coupling channels increases.
Being able to resolve the specific coupling mechanisms becomes a delicate and challenging task. At the same time,
the overall performance of the device is often decided by the collective performance of all of these channels rather
than by each of them separately. Because of that, it becomes essential to account for all of these mechanisms,
while modelling the system. As an example of such a system, a mechanical displacement sensor is presented,
whose principle is based on simultaneous evanescent coupling mechnisms between the TE- and TM-like modes. By
using the FEM approach, the coupling mechanisms between the waveguides (and modes) are analyzed separately
through 2-dimensional simulation and later combined and investigated globally in a series of 3-dimensional
simulations.
This paper describes a novel idea for reduction of speckle contrast in laser display projectors using the rotation of a diffraction pattern whose zeroth order has been canceled out without loosing power. The feasibility of the proposed method was investigated by illuminating gratings with a sinusoidal phase on two spatial light modulators (SLMs) in series for minimal intensity modulation, where the phase grating pattern was rotated with respect to the previous one on both SLMs. Two series of measurements were done with different periods of the sinusoidal grating. For each series, an image of the speckle pattern was recorded at discrete rotation angles of the phase grating, and then an average image was calculated. Experimental results were compared with a new theoretical model for speckle contrast of N partially correlated speckle patterns. The experimental measurement results compare well with the theoretical predictions resulting in a minimum speckle contrast of 0.36, with further reduction possible. Parameters necessary to achieve target contrast (0.08 or less) are discussed.
Optical vibration measurement systems are excellent tools for characterizing ultrasonic transducers. This paper
presents measurements on immersed arrays of capacitive ultrasonic transducers (CMUTs) using a heterodyne
interferometer. The interferometer allows measurements of vibrations from DC up to 1 GHz with a noise floor of
~1pm/√Hz. Previously CMUTs have been characterized in air. The transducer is intended for intravascular
use. Therefore the CMUTs were characterized in the transparent fluids kerosene and rapeseed oil that have
acoustic properties closer to blood. The optical measurements on immersed CMUTs were validated by assessing
the measurement errors caused by the acousto optic effects in the fluid.
When immersed there is significant cross coupling between individual CMUTs within an array. Simulations
presented here indicate that this causes an acoustic wave mode that is bound to the interface between the CMUTs
and the fluid. This is confirmed by measurements of the phase velocity and attenuation coefficient of this wave.
The measurement results indicate that the wave exists up to a maximum frequency and that the attenuation
constant increases with increasing frequency. Rapeseed oil causes a significantly larger attenuation coefficient
than kerosene, which most probably is due to a considerable difference in fluid viscosities. There was a mismatch
between the simulated and measured phase velocity for low frequencies. It is likely that the cause of this is
coupling between the fluid CMUT interface waves and Lamb waves in the substrate of the CMUT array.
Measurements performed with the heterodyne interferometer have confirmed the presence of dispersive waves
bound to the surface of the transducer by directly showing their propagation along the array. The setup has also
characterized the bound waves by measuring dispersion relations.
Multi-wire sawing of silicon wafers is a tribological process. Slurry consisting of small silicon carbide particles
embedded in polyethyleneglycol carries out the abrasive material removal process. During this process small silicon
chips are removed from the bulk material. Low coherence interferometry (LCI) is widely used for high accuracy surface
topography measurements of materials. This paper presents an application of LCI where the surface of a material
(silicon) is inspected from the inside. Light in the near infrared (NIR) wavelength region is used. High spatial resolution
is necessary to be able to observe the processes on the micro scale. Therefore a modified solid immersion approach is
suggested. That makes it possible to reach a spatial resolution in the range of the illumination wavelength. The
topography changes produced by the chippings are in the range of some micrometers. To be able to estimate the
volumes of the Si chippings interferometric phase measurements are applied.
Vulnerable plaques constitute a risk for serious heart problems, and are difficult to identify using existing methods. Hyperspectral imaging combines spectral- and spatial information, providing new possibilities for precise optical characterization of atherosclerotic lesions. Hyperspectral data were collected from excised aorta samples (n = 11) using both white-light and ultraviolet illumination. Single lesions (n = 42) were chosen for further investigation, and classified according to histological findings. The corresponding hyperspectral images were characterized using statistical image analysis tools (minimum noise fraction, K-means clustering, principal component analysis) and evaluation of reflectance/fluorescence spectra. Image analysis combined with histology revealed the complexity and heterogeneity of aortic plaques. Plaque features such as lipids and calcifications could be identified from the hyperspectral images. Most of the advanced lesions had a central region surrounded by an outer rim or shoulder-region of the plaque, which is considered a weak spot in vulnerable lesions. These features could be identified in both the white-light and fluorescence data. Hyperspectral imaging was shown to be a promising tool for detection and characterization of advanced atherosclerotic plaques in vitro. Hyperspectral imaging provides more diagnostic information about the heterogeneity of the lesions than conventional single point spectroscopic measurements.
Imaging modalities like hyperspectral imaging create large amounts of data. Time efficient, automated analytic
techniques are therefore required to enjoy the power of such methods. In this study it was investigated if
hyperspectral imaging followed by automated noise filtering and statistical image analysis is a suitable method for
characterization of the macroscopic structure of atherosclerotic lesions. Ten human aorta samples (6×8 cm) were
collected during autopsy. Hyperspectral white light and fluorescence images and 5 - 6 biopsies were collected from
each sample. The biopsies were stained (HES, Sudan red), and grouped according to histology. All images were
noise filtered and normalized. Fluorescence spectra were collected from all biopsied regions, and used to compute
average spectra for each histological group. Supervised classification was performed using Spectral angle mapping
(SAM) with the average spectra as endmembers. K-means- and ISO-data clustering was used for unsupervised
classification. The results show that noise filtering and normalization is essential for reliable classification.
Supervised classification was in general found to perform better than unsupervised classification. However, the
SAM results strongly depend on the variation in the spectra used to compute the average endmember spectra.
The analysis show that fatty deposits, calcifications, connective tissue and hemoglobin can be identified. The
lesions were found to have a complex structure where vulnerable regions could be found next to stabile regions
within the same lesion. In conclusion hyperspectral imaging, automated filtering and -analysis was found to be
a suitable tool to classify advanced atherosclerotic lesions.
The lack of commercial equipment for characterization of vibrating micro- and nanostructures has motivated the
development of a heterodyne interferometer. The setup is designed to measure phase and absolute amplitude in the entire
frequency range 0-1.2 GHz. Its transverse resolution is < 1 μm while the present sensitivity for vibrations is 3 pm/(Hz)1/2.
Capacitive micromachined ultrasonic transducers (CMUTs) are being developed for diagnostic imaging of vulnerable
plaques in the coronary arteries. The CMUTs have 5.7 μm radii, 100 nm membrane thickness and ~30 MHz center
frequency. Arrays of ~7500 CMUTs have been fabricated. Frequency scan measurements along a row of CMUTs reveal
a variation in resonance frequency. This may be due to variations of material properties, dimensions such as thickness
and transverse dimensions, and other manufacturing variance. The frequency scan revealed the fundamental mode and
two closely spaced higher order modes.
Modeling of individual CMUT elements was performed using the commercial program COMSOL. A finite element
model (FEM) based on symmetry assumptions predicted only one higher order mode. After closer analysis it was found
that the symmetry assumptions were insufficient. By using a complete physical model two higher order modes were
predicted in agreement with the measurements.
Simulations are able to predict transducer characteristics in great detail but are dependent on accurate input parameters.
The optical measurements contribute to validate or complement simulations and assumptions they rely on. The
heterodyne interferometer is therefore a valuable tool for quality control in the conception, design and manufacturing of
new acoustic devices.
Capacitor Micromachined Ultrasonic Transducers (CMUTs) are being developed and fabricated to be integrated
in a 1 mm diameter catheter, aiming to detect vulnerable plaques in the coronary arteries. The structure is built
up of an array of 72x104 CMUTs, where two linear arrays of CMUT cells are bonded together. The CMUTs
have resonance frequencies of about 30MHz. The radius of each CMUT is 5.7 &mgr;m and the vibration amplitude
is in the range 20pm-12nm. A heterodyne interferometer has been built for characterizing the CMUTs. It offers
the possibility of both phase and high resolution absolute amplitude vibration measurements. The setup can
measure vibrations from 0 to 1.2GHz. In this work we present interferometric measurements on the CMUTs and
compare them with electrical measurements performed by using a network analyzer. Using the interferometer
we are able to investigate individual CMUT cells, whereas the electrical measurements are based on a sum
of all currents in the CMUTs bonded together. In addition to a RF voltage at the operating frequency, the
CMUT is supplied with a bias voltage to vibrate. The CMUT resonance frequency can be tuned by varying this
DC voltage. In this article we have investigated the predicted linear relationship between applied AC voltage
and vibration amplitude. Other parameters investigated are the effects of temperature increase in addition to
traveling charges on the CMUT membrane. The interferometric setup can be used to characterize various devices
with small surface movements, such as MEMS- and SAW-devices.
A heterodyne interferometer has been built in order to characterize vibrations on Micro-Electro-Mechanical Systems (MEMS). The interferometer offers the possibility of both phase and high resolution absolute amplitude vibrational measurements, which is of great importance. A frequency shift is achieved by introducing acoustooptic (AO) modulation in one of the interferometer arms. By using a lock-in amplifier a narrow bandwidth detection regime is achieved. This factor improves the amplitude resolution. By using two AO-modulators and varying the frequency inputs of both, the setup is designed to measure vibrations in the entire frequency range 0 - 1.2GHz. The absolute amplitude is obtained by performing two measurements at each sample point. The first step is to measure the first harmonic of the object vibration. The second step is to measure the frequency components of the light reflected from the test device corresponding to the frequency without object modulation. This is obtained by mixing the detector signal with an external signal generator, and adjusting the frequency of the latter. By combining these two measurements we are able to determine the absolute amplitude of the vibration. The interferometric setup can be used to characterize various kinds of micro- and nanostructures. The system is here demonstrated on a Surface Acoustic Wave (SAW) device and on Capacitor Micromachined Ultrasonic Transducers (CMUTs). We have measured absolute amplitudes with picometer resolution.
A heterodyne interferometer with picometer sensitivity for non-destructive characterization of micro- and nanostructures has been built. The setup is designed to measure phase and amplitude in the entire frequency range 0-1.2GHz. The object can be scanned in the x- and y-direction with sub-micrometer precision. Absolute amplitude of vibration is determined by combining separate measurements of the carrier and sideband frequency of the detected signal. The detector signal is mixed with a signal from a generator. By adjusting the frequency of the signal generator, we can choose the carrier or sideband frequency.
We have performed measurements on capacitor micro-machined ultrasound transducers (CMUTs) which are being developed for diagnostic imaging of vulnerable plaques in arteries. Arrays of ~7500 CMUTs with a total area of 1.3mm x 0.9mm are planned used in an intravascular catheter. The CMUTs studied have typical radii of 5.7-12.5μm, membrane thickness of 100nm, and center frequencies 10-35MHz. Characterization of both single and arrays of CMUTs is important to optimize the manufacturing process and the design. Quality control during manufacture is also important to identify imperfect elements. Other structures have been characterized such as a piezoelectric element with excitation frequencies from a few kHz to several hundreds of kHz and a LiNbO3 surface acoustic wave (SAW) transducer with excitation frequencies from 20MHz to 30MHz. We have performed initial measurements of absolute amplitudes with picometer resolution. Theoretical calculations agree well with the measurements. The setup can be used to characterize a large range of micro- and nanostructures.
Electronic speckle pattern interferometry (ESPI) provides full field interferometric mapping of deformations, vibrations and density fluctuations in test objects. The interference fringes can be followed in real-time while phase-stepping techniques provide quantitative information. Interferometric measurement of biological objects by standard holography has so far been limited by fringe decorrelation due to microstructure changes. ESPI is based on video recording which allows us to monitor relatively rapid changes in biological objects. To illustrate the potentials of the technique, the gravitropical response of oat coleoptiles (seedlings) was studied. A coleoptile represents a fast growing, partly translucent biological object which is difficult to record interferometrically. However, growth and bending of the specimen were measured even on the tip of the coleoptile where the microstructure changes very rapidly. We also show how small temperature changes in transparent objects can be measured. In water, the sensitivity to temperature change is 0.7 multiplied by 10-3 K per meter. Due to a small difference in temperature between the bulk liquid and its droplet, it was possible to follow and measure movement of droplets in liquids.
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