Optical Path Length Changes Research Articles

Spatial refractive index measurement of porcine artery using differential phase optical coherence microscopy

We describe a methodology to record spatial variation of refractive index of porcine renal artery using differential phase optical coherence microscopy (DP-OCM). The DP-OCM provides quantitative measurement of thin specimen phase retardation and refractive index by measuring optical path-length changes on the order of a few nanometers and with a lateral resolution of 3 microm. The DP-OCM instrumentation is an all-fiber, dual-channel Michelson interferometer constructed using a polarization maintaining (PM) fiber. Two-dimensional en face dual-channel phase images are taken over a 150 x 200 microm region on a microscopic slide, and the images are reconstructed by plotting a two-dimensional refractive index map as the OCM beam is moved across the sample. Because the DP-OCM can record transient changes in the optical path-length, the system may be used to record quantitative optical path-length alterations of tissue in response to various stimuli. A fiber-based DP-OCM may have the potential to substantially improve in vivo imaging of individual cells for a variety of clinical diagnostics, and monitoring applications.

Changes of cerebral blood oxygenation and optical pathlength during activation and deactivation in the prefrontal cortex measured by time-resolved near infrared spectroscopy

To determine the alterations in optical characteristics and cerebral blood oxygenation (CBO) during activation and deactivation, we evaluated the changes in mean optical pathlength (MOP) and CBO induced by a verbal fluency task (VFT) and driving simulation in the right and left prefrontal cortex (PFC), employing a newly developed time-resolved near infrared spectroscopy, which allows quantitative measurements of the evoked-CBO changes by determining the MOP with a sampling time of 1 s. The results demonstrated differences in MOP in the foreheads with the subjects and wavelength; however, there was no significant difference between the right and left foreheads (p > 0.05). Also, both the VFT and driving simulation task did not affect the MOP significantly as compared to that before the tasks (p > 0.05). In the bilateral PFCs, the VFT caused increases of oxyhemoglobin and total hemoglobin associated with a decrease of deoxyhemoglobin, while the driving simulation task caused decreases of oxyhemoglobin and total hemoglobin associated with an increase of deoxyhemoglobin; there were no significant differences in evoked-CBO changes between the right and left PFC. The present results will be useful for quantitative measurement of hemodynamic changes during activation and deactivation in the adults by near infrared spectroscopy.

Time-resolved thermal lens measurements of the thermo-optical properties of glasses at low temperature down to 20 K

In this work the time resolved thermal lens spectrometry was applied to measure the absolute values of the thermo-optical properties of low silica calcium aluminosilicate and soda lime glasses at low temperatures, in the range between 20 and 300 K. The thermal relaxation calorimetry was used as a complementary technique to determine the specific heat. The results showed a marked decrease of the thermal diffusivity with the temperature rise, with a dependence similar to that of the mean free path $(\ensuremath{\sim}{T}^{\ensuremath{-}1})$ in the interval between 20 and 70 K, while in the range between 70 and 300 K the dependence was ${T}^{\ensuremath{-}(0.33\ifmmode\pm\else\textpm\fi{}0.02)}$. The marked variation of the temperature coefficient of the optical path length change with the temperature rise was attributed to the increase in the coefficient of the electronic polarizability. The results also showed that for the aluminosilicate glass the excess in the specific heat correlated to the so-called boson peak occurred at about 17 K, higher than that of soda lime, which occurs at about 12 K. In conclusion, our results showed the ability of the time resolved thermal lens to determine the thermo-optical properties of glasses at low temperatures, bringing possibilities for experiments in a wide range of optical materials.

Holographic chemical vapor sensor

A holographic interferometer senses vapor-induced optical path length changes in polymer or other chemically sensitive films. The interferometer is inherently sensitive to changes in chemical vapor content, self-compensates for drifts, and accommodates a large array of sensor elements. A sniff-locked-loop synchronous detection method takes advantage of the interferometer's rapid response to achieve vapor concentration sensitivity in the parts-per-billion (ppb, parts in 10(9)) range. We demonstrate, for example, 40 ppb sensitivity to ethyl alcohol using poly(N-vinyl pyrrolidone) with a measurement time of 5 s.

Spectroscopic study of ds/dT in commercial filter by using the thermal lens technique

In this work we report on the use of the Thermal Lens method in a spectroscopic form to investigate the dependence of the thermo-optical properties with the probe beam wavelength in a RG -610 commercial filter close to the absorption maximum (around 650 nm). The measurements were performed using a tunable dye laser as probe source and an Ar + laser at 514 nm as excitation one. Our results exhibit a strong dependence of the temperature coefficient of the optical path length change with the probe wavelength.

Photothermal effects in passive fiber Bragg grating resonators

Photothermal effects in passive Fabry-Perot resonators are caused by the conversion of circulating optical energy into heat as a result of absorption. This results in thermal change in the resonator's optical path length, the round-trip phase, and hence the resonance condition. We describe a simplified dynamic numerical model for photothermal effects in passive fiber Bragg grating resonators and present results of their experimental observation.

Parameter-optimized digital holographic microscope for high-resolution living-cell analysis

A parameter-optimized off-axis setup for digital holographic microscopy is presented for simultaneous, high-resolution, full-field quantitative amplitude and quantitative phase-contrast microscopy and the detection of changes in optical path length in transparent objects, such as undyed living cells. Numerical reconstruction with the described nondiffractive reconstruction method, which suppresses the zero order and the twin image, requires a mathematical model of the phase-difference distribution between the object wave and the reference wave in the hologram plane. Therefore an automated algorithm is explained that determines the parameters of the mathematical model by carrying out the discrete Fresnel transform. Furthermore the relationship between the axial position of the object and the reconstruction distance, which is required for optimization of the lateral resolution of the holographic images, is derived. The lateral and the axial resolutions of the system are discussed and quantified by application to technical objects and to living cells.

Thermo-optical properties measurements in chalcogenide glasses using thermal relaxation and thermal lens methods

In this work we determine the thermo-optical properties of two chalcogenide glasses (in mol%): 65 Ga2S3; 30 La2S3; 5 La2O3 (Ga:La:S) and 72.5 Ga2S3; 27.5 La2O3 (Ga:La:S:O). The thermal relaxation calorimetry and the thermal lens technique were combined so that the samples specific heat, thermal diffusivity, thermal conductivity, and the temperature coefficient of the optical path length change could be measured. Our results indicate that changes in thermal diffusivity (∼2.7×10−3cm2/s) and conductivity (∼4.8×10−3W/Kcm) observed when La2O3 is added in the glass compounds are less than the error in the data.

Electro-optical response of the ferroelectric relaxor poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer

The electro-optical (EO) properties of the ferroelectric relaxor poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer were investigated in the wavelength region from 3 to 5 μm. A large Kerr effect was observed in which a refractive index change of −2.6% can be induced under an electric field of 80 V/μm. When combined with the electrostrictive strain, the terpolymer film exhibits a total −5.6% optical path-length change under a field of 80 V/μm. Calculations based on density functional theory suggest that such a large EO effect was caused mainly by the reorientation of the C–F dipoles under external field in the crystalline region. Furthermore, the results support the notion that in the ferroelectric relaxor terpolymer, there exist nanopolar regions that undergo reorientation under external field and generate the observed electrostrictive strain and EO responses.

Measurement and noise characterization of optically induced index changes using THz interferometry

A Michelson interferometer designed for broadband single-cycle THz pulses is used to characterize optically induced index changes in semiconductors which result in submicron changes in optical path length. The interferometric measurements are compared both to standard THz time-domain spectroscopy (THz-TDS) and differential THz-TDS based on modulation of the sample. By analyzing noise contributions in THz spectroscopy systems, it is shown that the destructive interference achieved in THz interferometry reduces both some sources of random errors as well as errors due to system drift.

Nonperturbing measurements of spatially distributed underwater acoustic fields using a scanning laser Doppler vibrometer.

Localized changes in the density of water induced by the presence of an acoustic field cause perturbations in the localized refractive index. This relationship has given rise to a number of nonperturbing optical metrology techniques for recording measurement parameters from underwater acoustic fields. A method that has been recently developed involves the use of a Laser Doppler Vibrometer (LDV) targeted at a fixed, nonvibrating, plate through an underwater acoustic field. Measurements of the rate of change of optical pathlength along a line section enable the identification of the temporal and frequency characteristics of the acoustic wave front. This approach has been extended through the use of a scanning LDV, which facilitates the measurement of a range of spatially distributed parameters. A mathematical model is presented that relates the distribution of pressure amplitude and phase in a planar wave front with the rate of change of optical pathlength measured by the LDV along a specifically orientated laser line section. Measurements of a 1 MHz acoustic tone burst generated by a focused transducer are described and the results presented. Graphical depictions of the acoustic power and phase distribution recorded by the LDV are shown, together with images representing time history during the acoustic wave propagation.

Extending optics to 50 nm and beyond with immersion lithography

Numerical imaging simulations demonstrate the capability of immersion lithography to print features smaller than 45 nm (35 nm) with good depth of focus at a vacuum wavelength of 193 nm (157 nm). The optical impact of index variation of the immersion liquid is simulated and found to be a shift of focus of 1 nm for each 1 ppm change in the bulk index of the liquid. For an index which varies through the thickness of the liquid (e.g., due to nonuniform temperature), the focus shift is found to be proportional to the total change in optical path length (OPL), with a 1 nm change in OPL leading to a ∼1.5 nm focus shift at 1.3 numerical aperture. A focus offset of 1–3 nm can be expected due to heating during scanning exposure. The possible formation of nanobubbles at resist surfaces is also discussed. While simulations show that even 10 nm thick bubbles at the surface of the resist cause 30% modulation in the aerial image intensity, no evidence of bubbles is seen in open frame immersion exposures. Imaging of 100 nm features is shown using an immersion contact phase-edge technique, with no evidence of bubbles or adverse liquid–resist interactions. Finally, we describe progress in the search for low absorbance liquids for use at 157 nm. Liquid purity, including dissolved O2 and H2O, is found to be critical. The current absorbance record, 0.64±0.07 cm−1, held by perfluorotriglyme (CF3[OCF2CF2]3OCF3), is enough for a 350 μm working distance at 95% transmission.

Quantitative spectroscopy to determine the effects of photodegradation on a model polyester-urethane coating

During the accelerated exposure of a model polyester-urethane coating, measurements were taken at intervals to determine how the ultraviolet absorption, erosion, and chemical change proceeded during degradation. Infrared spectra showed that the carbonyl concentration increased with exposure as the material oxidized, but the ultraviolet results showed carbonyl peaks that diminished with exposure. Changes in optical path length, carbonyl concentration (both infrared and ultraviolet), and C–H concentration appeared linear with the exposure period. Knowing the chemical composition of the polyester and its reaction with isocyanurate made it possible to construct a representative model of the polymer network. Computational chemistry provided confirmation of the literature on which elements of the network were susceptible to degradation. As the polymer eroded, the aromatic content including some carbonyl was lost, but the remaining polymer was oxidized. Quantitative estimates were made for the mass lost and the change in concentration of the carbonyl and CH concentrations that agreed well with results from both the infrared and ultraviolet spectroscopy. Knowing the polymer composition allowed thickness change, a macroscopic quantity, to be connected to chemical changes via the polymer network model.

Imaging tissue response to electrical and photothermal stimulation with nanometer sensitivity.

Tissue response to thermal, electrical, or chemical stimuli are important in the health and survival of tissue. We report experimental results to assess tissue response to various stimuli using a low coherence differential phase interferometer. The optical system utilized to measure tissue response is a novel fiber-based phase sensitive optical low coherence reflectometer (PS-OLCR). Inasmuch as the PS-OLCR works with back-reflected light, noninvasive sensing of tissue response to stimuli is possible. In addition to high lateral (approximately 10 microm) and longitudinal (approximately 10 microm) resolution, PS-OLCR can measure sub-wavelength changes in optical path-length (Angstrom/nanometer range) by extracting the phase difference between interference fringes in two channels corresponding to orthogonal polarization modes. When light spatially splits into two polarization states, precise analysis of surface topography or tissue surface response such as swelling or collapse are possible. Time resolved measurements of nanometer-scale path length changes in response to electrical and thermal stimuli are demonstrated using longitudinally delayed polarization channels. Since PS-OLCR is a useful tool to detect ultra-small path length changes, the system has potential to aid scientists in investigating important phenomena in biomaterials and developing useful diagnostic and therapeutic imaging modalities. Applications include tissue surface profilometry, measurement of tissue, and cell response to various stimuli, high-resolution intensity and phase imaging.

Optical Measurement of Concentration Gradient Near Miscible Interfaces

A common path interferometer (CPI) system was developed to measure the diffusivity of transparent liquid pairs. The CPI is an optical technique that can be used to measure changes in the gradient of refractive index of transparent materials. The CPI is a shearing interferometer that shares the same optical path from a laser light source to the final imaging plane. Molecular diffusivity of liquids can be determined by using physical relations between changes in optical path length and liquid phase properties. In this paper, we present results obtained when the diffusivity depends on the local concentration of the miscible fluids.

Method for compensation of thermally induced modal distortions in the input optical components of gravitational wave interferometers

The next generation of interferometric gravitational wave detectors will employ laser powers approaching 200 W to increase shot-noise limited sensitivity. Optical components that transmit the laser light will exhibit increased thermal lensing induced by bulk absorption and concomitant changes in the material refractive index, resulting in significant changes in the modal characteristics of the beam. Key interferometer components such as electro-optic modulators and Faraday isolators are particularly at risk, since they possess relatively large absorption coefficients. We present a method for passive correction of thermally induced optical path length (ΔΛ) changes induced by absorption in transmissive optical components. Our method relies on introducing material in the optical path that possesses a negative index temperature derivative, thereby inducing a compensating opposite ΔΛ. We experimentally demonstrate a factor of 10 reduction in higher order spatial mode generation for terbium gallium garnet, a Faraday isolator material.

Inverse Piezoelectric and Electrostrictive Response in Freely Suspended FLC Elastomer Film as Detected by Interferometric Measurements

We report the electric field induced thickness variations of homeotropically oriented free standing films of a smectic (C* or A*) FLCE prepared from cross - linkable ferroelectric polysiloxanes. The changes in optical path length in free standing ferroelectric liquid crystal elastomer films have been detected by means of interferometric measurements at both the first and second harmonic of the exciting electric field (ω=33 Hz). The measured electrostrictive strain is above 2.7% (in the thickness direction) at a electric field around 1.5 MV /m. Our experiment reveal that the inverse piezoelectric and electrostrictive response increases sharply near the Sm-C* - Sm-A* phase transition temperature. Also X-ray reflection measurements on a spin cast FLCE film reveal the constriction of smectic layers.

Inverse Piezoelectric and Electrostrictive Response in Freely Suspended FLC Elastomer Film as Detected by Interferometric Measurements

We report the electric field induced thickness variations of homeotropically oriented free standing films of a smectic (C* or A*) FLCE prepared from cross - linkable ferroelectric polysiloxanes. The changes in optical path length in free standing ferroelectric liquid crystal elastomer films have been detected by means of interferometric measurements at both the first and second harmonic of the exciting electric field (ω=33 Hz). The measured electrostrictive strain is above 2.7% (in the thickness direction) at a electric field around 1.5 MV /m. Our experiment reveal that the inverse piezoelectric and electrostrictive response increases sharply near the Sm-C* - Sm-A* phase transition temperature. Also X-ray reflection measurements on a spin cast FLCE film reveal the constriction of smectic layers.

Continuous-wave near-infrared spectroscopy using pathlength-independent hypoxia normalization

A general physiological model for the hemodynamic response during altered blood flow, oxygenation, and metabolism is presented. Calculations of oxy-, deoxy-, and total hemoglobin changes during stimulation are given. It is shown that by using a global hyperoxic or mild hypoxic challenge it is possible to normalize the activation response in terms of the fractional changes in the cerebral blood volume, tissue oxygenation index, and oxygen extraction ratio, which are independent of the optical pathlength. Using a dual wavelength spectrometer, the method is validated by measuring pathlength-independent hemodynamic responses during mild hypercarbia in a rat model. Phantom experiments showed that the changes in optical pathlength were small as the hemoglobin concentration was varied over a wide range. The determination of quantitative parameters facilitates the use of continuous-wave transcranial methods by providing a means by which to characterize activation response across subjects.

Laser heterodyne method for high-resolution gas-density measurements

A new method for noncontact, high-resolution measurement of gas density is described. The method uses a two-frequency Zeeman-split He-Ne laser and cumulative phase-measuring electronics. The measurement is resolved in two dimensions and provides density that is averaged only along the length of the laser beam that passes through the test section. The technique is based on highly accurate measurement of the optical path-length change of the laser beam as it passes through a test cell (in principle, to within 0.001lambda, where lambda is the wavelength of the laser). The technique also provides a very large dynamic range (again, in principle, up to 10(10)), which makes the method additionally attractive. Although the optical path length through the test section is directly related to the index of refraction, and hence to the density of the gas, the method can also be used to measure temperature (if the gas pressure is known) or pressure (if the temperature is known).