Low-coherence Interferometer Research Articles

Polarized low-coherence interferometer based on a matrix CCD and birefringence crystal with a two-dimensional angle.

In this paper, an electronically scanned polarized low-coherence interferometer (PLCI) based on a matrix charge-coupled-device and birefringence crystal with a two-dimensional angle is proposed and demonstrated. By using a sensing interferometer composed of the fiber end face and the glass surface, the proposed system is applied to displacement measurement. The two-dimensional interference fringes are captured and comprehensive demodulated by different algorithms. The experimental results showed that, compared with a traditional PLCI system, the proposed system significantly expanded the measurement range (reach ~301 μm), and meanwhile ensured low measurement deviation (kept within ± 7 nm) and high resolution (2.52 nm).

Vibration-insensitive low coherence interferometer (LCI) for the measurement of technical surfaces

Abstract For the evaluation of operationally stressed surfaces containing microstructures, robot guided measurement systems need to be designed, that are able to automatically measure the integrity of these microstructures in a production related environment. Because drifts or vibrations between the robot mounted sensor and the sample surface noticeably influence measurement results, leading to usually insufficient results even large non-measured areas. The measurement system should be designed both to precisely measure the sample surface and to robustly handle the influences of the environmental vibration. In this paper, we introduce a robot guided low coherence interferometer (LCI) system, that can compensate inevitable drifts and vibrations using high-speed data acquisition and a phase correction method, which can analyze and correct distorted inferoferograms using Hilbert-Transform in the frequency domain. Utilizing these methods the measurement performance of this system can be improved in comparison with the conventional LCI, to reduce non-measured areas and to reach a low measurement deviation of less than 0.1 μm. The detailed capability of the proposed methods for the identification and the compensation of environmental vibration is shown based on experimental data.

Non-contact absolute internal distance measurement between two mirrors by using a low-coherence tandem interferometer (2st Report)

Non-contact absolute internal distance measurement between two mirrors by using a low-coherence tandem interferometer (2st Report)

Subaperture stitching test of convex aspheres by using the reconfigurable optical null

Subaperture stitching test in combination of the reconfigurable optical null we proposed recently provides flexible solutions to various surfaces including convex aspheres and even aspheres of large aperture. However it is challenging for the stitching optimization to get the real surface error because the surface error is strongly coupled with misalignment-induced aberrations in near-null subaperture measurements. Aiming at this challenge, we first figure out the property of aberrations induced by misalignment of optical null or test surface. It shows that identical misalignment of the optical null introduces nearly identical aberrations to subapertures with different off-axis distances, while misalignment of the test surface introduces little aberrations to the central subaperture. The stitching algorithm is then proposed with focus on decoupling surface error and induced aberrations. The major step is to calibrate out the effect of misaligned near-null optics before stitching optimization by using the central subaperture measurement. We also present the through-the-back null test for the purpose of cross test. The axial distance is precisely monitored by a low coherence interferometer, which enables accurate determination of the spherical aberration component of surface error. Final experimental results show consistent spherical aberration obtained by stitching test and by the through-the-back null test. It is a big step towards instrumentation of subaperture stitching test for aspheres with rather big amount of misalignments in surface metrology practice.

Measuring the difference in group delays between the cores of a multicore optical fiber by means of time-domain interferometry

A way of measuring the difference in group delays between the cores of multi-core optical fiber is proposed. The method is based on a low-coherence Mach–Zehnder interferometer and a broadband radiation source. The corresponding experimental setup is constructed and a seven-core optical fiber with identical cores manufactured at the Fiber Optics Research Center, Russian Academy of Sciences, is studied.

Comparison of Anterior Segment Measurements Obtained by Aladdin Optical Biometer and Sirius Corneal Topography.

Objectives: To assess the agreement of anterior segment parameter measurements derived from Aladdin optical biometer using optical low coherence interferometer and Sirius corneal topography using combined Scheimpflug-Placido disk.Materials and Methods: Data obtained using the Aladdin and Sirius systems from 110 eyes of 59 subjects who had no health problems other than refractive errors were retrospectively evaluated. Anterior chamber depth (ACD), flat (K1) and steep (K2) keratometry readings, and white-to-white distance (WTW) measurements taken with both devices were noted.Results: The mean age of the patients was 47.31±18.57 years (range, 25 to 79 years). Mean ACD was 3.35±0.4 mm using Aladdin and 3.42±0.44 mm using Sirius. Mean difference in ACD was 0.075 mm greater with Sirius than Aladdin (p<0.001). K1 measurement obtained by Aladdin was an average of 0.409 D higher (p<0.001). No statistically significant differences were detected between the two devices in respect to K2 and WTW measurements (p=0.18, p=0.85 respectively). Pearson correlation analysis showed high correlation between the two devices for all measurements (r=0.985, 0.895, 0.961 and 0.766 for ACD, K1, K2 and WTW respectively; p<0.001).Conclusion:Anterior segment parameters obtained by Aladdin optical biometer and Sirius anterior segment analysis system correlated well with each other and measurement differences between the devices were clinically negligible except for K1 values.

Cost-effective fiber multiplexing system based on low coherence interferometers and application to temperature measurement

Based on the low-coherence interferometric principles, a cost-effective all-fiber Mach-Zehnder multiplexing system is proposed and demonstrated. The system consists of two interferometers: sensing interferometer and demodulation interferometer. By scanning an optical tunable delay line back and forth constantly with a stable speed, sensing fibers with different optical paths can be temporal interrogated. The system is experimentally proved to have a high performance with a good stability and low system noises. The multiplexing capacity of the system is also investigated. An experiment of measuring the surrounding temperature is carried out. A sensitivity of 12 μm/°C is achieved within the range of 20°C to 80°C. This low cost fiber multiplexing system has a potential application in the remote monitoring of temperature and strain in building structures, such as bridges and towers.

Tilt performance of the ground settlement sensor configured in a fiber-optic low-coherent interferometer.

Ground settlement (GS) is one of the causes that destroy the durability of reinforced concrete structures. It could lead to a deterioration in the structural basement and increase the risk of collapse. The methods used for GS monitoring were mostly electronic-based sensors for reading the changes in resistance, resonant frequencies, etc. These sensors often bear low accuracy in the long term. Our published work demonstrated that a fiber-optic low-coherent interferometer configured in a Michelson interferometer was designed as a GS sensor, and a micro-meter resolution in the room environment was approached. However, the designed GS sensor, which in principle is based on a hydraulic connecting vessel, has to suffer from a tilt degeneration problem due to a strictly vertical requirement in practical installment. Here, we made a design for the GS sensor based on its robust tilt performance. The experimental tests show that the sensor can work well within a ±5° tilt. This could meet the requirements in most designed GS sensor installment applications.

Reticular lamina and basilar membrane vibrations in living mouse cochleae

It is commonly believed that the exceptional sensitivity of mammalian hearing depends on outer hair cells which generate forces for amplifying sound-induced basilar membrane vibrations, yet how cellular forces amplify vibrations is poorly understood. In this study, by measuring subnanometer vibrations directly from the reticular lamina at the apical ends of outer hair cells and from the basilar membrane using a custom-built heterodyne low-coherence interferometer, we demonstrate in living mouse cochleae that the sound-induced reticular lamina vibration is substantially larger than the basilar membrane vibration not only at the best frequency but surprisingly also at low frequencies. The phase relation of reticular lamina to basilar membrane vibration changes with frequency by up to 180 degrees from ∼135 degrees at low frequencies to ∼-45 degrees at the best frequency. The magnitude and phase differences between reticular lamina and basilar membrane vibrations are absent in postmortem cochleae. These results indicate that outer hair cells do not amplify the basilar membrane vibration directly through a local feedback as commonly expected; instead, they actively vibrate the reticular lamina over a broad frequency range. The outer hair cell-driven reticular lamina vibration collaboratively interacts with the basilar membrane traveling wave primarily through the cochlear fluid, which boosts peak responses at the best-frequency location and consequently enhances hearing sensitivity and frequency selectivity.

Comparison of ocular biometry using the of a new swept-source optical coherence tomography-based optical biometer and an optical low-coherence interferometer device in patients with clear lens.

Purpose: To compare the clinical measurements with the new IOLMaster-700 from those obtained with the Aladdin. Methods: A prospective, cross-sectional, observational Comparative study.24 eyes were assessed using the two devices. Measurements of axial length (AL), anterior chamber depth (ACD), mean keratometri (K m ) flattest (K f ), steepest keratometry (K s ) and white-to-white (WTW) were done with both devices in random order. Results: No statistically significant differences (p>0.05) between the two biometers were found for AL (mean difference 0.000 mm), ACD (mean difference 0.164 mm) and K f (mean difference 0.020 D). In K m (mean difference was 0.085 D), Ks (mean difference 0.151 D) and WTW (mean difference 0.29 mm) the differences was statistical different. Conclusions: Measurements with the new IOLMaster-700 correlated well with those obtained with the Aladdin in patients with clear lens.

Optically integrated trimodality imaging system: combined all-optical photoacoustic microscopy, optical coherence tomography, and fluorescence imaging.

We have developed a trimodality imaging system by optically integrating all-optical photoacoustic microscopy (AOPAM), optical coherence tomography (OCT), and fluorescence microscopy (FLM) to provide complementary information including optical absorption, optical back-scattering, and fluorescence contrast of biological tissue. By sharing the same low-coherence Michelson interferometer, AOPAM and OCT could be organically optically combined. Also, owing to using the same laser source and objective lens, intrinsically registered photoacoustic and fluorescence signals are obtained in a single pulse. Simultaneously photoacoustic angiography, tissue structure, and fluorescence molecular in vivo images of mouse ear were acquired to demonstrate the capabilities of the optically integrated trimodality imaging system, which can present more information to study tumor angiogenesis, vasculature, anatomical structure, and microenvironments in vivo.

Low-coherence interferometer for contact lens surface metrology

Contact lens performance depends on a number of lens properties. Many metrology systems have been developed to measure different aspects of a contact lens, but none test the surface figure in reflection to subwavelength accuracy. Interferometric surface metrology of immersed contact lenses is complicated by the close proximity of the surfaces, low surface reflectivity, and instability of the lens. An interferometer to address these issues was developed and is described here. The accuracy of the system is verified by comparison of glass reference sample measurements against a calibrated commercial interferometer. The described interferometer can accurately reconstruct large surface departures from spherical with reverse raytracing. The system is shown to have residual errors better than 0.05% of the measured surface departure for high slope regions. Measurements made near null are accurate to λ/20. Spherical, toric, and bifocal soft contact lenses have been measured by this system and show characteristics of contact lenses not seen in transmission testing. The measurements were used to simulate a transmission map that matches an actual transmission test of the contact lens to λ/18.

Low-coherence interferometry based roughness measurement on turbine blade surfaces using wavelet analysis

In this paper, a non-contact optical system, a low-coherence interferometer (LCI), is introduced for the purpose of measuring the surface roughness of turbine blades. The designed system not only possesses a high vertical resolution and is able to acquire the roughness topography, but also it has a large vertical scanning range compared to other commonly used optical systems. The latter characteristic allows us to measure turbine blades surfaces with large curvature without collisions between the lens and the measurement object. After obtaining the surface topography, wavelet analysis is applied to decompose the original surface into multiple bandwidths to conduct a multiscale analysis. The results show that the developed LCI system proofs a good performance not only in obtaining the surface topography in the roughness scale but also in being able to measure surfaces of objects that possess a complex geometry in a large vertical range. Furthermore, the applied biorthogonal wavelet in this study has performed good amplitude and phase properties in extracting the roughness microstructures from the whole surface. Finally, the traditional roughness parameters, such as the mean surface roughness Sa and the Root Mean Square (RMS) roughness Sq, are evaluated in each decomposed subband and their correlations with the scale of each subband are analyzed.

Non-contact absolute internal distance measurement between two mirrors by using a low-coherence tandem interferometer with optical prism mechanism (1st Report)

Non-contact absolute internal distance measurement between two mirrors by using a low-coherence tandem interferometer with optical prism mechanism (1st Report)

Wavelength calibration of dispersive near-infrared spectrometer using relative k-space distribution with low coherence interferometer

The commonly employed calibration methods for laboratory-made spectrometers have several disadvantages, including poor calibration when the number of characteristic spectral peaks is low. Therefore, we present a wavelength calibration method using relative k-space distribution with low coherence interferometer. The proposed method utilizes an interferogram with a perfect sinusoidal pattern in k-space for calibration. Zero-crossing detection extracts the k-space distribution of a spectrometer from the interferogram in the wavelength domain, and a calibration lamp provides information about absolute wavenumbers. To assign wavenumbers, wavelength-to-k-space conversion is required for the characteristic spectrum of the calibration lamp with the extracted k-space distribution. Then, the wavelength calibration is completed by inverse conversion of the k-space into wavelength domain. The calibration performance of the proposed method was demonstrated with two experimental conditions of four and eight characteristic spectral peaks. The proposed method elicited reliable calibration results in both cases, whereas the conventional method of third-order polynomial curve fitting failed to determine wavelengths in the case of four characteristic peaks. Moreover, for optical coherence tomography imaging, the proposed method could improve axial resolution due to higher suppression of sidelobes in point spread function than the conventional method. We believe that our findings can improve not only wavelength calibration accuracy but also resolution for optical coherence tomography.

1F35 1400帯低コヒーレンス光干渉計を用いたヒト皮膚における含水率分布マイクロ断層可視化システムの基礎的検討

Visco-elastic properties of skin tissue is strongly dependent on moisture content around epidermal tissue, which should cause skin wrinkles Therefore, an in vivo quantitative measurement of moisture content is necessary to clarify skin mechanics. In this paper, we propose OCM (Optical Coherence Moisturegraphy), which is a method of tomographic micro-visualizing moisture content using low coherence interferometer with 1400 nm band light source. The tomographic detection of the intensity attenuation due to light absorption of water can determine moisture content profile inside skin tissue. The rapid scanning optical delay line was implemented as a reference arm so as to visualize video-rate tomography of moisture content field. In order to verify the proposed system, respective OCT system with 1300 nm and 1400 nm light source were applied separately to a pig skin tissue covered with a moisturizing sheet. Consequently, the interference signal intensity of 1400 nm band, comparing with 1300 nm band, was observed tomograpically to be decreasing exponentially with water diffusion inside skin. It was concluded that OCM has promising potential of diagnosing tomographic moisture content inside skin tissue.

1310 nm Source Spectral-Domain Optical Coherence Tomography for Chinese Pigment and Jadeite Research

Optical Coherence Tomography (OCT) technique is a nondestructive optical detection technology based on low-coherence interferometer and it has become an attractive cultural heritage research method. A 1310 nm source spectral-domain optical coherence tomography (SD-OCT) system based on optical fiber Michelson interferometer and optical spectrum analyzer is proposed and demonstrated for Chinese cultural heritage research. The cross-section OCT images of Chinese pigment samples and jadeite samples can provide a lot of valuable microstructure information for the Chinese cultural heritage research and identification works.

高周波変調型低コヒーレンス干渉計を用いた微小循環血流速分布マイクロ断層可視化法の基礎的検討

高周波変調型低コヒーレンス干渉計を用いた微小循環血流速分布マイクロ断層可視化法の基礎的検討

The use of thin diamond films in fiber-optic low-coherence interferometers

In this paper we present the use of thin diamond films in fiber-optic low-coherence interferometers. Two kinds of diamond surfaces were used: undoped diamond film and boron- doped diamond film. They were deposited on glass plates as well as silicon layers. A conventionally used mirror was used as a reference layer. Diamond films were deposited using Microwave Plasma Enhanced Chemical Vapour Deposition (μPE CVD) system. Measurements were performed using two superluminescent diodes (SLD) with wavelengths of 1300 mm and 1550 mm. The optimal conditions for each layers were examined: the required wavelength of the light source and the length of Fabry-Perot interferometer cavity. Metrological parameters of Fabry-Perot interferometer with different mirrors were compared. The presented thin diamond films may be an interesting alternative to the commonly used reflective surfaces.

Birefringence-Dispersion-Induced Frequency Domain Nonlinearity Compensation for Polarized Low-Coherence Interferometry Demodulation

A high precision demodulation method in frequency domain is proposed for a polarized low-coherence interferometer with location-dependent birefringence dispersion. By minimizing the frequency nonlinearity caused by dispersion effects, the proposed method avoids the jump errors of traditional frequency domain analysis method and exactly retrieve the absolute phase at multiselected-wavenumber point simultaneously, which makes it superior in accuracy, stability, and measurement range. We carried out experiments with an optical fiber Fabry–Perot pressure sensing system to verify the effectiveness of the proposed method. The experiment result showed that the measured error was less than 0.049 kPa and the measurement range was widened to 285 kPa, which could be wider with suited experimental equipment and is no limit in theory.