Low-coherence Interference Research Articles

Three-dimensional scalar imaging in high-aperture low-coherence interference and holographic microscopes

In this paper we derive the three-dimensional scalar theory of the overall imaging process in a high-aperture interference, holographic, and correlation (heterodyne) microscope. The influence of low spatial and temporal coherence of light is completely described by means of a coherent transfer function. The results are compared with those for an ideal confocal microscope.

Velocity vector measurement based on correlation process between low coherence interference outputs

Velocity vector measurement by using fiber-optic low coherence interferometers is proposed and demonstrated. The interference outputs between the reference and backscattered light from the probing points in the flow are correlation processed. The probing points can be specified and are set on the x-, y-, and z-axis in the flow by using the fiber-optic low coherence interferometers. The direction (angles from each axis) and absolute value of the flow velocity can be measured from peak positions of correlations.

Phase-shifting digital holography with a low-coherence light source for reconstruction of a digital relief object hidden behind a light-scattering medium

Hiding image data with a light-scattering medium is effective as a basic data protection technique. The hidden image data can be observed only by using a low-coherence interference technique and is thus protected from unauthorized access. Unlike an intensity-distributed object, a digital relief object has no intensity distribution, making it possible to hide its existence by using a dilute light-scattering medium. To reconstruct the digital relief object through the light-scattering medium, we developed phase-shifting digital holography with a low-coherence light source. The experimental performance, including the spatial resolution and phase error of the reconstructed image, is estimated.

Development of a parallel demodulation system used for extrinsic Fabry-Perot interferometer and fiber Bragg grating sensors

A parallel demodulation system for extrinsic Fabry-Perot interferometer (EFPI) and fiber Bragg grating (FBG) sensors is presented, which is based on a Michelson interferometer and combines the methods of low-coherence interference and a Fourier-transform spectrum. The parallel demodulation theory is modeled with Fourier-transform spectrum technology, and a signal separation method with an EFPI and FBG is proposed. The design of an optical path difference scanning and sampling method without a reference light is described. Experiments show that the parallel demodulation system has good spectrum demodulation and low-coherence interference demodulation performance. It can realize simultaneous strain and temperature measurements while keeping the whole system configuration less complex.

Parallel demodulation system and signal-processing method for extrinsic Fabry–Perot interferometer and fiber Bragg grating sensors

A parallel demodulation system for extrinsic Fabry-Perot interferometer (EFPI) and fiber Bragg grating (FBG) sensors is presented that is based on a Michelson interferometer and combines the methods of low-coherence interference and Fourier transform spectrum. Signals from EFPI and FBG sensors are obtained simultaneously by scanning one arm of a Michelson interferometer, and an algorithm model is established to process the signals and retrieve both the wavelength of the FBG and the cavity length of the EFPI at the same time, which are then used to determine the strain and temperature.

Low-coherence interference microscopy with an improved switchable achromatic phase-shifter

We present results obtained with a computer-controlled low-coherence interference microscope featuring a modified optical layout that uses a pair of fast switchable achromatic phase-shifters. This layout makes it possible to use ferro-electric liquid-crystal devices made with readily available FLC materials having a switching angle of 45 degrees , and obtain phase shifts of 0 degrees and +/-90 degrees . The use of phase shifts of 0 degrees and +/-90 degrees simplifies calculations of the fringe visibility and the fractional fringe-order and yields maximum accuracy.

Determination of path length difference by low coherence interference spectrum

Path length difference is the key parameter in two-beam interferometer, especially in low coherence interferometer. It determines the visibility of the interference fringes. In this study, we present a method to determine the path length difference between two arms of a fiber optic Mach–Zehnder interferometer by evaluating the peaks of power distribution of the interference spectrum with a wide band light source. The experimental results are in close agreement with the theoretical calculations.

Analysis of low-coherence interference fringes by the Kalman filtering method.

Interferometers with low-coherence illumination allow noncontact measurement of rough-surface relief with a wide range of measurement definition by locating the visibility maxima of interference fringes. The problem is light scattering by the surface to be measured, which can cause distortion of low-coherence interferometric signals. We propose to use a stochastic fringe model and a Kalman filtering method for processing noisy low-coherence fringes dynamically. Prediction of the fringe's signal value at each discretization step is based on all the information available before this step; the prediction error is used for dynamic correction of the estimates of the fringe envelope and phase. The advantages of the Kalman filtering method consist in its immunity to noise, optimal fringe evaluation, and data-processing speed.

Low-coherence interference microscopy using a ferro-electric liquid crystal phase-modulator

We describe a computer-controlled low-coherence interference microscope, based on a Linnik interferometer configuration that can rapidly and accurately map the shape of micro-machined surfaces exhibiting steps and discontinuities. The novel feature of the system is a fast, switchable achromatic phase-modulator operating on the geometric phase, using a pair of ferro-electric liquid crystal devices.

Time-correlation analysis of low-coherence interference schemes with an arbitrary number of anisotropy defects in an optical path

Based on the time-correlation approach, a mathematical model is developed for analysis of the influence of anisotropy defects in an optical path on the correlation characteristics of low-coherence radiation. Several interference schemes of practical importance for optical coherence tomography (OCT) are analysed. The experimental data are presented which demonstrate the influence of anisotropy defects in an optical path on an output interference signal and the quality of two-dimensional images (tomograms) of biological objects obtained by the OCT method.

Measurements of velocity distribution along depth using low coherence interferometry

A new optical measurement method of a flow velocity distribution along the depth using low coherence interference techniques is proposed and demonstrated. In the proposed method, fluctuations of the interference signal formed with the reference and backscattering lights are detected by the optical heterodyne scheme. It is shown experimentally that the width of the spectrum of the interference signal is proportional to the flow velocity. The dynamic range is determined by the time constants of the fast Fourier transform (FFT) processor and lock-in amplifier used in the experiment.

Direct velocity sensing of flow distribution based on low-coherence interferometry

A new sensing method for measuring flow velocity distribution directly by using low-coherence interference techniques is proposed and demonstrated. In this method a temporally fluctuating signal, not the Doppler frequency shift, is detected. Theoretical analysis shows that a spectrum of light backscattered from a particle takes a Gaussian form whose width is simply proportional to the flow velocity. The measured velocity is in good agreement with the actual flow velocity derived from the flow rate. The dynamic range of this sensing method is governed by the frequency range of the fast-Fourier-transform processor used and is estimated to be 1.4×10-4–14 m/s. The depth position can be adjusted with an accuracy of approximately 30 μm, which is determined by the coherence length of the light source.

Growth of polarisation crosstalk along a silica-basedoptical waveguide

The authors report the length dependence of polarisation crosstalk for a 10 m long silica-based waveguide in which the total polarisation crosstalk is 8.9 × 10–3. This is achieved by numerically integrating the coupling strength obtained by using low coherence interference between excited and orthogonally coupled light waves along the length of the waveguide. A comparison of the measured and theoretical crosstalk curves leads to confirmation that the bends in the waveguide are the main origin of the crosstalk. The polarisation crosstalk per bent section is 4.7 × 10–5.

Polarization crosstalk dependence on length in silica-based waveguides measured by using optical low coherence interference

We describe a powerful method for precisely measuring polarization crosstalk dependence on length for birefringent waveguides which uses optical low coherence interference between excited and orthogonally coupled light waves. This method is applied to 10-m long silica-based waveguides with the total polarization crosstalks of 8.9/spl times/10/sup -3/ and 7.5/spl times/10/sup -3/. The spatial resolution is 10 cm and the measurement error for a waveguide part longer than 1 m is /spl les/10%. A comparison of measured and theoretical crosstalk curves for the waveguides enables us to confirm that the bends in the waveguides are the main origin of the crosstalk. The polarization crosstalk per bent section is /spl sim/4/spl times/10/sup -5/.

High sensitivity and submillimeter resolution optical time-domain reflectometry based on low-coherence interference

Characteristics of the low-coherence optical time-domain reflectometer (OTDR) are presented, and optical waveguide diagnosis using the OTDR is demonstrated. Ultrahigh sensitivity in the shot noise limited operation is discussed, and jagged-shaped Rayleigh-backscatter signals are predicted and observed experimentally. Signal smoothing by widening the resolution from 14 to 400 mu m can drastically reduce the jagging. In the experiment, polarization-sensitive and insensitive OTDRs using a fiber-coiled piezoelectric phase modulator are proposed. Measurements of beat length and bending loss, quality characterization of low-loss waveguides by polarization-sensitive OTDR, and cancellations of polarization fading in discrete backreflection and polarization jagging in Rayleigh backscattering by polarization-insensitive OTDR are successfully demonstrated. >

Digital signal-processing techniques for electronically scanned optical-fiber white-light interferometry

Several important digital processing techniques for optical-fiber sensor systems that use electronically scanned white-light interferometry are presented. These include fringe restoration, fringe-order identification, and resolution enhancement techniques. A pure low-coherence interference fringe pattern is restored by dividing, pixel by pixel, the beam intensity profile from the signal. The central (zero-order) fringe of the pattern is identified by using a centroid algorithm. A linear interpolation or a localized centroid algorithm is used to enhance further the phase resolution. Theoretical analyses, computer simulations, and experimental verifications have shown that these techniques are able to increase greatly the dynamic range of the measurement under a low signal-to-noise ratio environment.

Ultrahigh-sensitivity low coherence otdr using Er 1+ -doped high-power superfluorescent fibre source

The first-known demonstration of an Er3+-doped superfluorescent fibre source coupled to a high-resolution OTDR based on low coherence interference (low coherence OTDR) is reported. Linearly-polarised light with 14 mW output from the source achieves a – 146dB minimum detectable reflectivity at 3 Hz bandwidth. The spatial resolution determined by spectral FWHM is 28 μm for silica-based waveguides.

Jagged appearance of Rayleigh-backscatter signal in ultrahigh-resolution optical time-domain reflectometry based on low-coherence interference

Coherent jagging of the Rayleigh-backscatter signal has been reported by Healey [Electron. Lett. 18, 862 (1982); 20, 30 (1984)] in heterodyne optical time-domain reflectometry. We report that the same phenomenon occurs in ultrahigh-resolution optical time-domain reflectometry based on low-coherence interference. Signal smoothing is an easy way to reveal the desired backscatter signal profile. The bending loss measurement of a single-mode fiber is demonstrated by using the smoothed profile.

Rayleigh backscattering measurement of single-mode fibers by low coherence optical time-domain reflectometer with 14 μm spatial resolution

An optical time-domain reflectometer (OTDR) with −136 dB minimum detectable reflectivity and 14 μm spatial resolution is developed based on low coherence interference. The high sensitivity was accomplished by using a high-power superluminescent diode module with a 1.5 mW fiber output and a new system configuration, both for effectively operating the balanced heterodyne detection. The reflectivity of −136 dB is only 6 dB above the shot noise limit. The first observation of 1.3 μm wavelength Rayleigh backscattering in single-mode fibers was successfully made by the OTDR with a 17 dB dynamic range and 14 μm spatial resolution.

Resolution controllable optical time domain reflectometry based on low coherence interference

A low-coherence optical time domain reflectometer (OTDR) that can control spatial resolution is proposed to efficiently diagnose waveguides. This is achieved using a light source consisting of a superluminescent diode (SLD), an optical bandpass filter for truncating the SLD output, and an antireflection-coated laser diode booster amplifier. Two resolutions, 44 and 14 mu m, were experimentally demonstrated with and without the filter having a 2.7-nm full width at half maximum (FWHM), respectively. The length of the blind space and the minimum detectable reflectivity were respectively <7 mm and <-134 dB at a 3-Hz detection bandwidth, for both resolutions. The noise characteristics of the light source and the system performance are investigated theoretically and experimentally.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>