Surface roughness measurement based on fiber optic sensor

The multi-wavelength fiber sensor for measuring surface roughness and surface scattering characteristics were investigated. In this paper, specimens with different surface roughness were analyzed by using 650 nm, 1310 nm and 1550 nm laser as the light source, respectively. The working distance of 2 mm was chosen as the optimum measurement distance. The experimental results indicate that multi-wavelength fiber sensor can accurately measure surface roughness, and can effectively reduce the unsystematic error. The light scattering intensity ratio has a good linear relationship with the surface roughness. The minimum relative error of the surface roughness is 2.92%, the maximum relative error is 13.4%, and the average relative error is about 7.48%. The accuracy for measuring surface roughness by multi-wavelength fiber sensor is about twice as large as that by single-wavelength fiber sensor.     

Optical fiber distributed temperature sensor using short term Fourier transform based simplified signal processing of Raman signals

A simplified technique using short term Fourier transform to reduce the errors in distributed temperature measurement with a Raman scattering based optical fiber sensor system is presented. The two main sources of errors are differential attenuation to anti-Stokes and Stokes signal by fiber and local change in Stokes due to change in temperature. The proposed technique compensates these errors and extracts correct temperature profile in spite of practical difficulties encountered in applying the theoretical concept. Moreover proposed technique is less complex, self-reliant, can tolerate variation in laser power, requires less dead zone and suits automation using embedded solution. Results of measurement carried out, using the system developed at RRCAT, Indore, for two hot zones having spatial width of 1.9 m (kept at 56 °C) and 1.5 m (kept at 78 °C), located at 47 m and 85 m respectively, show that these parameters can be recovered with significantly small errors.     

Thermopower detection of single nanowire using a MEMS device

We present a MEMS-based device on a silicon nitride membrane in order to measure the thermoelectric properties of a single nanowire. A temperature gradient along a nanowire was generated by a nanoheater, and the temperature was measured by Pt thermometers. A thermal simulation using a finite element method was conducted to analyze the temperature distribution over the MEMS device. The validity of the MEMS device was established by testing the Pt nanowires which had different symmetry configurations. From the test results of Pt nanowires, a convincing temperature calibration method was proposed and applied to an actual case of Bi₂Te₃ nanowire. We measured a Seebeck coefficient of −53 μV/K and electrical conductivity of 2.23 × 10⁵ S/m for a single Bi₂Te₃ nanowire with a diameter of 70 nm at 300 K. Our solid design for thermoelectric measurements based on a membrane structure enables the fast and high-yield characterization of one-dimensional nanostructures.     

Nano force sensing using symmetric double stage micro resonator

A new approach is proposed to improve a graphical approach with considering intensity coupling loss coefficients in the analytical derivation of the optical transfer functions for a symmetric double stage vertically coupled microring resonator. An optimum transmission coupling condition is determined with considering terms of couplers intensity loss which leads to low insertion loss of 1.2 dB, finesse of 1525, the out of band rejection ratio of 61.8 dB. The resonating system is used as an optical force sensing system to make the benefit of the accuracy of measurements in micro and nano scales. The sensitivity of proposed force sensor in terms of wavelength-shift is 33 nm/nN and the limit of detection is 1.6 × 10⁻² nN. The proposed sensing system has the advantages of self-calibration and the low power consumption due to the low intensity.     

Simultaneous distributed measurement of the strain and temperature for a four-point bending test using polarization-maintaining fiber Bragg grating interrogated by optical frequency domain reflectometry

We demonstrate a simultaneous distributed strain and temperature measurement technique with the spatial resolution of 1 mm using fiber Bragg gratings inscribed in a polarization-maintaining and absorption-reducing fiber (PANDA-FBGs) and optical frequency domain reflectometry (OFDR). We conduct four-point bending tests in an environmental chamber. Using high birefringent PANDA-FBGs that are manufactured specifically for the simultaneous measurements, the uniform temperature distributions and the typical strain distribution profiles of the four-point bending tests were successfully obtained. The measurement errors of strain were from −31 με to 19 με, and of temperature were from −0.9 °C to 1.3 °C. The spatial standard deviation was 7.5 με and 0.9 °C. We also discussed the effect of the residual strain of the sensor-bonding procedures and the data averaging.     

Development of high-precision micro-roundness measuring machine using a high-sensitivity and compact multi-beam angle sensor

With recent development in advanced manufacturing, demand for nanometric accuracy in dimensional metrology has increased dramatically. To satisfy these requirements, we propose a high-accuracy micro-roundness measuring machine (micro-RMM) using a multi-beam angle sensor (MBAS). The micro-RMM includes three main parts: the MBAS, a rotary unit, and a bearing system. The MBAS has been designed and established in order to improve motion accuracy of the micro-RMM. The dimensions of the MBAS are 125(L) mm × 130(W) mm × 90(H) mm. Compared with other methods, an MBAS is less susceptible to spindle error (stage-independence) when detecting angles, can maintain high sensitivity with miniaturized size, and can be used conveniently at the factory level. The optical probe, reported in this paper, is based on the principle of an autocollimator, and the stability is improved when using the MBAS. Unlike multi-probe methods, the micro-RMM is constructed to realize roundness measurement by using only one probe, which is less susceptible to instrumental errors. Experimental results confirming the feasibility of the multi-beam angle sensor for roundness measurement are also presented.     

Parameters optimization on surface roughness improvement of Zerodur optical glass using an innovative rotary abrasive fluid multi-jet polishing process

With the advance of contemporary technology, high precision surface finishing techniques for optical glasses are of great concern and developing to meet the requirements of the effective industrialized processes. Not only the used tools but also process parameters have great influence on the surface roughness improvements. In this paper, surface roughness improvement of Zerodur optical glass using an innovative rotary abrasive fluid multi-jet polishing process has been presented. For the same purpose, a tool for executing ultra precision polishing was designed and manufactured. Taguchi's experimental approach, an L₁₈ orthogonal array was employed to obtain the optimal process parameters. ANOVA analysis has also been carried out to determine the significant factors. It was observed that about a 98.33% improvement on surface roughness from (R_a) 0.360 μm to (R_a) 0.006 μm has been achieved. The experimental results show that a surface finished achieved can satisfy the requirements for optical-quality surface (R_a < 12 nm). In addition, the influence of significant factors on surface roughness improvement has been discussed in this study.     

Subpixel edge location method proposed to improve the performance of optical fiber spherical coupling probe during dimensional measurement of micro-cavities with high aspect ratio

A subpixel edge location method based on orthogonal Jacobi–Fourier moments is proposed in this paper to improve the performance of optical fiber spherical coupling probe during dimensional measurement of micro-cavities with high aspect ratio. The effectiveness of the proposed method is proved through the performance test of a micro-hole measuring machine with optical spherical coupling probe. Test results indicate that a blind micro-hole of 400 μm in diameter can be experimentally measured at the depth of 2000 μm with a repeatability of 40 nm and an extremity resolution of 42 nm.     

Application of the continuous wavelet transform in periodic error compensation

This paper introduces a new discrete time continuous wavelet transform (DTCWT)-based algorithm, which can be implemented in real time to quantify and compensate periodic error for constant and non-constant velocity motion in heterodyne displacement measuring interferometry. It identifies the periodic error by measuring the phase and amplitude information at different orders (the periodic error is modeled as a summation of pure sine signals), reconstructs the periodic error by combining the magnitudes for all orders, and compensates the periodic error by subtracting the reconstructed error from the displacement signal measured by the interferometer. The algorithm is validated by comparing the compensated results with a traditional frequency domain approach for constant velocity motion. The algorithm demonstrates successful reduction of the first order periodic error amplitude from 4 nm to 0.24 nm (a 94% decrease) and a reduction of the second order periodic error from 2.5 nm to 0.3 nm (an 88% decrease). The algorithm also reduces periodic errors for non-constant velocity motion overcoming limitations of existing methods.     

Application of multispectral radiation thermometry in temperature measurement of thermal barrier coated surfaces

Ceramics coatings are materials widely used in gas turbines to provide thermal shielding of superalloy materials against excessive turbine temperatures. However, measurement of their surface temperatures using conventional radiation thermometers, more so in the presence of high ambient radiation and low emissivity is quite challenging. A multispectral method employing curve fitting technique to measure the temperature of such targets in the range of 800–1200 K and ambient temperature of 1273 K is implemented in this paper through simulation. Several simulated experiments were carried out to identify emissivity models best suited for multispectral radiation thermometry applicable to ceramic coatings. The best emissivity model applicable to yttria-stabilized zirconia of coating thickness of 330 μm in the wavelength range of 3.5–3.9 μm was found to predict temperature with an error of less than 1.5% in the presence and absence of background noise.     

Compact and inexpensive iodine-stabilized diode laser system with an output at 531 nm for gauge block interferometers

A compact and inexpensive iodine-stabilized diode laser system with an output at 531 nm has been applied to long gauge block measurements. Although the optical frequency of the output beam was widely modulated (modulation width of ∼22 MHz), the coherence length and interference phase stability are sufficiently long and high, respectively, for the interferometric measurement of long gauge blocks of up to 1000 mm in length. The effective uncertainty of laser frequency in the interferometric measurement was theoretically and experimentally confirmed to be less than 10⁻⁹.     

Absolute interferometric test for high numerical-aperture spherical concave surfaces: Gravitational effect

Spherical concave surfaces with high numerical apertures are required in industry for lithography optics at ultraviolet and X-ray wavelengths. Among the systematic errors in these spherical-surface test, the gravitational deformation has not been separated from the other optical aberrations. We utilized a two-surface comparison method to quantify the gravitational deformation in a vertical Fizeau interferometer. Certain aberrations vary with rotation around the optical axis. We averaged the ordinary aberrations and isolated the aberration caused by gravitational deformation. Experimental results show that a 4-in concave surface with an F-number 0.75 reveals to have a gravitational deformation of 7 nm peak-to-valley.     

Application of blue laser direct-writing equipment for manufacturing of periodic and aperiodic nanostructure patterns

This study presents the novel development of low cost, highly efficient blue laser direct-writing equipment for using mask-less laser lithography to manufacture periodic and aperiodic nanostructure patterns. The system includes a long-stroke linear motor precision stage (X, Y), a piezoelectric nano-precision stage (Y, θz), a 3-DOF (degrees of freedom) laser interferometer measurement system, and a blue laser direct-writing optical system. The 3-DOF laser interferometer measurement system gives the control system feedback for displacement (X, Y, θz) of the equipment. The laser processing equipment consists of a blue laser direct-writing optical head, a field-programmable gate array (FPGA) alignment interface, and an optical head servo controller. The optical head operates at a wavelength of 405 nm. Processing the nanostructures on thermo-reaction inorganic resists with precise control of the laser intensity, taking advantage of the threshold effect to exceed the limitations of optical diffraction, and reduces the nanostructure hole size. The equipment can be used to fabricate various periodic nanostructure patterns, aperiodic nanostructure patterns, and two-dimensional patterns. The equipment positioning accuracy is within 50 nm at a speed of 50 mm/s, and the minimum critical dimension can be achieved about 100 nm or so.     

Study on pump optimizing for Bi/Er co-doped optical fiber

Bi/Er co-doped optical fiber is one of the solutions for wave band extending technology which is very important for fiber amplifiers, lasers and communication system. Pump option can alter emission band of the Bi/Er co-doped fiber. In this paper, optimization of pump wavelength is proposed. A high Bi concentration co-doped Bi/Er optical fiber is tested as a sample to demonstrate pump wavelength influence to the emission spectrum band. An optical fiber measurement system is provided to measure out characteristics of active optical fibers. And some useful results and parameters of pump optimizing for Bi/Er co-doped optical fiber are discussed in detail. From this research, optimized pump wavelength is suggested around 1350 nm to get a wider continue spectrum covers from 1300 nm to 1600 nm.     

Upstream installation and misalignment effects on the performance of a modified electromagnetic flowmeter

The performance of a modified Danfoss 50 mm diameter electromagnetic flowmeter has been investigated when installed downstream from three different pipe diameters—50 mm, 55 mm and 45 mm. The effects of a 3 mm misalignment of the flowmeter, in both the vertical and horizontal planes, with respect to each of the three upstream pipe diameters has also been identified. The largest percentage errors are reported for the 45 mm upstream diameter pipe, with the flowmeter misaligned by 3 mm in the horizontal plane. The vertical and horizontal mean velocity and root-mean-square velocity profiles, measured within the flowmeter using laser Doppler anemometry, show significant variations in comparison with the ideal, fully developed profiles.     

Simultaneous interferometric measurement of the absolute thickness and surface shape of a transparent plate using wavelength tuning Fourier analysis and phase shifting

The absolute optical thickness and surface shape of optical devices are considered as the fundamental characteristics when designing optical equipment. The thickness and surface shape should be measured simultaneously to reduce cost. In this research, the absolute optical thickness and surface shape of a 6–mm-thick fused silica transparent plate of diameter 100 mm was measured simultaneously by a three-surface Fizeau interferometer. A measurement method combining the wavelength tuning Fourier and phase shifting technique was proposed. The absolute optical thickness that corresponds to the group refractive index was determined by wavelength tuning Fourier analysis. At the beginning and end of the wavelength tuning, the fractional phases of the interference fringes were measured by the phase shifting technique and optical thickness deviations with respect to the ordinary refractive index and surface shape were determined. These two kinds of optical thicknesses were synthesized using the Sellmeier equation for the refractive index of fused silica glass, and the least square fitting method was used to determine the final absolute optical thickness distribution. The experimental results indicate that the all the measurement uncertainties for the absolute optical thickness and surface shape were approximately 3 nm and 35 nm, respectively.     

Stable mounting of beamsplitters for an interferometer

The Basic Angle Monitoring (BAM) system for satellite GAIA (2012–2018) will measure variation on the angle between the lines-of-sight between two telescopes with 2.5 prad uncertainty. It is a laser-interferometer system consisting of two optical benches with a number of mirrors and beamsplitters. The optical components need to be stable with respect to each other within 0.17 pm in position and 60 nrad in angle during measurements over a period of 6 h with 0.1 mK thermal stability. This paper aims at finding the most suitable mounting plane of the fused silica beamsplitters mounted onto the silicon carbide optical bench in the BAM system. These beamsplitters must be clamped mechanically. Based on a force stability analysis, mounting in the plane of light is a more stable solution than mounting on the reflective surface. However, when making a conceptual design the difficulty is making a design which has sufficient alignment stability to survive launch vibrations and a cool-down trajectory is more difficult.     

Plasmonic effect of silver nanoparticles on the upconversion emissions of Sm3+-doped sodium-borosilicate glass

The effect of the localized surface plasmon resonance (SPR) on optical absorption and photoluminescence of Sm³⁺-doped sodium borosilicate glass containing reduced silver nanoparticles (NPs) is reported (Ag⁺ → Ag⁰). The interaction of ultraviolet light by metallic NPs and its effect on the optical properties of samarium in proposed glass were investigated by absorption and photoluminescence spectra analysis. The existence of the NPs was pursued by transmission electron microscopy technique, revealing the existence of Ag NPs with average size of ∼8–14 nm. The largest enhancement was achieved for emission at 561 nm. Such improvements were attributed and discussed by enhanced electric field around metallic NPs and energy transfer (ET) between Sm³⁺ ions and silver NPs.     

Performance evaluation of a cheap,open source,digital environmental monitor based on the Raspberry Pi

We report on the design, construction and evaluation of a low-cost digital environmental monitoring system based on a popular micro-computer board and mass market digital sensors. The system is based around the use of open source software and readily available digital sensors, providing key parameters required for environmentally-controlled calibration laboratories: air temperature, pressure and humidity. Each system logs data at set intervals with front-panel display, web page graphical display and email alerting when exceeding set tolerances. The sensors have been calibrated at the National Physical Laboratory using standards traceable to the SI. Long term stability of the system is estimated and in addition to monitoring of laboratory environments for regulatory purposes, the systems can also be used to provide on-demand values for local refractive index with an expanded (k = 2) uncertainty of 1.1 × 10⁻⁷ as required for many optical-based measuring systems.