Full C-band wavelength-tunable, 250 MHz repetition rate mode-locked polarization-maintaining fiber laser.

We demonstrate a full C-band wavelength-tunable mode-locked fiber laser with a repetition rate of 250 MHz, representing the highest repetition rate for C-band tunable mode-locked lasers thus far to the best of our knowledge. The polarization-maintaining fiber-based Fabry-Perot cavity enables a fundamental repetition rate of 250 MHz with a semiconductor saturable absorber mirror as a mode-locker. We observed a stable and single soliton mode-locking state with wide tunability of the center wavelength from 1505 to 1561 nm by adjusting the incident angle of a bandpass filter inside the cavity.

Linear-cavity Er-doped fiber mode-locked laser with large wavelength tunability.

A linear-type wavelength-tunable all-polarization-maintaining fiber mode-locked laser is proposed for the first time, to our knowledge, and is implemented with an Er-doped fiber and polarization-maintaining fiber components. The tuning range of the center wavelength is from 1533.7 nm to 1565.

A novel method to design and evaluate artificial neural network for thin film thickness measurement traceable to the length standard.

The artificial neural networks (ANNs) have been often used for thin-film thickness measurement, whose performance evaluations were only conducted at the level of simple comparisons with the existing analysis methods. However, it is not an easy and simple way to verify the reliability of an ANN based on international length standards. In this article, we propose for the first time a method by which to design and evaluate an ANN for determining the thickness of the thin film with international standards.

Optical method for simultaneous thickness measurements of two layers with a significant thickness difference.

In this study, an optical method that allows simultaneous thickness measurements of two different layers distributed over a broad thickness range from several tens of nanometers to a few millimeters based on the integration of a spectroscopic reflectometer and a spectral-domain interferometer is proposed. Regarding the optical configuration of the integrated system, various factors, such as the operating spectral band, the measurement beam paths, and the illumination beam type, were considered to match the measurement positions and effectively separate two measurement signals acquired using both measurement techniques. Furthermore, for the thickness measurement algorithm, a model-based analysis method for high-precision substrate thickness measurements in thin-film specimens was designed to minimize the measurement error caused by thin films, and it was confirmed that the error is decreased significantly to less than 8 nm as compared to that when using a Fourier-transform analysis.

Sub-100-nm precision distance measurement by means of all-fiber photonic microwave mixing.

The importance of dimensional metrology has gradually emerged from fundamental research to high-technology industries. In the era of the fourth industrial revolution, absolute distance measurements are required to cope with various applications, such as unmanned vehicles, intelligent robots, and positioning sensors for smart factories. In such cases, the size, weight, power, and cost (SWaP-C) should essentially be restricted.

Precise thickness profile measurement insensitive to spatial and temporal temperature gradients on a large glass substrate.

When manufacturing glass substrates for display devices, especially for large-sized ones, the time-varying spatial temperature gradient or distribution on the samples is remarkably observed. It causes serious degradation of thickness measurement accuracy due to the combination of thermally expanded thickness and temperature-dependent refractive index. To prevent or minimize the degradation in thickness measurement accuracy, the temperature distribution over an entire glass substrate has to be known in real time in synchronization with the thickness measurement to specify the refractive index of the sample based on an exact mathematical model of the temperature-dependent refractive index.

Simultaneous measurement method of the physical thickness and group refractive index free from a non-measurable range.

An optical method to resolve the non-measurable thickness problem caused by the overlap of optical path differences within a specific thickness range when measuring the physical thickness of a sample using a spectral-domain interferometer is proposed and realized. Optical path differences can be discerned by inserting a correction glass piece into the measurement path, thus increasing the measurement optical path length. To verify the proposed method, 0.

High-resolution angle sensor using multiple peak positions of a double slit interference pattern.

A high-resolution angle sensor which uses a double slit (DS) is proposed. By analyzing the positions of intensity peaks in the DS interference pattern, the incident angle of a collimated beam entering the DS is measured. The DS was designed to generate the multiple-order interference pattern with almost even modulation amplitude so that not only the central peak but also multiple side peaks could be used for the measurement.

A Hybrid Non-destructive Measuring Method of Three-dimensional Profile of Through Silicon Vias for Realization of Smart Devices.

Smart devices have been fabricated based on design concept of multiple layer structures which require through silicon vias to transfer electric signals between stacked layers. Because even a single defect leads to fail of the packaged devices, the dimensions of the through silicon vias are needed to be measured through whole sampling inspection process. For that, a novel hybrid optical probe working based on optical interferometry, confocal microscopy and optical microscopy was proposed and realized for enhancing inspection efficiency in this report.

Note: An absolute X-Y-Θ position sensor using a two-dimensional phase-encoded binary scale.

This Note presents a new absolute X-Y-Θ position sensor for measuring planar motion of a precision multi-axis stage system. By analyzing the rotated image of a two-dimensional phase-encoded binary scale (2D), the absolute 2D position values at two separated points were obtained and the absolute X-Y-Θ position could be calculated combining these values. The sensor head was constructed using a board-level camera, a light-emitting diode light source, an imaging lens, and a cube beam-splitter.

Wavelength measurement by Fourier analysis of interference fringes through a plane parallel plate.

When a diverging laser beam passed through a plane parallel glass plate, interference fringes were observed; analysis of these fringes provided accurate estimation of the source wavelength. The fringes had a unique angular range of uniform fringe density. Fourier transform of the fringes in this range directly provided wavelength information.

Total physical thickness measurement of a multi-layered wafer using a spectral-domain interferometer with an optical comb.

An interferometric method using an optical comb is proposed and realized to measure the total physical thickness of a multi-layered wafer even if the refractive index of each layer is not given. For a feasibility test, two-layered and three-layered silicon-on-glass wafers were chosen as samples and were measured. An uncertainty evaluation was conducted to estimate the performance capabilities of the proposed method.

An interferometric system for measuring thickness of parallel glass plates without 2π ambiguity using phase analysis of quadrature Haidinger fringes.

An interferometric system is proposed for measuring the thickness of parallel glass plates by analyzing Haidinger fringes. Although a conventional Haidinger interferometer can measure thickness without 2π ambiguity using positions of peak and valley points in the interferogram, measurement accuracy is directly affected by the number of these points involved in the calculation. The proposed method obtains phase values over the entire interferogram by analyzing the quadrature Haidinger fringes generated by a current modulated laser diode.

Measurement of refractive index dispersion of a fused silica plate using Fabry-Perot interference.

We used Fabry-Perot interferometry to measure the refractive indices of a fused silica plate at four different wavelengths ranging from 544 to 1550 nm, giving a detailed analysis on the uncertainty of this experimental method. Because of a small expanded uncertainty of 2.7×10(k=1.

Vibration-insensitive measurements of the thickness profile of large glass panels.

We propose and realize a modified spectral-domain interferometer to measure the physical thickness profile and group refractive index distribution of a large glass substrate simultaneously. The optical layout was modified based on a Mach-Zehnder type interferometer, which was specially adopted to be insensitive to mechanical vibration. According to the measurement results of repeated experiments at a length of 820 mm along the horizontal axis, the standard deviations of the physical thickness and group refractive index were calculated to be 0.

Fizeau-type interferometric probe to measure geometrical thickness of silicon wafers.

We developed an optical interferometric probe for measuring the geometrical thickness and refractive index of silicon wafers based on a Fizeau-type spectral-domain interferometer, as realized by adopting the optical fiber components of a circulator and a sheet-type beam splitter. The proposed method enables us to achieve a much simpler optical composition and higher immunity to air fluctuations owing to the use of fiber components and a common-path configuration as compared to a bulk-type optical configuration. A femtosecond pulse laser having a spectral bandwidth of 80 nm at a center wavelength of 1.

Quadrature laser interferometer for in-line thickness measurement of glass panels using a current modulation technique.

A thickness measurement system is proposed for in-line inspection of thickness variation of flat glass panels. Multi-reflection on the surfaces of glass panel generates an interference signal whose phase is proportional to the thickness of the glass panel. For accurate and stable calculation of the phase value, we obtain quadrature interference signals using a current modulation technique.

Vibration-insensitive measurement of thickness variation of glass panels using double-slit interferometry.

A technique which can measure thickness variation of a moving glass plate in real-time with nanometric resolution is proposed. The technique is based on the double-slit interference of light. Owing to the nature of differential measurement scheme, the measurement system is immune to harsh environmental condition of a production line, and the measurement results are not affected by the swaying motion of the panel.

Note: Nonlinearity error compensated absolute planar position measurement using a two-dimensional phase-encoded binary grating.

This Note presents a new absolute planar position measurement method using a two-dimensional phase-encoded binary grating and a sub-division process where nonlinearity error is compensated inherently. Two orthogonally accumulated intensity profiles of the image of the binary grating are analyzed separately to obtain the absolute position values in each axis. The nonlinearity error caused by the non-ideal sinusoidal signals in the intensity profile is compensated by modifying the configuration of the absolute position binary code and shift-averaging the intensity profile.

An optical absolute position measurement method using a phase-encoded single track binary code.

We present a new absolute position measurement method using a single track binary code where an absolute position code is encoded by changing the phase of one binary state representation. It can be decoded efficiently using structural property of the binary code, and its sub-division is possible by detecting the relative positions of the binary state representation used for the absolute position encoding. Therefore, the absolute position encoding does not interfere with the sub-division process and so any pseudo-random sequence can be used as the absolute position code.

Uncertainty improvement of geometrical thickness and refractive index measurement of a silicon wafer using a femtosecond pulse laser.

We have proposed a modified method to improve the measurement uncertainty of the geometrical thickness and refractive index of a silicon wafer. Because measurement resolution based on Fourier domain analysis depends on the spectral bandwidth of a light source directly, a femtosecond pulse laser having the broad spectral bandwidth of about 100 nm was adopted as a new light source. A phase detection algorithm in Fourier domain was also modified to minimize the effect related to environmental disturbance.

Precision depth measurement of through silicon vias (TSVs) on 3D semiconductor packaging process.

We have proposed and demonstrated a novel method to measure depths of through silicon vias (TSVs) at high speed. TSVs are fine and deep holes fabricated in silicon wafers for 3D semiconductors; they are used for electrical connections between vertically stacked wafers. Because the high-aspect ratio hole of the TSV makes it difficult for light to reach the bottom surface, conventional optical methods using visible lights cannot determine the depth value.

Note: high precision angle generator using multiple ultrasonic motors and a self-calibratable encoder.

We present an angle generator with high resolution and accuracy, which uses multiple ultrasonic motors and a self-calibratable encoder. A cylindrical air bearing guides a rotational motion, and the ultrasonic motors achieve high resolution over the full circle range with a simple configuration. The self-calibratable encoder can compensate the scale error of a divided circle (signal period: 20") effectively by applying the equal-division-averaged method.

Note: High speed optical profiler based on a phase-shifting technique using frequency-scanning lasers.

We present a high speed optical profiler (HSOP) using frequency-scanning lasers for three-dimensional profile measurements of microscopic structures. To improve upon previous techniques for implementing the HSOP, we developed frequency-scanning lasers and a compact microscopic interferometer. The controller of the HSOP was also modified to generate proper phase-shifting steps.

Visibility enhanced interferometer based on an injection-locking technique for low reflective materials.

We propose and demonstrate a novel method to enhance the visibility of an optical interferometer when measuring low reflective materials. Because of scattering from a rough surface or its own low reflectivity, the visibility of the obtained interference signal is seriously deteriorated. By amplifying the weak light coming from the sample based on an injection-locking technique, the visibility can be enhanced.