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. The wavelength-tunable high-repetition-rate mode-locked laser covering the full C-band is expected to be a compelling source for many frequency-comb-based applications, including high-precision optical metrology, broadband absorption spectroscopy, and broadband optical frequency synthesizers.

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. The original achievements of this work are to choose parameters of the ANN reasonably and to evaluate the training instead of a simple comparison with conventional methods. To do this, ANNs were built in 12 different cases, and then trained using theoretical spectra. The experimental spectra of the certified reference materials (CRMs) used here served as the validation data of each trained ANN, with the output then compared with a certified value. When both values agree with each other within an expanded uncertainty of the CRMs, the ANN is considered to be reliable. We expect that the proposed method can be useful for evaluating the reliability of ANN in the future.

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 1565.6 nm. The linear-type configuration makes the proposed laser simpler and more compact, allowing it to achieve the highest repetition rate of 126.5 MHz among C-band wavelength-tunable mode-locked lasers due to its short cavity length. Also, its polarization-maintaining fiber components provide reliable operating robustness. The significant wavelength tunability and high repetition rate of the proposed laser can be expected to make it an attractive resource for various applications, including optical communications, broadband spectroscopic LIDAR, and high-precision ranging.

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. The ability to undertake simultaneous thickness measurements of both layers using the proposed system was successfully verified on a specimen consisting of silicon dioxide thin film with nominal thicknesses of 100 nm and 150 nm and a 450 µm-thick silicon substrate, resulting in the exact separation between the two layers. From measurement uncertainty evaluation of a thin-film, a substrate in a thin-film specimen, and a single substrate, the uncertainties were estimated to be 0.12 nm for the thin-film, 0.094 µm for the substrate in a thin-film specimen, and 0.076 µm for the substrate. The measurement performance of thicknesses distributed on multi-scale was verified through comparative measurements using standard measurement equipment for several reference samples.

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. In this paper, sub-100 nm precision distance measurements based on an amplitude-modulated continuous-wave laser (AMCW) with an all-fiber photonic microwave mixing technique is proposed and realized potentially to satisfy SWaP-C requirements. Target distances of 0.879 m and 8.198 m were measured by detecting the phase delay of 15 GHz modulation frequencies. According to our measurement results, the repeatability could reach 43 nm at an average time of 1 s, a result not previously achieved by conventional AMCW laser distance measurement methods. Moreover, the performance by the proposed method in terms of Allan deviation is competitive with most frequency-comb-based absolute distance measurement methods, even with a simple configuration. Because the proposed method has a simple configuration such that it can be easily utilized and demonstrated on a chip-scale platform using CMOS-compatible silicon photonics, it is expected to herald new possibilities, leading to the practical realization of a fully integrated chip-scale LIDAR system.

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. In this paper, a measurement method for determining the thickness profile of a large glass substrate regardless of precise measurement of temperature distribution and the mathematical model of the refractive index was demonstrated. The widely used glass substrates with nominal thicknesses of 0.6 mm and 1.3 mm were measured at room and high temperatures. Through comparison of thickness profiles of hot glass substrates having large temperature gradients and those estimated through thermal expansion of thickness profiles measured at room temperature, it was confirmed that the proposed method can provide highly reliable thickness measurement results under such challenging conditions, unlike simple calculation from the optical thickness using the well-known 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, 0.2-mm-thick N-BK7 glass was used as a sample, with physical thickness and group refractive index measurements conducted according to three different correction glass elements with corresponding nominal thicknesses of 3.0 mm, 3.5 mm, and 4.0 mm. Through uncertainty evaluations according to the correction glass used, the physical thicknesses of the sample were found to be in good agreement within measurement uncertainties of less than 100 nm, results comparable to those of previous works which did not use any correction glass.

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. By averaging the incident angle values obtained from each peak position, the angle sensor achieved higher resolution and a smaller periodic nonlinearity error. The performance of the DS angle sensor was tested by comparison with a commercial autocollimator. The Allan deviation of the readout of the angle sensor was 0.0002 in. with the averaging time of 4 s, and the periodic nonlinearity error was evaluated to be less than 0.01 in.

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. The optical microscope was utilized for coarsely monitoring the specimen in a large field of view, and the other methods of interferometry and confocal microscopy were used to measure dimensions of small features with high speed by eliminating time-consuming process of the vertical scanning. Owing to the importance of the reliability, the uncertainty evaluation of the proposed method was fulfilled, which offers a practical example for estimating the performance of inspection machines operating with numerous principles at semiconductor manufacturing sites. According to the measurement results, the mean values of the diameter and depth were 40.420 µm and 5.954 µm with the expanded uncertainty of 0.050 µm (k = 2) and 0.208 µm (k = 2), respectively.

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. To obtain the uniform intensity profiles from the vignette scale image, we selected the averaging directions deliberately, and higher resolution in the angle measurement could be achieved by increasing the allowable offset size. The performance of a prototype sensor was evaluated in respect of resolution, nonlinearity, and repeatability. The sensor could resolve 25 nm linear and 0.001° angular displacements clearly, and the standard deviations were less than 18 nm when 2D grid positions were measured repeatedly.

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. Reference lasers were used to establish a calibration between the fringe density and wavenumber, with which we estimated the wavelength of a test laser. An accuracy of 4.5×10-5 was obtained, which is better than that of conventional grating spectrometers, while providing a much broader free spectral range. Our method has unique features, such as extreme simplicity of the setup, fast analysis, and low cost, which are great advantages in practical wavelength meter applications.

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. To verify the measured values, the wafers were also measured by a contact-type standard instrument. For the three-layered wafer, the total physical thickness distribution was determined in a selected area.

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. Therefore, it can achieve high speed measurement and nanometric resolution without mechanical rotation and thickness limitation of specimens. In the experiments, the standard deviation of repeated thickness measurement was evaluated as less than 0.3 nm, and the measured thickness profile of the proposed system agreed with that of a conventional thickness interferometer within ±15 nm. We also discussed the required accuracy of refractive index of specimens to implement the proposed method successfully and presented an exemplary measurement result of a multi-layer coated sample having a discontinuous thickness profile.

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 2.7×10-5(k=1.96) obtained using the experimental method, it was possible to make corrections to the existing Sellmeier formula [J. Opt. Soc. Am.55, 1205 (1965)JOSAAH0030-394110.1364/JOSA.55.001205] for our fused silica sample. The corrected Sellmeier formula resulted in a group index value larger than that evaluated using the Malitson's Sellmeier formula by 3×10-4. We verified this by comparing it with the group index measured with spectral domain interferometry at 1530 nm.

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 0.173 µm and 3.4 × 10(-4), respectively. To verify the insensitivity to vibration, the physical thickness values were monitored at a stationary point while the glass panel was swung at an amplitude exceeding 20 mm. The uncertainty components were evaluated, and the combined measurement uncertainty became 161 nm (k = 1) for a glass panel with a nominal thickness of 0.7 mm.

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 1.55 µm and an optical spectrum analyzer having a wavelength uncertainty of 0.02 nm were used to acquire multiple interference signals in the frequency domain without a mechanical phase-shifting process. Among the many peaks in the Fourier-transformed signals of the measured interferograms, only three interference signals representing three different optical path differences were selected to extract both the geometrical thickness and group refractive index of a silicon wafer simultaneously. A single point on a double-sided polished silicon wafer was measured 90 times repetitively every two seconds. The geometrical thickness and group refractive index were found to be 476.89 µm and 3.6084, respectively. The measured thickness is in good agreement with that of a contact type method within the expanded uncertainty of contact-type instruments. Through an uncertainty evaluation of the proposed method, the expanded uncertainty of the geometrical thickness was estimated to be 0.12 µm (k = 2).

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. The proposed system can measure a thickness profile with high speed and nanometric resolution, and obtain higher accuracy through real-time nonlinear error compensation. The thickness profile, measured by a transmissive-type experimental setup, coincided with a comparative result obtained using a contact-type thickness measurement system within the range of ±40 nm. The standard deviations of the measured thickness profiles and their waviness components were less than 3 nm with a scanning speed of 300 mm/s.

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. With the preliminary experimental setup with scanning speed of 100 mm/s, the measurement repeatability was 3 nm for the waviness component of the thickness profile, filtered with a Gaussian filter with cutoff wavelength of 8 mm.

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. Using an experimental setup, we measured a circular trajectory of 100 nm radius, and compared the measurement result with that of a laser interferometer. Applying the proposed compensation method, the nonlinearity error was reduced to less than 15 nm.

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. Because the proposed method does not require additional sensing part for the sub-division, it can be realized with a simple configuration and efficient data processing. To verify and evaluate the proposed method, an absolute position measurement system was setup using a binary code scale, a microscopic imaging system, and a CCD camera. In the comparison results with a laser interferometer, the measurement system shows the resolution of less than 50 nm and the nonlinearity error of less than ±60 nm after compensation.