A unique absolute length measurement method is proposed and demonstrated for the first time. Since it takes advantage of both the high-accuracy measurement capability of a pulse train interference method and the ability of a two-color method to compensate for environmental changes, the present method is expected to be useful for high-precision length measurement for not only the purposes of laboratory science but also for satisfying the requirements of industry. A length measurement was performed to demonstrate the feasibility of the proposed method.
We report a novel measurement method, referred to as the multiple pulse train interference (MPTI)-based method, to
measure the refractive index of air using a femtosecond optical frequency comb. The temperature change around the
region of interest is recorded based on the interference of multiple pulse trains, and this change is reflected as the
distance between MPTI fringes via the use of the conventional Fourier transformation method. The proposed method is
demonstrated using a sealed 600-mm cell containing a given volume of water. The results of this preliminary experiment
reveal the potential of the MPTI method towards obtaining a high-accuracy evaluation of the refractive index of air.
The fundamental importance of length measurement and traceability is clear. In July 2009, the national standard tool for
measuring length in Japan changed from an iodine-stabilized helium-neon (He-Ne) laser to a femtosecond optical
frequency comb (FOFC). Because of the great potential for a technological revolution in length measurement, FOFC
based length measurement has attracted much attention from physicists and engineers. This paper is intended to give a
description to the concept, the principle, and a demonstration of a new length measurement technique, called pulse
repetition interval-based Excess Fraction (PRIEF) method, which was developed for an arbitrary and absolute length
measurement that is directly linked to an FOFC. The basic idea of this new technique was inspired by the analogy
between the wavelength of a monochromatic laser source and the pulse repetition interval of an FOFC. Just as a
conventional Excess Fraction method can determine an arbitrary and absolute length of a gauge block based on the
wavelength of a monochromatic laser source, the same Excess Fraction method can range an arbitrary and absolute
length as a function of the pulse repetition interval of an FOFC. A demonstration of the proposed method is presented. A
literature review of pulse laser based length measurement is also performed. From the result of the preliminary
experiment and the literature review, it has been show the possibility that PRIEF method can be used for a high-accuracy
distant evaluation.
The interference measurement using the femtosecond optical frequency comb (FOFC) is in progress at present. We
analyzed the temporal coherence function (TCF) of an FOFC since which is the fundamental description of the
interference phenomenon. As a result, it has been understood that the same high coherence peak exists during the time
which is equal to the repetitions interval in the traveling direction of the FOFC. The theoretical derivation has been used
to model the TCF of an FOFC and shows good agreement with experimental measurements which is taken with a
combination of an ordinary Michelson interferometer and an unbalanced optical-path Michelson interferometer.
A super-heterodyne laser interferometer for sub-nanometer length measurement system is proposed. This interferometer
has a possibility to realize high resolution by using the self-zooming method and high accuracy by using external cavity
diode laser which is stabilized to femtosecond frequency comb(fs-comb) as an optical source. This length measurement
system is going to be applied for linear-encoder calibration system for national standards.
A low-coherence tandem interferometer with a single-mode optical fiber is developed for remote-measurement of length. The optical path difference provided in the first low-coherence interferometer is transmitted through the optical fiber to the second low-coherence interferometer. Low-coherence interference fringes are generated when the optical path difference in the second interferometer, which correspond to the length being measured, compensates that of the first interferometer. A gauge block of 50 mm long has been calibrated remotely through the single-mode optical fiber of 3 km length with a stnadard deviation of 0.12 μm.
High-accuracy long-distance is performed using a broad and stable femtosecond frequency comb. Cyclic-error, which has been the main source of inaccuracy in conventional measurements, is reduced more than tenfold, directly achieving high accuracies of 50 μm at 1-GHz frequency and 14 μm at 10-GHz frequency in a 240-m distance measurement using the phase measurement of intermode beats of a femtosecond frequency comb. Traceability of distance meters is discussed.
KEYWORDS: Interferometers, Calibration, Standards development, Temperature metrology, Analog electronics, Signal detection, Head, Data acquisition, Helium neon lasers, Light sources
NMIJ line standard interferometer has been modified for measurement of a linear scale module with analog output. The interferometer was developed for line standard calibration. The light source is a stabilized He-Ne laser. Before the modification, a line standard can be calibrated with an uncertainty of about 0.2 micrometer for the total length of 500 mm (k=2). After the modification, a linear scale module can be calibrated as well as a line standard. A linear scale is set in a support on a moving carriage. The displacement of the moving carriage is measured by the interferometer with a sampling frequency of 30 kHz to 300 kHz while the electronic output of the linear scale module is sampled with the same timing. The analog output of the line scale module is used instead of digital output because it is important to assure the simultaneous sampling of the displacement and the scale output.
A Fourier transform spectrometer with heterodyne modulation using a moving diffraction grating has been developed for the NIR region. The grating simultaneously acts as a beam splitter and a modulator, which realizes optical frequency shift of incident light for increasing the sensitivity of measurement by heterodyne modulation technique. The difference of diffraction angle is compensated by a collimating lens or a mirror and plane mirrors.
A new remote-measurement technique of length is developed, by using a low-coherence tandem interferometer and a single-mode optical fiber. The optical path length with a gauge block in the first low-coherence interferometer is transmitted through the optical fiber to the second interferometer. And then the interference fringes containing the length information are generated and detected with high accuracy.
A Novel setup for two-dimensional (2-D) parallel measurements of low-coherence heterodyne signal by using a tandem interferometer and a 2-D sensor has been developed. Generally speaking, it is difficult to obtain heterodyne signals whose frequencies are much higher than the frame rate of the coventional 2-D sensor. A mode-locked femtosecond laser (MLFL) is used as a light source.
The low-coherence interferometric technique is proposed for the in-situ measurement of the refractive index of dispersive samples with high accuracy. A tandem configuration of a Michelson interferometer and a triangular interferometer is used to compensate for the chirping effect which results from the broad spectrum of the light source. Thick samples can be successfully measured with the low- coherence interferometer, therefore the relative accuracy of the refractive index can be improved.
We have established four I2-stabilized Nd:YAG lasers to verify the frequency reproducibility of the lasers. The observed square root Allan variance of the four lasers was between 1 to approximately 4 X 10-14 depending on the obtained signal-to-noise ratio of the spectra, when the integration time is larger than 300 s. The observed frequency reproducibility of each laser was ranged from 9.1 X 10-14 approximately 1.5 X 10-13 (corresponding to frequency uncertainties of +/- 51 approximately 87 Hz). Frequency reproducibility of a group of lasers (four NRLM lasers) has been evaluated to be 8.2 X 10-13 (corresponding to a frequency uncertainty of +/- 640 Hz). One of the four NRLM lasers is a compact I2- stabilized Nd:YAG laser which is suitable to be transported to other laboratories for international frequency comparisons. Using this portable laser, we have accomplished frequency comparisons of Nd:YAG lasers between several metrological institutes in different countries. The absolute optical frequencies of the NRLM lasers were determined with an uncertainty of about 1.5 kHz by the frequency comparison between the NRLM and the JILA (formerly the Joint Institute for Laboratory of Astrophysics), Boulder, CO.
The use of an optical frequency comb generated by an ultrafast mode-locked laser has been realized as a promising method of the direct comparison between microwave and optical frequencies. We are currently investigating frequency control of a chirped-mirror-dispersion-controlled mode-locked Ti:Al2O3 laser. We stabilized the pulse repetition rate frep to a rf synthesizer locked to a cesium (Cs) clock to the Allan deviation of 4 X 10-12 in 1 s. We found that the position of the crystal, rotation of the chirped mirrors, and change of the pump-laser power can be used in controlling the carrier-envelope offset frequency fCEO. We extended the span of the comb to over one octave, i.e., from 530 nm to 1190 nm, at -20 dB using a photonic-crystal fiber made at the University of Bath. We are currently trying to measure the frequency of an iodine-stabilized Nd:YAG laser using a floating ruler of a f:2f frequency interval chain. We detected the self-referencing beat between the fundamental and second- harmonic frequencies of the comb, which will enable further precise comparison between microwave and optical frequencies through the control of the fCEO.
Heterodyne white-light interferometer that uses an optical grating to shift the optical frequency of the white light is
proposed. By moving the optical grating, the diffracted lights undergo the frequency shift. The dependency of diffraction
angle on wavelength can be resolved by using a spherical mirror, and then heterodyne detection of white-light can be
realized with the combination of the optical grating and the spherical mirrors. In practical uses, a tandem-configuration
interferometer is useful. The principle was demonstrated and the effect was confirmed experimentally for the first time
The surface profile of a step-like object was measured. The signal-to-noise ratio of heterodyne signal is increased by
thousand times from that of homodyne signal.
A novel low-coherence interferometer has been developed, based on the heterodyne technique for highly accurate and sensitive positioning of a three-dimensional (3-D) object, which uses two acousto-optic modulators (AOM's) and two spherical reflecting mirrors in a Michelson interferometer. By using this technique in a tandem interferometer, the profiles of diffusing and mirror-likes surfaces of the 3-D objects are measured with a high accuracy of 50 nm from the heterodyne signals of 200 kHz.
Optical Kerr gate with a function of light amplification and a femtosecond opening time in a special fast setup is applied to time-resolved imaging. Transient induced anisotropy created by a first pump pulse is canceled by the perpendicular-induced anisotropy which is created by a delayed pump. The transverse resolution of the image is better than 90 micrometers without degradation originated from optical Kerr effect or optical amplification process. Moreover, rather wide spectral band of the gate offers the possibility of spectroscopic imaging. 3D imaging of small- signal objects using the femtosecond amplifying optic al Kerr gate are demonstrated for several types of transmittance objects hidden behind light diffusers.
We have developed a laser interferometer to calibrate gauge block utilizing 2 lasers, 633 nm He-Ne and 1,064 nm Nd:YAG. At only 633 nm of wavelength, several kinds of practical He- Ne lasers have been developed and manufactured, while there is not practical laser light source for interferometer other than at 633 nm. To realize practical light source in addition to 633 nm, we stabilized the wavelength of 1,064 nm Nd:YAG laser, utilizing linear absorption of iodine at 532 nm. The iodine cell was inserted between two non linear crystals for second harmonic generation and first derivative signal was generated by dispersion effect near the absorption line of iodine, without electric circuit for modulation or for phase sensitive detection. Gauge blocks were measured with the two wavelengths of 633 and 1,064 nm. The fringe pattern was detected by CCD camera and the length was estimated by excess fraction method. It is easy to determine the integer part of the fringe order, since the wavelength difference between these two is large and the wavelength ratio is not expressed by a simple combination on integers. The results agreed with those obtained by conventional method with 198Hg isotope lamp.
The domestic round-robin testing for interferometric length measurements of gauge blocks was initiated in 1960 in Japan, to evaluate and also to improve the proficiency of calibration laboratories. It had been arranged by the National Research Laboratory of Metrology (NRLM) and Japan Precision Measuring Instrument Association 16 times prior to 1992. A 100 mm gauge block was used as the test object on each occasion and measurement result of the NRLM was used as the reference value. When the difference between the result of each laboratory and the NRLM was larger than 30 nm, the laboratory repeated the measurement and the reason for the discrepancy was investigated. About 10 laboratories, including those of local governments, private companies and semigovernment organizations, participated regularly and many of them maintained their proficiency in the measurement of gauge blocks.
A multi-point dynamic displacement probe was developed for measuring dynamic displacement and deformation. This method uses an acousto-optic deflector (AOD) as a diffractive scanner to allow the laser beam to scan a self-focusing microlens array (SMLA), and then uses the resulting focused spot to scan the diffuse surface. The focus information contained in the laser beam reflected from the diffuse surface is then transformed into the variations in shape of spot for detection. Due to the very high scanning speed of the AOD, the relative position of many points, and thus the dynamic deformation, can be measured in near real-time. This paper describes the method's principles, accuracy, and error sources.
A scanning beam collimation (SBC) method is developed for measuring dynamic angle variations in which an acousto-optic deflector is used as a diffractive scanner to compensate the angle variation in real time, and a position sensor is used to detect the position of the beam that is reflected from the reflector (i.e., object) to be measured. The angle variation can be measured at high speed due to the very high scanning speed available in the acousto-optic deflector. There was good agreement between the angle variation measured using the SBC method and that using a conventional electronic autocollimator. The repeatability, resolution, and linearity of the scanning beam collimation method were determined using a piezo translator to generate tilt angle; the results showing that high accuracy also can be achieved in dynamic angle variation measurement. The SBC method was applied to measure the tilt angle variation caused by the beating force of a dragonfly attached to one end of a gauge block. The results confirmed the effectiveness of this new method.
A new method has been developed to simultaneously measure the thickness and shape of a thin film, such as a dragonfly wing. The innovation in the method is the combining of a heterodyne interferometer and a laser triangulation displacement sensor into one optical system. We confirmed the accuracy of the method by measuring the displacement of a glass plate and the thickness variation generated by a rotated glass plate. The system has a relative accuracy of 1% in the shape measurement and 1.3% in the thickness variation measurement. The method was then applied to a dragonfly wing. The results indicated that the method is very effective in biomechanics studies, such as evaluating the flight performance of dragonflies. In such evaluations, it is essential to measure the high accuracy the variations in both shape and thickness of the wing simultaneously.
A high resolution distance meter has been made using optical wave-guide modulators. The modulation frequencies for the optical modulators are around 3.5 GHz, while the modulation frequencies of optical intensity are around 7 GHz because the modulation bias voltages are set to peak points of the electro-optic characteristic. The distance meter was evaluated in the NRLM tunnel. The measurement resolution is 15 micrometers of a standard deviation for a distance range of 0.5 m to 5 m after a correction of a cyclic error.
A new method for measuring vibrations using a quadrant position sensor has been developed. It allows noncontact measurement of natural frequencies of objects that are transparent and small or lightweight, such as dragonfly wings. It simultaneously measures natural frequencies in two dimensions without any reflective mark having to be placed on the object. The system consists of a laser light source, a quadrant position sensor, and a spectrum analyzer. The object is illuminated by divergent laser rays coming from a microscope objective lens. The vibration amplitudes of the object are magnified and detected by the position sensor. By adjusting the distance between the object and either the lens or the sensor, the sensitivity and linearity of the system can be changed to measure objects of different size. We measured the natural frequencies of a dragonfly wing along the chord and the span, using a vein as a mark to measure displacement during vibration. This allowed us to determine the natural frequencies of bending and torsional deformation. Our results show that this two-dimensional, noncontact method can be effectively applied to the field of millibioflight.
In a two-beam, white-light, or polychromatic-Iight interferometer, any path-length difference in the beamsplitter introduces wavelength-
dependent path delay because of the glass dispersion. A technique to align balanced path is described. The method is based on two- or three-wavelength simultaneous interferometry. Difference in intensity of fringes placed symmetrically around the zero interference order (ZIO) is used to measure and equalize the beamsplitter path (BP). As a polychromatic source of light. 660-, 840-, and 780-nm wavelength mixed beams of multimode laser diodes are applied. The technique allows us to balance the path difference with an accuracy better than ± 0.2 μm. Theoretical background and experimental verification of the method is presented.
A new method for measuring vibrations using a quadrant position sensor has been developed. It allows non-contact measurement of natural frequencies of objects which are small or light and transparent. The natural frequencies in two dimensions can be received at the same time. The system consists of a laser light source, a quadrant position sensor and spectrum analyzer. The object is illuminated by divergence laser rays coming from a microscope objective. The vibration amplitudes are magnified and detected by the position sensor. By adjusting the distance between object and microscope objective or detector, the sensitivity and linearity of the system can be changed. The natural frequencies of a plastic plate were measured by this method. The experimental results agreed well with the calculation results according to the theory of material mechanics. This method is very useful in the research field of millibioflight. The natural frequencies of the dragonfly wing, which is lighter than 0.1 mg, has been measured by this method. We can determine the natural frequencies of bending and torsional deformation by using the information of two dimensions.
In the evaluation of processed material such as metal, it is important to detect the geometrical surface of the material as well as inequalities in processing. In the case of optical evaluation, the penetration depth of light into the material is one of the most serious problems. This paper presents a method of optically evaluating the quality of processing, using ion beams, lasers, etc., with a resolution of nanometer order. The principle of the method is based on measurement of the difference between the phase changes of light reflected on the material for two colors. The optical system is composed of a two-color laser phase-locked interferometer.
This paper presents a two-color optical path-1ength-cdulated reversible fringe-counting interferometer system for iieasuring the change of the air refractive index. By measuring the changes of the optical path for infrared and visible light the interferoieter cancels the effect of the geoietrical distance change. This system is being tested over a 235 ii path in the tunnel at NRLM. 1.
The penetration depth of light into a processed surface is interferometrically ieasured by optically contacting it on a glass plate with a resolution of 3 urn. This principle is based on the ieasurement of the difference between the phase changes of light in reflections on the object and the glass plate. Also the depth is siiiultanecusly measured by an optical systeii with a phase-locked laser interferometer and an optical displacement sensor. 1 .
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