Reference Mirror Research Articles

Preliminary measurement performance evaluation of a new white light interferometer for cylindrical surfaces

This paper introduces a new design of a white light interferometer, suitable for measurement of cylindrical or quasi-cylindrical parts. A high precision 45° conical mirror is used to direct collimated light radially, making it possible to measure in true cylindrical coordinates. The image of the measurand, distorted by the conical mirror, is projected in a high resolution digital camera. A mapping algorithm is used to reconstruct the cylindrical geometry from the distorted image. The rest of the interferometer is quite similar to a conventional white light interferometer: A flat reference mirror is scanned through the measurement range while an algorithm is searching for the maximum contrast position of the interference pattern. The performance evaluation of a configuration suitable for measurement of external cylindrical surfaces is also presented in this paper. A master cylinder was used as reference. Uncertainties of about 1.0 µm were found at the present stage of development.

Improved phase modulation for an en-face scanning three-dimensional optical coherence microscope

We have previously described an inexpensive method for modulating the interferometer of an en-face scanning, focus-tracking, three-dimensional optical coherence microscope (OCM). In this OCM design, a reference mirror is mounted on a piezoelectric stack driven at a resonance frequency of about 100 kHz. We perform a partial discrete Fourier transform of the digitally sampled output fringe signal. In the original design, we obtained the amplitude of the backscattered light by summing the powers in the fundamental (1ω) and first harmonic (2ω) of the modulation frequency. We used the particular piezoamplitude that eliminates the effects of interferometer phase drift. However, as the reference mirror was stepped to scan at different sample depths, variations in the back-coupled reference power added noise to the fringe signal at the fundamental piezodriving frequency. We report here a technique to eliminate the effects of this piezowobble by using instead the sum of the 2ω and 3ω powers as a measure of the backscattered light intensity. Images acquired before and after this improvement are presented to illustrate the enhancement to image quality deep within the sample.

Head-Disk Spacing Measurement Using Michelson Laser Interferometry as Observed Through Glass Disk

A spacing measurement using Michelson laser interferometry was developed for measuring the head-disk spacing in a hard disk drive. This method utilizes the phase comparison of two kinds of interference fringe patterns formed respectively over the inner surface of a glass disk and the air bearing surface of a slider through the glass disk. To increase resolution and reproducibility, four principal techniques were newly introduced: an optical system on which a high-definition CCD (charge-coupled device) camera was mounted, external identification of the fringe interval by driving the reference mirror, a wobbling technique to freeze ridgelines at a prescribed position when capturing the fringe image, and a powerful method for extracting ridgelines from the fringe pattern. In this method, the phase shift generated by light reflection on the slider surface incurring multiple beam interference effects must be compensated. For this purpose, first the phase shift was quantified by simulating the multiple beam interferences over a stratified medium inclusive of the lubricating air film, the DLC (diamond-like-carbon) protective layer, the silicon underlayer and the substrate. Then, the shift was subtracted from the optical spacing to obtain the mechanical spacing. Finally, the repeatedly measured mechanical spacings were compared with the theoretically calculated spacings, showing a good accordance between them.

Measurement of a fiber-end surface profile by use of phase-shifting laser interferometry.

We describe a laser interferometric system in which two objectives are used to measure surface profile on a connectorized fiber-end surface. By the use of the proposed illumination design a He-Ne laser as a point light source is transformed to an extended light source, which is beneficial to localize interference fringe pattern near the test surface. To obtain an optimal contrast of the interference fringe pattern, the flat mirror with an adjustable reflection ratio is used to suit different test surfaces. A piezoelectric transducer attached on the reference mirror can move precisely along the optical axis of the objective and permits implementation of four-step phase-shifting interferometry without changing the relative position between the CCD sensor and the test surface. Therefore, an absolutely constant optical magnification can be accurately kept to capture the interference fringe patterns resulting from a combination of light reflected from both the reference flat mirror and the test surface. The experimental result shows that surface profile on a fiber-end with surface features such as a small fiber diameter of 125 microm and a low reflection ratio of less than 4% are measurable. Measurements on a standard calibration ball show that the accuracy of the proposed setup is comparable with that of existing white-light interferometers and stylus profilometers.

Dimensional Measurement at a Point on the Diffusing Surface by Using Speckle Pattern Interferometry

A collimated light beam from a He-Ne Laser, is transformed into two beams by using a concave lens and a beam-splitter cube, in a Twyman-Green-type interferometer. When a diffusing surface is placed at the focal point of one of the two beams, a diverging spherical wave front containing large-sized speckle patterns is scattered. If a smooth surface reference mirror is placed in the vicinity of the focal point of the other beam, the reflected beams have essentially the same shaped wave front. The two beams can be combined to form concentric interference fringes, which are fairly regular. By observing the concentric fringes flowing out or sinking in, it is shown that the normal component of the displacement of the diffused surface can be measured with an accuracy of a fraction of a half wavelength.

Low-frequency seismic spectrum measured by a laser interferometer combined with alow-frequency folded pendulum

A laser interferometer combined with a reference mirror suspended on a long-period foldedpendulum is presented to measure the low-frequency horizontal seismic spectrum. One ofthe reflecting mirrors is driven by a piezoelectric transducer to track anothermirror. The experimental results show that the horizontal seismic motion is about10−9 m Hz−1/2 at 1 Hz in our laboratory.

The LISA accelerometer

In the frame of investigating the fundamental nature of gravity, the Laser Interferometer Space Antenna (LISA) mission could open the way to a new kind of observations unreachable from ground. The experiment, based on a V-formation of six drag-free spacecraft, uses the cubic proof masses of inertial sensors to reflect the laser light, acting as reference mirrors of a 5 × 10 9 m arm length interferometer. The proof masses are also used as inertial references for the drag-free control of the spacecraft which constitute in return a shield against external forces. Derived from space electrostatic accelerometers developed at ONERA, such as GRADIO for the ESA ARISTOTELES and now GOCE mission (Bernard and Touboul, 1991), the proposed LISA sensor should shield its proof mass from any accelerometric disturbance at a level of 10 −15 ms −2 Hz − 1 2 . The accurate capacitive sensing of the mass provides its position relative to the satellite with a resolution better than 10 −9 m Hz − 1 2 in order to control the satellite orbit and to minimise the disturbances induced by the satellite self gravity or by the proof mass charge. The sensor configuration and accomodation has to be specifically optimised for the mission requirements. Fortunately, the sensor will benefit from the thermal stability of the LISA optical bench environment, i.e. 10 −6 K Hz − 1 2 , and of the selected materials that exhibit a very low coefficient of thermal expansion (CTE), ensuring a high geometrical stability. Apart from the modeling and the evaluation of the flight characteristics, the necessary indirect ground demonstration of the performance and the interfaces with the drag-free control will have to be considered in detail in the future.

High precision (1 part in 104) reflectivity measurement for the study of reflective materials used in solar collectors

Abstract A new method of reflectivity measurement for the study of reflective materials in solar energy technology is reported in this paper. The method employs a fast rotating reference mirror and certain geometry configuration to alternatively deliver the light via a sample mirror or by passing it to a photo detector. It has been proved to reach a precision of 10−4. With a good repeatability of the method, outdoor exposure tests of sample mirrors have been carried out in a relatively short period (1 month) and significant specular reflectance losses due to an aging process have been observed.

Normal incidence refractometer

A circularly polarized heterodyne light beam enters a modified Twyman-Green interferometer. An analyzer is used to extract the interference components of two interfering beams reflected normally from the test medium and a reference mirror in the interferometer. The phase of the interference signal is a function of the refractive index of the test medium and the azimuth angle of the analyzer. It can be measured with heterodyne interferometry. Then it is substituted into the specially derived equations from which the refractive index of the test medium is estimated. This method has many merits such as no measurement limitation, a simple optical setup, and easy operation in real-time manner.

Spectral interference Mirau microscope with an acousto-optic tunable filter for three-dimensional surface profilometry.

A nonmechanical scanning Mirau-type spectral interference microscope has been developed for the measurement of three-dimensional surface profiles of discontinuous objects. An acousto-optic tunable filter (AOTF) is used as a high-resolution spectral filter, which scans the optical frequency of the broadband light emitted from a superluminescent diode. To generate spectral fringes that make full use of the limited coherence length of the filtered light we unbalanced the Mirau interferometric system by positioning the reference mirror nearly halfway between the top and the bottom of the step height. When the frequency of the broadband light source is scanned by an AOTF, the interference fringes move in opposite directions on the top and the bottom of the object. To uniquely determine the sign of the fringe movement over the large area of the object, we developed a three-dimensional Fourier-transform technique, and from the detected sign of the fringe movement and phase information, we determined the three-dimensional step height. Experimental results of the measurement of 100-microm step height are presented. The main advantages of the proposed system are that it provides nonmechanical scanning and a large measurement range without ambiguity in the sign of the phase.

Quantum locking of mirrors in interferometers.

We show that quantum noise in very sensitive interferometric measurements such as gravitational-wave detectors can be drastically modified by quantum feedback. We present a new scheme based on active control to lock the motion of a mirror to a reference mirror at the quantum level. This simple technique allows one to reduce quantum effects of radiation pressure and to greatly enhance the sensitivity of the detection.

Optical Coherence Tomography用合成光源の基礎研究

We have studied on the light intensity ratio of synthesized light source (SLS) for the improvement of spatial resolution of optical coherence tomography (OCT). SLS consists of two LEDs with wavelength 691 nm, spectral width 99 nm and 882 nm, 76 nm. The small imaging interferometer also consists of plan-convex lens of focal length 6 mm and diameter 6 mm, beam splitter and reference mirror with PZT. The axial resolution was measured at 1.2 μm with the side lobe intensity 42 % on the condition of intensity ratio of 1:0.5. The irradiated power was 18 μW. This axial resolution was 57 % compared to the axial resolution using a single LED. The image of test pattern was measured using the phase shift method and two lines with the interval of 9.8 μm were clearly measured. The lateral resolution was calculated at 1.1 μm from wavelength and numerical aperture.

Novel Head-Disk Spacing Measurement Using Michelson Laser Interferometry through Glass Disk (Improvement in Accuracy by Considering Multiple Interferences over Stratified Slider Surface)

A novel method using the Michelson laser interferometry was developed for measuring the head-disk spacing in a hard disk drive. This method utilizes the phase comparison of two kinds of interference fringe patterns formed respectively over the inner surface of a glass disk and the air bearing surface of a slider through the glass disk. To increase the accuracy in identifying fringe parameters, the fringe pattern was equivalently magnified by only one set of the fringes was selected to be formed on CCD, and the fringe interval was determined from the displacement of a high precision piezo actuator attached to the reference mirror. In this method, a phase shift generated by light reflection on the slider surface incurring multiple interference effects must be compensated. For this purpose, first the phase shift was evaluated by considering the multiple beam interferences over a stratified medium inclusive of the glass disk, the lubricating air film, the DLC protective layer, the silicon under layer and the substrate. Then, the shift was subtracted from the optical spacing to obtain the mechanical spacing. Finally, the repeatedly measured mechanical spacings were compared with the theoretically calculated spacings, showing a good accordance between them and a good reproducibility of less than a standard deviation of 1 nm.

鉄鋼生産でのオンライン計測用赤外分光装置の開発

A new intrared spectrometer system for the steel sheet production was developed for the on-line measurement on the amount of the oxide films formed on the surface of 3% silicon iron alloy. Several characteristics which should be demanded in order to prepare the equipment for the on-line system were uniquely designed. A simple reflectivity correction procedure using the light polarization was de-veloped for GAR (Grazing Angle Reflectance) [RAS (Reflection Absorption Spectroscopy) in other words] measurement of a steel sheet to avoid the alternation of a reference mirror and samples. In ord-er to make the spectrometer robust against the vibrations and the mechanical shocks in the steel facto-ries, eight infrared band-pass filters, which are designed to reproduce the complex spectrum spreading from 800 to 1300 cm-1, were employed to eliminate fragile precise mechanism. Consequently the spectrometer newly developed here succeeded in performing a quantitative on-line measurement of the oxide film components in the actual steel production for the first time in the world.

Full-field optical coherence tomography with thermal light.

A simple optical coherence tomography system has been developed based on a white-light Linnik interferometric microscope with its reference mirror mounted on a piezoelectric translator. The geometrical extension of the optics allows efficient illumination of this device with a low-power (3-W) light bulb, yielding full-field interferometric images at 50 Hz with a fast CCD camera. Owing to the very broad spectral width of the light source and of the camera response, we achieved axial resolutions equal to 1.1 microm in free space and 0.7 microm through a standard microscope cover plate. Tomographic images of an epithelial cell smear and of an hematological sample are shown.

Preparation and characterization of a reference aluminum mirror.

The preparation and characterization of a reference mirror of protected aluminum (Al) is reported. The mirror is made of 50-60-nm-thick Al film, coated with several-nanometer-thick A12O5 and 30-nm-thick film of AIN. The mirror characterization is based on reliable and precise reflectance measurements relative to a silicon- (Si-) wafer reference mirror. The simple phenomenological Drude-Lorentz model is applied for modeling the dispersion relations n(lambda) and k(lambda) of the Al film. The reflection of the protected Al mirror is determined in the 400-800-nm spectral range with accuracy better than 0.01 for p- and s-polarized light at angles of incidence from 0 degrees to 70 degrees. The accuracy has been confirmed with an evaporated thin silver film with known n(lambda), k(lambda), and d derived by photometric measurements at normal light incidence.

Sinusoidal-wavelength-scanning interferometer with double feedback control for real-time distance measurement.

In addition to a conventional phase a the interference signal of a sinusoidal-wavelength-scanning interferometer has a phase-modulation amplitude Zb that is proportional to the optical path difference L and amplitude b of the wavelength scan. L and b are controlled by a double feedback system so that the phase alpha and the amplitude Zb are kept at 3pi/2 and pi, respectively. The voltage applied to a device that displaces a reference mirror to change the optical path difference becomes a ruler with scales smaller than a wavelength. Voltage applied to a device that determines the amplitude of the wavelength scan becomes a ruler marking every wavelength. These two rulers enable one to measure an absolute distance longer than a wavelength in real time.

Determination of a micromirror angular rotation using laser interferometric method

This paper presents an optical method for the measurement of the angular rotation of a micromirror driven by an electrostatic force. To detect the rotation of the micromirror, a He–Ne laser probe with a collimated light beam of 30 μm in diameter is directed onto the micromirror with an achromatic lens. The laser beam reflected off the micromirror falls on the achromatic lens and is combined with another laser beam from a reference mirror. The resulting interference fringe pattern is recorded and the angular rotation of the micromirror is determined from the change in the pitch of the fringe pattern. Experimental results obtained demonstrate the feasibility of the proposed method to be used for evaluating the angular rotation of a micromirror.

Phase modulation at 125 kHz in a Michelson interferometer using an inexpensive piezoelectric stack driven at resonance

Fast phase modulation has been achieved in a Michelson interferometer by attaching a lightweight reference mirror to a piezoelectric stack and driving the stack at a resonance frequency of about 125 kHz. The electrical behavior of the piezo stack and the mechanical properties of the piezo-mirror arrangement are described. A displacement amplitude at resonance of about 350 nm was achieved using a standard function generator. Phase drift in the interferometer and piezo wobble were readily circumvented. This approach to phase modulation is less expensive by a factor of roughly 50 than one based on an electro-optic effect.

Optical-feedback semiconductor laser Michelson interferometer for displacement measurements with directional discrimination

An optical-feedback semiconductor laser Michelson interferometer (OSMI) is presented for measuring microscopic linear displacements without ambiguity in the direction of motion. The two waves from the interferometer arms, one from the reference mirror and the other from the reflecting moving target, are fed back into the lasing medium (lambda = 830 nm), causing variations in the laser output power. We model the OSMI into an equivalent Fabry-Perot resonator and derive the dependence of the output power (and the junction voltage) on the path difference between the two interferometer arms. Numerical and experimental results consistently show that the laser output power varies periodically (period, lambda/2) with path difference. The output power variation exhibits an asymmetric behavior with the direction of motion, which is used to measure, at subwavelength resolution, the displacement vector (both amplitude and direction) of the moving sample. Two samples are considered in the experiments: (i) a piezoelectric transducer and (ii) an audio speaker.