Real-time displacement measurement using a multicamera phase-stepping speckle interferometer

Surface point displacements can be measured by using standard phase-stepping speckle interferometry, but the measured data are vulnerable to disturbances during the interferogram recordings. To overcome this an interferometer with a computational system has been developed to record three phase-stepped interferograms simultaneously and to calculate displacements. This system is evaluated by measurements of the out-of-plane rotation of a flat surface. The surface displacement is calculated at a rate of 25 times/s. If one reduces noise by filtering, hardware limitations decrease the speed to 12.5 displacement calculations/s. With this system displacements can be measured with an accuracy exceeding λ/55 if filtering is applied.     

Remote displacement measurement using a passive interferometer with a fiber-optic link

Remote displacement measurement is demonstrated using a Fabry-Perot cavity with a multimode optical fiber link. The sensing cavity modulates, as a function of its length, the spectrum of a light-emitting diode (LED). The light returns via the fiber and is analyzed by a tunable reference cavity. A closed-loop control causes the reference cavity to track the sensing cavity length within 2 x 10(-12) m. Displacement range is 2 x 10(-6) m. The reference cavity length is measured interferometrically, using a laser, to obtain the sensing cavity length. Advantages of this sensing technique include compatibility with multimode fiber-optic components, high immunity to optical losses, and large dynamic range.     

Real-time displacement measurement with a two-wavelength sinusoidal phase-modulating laser diode interferometer

A two-wavelength interferometer that uses two separate modulating currents with different phases but the same frequencies to detect a greater degree of object displacement in real time is proposed and demonstrated. The measurement error was 57 nm, roughly 1/80 of the synthetic wavelength. We have confirmed that this modulating technique enables us to equip our prototype interferometer with a simple feedback-control system that eliminates external disturbance.     

Sinusoidal phase-modulating interferometer insensitive to intensity modulation of a laser diode for displacement measurement

In sinusoidal phase-modulating laser diode interferometers, an injection current of a laser diode is sinusoidally modulated to scan the laser wavelength. However, the modulation of the injection current also involves an intensity modulation of the light, which increases the measurement error if a conventional signal processing is used. A novel signal processing for displacement measurement is proposed to eliminate the influence of the intensity modulation. Numerical simulation results and experimental results make it clear that an optimal depth of the sinusoidal phase modulation exists that can reduce the measurement error to a few nanometers.     

Vibration compensating beam scanning interferometer for surface measurement

Light beam scanning using a dispersive element and wavelength tuning is coupled with fiber-optic interferometry to realize a new surface measurement instrument. The instrument is capable of measuring nanoscale surface structures and form deviations. It features active vibration compensation and a small optical probe size that may be placed remotely from the main apparatus. Active vibration compensation is provided by the multiplexing of two interferometers with near common paths. Closed loop control of a mirror mounted on a piezoelectric transducer is used to keep the path length stable. Experiments were carried out to deduce the effectiveness of the vibration compensation and the ability to carry out a real measurement in the face of large environmental disturbance.     

Mach-zehnder interferometer as a spectral analyzer for molecular Doppler wind lidar

The theoretical performance of a Mach-Zehnder interferometer used as a spectral analyzer for wind-speed measurement by direct-detection Doppler lidar is presented. The interferometer is optimized for measurement of wind velocity from the signal backscattered by the molecules. Two arrangements are proposed, involving two detection channels (DMZ) or four detection channels (QMZ). Using the assumption of a pure molecular signal with a Gaussian spectral profile, we derive an analytic expression for the standard deviation of the measurement error for each arrangement. They are then compared with the ideal spectral analyzer (ISA) and with the double-edge Fabry-Perot (DFP) in the case of a shot-noise-limited signal. The DMZ measurement error is shown to be only 1.65 times that of the ISA and is 1.4 times lower than that given by the DFP. The QMZ arrangement provides a measurement that is insensitive to the aerosol scattering contribution but gives a measurement error that is 1.4 times higher than that of the DMZ.     

Fringe-imaging Mach-Zehnder interferometer as a spectral analyzer for molecular Doppler wind lidar

The theoretical performance of a Mach-Zehnder interferometer used as a spectral analyzer for wind-speed measurement by direct-detection Doppler lidar is presented. The interferometer is optimized for the measurement of wind velocity from the signal that is backscattered by the molecules. In the proposed fringe-imaging Mach-Zehnder (FIMZ) interferometer, a pattern of equally spaced linear fringes is formed and detected by two conventional detector linear arrays. Assuming a pure molecular signal with Gaussian spectral profile, an analytic expression for the standard deviation of the measurement error is obtained and compared with the Cramer-Rao lower bound given by an ideal spectral analyzer (ISA) in the case of shot-noise-limited signal. The FIMZ measurement error is shown to be 2.3 times that of the ISA and is comparable with the error given by previously developed multichannel spectral analyzers that are based on Fabry-Perot interferometers that, in contrast, have the disadvantages of producing unequally spaced circular fringes and requiring dedicated detectors. The optimal path difference for a FIMZ operating at 355 nm is approximately 3 cm. The interferometer is shown to match important lidar beam étendues without significant performance reduction.     

Simultaneous measurement of the velocity and the displacement of the moving rough surface by a laser Doppler velocimeter

A formula that can analyze the laser Doppler velocimeter spectrum was derived. The formula showed good agreement with computational experimental results. The use of a Fourier transform lens as detection optics was proposed and was found to be independent of the displacement of the object along the direction parallel to the optical axis. An experimental method was also proposed that can make the simultaneous measurement of velocity and displacement possible simply by adding a second detector. By using two detectors, not only can displacement of the surface be measured, but also the results of the speed measurement can be compensated for.     

角位移Fabry-Perot干涉测量

提出了一种基于Fabry-Perot板干涉的角位移测量新方法。此方法采用函数近似,只需将初始入射角确定在40°到50°之间,即可由角位移与干涉信号条纹数变化间的函数关系,高精度测量角位移。解决了采用F-P板干涉法测量角位移需精确确定入射光初始角的问题。使用计算机处理采集的干涉信号,对干涉条纹进行细分,实现干涉信号相位测量的高分辨力。理论模拟和实验结果得出本方法可以实现精度为10-5rad数量级的角位移测量。该方法的测量装置采用带尾纤的半导体激光作为光源,由自聚焦透镜准直,出射光束直径为0.5mm,使探测头为小光斑。该装置结构简单,能实现小型化。     

Polarization interferometer for measuring small displacement

A homodyne polarization laser interferometer is presented for high-speed measurement of small displacements. No modulation technique is used, so the opto-mechanical setup is relatively simple. The dual-beam arrangement enables the displacement to be measured while the use of polarization interferometry enables the determination of the directional nature of the displacement. Another feature of this interferometer lies in the fact that it is also suitable for the measurement of head-media spacing of a hard disk drive. Combined with the electronics used at present, sub-nanometer resolution is achievable. Its measurement bandwidth is limited only by the sampling rate of the A/D board being used     

Stabilization improvements of laser-diode closed-loop heterodyne phase-shifting interferometer for surface profile measurement

To stabilize the phase-shifting Fizeau-type interferometer against environmental disturbances (namely, vibration and temperature variations), the feedback scheme that uses the current-induced frequency modulation of a laser diode (lambda = 633 nm) and the two-frequency optical heterodyne method has been investigated, with particular attention to improvement of the achievable stabilization. It is demonstrated that introduction of the proportional-integral control into the feedback system improves stabilization against the proportional control case; e.g., stabilization is improved 5 times for 100-nm(p-p) vibration at the frequency range at 30 Hz. The surface profile measurement for a sample mirror was conducted with a reproducibility of 6.8 nm in the root mean square under the subwavelength-amplitude vibration at 100 Hz.     

远距离高精度多普勒位移测量

通过泛函、激光散斑理论和随机过程各态遍历的研究,导出激光多普勒信号强度与聚焦光斑直径、接收透镜通光口径、光电接收器响应等参数间的关系,在此基础上设计出一种适用于远距离处面内位移测量的光路。此光路将高斯光束束腰聚焦在被测体上,实现最小聚焦光斑和平面波叠加,此外采用大口径透镜接收散射光,用响应度高的光电接收器转换光电信号等措施获得高强度高信噪比的测量信号。将此光路用于 100m 处面内位移(49.70mm)测量,其精度可达 2%。此设计方法能用于振动或地震波的高精度检测。     

Beam splitter for a wide-angle Michelson Doppler imaging interferometer

The design and construction of a hexagonal interferometer beam splitting prism is described in which the light falls onto the all-dielectric partially reflecting coating at an angle of 300 with respect to the normal. For a beam that undergoes one transmission and one reflection by the device, the average effective transmittance 1/2 (T(parallel)R(parallel) + T(perpendicular)R(perpendicular)) in the 0.55-0.75-microm spectral region is approximately 0.23. This quantity is not strongly dependent on the plane of polarization of the incident light, nor on small changes of the angle of incidence from the design angle of 30 degrees. Ways to improve further the performance of the beam splitter are indicated. Coating designs for other angles of incidence are also given.     

Holographic common-path interferometer for angular displacement measurements with spatial phase stepping and extended measurement range

A novel technique for extending the unambiguous measurement range for differential measurements of angular deflections is presented. The technique utilizes a common-path interferometer that simultaneously probes the out-of-plane displacement of three points on the object surface. The system is based on a single laser diode, and all the optical functions of the system are implemented in a dedicated holographic optical element (HOE). The HOE automatically provides spatially phase-stepped interference signals for real-time phase measurement. It is therefore not necessary to employ any polarizing optics or active elements to introduce the phase stepping. The common-path scheme combined with the HOE provides a system that is inherently stable, since the HOE operates as both transmitter and receiver in the system. The system is compact, is robust, and has the potential for being mass-produced at a low cost and is thus well suited for industrial use, such as in commercial vibrometers. The technique is demonstrated in a system for measuring angular deflections of a plane mirror. The technique, however, is not restricted to this use alone and can easily be configured to probe other types of surface displacements, e.g., the deflection of a diaphragm. In the present configuration, the system can measure angular deflections with a sensitivity of 2.5 x 10(-7) rad over a measurement range that is approximately 3.5 x 10(-3) rad, i.e., a dynamic range of approximately 1:14,000. Furthermore, the system can easily be reconfigured for a desired angular sensitivity and measurement range.     

All-sky Doppler interferometer for thermospheric dynamics studies

An efficient, all-sky input optical system has been mated to a 100-mm-aperture Fabry-Perot interferometer that employs a cooled (-150 °C) CCD as a photon detector to create an all-sky Doppler interferometer. The instrument is capable of simultaneously measuring Doppler shifts and widths of nightglow emission lines from many different points in the sky, thereby providing determinations of upperatmosphere neutral wind and temperature fields over a large region (to ≈2000 km in extent). For OI 630-nm (thermosphere) and OH 799.6-nm (mesopause) nightglow emissions, exposure times of 5-15 min provide good-quality interferometric images. The capability of the all-sky Doppler interferometer is illustrated by examples of thermospheric wind and temperature fields measured over Millstone Hill, Massachusetts.     

Assessment of a multibeam Fizeau wedge interferometer for Doppler wind lidar

The Fabry-Perot interferometer is the standard instrument for the direct detection Doppler lidar measurement of atmospheric wind speeds. The multibeam Fizeau wedge has some practical advantages over the Fabry-Perot, such as the linear fringe pattern, and is evaluated for this application. The optimal Fizeau must have a resolving power of 10(6) or more. As the multibeam Fizeau wedge is pushed to such high resolving power, the interference fringes of the device become complicated by asymmetry and secondary maxima. A simple condition for the interferometer plate reflectance, optical gap, and wedge angle reveals whether a set of parameters will yield simple, Airy-like fringes or complex Fizeau fringes. Tilting of the Fizeau wedge improves the fringe shape and permits an extension of the regime of Airy-like fringes to higher resolving power. Sufficient resolving power for the wind lidar application is shown to be possible with a large-gap, low-finesse multibeam Fizeau wedge. Liabilities of the multibeam Fizeau wedge in the wind lidar application include a smaller acceptance solid angle and calibration sensitivity to localized deviations of the plates from the ideal.     

Vibration-compensated interferometer for surface metrology

An advanced interferometer was built for surface metrology in environments with severe vibration. This instrument uses active control to compensate for effects of vibration to allow surface measurement with high-resolution phase-shifting interferometry. A digital signal processor and high-speed phase control from an electro-optic modulator allows phase measurements at 4000 Hz. These measurements are fed back into a real-time servo in the digital signal processor that provides a vibration-corrected phase ramp for the surface measurements taken at video rates. Unlike fringe locking, which compensates vibration to keep the phase constant, we show a true phase servo that allows the phase to be stabilized while it is ramped, enabling surface measurements using phase-shifting interferometry that requires multiple images with controlled phase shifts.