Interferometric thickness measurement of glass plate by phase-shifting analysis using wavelength scanning with elimination of bias phase error

Optical Review - Phase-shifting fringe analysis using wavelength scanning has been broadly applied to the interferometric profiling of the thickness variation of glass plates. However, the...     

Simultaneous measurement of several near-parallel surfaces with wavelength-shifting interferometry and a tunable phase-shifting method

Wavelength-shifting interferometry can distinguish in frequency space interference signals from different surfaces, and therefore allows the measurement of optical thickness variation between several quasi-parallel surfaces of a composite transparent object. Frequency analysis of the signal spectrum with a tunable phase-shifting formula can then detect the phase of the individual signals. We have devised a tunable phase-shifting method which uses a freely adjustable number of intensity samples and can be adapted to any frequency spectrum. To extract the signal reliably, two properties of the phase-shifting method are particularly important: it should suppress cross-talk from unwanted frequencies, and it should allow for some variation in the signal frequency. We show that a carefully designed sampling function envelope will combine these benefits, and demonstrate the technique in measurements of three different composite objects each consisting of three reflecting surfaces. The importance of phase-shift linearisation is discussed, and methods for selecting optimal set-up parameters are given.     

Simultaneous wavelength tuning and amplitude modulation in QPM optical parametric amplifiers

We repot simultaneous optical parametric amplifiers (OPA) and amplitude modulation of signal pulse in a monolithic periodically poled KTP (PPKTP) by electrically controlling the relative phase between the pump, signal and idler waves in a KTP dispersion section sandwiched between two PPKTP gratings, theoretically. The controlling electric field can be arbitrary direction. For an ultrashort pulse group velocity mismatch, group dispersion and diffraction must be regarded, simultaneously. The solutions of the truncated equations, including the above effects in the two PPKTP gratings of simultaneous frequency tuning and amplitude modulation OPA, were analyzed. It is shown that the intensity of output signal pulse of OPA depends on the thickness of the dispersion section, the magnitude and azimuth of controlling electric field and one evidences periodicity. For the certain angular dispersion there exist the largest frequency bandwidths of optical parametric amplifiers. Certain angular dispersion of a signal pulse can increase the frequency bandwidth, and broadband amplification of ultrashort pulse can be performed effectively.     

Optical surface profile measurement using phase retrieval by tuning the illumination wavelength

In this paper, we present a method for measuring the surface profile of an object by using diffraction intensity patterns recorded at different illumination wavelengths. The main advantages of this technique are: simple optical set-up, high immunity to noise and environmental disturbance, since no reference beam (like in holography) or additional moving parts are needed. Two iterative calculations are synchronously performed using two sequences of diffraction intensity patterns, producing fast convergence to the expected result. The effects of different parameters on the accuracy and efficiency of the method are investigated. Simulation and experimental results are presented.     

大口径光学玻璃光学均匀性干涉绝对测量方法

在光学透射材料均匀性测量的各个方法中,干涉测量方法作为绝对测量方法,能摈除干涉仪标准面及待测元件的面形影响,具有很高的测量精度而逐渐被广泛使用。详细研究了使用干涉手段测量透射材料均匀性的方法,对其中材料切割角度所引入的误差进行了详细分析,并提出修正方法。同时研究了测量光学材料均匀性的拼接算法,实验表明：该方法可以实现用小口径干涉仪测量大口径玻璃材料的光学均匀性的目的,而且其测量精度很高。     

Optical substrate thickness measurement system using hybrid fiber-freespace optics and selective wavelength interferometry

Proposed and demonstrated is a simple few components non-contact thickness measurement system for optical quality semi-transparent samples such as Silicon (Si) and 6H Silicon Carbide (SiC) optical chips used for designing sensors. The instrument exploits a hybrid fiber-freespace optical design that enables self-calibrating measurements via the use of confocal imaging via single mode fiber-optics and a self-imaging type optical fiber collimating lens. Data acquisition for fault-tolerant measurements is accomplished via a sufficiently broadband optical source and a tunable laser and relevant wavelength discriminating optics. Accurate sample thickness processing is achieved using the known material dispersion data for the sample and the few (e.g., 5) accurately measured optical power null wavelengths produced via the sample etalon effect. Thicknesses of 281.1 μm and 296 μm are measured for given SiC and Si optical chips, respectively.     

Alternative method of wavelength drift free dual-wavelength heterodyne interferometry for the absolute distance measurement

Based on the technique of dual-wavelength and principle of heterodyne interferometry, a modified method for measuring the absolute distance is proposed. Because two test lights suffer from the same influence of wavelength drift in the measurement setup, the minus effect coming from the wavelength drift can be offset. Therefore, the measurement accuracy can be significantly increased. The feasibility of this method was demonstrated with a measurement resolution of about 1.50 μm. This method provides the advantages of a simple optical setup, easy operation and rapid measurement.     

Evaluation and correction of aberrations in an optical correlator by phase-shifting interferometry

We propose a new method for evaluating and correcting aberrations in a Vander Lugt correlator. The technique is achieved with liquid-crystal displays of the correlator and allows the task to be performed in situ. We present the theory on which the method is based and the experimental results that we obtained by applying it in a convergent correlator.     

An optical measurement of vortex shape at a free surface

We have proposed an optical method of vortex shape measurement based on Fourier transform profilometry (FTP) and verified it by experiment. The results of our experiment proposed in this paper show that FTP can efficiently reconstruct the vortex shape at a free surface and this method is suitable for wide use in studying such problems as liquid shear flow, wake of an object, flow behind a bluff body, and wetting angle.     

Simultaneous measurement of the phase and group indices and the thickness of transparent plates by low-coherence interferometry

We propose and demonstrate a novel technique for simultaneous measurement of the phase index, n(p) , the group index, n(g) , and the thickness, t , of transparent plates by use of a low-coherence interferometer. The output light from a superluminescent diode is focused upon the front plane of a transparent plate that is used as the sample. The sample stage is subsequently moved until the light is focused upon the rear plane of the plate. Measurement of the stage movement distance and the corresponding optical path difference allows us to determine both n(p) and n(g) . By placing the sample between two glass plates, we measured n(p) , n(g) , and t simultaneously, with an error of 0.3% or less, for nearly 1-mm-thick transparent plates, including glass and electro-optic crystals.     

Review of surface profile measurement techniques based on optical interferometry

With the fast development of modern science and technology, two or three-dimensional surface profile measurement techniques with high resolution and large dynamic range are urgently required. Among them, the techniques based on optical interferometry have been widely used for their good properties of non-contact, high resolution, large dynamic measurement range and well-defined traceability route to the definition of meter. A review focused on surface profile measurement techniques of optical interferometry is introduced in this paper with a detailed classification sorted by operating principles. Examples in each category are discussed and analyzed for better understanding.     

Two-frame phase-shifting interferometry for retrieval of smooth surface and its displacements

A new two-frame interferometric method with a blind phase shift of a reference wave is proposed for the reconstruction of smooth surface relief areas. In this method, a correlation approach based on the determination of correlation coefficient between intensity distributions of two interferograms is applied for the phase shift extraction. An algorithm for this method realization is developed and its performance is verified on test surfaces. An experimental setup containing a Twiman-Green interferometer was used in the reconstruction of smooth surfaces of metal specimens using the developed algorithm. Surface displacement phase fields were generated in case of a CT-notched specimen made of an aluminum alloy by the interactive procedure of element stitching of two surface reliefs and subtracting the surface relief after cyclic loading from the initial one. The received surface displacement fields allow us to determine the studied specimen fatigue process zone (FPZ) and FPZ size.     

Investigation of glass panels thickness profile by all-fiber digital heterodyne interferometry

All-fiber digital heterodyne interferometry is a laser metrology technique employing pseudo-random codes phase modulated onto an optical carrier signal. In this heterodyne interferometer system, the optical signal includes signal reflected and transmitted from the sample, respectively. Compared with the conventional heterodyne interferometry, this enhanced optical system has much higher measurement sensitivity, and can distinguish the signal which reflected from the front and the rear surface of the sample. Analysis and simulation for the digital heterodyne interferometry are presented. It takes approximately 4 s to scan the whole surface with the diameter of 300 mm. The thickness profile of the sample is obtained in the experiment. The discussion about the experiment is given finally.     

Measuring small wavelength differences by heterodyne interferometry and optical activity of chiral crystal

The optical activity of a chiral crystal and common-path heterodyne interferometry are used in a simple measurement technique that was developed to measure small wavelength differences. When circularly polarized heterodyne light passes through a chiral crystal, the plane of polarization rotates. The phase difference between the right- and left-circular lights is directly proportional to the angle of rotation. The rotation angle depends strongly on the wavelength. The phase difference can be accurately detected and substituted into specially derived equations to estimate wavelength variations. The feasibility of this method was demonstrated and the wavelength sensitivity was about 0.00812 nm. This method provides the advantages of a simple structure, ease of operations, a large wavelength measurement range and high sensitivity.     

基于合成波长法的飞秒激光外差干涉测距方法

本文提出了一种基于合成波长法的飞秒激光外差干涉测距方法.系统采用两个带通滤波器产生两个具有一定波长差的单波长,用于产生合成波长.本方法结构简单,能量利用率高.与双频激光干涉仪在40 mm范围内的比对结果表明,该方法比对残差的标准差为91 nm.     

Precise wavelength tuning of a dye laser using an active diffractive optical element

In this work, an active, reflecting diffraction grating was developed and characterized. A grating was holographically imposed onto the surface of a ceramic which could be deformed via the transverse, inverse piezoelectric effect, thus allowing active fine tuning of the grating constant. The tuning characteristics of this novel optical element were studied and the results compared with the linear theory of the piezoelectric effect. To demonstrate its applicability, the grating was installed in a dye laser and used to effect precise tuning of the output wavelength of the laser within a range of 220 pm. The possibility of quasi-static to high frequency operation of the system will allow this element to find use in a wide variety of applications.     

Measurement of surface resistivity and surface conductivity of anodised aluminium by optical interferometry techniques

Optical interferometry techniques were used for the first time to measure the surface resistivity and surface conductivity of anodised aluminium samples in aqueous solution, without any physical contact. The anodization process (oxidation) of the aluminium samples was carried out in different sulphuric acid solutions (1.0–2.5% H₂SO₄), by the technique of electrochemical impedance spectroscopy (EIS), at room temperature. In the mean time, the real-time holographic interferometric was carried out to measure the thickness of anodised (oxide) film of the aluminium samples during the anodization process. Then, the alternating current (AC) impedance (resistance) of the anodised aluminium samples was determined by the technique of electrochemical impedance spectroscopy (EIS) in different sulphuric acid solutions (1.0–2.5% H₂SO₄) at room temperature. In addition, a mathematical model was derived in order to correlate between the AC impedance (resistance) and to the surface (orthogonal) displacement of the samples in solutions. In other words, a proportionality constant (surface resistivity or surface conductivity=1/surface resistivity) between the determined AC impedance (by EIS technique) and the orthogonal displacement (by the optical interferometry techniques) was obtained. Consequently the surface resistivity (ρ) and surface conductivity (σ) of the aluminium samples in solutions were obtained. Also, electrical resistivity values (ρ) from other source were used for comparison sake with the calculated values of this investigation. This study revealed that the measured values of the resistivity for the anodised aluminium samples were 2.8×10⁹, 7×10¹², 2.5×10¹³, and 1.4×10¹²  Ω cm in 1.0%, 1.5%, 2.0%, and 2.5% H₂SO₄ solutions, respectively. In fact, the determined value range of the resistivity is in a good agreement with the one found in literature for the aluminium oxide, 85% Al₂O₃ (5×10¹⁰ Ω cm in air at temperature 30 °C), 96% Al₂O₃ (1×10¹⁴  Ω cm in air at temperature 30 °C), and 99.7% Al₂O₃ (>1×10¹⁴ Ω cm in air at temperature 30 °C).