Dejiao Lin

High stability multiplexed fibre interferometer and its application on absolute displacement measurement and on-line surface metrology

We propose a self-reference multiplexed fiber interferometer (MFI) by using a tunable laser and fiber Bragg grating (FBG). The optical measurement system multiplexes two Michelson fiber interferometers with shared optical path in the main part of optical system. One fiber optic interferometer is used as a reference interferometer to monitor and control the high accuracy of the measurement system under environmental perturbations. The other is used as a measurement interferometer to obtain information from the target. An active phase tracking homodyne (APTH) technique is applied for signal processing to achieve high resolution. MFI can be utilized for high precision absolute displacement measurement with different combination of wavelengths from the tunable laser. By means of Wavelength-Division-Multiplexing (WDM) technique, MFI is also capable of realizing on-line surface measurement, in which traditional stylus scanning is replaced by spatial light-wave scanning so as to greatly improve the measurement speed and robustness.

Roll angle interferometer by means of wave plates

A novel roll angle interferometer using a transverse Zeeman laser (TZL) is proposed in this paper. The orthogonally linearly-polarized light from TZL is changed to be slightly elliptically-polarized light by a quarter wave plate (QWP) when the principal axis of the QWP is at a small angle with the basis of the direction of polarization. The beam then passes through a half wave plate (HWP) that is employed as a sensor. After that it is reflected by a right angle prism (RAP) and returns to the HWP again. Owing to the reasonable transformation of coordinates by the prism, the change of direction of polarization is accumulated twice when the beam passes forward and back through the HWP. Theoretical analysis and experimental results show that the sensitivity of roll measurement can reach sub-second and be improved four times than that we had reported. This system of implementing roll measurement takes the advantage of compactness, stable performance, low cost and robust to the environment disturbance.

Straightness/coaxiality measurement system with transverse Zeeman dual-frequency laser

The characterization of straightness/coaxiality is a fundamental geometric tolerance in modern mechanical industries. Straightness measurement over a long range is difficult to implement due to atmospheric disturbance. A novel straightness/coaxiality measurement system that combines a transverse Zeeman dual-frequency laser with a pair of Wollaston prisms is presented in this paper. Orthogonally linearly polarized light from the laser output is divided into two beams by a Wollaston prism, which simultaneously acts as a sensor to measure the straightness of a guideway. The two beams become parallel to each other when they pass through the other Wollaston prism and then are reflected by a rectangular reflector. The measuring signals are processed by means of a phase measurement method with a resolution of 0.1°, which corresponds to 1 µm in straightness measurement. The system meets the principle of self-adaptation so that it is capable of overcoming disturbances in the air. It also has the advantages of compact structure, long-range measurement and good stability. Possible errors of the system are analysed theoretically. A calibration experiment of the straightness measurement over a range of 16 m demonstrated high accuracy with a stability of 3.6 µm.

Step-height measurement by means of a dual-frequency interferometric confocal microscope

A novel instrument, the dual-frequency interferometric confocal microscope (DICM), which facilitates the measurement of step features, is investigated. It combines the advantages of the high resolution (subnanometer) of heterodyne interferometry and the relatively large measurement range (approximately 5 microm) of confocal microscopy. The axial response curves of the confocal microscopy system are compared in experiments in which microscopic objects with various numerical apertures and magnifications are used. The results prove that the variation in light intensity is enough to permit discrimination of different orders of interference fringes. The DICM has been successfully utilized to measure the step height of a standard mask, and the experimental results agree well with those measured by scanning probe microscopes. The results also show that the system has good repeatability, with a maximum deviation of 5 nm.

Investigation of some critical aspects of on-line surface measurement by a wavelength-division-multiplexing technique

Some critical aspects of a new kind of on-line measurement technique for micro and nanoscale surface measurements are described. This attempts to use spatial light-wave scanning to replace mechanical stylus scanning, and an optical fibre interferometer to replace optically bulky interferometers for measuring the surfaces. The basic principle is based on measuring the phase shift of a reflected optical signal. Wavelength-division-multiplexing and fibre Bragg grating techniques are used to carry out wavelength-to-field transformation and phase-to-depth detection, allowing a large dynamic measurement ratio (range/resolution) and high signal-to-noise ratio with remote access. In effect the paper consists of two parts: multiplexed fibre interferometry and remote on-machine surface detection sensor (an optical dispersive probe). This paper aims to investigate the metrology properties of a multiplexed fibre interferometer and to verify its feasibility by both theoretical and experimental studies. Two types of optical probes, using a dispersive prism and a blazed grating, respectively, are introduced to realize wavelength-to-spatial scanning.

New advance in confocal microscopy

Step height and line-width are two key parameters in the metrology of micro-electronic masks. A novel common-path heterodyne interferometric confocal measuring system is presented to measure the step height of masks. It combines both the methods of heterodyne interferometry and confocal microscopy. The resolution is 0.01 nm and the measurement range is around 8 µm. The procedure is direct by the integration of the measurement of intensity and phase, hereby faster than a normal scanning microscope. For the line-width measurement of masks, a polarization heterodyne interferometric confocal microscope is proposed, which combines a polarization interferometer with a confocal microscope. An ideal beam spot is obtained and precise focus is realized by using the confocal technique. The phase shifts of the two orthogonal polarization beams differ from each other when they are reflected at the edge of a sample. The experimental results show that the uncertainty of line-width measurement is 21 nm. Both of the systems satisfy the common-path principle, so as to get high ability of resistance to environment disturbances.

A double common-path heterodyne interferometer for the measurement of flying height modulation

A double common-path differential heterodyne interferometer is proposed to measure the flying height modulation (FHM) of a slider in a hard disk drive. A transverse Zeeman He–Ne laser is employed as the light source and high-speed phase measurement is applied for signal processing. The resolution and the sampling frequency of FH measurement are 0.1 nm and 500 kHz, respectively. According to the symmetrical optical path design, the environmental noise and Abbe error from the disk can be adaptively compensated. The experimental results show that the stability of the system is sub-nanometer in a normal laboratory without air conditioning, and the FHM of ±3 nm is achieved when the pitch of the rotating disk is around 1.2 µm.

Design and detection of superprecision positioning stage with nanometer resolution

A precision positioning system with a high displacement resolution has been widely required for modern industrialized applications, such as microelectronics, super-precision manufacturing etc. This paper discusses the design and the features of a new piezo driven precision micro positioning stage utilizing flexure hinges. Theoretical analysis for the stiffness of the flexure hinge is also given briefly. A piezoelectric ceramic is applied to drive the precision state, whose displacement can reach 5 micrometers when employed with 1000 voltage power. In order to testify the robust and measurement stability of the precision sta, three kinds of PZT produced in Germany, Japan and China respectively are utilized. A dual-frequency interferometer with nanometer resolution and accuracy is adopted to evaluate the mechanical characteristics of the positioning stage. The experimental result shows that the open loop control of the stage provides 0.2nm positioning resolution along the moving direction.

Frequency stabilization of transverse Zeeman He–Ne laser by means of model predictive control

A model predictive control (MPC) technique is proposed for the frequency stabilization of transverse Zeeman He–Ne laser. The beat frequency locking method is applied as the criterion of frequency stabilization. When heated by the voltage output of the feedback electronic circuit, the copper wire twisted around the laser tube is applied to maintain the length of the laser cavity and stabilize the frequency of the laser. Four steps for MPC design, i.e., model setup, output prediction, selection of reference path, and calculation of control quality, have been introduced. By this means, a single-chip microprocessor (80C196) generates a pulse width modulation wave that is transferred into a heating signal. The calibration result shows that the frequency stability of our proposed transverse Zeeman laser reaches 5.5×10−11 by means of the MPC method.

Dual-frequency laser using anisotropic bireflectance film

A kind of bireflectance thin film on the window plate of a 633nm He-Ne laser is presented in this paper. The film of non quarter-wave-stack is coated on the substrate with the application of external load on it. The load on the substrate is removed after the coating has been accomplished, then the strain on the substrate will transfer to the multilayer film. Due to photoelastic effect, the multilayer film becomes an anisotropic film. Selecting appropriate film structure and suitable center wavelength, a high phase dispersion with a nearly constant reflectivity will be obtained around the working wavelength. For normal incidence, a phase shift difference between the two orthogonal polarization states of the reflected wave will produce. As a result, a dual-frequency laser with a beat frequency of 4-5 megahertz can be carried out by using this kind of bireflectance thin film. Based on this principle, a He-Ne dual-frequency laser equipped with bireflectance cavity mirror is described. The model coupling is reduced by utilizing transverse Zeeman effect so that two linear and orthogonal polarization components with 5MHz beat frequency are generated. The effect of the magnetic fields direction on the dual-frequency as well as the polarization property of the laser are investigated by experiments. After stabilizing the frequency, the laser is calibrated with the iodine frequency stabilization laser at Chinese National Institute of Metrology. Experimental results indicate that the expanded uncertainty of wavelength in vacuum is 1 X 10-7 with the frequency stabilization of 6.6 X 10-10.