Zhaogu Cheng

Study of a dual-frequency laser interferometer with unique optical subdivision techniques

A dual-frequency laser interferometer has been developed based on a low-performance commercial interferometer. An optical resolution of 1.24 nm and a nanometer-scale accuracy have been achieved by using unique techniques to obtain an optical subdivision factor of 1/8. A method for reducing static positioning errors was also shown. The measurement of a free-falling body was performed to test the maximum achievable target velocity of the device. The experimental setup for measuring the static positioning errors was also given. The new interferometer could be widely used in nanometer-scale fabrications and measurements.

Study on high-accuracy displacement interferometer for lithography application

A measurement interferometer available for wafer stage metrology of lithography has been investigated by means of resolution-extending of optical subdivision based on commercial interferometers. Factors that determine the accuracy, linearity and repeatability of nanometer-scale measurements of displacements exceeding hundreds of millimeters and of target velocities exceeding hundreds of millimeters per second are also discussed.

Modelling and simulation of the second-order Doppler error of a laser dual-frequency interferometer

Only the first-order Doppler frequency shift is considered in current laser dual-frequency interferometers; however; the second-order Doppler frequency shift should be considered when the measurement corner cube (MCC) moves at high velocity or variable velocity because it can cause considerable error. The influence of the second-order Doppler frequency shift on interferometer error is studied in this paper, and a model of the second-order Doppler error is put forward. Moreover, the model has been simulated with both high velocity and variable velocity motion. The simulated results show that the second-order Doppler error is proportional to the velocity of the MCC when it moves with uniform motion and the measured displacement is certain. When the MCC moves with variable motion, the second-order Doppler error concerns not only velocity but also acceleration. When muzzle velocity is zero the second-order Doppler error caused by an acceleration of 0.6g can be up to 2.5 nm in 0.4 s, which is not negligible in nanometric measurement. Moreover, when the muzzle velocity is nonzero, the accelerated motion may result in a greater error and decelerated motion may result in a smaller error.

Self-adaptive optical systems for long-distance flying optics

The focusing characteristics of long-distance flying optics were studied systemically for TEM(mn) Gaussian beams. The results show that the ABCD law of parameter q can be extended to Gaussian modes of any order when waist radius w in the imaginary part of parameter q is replaced by Rayleigh range Z(R) of a certain resonator in the equation. The difference between the real focal length and the geometric focal length, defined as Df, was calculated for laser applications. A novel self-adaptive optical system was demonstrated for precisely controlling the focusing characteristics of long-distance flying optics. Theoretical analyses and experimental results were consistent.

Novel adaptive laser processing system for flying optics

The focusing characteristics of higher-order mode Gaussian beams in flying optics were investigated in detail. On the basis, a novel adaptive laser processing system for flying optics was developed. In the system, a lens with long focal length and two adaptive deformable mirrors controlled by hydraulic were employed. The lens was near the laser source for some distance and the two adaptive mirrors were integrated together with parabolic focusing mirror. The system has the advantages of compact structure and easy control. The system can keep the focus position and the focus radius constant for long distance laser processing.

The effect of resonator thermal deformation on focusing characteristics and the compensation for a high power CO 2 laser

Research on change of Rayleigh range ZR of output beams and effect of focusing characteristics caused by the dynamic process of resonator deformation especially for the HR coupler in high power CO2 Laser is given in this paper. An adaptive optics system which can compensates the change of Rayleigh range ZR was used. The real-time control of focus position was achieved.