Abstract
The phase quadrature measurement method is capable of measuring nonlinearity in heterodyne laser interferometers with picometer accuracy whereas it cannot be applied in the new kind of heterodyne interferometers with bidirectional Doppler frequency shift especially in the condition of non-uniform motion of the target. To solve this problem, a novel measurement method of nonlinearity is proposed in this paper. By employing double-channel quadrature demodulation and substituting the external reference signal with internal ones, this method is free from the type of heterodyne laser interferometer and the motion state of the target. For phase demodulation, the phase differential algorithm is utilized to improve the computing efficiency. Experimental verification is carried out and the results indicate that the proposed measurement method achieves accuracy better than 2 pm.
Highlights
Heterodyne laser interferometers are widely applied in precision metrology, nanotechnology, and lithography due to their high accuracy and robustness [1,2,3]
This paper presents a novel measurement method for nonlinearity in heterodyne laser interferometers, which adopts the architecture of double-channel quadrature demodulation with internal references and, thereby, is able to break through the limits in type of heterodyne laser interferometers and in motion state of the target
The measurement of the nonlinearity is performed by the measuring circuit in which the two signals from the waveform generator are first converted into digital signals by the analog-to-digital converters (ADC) (AD9446, Analog Devices, Norwood, MA, USA) and are processed in the FPGA (EP3C120F780C8, Altera Corporation, Santa Clara, CA, USA)
Summary
Heterodyne laser interferometers are widely applied in precision metrology, nanotechnology, and lithography due to their high accuracy and robustness [1,2,3]. Benefited from lock-in amplification, the phase quadrature measurement method [10] is promising in a nonlinearity measurement up to picometer accuracy In this method, the reference signal of the interferometer serves as the external. For the generation heterodyne laser interferometers [12,13], most of them adopt an optical configuration with bidirectional Doppler frequency shift (DFS), i.e., the measurement and reference signals have equal DFS but come with an opposite sign. When the target is in fast and non-uniform motion, the frequency of the reference signal will change rapidly In this case, the existing phase quadrature measurement method is not applicable. Figure of the measurement method for nonlinearity in heterodyne laser interferometers basedon onSchematic double-channel quadrature demodulation. By utilizing double-channel quadrature demodulation, the method can be applied extensively to the new type heterodyne laser interferometers with bidirectional DFS. The phase differential algorithm is employed to improve computing efficiency
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