We present a high-resolution heterodyne interferometric system using a high-output-power, three-longitudinal-mode He-Ne laser. In this system, an interferometric wavelength of λ/2 is synthesized with the super-heterodyne detection of the three modes, resulting in twice the sensitivity for the target displacement in comparison with the conventional two-frequency interferometer. In the experiment, the achievement of subnanometre resolution has been confirmed.

https://iopscience.iop.org/article/10.1088/0957-0233/12/2/305/pdf

A subnanometre heterodyne interferometric system with improved phase sensitivity using a three-longitudinal-mode He-Ne laser

Previously, the authors have developed a subnanometre heterodyne interferometric system known as a three-mode heterodyne interferometer (TMHI). By converting the target displacement into twice the phase variation of a conventional interferometer, the TMHI has achieved position resolution of 0.044 nm even with a single-pass interferometer. However, the heterodyne beat frequency in TMHI is quite low, of the order of several hundred kilohertz, which limits the measurable target speed to several tens of millimetres per second. In this paper, we present a method to extend the measurable target speed of the TMHI without increasing the heterodyne beat frequency of a light source. Keeping the excellent position resolution of the TMHI, the proposed method provides sufficient response to high speed movement of the target. The system comprises a pair of TMHI and utilizes two conjugate signals generated by them selectively according to the direction of movement of the target. Possible errors in switching the two signals are analysed by introducing an error zone model. The switching function was demonstrated using the mechanical vibration of the interferometer and proved to be free from erroneous counts. It was also confirmed that the system can respond at a speed that exceeds that determined by the beat frequency.

https://iopscience.iop.org/article/10.1088/0957-0233/16/9/017/pdf

High-speed subnanometre interferometry using an improved three-mode heterodyne interferometer

Highest intensity, single mode He-Ne laser can be obtained by combination of 3-modes operation of a 3∼4 modes oscillating laser tube and a polarizer for selection of a single mode. In this experiment, as a 3∼4 modes oscillating laser tube, a tube whose cavity length is 35cm and output intensity is 5mW was employed. By selectively using the center mode of 3-modes oscillation as a single mode, highest intensity, single mode He-Ne laser has been realized. For a particular mode stabilization, the polarization stabilization method whose control signal is intensity is usually used. However, mode stabilization at the gain curve center is difficult in this method. Moreover, detection of the intensity is uncertain due to temperature drifts in the detector sensitivity and amplifiers, and contamination in the cavity mirror and detector surface, resulting in decreasing the frequency stability. In this experiment, intermode beat frequency change by Frequency Pulling which is strongly related to the mode position was utilized for the stabilization. The difference frequency of two intermode beats generated by the center mode and two other modes which oscillate at each side of the center mode is used as a control signal. It is convenient for the stabilization that the position of this frequency variation corresponding to the most intense center mode oscillation is at middle of its slope. Because the control signal based upon the frequency can be processed digitally, this stabilization method is free from uncertainties which is inevitable in the polarization stabilization method.

T. Araki

Papers

High Intensity, Single Mode He-Ne Laser as a Light Source of Multi-Dimension Interferometer