1. Lithium Niobate Waveguide Phase Modulator for Roll Angular
Displacement Measurements
Ruey-Ching Twu*, Kuei-Wen Wang, and Chia-Wei Hsueh
Department of Electro-Optical Engineering, Southern Taiwan University
1, Nan-Tai St., Yongkang District, Tainan 71005, Taiwan R.O.C.
Abstract
A Zn-indiffused lithium niobate phase modulator (ZIPM) was proposed for measuring the roll angular displacements based on the optical heterodyne technique. The homemade ZIPM with a low driving voltage and stable phase modulation was demonstrated successfully to achieve an angular displacement resolution of around 3.57×10-5 deg.
Introduction
Roll angular displacement measurement has been widely studied for the Faraday effect [1] and optical activity media [2]. The linearly polarized light is rotated while passing through the optical activity media. The induced phase variation can be measured in a polarization interferometer. Although an intensity interrogation is an easy arrangement, the laser power fluctuations and environmental noises will make low measurement resolutions. A phase interrogation technique improves the measurement resolution by utilizing optical phase or frequency modulations. Therefore, the angular displacement can be measured through the determinations of phase variations in the common-path polarmetric interferometer. Usually, it needs two-channel signals to compare the phase difference between them in a heterodyne metrology [2]. In the acousto-optic modulation (AOM) heterodyne setup [3], it needs more optical elements, and the light splitting and combing is disturbed easily by environmental functions. Besides, the heterodyne light can be generated by employing electro-optic modulation (EOM) [2], it is a more simple arrangement for obtaining the probe light. However, the driving voltages of over 100V are necessary for both of the AOM and EOM schemes.
In this paper, a Zn-indiffused lithium niobate phase modulator (ZIPM) was proposed for measuring the roll angular displacements based on the optical heterodyne technique [4]. The homemade ZIPM with a low driving voltage and stable phase modulation was demonstrated successfully to achieve a measurement resolution of 3.57×10-5 deg.
Exprimental Setup 、
Fig. 1 Schematic diagram of the optical activity measurement setup.
Experimental Results and Discussion
(a)
(b)
Fig. 2 The measured modal profiles for the (a) TE and (b) TM modes.
Fig. 3 The measured optical response and applied voltage.
(a)
(b)
(c)
Fig. 4 (a) Phase variation and (b) sensitivity versus roll angle for different phase retardations.
(c) Maximum sensitivity and FWHM versus phase retardation of δWP.
Fig. 5 Phase variation versus roll angular displacement.
Conclusion
The proposed homemade ZIPM has been successfully applied in the heterodyne-based roll angular displacement measurement. Finally, the results show the system measurement resolution can reach to 3.57×10-5 deg.
References
[1] P. S. PERSHAN, “MAGNETO-OPTICAL EFFECTS,” J. APPLIED PHYSICS 38, 1482–1490 (1967).
[2] J. Y. Lin and D. C. Su, “A new method for measuring the chiral parameter and the average refractive index of a chiral liquid,” Opt. Commun., 218, (2003).
[3] C. M. Wu and T. T. Chuang, “Roll angular displacement system with microradian accuracy,” Sensors and Actuator A, 116, (2004).
[4] R. C. Twu, H. H. Hong, and H. H. Lee, “Dual-channel optical phase measurement system for improved precision,” Opt. Lett. 33, (2008).
[5] Z. Liu, D. Lin, H. Jiang, and C. Yin, “Roll angle interferometer by means of wave plates,” Sensors and Actuator A, 104, (2003).