Investigation of a polarization controller in Ti:LiNbO3 near 1530 nm wavelength
Introduction
Polarization control in optical communication systems has long been an issue of concern [1], [2], [3], [4], [5]. Many efforts have been made for developing optical polarization controllers including the use of retardation plates [2], [4], liquid crystals [5], [6], [7], fiber squeezers [8], [9] and photonic waveguides [1], [10], [11], [12]. Among those the electro-optic approach utilizing waveguides in birefringent materials, such as lithium niobate (LiNbO3), is attractive for rapid stabilization of fluctuating polarizations aspects.
To control the state of polarization (SOP) in any type of optical waveguide device requires mechanisms for manipulating the phase difference between the orthogonal transverse electric (TE) and transverse magnetic (TM) components of the guided lightwave, as well as their relative strength. In LiNbO3, these can be accomplished by exploiting the linear electro-optic (Pockels) effect of the material and offers fast control speeds. Furthermore, with applied external voltage controls, it is possible to transform any incident polarization into any desired SOP at the output.
This paper reports the results of investigations on a polarization controller configuration produced on LiNbO3 that was conceived and developed for integration with an arsenic trisulfide (As2S3) ring resonator on a hybrid platform and successfully implemented in a frequency discriminator demonstration aimed toward enhancing the frequency response linearity in microwave photonic link applications [13], [14]. It is composed of two polarization converters with a phase shifter centered between them, and all integrated on a single titanium (Ti) diffused channel waveguide. The polarization converters provide a scheme for adjusting the relative strength between the orthogonal TE and TM components, while the phase shifter offers a means to alter their relative phase. A simple approach for making integrated optics frequency discriminators is based on the use of Mach–Zehnder interferometer (MZI) configuration with a phase shifter on one arm and a ring resonator on the other arm. On the hybrid platform with a single Ti-diffused channel waveguide and an As2S3 ring resonator that exhibits a strong preference for coupling TM polarization, the polarization converters provide a means for tuning the splitting ratio, while the phase shifter offers selectivity for the center wavelength. For the purpose of addressing the integrated hybrid platform application needs, all experimental characterizations were initially directed only toward the conversion of orthogonal TE or TM input SOPs to either TM or TE output. The investigations were then extended for the characterization of this configuration as a polarization controller for the transformation of arbitrary incident SOPs to either TE or TM output. In all measurements, the power in the unwanted output SOP was suppressed by greater than 35 dB (up to ~40 dB). Simulations results indicate endless polarization transformation between input and output SOPs could be realized.
Section snippets
Design and operating principle
A schematic illustration of the device configuration on x-cut LiNbO3 is shown in Fig. 1. The waveguide is single mode for both TE and TM polarizations. The interdigitated electrode sets on the surface of substrate near the two end sections are for controlling the relative strength between the orthogonal TE and TM polarization components of the guided mode, which can be accomplished by applying voltages to introduce an x-directed electric field and promote coupling electro-optically through a
Analytical considerations
The response of the polarization converter depicted in Fig. 1 can be described by using a matrix approach for determining the output orthogonal field vector components. Expressing the contribution of each element by Jones calculus, the overall transfer matrix can be obtained from the product of the individual matrices. The analysis assume single mode for both TE and TM polarizations, and all losses are ignored.
Experimental results
Devices with the configuration of Fig. 1 were fabricated on x-cut y-propagating LiNbO3 substrates. Single-mode channel waveguides for both TE and TM polarizations were produced by diffusing 93-nm-thick 7-μm-wide stripes of sputtered Ti film at 1030 °C for 15 h in wet ambient. Next, 160-nm-thick silicon dioxide (SiO2) buffer layer was deposited on the surface by RF sputtering, and annealed at 900 °C for 25 min. Finally electrode patterns were delineated in a DC magnetron sputtered 160 nm-thick
Conclusion
A simple configuration for an electro-optic polarization controller on LiNbO3 has been investigated analytically and experimentally. All experimental characterizations were carried out on devices that were produced with a phase shifter centered between the two polarization converters along the waveguide, and were initially directed solely toward TE↔TM conversion. The choice of that configuration was made to accommodate its integration on LiNbO3 as hybrid platform with a high-index vertically
References (17)
- et al.
IEEE Photonics Technol. Lett.
(2008) - et al.
IEEE Photonics J.
(2010) - et al.
IEEE J. Sel. Top. Quantum Electron
(2010) - et al.
Opt. Express
(2006) - et al.
J. Lightwave Technol
(2006) - et al.
Opt. Express
(2012) - et al.
J. Lightwave Technol.
(1990) - et al.
J. Lightwave Technol
(1988)