Design and fabrication of optical tunable filters

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Design and fabrication of optical tunable filters

 

Author: Lam, Man Ying
Title: Design and fabrication of optical tunable filters
Degree: M.Phil.
Year: 2014
Subject: Tunable lasers.
Light filters.
Light emitting diodes.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Applied Physics
Pages: xix, 104 leaves : illustrations (some color) ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2762874
URI: http://theses.lib.polyu.edu.hk/handle/200/7729
Abstract: This research study aims to develop optical tunable filters (OTFs) for next-generation telecommunication networks and microfluidic systems using different tuning mechanisms. Two types of OTFs have been investigated: PZT-actuated OTF and electrolyte-capacitor (EC) OTF. The former utilizes a well established tuning method but tries to improve the specifications for practical industrial applications. The latter explores a new tuning mechanism and focuses more on scientific study. The PZT-actuated OTF consists of two mirrors, each has high reflection (HR) coating (reflectivity > 95% over the C-band) in one surface and anti-reflection (AR) coating (reflectivity < 0.01%) on the other surface. One of the mirrors is attached to a miniature PZT actuator mounted on an L-shaped brass holder, while the other one is fixed to another brass holder directly. Tuning of the transmission wavelengths is achieved by applying a bias voltage to the PZT-actuator to change the cavity length. In this project, 20 miniaturized tunable filters have been fabricated and packaged, with typical dimensions of 8 x 5 x 5 mm³, a wavelength shift of ± 1 nm/ V within the bias of ± 20 V, an insertion loss of 0.5 dB and a response time of 1 ms. The size is small enough to fit into the butterfly package box (25 mm x 10.7 mm x 9.5 mm) that houses the narrow-linewidth tuning laser developed for the industrial partner, and the tuning performance is superior to the available products. Moreover, the mechanical stability and the PZT hysteresis problem have been investigated in detail, and different designs of brass holders and packaging methods have been explored. To deal with the hysteresis problem between the voltage and the PZT displacement (displacement error > 20%), a current injection actuation technique has been developed to reduce the error to < 2% in open loop. As a result, it simplifies the feedback control of laser wavelengths. The packaged OTFs have been successfully used in the butterfly-packaged narrow-linewidth tunable lasers, which demonstrated superior tunability and spectral purity.
On the scientific side, this research proposes a new tuning method the electrolyte-capacitor method. This is accomplished by constructing the Fabry-Perot etalon in a structure of electrolyte capacitor and by filling the FP cavity with ionic liquid. Under the bias voltage, the ion concentration and thus the refractive index of the ionic liquid could be tuned significantly. In the experiment, one of the mirror of the FP etalon (called electrode mirror) has half of its HR surface coated with a gold thin film, whereas the other mirror (called capacitor mirror) has half of its HR surface coated with a silver thin film covered by a layer of SiO₂. Then, both mirrors are put together with their coated parts facing each other, and the FP resonant cavity is filled with BMIM-PF₆ ionic liquid. In the FP etalon, the coated non-transparent half region is for electrical tuning and the uncoated transparent half region is for light propagation. At the beginning, the FP cavity only allows a comb of wavelength to pass through. When a bias voltage is applied to the coated parts, the electric field is built up across the SiO₂ layer; and ions are attracted to the capacitor side. As a result, the ion concentrations in the uncoated transparent region are reduced accordingly, leading to a change of the refractive index and thus the optical path length of the FP cavity. Therefore, the transmission peaks of the tunable filters are shifted. For experimental demonstration, we have fabricated the device and have obtained a wavelength shift at about 0.17 nm/ V within the bias 017 V. Apart from BMIM-PF₆, a varieties of electrolytes have examined, including salt solution (NaCl) and conductive polymers (PEDOT and Nafion). BMIM-PF₆ solution presents the best performance in terms of insertion loss, contrast and tunability. Although the performance has a lot to be improved, this tunable filter demonstrates a new working principle and may be applicable to microfluidic systems that require on-chip optical filters in fluidic environment. In conclusion, this research study has successfully developed two types of optical tunable filters, the PZT-actuated tunable filters and the electrolyte-capacitor tunable filters. For industrial applications, the PZT-actuated tunable filters have small dimensions, large tuning range, convenient electrical control, high speed and good linearity, and meet the stringent requirements of the narrow-linewidth tunable lasers under development for commercial uses in next-generation high-speed coherent communication networks. For scientific study, the electrolyte-capacitor (EC) tunable filters utilize a new mechanism and feature no mechanical movement and easy integration with microfluidic systems. All these help provide new tunable filters for telecommunication and microfluidic systems.

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