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UBC Theses and Dissertations

Wide band electro-optic intensity modulator frequency response measurement using optical heterodyne, Down-conversation Technique Lam, Alfred Kan Min


In this thesis a novel system for measuring the frequency responses of electrooptic intensity modulators in the millimeter-wave regime is described. As the frequency responses of modulators increase, wide band, and cost effective methods to measure these responses need to be developed. The system described in this thesis uses optical heterodyning and a wide band photodiode to generate millimeter-waves to drive the device under test (DUT). The need for a wide band microwave sweeper can therefore be eliminated. Moreover, an inexpensive frequency shifter consisting of a Mach-Zehnder modulator and an optical bandpass filter is used to down-convert the output of the DUT to a low microwave frequency at which the output is easily measured. The system measures | S₂₁ | in dBe. The system can measure the frequency responses of different types of electrooptic intensity modulators such as Mach-Zehnder modulators, mode-converter modulators, electro-optic polymer modulators, and should also work for electroabsorption modulators. In this thesis, the frequency response measurement system is described, and mathematical expressions for the optical fields at the output of an electro-optic modulator are developed when the DUT is biased or operated at its maximum, quadrature, and minimum points. High frequency measurements on a 10 Gb/s LiNbO₃ Mach-Zehnder type modulator and a 40 Gb/s GaAs mode-converter type modulator are made. The measurements show close agreement with | S₂₁ | measurements made using vector network analyzers and calibrated wide band photodiodes. In addition, the harmonic rejection feature of our system is described. Our system "rejects" higher order harmonics, due to the non-linearities, generated in the DUT with proper selection of the modulation and the shift frequencies. This feature means that our system can measure the linear frequency response of the DUT even if it is operated, or biased, in a non-linear region.

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