Study on high-speed optical frequency shifter utilizing polarization-reversal structure

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An optical frequency shifter (OFS) is an important device in many optoelectronics systems, such as, optical communication systems, optical signal processing, optical measurements, laser spectroscopy, laser cooling, photochemistry, and so on. However, the operation frequencv of a commerciallv available OFS by acousto-optic effects is in several hundred MHz, therefore, a high-speed OFS with the operation frequency over GHz is expected in many application fields.In this research project, we have studied a novel electrooptic (EO) high-speed OFS by using a new guided-wave optical circuit, which we call as '3-branch waveguide interferometer' and the polarization-reversal techology of ferroelectric materials. This OFS is very compact and can be operated in very high frequency (1-100GHz) with only a single RF modulation signal and DC bias voltages. A continuous sweep of optical frequency over several GHz is possible, and the direction of a frequency shift (upper or lower) can be switched by tuning DC bias voltages.The operation of the proposed OFS has been analyzed in detail and proto-type devices has been designed and fabricated by use of z-cut LiTaO_3. In the experiment, basic operations of the fabricated OFS have been confirmed successfully around the designed operational frequency of 15GHz and 25GHz.The applications of the fabricated OFS to a wavelength-division-multiplexing (WDM) optical fiber communication system have carried out and optical frequencies of several muitiplexed signals have been shifted simultaneously by using the fabricated OFS. The applications to laser spectroscopy have also carried out and absorption spectrum of a standard gas-cell has been measured by using the fabricated OFS.