We demonstrate for the first time the continuous operation of a Bragg diffraction type electrooptic (EO) frequency shifter using a 16？GHz modulation signal. Because frequency shifting is based on the Bragg diffraction from an EO traveling phase grating (ETPG), this device can operate even in the millimeter-wave (>30？GHz) range or higher frequency range. The ETPG is generated based on the interaction between a modulation microwave guided by a microstrip line and a copropagating lightwave guided by a planner waveguide in a domain-engineered LiTaO3 EO crystal. In this work, the modulation power efficiency was improved by a factor of 11 compared with that of bulk devices by thinning the substrate so that the modulation electric field in the optical waveguide was enhanced. A shifting efficiency of 65% was achieved at the modulation power of 3？W. 1. Introduction Coherent optical frequency conversion based on external modulation is an important technique not only for the optical communication and optical measurement but also for microwave and millimeter-wave photonics because it corresponds to an upconversion from RF to optical domain. Photomixing of two optical modulation sidebands generated by an electrooptic phase modulator (EOM) or a frequency comb generator has been used to generate low phase noise coherent microwaves or millimeter-waves, which are desirable for many applications such as the radar [1], sensing [2–4], and wireless communications [5]. The advantages of the external modulation method over other methods such as optical injection locking, optical phase-locked loop, and dual-wavelength laser source are the system’s simplicity, stability, and frequency tunability. However, typical sinusoidal phase modulation is an inherently inefficient method of frequency conversion. At best, the fraction of the power in the first-order sideband generated by normal phase modulation is theoretically , where is a first-order Bessel function of the first kind and is the modulation depth. The conversion efficiency of 34% corresponds to an extra loss of about 5？dB. Because the noise figure (NF) of an optical amplifier is relatively poor compared to electronic amplifiers, extra loss due to a low conversion efficiency impacts most key aspects of microwave and millimeter-wave photonics in which low-loss 1550？nm components should be used [6, 7]. The lower conversion efficiency results in a lower carrier-to-noise ratio (CNR). The higher conversion efficiency is essential not only for microwave and millimeter-wave photonics applications but also for other photonic
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