Study Of Low Insertion Loss LNOI Electro-optic Modulator

With the rapid development of the information age,the research on electro-optical modulators is gradually developing towards the direction of miniaturization,low loss and large bandwidth.Commercial electro-optic modulators are mostly based on the bulk material lithium niobate,which has disadvantages such as high half-wave voltage,large device size,and difficulty in integration.The thin-film lithium niobate(LNOI)material not only inherits the excellent electro-optical properties of the bulk material lithium niobate,but also has the advantages of small size,low power consumption,and easy integration.It has become an ideal material for the preparation of electro-optical modulators.Based on this,theoretical and experimental researches are carried out in this paper based on LNOI electro-optic modulator.It has been reported that the waveguides of LNOI electro-optic modulators mostly adopt ridge structures.Considering the limitations of processing technology,and in order to facilitate the monolithic integration with other LNOI devices(such as resonant rings,polarization beam splitters,arrayed waveguide gratings),etc.,a rectangular LNOI waveguide structure is adopted in this paper.An electro-optic modulator based on the Mach-Zehnder interferometer(MZI)structure was designed using this waveguide.Using the traveling wave electrode theory,the coplanar waveguide(CPW)traveling wave electrode of the device is designed and optimized.The simulation results show that when the length of the modulation region is 5 mm,the halfwave voltage is 4.2 V,the modulation efficiency is 2.1 V cm,and the electrical 3 dB bandwidth is >100 GHz.When the input light wavelength is 1550 nm,the extinction ratio is about 35 dB.Without considering coupling loss and transmission loss,the on-chip insertion loss is only 0.36 dB.In order to further increase the modulation bandwidth,this paper optimizes the traveling wave electrode of the device,and designs a traveling wave electrode with a microstructure.On the premise of ensuring wave speed matching and impedance matching,the 3 dB bandwidth can be greater than 200 GHz.In the experiment,the preparation of the LNOI electro-optic modulator was completed,and the SEM and optical characterizations of the device were carried out.A test platform is built to test the LNOI modulator.The experimental results show that the half-wave voltage of the modulator chip is 4.3 V.The length of the modulation region is 5 mm,the modulation efficiency is 2.15 V cm,and the extinction ratio is greater than 20 dB.Using a lensed fiber for fiber-to chip coupling,the single-end coupling loss is about 5 dB.The measured3 dB bandwidth of the electrode is about 20 GHz,and the return loss S11 <-10 dB.In order to obtain lower coupling loss and thus lower insertion loss,three low-loss fiber-to chip coupler structures are simulated and designed in this paper.The simulation results show that the coupling loss of the two couplers using silicon oxynitride cladding is less than 0.5 dB;the coupling loss of the coupler using the subwavelength grating structure is less than 1.5 dB,and has the advantages of wide bandwidth and compact structure.