Optical Contants And Y-branch Waveguides Of Single-crystal Lithium Niobate Thin Film

Lithium niobate?LiNbO₃,LN?is one of the most widely used optical crystals,due to its excellent electro-optical,acoustic-optical,piezoelectric,pyro-electric,photorefractive,and non-linear optical properties.In addition,it has stable mechanical properties,and it is resistance to corrosion.LN is easy to grow with abundant raw material sources.In recent years,single-crystal LN thin film?lithium niobate on insulator,LNOI?,which is fabricated by the crystal ion slicing and the wafer bonding technologies,has become a hot research direction in the integrated optics.Due to the high refractive index contrast and strong confinement of the light,waveguide with small cross-section and bending loss can be made on LNOI,which can greatly improve the integration of the integrated optics,reduce the volume of devices,power consumption,and threshold value.The absorption coefficient and the refractive index of LNOI in the near UV to visible wavelength are important physical parameters in the research field such as design and fabricate electro-optic modulator,and non-linear phase matching devices,but there is little experimental data reported on these parameters currently.We use transmission spectroscopy to measure the absorption coefficient and the refractive index of LNOI.The transmission spectroscopy has the advantages of wide wavelength range,the simple test procedure,the high precision,and no damage to the sample.It is based on the measurement of the extremes in the interference fringes.By fitting the envelope of maxima and minima of the interference fringes of the transmission spectra,the real and imaginary parts of the complex refractive index,absorption coefficient and also film thickness in both weak absorption region and transparent region can be obtained.We choose quartz as substrate for LNOI,because quartz has wide transmittance range in the near UV to infrared band.LN thin film is Z-cut,because the Z-cut sample has no birefringence effect when the transmission spectrum is measured.Optical communication plays an important role in modern society,and it has changed the way that people communicate,access to and share the information.The electro-optic modulator is an important device in optical fiber communication.Especially,high-speed Mach-Zehnder?M-Z?electro-optical?E-O?modulator with low driving voltage and wide modulation bandwidth,is always one focus of the optical research area.LiNbO₃ is used as the waveguide material for high-speed M-Z electro-optical modulator,due to its large electro-optical coefficient,and it has a good transmittance in the visible and near-infrared band.The optical path of M-Z electro-optical modulator is composed of two symmetrical Y-branch waveguides.The electrode is using traveling wave electrode structure.When the light wave and the microwave have the same velocity,a high-bandwidth modulation can be achieved.We have cooperation with a USA company,and Helmut Schmidty University of Germany to develop high-bandwidth modulator.We are responsible the LN thin film fabrication and design of optical path of the M-Z modulator.To increase the velocity of the microwave to match the velocity of the light,quartz is selected as the substrate,due to its low dielectric constant.In this way,EO modulators with bandwidth over 200 GHz are expected.The main content of this paper includes two parts:1.The measurement of refractive indices and absorption coefficients of single-crystal LiNbO₃ thin film by transmission spectroscopy.2.Y branch design of Ti-diffused waveguide in LNOI for high-speed M-Z modulator.1.The measurement of refractive indices and absorption coefficients of single-crystal LiNbO₃ thin film.A 3-inch single crystal LN thin film on quartz was fabricated by the ion implantation and the wafer bonding technologies.The transmission spectrum of the Z-cut LNOI was measured by a dual optical path UV-visible spectrophotometer.The envelope method was used to calculate the ordinary refractive index of the films at different wavelengths,and the Sellmeier formula was obtained.Compared with that of the congruent bulk LN,the refractive index of the film was slightly smaller.The LN thickness was about 520.7 nm and the absorption edge was about 314 nm.Using the prism coupler,the thickness of the LN thin film was measured to be 521.5 nm,which was well matched with the data?520.7?measured by transmission spectrum.The ordinary refractive index of LN thin film was 2.279 and 2.2002 at wavelength of 633 nm and 1539 nm,respectively,which was very close to the values measured by transmission spectrum.2.Design the LN thick:ness in Mach-Zehnder modulator By simulartion with Rsoft software,the effective refractive index of Ti-diffused waveguides with different LN thin film thicknesses was calculated.It was found that when the LN thin film thickness was approximately 7.8 ?m,the light wave velocity matched well with the microwave velocity,and the modulator would have high bandwidth.3.Study of Ti-diffused waveguides in LNOIThe Ti-diffused waveguide modes were simulated by Mode Solution software.It was found that the Ti stripe width on LNOI for single-mode conditions for TE,TM modes were less than 0.6 ?m and 1 ?m,respectively,while the Ti stripe width on the bulk LN material for TE,TM modes were both less than 6.5 ?m.The number of the waveguide modes in the LNOI was larger than that in the bulk LN material.4.Study of Ti-diffused Y-branch waveguides in LNOIThe relationship between the bending loss and the radius of the waveguide at a wavelength of 1.55 ?m was simulated.When the bending radius was greater than 1 cm,the loss was below 1E-3 dB/cm.The relationship between the transmittance of the Y-branch waveguide and the width of the Ti strip was simulated by Rsoft.When the Ti strip width was less than 5 ?m,the Y-branch output power in LNOI was higher than that in bulk LN.The highest Y-branch output power was up to 90%.When a fiber was used to couple the light into the Y-branch,taking the end-face reflection into account,the highest Y-branch output power was up to 75%.