Design, Fabrication, and Characterization of Monolithically Integrated Acoustic and Photonic Devices on Lithium Niobate Over Insulator (LNOI) Platform
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Integration of acoustics and photonics devices on the same chip will enable various applications including: building miniaturized sensors, on-chip filtering and optical signal processing, high speed modulation, as well as non-linear optical devices. As an example of the capabilities enabled by such integration, we target the development of a rotation sensor gyroscope based on the acousto-optic effect. The gyroscope components are integrated on a Lithium Niobate Over Insulator (LNOI) substrate because it is a unique platform that exhibits exceptional acoustic as well as photonic properties. However, acoustics and photonics have never been integrated on such substrate, which required the development of a new fabrication process and the design of novel components.. The main challenges we had to overcome and resulted in innovative demonstrations of fabrication processes and devices are: Developing a robust fabrication process for etching lithium Niobate (LN) waveguides and integrating them with acoustic transducers: A robust fabrication process was developed on the LNOI platform, which can integrate patterning sub-micron features together with microscale ones on the same 3’’ substrate. Furthermore, the developed fabrication process enabled integrating metallic Al electrodes together with etched LN waveguides, which is required for building various components like electro-optic modulators and acousto-optic modulators. Coupling light in and out of chip: Gratings couplers were designed for optimum coupling of the TE polarized light. The optimization was based on FDTD simulation on LUMERICAL. The grating couplers realization enabled estimation of the light coupling loss in and out of the chip. The measured coupling loss was about 9 dB per coupler in the best case which is much more than the estimated from simulation. That difference is attributed to the alignment accuracy of the photonic chip. Integrating photonic waveguides/resonators and coupling light between them: LNOI waveguides and photonic resonators were designed and built. The photonic resonators enabled extraction of the losses of waveguides by monitoring the photonic resonator Quality factor, Q, or Finesse (F). Directional couplers (DCs) are commonly used as coupling elements to photonic resonators. However, etching narrow gaps in LN is a challenge that we avoided by using multi-mode interference (MMI) couplers, where butterfly MMI couplers were designed as coupling element to photonic racetrack (RT) resonators aiming for critical coupling condition. Additionally 3-dB MMI couplers were designed to be used as beam combiners in the Mach-Zehnder interferometer (MZI). The built RT resonators enabled extraction of the propagation losses in the etched LNOI photonic waveguides, which were found to be equal to2.5 dB/cm. Building high efficiency electro-optic modulators (EOMs): The EOM is used in the AOG to compensate for temperature variations and other environmental variation affecting the rotation measurement. The EOM realization enabled extraction of the electro-optic (EO) coefficient for the LN thin film, which permits to evaluate the magnitude of the control voltages required to stabilize the system. EOMs of two different types were demonstrated, one is based on a photonic RT while the other is based on an Asymmetric MZI (AMZI). The RT EOM represents the first demonstration for such device with etched waveguides in Y cut LNOI platform. Modulation bandwidth of 4 GHz, wavelength tuning rate of 0.32 pm/V and an ER of more than 10 dB were experimentally measured for the RT EOM. For the AMZI, a half wave voltage length product of 16.8 Vcm was experimentally measured. Although, it is not the best we can get from this LNOI platform because of our wide waveguides, feeding that EO coefficient to the AOG system model ensures that the temperature variation from -54 oC to 25 oC can be compensated by applying a maximum voltage of 64.5 V. Building efficient acousto-optic modulators (AOMs): The AOM enabled the extraction of the acousto-optic (AO) coefficient, which directly impacts the AOG scale factor (SF). Additionally, two different types of AOMs were demonstrated, one is based on an MZI embedded inside a SAW cavity while the other is based on a photonic RT whose coupling condition is under EO control. For the MZI AOM, the SAW resonator enhances the modulation efficiency due to the resonator Q such that the phase shift per square root of power extracted from the measurements is a factor of 3x higher than what previously reported on a GaAs platform, which makes it, to the author’s knowledge, effectively the highest AO modulation ever attained on chip. On the other hand, the EO tuned RT AOM showcases integration of various functionalities on same platform to build efficient AOM that can be operated at the desired wavelength. The EO tuning not only changes the operating optical wavelength but also ensures the critical coupling condition needed for efficient modulation. This design takes advantage of the unique AO and EO properties of LN, hence showcasing important building blocks for RF-photonic applications. By addressing all the previous challenges through the demonstration of high performance components, we were able to prototype the first acousto-optic gyroscope. That prototype represents the first demonstration of a novel rotation sensing technique, which combines the following advantages: (i) large mass (there is no suspended mass in the sensing mechanism and hence no limits on increasing the mass and no concerns about stiction issues during fabrication), and (ii) high shock resistance (since the sensing mechanism is strain based, the AOG has no moving parts that would not survive high G accelerations). The AOG SF is estimated comparing three photonic phase sensing techniques which are MZI, RT as well as RT coupled to MZI (MZI/RT). The phase sensitivity is estimated in terms of the cavity F for each technique. That theoretical analysis is verified by experimental measurement for the SF for both the MZI and the RT AOGs. The measured SF for the MZI is 48 nv/(o/sec) while it is about 9 nv/(o/sec) for the RT AOG. The SF is lower for the RT AOG because the Finesse (F~6) of the RT is not as high as expected. Nevertheless, these prototypes represent a proof of concept for our novel method for sensing rotation. Future work could prove that this AOG concept could be disruptive. Reducing the losses in the LNOI waveguide is a key challenge that can be overcome and has been already demonstrated by other groups showcasing 100x lower propagation loss. The estimated F from our model in that case would increase by approximately 50x, hence improving the gyroscope SF by the same factor. Further improvement of 100x is possible by increasing the SAW wavelength and Q. A separate challenge that needs to be addressed is the laser and photodetector integration on chip, which will reduce the coupling loss and the sensitivity to optical alignment.