Florent Bassignot

High efficiency frequency doubling in fully diced LiNbO3 ridge waveguides on silicon

Nonlinear periodically poled ridge LiNbO3 waveguides have been fabricated on silicon substrates. Components are carved with only the use of a precision dicing machine without the need for grinding or polishing steps. They show efficient second harmonic generation at telecommunication wavelengths with normalized conversion reaching 204%/W in a 15 mm long device. The influence of the geometrical non uniformities of waveguides due to fabrication processes is asserted. Characteristics of the components are studied; notably their robustness and tunability versus temperature.

Acoustic resonator based on periodically poled lithium niobate ridge

The constant improvement of industrial needs to face modern telecommunication challenges leads to the development of novel transducer principles as alternatives to SAW and BAW solutions. The main technological limits of SAW (short-circuit between electrodes) and BAW (precise thickness control) solutions can be overcome by a new kind of transducer based on periodically poled ferroelectric substrate. The approach proposed in this paper exploits a ridge structure combined with a periodically poled transducer (PPT), allowing for the excitation of highly coupled modes unlike previously published results on planar PPTs. High-aspect-ratio ridges showing micrometer dimensions are achieved by dicing PPT plates with a diamond-tipped saw. An adapted metallization is achieved to excite acoustic modes exhibiting electromechanical coupling in excess of 15% with phase velocities up to 10 000 m·s-1. Theoretical predictions show that these figures may reach values up to 20% and 18 000 m·s-1, respectively, using an appropriate design.

Laterally coupled narrow-band high overtone bulk wave filters using thinned single crystal lithium niobate layers

This work is devoted to the development of laterally coupled filters built on compound single crystal substrates. High overtone bulk acoustic resonators are built on LiNbO3 thinned films bonded on thick LiNbO3 or Quartz substrates. Two resonators are coupled via a narrow gap between their electrodes, yielding the possibility for their modes to interact and to produce coupled mode resonance conditions. The coupling efficiency is shown to be dependent on the frequency (wavelength), yielding well defined coupled-mode filters at intermediate frequencies (300–800 MHz) and single mode transfer functions above 1 GHz. The implementation of an oscillator stabilized by such a filter at 1.7 GHz is demonstrated.

Fast-beam self-trapping in LiNbO(3) films by pyroelectric effect.

We report light-beam self-trapping triggered by the pyroelectric effect in an isolated ferroelectric thin film. Experiments are performed in an 8-μm-thick congruent undoped LiNbO(3) film bonded onto a silicon wafer. Response time two orders of magnitude faster than in bulk LiNbO(3) is reported. The original underlying physics specific of this arrangement is discussed.

Wideband Lithium Niobate FBAR Filters

Filters based on film bulk acoustic resonators (FBARs) are widely used for mobile phone applications, but they can also address wideband aerospace requirements. These devices need high electromechanical coupling coefficients to achieve large band pass filters.The piezoelectric material LiNbO3 complies with such specifications and is compatible with standard fabrication processes. In this work, simple metal--LiNbO3--metal structures have been developed to fabricate single FBAR elements directly connected to each other on a single chip. A fabrication process based on LiNbO3/silicon Au-Au bonding and LiNbO3 lapping/polishing has been developed and is proposed in this paper. Electrical measurements of these FBAR filters are proposed and commented exhibiting filters with 8% of fractional bandwidth and 3.3 dB of insertion losses. Electrical measurements show possibilities to obtain 14% of fractional bandwidth. These devices have been packaged, allowing for power handling, thermal, and ferroelectric tests, corresponding to spatial conditions.

A new acoustic resonator concept based on acoustic waveguides using silicon/periodically poled transducer/silicon structures for RF applications

We propose a new acoustic resonator concept based on a periodically poled transducer (PPT) in a piezoelectric substrate (LiNbO3 or LiTaO3), embedded between two guiding substrates in order to create an acoustic waveguide. A resonator operating at 131MHz have been successfully fabricated and used in order to stabilize an oscillator. However the fabricated resonator presents a significant thermal sensitivity. The following experiments have consisted in studying a Si/thinned PPT layer/Si in order to reduce the thermal sensitivity.

An acoustic waveguide using doubly-bonded silicon/thinned PPT/silicon structures for RF applications

In this paper, we present new results on the development of piezoelectric transducers based on periodically poled ferroelectric domains in a lithium niobate plate bonded between two silicon wafers. The fabrication of the periodically poled transducers operating in the range 50 – 500 MHz has been achieved on a 3 inches 500 µm thick wafer. These devices then have been bonded on silicon wafers to fabricate a waveguide. Guided elliptic as well as partially guided longitudinal modes are excited. The experimental responses of the tested devices are compared to predicted harmonic admittances, showing a good agreement between both results and allowing for a reliable analysis of the nature of the excited modes. We also show interesting studies of material combinations used to guide ultrasonic waves. Dispersion properties have also been studied for a structure Si/PPT/Si. Operating points corresponding to a specific thickness/period ratio are found. Therefore a new conception with a Si/thinned PPT/Si structure is fabricated.

Periodically poled LiNbO3 ridge waveguides on silicon for second-harmonic generation

Nonlinear periodically poled ridge LiNbO3 waveguides have been fabricated on silicon substrates. Components are micromachined with a precision dicing machine and/or by grinding or polishing steps. They show efficient second harmonic generation at telecommunication wavelengths with normalized conversion reaching 600%/W in a 20mm long device. Influence of geometrical non uniformities of waveguides due to fabrication process is asserted. Components characteristics are studied notably their robustness and tunability versus temperature.

Ridge-shaped periodically poled transducer for wide band R-F filter

Transducer on Periodically Poled Lithium Niobate (PPLN) exhibiting an electromechanical coupling about 20% and an equivalent phase velocity about 10 km.s-1 were shown in [1]. These transducers are 3D structured to overcome Surface Acoustic Wave (SAW) and Bulk Acoustic Wave (BAW) technological limits, such as short circuit between electrodes for SAW and precise thickness control for BAW. The present work shows the use of these 3D structured transducers for Radio Frequency (RF) filtering. An acoustic filter showing a band pass of about 5MHz has been realized on Lithium Niobate at 250MHz. This filter is made of PPLN exhibiting a poling period of 14.9μm.

New acoustic resonator based on periodically poled transducer in lithium niobate or tantalate

In this paper, we propose a new concept of acoustic resonator based on a waveguide structure. In the telecommunication market, composants used are indeed generally bases on Surface Acoustic Wave (SAW) or Bulk Acoustic Wave (BAW) devices. However, those systems present technological limits as short-circuits between the electrodes of the interdigital transducers (for SAW device) or the precise control of the piezoelectric material thick resonator (for BAW device). We suggest a new concept based on a periodically poled transducer (PPT) in a ferroelectric substrate (LiNbO3 or LiTaO3), embedded between two guiding substrates in order to create an acoustic waveguide. Periodically poled transducers have been investigated recently as an alternative to classical inter-digital transducers for the excitation and detection of guided acoustic waves. A resonator operating at 131MHz has been successfully fabricated and used in order to stabilize an oscillator at this frequency. However, the developed resonator presents a significant thermal sensitivity. The following experiments have consisted in studying a Si/thinned PPT layer/Si structure in order to reduce the thermal sensitivity.