Polymer waveguide optical phased array (OPA) beam scanners can achieve stable beam steering with low driving power owing to the large TO coefficient and low thermal conductivity of the polymer material. However, polymeric OPAs with low refractive index contrast exhibit narrow field-of-view (FOV) because of the wide pitch of the output waveguide channels compared to their counterparts of Si and silicon nitride (SiN) photonic integrated circuit platforms. Meanwhile, SiN waveguides offer stronger mode confinement with smaller waveguide core sizes than polymer waveguides, and they can handle high optical power without nonlinear effects. Through monolithic integration of polymer and SiN waveguides using adiabatic transition tapers, a high-performance phase modulator (PM) is achieved for the purpose of demonstrating an OPA device with low power consumption, wide scanning angle, and fast response.
A hybrid structure of Si-LiNbO3 micro-ring resonator was fabricated. Free standing single crystal LiNbO3 microplatelets
(mm long and 1 um thick) were obtained from a bulk LiNbO3 wafer by ion implantation and thermal shock.
They were then transferred, positioned and bonded to Si micro-ring structure. In this hybrid structure, a large portion of
the TM field is located above and below the Si waveguide. Then, the effective index of the Si waveguide can be changed
by varying the refractive index of the LiNbO3 cladding layer. Theoretical calculation with finite difference method
proved that the ratio between effective index change of the Si waveguide and index change of LiNbO3 cladding layer
was 0.31 (ΔnSi/ΔnLNO=0.31) for TM mode. Then, calculated ΔnSi was about 1×10-4 with 3 V. The effective r coefficient
of Si and tuning sensitivity were about 7.2 pm/V and 2.55 GHz/V, respectively. These values are comparable to current
active Si photonics with plasma dispersion methods. In addition, high speed modulation (over 40 GHz) is possible in this
hybrid structure. This demonstration of a single crystalline LiNbO3 acting as the upper cladding shows the possibility of
integrating a very good EO and NLO material into the silicon-on-insulator photonics technology.
A refractive index sensor based on a three-layered polymeric waveguide was proposed and demonstrated. A high-index
thin film in TiO2 was placed on top of the waveguide in the sensing region, playing the role of strengthening the
evanescent field to enhance the sensitivity of the sensor. The refractive index of the analyte applied to the surface of the
sensor was estimated by observing the count of the polarimetric interference between the TE and TM polarizations,
which is manifested as a periodic variation in the optical output of the sensor. For a fabricated sensor involving a 20 nm
thick TiO2 film, the sensitivity was found to be equivalent to 1.8x10-3 RIU. It was found to be enhanced by increasing the
thickness of the high-index overlay to a certain degree.
We present design and fabrication considerations for a vertically integrated electro-optic polymer modulator. The hybrid design incorporates both passive and active core segments for optimized transmission and modulation of an optical signal. When compared to traditional structures, this vertically integrated modulator potentially reduces fiber coupling and propagation losses by more than 10 dB for a 6 cm structure while maintaining a minimized V(pi ).
The full potential of second order nonlinear polymers can be utilized in electro-optic polymer modulators with a DC biased operation scheme to greatly reduce the V(pi ). This technique makes use of the total achievable electro-optic coefficient, which can be more than three times as high as the residual value after the fast partial relaxation following corona or contact poling. As the result of the DC bias and with high (mu) (beta) chromophores, a low V(pi ) of 1.5 V was achieved with 2 cm long birefringent waveguide modulators at the wavelength of 1.3 micrometer. Results of 200 degrees Celsius stability experiment indicate that this scheme also enables electro-optic polymer devices to meet the stability required for high temperature hermetic sealing because the polymer does not need to be poled before device packaging.
We present design considerations and fabrication results for a vertically integrated waveguide polarization splitter. Fabrication techniques of shadow reactive ion etching (RIE) and variable photolithography exposure produced the required vertical waveguide structures. The fabricated vertical waveguide bends exhibit excess loss of only 0.2dB. By constructing this vertical bend with a birefringent polyimide, simulation results show the possibility of a polarization splitter with an extinction ratio of over 15dB. We demonstrate preliminary waveguide experiments showing the practicality of these structures as three dimensionally integrated optical devices.
A digital optical switch with a modified coupling region is designed by using the beam propagation method, and it is fabricated by employing an electro-optic polymer rib waveguide formed by the reactive ion etching method. The structure of the modified coupling region and its refractive index profile are designed to optimize the mode coupling in the Y-branch, since the coupling determines the switching performance. Therefore, the switching performance of the device may be enhanced with a fixed device length. The practical optimization of coupling region is performed by using the photobleaching method after the device fabrication. The crosstalk has been experimentally improved by as great as 4 dB by introducing the modified coupling region. The drive voltage is also reduced by more than 30 percent.
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