Sang-Shin Lee

Three-dimensional integrated optics using polymers

Some of the key components are demonstrated to make three-dimensional (3-D) optical integrated circuits possible using polymers. Fabrication techniques of shadow reactive ion etching, shadow photolithography, and gray-level photolithography to produce complex 3-D integrated optic structures are demonstrated. Vertical waveguide bends exhibit excess losses of <0.3 dB, and vertical power splitters possess predictable output splitting ratios between multiple core levels with excess losses of <0.5 dB. Vertical polarization splitters exhibit power extinction ratios of 15 dB between the output core layers. A 1/spl times/4 vertical-horizontal power splitter is also demonstrated. Additionally, these techniques are used to integrate different polymer materials into the same optical circuit while easily solving the mode mismatch problem. To show the technique, a polymer electrooptic modulator is vertically integrated with a low-loss waveguide.

Polymer electro-optic devices for integrated optics

Abstract Recent advances in polymer electro-optic polymers and in fabrication techniques have made possible advances in polymer optical guided wave devices which bring them much closer to system ready. The processing of a new thermal set FTC polymer and its incorporation into a high-frequency, low-Vπ optical amplitude modulator are reviewed. The design and fabrication of 100 GHz modulators and their integration with rectangular metal waveguides using an anti-podal finline transition with a flexible Mylar substrate is discussed. High-speed polymer modulators with balanced outputs and the in situ trimming of the output coupler is described. More complex guided wave devices using polymers are demonstrated by the photonic rf phase shifter. Techniques for integrating both passive and active polymers into the same optical circuit without the need for mode matching is presented and demonstrated. To reduce the Vπ of a polymer amplitude modulator to 1 V or under, a technique of constant-bias voltage is demonstrated. Finally, a technique to directly laser write electro-optic polymer devices is reviewed.

Demonstration of a photonically controlled RF phase shifter

Integrated photonic radio frequency (RF) phase shifters with dc voltage control have been realized using a nested dual Mach-Zehnder modulator configuration in a new nonlinear optical polymer, CLD2-ISX. These modulators have a V/sub /spl pi// of 10.8 V and exhibit excellent frequency performance measured up to 20 GHz. A near linear phase shift exceeding 108/spl deg/ was obtained for a 16-GHz microwave signal by tuning the dc control voltage over a 7.8-V range. It is expected that these integrated polymer phase shifters will find widespread applications in new types of lightweight optically controlled phased array systems.

Vertically integrated waveguide polarization splitters using polymers

Design considerations, fabrication procedures, and experimental results are presented for vertically integrated waveguide polarization splitters. These devices separate the TE and TM polarizations onto different vertical levels of a three-dimensionally integrated optic circuit. This provides increased integration density for polarization diverse structures. The vertical polarization splitters utilize birefringent polymer materials to create an adiabatic structure. Experimental results yielded extinction ratios as high as 22 and 12 dB for TE and TM polarizations, respectively.

Optical intensity modulator based on a novel electrooptic polymer incorporating high /spl mu//spl beta/ chromophore

We have synthesized a novel electrooptic (EO) polymer based on a high /spl mu//spl beta/ chromophore incorporating tricyanobutadiene acceptors. A crosslinked polyurethane network was also adopted to enhance its thermal stability. In order to find the optimum poling condition for the polymer, the influence of the electric poling profile on optical characteristics such as the EO effect, thermal stability, and damage was investigated. Then a high-speed intensity modulator using the EO polymer was designed and fabricated. The measured half-wave voltage V/sub /spl pi// was 4.5 V at the wavelength of 1.31 /spl mu/m. Accordingly, the achieved EO coefficient r/sub 33/ was as high as 25 pm/V, and the thermal stability of the poled polymer was as high as 95/spl deg/C. Finally, the modulator was successfully operated up to 40 GHz.

High-frequency polymer modulators with integrated finline transitions and low V/sub π/

Ultrahigh-speed integrated electrooptic polymer phase modulators have been fabricated and tested. They are made from a new nonlinear optical polymer, amino phenylene isophorone isoxazolone (APII), and are incorporated with integrated high-speed electrode transitions for W-band (75-110 GHz) operation. This new polymer has also been used to fabricate Mach-Zehnder modulators. These devices show good performance over a wide frequency band ranging to 40 GHz and have a V/spl pi/<10 V. The measurements establish APII, and other chromophores specially designed to minimize chromophore-chromophore interaction, as strong contenders for fabricating modulators for commercial and military applications.

Hybrid Si-LiNbO 3 microring electro-optically tunable resonators for active photonic devices

Hybrid Si-LiNbO₃ electro-optic tunable ring resonators have been proposed and demonstrated as a path to achieving ultracompact and high-speed electro-optic devices. Free standing single crystal LiNbO₃ microplatelets (~mm long and ~1 μm thick) were obtained from a z-cut LiNbO₃ substrate by ion implantation and thermal treatment. The platelets were transferred and thermally bonded on top of Si resonators that were fabricated in a Si-on-insulator platform by a 0.18 μm standard complementary metal-oxide-semiconductor process. For the hybrid microring resonator, a free spectral range of 16.5 nm, a finesse F of ~1.67 × 10², a Q-factor of ~1.68 × 10⁴, and an effective r coefficient of ~1.7 pm/V were achieved for the TE mode. These values are in good agreement with the calculated results.

DC biased electro-optic polymer waveguide modulators with low half-wave voltage and high thermal stability

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 half-wave voltage. This technique makes use of the total achievable electro-optic coefficient, which can be more than three times the value that is used by the conventional devices of poled electro-optic polymer. As the result of the DC bias and with high-μβ chromophores, a low half-wave voltage of 1.5 V was achieved with 2-cm-long birefringent waveguide modulators at the wavelength of 1.3 μm. Results of a 200°C stability experiment indicate that this scheme also enables electro-optic polymer devices to meet the short-term high-temperature stability requirement because the polymer does not need to be poled prior to high-temperature steps.

TM-pass polarizer based on a photobleaching-induced waveguide in polymers

A polymer TM-pass polarizer is demonstrated by integrating photobleaching (PB) waveguides, which support only the TM mode, with mode-matched rib waveguides formed by reactive ion etching (RIE). The input and output rib guides support both TE and TM modes. The PB can be controlled to closely match the mode profile of the photobleached guide to the mode profile of the input and output rib waveguides in order to reduce the insertion loss. The polymer selected for photo-bleaching was PMMA-DRI. This electrooptic polymer has a PMMA backbone with the azo dye Disperse Red I side chain attached to the backbone.

Integrated optic vertical polarization splitters using polymers

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.