Improvement of electro-optic effect and novel waveguide structure in hybrid polymer/sol-gel modulators

Abstract

A thermally stable crosslinkable electro-optic (EO) polymer is successfully contact-poled in a hybrid EO polymer/sol-gel waveguide modulator. The highest EO coefficient is demonstrated when the EO polymer is poled with a low resistivity sol-gel cladding layer. We achieve the highest in-device EO polymer r 33 coefficient of 170pm/V at 1.55µm, which has not been possible with previously reported polymer claddings. A sol-gel waveguide/cladding plays the main role in our hybrid approach not only for poling efficiency, but also for low coupling loss, and stable coupling and waveguiding. The coupled light from the optical fiber propagates through the sol-gel input waveguide and adiabatically vertically transits between the sol-gel core and the EO polymer core. A phase modulator and a Mach-Zehnder modulator with a 2.4cm-long electrode and an inter-electrode distance of 15µm demonstrated a half wave voltages (V π ) of 2.5V and 1.0V at 1.55µm, respectively. We also examine a push-pull poled Mach-Zehnder waveguide modulator. INTRODUCATIONPolymeric electro-optic (EO) modulators have been demonstrated to operate up to 110GHz 1, 2 due to the low dispersion of their dielectric constants, compared to those of inorganic materials. Polymeric EO modulators have been reported using an all polymeric approach that consist of an EO polymer core and a passive polymer cladding to achieve low half wave voltages 3-8 (V π ). Until recently, the highest reported EO coefficients in polymer waveguide modulators were 36pm/V at 1.55µm 6, 7 (near the 30pm/V of commercially available inorganic LiNbO 3 ) and 58pm/V at 1.3µm 4, 5 . The lowest V π at 1550nm for Mach-Zehnder modulator using the push-pull poling technique 3-7 was 1.8V with an electrode length (L) of 20mm and relatively small inter-electrode distance (d) of 7.5µm. Since present generation EO polymers have an absorption peak near 800nm , the V π at the near resonance wavelength of 1.3µm have shown lower values (0.8 V 4, 5 and 1.2 V 7 for L of 30mm and 20mm, respectively) than at 1.55µm.Even though single layer EO polymer films have demonstrated r 33 of 92 pm/V at 1.06µm (near resonance), smaller r 33 values exist at off resonance wavelengths, which resulted in V π of 3.7V at 1550nm with L of 20mm and d of 9.0µm 8 . Previous EO coefficients in all-polymeric approaches have shown low poling efficiencies 9 of ~30% for an EO polymer core sandwiched between low conductivity polymeric-cladding-layers, despite the fact that single EO polymer films have shown high EO coefficients when poled between electrodes without other layers. Moreover, coupling losses, thermal stability and optical stability were reported as critical issues, due to the smaller mode field diameter and its modification deriving from index modification of the EO polymers caused by the EO polymers' thermal and photochemical reactions 10 .On the other hand, the hybrid approach has now demonstrated the highest poling efficiency 16,17 in a waveguide modulator device, following rapidly after the first hybrid EO polymer/sol-gel mod...

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