60GHz Polymer Electro-optic Phase Modulators

Direct modulation has a limited bandwidth and requires high quality, low chirp semiconductor lasers. External optical modulation permits the development of low cost, high frequency optical fiber data links using relatively simple semiconductor lasers. As the volume of communication increases, millimeter wave frequency modulation is desired. LWO3 and semiconductor modulators become increasingly difficult to fabricate at these frequencies. Organic nonlinear optical(NL0) materials have the potential to become very important for making photonic devices that are inexpensive to produce, capable of being integrated with other electronics, and suitable for ultrahigh frequency broadband operation[ 13. We have reported fabrication of traveling wave electro-optic (E-0) phase modulators using a themally cross-linked NLO polymer, PUR-DR19[2, 3, 41. The devices have shown temporal stability over 2 years, and good power handling capability. In this paper, we report new measurement results up to 60GHz using an optical heterodyne detection technique. This is the highest modulation frequency reported so far for polymer E-0 modulators. Our devices have a three layer stack structure with buried straight channel optical waveguides, integrated with a 50Q gold microstrip line circuit. A fused silica slide covered with a gold ground plane was used as the substrate, and commercial polyurethane was used as the cladding layers. Corona poling and thermal cross-linking were performed in the same temperature ramp. The channel optical waveguide structure was defined using standard photolithography and oxygen reactive ion etching on the poled and cross-linked polymer layer. An etching depth of 0 . 6 ~ and waveguide width 8pm were chosen to compromise the mode quality and waveguide loss. Single mode operation was achieved by proper launching. The end surfaces of the device were cut with dicing saw and carefully polished for end-fire coupling. Two Wiltron V launchers were used for coaxial component connections. The devices typically have about 13dB optical insertion loss with a pair of 20x microscope objectives, and an on-off ratio of better than 13dB. We have used 30mW at wavelength of 1.3 19pm of input optical power to test the device durability for 24 hours, which is equivalent to an optical power density of about 10w/cm2 inside the waveguide, and no change in half-wave voltage was observable within our experiment error. Optical power levels from 10 to 20 mW have frequently been used for system alignment, and up to 40-5OmW were used for short times in other measurements where high signal to noise ratios are desired. We have devices that are made of films poled over 2 years ago and have been under test for over 12 months in ambient conditions. No change in half-wave voltage was observed for these devices. The device frequency response was obtained initially up to 40GHz from direct detection using fast photodetectors and external microwave mixers in a polarization modulator configuration[5]. For higher frequency measurements where commercial photodetectors and mixers start to degrade, we used an optical heterodyne technique to directly observe the phase modulated optical signal[6]. The beam of the first diode pumped Nd:YAG laser at 1 . 3 1 9 ~ was end-fie coupled in and out of the modulator with microscope objectives. The second laser, used as the local oscillator, was aligned co-linearly with the phase modulated output light from the modulator to a low frequency detector. The photo current excited in the detector, for small signal modulation, has a term that is proportional to