Advantages of Modal Dispersion Phase-Matching and Materials Requirements for Polymeric Devices Using Efficient Second Ha
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ABSTRACT Modal Dispersion Phase Matching appears to be currently much better adapted to parametric mixing in polymeric material waveguides than Quasi Phase Matching. For second harmonic generation at telecommunication wavelengths, using organic materials should allow better performance than with ferroelectric crystals. Promising results are expected in view of theoretical expectations and continuously improving experimental past and current results. INTRODUCTION Polymers have recently penetrated photonics in a number of areas. Notable examples are electro-optic modulators operating around 100 GHz, photorefractive media with record writing efficiencies, and now electroluminescent and even lasing devices. Parametric mixing, most specifically second harmonic generation was initially tried as early as 1990 with polymers for doubling diode lasers into the blue. However, for this application, the transparency-nonlinearity tradeoffs proved problematic and, although there were sporadic reports of second harmonic generation, this type of parametric mixing in poled polymers was effectively abandoned. For incident wavelengths in the communication bands, however, and most specifically at 1550 nm, the transparency-nonlinearity tradeoffs are very favorable for poled polymers. In this wavelength range there are many applications ranging from frequency shifting in Wavelength Division Multiplexing (WDM), to cascading for all-optical signal processing and to tunable frequency generation via optical parametric generation and amplification. The primary prerequisite for these applications is to obtain very efficient SHG. Quasi-phase-matched LiNbO 3 channel waveguides are currently the material of choice for these applications but this kind of device is near its theoretical limits, which could be by far surpassed by doped polymer based components. This is the main motivation for our investigation in this field. In this paper, we shall first briefly recall the basic equations governing second harmonic generation (SHG) and the associated figures of merit (FOMs). Two phase-matching techniques were considered as potentially useful, namely Quasi Phase Matching (QPM) and Modal Dispersion Phase Matching (MDPM). Then, we shall discuss how poled polymer devices were realized and characterized. These devices were based on the well-known stilbene DANS, (4dimethylamino-4'-nitrostilbene) and an azobenzene DR (disperse red) chromophores. Finally, we will speculate about the ultimate potential of poled polymers for parametric optical processes at telecommunications wavelengths, and the materials challenges that must be met for these applications to become a reality. 179
Mat. Res. Soc. Symp. Proc. Vol. 488 ©1998 Materials Research Society
THEORETICAL EXPECTATIONS Second-Harmonic Generation The SHG process is usually studied first in the limit of low conversion for the fundamental beam. In a channel waveguide capable of two-dimensional confinement of the optical beams, the wavevector detuning is defined as A = 13(20o) - 213(o) = 44[N(2co)-N(o))]/X, w
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