Low Halfwave Voltage Electrooptic Polymer Modulator Technology
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2,
(1)
where ∆neff is the effective index change in the waveguide, L is the interaction length, and λ is the optical wavelength. The halfwave voltage of a modulator depends on the device architecture and materials used in device fabrication. A typical EO polymer modulator consists of a triple-stack polymer waveguide structure sandwiched between a pair of driving electrodes, as shown in Figure 1. The waveguide structure can be either a straight channel birefringence or a Mach-Zehnder interferometer type modulator.
V
ng
d
n3
∆
h
n1 substrate
Figure 1. The cross section view of a triple-stack polymer channel waveguide between modulation electrodes. When the modulation electric field is approximately uniform across the optical waveguide, halfwave voltage Vπ can be expressed as Vπ =
λh , ng3reff L Γ
(2)
where h is the separation between the driving electrodes, ng is the index of refraction of guiding layer, Γ is the overlap integral of the optical and modulation fields over active waveguide area, and reff is a generalized effective EO coefficient which may include the contributions from device architecture. For a given optical wavelength, obviously Vπ can be lowered by increasing effective EO coefficient reff, interaction length L, overlap integral Γ, or by reducing electrode separation h. Due to the modulation bandwidth, electrode impedance, and device insertion loss requirements, the changes in interaction length and electrode separation are limited. Therefore, the research focus is on improving the effective EO coefficient and overlap integral. Our goal here is to maximize the effective EO coefficient and the overlap integral through modulator architecture design and optimization. The influence of modulator architecture on the effective EO coefficient is listed in Table 1 for two typical EO polymer modulator designs. Table 1. Modulator architecture and effective EO coefficient Parameter reff Vπ
Straight Channel Birefringence Modulator r33-r13 Vπ =
λh ng3 (r33 − r13 )L Γ
OPTICAL PUSH-PULL DESIGN
Mach-Zehnder Interferometer Modulator r33 Vπ =
λh ng3 r33 L Γ
EO polymer allows us to define multiple alignment orientations inside a single film during electric field poling. A single driving electrode push-pull (optical push-pull) modulator can be implemented when two arms of a Mach-Zehnder modulator are aligned in opposite directions. Several optical push-pull EO polymer modulators have been demonstrated [2-4]. In our modulator design, we have used a single microstrip line electrode optical push-pull architecture for ultra-wide bandwidth applications [4]. As shown in the cross-section view of our EO polymer modulator structure and electric field poling scheme (Figure 2), the driving field is always along the poling direction in one arm and opposite in the other arm. The total phase change is twice larger than that in a non-push-pull modulator. Without modifying Eq. 2, we can use a generalized reff=2r33 to calculate Vπ value for the optical push-pull configuration. Obviously when compared with single arm
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