Once we had demonstrated that the basic idea of matching the phase velocities using antenna-coupling, we wanted to demonstrate a more practical design.
The prototype designs at 10 GHz and 60 GHz were narrowband phase modulators operating at 0.633 pm. In practice one would like to see broadband amplitude modulators at 1.3 pm or 1.5 pm. So we had three changes to make:
~ ~ t - l - l - l - ~ ~ ~ '
Dimensions flier ofis
Figure 6.1 60 G H z Dipole Modula?or
Gap between electrodes may be difficult t o r e s o l v e a t t h i s scale,
design a broadband antenna/ transmission line segment; design an amplitude modulator instead of a phase modulator; change the optical waveguide for 1.3 or 1.5 pm optical wavelength.
It was relatively easy to design the modulator for the new optical wavelength, requiring only a change in the dimensions of the optical waveguide (see Appendix C), although we did not yet have a suitable laser source for use with the new modulator. Designing an amplitude modulator meant splitting the optical signal into two paths to make a Mach-Zehnder amplitude modulator.
The optical splitting and recombining is achieved using Y-junctions. The Y- junction is a transformation of the optical waveguide from a single guide into two separate waveguides (or vice-versa), with the power split equally between them. This is done by first gradually widening the input waveguide, then splitting it in the center and gradually moving the two halves away from each other. The angle at which the two sides diverge is usually less than lo in order to minimize optical losses. The waveguides must be separated by enough distance to ensure that there will be no cross-coupling between them, and because of the small divergence angle this can take considerable distance -
several millimeters, in fact. Consequently the Y-junction takes up quite a bit of the space on the 25 mm modulator substrate. Again, however, this is not a difficult design issue.
There were two difficult design issues: first, designing a broadband antenna/transmission line segment; second, arranging for DC bias to be applied to each of the segments. Using the theoretical model developed for the 10 GHz prototype, I established that if the antenna could be made to have a constant
impedance which was similar to the characteristic impedance of the transmission line electrode, the bandwidth of the antenna/transmission line segment would be large. Constant-impedance or near-constant-impedance antennas exist as frequency-independent antennas or nearly-frequency- independent antennas such as spiral antennas and log-periodic antennas.
However, it turns out that these antennas are too big for this application. The antennas are so large that very few antenna/transmission line segments would fit on the substrate. Instead we chose to use short bow-tie antennas. Long bow-tie antennas are frequency-independent, but have no main beam on the axis of symmetry, and so would not be very useful here. Compton et al. [l]
have pointed out that long bow-tie antennas are traveling-wave antennas, and each half of the antenna transmits its own beam into the substrate. Essentially the traveling wave in each half of the antenna radiates downward into the substrate at an angle to the direction of propagation of the wave in the antenna arm. Short bow-tie antennas a,re not frequency-independent, but are quite broadband [I] and have antenna patterns similar to dipole antennas. In addition, these antennas are reported to be less sensitive to perturbations such as bias-connections made at their ends.
The length of the transmission line electrode no longer had to be chosen to resonate with the antenna at the center frequency. However, the choice of length was limited by phase-velocity mismatch considerations, which prevented any significant increase in the length of the electrode. We used electrodes whose length was X/2, where X is the effective wavelength in the electrodes, i.e., X,/3.8.
The substrate to be used was X-cut, which meant that the optical waveguides needed to be modulated with a field parallel to the surface of the substrate.
With only two conductors in the transmission line electrode, we chose to position the antenna/transmission line segments so that one optical waveguide ran along the center of the transmission line electrode gap (as in Figure 5.3), with the other optical waveguide running under the metal of one of the conductors, well away from the gap. The electric field in this second region is very small.
The design of the 60 & 94 GHz broadband amplitude modulator metal masks is shown in Figure 6.2. The figure shows close-up views of individual segments as well as each entire modulator. The close-up view shows the bow-tie antenna and electrode (the gap between the electrode conductors cannot be seen at this scale). At the ends of the bow-tie antenna the antenna does not end abruptly, but is continued at each end by metallization whose width is equal to the width of the end of the antenna. This is intended to reduce the amplitude of the reflection at the end of the antenna, thereby increasing the bandwidth as much as possible. The end of this metallization is believed to have negligible effect on the antenna's behavior (because the RF is expected to have radiated most of its power before reaching this point), so it is connected to a bias-pad by a thin metal strip, as shown in the view of the entire modulator mask. The performance of these modulators will be discussed in a later section.