There are many frontiers left in the development of high-frequency, high-fidelity analog modulators, both theoretical and experimental. Some of these are listed below.
Experimental Work
On the experimental side of high-fidelity optical links, one is struck by how few experimental demonstrations have been made compared to the number of theoretical publications that have appeared. Certainly, a fruitful area of development would be simply to confirm experimentally the predictions of this thesis and other published schemes. It is clear that to do so, one must have one’s own modulator chip fabrication facility, one that is capable of precise parameter control, such as the coupling constant in lithium niobate waveguide directional couplers.
Without such careful control, it would be difficult to proceed. However, with such capability, it would be interesting to explore several areas experimentally as listed below.
The basic idea of making an antenna-coupled millimeter-wave modulator based on a directional coupler optical structure was proven very early on, albeit with a defective optical directional coupler structure (the “mouse bites”), Ref. [6.3]. This program was aimed at improving and optimizing the structure (mainly by removing these serious defects that were discovered in the original design). Unfortunately, we were unable to demonstrate our improvements, due to limited chip fabrication availability.
In the meantime, a new directional coupler structure has come to light that would be a much better choice for an antenna-coupled structure, the Y-fed DCM, Ref. [3.9]. This structure was described in detail in Chapter 4 of this thesis, since it is also a good candidate for linearization.
The Y-fed DCM is simply a DCM with equal optical powers fed to the two input arms, rather than all of the optical input fed into one arm. Due to this symmetrical feed, the transfer function changes dramatically. As discussed in Chapter 3, “zero” is automatically the correct bias for this modulator. This means that an antenna-coupled modulator need only the antenna-plus-transmission-line-segments to couple the r-f into the modulator. No extra bias electrodes are required (or desired, for that matter). This structure is a “natural” for antenna- coupled devices. Any future demonstration of antenna-coupled directional coupler modulators should use this much simpler configuration.
Another experimental area would be to confirm the predictions in this thesis for the improved bandwidth of the reflection modulators. These are relatively simple to make in the MZM versions, and should offer a significant improvement at low cost.
Once the directional coupler fabrication precision is established, an experimental demonstration of the “band-pass” characteristics of the DCM and YFDCM would be in order.
Finally it should be noted that the accuracy of photolithography defining the antenna structures is not even challenged at 100 GHz. It would be interesting to try modulating light at higher frequencies using the antenna-coupled schemes up to the point where lithium niobate has significant absorption, 0.5 THz.
One could also develop variations on the antenna elements themselves, such as “Vee”
antennas with gain replacing simple dipoles, as suggested in Sheehy’s thesis (but not demonstrated) could be investigated experimentally.
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CH-1
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