Embodiments of the invention may relate to millimeter-wave electro-optic modulators, particularly such modulators fabricated using lithium niobate.
Existing electro-optic modulators have been limited by their structure to 110 GHz bandwidths or to narrow operational band widths at higher frequencies in the millimeter-wave range (30-300 GHz). In particular, known lithium niobate (LiNbO3: hereinafter, “LN”) electro-optic modulators are often limited in bandwidth by poor index matching and/or radio frequency (RF) energy leaking into the substrate. It would be desirable in some applications, such as high-speed data transfer and millimeter-wave imaging, to be able to overcome such issues, in order to obtain electro-optic modulators that operate at significantly higher speeds.
Various embodiments of the invention may involve several techniques, such as ridged co-planar waveguide (CPW) structure and/or thin LN substrate, for obtaining an electro-optic phase modulator. As a result of such techniques, a very good optical and RF index matching may be achieved, and substrate modes may be reduced or eliminated, allowing, for example, a 300 GHz operational bandwidth. Embodiments of the invention may include such modulators and/or methods for fabricating such modulators.
Various embodiments of the invention will now be described in conjunction with the accompanying drawings, in which:
Various embodiments of the invention may include a lithium niobate (“LN”) electro-optic phase modulator that may provide wide bandwidths, for example, but not limited to, a 300 GHz modulation bandwidth. Embodiments of the modulator may be based on a traveling wave electrode, which may, e.g., be made of gold, and which may be built on top of a waveguide, e.g., a titanium (Ti) waveguide, that may be diffused in a LN substrate.
In some embodiments, it may be desirable to operate over the full millimeter-wave spectrum (30-300 GHz). In order to operate at back end of the millimeter-wave (mmW) spectrum at 300 GHz, it may be necessary for modulated light traveling in the waveguide and a radio-frequency (RF) modulating signal propagating on the traveling wave electrode to travel at the same precise speed. Moreover, it may be desirable to eliminate, to the extent possible, substrate modes resulting from coupling of the RF energy into the substrate, which may ensure an optimal interaction between RF and optical signals.
As discussed above, and as shown in
The phase change may be directly proportional to the integral of the electrical field crossing the optical waveguide over the length of the signal electrode. As a result, the conversion efficiency of the device may depend on the strength of the interaction between the electrical field and the optical field along the length of the electrode. Therefore, one may wish, ideally, for both fields to travel at the same speed during their entire interaction. However, this velocity matching may represent a challenge, as the effective index of the mmW in LN is typically about 6, whereas it may be only 2.19 for the optical field for a 1550 μm wavelength in LN. This discrepancy in effective index may be addressed, in embodiments of the invention, by means of a ridge structure combined with thick electrodes and the deposition of a silicon dioxide layer between the signal electrode and the optical waveguide, shown as the “Buffer Oxide” in
The buffer layer may also contribute to reducing the mmW effective index, as well as preventing the optical field from scattering off the RF electrode. However, the buffer layer may also reduce the strength of the electric field crossing the optical waveguide. Therefore, one may need to set a thickness of the buffer layer to allow optimal interaction between the fields as well as index matching, which may be determined based on analytical and/or empirical methods.
Other parameters than index matching may be optimized in order for the modulator to operate at the desired bandwidth. One may wish to have the input impedance of the modulator be as close as possible to 50Ω to minimize the radio frequency (RF) insertion loss. Moreover, one may wish to keep the half-wave voltage Vπ and conduction and dielectric losses as low as possible. An optical and an electrical analysis may be performed to optimize the profile of the modulator structure in terms of efficiency. In an example of one such analysis by the inventors, to which the invention is not limited, it was determined that the ridge height H, the ridge width R, the electrode height T, the electrode width S, the gap G, the buffer layer B may be set to 3.6 μm, 11 μm, 24 μm, 8 μm, 25 μm and 0.9 μm, respectively. The LN substrate may also be thinned down, which may help to eliminate substrate modes coupling; this will be discussed further below.
In order to obtain such a modulator, a technique, such as the process shown in
a shows an example of a resulting CPW gold-plated structure with the LN substrate etched on each side thereof.
Following the above process, resist and seed layers may be removed, as shown, e.g., in
Returning to
Optical fibers may be attached to the resulting modulator. In an embodiment of the invention, polarization maintained optical fibers may be aligned and bonded to both end faces of the modulator using UV curable epoxy.
It is noted that variations on some of the above-noted techniques may be possible and that the order of operations may be varied under some circumstances.
Various embodiments of the invention have now been discussed in detail; however, the invention should not be understood as being limited to these embodiments. It should also be appreciated that various modifications, adaptations, and alternative embodiments thereof may be made within the scope and spirit of the present invention.
This work was supported by the Defense Advanced Research Projects Agency-Microsystems Technology Office, under Contract No. ______, and by the Office of Naval Research-C4ISR Applications Division, under Contract No. ______. The Government has certain rights in the invention.