Design and characterization of modules for millimeter-wave imaging applications
Interest in the millimeter-wave spectra is growing rapidly as numerous radio frequency (RF) applications are migrating to higher frequencies. In particular, imaging receivers are finding new applications that promise unique advantages and improved performance. However, the successful implementation of millimeter-wave frequencies depends on how researchers resolve the new set of challenges and constraints associated with these applications. To address some of these challenges, I present my work on the development of W-band modules for millimeter-wave detection at 95 GHz that opens the door for imaging applications in a simpler and cost-effective way. Accordingly, an unique approach used a broadband modulator to upconvert the W-band signals to the optical domain. This modulator must be packaged in such a way that it can be coupled to an antenna efficiently.
In this regard, I present two different transitions from the modulator coplanar waveguide (CPW) to rectangular waveguide for the packaging. The first method is applicable to the vertical integration of a horn antenna. Full W-band operation was achieved using a slot coupled stacked patch antenna to excite the fundamental waveguide mode. The second model was designed for an in-line transition using a cosine shaped metallic fin to gradually lift and rotate the electric field of the CPW to match the field of the waveguide. These two transitions were developed to individually package the modulators. However, the key to successful integration of detector elements, especially in the case of large imaging arrays (∼200 elements), is to use multi-chip module (MCM) integration.
The MCM technique allows for the integration of multi-functional chips in a single package and thus it not only reduces the size and weight of a system but also brings down the packaging cost. A scheme for integrating the modulator with other imager circuit elements was developed using a ribbon bonding technique. This involved the proper choice of material for the chip carrier substrate and an investigation of the CPW and ribbon bonding parameters. Following this, a transition to the rectangular waveguide was introduced using a rectangular probe to couple the electric field from a microstrip to the waveguide. Metallic vias were used to synthetically reduce the electrical size of the substrate, which facilitated direct integration of the probe on a much wider chip substrate without exciting the unwanted modes.
Further reduction of the volume can be achieved by using a distributed aperture technique, which provides a large effective aperture for high resolution images without the associated volumetric scaling. However, the bandwidth of such a system is limited by the maximum baseline of the aperture and field of view. Therefore, to reject the undesired signal and improve the image contrast, a bandpass filter was designed.
A major part of this research work was carried out for waveguide-based packaging of lithium niobate (LiNbO3) modulators for direct connection to the horn antennas. However, when the operating frequency is above 100 GHz, the machining of the waveguides and horn antennas becomes expensive due to the high mechanical tolerance. Therefore, an alternate approach is demonstrated for direct integration of the modulator with a planar antenna. To obtain compatible gain patterns as the horn antenna, a silicon lens was incorporated onto the back surface of the substrate.