Monday, November 3, 2014

University of Birmingham-Terahertz waveguide circuits laboratory

Terahertz radiation is electromagnetic radiation with a frequency above the RF and microwave region and extending towards the optical. It is an area of the electromagnetic spectrum which is under used at the moment due to the difficulties in producing practical components and systems. However, it is well known that terahertz will be important in the future for many applications. In this laboratory terahertz circuits are designed and tested.
The laboratory is led by Professor Lancaster.
Terahertz radiation has five primary properties for applications: (i) it is able to pass through dielectrics such as paper, plastic, cloth, wood, ceramics and silicon, which are also common packing materials, (ii) metals are highly reflective in the terahertz region; (iii) many chemical and biological agents have unique spectral fingerprints in the terahertz frequency region and (iv) compared with X-rays, terahertz radiations do not present health hazard to people being scanned or to people operating the scanned systems (v) Large bandwidths are available for communications systems. Due to these advantages, there is an increasing interest in terahertz frequencies and many companies and universities are working towards real useful applications.
Waveguide technology is a desirable choice for terahertz wave devices, mainly due to its low loss characteristics. The conventional way of making waveguide components, is precisely controlled CNC metal milling. However, with the increase in the frequency it is more and more challenging to machine out the small features and sometimes it is impossible to achieve complicated internal waveguide structures. Recently various micromachining techniques have been developed to fabricate such devices with higher precision and possibility at a low cost. Among them, thick layer SU8 photoresist technology affords good dimensional accuracy and at the same time only requires standard ultraviolet photolithography, therefore making it a highly desirable choice for high precision and high performance applications. More information on this technology is available elsewhere on the website.
Terahertz filter
This technology has been employed by our research group to demonstrate waveguide filters operating at W-band (75-110 GHz), WR-3 band (220-325 GHz) and WR-1.5 band (500-750 GHz). Here the WR-1.5 filter will be discussed in detail.

Figure 1 Diagrams of the WR-1.5 band filter formed of three SU8 layers with a same thickness. (a) Illustration of the filter. The standard UG-387 waveguide flange dowel pins holes and screws holes are shown. (b) Front view of one SU8 layer. (c) Diagram of the filter structure, which is thefunctional bluepart also shown in (a). (d) A schematic front-view diagram of the filter structure. The first and third resonators are represented using red rectangles, whereas the blue rectanglerepresents thesecond resonator; the offset determines the filter properties.
As shown in Figure 1, the filter is composed of three silver-coated SU8 layers, each of the same nominal thickness of 191 µm. Rather than placing the resonators in alignment and controlling the couplings through irises, this WR-1.5 filter shifts the relative positions of resonators to achieve the desired specified external and internal coupling coefficients and therefore the desired frequency response. This is shown in Figure 1 (c)-(d). This novel structure is ideally suitable for the layered SU8 micromachining process as it avoids irises features within a layer, and is thereby more robust for fabrication. It also has a very accurate flange for connecting to the measurement equipment. Figure 2 shows a photograph of several silver-coated SU8 layers before they are put together into the final filter.

Figure 2 Photograph of a few silver-coated SU8 layers.
These SU8 layers are delicate due to their small thickness, and carefully mounted onto a separate metal straight though waveguide section and then inserted between the two ports of a network analyser to perform the measurement as shown in Figure 3 (a).
Measurements of the filter were performed using an Agilent N5247A Network Analyzer with a pair of VDI (Virginia Diodes Inc.) extension modules. During the measurement the SU8 filter and the waveguide section were placed in the middle of two standard WR-1.5 waveguide flanges (i.e. UG-387), as shown in Figure 3 (b). The four alignment pins of the waveguide flanges addressed both the accuracy to which the three SU8 layers were aligned and the accuracy to which the micromachined filter was aligned to flanges of network analyzer.

Figure 3 (a) SU8 shims mounted to a 1-in long straight through waveguide section. This prevents the SU8 shims from bending or wrapping. (b) Test setup for the micromachined SU8 waveguide filter.

Figure 4 Measurement results of the WR1.5 SU8 filter.
The measurement results of the SU8 filter together are shown in Figure 4, which exhibits a 3 dB bandwidth of 53.7 GHz at a centre frequency of 671 GHz. The median passband insertion loss is measured to be 0.65 dB, which is close to the theoretical value of 0.28 dB obtained from a simulation using the conductivity of silver The measured return loss is better than 11 dB across the whole passband. These are excellent results and this filter is one of a very few demonstrated at this frequency in the world.
Terahertz antenna
In addition to filters terahertz antennas are an important area for the EDT group. Rectangular waveguide slot antennas have been chosen and are widely used in the field of millimetre-wave applications and radar systems due to having high gain, inherent low transmission losses, and simplicity in fabrication. They also offer significant advantages in terms of weight, volume, and radiation characteristics. These antennas are very attractive due to their planar, compact, and rugged construction and a made by slots in a waveguide. The dimensions of the slots in the waveguide walls can be controlled to realise the desired pattern shape.
A Micromachined 300-GHz slotted waveguide antenna is demonstrated here using a simple fabrication technique based on metal-coated SU-8 thick resist. The configuration of the design is shown in Figure 5. The top layer contains 8 slots which are positioned at the centre of the narrow-wall of the waveguide. The next three layers form the rectangular waveguide, and the whole design is enclosed by the last layer (layer 5).

Figure 5. Illustration of the design of 8-slots in the narrow-wall of the waveguide with the H-bend input port. The dark blue shows the extent of the air filled waveguide and slot sections.
An embedded five layer H-plane bend is designed in order to connect the device with the waveguide flange easily and accurately, as shown in Figure 5. The effect of the bend on the performance of the device is negligible. Figure 6 shows the assembled antenna.

Figure 6: Diagrams of the assembled antenna seen from (left) the radiation side and (right) the feed side. The holes are for (A) precision alignment pins, (B) flange dowel pins, (C) flange screws, and (D) pressure screws.

Figure 7: Connection of the 300-GHz slotted waveguide antenna with the test port flange for measurement.
The radiation patterns of the antenna were measured at the Rutherford Appleton Laboratory, in an anechoic chamber with a WR-2.8 corrugated feed horn. The mm-wave source module and the detector were connected to a network analyser as shown in Figure 7. At least 40 dB dynamic range was maintained during the measurement. Figure 8 shows the measured normalized H pane radiation pattern which agrees very well with the simulation. This indicates good dimensional accuracy for the radiation slots, rendered by the lithography-based fabrication process.

Figure 8. Measured H-plane radiation patterns in comparison to simulations. For “measurement-1,” no absorbing material is applied to the antenna. For “measurement- 2,” a sheet absorber is attached to the brass plate on the radiation side. Again, the simulation model includes the brass plates and some metal cylinders representing the effects of the screws.

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