http://www.birmingham.ac.uk/research/activity/eese/systems-devices/edt/terahertz-waveguide-circuits-lab.aspx
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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