Novel 3D printed synthetic dielectric substrates

This letter presents dielectric properties of air filled synthetic substrates fabricated in a single process using three‐dimensional printing. The permittivity and loss tangent of a given sized substrate can be changed by controlling the air infill volume fraction. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:2344–2346, 2015


INTRODUCTION
Additive manufacturing (AM) technology constructs successive layers of materials to create three-dimensional (3D) objects. With computer aided design (CAD) and computer aided manufacturing (CAM), it is possible to build rapid prototypes in almost any geometry and internal structure. The advances in digital AM equipment and new materials enables 3D printing to produce a wide range of products in a variety of fields including biology, aerospace, electronics, and electromagnetics (EM) [1][2][3][4][5].
There are a number of EM applications that require complicated shapes and 3D internal structures, such as lens antennas and metamaterials, to achieve bespoke EM properties. Traditional mechanical machining and micromachining have been up to now the dominant approach for fabrication [6,7]. These machining techniques remove or shape parts of raw materials using operations such as drilling and milling. This is time costly and also generates material wastage. Moreover, it is difficult to create complicated internal structures using machining techniques in a single process. With 3D printing, the final shape is successively constructed layer by layer and machining is not required. Therefore, there is no waste of materials for 3D printing, and it is easy to generate complicated internal structures. 3D printed dielectric materials which are cost efficient and can be rapidly prototyped are becoming increasingly attractive in antenna design and fabrication. The specifications of dielectric laminates, such as thickness and relative permittivity, are usually determined by the manufacturers, and therefore, antenna engineers' designs are restricted. Using 3D printing in dielectric materials, fabrication allows engineers to customize the substrate to desired dimensions, for instance, conformal antenna applications. Furthermore, using 3D printing can create multiple-material objects in a single process which means assembly is not required. Therefore, it is much easier to produce synthetic dielectric materials with combinations of diverse materials such as plastics, polymers, nylons, and ceramics using 3D printing. This letter demonstrates the combination of nontoxic material polylactic acid (PLA) and air to spawn novel dielectric substrates. The dielectric properties of materials such as permittivity and loss factor can be tailored with different air to PLA percentage ratios.

3D PRINTED DIELECTRIC SUBSTRATES
In this work, a fused deposition modeling (FDM) MakerbotV R Replicator TM 2X 3D printer was used to print the dielectric substrate samples. The 3D models were designed using CAD tools and subsequently sliced into successive layers. The heated printer nozzle extruded the thermal material and created the object layer by layer from the bottom upwards. Each layer thickness was 0.2 mm, and the extrusion temperature of the printer nozzle for the PLA material was 2008C.
The 3D printed substrates were constructed of three parts: top lid, bottom base, and the nonsolid infill pattern in the middle. The lid and base were printed using 100% infill PLA. The middle part was printed with various infill patterns with air voids. Three internal infill structures including waffle, honeycomb, and empty were used here for examining the effect of infill patterns on the dielectric properties. Figure 1 shows the CAD models of the waffle and honeycomb lattice infill patterns. To reveal the internal structures, the top lids are not shown. The waffle infill is shown in in Figure 1(a). The vertical interior walls were at 458 angles in relation to the outer walls. Figure  1 The infill percentage indicated the PLA volume fraction (VF) which was the volume ratio of PLA material to the whole printed sample, excluding the four exterior walls. The PLA VF for a solid substrate was 100%. The VF was varied by changing the infill patterns. Smaller patterns resulted in higher PLA VF. The PLA VF included the top lid and bottom base. Thus, the hollow infill sample had 33% PLA VF.

RESULTS
Six dielectric substrate samples were printed. A commercially available, split postdielectric resonator from QWED (www. qwed.com.pl) was used for measuring the relative permittivity and loss tangent of these samples at 2.4 GHz [8]. The details of the infill structure and measured results are shown in Table 1. Sample A with 100% infill had a measured permittivity of 2.72 and loss tangent of 0.008. Both samples B and C used waffle infill with different PLA VF. The waffle squares in sample B were smaller than sample C and, therefore, sample B had higher PLA VF. A larger PLA volume showed an increase in both permittivity and loss tangent. Sample D had the same PLA volume fraction as sample C but used the honeycomb infill. The  measured results indicated that the infill shape did not greatly affect the dielectric properties.
Although sample E had no infill, the inclusion of the lid and base made up 33% of the total volume. To examine the effects of the external walls, sample F was printed with the same honeycomb infill with thinner bottom base but without the top lid. The thicknesses of the base and the honeycomb infill part of sample F were 0.2 mm and 2.2 mm, respectively. The measured results showed that sample F had a lower permittivity and loss tangent value than sample E, due to its lower PLA volume fraction.

CONCLUSION
This letter has presented the feasibility of creating low loss dielectric substrates with various relative permittivities and loss tangent values using conventional 3D printing. Voids were introduced inside the substrates which were printed in one process. The volume fraction of air in the host material affected the dielectric properties more significantly than the infill shape. The permittivity and loss factor of the substrate were reduced by the increasing air volume fraction. Therefore, the permittivity and loss tangent of the dielectric substrate can be tailored to the desired values by extrapolating from the sample results produced here.
These highly customisable dielectric materials will improve the flexibility of antenna design and related EM applications. The automatic fabrication process also allows the dielectric properties to be graded within one structure. ABSTRACT: In this article, a novel idea of a hidden bit channel established in the downstream passive optical network (PON) frames is proposed. As the downstream frames in time-division multiplexed, PONs are broadcasted and delivered to all active end-point optical network units (ONUs), and this bitstream could be potentially used for creating an additional hidden bit channel containing bits for all ONUs. Therefore, the downstream frames could simply transmit standard information fields and ONU contributions according to the standard PON principles; however, these bits could be also reused multiple times to carry additional data for each ONU in a form of segmented bit sequences spread within the entire bitstream. To establish effective bit channel with positive bit gain, an effective algorithm for addressing its bits is necessary. The most promising bit addressing algorithm is proposed within this article together with the results obtained by its simulations and performance evaluations. Key words: bit addressing; bitstream; passive optical networks; security; XG-PON

INTRODUCTION
The passive optical networks (PONs) are now being widely deployed in many countries as their present generation can reach shared transmission rates up to 10 Gbps [1]. The previous types, such as Ethernet PON (EPON) and Gigabit PON (GPON) according to the IEEE 802.3ah standard [2] and ITU-T G.984 recommendation [3], are currently being replaced by updated PON solutions, 10GEPON presented within IEEE 802.3av std. [4] and XG-PON in ITU-T G.987 [5] rec. All these PON types mentioned above are based on time-division multiplex (TDM) principle while both upstream and downstream TDM frames consist of multiple contributions to/from all active end-points optical network units (ONUs) [6]. Therefore, a part of the optical distribution network (ODN) is always shared by multiple ONUs. The continuous evolution of new PON generations and the progress of their development in the future is now pointed toward applying wavelength multiplexing techniques [7,8], various modulation formats [9], optical code multiplexing [10], and so forth.
The ODN in case of pure PONs is always passive; therefore, the downstream optical signals are split by passive optical