The fast development of terahertz (THz) technology holds significant importance in numerous applications where imaging systems are indispensable. Moreover, advanced imaging systems increasingly demand the capability to image transparent objects. This is particularly challenging due to the relatively low efficiency of THz sources, leading to low power in THz systems. Additionally, parasitic reflections in mostly very coherent THz optical systems result in the occurrence of high speckle noise. One of the solutions for the enhancement of the imaging contrast of transparent objects is the application of spatial filtering (SF) methods in the system, which we implement here via the following methods: positive phase contrast (PPC), negative phase contrast (NPC), and dark field (DF). In this study, the authors report the implementation of all three methods in the 4f THz system for relatively long-distance imaging of 1200 mm. The system was tested with the numerical simulation. The detailed analysis of the results allowed for the selection of the recommended size of the spatial phase filter for the implementation of the SF methods in the experimental optical setup. The filter and the object were manufactured using fused deposition modeling (FDM) 3D printing technology. Subsequently, the 4f system facilitating the SF methods was built and examined experimentally. The analysis of the experimental results indicates the enhancement of the signal-to-noise ratio (SNR) by almost 14 times with the use of the PPC method compared to the system without the inserted filter. Thus, the improvement of the image contrast by the implementation of SF method is unquestionable.
Increasing demand for high data transmission rates and bandwidth availability driven by the rapid development of the broadband services becomes challenging in the context of constraints imposed by nowadays exploited telecommunication optical fibers. Based on single-mode fibers and multiplexing in the wavelength domain (wavelength division multiplexing, WDM), the current technology seems to have reached its fundamental limits. At present, two promising technologies are a subject of intensive research. One of them is the increase of the data transmission through multiplying the number of fiber cores, thus implementing the concept of Multi-Core Fibers (MCF), where each core is used as a separate data transmission channel. The second widely investigated technique is based on the idea of mode division multiplexing (MDM), where different transverse modes of a Few-Mode Fiber (FMF) can be used as different carriers for data channels.
In this work, we demonstrate the results of R&D works and the comprehensive tests of the few-mode fibers developed within the project NMKM+. Developed few-mode fibers (both passive and active) and their commercially available counterparts have been tested with respect of transmission parameters and applicability to the real telecom systems. In particular, guided mode profiles, numerical apertures, OTDR, and dispersion characteristics for the passive fibers have been recorded and compared. Also, methods of selective excitation and detection of singular modes have been discussed and partially verified. The results of data transmission experiments have been performed and discussed with respect to the quality of transmitted signals. These have been complemented by the amplification experiments with the use of erbium-doped few-mode fibers in “classical” and microstructural geometry. The results were studied and analyzed, showing the potential for future optimization.
Acknowledgement: This work has received support from the National Centre for Research and Development through project NMKM+ (TECHMATSTRATEG1/348438/16/NCBR/2018).
Designing of diffractive optical elements (DOEs) requires knowledge about possible methods of calculating and simulating their performance, possible materials and characteristics of the particular range of radiation. The demand for compact and lightweight setups intuitively leads to the application of diffractive elements, which are characterised by both these features, having though one significant drawback – large chromatic aberration. As DOEs are meant to introduce specific phase shift, they are related to one particular design wavelength (DWL). However, thanks to different design approaches (e.g. kinoforms of higher order), elements functioning also for broader spectral ranges can be created. They are thicker, thus usage of appropriate material, having small attenuation coefficient or adjusting structure height during design process is required. Here, a simple method of designing diffractive lenses working in on- and off-axis regimes is presented. Using 3D printing for manufacturing is possible because different materials, polyamide, wax or chocolate, are relatively transparent below 0.5 THz. Each material has its own limitations like hardness, thermal resistance or ability for mechanical processing that have to be considered. Thus, using such simple methods of manufacturing for DOEs working for frequencies larger than 0.5 THz can be achieved using different design approach and ordinary devices with easily accessible materials (e.g. paraffin). It seems very important to create a method of producing diffractive elements that will be available in many laboratories to show the advantage of using such optical structures.
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