Advances in Front-end Enabling Technologies for Thermal Infrared ‘THz Torch’ Wireless Communications
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The thermal (emitted) infrared frequency bands (typically 20–40 and 60–100 THz) are best known for remote sensing applications that include temperature measurement (e.g. non-contacting thermometers and thermography), night vision and surveillance (e.g. ubiquitous motion sensing and target acquisition). This unregulated part of the electromagnetic spectrum also offers commercial opportunities for the development of short-range secure communications. The ‘THz Torch’ concept, which fundamentally exploits engineered blackbody radiation by partitioning thermally generated spectral radiance into pre-defined frequency channels, was recently demonstrated by the authors. The thermal radiation within each channel can be independently pulse-modulated, transmitted and detected, to create a robust form of short-range secure communications within the thermal infrared. In this paper, recent progress in the front-end enabling technologies associated with the THz Torch concept is reported. Fundamental limitations of this technology are discussed; possible engineering solutions for further improving the performance of such thermal-based wireless links are proposed and verified either experimentally or through numerical simulations. By exploring a raft of enabling technologies, significant enhancements to both data rate and transmission range can be expected. With good engineering solutions, the THz Torch concept can exploit nineteenth century physics with twentieth century multiplexing schemes for low-cost twenty-first century ubiquitous applications in security and defence.
KeywordsThermal infrared THz Torch Blackbody radiation Wireless communications Frequency division multiplexing Frequency-hopping spread-spectrum
Optical wireless communications have been widely used in modern communication systems. Transmission windows having relatively low atmospheric attenuation support the use of optical sources at specific wavelengths (e.g. 800 and 1550 nm and 10 μm) . For 800-nm wireless links, low-cost light-emitting diodes (LEDs) and silicon-based photodiodes can be employed in the front-end of the transmitter and receiver, respectively. The main drawback for 800-nm technology relates to eye safety: it can potentially induce thermal damage to the retina . The cornea is opaque to radiation at wavelengths above ∼1.4 μm, greatly reducing ocular hazards. As a result, 1550-nm technologies are preferred from a safety perspective. However, detectors available for this range, which are usually made from germanium (Ge) or indium gallium arsenide (InGaAs), are more expensive when compared to silicon photodiodes. Moreover, both 800 nm and 1550 nm experience atmospheric absorption, scattering losses and scintillation effects , limiting their applications to indoor short-range communications.
By moving to the mid- or long-wavelength infrared (MWIR or LWIR) regions, improved link and increased transmission distance can be achieved, due to a lower susceptibility to atmospheric affects. For example, the 3–5- and 8–14-μm atmospheric transmission windows can be used, for their superior penetration of atmospheric obscurants such as fog, smoke and dust . Within the MWIR (3–8 μm or 37–100 THz) and LWIR (8–15 μm or 20–37 THz), quantum cascade lasers (QCLs) [5, 6, 7] and 10 μm (or 30 THz) CO2 lasers  are generally employed. However, these laser sources would be considered extravagant (non-ubiquitous) products for the domestic market, and only high-end users (e.g. scientific or military) can justify their high cost and, with the latter, physically accommodate their large size.
On the other hand, there have only been a limited number of enabling technologies using thermal sources ‘beyond the THz horizon’ (3–30 μm or 10–100 THz) to support wireless communications. This offers opportunities for developing such systems within this largely unregulated part of the electromagnetic spectrum. The ‘THz Torch’ concept was recently introduced by the authors for providing secure wireless communications over short distances within the thermal infrared [8, 9, 10, 11, 12, 13]. Both single- and multi-channel wireless links have been reported; the resilience to interception and jamming with 2.56 kbps frequency division multiplexing (FDM) and 0.64 kbps frequency-hopping spread-spectrum (FHSS) systems has been demonstrated experimentally . Furthermore, the radiation mechanisms for the basic thermal source transducers , as well as the power link budget analysis for the system were previously reported . In this paper, we discuss recent advancements in the front-end enabling technologies associated with the THz Torch concept for wireless communications, which go beyond what has already been published.
2.1 Single-Channel Wireless Link
Compared to optical wireless communication systems operating at other wavelengths, our thermal-based system is inherently low cost and potentially has a high level of resilience to interference and jamming . With the former, the cost for the complete system is significantly reduced, as thermal sources and detectors have been employed. Systems based on 800-nm LEDs have similar cost and complexity, but LEDs are not spectrally tunable, and the output power has to be limited under a specific safety threshold. In contrast, the output power level and radiance peak for thermal sources can be controlled simply by adjusting the bias current, although this may come with the penalty of lowering spectral power conversion efficiency (from DC). Moreover, our wireless communication system can be considered to be eye-safe, as the shortest operation wavelength is >3 μm. With the latter, it has been experimentally verified that in order to undermine the inherent immunity to interference and interception, both the jammer and intruding receiver, respectively, must be designed to have (1) a significant amount of overlapping spectral channel bandwidths, (2) similar modulation frequency, (3) line-of-sight detection and (4) synchronized hopping pattern (with FHSS) . The main drawbacks of using thermal-based sources are the inherently low switching speed (requiring a separate high-speed modulator) and lack of signal coherency, leaving only the power intensity of the thermal source to be modulated and measured.
2.2 Multi-channel Thermal Infrared THz Torch System
With modern communication systems, multiplexing schemes can offer important benefits, including increased overall end-to-end data rates (with band-limited channel assignments) and higher levels of security. Both frequency division multiplexing [10, 11] and frequency-hopping spread-spectrum  schemes have been demonstrated with four-channel implementations. To define the four non-overlapping frequency bands, within the thermal infrared, 1-mm-thick optical coating band-pass and long-pass filters are used. The measured transmittances for all channel filters are given in Fig. 2, from 20 to 100 THz, for channels A, B, C and D.
3 Limitations and Engineering Solutions for THz Torch Technology
Until now, the maximum measured data rate reported for current multi-channel thermal infrared THz Torch wireless communication systems was 0.64 kbps per channel. This data rate was to a greater extent limited by the rotational speed of the large bespoke (stepper motor-based) mechanical chopper and to a lesser extent by the spectral location and bandwidth of the channel filters . The 25–50 THz (70 % transmittance bandwidth) single-channel system is used to investigate the fundamental performance limitations for the thermal-based wireless link; possible engineering solutions for each limitation are proposed and verified.
3.1 Spreading Loss, Collimating Lenses and Reflectors
Spreading loss has been found to be one of the most significant fundamental limitations, which can be compensated for. Unlike laser sources, which have very narrow beamwidths, radiation from open thermal sources is isotropic. From the Friis equation, in the far field, received power density is inversely proportional to the square of the transmission range . With the previously demonstrated multiplexing systems setup , having a range set to 3 cm and bias current of 80 mA (giving a source temperature of 772 K, or total output power of 97.5 mW), the measured bit error rate (BER) was ∼10−3.
With thermal sources, the most obvious partial-solution is to introduce a surface aperture, from which energy can be radiated. For example, a high reflectance parabolic surface with cylindrical extension is an efficient solution for maximizing the forward radiated power. A high transmittance collimating lens can then be employed. With the former, reflectors have been widely used in many thermal sources to maximize the output level of radiation from the package. However, to date, no reflector has been used with our experiments. It is believed that future systems will adopt metal-coated parabolic reflectors to enhance the directed power—improving performance significantly.
3.2 Lens Materials for Thermal Infrared Applications
3.3 Increased Channel Density
Tungsten filaments are not considered good thermal emitters, due to the large thermal time constant and low emissivity. Moreover, having glass envelopes (e.g. with the miniature COTS incandescent light bulbs used in our earlier proof-of-concept demonstrators), the output radiated power is significantly reduced because of the high absorption of the window glass below ∼65 THz . For these reasons, more efficient but also more expensive miniature COTS thermal emitters can be used. When compared to miniature incandescent light bulbs , they have higher emissivity (>80 %) elements, similar emitting areas and do not need glass windows . This significantly improves the transmitter’s power conversion efficiency. Furthermore, since they can be modulated at higher frequencies (up to 100 Hz), mechanical choppers are no longer needed, making the complete system more compact and lightweight. In addition, real (or pseudo-random binary sequence) data can be transmitted at relatively much higher rates since inertia limitations associated with stepper motor-based mechanical choppers are avoided.
Moreover, with the considerably more thermally radiated output power available from COTS thermal emitters, with sizes comparable to incandescent light bulbs, the channel bandwidth can be reduced to maintain the minimum required output power to meet operational needs. As a result, more channels can be introduced within the same spectral range of interest. Without the mechanical chopper, each channel operates at the lower speed of 100 Hz but can still maintain or increase the end-to-end data rate.
Sixteen-channel thermal infrared THz Torch systems are currently being developed for a variety of different applications. For example, with coarse spectroscopy, 16 matched pairs of COTS narrowband BPFs, within the spectral range from 20 to 80 THz, provide 16 discrete spectral data points. As can be seen from Fig. 10b, the filter specifications and spectral coverage are not ideal, due to the inherent limitations of optical coating filters. However, it was still possible to sample the spectral range of interest. The measured power ratio (power transmittance in transmission mode or reflectance in reflection mode) with and without a sample under test can be compared with pre-characterized spectral signatures with a material database. The resulting spectrometer can operate at room temperature, be lightweight, compact and at a very low cost [26, 27]. Unfortunately, optical coating filters are not frequency-scalable, which limits filter specifications and spectral coverage availability.
3.4 Integrated Frequency-Selective Thermal Emitters
In principle, the thermally radiated output power from a blackbody source can be spectrally filtered by an inductive (band-pass) metal mesh filter . However, at thermal infrared frequencies, free-standing metal mesh filters are not mechanically robust, as the thickness of the metal layer has to be kept thin in order to avoid waveguide modes, while substrate-based filters will have a Fabry–Pérot etalon, since the thickness of substrate is comparable to the wavelength inside. Therefore, at thermal infrared frequencies, both types of filters face challenges in terms of design and fabrication.
As an alternative to using wideband thermal emitters (such as blackbody sources), frequency-selective thermal emitters can be employed, having a dielectric spacer layer that is thin enough to avoid in-band Fabry–Pérot resonances. According to Kirchhoff’s law of thermal radiation, the absorptance of a material at thermal equilibrium is equal to its emissivity. Therefore, high absorptance (emissivity) can be obtained from a structure that minimizes both transmittance and reflectance. Cross-shaped capacitive metal mesh structures have been used to create near-blackbody radiators/absorbers with frequency-selective properties within the terahertz , mid-infrared , offering the ability to engineer the desired spectral emissivity profile within the thermal infrared.
One of the advantages of using frequency-selective thermal emitters is that each channel source can have a be-spoke design, in terms of resonance frequency, emissivity and fractional bandwidth. This removes the need for commercial emitters and optical coating filters. Moreover, the design and implementation of frequency-selective emitters can be arrayed onto a single substrate, leading to an integrated module that is lightweight, compact and low cost. One of the challenges for these frequency-selective thermal emitters is that in order to obtain a high output power, the temperature of the emitter has to be increased (e.g. to ∼500 K), and the material deformation and degradation have to be considered.
In this paper, we first briefly introduced the THz Torch technology, along with its hardware requirements. Fundamental limitations associated with the front-end hardware were identified; engineering solutions are then proposed and verified. For example, free-space spreading loss can be compensated for by employing reflectors (e.g. Winston cone) and collimating lenses made from suitable materials. Moreover, channel density can be increased by using commercially available thermal sources or be-spoke frequency-selective emitters.
New simulation/experimental results are given to quantify the effects of introducing engineering improvements in a number of enabling front-end technologies (not previously demonstrated with our specific THz Torch application); while comparing and contrasting other technologies act as a useful review. Collectively, we believe that our paper points to a raft of engineering solutions that will significantly improve the performance of future systems and thus further promote the new application.
These practical engineering solutions are expected to be implemented in next-generation wireless communication systems; improving both the data rate and transmission range. Moreover, with the continual advancements being made in front-end thermodynamics, back-end electronics and signal processing, other THz Torch applications are beginning to emerge, such as spectroscopy [26, 27] and imaging (with 2D scanners and 3D tomography). With these applications, purely reflective optics can be used in the experimental setups, which are low loss, more broadband and cheaper than having IR lenses.
This work was in part supported by the China Scholarship Council (CSC). The authors would like to thank Dr Mark Bailey at Northumbria Optical Coatings Ltd. for the kind donation of the 16 pairs of narrow band-pass filters and associated measured data.
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