Dispersion-limited versus power-limited terahertz communication links using solid core subwavelength dielectric fibers Download: 671次
1. INTRODUCTION
The terahertz (THz) frequency spectrum (0.1–10 THz) holds high promises for many applications that include communications [1], imaging [2], sensing [3], and spectroscopy [4]. In communications, in order to meet the bandwidth demand set by the next generation of wireless systems, a shift in the carrier frequency toward the THz band is unavoidable [4,5]. THz communications have been already demonstrated in the context free space wireless links that profit from the presence of several low-/modest-loss atmospheric transmission windows [6]. Although there are many advantages of wireless communications including convenience in mobility for the end user, ease in scaling up the network, flexibility of device interconnectivity, etc., they also possess many challenges. Particularly, due to high directionality of the THz beams, THz wireless links are known for their high sensitivity to alignment errors, thus requiring careful positioning of the transmitters and receivers [7]. Moreover, reliable communications in non-static environments (ex., between moving objects) require complex beam steering solutions. The situation is further exacerbated in geometrically complex environments (such as inside vehicles and buildings), where highly complex channel modeling is required. Moreover, free space links have higher chances of eavesdropping, thereby increasing the risks for secure communications [8]. Finally, atmospheric weather conditions such as rain, snow, and fog, play a major role in affecting the performance and reliability of the wireless THz links. In view of these limitations of wireless THz communications, short-range THz fiber links () can offer an alternative solution as THz fibers present a closed highly controlled propagation environment, can span complex geometrical paths, and can offer reliable coupling to receiver and transmitter for both static and dynamic applications. One interesting area of application for THz fiber links is in reliable onboard connectivity and intra-vehicle communications for military and civil transportation. In Fig.
Fig. 1. Schematic of the THz wireless and fiber communication links for reliable and versatile intra-/inter-vehicle communication applications.
This type of scenario in which THz signals are detected using multiple antennas and then are relayed over the complex geometrical paths to a central processing unit can profit greatly from flexible THz fiber links. Another scenario is using THz fiber links for reliable delivery of high-speed data through partially blocked or geometrically complex areas, which is of importance for hard-to-reach or highly protected environments such as enclosures for aggressive environments (ex., bio-enclosures) and protected structures (ex., shelters and bunkers), as well as for intra- and inter-device THz communications in which different parts of the same system can be conveniently linked using flexible fibers. Finally, THz fiber links can be used as a backup solution for short-range wireless communications in case of sudden deterioration of the atmospheric conditions, which can be of particular importance for places with harsh weather.
In designing an efficient THz fiber communication link, the fiber parameters such as transmission loss, bend loss, dispersion, coupling strength, and ease of handling play a significant role. Furthermore, the degree of complexity in the fiber fabrication process determines the cost and commercialization opportunities. While the fiber loss and coupling strength limit the communication link distance, the maximum achievable bit rate can also be limited by the fiber dispersion. Therefore, low transmission loss and low dispersion are the primary concerns for the THz fiber designs. We start by reviewing several types of existing THz fibers. The choice of waveguide material is one of the key factors in achieving THz guidance with low loss and low dispersion. In the case of metallic waveguides, the finite conductivity of metallic layers leads to ohmic losses, whereas in dielectric waveguides the loss is mainly due to material absorption. Independently of the materials used, longer THz waveguides (over 1 m) are frequently designed to use modes predominantly guided in the low-loss dry air region. Most recently in Ref. [10], bare metal wires in air were proposed as open waveguides for 5G communication applications; however, such waveguides suffer from high coupling losses and difficulty in mechanical handling due to wire waveguide open structure. Low-loss, low-dispersion, and efficient coupling can be achieved using two-wire plasmonic THz waveguides; however, longer (over 1 m) two-wire waveguides are inconvenient in practice due to challenges in packaging and handling [11]. This is because in the two-wire plasmonic waveguides, the air gap between the two metallic wires should be precisely maintained along the whole fiber length, which is difficult to achieve in long fiber links. While encapsulating the two metallic wires within a porous dielectric cladding using fiber drawing offers a solution to the mechanical stability and handling problem, this also leads to additional losses and dispersion due to coupling of a plasmonic mode to the dielectric cladding [12].
Alternatively, by selecting proper dielectric materials with low absorption loss [Teflon, polyethylene, polypropylene (PP), cyclic olefin copolymer, to name a few] and engineering the waveguide structure to expel the mode into the low-loss dry air region, highly efficient THz waveguides can be demonstrated [13
In solid core THz fibers, the transmission bandwidth is much larger than that in the hollow core fibers as the propagation mechanism is TIR. However, the transmission loss in such fibers is much higher and is generally comparable to the absorption loss of the fiber material. In order to minimize the transmission loss, one usually resorts to either subwavelength core dielectric fibers that are simple rod-in-air fibers or rod-in-foam fibers [26,41] or small solid core photonic crystal fibers (PCFs) with porous claddings [42]. The rod-in-air/foam THz fibers with subwavelength size cores offer low loss and low dispersion guidance as a large fraction of the modal fields in such waveguides is guided in the low-loss air or foam regions [2628" target="_self" style="display: inline;">–
As discussed earlier, although simple rod-in-air subwavelength fibers are easy to fabricate and potentially offer low propagation loss and low dispersion, mechanical manipulation of such fibers and their integration into systems are problematic due to significant extent of the modal fields into air [47]. Therefore, for practical applications, such fibers have to be encapsulated in such a way as not to significantly affect the weakly core-bound guided modes, while allowing direct mechanical handling of the fibers. One solution to this problem is a wagon-wheel structure in which solid core is suspended in air using several deeply subwavelength bridges [27]. Fabrication of such fibers is, however, challenging due to complexity of the fiber cross section. Alternatively, solid subwavelength cores can be encapsulated using low-loss, low-RI () dielectric foams, resulting in what we call throughout the paper rod-in-foam fibers (see Fig.
Fig. 2. (a) Schematic of the rod-in-foam subwavelength THz fiber. Fiber outer diameter is chosen to accommodate of the power guided by the identical rod-in-foam waveguide with infinite cladding. (b) Photograph of the rod-in-foam fiber.
Now, we briefly review some of the recent demonstration of THz communications using THz fibers [49
In this work, we aim at deeper understanding of the limitations of the fiber link quality posed by the combined effects of the modal loss and dispersion. Without the loss of generality, we concentrate on a pure system of rod-in-air dielectric THz subwavelength fiber for short-range () communication links with up to 6 Gbps data speeds. In fact, the rod-in-air fiber can be used in further studies as a performance benchmark for the more practical fibers such as rod-in-foam and suspended core fibers. In the following, we fix the carrier frequency at 128 GHz, while using fibers of various diameters to realize power-limited or dispersion-limited transmission regimes. The dielectric fibers are made of low-loss PP material with three different diameters of 1.75 mm, 0.93 mm, and 0.57 mm. Both theoretical and experimental studies are then carried out, and a comparative analysis of the two is presented. Experiments were conducted using a photonics-based THz communication system reported in Refs. [64,65]. We then demonstrate that the limitation in the error-free link distance is mainly due to the modal loss for the 1.75-m-diameter fiber, while for the 0.93 mm and 0.57 mm diameter fibers, the link distance is limited due to modal dispersion. By optimizing the decision threshold, an error-free -long link at 4 Gbps is achieved with the 0.57-mm-diameter fiber, while the argument is made for over 10 Gbps fiber links with over 10 m length when designing the fiber to operate near zero dispersion frequency (ZDF). Furthermore, study of the bending losses of the rod-in-air fibers is presented, in which we conclude that even relatively tight bends of sub-10-cm radius can be well tolerated by such fibers. Finally, the power budget of the fiber-based link is compared with that of the free space links, and the case is made for the strong potential of the rod-in-air fibers in short-range communications. To the best of our knowledge, this is the first comprehensive study of all the major limiting factors and comparative advantages that relate to design and operation of short-range fiber-assisted THz communications links.
2. THEORY OF ROD-IN-AIR DIELECTRIC THz FIBERS
Many polymers possess almost constant RI and low absorption losses at lower THz frequencies (). PP, in particular, has one of the lowest losses over the wide THz frequency range ( below 1 THz) [6668" target="_self" style="display: inline;">–
2.2 A. Effective Index, Modal Losses, and Excitation Efficiency
The normalized electric field distribution of the fundamental mode (normalized to 1 W of carrying power) for the PP fibers of different diameters at 128 GHz is shown in Figs.
Fig. 3. Normalized electric field profile of the fundamental mode at the carrier frequency of 128 GHz: (a) 1.75 mm fiber, (b) 0.93 mm fiber, and (c) 0.57 mm fiber. (d) The power fraction of the fundamental mode within the aperture of a variable diameter. (e) The effective refractive indices of the guided modes, and (f) the corresponding modal absorption losses for the rod-in-air fibers of different diameters at the carrier frequency of 128 GHz. As a reference, the bulk refractive index and absorption loss of the fiber polypropylene core are 1.485 and 2.36 dB/m, respectively, at 128 GHz.
We next study excitation efficiency of the fiber fundamental modes using external THz sources. Generally, the excitation efficiency is maximized when the size of the source field distribution is comparable to that of the fiber mode. In our experiments, the subwavelength fibers are butt coupled to the conical horn antenna that is connected to the WR-6 waveguide flange of the THz emitter. The horn antenna is a mode converter with the tapered structure that converts the fundamental mode of a rectangular waveguide to the Gaussian-like mode at the output. Coupling efficiency can be further optimized by properly positioning the fiber input end inside of the horn antenna. While many coupling scenarios have been considered in the literature [56,69,70,71], they all essentially result in a similar coupling efficiency as achieved by a simple free space coupler that uses a single plano–convex lens. The schematic of such a coupler is shown in the inset of Fig.
Fig. 4. Excitation efficiency by power of the fundamental mode of a rod-in-air fiber of three different diameters. (a) Excitation efficiency versus Gaussian beam diameter. (b) Excitation efficiency as a function of frequency for the optimized Gaussian beam diameter. Inset in (a), schematic of a simple free space coupler.
Next, we use the transfer matrix theory and a mode matching technique to estimate excitation efficiency of the fiber fundamental mode with the focused linearly polarized Gaussian beam generated by such a coupler/taper at the input facet of a fiber [72,73]. In Fig.
Table 1. Maximum Excitation Efficiency and Its Corresponding Gaussian Beam Size for the Fibers of Different Diameters
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From the data presented earlier in this section, we can now estimate the maximal fiber link distance given a power budget that is typical for our optics-based THz communication system (for the moment we ignore modal dispersion and bending loss). Thus, received power at the end of the fiber link can be estimated using the following expression: where is the power at the receiver end, is the power at the transmitter end, is the input/output power coupling efficiency per facet (see Fig.
Fig. 5. Power budget considerations for the fiber links of variable distance and 6 Gbps data transmission rate used in our experiments. Transmitter THz power is −6.6 dBm ( ). The signal loss level for the error-free data transmission is experimentally found at , and the absolute noise floor is .
The signal loss level for the error-free transmission of data with the bit rate of 6 Gbps is found to be (10 μW), while the absolute noise level below which transmission is impossible is found to be at (0.4 μW) (see Appendix
2.3 B. Bending Losses
The bending losses of the guided modes of the rod-in-air THz fibers are calculated using both numerical simulation and analytical approximations. The numerical simulations were carried out using the 2D axis-symmetric model in COMSOL Multiphysics. Here, the radiating wave propagates in the azimuthal direction, , and the electric field is expressed as where is the fiber bending radius, while is the leaky mode propagation constant. The computational cell is a rectangle with a circular fiber core positioned at from the axis of rotation, while the other boundaries are perfectly matched layers terminated with a perfect magnetic conductor. Furthermore, we use reflection symmetry with respect to the horizontal plane crossing the fiber center together with the perfect electric conductor or perfect magnetic conductor boundary conditions at the plane to calculate bent fiber modes of two polarizations [electric field at the symmetry plane directed parallel (Y polarization) or perpendicular (X polarization) to the bend axis]. Fiber materials are considered lossless for this simulation.
Bending losses of the fiber fundamental mode in dB/m are computed for the bending radii in the range of 4–30 mm. The calculated and fitted values of the bending loss are presented on the logarithmic scale in Fig.
Fig. 6. (a) Bending losses of the 1.75 mm, 0.93 mm, and 0.57 mm fibers for different bending radii and polarizations. The solid curve corresponds to the X -polarized leaky mode, and the dashed curve corresponds to the Y -polarization leaky mode of a bend modeled using COMSOL software. The dotted lines correspond to the analytical estimations of the bending loss given by Eq. (3 ). (b) The group velocity dispersion of the fundamental X -polarized leaky mode of the 1.75 mm fiber as a function of the bending radius. (c) The field distributions correspond to those of the bent leaky modes for fibers of different diameters and bending radius of 3 cm.
Bending loss can also be estimated analytically using the classical expression for the bending loss of step-index fibers in the regime of weak modal confinement [74,75], where is the power loss coefficient, a is the fiber radius, is the bending radius, is the modal propagation constant in a straight fiber, and represents the modified Bessel functions where is the azimuthal mode number corresponding to the subscript . As corresponds to the mode within scalar approximation, we use in Eq. (
2.4 C. Modal Group Velocity Dispersion and Maximal Bit Rate Estimation
Next, we study modal group velocity dispersion and maximum error-free bit rate for the three fibers assuming a 10-m-long fiber link. The link length of 10 m is chosen to be long enough to be of practical importance, while making sure that all the fibers have no more than 25 dB loss (by power) over the link distance. In general, the maximum bit rate in the communication link is limited by the pulse dispersion and propagation loss. The dispersion parameters and are the second- and third-order derivate of the propagation constant () with respect to the angular frequency , which is used to characterize the degree of pulse broadening in fibers.
Particularly, considering only the second-order modal dispersion, the maximum bit rate (for ASK modulation) supported by the fiber of length can be estimated using Eq. (
Fig. 7. (a) Second-order dispersion of the fundamental mode for 1.75 mm, 0.93 mm, and 0.57 mm fibers. The dashed vertical line corresponds to the single mode cutoff frequency of respective fibers. (b) The maximum bit rate supported by the fibers in a 10 m link with zero modal loss.
It is important to mention that 1.75-mm-diameter fiber has a ZDF (at which ) in the immediate vicinity of 128 GHz carrier frequency used in our experiments. The maximum error-free bit rate supported by such a fiber at ZDF is, thus, limited by the third-order dispersion , and can be estimated using Eq. (
Table 2. ZDF for 1.75 mm, 0.93 mm, and 0.57 mm Fibers and Their Maximal Supported Bit Rates Estimated Using Third-Order Dispersion
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Finally, we note that dispersion of a bent fiber can be different from that of a straight fiber, especially at smaller bending radii. In Fig.
3. EXPERIMENTAL CHARACTERIZATION OF THE ROD-IN-AIR SUBWAVELENGTH FIBERS
3.1 A. THz Communication System, Fiber Holding Method, and Principal Measurement Challenges
The experimental characterization of the fibers was carried out using an in-house photonics-based THz communication system reported earlier in Refs. [64,65]. The schematic and the experimental setup of the THz communication system are shown in Figs.
Fig. 8. (a) Schematic of the photonics-based THz communication system. Inset, butt coupling of the THz fiber with the horn antenna using fisherman’s knot assembly. (b) Photograph of the 6-m-long 1.75 mm diameter rod-in-air fiber THz communication link.
The largest rod-in-air fiber used in our experiments is the 1.75 mm fiber, which is a commercial 3D printing PP filament (Verbatim) with a diameter of . The 0.93 mm fiber was fabricated by reducing the diameter of the 1.75 mm PP filament using a 3D printer (Raise3D Pro2). The temperature of the extruder was set to 220°C, and a 1 mm nozzle was used for the extrusion. A motorized spinning tool is used to precisely control the diameter of the filament that is extruded from the nozzle by adjusting the drawing speed. Thus, fabricated fiber had diameter along the 8 m length. Similarly, a 10-m-long fiber is fabricated by using the same 1 mm nozzle at increased drawing speed. While characterizing fiber links, a stable and consistent fiber coupling must be used. This is of particular importance for subwavelength fibers that can have significant modal presence in the air cladding. In our experiment, we used butt coupling with the fiber ends judiciously positioned inside the horn antenna. To counter the weight of the free-hanging fiber, the fiber is held tightly using an arrangement of two fisherman’s knots made of thin threads and positioned at both emitter and detector ends [see inset of Fig.
3.2 B. Measuring Fiber Propagation Loss Using Cutback Technique
The modal propagation loss of the PP fiber is studied experimentally using the THz communication system and a cutback method, which is then compared to the theoretical values [see Fig.
Fig. 9. Measuring propagation losses of a 1.75 mm fiber using cutback technique. (a) Measured eye amplitude for 1 Gbps, 3 Gbps, and 6 Gbps signals as a function of the fiber length. (b) Power loss estimation using detector pre-calibration and recorded eye amplitude.
3.3 C. Modal Field Extent in the Air
As mentioned earlier, subwavelength fibers can have significant modal presence in the air cladding. In practice, one has to choose the size of fiber cladding (foam, for example) in such a way as to incapsulate most of the modal field. Experimentally, to measure the extent of modal field into air, we place the fiber in the center of a circular metallic aperture of the variable diameter (1–25 mm). The eye amplitude for 1 Gbps bit rate is then recorded as a function of the aperture diameter, from which the fraction of the received THz power is estimated [see Fig.
Fig. 10. (a) Fraction of the modal power inside the aperture of a variable diameter. Inset, circular aperture centered around the rod-in-air fiber. Photograph of the THz subwavelength fibers with polystyrene foam cladding: (b) 1.75 mm fiber with 5 mm diameter foam cladding (100% of power); (c) 0.93 mm fiber with 6 mm diameter foam cladding (90% of power); (d) 0.57 mm fiber with 45 mm diameter foam cladding (90% of power).
As mentioned earlier, one of the major challenges posed by the rod-in-air subwavelength fibers is the difficulty in their handling to significant presence of the modal field in the air cladding. To counter this problem, and to enable easy handling and manipulation of such fibers in practical installations, one can insert the subwavelength THz fiber core, for example, in a circular/square-shaped low-loss foam that features RI close to that of air. In our experiments, we realized some of such fibers using polystyrene foams with RI of 1.0104 and losses [11]. In Figs.
4. BIT ERROR RATE MEASUREMENTS
The BER measurements were carried out to study the fiber link performance under the laboratory environment. In the measurements, the emitter photocurrent was set to 7.5 mA, which corresponds to the THz power of (). A non-return to zero (NRZ) PRBS with the bit rates between 1 Gbps and 6 Gbps and a pattern length of was used as a baseband signal. For the target BER of (error-free detection), the duration of a single measurement was . Furthermore, the decision threshold is optimized so that insertion error (digital 0 is mistaken as digital 1) and omission error (digital 1 is mistaken as digital 0) are approximately the same.
4.2 A. BER Measurement for the 1.75 mm and 0.93 mm Fibers at 8 m Link Length
First, we identify the maximal fiber length of the 1.75 mm fiber for BER measurements in our system. For that, we consider the eye pattern and observe that beyond 8 m of fiber length, the eye amplitude becomes comparable to the 0 and 1 noise levels, thus resulting in impractical BER values. Therefore, the maximum link length is fixed at 8 m. Similar to the modal loss measurement, an 8-meter-long 1.75 mm fiber is butt coupled to the emitter and detector units using fisherman’s knots to hold the fiber in place. The BER measurement is carried out for the 1.75 mm fiber by varying the bit rate from 1 Gbps to 6 Gbps. At each bit rate, the decision threshold is optimized so that both insertion and omission errors are the same.
The total BER of the 8 m link is presented in Fig.
Fig. 11. Measured BER versus bit rate for the 1.75 mm and 0.93 mm fibers, and the link length of 8 m. Inset, eye patterns for the two fibers at various bit rates.
From the eye patterns, we can judge the effects of modal loss and modal dispersion on the fiber link performance. Thus, in the case of 1.75 mm fiber, the eye amplitude is much smaller than that for the 0.93 mm fiber indicating that 1.75 mm fiber link performance is limited by the modal absorption loss. The error-free operation for the 1.75 mm fiber is observed for link lengths shorter than 5 m (input THz power is ) and bit rates of up to 6 Gbps [currently limited by the THz communication system, and the input THz power is ()]. At the same time, even at the link distance of 8 m, the measured BER of is well below the forward error correction (FEC) limit (). As the eye patterns for the 1.75 mm fiber stay relatively symmetric even for longer fiber links (8 m) and at high bit rates (6 Gbps), we believe that such a fiber can support at least 9 Gbps up to 10 m (as predicted theoretically), by compensating modal absorption losses with higher input powers [above ()]. In contrast, for the case of 0.93 mm 8-m-long fiber, although its absorption loss is much lower than that of the 1.75 mm fiber due to much higher group velocity dispersion in such fibers [], the maximal bit rate is limited to only 3 Gbps. This is also confirmed by the shapes of the eye patterns for the 0.93 mm fiber that show significant shape degradation at higher bit rates, while also featuring almost 10 times higher powers of the received signals in the case of the 1.75 mm fiber.
4.3 B. BER Measurement for the 0.57 mm Fiber at 10 m Link Length
We now consider the 0.53 mm fiber that is theoretically predicted to have a very small absorption loss and a relatively small value of dispersion []. As predicted theoretically, an error-free transmission of up to 4.7 Gbps can be achieved using 0.57 mm 10-m-long straight fiber link. Experimentally, we use the same arrangement as discussed earlier, and then conduct BER measurements for data bit rates between 1 Gbps and 6 Gbps and a fiber length of 10 m. Error-free transmission with an optimized decision threshold is observed up to 4 Gbps (measured in steps of 1 Gbps) as shown in Fig.
Fig. 12. Measured BER versus bit rate for the 0.57 mm fiber and the link length of 10 m. Inset, eye patterns for 1, 2, and 6 Gbps bit rates.
Fig. 13. Measured BER for the 90° bending of 1.75 mm fiber with the bending radius of 6.5 cm versus bit rate. The schematic and experimental setup of the bent fiber are shown in the inset.
4.4 C. BER Measurement for the 1.75 mm Fiber and a 90° Bend
The effect of bending on performance of the 8-m-long 1.75 mm fiber is studied using a 90° bend of 6.5 cm bending radius. The BER measurement (see Fig.
5. POWER BUDGET COMPARISON OF THE ROD-IN-AIR THz FIBER LINKS WITH THE FREE SPACE COMMUNICATION LINK
In this section, we highlight the advantages of fiber-based communication links by comparing them with the free space communications assuming a simple ASK modulation scheme. In free space optics, the power received at a distance from the source is given by where is the received power, is the transmitter power, and and are the aperture size of the source and detector antenna (lens diameter or horn antenna aperture, for example), respectively. The angle is a full divergence angle of the beam, is the beam waist size at the transmitter end, and is the air attenuation coefficient. The equation is valid in the far field region, i.e., at distances . A typical power attenuation coefficient in air at the carrier frequency of 128 GHz is [77]. The received THz power using the fiber link can be estimated using Eq. (
By using a large-area lens or parabolic reflector antenna, it is possible to collect most of the transmitted energy. However, using large collecting optics is not favorable in many space-limited applications. For the emitter power of 0 dBm (1 mW), the received power after distance for both fiber and free space links is shown in Fig.
Fig. 14. Comparison between free space and rod-in-air fiber (straight)-based THz communication links at 128 GHz carrier frequency. The emitter power is set at 0 dBm.
In Table
Table 3. Maximal Bit Rate (ASK Modulation) at Different Link Distances and Required Emitter Power to Result in the Signal Power (Error-Free Transmission) at the Receiver End for Both Free Space and Fiber Communication Linksa
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6. CONCLUSION
In this work, we presented a comprehensive theoretical and experimental study of simple, yet practical dielectric rod-in-air/foam THz fibers in view of their potential applications in short-range THz communication applications. The THz fibers under study were made of PP and featured three different core diameters of 1.75 mm, 0.93 mm, and 0.57 mm. Furthermore, THz communication link performance was characterized with fibers of lengths 8 m and 10 m as a function of the variable data bit rates from 1 Gbps to 6 Gbps at the carrier frequency of 128 GHz. Our main conclusion was that, depending on the fiber diameter, the communication links were operating either in the power-limited or dispersion-limited regime. Thus, the 1.75 mm fiber featured loss and zero dispersion at the carrier frequency, and it could carry the highest bit rate of 6 Gbps up to the maximal distance of 8 m only limited by the fiber absorption loss, while error-free transmission with such a fiber was observed up to 5 m link length. Modal field extent of the core-guided mode into air cladding was only several mm deep due to relatively strong confinement of the modal field in the fiber core. As a result, the 1.75 mm fiber was also well tolerant to bending with virtually no degradation in the link performance when inserting a 90° tight bend of 6.5 cm radius. Further encapsulation of the fiber with polystyrene foam of sub-1-cm diameter makes such a fiber an excellent candidate for practical short-range THz communication links due to its ease of handling and installation, as well as good optical properties and tolerance to perturbations such as bending. Similarly, the 0.57 mm straight fiber featured a very low absorption loss and a relatively small dispersion of . The resultant performance was similar to that of a 1.75 mm fiber; however, maximal link length was rather limited by dispersion than by the modal loss. As a result, error-free transmission was realized for a 10 m link with up to 4 Gbps data rates, while signal strength was considerably higher than the noise level. One of the major disadvantages of this fiber is high sensitivity to bending and several-cm-deep penetration of the modal fields into the air cladding, thus making even the foam-cladded fibers inflexible and somewhat difficult to handle. Finally, the 0.93 mm fiber, while featuring relatively small absorption loss of , also featured relatively high dispersion of , thus significantly limiting the maximal supported bit rate even for a good signal strength. Aa a result, a maximal bit rate of only 2.4 Gbps was demonstrated for an 8 m fiber link. Finally, we compared the THz fiber communication links with free space links that use relatively small focusing optics (up to 5 cm diameter) and concluded that in this case fiber links are generally more efficient in terms of the power budget for short-range communications up to several tens of meters. Fiber links are also more reliable and easier to install, maintain, and reconfigure than free space links, especially in complex communication environments (on-board communications, for example). At the same time, free space communications outperform fiber-based links in terms of maximal bit rate as air features significantly lower dispersion than fiber.
7 Acknowledgment
Acknowledgment. We thank technician Mr. Jean-Paul Levesque for his assistance.
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Article Outline
Kathirvel Nallappan, Yang Cao, Guofu Xu, Hichem Guerboukha, Chahé Nerguizian, Maksim Skorobogatiy. Dispersion-limited versus power-limited terahertz communication links using solid core subwavelength dielectric fibers[J]. Photonics Research, 2020, 8(11): 11001757.