In various applications, effective communication among devices is crucial for smooth operation. This is particularly true in systems with rotating components or in environments where limited space necessitates hollow internal configurations. In medical computed tomography (CT) scanners, the CT slip ring is a critical component of the CT imaging system. It is used to transmit data from the X-ray scan of the human body to a computer for image reconstruction. As illustrated in Fig. 1, data detected by the X-ray is emitted as optical signals by a transmitter. These optical signals are then coupled to a detector through a convergence lens. Finally, the signals are transmitted back to the computer system via a data cable for image processing. Both the transmitter and the convergence lens are mounted on a rotating platform, while the detector remains stationary. The slip ring communication technique is widely used in various applications, including optical signal transmission for medical CT^[1], high-speed dome cameras, sweeper radars, rotating parts of robots^[2], wind power generators^[3], and for the signal transmission in the photoelectric pods of drones and airplanes^[4]. At present, slip ring communication technology includes brush-type slip rings, capacitive coupling slip rings, and optical slip rings. Brush-type slip rings suffer from high wear and low transmission rates^[5]. Capacitive coupling slip rings are prone to electromagnetic interference^[6], and optical slip rings are costly and challenging to align optically. Therefore, there is a need for a new slip ring communication method to address these existing challenges.

In this paper, we propose a new slip ring communication technology—fiber side-emitting communication (FSEC). FSEC uses the side-emitting fiber (SEF) as the transmitter in communication. SEFs are designed to promote leakage of the core-transmitted radiation via their side surfaces^[7]. Figure 2 shows the architecture of the FSEC system: a laser diode (LD) couples optical signals into an SEF, and the light emerging laterally from the fiber is collected through an optical filter and convergence lens before detection, thus completing data transmission. Figure 3 illustrates the structure of the FSEC system applied to a CT communication slip ring, where the SEF is secured on the rotor, while a photodetector is fixed on the stator. Data from the X-ray scanning of the human body is converted into an optical signal through the drive module and coupled into the SEF. The optical signal is transmitted via side light to the detector and then sent to the computer to construct a detailed tomographic image of the human body. Unlike conventional slip ring communication methods, the slip ring system employing an SEF as the transmitter is immune to electromagnetic interference and leverages the high-speed, high-capacity advantages of wireless optical communication^[8]. This innovative approach offers a more reliable and cost-effective solution for slip ring communication. To our knowledge, this is the first time that the SEF has been applied to slip ring communication.

There is limited research on the application of SEFs in optical communication. Three major limiting factors hinder their practical implementation. 1) A comprehensive transmission model for SEF has yet to be established. Unlike the light sources used in conventional optical fiber and wireless optical communication—which typically emit collimated light—SEF functions as a curved surface light source. Since SEFs have been primarily applied in illumination, a dedicated transmission model for communication purposes has not been developed. 2) The coupling efficiency of the side-emitting light is low. The light is emitted omnidirectionally (360°), whereas the photodetector is typically positioned on one side of the fiber, making it difficult for most of the light to couple into the detector. 3) The effective optical power available for communication is relatively limited. This is due to both the restricted output of the light source and the low side-emitting power of SEF, which together result in a low signal-to-noise ratio. Fiber side emitting is used in biomedical applications for microalgae culture, photo-sterilization^[9], photothermal chemotherapy^[10], and laser surgery^[11], and in illumination and decoration applications for fiber optic lighting^[12] and fiber array displays^[13]. In 2021, Teli et al. proposed and demonstrated a novel wireless communication link using SEF as a transmitter in optical camera communications. In 2023, Eöllős et al. introduced a novel application for SEFs^[14], employing them as distributed transmitters. By OOK modulation, they achieved transmission rates of 3.54 and 5.28 kHz over distances of up to 35 and 40 m.

To address these limitations, we propose and experimentally validate an optical design that overcomes the low coupling efficiency issue. Moreover, by optimizing the system design, the FSEC system achieves data communication with a bit error rate (BER) below 1×10−12 at 1.25 Gbps. Additionally, we have developed a mathematical transmission model for the SEF. The contributions of this paper can be concluded as follows. 1)An optical intensity and irradiance model for SEFs was developed, providing a theoretical basis for studying side-emitting attenuation and side-emitting communication.2)Through optical design and simulation validation, we compared various optical lens coupling methods and designed an optical antenna with a coupling gain of 9.6 dB, effectively addressing the issue of low coupling efficiency of side-emitting light into photodetectors.3)We designed a higher-power drive light source and an SEF with enhanced side emission intensity. Experimental results validated that the FSEC system achieved real-time communication with a BER below 1×10−12 at a data rate of 1.25 Gbps.4)We proposed and validated the communication capability of fiber side emission, expanding the communication scenarios and application methods of optical fibers.

The loss in optical fibers arises from various factors, including absorption, scattering, and bending. In the case of SEFs, the optical light primarily propagates along the axial direction and undergoes lateral scattering. Therefore, in the model presented in this paper, the side emitting light is assumed to propagate through these two primary paths: axial transmission and side scattering. This simplification allows for a more focused analysis of the intensity and irradiance associated with these specific light paths, which dominate the behavior of SEFs.

As in Fig. 4, following the ideas discussed by Spigulis and Endruweit et al.^[15,16], the intensity Io after crossing a segment of the fiber length of Δx is Io=(4π)−1Iiexp(−kλΔx),(1)where Ii,Io,andIs are the incident intensity, outgoing intensity, and scattered intensity, respectively. It is the intensity at the incoming end of the fiber, and kλ is the attenuation coefficient.

The attenuation coefficient kλ is expressed as kλ=kaλ+keλ+kbλ,(2)where kaλ is the absorption attenuation coefficient due to the fiber material, keλ is the side-emitting attenuation coefficient, and kbλ is the bending loss coefficient due to the fiber bending. According to the previous analysis, we neglect absorption and bending loss, so Eq. (1) can be expressed as Io=(4π)−1Iiexp(−keλΔx),(3)Is=(4π)−1Ii[1−exp(−keλΔx)].(4)

Therefore, Is(x0,Δx)=(4π)−1Itexp(−keλx0)[1−exp(−keλΔx)].(5)

Equation (5) gives the fiber length as a function of intensity.

In the FSEC system, the optical signal from the fiber side carrying information is coupled into the detector. Since the photodetector’s receiving area is a crucial factor, it is essential to explore the optical power of the fiber side light per unit area.

As shown in Fig. 5, according to the Rambling cosine law, the expression of the side light intensity at (r,x0), angle (ζ,η), is Is(r,x0,η,ζ)=Aexp[−keλ(x0+ρcosη)]cosθ,(6)where, according to the law of cosines, r2+ρ2cos2η=R2+ρ2−2Rρcosθ.(7)

Therefore, cosθ=R2−r2+ρ2(1−cos2η)2Rρ,(8)ρ=rcosζ−r2(cos2ζ−1)+R2sinη.(9)

Then, Eq. (6) can be expressed as Is(r,x0,η,ζ)=Aexp[−kλ(x0+ρcosη)]R2−r2+ρ2(1−cos2η)2Rρ.(10)

The irradiance E of light incident from all directions ex, over all solid angles dΩ, onto a receiving surface element dA can be expressed as E=1A∫A∫ΩIsdAexdAnAdΩdA,(11)where the area of the light receiving surface is A, the normal is nA, and exdAnA is the cosine of the angle between the normal of the receiving surface and the side light intensity. If the receiving surface area is small and IsdA is approximately constant over A, Eq. (11) can be replaced by E=∫ΩIsexnAdΩ,(12)and dΩ=dAr2=sinηdηdζ, IsexnA=Iλsinηcosζ, so Eq. (12) can be replaced by E=∫ζ∫ηIssin2ηcosζdηdζ.(13)

In the FSEC system, the choice of SEF is governed by coupling efficiency, diameter, numerical aperture, and refractive index. A smaller fiber diameter reduces intermodal dispersion; however, if the diameter is too small, it hinders efficient coupling from the light source into the SEF, thereby lowering the overall coupling efficiency. Consequently, we selected an SEF with a 2 mm diameter. Furthermore, the difference between the core and cladding refractive indices determines both the intensity of the side emitted light and the numerical aperture. Based on experimental tests, we chose the CF2.0 SEF model, which features a core refractive index of 1.49 and a cladding refractive index of 1.38, resulting in a relatively high coupling efficiency. This CF2.0 fiber optic comes from Nanjing Chunhui company. The parameters are shown in Table 1.

The FSEC system employs a step-index plastic optical fiber (SI-POF), whose transmission loss varies with wavelength. Figure 6 shows the attenuation spectrum obtained from three measurements of a standard fiber of class A4a by the POF-AC (POF Application Center, Germany)^[17]. The 400–600 nm band is the low-loss interval for PMMA-SI-POF, with loss ranging from 0.07 to 0.1 dB/m at fiber diameters greater than 750 µm.

The length of fiber we used is 1 m, and the loss of 1 m is 0.1 dB when the light is 450 nm. We used a 450 nm light source, and the photodetector is selected as an avalanche photodiode (APD) (S8664-05K).

SEF, serving as a transmitter in slip ring communication, involves two crucial optical coupling aspects: the coupling of the LD to the fiber and the coupling of side light to the APD. The former efficiency depends on the divergence angle of the LD, the NA of the optical antenna, and the NA of the SEF. The coupling of side light to the APD is a critical issue, as efficiency depends on the utilization rate of the fiber side light and the area of the photodetector’s photosensitive surface. Due to the omnidirectional (360°) emission of the fiber’s side light, and the photodetector being positioned on only one side of the fiber, it is challenging for most of the side light to effectively couple into the photodetector, resulting in low utilization of side light. On the other hand, the bandwidth of a photodetector is related to its parasitic capacitance and the area of the photosensitive surface. A larger photosensitive surface area enhances optical power reception, yet it concurrently elevates parasitic capacitance, ultimately diminishing bandwidth. In scenarios involving low-speed communication, opting for a larger photosensitive surface is a viable choice. Conversely, high-speed communication mandates a photodetector with minimal parasitic capacitance and high bandwidth. Hence, achieving effective coupling between the side light and the photosensitive surface becomes a critical concern, given the fiber’s relatively weak side light and the limited photosensitive surface of high-speed detectors.

The geometric image analysis in ZEMAX software is an effective tool for calculating fiber coupling efficiency. We simulated the parameters of a CFP2.0 fiber. We modeled a POF with a core diameter of 1960 µm, a cladding diameter of 2000 µm, and an NA of 0.65. The core material is polymethyl methacrylate (PMMA), with a refractive index of 1.49. We employ the ray-tracing method to simulate the number of rays reaching the detector. “Total Number of Rays” refers to the predefined total number of rays in the simulation, whereas “Total Hits” denotes the number of rays that actually impact the detector. Coupling efficiency is determined by the ratio of total hits to the total number of rays. 1)Coupling Light into Fiber. We simulated several external lenses, their ball lens, aspheric mirror, and plano-convex lens. The total hits of the direct coupling way are 94139 and the coupling efficiency is 94.14%. The total hits for the ball-lens coupling way are 99970, and the coupling efficiency is 99.97%. Table 2 is a comparison of different coupling ways.The coupling efficiency of both the direct coupling and lens coupling is greater than 94%. Since lens coupling requires designing and installing more complex lens structure components, we used direct coupling.2)Coupling Side Light into Detector. In the FSEC system, the coupling efficiency of side light coupling to the detector is low for three reasons: i) The main light in the fiber transmission is transmitted along the axial direction and the optical power used for side emitting is low; ii) SEF around the light, and the detector can only receive part of the light, and side light utilization efficiency is low; iii) High-speed photoelectric detector junction capacitance is small, corresponding to the photosensitive surface area also being small, thereby limiting the efficiency of optical coupling into the detector.Table 2.

Comparison of Different Coupling Ways

To enhance the coupling efficiency, we conducted simulations using ZEMAX to evaluate lens gain. We simulated several commonly used converging lenses, including the plano-convex lens, aspheric mirror, ball lens, and cylindrical lens. To enhance the utilization of side light efficiency, we employed a reflector to redirect the side light. After conducting simulations and comparisons, a parabolic mirror has the highest efficiency. Figure 7 illustrates the structural diagram and simulation results for direct coupling. Figure 8 depicts the system’s structure and simulation outcomes with the side light passing through a parabolic mirror and cylindrical lens. The fiber length in the simulation is 16 mm, and the total number of rays is 5×106. The ray trace in ZEMAX is used to determine the coupling efficiency, and the number of rays hitting the detector surface under different lenses is obtained. Table 3 shows the comparison of varying coupling ways.

Given the detector’s small surface, focusing side light both vertically and horizontally was crucial, leading us to employ a combination of lenses. A cylindrical lens focuses light parallel to the fiber, whereas a ball or parabolic lens handles vertical focusing, greatly enhancing coupling efficiency.

The direct coupling efficiency of fiber side light into the APD is remarkably low, yielding only 3612 detected rays. In contrast, a parabolic mirror plus cylindrical lens assembly delivers a 9.6 dB improvement over direct coupling. However, due to the high cost of custom optical components, we employed a ball-lens coupling configuration, which achieved a 3.1 dB gain. Consequently, the coupling efficiency increased from 0.072% to 0.664%.

The communication system designed in this paper uses on-off keying (OOK) modulation. The general design block diagram of the system is shown in Fig. 9. The output signal of the bit error rate tester (BERT) is an OOK signal generated according to the generating polynomial of 231−1. The signal enters the transmitting unit, and the electrical signal is converted into an optical signal, which is emitted through the LD and coupled into the SEF in free space. The fiber side light is coupled into the APD through the ball lens. It enters the receiving unit, where the optical signal is converted into an electrical signal, and then enters the clock and data recovery (CDR) circuit. The CDR recovers the clock from the signal and forwards the recovered signal to the BERT for BER testing.

To simulate slip ring communication, we conducted an experiment on the communication rate of the FSEC system. In this setup, the SEF functioned as the rotor, attached to the rotary table, while the receiver served as the stator, fixed to an optical pedestal. In slip ring applications, the circumference of the slip ring rotor typically ranges from several centimeters to several tens of centimeters^[18]. Given the increased attenuation of side light from the side, we positioned the SEF 90 to 100 cm from the LD module, secured to the rotary table. Successful communication at this range demonstrates the SEF’s effective communication capability for distances up to 1 m.

As shown in Fig. 10, BERT inputs the OOK signal into the LD module through the coaxial cable and then couples into the SEF. The side light is coupled into the receiver through the ball lens fixed on the receiver and then returns to BERT for BER testing after CDR. The experimental parameters are listed in Table 4.

We conducted communication rate tests on the FSEC system, with the results presented in Fig. 11(a), which displays the BER versus communication rate, while Fig. 11(b) shows oscilloscope eye diagrams for rates of 1.25 Gbps. Notably, the FSEC system achieved a BER of less than 10−12 at 1.25 Gbps. Throughout the rate testing, the rotary table was used to simulate the motion state of the slip ring rotor.

In slip ring applications, the distance between the stator and rotor is very small, with the air gap typically in the millimeter range^[19]. This air gap represents the communication distance from the fiber to the detector. We conducted BER tests across communication distances from 2 to 35 mm.

We explored the effect of communication distance on received optical power and BER. At 1.25 Gbps, we obtained the received optical power and BER as a function of the distance from the fiber side to the APD. The experimental results are shown in Fig. 12. The horizontal axis represents the distance between the fiber and the detector, while the vertical axis represents the received optical power (dBm) and BER. The experimental results show that the communication distance between the rotor and stator of the FSEC system can achieve a BER of less than 10−12 within 5 mm.

After finishing the above tests, we verified the communication performance of the slip ring system when the rotor moves. To facilitate the experiment, the fiber was securely fastened to a rotary table, with fixed positions for both the rotary table and the APD. By rotating the rotary table, we accomplished fiber side-emitting communication during fiber movement. The fiber segment for communication is 10 cm long. At this time, the communication rate is 1.25 Gbps, and the distance between the rotor and stator is 1 mm. This experiment verified that FSEC can be applied to optical slip ring communication, and it is also the first time that the SEF is applied to slip ring communication.

The side-emitting attenuation of a 100 mm fiber segment was experimentally characterized. The total attenuation for this 10 cm fiber amounted to 6.6 dB, averaging 0.66 dB per centimeter. Figure 13 presents the irradiance profile alongside the theoretical decay curve computed from Eq. (13). The irradiance does not maintain a decreasing trend in the 95 to 97 cm section of the graph due to measurement errors.

In this paper, we proposed a novel application of fiber side emitting for data communication and designed an FSEC system to achieve real-time transmission at 1.25 Gbps. Our design overcomes the limitations of conventional slip ring communications, such as electromagnetic interference, limited data rates, and high costs, while also addressing challenges inherent to SEF applications, including the absence of an established communication model and low coupling efficiency.

We established mathematical models for side emission, encompassing both the intensity and irradiance models. To tackle the challenge of low coupling efficiency between the fiber’s side emitted light and the receiving unit, an optical system was designed using ZEMAX to enhance the optical gain, achieving an optical antenna gain of 9.6 dB. Furthermore, we developed a higher-power drive light source and employed an SEF with enhanced side emission intensity. Experimental results validated that the FSEC system achieved real-time communication with a BER below 1×10−12 at a data rate of 1.25 Gbps. By rotating the fiber on a fixed-position rotary table, the system maintained effective side-emitting communication under motion, thereby demonstrating its suitability for slip ring communication applications. This research not only extends the applications of fiber optic technology but also offers a cost-effective and high-speed solution to slip ring communication challenges, with potential applications in medical imaging, industrial robotics, and wind turbine control systems.