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Abstract
Infrared optical wireless communication system can achieve ultrahigh capacity and high privacy data transmission. However, for using narrow infrared laser beam as carrier to transmit signal, the high-speed data transmission can only be achieved by point-to-point connection. With the rapid number increasement of consumer electronic devices, such connection method puts a heavy burden on the number of transmitters. Thus, the transmitting end with the point-to-multipoint capability or multi-user accessibility is required. In this paper, we present a multi-user accessible indoor infrared optical wireless communication system employing passive diffractive optics based on a virtually imaged phased array (VIPA). Multiple beams can be generated in a point-to-multipoint scheme by using VIPA-based beam-steering antenna (BSA). On the other hand, by tuning wavelength of laser source, fast 2D steering of multiple beams with the same steering trajectory is supported, which can be used for user ends with changing locations. In the experiment, 5 beams are generated by utilizing only one transmitter. All five beams can realize 12.5 Gb/s on-off-keying (OOK) data rate transmission. Free-space optical wireless transmission at 3.6-m communication distance is demonstrated for system performance verification and evaluation. a total 3.44°×7.9° scanning field of view of five beams is achieved.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
With the explosive growth of consumer electronic products and smart living technologies, the demand for high-speed, high-capacity and high-privacy wireless communication is proliferating. Conventional radio frequency (RF) communications are facing the serious challenges of limited data rates and user congestion [1–3]. As a short-range communication technology, optical wireless communication (OWC) attracts tremendous attentions, which offers serval advantages, such as unlicensed, high aggregate capacity, insensitive to electromagnetic interference (EMI) and high bandwidth [4]. Generally, both visible light and infrared laser are widely applied in indoor signal transmission while ultraviolet (UV) light is used for solar-blind or diffuse communication [5]. Visible light communication (VLC) technology usually uses LEDs as light source due to its low cost, high reliability and safe characteristics. Visible light laser diodes are normally used combined with a diffuser for the consideration of eye-safety [6]. In the infrared OWC systems, lasers are employed with central wavelengths at 850 nm, 1310 nm, and 1550 nm, which can easily achieve a higher transmission data rate if compared with LEDs. But the laser beam has a respectively narrow beam diameter compared with an LED. Thus, the infrared OWC system is usually implemented in line-of-sight link configuration and point-to-point strict alignment between transceivers is needed. Hence, it is a challenge to connect a mobile user or the users at different locations. Using beam-steering technology in infrared OWC systems is a promising solution to this issue [7]. On the one hand, beam-steering devices can improve the coverage area of the transmission beam. On the other hand, they can enhance the connectivity between the system and enable moving users by changing the beam direction according to the users’ position. Mechanical beam-steering systems, which have moving components, such as polygon mirrors and micro-electro-mechanically-systems (MEMS) mirrors, have been utilized in IR OWC [8–10]. The mechanical beam-steering devices have the advantages of large scanning field of view (FoV) and mature technology. However, they suffer from large size, high cost, vibrations, inherent slowness, and incapability of random beam-steering due to inertia [11]. Hence, solid-state beam-steering devices without moving components attracts much more attention. For example, optical phased array (OPA) is employed in OWC system for steering with the advantages of chip size and fast beam-steering speed [12,13]. while high coupling loss, high power consumption and fabrication difficulty have hindered its application in OWC systems [14]. Spatial light modulator (SLM) based on liquid crystal on silicon (LCOS) is another solid-state beam scanner for OWC systems with high capacity, wide scanning field of view but slow speed and complex configuration [15–17]. Another useful approach is spectral scanning utilizing diffractive optics, such as grating and array waveguide guide (AWG) with a wavelength-swept laser [18,19]. It has the advantages of inertia-free, stable and fast beam-steering [20]. However, current systems based on two cascaded gratings has the drawbacks of sparse scan line, which will bring many communication dead zones within the transmission coverage area. AWG-based beam-steering antenna offers a limited number of access points and it has the issues of signal crosstalk.
The beam-steering technologies discussed above offer an excellent solution to user movement and expand the signal transmission coverage area. On the other hand, the point-to-point connection method puts a heavy burden on the number of transmitters as the requirements of data transmission device are increasing. Thus, it is of significance to develop the technology which can support point-to-multipoint connection. However, serving multiple users at different locations simultaneously is still a huge challenge to overcome. For example, mechanical or OPA beam-steering systems need to split the signal beam into multiple beams. For AWG-based or grating-based beam-steering technologies, multiple transmitters working at different wavelengths are required to achieve multi-user access [21,22]. Recently, a system using SLM to achieve point to multipoint scheme is reported [23], which addresses a large number of end-users. But the operation of SLM is relatively complicated and the cost of SLM is high. Besides, a quasi-passive optical beam switch is reported for indoor OWC systems, which has the advantages of zero power consumption when users are still and can serve multiple users in different positions at the same time [24]. However, since beam-steering is realized by an optical switch, only four signal beams with fixed output direction can be generated.
In this work, we propose a multi-user accessible indoor infrared OWC system based on a passive diffractive beam-steering antenna. It consists of a VIPA and a transmission grating. Benefiting from the special diffractive property of VIPA, BSA can support point to multipoint connection which is utilized for multi-user access. It is a good solution to address the large number of user ends. On the other hand, the generated multiple beams can achieve 2D spectral beam-steering with the same steering trajectory by simply tuning the wavelength of laser source. Compared with other spectral beam-steering techniques, the VIPA-based BSA offers uniquely dense scanning lines (61 scanning lines within 3.44°×7.9° scanning FoV) that can provide precise signal beam steering for system. By combining spectral scanning technique and time division multiple access (TDMA) technique, five user ends with changing locations can also keep their connection. We experimentally generate 5 beams by using a single transmitter, each of whom can support transmission data rate up to 12.5 Gb/s using on-off-keying (OOK) modulation. The total scanning FoV is 3.44°×7.9°. A free-space optical wireless transmission distance of 3.6 m is demonstrated.
2. Working principle and system design
2.1 System structure
Figure 1 shows the architecture of the proposed multi-user accessible OWC system in two indoor scenarios. A BSA in installed on the ceil and it is fed by an optical fiber from central control system (CCS). The BSA transmit the high-capacity signal beam to the users ends near floor. Here, we introduce two application scenes in Fig. 1. As demonstrated in application scene 1 in Fig. 1, the implemented BSA can achieve point to multipoint scheme. By using only one transmitter with single wavelength, the large number of users ends can realize high date rate transmission connection simultaneously by using time division multiple access (TDMA) technique [25]. The transmission wavelength and data sequence diagram are shown in Fig. 2(a) and (b), respectively. The wavelength is fixed, and the 5 users can access to the system during a certain time slot in alternate frames. Also, other multiple access technology, such as orthogonal frequency division multiple access (OFDMA) or non-orthogonal multiple access (NOMA) [26,27] can be used. Such multi-user accessible method greatly addresses traffic demand distributed over several user ends. However, due to the diffraction characteristics of VIPA, the served multiple users ends by a transmitter should be placed aligned in a row as shown in Scene 1 in Fig. 1. And the space interval between users ends is determined by the free angle range which will be discussed in the following section. Thus, such configuration of BSA is very suitable for applications in indoor scenes where there are many stationary communication devices, such as computer rooms, classrooms, and offices. On the other hand, when user ends change their position within the whole coverage area of the BSA’s FoV, the proposed system can still steer the signal beams into the mobile user ends, combining with user localization method [28–30] and time division multiple access (TDMA) technique. As shown in corresponding scene 2 in Fig. 1, five nomadic user ends can keep their connections. For example, user #1 and user #2 are located along a row and requires the same wavelength but are served by different diffraction orders; user #1 and user #2 are located along a column and are served by the same diffraction order but requires different wavelength. The corresponding transmission wavelength and data sequence diagram are illustrated in Fig. 2. We assume that the locations of five users are already obtained. Then the corresponding wavelengths for the 5 users are changed from ${\lambda _1}$ to ${\lambda _4}$ respectively. These 4 wavelengths are sequentially tunned during a data frame as shown in Fig. 2(c). And each data frame is divided into 5 time slots which are synchronized with the wavelength tuning as depicted in Fig. 2. (d). There is a switching time $\Delta t$ in each time slot, which is reserved for wavelength tuning between two nearby time slots. The speed of switching time only relies on the tuning speed of the laser source, which has already been confirmed in various spectrally scanning LiDAR systems [31–34]. For example, the beam-steering frame rates can reach 10 kHz within a tuning bandwidth of 40 nm by using a MEMS-based VCSEL [32].
Fig. 1. Architecture of point-to-multipoint scheme in the proposed multi-user accessible OWC system. Left: stationary 5 users aligned in a row; Right: nomadic 5 users; BSA: beam-steering antenna. The shade areas with different colors represents the scanning coverage area of generated multiple beams respectively. The dotted red beams represent the remaining unused beams.
Fig. 2. The working principle of multi-user access by using fast spectral scanning technique and TDMA technique. (a)(c) spectral scanning process in two scenes. (b)(d) data stream is divided into multiple frames.
In the following subsections 2.2 and 2.3, the principle and design of the VIPA-based BSA for multi-user situation will be introduced.
2.2 Structure of beam-steering antenna
The beam-steering antenna based on VIPA requires multiple beams output ability to support multiple users. In our previous work, we have utilized such beam-steering device in light detection and range (LiDAR) system as a single-beam scanner [33]. Here, the VIPA-based BSA can support point to multipoint scheme, which is utilized for multi-user access. The structure of BSA based on VIPA is illustrated in Fig. 3(a). A cylindrical lens is used to focus the light into the VIPA cavity. After going through VIPA, multi-order laser beams are generated with different output angles along the y-axis. Such special diffractive character is used to support point-to-multipoint scheme and then multi-user accessibility can be realized. For the realization of beam-steering function, a wavelength-swept laser source is utilized. Along with the wavelength-swept of the incident light, the generated laser beams will scan repeatedly along the y-axis, which can be seen in Fig. 3(b). Then a transmission grating with orthogonal dispersion direction to the VIPA’s is used to separate these repeatedly scanning beams spatially along the x-axis direction. As demonstrated in Fig. 3(c)(d), the generated multiple beams would scan along the trajectory drawn in the figure at the same time. During the scanning process, the generated multiple beams always have the same output angles along the x-axis direction. The output angle difference between nearby beams is named free angle range (FAR) which is decided by the parameters of VIPA. Such property means the proposed BSA can be used in the indoor scene where there are many user ends with fixed position such as computer room, class room or offices. On the other hand, the beam-steering antenna can also steer signal beams to user end whose position is always changing but within the scanning FoV of BSA.
Fig. 3. (a) The structure of beam-steering antenna based on VIPA. (b) The 2D spatial map by tunning laser wavelength before going through grating. (c) 2D beam-steering by a VIPA-based BSA and tuning wavelength. (d) Beam-steering performance in practical scene. CL: cylindrical lens. FAR: free angle range.
2.3 Theoretical analysis and design of beam-steering antenna
For the BSA used in the proposed system, the beam-steering coverage area (scanning field of view), scanning line density and the number of accessible users is of the greatest importance. As discussed in subsection 2.2, the performance of the beam-steering antenna mainly depends on the VIPA and grating. In this subsection, a theoretical analysis design of beam-steering antenna based on VIPA and grating is conducted. The grating is low-order diffractive optics with simple dispersion characteristic. As a high-order dispersive optics, the working principle and dispersion characteristic of VIPA is more complicated. Four parameters (the tilt angle, cavity material, thickness and F-number) of VIPA will be discussed.
2.3.1 Working principle: grating
A commercial blazed grating is used here for x-axis dispersion, whose dispersion is ruled by grating equation
where m is the diffractive order which is −1; $\lambda $ is the light wavelength; $l$ is the line density of grating. The grating decides the FoV along the x-axis of the BSA. From Eq. (1), the FoV along the x-axis within the wavelength range from ${\lambda _1}$ to ${\lambda _2}$ is given by [33]:For most blazed grating, the incident angle is specified to ensure high efficiency. Thus, $FO{V_x}$ are mainly decided by the line density of grating and the wavelength-swept range. Large wavelength-swept range and dense grating line density can improve the scanning $FO{V_x}$.
2.3.2 Working principle: VIPA
Figure. 4(a) shows the side view of a VIPA and the propagation process of light in the VIPA cavity. The reflectivities of the VIPA cavity left surface R and right surface r are usually 100% and 95% respectively. Once light incidents into the cavity, it will experience multiple reflections, and multiple virtual light sources are formed. These virtual light sources have the same beam profiles and interfere with each other. Thus, angular dispersion is achieved. Based on paraxial wave theory [33,35], the output light field can be expressed as
Fig. 4. (a) Schematic Diagram of a VIPA. (b) The simulated intensity distribution at y-axis. The focus length of cylindrical lens is 5 cm; The beam diameter is 3 mm; the material in VIPA cavity is silicon (the refractive index at 1550 nm is 3.48); the thickness of VIPA cavity is 7 mm; the incident angle is 5.7 degree.
2.3.3 Design of beam-steering antenna
Transmission coverage area, scanning line density, and the number of accessible users are the most critical performances of the beam-steering antenna. Correspondingly, a large scanning field of view, dense scanning lines and multi-beams output is guaranteed by the design of beam-steering antenna. These three performance metrics will be discussed as follows.
- (i) Transmission coverage area. The transmission coverage area is also the scanning FoV. At first, the dispersion along the x-axis can be analyzed by Eqs. (1) and (2). A large transmission coverage area along x-axis is achieved by using two methods. The first is using a grating with dense line density, which offers a large angular dispersion. The second is using a large wavelength-swept range, which is decided by the tunable laser source. As demonstrated in Eq. (3) and Fig. 5, a large width of intensity Gaussian envelope can maximize the total scanning FoV along the y-axis. Here the 1/e2 width of the envelope is regarded as the maximum total FoVy. It can be derived from Eq. (3) that the total FoVy is decided by w/f, where beam radius w is determined by the used collimator and f is the focal length of the cylindrical lens. Thus, a large beam radius and a cylindrical lens with a short focal length makes a large scanning FoVy.
- (ii) Scanning line density. As illustrated in Fig. 4(b), output signal beams with different diffraction orders support different users’ data transmission. The laser beam at each order owns its sub-scanning FoVy. Here, such sub-scanning FoVy is named as free-angle range (FAR). As demonstrated in Fig. 5, the corresponding wavelength tuning range to scan across FoVy is defined as free spectral range (FSR). the large scanning pattern slopeξmeans the distance between the scanning lines is small and the line density is large. Thus, the scanning pattern slope ξis used to evaluate the scanning line density, which is expressed as$$\xi = \frac{{{\raise0.7ex\hbox{${d{\theta _{y,out}}}$} \!\mathord{\left/ {\vphantom {{d{\theta _{y,out}}} {d\lambda }}}\right.} \!\lower0.7ex\hbox{${d\lambda }$}}}}{{{\raise0.7ex\hbox{${d{\theta _{x,out}}}$} \!\mathord{\left/ {\vphantom {{d{\theta _{x,out}}} {d\lambda }}}\right.} \!\lower0.7ex\hbox{${d\lambda }$}}}}.$$
From Eq. (4) and (1), ${\raise0.7ex\hbox{${d{\theta _y}}$} \!\mathord{\left/ {\vphantom {{d{\theta_y}} {d\lambda }}} \right.} \!\lower0.7ex\hbox{${d\lambda }$}}$ under first-order approximation [33] and ${\raise0.7ex\hbox{${d{\theta _x}}$} \!\mathord{\left/ {\vphantom {{d{\theta_x}} {d\lambda }}} \right.} \!\lower0.7ex\hbox{${d\lambda}$}}$ can be respectively expressed as
$${\raise0.7ex\hbox{${d{\theta _y}}$} \!\mathord{\left/ {\vphantom {{d{\theta_y}} {d\lambda }}} \right.} \!\lower0.7ex\hbox{${d\lambda }$}} = - {\frac{{{n^2}}}{{\lambda \theta }}_i},$$$${\raise0.7ex\hbox{${d{\theta _x}}$} \!\mathord{\left/ {\vphantom {{d{\theta_x}} {d\lambda }}} \right.} \!\lower0.7ex\hbox{${d\lambda }$}} = \frac{{ml}}{{\cos ({\theta _{x,out}})}}.$$Therefore,ξ is given by
The working order m in Eq. (8) of the grating is -1. The scanning line density can be increased by enlarging $\xi $. According to Eq. (8), dense scanning line density is available when using a VIPA with high-refractive-index cavity material and small incident angle, or using a grating with relatively sparse line density. On the other hand, the ratio between the total wavelength-swept bandwidth F and the FSR of VIPA determines the number of scanning lines. The linearly approximated version of FSR can be derived from Eq. (4) which is [33]
Thus, the number of scanning lines can be increased by using VIPA with thin cavity and high refractive index material in the cavity.
- (iii) The number of accessible users. As demonstrated in Fig. 5, the 1/e2 width of the intensity envelope decides the maximum FoVy. In order to ensure the scanning line density, the parameters of VIPA are determined as discussed in (ii), and the angle interval between two nearby orders can be obtained. Thus the number of accessible users can be calculated by $N = FO{V_y}/FAR$. The most effective method to increase the number of output beams is enlarging the total FoVy, which is in agreement with the discussion results in (i).
3. Experimental results
3.1 Performance of beam-steering antenna
In this subsection, the parameters of the beam-steering antenna used in the experiment and its performance are demonstrated. A customized VIPA (LightMachinery Inc) with fused silica in the cavity with 0.5-mm thickness is used. A transmission grating (T-940CL, LightSmyth Technologies) with 940 lines/mm line density and above 90% efficiency is utilized. A 5 cm focal length of cylindrical lens is used. Figure 6(a) shows theoretical 2D beam scanning pattern obtained by using Eq. (4). The wavelength-swept range is from 1500 nm to 1600 nm. The total scanning FoV is then 3.44° × 7.90°. Five signal beams are generated simultaneously by a single incident laser beam. In order to check output beams, a laser viewing card is used to observe the laser beams at 1577.4 nm. As shown in Fig. 6(b), five beams can be clearly observed on the left picture which agrees with the theoretical result shown in Fig. 6(a). Due to the intensity envelope of VIPA, the power of the output beam is relatively small while the output angle is away from the central position θ y,out=0. For example, the output angle of signal beam #1 is close to the boundary of 1/e2 width of the intensity envelope, and the laser beam observed in Fig. 6(b) is darker compared with other laser beams. the power of signal beam is reduced about −8.7 dB from the central position ${\theta _{y,out}}$ to the FoV boundary. In addition, VIPA leads to 3-dB insertion loss. Such loss can be compensated by increasing the total output power. The beam divergence depends on the collimator. In this experiment, a collimator with 0.032 degrees full divergence angle is used. Within 3.44° × 7.7° transmission coverage area, 61 scanning lines are obtained, which is close to a ten-fold improvement compared with previously demonstrated grating based passive beam-steering device. The detailed design method of such 2D disperser for fast beam-steering can be found in our earlier work [32,33].
Fig. 6. (a) The simulated 2D beam-steering pattern. (b) Intensity distribution along VIPA dispersion direction and the left photo shows the multiple output beams at 1577.4 nm observed by a visible card.
3.2 Performance of high-speed data transmission
The experimental setup for the proposed multi-user accessible high-speed indoor OWC system is demonstrated in Fig. 7. The laser beam from a tunable laser is modulated by a Mach-Zehnder modulator (FTM7938EZ, Fujitsu) with non-return-to-zero on–off keying (NRZ-OOK) modulation format. A bit error rate tester (BERT, MP2100B, Anritsu) is used to generate a pseudorandom bit sequence (PRBS) signal with a length of 231 −1. The pulse pattern generator (PPG) model integrated in the BERT can only generate peak-to-peak 0.8 Vpp. Thus, the NRZ-OOK signal needs to passe through an amplifier (DR-DG-20-MO, iX-blue) before it enters into the Mach-Zehnder modulator, which has a 7-dB loss. An avalanche photodiode (KY-PRM-18G, KY Photonics) module is used to capture the signal. The received digital baseband signal is analyzed by an error detector module to measure the bit error rate (BER). A real-time oscilloscope is used to observe the eye pattern. The collimator (F280APC-1550, Thorlabs) used in the transmitter has 3.6-mm output beam waist diameter and 0.032-degree full angle divergence. In the receiver, a zoom fiber collimator (ZC618FC-C, Thorlabs) with 6–18 mm adjustable focal length and 1.1–3.3 mm input beam waist diameter is used to optimize receiving coupling efficiency. In the experiment, single mode fiber is used in the receiver and a maximum coupling efficiency of 90% can be achieved. The beam diameter before the receiver collimator is 3.8 mm. The total emitted optical power of the generated 5 beams is around 20 mw. At different wavelength, the generated 5 beams have different power distribution. The distance between the transmitter and receiver is 1.5 m.
Fig. 7. Experimental setup for multi-user high speed data transmission. Both the downlink and uplink experimental setup are shown. MZM: Mach-Zehnder modulator.
As shown in Fig. 8, the results of a bidirectional optical wireless transmission are demonstrated. This experiment is conducted by changing the position of the receiver and transmitter. BER of five signals which serve five different users at three different optical carrier wavelengths is measured. The data transmission rate is 12.5 Gbit/s. It is clear that both downlink and uplink transmissions can achieve error free (BER< 10−9) operation. Besides, different signal beams serving different users have the same transmission performance. The only difference between beams at different orders is the optical power. The open-eye diagrams captured at 1550 nm optical carrier channel exhibit good received signal quality as shown in insets of Fig. 8. It further suggests that our proposed beam-steering antenna is a promising candidate for a full-duplex indoor OWC system.
Fig. 8. Bit error rate measurement results with 12.5 Gbps data transmission and typical eye diagram. Downlink and uplink measurements of five users at three different optical carrier wavelengths of 1520 nm, 1550 nm and 1580 nm.
The BER values versus the received sensitivity is presented in Fig. 9. With a data transmission rate of 12.5 Gbit/s, diffracted links at 1520 nm, 1550 nm and 1580 nm are also measured. Eye diagrams for 1550-nm channel at BER 10−3 and 10−10 are both shown in insets of Fig. 9 to demonstrate the signal quality. Different diffracted channels have a similar performance. When the received power is larger than −24 dBm, the BER is under the forward error correction (FEC) criterion of 3.8 ×10−3. In order to measure the performance at long distance with high data transmission, two silver-coated mirrors are used to achieve 3.6 m total length transmission. The BER performance is demonstrated in Fig. 10. 12.5 Gbit/s data transmission at 3.6 m is realized for the OWC system. The receiver optical power is required to be larger than −20 dBm at 3.6 m when BER is under the forward error correction (FEC) criterion of 3.8 ×10−3.
Fig. 9. BER performance versus received power at three different optical carrier channels of 1520 nm, 1550 nm and 1580 nm.
4. Discussion
By adopting a wavelength-swept laser source, the OWC system can support a 3.44°×7.9° transmission coverage area within 100 nm optical carrier wavelength range. Within this area, a total amount of 61 scanning lines is realized. Using a bandwidth limited transceiver, 12.5 Gbit/s transmission data rate is demonstrated. In the meanwhile, five users can be supported by using only one transmitter. However, the performance of the proposed system can be further improved by optimizing the parameters of the beam-steering antenna and transmitter and receiver. In this subsection, a discussion of potential improvements on scanning field of view, the number of accessible user ends, and transmission data rate are presented. In addition, the eye-safety issue of our proposed system and optical crosstalk are also been discussed.
4.1 Spectral response
Due the speed limitation of the used BER tester, only 12.5 Gbit/s transmission data rate is demonstrated. The proposed system relies on wavelength-control to achieve beam-steering. The dispersion characteristic of the VIPA-and-grating-based 2D disperser together with the limited aperture of the collimator in the receiver act as a bandpass filter. The spectral passband width limits the maximum transmission data rate of the corresponding channel [19]. Here, an amplified spontaneous emission (ASE) light source and an erbium-doped fiber amplifier (EDFA) are utilized to measure the spectral response of channels. In the receiver, an optical spectrum analyzer (OSA) is used to record the data. As shown in Fig. 11, three wavelength channels 1530 nm, 1540nm and 1550 nm are measured and all channels have more than 20 GHz −3dB spectral response. The results show that the proposed OWC system can support an effective transmission bandwidth more than 20 GHz. However, the multiple output signal beams generated by a tunable laser share the total bandwidth from the same transmission channel. Thus in real application, the sub-bandwidth for each user is decreased based on the number of accessible users when using multiple access technology, such as orthogonal frequency division multiple access (OFDMA) or non-orthogonal multiple access (NOMA) [26,27].
Fig. 11. Spectral response of different channels at 2 m. FWHM: Full Width at Half Maximum; BW: bandwidth.
4.2 Scanning performance and the number of accessible users
As discussed in section 2, increasing the 1/e2 width of the intensity envelope can enlarge the scanning field of view and increase the number of output beams. As experimentally demonstrated, using a cylindrical lens with 5 cm focal length, five signal beams within 3.44°×7.9° scanning field of view are realizable. To prove the feasibility of increasing FoV and the number of accessible users, we employ a cylindrical lens with a shorter focal length of 4 cm to further improve the FoV and increase the number of accessible users. As shown in Fig. 12, it is clearly that 7 output signal beams are generated within the scanning FoV 4.3°×7.9°. By further increasing the beam diameter of collimator and using lens with shorter focal length, the number of accessible users can be kept increasing. When a large number of laser beams are generated, the power of each beam is reduced. But such loss can be compensated by using an EDFA to amplify the power of signal beams. If multiple lasers with different wavelength are used, more users can be served.
As discussed in section 2, FoVx can be further improved by using grating with denser line density. For example, 1200-lines/mm grating is a suitable option that can offer 21.43° FoVx. If silicon (n=3.48) VIPA is utilized, the scanning line density can also be improved.
4.3 Infrared radiation and eye-safety
In our proposed OWC system, infrared (IR) lasers are employed to generated beam. The power of the laser beam is more concentrated compared with LED. In addition, IR radiation is non-visible. Thus, the eye-safety issue is of importance to analyze.
IR light with wavelength larger than 1400 nm can hardly reach to retina of human. Therefore, the eye-safety compliant power of the infrared light is relatively large. Based on the eye-safety regulation from IEC 60825 and ANSI Z1136 [7], the maximum transmitted power of infrared laser within the eye-safety constraint is 10 mw. As demonstrated in Fig. 4(b), the intensity envelope which is imposed by VIPA causes the non-uniform optical power distribution of generated multiple beams. Thus, the proposed system has the additional requirements for beam power. The output power of the beam with the maximum power should smaller than 10 mw limitation and the output power of beam with the minimum power should meet the demand for power in communication.
As demonstrated in subsection 2.3.3, the 1/e2 width is used to define the illumination range which means the power difference between the maximum value and the minimum value is around 8 dB. In our experiment, the generated maximum output power of beam is 7 mw which is smaller than 10 mw limitation. The minimum power of beam which close to the edge of 1/e2 width is around 1 mw (0dBm), which can still support error free data transmission, which can be confirmed in Fig. 9. The receiver power, which can support error free data transmission, can reach −16 dBm. Thus, the maximum output power can be further decreased to −9 dBm, and the eye-safety can be guaranteed in our proposed system.
4.4 Optical crosstalk
Compared with LED, infrared laser beam has good directivity and collimation characteristics. The optical crosstalk could happen when two laser beams overlap each other spatially. Our proposed system can support point-to-multipoint scheme. Thus, the optical crosstalk between generated multiple beams is needed to be discussed.
Figure 13 shows the cross-section pattern of two adjacent beams. In our experiment, the FAR is 0.68 degree. The beam divergence of the output beam is 0.032 degree and the beam waist diameter is 3.6 mm, which is decided by the output collimator (F280APC-1550, Thorlabs). Based on the above parameters, the generated multiple beams would overlap with each other spatially only if the transmission distance is less than 30 cm, which gives the minimum transmission distance. However, in practical application, the BSA is installed on the ceil and the free space transmission distance is usually 2-4 m. Such cross talk can hardly happen.
5. Conclusion
In this work, we proposed an OWC system with multi-user accessible ability. Benefiting from the particular diffractive property of VIPA, the VIPA-based BSA can support point to multipoint scheme which offers the system with multi-user accessible ability. It greatly addresses traffic demand distributed over several user ends. As a dispersive device, VIPA has the advantages of large angular dispersion ability and polarization insensitivity. On the other hand, the beam-steering function is achievable by adopting a tunable laser, that can provide the system with the ability of steering signal beam to user ends with changing location. By combining spectral scanning technique and time division multiple access (TDMA) technique, five nomadic user ends can also access to the system. We experimentally set up the VIPA based infrared optical wireless communication system, which can support five user ends by using a single transmitter. Each signal beam can achieve a 12.5 Gbit/s NRZ-OOK data rate. We also extend the transmission distance to 3.6 m while maintaining the communication quality. Based on the measurement, our proposed OWC system possesses more than 20-GHz bandwidth. An optimized method is also given to further improve the performance of beam-steering antenna such as the servable numbers of users ends and scanning FoV. In short, the proposed OWC system offers an easy and feasible point to multipoint scheme and a good solution to address the traffic demand distributed over several user ends. On the other hand, the beam-steering function is available by using a tunable laser. When user ends change their position within the coverage area of BSA, the proposed system can still steer the signal beams into the mobile user ends at different location by combining fast spectral scanning technique and TDMA technique.
Funding
Shenzhen Municipal Science and Technology Innovation Council (WDZC20200820160650001, JCYJ20180507183815699); Overseas Research Cooperation Fund of Tsinghua Shenzhen International Graduate School (HW2020006); Guangdong Basic and Applied Basic Research Foundation (2021A1515011450).
Acknowledgement
The Authors would like to express sincere thanks to the Shenzhen Technology and Innovation Council (WDZC20200820160650001, JCYJ20180507183815699), Overseas Research Cooperation Fund of Tsinghua Shenzhen International Graduate School (HW2020006) and Guangdong Basic and Applied Basic Research Foundation (2021A1515011450).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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