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Abstract
White light generated by mixing the red, green, and blue laser diodes (RGB LDs) for simultaneous high-speed underwater wireless optical communication (UWOC) and high-efficiency underwater solid-state lighting (SSL) was proposed and demonstrated experimentally for the first time. The allowable maximum real-time data transmission rates of 3.2 Gbps, 3.4 Gbps, and 3.1 Gbps for RGB LDs with corresponding BERs of 3.6 × 10−3, 3.5 × 10−3 and 3.7 × 10−3 were obtained at a 2.3 m underwater transmission distance using an on-off keying (OOK) modulation scheme, respectively. And the corresponding UWOC aggregate data rate of 9.7 Gbps was achieved based on RGB LDs-based wavelength-division multiplexing (WDM) UWOC. Moreover, UWOC and underwater SSL by using RGB LDs mixed white light were investigated at different scenarios over an underwater link of 2.3 m. The RGB LDs mixed white light-based UWOC system without optical diffusers yielded a maximum allowable data rate of 8.7 Gbps with Commission International de l’Eclairage coordinates (CIE) of (0.3154, 0.3354), a correlated color temperature of 6322 K, a color rendering index of 69.3 and a corresponding illuminance of 7084 lux. Furthermore, optical diffusers were employed to provide large-area underwater SSL. The LDs mixed white light-based UWOC system with line and circle optical diffusers implemented data rates of 5.9 Gbps and 6.6 Gbps with CIE coordinates of (0.3183, 0.3269) and (0.3298, 0.3390), respectively. This work suggests the potential of LDs for applications in high-efficiency underwater white-light SSL and high-speed UWOC.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The demand of ocean source exploration and underwater human activities has accelerated the development of underwater wireless communication (UWC) technology. UWC has always been an attractive topic to researchers worldwide due to the convenience and efficiency without requiring large stationary devices and engineering maintenance issues. Traditional UWC technologies of acoustic communication and radio-frequency (RF) communication can hardly meet the growing requirements of the high data rate due to the low bandwidth and low propagation speed of the acoustic wave and high attenuation of the RF wave in water. In contrast, optical wave in the blue-green region, featuring high bandwidth, low attenuation and low latency enables higher capacity for underwater wireless optical communication (UWOC) [1, 2]. Therefore, the UWOC has gradually emerged as a new UWC direction to meet the increasing requirement of the high-speed UWC. UWOC systems based on light-emitting diodes (LEDs) have been proposed by many researchers [3–6]. Nevertheless, relatively large beam divergence angle and low modulation bandwidth of the LEDs constrain the achievable transmission distance and data rate. In comparison, in applications of high-speed UWOC, lasers diode (LDs) demonstrate advantages of higher modulation bandwidth, higher efficiency, higher intensity of light output power, narrower spectral width and smaller divergence [7–9]. UWOC performance using LDs as transmitter has been experimentally demonstrated in water tank [10–15]. Chen et al. presented 26 m/5.5 Gbps air-water optical wireless communication based on a 520-nm LD [11]. Recently, our group demonstrated a long underwater transmission distance of 34.5 m at a high data rate of 2.7 Gbps based on a 520-nm LD with a very smaller beam divergence angle of 0.35° [10]. However, few work reported UWOC based on wavelength division multiplexing (WDM) technology, which can increase the data rate significantly and also can be combined with underwater solid-state lighting (SSL).
With the growth of smart lighting, LEDs or LDs as front-end transmitters have been proposed as key devices for both free-space visible light communication (VLC) and SSL, especially for indoor applications. Research on underwater SSL has not been well developed. LEDs have been commonly used [16–18] but LDs-based underwater SSL has not been reported. Due to the light attenuation of the water, underwater visibility can be greatly decreased. So high-efficiency and high-quality underwater SSL is indispensable for underwater ocean source exploration and underwater human activities e.g. marine biology, geology and oceanography, in particular, documenting videos of the underwater world. It not only requires high-efficiency but also large-area underwater SSL. Similar to SSL in free-space, the long-distance and large-area SSL can increase effective operation area for underwater activities. Long-distance large-area underwater lighting is challenging due to the light attenuation of the water and difficulty of the underwater energy supply. However, LEDs suffer from efficiency droop, i.e. the external quantum efficiency (EQE) decreases at a high injection current due to Auger recombination, electron leakage, defect-assisted recombination and so on [19, 20]. Besides, micro-LEDs have been used for UWOC due to the high modulation bandwidth at current densities of kA/cm2 [3], at which the EQE already dropped to a pretty low value [19, 20]. In comparison, the EQE of the LDs keeps increasing and saturates at high current densities without efficiency droop problem. Through employing high-efficiency LDs, we expect that the energy consumption of the underwater SSL can be greatly reduced, which can contribute significantly to alleviating the problems of long-duration deployments for underwater vehicles. Thus, red, green and blue LDs (RGB LDs) have been used in this work to demonstrate the high-efficiency underwater laser-based white-light SSL for the first time.
Furthermore, we proposed several scenarios of underwater applications including systems of long-distance and large-area UWOC and underwater SSL by mixing RGB LDs. The corresponding UWOC performances and underwater SSL properties were presented, and several advantages of the system can be found. Firstly, the EQE keeps above 10% even at high currents to maintain the high efficiency for underwater SSL. Secondly, the low water attenuation of the light output power of the LDs suggests the potential of long-distance SSL and UWOC. The high-quality white light strongly depends on the distance due to the different light attenuation at different emission wavelengths in the water. We have achieved white light at a distance of 2.3 m, and at different underwater transmission distances white light should be obtained by adjusting the light output power of each LD. Thirdly, high modulation bandwidth of 1.4 GHz of all the LDs also indicates significant advantages of the LDs over LEDs in high-speed UWOC. Finally, we only used three LDs for UWOC, and due to the narrow spectral width of the LDs, more LDs can be used for WDM UWOC in the visible light range to achieve much higher data rate. Our work demonstrated the potential of laser lighting in high-efficiency underwater SSL and high-speed UWOC at a long distance, which paves the way to underwater smart lighting applications including underwater sensor network and underwater Internet of Things. It is worth noting that different SSL characteristics are required for different underwater activities, such as mobile lighting, proximate short-range lighting, and long-range guidance lighting. For example, distance of several meters and area of dozens of square meters are typically required for underwater SSL [21], in which scenario the UWOC performance needs to be sacrificed. As a consequence, a trade-off needs to be made between SSL and UWOC according to the actual underwater requirements.
2. Experimental details
In this study, a commercial GaN-based 450-nm blue LD, a commercial GaN-based 520-nm green LD and a commercial AlGaInP 660-nm red LD were employed in our proposed RGB LDs-based underwater SSL and WDM UWOC system. They are single transverse mode semiconductor lasers with miniaturized TO38 ICut packages. However, due to the differences in the LDs’ epitaxial material, device processing and packaging techniques, the LDs have various characteristics such as the different threshold voltage. In order to stabilize the light-output power of the LDs, the use of Peltier thermoelectric cooler keeps working at temperature of ~25°C. The details of the experimental setup are similar to those in our previous work [10]. The transmitter lens (Tx lens) for RGB LDs has a size of M7 × 5.5 mm and a focal length of 9.8 mm. The receiver lens (Rx lens) is aspherical lens with a size of 63 mm and a focal length of 63 mm. A series of neutral density (ND) filters were exploited to adjust the light output power from the blue and green LDs to achieve high-quality white-light for underwater SSL, and thus optimized direct current (DC) bias could be kept with a high modulation bandwidth for high-speed UWOC. Long-pass dichroic mirrors (DMLP567 and DMLP490) were utilized to pass long-wavelength emission light and reflect short-wavelength emission light to acquire white-light generated by mixing RGB LDs. For instance, dichroic mirror of DMLP567 has a cut-off wavelength of 567 nm which passed the 660-nm red light and reflected the 520-nm green light. Optical diffusers (ED1-C20-MD and ED1-L4100-MD), with a circular divergence angle of 20° and a line divergence angle of 0.4°×100°, respectively, were used to diffuse and diverge the collimated laser beam for practical applications of the large-area underwater SSL and UWOC. All used optical elements are shown in Fig. 1.
Fig. 1 Experiment setup of the proposed RGB LDs-based white-light system for underwater SSL and WDM UWOC.
The schematic of the experimental setup of the mixed RGB LDs-based white light source for both underwater SSL and WDM UWOC is illustrated in Fig. 1. The DC sources of Yokogawa GS610 and Keithley 2614B were used to drive the LDs. Signals of pseudorandom binary sequences (PRBS), with a pattern length of 27-1 from a pulse pattern generator (PPG) module built in an Anritsu MP1800 signal quality analyzer, were superimposed on the DC for each LD by bias-tee (ZFTB-6GW + , Mini-Cicuits) to achieve WDM UWOC. The emission light of each LD was collimated with Tx lenses at the transmitter and combined in a single path with two long-pass dichroic mirrors to generate white-light. The light outputs of the green and the red LDs were aligned assisted by dichroic mirror of DMLP567. The red light emission was mixed with the green light emission to generate yellow light emission. Subsequently, the light beam of the blue LD was mixed with the yellow light beam using another optical long-pass dichroic mirror of DMLP490. Through adjusting the light output power of the RGB LDs white light beam was generated. To achieve large-area underwater laser lighting, the white-light beam was expanded by the optical diffusers. Afterwards, the white-light was transmitted through a 2.3 m water tank filled with tap water to simulate the underwater channel. The transmitted emission light was focused on a PIN photodetector (1.4GHz, Femto) with a photosensitive diameter of 1.0 mm or an avalanche PD (APD210, 1 GHz, Throlabs) with a photosensitive diameter of 0.5 mm by a Rx lens. The optical signal from each LD was converted into electrical signal for bit error rate (BER) analysis. In our system, the experiments under different scenarios were investigated, where underwater propagation links were not exactly same. Therefore, the experimental details of each scenario will be described in more detail in the following sections. In a practical implementation of the WDM, the RGB light emission would be detected, respectively, by adding filters in front of the PDs [15, 22]. As all the LDs have quite narrow spectra width, we expect that more LDs can be used for underwater SSL and UWOC with different emission wavelengths, and thus SSL with higher power and UWOC with higher data rate can be achieved [23]. However, in our study, the Anritsu MP1800 signal quality analyzer can only modulate one LD at a time. Similar to the reported work [7, 22, 23], the transmission data rates of the RGB LDs were separately measured and then aggregated.
The characteristics of light-output power versus current were measured by a current source and a standard silicon photodiode power detector (S120VC). And then the EQEs of the LDs can be calculated. Frequency response was measured by an Agilent network analyzer (N5225A, 10 MHz-50 GHz). The high-speed PIN photodetector (1.4 GHz, Femto) was used as a receiver to measure frequency response, as well as the UWOC performance of the RGB LDs-based white-light system without the optical diffusers. When the optical diffusers were employed to diffuse the white light for large-area underwater SSL, the density of the light output power of the white light decreased after a 2.3 m transmission through the water. And a high-sensitivity APD (Throlabs APD210) was employed to test the UWOC performance. The eye diagrams were captured by an Agilent wide-bandwidth oscilloscope (86100A, 14 GHz) and BER was analyzed by an error detector module (ED, 0.1-14 Gb/s) from the MP1800 signal quality analyzer to evaluate the data transmission speed. The illumination performances of the white light were measured using a spectral flickering irradiance meter (SFIM-300, EVERFINE). The measured spectra were processed using the SFIM-300 software to obtain characteristics of Commission International de l’Eclairage coordinates (CIE), correlated color temperature (CCT), color rendering index (CRI), and illuminance.
3. Results and discussions
3.1 Characteristics of the RGB LDs for underwater SSL and UWOC
The essential properties of the individual RGB LDs for underwater SSL and UWOC applications will be demonstrated, including the quantum efficiency, modulation bandwidth and the water attenuation of light output power.
The EQEs of the RGB LDs are shown in Fig. 2. It can be seen that after the stimulated emission the EQEs of the RGB LDs kept increasing with higher current and reached saturated values of 0.14, 0.10 and 0.20, respectively. In contrast to the efficiency droop phenomenon of the LEDs at higher currents, LDs have much higher external quantum efficiency and no efficiency droop can be observed at higher currents, making them a better candidate for high-efficiency underwater SSL application. We also noticed that the EQE of the green LD is lowest due to the “green-gap” problem as discussed in the references [24, 25]. Although green LD has the lowest attenuation coefficient in the latter discussions, the relatively low EQE of the green LD may limit the high-power green LD development and further the high-power white-light source generated by RGB LDs [24–26]. However, the high-efficiency laser-based underwater SSL still shows great potential in underwater SSL.
The −3dB electrical-to-optical (E-O) modulation bandwidth of the RGB LDs was attained by characterizing the frequency response performance. Figure 3 shows the normalized frequency response characteristics of the RGB LDs at injection currents of 83 mA, 69 mA and 37 mA. Note that these currents are optimal points for UWOC in latter discussions. The −3 dB bandwidth level is marked in dash line. The corresponding extracted −3 dB E-O modulation bandwidth of the RGB LDs are all around 1.4 GHz in Fig. 3, which is limited by the 1.4 GHz bandwidth of the PIN photodetector. We expect the actual −3 dB E-O modulation bandwidth of the RGB LDs may be higher than 1.4 GHz. Further improvements of measuring the bandwidth may be achieved by a photodetector with higher bandwidth. The high bandwidth of the RGB LDs shows significant advantages over LEDs for high-speed UWOC [1–3].
Fig. 3 Normalized frequency responses of the RGB LDs at injection currents of 83 mA, 69 mA and 37 mA, respectively. The dashed line represents the −3 dB bandwidth.
Since the light-output power in the UWOC system is one of the important parameters affecting the transmission distance and data rate [10], we measured the light-output power of the RGB LDs versus the injection current at the 2.3 m underwater transmission distance without water in the tank and with water in the tank under collimating laser beams, as shown in Figs. 4(a)-4(c). The attenuation coefficients for the RGB LDs through the 2.3 m underwater link can be calculated by equation [27]:
where is the received light-output power with water in the tank, is the received light-output power without water in the tank, is the attenuation coefficient and is the underwater transmission distance of 2.3 m. Then, the attenuation coefficients of the RGB LDs through tap water are calculated via the following equation:Fig. 4 Received light-output power versus injection current of the (a) red, (b) green and (c) blue LDs at a 2.3 m transmission distance without and with water in the tank.
As described in latter discussion, the optimal injection currents of 83 mA, 69 mA and 37 mA for data communication were found for the RGB LDs, respectively. The corresponding received light-output power of and can be obtained from Fig. 4. According to the Eq. (2), the attenuation coefficients of , and for the RGB LDs were calculated to be 0.47, 0.12 and 0.17, respectively. These coefficients are relatively low compared with the reported values, demonstrating the advantages of our UWOC link [2]. It mainly benefits from the optimized design of optical antennas and the light beam collimation resulting in decreased loss of the light-output power. The lower attenuation coefficients can decrease the loss of the light-output power, so we could obtain higher received power at longer distances for SSL and UWOC. Therefore, the lower coefficients benefit the improvement of underwater SSL and UWOC distance using our proposed laser-based white-light system.
In our study, in order to appropriately mix RGB LDs to generate high-quality white-light for underwater SSL, the light output power was mainly adjusted by the ND filters. The DC bias was only slightly adjusted around the optimal operation current to achieve high modulation bandwidths of LDs and high-speed UWOC. As the attenuation coefficients in water are different for RGB LDs, the laser-based white-light should be achieved by adjusting the light output power of the RGB LDs according to the distances from the laser sources, e.g. 2.3 m in this work.
First, for the sake of obtaining the ratio of the light-output power of the RGB LDs to generate white-light, the characteristics of the white-light source obtained by mixing RGB LDs in free-space are shown in Fig. 5. The corresponding driven current and ratio were shown in Table 1. The CIE coordinates of (0.3168, 0.3336) for the white-light was obtained from Fig. 5. The dark line stands for the color temperature of the light source. The inset of Fig. 5 presented the corresponding white-light emission spectrum by mixing RGB LDs. It can be observed that the corresponding peak emission wavelengths are 664.0 nm, 516.5 nm and 442.8 nm for RGB LDs, respectively. The light intensity of the red light, green light and blue light was evaluated by integrating the spectrum in the inset of Fig. 5, and then the ratio of about 8.8: 2: 1 can be calculated. Meanwhile, the driven current was attained and the light-output power density was measured for each laser. All the parameters are listed in Table 1. From Fig. 5 and Table 1, we are able to precisely control the light-output power to achieve high-quality white light and high-speed communication under different scenarios, which will be demonstrated as follows.
Fig. 5 CIE coordinates of the generated white-light mixing RGB LDs in free-space. Inset: emission spectrum of the white-light.
Table 1. The parameters of the RGB LDs for generating white light
In order to achieve the optimal performance of UWOC system, optimal operation points of the individual RGB LDs were measured as shown in Fig. 6. Through adjusting the injection current and the modulation depth for the red-light beam, green-light beam and blue-light beam transmitting through the 2.3 m underwater link, the optimal operation points were obtained. The characteristics of BERs versus injection current and peak-to-peak voltage (Vpp) of modulation signal (i.e. modulation depth) for the individual RGB LDs are shown in Figs. 6(a)-6(c), respectively. Figure 6(a) presents curves of the BER versus injection current and modulation depth at data rates of 2.8 Gbps and 2.9 Gbps, respectively, for the red LD. The BER performances of the green LD are displayed in Fig. 6(b) at data rates of 3.2 Gbps at different injection currents and 2.8 Gbps with different modulation depths. Figure 6(c) shows the BER characteristics of the blue LD at a data rate of 2.9 Gbps with different injection currents and different modulation depths. It can be found that a better BER was obtained with increasing the injection current and modulation depth at the beginning. However, when the injection current and modulation depth continued to increase, the BER performance gradually degraded. Therefore, in order to achieve high-speed communication, moderate driving current and modulation depth should be set for the individual RGB LDs. From Fig. 6, the injection currents of 83 mA (2.45 V), 69 mA (5.8 V), and 37 mA (4.5 V) were considered to combine with the corresponding signal with voltage swings of 1 V, 1.5 V and 1.3 V to drive the RGB LDs, respectively, in the laser-based white-light system. The optimal operation points are estimated values, which are related to many factors including the photodetector properties, signal-to-noise ratio (SNR), DC bias and modulation depth.
Fig. 6 BER characteristics of the (a) red, (b) green and (c) blue LDs under different injection currents (top) and various modulation depths (bottom).
Based on the above results, compared with LEDs, LDs with higher efficiency, higher bandwidth and lower attenuation coefficient enable simultaneously long-distance high-speed UWOC and high-efficiency underwater SSL.
3.2 Long-distance SSL and UWOC based on RGB LDs
To better examine the practical applications of the laser-based WDM system for underwater SSL and UWOC, different scenarios were investigated based on the schematic of the experiments in Fig. 1. First, we demonstrated WDM UWOC system without ND filters and optical diffusers (referred as I-RGB), without considering the white light generation. All the RGB LDs were kept at the optimized operating condition. Secondly, ND filters were added to further generate white light based on the RGB LDs for both WDM UWOC and underwater SSL without optical diffusers (referred as W-RGB). It is worth noting that under white-light scenarios, in order to obtain high power for long-distance and larger-area SSL, the red LD was driven at a high DC bias of 153 mA, which is different from that of the red LD in I-RGB scenario.
Figure 7 presents the images of the proposed W-RGB system for long-distance underwater SSL and UWOC. Figure 7(a) shows the UWOC link of the laser-based white-light system including a transmitter, a light propagation channel of a 2.3 m water tank with tap water inside, and a receiver with a Rx lens and a spectral flickering irradiance meter. The transmitter consists of the packaged RGB LDs with three Tx lenses and two dichroic mirrors, as shown in Fig. 7(b). The receiver is a spectral meter for SSL test or an APD/PIN for UWOC test with a focus lens. Figure 7(c) depicts the picture of the white light beam through the water in the 2.3 m tank filled with tap water. The size of the white light beam was narrow in absence of the optical diffuser, and thus less attenuation of the light-output power was expected for long-distance applications. The chromaticity diagram coordinates of the W-RGB at the 2.3 m underwater transmission distance are shown in Fig. 7(d). Without the optical diffuser, W-RGB exhibited CIE coordinates, a CCT value, a CRI and an illuminance of (0.3154, 0.3354), 6322 K, 69.3 and 70840 lux, respectively. It can be seen that through mixing RGB LDs, high-quality white-light was obtained.
Fig. 7 Images of the proposed RGB LDs-based WDM UWOC and underwater SSL. Diffusers were not used. Pictures of (a) the system of long-distance UWOC link and underwater SSL, (b) the magnified transmitter consisting of the packaged RGB LDs with the Tx lenses and dichroic mirrors, and (c) collimated light beam through the 2.3 m underwater transmission channel. (d) CIE 1931 chromaticity diagram of the W-RGB.
The UWOC performances of the I-RGB and W-RGB at an underwater transmission distance of 2.3 m are shown in Fig. 8. Figures 8(a) and 8(b) present the BER performance versus data rate for I-RGB and W-RGB systems. The dash line represents the forward error correction (FEC) threshold of 3.8 × 10−3. Note that the BER performance degraded with increasing the data rate. Figure 8(a) shows the BER characteristics of I-RGB. With BER meeting the FEC criteria, the maximum achievable data rates of up to 3.2 Gbps, 3.4 Gbps and 3.1 Gbps for the RGB LDs were obtained at the underwater distance of 2.3 m with corresponding BERs of 3.6 × 10−3, 3.5 × 10−3 and 3.7 × 10−3, respectively. Aggregate data rate of 9.7 Gbps was successfully achieved. Figure 8(b) displays BER curves of the W-RGB. It can be seen that the maximal data rate of 2.7 Gbps with a BER of 3.7 × 10−3 was achieved for the red LD. Data rates of 3.3 Gbps and 2.7 Gbps were achieved with BERs of 3.7 × 10−3 and 3.6 × 10−3 for the green and blue LDs, respectively. This scenario provided an aggregate data rate of up to 8.7 Gbps for the proposed laser-based white-light system. In addition, it can be seen that in comparison with I-RGB scenario [Fig. 8(a)], the communication performance of the W-RGB scenario degraded [Fig. 8(b)]. For the red LD, the degradation of achievable data rate was attributed to deviation of the DC from the optimal operation point. The different communication performances of the green and blue LDs in the I-RGB and W-RGB scenarios were ascribed to the attenuation of the light-output power resulting from the ND filters used in W-RGB scenario.
Fig. 8 The UWOC performances of the I-RGB and W-RGB at the underwater transmission distance of 2.3 m. BER versus data rate of (a) I-RGB and (b) W-RGB. The FEC threshold is marked in dash line. Eye diagrams of the (c) I-RGB scenario at data rates of 2.5 Gbps, 3.0 Gbps and 2.3 Gbps for RGB LDs, respectively, and (d) W-RGB scenario at data rates of 2.5 Gbps, 3.0 Gbps and 2.3 Gbps for RGB LDs, respectively.
The captured eye diagrams of the I-RGB and W-RGB at the 2.3 m underwater transmission distance are shown in Figs. 8(c) and 8(d), which also allow us to evaluate the data transmission performance of the proposed laser-based white-light system. Eye diagrams at data rates of 2.5 Gbps, 3.0 Gbps and 2.3 Gbps for RGB LDs in I-RGB scenario were shown in Fig. 8(c), respectively. In Fig. 8(c), the captured “eyes” are open and clear, and the corresponding BERs are zero, indicating that the error-free communication was achieved with an aggregate data rate of 7.8 Gbps at an underwater communication distance of 2.3 m. Figure 8(d) presents the eye diagrams of the W-RGB scenario with the same data rates for RGB LDs, respectively. It can be seen that the “eyes” become smaller and the corresponding BERs increased from the 0 to 2.8 × 10−5, 5.0 × 10−9 and 4.2 × 10−4 for RGB LDs, respectively. The trend coincides with that of BER curves in Figs. 8(a) and 8(b).
3.3 Large-area SSL and UWOC based on RGB LDs
In order to verify the proposed RGB LDs-based WDM UWOC system can implement long-distance and high-speed UWOC and accomplish the large-area underwater SSL, the laser-based white-light systems with various diffusers and ND filters were illustrated in Fig. 9. Figures 9(b) and 9(c) presented the magnified images of the transmitter. In comparison with Fig. 7(b), optical diffusers were employed in Fig. 9(b). Figure 9(c) depicts the generated divergent white-light by mixing the RGB LDs through the optical diffuser. Figure 9(d) presents the RGB LDs-based divergent white light dispersed by the line diffuser. Similarly, Fig. 9(e) exhibits the RGB LDs-based diverged white light through the water observed from the receiving end, which was diffused by the circle diffuser with 20° divergent angle. Compared with Fig. 7(c), the light beam was diffused and diverged through the optical diffusers. Therefore, the proposed RGB LDs-based white-light is expected to be able to provide large-area underwater SSL, which exhibits simultaneously ability to transmit data in water. It is worth noting that the light spot in the middle in Figs. 9(d) and 9(e) is the divergent white light beam, others are the reflection beams by the water/glass and the water/air interface.
Fig. 9 Images of the proposed RGB LDs-based WDM UWOC and underwater SSL system with various diffusers. Pictures of (a) the UWOC link, (b) the magnified transmitter including RGB LDs with three Tx lenses, two ND filters, two dichroic mirrors, and optical diffusers, (c) diffused and divergent white light with the optical diffuser, (d) the RGB LDs-based white light diverged by the line diffuser through the water, and (e) the RGB LDs-based white light diverged by the circle diffuser through the water.
The line diffuser was employed to construct the RGB LDs-based WDM UWOC and white-light system (denoted as WL-RGB), which is expected to provide large-area underwater SSL and UWOC [Fig. 9(d)]. In comparison with the WL-RGB, we further investigate underwater SSL and UWOC of the laser-based white-light system using the circle diffuser with 20° divergent angle [Fig. 9(e)] to diffuse the collimated white-light (denoted as W20-RGB).
The angle distribution of the white-light illuminance and CIE coordinates of the WL-RGB and W20-RGB at a 2.3 m underwater transmission distance are presented in Fig. 10. Figure 10(a) shows the white-light properties of the WL-RGB with CIE coordinates, a CCT value and an illuminance of (0.3183, 0.3269), 6216 K, and 11.4 lux, respectively. The W20-RGB has CIE coordinates, a CCT value and an illuminance of (0.3298, 0.3390), 5617 K and 11.7 lux, respectively, as shown in Fig. 10(b). The lower illuminance in Figs. 10(a) and 10(b) is ascribed to the decrease of light-output power resulting from the diffused white-light and attenuation of the water and ND filters. The left images in Figs. 10(a) and 10(b) present the angle distribution of white-light illuminance for WL-RGB and W20-RGB, respectively. Due to the dimension limitation of the water tank, we investigated the illuminance distribution under limited angle region. It can be found that the illuminance changes insignificantly in the range of 15°, which indicates that our proposed RGB LDs-based white light is well applicable to the large-area underwater SSL. The corresponding white-light spectra of the WL-RGB and W20-RGB are shown in the insets of Figs. 10(a) and 10(b), respectively. In our study, the insufficient light-output power of the red LD required for white-light generation limited the overall light-output power of the white light. And the green and blue lights were attenuated to generate white light. Thus, we have to make a trade-off to achieve both high-speed UWOC and high-quality SSL.
Fig. 10 Angle-dependent illuminance distribution of the white light and CIE 1931 coordinates of the (a) WL-RGB and (b) W20-RGB system. Inset: the corresponding white-light spectra of WL-RGB and W20-RGB, respectively.
Figure 11 presents the UWOC performances of the WL-RGB and W20-RGB at a 2.3 m underwater transmission link. The BER performance as a function of data rate for WL-RGB and W20-RGB is shown in Figs. 11(a) and 11(b), respectively. The FEC threshold of 3.8 × 10−3 is marked in dash line. With the line diffuser diverging the white light, the BERcharacteristics of the WL-RGB are displayed in Fig. 11(a). It can be found that data rates of 2.4 Gbps, 1.5 Gbps and 2.0 Gbps were achieved for the RGB LDs with corresponding BERs of 2.2 × 10−3, 3.0 × 10−3 and 3.5 × 10−3, respectively, which demonstrated an aggregate transmission data rate of 5.9 Gbps for the proposed RGB LDs-based white-light WDM UWOC system. Similar to Fig. 11(a), the BER performances of the W20-RGB with the circle diffuser are presented in Fig. 11(b). With the BER underneath the FEC limit of 3.8 × 10−3, the maximal allowable data rates of 2.4 Gbps, 2.5 Gbps and 1.7 Gbps with the BERs of 3.4 × 10−3, 3.4 × 10−3 and 3.3 × 10−3 were achieved, respectively. It delivered an aggregate data rate of 6.6 Gbps at this scenario for the laser-based white-light system we proposed. The corresponding eye diagrams of the WL-RGB and W20-RGB at a 2.3 m underwater transmission link are shown in Figs. 11(c) and 11(d). In comparison with Fig. 8(c), owning to the attenuation of the light-output power, a remarkable drop in eye height, even at lower data rates than those in Fig. 8(c), can be observed. The eye diagrams at data rates of 2.4 Gbps, 1.5 Gbps and 2.0 Gbps are shown for RGB LDs in the WL-RGB scenario in Fig. 11(c), respectively. Figure 11(d) presents the eye diagrams of the W20-RGB with data rates of 2.4 Gbps, 2.5 Gbps and 1.7 Gbps for RGB LDs, respectively. It can be seen that the “eyes” are almost closed, and the corresponding BERs of all are up to 10−3 but are less than the FEC of 3.8 × 10−3. These results imply that the high-speed communication is realized in our proposed RGB LDs mixed white-light underwater SSL and WDM UWOC system.
Fig. 11 The UWOC performances of the WL-RGB and W20-RGB at an underwater transmission distance of 2.3 m. BER versus data rate of the (a) WL-RGB and (b) W20-RGB system. The dash line represents the FEC threshold. Eye diagrams of the (c) WL-RGB at data rates of 2.4 Gbps, 1.5 Gbps and 2.0 Gbps, respectively, and (d) W20-RGB at data rates of 2.4 Gbps, 2.5 Gbps and 1.7 Gbps for RGB LDs, respectively.
3.4 Comparison of the different scenarios
The communication performances in I-RGB, W-RGB, WL-RGB and W20-RGB scenarios are different after comparing the achievable data rates of RGB LDs in Fig. 8 and Fig. 11. The variation is mainly ascribed to the differences of received light-output powers of RGB LDs and the responsivities of the PDs, based on which we can calculate the converted electrical signal strength for LDs with different wavelengths. Higher signal strength will improve the SNR significantly and thus benefit a higher achievable data rate which can be employed to explain the trends of the BER variation of the RGB LDs in Fig. 8 and Fig. 11. The theoretically calculated results agree with our experimental results, except the red LD in Fig. 8 (b) and the green and blue LDs in Fig. 11(a). We expect that other factors, including the nonlinearity of the LDs, multipath effects and optical antennas may influence the communication performance and cause the trend variation of the BERs for RGB LDs.
Table 2 summarizes the UWOC and underwater SSL performances at an underwater transmission distance of 2.3 m for various light sources by mixing RGB LDs. The RGB LDs arrangement was exploited to establish an underwater white-lighting communication system. The RGB LDs-based UWOC with corresponding aggregate transmission data rate of 9.7 Gbps was demonstrated with an allowable underwater transmission distance over 2.3 m. Subsequently, ND filters were used to generate white light. The allowable aggregate data rate of up to 8.7 Gbps was achieved. Afterwards, the light diffusion and beam divergence were demonstrated using optical diffusers. The illuminance performance of the white light generated by the RGB LDs was measured at an underwater distance of 2.3 m. We obtained the CIE coordinates of (0.3154, 0.3354), the CCT of 6322 K, the CRI of 69.3 and corresponding illuminance of 7084 lux. When a line diffuser was used, the diffused and diverged white light mixed by the tricolor RGB LDs exhibited an aggregate data rate of 5.9 Gbps over a 2.3 m underwater environment. The CIE coordinates of (0.3183, 0.3269) and the CCT value of 6216 K were attained. Similarly, a circle diffuser replaced the line diffuser to generate the diffused and divergent white light. The proposed W20-RGB system exhibited the aggregate data rate of 6.6 Gbps and CIE coordinates of (0.3298, 0.3390). The CCT decreased from 6216 K to 5617 K using the circle diffuser replacing the line diffuser. Consequently, the white-light performance can be improved by exploiting higher-power RGB LDs and increasing the DC bias of individual RGB LDs. These achievements indicate that our proposed system exhibits a high performance in applications of UWOC and underwater SSL.
Table 2. Performances of the proposed RGB LDs-based WDM UWOC and underwater SSL system
4. Conclusion
Simultaneous implementation of the RGB LDs based white-light underwater SSL and white-light WDM UWOC was reported for the first time in this study. We demonstrated experimentally and analyzed systematically the underwater white-light and UWOC performances based on the laser-based white-light system. In our experiments, using OOK modulation scheme for each RGB LD, we have achieved maximum data rates of 3.2 Gbps, 3.4 Gbps, and 3.1 Gbps for RGB LDs with corresponding BERs of 3.6 × 10−3, 3.5 × 10−3 and 3.7 × 10−3, respectively, and a high aggregate data rate of 9.7 Gbps. Moreover, in the laser-based white-light system, the maximal allowable aggregate data rate of 8.7 Gbps was realized with the CIE coordinates of (0.3154, 0.3354), CCT value of 6322 K, CRI of 69.3 and illuminance of 7084 lux. The line and circle optical diffusers were employed to obtain the large-area underwater SSL, and the maximum achievable aggregate data rates of 5.9 and 6.6 Gbps have been implemented, meanwhile, exhibiting the high white-light quality with the CIE values of (0.3183, 0.3269), (0.3298, 0.3390), respectively. The above results reveal that such a high-performance laser-based white-light system can provide both high-speed underwater wireless communication and white lighting in a water environment, which implicates the potential applications in underwater wireless networks and underwater Internet of Things.
Funding
National Natural Science Foundation of China (NSFC) (61705041 and 61571135); Shanghai Sailing Program (17YF1429100); State Key Laboratory of Intense Pulsed Radiation Simulation and Effect Funding (SKLIPR1607); National Key Research and Development Program of China (2017YFB0403603).
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