To enable high-speed long-distance underwater optical wireless communication (UOWC) supplementing traditional underwater wireless communication, a low-power 520 nm green laser diode (LD) based UOWC system was proposed and experimentally demonstrated to implement maximal communication capacity of up to 2.70 Gbps data rate over a 34.5 m underwater transmission distance by using non-return-to-zero on-off keying (NRZ-OOK) modulation scheme. Moreover, maximum data rates of up to 4.60 Gbps, 4.20 Gbps, 3.93 Gbps, 3.88 Gbps, and 3.48 Gbps at underwater distances of 2.3 m, 6.9 m, 11.5 m, 16.1 m and 20.7 m were achieved, respectively. The light attenuation coefficient of ~0.44 dB/m was obtained and the beam divergence angle is 0.35°, so the aallowable underwater transmission distance can be estimated to be ~90.7 m at a data rate of 0.15 Gbps with a corresponding received light-output power of −33.01 dBm and a bit-error rate (BER) of 2.0 10−6. In addition, when the data rate is up to 1 Gbps, the UOWC distance is predicted to be ~62.7 m for our proposed UOWC system. The achievements we make are suitable for applications requiring high-speed long-distance real-time UOWC.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
High-speed long-distance underwater wireless communications are strongly demanded given the growing underwater activities including oceanography investigation, offshore oil exploration, and sea floor monitoring. Traditional underwater acoustic communication suffers from low data rate of tens of kbps over km range. Radio frequency (RF) communication distance is limited in a range less than 10 m. For underwater optical wireless communication (UOWC), optical signal attenuation in the blue-green spectrum region is relatively small [1–3]. In addition, the blue and green-emitting GaN-based LEDs and laser diodes (LDs) have been developed significantly recently which offers high quantum efficiency and high modulation bandwidth, and thus has been studied for high-speed free-space communication and UOWC [3–17]. Theoretical work has predicted that high-speed UOWC can be achieved at a long distance of up to hundreds of meters with data rate of Mbps in pure water, and the radiation angle and light intensity distribution of the light sources affect the UOWC distance significantly . However, the experimental UOWC distance is still limited to be less than 21 m based on LDs with data rate of several Gbps , and the distance has been extended to 50 m with data rate of several Mbps based on LEDs . High-power LEDs of several Watts lead to longer UOWC distance, but high-power LEDs suffer from low modulation bandwidth which limits the maximum data rate [9, 10]. And micro-LEDs with high bandwidth have been proposed to enhance the data rate to close to Gbps . Comparatively, the LDs offer much narrower beam and higher bandwidth than LEDs, and thus we expect that a low-power LD can lead to a high-speed long-distance UOWC.
The allowable data rates and related transmission performances of the LD-based UOWC system proposed in previous reports are summarized and compared in Table 1. The longest underwater channel of 21 m with a data rate of 5.5 Gbps based on an orthogonal frequency division multiplexing (OFDM)-modulated 520 nm laser diode was studied by Y. Chen et al. . For practical UOWC applications, the complexity of the system including arithmetic and circuit design may be increased using OFDM scheme for real-time communication. In addition, high signal-to-noise ratio (SNR) is required for OFDM which limits the maximum achievable UOWC distance. A UOWC link of 20 m based on 450 nm LDs using NRZ-OOK modulation was proposed by C. Shen et al., with a relatively low real-time data rate of 1.5 Gbps . The present study demonstrated that, based on a green LD with effectively used light-output power of 5.27 mW, we have achieved UOWC distance of 34.5 m with a data rate of 2.70 Gbps. Moreover, the transmitted light beam divergence angle is 0.35° in the horizontal direction and can be neglected in the vertical direction, so we expect that the light attenuation coefficient of ~0.44 dB/m would not increase significantly with increasing distance. Thus, the maximum underwater distances are predicted to be 90.7 m at a data rate of 0.15 Gbps and 62.7 m at a data rate of 1 Gbps. Up to now, Gbps UOWC at distances more than 30 meters based on a LD has not been experimentally demonstrated. Based on the experimental study and the theoretical prediction in this work, through further optimization, e.g. using high-power LD and adjusting the optical lenses to further reduce the radiation angle, long-distance UOWC up to hundreds of meters may be achieved in the future.
The schematic diagram of the experimental setup for LD-based UOWC using NRZ-OOK modulation scheme is illustrated in Fig. 1. A 520 nm green LD was driven by a bias-tee combining a direct current (DC) from a Yokogawa GS610 current source with pseudo-random binary sequences (PRBS) from a pulse pattern generator (PPG) module built in an Anritsu MP1800 signal quality analyzer. The green-emission light was collimated by a transmitter lens (Tx lens), and then transmitted through a water tank with a length of 2.3 m filled with tap water to simulate the underwater channel. High-reflectivity mirrors were mounted on the sidewalls of the water tank to reflect the light, so the UOWC distance was extended and n represents the reflection time of the light beam in Fig. 1. The transmitted emission light was focused on an APD (Throlabs, APD210, 1 GHz or Hamamatsu, APD12702, 100 MHz) or a PIN photodetector (Alphalas UPD-50-UP, 7 GHz, or Femto, 1.4 GHz) by a receiver lens (Rx lens), and the optical signal was converted into electrical signal. The electrical signal was captured to obtain the frequency response by an N5225A network analyzer or analyze the BER by an error detector module from the MP1800 signal quality analyzer, as shown in red line and black line in Fig. 1, respectively. The blue line in Fig. 1 illustrates the setup of capturing eye diagrams by an 86100A wide-bandwidth oscilloscope.
The images of the 520 nm green LD-based UOWC system are shown in Fig. 2. The communication link consists of an APD receiver with a focus lens in Fig. 2(a), a light propagation channel for UOWC without water in the tank in Fig. 2(b), and a packaged green LD with a transmitter lens in Fig. 2(c). The laser spots can be seen on the mirrors fixed on the sidewalls of the tank in Fig. 2(b). The focus lens at the receiver is a Fresnel lens with a diameter of 10 cm and focal length of 10 cm. The collimating lens at the transmitter is an application-specific lens for the LD with a size of M7 5.5 mm and a focal length of 9.8 mm. Figure 2(d) shows the temperature-controlled packaged 520 nm green GaN-based LD with a miniaturized TO38 ICut package, which is a single transverse-mode semiconductor laser. Due to the light output power of the LD is sensitive to the ambient temperature, a Peltier thermo-electric cooler was employed to maintain the operation temperature of ~25°C and copper foils were used to fill the gaps between the Peltier cooler and the LD. Figure 2(e) shows the picture of 15 green light beams through the water and thus UOWC distance of 34.5 m was obtained. The used mirror with a size of 10 cm 10 cm was coated with 2-mm high-reflectivity Ti3O5 film to reflect the light beams. The weaker light beams with increasing transmission distance and the scattering of the green light indicate the strong water attenuation.
3. Results and discussions
The I-V and light-output power versus current (L-I) characteristics of the 520 nm LD are shown in Fig. 3(a). It can be seen that the threshold current of the LD is around 44 mA, and the corresponding voltage is 5.6 V. In addition, the operation current of 90 mA for LD-based UOWC is set in order to achieve optimal UOWC performance and the corresponding light-output power is 19.40 mW. The electroluminescence (EL) spectra in Fig. 3(b) were measured by using an Ocean Optics USB4000 Spectrometer. The peak wavelength at 90 mA is around 523.9 nm with full-width at half-maximum (FWHM) of ~1.7 nm for the green LD and is slightly red-shifted with increasing driven current.
Figure 4 illustrates frequency response and extracted −3 dB modulation bandwidth characteristics of the 520 nm green LD at different injection currents from 44 mA to 145 mA. The dashed line indicates the −3 dB bandwidth level. Note that the PIN photodetector used for frequency response test has a bandwidth of 1.4 GHz, which limits the maximum measured −3 dB modulation bandwidth of the green LD. In addition, the ultrafast Si PIN photodetector was employed to measure the frequency response of the LD. However, due to the low sensitivity of the ultrafast Si PIN photodetector, the frequency response cannot be able to be effectively obtained. In Fig. 4(a), it can be seen that the modulation bandwidth increases as the injection current increases from 44 mA to 90 mA, when the injection current continues to increase from 90 mA to 145 mA the bandwidth fluctuates around 1.4 GHz. Figure 4(b) summarizes the characteristics of the −3 dB modulation bandwidth, from which the dependence of the modulation bandwidth on the driven current is clearly observed.
Optimum operation point is the crucial parameters for the LD which influences the transmission capacity [6, 8]. Through adjusting the DC injection current and the modulation signal amplitude, the optimal operation point is obtained. The characteristics of BERs versus injection current and peak-to-peak voltage of modulation signal are shown in Fig. 5. At the beginning, the better BER performance was obtained with increasing the injection current and modulation depth. However, when the injection current and modulation depth are too high, the BER performance gradually degrades. Therefore, in order to be able to achieve longer underwater transmission distance and higher transmission speed, moderate driving current should be chosen. In the UOWC system we proposed, the injection current of 90 mA (6.2 V) was selected combining the small signal with the voltage swing of 1.5 V to drive the 520 nm green LD.
To experimentally demonstrate the transmission distances and the achievable data rate of the 520 nm green LD-based UOWC system, we have systematically studied the maximal achievable data rate at different underwater communication distances of 2.3 m, 6.9 m, 11.5 m, 16.1 m, 20.7 m and 34.5 m. The measured BER as a function of data rate at various underwater distances are shown in Fig. 6. The dash line represents the forward error correction (FEC) threshold of 3.8 10−3. To extend the underwater light propagation distance, high-reflectivity reflectors are employed to reflect the light beam within the 2.3 m water tank. It can be seen that the BER performance degrades with increasing the data rate. Moreover, the penalty for extending the underwater transmission distance is a decrease in the data rate. Under the FEC threshold, the maximum achievable data rate of up to 4.60 Gbps, 4.20 Gbps, 3.93 Gbps, 3.88 Gbps, 3.48 Gbps and 2.70 Gbps were obtained at underwater distances of 2.3 m, 6.9 m, 11.5 m, 16.1 m, 20.7 m and 34.5 m with the corresponding BERs of 2.1 10−3, 3.6 10−3, 2.2 10−3, 3.1 10−3, 3.3 10−3 and 3.4 10−3, respectively. It is worth noting that at underwater distances of 2.3 m and 6.9 m, the ultrafast Si PIN photodetector with a bandwidth of over 7 GHz was employed to attain maximal allowable data rate. Increasing the distance from 6.9 m to 20.7 m, a high-speed PIN photodetector with a −3 dB bandwidth of 1.4 GHz was used to detect the relatively weak signal to evaluate maximum data rate. Continuously extending the underwater distance into 34.5 m, the PIN photodetector was replaced with an APD (Throlabs, APD 210) with a 1 GHz bandwidth for improving the detectivity. The phenomenon is attributed to the attenuation of light-output power of the LD with increasing the UOWC distance. Note that the increasing underwater distance led to additional dispersion, multi-path propagation loss and reflective loss which degraded the BER performance. An inflection point of the characteristics of BER vs. data rate can be seen from the curve at the 16.1 m UOWC distance at high data rates, which is possibly related to the degradation of the SNR resulting from multi-path effect at high data rates with BERs approaching to the FEC threshold. As the OFDM modulation can alleviate multi-path effect [18,19], we expect that using an OFDM modulation scheme in our proposed UOWC system the data rate can be further improved in our future study.
The eye diagrams captured at different underwater communication distances from 2.3 m to 34.5 m are shown in Fig. 7. Figures 7(a) and 7(b) show the eye diagrams at underwater distances of 2.3 m and 6.9 m at a data rate of 4 Gbps using the ultrafast PIN photodetector as a receiver. The eyes are open and clear and the resultant BERs are 5.0 10−7 and 6.5 10−4. The relatively low amplitude of the eye diagrams in Figs. 7(a) and 7(b) is due to the lower detection sensitivity of the ultrafast PIN photodetector. The light beam shows increased attenuation as the underwater distance extends so as to not be able to detect, hence the high-sensitivity photodetector is employed. The eye diagrams measured at underwater distances of 11.5 m, 16.1 m and 20.7 m using the 1.4 GHz PIN photodetetor are shown in Figs. 7(c)-7(e). The open eyes are observed with data rates of 3.60 Gbps, 3.50 Gbps and 3.30 Gbps with the BERs of 4.8 10−9, 6.9 10−4 and 3.0 10−4. When the underwater transmission distance is lengthened to be 34.5 m, the received light-output power is only 150 μW. The high-sensitivity APD (1 GHz) is used to capture the eye diagram at a data rate of 2.50 Gbps, which is shown in Fig. 7(f). The results in Fig. 7 further confirm that the communication performance degrades with the distance increase, consistent with the BER performance in Fig. 6.
Table 2 summarizes the maximal allowable data rates and the corresponding BERs at different UOWC distances. To enhance the applicability of the 520 nm green LD-based UOWC system, the underwater communication distances were increased to 2.3 m, 6.9 m, 11.5 m, 16.1 m, 20.7 m, and 34.5 m to analyze the maximum achievable communication capacities. A data rate of up to 3.48 Gbps was guaranteed within a 20.7 m UOWC link using NRZ-OOK modulation scheme. The distance was extended to 34.5 m by using eight high-reflectivity mirrors installed on the water tank to reflect light beams, maintaining high data rate of 2.70 Gbps. Compared with the previous reported UOWC [5–8], record UOWC distance over 34.5 m with real-time data rate of 2.70 Gbps based on LD UOWC with simplest NRZ-OOK modulation scheme has been experimentally successfully demonstrated. It is noteworthy that the 0.6 m width of the water tank affords 14 reflections of light beam. Therefore, to further investigate the allowable communication capacity at longest distance theoretically, we discuss the characteristics of the BER versus light output power in the following part.
Due to the limitation of the water tank length, our study experimentally demonstrated a UOWC distance of 34.5 m with maximum data rate of 2.70 Gbps and the corresponding received light-output power is −8.24 dBm (150 μW). In order to evaluate the longest achievable underwater transmission distance of the LD-based UOWC with NRZ-OOK modulation scheme, we further studied the dependence of the BER on the received light-output power. Through measuring the light-output power versus the injection current at 34.5 m underwater transmission distance without water in the tank and with water in the tank, the light-output attenuation of the laser beam in tap water is calculated to be ~0.44 dB/m from Fig. 8(a). The attenuation of 0.44 dB/m is slightly higher than the reported value of 0.39 dB/m . The light-output power in the UOWC system is one of the critical parameters affecting the transmission distance and data rate. The low water attenuation in this work comes from the little beam divergency in our UOWC system, and thus less water scattering can be expected, compared with UOWC using LEDs . Figure 8(b) shows the measured BER performance as a function of the received light-output power under different data rates. A series of neutraldensity filters were employed in front of the photodetector to adjust the received light-output power. From Fig. 8(b), we can evaluate the minimum required received light-output power for the LD-based UOWC system at a certain data rate with a corresponding BER below the FEC threshold of 3.8 10−3. It can be seen that a data rate of 0.15 Gbps was achieved at a received light-output power of −33.01 dBm (0.50 μW) with a BER of 2.0 10−6. In order to detect such weak signal, the high-sensitivity detector of APD (APD12702, 100 MHz) was employed. Based on the attenuation coefficient of 0.44 dB/m, we estimate the maximum achievable UOWC distances in our proposed system as follow:Fig. 8 and Eq. (1), the maximum achievable UOWC distance is predicted to be 90.7 m. Therefore, for UOWC with extended distance, a data rate of 0.15 Gbps at UOWC distance of 90.7 m can be achieved. Moreover, the received light-output power of −20.66 dBm (8.59 μW) is required when the certain data rate is 1 Gbps with a BER of 3.7 10−3. Similarly, the predicted maximum UOWC distance is calculated to be 62.7 m at a data rate of 1 Gbps. Thus, for applications requiring high-speed UOWC larger than 1 Gbps, the UOWC distance of 62.7 m can be achieved.
We have measured the laser beam sizes at different distances and light beam divergence can be calculated as shown in Fig. 9. Figures 9(a) and 9(b) show the pictures of laser spots with horizontal sizes of 5 mm at the transmitter and 12 mm at the receiver with UOWC distance of 11.5 m, and the corresponding light beam divergence is observed in Fig. 9(c). According to the conception of light beam divergence, divergence angle of θ is calculated as follow:Fig. 9(c) and Eq. (2). The small horizontal beam expansion is possibly caused by the light intensity distribution of the green LD or the slightly tilted optical antenna at the transmitter. In addition, the light beam in the vertical direction shows little beam expansion. These results suggest that the 0.44 dB/m attenuation coefficient will not fluctuate significantly at longer distances and our estimation of 90.7 m UOWC is reasonable . In addition, in our UOWC system, the reflectivity of the mirror is not 100% in the water, however, in actual applications, without the reflection losses we expect longer distance of more than 90.7 m may be achieved with similar water attenuation coefficient. To the best of our knowledge, 2.70 Gbps is the highest data rate achieved for UOWC at distances longer than 30 m, and data rates of 0.15 Gbps at a distance of 90.7m and 1 Gbps at 62.7 m are also expected based on our system. In addition, based on our study of high-speed and long-distance UOWC in tap water, further work on our proposed UOWC system may be demonstrated considering realistic underwater environment, e.g. coast ocean and turbid harbor.
In summary, record high-speed long-distance UOWC system based on a low-power 520 nm green LD directly modulated by NRZ-OOK modulation scheme has been successfully demonstrated. We systematically studied the communication capacities at different underwater transmission distances of 2.3 m, 6.9 m, 11.5 m, 16.1 m, 20.7 m, and 34.5 m. To fit within the threshold of the FEC criterion of 3.8 10−3, the corresponding maximal allowable real-time data rates are up to 4.60 Gbps, 4.20 Gbps, 3.93 Gbps, 3.88 Gbps, 3.48 Gbps and 2.70 Gbps, respectively. In particular, we experimentally realized a transmission distance of over 34.5 m at a record data rate of up to 2.70 Gbps UOWC. To the best of our knowledge, 2.70 Gbps is the highest data rate achieved for UOWC at distances longer than 30 m. Moreover, the maximal achievable underwater distances are estimated to be 90.7 m at a data rate of 0.15 Gbps and 62.7 m at a data rate of 1 Gbps for the LD-based UOWC we proposed which benefits from the little light beam divergence of 0.35° and the low light attenuation coefficient of 0.44 dB/m. The results we achieved indicate that a progress has been made for exploiting high-speed and long-distance UOWC.
National Natural Science Foundation of China No. 61571135; Shanghai Sailing Program 17YF1429100; State Key Laboratory of Intense Pulsed Radiation Simulation and Effect Funding SKLIPR1607 and State Key Laboratory of Luminescence and Applications funding SKLA201619.
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