We experimentally demonstrate a record high-speed underwater wireless optical communication (UWOC) over 7 m distance using on-off keying non-return-to-zero (OOK-NRZ) modulation scheme. The communication link uses a commercial TO-9 packaged pigtailed 520 nm laser diode (LD) with 1.2 GHz bandwidth as the optical transmitter and an avalanche photodiode (APD) module as the receiver. At 2.3 Gbit/s transmission, the measured bit error rate of the received data is , well below the forward error correction (FEC) threshold of required for error-free operation. The high bandwidth of the LD coupled with high sensitivity APD and optimized operating conditions is the key enabling factor in obtaining high bit rate transmission in our proposed system. To the best of our knowledge, this result presents the highest data rate ever achieved in UWOC systems thus far.
© 2015 Optical Society of America
The motivation for this work stems from the need to demonstrate high data rate and error-free underwater wireless communication system for practical applications such as real-time video transmission for oceanography studies, offshore oil exploration, sea floor survey and monitoring, etc. Currently, acoustic technology is the most widely used underwater wireless communication method but it suffers from both low bandwidth (2-30 kHz) and high latency . Also, reliably implementing radio frequency (RF) communication in underwater is severely limited due to high attenuation (> 150 dB/m at 100 MHz) of RF waves in seawater . The other method is the fiber optic channel which offers long range and high-bandwidth underwater communications. However, the fiber optic technology is inadequate for remotely operated vehicles (ROV) and autonomous underwater vehicles (AUV) because the transmitter and the receiver have to be physically connected with a cable. Owing to its high bandwidth and stealthiness, underwater wireless optical communication (UWOC) has attracted considerable attention from scientific, commercial and military communities [3–9 ] as a promising alternative or complementary to acoustic and RF techniques. Exploiting the low absorption window of seawater in blue-green portion of the electromagnetic spectrum, UWOC is expected to play an important role by offering secure, efficient and high data rate communications among submarines, unmanned underwater vehicles (UUV), ships, divers, buoys, and underwater sensors within short range (< 100 m). In addition, UWOC can be utilized to investigate climate change as well as monitor ecological and biogeochemical changes in the ocean, sea, and lake environments [10,11 ].
However, the underwater environment is optically very challenging. The propagation of light in water is affected by attenuation which is a combined effect of absorption and scattering . Absorption is the process in which the photon energy is lost due to the transfer of energy during the interaction with water molecules and particles. In scattering, the photons are scattered away from the initial path after interacting with particulate matter in the water. The effect of multiple scattering especially in turbid harbor and coastal waters strongly degrades bit error rate (BER) performance for high data rate UWOC systems with on-off keying modulation . Therefore, developing a high-speed and long-range underwater wireless optical communication system is important.
A prior experimental demonstration used an externally modulated laser at 1064 nm, frequency doubled to 532 nm in a periodically poled lithium niobate (PPLN) crystal to establish 1 Gbit/s link in a 2 m water tank . Very recently, Nakamura et al. have demonstrated optical wireless transmission of 405 nm, 1.45 Gbit/s optical intensity modulation/direct detection-orthogonal frequency division multiplexing (IM/DD-OFDM) signals through a 4.8 m underwater channel . In this paper, we report on the demonstration of a record high-speed UWOC link operating at data rates of up to 2.3 Gbit/s over a 7 m distance using OOK-NRZ modulation scheme. The communication link uses a TO-9 packaged pigtailed 520 nm LD as the optical transmitter and an avalanche photodiode (APD) module as the receiver. Performance BER of which is well below the FEC limit of and open eye diagram were successfully achieved. Our system uses OOK, which is the simplest and most cost-effective modulation technique and outperforms all previously reported UWOC systems.
2. Experimental setup
Figure 1 shows the schematics of the experimental setup used to measure the underwater transmission of light. The transmitter is a low cost commercially available 15 mW and single mode pigtailed laser diode (Thorlabs LP520-SF15) with the emission peak wavelength at around 517 nm at bias voltage of 7.1 V. The water tank is 1 m long with 6 cm × 6 cm optical windows on each end. The optical path length of the modulated laser light passing through the tank was extended up to 7 m using reflective mirrors. The tank was filled with coastal water with an attenuation coefficient of 0.568 m−1 at 517 nm . Compared to an attenuation coefficient of 450 nm laser of 0.619 m −1, 517 nm green laser is more suitable for low-loss transmission in coastal waters. It should be noted that the walls inside the water tank were painted black in order to prevent light from reflecting off. The direct intensity modulated laser light was collimated by the projection optics of the transmitter and entered the tank through a window at one end. At the other end of the tank, a high sensitivity silicon APD (Menlo Systems APD210) receiver with an active diameter of 0.5 mm, responsivity of around 13 A/W at 520 nm and a noise equivalent power (NEP) of 0.4 pW/Hz1/2, was precisely aligned with the transmitted beam for measuring its power after 7 m of length.
A variable attenuator and neutral density filters were used to control the intensity level of the transmitted laser beam. The receiver system included the APD and collection optics. The collection optics consists of a 50 mm diameter and 75 mm focal length lens. No optical interference filter was used to suppress the ambient light.
4. Results and discussion
The light-current-voltage characteristics of the LD at 25 °C are shown in Fig. 2(a) in which the threshold is 50 mA and the peak optical power is around 12 mW at a bias current of 135 mA. The slope efficiency is 16.7%. In Fig. 2(b), we show the optical spectra at 25 °C under different drive currents measured using an Ocean Optics HR4000 Spectrometer. The spectral width (full-width at half-maximum) of the laser is around 0.5 nm. The peak emission wavelength at 60 mA is around 514.5 nm and slightly changes with increasing drive current.
As a figure of merit, the overall frequency response of the system which includes the laser driver, the LD, the underwater channel, and the photodetector was characterized under different bias currents and shown in Fig. 3 . As clearly seen from the figure, the frequency response and the modulation bandwidth are dependent on the bias current. Note that as the bias current level increases, the bandwidth of the LD is extended due to up-shift of its relaxation oscillation frequency and gain. The maximum 3 dB attenuation occurs around 1.2 GHz, as indicated by the dashed line. The frequency response was measured using a vector network analyzer and the APD. The APD is capable of measuring up to 1.6 GHz response.
Before the underwater high speed transmission measurements, a throughput optimization of the system was performed by adjusting both the laser DC bias and the modulation signal amplitude. The highest data rate was achieved when the bias level of the LD and the modulation depth was set to Vbias = 6.77 V (Ibias = 81 mA) and Vpp = 2.1 V, respectively. The performance of the UWOC system was analyzed using a high-performance bit error rate tester (Agilent J-BERT N4903B) and a digital communication analyzer (Agilent DCA-86100C) of 50 GHz bandwidth. A pseudorandom binary sequence (PRBS 210-1) OOK-NRZ data stream was used to modulate the laser. The 210-1 long PRBS pattern is consistent with data pattern length found in applications such as Gigabit Ethernet and SATA 1 which use 8b/10b and other related encodings. The OOK-NRZ data was electrically pre-amplified with an ultra-broadband amplifier (Picosecond Pulse Labs, 5865) of 26 dB gain to increase the RF signal power and improve the extinction ratio (ER). Eye diagrams at 1 Gbit/s and 2.3 Gbit/s measured using the DCA are shown in Fig. 4 . As can be seen from the figure, open eyes are observed up to 2.3 Gbit/s which confirms the potential of visible LDs for high-speed UWOC applications. The system could be further optimized to achieve higher data rates. For example, a preamplifier can be inserted to improve sensitivities, or a photodetector with larger bandwidth can be used. In addition, the effect of multiple reflection losses could be addressed by using glass windows with anti-reflection coatings.
Figure 5 shows the measured bit error rate (BER) performance versus received optical power of the UWOC system at 7 m transmission distance when the system data rate varies from 1.0 Gbit/s to 2.3 Gbit/s. This figure aims to determine the minimum required received optical power for a certain bit rate. For example, for the 1 Gbit/s, received power of −29 dBm is required to achieve a BER of . When the bit rate increases to 2.3 Gbit/s, a minimum received power of –14 dBm is required to achieve similar performance. We attribute this power penalty to the limited bandwidth of the system which is a combination of limitations imposed by the laser driver and the LD. It can be seen that the BER values are below for data rate up to 1.5 Gbit/s. At 2.3 Gbit/s, the system BER is , which is well below the FEC limit criterion of .
Recent numerical study suggests that the effect of multiple scattering in coastal and harbor waters introduces temporal pulse spread (intersymbol interference) which strongly degrades the BER performance for high data rate UWOC systems with OOK modulation . We thus investigated the effect of intersymbol interference (ISI) on the system performance. Figure 6 shows BER versus link distance for various data rates. It should be noted that the received optical power was kept constant by using a variable attenuator as the link distance was increased from 1 m to 7 m. As shown in the figure, a relatively flat BER is observed for all three data rates studied. Note that the BER performance may be degraded when the link distance increases to 40 m as found in . However, our experimental results suggest that ISI has no effect on BER performance for high data rate UWOC systems with OOK modulation for link ranges up to 7 m in coastal waters.
In this paper, we have demonstrated a record 2.3 Gbit/s OOK-NRZ underwater wireless optical communication link over a 7 m distance. The communication system uses a commercially available TO-9 packaged pigtailed 520 nm LD as the transmitter and an APD module as the receiver. The LD has a threshold current of 50 mA, a peak wavelength of 520 nm and an output power of 12 mW at 135 mA. At 125 mA bias current, the maximum −3 dB optical bandwidth is found to be 1.2 GHz. Underwater data transmission experiments were carried out from 1 to 2.3 Gbit/s using a BERT and a DCA. Open eye diagrams and FEC compliant BER () results for data rates up to 2.3 Gbit/s were successfully achieved. The high bandwidth of the LD coupled with high sensitivity APD and optimized operating conditions enabled the high data rate transmission demonstrated in this study. Our communication system stands as a simple and cost-effective solution for next-generation high speed underwater wireless optical links.
This work is supported by KAUST baseline funding; KAUST Competitive Center Funding (Red Sea Research Center), and KACST TIC (Technology Innovation Center) for Solid State Lighting at KAUST.
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