1. Introduction

The research on underwater wireless communication (UWC) has boomed up over the past decade for applications such as underwater navigation, underwater sensing network, ocean study, offshore oil expedition, and aquaculture. The UWC technology primarily includes underwater acoustic communication (UAC), underwater radio frequency (RF) communication, and underwater wireless optical communication (UWOC). Among them, UAC is the most reliable method to transmit data over a range of tens of kilometers underwater [1]. It only transmits data at a low data rate of tens of kb/s based on the underwater acoustic characterization. Even though underwater RF communication’s data rate achieves Mb/s [2], the transmit distance is shorter than 10 m due to the heavy attenuation in the conductive seawater [3,4]. Considering the disadvantages of UAC and underwater RF communication, UWOC is proposed to balance high speed exceeding Gb/s [5] and long transmit distance up to hundreds of meters [6]. Its compatibility between high-speed and long-distance makes the real-time underwater high-resolution video streaming transmission with a short latency into reality [7]. Moreover, the transceivers of UWOC are low-cost and energy-efficient in contrast to UAC and underwater RF communication [3].

Laser diodes (LDs) and light-emitting diodes (LEDs) are commonly employed as optical transmitters in the UWOC system [8]. Because the bandwidth of the UWOC system is primarily restricted to the electrical to optical bandwidth of the optical transmitter, the LDs become more suitable for the UWOC system, whose modulation bandwidth is in excess of GHz [9–11]. Because of the resonant cavity, LDs have much higher coherence and collimation than LEDs. Moreover, out of alignment is a key issue in the LD-based UWOC system because underwater movement easily exerts a negative impact on the optical link. Therefore, the LD-based UWOC system urgently requires a strict alignment system [5,12–14]. Here, we put forward a novel method to enlarge the receiver FOV to $120^{\circ }$ and enhance the robustness of the LD-based UWOC system. A CMOS monolithic photodetector with a built-in 2-dimensional light direction sensor was integrated into a single chip which is fabricated by a standard 0.5 μm CMOS process. The results demonstrated that the system has good sensitivity to the incident angle and achieves the alignment accuracy of $1.9^{\circ }$ over a range of $120^{\circ }$. This allowed for a high data rate UWOC using a laser diode and NRZ-OOK, insensitive to the optical misalignment achieving 110 Mbps at a bit error rate (BER) of 10$^{-10}$ below the limit of forwarding error correction (FEC).

2. Device characterization

A CMOS monolithic photodetector with a built-in 2-dimensional light direction sensor aims to track the incident light and point the light direction as shown in Fig. 1(a). An elementary cell of the sensor is presented in Fig. 1(b). The whole sensor consists of a number of basic cells. A metal wall created by stacking metal layers, contacts, and vias available in the process is employed to generate an on-chip micro-scale shadow. The bandgap voltage of silicon-based material is about 1.12 V. Thus, the sensor can absorb the light wavelength below 1.1 μm, which covers 450 nm blue light emitted by an LD. However, the distance between two adjacent metal walls is 30 μm, while the height of the metal wall is 12 μm. The physical dimensions are much larger than the wavelength of the absorbed light, so the diffraction has little impact on the performance of the sensor.

 

Fig. 1. (a) Photograph of a CMOS monolithic photodetector with a built-in 2-dimensional light direction sensor. (b) Structure of an elementary sensor cell.

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As shown in Fig. 1(b), two identical photodiodes are put onto two sides of the metal wall. $D_L$ denotes the left side diode and $D_R$ denotes the right side diode. The equivalent current sources along with each diode present that the photocurrents are generated by the corresponding photodiodes. The angle between the metal wall and the light direction is $\theta$. When the light comes right above the wall, namely $\theta$ = 0, two photodiodes are illuminated equally and generate the same currents. When the light comes from one side above the wall, namely $\theta$ > 0 or $\theta$ < 0, the metal wall shadows part of the light to one photodiode so that it produces less current than the other photodiode. Therefore, the relationship between $I_{DL}$ and $I_{DR}$ is correlated to the incident light direction. This experiment was conducted in a room with the ambient light power density of 11.4 μW/cm$^2$. Figure 2 shows 2-dimensional photocurrents versus incident light angles under the LD’s power density of 600 mW/cm$^2$, which is applied to the alignment system. It indicates that a 2-dimensional light direction sensor can cover the range of $120^{\circ }$ for both dimensions. If the power density decreases to 33 μW/cm$^2$, the 2-dimensional light direction sensor can still cover the range of $120^{\circ }$ for both dimensions. Based on those two sets of photocurrents, we can distinguish the incident light angles with $1.9^{\circ }$ alignment accuracy.

 

Fig. 2. (a) Vertical photocurrents versus incident vertical light angles under the LD’s power density of 600 mW/cm$^2$. (b) Horizontal photocurrents versus incident horizontal light angles under the LD’s power density of 600 mW/cm$^2$. It indicates that 2-dimensional light direction sensor can cover the range of $120^{\circ }$ for both dimension.

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3. Experimental details

Figure 3 and Fig. 4 show the schematic and the photograph of the experimental setup for the UWOC system within 50 cm long water tank measurements using a 450 nm LD, a CMOS monolithic photodetector, and NRZ-OOK modulation scheme. At the transmitter, a TO-56 packaged single-mode blue LD with a collimation lens was mounted on a thermoelectric cooler (TEC) module (SaNoor SN-LDM-T-P). The LD worked at room temperature. The pattern generator in the J-BERT (Agilent E4832A) was used to generate the pseudorandom binary sequence (PRBS) 2$^{15}$-1 pattern as a digital input signal. A direct current (DC) bias (SaNoor Laser Driver-5A) was added to the digital input signal by a Bias-Tee (Mini-Circuits ZFBT-6GW) driving the blue LD. It transmitted through a 50 cm long water tank. At the receiver, a CMOS monolithic photodetector collected the light and generated the photocurrent. The output signal was amplified by a power amplifier (Mini-Circuits ZHL-6A-S+) and filtered by an 80 MHz low pass filter. The eye diagram was analyzed by a wide-bandwidth oscilloscope (Agilent 86100A and HP 83483A). The bit error rate (BER) and data rate were measured using J-BERT (Agilent E4832A). A parametric network analyzer (ROHDE and SCHWARZ Vector Network Analyzer ZVB 8) was used for the small-signal system bandwidth measurement. The optical power-current-voltage (LIV) characterization setup involved a precision source meter (Keithley B2902A) and optical power monitor (Thorlabs PM100D).

 

Fig. 3. Schematic of the experimental setup for UWOC system within 50 cm long water tank measurements using a 450 nm LD, a CMOS monolithic photodetector, and NRZ-OOK modulation scheme

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Fig. 4. Photograph of the experimental setup for UWOC system within 50 cm long water tank measurements using a 450 nm LD, a CMOS monolithic photodetector, and NRZ-OOK modulation scheme. (a) The CMOS monolithic photodetector with a built-in 2-dimensional light direction sensor. (b) The packaged LD and the Bias-Tee. (c) The UWOC transmission channel based on a LD and a CMOS photodetector within the water tank.

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4. Results and discussion

The LIV characteristics of a 450 nm LD are shown in Fig. 5(a). The threshold current is 80 mA while the corresponding voltage is 3.6 V. At the forward current of 650 mA, the LD’s optical output power is 600 mW. When a small signal is modulated to the LD, it would result in the varied optical power emitted by the LD. As shown in Fig. 5(b), the normalized frequency response of UWOC links is measured at different bias currents. When bias current increases from 550 mA to 750 mA, the 3-dB system bandwidth keeps constant approximately at 52.4 MHz due to the bandwidth limit of the CMOS monolithic photodetector. In addition, the frequency response of UWOC links drops rapidly beyond the 3-dB bandwidth frequency because of the 80 MHz low pass filter. We will do some further research to broaden the system bandwidth. The PIN CMOS monolithic photodetector would be utilized for a higher speed data transmission.

 

Fig. 5. (a) Optical power-current-voltage characteristics of 450 nm LD. (b) Normalized frequency response of UWOC links at different bias currents.

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The optimal operating point of the presented UWOC links is captured from Fig. 6. The UWOC links worked at a data rate of 110 Mb/s. Because the typical forward current of 450 nm LD is 650 mA from the datasheet, we set 650 mA DC bias current as of the original operating point in order to obtain the optimal modulation voltage. When the modulation voltage increased from 1.75 V to 3.5 V, the BER decreased gradually from 8.33$\times$10$^{-6}$ to 2.31$\times$10$^{-10}$. So the optimal modulation voltage at 650 mA DC bias current is 3.5 V. In order to obtain the optimal bias current, the DC bias current firstly increased from 550 mA to 650 mA, the BER went down from 1.15$\times$10$^{-8}$ to 2.31$\times$10$^{-10}$. When the DC bias current kept increasing to 775 mA, the BER turned worse to 1.03$\times$10$^{-3}$. Fig. 6 also demonstrates the location of the optimal operating point of the UWOC links in the contour plot. Therefore, 650 mA DC bias current and 3.5 V modulation voltage is the optimal operating point of the presented UWOC links.

 

Fig. 6. Optimal operating point of the presented UWOC links is 650 mA DC bias current and 3.5 V modulation voltage.

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Figure 7 shows that the BER becomes worse exponentially when the received optical power becomes weaker from 600 mW to 230 mW in the UWOC link. For example, the UWOC links worked at a data rate of 110 Mb/s. The BER kept stable at 2.31$\times$10$^{-10}$ when the received optical power was 600 mW. By adding the neural density filter in the path, the received optical power decreased to 230 mW, thus the BER became worse to 8$\times$10$^{-2}$. It illustrates that the performance of the presented UWOC links is strongly correlated to the received optical power especially for the point-to-point LD-based UWOC links. If the proposed UWOC link reaches a data rate of 110 Mbps at a BER of the FEC threshold, the estimated maximum transmission distance of the system is 4.67 m when the attenuation coefficient of the tested water is 0.746 dB/m.

 

Fig. 7. BER versus received optical powers from 600 mW to 230 mW at data rates of 90 Mb/s, 100 Mb/s, and 110 Mb/s.

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As shown in Fig. 8, the incident angles at the receiver side change the collection of the received optical power, similar to the mechanism by using a neutral density filter in the path in Fig. 7. When the 2-dimensional angles ranged from 0$^{\circ }$ to 40$^{\circ }$ and the presented UWOC links were misaligned gradually, the performance of the UWOC links at a data rate of 110 Mb/s dropped dramatically from 2.3$\times$10$^{-10}$ to 1$\times$10$^{-2}$. If the 2-dimensional angles ranged from 40$^{\circ }$ to 60$^{\circ }$ and the UWOC links were out of alignment, the CMOS monolithic photodetector can not receive any data from the transmitter. If the channel loss was greater and the received optical power at 0$^{\circ }$ was attenuated, the overall BER would increase and the system can not detect the data even the 2-dimensional angles ranged less than 40$^{\circ }$. But the 2-dimensional light direction sensor onto the CMOS monolithic photodetector in Fig. 1 can drive the tracking system, which eliminated the misalignment away from the point-to-point transmission by 60$^{\circ }$ and enhanced the UWOC’s performance exponentially.

 

Fig. 8. BER versus the vertical and horizontal angles from -40$^{\circ }$ to 40$^{\circ }$.

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5. Conclusion

The UWOC can be employed to realize a high-speed underwater wireless network capable of competing with UAC and URF systems. Additionally, LDs offer key qualities ideal for the implementation of UWOC applications. But LDs have the features of high collimation and the LD-based UWOC links need a strict alignment. The UWOC links using a 450 nm LD and a CMOS monolithic photodetector performed at a data rate of 110 Mbps and a BER of 2.3$\times$10$^{-10}$ within an underwater distance of 50 cm. We proposed a CMOS monolithic photodetector with a built-in 2-dimensional light direction sensor for LD-based UWOC links to increase BER by orders and enlarge the FOV of the receiver to $120^{\circ }$ when it was out of alignment. Therefore, the LD-based UWOC links in our work became robust and reliable.