With the expanding marine exploitation, high-speed long-reach underwater wireless communication has caught extensive attention. Compared with underwater acoustic communication and radio frequency communication, underwater wireless optical communication (UWOC) features a high data rate, great confidentiality, and large data capacity. Nevertheless, the challenging underwater environment exerts a significant effect on underwater light transmission. Absorption, scattering, and communication distance change may lead to a great variety of detection optical intensity, and the link misalignment between the transmitter and receiver caused by turbulence can also realize this. Nonlinear distortion or loss of receiver signal will be resulted in. Optical receivers generally employ variable gain amplification circuits to process dynamic signals to mitigate these effects. However, existing UWOC systems have some shortcomings. Limiting amplifier is generally applicable to optical communication systems of on-off keying modulation digital transmission. Automatic gain control (AGC) circuit generally adopts a detector to detect the changes in output signal amplitude to form a gain control feedback loop. In this feedback loop, the time to reach a stable operating state is affected by its characteristics, and the AGC adjustment time is fixed, with hysteresis in gain control. We propose an adaptive light intensity detection circuit, which utilizes automatic gain control technology to adjust the amplification of the received signal and output stable electrical signals. This adaptive light intensity detection circuit has the advantages of fast response time and better real-time performance. We hope that our proposed method can improve the practicality of UWOC systems, aiming for optical communication systems in complex environments.

Our proposed circuit is based on the characteristics of the direct-current-biased optical optial orthogonal frequency division multiplexing (DCO-OFDM) signal. The circuit takes advantage of the proportional relationship between the amplitude of the DC signal and the AC signal in the DCO-OFDM signal, and logarithmically amplifies the DC signal to form a control voltage to adjust the amplification of the AC amplifier circuit. The circuit leverages the AGC technique to adjust the amplification of the received signal and output a stable electrical signal. First, relevant parameters of the adaptive light intensity detection circuit are analyzed theoretically and verified by simulations, and then the circuit is designed and tested experimentally for air and underwater channels. The output signal of the adaptive light intensity detection circuit and the relationship between the control voltage formed by the logarithmic amplifier circuit and the gain of the circuit are investigated and compared with theoretical values. Then, this circuit is applied to an optical communication system for ethernet communication experiments at different distances.

Our proposed circuit can realize automatic gain adjustment to output stable electrical signals, thereby expanding the optical communication system receiver's dynamic range of signal processing. Preliminary experiments are carried out to obtain the relationship between the AC signal and DC signal (Fig. 7). According to this relationship, we adjust the parameters of the designed circuit and further investigate the performance of the UOWC system through this circuit. The results of the air channel experiment show that when the light intensity changes, and the DC voltage varies from 18 mV to 1300 mV, and the output signal's peak-to-peak value in the variable gain amplifier circuit can be stabilized around the set value with a fluctuation range of -3.3%-3.3% (Table 3). As depicted in Figs. 8(a) and 8(b), when the DC signal changes, the AC signal is stable. Then the logarithmic amplifier circuit's parameters are tested. Table 4 lists the theoretical and measured values of the control voltage. The results show that the designed logarithmic amplifier circuit realizes the desired function. The DC signal in the DCO-OFDM signal generates a gain control voltage to the AC signal (Fig. 9). Experiments for the underwater channel indicate that when the communication link is misaligned and the signal strength changes, the DC signal varies in the range of 356-890 mV and the peak-to-peak value of output signal is stable with a fluctuation range of -3.3%-3.3% (Table 6). Finally, we experimentally demonstrate a 45 Mbit/s optical communication link over a 6 m air channel through the adaptive light intensity circuit in an optical communication system receiver (Fig. 14).

We design an adaptive light intensity detection circuit based on the characteristics of the signal from an underwater optical communication system using DCO-OFDM modulation. The performance of the optical communication system in terms of received signals is experimentally analyzed for air and water as channel media. The feasibility of the circuit is demonstrated and verified in the experiment. The optical communication system which adopts an adaptive light intensity circuit shows good communication performance and robustness in various channels. When channel conditions such as link misalignment change, the peak-to-peak value of the circuit output signal is stable, and the maximum difference between the actual working gain and the ideal gain of the circuit is 0.29 dB. It is inferred from the experimental results that the circuit has better processing capability for dynamic signals. The adaptive optical intensity detection circuit employs DC signals to achieve gain control of AC signals with the advantages of simple structure, fast response, low circuit cost, and low output signal fluctuation. Additionally, it is expected to be widely applied in optical communication receivers to improve system performance.