1：Experimental Design and Method Selection:

The system employs a PAM4-modulated 405-nm blue-light injection-locked laser diode transmitter. It integrates free-space optical (FSO) link, plastic optical fiber (POF) transportation, and underwater wireless optical communication (UWOC) channel. Key components include doublet lenses for FSO link, GI-POF for extension, and an optical beam reducer for UWOC performance improvement.

2：Sample Selection and Data Sources:

A pseudorandom bit sequence (PRBS) pattern generator produces two

3：5 Gb/s NRZ signals with a size of 215-The clear ocean water in a 2 m-long tank has an attenuation coefficient of 151 m?List of Experimental Equipment and Materials:

Includes laser diodes (LD1 and LD2), variable optical attenuator, doublet lenses (lens1 and lens2), 30 m GI-POF with 1-mm core diameter,

4：2-dB total power loss, and 3-GHz 3-dB bandwidth, fiber collimators (350-700 nm operating wavelength), convex lenses (focal lengths 50 mm and 4 mm), optical beam reducer, photodiode with trans-impedance amplifier (TIA) receiver (10 GHz 3-dB bandwidth), electrical equalizer, PAM4 decoder, BER tester, and digital storage oscilloscope. Experimental Procedures and Operational Workflow:

PRBS signals are converted to a 9 Gb/s PAM4 signal using a PAM4 converter and linear driver. The signal modulates LD1, which injection-locks LD

5：Light from LD2 passes through a variable optical attenuator, 50 m FSO link with doublet lenses, 30 m GI-POF, and 2 m underwater channel with optical beam reducer and convex lenses. The received signal is detected by PD with TIA, equalized, decoded back to NRZ signals, and analyzed for BER and eye diagram. Data Analysis Methods:

BER values are measured in real-time using a BER tester. Eye diagrams of NRZ signals are captured with a digital storage oscilloscope. Frequency responses are analyzed to determine 3-dB bandwidths.