The Investigation of Underwater Wireless Optical Communication Links Using the Total Reflection at the Air-Water Interface in the Presence of Waves

Round 1

Reviewer 1 Report

The manuscript proposes UOWC links that utilize reflections at the air-water interface in the presence of waves. Underwater IoT is addressed as a potential application, obviously extending from the seafloor to the surface since otherwise seabed rocks would not be harmful (see Fig. 1). For this purpose, a green InGaN-based micro-LED with an optical power as low as 80 micro-W is used.  Laboratory experiments are conducted in a tiny basin. A transmission range of several 10 cm is reported, obviously for clean water. Unfortunately, the overall setup is not realistic for underwater IoT applications and related UOWC applications. Therefore, the manuscript is not publishable as is. Before re-submission, an experimental verification in oceanic waters with realistic distances and real waves is needed. 

Author Response

Thanks for pointing out this issue of the low optical power of the single Micro-LED. First of all, we utilized the single green Micro-LED in the UWOC system to prove that the UWOC links based on the total reflection at the air-water interface offer an alternative way to transmit data when interrupted by unforeseen reasons such as fish schools or oceanic turbulence. Moreover, we also noticed in this experiment that a Micro-LED offers higher optical-to-electrical bandwidth than an LED, but it lacks optical power; thus, the transmission range is 1.8 m for clean water, as reported in this paper.

Therefore, we put forward a UWOC system using GaN Micro-LED arrays including 5000 pixels packaged by flip-chip technology, as shown in Fig.1. Moreover, the pitch size of Micro-LED is 15 μm by 15 μm. The GaN Micro-LED arrays have an optical power of up to 3.5 mW at a 300 mA drive current. The system achieved a high data rate of 92 Mb/s with a BER of 3.41×10^-3 and was estimated to reach 50 m in tap water and 10 m in seawater. In the end, we balanced the high bandwidth and the optical power in UWOC links.

Author Response File: Author Response.docx

Reviewer 2 Report

I think the work described in the article is a valuable contribution in the field of photonics with very innovative ideas. However, before the work can be published, the authors need to address the following issues:

• Authors use a LED with peak wavelength at 522 nm. Why do they choose these WLED with these wavelength? Red wavelength easy propagate in water and even tissues (for example animals in the water). Did they consider other wavelengths before choosing thin LED?
• They can talk in the paper (maybe in the introduction) about the propagation of different wavelengths of light in the water. In this way, the paper should be complete, consisted and attractive for the readers.

Author Response

Thanks for pointing out this issue in this paper. Optical waves, indeed, can provide high-speed transmission and low propagation, with the drawback of a limited communication range (10 to 100 meters). When targeting short-range UWC, it is possible to exploit the low-attenuation window in the EM spectrum, which lies in the visible region around the green wavelength at 522 nm, as shown in Fig. 1. We will add some more details in the introduction part.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The manuscript proposes UOWC links that utilize reflections at the air-water interface in the presence of waves. Underwater IoT is addressed as a potential application, extending from the seafloor to the surface. For this purpose, a green InGaN-based micro-LED array with a low optical power is used. Laboratory experiments are conducted in a tiny basin. 

In my first review I have already mentioned that "an experimental verification in oceanic waters with realistic distances and real waves is needed".  Without oceanic experimentation, the claim that UOWC links can utilize the reflections at the air-water interface in the presence of waves is not provable.  Currently, this zone is often avoided in UOWC because of detrimental effects due to ambient light.  Sorry, but the manuscript is not publishable.

 

Author Response

Point 1: The manuscript proposes UOWC links that utilize reflections at the air-water interface in the presence of waves. Underwater IoT is addressed as a potential application, extending from the seafloor to the surface. For this purpose, a green InGaN-based micro-LED array with a low optical power is used. Laboratory experiments are conducted in a tiny basin.

Response 1: Thanks for pointing out this issue of the low optical power of the single Micro-LED. We put forward a UWOC system using GaN Micro-LED arrays, including 5000 pixels packaged by flip-chip technology. The GaN Micro-LED arrays have an optical power of up to 3.5 mW at a 300 mA drive current. Thus, we balanced the high bandwidth and the optical power in UWOC links.

Point 2: In my first review I have already mentioned that "an experimental verification in oceanic waters with realistic distances and real waves is needed". Without oceanic experimentation, the claim that UOWC links can utilize the reflections at the air-water interface in the presence of waves is not provable. Currently, this zone is often avoided in UOWC because of detrimental effects due to ambient light. Sorry, but the manuscript is not publishable.

Response 2: Thanks for your suggestions! However, most UWOC research has been done in the lab other than the oceanic experimentation, such as the absorption effects, the scattering effects, the turbulence effects, etc. Moreover, the ambient light is Gaussian noise in these UWOC links. Meanwhile, the UWOC links transmit data rates at high frequencies. Thus, this zone is employed in UWOC, and ambient light would not determine the performance.

Author Response File: Author Response.docx