[Research] Fiber Communication Test Bench

[RuAlMac]

Fiber Communication Test Bench

Difficulty: Medium/Hard

Project Summary

We currently use Wet-Conns to connect to SubjuGator when it's underwater. However, using a fiber-optic connection over Wet-Conns has many advantages. Eventually, we want to use fiber with our SubjuGator. Design a PCB (or find components to purchase) that can accept a water-proof fiber connection and convert it somehow to ethernet. A PCB would be really neat (highly encouraged), but if you can find an off-the-shelf product that's okay too.

Getting Started

Research!

There are lots of existing products out there that "convert" fiber to ethernet. Do some Googling, find out what specialty hardware you need, etc. Also, feel free to ask ChatGPT for some starting places (but don't rely entirely on it). I attached an example of a question I asked below.

Milestones

1) Create an action plan (1-2 weeks)
Once you have a general idea of how you want to go about this project, find me (Russell) and talk about your plans. I'll give you some tips and recommendations on how to proceed. Please talk to me before investing a lot of time into this project.

2) Design (2-4 weeks)
Once you have a plan, it's time to design. If you're designing a PCB, create a full schematic and PCB layout. If you don't want to go the PCB route, figure out exactly what off-the-shelf component(s) you want to buy and how they'll work together).

3) Bill Of Materials (1 week)
After you've created a general design, put together a list of everything we need to buy. You're NOT using your own money to fund this project, MIL will provide the funds. This should be a detailed list containing information on what product to buy, its part/serial number, the vendor, and other relevant information. If you've designed a PCB, please include the Part Designator number next to each part.

4) Assembly (1-2 weeks)
Once parts have been ordered, put your project together!

5) Testing (2-? weeks)
Ideally, everything works perfectly the first time. Realistically, that probably won't be the case, especially if you go the PCB route. That's okay! Figure out what works and what doesn't. Then iterate. Redesign, reorder, retest. Keep working on it, and eventually you'll get there. Talk to leads and your peers, don't be afraid to ask for help!

6) Documentation (1 week)
Once you've gotten your project working, you'll need to document your design process and how it works. Create a page on the MIL Wiki so that any future member can learn why you did what you did. Your Wiki page doesn't need to be extremely detailed, but it should discuss the key parts of your project, major design choices, and how to use it. Also be sure to provide links to your parts' datasheets.

[RuAlMac]

Example ChatGPT research question

My Question

I'm interested in working with a fiber-optic cable for communicating with a device. Ideally, I'd like to create a PCB that takes in a fiber connection and converts it to an ethernet connection. Is there a product or integrated circuit / receiver component I can find that I could put onto a PCB?

ChatGPT's Response:

Yes — and the easiest/most “PCB-friendly” way to do this is not to terminate bare fiber directly on your PCB, but to use a pluggable fiber transceiver module (SFP/SFP+) and convert that to copper Ethernet.
There are basically 3 common ways to build what you described:

Option A (Most common + easiest): SFP cage + Ethernet PHY (media converter design)

What you put on the PCB:

• SFP cage + connector (LC fiber plugs into an SFP optical transceiver module, not directly into your PCB)
• A copper Ethernet PHY for the RJ45 side (10/100 or 10/100/1000)
• An RJ45 with magnetics (or discrete magnetics)

Why this is great:
-You don’t have to design any optical analog front-end.

• You can pick the fiber type later by swapping the SFP module:
  -- 1000BASE-SX (850nm multimode)
  -- 1000BASE-LX (1310nm single-mode)
  -- BiDi, etc.
• SFP cages on PCBs are a well-established approach.

✅ This is how many commercial “fiber-to-ethernet” media converters work (they just hide it in a little box).

What “connects” the SFP to your logic/PHY?
Most SFP modules present a SERDES interface (typically 1000BASE-X / SGMII electrical). That means you generally need:

• a switch/MCU/FPGA/SoC with SGMII / 1000BASE-X
• or a PHY / bridge solution that supports fiber-side serial

Option B: Use a PHY that can directly do Copper ↔ Fiber (no big MAC needed)

Some Ethernet PHYs can be used in “media converter” modes.
Example family:

• Marvell Alaska 88E151x gigabit PHYs — these support RGMII/SGMII and are commonly used in fiber/copper bridging designs.
• Texas Instruments also publishes complete reference designs for copper↔fiber converters:
  -- Example: TI design using DP83849 for 10/100 copper-to-fiber media conversion.

✅ This path is great if you want a board that is basically a “transparent bridge” without a full CPU doing packet handling.

Option C (Simplest electrically): Fiber module that outputs RJ45 Ethernet directly

This is basically buying/embedding an off-the-shelf media converter module (some vendors sell board-level converter modules), but it’s less flexible for custom PCB integration.

What you probably want (practical recommendation)
If your goal is “fiber in, RJ45 ethernet out” on a custom PCB, do this:
✅ Recommended architecture
Fiber LC → SFP optical transceiver → (serial electrical) → Ethernet PHY / switch chip → RJ45
That means your PCB includes:

• SFP cage footprint
• A PHY / switch / SoC that can talk to it (SGMII / 1000BASE-X)
• Copper Ethernet PHY + RJ45 magnetics (if not integrated)

This is the standard approach used in industry.

Parts you’ll commonly shop for

1. SFP cage (PCB connector)
   Search terms:

• “SFP cage connector with integrated EMI springs”
• “SFP+ cage (works for 1G too)”

2. SFP optical transceiver module (pluggable)
   Search terms:

• “1000BASE-SX SFP LC”
• “1000BASE-LX SFP LC”

You can prototype your design immediately using an off-the-shelf media converter that accepts an SFP (just to prove your optics link works).
3) Copper Ethernet PHY (RJ45 side)

For 10/100: Microchip LAN8720A (RMII) is a common choice.
For Gigabit: Marvell Alaska / TI gigabit PHYs are common.

One important reality check

To convert fiber ↔ ethernet, you are not just converting “signal format” like UART level shifting.
You’re bridging between:

Optical Ethernet PHY on the SFP side (1000BASE-X / SERDES) and Copper Ethernet PHY on RJ45 (1000BASE-T)

So you usually need a device in the middle that can handle the proper MAC/PCS interface (or you use a PHY that supports special converter modes).

If you tell me these 3 things, I can point you to a concrete chip+block diagram

• Target speed: 100 Mbps or 1 Gbps?
• Fiber type: multimode (short) or single-mode (long)?
• Do you want it to be a “dumb bridge” (no CPU), or do you already have a processor/FPGA?
  And I’ll recommend a realistic BOM (SFP cage + PHY/switch + RJ45 magjack) and the usual layout gotchas.