See the light.
Design the link.
Turn optical power, fiber loss, and coupler ratios into a network you can understand. Explore two CATV designs, one calculation at a time.
1310 & 1550 nmInteractive design toolsGuided practiceThree drops. One balanced bus.
40 analog channels · minimum CNR 51 dB · 1310 nm · SC/APC
2Follow the optical path
3Work back toward the transmitter
Go farther with an EDFA.
77 channels + digital · 1550 nm · a long-haul branch and two local receivers
The two branches
Backward power budget
A small split makes a big difference.
Explore the actual insertion-loss values in the workbook, rather than an ideal splitter model.
Why the real loss is higher
An ideal 50/50 split loses 3.01 dB per output. The workbook specifies 3.4 dB per output. Use the tabulated component loss for these exercises.
Output power (dBm) = input power − insertion loss (dB)
Show all 50 workbook split ratios
| Split | First loss | Second loss | Delta* |
|---|
*Delta is calculated from the displayed rounded losses. The workbook delta column occasionally differs by 0.1 dB because of rounding.
Find the receiver’s starting point.
Read the CNR curve first. Then use the receiver power in your link budget.
Illustrative reconstruction of the workbook’s OTPN-1000 graph using sampled points and linear interpolation. Readings are approximate; no extrapolation beyond −8 to +2 dBm. This is a training aid, not a current product specification.
dBm ↔ milliwatts
dBm = 10 × log₁₀(mW)
dBm expresses an absolute power relative to 1 mW. dB expresses a ratio, such as fiber or coupler loss. Subtract dB losses from dBm power to obtain the remaining dBm.
Quick reference
| 0 dBm | 1 mW |
| +3 dBm | ≈ 2 mW |
| +10 dBm | 10 mW |
| −10 dBm | 0.1 mW |
Build your engineering intuition.
Check the method, then download original diagrams for your own tutorial pages.
Three quick checks
Your visual toolkit
Original scalable illustrations, with clear labels and no embedded workbook screenshots.
The design method
- Read the required receiver optical power from the channel-loading curve.
- Add span loss: distance × attenuation.
- Try each coupler ratio and calculate the input needed by both branches.
- Choose the ratio with the smallest worst-case input requirement.
- Continue upstream, then add practical losses and design reserve.
Use appropriate laser safety procedures in a physical lab. Never look into a fiber or connector.