How to determine the splitting ratio of a coupler

The splitting ratio of a coupler is determined by measuring the output power at each port relative to the input power and expressing it as a percentage of total output.

Definition of Splitting Ratio

The splitting ratio (SR) defines how optical power is distributed among the output ports of a coupler. For a 1×N coupler, the sum of all output powers (neglecting excess loss) equals 100%. It is commonly expressed in the format X%|Y%, where X and Y represent the percentage of input power delivered to each output port .

Measurement Method

1. Set up the coupler with a stable light source connected to the input port.
2. Measure the output power at each port using an optical power meter. Ensure all measurements are in the same units (mW or dBm) .
3. Calculate the splitting ratio for each port: $$S{R}_i=\frac{P_{\text{out},i}}{{\sum }_{j=1}^N{P}_{\text{out},j}}\times 100\%$$ where $P_{\text{out},i}$ is the measured power at port i, and the denominator is the sum of all output powers .
4. Check for consistency: The sum of all SR values should be close to 100%, with deviations indicating excess loss or measurement errors.

Considerations

• Insertion Loss (IL): The ratio of input power to output power at a single port, including splitting and internal losses. IL can be calculated as: $$IL=10·{\log }_{10}\left(\frac{P_{\text{in}}}{P_{\text{out}}}\right)$$ Values close to the ideal splitting loss indicate a healthy coupler .
• Excess Loss (EL): Quantifies energy lost inside the coupler beyond the ideal split: $$EL=10·{\log }_{10}\left(\frac{P_{\text{in}}}{\sum P_{\text{out}}}\right)$$ Higher EL may indicate contamination, microbends, or wavelength mismatch .
• Tap Ratio: For tap couplers, a small fraction of power is diverted for monitoring, while the majority passes through. The same measurement principles apply, but the ratio is typically asymmetric (e.g., 10%|90%) .

Theoretical Determination

For directional couplers, the splitting ratio can also be predicted using coupled-mode theory. The fraction of power transferred depends on the coupling coefficient (C), the interaction length (L), and the supermode effective indices of the waveguides. The coupled power is given by:

where $P_0$ is the input power . This approach is useful for designing couplers with specific ratios (50/50, 75/25, etc.) before fabrication.

Practical Tools

Online calculators and lab software can simplify the process by converting measured powers to mW or dBm, computing insertion and excess loss, and reporting the splitting ratio for 1×2, 1×3, or 1×4 couplers . By combining accurate measurements with theoretical predictions, you can reliably determine the splitting ratio and ensure your coupler meets design specifications.