Basic understanding on Tap ratio for Splitter/Coupler

Fiber splitters/couplers divide optical power from one common port to two or more split ports and combine all optical power from the split ports to one common port (1 × N coupler). They operate across the entire band or bands such as C, L, or O bands. The three port 1 × 2 tap is a splitter commonly used to access a small amount of signal power in a live fiber span for measurement or OSA analysis. Splitters are referred to by their splitting ratio, which is the power output of an individual split port divided by the total power output of all split ports. Popular splitting ratios are shown in Table below; however, others are available. Equation below can be used to estimate the splitter insertion loss for a typical split port. Excess splitter loss adds to the port’s power division loss and is lost signal power due to the splitter properties. It typically varies between 0.1 to 2 dB, refer to manufacturer’s specifications for accurate values. It should be noted that splitter function is symmetrical.

where IL = splitter insertion loss for the split port, dB

Pi = optical output power for single split port, mW

PT = total optical power output for all split ports, mW

SR = splitting ratio for the split port, %

Γe = splitter excess loss (typical range 0.1 to 2 dB), dB

Common splitter applications include

• Permanent installation in a fiber link as a tap with 2%|98% splitting ratio. This provides for access to live fiber signal power and OSA spectrum measurement without affecting fiber traffic. Commonly installed in DWDM amplifier systems.

• Video and CATV networks to distribute signals.

• Passive optical networks (PON).

• Fiber protection systems.

Example with calculation:

If a 0 dBm signal is launched into the common port of a 25% |75% splitter, then the two split ports, output power will be −6.2 and −1.5 dBm. However, if a 0 dBm signal is launched into the 25% split port, then the common port output power will be −6.2 dBm.

Calculation.

Launch power=0 dBm =1mW

Tap is  25%|75%

so equivalent mW power which is linear  will be

0.250mW|0.750mW

and after converting them ,dBm value will be

-6.02dBm| -1.24dBm

Some of the common split ratios and their equivalent Optical Power is available below for reference.