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When laser source is launched into two 1x2 50/50 fiber optic couplers connected as below the output power constantly fluctuates in range of 70 uW. The fluctuation happen roughly one to two times per second. Why does this happen?

            -------
Laser -----|       |----- Power Meter
            ------- 

However, when the output power of each coupler is measured individually (as below) on each leg the output power is stable on every leg of every coupler.

            ------- Power Meter
Laser -----|       
            ------- Power Meter

I initially suspected that this may be caused by laser source instability due to back reflections, however after I used a source with an optical isolator, the output power still fluctuated as before. I tried several laser sources, and several couplers with the same fluctuating result.

I tried with 1,3,4 and 5 mW laser input power and 1310nm and 1550nm wavelengths. I also tried DFB abd FP laser diodes with the same result.

When laser source is launched into two 1x2 50/50 fiber optic couplers connected as below the output power constantly fluctuates in range of 70 uW. The fluctuation happen roughly one to two times per second. Why does this happen?

            -------
Laser -----|       |----- Power Meter
            ------- 

However, when the output power of each coupler is measured individually (as below) on each leg the output power is stable on every leg of every coupler.

            ------- Power Meter
Laser -----|       
            ------- Power Meter

I initially suspected that this may be caused by laser source instability due to back reflections, however after I used a source with an optical isolator, the output power still fluctuated as before. I tried several laser sources, and several couplers with the same fluctuating result.

I tried with 1,3,4 and 5 mW laser input power and 1310nm and 1550nm wavelengths. I also tried DFB abd FP laser diodes with the same result. The legs of the couplers are 1m each.

When laser source is launched into two 1x2 50/50 fiber optic couplers connected as below the output power constantly fluctuates in range of 70 uW. The fluctuation happen roughly one to two times per second. Why does this happen?

            -------
Laser -----|       |----- Power Meter
            ------- 

However, when the output power of each coupler is measured individually (as below) on each leg the output power is stable on every leg of every coupler.

            ------- Power Meter
Laser -----|       
            ------- Power Meter

I initially suspected that this may be caused by laser source instability due to back reflections, however after I used a source with an optical isolator, the output power still fluctuated as before. I tried several laser sources, and several couplers with the same fluctuating result.

I tried with 1,3,4 and 5 mW laser input power and 1310nm and 1550nm wavelengths.

When laser source is launched into two 1x2 50/50 fiber optic couplers connected as below the output power constantly fluctuates in range of 70 uW. The fluctuation happen roughly one to two times per second. Why does this happen?

            -------
Laser -----|       |----- Power Meter
            ------- 

However, when the output power of each coupler is measured individually (as below) on each leg the output power is stable on every leg of every coupler.

            ------- Power Meter
Laser -----|       
            ------- Power Meter

I initially suspected that this may be caused by laser source instability due to back reflections, however after I used a source with an optical isolator, the output power still fluctuated as before. I tried several laser sources, and several couplers with the same fluctuating result.

I tried with 1,3,4 and 5 mW laser input power and 1310nm and 1550nm wavelengths. I also tried DFB abd FP laser diodes with the same result.

When laser source is launched into two 1x2 50/50 fiber optic couplers connected as below the output power constantly fluctuates in range of 70 uW. Why does this happen?

            -------
Laser -----|       |----- Power Meter
            ------- 

However, when the output power of each coupler is measured individually (as below) on each leg the output power is stable on every leg of every coupler.

            ------- Power Meter
Laser -----|       
            ------- Power Meter

I initially suspected that this may be caused by laser source instability due to back reflections, however after I used a source with an optical isolator, the output power still fluctuated as before. I tried several laser sources, and several couplers with the same fluctuating result.

I tried with 1,3,4 and 5 mW laser input power and 1310 and 1550nm.

When laser source is launched into two 1x2 50/50 fiber optic couplers connected as below the output power constantly fluctuates in range of 70 uW. The fluctuation happen roughly one to two times per second. Why does this happen?

            -------
Laser -----|       |----- Power Meter
            ------- 

However, when the output power of each coupler is measured individually (as below) on each leg the output power is stable on every leg of every coupler.

            ------- Power Meter
Laser -----|       
            ------- Power Meter

I initially suspected that this may be caused by laser source instability due to back reflections, however after I used a source with an optical isolator, the output power still fluctuated as before. I tried several laser sources, and several couplers with the same fluctuating result.

I tried with 1,3,4 and 5 mW laser input power and 1310nm and 1550nm wavelengths.