I have tried to switch laser modulation frequency to lower frequency.

I have tested several frequencies. The lowest frequency still was working fine is 25 MHz. I found that amplitude of signal decrease to 0.65 of initial value (at 150 MHz).

I think that I need to make additional configuration of Si5351 PLL to set modulation frequency to lower frequency. My target is to set modulation frequency to 15 Mhz.

Why I need lower frequency? To increase unambiguity distance - at 15 MHz is would be 10 m.