I would use a PLL like this. Should give you a -5 to +20 Mhz bandwidth. In this system you are basically locking the laser to produce a 5 MHz beat note. The reference is the 5 MHz crystal oscillator.
If you really need a +/- 50 MHz bandwidth, you can do it too (with a different circuit)–you would need to upconvert the beat note out of the photodiode to 50 Mhz, and use a 50 MHz crystal oscillator. You would need to convert the 5 Mhz beat note to 50 MHz using an image reject mixer and another local oscillator (a second crystal oscillator) at 45 Mhz.
In any case, you want to make a system that locks up producing a 5 Mhz beat note, but not being able to lock up with a -5 Mhz beat note!
Vout would be ~proportional to the instantaneous deviation from the desired 5 MHz beat note as long as the loop stayed locked.
There are probably other phase frequency detector chips out there too. There is a 50/50 chance you would have to swap the “U” and “D” pins to get it to lock. And you need to choose the R and C values to make a control loop with some phase margin.