How to synchronize two different precision measurement instruments?
A better laser regulates intensity by current with a PD in a feedback loop detecting a weak sample of the beam internal to the laser. Location and stability of the optics for the PD are critical to stability and are best done internally unless you have a large budget.

PD's are very stable (PT's are not) but temperature can also be regulated to improve this laser stability.

I would be more likely to measure turbidity by ; 


return loss (reflection) which would be far more sensitive but requires special design of apparatus from stray contain reflections and use a very small aperture to measure a linear reflection rather than a radial reflection. This would apply to low torbidity clear liquids, otherwise ...
scattering by diffusion around the container with the beam diffused to multiple detectors.
then transmission loss for heavy turbidity (really dirty)
multiple wavelengths for sensitivity to particle size from IR to UV depnding on clarity and particle types for photoemission or reflectivity of contaminants.
if the non-turbid fluid is not clear, then I would use a dual beam approach with filtered fluid.