My recommendation:
Look into some laser diode´s datasheets.
Step through every specified item and decide if it is of interst for your application. And then decide which limit you specify for your desisred laser diode.
Generally:
Safety. Which laser class you want to use? How to achieve this?
Laser wavelength should meet your sensor´s wavelength.
Modulation, speed.
optic, beam angle
Just to experiment with, consider an inexpensive laser pointer (often sold as a 'pet toy'). They are low power and visible to the human eye. Messages on a forum said it does not permanently injure our eyes although we should not make it a habit to look directly at it.