Why is the output from my photodiode not linearly related to laser power?
With 100mA limit at a peak response of 0.65A/W
You can expect a max input of  ~ 150 mW optical input.

Also you can expect efficacy of your laser to decline with increasing power due to more losses in bulk resistance  or I^2 * ESR.  

You can verify Pd vs If in your laser.

Take care in temperature rise which can also reduce efficacy of the emitter.