Direct CW Modulation

A CW laser will produce a fixed wavelength after a transient time of ~10 ms.  Therefore you can modulate the laser with a signal slower than 100 Hz and expect the output wavelength to follow the input current with the relation measured in its datasheet.

CW lasers can also be modulated more quickly. The emitted power will follow the current amplitude faithfully at high speeds; for reference you can see this paper describing a free-space link functioning at 330 MHZ.

However the wavelength modulation, being a thermal effect,  
will be suppressed at speeds exceeding 1 MHz and will decrease monotonously between 100 Hz and 1 MHz. The following graph shows data for an amplitude for a specific laser; the exact values will vary from one laser to the next.

Click to enlarge