You can pre-treat with all sorts of stuff, not just sodium bicarbonate.
Try a light wash of acrylic house paint. Dilute 1:1 with water, wipe on, wait 30 secs - 2 mins - try different times - and wipe off, let dry, light sand/wire wool, laser.
Table salt with teabags used to give any required colour is another. 15mg > 250ml.
It very much depends on the finish you want.
If you are just looking to get darker engraving, give it a wipe with thin shellac before engraving, then a wipe over with a cloth with thin dye after. The shellac will protect the unburned wood from the dye, and you can sand it off easily,
Some woods engrave really dark, some don’t. I wouldn’t get hung up on getting perfectly dark images from the laser, at the expense of a loss of detail or other changes. You can always mask and spray. And it’s truckloads faster, too.