Really thin wood will never be flat, no matter where it comes from or what it's called. It's just the nature of thin wood, and thin Baltic birch is no exception. 

Each sheet will have a cup between two opposite corners because it's too thin and too wide to lay flat. 

Fortunately, there are some good, tried and true tricks for getting the material to lay flat in your laser cutter.

  1. Use double sided tape to adhere the sheet to MDF or another flat substrate
  2. Create a frame or clamping mechanism that holds the edges down on the bed
  3. Use magnets if the bed of your laser cutter is steel

A Little More Info

Baltic birch and other plywoods that are 1/2, 3/8", 1/4" or 1/8" in thickness are manufactured for things like drawer bottoms, cabinet backs, and photo and art frame backers, and similar products. In the final product, they're captured within a groove or stapled in place.

Likewise, to use these sheets in a laser cutter, you just need to learn some tricks to get the best results. 

Or switch to a material that is guaranteed to be flat, such as fiberboard, or medium-density fiberboard.