A method and apparatus for spatial mode selection of planar waveguide and thin slab lasers in the laser's large cross section size direction. The planar laser has a small cross section size direction orthogonal to a major axis thereof and a large cross section size direction orthogonal to the small cross section size direction and to the major axis. A planar reflector is disposed at one end of the planar laser, the planar reflector having a reflecting surface which is arranged orthogonally to the major axis and parallel to both the small and large cross section size directions of the laser. A Bragg grating is disposed in the optical track of the laser, the Bragg grating having a grating vector which is arranged in the plane normal to said small cross section size direction and at the angle between 0.degree. and 90.degree. to said large cross section size direction for suppression of higher order optical modes in said large cross section size direction.