Apparatus and method for realizing tubular optical waveguides in glass by femtosecond laser direct writing. Irradiation in glass with focused femtosecond laser pulses leads to decrease of refractive index in the modified region. Tubular optical waveguides of variable mode areas are fabricated by forming the four sides of the modified regions with slit-shaped femtosecond laser pulses, ensuring single mode waveguide with a mode field dimension compatible with direct coupling to single-mode optical fibers.