Lateral current spreading is experimentally and theoretically investigated in ridge waveguide laser diodes having various residual guide thickness outside the ridge region. It is found that a critical residual thickness exists below which the lasers emit in a single mode with a low threshold current. Above this critical value, the threshold rises rapidly and the lasers oscillate simultaneously in the two lowest order lateral modes. This critical thickness can be used to experimentally determine an average doping level of the upper waveguide layer. This doping level permits the control of the threshold current and series resistance.