This study proposes a method for reducing signal crosstalk between LP11a and LP11b spatial modes via digital orthogonal filtering. The experimental results of testing the intensity-modulation direct-detection (IM-DD)-mode-division multiplexing transmission based on mode-selective photonic lanterns demonstrate that the bit error rate of the PAM8 signal can be reduced by at least two orders of magnitude via digital orthogonal filtering and conventional interference cancellation in back-to-back, 500-m few-mode fiber transmission. With respect to signal processing, this method provides a solution for modal crosstalk mitigation in low-cost, short-distance IM-DD-mode-division multiplexing transmission.