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We propose and experimentally demonstrate a robust mode converter based on liquid crystal-on-silicon (LCOS), which is not placed at the focal plane of the collimating lens. We present theoretical investigations to evaluate the performance of proposed configuration, and our experimental results verify the favorable function of mode-selective conversion with orthogonal linear polarizations, as well as two-mode multiplexing. The robust device configuration with an insertion loss of less than 3 dB, together with cascade capability, can be used for multidimensional fiber-optical transmission system. In particular, in terms of LP11 mode conversion, the off-focus operation allows the range of position offset for input fiber with graded-index (GRIN) lens, LCOS, and few-mode fiber to be 7.5 mm, 151 μm, and 40.4 mm, respectively, indicating easy alignment and maintenance.