In this paper, we consider the polarization division multiplexed (PDM) coherent optical-orthogonal frequency division multiplexing/offset-quadrature amplitude modulation (CO-OFDM/OQAM) system. The problem of channel estimation for this system is investigated. Firstly, we introduce the sparse preamble channel estimation method for PDM CO-OFDM/OQAM. We then propose a novel sparse preamble pattern. The interference approximation method is also taken into account for comparison. Simulation results, which are in accordance with the analysis, show that the proposed sparse preamble not only achieves significant gains in terms of bit-error rate (BER), but also offers lower peak-to-average power ratio (PAPR) and preamble overhead compared to the available solutions.