We report on the development of a 20 µm-core polarization maintaining endlessly single-mode photonic crystal fiber for use in the delivery of high-power single-frequency lasers, especially for quantum physics experiments. A complete numerical study is used to determine the optimal geometry of the fiber to be single mode over the 600-1700nm spectral range with a polarization extinction ratio greater than 20 dB. Three versions of this fiber with different pitch and hole size ratios were fabricated and experimentally characterized at 780 nm, 1064 nm, and 1550 nm. The Brillouin threshold was also measured at 1064 nm and compared to that obtained with commercial fibers.