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Precision measurements of modal birefringence in polarization-maintaining single-mode fibers and its stretch-dependence are described. An orthogonally polarized two-frequency laser beam is launched into some length of a fiber in accordance with the principal axes of the fiber. After a passage through the fiber, its optical heterodyne detection processes are made to produce a beat-photocurrent signal whose phase is linear to modal birefringence. The modal birefringence is accurately measured by detecting the phase change of the beat signal resulting from a short cut of the fiber. The stretch-dependent change in modal birefringence is also examined in detail and is recognized to be in proportion to the longitudinal strain of the fiber.