  Optical interferometers are extensively used in fundamental physics test,
gravitational wave detection, quantum metrology, topological photonics, and
quantum information processing. Fiber-based interferometers are compact, robust
and cheap, thus are ubiquitously deployed. However, the optical phase in fiber
interferometers is sensitive to ambient perturbation, resulting in compromised
phase sensing precision. Therefore, phase control, shifting and stabilization
of fiber interferometers is essential. Methods to create stable interference
patterns and to lock a fiber interferometer at arbitrary phase have been shown,
which however are sophisticated, bulky and delicate, preventing wider
application in harsh environment outside laboratories or in space. Here we
demonstrate a new method for arbitrary-phase locking of fiber unbalanced
Mach-Zehnder interferometers. Compared to existing method, our method is
simpler, more robust and more compact. We showcase the preparation and
characterization of narrow-band energy-time-entanglement photon state generated
in integrated nonlinear microresonators, where two-photon interference
visibility reaching 0.993(6) is enabled. Our method constitutes a critical
building block for photonic quantum network, and is useful to emerging
single-photon interference in curved space-time that facilitates exploration of
the interface of quantum mechanics and general relativity.
