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arXiv:2312.10145 · Applied Optics

Prescriptive method for fiber polarization compensation in two bases

Teodor Strömberg, Peter Schiansky, Philip John Walther

Published Feb 6, 2024• 7 citations
arXivHTMLPDF

Abstract

Single-mode optical fibers exhibit a small but non-negligible birefringence that induces random polarization rotations during light propagation. In classical interferometry these rotations give rise to polarization-induced fading of the interferometric visibility, and in fiber-based polarimetric sensors as well as quantum optics experiments, they scramble the information encoded in the polarization state. Correcting these undesired rotations is consequently an important part of many experiments and applications employing optical fibers. In this Lab Note we review an efficient method for fully compensating fiber polarization rotations for general input states. This method was not originally devised by us, but does, to the best of our knowledge, not appear in the literature, and our interactions with the community have indicated that it is not well known.

DOI
10.1364/ao.515768

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Single-mode optical fibers exhibit a small but non-negligible birefringence that induces random polarization rotations during light propagation. In classical interferometry these rotations give rise to polarization-induced fading of the interferometric visibility, and in fiber-based polarimetric sensors as well as quantum optics experiments, they scramble the information encoded in the polarization state. Correcting these undesired rotations is consequently an important part of many experiments and applications employing optical fibers. In this Lab Note we review an efficient method for fully compensating fiber polarization rotations for general input states. This method was not originally devised by us, but does, to the best of our knowledge, not appear in the literature, and our interactions with the community have indicated that it is not well known.
Single mode fiber (SMF) birefringence effects have been a limiting factor for a variety of Sagnac applications over longer distanceSMF links. In this report, we present a new concept of the SMF birefringence compensation in a Sagnac interferometric setup, based on a novel polarization control system. For the destructive interference, our control system guarantees a perfect compensation of both the SMF birefringence and imperfect propagation times matching of the setup's components. For the stabilization of the constructive interference, we have applied a fiber stretcher and a simple proportional-integral-derivative (PID) controller. The enclosed experimental data of the setup's visibility confirm validity of our polarization control system. We have also showed that the SMF birefringence model used in a "plug & play" interferometric setup [19], widely cited in the papers on quantum key distribution [11, 12, 13], cannot be applied in SMF Sagnac interferometric setup. However, the SMF birefringence model based on the Kapron equivalence well describes SMF Sagnac.
Birefringence in optical fibers is compensated by applying polarization modulation at a receiver. Polarization modulation is applied so that a transmitted optical signal has states of polarization (SOPs) that are equally spaced on the Poincare sphere. Fiber birefringence encountered in propagation between a transmitter and a receiver rotates the great circle on the Poincare sphere that represents the polarization bases used for modulation. By adjusting received polarizations, polarization components of the received optical signal can be directed to corresponding detectors for decoding, regardless of the magnitude and orientation of the fiber birefringence. A transmitter can be configured to transmit in conjugate polarization bases whose SOPs can be represented as equidistant points on a great circle so that the received SOPs are mapped to equidistant points on a great circle and routed to corresponding detectors.
Birefringence in optical fibers poses a challenge to controllably delivering polarized light. Strain-induced birefringence caused by bends in the fiber, vibrations, or a large temperature gradient can significantly alter the polarization, making it particularly difficult to deliver polarization states to low-temperature environments by fiber. In this paper, we investigate the transmission of polarized light through a fiber and discuss a method we have developed for delivering arbitrarily polarized light to the base stage of a dilution refrigerator using a standard optical fiber. We have created a compact, cryogenic optical system to identify the polarization of the delivered light, while room-temperature waveplates and a mathematical fiber model are used to fully characterize and compensate for the fiber's birefringent effects. We show here that we are able to deliver horizontal, vertical, diagonal, anti-diagonal, right circular, and left circular polarization states to milli-Kelvin temperatures, with state fidelities of greater than 0.96 being achieved in all cases. Additionally, we demonstrate that we can deliver randomly selected elliptical states through a standard fiber to the refrigerator. This opens up new opportunities for fiber-based optical experiments using polarized light, such as quantum information experiments using quantum states encoded in the polarization of single photons.
The polarization of light is critical in various applications, including quantum communication, where the photon polarization encoding a qubit can undergo uncontrolled changes when transmitted through optical fibers. Bends in the fiber, internal and external stresses, and environmental factors cause these polarization changes, which lead to errors and therein limit the range of quantum communication. To prevent this, we present a fast and automated method for polarization compensation using liquid crystals. This approach combines polarimetry based on a rotating quarter-waveplate with high-speed control of the liquid-crystal cell, offering high-fidelity compensation suitable for diverse applications. Our method directly solves for the compensation parameters, avoiding reliance on stochastic approaches or cryptographic metrics. Experimental results demonstrate that our method achieves over 99% fidelity within an average of fewer than six iterations, with further fine-tuning to reach above 99.5% fidelity, providing a robust solution for maintaining precise polarization states in optical systems.
Polarization-sensitive receivers for single photons are of crucial importance in various applications within the fields of quantum communication and quantum sensing, and are more commonly implemented in free-space optics rather than in optical fibers. This is primarily due to the unpredictable and varying birefringence in single-mode optical fibers. We present a method for birefringence compensation in an all-fiber detection setup that relies solely on coincidence measurements of a polarization-entangled state, or known correlations in a prepare-andmeasure scenario. We define coincidence entropies as functions of the measured coincidence counts. These quantify the randomness of measurement outcomes, remain independent of the transmitted Bell state, and serve as indicators of the degree of entanglement. By leveraging coincidence entropy as a cost function in a gradient descent algorithm, we are able to align the polarization bases between two distinct polarization-sensitive receivers. Additionally, coincidence entropies can be employed to monitor the quality of entanglement transmission, thereby enhancing the system’s ability to detect potential eavesdropping attempts, such as intercept-resend quantum attacks.