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Single-polarization Fibers

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

Definition: optical fibers which transmit only light with a certain polarization

Alternative term: polarizing fibers

Category: article belongs to category fiber optics and waveguides fiber optics and waveguides

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DOI: 10.61835/7bx Cite the article: BibTex plain text HTML Link to this page! LinkedIn

Single-polarization fibers (also called polarizing fibers) are specialty optical fibers which can transmit light with a certain linear polarization direction, whereas light with the other polarization direction is either not guided or at least experiences strong optical propagation losses. Such fibers should not be confused with polarization-maintaining fibers, which guide light with any polarization state, but can preserve a linear polarization state when the polarization direction is properly aligned with the birefringence axis.

In many cases, single-polarization guidance occurs in only a limited wavelength range. Outside that range, both polarization directions or no light at all may be guided. Also, some fibers exhibit a limited extinction ratio.

Operation Principles for Single-mode Guidance

Different principles of operation can be utilized for single-polarization fibers, which generally involve the generation of strong birefringence with some kind of design asymmetry. One obtains a polarization dependence of the cut-off wavelength, so that only light with one polarization direction is guided, whereas the fiber is a leaky waveguide for the other polarization. In any case, the strong birefringence also serves to reduce the coupling of the two polarization directions, and thus excessive loss for the wanted polarization state.

A common approach is the use an elliptical fiber core. Alternatively or in addition, the rotational symmetry may be broken by structures around the fiber core, such as air holes on two sides (side-hole fibers, hole-assisted fibers), or a bow-tie geometry. Some built-in mechanical stress is often utilized.

There are also various types of single-polarization photonic crystal fibers. Here, a suitable arrangement of microscopic air holes breaks the rotational symmetry and introduces polarization-dependent guiding properties.

Generally, the required fabrication techniques are more sophisticated than for standard fibers or polarization-maintaining fibers, leading to higher cost. The propagation losses may also be higher than in standard fibers.

Applications

Used in fiber lasers, single-polarization fibers guarantee polarized laser emission. There are also various uses in the field of fiber-optic sensors, where polarization effects in standard fibers can lead to unwanted effects, and polarization purity can thus be helpful. For example, one uses polarizing fibers in fiber-optic gyroscopes and interferometric sensors.

Comparing with polarization-maintaining fibers, single-polarization fibers are a small niche market, although for some applications they are critical.

More to Learn

Polarization-maintaining fibers

Photonic crystal fibers

Polarizers

Suppliers

The RP Photonics Buyer's Guide contains four suppliers for single-polarization fibers. Among them:

Fibercore

Fibercore Zing™ speciality optical fiber is an all-fiber polarizer, guiding only a single-polarization state, providing practical real world polarizing performance. Straight or coiled, Zing™ delivers the high Polarization Extinction Ratio (PER) and broad, stable operating window that applications demand.

Exail

Exail (formerly iXblue) polarizing fiber (PZ) is designed so that only one state of polarization is guided along the fiber; any other state of polarization will be lost rapidly, thus yielding a high built-in polarization extinction ratio. This particular mechanism is obtained through a specific waveguide design and a careful optimization of the glass composition resulting in both high birefringence and leakage behavior.

PZ fibers are available for different wavelengths with a broad polarizing window (typically larger than 100 nm), low attenuation and high extinction ratio (≥30 dB), that can be tuned by coiling the proper fiber length at the appropriate coil diameter.

Exail also offers ready-to-use polarizing solutions based on PZ fibers.

Bibliography

[1]	V. Ramaswamy et al., “Single polarization optical fibers: exposed cladding technique”, Appl. Phys. Lett. 33, 814 (1978); https://doi.org/10.1063/1.90538
[2]	T. Hosaka et al., “Low-loss single polarization fibers with asymmetrical strain birefringence”, Electron. Lett. 17, 530 (1981); https://doi.org/10.1049/el:19810371
[3]	K. Okamoto et al., “Polarization properties of single-polarization fibers”, Opt. Lett. 7 (11), 569 (1982); https://doi.org/10.1364/OL.7.000569
[4]	T. Katsuyama et al., “Propagation characteristics of single polarization fibers”, Appl. Opt. 22 (11), 1748 (1983); https://doi.org/10.1364/AO.22.001748
[5]	D. A. Nolan, “Single-polarization fiber with a high extinction ratio”, Opt. Lett. 29 (16), 1855 (2004); https://doi.org/10.1364/OL.29.001855
[6]	D. T. Walton et al., “Challenges in single-polarization fibers”, Proc. SPIE 5709, 316 (2005); https://doi.org/10.1049/el:19810371
[7]	T. Schreiber et al., “Stress-induced single-polarization single-transverse mode photonic crystal fiber with low nonlinearity”, Opt. Express 13 (19), 7621 (2005); https://doi.org/10.1364/OPEX.13.007621
[8]	M.-J. Li et al., “High bandwidth single polarization fiber with elliptical central air hole”, J. Lightwave Technol. 23 (11), 3454 (2005); https://doi.org/10.1109/JLT.2005.855856
[9]	J. R. Folkenberg et al., “Broadband single-polarization photonic crystal fiber”, Opt. Lett. 30 (12), 1446 (2005); https://doi.org/10.1364/OL.30.001446
[10]	X. Chen et al., “Wide band single polarization and polarization maintaining fibers using stress rods and air holes”, Opt. Express 16 (16), 12060 (2008); https://doi.org/10.1364/OE.16.012060
[11]	K. K. Y. Lee et al., “Design strategies and rigorous conditions for single-polarization single-mode waveguides”, Opt. Express 16 (19), 15170 (2008); https://doi.org/10.1364/OE.16.015170
[12]	M. Chen and Y. Zhang, “Improved design of polarization-maintaining photonic crystal fibers”, Opt. Lett. 33 (21), 2542 (2008); https://doi.org/10.1364/OL.33.002542

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