polarization scramblers (original) (raw)

Definition: devices used for depolarizing light by temporal polarization modulation

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Related: polarization of lightdepolarizerselectro-optic modulators

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Contents

What are Polarization Scramblers?

Polarization scramblers are devices which are widely used to reduce or eliminate the degree of polarization of a light beam — that is, to convert polarized light into unpolarized light. They achieve this by dynamically and rapidly modulating the state of polarization (SOP) in such a way that, when observed over the relevant timescale, the light appears unpolarized. This time-domain randomization distinguishes scramblers from other depolarizing technologies.

It is important to note that the term polarization scrambler is not always used consistently in the scientific literature. Here, we define polarization scramblers as devices that depolarize light by modulating its polarization in time. In contrast, there are various kinds of depolarizers which use other methods of pseudo-depolarization based on frequency-dependent or spatially varying polarization changes, but which are also sometimes called polarization scramblers.

Electro-optic Polarization Scrambling

If the input polarization is linear in a fixed direction, the simplest approach is to use an electro-optic modulator where the birefringent axes are oriented at 45° against the input polarization. The modulator should be driven with a strong signal, leading to large relative phase shifts. Ideally, it is not sinusoidal but (quasi-)random.

For more thorough depolarization, and for minimizing the dependence on input polarization, one may use multiple such modulators with different orientations and driven with different signals.

Electro-optic polarization scramblers can be of bulk-optical form or made in waveguides, e.g. based on lithium niobate (LiNbO3). Waveguide-based devices, which can then be part of photonic integrated circuits (PICs), are best suited for applications in fiber optics, especially in fiber optic communications. The small dimensions are also favorable for high-bandwidth scrambling without using excessively large electrical voltages. However, fiber-to-waveguide coupling may introduce a significant amount of insertion loss.

Fiber Squeezers

Some fiber-optic piezo-optomechanical polarization scramblers apply physically stress to the fiber at different axes to rapidly vary the fiber's local birefringence. In that way, one can rapidly modulate the state of polarization. Multiple such devices can be combined [11].

A variant uses a fiber wound around a piezoelectric cylinder, which is driven at a mechanical resonance to achieve strong modulation. One or several of such cylinders can be used.

Key advantages of the piezo-based method against the electro-optic method are substantially lower cost and low insertion loss. However, the scrambling can by far not be as rapid as with electro-optic devices.

All-optical Methods

Polarization modulation can also be achieved with all-optical methods involving nonlinear optical effects (mostly using the Kerr effect), e.g. in optical fibers [13, 17]. That can result in very fast quasi-random polarization modulation, but the setup is considerably more complex, typically limited to light with certain characteristics, and often requires high optical powers.

Ideal and Practical Characteristics

An ideal polarization scrambler would exhibit the following characteristics:

In reality, polarization scramblers can only approximate these ideals:

For a specific application, one should consider the actually required depolarization characteristics and find the best compromise between those and other factors such as size, power requirements and cost.

Applications of Polarization Scramblers

Fiber Communications

Polarization scramblers are mainly used in optical fiber communications. They can help to avoid or mitigate problems associated with polarization-dependent gain and loss, polarization mode dispersion, and polarization hole burning in erbium-doped fiber amplifiers (EDFAs). Scrambling the SOP ensures signal statistics compatible with system assumptions and stabilizes link performance when transmitters emit highly polarized light.

Note that telecom transmitters usually start with fully polarized light, e.g. the output of an electro-optic data modulator, and may then need to apply a method of depolarization.

Fiber Sensing

Distributed and point fiber-optic sensors (for example, for structural health monitoring, distributed temperature or strain sensing, and similar sensor applications) can suffer from polarization fading or variable sensitivity linked to SOP fluctuations. Scramblers maintain consistent system response by averaging out polarization-dependent effects.

Other Optical Measurements

Optical spectrum analyzers and other measurement instruments often exhibit polarization sensitivity, for example due to the use of diffraction gratings or polarizing components. Scrambling the input polarization prevents measurement artifacts, producing more reliable and reproducible results.

Device Characterization

For devices that are sensitive to polarization, using a polarization scrambler enables SOP-averaged characterization, which can be useful in certain application areas where unpolarized light is used.

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Bibliography

[1] N. G. Walker and G. R. Walker, “Polarization control for coherent communications”, J. Lightwave Technol. 8 (3), 438 (1990); doi:10.1109/50.50740
[2] P. Chien and C. Pan, “Fiber-optic gyroscopes based on polarization scrambling”, Opt. Lett. 16 (3), 189 (1991); doi:10.1364/OL.16.000189
[3] F. Bruyere et al., “Demonstration of an optimal polarization scrambler for long-haul optical amplifier systems”, IEEE Photon. Technol. Lett. 6 (9), 1153 (1994); doi:10.1109/68.324696
[4] F. Heismann and K. L. Tokuda, “Polarization-independent electro-optic depolarizer”, Opt. Lett. 20 (9), 1008 (1995); doi:10.1364/OL.20.001008
[5] F. Heismann, “Compact electro-optic polarization scramblers for optically amplified lightwave systems”, J. Lightwave Technol. 14 (8), 1801 (1996); doi:10.1109/50.532017
[6] D. Sandel et al., “Some enabling techniques for polarization mode dispersion compensation”, J. Lightwave Technol. 21 (5), 1198 (2003)
[7] P. J. Leo, G. R. Gray, G. J. Simer and K. B. Rochford, “State of polarization changes: classification and measurement”, J. Lightwave Technol. 21 (10), 2189 (2003)
[8] C. Xie, D. Werner and H. F. Haunstein, “Dynamic performance and speed requirement of polarization mode dispersion compensators”, J. Lightwave Technol. 24 (11), 3968 (2006)
[9] Y. K. Lizé et al., “Fast all-fiber polarization scrambling using re-entrant Lefèvre controller”, Opt. Commun. 279 (1), 50 (2007); doi:10.1016/j.optcom.2007.06.050
[10] J. Kim, S. Park, W. Chu and M. Oh, “Integrated-optic polarization controllers incorporating polymer waveguide birefringence modulators”, Opt. Express 20 (11), 12443 (2012); doi:10.1364/OE.20.012443
[11] L. Yao et al., “A novel scheme for achieving quasi-uniform rate polarization scrambling at 752 krad/s”, Opt. Express 20 (2), 1691 (2012); doi:10.1364/OE.20.001691
[12] M. Guasoni, J. Fatome and S. Wabnitz, “Intensity noise-driven nonlinear fiber polarization scrambler”, Opt. Lett. 39 (18), 5309 (2014); doi:10.1364/OL.39.005309
[13] M. Guasoni et al., “Fast and chaotic fiber-based nonlinear polarization scrambler”, IEEE J. Sel. Top. Quantum Electron. 22 (2), 88 (2016); doi:10.1109/JSTQE.2015.2501382
[14] A. A. E. Hajomer, L. Zhang, X. Yang and W. Hu, “Accelerated key generation and distribution using polarization scrambling in optical fiber”, Opt. Express 27 (24), 35761 (2019); doi:10.1364/OE.27.035761
[15] P. Huang et al., “Secure key generation and distribution scheme based on two independent local polarization scramblers”, Appl. Opt. 60 (1), 147 (2021); doi:10.1364/AO.413171
[16] D. Xu, O. Lopez, A. Amy-Klein and P. Pottie, “Polarization scramblers to solve practical limitations of frequency transfer”, J. Lightwave Technol. 39 (10), 3106 (2021)
[17] Y. Yu et al., “All-optical polarization scrambler based on polarization beam splitting with an amplified fiber ring”, Opt. Express 32 (11), 19210 (2024); doi:10.1364/OE.510422

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