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Lightning protection demonstrated through usage of EPS1000 polarization scrambler

The EPS1000 has been used to demonstrate lightning protection https://www.infinera.com/ice5-innovation/ of coherent optical data transmission at the OFC2018 exhibition.

Schematic of EPS1000 polarization scrambler

	Waveplates in EPS1000-20M and EPS1000-10M

	Waveplates in depolarizer-version of EPS1000-50M

Waveplates in EPS1000-XXM (20 < XX ≤ 80; standard since 02/2026). Ideally suited for precise testing of coherent receivers at highest scrambling speeds, up to 80 Mrad/s.

Electrooptic rotating quarterwave plates (QWP0...QWP5) and a halfwave plate (HWP) are traversed by the light. This way arbitrary input polarizations are scrambled. In all Poincaré sphere representations here below the HWP rotates fast compared to QWP1...QWP4, and QWP0, QWP5 are not implemented.

Rotating QWPs have time-variable orientation. When also retardation is time-variable a QWP becomes a linear retarder (RET). One can switch between QWP and RET operation. RET operation improves scrambling randomness. Therefore RET1, RET2, HWP, RET3, RET4 behave very siimilar to the established QWP0, QWP1, QWP2, HWP, QWP3, QWP4, QWP5. But with 5 rather than 7 waveplates, EPS1000-XXM (20 < XX ≤ 80) is a lot faster than the traditional EPS1000-20M.

Function of waveplates in EPS1000 polarization scrambler

A polarization transformer made from LiNbO₃ is configured such that it contains cascaded electrooptic waveplates. Each electrical period of waveplate voltages corresponds to half a mechanical turn of a waveplate. As the scrambling speed we define that speed on the Poincaré sphere which occurs if a waveplate generates rotating linear polarization.

QWP0 to QWP2 scramble the unknown input polarization.

Dependent on this, the fast HWP generates circles in parallel planes of the Poincaré sphere with radii between 0 and 1, having an arithmetic mean of π/4. In each period of the HWP voltages such circle is traversed twice. At radius 1, corresponding to rotating linear polarization on the Poincaré sphere equator, a trajectory of length 4π is thus generated in each electrical period. The polarization change speed of the HWP, adjustable from -10,000 to +10,000 krad/s in steps of 0.01 krad/s (6 decades), is referred to this case. For equidistributed input polarization the root-mean-square speed is √2/3 = 0.816 times as large.

QWP3 to QWP5 subsequently scramble the orientations of the circles and thereby distribute the polarization changes and their directions equally over the Poincaré sphere. A QWP can likewise generate rotating linear polarization, from circular input polarization. In doing so the Poincaré sphere equator is traversed once in each electric period of the QWP voltages, corresponding to a trajectory of length 2π. The polarization change speed of the QWPs, individually adjustable from -1,000,000 to +1,000,000 rad/s in steps of 0.01 rad/s (8 decades), is referred to this case. For input polarizations having ellipticity angle magnitudes <π/8 the polarization change speed of a QWP is up to √2 times as large. For equidistributed input polarization the root-mean-square speed is 2/√3 = 1.15 times as large as for circular input polarization.

Poincaré sphere representations, recorded behind EPS1000 polarization scrambler

				QWP0 and QWP5 are not implemented here. All other waveplates are running: QWP1, QWP2, HWP, QWP3, QWP4. Rightmost: HWP rotates fast. Everywhere else, also below: HWP rotates slowly.

			QWP1, QWP2 at the front transform the input polarization of the HWP which therefore generates circles of varying sizes in parallel planes. QWP3, QWP4 at the rear are switched off.

QWP1, QWP2 at the front are switched off. The HWP generates large (left) or small (right) circles. These are transformed by QWP3, QWP4 at the rear into varying orientations.

Polarization scrambling at 80 Mrad/s

		Halfwave plate witn locally linear polarization running at 80 Mrad/s in EPS1000-80M

		Semilogarithmic speed distribution generated by halfwave plate with locally linear polarization, running at 80 Mrad/s in EPS1000-80M

		Peaked speed distribution 80 Mrad/s generated by RET1, RET2, HWP, RET3, RET4 in EPS1000-80M. Speed distribution is calibrated up to 80 Mrad/s. Peaked disribution is dominated by fast speeds. This allows fast testing of coherent receivers designed for a certain control speed.

Similar to above, but 100 krad/s

Step response of EPS1000-80M polarization scrambler

Electroptic step response 10 ns of EPS1000-80M, measured with photodiode behind polarizer. Horizontal: 10 ns/div. Vertical: not to scale.

Polarization-dependent loss (PDL) measurement

For details see Section IV. here. PDL measurement requires an EPS1000 equipped with 1 or 2 optical power detectors. For various generated polaization states the optical power behind the device under test (DUT) is measured. The averaging interval is selectable (80 ns, 160 ns, 320 ns, ... 2.68 s). With default settings, one set of measurements plus data transfer takes about 30 s. Reference measurements allow the losses of the LiNbO₃ polarization transformer inside the EPS1000 to be mathematically eliminated. Minimum, mean and maximum attenuation and PDL of the DUT are thus determined. Typical results follow.