hledat:

Impact of Polarization State on High-Speed Transmission Formats in Laboratory and Real Transmission Line

CESNET technical report 16/2010

Pavel Škoda, Josef Vojtěch

Received 29. 11. 2010

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Abstract

The cutting edge optical transmission technologies utilize signal phase to transmit data rather then signal amplitude. With polarization multiplexed technologies emerging on the horizon, we prepared tests to analyse an impact of changes in state of polarization on a 40Gbps DPSK transmission performance. We tested the 40Gbps DPSK transmission performance against fast 150μs state of polarization changes and fast scan of all polarization states. Although we found a 40Gbps DPSK system resilient to changes in polarization, the system sensitivity on chromatic dispersion implies careful planning of DPSK systems deployment.

Keywords: state of polarization, 40G DPSK, polarization scanning

1  Introduction

The boom in transmission capacity is driven by new applications for end-users and also market policy of internet service providers. The ultra-fast broadband of 100Mbit/s is foretold to reach 50% coverage of households by 2020 [2] and current 10Gbit channels in core lines may not meet the demand for transmission rate. The most straightforward solution to increase transmission rate four times is to reduce bit interval from 100 to 25 ps. Such a 40Gbit/s OOK system suffers from Chromatic Dispersion (CD) sensitivity worsened by a factor of 16 and PMD by a factor of 4 compared with 10Gbit/s [3]. Another option is the phase modulated signal that utilizes signal phase to represent bits rather then signal amplitude. One of the proposed phase modulations is Differential Phase-Shift Keying (DPSK) that transmits ones and zeroes as ±π/2 shift in signal phase space. Although DPSK is the modulation of phase, the influence of states of optical polarization on transmission performance is not clear in depth, for example through polarization depended attenuation or gain. We therefore proposed to analyze the impact of State of Polarization (SOP) on the transmission system performance and also system performance sensitivity to rapid changes in SOP. This analysis should be very useful for future examination of influence of state of polarization of 100Gbit/s DP-DQPSK transmission which is successor of 40Gbit/s speeds [4].

2  Polarization representation

Generally the polarization of light describes the orientation of light oscillation that is perpendicular to light propagation. It is worth of noting that light does not have to be completely polarized. This fact is described by Degree of Polarization (DOP) expressed in percentage of polarized light. The unpolarized light changes its oscillation orientation during propagation in random manner. The polarized light can be classified into three main SOP: linearly polarized, circularly polarized and elliptically polarized. All SOPs can be depicted at the surface of the Poincaré Sphere (PS) in Figure 1.

Linearly polarized light oscillate just in one plane perpendicular to propagation and can be found at equator of Poincaré Sphere. The plane of oscillation of circularly polarized light rotates with propagation and the envelope creates symmetrical helix. This special type of elliptical polarization is either clockwise (right-hand) or counterclockwise (left-hand) depending at direction of rotation and can be found at poles of Poincaré Sphere. The last and the most common case is the elliptically polarized light that represents all combination of circular and linear polarization. The elliptical polarization covers Poincaré Sphere except its poles and equator.

2.1  Polarization state formation

The light emitted by laser is created by stimulated emission and share the same parameters like direction, phase and also polarization. Lasers usually produce linearly polarized light if we exclude some special types of lasers (i.e. fibre lasers). Moreover a polarizer can be used to alter polarization of light. The linear polarizer can utilize dichroic material that preferably absorb light in a particular direction or grid structure to pass linear polarized light and reflect the rest. An easy way to create circularly polarized light is the use of a quarter-wave plate after the 45-degrees polarizer according to Figure 2.

It is important that the linearly polarized light that enters a quarter-wave plate arrive at 45° to fast and slow axis of quarter-wave plate. When the light leaves the quarter-wave plate then part of the light traveling along the fast axis will have one quarter of wavelength lead and that will result in circular polarization of the outgoing light. We used both linear and circular polarizer in our experiments. New testing options provide Deterministic Polarization Controller DPC5500 from ThorLabs that offers polarization scrambling even hundred times per second. The device is based on a electrically controlled polarization controller followed by a precise polarimeter. The polarimeter utilizes the novel structure with tilted fiber Bragg gratings and fiber quarter-wave plate as depicted in Figure 3. Fast feedback loop and computer control make up the system into a powerful R&D tool.

3  High-speed transmission system

We used a new 40G DPSK transmission system OTS-4000 from OpNext. Module OTS-4011 DPSK provides multiplexing of four 10G data flows into one 40G DPSK signal that fits even the 50GHz wavelength grid and can coexist with current 10G DWDM systems. We decided to run a 10G BER test as one data flow that will be multiplexed in OTS to observe possible transmission errors. OTS user interface also monitors errors in transmission, but does not calculate the BER.

4  40G static polarization test

During the static polarization test the 40G DPSK signal was polarized and sent through several types of transmission lines. The test was set up in the CESNET optical laboratory according to Figure 4.

10G BERT was fed into OpNext 40G DPSK system. The polarization of 40G signal was selected by the dichroic plate polarizer placed in the fibre bench. The signal polarization was set by the polarization controller to maximize the transmitted signal. The transmitted signal power was measured at the 5% tap by the power meter (PM). The polarized 40G DPSK signal went through fibre line and was amplified by EDFA preamplifier before dispersion compensation. The amplified and compensated 40G DPSK signal was demultiplexed in OpNext and 10G flow was fed back to 10G BERT to analyze BER. We tested transmission of 40G DPSK signal over multiple laboratory fibre spools and the real fibre line between CESNET and ÚFE AVČR whose the Optical Time-Domain Refractometry (OTDR) measurement is showed in Figure 5.

Details about tested lines are gathered in Table 1. The table contains types of fibers for each test, where “MC-SMF” represents standard fibres with CD of 16ps/nm/km and “TW-SMF” stands for TrueWave fibre with flattened dispersion of 4.5ps/nm/km. The above mentioned numbers of CD are evaluated for our test wavelength of 1550nm. The “combined” line type is composition of MC and TW types. The “real” line type represents real deployed line between CESNET and ÚFE AVČR. The length of tested lines is marked by L and expressed in kilometers. Next three columns show evaluated CD of tested lines, amount of CD compensation and residual amount of CD at each test line. Last column presents issues that arise during test, where “Loss” stands for the Loss of Frame error of the OTS system probably because of accumulated CD in the link. “OK” represents error-free transmission for all tested SOPs.

5  40G dynamical polarization test

The dynamical polarization test aims to change polarization of 40G DPSK signal at fraction of millisecond scale to see the impact on the transmission performance. Polarization controller and fiber bench was replaced by Deterministic Polarization Controller (DPC) as can be seen in Figure 6.

Test system was again set for all tested lines according to Table 1 and SOPs were changed in two modes. At first in rapid manner, where the time between different SOPs were about 150μs, and then in scanning manner, where DPC changed the polarization to cover whole Poincaré Sphere in about 2 minutes as can be seen in DPC control panel in Figure 7.

The DPSK transmission system worked without error even for the most dynamical polarization changes in all tests with result “OK” in Table 1. Failures represented by “Loss” were caused by insufficient CD compensation.

In the latest test the output of DPC was connected directly back to the OpNext DPSK system to analyze the resilience of receiver (especially delay line interferometer) of the OpNext system on polarization changes. The detection optic showed no errors as a reaction on dynamical polarization changes.

6  Conclusions

We realized the transmission test of the 40G DPSK signal over various fibre lengths in laboratory environment and also real fibre line. Bulk optical components were used to set up the static polarization state of the DPSK signal. Linear and circular polarization states were tested, but we observed no impact of the polarization states on the transmission performance. This implies that neither polarization dependent components nor polarization sensitive DPSK detection was used in the test. Also real line conditions did not raise any polarization dependent issues. The dynamical polarization tests were conducted in two modes. Firstly by fast polarization scrambling, where the change between any two SOPs was about 150μs, and secondly by polarization scanning, where change of SOP was controlled to systematically cover whole Poincaré Sphere. We found no impact of fast polarization changes on transmission performance as well as on detection optics of 40Gbit/s DPSK transmission system. We also verified that DPSK modulation is extremely sensitive to chromatic dispersion and as little as 150ps/nm of accumulated chromatic dispersion can easily break down the transmission, but when properly compensated the system can overcome more then 200 km in single span. Future test of polarization multiplexed modulation formats like the Polarization Multiplexed Differential Quadrature Phase-Shift Keying would be interesting. Also future exploration of transmission limits for 40G DPSK over single span may need precise characterization of chromatic and polarization dispersion.

References

[1]	Circular Polarizer. In Wikipedia, The Free Encyclopedia. [cit. 2010-11-29]. Available online.
[2]	YARDLEY, M. Digital Agenda for Europe: the green light for fibre? Analysis Mason, 4 November 2010 [cit. 2010-11-29]. Available online.
[3]	A comparison of next-generation 40-Gbps technologies. White paper, Nortel, 2008 [cit. 2010-11-29]. Available online.
[4]	LYUBOMIRSKY, I. Quadrature Duobinary for High-Spectral Efficiency 100G Transmission. Journal of Lightwave Technology, 2010, vol. 28, no. 1, p. 91–96.