Vitali Mirvoda

PMD in high-bit-rate transmission and means for its mitigation

Polarization-mode dispersion (PMD) prevents the cost-effective upgrading of fiber networks to 40 and sometimes even to 10 Gbit/s. This paper reviews recent progress in its mitigation and compensation and points out where more research is needed. Electronic PMD mitigation is preferable at 10 Gbit/s, due to its low cost, even though it is accompanied by a considerable residual penalty. A lot of work takes place in the field of optical PMD compensation. Among the numerous detection methods for first-order PMD, we prefer a purely electronic, hence low-cost, arrival time detection method, with a linear readout and ps-sensitivity. Surprisingly, the most easily detectable higher order of PMD is the third order, indicated by a slope steepness difference. Both methods rely on a polarization scrambler at the transmitter side, which can be shared. Regarding PMD compensators, LiNbO/sub 3/ devices are probably needed to guarantee a sufficient speed. A distributed PMD compensator allows to integrate a number of polarization transformers and differential group delay sections on one chip, thereby exactly emulating the way how the fiber accumulates, but in reverse order and orientation. We report on progress in using these devices, including their use for PMD compensation in a 40-Gbit/s carrier-suppressed return-to-zero differential phase-shift keying experiment. More work is needed to perfection the device, and to implement a fast endless polarization control. The theory of the distributed PMD compensator lends itself to a new definition of higher order PMD by a Fourier expansion of mode conversion, as an alternative to the familiar Taylor expansion of the PMD vector.

Polarization mode dispersion detected by arrival time measurement of polarization-scrambled light

Polarization mode dispersion (PMD) limits optical fiber capacity. PMD compensators usually minimize the associated eye closure. This measure scales with the square of the differential group delay (DGD) and makes it difficult to detect low DGDs. However, light with a low-speed polarization modulation suffers arrival time variations, in the presence of PMD, that are proportional to the DGD. These are detected by integrating the voltage-controlled oscillator (VCO) input signal of the clock recovery phase-locked loop (PLL). This novel method has been demonstrated for 40 Gb/s nonreturn-to-zero (NRZ) and for 2/spl times/40 Gb/s return-to-zero (RZ) polarization division multiplex transmission. PMD detection sensitivities range between 2 ps and 84 fs.

Optical Endless Polarization Stabilization at 9 krad/s With FPGA-Based Controller

We demonstrate endless polarization stabilization with a control speed of up to 9 krad/s, over random Poincare sphere trajectories. These are in total >35 Mrad long, are composed in particular of difficult-to-track circles with all radii and orientations, and thereby include all possible worst cases. The maximum polarization mismatch and relative intensity errors are 0.13 rad and 0.43%, respectively. The controller runs on a field-programmable gate array and uses a commercial multistage LiNbO3 polarization transformer as the control device.

Record 59-krad/s Polarization Tracking in 112-Gb/s 640-km PDM-RZ-DQPSK Transmission

We present results obtained with a fast optical polarization controller. Tracking errors were <; 0.208 rad on the Poincaré sphere during 282 hours of endless polarization scrambling at up to 60 krad/s speed. Fifty-kilorad/second polarization changes were tracked at wavelengths between 1513 nm and 1600 nm. With this subsystem we optically demultiplexed the two polarizations of a 112-Gb/s PDM-RZ-DQPSK signal transmitted over 640 km of fiber at 59 krad/s. This is the fastest polarization control reported for this or higher bitrate and direct detection.

40-krad/s Polarization Tracking in 200-Gb/s PDM-RZ-DQPSK Transmission Over 430 km

We present real-time transmission of 50-Gbaud polarization-division-multiplexed (PDM) return-to-zero differential quadrature phase-shift keying (4 bit/symbol) over five fiber spans, 430 km in total. The two PDM channels are demultiplexed using a beam splitter and automatic optical polarization control with interference detection. For the first time, PDM signal tracking speeds of up to 40 krad/s are demonstrated.

Standard (NRZ 1 x 40 Gb/s, 210 km) and polarization multiplex (CS-RZ, 2 x 40 Gb/s, 212 km) transmissions with PMD compensation

A single-waveplate polarization scrambler at the transmitter (TX) generates pulse arrival time fluctuations in the presence of polarization-mode dispersion. These were detected with a 680-fs sensitivity in the receiver clock recovery phase-locked loop, thereby enabling a polarization-mode dispersion (PMD) compensated 210-km nonreturn-to-zero transmission. For polarization division multiplex, polarizations were scrambled by an interchannel phase modulation which enabled PMD-compensated CS-RZ 212-km transmission.

Polarization mode dispersion compensation at 20 Gb/s with a compact distributed equalizer in LiNbO/sub 3/

PMD of existing fibers impairs transmission at rates greater than 10 Gb/s. We present a 43 ps PMD compensator in X-cut, /spl gamma/-propagation LiNbO/sub 3/ with cascaded TE-TM converters and demonstrate successful operation at 20 Gb/s.

High-speed endless optical polarization stabilization using calibrated waveplates and field-programmable gate array-based digital controller.

A truly endless polarization stabilization experiment with a tracking speed of 15 krad/s is presented. The high-speed polarization tracking is realized by using calibrated lithium niobate linear retardation waveplates as the polarization transformers combined with a very fast digital controller running on a field-programmable gate array (FPGA).

38-krad/s 3.8-Grad Broadband Endless Optical Polarization Tracking Using LiNbO

We demonstrate automatic endless optical polarization tracking over 3.8 Grad at up to 38-krad/s control speed with mean/maximum polarization errors of 0.068/0.185 rad. Without polarization fluctuations, mean/maximum polarization errors are 0.05/0.1 rad. Small-signal control time constant is about 2 mus. Function is maintained over the wavelength range 1505-1570 nm.

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Following an initial discussion of control error signal generation, we present new developments and applications of automatic endless optical polarization control based on a commercial electrooptic LiNbO(3) polarization transformer: (i) Fast tracking and subsequent demultiplexing of DPSK/DQPSK/QAM polarization channels was hitherto limited to a fairly fixed optical input power. With APD photoreceivers used for residual interference detection, we demonstrate here an optical level tolerance of at least 7 dB, compared to only 3 dB for PIN photoreceivers. DPSK channel polarizations are tracked at up to 40 krad/s and higher speed on the Poincaré sphere. (ii) High-order optical modulation schemes require increased accuracy of the polarization controller in the demultiplexer. This is possible at the expense of a reduced tracking speed. We achieve a mean polarization extinction ratio of >40 dB or <0.02 rad error while tracking arbitrary endless polarization changes of up to 1000 rad/s. (iii) While electronic polarization tracking in coherent receivers is currently limited to a symbol rate of about 28 GBaud we show optical polarization tracking of a signal with 1 THz bandwidth.