D. Sandel

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.

5.94-Tb/s 1.49-b/s/Hz (40/spl times/2/spl times/2/spl times/40 Gb/s) RZ-DQPSK polarization-division multiplex C-band transmission over 324 km

The combination of return-to-zero differential quadrature phase-shift keying with polarization-division multiplex, a 16-ary modulation scheme, allows for ultimate spectral efficiency. We raise C-band fiber capacity with phase-shift keying transmission beyond previously reported figures, achieving forward-error correction limit performance over four fiber spans.

Coherent Digital Polarization Diversity Receiver for Real-Time Polarization-Multiplexed QPSK Transmission at 2.8 Gb/s

This letter presents a coherent digital polarization diversity receiver for real-time polarization-multiplexed synchronous quadrature phase-shift keying transmission with distributed feedback lasers at a data rate of 2.8 Gb/s. The tolerance against fast polarization changes and polarization-dependent loss is evaluated for different filter widths in the carrier recovery circuit. The minimum achieved bit-error rate is 3.4 times 10-7.

Crosstalk detection schemes for polarization division multiplex transmission

Polarization division multiplex (PolDM) is a bandwidth-efficient and sensitive modulation format suitable for upgrading bandwidth-limited trunk lines. We show how control signals for polarization demultiplex can be obtained efficiently. For interleaved return-to-zero (RZ) signals, incoherent crosstalk has to be detected and minimized. In other cases, in particular for non-return-to-zero (NRZ) signals, coherent crosstalk senses penalties much better and should be detected instead. NRZ transmission experiments with either scheme are presented at a data rate of 2/spl times/10 Gb/s, with endless polarization tracking. Polarization mode dispersion (PMD) tolerance is also assessed.

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.

1.6-b/s/Hz 160-Gb/s 230-km RZ-DQPSK polarization multiplex transmission with tunable dispersion compensation

A 160-Gb/s (4/spl times/40 Gb/s) return-to-zero differential quadrature phase-shift keying polarization-division multiplex transmission is demonstrated with Q>15.6 dB in one of eight 100-GHz-spaced wavelength-division-multiplex channels after 230 km of fiber. Residual chromatic dispersion (CD) is equalized by a thermally tunable CD compensator for the 192.5-THz channel. Polarizations, in-phase and quadrature data channels are demultiplexed using a LiNbO/sub 3/-based automatic polarization control and a 1-bit interferometer, respectively.

Novel nonmagnetic 30-dB traveling-wave single-sideband optical isolator integrated in III/V material

Various attempts have been made to fabricate waveguide-type isolators in III/V material by implanting magnetic materials, but none of them has so far resulted in a commercial product. Here, we report for first time on an integrated optical isolator implemented in III/V material. It consists of a single-sideband electrooptic modulator where traveling electrical waves make the transmission direction-dependent. Isolation is 30 dB, excess insertion loss is 8 dB. Residual rms ripple is 7% for peak-to-peak RF driving amplitudes of 3.5 V at 4.0 GHz. The estimated transmission penalty for 40 Gb/s return-to-zero differential phase shift keying (RZ-DPSK) signals is 0.2 dB (0 dB measured).

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.

Optical network analysis and longitudinal structure characterization of fiber Bragg grating

A method for polarization-resolved optical fiber Bragg grating (FBG) characterization is reported on. The complete reflectance Jones matrix is measured interferometrically. Required polarization transformers need not be accurate, just to operate reproducibly, because redundant measurements yield pairs of orthogonal polarizations. Local dichroic reflectivity and birefringence of a sampled grating was derived from this data. Knowledge of these quantities should allow improvement of the ultraviolet (UV) illumination process and to effectively correct phase mask errors by longitudinally selective UV light postprocessing.

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.