RZ Signals Research Articles

Direct UV written Michelson interferometer for RZ signal generation using phase-to-intensity modulation conversion

An integrated Michelson delay interferometer structure making use of waveguide gratings as reflective elements is proposed and fabricated by direct ultraviolet writing. Successful return-to-zero alternate-mark-inversion signal generation using phase-to-intensity modulation conversion is demonstrated up to 40 Gb/s using the device. Compared to other implementations, the device is compact, inherently stable, and allows for easy customization of the pulsewidth by proper positioning of the gratings on a single coupler structure.

On the transition to pseudo-linear regime in dispersion managed systems with NRZ, RZ and duobinary signal formats

The transition to the pseudo- or quasi-linear regime in single channel transmission for non-return-to-zero, return-to-zero and duobinary signal formats is investigated. An expression for the transition bit rate to such regime, valid for the three different formats, is proposed. Several duty cycles and pulse shapes are assessed in the case of the return-to-zero format. Dependencies of the transition bit rate on fibre parameters are presented in an explicit form and confirmed using extensive simulation for standard singlemode fibre and nonzero dispersion shifted fibre. Using the obtained expression, the origin of the main transmission impairments in single channel transmission can be assessed and conclusions about the optimum post-compensating dispersion managed scheme can be drawn.

Performance Study of 40-Gb/s RZ Signals Through Cascaded Thin-Film Filters with Large Dispersion Slope

The performance of 40-Gb/s RZ signals through cascaded thinfilm filters is investigated by both numerical and analytical means. It is observed that the filtering effects can reduce the eye closure penalties caused by the large dispersion slope of the thin-film filters. In addition, the performance can be further improved by proper frequency detuning between the signal and the center of the filter. The combined effects of dispersion slope and filtering on 40-Gb/s signals are investigated analytically and explained for typical bit patterns.

Simplified optoelectronic 3R regenerator using nonlinear electro-optical transformation in an electroabsorption modulator

A simplified optoelectronic 3R regenerator without electrical signal processing is demonstrated by utilizing the nonlinear electro-optical transfer function of an electroabsorption modulator. 3R regeneration of impaired 10-Gb/s RZ signals is demonstrated, verifying the proposed concept.

Planar lightwave circuit dispersion compensator using a compact arrowhead arrayed-waveguide grating

An ultra-compact chromatic dispersion compensator, based on a compact arrowhead arrayed-waveguide grating (AWG), has been proposed and fabricated. A dispersion compensating mirror, monolithically integrated into the second slab of the AWG, modulates the phase of each spectral component of the input light. The use of the compact arrowhead structure provides a dispersion compensator with a small footprint. The chromatic dispersion, the bandwidth and the insertion loss of the dispersion compensator based on the 8ch, 12.5GHz-spacing high-resolution arrowhead AWG are 123ps/nm, 70GHz and 7.5dB, respectively. Dispersion compensation experiments for 40Gbit/s, NRZ and RZ signals have been successfully demonstrated.

A Study on 10-GHz Clock Signal Extraction from 160-Gbit/s RZ Optical Signals

A 10-GHz clock signal is successfully extracted from a 160-Gbit/s optical RZ signal using devices operable up to 40 GHz. The down-conversion of the 160-Gbit/s input signal into 40 Gbit/s signal is performed by modulating the input optical signal by an LN modulator that is driven by a RF signal with an amplitude of 3Vπ and a frequency of 40 GHz. Extraction of 10-GHz clock from the down-converted signal is done by an injection locking technique. An electrical PLL circuit is also examined for a reference. Regarding the recovered clock, basic properties of hold-in frequency range, frequency response, and phase jitter are found to be satisfactory. The faultlessness of the extracted clock signal was confirmed by bit error rate measurements.

40 Gbit/s signal format conversion from NRZ to RZ using a Mach-Zehnder delay interferometer

40Gbit/s signal format conversion from NRZ to RZ using a Mach-Zehnder delay interferometer has been demonstrated for the first time. The converted RZ signal has high receiver sensitivity and shows significantly improved transmission performance.

Optimum dispersion map for Raman amplified 160 km NZDSF+DCF fibre spans with 40 Gbit/s RZ signals

Pre-, post- and symmetrical dispersion compensating schemes for 160 km NZDSF+DCF fibre spans are experimentally compared for a 40 Gbit/s optical transmission system using combined EDFA and Raman amplification. Good performance for this 40 Gbit/s system is demonstrated, and it is shown that the symmetrical compensation scheme results in the best performance.

10 Gbit/s repeaterless transmission over 265 km SMF-28: using a modified duo-binary RZ signal generated by one dual-drive LiNbO 3 modulator

We will experimentally demonstrate 10 Gbit/s repeaterless transmission over a record length of standard fiber (SMF-28) using a modified duo-binary RZ signal generated by one dual-drive LiNbO 3 modulator. Our results show that the MD-RZ signal can tolerate a certain dispersion tolerance range.

Generation of modified duobinary RZ signals by using one single dual-arm LiNbO3 modulator

We will demonstrate a simple method to generate modified duobinary return-to-zero (MD-RZ) signals by using only one single dual-arm Mach-Zehnder LiNbO/sub 3/ modulator (LN-MOD). Different duty cycle signals can be generated only by changing the time delay between two-arm electrical signals added on the LN-MOD. MD-RZ signals show high receiver sensitivity relative to duobinary nonreturn-to-zero signals.

40-Gb/s RZ signal transmission in a transparent network based on wavelength-selective optical cross connect

We investigate 40-Gb/s return-to-zero signal transmission in a transparent optical network equipped with wavelength-selective optical cross connects (WSOXCs). The investigation is performed in a recirculating loop setup consisting of 4100-km spans of transmission fiber. A WSOXC is placed inside the loop to mimic cascaded WSOXCs with a spacing of 400 km. We study various degradation effects in the system, particularly those of node loss and optical filtering. Q value measurements of the received signal were carried out at different transmission distances and WSOXC loss conditions. A Q value of 15.5 dB was measured at 1200-km distance with two cascaded WSOXCs of which the node loss is 17 dB.

All-optical RZ to NRZ data format and wavelength conversion using an injection locked laser

All-optical RZ-to-NRZ data format conversion using a repetition-rate doubler and an injection-locked Fabry–Perot laser diode is demonstrated at 10 Gb/s. Conversion of signal wavelength is achieved simutaneouly. At a bit-error-rate of 10 −9, a power penalty of 2.5 dB was observed in the format conversion. The spectral width of the data-format converted NRZ signal (14 GHz) was reduced compared with the input RZ signal (25 GHz), making the converted signal more spectrally efficient for use in dense-wavelength-division-multiplexed (DWDM) access networks. We also describe numerical analysis which indicates the optimum delay in the repetition-rate doubler for format conversion.

40 Gbit/s×25 WDM 306 km unrepeatered transmission using 175 µm2-Aeff fibre

The authors have successfully demonstrated 40 Gbit/s-based terabit-capacity unrepeatered transmission over 306 km. They transmitted 100 GHz-spaced 25×40 Gbit/s RZ signals with Q-factors all better than 15.6 dB using a hybrid transmission line configuration of 175 µm2-Aeff (effective core area) fibres and NZ-DSF and using a remote EDFA that requires no dedicated pump light transmission fibre. The attained transmission distance was over 306 km for 40 Gbit/s-based unrepeatered transmission.

Regeneration capabilities of passive saturable absorber-based optical 2R in 20 Gbit/s RZ DWDM long-haul transmissions

A semiconductor saturable absorber-based optical 2R regenerator is implemented in a 20 Gbit/s WDM-emulated dispersion-managed loop transmission with ‘standard’ 50% RZ signals. When reducing the channel spacing from 200 down to 50 GHz, the optical 2R regenerator is seen to ensure Q-factors at 9000 km distance of 14.5 and 11.2 dB, respectively.

Compact-spectrum RZ signal generated by single-stage push–pull type Mach-Zehnder modulator for DWDM transmission

A novel compact-spectrum RZ signal is proposed; its bandwidth is approximately twice the bit rate. It is generated by a single-stage Mach-Zehnder modulator with 2:1 multiplexing function. 50 GHz-spaced 8×20 Gbit/s DWDM transmission is realised.

Low frequency band monitoring method for PMD compensation

A signal monitoring method using a low frequency band is proposed for polarisation mode dispersion (PMD) compensators. By introducing the low frequency band monitoring, a wide range of PMD including differential group delay (DGD) equivalent to a bit period was successfully compensated for the 40 Gbit/s RZ signals.

Fiber transmission systems with semiconductor optical amplifiers without in-line filtering

10 Gbit/s RZ signal transmission at 1.3 μm in a fiber transmission system with in-line semiconductor optical amplifiers is investigated using a recirculating fiber loop setup. We found that under certain conditions the system operates without in-line filtering and with strongly saturated amplifiers, provided the signal is extracted from the amplified spontaneous emission by means of a narrow optical filter in front of the receiver. These results are compared with a transmission scheme with in-line filtering.

40 Gbit/s × 25 WDM 306 km unrepeatered transmissionusing 175 µm 2 -Aeff fibre

40 Gbit/s-based terabit-capacity unrepeatered transmission over 306 km has been demonstrated for the first time. 100 GHz-spaced 25 × 40 Gbit/s RZ signals were transmitted using 175 µm2-Aeff fibre with Q-factors all better than 15.6 dB without using a dedicated pump light transmission fibre for a remote-EDFA.

Fibre grating laser-based RZ pulse source for 40Gbit/s OTDMtransmission systems

The performance of an actively modelocked fibre grating laser (FGL) used to generate near-transform-limited pulses (/spl Delta/v/spl Delta//spl tau/=0.5-0.65) with FWHM of 12-18 ps at 10 GHz repetition rate is described. 40 Gbit/s OTDM experiments using the modelocked FGL as a pulse source yielded less than 1 dB power penalty with respect to a 10 Gbit/s RZ signal.

50 GHz spaced 40 Gbit/s × 25WDM transmissionover 480 kmusing bandlimited RZ signals

50 GHz spaced 40 Gbit/s × 25WDM transmission over 480 km has been successfully demonstrated using bandlimited RZ signals and an SMF-based dispersion-flattened transmission line. What the authors believe is the longest distance transmission for a spectral efficiency of 0.8 bit/s/Hz was achieved without using polarisation demultiplexing.