Experimental demonstration of 10-gb/s data format conversions between NRZ and RZ using SOA-loop-mirror

We have demonstrated an extinction ratio improvement of more than 3 dB at 2.5 Gb/s with a method based on four-wave mixing in a semiconductor optical amplifier. A wavelength conversion and a data format conversion from nonreturn-to-zero (NRZ) to return-to-zero (RZ) have been simultaneously achieved.

Investigation of data format conversions based on MZI-SOA

We investigate return-to-zero (RZ) to non-return-to-zero (NRZ) format conversion by means of the linear time-invariant system theory. It is shown that the problem of converting random RZ stream to NRZ stream can be reduced to constructing an appropriate transfer function for the linear filter. This approach is then used to propose novel optimally-designed single fiber Bragg grating (FBG) filter scheme for RZ-OOK/DPSK/DQPSK to NRZ-OOK/DPSK/DQPSK format conversion. The spectral response of the FBG is designed according to the optical spectra of the algebraic difference between isolated NRZ and RZ pulses, and the filter order is optimized for the maximum Q-factor of the output NRZ signals. Experimental results as well as simulations show that such an optimally-designed FBG can successfully perform RZ-OOK/DPSK/DQPSK to NRZ-OOK/DPSK/DQPSK format conversion.

All-optical non-return-to-zero (NRZ) -to- return-to-zero (RZ) data-format conversion has been successfully demonstrated using a semiconductor optical amplifier in a fiber-loop mirror (so-called SOA-loop mirror) with a continuous-wave (CW) holding beam. The converted RZ signal after pulse compression has been used to create a 40 Gb/s OTDM (Optical Time Division Multiplexing) signal. Here is proposed an NRZ-to-RZ conversion method without any additional optical clocks, unlike conventional methods based on optical AND logic. In addition, it has the merit of operating at various bit-rate speeds without any controlling device. Moreover, it has a simple structure, and it can be used for all-optical bit-rate-flexible clock recovery.

A tunable modulation format converter based on spectral line-by-line pulse shaper is proposed to realize different format conversions. The pulse shaper works as a format converter by setting its frequency response equivalent to the transform function between two formats. The working principles show that the format converter is suitable for different formats by adjusting its frequency response. Examples of format conversion from return to zero differential phase-shift keying (RZ-DPSK) to on-off keying (OOK) with different data packets and from return to zero (RZ) to non-return to zero (NRZ) are given. The results show that the format converter is not only suitable for different formats but also for random data packets.

Under an optical nonreturn-to-zero (NRZ) data injection at 10 Gbit/s, the 10-GHz mode-locking and pulsed return-to-zero (RZ) clock extraction from a semiconductor optical amplifier (SOA) based fiber ring is investigated in this paper. The diagnoses on gain and intracavity-power-controlled anomalous blueshifted spectrum and subpicosecond timing jitter are demonstrated. By increasing the injecting power of the optical NRZ data from -3 to 8 dBm into the SOA bias at different currents, the mode locking is completed with a dc level greatly decreasing from 480 to 50 ¿W (only 1.5% of the mode-locked pulse power at 3 mW), corresponding to a pulse/dc amplitude contrast ratio up to 18 dB. Increasing the SOA bias current up to 350 mA significantly suppresses the timing jitter from 1.8 ps to 345 fs, and the extracted RZ clock pulse is shortened from 55 to 27 ps. The pulsewidth of the amplified SOAFL is compressed from 11 ps to 836 fs after dispersion compensation. At constant data injection level, the increasing SOA bias or gain oppositely redshifts the mode-locked SOA fiber ring laser (SOAFL) spectrum by 5 nm. The amplifier spontaneous emission of SOA at short wavelength region ( ~ 1520 nm) is eliminated with increasing NRZ data power, whereas the mode-locking gain peak arises and blueshifts from 1558 to 1552 nm due to the band-filling effect. Such a blueshift in mode-locking spectrum becomes more significant in SOA at lower bias (or gain) condition. A theoretical model interprets the correlation between the nonlinear gain suppression-induced variation of electron-hole plasma in SOA and the blueshifted mode-locking SOAFL spectrum, which is occurred when the gain saturation condition for the SOA becomes more pronounced.

Simultaneous all-optical multi-channel RZ and CSRZ to NRZ format conversion

We propose a novel all-optical format conversion from nonreturn-to-zero (NRZ) to return-to-zero (RZ) based on single semiconductor optical amplifier (SOA) and a detuning optical bandpass filter (OBF). By adopting the ultrafast SOA model associated with intraband mechanism, the 40Gb/s NRZ-to-RZ format conversion is successfully demonstrated with numerical simulation. We investigate the influence of the filter detuning, extinction ratio, conversion efficiency, output pulsewidth, and filter bandwidth. These simulation results confirm the feasibility of our scheme. The proposed scheme is robust and potential for applications in future optical networks.

Based on the Sagnac interferometric structure, a simple novel ultrafast scheme of an all-optical nonreturn-to-zero (NRZ) to return-to-zero (RZ) is proposed. The operations of this scheme at 40 Gbit/s 2 7 -1 PRBS sequences are simulated correctly with an output extinction ratio of more than 19.1 dB. Through a built theoretical model and numerical analysis, the operating characteristics of the scheme are illustrated. Furthermore, the carrier recovery time of the semiconductor optical amplifier (SOA) is no longer a crucial parameter to restrict the operation speed of this scheme. This scheme is potentially capable of all-optical NRZ-to-RZ format converter operation speeds to 80 Gbit/s thus far.

Wavelength conversion using a nonlinear optical loop mirror (NOLM) is investigated. Our interest is in considering nonreturn-to-zero (NRZ) and return-to-zero (RZ) signals with a high-duty ratio resulting in a nonlinear phase shift of the counterpropagating wave that is not negligible and reduces the extinction ratio considerably compared to signals with a low-duty ratio. It is shown how the NOLM can be used for high-duty signals anyhow by configuring/adjusting the NOLM, particularly the polarization controller inserted in the loop. This paper gives a mathematical description of the NOLM utilizing the Jones calculus and considers different ways of adjusting the polarization controller. The nonlinear phase shifts are calculated and a reasonable confinement for the walk-off is given. Furthermore, the probability density function of the extinction ratio depending on the duty ratio of the control signal and the configuration of the NOLM for either parallel or orthogonal polarizations of the counterpropagating signal waves is derived and evaluated for some exemplary parameters.

Theoretical study of all-optical RZ to NRZ format conversion based on cascaded nanoporous silicon waveguides

Polarization-multiplexing quadrature phase shift keying (PM-QPSK) has become a key ingredient in the design of 100 and 200-Gb/s dense wavelength-division multiplexed (DWDM) networks. The performance of the system using these formats varies according to the shape of the pulses employed by the optical carrier: non-return to zero (NRZ), return to zero (RZ) or carrier-suppressed return to zero (CSRZ). In this paper, we analyse the tolerance of PM-QPSK to linear and nonlinear optical impairments, including amplified spontaneous emission (ASE) noise, crosstalk, distortion by optical filtering, chromatic dispersion (CD), polarization mode dispersion (PMD) and fibre Kerr nonlinearities. The simulation results show that RZ formats with a low-duty cycle have a higher tolerance to CD, PMD and intrachannel nonlinearities due to the reduction of pulse-to-pulse interaction. Conversely, the PM–NRZ-QPSK format has a higher spectral efficiency (SE) than other options and shows better tolerance to filtering distortion and CD in filterless networks.

In this work we study all-optical multi-channel return-to-zero (RZ)–on-off keying (OOK) to nonreturn-to-zero (NRZ)–OOK format conversion in single uniform fiber Bragg grating (FBG) for mixed line-rate dense wavelength-division multiplexing systems using mathematical simulations. Forty and 20 Gbit/s RZ–OOK signals with 33% and 50% duty cycles are converted to NRZ–OOK signals in single uniform FBG with 21% reflectivity. Impact of amplitude noise from FBG contrast profile on modulation format conversion efficiency is also studied.

Simultaneous multiple channel all-optical NRZ to CSRZ and RZ to CSRZ format conversion using an SOA-NOLM

We have developed two modulator driver ICs that are based on the functional distributed circuit (FDC) topology for over 40-Gb/s optical transmission systems using InP HBT technology. The FDC topology enables both a wide bandwidth amplifier and high-speed digital functions. The none-return-to-zero (NRZ) driver IC, which is integrated with a D-type flip-flop, exhibits 2.6-Vp-p (differential output: 5.2Vp-p) output-voltage swings with a high signal quality at 43 and 50Gb/s. The return-to-zero (RZ) driver IC, which is integrated with a NRZ to RZ converter, produces 2.4-Vp-p (differential output: 4.8Vp-p) output-voltage swings and excellent eye openings at 43 and 50Gb/s. Furthermore, we conducted electro-optical modulation experiments using the developed modulator driver ICs and a dual drive LiNbO3 Mach-Zehnder modulator. We were able to obtain NRZ and RZ clear optical eye openings with low jitters and sufficient extinction ratios of more than 12dB, at 43 and 50Gb/s. These results indicate that the FDC has the potential to achieve a large output voltage and create high-speed functional ICs for over-40-Gb/s transmission systems.