Tunable modulation format conversion based on spectral line-by-line pulse shaper for all-optical signal processing

This paper describes the demonstration of a simple all-optical data format conversion scheme between return-to-zero (RZ) and nonreturn-to-zero (NRZ) that employs a semiconductor optical amplifier (SOA) in a nonlinear optical loop mirror. The format conversion has been performed between the most widely used data formats-NRZ and RZ formats. The format conversion scheme is based on gain variation by an intensity-dependent phase change in an SOA-loop mirror. The input data stream acts as a control signal that induces the phase differences between clockwise- and counterclockwise-propagating data inside an SOA-loop mirror. It is possible to change the data format of the output data stream by controlling the phase differences of the clockwise and counterclockwise pulse in an SOA-loop mirror appropriately. For the converted NRZ data from RZ data, 10-Gb/s error-free transmission up to 78 km over standard single-mode fiber has been obtained. By comparing the conventional NRZ transmission with the Mach-Zehnder modulation scheme, the proposed RZ-to-NRZ conversion shows an improved transmission performance. The NRZ-to-RZ conversion has clear eye openings up to 78 km. On the contrary, the conventional RZ binary data from a mode-locked laser has a nearly closed eye even at 52 km. The converted RZ data has a 2-dB conversion power margin to the injected NRZ data, which indicates an increase in the receiver sensitivity due to the signal format conversion. The improved transmission distance of the converted RZ signal is due to the duobinary coding effect of the SOA-loop mirror. The SOA has the possibility of high-speed operation over 40 Gb/s, and the SOA-loop mirror has the capabilities of format and wavelength conversions. Therefore, the SOA-loop mirror can be a universal building block in future all-optical networks. In addition, the proposed format conversion scheme can serve as an important format converter between the ultrafast optical-time-division-multiplexed networks and the lower line-rate wavelength-division-multiplexed networks.

Choice of modulation format plays a critical role in the design and performance of fiber optic communication systems. We discuss the basic physics of electro-optic phase and amplitude modulation and derive model transfer functions for ideal and non-ideal Mach-Zehnder modulators. We describe the generation and characteristics of the standard nonreturn-to-zero (NRZ) modulation format, as well as advanced formats such as return-to-zero (RZ), carrier-suppressed RZ (CSRZ), duobinary, modified duobinary, differential phase-shift keyed (DPSK), and return-to-zero DPSK (RZ-DPSK). Finally, we discuss the relative merits of these formats with respect to a variety of system impairments.KeywordsFiber opticsModulation formatsOptical communication systemModeling

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.

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.

We report a novel phase-erased demodulation of differential phase-shift keying (DPSK) by exploiting cascaded second-harmonic generation and difference-frequency generation (cSHG/DFG) in a periodically poled lithium niobate (PPLN) waveguide. Analytical solutions are derived to clearly describe the operation principle. The binary optical phase information carried by the conventional DPSK demodulation outputs is removed thanks to the cSHG/DFG in a PPLN waveguide. PPLN-assisted phase-erased wavelength conversion and demodulation of 40 Gbit/s non-return-to-zero DPSK (NRZ-DPSK), return-to-zero DPSK (RZ-DPSK), and carrier-suppressed return-to-zero DPSK (CSRZ-DPSK) are demonstrated in the experiment. Moreover, the accompanying all-optical format conversions from optical duobinary (ODB) to NRZ and from ODB/alternate-mark inversion (AMI) to RZ are also substantiated in the experiment. In addition, the calculated theoretical results including optical spectra, temporal waveforms, and phase diagrams also confirm the successful implementation of PPLN-assisted 40 Gbit/s NRZ-DPSK/RZ-DPSK/CSRZ-DPSK phase-erased wavelength conversion, demodulation, and ODB-to-NRZ and ODB/AMI-to-RZ format conversions.

Investigation on wavelength re-modulated bi-directional passive optical network for different modulation formats

The present paper proposes the optimal transmission parameters for non-return-to-zero (NRZ), return-to-zero differential phase shift keying (RZ-DPSK), carrier-suppressed RZ-DPSK (CSRZ-DPSK) and duobinary (DB) modulation formats in 16-channel 10 Gbit/s wavelength division multiplexed passive optical network (WDM-PON) transmission system. The paper investigates influence of laser output power, length of dispersion compensating fiber (DCF) and application of erbium-doped fiber amplifiers (EDFA), as well as impact of optical components configuration on received signal quality. It compares demand of these modulation formats for amplification and dispersion compensation with the increase of transmission distance, determining the optimal conditions for achieving maximal network reach for each of them. It is shown, that DB is the most resistant to chromatic dispersion (CD) and is able to achieve the longest transmission distance (100 km) without dispersion compensation. Whereas RZ-DPSK and CSRZ-DPSK modulation formats demonstrated maximal achievable reach (180 km) using DCF and EDFA.

We propose logic XOR gate and format conversion for carrier-suppressed return-to-zero differential phase-shift keying (CSRZ-DPSK) signals using non-degenerate four-wave mixing (FWM) in a highly nonlinear fiber (HNLF). Due to the linear relationship of the complex amplitudes between output and input optical waves, the three generated idler waves by three non-degenerate FWM processes take the return-to-zero differential phase-shift keying (RZ-DPSK) format and the logic XOR result of three input CSRZ-DPSK signals. All-optical 40 Gbit/s multicasting CSRZ-DPSK logic XOR operation and CSRZ-DPSK to RZ-DPSK format conversion are simultaneously demonstrated in the experiment.

Delay-interferometer based timing alignment monitoring for data and clock signal in optical RZ-DPSK and RZ-OOK systems

Receiver Modeling for Optically Amplified Communication Systems

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

This paper reviews technologies and techniques that have been used in deployed long-haul wavelength division multiplexing (WDM) systems and emerging technologies that could be used for the next generation of cost-reduced systems. The overview of current generation technologies starts with a discussion of modulation formats, focusing on the superior properties of the chirped return to zero on-off shift keying (CRZ-OOK) modulation format. The use of 10-Gb/s CRZ-OOK modulation format together with advanced fiber types, more powerful forward error correction (FEC), and broadband erbium-doped fiber amplifiers resulted in the deployment of dense WDM systems with capacities per fiber in terabits per second range and trans-Pacific reach. Demand for the systems with large design capacity led to further development of broadband optical amplifiers. Laboratory demonstrations successfully expanded transmission into the full C-band and later in the C and L transmission bands. The current market conditions dictate the need for reducing the first cost of an installed system rather than reaching record capacity per fiber. Reducing first cost can be achieved by reducing the amount of optical amplifiers in the cable by utilizing an excess performance margin provided by available RZ-OOK technology. Further improvements would be possible if more powerful FEC and modulation formats with better receiver sensitivity are used. For example, the RZ differential phase-shift keying (RZ-DPSK) modulation format with 3-dB better receiver sensitivity and better nonlinear tolerance to large amounts of accumulated dispersion is a very promising technology. This paper will review long-haul transmission results using RZ-DPSK and will compare the transmission properties of RZ-DPSK signals versus RZ-OOK signals. Due to superior receiver sensitivity, the RZ-DPSK modulation format can be an enabling technology for 40-Gb/s per channel transoceanic transmission

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

In this article optical minimum-shift keying (MSK) modulation format in 9×40 Gbit/s wavelength-division multiplexing (WDM) spacing of 100 GHz transmission systems is investigated and compared with return-to-zero differential phase-shift keying (RZ-DPSK) and return-to-zero (RZ). The performance of optical MSK in presence of amplification noise, chromatic dispersion and nonlinear effects is analytically determined. Operations over a wide range of parameters, such as span input power, precompensation value, and chromatic dispersion parameter are performed; experimental results show that optical MSK demonstrates special performance in transmission distance, nonlinear effects, and dispersion tolerance.