André Albuquerque

Pump-linewidth-tolerant optical wavelength conversion for high-order QAM signals using coherent pumps

Optical wavelength conversion (OWC) is expected to be a desirable function in future optical transparent networks. Since high-order quadrature amplitude modulation (QAM) is more sensitive to the phase noise, in the OWC of high-order QAM signals, it is crucial to suppress the extra noise introduced in the OWC subsystem, especially for the scenario with multiple cascaded OWCs. Here, we propose and experimentally demonstrate a pump-linewidth-tolerant OWC scheme suitable for high-order QAM signals using coherent two-tone pumps. Using 3.5-MHz-linewidth distributed feedback (DFB) lasers as pump sources, our scheme enables wavelength conversion of both 16QAM and 64QAM signals with negligible power penalty, in a periodically-poled Lithium Niobate (PPLN) waveguide based OWC. We also demonstrate the performance of pump phase noise cancellation, showing that such coherent two-tone pump schemes can eliminate the need for ultra-narrow linewidth pump lasers and enable practical implementation of low-cost OWC in future dynamic optical networks.

Numerical Comparison of WDM Interchannel Crosstalk in FOPA- and PPLN-Based PSAs

We propose a model and numerical simulations to calculate crosstalk (XT) between multiple wavelength-division-multiplexing (WDM) channels in copier + phase sensitive amplifier (PSA) chains based on periodically poled lithium niobate (PPLN). The calculated XT is compared with XT generated in copier + PSA chains based on highly nonlinear fiber (HNLF). For ordinary device parameters, the results show that for an increasing channel number, the growth of interchannel XT in PPLN is less rapid compared with HNLF, as a consequence of the finite quasi-phase matching band of PPLN. This ensures lower XT in PPLN-based chains for 18 channels, whereas HNLF chains show better XT performance for 4 channels.

Phase-sensitive amplification in a single bi-directional PPLN waveguide.

We investigate phase-sensitive amplification (PSA) and phase regeneration of a binary phase-shift keying (BPSK) signal using a single periodically poled lithium niobate (PPLN) waveguide. The PPLN is operated bi-directionally in order to simultaneously achieve phase correlated signals and phase-sensitive (PS) operation. We use injection-locking for carrier phase recovery and a lead zirconate titanate (PZT) fiber stretcher to correct path length deviations in the in-line phase regenerator. We observe a trade-off between high PS gain provided by high pumping power and stability of the device.

Transfer Matrix and Fourier Transform Methods for Simulation of Second-Order Nonlinear Interactions in a PPLN Waveguide

A new method for the simulation of nonlinear processes of sum-frequency generation (SFG), difference-frequency generation (DFG) and cascaded SFG/DFG (cSFG/DFG) in complex periodically poled lithium niobate (PPLN) gratings for continuous wave signals is proposed and numerically validated. It is based on the transfer matrix method and shows better performance in comparison with traditional numerical solving methods of ordinary differential equations, in particular when nonuniform PPLN are considered. In addition, a Fourier transform method to compute SFG, DFG, cSFG/DFG, and also some cascaded second-harmonic generation/difference-frequency generation interactions of modulated signals is presented. This new algorithm considerably decreases the computing time in comparison with well-known finite-differences methods.

Investigation of PPLN-Based PSAs for High-Gain Optical Amplification

In this paper, different configurations of phasesensitive amplifiers (PSAs) built with periodically poled lithium niobate (PPLN) devices are theoretically and numerically investigated, focusing on their application for amplification in optical communications systems. Singleand dual-pump configurations of one-, two-, and four-mode PSAs are discussed. For each configuration, analytical expressions for the maximum and minimum gain of the amplifiers are provided, showing the influence of the power of the pump and signal waves, as well as the length and efficiency of the PPLN waveguide. The analytical expressions are numerically validated by solving the coupled differential equations describing the nonlinear interactions in the PPLN. The obtained results show that the gain of all PSA configurations exponentially increases with the power of the pump waves, and the length and efficiency of the PPLN device, whereas it is almost independent of the power of the signal wave. In addition, it is shown in this paper that PSA configurations where an intermediate interaction is necessary to generate waves at the second-harmonic band have a gain penalty of 6 dB. It is also shown that no significant difference in terms of gain bandwidth is observed for the singleand dual-pump configurations of two-mode PSAs with an intermediate interaction. Finally, it is shown that a four-mode PSA can only be implemented under very strict conditions, with no gain advantage over two-mode PSAs.

Investigation of black-box phase regeneration using single bi-directional PPLN waveguide

We investigate a novel in-line phase regeneration set-up using a single bi-directional PPLN waveguide for both generation of phase correlated signals and phase-sensitive regeneration and injection-locking for carrier phase recovery.

Pump-phase-noise-free optical wavelength data exchange between QAM signals with 50-GHz channel-spacing using coherent DFB pump.

An important challenge for implementing optical signal processing functions such as wavelength conversion or wavelength data exchange (WDE) is to avoid the introduction of linear and nonlinear phase noise in the subsystem. This is particularly important for phase noise sensitive, high-order quadrature-amplitude modulation (QAM) signals. In this paper, we propose and experimentally demonstrate an optical data exchange scheme through cascaded 2nd-order nonlinearities in periodically-poled lithium niobate (PPLN) waveguides using coherent pumping. The proposed coherent pumping scheme enables noise from the coherent pumps to be cancelled out in the swapped data after WDE, even with broad linewidth distributed feedback (DFB) pump lasers. Hence, this scheme allows phase noise tolerant processing functions, enabling the low-cost implementation of WDE for high-order QAM signals. We experimentally demonstrate WDEs between 10-Gbaud 4QAM (4QAM) signal and 12.5-Gbaud 4QAM (16QAM) signal with 3.5-MHz linewidth DFB pump lasers and 50-GHz channel spacing. Error-free operation is observed for the swapped QAM signals with coherent DFB pumping whilst use of free-running DFB pumps leads to visible error floors and unrecoverable phase errors. The phase noise cancellation in the coherent pump scheme is further confirmed by study of the recovered carrier phase of the converted signals. In addition to pump phase noise, the influence of crosstalk caused by the finite extinction ratio in WDE is also experimentally investigated for the swapped QAM signals.

Experimental investigation of phase-sensitive amplification of data signals in a four-mode fiber-based PSA

In this Letter, we investigate the influence of the phase and power of pump and signal waves on the gain of a four-mode phase-sensitive amplifier (PSA) built with a highly nonlinear fiber (HNLF), using a copier + PSA scheme to generate phase- and frequency-correlated idler waves. Using such an amplifier, low-noise amplification of a 10 Gsymbol/s quadrature phase-shift keying (QPSK) signal, with net gain of ∼20  dB and less than 1 dB optical signal-to-noise ratio (OSNR) penalty at a bit error ratio (BER) of 10(-3), was achieved. We also verified an additional net gain of 11.6 dB when switching from phase-insensitive to phase-sensitive operation, which is in good agreement with theoretical predictions of 12 dB.

PPLN Poling Design Based on a Discrete Layer Peeling Algorithm Combined With a Deleted-Reversal Method

In this paper, we propose a method to design the poling pattern of periodically poled lithium niobate (PPLN) devices according to a target spectral response of the selected nonlinear interaction. This method combines a discrete layer peeling algorithm with the deleted-reversal method. The main advantages and limitations of the proposed method in terms of fabrication feasibility of the resulting poling patterns are discussed. The effectiveness of the proposed method is shown by designing PPLNs with two different types of spectral responses: quasi-rectangular bandpass filtering and multichannel filtering. We experimentally demonstrate the method by designing and producing a PPLN which allows performing wavelength conversion within a spectral response approximately given by a 400 GHz quasi-rectangular filter.

All-optical signal processing techniques with fiber based devices

The development of new optical devices and techniques has boosted new possibilities in many different topics. In these, the field of communications has been one of the most prolific, with the exponential growth of optical communications, fuelled by the explosion of the internet and the increased demand for “broadband for all”. However, todays optical networks are fairly static and operate within well-defined specifications. The addition of new nodes or the upgrade of existing links demands an enormous expenditure. A cost-effective implementation of future optical networks should accommodate old static networks, as well as new highly-efficient networks. It also should be intelligent, self-managed, monitored and dynamically-reconfigurable and should be able to accept new nodes in a plug-and-play manner. All-optical processing devices and techniques are a cost-effective solution for the implementation of these new paradigms in future optical networks. Such devices allow surpassing some of the limitations inherent to electric devices by keeping the signal in the optical domain, avoiding electrical-optical-electrical (OEO) conversions. In order to enable the reconfigurability of the network, all-optical devices should be transparent to modulation format, bit rate, protocol, as well as other requirements. Fiber based devices offer a cost-effective solution when compared to integrated technologies, maintaining the advantages of all-optical processing. For example, fiber Bragg gratings (FBG) are passive optical devices that can be designed with custom transfer functions for different applications such as optical filtering, dispersion compensation [1], pulse shaping etc. For example, the transfer function of a sinc-shaped FBG with a shading function in the spatial domain is a super-Gaussian with almost flat group delay response. This filter is a good candidate for format conversion between double sideband modulation (DSB) and vestigial sideband modulation (VSB). For this purpose, a grating was produced using a CW Ar ion laser emitting at 244 nm and an interferometric setup for the production of the UV fringes in a hydrogenated SMF fiber. all-optical processing, fiber Bragg gratings, optics communications.