Francesca Bontempi

Optical linear feedback shift register

Linear feedback shift registers (LFSRs) are essential components of communications systems such as pseudo random bit sequence (PRBS) generators, convolutional encoders, scramblers and randomizers [1, 2]. Fig.1 (left) shows a LFSR scheme in which the XOR gate is operated among two tapped bits i and i+k within the shift register. The result bit is fed to the shift register at the last position (n) and the sequence in the register is 1-bit shifted as well. The experimental setup for demonstrating an all-optical LFSR is shown in Fig.1 (middle). The main components of the scheme are an optical buffer [3], a bit selecting circuit, and a XOR gate. The data vector (A) is generated by modulating the continuous wave (CW) λs with a 10Gbps non-return-to-zero (NRZ) electrical pattern whose period is TP=33.3168µs. Only 5 bits are used as the initial sequence for the shift register. The travelling time of the packet in the buffer is TL=1.85 µs, meaning that up to 18 round trips can be accommodated by the patterns period TP but one of those should be preserved for erasing the buffer before it is fetched a new sequence. The initial sequence enters the buffer as well as propagates directly to the bit selecting circuit. Every round trip, a new sequence as a replica of the previous one plus the feedback bit comes out. At the last round trip, a pump signal inserted into the buffer through the circulator will strongly reduce the gain of the semiconductor optical amplifier (SOA). Both the circulating packet and the accumulating in-band amplified spontaneous emission (ASE) noise will be suppressed. The buffer is now completely cleaned for a new cycle. The first and the last bits of the 1-bit shifted sequence are selected for the XOR operation. Referring to Fig.1 (left), i=1 and k=4, n=5. With this setup, the bit selecting circuit requires 2 clock signals (C) for tap the proper bits from the incoming sequence. The sampling process is done by exploiting four-wave-mixing (FWM) in a 250m-long highly nonlinear fiber (HNLF). Since at every round trip the sequence must be 1-bit shifted, the sampling clocks must be accordingly delayed for 1 bit, resulting the clock period of TC=TL+TB where TB=100ps is the bit time. The outputs of the bit selecting circuit are fed as pump signals to port a and d of a SOA Mach Zehnder interferometer (MZI). They are synchronized with a clock at λs used as the probe signal (F) at port b. The output of the XOR gate is then fed back into the buffer. The feedback bit should be precisely allocated at the end of the packet circulating inside the buffer, meaning that the feedback time should be TF=TL+TB+Tseq in which Tseq is the original sequence duration and equal to 500ps in our setup. Fig.1 (right) depicts the results at the 10th round trip for two different input initial sequences (10001) and (10011) taken at point (B). The equalization between bits in the sequence and the quality of the pulses is strongly limited by the accumulating ASE noise inside the buffer, the long fiber-based structure and the bits polarization. The fiber-based setup also causes misalignment between the clocks, spoils the essential synchronism and degrades the performance of the FWM and the XOR operation. One possible solution for solving all of these problems and hence improving the performance is the integration approach. The bit selection could be implemented exploiting nonlinear effects into an SOA or a periodic-poled lithium niobate (PPLN) instead of using a long HLNF. Moreover the extinction ratio of the pulses at the SOA-MZI XOR output (<13.5dB) could be improved, e.g. with a saturable absorber at its output, thus increasing the number of allowed round trips into the shift register and reducing the noise on the final bit sequence.

An InP Monolithically Integrated Unicast and Multicast Wavelength Converter

We experimentally demonstrate a novel Indium Phosphide monolithically integrated optical circuit for all-optical wavelength conversion. The circuit exploits two cascaded cross gain modulation (XGM) interactions in semiconductor optical amplifiers. In a first XGM stage, the input signal and a continuous wave light at the desired output wavelength generate a wavelength converted yet not ideal copy of the input signal, whereas, in the second XGM stage, the quality of the wavelength converted signal is significantly improved because of the cross gain compression effect. We report 10 Gb/s wavelength conversion performance for both the unicast (single output wavelength) and the multicast (multiple output wavelengths) conversion. Experimental results confirm the effectiveness of the present photonic integrated circuit in both cases, showing only a moderate bit error rate power penalty.

A wavelength-preserving photonic integrated regenerator for NRZ and RZ signals

This paper presents a novel Indium Phosphide based photonic integrated circuit (PIC) for all-optical regeneration of both nonreturn-to-zero (NRZ) and return-to-zero (RZ) on-off-keying (OOK) signals. The PIC exploits cross gain compression in two semiconductor optical amplifiers to simultaneously obtain a wavelength-preserved and reshaped copy, and a wavelength-converted yet inverted copy of the input signal. Regeneration of 10 Gb/s signals on multiple wavelengths is demonstrated, showing a Q-factor improvement from 1.5 to 4 for NRZ-OOK signals and from 2.3 to 3.6 for RZ-OOK signals, and a BER improvement up to 1.5 decades.

Multifunctional Current-Controlled InP Photonic Integrated Delay Interferometer

We demonstrate a novel and flexible InP monolithically integrated optical circuit with multiple applications. The circuit is an interferometric delay loop and its particular configuration allows to perform a number of different functions, namely, optical buffering, differential phase-shift keying (DPSK) demodulation, intensity modulation, and differential XOR logic operation. By properly controlling the current supplied to the active elements in the circuit loop (i.e., a semiconductor optical amplifier and a variable optical attenuator), the relative phase of the propagating signals can be adjusted, changing the interference condition between the input signal and its delayed copies. In this way the four different functions are enabled. Experimental results are reported for all the different functions of the device. When the circuit is operated in buffer configuration, up to 13 circulations (corresponding to a 1.62 ns delay) of an input bit at 12.5-Gb/s data rate are shown before the output signal degrades due to excessive ring losses. Its behavior as a current-controlled DPSK demodulator is demonstrated for 8-Gb/s signals, and the resulting bit error rate measurements are compared with those of a thermally tuned commercial demodulator, showing no significant power penalty. A proof-of-concept demonstration as an intensity modulator is reported for relatively low-frequency signals at 10 and 20 MHz, being limited by electrical components of the setup. Finally, error-free operation for a 1-, 2-, and 4-b differential XOR logic gate has been demonstrated at 8, 16, and 32 Gb/s, respectively.

InP monolithically integrated coherent transmitter

A novel InP monolithically integrated coherent transmitter has been designed, fabricated and tested. The photonic integrated circuit consists of a distributed Bragg reflector laser and a modified nested Mach-Zehnder modulator having tunable input power splitters. Back-to-back coherent transmission for PDM-QPSK signals is reported up to 10 Gbaud (40 Gb/s) using the integrated laser and up to 32Gbaud (128 Gb/s) using an external low phase noise laser.

Current-controlled InP monolithically integrated DPSK demodulator

A monolithically integrated InP optical circuit performing current-controlled DPSK demodulation is reported. The circuit consists of an interferometric structure with a 1-bit delay SOA-amplified loop. Operation at 8 Gb/s in C-band is reported.

Integrated Reconfigurable Coherent Transmitter Driven by Binary Signals

A novel InP monolithically integrated transmitter composed of a tunable distributed Bragg reflector laser and a reconfigurable IQ modulator with tunable power splitters has been designed, fabricated and tested. The transmitter architecture allows, in principle, generation of offset-free phase-amplitude constellations such as QPSK and 16-QAM by employing simple binary signals with equal peak-to-peak amplitude. The adoption of tunable splitters enables reconfiguration of the output constellation and compensation for imperfections related to fabrication. The solution presented in this paper minimizes the complexity of the employed architecture together with the one of the driving signals. Numerical analysis has been conducted to predict system behavior and evaluate the impact of sub-optimum settings that might occur in a real implementation. Experimental results show the generation PM-QPSK signals at 10 Gbaud when using the integrated DBR laser and at 32 Gbaud when using an external low-phase-noise laser. Unfortunately, the generation of 16-QAM signals, which can be in principle realized with this novel PIC, was not possible in practice because of significant spurious amplitude modulation in the electro-optic phase modulators.

A novel photonic integrated regenerator

We demonstrate a novel InP-PIC for all-optical regeneration. The PIC exploits cross-gain compression in SOA obtaining, at the same time, wavelength-preserving regeneration and wavelength conversion. 10 Gb/s operation is shown by Q-factor and BER improvement.

Photonic Integrated wavelength converter based on double stage cross gain modulation in SOAs

We report a new monolithic InP Photonic-Integrated-Circuit (PIC) for all-optical wavelength conversion. The PIC exploits double stage cross-gain-modulation in semiconductor optical amplifiers (SOA)s and an integrated DBR laser. We report device characterization and 10 Gb/s wavelength conversion operation.

All-Optical Distribution Node for Long Reach PON Downlink

A simple photonic circuit is proposed for all-optical distribution of long-reach passive optical networks (PONs) aggregated in a WDM distribution trunk. The circuit can distribute a WDM comb in C-band carrying G- and/or XG-PON channels into a number of downlink fibers and it is also able to convert the signals to standard PON wavelengths (1500 nm for G-PON and 1570 nm for XG-PON). The circuit is based on parallel cross gain modulation between WDM signals in C-band and continuous wave probes at destination downlink wavelengths. A proof-of-concept experimental demonstration is reported showing a maximum 3-dB power penalty for operation at 10 Gb/s in the counter-propagating case. Broadband operation is also reported for input signals in the whole C-band.