K. Pradeep Kumar

Radial Basis Function Neural Network Nonlinear Equalizer for 16-QAM Coherent Optical OFDM

We propose a radial basis function neural network (RBFNN)-based nonlinear equalizer (NLE) for coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. The hidden layer neuron weights of the RBFNN-NLE are calculated using the K-means clustering algorithm and the output layer weights are updated using the least mean square algorithm. With only 3% overhead in training, the proposed RBFNN-NLE was found to provide up to 4-dB performance improvement in terms of Q-factor for 70-Gb/s 16-QAM CO-OFDM transmission over 1000 km (10 × 100 km) fiber. Numerical results show that the operating data rate is 80 Gb/s at Q = 6.25 dB with the proposed RBFNN-NLE, compared with previously reported value of 70 Gb/s with artificial neural network-based NLE.

Decoy-pulse protocol for frequency-coded quantum key distribution

We propose a decoy-pulse protocol for frequency-coded implementation of B92 quantum key distribution protocol. A direct extension of decoy-pulse method to frequency-coding scheme results in security loss as an eavesdropper can distinguish between signal and decoy pulses by measuring the carrier photon number without affecting other statistics. We overcome this problem by optimizing the ratio of carrier photon number of decoy-to-signal pulse to be as close to unity as possible. In our method the switching between signal and decoy pulses is achieved by changing the amplitude of RF signal as opposed to modulating the intensity of optical signal thus reducing system cost. Our simulation results show a key generation rate of 1.5×10-4/pulse for link lengths up to 70 km. Finally, we discuss the optimum value of average photon number of signal pulse for a given key rate while also optimizing the carrier ratio.

Pump suppressed four-wave mixing in optical fibers for correlated photon generation using feedback technique

In this paper we propose and demonstrate a novel technique to suppress pump in four-wave mixing experiments. The residual pump powers at the fiber output are reflected by fiber-Bragg gratings (FBGs), amplified by an EDFA to compensate for pump losses in feedback path, and fed back to the fiber. With the increase in total input power to the fiber, the ratio of signal and idler to pump increases. Additional optical filters can then be used for further pump suppression. In our experiments, two pump waves of wavelengths 1549.70nm and 1549.85nm are combined using a 3dB coupler and fed to a highly-nonlinear fiber (HNLF) of length 1km, nonlinear coefficient of 12.4/W-km, and zero dispersion wavelength (ZDW) of 1513nm. Without feedback, we obtained the signal and idler to pump ratios of -21dB and -20.6dB respectively. After filtering by FBGs of 95% reflectivity and unity gain feedback, the ratio becomes - 14.1dB and -12.2dB respectively. When the residual pumps are amplified and fed back to the HNLF, the ratio improves to -7.5dB and -8.6dB indicating the potential of our method.

RF pulse shaping for QPSK, PM-QPSK and QAM formats

We propose and evaluate the performance of a novel RF pulse against rectangular pulse for three modulation formats: QPSK, PM-QPSK, and Star 16 QAM. The proposed pulse shows BER improvement by over an order of magnitude even for large phase and pre-amplifier noise.

RF pulse shaping for higher-order optical modulation

We propose a novel RF pulse shape and evaluate its performance for QPSK, PM-QPSK and 16 star QAM modulation formats under the effects of phase noise, preamplifier noise, dispersion and nonlinearity. Simulation of a 100 Gbps system with proposed pulse shape shows significant BER reduction compared to the system with rectangular pulse shape.

RF pulse shaping for coherent optical OFDM

We describe a novel RF pulse shape for CO-OFDM systems. Simulation results of a 100 Gbps system show significant BER reduction even at rather large phase noise and chromatic dispersion compared to rectangular pulse shaping.