Pradeep Kumar Krishnamurthy

Phase Noise Tracking and Compensation in Coherent Optical Systems Using Kalman Filter

The performance of phase-modulated coherent optical communication systems is often limited by laser and nonlinear phase noises (PN and NLPN). The laser PN and the NLPN at high launch powers degrade system performance severely. In this letter, we investigate the use of Kalman filter (KF) to estimate and track both the laser PN and the NLPN in 100-Gb/s single channel coherent optical phase-modulated systems. Initially, the KF is operated in the training mode with pilot symbols; the filter is switched to decision directed mode after training. Simulation results for single-polarization, 100-Gb/s QPSK transmission over 20 × 80 km fiber with in-line dispersion compensation show that KF can successfully mitigate NLPN at 8-dBm launch power with 1-MHz laser linewidth. Comparison with digital backpropagation algorithm (one step/span) shows that KF provides 6-dB improvement in Q-factor. Kalman estimates corresponding to the time evolution of PN and NLPN are also provided, which shows optimal tracking of both the noise processes, thereby validating our proposed scheme.

Bidirectional Pumping for Entangled Photons

We study a bidirectional FWM scheme for entangled photon generation. Simulation shows 26% increase in signal power and a 33% decrease in pump power compared to unidirectional FWM.

Tracking linear and nonlinear phase noise in 100G QPSK modulated systems using Kalman filter

We investigate the use of Kalman filter to estimate both linear and nonlinear phase noise in 100G QPSK modulated systems. Simulations show a considerable improvement in BER with Kalman filter compared to digital backpropagation algorithm.

Generation of CW squeezed light at 1550nm using optical phase conjugation in fiber

We propose and numerically study a technique to generate squeezed light using optical phase conjugation in fiber. At 25mW of pump power, a maximum noise reduction of 36dB at 1550nm is obtained.

Phase conjugation without frequency shift using dual pumped bidirectional FWM in optical fibers

We describe a novel method of polarization and frequency-shift free optical phase conjugation in fibers. Theoretical analysis based on NLSE and simulations show perfect phase conjugation and spectral inversion.

Nonlinearity Mitigation in Coherent Optical Communication Systems: All-Optical and Digital Signal Processing Approaches

Information transmission through fiber-optic channel is subjected to several impairments such as chromatic and polarization mode dispersion, nonlinear phase noise due to interaction of amplifier noise with fiber Kerr nonlinearity, and nonlinear effects such as self- and cross-phase modulation. In addition, laser phase noise and frequency offset between signal and local oscillators also degrade the received signal quality. Unless these impairments are mitigated, the performance of high data rate optical communication systems is degraded. We describe two approaches to mitigate fiber impairments in high data rate coherent optical communication systems. In the first approach, nonlinearity and dispersion in either fibers or semiconductors is used to undo the effects of transmission fiber on the optical carrier. We propose the use of mid-span spectral inversion, realized using counter-propagating dual pumped four-wave mixing in fibers, to mitigate dispersion and nonlinearity in 40 Gbps QPSK systems. We describe our work on realizing optical phase conjugation in semiconductor optical amplifiers. In the second approach, the optical signal is sampled after coherent reception and processed using digital signal processing algorithms to mitigate dispersion and nonlinearity. We describe Kalman filters to estimate and track phase noise in 100 Gbps QPSK systems. We also describe radial basis function neural network equalizer to mitigate nonlinearity in 80 Gbps 16 QAM CO-OFDM systems.

A novel scheme of cascaded four-wave mixing for phase-sensitive amplification in nonlinear optical fibre

Abstract We propose and numerically verify a scheme of phase-sensitive amplifier (PSA) using four-wave mixing (FWM) in cascaded highly nonlinear fibres (HNLFs), without requiring initial phase-locking between signal and pump. The first HNLF is used to generate two phase-conjugate waves, which act as two pumps for FWM process in second HNLF. We feed the two pumps from opposite ends of second HNLF and a signal co-propagating with one of the pumps. We keep the signal frequency in the middle of two pump frequencies to obtain phase-conjugate wave at the same frequency as the signal by FWM process in second HNLF. Signal and phase-conjugate wave appear at opposite ends of the second HNLF and combined to obtain PSA. The frequency-shift-free operation of phase conjugation helps in preserving the frequency of input signal during phase-sensitive amplification. We derive the expression for PSA signal output and PSA gain and show analytically that PSA gain depends upon signal phase only, as the two pumps are phase conjugate to each other. Thus, eliminating the need of phase locking between signal and pump waves. We show that PSA provides high gain for in-phase component and almost cancellation for quadrature-phase component of signal. We show the broadband nature of PSA due to minimum effect of group velocity and group velocity dispersion owing to counter-propagating nature of signal and conjugate waves. We study the performance of PSA under the effects of pump-signal detuning, amplifier length and input signal phase. Simulation results show that PSA output is forced to attain 0 or phase regardless of large variation of phase in the input signal. Nonlinear phase noise reduction of 100 Gbps DPSK signal transmitted over 1000-km standard single-mode fibre confirms phase regeneration by PSA.

Continuously Tunable Multi-wavelength Actively Mode-locked EDFRL Using Intra-cavity Birefringence

We report on a continuously tunable multi-wavelength actively mode-locked EDFRL based on induced birefringence in cavity. A tuning range of 30nm over 1530-1560 nm and simultaneous operation of up to three wavelengths was achieved.

Frequency-shift free MSSI for dispersion compensation of advanced modulation formats using dual pump bidirectional FWM

We propose and numerically study frequency-shift free MSSI using fiber OPC for dispersion compensation. Simulation of 20Gbps QPSK data over 320km fiber shows complete dispersion compensation and ≈ 10−8 BER at 25dB transmit OSNR.

Quantum Key Distribution Using 8-ary PSK Modulation and Coherent Detection

We describe a QKD protocol using 8-ary phase-encoding of single-photons and coherent detection that allows exchange of two key bits/photon. An average QBER of 43.75% for intercept/resend attack makes the scheme robust and secure.