Graham Hesketh

Polarization-Assisted Phase-Sensitive Processor

We propose and experimentally demonstrate a phase-sensitive optical processor, capable of generating two codirectional and π-phase-shifted phase-sensitive amplifiers (PSAs) in a single device. Phase-sensitive operation is obtained by polarization mixing a phase-locked signal/idler pair generated in a degenerate dual-pump vector parametric amplifier based on four-wave mixing in a highly nonlinear fiber. We refer to this configuration as a polarization assisted PSA and demonstrate some of the applications that it may find. First, we experimentally demonstrate the regeneration of a binary phase shift keying signal in a system that requires only a very low nonlinear phase shift of 0.35 rad. Second, we decompose a quadrature phase shift keying (QPSK) signal into its in-phase and quadrature components. While application to a QPSK signal is shown in our demonstration, we demonstrate numerically that any complex modulation format signal can be decomposed using this approach. Finally, we use our processor to regenerate QPSK signals in a single nonlinear device.

Reducing bit-error rate with optical phase regeneration in multilevel modulation formats

We investigate theoretically the benefits of using all-optical phase regeneration in a long-haul fiber optic link. We also introduce a design for a device capable of phase regeneration without phase-to-amplitude noise conversion. We simulate numerically the bit-error rate of a wavelength division multiplexed optical communication system over many fiber spans with periodic reamplification and compare the results obtained with and without phase regeneration at half the transmission distance when using the new design or an existing design. Depending on the modulation format, our results suggest that all-optical phase regeneration can reduce the bit-error rate by up to two orders of magnitude and that the amplitude preserving design offers a 50% reduction in bit-error rate relative to existing technology.

Optical phase quantizer based on phase sensitive four wave mixing at low nonlinear phase shifts

We propose and demonstrate a new phase-sensitive (PS) scheme based on four-wave mixing followed by a polarizer to achieve an ideal binary step-like phase transfer function at nonlinear phase shifts as low as 0.3 rad by significantly increasing the parametric deamplification component. PS operation is obtained by polarization mixing the phase-locked and orthogonally polarized signal and idler, which is generated in a degenerate dual-pump vector parametric amplifier.

An optical phase quantiser exhibiting suppressed phase dependent gain variation

We experimentally demonstrate an all-optical phase quantiser based on phase-sensitive amplification which alleviates phase noise to amplitude noise conversion. Phase transfer functions are measured for the very first time using a novel scheme.

Quadrature decomposition of optical fields using two orthogonal phase sensitive amplifiers

We propose a new technique to optically process coherent signals by simultaneously extracting their two (I and Q) quadrature components into two orthogonal polarizations at the same frequency. Two possible implementations are demonstrated.

Full quadrature regeneration of QPSK signals using sequential phase sensitive amplification and parametric saturation

We demonstrate all-optical regeneration of both the phase and the amplitude of a 10 GBaud quadrature phase shift keying (QPSK) signal using two nonlinear stages. First we regenerate the phase using a wavelength converting phase sensitive amplifier and then we regenerate the amplitude using a saturated single-pump parametric amplifier, returning the signal to its original wavelength at the same time. We exploit the conjugating nature of the two processing stages to eliminate the intrinsic SPM distortion of the system, further improving performance.

Optimisation of amplitude limiters for phase preservation based on the exact solution to degenerate four-wave mixing

Adopting an exact solution to four-wave mixing (FWM), wherein harmonic evolution is described by the sum of two Bessel functions, we identify two causes of amplitude to phase noise conversion which impair FWM saturation based amplitude regenerators: self-phase modulation (SPM) and Bessel-order mixing (BOM). By increasing the pump to signal power ratio, we may arbitrarily reduce their impact, realising a phase preserving amplitude regenerator. We demonstrate the technique by applying it to the regeneration of a 10 GBaud QPSK signal, achieving a high level of amplitude squeezing with minimal amplitude to phase noise conversion.

Investigation into the role of pump to signal power ratio in FWM-based phase preserving amplitude regeneration

We carry out a detailed experimental characterization of a four-wave mixing based amplitude limiter in highly nonlinear fiber based on the Bessel-like power transfer characteristics and highlight trade-offs for phase preserving capabilities.

All-optical phase regeneration of multi-level amplitude and phase shift keyed signals

All-optical phase regeneration of multi-level phase shift keyed signals in fiber optic communications systems reduces the impact of phase noise induced by, e.g., self/cross-phase modulation of amplitude varying symbols, and has been previously simulated and experimentally demonstrated [1-3]. Phase regeneration is achieved through a judicious coherent addition of phase harmonics, bearing integer multiples of the signal phase, to the signal via four-wave mixing in a highly nonlinear fiber (HNLF), such that a staircase is realised in the signal phase transfer function of the HNLF. Phase regeneration may allow for greater transmission capacity and optical regeneration has the potential to work faster and consume less power than electronic compensation.

Phase and amplitude regeneration through sequential PSA and FWM saturation in HNLF

All-optical phase and amplitude regeneration of a QPSK signal using only two nonlinear stages is achieved by combining a PSA with a saturated pump-degenerate FWM based amplitude squeezer. EVM, phase noise and magnitude noise are all reduced by 50%.