Phase-preserving Amplitude Regeneration Research Articles

Phase-preserving principle of all-optical regenerators with applications to MZI-nested NOLM structure.

The principle of phase-preserving regeneration is revealed by a simple theoretical model, that is, in the regenerated signals the linear phase shift component is dominant over the nonlinear counterpart for phase-preserving amplitude regeneration (PPAR). A Mach-Zehnder- interferometer (MZI)-nested nonlinear optical loop mirror (NOLM) PPAR scheme is proposed and verified by theory and experiment. Our experiment shows that for QPSK regeneration the noise reduction ratio in terms of error vector magnitude (EVM) is linearly dependent on the input signal-to-noise ratio (SNR) with the slope of 0.78 and the average phase disturbation is 4.37 degree, close to the theoretical value of 3.8 degrees. The influence of the optical couplers on the PPAR performance is in detail discussed.

All-optical phase-preserving amplitude-regeneration technology based on bidirectional orthogonal-pumped semiconductor optical amplifier configuration

The phase-preserving amplitude regeneration scheme based on the bidirectional orthogonal-pumped semiconductor optical amplifier (SOA) is proposed in this work. Experimental investigation into the multiple four-wave mixing (FWM) process from the pump, the signal and their corresponding reflective fields is carried out in detail. The regeneration performance obtained from the product between co-propagating fields is also discussed, including its dependence on the signal launch power and the signal quality, to quantify the amplitude regeneration and the phase preserving behaviors. The amplitude distortion is suppressed by 2.2 dB experimentally, confirming the regeneration capability of the proposed scheme. Moreover, the regeneration performance is further investigated for multiple phase shift keying (MPSK) signals through the simulation. According to the numerical results, the operational parameters of the regenerator are the same for advanced modulation formats, proving the robust operation of the proposed bidirectional orthogonal-pumped SOA configuration.

On-chip Mach Zehnder interferometer-based all-optical amplitude regenerator for optical 16-QAM signals.

We propose an on-chip all-optical multilevel amplitude regenerator scheme over a Mach Zehnder interferometer (MZI) configuration, enabling multiple amplitude-noise suppression on 16-QAM signals. Joint parameter optimization is carried out based on the general nonlinear model of the proposed scheme to significantly reduce the phase distortion caused by the nonlinear interferometer, which is the key to perform the phase preserving operation. The full function of the phase-preserving amplitude regeneration (PPAR) is verified by an experiment on an on-chip nonlinear waveguide with the length of the 2.31 cm. Furthermore, we perform thoughtful investigations on the oscillatory behavior achieved by the silicon MZI regenerator, enabling the full PPAR on 16-QAM signals through the optimized multiple power plateaus. A maximum 1.6 dB improvement of signal quality is achieved by the proposed on-chip amplitude regenerator at the input signal-to-noise ratio (SNR) of 25 dB. The impact from the two-photon absorption (TPA) effect as an positive role in the regenerator is also well discussed.

Optimization method for universal NOLM-based phase-preserving amplitude regenerator with application to Att-NALM scheme

We theoretically investigate the conditions of achieving phase-preserving amplitude regeneration for the universal nonlinear optical loop mirror (NOLM) structure and present a method of determining the optimal working point for amplitude shaping. A normalized amplitude-phase transfer parameter is introduced to measure the phase-preserving performance at the working points of multilevel regeneration. Then, a new NOLM (so-called Att-NALM) structure, consisting of an optical attenuator and a bidirectional optical amplifier, is taken into account by the optimization method. The optimal working point can respectively be designed on the first or second plateau of the optical power transfer function for phase-preserving and amplitude regeneration of QPSK/8PSK or two-amplitude 8QAM signal. Our simulation shows that, the corresponding error vector magnitude (EVM) has a maximal reduction with the noise reduction ratio of 5.44dB, 4.7dB and 3.7dB for only amplitude noise, and 1.86dB, 1.58dB and 1.4dB for simultaneous amplitude and phase noise, respectively. We also list the design parameters of the existing NOLM structures for phase-preserving amplitude regeneration.

All-optical rectangular-QAM regenerators based on phase-preserving I/Q shaping

We put forward a new all-optical I/Q regeneration scheme for quadrature amplitude modulation (QAM) signals with rectangular constellation. In the proposed scheme, two phase-preserving amplitude and phase shift key (PP-APSK) regeneration units are built up by optimizing the nonlinear amplified fiber loop mirror. The phase-preserving amplitude regeneration characteristics obtained by simulation are identical with the theoretical results. Our simulation shows that the maximum noise reduction ratios of the proposed regenerators are up to 3 dB, 6.9 dB, and 4.5 dB at the input optical signal-to-noise ratio of about 26 dB for 8QAM, 16QAM and 32QAM signals, respectively.

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.

Fiber-Optical Parametric Amplification of Sub-Picosecond Pulses for High-Speed Optical Communications

This article reviews recent results of amplification of short optical pulses using fiber-optical parametric amplifiers. This includes chirped-pulse amplification of 400 fs pulses, error-free amplification of a 640-Gbit/s optical time-division multiplexed signal with less than a 1-dB power penalty, and all-optical phase-preserving amplitude regeneration of a 640-Gbit/s return-to-zero differential phase-shift keying optical time-division multiplexed signal.

Cascaded phase-preserving multilevel amplitude regeneration.

The performance of cascaded in-line phase-preserving amplitude regeneration using nonlinear amplifying loop mirrors has been studied in numerical simulations. As an example of a spectrally efficient modulation format with two amplitude states and multiple phase states, the regeneration performance of a star-16QAM format, basically an 8PSK format with two amplitude levels, was evaluated. An increased robustness against amplified spontaneous emission and nonlinear phase noise was observed resulting in a significantly increased transmission distance.

All-optical phase-preserving multilevel amplitude regeneration.

The possibility of all-optical phase-preserving amplitude regeneration for star-8QAM is demonstrated using a modified nonlinear optical loop mirror. Experiments show a reduction in amplitude noise on both amplitude levels simultaneously, considering two different types of signal distortions: deterministic low-frequency amplitude modulation and broadband amplitude noise. Furthermore, using this amplitude regeneration, the robustness against nonlinear phase noise from fiber nonlinearity in a transmission line is increased. The scheme suppresses the conversion of amplitude noise to nonlinear phase noise. This is shown for simultaneous amplitude regeneration of the two amplitude states as well as for amplitude regeneration of the high-power states only. If the transmission is limited by nonlinear phase noise, single-level operation at the more critical higher-power state will benefit because of the wider plateau region. Numerical simulations confirm the experimental results.

Polarization-Independent All-Optical Regenerator for DPSK Data

We demonstrate polarization-independent simultaneous all-optical phase-preserving amplitude regeneration and wavelength conversion of NRZ differential phase shift keying (DPSK) data by four-wave mixing (FWM) in a semiconductor optical amplifier (SOA). The dependence upon polarization state of the signals is eliminated by using a co-polarized dual-pump architecture. Investigation on the regenerative capability vs. pumps detuning shows significant BER threshold margin improvement over 6 nm conversion range.

On multi-channel operation of phase-preserving 2R amplitude regenerator

We analyze multi-channel operation of a phase-preserving amplitude regenerator based on a nonlinear amplifying loop mirror (NALM) with a dispersion-managed nonlinear medium. We show that, under the conditions similar to those of the previously studied multi-channel Mamyshev regenerator, the multi-channel performance of the NALM-based regenerator with phase-encoded signals is considerably worse than that of Mamyshev regenerator with on-off-keying signals. This is explained by a weaker, compared to a timing jitter׳s, dependence of nonlinear phase jitter on the inter-channel spacing. We show that the multi-channel NALM performance can be significantly improved by using the dispersion map that includes a larger number of fiber sections separated by periodic group-delay devices.

Parametric amplification and phase preserving amplitude regeneration of a 640 Gbit/s RZ-DPSK signal

We report the first experimental demonstration of parametric amplification and all-optical phase-preserving amplitude regeneration for a 640 Gbit/s return-to-zero (RZ) differential phase-shift keying (DPSK) optical time division multiplexed (OTDM) signal. In the designed gain-flattened single-pump fiber optical parametric amplifier (FOPA), 620 fs short optical pulses are successfully amplified with 15 dB gain with error-free performance and less than 1 dB power penalty. Phase-preserving amplitude regeneration based on gain saturation in the FOPA is carried out for optical signals with degraded optical signal-to-noise ratio. An improvement of 2.2 dB in receiver sensitivity at a bit-error-ratio of 10(-9) has been successfully achieved after regeneration, together with 13.3 dB net gain.

Polarization-Insensitive Phase-Preserving Regenerator Based on a Fiber Optical Parametric Amplifier With Dual Orthogonal Pumps

We investigate the gain-saturation characteristics of a dual-orthogonal-pump fiber optical parametric amplifier. Nearly uniform power transfer functions have been obtained for various input signal polarizations by adjusting the total pump power. Based on this physical behavior, we demonstrate polarization-insensitive phase-preserving amplitude regeneration of 40-Gb/s nonreturn-to-zero differential phase-shift keying signals. The receiver sensitivity has been improved by a maximum value of 4.7 dB.

Regeneration of DPSK Signals in a Saturated SOA

We investigate and experimentally demonstrate a simple method for phase-preserving amplitude regeneration of constant envelope phase-coded signals. This scheme exploits nonlinear interaction between noisy data and a continuous wave beam at a different wavelength in a saturated semiconductor optical amplifier (SOA). We show that proper balancing of the input signals' power allows us to exploit the amplitude limiting effect of SOA saturated gain without introducing significant excess phase noise due to suppression of the α-factor in the amplifier. In a 10-Gb/s nonreturn-to-zero differential phase-shift-keying experiment, both four-wave-mixing (FWM) and pass-through signals are remarkably improved with respect to input data in terms of Q-factor and bit error ratio threshold margin, demonstrating wavelength preserving and wavelength converting regeneration. In particular, the FWM signal exhibits better regenerative performance over a broader range of degraded input data and for lower input overall power levels.

All-Optical 2R Regeneration With a Vertical Microcavity-Based Saturable Absorber

This paper gives an overview of recent demonstrations of optical 2R regeneration achieved by vertical microcavity mirror-based multiple-quantum-well saturable absorber (SA). The potential of the device to perform wavelength division multiplexing regeneration is first demonstrated through the first pigtailed SA chip implemented with eight independent fibers using a cost effective coupling technique. The cascadability and wavelength tunability assessment of this module associated with a power limiter fiber-based function has been experimentally demonstrated at 42.6 Gbit/s. Because this method of power limiting is not a suitable solution for all-optical multichannel 2R regeneration, a new SA structure allowing a power limiting function was proposed. We describe and characterize such a structure in this paper. This new SA opens the door to a complete passive all-optical 2R regeneration relying upon a single technology, as shown in this paper through the use of two SA: SA.0 for extinction ratio enhancement and SA.1 for high-power level equalization allowing receiver sensitivity (up to 3.5 dB) and Q-factor (up to 1.4 dB) improvement for a RZ signal at 42.6 Gbit/s. The limitation of SA.1 when the regenerator must be cascaded a large number of times is also described, leading to the observation that SA.1 should be more suitable for phase encoded formats that are more spectrally efficient than ON-OFF keying formats. A SA.1 used as a phase-preserving amplitude regenerator in a 42.6 Gbit/s RZ differential quadrature phase-shift keying transmission system is, therefore, assessed . A fiber launched power margin of 2 dB and a receiver sensitivity improvement of 5.5 dB are obtained. Finally, we use, for the first time an SA.1 as a phase-preserving amplitude regenerator of RZ differential quadrature phase-shift keying signals. The regenerator is assessed in a recirculating loop at 28 Gbaud. The system tolerance to nonlinear phase noise is enhanced by 3 dB and the distance improvement factor was 1.3 for a bit error rate = 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> .

Multilevel Phase-Preserving Amplitude Regeneration Using a Single Nonlinear Amplifying Loop Mirror

A possibility of multilevel phase-preserving amplitude regeneration using a nonlinear amplifying loop mirror (NALM) is presented for the optical star-8 quadrature amplitude modulation (QAM) transmission format as an example. Two significantly different state power ratios for the QAM signal, 1:3 and 1:7, were investigated. After the optimization of the coupler splitting ratio and the directional phase bias in the NALM, amplitude noise can be efficiently suppressed at both signal power levels simultaneously. Bit-error-ratio (BER) simulations have shown that in a system limited by nonlinear phase noise, the deployment of the NALM allows an increase of the fiber launch power by 1.9 and 2.2 dB at a BER of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> for a state power ratio of 1:3 and 1:7, respectively. The regeneration limits due to imperfections of the power transfer characteristic are also discussed.

Phase-Preserving Amplitude Regeneration in DPSK Transmission Systems Using a Nonlinear Amplifying Loop Mirror

A phase-preserving 2R regenerator based on a nonlinear amplifying loop mirror was implemented in a RZ-DPSK transmission system. Its performance has been investigated in numerical simulations and experimentally. The results show that amplitude regeneration using a NALM can efficiently prevent accumulation of nonlinear phase noise in a 10 Gb/s DPSK transmission system. In the experiments, significant improvements of eye opening and of BER as well as a 3 dB increase in fiber launch power have been demonstrated. Simulations at 10 Gb/s and 100 Gb/s indicated that the enhancement of the transmission quality is smaller at 100 Gb/s. The reason is that at 100 Gb/s nonlinear intra-channel effects rather than pure nonlinear phase noise are the main limiting factor and the NALM can only reduce the accumulation of amplitude noise in that case.

Nonlinear Phase Noise Reduction of DQPSK Signals by a Phase-Preserving Amplitude Limiter Using Four-Wave Mixing in Fiber

It is experimentally shown that phase-preserving amplitude regeneration by an all-optical amplitude limiter using saturation of four-wave mixing in a highly nonlinear fiber can enhance differential quadrature phase-shift keying (DQPSK) transmission performance. The limiter suppresses amplitude fluctuations of the signal, by which the nonlinear phase variance caused by self-phase modulation in the transmission fiber is reduced. The 10-GSymbol/s short-pulse DQPSK transmission experiment over densely dispersion-managed fiber shows that the amplitude limitation after an imperfect transmitter increases maximum transmitted signal power by 5 dB.

Phase-preserving amplitude regeneration for a WDM RZ-DPSK signal using a nonlinear amplifying loop mirror

We propose a modified nonlinear amplifying loop mirror (NALM) for phase-preserving 2R regeneration of wavelength division multiplexed (WDM) return-to-zero differential phase-shift-keyed signals. As proof of principle the regeneration capability of this NALM setup has been investigated experimentally for two 10 Gbit/s wavelength channels. A significant eye-opening improvement and a negative power penalty of 1.2 dB have been observed in both channels.

Performance improvement of DPSK signal transmission by a phase-preserving amplitude limiter

It is theoretically and experimentally shown that phase-preserving amplitude regeneration by an all-optical amplitude limiter using saturation of four-wave mixing in a nonlinear fiber can enhance DPSK transmission performance. The limiter suppresses amplitude fluctuations of the signal, by which the nonlinear phase noise caused by self-phase modulation of the transmission fiber is reduced. A 10-Gbit/s short-pulse DPSK transmission experiment shows that the limiter inserted either after a transmitter or inside a recirculating transmission loop can enhance the performance. Theoretical expressions for the linear and nonlinear phase noise are derived, with which the influence of imperfections of the limiter is examined.