Reconfigurable optical generation of nine Nyquist WDM channels with sinc-shaped temporal pulse trains using a single microresonator-based Kerr frequency comb.

Sinc-shaped temporal pulse trains have a spectrally efficient, rectangular Nyquist spectrum. We demonstrate the simultaneous and reconfigurable optical generation of multiple Nyquist-shaped wavelength-division-multiplexed (WDM) channels having temporal sinc-shaped pulse trains as data carriers. The channels are generated through the insertion of coherent lines using cascaded continuous-wave amplitude modulation around the spectral lines of a microresonator-based Kerr optical frequency comb.

Performance enhancement of an optical high-order QAM channel by adding correlated data to robust neighboring BPSK or QPSK channels.

We numerically simulate and experimentally demonstrate an approach to potentially enhance the performance of a high-order quadrature amplitude modulation (QAM) channel by adding correlated data to other robust binary-phase-shift-keyed (BPSK) or quadrature-phase-shift-keyed (QPSK) channels. The correlated data are introduced by optically multiplying the BPSK or QPSK channels, already modulated with their own data, by the target high-order QAM data of the same baud rate. After joint detection and signal processing, a ∼3  dB optical signal-to-noise (OSNR) improvement is observed in simulations by comparing the performance of the target QAM channel (from 4QAM to 256QAM) with and without the use of channel correlation.

Experimental utilization of repeated spatial-mode shifting for achieving discrete delays in a free-space recirculating loop.

We demonstrate an optical recirculating delay loop by shifting the spatial mode order of orbital-angular-momentum (OAM) beams in the free-space. The desired delay can be selected at the loop output by exploiting the orthogonality of the OAM modes. When sending a 20-Gbaud quadrature-phase-shift-keyed (QPSK) signal through the delay system, three recirculations are demonstrated, each with an additional delay of 2.

Raman-assisted phase sensitive amplifier using a fiber Bragg grating-based tunable phase shifter.

A low-loss Raman-assisted phase sensitive amplifier (PSA) with a ∼20  dB signal net gain is experimentally demonstrated. The amplitude and phase adjustment for PSA are achieved by using non-uniform Raman gain and a tunable fiber Bragg grating (FBG), respectively. The total component loss of the system is measured to be ∼8  dB.

Effects of erbium-doped fiber amplifier induced pump noise on soliton Kerr frequency combs for 64-quadrature amplitude modulation transmission.

We experimentally investigate the effects of erbium-doped fiber amplifier induced pump noise on soliton Kerr frequency combs for 64-quadrature amplitude modulation (QAM) transmission. We find that the optical carrier-to-noise ratios (OCNRs) of the comb lines across the C-band almost linearly depend on the pump OCNR and are similar for a constant input pump power and noise. For a specific three-soliton state, despite higher comb line power, there is no noticeable OCNR improvement compared to the single-soliton comb.

Tunable insertion of multiple lines into a Kerr frequency comb using electro-optical modulators.

We experimentally insert a flexible number of electro-optical (EO) comb lines into a Kerr frequency comb via EO modulation and demonstrate the use of combined Kerr and EO combs as light sources in coherent communications. The number of EO lines inserted into the Kerr comb can be varied by changing the modulation frequency. Additionally, the inserted EO comb is found to have similar coherence to that of the Kerr comb, as indicated by their linewidths.

Pump-linewidth-tolerant wavelength multicasting using soliton Kerr frequency combs.

We experimentally demonstrate pump-linewidth-tolerant wavelength multicasting using microresonator-based soliton Kerr frequency combs. When Kerr comb lines serve as coherent pumps in a periodically poled lithium niobate waveguide, the linewidth of the multicast signal almost remains that of the original signal at different linewidths of Kerr combs, ranging from 100 kHz to 1 MHz. However, in conventional multicasting where free-running (FR) pumps are used, the linewidth of the converted signal significantly increases.

Pilot-tone-based self-homodyne detection using optical nonlinear wave mixing.

An all-optical pilot-tone-based self-homodyne detection scheme using nonlinear wave mixing is experimentally demonstrated. Two scenarios are investigated using (1) multiple wavelength-division-multiplexed channels with sufficient power of the pilot tones and (2) a single channel with a low-power pilot tone. The eye diagram and bit error rate of the system are studied by tuning various parameters such as pump power, relative phase, and pilot-to-signal ratio.

Dependence of a microresonator Kerr frequency comb on the pump linewidth.

We experimentally investigate the dependence of Kerr comb generation, comb linewidth, and coherent system performance on the pump linewidth in a microresonator. We find that the generation of the primary comb can have a larger tolerance to the pump linewidth compared with that of the low-phase-noise comb. In addition, the linewidths of the generated combs are almost linearly dependent on the pump linewidth in the primary and low-phase-noise states.

Dual-pump generation of high-coherence primary Kerr combs with multiple sub-lines.

We experimentally generate high-coherence primary Kerr combs with multiple sub-lines by using dual pumps and demonstrate the application of a primary comb state in multichannel communications. We find that more than 10 primary comb lines can be generated within the spectrum of modulation instability gain in our microring resonator. The generation is also verified by numerical simulations and the measured linewidth confirms the high coherence of the generated primary comb lines.

Simultaneous all-optical phase noise mitigation and automatically locked homodyne reception of an incoming QPSK data signal.

Simultaneous phase noise mitigation and automatic phase/frequency-locked homodyne reception is demonstrated for a 20-32 Gbaud QPSK signal. A phase quantization function is realized to squeeze the phase noise of the signal by optical wave mixing of the signal, its third-order harmonic, and their corresponding delayed variant conjugates, converting the noisy input into a noise-mitigated signal. In a simultaneous nonlinear process, the noise-mitigated signal is automatically phase- and frequency-locked with a "local" pump laser, avoiding the need for feedback or phase/frequency tracking for homodyne detection.

Experimental demonstration of phase-sensitive regeneration of a binary phase-shift keying channel without a phase-locked loop using Brillouin amplification.

All-optical phase regeneration of a binary phase-shift keying signal is demonstrated at 10-30 Gb/s without a phase-locked loop in a phase-sensitive amplification-based system using Brillouin amplification of the idler. The system achieves phase noise reduction of up to 56% and up to 11 dB OSNR gain at 10 bit error rate for the 10 Gb/s signal. The system's sensitivity to different parameters and stability is also evaluated.

Demonstration of optical multicasting using Kerr frequency comb lines.

We experimentally demonstrate optical multicasting using Kerr frequency combs generated from a SiN microresonator. We obtain Kerr combs in two states with different noise properties by varying the pump wavelength in the resonator and investigate the effect of Kerr combs on multicasting. Seven-fold multicasting of 20 Gbaud quadrature phase-shift-keyed signals and four-fold multicasting of 16-quadrature amplitude modulation signals have been achieved when low-phase-noise combs are input into a periodically poled lithium niobate waveguide.

Reconfigurable optical inter-channel interference mitigation for spectrally overlapped QPSK signals using nonlinear wave mixing in cascaded PPLN waveguides.

A reconfigurable all-optical inter-channel interference (ICI) mitigation method is proposed for an overlapped channel system that avoids the need for multi-channel detection and channel spacing estimation. The system exhibits a 0.5-dB implementation penalty compared to a single-channel baseline system.

Experimental demonstration of tunable homodyne detection of WDM and dual-polarization PSK channels by automatically locking the channels to a local pump laser using nonlinear mixing.

This Letter proposes a method for tunable automatically locked homodyne detection of wavelength-division multiplexing (WDM) dual-polarization (DP) phase-shift keyed (PSK) channels using nonlinear mixing. Two stages of periodically poled lithium niobate (PPLN) waveguides and an LCoS filter enable automatic phase locking of the channels to a local laser.

Effect of a breather soliton in Kerr frequency combs on optical communication systems.

In this study, we numerically investigate the effect of Kerr-comb-generated breather soliton pulses on optical communication systems. The breather soliton pulse amplitude and spectrum envelope oscillate periodically in time. Simulations show that the spectrum of each comb line in the breather soliton state has multiple sub-teeth due to the periodic oscillation of the comb spectrum.

All optical signal level swapping and multilevel amplitude noise mitigation based on different regions of optical parametric amplification.

All optical signal level swapping and multilevel amplitude noise mitigation are experimentally demonstrated using the three gain regions of optical parametric amplification, i.e., linear, saturation, and inversion.

Optical channel de-aggregation of quadrature-phase-shift-keying and eight-phase-shift-keying data using mapping onto constellation axes.

An eight-phase-shift-keying signal is experimentally de-aggregated onto two four-pulse amplitude modulation signals using nonlinear processes in optical elements. Quadrature-phase-shift-keying signals are similarly de-multiplexed into two binary phase shift keying signals by mapping the data points onto the constellation axes. De-multiplexing performance is evaluated as a function of the optical signal-to-noise ratio of the incoming signals.

Phase noise mitigation of QPSK signal utilizing phase-locked multiplexing of signal harmonics and amplitude saturation.

We demonstrate an all-optical phase noise mitigation scheme based on the generation, delay, and coherent summation of higher order signal harmonics. The signal, its third-order harmonic, and their corresponding delayed variant conjugates create a staircase phase-transfer function that quantizes the phase of quadrature-phase-shift-keying (QPSK) signal to mitigate phase noise. The signal and the harmonics are automatically phase-locked multiplexed, avoiding the need for phase-based feedback loop and injection locking to maintain coherency.

Tunable radio frequency photonics filter using a comb-based optical tapped delay line with an optical nonlinear multiplexer.

A radio frequency (RF) photonic filter is experimentally demonstrated using an optical tapped delay line (TDL) based on an optical frequency comb and a periodically poled lithium niobate (PPLN) waveguide as multiplexer. The approach is used to implement RF filters with variable bandwidth, shape, and center-frequency.

Experimental demonstration of robustness and accuracy of a DLI-based OSNR monitor under changes in the transmitter and link for different modulation formats and baud rates.

We experimentally studied the performance of a delay-line interferometer-based optical signal-to-noise ratio (OSNR) monitor that is pre-calibrated in optimal conditions for 25-Gbaud pol-muxed quadrature-amplitude-modulation (QAM) signals, when unpredicted changes outside the monitor occurred either in the transmitter or the link.

Optical Nyquist channel generation using a comb-based tunable optical tapped-delay-line.

We demonstrate optical Nyquist channel generation based on a comb-based optical tapped-delay-line. The frequency lines of an optical frequency comb are used as the taps of the optical tapped-delay-line to perform a finite-impulse response (FIR) filter function. A single optical nonlinear element is utilized to multiplex the taps and form the Nyquist signal.

Reconfigurable 2D optical tapped-delay-line to perform correlation on images.

We demonstrate a reconfigurable 2D optical tapped-delay-line (OTDL) to correlate quadrature-phase-shift-keying (QPSK) 20 Gbaud data. By implementing two independent tapped-delay-lines which performs correlation over rows (inner sum), along with a coherent multiplexing of the correlated rows which performs correlation over columns (outer sum), a 2D correlator is achieved with average error vector magnitude (EVM) of ∼7.8%.

Demonstration of tunable optical generation of higher-order modulation formats using nonlinearities and coherent frequency comb.

We demonstrate a tunable, optical generation scheme of higher-order modulation formats including pulse amplitude modulation (PAM) and quadrature amplitude modulation (QAM). Using this method, 100.4 Gbit/s 16-QAM and 120 Gbit/s 64-QAM were generated from 50.

Demonstration of all-optical phase noise suppression scheme using optical nonlinearity and conversion/dispersion delay.

We propose and demonstrate an all-optical phase noise reduction scheme that uses optical nonlinear mixing and tunable optical delays to suppress the low-speed phase noise induced by laser linewidth. By utilizing the phase conjugate copy of the original signal and two narrow-linewidth optical pumps, the phase noise induced by laser linewidth can be reduced by a factor of ∼5 for a laser with 500-MHz phase noise bandwidth. The error-vector-magnitude can be improved from ∼30% to ∼14% for the same laser linewidth for 40-Gbit/s quadrature phase shift keying signal.