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This paper highlights certain optical signal processing (OSP) functions that can be achieved when using optical frequency combs. An optical frequency comb can provide many narrow-linewidth, mutually coherent, and equidistant optical carriers, making it a potentially useful tool for facilitating efficient signal processing functions. In this paper, we explore the following general topics: (a) tailoring a comb for enhanced usability in OSP, such as inserting more lines, generating Nyquist pulse trains, and regenerating a comb at a distance; (b) achieving a tunable linear and nonlinear filter for correlation, equalization, and Volterra filtering, and (c) enabling various transmission and networking functions, such as multicasting, format conversion, and dynamic bandwidth allocation.

A light beam’s frequency can blueshift when the beam travels through a medium that exhibits a time-dependent decrease in the refractive index. Here we show that a metasurface made of a plasmonic an...

The remote control and monitoring of a tunable optical correlator based on nonlinear wave-mixing is demonstrated. The correlator is remotely controlled via the bidirectional transmission of multiple waves over an optical link. The effects of the temperature-drift and link backscattering are mitigated at the transmitter.

We experimentally investigate the tunable Doppler shift in an 80 nm thick indium-tin-oxide (ITO) film at its epsilon-near-zero (ENZ) region. Under strong and pulsed excitation, ITO exhibits a time-varying change in the refractive index. A maximum frequency redshift of 1.8 THz is observed in the reflected light when the pump light has a peak intensity of ∼ 140 G W / c m 2 and a pulse duration of ∼ 580 f s , at an incident angle of 40°. The frequency shift increases with the increase in pump intensity and saturates at the intensity of ∼ 140 G W / c m 2 . When the pump pulse duration increases from ∼ 580 f s to ∼ 1380 f s , the maximum attainable frequency shift decreases from 1.8 THz to 0.7 THz. In addition, the pump energy required to saturate the frequency shift decreases with the increase in pump pulse duration for ∼ x < 1 p s and remains unchanged for ∼ x > 1 p s durations. Tunability exists among the pump pulse energy, duration, and incident angle for the Doppler shift of the ITO-ENZ material, which can be employed to design efficient frequency shifters for telecom applications.

We experimentally demonstrate tunable de-aggregation of two data carrying optical channels using nonlinear wave mixing. For each channel, the signal and its conjugate are coherently added and subtracted in order to map the constellation points onto real and imaginary axes, respectively. Each of two 10 and 15 Gbaud optical channels with a format of 16-quadrature amplitude modulation (16-QAM) are de-aggregated into two 4-level pulse amplitude modulation (4-PAM) signals. We use the same technique to de-aggregate quadrature phase shift keying (QPSK) signals into two binary phase shift keying (BPSK) signals at baud rates of 10 and 15 Gbaud for both channels. System performance is evaluated in terms of the optical signal-to-noise ratio (OSNR), and penalty of ∼ 0.7 dB is observed at bit error rate (BER) of 10−3.

Optical mitigation of interchannel interference (ICI) for multiple spectrally overlapped data channels is experimentally demonstrated without multichannel detection and channel spacing estimation. The ICI mitigation takes place in three stages of periodically poled lithium niobate (PPLN) waveguides. In the first PPLN stage, the conjugate copies of channels are generated. In the second stage, the overlapped signals are coherently multiplexed with different complex taps. In the third stage, the concurrent nonlinear processes and the coherent multiplexing of the signals with their delayed copies compensate the ICI. The bit error rate and the constellation diagrams of wavelength division multiplexed (WDM) channels carrying quadrature phase-shift keying (QPSK) or 16 quadrature amplitude modulation (16-QAM) formats demonstrate the potential capability of the proposed method to reduce the ICI and its possible modulation format transparency. The effect of channel spacing on the performance of the method is also demonstrated. After optical ICI mitigation, a reduction of almost 4 dB is achieved for the value of optical signal-to-noise ratio at BER of 10–3 for 20-GBd QPSK signals with a channel spacing of 17.5 GHz. The overlapped WDM system of 20-GBd 16-QAM signals with channel spacing of 17.5 GHz is also ICI mitigated and error vector magnitudes are reduced by almost 28%.

This tutorial highlights challenges and opportunities in achieving efficient flexible optical networks. Optical signal processing may potentially increase network flexibility because of its functions’ transparency, tunability, and reconfigurability. We review recent advances in high-speed optical signal processing techniques that might enable flexible networks. Various optical approaches that enable key functions are discussed, including format conversion, increases in spectral efficiency, and phase-sensitive operations. We also discuss the potential utilization of basic enabling technologies, such as optical frequency combs and optical nonlinear devices.

An optical tapped-delay-line based on a cascade of MZI under non-ideal operation is simulated. The effect of different non-ideal conditions on FIR filtering metrics and analog or digital signal qualities is investigated.

We experimentally demonstrate a remotely controlled and powered optical switch. The data channel, switching control signal and bias power are wavelength multiplexed and transmitted through 7.66 km SMF to the remote switch. We transmit a 20 Gbaud QPSK signal that is switched at 2 Mb/s rate.

We experimentally demonstrate a 2-4 tap QPSK correlation using a cascade of MZIs which are remotely controlled through an optical link. The delivered power to the correlator is tuned and different target patterns are tailored.

We simulate and experimentally demonstrate a feedback-based probabilistic constellation shaping (FB-PCS) of a 10 Gbaud 16-ary quadrature amplitude modulation (16-QAM) signal. Our approach is to adaptively modify the distribution of transmitted symbols based on errors at the receiver, and assumptions about the channel model are not required. Specifically, the error feedback enables solving an optimization problem to find the distribution that maximizes the mutual information between the input and output of the channel without knowledge of the channel itself. A known training sequence with uniform distribution is transmitted, and the errors at each constellation point are counted at the receiver. This information is relayed to the transmitter, which then updates the data constellation with a new probability distribution such that constellation points with more errors are used less frequently. We examine four different system scenarios in simulation and one scenario in experiment. In simulation, we find that FB-PCS (a) reduces the number of errors when compared to uniform shaping for the four scenarios, and (b) reduces symbol error rate (SER) by approximately an order of magnitude or has similar SER compared to conventional Maxwell–Boltzmann (M–B) shaping. Moreover, we demonstrate that FB-PCS can lead to an SER reduction of ∼ 50 % .

We experimentally demonstrate Kramers–Kronig detection of four 20 Gbaud 16-quadrature-amplitude-modulated (QAM) channels after 50 km fiber transmission using two soliton Kerr combs as signal sources and local oscillators. The estimated carrier phase at the receiver for each of the channels is relatively similar due to the coherence between the frequency comb lines. The standard deviation of the estimated carrier phase difference of the channels is less than 0.08 rad after 50 km single-mode fiber (SMF) transmission. This enables the carrier phase recovery derived from one channel to be shared among multiple channels. In the back-to-back scenario, the bit error rate (BER) performance for shared carrier phase recovery shows an optical signal-to-noise ratio penalty of ∼ 0.5 d B compared to the BER performance for carrier phase recovery when derived for each channel independently. BERs below the forward error correction threshold are achieved after 50 km SMF transmission with both independent and shared carrier phase recovery for four 20-Gbaud 16-QAM signals.

A technique for probabilistic constellation shaping based on distribution learning from a training sequence is investigated. In this approach, the probability distribution is optimized such that it can maximize the mutual information. The effectiveness of this approach is verified by shaping 10 Gbaud 16QAM in simulation and experiment.

We experimentally demonstrate all-optical phase-sensitive quantization of a 10–20 Gbaud quadrature phase-shift keying (QPSK) signal’s phase without an active phase-locked loop (PLL) by utilizing Brillouin amplification (BA) to amplify the fourth-harmonic idler without path separation, allowing the pump, signal, third-harmonic idler and amplified fourth-harmonic idler to remain synchronized in the phase. We observe up to 65% reduction in phase noise and 4.6 dB gain at a bit-error rate (BER) of 1 0 − 4 over 36 min of continues operation.

A scheme for reconfigurable channel slicing and stitching is proposed and experimentally demonstrated. By employing optical nonlinear wave mixing and a coherent frequency comb, a single channel spectrum is sliced and redistributed into fragmented frequency slots, which can be stitched together to recover the original channel at the receiver. This approach is verified through a single channel experiment with the modulation formats of quadrature phase-shift keying and 16 quadrature amplitude modulation. The system exhibits less than 1.5% error-vector-magnitude deterioration and no more than 2-dB optical signal-to-noise ratio penalty, compared to a back-to-back baseline. To demonstrate robustness of the scheme, different parameters of the channel slices are varied, such as relative phase offset, relative amplitude, and the number of slices. A 10-km transmission experiment is also conducted and the additional system penalty is negligible. This scheme is used to experimentally demonstrate fragmented channel bandwidth allocation in a dense 6-channel wavelength-division-multiplexing system. The incoming 20-Gbaud optical channel is successfully reallocated into two fragmented frequency slots and reconstructed at the receiver.

We experimentally demonstrate remotely powered, controlled, and monitored optical switching. The control signal of the switch is modulated on an optical wave and sent from a transmitter. At the switch location, the control signal is converted from an optical to an electrical signal to drive the switch. In addition, to provide electrical power at the switch location, optical power is sent from a distance and converted to electrical power using a series of photodiodes. We experimentally demonstrate (a) 1 Gb/s on-off keying data channel transmission and switching with a 1 MHz optically delivered control signal, and (b) 40 Gb/s quadrature phase-shift keying data channel transmission and remotely monitoring switch state and bias drift. The switching function is demonstrated without using any local electrical power supply. Moreover, the monitoring tones are transmitted to the remote switch and fed back to the transmitter to realize a switch state and detect the bias drift.

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. For each of nine Kerr frequency comb lines, we insert sub-groups of uniform and coherent lines to generate nine WDM channels. The deviations from ideal Nyquist pulses for the nine channels at repetition rates of 6 and 2 GHz are between 4.2%-6.1% and 2%-4.5%, respectively. Each WDM channel is modulated with on-off keying (OOK) at 6 Gbit/s. In addition, we show the reconfigurability of this method by varying the number of WDM channels, the generated sinc-shaped pulse train repetition rates, the duration, and the number of zero-crossings. (C) 2019 Optical Society of America

We experimentally demonstrate three-fold wavelength multicasting of a 64-quadrature-amplitude-modulation (QAM), 120-Gbit/s data channel using a microresonator Kerr frequency comb and nonlinear wave mixing. The multicasting is achieved with a data signal and four comb lines serving as the pump lasers in a periodically poled lithium niobate (PPLN) waveguide. Minimal extra phase noise from the pumps is introduced into the multicast copies due to the mutual coherence between the Kerr comb lines. All three multicast copies achieve a bit-error rate (BER)

A tunable optical single-sideband generation using optical frequency combs and nonlinear wave-mixing is experimentally demonstrated for 20-Gbit/s OOK and PAM4 channels. The system performance is investigated by tuning the number and spacing of comb lines.

We experimentally achieve optical time multiplexing of multiple Nyquist channels. A single microresonator Kerr comb allows the generation of multiple Nyquist sinc shaped pulse trains. TDM of pulse trains are obtained through wavelength conversion inside a PPLN.

A tunable and reconfigurable optical Nyquist channels aggregation is experimentally demonstrated. The proposed method two aggregate lower order modulation formats into a single higher order one. Two 16 Gbaud QPSK Nyquist pulses aggregated to a 16 QAM Nyquist pulse.

We experimentally demonstrate a tunable optical second-order Volterra filter using wave mixing and delays. Wave mixing is performed in a periodically poled lithium niobate waveguide with the cascaded sum-frequency generation and difference-frequency generation processes. Compared to conventional optical tapped delay line structures, second-order taps are added through the wave mixing of two signal copies. We measure the frequency response of the filter by sending a frequency-swept sinusoidal wave as the input. The tap weights are tuned with a liquid-crystal-on-silicon waveshaper for different filter configurations. With the additional second-order taps, the filter is able to perform a nonlinear function. As an example, we demonstrate the compensation of a nonlinearly distorted 10–20 Gbaud 4-amplitude and phase shift keying signal.

We experimentally demonstrate tunable optical single-sideband (SSB) generation using a tapped-delay-line (TDL) optical filter for 10 and 20 Gbit/s on/off-keying (OOK) signals and a 20 Gbit/s four-level pulse-amplitude-modulated (PAM4) signal. The optical SSB filter is realized by using an optical frequency comb, wavelength-dependent delay, and nonlinear wave-mixing to achieve the TDL function. Moreover, SSB tunability is achieved by adjusting the amplitude, phase, frequency spacing, and number of selected optical frequency comb lines. We show that the one-sideband suppression of a double-sideband (DSB) channel can be enhanced as the number of taps is increased; however, we do measure a ∼ 1.5 % error-vector-magnitude penalty. Furthermore, we demonstrate that the chromatic-dispersion-induced penalty after 80 km standard-single-mode-fiber transmission of a 10 Gbit/s SSB OOK signal without chromatic dispersion compensation has been reduced by > 3 d B when compared to DSB.