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Phase manipulation is crucial for radio frequency (RF) and optical signal processing. In this work, we introduce an interferometric phase-reduction scheme to flexibly suppress the phase change while simultaneously amplifying the signal. We develop a numerical model to evaluate the characteristics of this processing scheme. The concept is further demonstrated with a proof-of-principle experiment that modifies the phase and gain profiles induced by stimulated Brillouin scattering in an optical fiber. This scheme provides an additional freedom to control the signal phase and amplitude in RF/optical processing functionalities.

Unlike the scenario in traditional high-order QAM signals, Viterbi and Viterbi (V&V) algorithm becomes more powerful for carrier phase recovery (CPR) in probabilistically shaped (PS) signals. The reason relies on that the PS signals assign much higher probability to the innermost 4 constellation points, which are qualified for V&V algorithm. In this article, we compare the performance of V&V algorithm to that of blind phase search (BPS) algorithm for PS-64 QAM and PS-256 QAM in the aspects of window size and signal to noise ratio under the constraint of different laser linewidths. Besides, the impact of signal entropy on both algorithms is investigated. The computational complexity and running time for both algorithms are also summarized. Results show that the two algorithms perform similarly for PS-64 QAM signals when the linewidth is less than 1 MHz while the running time of BPS algorithm is at least 45 times longer than that of V&V algorithm. Finally, we conduct a coherent detection based optical back-to-back experiment where the laser linewidths of both transmitter laser and local oscillator are 100 kHz. For PS-64 QAM, both V&V and BPS algorithms show negligible penalty compared to the performance of homodyne detection, verifying the feasibility of applying V&V algorithm in the CPR of PS-64 QAM.

We demonstrate an effective wavelength multicast scheme to generate 16 new channels with low penalty and high spectral efficiency using a single optical pump. The scheme capitalizes on cross-phase modulation of an input signal by a rate-multiplied pump pulse train to generate multiple wavelength channels. The pulse rate is multiplied in an energy-efficient way via the temporal Talbot effect. In the experiment, we have successfully achieved more than 20 multicast copies of a 40-Gb/s QPSK OFDM single-sideband signal. The copied channels are spaced by 50 GHz which is determined by five times multiplication on the repetition rate of a 10-GHz input pump pulse train. The BER performances of the central 16 channels are evaluated, showing less than 1.3-dB OSNR difference among the channels. Compared to the original single-sideband signal, the central channel experiences only a 0.1-dB OSNR penalty in our approach. The result verifies the effectiveness of our scheme in realizing spectrally efficient and high-channel-count wavelength multicast by using a single pump generated with relatively low-speed electronic resources.

In this letter, we investigate the performances of different learning rate decay schedules in least mean square (LMS) based equalization. The required iteration numbers for convergence are 275, 150 and 150 for time-based, staircase and exponential decay schedules respectively, which are much less than 1000, the required iteration number for constant learning rate schedule in processing our experimental data. Meanwhile, the corresponding training times are reduced to shorter than one third, one fifth and one fourth of that in constant schedule. The system performances in the aspects of carrier to signal power ratio and optical signal to noise ratio are measured in a 110 Gbit/s probabilistically shaped 64 QAM direct detection system to evaluate different learning rate decay schedules.

We experimentally demonstrate an optical phase conjugation (OPC) enhanced transmission system based on the Kramers–Kronig direct detection scheme. The single-sideband signal is modulated at 80-Gb/s 16-QAM with orthogonal frequency-division multiplexing. In the 180-km transmission experiment, the OPC surpasses digital chromatic dispersion compensation in Q factor, optimum launched optical power (LOP) and optical signal-to-noise ratio (OSNR) sensitivity. The improvement in Q factor, optimum LOP and OSNR sensitivity are 0.9 dB, 1.0 dB, and 1.7 dB respectively. The superiority of OPC over pure optical dispersion compensation is also analyzed by simulation. The results indicate that the OPC is an effective means to enhance the transmission performance of single-sideband signals.

The programmable envelope shaping technique of high-repetition-rate optical pulse sequences with low-speed modulation is summarized in this letter. First, we reviewed our work on amplitude-only modulation approach for generating symmetric envelopes, including parabolic, step-like and binary patterns. Next, we reported our recent demonstration of asymmetric-enveloped pulse generation achieved by applying simultaneous amplitude and phase modulations. The asymmetric patterns we demonstrated include a trapezoid envelope and a random optical pattern. Finally, we analyze the influence of practical issues of the implementation on the generated optical pulse trains through numerical simulation. The newly proposed scheme provides flexibility in envelope processing and enriches the diversity of temporal pulse shapes.

In this paper, we report the first demonstration of using silicon photonic integrated circuits for 62 Gb/s 4-quadrature-amplitude-modulation (4-QAM) orthogonal frequency-division multiplexing (OFDM) and 112 Gb/s 16-QAM OFDM over an 80-km standard single-mode fiber (SSMF) transmission with the Kramers–Kronig (KK) direct detection. Single sideband optical signals are generated by two silicon micro-ring modulators, and directly detected by a single integrated germanium-on-silicon waveguide photodiode fabricated in a multi-project wafer run. The complex optical field can be fully re-constructed by KK detection after an 80-km SSMF transmission. The integrated transceivers with KK detection are suitable for future low-cost, low-power, and small-footprint data center interconnects in the short and medium reach.

In this letter, we propose a novel approach for optical single-sideband (SSB) generation by utilizing four-wave mixing Bragg scattering (FWM-BS). The scheme comprises three stages: the initial FWM-BS stage, the spectral phase control stage, and the final FWM-BS stage. Constructive or destructive interference can occur between the sidebands generated in the two FWM-BS stages, depending on the spectral phase mask applied between the stages. By applying an appropriate phase mask, one sideband can be eliminated by destructive interference, while the other sideband is magnified by constructive interference, thus realizing SSB generation. Both upconverted and downconverted SSB generations can be achieved. To verify the scheme, we experimentally demonstrate 50-GHz optical frequency upconversion and downconversion separately for multi-channel 16-Gbaud QPSK signals.

We demonstrate the generation of optical frequency combs with tunable spacing at the hundred-GHz range in the 1550-nm window. The widely spaced combs are realized through silicon-based cross-phase modulation. The optical pump is prepared by multiplication of a 10-GHz train of 1.7-ps pedestal-free pulses. Energy-efficient temporal Talbot processing is used to multiply the repetition rate by a factor of up to 20. In our approach, the multiplication factor can be flexibly controlled by tuning the temporal dispersion inside an optical processor. Optical frequency combs with spacings ranging from 140 to 200 GHz have been successfully generated with a maximum carrier-to-noise suppression ratio of ∼45 dB.

Using modulation-assisted temporal Talbot processing followed by cross-phase modulation in a silicon-on-insulator (SOI) waveguide, we multiply the repetition rate of a 10-GHz optical pulse train and generate widely spaced optical frequency combs with programmable spacing.

Probabilistically shaped signals with intermediate constellation sizes are found to be relatively tolerant to laser phase noise. We demonstrate experimentally that PS-100 QAM is superior to PS-256 QAM for 200-kHz sum linewidth heterodyne detection.

Using a silicon-based four-wave mixing interferometer, we apply phase-amplification to magnify the optical delay induced by stimulated Brillouin scattering slow light. The delay can be effectively tuned by controlling the power of a reference pulse.

We investigate high-speed optical sampling based on temporal lenticular lens. In our scheme, the sampling rate is enhanced by applying multi-wavelength time-interleaved processing. The effective number of bits ranges from 4.2 to 3.5 over the frequency range of 40 – 80 GHz.

Optical sampling based on the temporal lenticular lens can eliminate the requirement of high-power laser pulses and the need of wavelength conversion. In this letter, we improve the spectrally efficient sampling scheme to process wideband optical signals applicable for communications. A parallel architecture based on time-interleaved sampling is demonstrated to increase the effective sampling rate. A 20-Gb/s signal is experimentally sampled and recovered at 80 GSample/s. The sampling system performance is measured and analyzed. The input optical frequencies from 40 to 80 GHz are characterized with an effective number of bits ranging from 4.2 to 3.5.

Four-wave-mixing (FWM) crosstalk in frequency nondegenerate phase-sensitive fiber optical parametric amplifiers (FOPAs) applied for wavelength-division-multiplexed (WDM) signals is investigated experimentally and theoretically. We analyze significant FWM interactions that can generate crosstalk components in the signal band. Approximate expressions are developed for the FWM crosstalk to conveniently estimate how it scales with the system parameters. The analytical results agree well with the experimental results and simulations. The results show that the FWM crosstalk grows dramatically with the increase of the system parameters, including input signal power per channel, signal gain, the length of nonlinear fiber, and signal channel number. We also compare the FWM crosstalk in the phase-sensitive FOPA and the phase-insensitive FOPA experimentally and explain the difference of their crosstalk behaviors through theoretical analysis and simulations. The results show that, due to the presence of idlers at the input, the phase-sensitive FOPA produces higher crosstalk than the phase-insensitive FOPA even when they have equivalent signal gains. It indicates that the crosstalk in the phase-sensitive FOPA is more detrimental so that careful design of the amplifier to suppress the impact of the FWM crosstalk is necessary. The analytical solutions obtained in this paper are useful in the design of PS-FOPAs for amplifying WDM signals.

Support vector machine (SVM) based differential pulse-width pair Brillouin optical time domain analyzer (DPP-BOTDA) has been proposed and experimentally demonstrated. With only one SVM model, temperature distribution along 5 km fiber under test has been successfully extracted from differential Brillouin gain spectrum (BGS) measured under different spatial resolution in DPP-BOTDA. The temperature accuracy by SVM is better than that by Lorentzian curve fitting (LCF), especially when the pump pulse width difference and the number of trace averaging used in the measurement are small, indicating larger tolerance of SVM to high spatial resolution and low signal-to-noise ratio. SVM is also more robust to a wide range of frequency scanning steps and has less accuracy degradation under large frequency scanning step. To extract temperature from 50 000 differential BGSs, 133.17 and 1.12 s are consumed by SVM-0.1 and SVM-1 °C, respectively, both of which are much shorter than that by LCF. The data processing time of SVM is further shortened with the help of principle component analysis for data dimension reduction. SVM for measurand extraction would be especially helpful in the scenario of DPP-BOTDA where high data sampling rate is required to resolve plenty of submeter scale sensing points.