We demonstrate a low latency delay of a radio frequency (RF)–linear frequency-modulated (LFM) pulse by modulating it onto optical carriers from a Kerr comb and sending the signal through a concatenation of off-the-shelf linearly chirped fiber Bragg gratings (LC-FBGs) and chirped-and-sampled FBG (CS-FBG). We characterize the frequency response and latency of the LC-FBG and CS-FBG. Then, experimentally, the LFM pulse performance is characterized by measuring the peak sidelobe level (PSL) at the output of the tunable delay system. The experiment, performed with an LFM pulse of 1 GHz bandwidth at a 10 GHz center frequency, shows a PSL better than 34.4 dB, attesting to the high quality of the buffer RF transfer function. Thus, the proposed optical memory buffer architecture, utilizing compact devices based on a Kerr comb and FBGs, offers several benefits for delaying LFM pulses, including (i) a larger tunable delay range, (ii) low latency, (iii) wide bandwidth, and (iv) high PSL.

We demonstrate a tunable optical tapped delay line (OTDL) using delays based on orbital angular momentum (OAM) modes to perform data correlation and equalization for a quadrature phase-shift-keyed (QPSK) signal. The system: (i) creates N signal replicas as N taps on different OAM modes through mode multicasting, (ii) tunes the complex tap weights (i.e., amplitude and phase of the modes), (iii) applies a tunable mode-dependent delay to each replica, and (iv) coherently combines the taps in order to realize the OTDL functions of correlation and equalization. The experiment results: (i) identify the correlation occurrence of a sequence of 2 or 3 symbols in an incoming 10 or 20 Gbaud QPSK signal, and (ii) equalize the distortion caused by chromatic dispersion for a 20-Gbaud QPSK data signal and reduce error vector magnitude (EVM) from ∼25.4 to ∼12.9%.

We present a 4λ×112 Gb/s/λ hybrid-integrated silicon photonic TX suitable for 400G Ethernet modules and co-packaged optics. The photonic IC (PIC) uses cascaded micro-ring modulators (MRMs) with integrated heaters for efficient wavelength division multiplexing (WDM). The 28nm CMOS electronic IC includes PAM4 MRM drivers with nonlinear FFE and control circuits to stabilize MRM performance against process and temperature variations. A thermal control scheme based on sensing MRM photocurrents is used to minimize monitoring hardware in the PIC. Measured results demonstrate 112 Gb/s PAM4 operation with <0.7 dB TDECQ from each of the 4 channels. To our best knowledge, this is the highest per-λ data rate reported for an O-band ring-based WDM transmitter.

We demonstrate a silicon-photonic dual-polarization transmitter with an integrated on-chip laser, transmitting 260 Gb/s PAM-4 data on a single wavelength carrier, and a single-chip polarization demultiplexer, recovering the polarization multiplexed signals with a TDECQ of 3.0 dB for both polarizations.

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 demonstrate a silicon micro-ring modulator supporting 128 Gb/s NRZ modulation with SNR=5.2, ER=3.8dB, 0.8 Vpp drive swing, and 5.3 fJ/bit power consumption. We have also achieved 192 Gb/s PAM-4 modulation with TDECQ of 2.5dB.

We demonstrate an all-silicon micro-ring resonant photodetector with a responsivity of 0.23 A/W and dark current <100nA capable of detecting 112 Gb/s PAM4 signal with an eye closure penalty of <1.0 dB.

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 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 ${\sim}{0.5}\;{\rm dB}$∼0.5dB 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.

We demonstrate a tunable OTDL using the orbital angular momentum modal domain to create the taps through mode-multicasting. We demonstrate 2-3 tap correlation and equalization. Equalization results show EVM improvement for a 20-Gbaud QPSK with chromatic dispersion of 20-km from EVM=26.3% to EVM=11.4%

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 >3dB when compared to DSB.

We demonstrate an optical second-order Volterra filter using wave mixing and delays. We measure the frequency response and perform the compensation of a nonlinearly distorted 20Gbaud 4-APSK signal with BER reduction from 8.2×10-3 to 3.2×10-3.

We experimentally demonstrate the use of a high-coherence hybrid silicon (Si)/III–V semiconductor laser as the light source for a transmitter generating 20 Gbaud 16- and 64- quadrature amplitude modulated (QAM) data signals over an 80 km single-mode fiber (SMF) link. The hybrid Si/III–V laser has a measured Schawlow–Townes linewidth of ∼ 10 k H z , which is achieved by storing modal optical energy in low-loss Si, rather than the relatively lossy III–V materials. We measure a received bit error rate (BER) of 4.1 × 10 − 3 when transmitting the 64-QAM data over an 80 km SMF using the hybrid Si/III–V laser. Furthermore, we measure a BER of < 1 × 10 − 4 with the Viterbi–Viterbi digital carrier phase recovery method when transmitting the 16-QAM data over an 80 km SMF using the hybrid Si/III–V laser. This performance is achieved at power penalties lower than those obtained with an exemplary distributed feedback laser and slightly higher than those with an exemplary narrow-linewidth external cavity laser.

We experimentally demonstrate tunable modulation format conversion of quadrature phase shift keying (QPSK) to four-level pulse amplitude modulation (PAM4) using nonlinear wave mixing in periodically-poled Lithium-Niobate (PPLN) waveguides. The conversion operation is accomplished by: (a) rotating the data constellation by applying a phase offset, and (b) offsetting the data constellation in the radial direction by adding a constant bias optical power. In this manner, the constellation data points can be tailored to have different amplitude levels, thus realizing the PAM4 format. In a tunable fashion, we convert 20- and 30-Gbit/s QPSK channels to PAM4, and open eye diagrams and low bit-error-rates (BER) are obtained at the receiver. We investigate the impact of phase rotation on the eye diagram of the received signal, and we determine that a phase rotation of ∼71°provides substantially good performance. Wavelength tunability of the output PAM4 is demonstrated by tuning the wavelength of the pump. In addition, open eye is also obtained when a high phase noise laser is used for modulating QPSK signal.

A tunable and reconfigurable optical aggregation system is experimentally demonstrated. Optical Nyquist pulses are generated on multiple channels using a microresonator-based Kerr optical frequency comb and insertion of uniform lines by an intensity modulator. Data are modulated on optically generated Nyquist pulses and aggregated through nonlinear wave mixing in a periodically poled lithium niobate (PPLN) waveguide. Two quadrature-phase-shift-keying (QPSK) channels are aggregated to a single 16-quadrature amplitude modulation (16-QAM) channel of Nyquist pulses. To demonstrate the system tunability, we perform aggregation over different baud rates and different modulation formats. The reconfigurability of the system is demonstrated by aggregating two binary-phase-shift-keying (BPSK) channels into a QPSK or a 2-level amplitude-shift keying and a 2-level phase-shift keying (2-ASK/2-PSK) channel by tuning the relative phase and amplitude of the inputs. Furthermore, three BPSK channels are aggregated into one 4-ASK/2-PSK channel. The quality of the aggregated channel is investigated using two different approaches for wave mixing in the PPLN waveguide.

Coherent optical communication is considered as an indispensable solution to the ever-increasing demand for higher data rates. To reduce the cost and form factor of coherent transceivers, full integration of photonic devices including lasers, modulators, amplifiers, photodetectors, and other components is necessary. However, as fabricating optical isolators on chip remains extremely challenging, optical feedback, which can degrade the coherence of semiconductor lasers, becomes the main obstacle, thwarting large-scale photonic integration. An appealing solution to such a problem is to use semiconductor lasers with intrinsic insensitivity to optical feedback as the integrated light sources. The heterogenous Si/III-V lasers, with their built-in high-Q resonators, are expected to possess a robustness to optical feedback which exceeds by several orders of magnitude compared to commercial III-V distributed feedback (DFB) lasers, which will be validated here. We present data showing that the heterogeneous Si/III-V lasers can preserve their phase coherence under much larger optical feedback and therefore function without severe degradation in isolator-free coherent optical communication systems.

We by simulation and experiment investigate appropriate dispersion values and numbers of the dispersion elements for a phase retrieval based direct-detection system. A 149.5-Gbit/s QPSK transmission using phase retrieval with two dispersion elements is demonstrated in a WDM system.

We numerically and experimentally report flexible spectrum sharing of two asynchronous phase-shift keying (PSK) signals using power division multiplexing. We show that a hybrid quadrature-amplitude-modulated signal is generated when two PSK signals with different power levels are superposed. By using successive interference cancellation, a 20 Gbaud "strong" signal combined with a 9 or 4 Gbaud "weak" signal can be recovered sequentially with bit-error rate performance below the forward error correction threshold. In addition, we show the dependence of system performance on the power ratio between the strong and weak signals. These two signals can contain different baudrates, pulse shapes, and modulation formats.

We experimentally demonstrate a delay subsystem that utilizes a combination of three types of fiber-Bragg gratings (FBGs), wavelength conversion, and wavelength multicasting using a frequency comb source. While the linearly chirped FBG together with a channelized dispersion compensation one, provide the continuous delay, an array of standard FBGs utilizes the multicast copies to extend the delay in steps determined by the array geometry. Following the characterization of the group-delay dispersion performance of the FBGs, a 10 Gbaud quadrature phase-shift keying (QPSK) signal is continuously delayed by 2.2 ns and discretely delayed in 3 steps of similar to 2 ns, to provide a total gap-less delay of 8.1 ns. The delayed signal is finally recovered to its original wavelength with an optical signal-to-noise ratio (OSNR) penalty of similar to 2.3 dB. Furthermore, we show that the penalty can be reduced to 1.2 dB after equalization and report delaying a 20 Gbaud QPSK signal. The demonstrated high-quality delay performance is quite scalable. The continuous delay can be extended to tens of nanoseconds, limited only by the performance of the used components, while the discrete delay is determined by the number of multicast copies and propagation distance between the reflections in the array of FBGs.