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. Open eye-diagrams are obtained for in-phase and quadrature-phase components of the signal and ∼2 dB OSNR gain is achieved at BER 10-3.

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. Experiments using a dual-carrier QPSK overlapped system with both 20G-baud and 25G-baud under different channel spacing conditions evaluate the performance of the method. Improved signal constellation and receiver sensitivity demonstrate the effectiveness of this approach. This results in over 4-dB OSNR of benefit when the system Q-factor reaches a forward error correction (FEC) threshold of 8.5 dB under less-than-baudrate channel spacing conditions.

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.

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. The two-amplitude-shift-keying and eight-quadrature-amplitude-modulation optical communication systems with baud rates of both 10 and 20 Gbaud have been employed to demonstrate the proposed approaches. Less than 1% error-vector-magnitude degradation is observed after signal level swapping. For amplitude noise mitigation, a more than 20% decrease in amplitude error is confirmed.

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. The residual phase noise converts to amplitude noise in the quantizer stage, which is suppressed by parametric amplification in the saturation regime. Phase noise reduction of ∼40% and OSNR-gain of ∼3 dB at BER 10(-3) are experimentally demonstrated for 20- and 30-Gbaud QPSK input signals.

Three-input optical addition and subtraction of quaternary base numbers.

We present an approach to implementing three-input addition and subtraction of quaternary base numbers in the optical domain using multiple non-degenerate four-wave mixing (FWM) processes in a single highly nonlinear fiber (HNLF) and differential quadrature phase-shift keying (DQPSK) signals. By employing 100-Gbit/s three-input return-to-zero DQPSK (RZ-DQPSK) signals (A, B, C), we demonstrate 50-Gbaud/s three-input quaternary hybrid addition and subtraction (A + B-C, A + C-B, B + C-A). Moreover, by adding a conversion stage from C to -C via conjugated degenerate FWM, we also demonstrate 50-Gbaud/s three-input quaternary addition (A + B + C). The power penalties of three-input quaternary addition and subtraction (A + B-C, A + C-B, B + C-A, A + B + C) are measured to be less than 6 dB at a bit-error rate (BER) of 10(-9). In addition, no significant degradations are observed for RZ-DQPSK signals (A, B, C or -C) after the operations of quaternary addition and subtraction.

2 Tbit/s free-space data transmission on two orthogonal orbital-angular-momentum beams each carrying 25 WDM channels.

We demonstrate a 2 Tbit/s free-space data link using two orthogonal orbital angular momentum beams each carrying 25 different wavelength-division-multiplexing channels. We measure the performance for different modulation formats, including directly detected 40 Gbit/s nonreturn-to-zero (NRZ) differential phase-shift keying, 40 Gbit/s NRZ on-off keying, and coherently-detected 10 Gbaud NRZ quadrature phase-shift keying, and achieve low bit error rates with penalties less than 5 dB.

Simultaneous subchannel data updating for multiple channels of 16-quadrature amplitude modulation signals using a single periodically poled lithium niobate waveguide.

Subchannel data updating of high-order modulation format signals using cascaded sum- and difference-frequency generation in a single periodically poled lithium niobate waveguide is demonstrated. One quadrature phase-shift-keying subchannel of a 16-quadrature amplitude modulation (QAM) signal at 40 Gbit/s is successfully updated, with an optical signal-to-noise ratio (OSNR) penalty of ~2 dB for return-to-zero and ~4 dB for non-return-to-zero at a bit-error rate (BER) of 2×10(-3). Simultaneous processing of four wavelength-multiplexed 16-QAM signals with an average OSNR penalty of 4.5 dB at a BER of 2×10(-3) is also demonstrated.

Efficient generation and multiplexing of optical orbital angular momentum modes in a ring fiber by using multiple coherent inputs.

We propose an approach to efficiently generate and multiplex optical orbital angular momentum (OAM) modes in a fiber with a ring refractive index profile by using multiple coherent inputs from a Gaussian mode. By controlling the phase relationship of the multiple inputs, one can selectively generate OAM modes of different states l. By controlling both the amplitude and phase of the multiple inputs, multiple OAM modes can be generated simultaneously without additional loss coming from multiplexing. We show, by simulation, the generation of OAM modes (OAM state |l|<3) with mode purity greater than 99%. The power loss of generating and multiplexing seven modes is about 35%. A transmitter for an OAM-based mode-division multiplexing system is proposed based on the discrete Fourier transform between the data carried by the multiple inputs and the data carried by the OAM modes. The experimental implementation of the proposed approach could be achieved by integrating ring fiber, multicore fiber, and photonic integrated circuit technology.

Octave-spanning supercontinuum generation of vortices in an As2S3 ring photonic crystal fiber.

We propose As(2)S(3) ring photonic crystal fiber (PCF) for supercontinuum generation of optical vortex modes. Due to the large material index contrast between As(2)S(3) and air holes in the designed ring PCF, there is a two-orders-of-magnitude improvement of the difference between the effective refractive indices of different vortex modes compared with regular ring fiber. The design freedom of PCFs enables a low dispersion (<60 ps/nm/km variation in total) over a 522 nm optical bandwidth. Moreover, the vortex mode has a large nonlinear coefficient of 11.7/W/m at 1550 nm with a small confinement loss of <0.03 dB/m up to 2000 nm. An octave-spanning supercontinuum spectrum of the vortex mode is generated from 1196 to 2418 nm at -20 dB by launching a 120 fs pulse with a 60 W peak power at 1710 nm into a 1 cm long As(2)S(3) ring PCF.

High-speed addition/subtraction/complement/doubling of quaternary numbers using optical nonlinearities and DQPSK signals.

We propose an innovative approach to implementing multiple arithmetic functions of quaternary numbers using optical nonlinearities and differential quadrature phase-shift keying (DQPSK) signals. By adopting 100 Gbit/s DQPSK signals (A, B) and exploiting nondegenerate four-wave mixing (FWM) for addition/subtraction and degenerate FWM for complement and doubling in a single highly nonlinear fiber, we demonstrate 50 Gbaud/s simultaneous quaternary addition (A+B), dual-directional subtraction (A-B, B-A), complement (-A, -B), and doubling (2B). Power penalties less than 4 dB (addition), 3 dB (dual-directional subtraction), 2 dB (complement), and 3.1 dB (doubling) are observed at a bit-error rate of 10(-9).

UWB monocycle pulse generation using two-photon absorption in a silicon waveguide.

We propose and experimentally demonstrate ultrawideband monocycle pulse generation using nondegenerate two-photon absorption in a silicon waveguide. The free-carrier absorption induced pulse tail at the rising edge of inverted probe pulse is largely compensated by the overlapped pump pulse and results in a symmetric negative monocycle pulse. A 143 ps Gaussian monocycle pulse is successfully obtained with a 131.7% fractional 10 dB bandwidth using a 68 ps pulsed pump. The 10 dB bandwidth and center frequency of the RF spectrum for the generated monocycle pulse can be largely tuned using an optical delay line. An operational bandwidth of 30 nm is demonstrated experimentally with stable performance, and larger optical bandwidth is expected.

Fiber structure to convert a Gaussian beam to higher-order optical orbital angular momentum modes.

We propose a fiber structure of a square core and ring refractive index profile that converts an input circular polarized Gaussian mode into optical orbital angular momentum (OAM) modes. By breaking the circular symmetry of the waveguide, the input circularly polarized fundamental mode in the square core can be coupled into the ring region to generate higher-order OAM modes, corresponding to the transference of spin angular momentum and orbital angular momentum. We show, by using simulation, the generation of OAM modes with a topological charge l up to 9 using <10 mm long fiber. The mode purity is above 96.4% and the extinction ratio can be 30 dB.

Nondegenerate four-wave-mixing-based radio frequency up/downconversion using a parametric loop mirror.

We propose a method of reconfigurable radio frequency up/downconversion using a parametric optical loop mirror. The baseband optical signal can be unconverted to a carrier-suppressed radio-over-fiber signal at a wide range of frequencies without the help of optical filters to remove the optical carrier. Upconversion of up to 60 GHz and downconversion from 40 GHz for both single-channel and wavelength-division-multiplexing signals using the same setup are demonstrated.

Fiber coupler for generating orbital angular momentum modes.

We propose a fiber coupler consisting of a central ring and four external cores to generate up to ten orbital angular momentum (OAM) modes. Four coherent input lights are launched into the external cores and then coupled into the central ring waveguide to generate OAM modes. By changing the size of the external cores, one can selectively excite a high-order OAM mode. The quality of the generated OAM modes can be enhanced by adjusting the polarization state and the phase of input lights. We show the generation of OAM modes with odd charge numbers of -9 to +9 (i.e., 10 modes totally) with mode purity of >99% using <2 mm long fiber. This fiber coupler design can be extended to enable all-fiber spatial-mode (de)multiplexing.

Time-stretch oscilloscope with dual-channel differential detection front end for monitoring of 100 Gb/s return-to-zero differential quadrature phase-shift keying data.

Optical performance monitoring of high-capacity networks is one of the enabling technologies of future reconfigurable optical switch networks. In such networks, rapid performance evaluation of data streams becomes challenging due to the use of advanced modulation formats and high data rates. The time-stretch enhanced recording oscilloscope offers a potential solution to monitoring high-rate data in a practical time scale. Here we demonstrate an architecture with a differential detection front end for simultaneous I/Q data monitoring of a 100 gigabits/s return-to-zero differential quadrature phase-shift keying signal. This demonstration shows the potential of this technology for rapid performance monitoring of high-rate optical data streams that employ advanced modulation formats.

Experimental demonstration of packet-rate 10-Gb/s OOK OSNR monitoring for QoS-aware cross-layer packet protection.

A cross-layer network platform may enable introspective access to the physical layer, allowing optical performance monitoring measurements to feedback to higher layers for packet rerouting and protection. We experimentally demonstrate quality-of-service-aware packet protection that leverages cross-layer signaling based on the monitoring of packets' optical-signal-to-noise ratio. In order to detect degraded data streams, the monitoring system is based on a delay-line Mach-Zehnder interferometer and a field-programmable gate array. The system is realized in an experimental cross-layer enabled optical packet switched fabric, measuring the optical-signal-to-noise ratio for 10-Gb/s OOK streams. The packet protection scheme uses the dynamic performance measurements to actuate a rerouting of high-quality-of-service packets. 8 × 10-Gb/s wavelength-striped optical messages are rerouted through the fabric error-free (bit-error rates less than 10(-12)).

Reconfigurable 2.3-Tbit/s DQPSK simultaneous add/drop, data exchange and equalization using double-pass LCoS and bidirectional HNLF.

We propose a reconfigurable Tbit/s network switching element using double-pass liquid crystal on silicon (LCoS) technology accompanied by bidirectional degenerate four-wave mixing (FWM) in a single highly nonlinear fiber (HNLF). We demonstrate the LCoS + HNLF-based 2.3-Tbit/s multi-functional grooming switch which performs simultaneous selective add/drop, switchable data exchange, and power equalization, for 23-channel 100-Gbit/s return-to-zero differential quadrature phase-shift keying (RZ-DQPSK) signals. Less than 1.5-dB power penalty is observed for power equalization at a bit-error rate (BER) of 10⁻9. Selective single-/two-channel add/drop are achieved with power penalties less than 1.2 dB. Switchable two-channel data exchange and simultaneous six-channel data exchange are implemented with power penalties less than 5 dB.

Optical monitoring of PMD accumulation on a Pol-MUX phase-modulated signal using degree-of-polarization measurements.

We experimentally demonstrate a chromatic dispersion insensitive monitoring technique for monitoring of polarization mode dispersion and time misalignment in a 80 Gbit/s polarization-multiplexed return-to-zero differential phase-shift-keying (Pol-MUX RZ-DPSK) signal using a polarimeter and degree-of polarization (DOP) measurements. This technique is modulation format independent (i.e., applicable to both the phase- and amplitude-modulated data) and capable of measuring the fast change of polarization effects caused by vibration or other fast disturbances in the fiber link. We show that the monitored DOP of this signal decreases by 10.8% with differential group delay of 0-12 ps and decreases by 20% with a maximum misalignment of 12.5 ps between two orthogonally polarized RZ-DPSK channels. These measurements are less sensitive to chromatic dispersion of 0-400 ps/nm.

Multi-channel 100-Gbit/s DQPSK data exchange using bidirectional degenerate four-wave mixing.

We propose a simple approach to performing simultaneous multi-channel data exchange using bidirectional degenerate four-wave mixing (FWM) in a single highly nonlinear fiber (HNLF) assisted by optical filtering. Data exchange between two-channel 100-Gbit/s return-to-zero differential quadrature phase-shift keying (RZ-DQPSK) signals with a variable channel spacing is implemented with a power penalty of less than 4.2 dB at a bit-error rate (BER) of 10(-9). Moreover, simultaneous ITU-grid-compatible four-channel 100-Gbit/s RZ-DQPSK data exchange is demonstrated with a power penalty of less than 4.7 dB at a BER of 10(-9).