[show abstract][hide abstract] ABSTRACT: We present a compact (130 μm cladding diameter) trench-assisted multi-orbital-angular-momentum (OAM) multi-ring fiber with 19 rings each supporting 22 modes with 18 OAM ones. Using the high-contrast-index ring and trench designs, the trench-assisted multi-OAM multi-ring fiber (TA-MOMRF) features both low-level inter-mode crosstalk and inter-ring crosstalk within a wide wavelength range (1520 to 1630 nm), which can potentially enable Pbit/s total transmission capacity and hundreds bit/s/Hz spectral efficiency in a single TA-MOMRF. Moreover, the effective refractive index difference of even and odd fiber eigenmodes induced by the ellipticity of ring and fiber bending and their impacts on the purity of OAM mode and mode coupling/crosstalk are analyzed. It is found that high-order OAM modes show preferable tolerance to the ring ellipticity and fiber bending. The designed fiber offers favorable tolerance to both small ellipticity of ring (<-22 dB crosstalk under an ellipticity of 0.5%) and small bend radius (<-20 dB crosstalk under a bend radius of 2 cm).
[show abstract][hide abstract] ABSTRACT: We propose a simple approach to broadband generation of orbital angular momentum (OAM) carrying vector beams based on compact metamaterials. It consists of two concentric rings in a gold film, where each ring is composed of subwavelength rectangular apertures with gradually varied orientation. The subwavelength rectangular aperture serves as a localized spatial polarizer. We show the generation of different OAM-carrying vector beams with OAM charge number and polarization order varying from -3 to +3 using a 11.2×11.2 μm device. The extinction ratio can exceed 20 dB, and the operation bandwidth (1500 nm) can cover from 1000 to 2500 nm (from near-infrared to mid-infrared). The device provides three degrees of freedom (polarization order l, polarization of input beam σ, and initial orientation angle α<sub>0</sub>) to flexibly generate different OAM-carrying vector beams. We can use a single device to generate two OAM-carrying vector beams with opposite charge sign of OAM by simply controlling the polarization of the input beam. We further study the performance dependence of the designed metamaterials on the offset of the initial orientation angle, length, and width of the rectangular apertures. The obtained results indicate favorable fabrication tolerance.
[show abstract][hide abstract] ABSTRACT: We propose an approach to implementing wavelength- and time-selective optical switching, dropping and swapping based on the sum-frequency generation (SFG) or cascaded sum- and difference-frequency generation (cSFG/DFG) in a periodically poled lithium niobate (PPLN) waveguide. Analytical solutions are derived, showing the parametric depletion effect for optical switching and the narrow-band operation due to quasi-phase matching (QPM) condition of PPLN. Using parametric depletion effect of SFG process, we demonstrate wavelength- and time-selective optical switching for ITU-grid compatible 40-Gbit/s wavelength-division multiplexed (WDM) signals with a channel spacing of 100 GHz. Less than 1-dB power penalty at a bit-error rate (BER) of 10<sup>-9</sup> is measured for the wavelength- and time-selective switching channel. Negligible impacts are observed on other channels of WDM signals. Using combined effects of parametric depletion and wavelength conversion of cSFG/DFG processes, we demonstrate wavelength- and time-selective optical dropping for ITU-grid compatible 100-GHz-spaced 40-Gbit/s WDM signals. Moreover, we demonstrate optical swapping between two 100-GHz-spaced 40-Gbit/s signals. The obtained theoretical and experimental results confirm single-PPLN-assisted wavelength- and time-selective optical switching, dropping and swapping for 100-GHz-spaced WDM signals, which might potentially be extended to WDM signals with narrower channel spacing.
[show abstract][hide abstract] ABSTRACT: 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<sup>-9</sup>. In addition, no significant degradations are observed for RZ-DQPSK signals (A, B, C or -C) after the operations of quaternary addition and subtraction.
[show abstract][hide abstract] ABSTRACT: A compact low-crosstalk multi-ring fiber transmitting multiple orbital angular momentum (OAM) modes is presented. The multi-OAM-mode multi-ring fiber (MOMRF) consists of 7 rings, each supporting 22 modes with 18 OAM ones (i.e., 154 channels in total), which can be used for high-density space-division multiplexing. The employed high-contrast-index ring structure benefits tight light confinement and large effective refractive index difference of different OAM modes , featuring both low-level inter-ring crosstalk ( 30 dB for a 100-km-long fiber) and intermode crosstalk over a wide wavelength range (1520-1580 nm). The designed MOMRF is also compatible with wavelength-division multiplexing technique (e.g., 75 ITU-grid wavelengths from 1520.25 to 1579.52 nm with 100-GHz spacing) and advanced multilevel amplitude/phase modulation formats (e.g., 16-ary quadrature amplitude modulation), which might be used to achieve petabit-per-second total transmission capacity and hundred bits-per-second-per-hertz aggregate spectral efficiency.
[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: We propose an approach to generate and multiplex 7 OAM spatial modes using a ring fiber launched by the light from multiple single-mode fibers. The purity of each OAM mode is >99% and the total power efficiency of multiplexing is 70%.
[show abstract][hide abstract] ABSTRACT: We described recent progress on optical communications using light beams carrying orbital angular momentum (OAM). We achieved a 25.6-bit/s/Hz spectral efficiency using 16-QAM signals over pol-muxed four OAM beams.
[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: It has fundamental meaning to find the elements influencing the laser-induced damage threshold (LIDT) of KH(2)PO(4) (KDP) crystal and to provide suitable characterization parameters for these factors in order to improve the LIDT of KDP. Using single-point diamond turning (SPDT) to process the KDP crystal, the machined surface quality has important effects on its LIDT. However, there are still not suitable characteristic parameters of surface quality of KDP to correspond with the LIDT nowadays. In this paper, guided by the Fourier model theory, we study deeply the relationship between the relevant characteristic parameters of surface topography of KDP crystal and the experimental LIDT. Research results indicate that the waviness rather than the roughness is the leading topography element on the KDP surface machined by the SPDT method when the LIDT is considered and the amplitude of micro-waviness has greater influence on the light intensity inside the KDP crystal within the scope of dangerous frequencies between (180 μm)(-1) and (90 μm)(-1); with suitable testing equipment, the characteristic parameters of waviness amplitude, such as the arithmetical mean deviation of three-dimensional profile S(a) or root mean square deviation of three-dimensional contour S(q), are able to be considered as suitable parameters to reflect the optical quality of the machined surface in order to judge approximately the LIDT of the KDP surface and guide the machining course.
Journal of Applied Physics 12/2011; 110(11):113103-1131037. · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: Optical signal processing techniques employ a wide range of devices and various nonlinearities to achieve multiple network functionalities. The choice of nonlinearity can also impact the relative efficiency, both in terms of energy and material consumption, of the signal processing function being implemented. Techniques for some of the important functionalities, wavelength multicasting, wavelength-division multiplexing to time-division multiplexing, add-drop multiplexing, and wavelength exchange are compared in terms of the used optical spectrum, number of pumps required, and optical energy consumed. These include varieties of four-wave mixing, cross-phase modulation, Kerr-effect-based polarization rotation in optical fibers, and three-wave mixing in lithium niobate waveguides (WGs). Future possibilities of greener optical signal processing using on-chip WG technologies are discussed within the scope of recent developments in the dispersion tailored, highly nonlinear WGs.
IEEE Journal of Selected Topics in Quantum Electronics 05/2011; · 4.08 Impact Factor
[show abstract][hide abstract] ABSTRACT: We experimentally demonstrate a tunable optical generation of up to 40-Gbaud 16-QAM from a QPSK signal using a variable DGD element based on vector addition between two orthogonal polarization states of the input QPSK signal.