Long-distance temporal quantum ghost imaging over optical fibers.

Since the first quantum ghost imaging (QGI) experiment in 1995, many QGI schemes have been put forward. However, the position-position or momentum-momentum correlation required in these QGI schemes cannot be distributed over optical fibers, which limits their large-scale geographical applications. In this paper, we propose and demonstrate a scheme for long-distance QGI utilizing frequency correlated photon pairs. In this scheme, the frequency correlation is transformed to the correlation between the illuminating position of one photon and the arrival time of the other photon, by which QGI can be realized in the time domain. Since frequency correlation can be preserved when the photon pairs are distributed over optical fibers, this scheme provides a way to realize long-distance QGI over large geographical scale. In the experiment, long-distance QGI over 50 km optical fibers has been demonstrated.

Generation of hyper-entanglement on polarization and energy-time based on a silicon micro-ring cavity.

In this paper, hyper-entanglement on polarization and energy-time is generated based on a silicon micro-ring cavity. The silicon micro-ring cavity is placed in a fiber loop connected by a polarization beam splitter. Photon pairs are generated by the spontaneous four wave mixing (SFWM) in the cavity bi-directionally. The two photon states of photon pairs propagate along the two directions of the fiber loop and are superposed in the polarization beam splitter with orthogonal polarizations, leading to the polarization entanglement generation. On the other hand, the energy-time entanglement is an intrinsic property of photon pairs generated by the SFWM, which maintains in the process of the state superposition. The property of polarization entanglement is demonstrated by the two photon interferences under two non-orthogonal polarization bases. The property of energy-time entanglement is demonstrated by the Franson type interference under two non-orthogonal phase bases. The raw visibilities of all the measured interference fringes are higher than 1/2, the bench mark for violation of the Bell inequality. It indicates that silicon micro-ring cavity is a promising candidate to realize high performance hyper-entanglement generation.

Telecom-band degenerate-frequency photon pair generation in silicon microring cavities.

In this Letter, telecom-band degenerate-frequency photon pairs are generated in a specific mode of a silicon microring cavity by the nondegenerate spontaneous four-wave mixing (SFWM) process, under two continuous-wave pumps at resonance wavelength of two different cavity modes. The ratio of coincidence to accidental coincidence is up to 100 under a time bin width of 5 ns, showing their characteristics of quantum correlation. Their quantum interference in balanced and unbalanced Mach-Zehnder interferometers is investigated theoretically and experimentally, and the results show potential in quantum metrology and quantum information.

Generation of 1.5 µm discrete frequency-entangled two-photon state in polarization-maintaining fibers.

In this Letter, the generation of a 1.5 µm discrete frequency-entangled two-photon state is realized based on a piece of commercial polarization-maintaining fiber (PMF). It is connected with a polarization beam splitter to realize a modified Sagnac fiber loop (MSFL). Correlated two-photon states are generated through a spontaneous four-wave-mixing process along the two propagation directions of the MSFL, and output from the MSFL with orthogonal polarizations. Their quantum interference is realized through a 45° polarization collimation between polarization axes of PMFs inside and outside the MSFL, while their phase difference is controlled by the polarization state of the pump light. The frequency-entangled property of the two-photon state is demonstrated by a spatial quantum beating experiment with a fringe visibility of 98.2±1.3%, without subtracting the accidental coincidence counts. The proposed scheme generates a 1.5 µm discrete frequency-entangled two-photon state in a polarization-maintaining way, which is desired in practical quantum light sources.

The impact of nonlinear losses in the silicon micro-ring cavities on CW pumping correlated photon pair generation.

In this paper, 1.5 µm correlated photon pairs are generated under continuous wave (CW) pumping in a silicon micro-ring cavity with a Q factor of 8.1 × 10(4). The ratio of coincidences to accidental coincidences (CAR) is up to 200 under a coincidence time bin width of 5 ns. The experiment result of single side photon count shows that the generation rate does not increase as the square of the pump level due to the nonlinear losses in the cavity which reduce the Q factor and impact the field enhancement effect in the cavity under high pump level. Theoretical analysis shows that the photon pair generation rate in the cavity is proportional to the seventh power of the Q factor, which agrees well with the experiment result. It provides a way to analyze the performance of CW pumping correlated photon pair generation in silicon micro-ring cavities under high pump levels.

Energy-time entanglement generation in optical fibers under CW pumping.

In this paper, the energy-time entangled photon-pairs at 1.5 µm are generated by the spontaneous four wave mixing (SFWM) in optical fibers under continuous wave (CW) pumping. The energy-time entanglement property is demonstrated experimentally through an experiment of Franson-type interference. Although the generation rates of the noise photons are one order of magnitude higher than that of the photon-pairs under CW pumping, the impact of noise photons can be highly suppressed in the measurement by a narrow time domain filter supported by superconducting nanowire single photon detectors with low timing jitters and time correlated single photon counting (TCSPC) module with high time resolution. The experiment results show that the SFWM in optical fibers under CW pumping provides a simple and practical way to generate energy-time entanglement at 1.5 µm, which has great potential for long-distance quantum information applications over optical fibers.

1.5 µm polarization entanglement generation based on birefringence in silicon wire waveguides.

In this Letter, the 1.5 µm polarization entanglement generation is realized in a silicon wire waveguide utilizing its birefringence. In this scheme, two orthogonal polarized correlated states are generated by scalar processes of spontaneous four-wave mixing (SFWM) in the quasi-transverse electrical and quasi-transverse magnetic modes, respectively. Meanwhile, the vector processes of SFWM are suppressed by the group birefringence in the waveguide. The maximum polarization entangled state is generated by optimizing the pump polarization, which is demonstrated by the experiments of two-photon interference and polarization indistinguishability at one side. The fringe visibilities of two-photon interferences are 96.8±4.7% and 86.0±3.7% under two nonorthogonal polarization detection settings, respectively. This scheme provides a simple way to realize silicon integrated sources for 1.5 µm polarization entanglement generation.

Polarization entanglement generation at 1.5 µm based on walk-off effect due to fiber birefringence.

In this Letter, a linear scheme to generate polarization entanglement at 1.5 µm based on commercial polarization maintained dispersion shifted fiber (PM-DSF) is proposed. The birefringent walk-off effect of the pulsed pump light in the PM-DSF provides an effective way to suppress the vector scattering processes of spontaneous four-wave mixing. A 90 deg offset of fiber polarization axes is introduced at the midpoint of the fiber to realize the quantum superposition of the two correlated photon states generated by the two scalar processes on different fiber polarization axes, leading to polarization entanglement generation. Experiments of the indistinguishable property on single-side and two-photon interference in two nonorthogonal polarization bases are demonstrated. A two-photon interference fringe visibility of 89±3% is achieved without subtracting the background counts, demonstrating its great potential in developing highly a efficient and stable fiber based polarization-entangled quantum light source at the optical communication band.

Single polarization transmission in pedestal-supported silicon waveguides.

In this Letter, properties of a pedestal-supported silicon waveguide are investigated, showing that it supports single polarization transmission. The pedestal is fabricated easily through a wet-etching process on strip waveguides. Theoretical analysis shows that this property is due to the leakage of quasi-TM mode when the pedestal width is small. A polarization extinction ratio larger than 20 dB at 1550 nm is measured in the pedestal waveguide sample, demonstrating single polarization transmission property experimentally. Thanks to its large single polarization transmission bandwidth, robustness in fabrication tolerance, and simple fabrication process, pedestal waveguides will have potential applications as simple silicon-integrated polarizers.

Control of modes coupling in photonic crystal waveguides by introducing asymmetry and long periodicity.

The control of modes coupling in photonic crystal waveguides (PCWGs) is quite important because it's the basic working mechanism of many devices in optical integrative circuits, such as filters, switches, optical add drop multiplexers (OADMs), etc. Up to now, the researches of this area mostly focus on the modes coupling between two parallel PCWGs or between PCWGs and resonance cavities. In this paper, we proposed a new way of controlling modes coupling in PCWGs by introducing asymmetry and long periodicity. Because of the presence of asymmetry and long periodicity in PCWGs, some interesting modes coupling phenomena, which used to be forbidden in normal PCWGs, happen. Then a filter with a 1.42 nm full-width at the half value (FWHM) and an OADM with a 1.31 nm FWHM and a 0.34 dB insertion loss have been designed by utilizing the new modes coupling phenomena. Our researches not only provide a new way of controlling modes coupling in PCWGs but also benefit the design of many devices in optical integrative circuits greatly.

Noise performance comparison of 1.5 microm correlated photon pair generation in different fibers.

In this paper, the noise performances of 1.5 microm correlated photon pair generation based on spontaneous four wave-mixing in three types of fibers, i.e., dispersion shifted fiber, traditional highly nonlinear fiber and highly nonlinear microstructure fiber are investigated experimentally. Result of the comparison shows that highly nonlinear microstructure fiber has the lowest Raman noise photon generation rate among the three types of fibers while correlated photon pair generation rate is the same. Theoretical analysis indicates that the noise performance is determined by the nonlinear index and Raman response of the material in fiber core. The Raman response rises with increasing doping level, while, for the nonlinear index, the impact of doping level is weak. As a result, highly nonlinear microstructure fiber with pure silica core has the best noise performance and great potential in practical sources of correlated photon pairs and heralded single photons.

Refractive index dependence of the coupling characteristics between long-range surface-plasmon-polariton and dielectric waveguide modes.

The coupling characteristics of a hybrid coupler comprised of a long-range surface-plasmon-polariton (LRSPP) waveguide and a dielectric waveguide is analyzed with different detecting layers. Calculation results show that the coupling strength between the LRSPP mode and the dielectric-waveguide mode is rather sensitive to the refractive index of the detecting layer. This is promising to realize an integrated refractive index sensor with high resolution better than 4 x 10(-7) refractive index units or a modulator with rather low driving power and insert loss.

Polarization-entangled Bell states generation based on birefringence in high nonlinear microstructure fiber at 1.5 microm.

Polarization-entangled photon pair generation based on two scalar scattering processes of the vector four- photon scattering has been demonstrated experimentally in high nonlinear microstructure fiber with birefringence. By controlling pump polarization state, polarization-entangled Bell states can be realized. It provides a simple way to realize efficient and compact fiber-based polarization-entangled photon pair sources.

Influences of pump wavelength and environment temperature on the dual-peaked Brillouin property of a small-core microstructure fiber.

The influences of the pump wavelength and temperature on the dual-peaked Brillouin property of a piece of a small-core microstructure fiber are investigated using the heterodyne method. The experimental results indicate that the dispersion characteristics of the acoustical modes participating in the Brillouin scattering are formed by the coupling of two acoustical modes with different acoustical field distributions. The influence of the pump wavelength can contribute to the variation of the acoustical mode wave vectors. On the other hand, the influence of the environment temperature can contribute to the shift of acoustical mode dispersion map by the variation of the material mechanical parameters and thermal expansion. The model and experiment results demonstrate that the height ratio of the two Brillouin peaks can be controlled by adjusting the pump wavelength or environment temperature.

Opposite Goos-Hänchen shifts for transverse-electric and transverse-magnetic beams at the interface associated with single-negative materials.

Goos-Hänchen shifts are investigated when total reflection occurs at the interfaces associated with single-negative materials (SNMs). A general rule for judging the direction of the Goos-Hänchen lateral shift concerning lossless media is obtained: Whether the lateral shift is positive or negative depends on the sign of micro1micro2 for TE-polarized incident beams and epsilon1epsilon2 for TM-polarized incident beams. It was theoretically demonstrated that, at the interface associated with SNMs, TE- and TM-polarized incident beams experience opposite Goos-Hänchen lateral shifts. An effective and simple approach to discriminating epsilon-negative material and micro-negative material is proposed.

Broadband source generated by stimulated Raman scattering and four-wave mixing in a highly nonlinear optical fiber ring cavity.

A novel high-power broadband source based on the combined action of stimulated Raman scattering and parametric four-wave mixing in a highly nonlinear dispersion-shifted fiber ring cavity is investigated experimentally. An output spectrum of 1510-1580 nm with a stable power of 91 mW is demonstrated with a dual-wavelength pumping scheme.

A novel design for all-solid silica Bragg fiber with zero-dispersion wavelength at 1550 nm.

A novel all-solid Bragg fiber composed entirely of silica material is proposed in this paper. The core of this Bragg fiber is composed of conventional silica, and the cladding is formed by a set of alternating layers of up-doped and down-doped silica. This all-solid silica Bragg fiber is technically feasible and can simplify the fabrication technique. Dispersion properties of this silica Bragg fiber are investigated, and simulations show that zero dispersion wavelength lambda0 near 1.55 m with nonlinear coefficient gamma about 50 W-1km-1 can be obtained in silica Bragg fiber.

Wide-angle beam splitting by use of positive-negative refraction in photonic crystals.

We present a positive-negative refraction effect in which, under certain conditions, an incident plane wave launched into a photonic crystal excites a positive-refracted Bloch wave and a negative-refracted Bloch wave simultaneously, both of which maintain the polarization. By utilizing this phenomenon, wide-angle beam splitting can be realized at the microscale level. Numerical simulations are employed to demonstrate this anomalous refraction behavior.