Signal Optical Field Research Articles

Research on dynamic polarization control in continuous variable quantum key distribution systems

In a commercial fiber-based quantum key distribution system, the local and signal optical fields are transmitted through long distance fibers by using time division multiplexing and polarization multiplexing. The state of polarization of the optical field is inevitably disturbed by random birefringence of the standard single-mode fiber caused by external complex environments. This drift of the state of polarization significantly affects the balanced homodyne detection results and the secret key rate. Therefore, the key technology of the dynamic polarization control unit is crucial for the system in a large-scale commercial application. We theoretically analyze and prove that the polarization control unit only needs the combination of two degrees of freedom when considering the result of an arbitrary polarization extinction ratio at the receiver of the system. To overcome the influence of polarization variations, we propose a chaotic monkey algorithm based on Bayesian parameter estimation method and implement intelligence algorithm on field programmable gate array (FPGA) hardware under pulsed light with an integral-type detector for the dynamic polarization control unit. The simulation results show that the optimal combination is four degrees of freedom and the optimal prior distribution is an exponential distribution among various distributions in the dynamic polarization control unit. According to the simulation results, the experimental results show that the achieved polarization extinction ratio is over 30 dB and the average time of polarization control is 400 μs for a single random polarization scrambling. By combining the dynamic polarization control unit with the system, we demonstrate the continuous variable quantum key distribution (CV-QKD) under a continuous polarization scrambling scope of 0－2 krad/s and verify its effectiveness. In addition, the methods presented will improve the performance of the system and expand the range of applications even under strong external disturbance.

Feasibility of Plasmonic Circuits in Nanophotonics

The configuration of plasmonic circuits comprising SiO2-load waveguides and their characteristics within the nanophotonic range are presented and compared with electronic and lightwave circuits in 1300 and 1550 nm wavelength bands. In the nanophotonic range of less than $1~\mu \text{m}$ , plasmonic signals propagate in narrow waveguides with cross-sections less than a few hundred square nanometers, while lightwaves exhibit only slight propagation in high-refractive-index (i.e., Si) waveguides owing to the transmission loss increase via the cut-off wavelength of the waveguide. Additionally, the plasmonic signal transmission loss is lower than that of electric signals for transmission lengths less than a few hundred micrometers. During signal transmission, a narrow spectral width of the plasmonic signals is needed to suppress any signal shape deformation induced by the frequency dependence of the collective oscillation of electrons in plasmonic signals. In the nanophotonic range, the degree of integration for plasmonic circuits is not governed by the transmission loss but by the leak distance of the plasmonic signal optical field from the side-walls of the waveguides and components. Employing a metal/insulator/metal structure in plasmonic circuits is a valid way to heighten the integration density, and its effectiveness is numerically and experimentally confirmed in a plasmonic multiplexer less than $1~\mu \text{m}$ in length. On the basis of these results, the feasibility of plasmonic circuits is discussed and it can be said that plasmonic circuits including waveguides and components are promising photonic techniques for signal transmission in the nanophotonic range

Cavity-enhanced Raman heterodyne spectroscopy in Er3+:Y2SiO5 for microwave to optical signal conversion

The efficiency of the frequency conversion process at the heart of Raman heterodyne spectroscopy was improved by nearly four orders of magnitude by resonant enhancement of both the pump and signal optical fields. Our results using an erbium doped Y$_2$SiO$_5$ crystal at temperatures near 4K suggest that such an approach is promising for the quantum conversion of microwave to optical photons.

High sensitivity quantum Michelson interferometer

Michelson interferometer can be applied to not only the building block of the fundamental research of physics, but also the precise measurement, such as the direct observation of gravity wave signal. Therefore, high performance Michelson interferometer is the key step towards the implementation of direct observation of weak gravity wave signal. Recently, the vacuum noise was reduced by injecting squeezed vacuum into the unused port of Michelson interferomter, and the phase signal optical field in Mach-Zender interferometer is amplified based on the four-wave mixing in hot Rubidium atom. Here we study high sensitivity quantum Michelson interferometer. In the Michelson interferometer, the linear optical beam splitter is replaced by a non-degenerated optical parametric amplifier to realize the splitting and combining of optical fields, and the squeezed vacuum is also injected into the unused port of interferomter, so that the high signal-to-noise ratio and high sensitivity of phase measurement can be realized. Due to the inevitable optical losses, the losses inside and outside the Michelson interferometer are considered in our theoretical model. We investigate the influences of the losses inside and outside the Michelson interferometer on the sensitivity of phase measurement. By theoretical calculation, we analyze the dependence of sensitivity of phase measurement on system parameters, such as intensity of optical fields for phase sensing, gain factor of non-degenerated optical parametric amplifier, the losses inside and outside the Michelson interferometer, and the squeezing parameter of input squeezed vacuum, and thus the condition of high sensitivity nonlinear Michelson interferometer can be obtained. In a broad system parametric range, the quantum Michaleson interferometer can surpass standard quantum limit, and the nonlinear Michaleson interferometer with squeezed state injection can provide the optimal sensitivity for phase measurement. The nonlinear Michelson interferometer with squeezed state is suitable for weak signal measurement. While the gain factor of non-degenerated optical parametric amplifier is large enough, the nonlinear Michelson interferometer without injecting the squeezed vacuum can still reach the optimal sensitivity, which reduces the use of quantum resources. When the phase sensing optical field is strong, the linear Michelson interferometer with injecting the squeezed vacuum can also reach the optimal sensitivity, and the sensitivity is robust for both losses inside and outside the interferometer. All the kinds of interferometers are more sensitive to the loss inside the interferometer than outside the interferometer, and the sensitivity of phase measurement can be improved by reducing the loss inside the interferometer. Our result provides direct reference of experimental implementation of high performance interferometer for high precision quantum metrology.

Coherent frequency up-conversion of microwaves to the optical telecommunications band in an Er:YSO crystal

The ability to convert quantum states from microwave photons to optical photons is important for hybrid system approaches to quantum information processing. In this paper we report the up-conversion of a microwave signal into the optical telecommunications wavelength band using erbium dopants in a yttrium orthosilicate crystal via stimulated Raman scattering. The microwaves were applied to the sample using a 3D copper loop-gap resonator and the coupling and signal optical fields were single passed. The conversion efficiency was low, in agreement with a theoretical analysis, but can be significantly enhanced with an optical resonator.

Compact Intracavity Singly-Resonant Optical Parametric Oscillator Pumped by GaSb-Based Vertical External Cavity Surface-Emitting Laser: Concept and the Main Operational Characteristics

The concept of an intracavity singly-resonant optical parametric oscillator pumped by a GaSb-based vertical external cavity surface-emitting laser has been proposed. The steady-state characteristics of the parametric oscillator with the joint cavity shared by the pump and signal optical fields have been numerically analyzed. Using a few millimeter long orientation-patterned quasi-phase-matched GaAs nonlinear crystal in such a cavity allows fairly compact ( ~15-mm long) device working in the mid-infrared range (wavelength of 16.5 μm) to be built.

Adjustable electromagnetically induced transparency and absorption, optical controlled-phase gate in semiconductor quantum wells

We investigate theoretically both the linear and nonlinear properties of the probe and signal optical pulsed fields in a system of four-level coupled GaAs/AlGaAs quantum wells with spontaneous decay of the longitudinal optical phonons (SDLOP) between the lower excited state and the ground state levels. In the linear range, we predict the existence of electromagnetically induced absorption (EIA), and that it is possible to mutually convert from electromagnetically induced transparency to EIA by adjusting the coherent control of the control optical field and the SDLOP. In the nonlinear range, it is shown that by taking into account the influence of the SDLOP, the cross-Kerr nonlinearity of the probe and signal optical fields can be tremendously enhanced. Simultaneously, we obtain the presence of the suppression of self-Kerr optical absorption of the probe field. These characteristics are advantageous to enable the realization of efficient photon-photon entanglement, and generate conditional nonlinear phase shifts of order π. Thereafter, we present a feasible scheme to carry out a two-qubit optical controlled-phase gate by encoding the polarization state of the probe and signal optical fields. Utilizing the linear optics components, we can discriminate the maximal polarization-entangled state of such a two-qubit system. This proposal is potentially applicable to facilitate the realization of solid states mediated all-optical quantum computation and information processing.

Optical OFDM: A promising high-speed optical transport technology

Orthogonal frequency division multiplexing (OFDM) is a widely used modulation/multiplexing technology in wireless and data communications. Leveraging recent advances in high-speed complementary metal-oxide semiconductor (CMOS) technologies and optical modulation and detection technologies, optical OFDM at a 40Gb/s or even a 100Gb/s information rate becomes feasible. At the optical transmitter, OFDM is realized by a digital signal processor (DSP) using inverse fast Fourier transformation (iFFT) with subsequent digital-to-analog conversion and Cartesian electro-optic modulation. At the receiver, the signal optical field is first reconstructed, e.g., by coherent detection. Then reverse signal processing is applied to recover the original data. OFDM enables efficient compensation of transmission effects such as chromatic dispersion and polarization mode dispersion that often are prohibiting impairments to cost-effective realization of high-speed optical transport systems. In light of the emerging demand for a 100+ Gb/s data rate in future optical transport systems, optical OFDM is considered to be a promising enabling technology. In this paper, the optical OFDM architectures will be reviewed, and their performance under various system conditions will be discussed and compared with alternative technologies. The challenges in the implementation of optical OFDM will also be discussed.

PMD Compensation/mitigation techniques for high-speed optical transport

In high-speed long-haul optical transport systems with channel data rates at 40Gb/s and beyond, polarization mode dispersion (PMD) is one of the major obstacles that limits system performance. It is desired or in most cases required to increase the system tolerance to PMD through PMD compensation and mitigation. In this paper, we review the techniques of PMD compensation and mitigation, including per-channel based optical PMD compensators (PMDCs), analog and digital electronic PMDCs, and all-channel PMD mitigation using distributed fast polarization scrambling and forward error correction (FEC). With the emergence of digital coherent detection, the signal optical field becomes digitally available, enabling powerful digital PMDC techniques to be applied. Digital PMDC techniques in optical single carrier coherent systems and orthogonal frequency division multiplexing (OFDM) systems are briefly reviewed.

III-nitride-based planar lightwave circuits for long wavelength optical communications

Planar lightwave circuits based on III-nitride wide-bandgap semiconductors are proposed and the feasibility of developing III-nitride-based novel photonic integrated circuits for applications in fiber-optical communications is discussed. III-nitrides have low attenuation in the near-infrared wavelength region because of their wide bandgaps, while as semiconductors their refractive indexes can be modulated by carrier injection. III-nitrides are also well known for their ability to operate at high temperatures, high power levels and in harsh environments. These characteristics make III-nitrides ideal candidates for tunable optical phased-array (PHASAR) devices for optical communications. We have characterized the optical properties of Al/sub x/Ga/sub 1-x/N epilayers in the 1550-nm wavelength region, including the refractive indexes and the impact of Al concentrations. Single-mode ridged optical waveguide devices using GaN-AlGaN heterostructures have been designed, fabricated and characterized for operation in the 1550-nm wavelength window. The birefringence of wurtzite GaN grown on sapphire substrate has been observed. Refractive indexes were found to be different for signal optical field perpendicular and parallel to the crystal c axis (n/sub /spl perp// /spl ne/ n/sub ///). More importantly, we found an approximately 10% change in the index difference /spl Delta/n=n/sub ///-n/sub /spl perp// with varying the waveguide orientation within the c plane, and a 60/spl deg/ periodicity was clearly observed. This is attributed to the hexagonal structure of the nitride materials. Various functional waveguide devices have been realized, including 2/spl times/2 directional couplers and eight-wavelength array-waveguide gratings. Theoretical predictions of temperature sensitivity and the efficiency of carrier-induced refractive change are provided.

Birefringence of GaN/AlGaN optical waveguides

We have experimentally studied the birefringence of wurtzite GaN grown on a sapphire substrate. The measurements were done with single-mode GaN/AlGaN planar optical waveguides on c-plane grown heterostructure films. The refractive indices were found to be different for signal optical field perpendicular or parallel to the crystal c axis (n⊥≠n∥). More importantly, we found an approximately 10% change in index difference Δn=n∥−n⊥ with variation of the waveguide orientation in the a–b plane, and a 60° periodicity was clearly observed. This is attributed to the hexagonal structure of nitride materials.

Optical spectral analysis using moving space-charge field effects in photoconductive semiconductors

The interference of two mutually coherent optical fields with unequal center frequencies generates moving space-charge electric fields inside photoconductive semiconductors that contain deep level traps. The motion of the internal space-charge field drifts free charge carriers out of the photoconductive sample and results in measurable photocurrents that consist of ac and dc components even in the absence of external bias electric fields. The dc photocurrent can be expressed as a convolution integral between the baseband power spectrum of the signal optical field and the characteristic impulse response of the photoconductive crystal, provided that the local oscillator has negligible optical linewidth. We have demonstrated experimentally a new kind of optical spectrum analyzer with a resolution bandwidth smaller than 15 kHz by sweeping the local-oscillator center frequency over the spectral region of interest and by subsequent deconvolution of the recorded photocurrent with the characteristic response of the crystal. The scanning range of the analyzer is limited only by the tuning range of the local oscillator.

Homodyne detection using photorefractive materials as beamsplitters

The use of two-wave mixing in photorefractive (PR) materials in a homodyne detection receiver structure is studied. The interaction between a strong local oscillator optical field and a weak signal optical field in the PR material results in the formation of a volume index of refraction phase grating. The grating automatically aligns such that it coherently couples the signal and local oscillator fields just like a fixed beamsplitter in a conventional homodyne receiver structure. The slow response of the PR material automatically compensates for slow changes in the angle of arrival of the signal beam but acts as a fixed grating with respect to rapid phase modulation of the signal beam. The performance of a homodyne receiver that uses a PR material as a beamsplitter, is shown to be up to 3 dB superior to the performance of a conventional homodyne receiver that uses a fixed 50-50 beamsplitter. Furthermore, a PR receiver can adjust and retain its optimal performance when changes in the received signal power occur, whereas a conventional receiver cannot.