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We demonstrate an acousto-optic mode converter based on a tapered optical fiber to efficiently generate orbital angular momentum states of light. In our scheme an acoustic wave is deployed to the waist of tapered optical fiber where two degenerate HE21 modes leading to +1 and −1 orbital angular momentum eigen-modes are resonantly excited. The excitation of TM01 and TE01 modes is suppressed by enlarging the intermodal index difference between near-degenerate spatial modes. Numerical calculation for optimization of the taper diameter is provided. The experimental characterization of generated states is performed by analyzing the output far-field pattern and the spatial interference fringes with a uniform reference beam.

This letter presents an experimental method to compensate for the differential group delay between the core modes in a multi-core fiber. Optical fiber sections are sequentially spliced with azimuthal offsets such that light initially launched to any cores can propagate through the combination of the same lengths of different cores, resulting in ideally uniform group delay. The differential group delays before and after the compensation are measured by wavelength-domain Mach–Zehnder interferometry through multi-core fiber sections. The experimental results show that the group delay compensation is practically limited by non-uniformity of the multi-core fiber.

We demonstrate time-multiplexing of heralded single-photon sources using switchable optical fiber d elay l ines a ided b y p olarization-space m ultiplexing. Relative-multiplexing HOM interferometry tests the indistinguishability of output photons.

We report the measurement of bending-induced birefringence in the presence of large intrinsic birefringence in a hollow-core photonic crystal fiber (HC-PCF). The fast axis of bending-induced birefringence was found to be normal to the bending plane, in contrast to the conventional fiber case. The dependence of the induced birefringence on the bending radius was also different from the typical inverse square law. Possibilities and design criteria for polarization controllers using bending-induced birefringence in HC-PCFs are presented.

We experimentally demonstrate a time-multiplexing method for pulse-pumped heralded single-photon sources using switchable optical fiber delay lines. The optical fibers constitute a 32-channel binary-division photon buffer that can multiplex time slots into single temporal mode to enhance single-photon generation rate without increasing multi-photon noise. Relative time multiplexing using the same actively controlled variable delay realizes Hong-Ou-Mandel interferometry by two photons produced in arbitrary two time-bins. The interference dip depth in good agreement with theory verifies path length matching within the photonic coherence length and dispersion compensation of the optical fiber delay sections. We independently confirm by low-coherence interferometry that the internal optical path lengths of the optical switches are constant during the switching operations.

We calculate the operation wavelength range of polarization controllers based on rotating wave plates such as paddle-type optical fiber devices. The coverages over arbitrary polarization conversion or arbitrary birefringence compensation are numerically estimated. The results present the acceptable phase retardation range of polarization controllers composed of two quarter-wave plates or a quarter-half-quarter-wave plate combination, and thereby determines the operation wavelength range of a given design. We further prove that a quarter-quarter-half-wave-plate combination is also an arbitrary birefringence compensator as well as a conventional quarter-half-quarter-wave-plate combination, and show that the two configurations have the identical range of acceptable phase retardance within the uncertainty of our numerical method.

We present a practical calibration method of the detection efficiency (DE) of single photon detectors (SPDs) in a wide wavelength range from 480 nm to 840 nm. The setup consists of a GaN laser diode emitting a broadband luminescence, a tunable bandpass filter, a beam splitter, and a switched integrating amplifier which can measure the photocurrent down to the 100 fA level. The SPD under test with a fibre-coupled beam input is directly compared with a reference photodiode without using any calibrated attenuator. The relative standard uncertainty of the DE of the SPD is evaluated to be from 0.8% to 2.2% varying with wavelength (k = 1).

Fibre transport of multi-dimensional photonic quantum states promises high information capacity per photon without space restriction. This work experimentally demonstrates transmission of spatial ququarts through multi-core optical fibres and measurement of the entanglement between two fibres with quantum state analyzers, each composed of a spatial light modulator and a single-mode fibre. Quantum state tomography reconstructs the four-dimension entangled state that verifies the nonlocality through concurrences in two-dimensional subspaces, a lower bound of four-dimensional concurrence and a Bell-type CGLMP inequality.

We demonstrate a highly efficient acoustooptic mode converter for selective excitation of the higher-order modes in a four-mode fiber which guides the LP01, LP11, LP21, and LP02 modes at telecommunication wavelengths. The coupling efficiency and the mode extinction ratio are measured by analyzing the output far-field patterns. The acoustic resonance frequencies and the operating bandwidths are compared with the theoretical results, by which the modal characteristics, the core diameter, and the core-cladding index difference are estimated.

We demonstrate the measurement of detection efficiency, the probability for the successful detection of a single photon incident on a detector, in a wide wavelength range from 250 nm to 1000 nm. The method is based on the responsivity comparison of two detectors by using a tunable monochromatic light source at a radiant power controlled to the pico-Watt level. As the reference detector, a photodiode with a switched integrating amplifier is used, whose spectral responsivity is calibrated at a photo-current level of pico-Ampere. A fibre-based spatial filter is used to form a beam focus that is focused to a diameter of less than 100 µm at the detector under test. The typical uncertainty of the detection-efficiency measurement for a Si-based single photon avalanche photodiode is evaluated to be less than 1% as a relative standard uncertainty (k = 1), which is currently limited by the characteristics of the tunable source.

We demonstrate an experimental method to control four-dimensional spatially entangled states between two multi-core optical fibers based on spatial beam shaping of a laser that pumps spontaneous parametric down-conversion.

We propose a linear optical scheme that can conditionally generate high NOON states using polarization modes. This scheme provides advantages over the previous proposals on path-entangled NOON states in view of success probability or required resources of optical elements. We also investigate two experimental schemes feasible within existing technology that can produce the NOON-like or the NOON state for N = 4., Published version, 5 pages, 4 figures

We describe two methods for evaluating the dead time of a time-to-amplitude converter (TAC). The dead time is obtained by measuring either the corresponding time interval in an oscilloscope trace or the relation between the single count rate and the coincidence count rate. Values for the TAC dead time are obtained in the range from 3.4 µs to 14.3 µs for the two methods with respective standard uncertainties of 2.9 × 10−8 s and 3.3 × 10−9 s. The TAC dead time is applied to the calibration of coincidence-counting measurements of optical transmission and photon-heralding efficiency.

We propose and demonstrate an experimental technique that generates and measures arbitrary superpositions of the core modes in a multi-core fiber. This technique is potentially useful for transmission of high-dimensional quantum states through optical fiber.

We demonstrate an experimental technique to generate and measure arbitrary superpositions of core modes in a multi-core fiber. Two spatial light modulators couple the fundamental mode of a single-mode fiber with multiple-core modes of the MCF to constitute a Mach-Zehnder-type multi-path interferometer. The phase tunability of each path is verified by comparing two-, three-, and four-path interference patterns with the theory. Interference fringes in the wavelength domain estimates the inter-core group index differences with a resolution of 10(-5) using a fiber length of 1 m.

This paper describes a novel non-contact laser-ultrasound system which is compact and has a high level of immunity to ambient vibration, based on the use of an optical fibre Fabry–Perot interferometer as an acoustic detector. The new optical acoustic detector was employed for measuring a high-frequency acoustic wave of small amplitude generated by a relatively low output power source using an erbium-doped fibre amplifier. Together with the characteristics of the proposed detector, the preliminary results of 80 MHz laser-ultrasound experiments on an aluminium film are described.

Optical fibre techniques for detecting ultrasonic waves are, in principle, well established. The ultrasonic wave could impose a pressure and/or strain field on an optical fibre in its path and this pressure and/or strain modulation would in turn be observed as a change in optical delay along the fibre length. The delay change may be detected interferometrically, polarimetrically or through using the induced strain changes in Bragg gratings. The resulting fibre optic detection system can perform as well in terms of signal to noise ratio over specified bandwidths as more established piezoelectric ceramic detectors. Additionally fibre systems can also perform some signal processing, particularly beam forming, and have inherently much higher spatial and temporal bandwidth than the piezoceramic precursor. This paper explores some of the options available for the use of fibre optics in detecting ultrasound and demonstrates the elements of basic signal processing techniques including examples of source location (equivalent to fault detection) and optical fibre beam forming. Whilst these functions can be performed using piezoceramics, and indeed can be made more adaptable through electronic element by element processing on large arrays, fibre optic systems in contrast offer substantial benefits in terms of a much simplified array interconnect (only the fibre itself is required) and material compatibility especially for high performance high strain range mechanical systems.

Utilizing time multiplexing of a heralded single-photon source based on spontaneous parametric downconversion pumped by periodic laser pulses, we observed large enhancements in the single-photon probability.

In this paper, we demonstrate a novel all-fiber wavelength-tunable acoustooptic switch utilizing intermodal coupling in a two-mode fiber (TMF). Its all-fiber configuration consisting of a fiber acoustooptic tunable filter and a mode-selective coupler results in the low loss ( 50 nm, the switching time of 40 /spl mu/sec, and the crosstalk of 20 dB were achieved. By controlling the design parameters of the two-mode fiber, the 3-dB bandwidth of the switched signal was varied from 2.5 nm to >35 nm. A novel all-fiber dynamic optical add-drop multiplexer is also demonstrated using two acoustooptic switches in series.

A novel tunable polarization-dependent-loss (PDL) element is realized using two rotatable birefringent prisms and a half-wave plate. The design is favorable for interferometric stability and is operable over a wide wavelength range. The PDL can be tuned from 0 to 35 dB for a wavelength of 780 nm. An interference visibility of 0.98 between the two principal polarizations for 0-dB loss is achieved by compensating for thickness mismatch between the prisms.

We propose and experimentally demonstrate a novel configuration for an acoustooptic tunable bandpass filter based on intermodal coupling in a two-mode fiber. The double-pass scheme provides zero frequency-shift, bandwidth narrowing, and enhanced extinction ratio. A 3-dB bandwidth of 2.0 nm, insertion loss of 5.6 dB, and sidelobe suppression ratio of 14 dB were achieved. The wavelength tuning range was greater than 90 nm for the transmission dynamic range larger than 20 dB. Multiwavelength operation of a single device is achieved by simply applying multifrequency electrical signal. Unwanted intensity modulation of the multiwavelength filter is also investigated

A fiber-optic analogue to an externally driven three-level quantum state is demonstrated by acousto-optic coupling of the spatial modes in a few-mode fiber. Under the condition analogous to electromagnetically induced transparency, a narrow-bandwidth transmission within an absorption band for the fundamental mode is demonstrated. The presented structure is an efficient converter between the fundamental mode and the higher-order modes that cannot be easily addressed by previous techniques, therefore can play a significant role in the next-generation space-division multiplexing communications as an arbitrarily mode-selectable router. (C) 2014 Optical Society of America

Beam recombination characteristics were numerically investigated in array laser amplification using stimulated Brillouin scattering phase conjugation. To clarify the effect of piston errors due to imperfect phase locking, spatial intensity profiles of the beam recombination output were calculated in both the near and the far field on the basis of Rayleigh-Sommerfeld diffraction theory. The analyses indicate that piston errors are seriously detrimental to the quality of a beam recombination output and should be eliminated by a proper phase locking. It is also found that the gap between the beam splitting-combining wedges has a negligible effect.

We demonstrate a narrow-bandwidth acoustooptic tunable filter with very low polarization-sensitivity using a dispersion-compensating fiber that has a large group index difference between the core and the cladding modes with a specific stress distribution. The 3-dB bandwidth of the filter is 0.8 nm with the interaction length of 10 cm, and the polarization-dependent center-wavelength splitting is 0.04 nm at the wavelength around 1550 nm. The origin of the low polarization-dependence is successfully explained through the analysis on the stress field in the fiber.

We propose a multi-photon interferometer with a generalized C-SWAP operation that can estimate Tr[rho(1)rho(2)center dot center dot center dot rho(n)], a nonlinear functional of n photonic density matrices. The scheme is demonstrated for three single-photon states whose overlap is experimentally measured as the interference visibility of a control qubit encoded into photonic paths. The validity of this method is verified by comparing the visibility with the results of Hong-Ou-Mandel experiments. (C) 2013 Optical Society of America

We report the experimental demonstration of the Young's double-slit type spatial quantum interference of the three-photon N00N state, showing three times denser non-classical spatial interference fringes than that of the single-photon state.

We demonstrate a novel tunable narrow-bandwidth optical filter based on an acoustically modulated fiber Bragg grating (FBG). A flexural acoustic wave applied to an off-centered core FBG modulates the period of the grating, producing sidebands tunable both in wavelength and magnitude.

We demonstrate a novel optical frequency-domain reflectometry system with compactness and short measurement time based on the use of a wavelength-swept mode-locked fiber laser. The optical source uses an intracavity tunable Fabry-Perot filter as a tuning element. The fiber laser sweeps 20 nm in less than 10 ms. Spatial resolution of 100 /spl mu/m and total measurement range of several centimeters are demonstrated.

We experimentally demonstrate the entanglement of spatial modes between two photons propagating through separate few-mode optical fibers. Quantum states over the two lowest-order spatial modes are measured with highly efficient spatial-mode analyzers based on acousto-optics. Quantum state tomography verifies the entanglement of the spatial-domain Bell state.

We demonstrate a novel all-fiber add-drop multiplexer by using a tilted Bragg grating written on a two-mode fiber and a mode-selective coupler. The device can be made potentially with low loss and low cost, useful for add-drop of a small number of channels in wavelength-division-multiplexing (WDM) communication systems.

Spatial interference of quantum mechanical particles exhibits a fundamental feature of quantum mechanics. A two-mode entangled state of N particles known as N00N state can give rise to non-classical interference. We report the first experimental observation of a three-photon N00N state exhibiting Young's double-slit type spatial quantum interference. Compared to a single-photon state, the three-photon entangled state generates interference fringes that are three times denser. Moreover, its interference visibility of $0.49 \pm 0.09$ is well above the limit of 0.1 for spatial super-resolution of classical origin. The demonstration of spatial quantum interference by a N00N state composed of more than two photons represents an important step towards applying quantum entanglement to technologies such as lithography and imaging.

A tunable entanglement distiller for arbitrary two-photon polarization qubits is demonstrated using novel tunable local filters constructed with rotatable birefringent prisms. The distillation is confirmed by comparing the concurrences of the two-photon states.

A heralded three-photon NOON state is generated from two pairs of photons produced by spontaneous parametric down-conversion (SPDC). The measured four-photon coincidences reveal fringes three times finer than single-photon interference.

We propose a novel approach to optical generation of superresolution atomic state density distribution based on interaction between Lambda- type atoms and temporally-cascaded driving fields. The scheme effectively inscribes multiplication of optical intensity profiles on atomic state density distribution, which can produce arbitrary patterns beyond the diffraction limit. The degree of resolution enhancement can be increased, not depending on the intrinsic nonlinearity of the photographic medium or the number of simultaneously interacting fields unlike the previous schemes. Procedures for drawing arbitrary two-dimensional patterns and effects of atomic state decoherence are described.

We present a CW optical parametric oscillator (OPO) based on quasi-phase-matching. Pumped at 532 nm, the OPO provides coherent radiation from 800 nm to 900 nm and from 1300 nm to 1600 nm for signal and idler, respectively. Application for calibrating the spectral responsivity of Si and InGaAs photodiodes is demonstrated.

We propose and demonstrate a novel apodization technique for an elliptical-core two-mode fiber acousto-optic filter by twisting the fiber and adjusting the acoustic polarization. The sidelobe suppression in the filter spectrum was improved from -9.7 to -15.5 dB. A theoretical analysis that includes acoustic birefringence explains the experimental results with good agreement.

We describe a novel optical acoustic detector based on a bias-controlled fiber Fabry-Perot interferometer. The detector has a broad bandwidth from 10 Mhz to a few gigahertz and higher sensitivity than conventional systems, which are useful for noncontact characterization of microsamples based on laser ultrasound.

We demonstrate a new apodization technique for an elliptic-core two-mode fiber acousto-optic tunable filter based on controlling the acoustic polarization. The intensity of the sidelobes in the filter spectrum was reduced by almost 6 dB using this apodization technique.

We demonstrate a narrow-bandwidth all-fiber acousto-optic tunable filter with low polarization dependence using a dispersion compensating fiber. The 3-dB bandwidth at 10-dB notch was 0.66 nm, and the polarization-dependent wavelength shift was 0.04 nm.

We describe a novel optical acoustic detector based on a bias-controlled fiber Fabry-Perot interferometer. Broad bandwidth as 15 MHz ~ a few GHz and higher sensitivity than that of conventional systems were demonstrated. The minimum detectable phase was 4 x 10-8 rad/Hz1/2, which was 3 times larger than the quantum-limited value.

We describe an all-fiber add-drop multiplexer constructed with readily available components: a tilted fiber Bragg grating in a two-mode fiber and mode-selective directional couplers. The simple configuration allows the implementation of standard fiber Bragg grating technologies for improved performance.

We describe a novel all-fiber wavelength-tunable acousto-optic switch utilizing intermodal coupling in a two-mode fiber. Low loss (/spl sim/2 dB) operation was possible due to its all-fiber configuration with a fiber acousto-optic tunable filter and a mode-selective coupler.