High-resolution microwave frequency dissemination on an 86-km urban optical link

We report the first demonstration of a long-distance ultra-stable frequency dissemination in the microwave range. A 9.15-GHz signal is transferred through an 86-km urban optical link with a fractional frequency instability of 1.3×10⁻¹⁵ at 1-s integration time and below 10⁻¹⁸ at one day. The optical link phase noise compensation is performed with a round-trip method. To achieve such a result we implement light polarisation scrambling and dispersion compensation. This link outperforms all the previous radio-frequency links and compares well with recently demonstrated full optical links.     

Nonadiabatic nondegenerate excitation by optical near-field and its application to optical pulse-shape measurement

Using DCM dye grains and light of different wavelengths generated by two CW laser diodes that oscillate in the near-infrared wavelength region, visible light emission from dye grains due to near-infrared excitation based on a nonadiabatic, nondegenerate excitation process was observed for the first time. Unlike sum-frequency generation with nonlinear polarization, the difference in polarization angles of the two beams did not affect the emitted light intensity. Optical sampling based on this nonadiabatic, nondegenerate excitation principle was demonstrated for the first time. The optical pulse shape in the wavelength band of λ=1250–1350 nm, which is close to the wavelength range used for optical fiber communications, was measured with a temporal resolution of 0.8–1.1 ps.     

Non-collinear optical parametric chirped-pulse amplifier for few-cycle pulses

Modified parametric coupled equations for three-wave interaction in non-collinear optical parametric chirped-pulse amplification (OPCPA) are presented, in which the effects from the non-collinear configuration have been taken into account. By utilizing this new model, a two-stage OPCPA system based on a BBO crystal is numerically calculated with a split-step Fourier transform algorithm. Tracing the dynamics of the signal wave in the crystal reveals that in the first stage spectral gain narrowing occurs due to the weak input signal intensity and the non-uniform temporal distribution of the pump light. However, in the saturation regime the spectrum of the signal will be broadened as a consequence of back conversion. The simulation shows that it is crucial to correctly control the experimental parameters to balance both processes. For maximizing the energy-bandwidth product (EBP), an optimized configuration is sought by examining several parameters such as pump intensity, seed energy and crystal length. With a pump intensity of 7.75 GW/cm² and a crystal length between 3.75 and 4.25 mm pulses with a sub-10 fs duration can be amplified with a total gain of up to 1.7×10⁶.     

Quasi-phase-matched optical activity effect in “gyroelectric” crystals and its applications

“Quasi-phase matching” is first introduced to the optical activity (OA) effect and a wave coupling theory for the quasi-phase-matched (QPM) OA effect in periodically poled “gyroelectric” crystals is developed. The OA effect is observed clearly even though the propagating direction of light deviates far from the optical axis in the QPM crystal with both optical activity and natural birefringence. The QPM OA effect provides a special way to determine the gyration coefficients that cannot be observed by the normal OA effect, and it provides a principle for building optical filters without external field.     

A new polarimeter based on optical non-reciprocity in gratings with two-dimensional chirality

We have studied the optical transmission properties of a number of different dielectric diffraction gratings that each exhibit two-dimensional chirality in their grating elements. We have found that the diffracted beams transmitted through these structures exhibit large anisotropic polarization changes and polarization-dependent intensity modulations even though the chiral elements are arranged in square grating arrays. The strength and functional form of both of these effects differ for each diffracted beam. They are also dependent on the chirality of the surface patterning and the thickness of the patterned dielectric film. We show how these properties could be exploited to provide a new and more versatile form of polarimeter, and how the operation of such a device is inherently dependent on the optical non-reciprocity of these chiral gratings.     

Nontransparency and optical phase erasure characteristic of four-wave mixing

Four-wave mixing (FWM) is a well-known nonlinear optical phenomenon and has attracted great interest in nonlinear all-optical signal processing owing to its ultrafast and transparent properties. Here we report the experimental observation via spectra on the nontransparency and optical phase erasure characteristic of FWM. The distinctive feature of “optical phase erasure” is a consequence of square relationship between the converted idler and input signal. It is interesting to find that the optical phase information carried by the input signal is erased in the converted idler by exploiting FWM. Such new attribute of FWM enables optical phase information removal and all-optical format conversion from carrier-suppressed return-to-zero (CSRZ) to return-to-zero (RZ). Moreover, similar phenomena are observed in the higher-order FWM. In addition, the nontransparency phenomenon and optical phase erasure characteristic are also demonstrated by theoretical analyses with optical spectra and phase diagrams.     

Birefringent modifications induced by femtosecond filaments in optical glass

We present a study of the refractive index modifications spontaneously induced by infrared femtosecond filaments propagating in the bulk of transparent solids. It was found that extended modified refractive index channels up to several mm in length can be formed under loose focusing conditions in fused silica. The observed birefringence zone at the beginning of these channels is attributed to the anisotropic stress induced via nonlinear losses in the high intensity region of the filament.     

Dynamical coupling between a Bose–Einstein condensate and a cavity optical lattice

A Bose–Einstein condensate is dispersively coupled to a single mode of an ultra-high finesse optical cavity. The system is governed by strong interactions between the atomic motion and the light field even at the level of single quanta. While coherently pumping the cavity mode the condensate is subject to the cavity optical lattice potential whose depth depends nonlinearly on the atomic density distribution. We observe optical bistability already below the single photon level and strong back-action dynamics which tunes the coupled system periodically out of resonance.     

Hierarchy in optical near-fields based on compositions of nanomaterials

Optical near-field interactions exhibit hierarchical responses in the nanometer scale allowing unique functions in nanophotonic systems. Such hierarchical properties in optical near-fields originate various physical entities in the nanometer scale. Engineering nanomaterial compositions, while maintaining geometrically equivalent conditions, leads to characteristic hierarchical responses. We experimentally demonstrate such material-dependent optical near-field hierarchy using core–shell-type nanostructures composed of gold and silver.     

Compact and widely tunable terahertz source based on a dual-wavelength intracavity optical parametric oscillation

In this paper, a continuously tunable terahertz (THz) source is obtained using a compact intracavity pumped dual-wavelength optical parametric oscillation operating around 2.1 μm as difference-frequency generation pump source. The tuning range of the THz-wave frequency covers from 0.147 THz to 3.651 THz. Based on the collinear difference-frequency generation in the GaSe crystal, the experiment result shows that our schematic is a good option to construct a compact and portable terahertz source with widely tunable range.     

Nonlinear optical absorption properties of porphyrins confined in Nafion membrane

The nonlinear absorption (NLA) properties of five different types of porphyrins were studied using the Z-scan technique. The porphyrins under investigation were confined into Nafion column matrix membrane in order to protect them from possible degradation. The results of the experiments have indicated that all the porphyrins tested exhibited interesting NLA properties. The nonlinear absorption coefficients (β’s) were determined at different porphyrin concentrations by comparing the Z-scan data with the theoretical functions.     

Nonlinear optical absorption and refraction in Ru, Pd, and Au nanoparticle suspensions

We present the results of studies of the nonlinear optical properties of Pd, Ru, and Au nanoparticles. We studied the nonlinear refraction and nonlinear absorption of suspensions of these nanoparticles at 1064-nm wavelength. A relatively strong nonlinear absorption of the Pd nanoparticles was observed in the case of 1064-nm, 50-ps pulses (β=2×10⁻⁹ m W⁻¹). The Ru and Pd nanoparticles showed weak negative nonlinear refraction (γ∼−(6–8)×10⁻¹⁶ m² W⁻¹) in this spectral range. In the case of the Au nanoparticles, a saturated absorption at 532 nm dominated over other nonlinear optical processes.     

A terabit capacity passive polymer optical backplane based on a novel meshed waveguide architecture

An optical backplane based on a meshed polymer waveguide architecture enabling high-speed board-to-board optical interconnection is presented. This planar array of multimode polymer waveguides can provide passive strictly non-blocking links between server line cards fitted with optical transmitter and receiver arrays. This architecture offers a scalable and low-cost solution to the bandwidth limitations faced by electrical backplanes and is suitable for PCB integration. The reported backplane demonstrator uses a matrix of 100 waveguides each capable of 10 Gb/s operation to interconnect 10 cards for a total capacity of a terabit per second aggregate data rate in multicast mode. Characterisation of the backplane demonstrator reveals low link losses of 2 to 8 dB for a multimode fibre input and crosstalk values below −35 dB. Error free data transmission at 10 Gb/s is achieved with a power penalty of only 0.2 dB at a bit-error-rate of 10⁻⁹. Additionally, lossless operation of a Gigabit Ethernet link over the backplane is achieved even when using the worst-case highest loss links.     

A pump-resonant signal-resonant optical parametric oscillator for spectroscopic breath analysis

We report on a widely tunable continuous-wave single-frequency pump-resonant signal-resonant optical parametric oscillator (PRSRO) delivering 20 mW to 50 mW of idler power spanning an octave bandwidth in the mid-infrared (1.7 μm and 3.5 μm). The PRSRO is pumped by a Ti:Al₂O₃ ring laser delivering 760 mW around λ _p =795 nm, and thanks to the pump enhancement a threshold as low as 110 mW is achieved with a multi-grating temperature-tuned MgO-doped periodically-poled LiNbO₃. This PRSRO will be used for multi-species trace gas sensing based on cavity ring-down spectroscopy.     

Bundling influence on ultrafast optical nonlinearities of single-walled carbon nanotubes in suspension and composite film

Nonlinear optical characteristics of single-walled carbon nanotubes (SWCNTs) dispersed in dichlorobenzene and imbedded in polymer were investigated at 800 nm using the time-resolved optical Kerr gate technique. For systematic study of the influence of SWCNT bundling on optical nonlinearities, SWCNT solutions with different concentrations and a series of SWCNT/polymer composites deposited on glass substrates with different concentrations and thicknesses were prepared. The nonlinear response was comparable to the pulse duration of the laser used (～90 fs) both in SWCNT solutions and SWCNT/polymer composites. Over three orders of magnitude enhancement was observed in the third-order nonlinear susceptibility of SWCNT/polymer composite film compared with that of SWCNT solution. An appreciable reduction of microscopic and macroscopic nonlinearities was observed with increasing SWCNT concentrations due to stronger bundling of SWCNTs.     

Fabrication and characterization of hollow metal waveguides for optical interconnect applications

As data rates continue to increase in high-performance computer systems and networks, it is becoming more difficult for copper-based interconnects to keep pace. An alternative approach to meet these requirements is to move to optical-based interconnect technologies which offer a number of advantages over the legacy copper-based solutions. In order to meet the stringent requirements of high performance and low cost, manufacturable waveguide technologies must be developed. Past solutions have often employed polymer waveguide technologies, which can be expensive and limited by modal dispersion. In the present work, hollow metal waveguides (HMWGs) are investigated as a potential alternative. These waveguides demonstrate very low optical losses of <0.05 dB/cm and the capability to transmit at extremely high data rates. The fabrication, modeling, characterization of the HMWGs are discussed to enable photonic interconnect solutions for future generations of computer and server products.     

A novel subwavelength plasmon-polariton optical filter based on tilted coupled structures

This work proposes a completely new approach for the design of resonant structures aiming at wavelength-filtering applications. The structure consists of a subwavelength metal-insulator-metal (MIM) waveguide presenting tilted coupled structures transversely arranged in the midpoint between the input and output ports. The cavity-like response of this device has shown that this concept can be particularly attractive for optical filter design for telecom applications. The extra degree of freedom provided by the tilting of the cavity has proved to be not only very effective on improving the quality factor of these structures, but also to be an elegant way of extending the range of applications for tuning multiple wavelengths, if necessary.     

Robust free space board-to-board optical interconnect with closed loop MEMS tracking

We present a free-space optical interconnect system capable of dynamic closed-loop optical alignment using a microlens scanner with a proportional integral and derivative controller. Electrostatic microlens scanners based on combdrive actuators are designed and characterized with vertical cavity surface emitting lasers (VCSELs) for adaptive optical beam tracking in the midst of mechanical vibration noise. The microlens scanners are fabricated on silicon-on-insulator wafers with a bulk micromachining process using deep reactive ion etching. We demonstrate dynamic optical beam positioning with a 700 Hz bandwidth and a maximum noise reduction of approximately 40 dB. Eye diagrams with a 1 Gb/s modulation rate are presented to demonstrate the improved optical link in the presence of mechanical noise.     

Realization of KaptonTM based optical interconnect by KMnO4 wet etching

In the paper, based on KMnO₄ wet-etching technology PDMS (polydimethylsiloxane) based flexible optical interconnect packaged with Kapton^TM (Dupont) foils was successfully realized through a stable and effective bonding technology. The optical and mechanical properties of the PDMS waveguide layer remained unchanged before and after packaging with KMnO₄ etched Kapton foils. The mechanical stability limit of the tested optical interconnects is determined only by the intrinsic mechanical stability of the used PDMS materials. The optical loss at 850 nm is <0.05 dB/cm even after temperature treatments up to lead-free soldering temperatures of 260°C. In addition, the main mechanism of forming a good bonding between PDMS and Kapton foil was identified and analyzed as well.     

Nonlinear optical and optical limiting studies of alkoxy phthalocyanines in solutions studied at 532 nm with nanosecond pulse excitation

We report our results on the nonlinear optical and optical limiting properties of two alkoxy phthalocyanines namely 2,3,9,10,16,17,23,24-octakis-(heptyloxy) phthalocyanine and 2,3,9,10,16,17,23,24-octakis-(heptyloxy) phthalocyanine zinc(II) studied at a wavelength of 532 nm using 6 ns pulses. Using the standard Z-scan technique we observed that both the phthalocyanines exhibited negative nonlinearity as revealed by the signature of closed aperture data. The magnitude of the nonlinear refractive index n₂ evaluated from the closed aperture data was ∼ 1.61×10^-11 cm²/W for the free-base phthalocyanine and ∼ 1.56×10^-11 cm²/W for the metallic phthalocyanine. Open aperture Z-scan data indicates strong nonlinear absorption in both the phthalocyanines with measured nonlinear coefficients of ∼ 1650 cm/GW and ∼ 1850 cm/GW respectively. We also report optical limiting properties of these phthalocyanines with limiting thresholds (I_1/2) of ∼ 0.5 J/cm². Our studies suggest that these phthalocyanines are one of the best molecules for nonlinear optical applications studied recently. PACS 42.65.-k; 42.70.Jk, 42.65.Jx