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Orbital angular momentum (OAM), inherent to light beams characterized by helical phase fronts, represents an alternative spatial degree of freedom for photons. In recent years, OAM-based communication has gained significant attention as a potential solution to increase the capacity and spectral efficiency of optical fiber communication systems. In optical fibers, OAM modes can be generated and manipulated by utilizing specialized optical components and techniques. One approach involves using specially designed optical fibers, such as ring–core fibers, which have the capability to support multiple OAM modes. In the manufacturing process, the ring fiber is hardly made perfectly, especially the doped-cores. The introduced defect breaks the symmetry of the ring–core, which has a serious impact on OAM mode transmission. Using a material with a slightly higher refractive index than the cladding to wrap the ring–core is effective in reducing impairment of defects. After numerical simulation, the proposed method improves the walk-off length by a factor of 2–40. [ABSTRACT FROM AUTHOR]

Mode division multiplexing technology can regard each eigenmode in optical fibers as an independent channel to carry different data information, helpful for improving the transmission capacity of optical fiber communication systems. The spatial distribution, state of polarization and propagation constant of guided optical eigenmodes in the magneto-optical fiber are derived theoretically by the analysis of linear polarization and exact modes. The transverse distribution of the eigenmodes in the MO fiber can be constructed by those of exact modes in the conventional fiber. The effect of applied magnetic field on the eigenmodes and their effective refractive indices is studied. Simulation results show that, the eigenmodes in the MO fiber may be represented by the pseudo-modes of circular polarization ( CP ℓ n R / L ) or the true modes of orbital angular momentum ( OAM ± ℓ , n R / L ) when applied magnetic field is sufficiently large. The measurement method for weak magnetic field based on Stokes parameters is proposed. The magnetic tunability of OAM mode selective couplers is also analyzed. [ABSTRACT FROM AUTHOR]

Owing to the rapid development of coherent optical fiber communication, free-space optical communication systems typically have a fiber-based coupling structure. In such systems, the received light field is coupled to a single-mode fiber or a few-mode fiber (FMF). In this study, we respectively establish an expectation model of FMF coupling efficiency under optical system aberration (OSA) and fiber positioning error (FPE) conditions, based on the Kolmogorov power-law spectrum and plane wave. The performance of the FMF with different coupling geometry parameters is analyzed. To the best of our knowledge, this is the first time the impacts of OSA and FPE on the coupling efficiency of a FMF under different turbulence strengths and coupling geometry parameters have been analyzed. [ABSTRACT FROM AUTHOR]

Solitons are ubiquitous phenomena that appear, among others, in the description of tsunami waves, fiber-optic communication and ultracold atomic gases. The latter systems turned out to be an excellent playground for investigations of matter-wave solitons in a quantum world. This tutorial provides a general overview of the ultracold contact interacting Bose and Fermi systems in a one-dimensional space that can be described by the renowned Lieb–Liniger and Yang–Gaudin models. Both the quantum many-body systems are exactly solvable by means of the Bethe ansatz technique, granting us a possibility for investigations of quantum nature of solitonic excitations. We discuss in details a specific class of quantum many-body excited eigenstates called yrast states and show that they are strictly related to quantum dark solitons in the both considered Bose and Fermi systems. [ABSTRACT FROM AUTHOR]

In this paper, the gain engineering of a semiconductor quantum dots fiber amplifier (SQDFA) is studied. The optical amplification is realized by exciting the PbS quantum dots coated around the tapered single-mode fiber coupler with an evanescent wave. The gain spectra and saturation properties of the SQDFA are obtained via solving the rate and coupled power equations based on the two-level model. Then, the effect of the signal wavelength, pump power and amplifier length on the saturation characteristics is numerically simulated. The obtained results show that the proposed model is in a good agreement with the previous experimental results. Unique characteristics of the SQDFA such as the broadband optical gain and smooth bandwidth are of great interest for dense wavelength-division multiplexing systems. Therefore, the proposed model show promising applications in the future fiber-optic communications. [ABSTRACT FROM AUTHOR]

In the article, we report the photoluminescence (PL) properties of D and S defects induced by Si+/Ni+ ions co-implanting into the top Si film of the silicon-on-insulator (SOI) wafer. Variable-temperature PL spectra of these co-implanted SOI samples indicate that the light emitting from the D defects can be observed as high as 273 K. In comparison with the other ion-implantation, the Si+/Ni+ ion-co-implantation optimizes luminescent temperature stability of the both D and S defects and purifies the S defect type in silicon then effectively restrains the spectral broadening of the S-line in PL spectra. The depth distribution of the D and S defects along the normal direction of SOI surface at the corresponding ion-implantation energy has been well depicted by detecting the PL signals of the layer-by-layer etched SOI surface, respectively. These results provide valuable information to fabricate SOI-based infrared light sources for optical fiber communications. [ABSTRACT FROM AUTHOR]

Nanoparticle materials have many potential applications in the area of electronics and optoelectronics due to their unique and versatile properties. In particular, lead sulfide nanoparticles (PbS NPs) have shown excellent ultrafast photonics and can be applied to communication systems because of their low bandgap, high thermal damage threshold and stability. The wavelength division multiplexor (WDM) technique is vital to fiber optical communication, which allows the transmission of many different-wavelength signals in one fiber cable. However, PbS NPs for multi-wavelength pulse generation has not been reported until now. In this work, PbS NPs have been investigated and successfully applied in an Er-doped fiber laser as a saturable absorber (SA) to generate a dual-wavelength short pulse for the first time. A picosecond-level ultrashort pulse at center wavelengths of 1545 and 1585 nm can be achieved simultaneously or respectively. It is worth mentioning that the two wavelengths are separated up to 40 nm, which can significantly expand the optical communication capacity. The result suggests that PbS NPs as smart nonlinear optical components have wide applications in optical communications, short-pulse lasers, and even high-performance photodectors. [ABSTRACT FROM AUTHOR]

Quantum-mechanically indistinguishable photons produced by independent (or equivalently, mutually phase incoherent) light sources are essential for distributed quantum information processing applications. We demonstrate heralded generation of such photons in two spatially separate telecom-fiber spools, each driven by pulsed pump waves that are measured to have no mutual phase coherence. Through Hong–Ou–Mandel experiments, we measure the quantum interference visibility of those photons to be . Our experimental results are well predicted by a quantum multimode theory we developed for such systems without the need for any fitting parameter. [ABSTRACT FROM AUTHOR]

To make use of the spectrum of a step-index polymer optical fiber (SI-POF) channel, a hybrid modulation scheme is proposed combining discrete multi-tone modulation and M-ary position phase shift keying. The discrete line spectrum interference and sideband interference components of the new modulation scheme are analyzed. The waveform optimization factor and a new numerical symbol generating approach are introduced to strengthen the anti-interference capability of the system. Offline processing with an arbitrary waveform generator and digital storage oscilloscope is used to investigate the transmission performance. A transmission rate of 510.5 Mbps is achieved for a 50 m SI-POF in an intensity modulated direct detection system using eye-safe resonant cavity light-emitting diodes and a PIN diode at red 650 nm. The experimental verification demonstrates that, using the new methodology, an additional bit rate of 19.5 Mbps in the low signal-to-noise ratio region is obtained. Furthermore there 6 dB power is saved at a bit error rate (BER) of 1×10−4, so better spectral efficiency can be achieved. It is demonstrated that the proposed scheme can offer better BER performance and make a better compromise between transmission rate, power consumption, spectral efficiency and BER. [ABSTRACT FROM AUTHOR]

In this paper, we have experimentally demonstrated time domain characteristics of all-optical intensity modulation using cross-gain modulation induced by side-mode injection locking in a single-mode Fabry–Pérot laser diode (SMFP-LD). Both bistable hysteresis and injection locking behaviors are observed and discussed for different wavelength detuning between the injection beam and the selected side mode, which is the direct physical mechanism to successfully perform optical intensity modulation in the laser chip. Both the modulation depth of the injection signal and wavelength detuning have very important influences on the optical characteristics, including the modulation depth, and rise and fall time of the output signal at the dominant mode. The obtained modulation depth in the output signal can reach as high as ∼25 dB when a pulsed injection beam with a time duration of 8 ns is introduced into the laser chip under conditions of a bias current of 12 mA and an operating temperature of 16 ° C. [ABSTRACT FROM AUTHOR]

The possibility of using optical vortices with different values of topological charge l, and hence with different values of the orbital angular momentum (OAM), for mode-division multiplexing in optical fibres is here investigated. Two OAM modes with l equal to 0 and 1, respectively, are multiplexed in a few-mode fibre and then demultiplexed after a fibre propagation of 200 m. Such modes are spatially separated at the two output ports of a Mach–Zehnder interferometer with mutually 90°-rotated Dove prisms in the two arms. It is also shown how to generalize this demultiplexing scheme, which is all-optical, passive and in principle without splitting losses, in order to deal with a higher number of vortices propagating in the fibre. Therefore the proposed mode-division multiplexing technique based on OAM modes is very promising for increasing the capacity of fibre-optic transmission systems in an energy-saving efficient way, without the high power consumption of modal demultiplexing exploiting real-time electronic post-processing. [ABSTRACT FROM AUTHOR]

Multimode 850 nm vertical cavity surface-emitting lasers (VCSELs) suitable for high bit rate operation are studied. VCSELs with oxide aperture diameters of 5-7 μm show a high -3 dB modulation bandwidth (∼20 GHz) and D-factor (∼ 8 GHz mA-1/2). To allow low capacitance a multiple layer oxide-confined aperture design was applied. Eye diagrams are clearly open up to 35 Gbit s-1 at the temperature of 25 °C. Using 35 μm diameter PIN photodiodes and 6 μm oxide aperture diameter VCSELs error-free 25 Gbit s-1 (defined as a bit error ration of ⩽1 × 10-12) optical fiber communication links were tested over 100 m of standard OM3 multimode optical fibers at 25 °C and 85 °C. The received optical power for error-free operation was below -4 dBm at both temperatures. A VCSEL reliability study at 95 °C was performed at the high current densities (∼18 kA cm-2) needed for error-free 25 Gbit s-1 operation at elevated temperatures. After 6000 h a slight increase (less than 5%) of the output optical power at a constant current was observed and most likely due to an ohmic contact burn in effect within the first 2000 h of the study. The results clearly indicate that 25 Gbit s-1 850 nm oxide-confined VCSELs with a complex AlGaO multilayer aperture design and with step-graded Al-compositions have the potential for reliable operation. [ABSTRACT FROM AUTHOR]

This paper presents an all-fiber tunable comb filter based on high-order mode fiber (HOMF). The filter is constructed by splicing a segment of HOMF between two sections of conventional single-mode fibers (SMFs), with an intentionally introduced offset at the first joint between the SMF and HOMF, and without an offset at the second joint between the HOMF and SMF, to optimize the visibility of the transmission spectrum. The optimal transverse offset of the fibers at the first joint and the tunable polarization-independent properties of the filter were studied theoretically and experimentally. An example of using this tunable comb filter in the application of a sensor, based on demodulating the wavelength shift of a fiber Bragg grating, has also been demonstrated. The particular advantages of this all-fiber tunable filter make it valuable in various applications. [ABSTRACT FROM AUTHOR]

We present a novel single optical packaging and encryption (SOPE) procedure for multiple inputs. This procedure is based on a merging of a 2f scheme with a digital holographic technique to achieve efficient handling of multiple data. Through the 2f system with a random phase mask attached in its input plane, and the holographic technique, we obtain each processed input. A posteriori filtering and repositioning protocol on each hologram followed by an addition of all processed data, allows storing these data to form a single package. The final package is digitally multiplied by a second random phase mask acting as an encryption mask. In this way, the final user receives only one encrypted information unit and a single key, instead of a conventional multiple-image collecting method and several keys. Processing of individual images is cast into an optimization problem. The proposed optimization aims to simplify the handling and recovery of images while packing all of them into a single unit. The decoding process does not have the usual cross-talk or noise problems involved in other methods, as filtering and repositioning precedes the encryption step. All data are recovered in just one step at the same time by applying a simple Fourier transform operation and the decoding key. The proposed protocol takes advantage of optical processing and the versatility of the digital format. Experiments have been conducted using a Mach–Zehnder interferometer. An application is subsequently demonstrated to illustrate the feasibility of the SOPE procedure. [ABSTRACT FROM AUTHOR]

In this paper, we propose the design of tunable and robust wavelength converters which can perform the polarity-preserved or polarity-inverted wavelength conversion by using the cross-phase modulation (XPM) in a dispersion-flattened highly nonlinear photonic crystal fiber (DF-HNL-PCF). The newly designed wavelength converter consists of a DF-HNL-PCF and an optical band-pass filter in the simple straight-line configuration, where XPM leads to broadening the spectrum of a continuous-wave probe light and the polarity-preserved, tunable wavelength conversion is then implemented by filtering out the blue-chirped or red-chirped spectral component of a probe light. The experimental results show that the polarity-preserved wavelength converter using a DF-HNL-PCF can achieve the error-free conversion of 10 Gbit s-1 optical data signals with the maximum power penalty of less than 2.1 dB over a wavelength range of nearly 34 nm. This wavelength tunability is actually limited by an operational wavelength range of the tunable optical band-pass filter that is used to extract the wavelength-converted signal in our experiment. Moreover, the polarity-inverted wavelength conversion is observed at the XPM-based wavelength converter using a DF-HNL-PCF, for the first time. This is achieved by tuning the central wavelength of an optical band-pass filter to coincide with that of a chirped probe light. [ABSTRACT FROM AUTHOR]

We present a detailed analysis of a gigahertz clock rate environmentally robust phase-encoded quantum key distribution (QKD) system utilizing several different single-photon detectors, including the first implementation of an experimental resonant cavity thin-junction silicon single-photon avalanche diode. The system operates at a wavelength of 850 nm using standard telecommunications optical fibre. A general-purpose theoretical model for the performance of QKD systems is presented with reference to these experimental results before predictions are made about realistic detector developments in this system. We discuss, with reference to the theoretical model, how detector operating parameters can be further optimized to maximize key exchange rates. [ABSTRACT FROM AUTHOR]

Faithful long-distance quantum teleportation necessitates prior entanglement distribution between two communicated locations. The particle carrying on the unknown quantum information is then combined with one particle of the entangled states for Bell-state measurements, which leads to a transfer of the original quantum information onto the other particle of the entangled states. However in most of the implemented teleportation experiments nowadays, the Bell-state measurements are performed even before successful distribution of entanglement. This leads to an instant collapse of the quantum state for the transmitted particle, which is actually a single-particle transmission thereafter. Thus the true distance for quantum teleportation is, in fact, only in a level of meters. In the present experiment we design a novel scheme which has overcome this limit by utilizing fiber as quantum memory. A complete quantum teleportation is achieved upon successful entanglement distribution over 967 meters in public free space. Active feed-forward control techniques are developed for real-time transfer of quantum information. The overall experimental fidelities for teleported states are better than 89.6%, which signify high-quality teleportation. [ABSTRACT FROM AUTHOR]

Two protocols of quantum direct communication with authentication [Phys. Rev. A 73 (2006) 042305] were recently indicated to be insecure against the authenticator Trent attacks [Phys. Rev. A 75 (2007) 026301]. We present two efficient protocols by using four Pauli operations, which are secure against inner Trent attacks as well as outer Eve attacks. Finally we generalize them to multiparty quantum direction communication. [ABSTRACT FROM AUTHOR]

A class of unlockable bound entangled states and their applications are presented. They can be considered as quasi generalized Smolin states [Phys. Rev. A 63 (2001) 032306], which are the states of N + N qubits. No pure entanglement can be distilled from this class of states by local quantum operations and classical communications. However, if certain parties group together, they become distillable. Although they are bound entangled states, they could be used to achieve some non-trivial tasks, such as quantum secret sharing shown in the study. [ABSTRACT FROM AUTHOR]

The output characteristics of optical feedback in a helium-neon laser with a birefringent internal cavity are studied systematically in five different regions of the gain curve for the two orthogonally polarized modes. When the laser operates in the two end regions of the laser gain curve, one of the two orthogonally polarized modes will be a leading one in optical feedback. Strong mode competition can be observed. However, when the laser operates in the middle region of the laser gain curve, the two modes can oscillate equally with optical feedback. Besides the intensity of the two polarized lights, the total light intensity is also studied at the same time. M-shaped optical feedback curves are found. Particularly, when the average intensities of the two lights are comparable, the intensity modulation curve of the total light is doubled, which can be used to improve the resolution of an optical feedback system. [ABSTRACT FROM AUTHOR]

Optical fibers are seen in the optical sensing and optical fiber communication. Simultaneous propagation of optical pulses in an inhomogeneous optical fiber is described by a coupled time-dependent coefficient fourth-order nonlinear Schrödinger system, which is discussed in this paper. For such a system, we work out the Lax pair, Darboux transformation, and corresponding vector semi-rational nonautonomous rogue wave solutions. When the group velocity dispersion (GVD) and fourth-order dispersion (FOD) coefficients are the constants, we exhibit the first- and second-order vector semi-rational rogue waves which are composed of the four-petalled rogue waves and eye-shaped breathers. Both the width of the rogue wave along the time axis and temporal separation between the adjacent peaks of the breather decrease with the GVD coefficient or FOD coefficient. With the GVD and FOD coefficients as the linear, cosine, and exponential functions, we respectively present the first- and second-order periodic vector semi-rational rogue waves, first- and second-order asymmetry vector semi-rational rogue waves, and interactions between the eye-shaped breathers and the composite rogue waves. [ABSTRACT FROM AUTHOR]

We present an all-fiber-based method for generating orbital angular momentum (OAM) modes by using photonic lanterns in an adaptive-spatial mode control system. The device consists of a phase control part, a photonic lantern and a mode decomposition part. The use of photonic lanterns as adiabatic mode converters has been elaborated upon mathematically. To lock the desired OAM mode, the stochastic parallel gradient descent algorithm is used to control the phase modulators. The purities of the generated OAM modes with different topological charge are higher than 90% at 1550 nm, which has immediate utility for free-space propagation systems, and the superposition of different OAM modes at a specific ratio can be obtained to meet the requirements of mode multiplexing in optical fiber communication systems. [ABSTRACT FROM AUTHOR]

Optical fibre communication enables the global internet, but few youths ever learn about how it works, even at a basic level, until tertiary education. While some middle school curricula might include simple geometrical optics concepts like reflection and refraction, they often lack contextual linkage to worldwide telecommunications. Through our studies, we have found that students are more engaged in the learning process when the material directly relates to real life. To address this gap, and moreover to tackle the issue of the STEM skills shortage, we have designed a self-contained lesson to introduce youths to this topic. It is modularised into three parts, beginning with using light to communicate Morse code, and then covering advanced themes such as multiplexing and fibre guidance based on total internal reflection. The modules can be taught sequentially or individually depending on educational level. They emphasise a more phenomenological than theoretical approach and include hands-on activities using easily obtainable materials. We outline the lesson and pedagogical guidelines for classroom settings, as well as evaluate actual classes run. This lesson can be flexibly implemented in formal classes or through educational outreach programmes. [ABSTRACT FROM AUTHOR]

Remote sensing remains a challenge due to its demand for high sensitivity, convenient sampling and rapid response time. Surface enhanced Raman scattering (SERS) spectroscopy is a powerful analytical method for the detection of various samples. Here, aiming at increasing the sensitivity, a novel strategy for the preparation of a SERS probe is demonstrated by using hollow optical fiber tips decorated by layer-by-layer assembly of two kinds of nanoparticles. Specifically, Au@Ag core–shell nanorods and Ag nanospheres with opposite surface charge were assembled layer-by-layer on the tip of hollow optical fibers through electrostatic interaction. Then, much more hotspots are generated due to the close gap between the nanorods and nanospheres in the resultant 3D structure, which can lead to a dramatically enhanced SERS activity of the probe compared with that fabricated by pure silver sphere nanoparticles or nanorods. On the other hand, taking the advantages of the vibration spectroscopic fingerprints property of SERS spectra and the long-distance communication capacity of optical fibers, the remote online detection of biological species including proteins, funguses and cells can be easily achieved within a few minutes. Therefore, such a novel kind of optical fiber-SERS sensor holds great potential for the rapid detection of a wide range of samples due to its superiority of simplicity and high sensitivity. [ABSTRACT FROM AUTHOR]

The laser-produced plasma and laser-induced breakdown spectroscopy (LIBS) of two steels from a nuclear power plant, including the Z3CN20-09M steel used for the main reactor coolant pipes, and the 16MND5 steel used for nuclear reactor pressure vessels. These were investigated based on a fiber-optic LIBS system. Under the same input laser energy, the laser-produced plasma from the Z3CN20-09M steel had a weaker emission intensity compared to the 16MND5 steel, probably due to a greater content of Cr. Thus, plasma from the Z3CN20-09M steel had a larger kinetic energy and induced a faster expansion shockwave in air. Calibrations of the fiber-optic LIBS system by the means of internal standardization, support vector regression and random forest regression were set up. For the Cr element, as a major element in Z3CN20-09M (19.33wt.%), it was determined with the relative error controlled under 3.5%; as a trace element (0.11wt.%) in 16MND5 it was measured with the best root-mean-square error of prediction as 0.032wt.%, and the Mn content (1.19–1.51wt.%) in both steels was measured with the relative error around 10%. [ABSTRACT FROM AUTHOR]

This work presents the development of a three-waveguide polarization-independent 1 × 2 power splitter using the silicon-on-insulator platform. Design of the device was accomplished by the effective index based matrix method and coupled mode theory. The component was fabricated using a readily available silicon-on-insulator substrate of a 5 μm thick device layer and characterized at 1.55 μm wavelength of light. The fabricated device has an insertion loss of 11.43 dB for transverse electric (TE) polarization and 11.80 dB for transverse magnetic (TM) mode, which indicates its polarization-independent behavior. The power splitting shows an imbalance of 0.23 dB for the TE mode and 0.82 dB for TM mode, which is due to the fabrication error of the separation of outer arms from the central input waveguide. The design can be extended for any 1 × 2N 3-dB power splitter, which will be very useful for optical interconnects and fiber-optic communication networks. [ABSTRACT FROM AUTHOR]

Digital data communication, i.e. the transmission of discrete encoded information, is already pervasive, but possesses an ever-growing importance for everyday life in contemporary societies. The underlying physics can vary vastly, e.g. between fibre-optic communication and wireless mobile cellular networks. However, at least three unifying elements can clearly be identified: modulation, transmission and demodulation. Here, we present a setup for demonstrating live as well as teaching the basic elements of data communications. Our setup comprises two identical piezoelectric transducers for actuating and sensing ultrasonic waves on spatially separated plates. The two thin metallic plates, which are bridged by a movable solid piece, constitute the physical transmission medium. Especially the choice of guided ultrasound waves, which are mechanical waves and equivalent to ordinary sound, should facilitate comprehension by students. In order to further reduce abstraction, the usage of a manifest movable link, which allows for disruption of the data flow, is accompanied by introducing the metaphor of a ‘drawbridge’. The presented didactic demonstration should be suitable for stimulating the exploration of more advanced topics in the lecture hall, such as source and channel coding, multiple-carrier and multiple-access methods. [ABSTRACT FROM AUTHOR]

Optical fibre sensors (OFS), as a result of their unique properties such as small size, no interference with electromagnetic radiation, high sensitivity and the ability to design multiplexed or distributed sensing systems, have found applications ranging from structural health monitoring to biomedical and point of care instrumentation. While the former represents the main commercial application for OFS, there is body of literature concerning the deployment of this versatile sensing platform in healthcare. This paper reviews the different types of OFS and their most recent applications in healthcare. It aims to help clinicians to better understand OFS technology and also provides an overview of the challenges involved in the deployment of developed technology in healthcare. Examples of the application of OFS in healthcare are discussed with particular emphasis on recently (2015–2017) published works to avoid replicating recent review papers. The majority of the work on the development of biomedical OFS stops at the laboratory stage and, with a few exceptions, is not explored in healthcare settings. OFSs have yet to fulfil their great potential in healthcare and methods of increasing the adoption of medical devices based on optical fibres are discussed. It is important to consider these factors early in the device development process for successful translation of the developed sensors to healthcare practice. [ABSTRACT FROM AUTHOR]

In this report, we present a quantum theory describing the propagation of the electromagnetic radiation in a fiber in the presence of the third order dispersion coefficient. We obtained the quantum photon-polariton field, hence, we provide herein a coupled set of operator forms for the corresponding nonlinear Schrödinger equations when the third order dispersion coefficient is included. Coupled stochastic nonlinear Schrödinger equations were obtained by applying a positive P-representation that governs the propagation and interaction of quantum solitons in the presence of the third-order dispersion term. Finally, to reduce the fluctuations near solitons in the first approximation, we developed coupled stochastic linear equations. [ABSTRACT FROM AUTHOR]

A polymer mode multiplexer based on asymmetric couplers is theoretically designed and experimentally demonstrated. The proposed X-junction coupler is formed by waveguides overlapped with different crossing angles in the vertical direction. A beam propagation method is adopted to optimize the dimensional parameters of the mode multiplexer to convert LP01 mode of two lower waveguides to LP11a and LP21a mode of the upper waveguide. The ultraviolet lithography and wet chemical etching are used in the fabrication process. A conversion ratio over 98% for both LP11a and LP21a mode in the wavelength range from 1530 to 1570 nm are experimentally demonstrated. This mode multiplexer has potential in broadband mode-division multiplexing transmission systems. [ABSTRACT FROM AUTHOR]

We develop an efficient approach to the design of families of single solitons and soliton molecules most suited to a given fiber system. The obtained solitonic entities exhibit very high stability, with a robustness which allows them to propagate over thousands of kilometers and to survive collisions with other solitonic entities. Our approach enables the generation of a large number of solitonic entities, including families of single solitons and two-soliton molecules, which can be distinguished sufficiently by their respective profiles or energy levels, and so can be easily identifiable and detectable without ambiguity. We discuss the possible use of such solitonic entities as symbols of a multi-level modulation format in fiber-optic communication systems. [ABSTRACT FROM AUTHOR]

We present a scheme of frequency-degenerate mid-span spectral inversion (MSSI) for nonlinearity compensation in fiber-optic transmission systems. The spectral inversion is obtained by using counter-propagating dual pump four-wave mixing in a semiconductor optical amplifier (SOA). Frequency-degeneracy between signal and conjugate is achieved by keeping two pump frequencies symmetrical about the signal frequency. We simulate the performance of MSSI for nonlinearity compensation by scrutinizing the improvement of the Q-factor of a 200 Gbps QPSK signal transmitted over a standard single mode fiber, as a function of launch power for different span lengths and number of spans. We demonstrate a 7.5 dB improvement in the input power dynamic range and an almost 83% increase in the transmission length for optimum MSSI parameters of −2 dBm pump power and 400 mA SOA current. [ABSTRACT FROM AUTHOR]

A fiber-based, star-shaped joint time and frequency dissemination scheme is demonstrated. By working in cooperation with the existing commercial telecommunication network. Our scheme enables the frequency, time, and digital data networks to be integrated together and could represent an ideal option of interconnection among scientific institutions. The compensation functions of the time and frequency transfer scheme are set at the client nodes. The complexity of the central node is thus reduced, and future expansion by the addition of further branches will be accomplished more easily. During a performance test in which the ambient temperature fluctuation is 30 °C/day, timing signal dissemination stability is achieved to be approximately ±50 ps along 25-km-long fiber spools. After calibration, a timing signal synchronization accuracy of 100 ps is also realized. The proposed scheme offers an option of the construction of large-scale fiber-based frequency and time transfer networks. [ABSTRACT FROM AUTHOR]

We demonstrate a platform for implementing quantum walks that overcomes many of the barriers associated with photonic implementations. We use coupled fiber-optic cavities to implement time-bin encoded walks in an integrated system. We show that this platform can achieve very low losses combined with high-fidelity operations, enabling an unprecedented large number of steps in a passive system, as required for scenarios with multiple walkers. Furthermore the platform is reconfigurable, enabling variation of the coin, and readily extends to multidimensional lattices. We demonstrate variation of the coin bias experimentally for three different values. [ABSTRACT FROM AUTHOR]

Advances in single-photon creation, transmission, and detection suggest that sending quantum information over optical fibers may have losses low enough to be correctable using a quantum error correcting code (QECC). Such error-corrected communication is equivalent to a novel quantum repeater scheme, but crucial questions regarding implementation and system requirements remain open. Here we show that long-range entangled bit generation with rates approaching 108 entangled bits per second may be possible using a completely serialized protocol, in which photons are generated, entangled, and error corrected via sequential, one-way interactions with as few matter qubits as possible. Provided loss and error rates of the required elements are below the threshold for quantum error correction, this scheme demonstrates improved performance over transmission of single photons. We find improvement in entangled bit rates at large distances using this serial protocol and various QECCs. In particular, at a total distance of 500 km with fiber loss rates of 0.3 dB km−1, logical gate failure probabilities of 10−5, photon creation and measurement error rates of 10−5, and a gate speed of 80 ps, we find the maximum single repeater chain entangled bit rates of 51 Hz at a 20 m node spacing and 190 000 Hz at a 43 m node spacing for the and QECCs respectively as compared to a bare success rate of 1 × 10−140 Hz for single photon transmission. [ABSTRACT FROM AUTHOR]

We propose a technique to realize a tachyonic band structure in a coherent network, such as an array of coupled ring resonators. This is achieved by adding ‘PT symmetric’ spatially balanced gain and loss to each node of the network. In a square-lattice network, the quasi-energy bandstructure exhibits a tachyonic dispersion relation, centered at either the center or corner of the Brillouin zone. There is one tachyonic hyperboloid in each gap, unlike in PT-symmetric tight-binding honeycomb lattices where the hyperboloids occur in pairs. The dispersion relation can be probed by measuring the peaks in transmission across a finite network as the gain/loss parameter is varied. [ABSTRACT FROM AUTHOR]

A global positioning system (GPS) carrier-phase frequency transfer link along a baseline of 450 km has been established and is characterized by comparing it to a phase-stabilized optical fiber link of 920 km length, established between the two endpoints, the Max-Planck-Institut für Quantenoptik in Garching and the Physikalisch-Technische Bundesanstalt in Braunschweig. The characterization is accomplished by comparing two active hydrogen masers operated at both institutes. The masers serve as local oscillators and cancel out when the double differences are calculated, such that they do not constitute a limitation for the GPS link characterization. We achieve a frequency instability of in 30 s and for long averaging times. Frequency comparison results obtained via both links show no deviation larger than the statistical uncertainty of . These results can also be interpreted as a successful cross-check of the measurement uncertainty of a truly remote end fiber link. [ABSTRACT FROM AUTHOR]

We proposed and fabricated a long period fiber grating (PS-LPG) for simultaneous measurement of strain and temperature. The is fabricated by inscribing micro-tapers on a pre-tapered fiber using the filament heating method. When the temperature or strain varies, the wavelengths of both transmission notches shift in the same direction, whereas the depths of the notches change in different directions. Therefore, the sensitivity of the PS-LPG can be improved by detecting the depth difference between the two notches, and simultaneous temperature and strain measurements can be achieved from the variation of the wavelength and the depth difference. Experimental results show that the wavelength sensitivities can reach a maximum of respectively −1.5 nm mϵ−1 and for strain and temperature, and the depth sensitivities are as high as −4.6 dB mϵ−1 and −0.06 dB °C−1, respectively. [ABSTRACT FROM AUTHOR]

This paper reports a new design of optical time-division multiplexed (OTDM) systems that possess a functionality of simultaneous time demultiplexing and wavelength multicasting based on the cascaded four-wave mixing in a dispersion-flattened highly nonlinear photonic crystal fiber (DF-HNL-PCF). A module of OTDM demultiplexing and wavelength multicasting can be feasibly implemented by using a 3 dB optical coupler, a high-power erbium-doped fiber amplifier, a short-length DF-HNL-PCF, and a wavelength demultiplexer in the simple configuration. We also carry out an experiment on the proposed system to demonstrate the 100–10 Gbit s−1 OTDM demultiplexing with wavelength conversion simultaneously at 4 multicast wavelengths. It is shown that error-free wavelength multicasting is achieved on two wavelength channels with the minimum power penalty of 3.2 dB relative to the 10 Gbit s−1 back-to-back measurement, whereas the bit error rates of other two multicasting channels are measured to be about 10−6–10−5. Moreover, we propose the use of a proper error-correcting code to improve the multicasting performance of such an OTDM system, and our work reveals that the resulting system can theoretically support error-free multicasting of the OTDM-demultiplexed signal on four wavelength channels. [ABSTRACT FROM AUTHOR]

A 32 Gb/s monolithically integrated electroabsorption modulated laser is fabricated by selective area growth technology. The threshold current of the device is below 13 mA. The output power exceeds 10 mW at 0 V bias when the injection current of the distributed feedback laser is 100 mA at 25°C. The side mode suppression ratio is over 50 dB. A 32Gb/s eye diagram is measured with a 3.5Vpp nonreturn-to-zero pseudorandom modulation signal at −2.3 V bias. A clearly opening eyediagram with a dynamic extinction ratio of 8.01 dB is obtained. [ABSTRACT FROM AUTHOR]

In this paper, the positive influence of a uni-traveling-carrier (UTC) structure to ease the contract between the responsivity and working speed of the InP-based double hetero-junction phototransistor (DHPT) is illustrated in detail. Different results under electrical bias, optical bias or combined electrical and optical bias are analyzed for an excellent UTC-DHPT performance. The results show that when the UTC-DHPT operates at three-terminal (3T) working mode with combined electrical bias and optical bias in base, it keeps a high optical responsivity of 34.72 A/W and the highest optical transition frequency of 120 GHz. The current gain of the 3T UTC-DHPT under 1.55-μm light illuminations reaches 62 dB. This indicates that the combined base electrical bias and optical bias of 3T UTC-DHPT can make sure that the UTC-DHPT provides high optical current gain and high optical transition frequency simultaneously. [ABSTRACT FROM AUTHOR]

A new polarization rotator based on the silica photonic crystal fiber is proposed. The proposed polarization rotator photonic crystal fiber (PR-PCF) possesses a triangle jigsaw-shape core region. The full-vector finite-element method is used to analyze the phenomenon of polarization conversion between the quasi-TE and quasi-TM modes. Numerical simulations show that the wavelengths of 1.31 μm and 1.55 μm are converted with a nearly 100% polarization conversion ratio with their matched coupling length and has a relatively strong realistic fabrication tolerance −100 nm on the y axis and 50 nm on the x axis. The full vectorial finite difference beam propagation method is used to confirm the performance of the proposed PR-PCF. [ABSTRACT FROM AUTHOR]

In this paper, we theoretically study the effects of doping concentration ND and an external electric field on the intersubband transitions in InxAl(1 − x)N/InyGa(1 − y)N single quantum well by solving the Schrödinger and Poisson equations self-consistently. Obtained results including transition energies, the band structure, and the optical absorption have been discussed. The lowest three intersubband transitions (E2 − E1), (E3 − E1), and (E3 − E2) are calculated as functions of doping concentration ND. By increasing the doping concentration ND, the depletion effect can be reduced, and the ionized electrons will compensate the internal electric field which results from the spontaneous polarization. Our results show that an optimum concentration ND exists for which the transition 0.8 eV (1.55 μm) is carried out. Finally, the dependence of the optical absorption α13(ω) on the external electric field and doping concentration is studied. The maximum of the optical absorption can be red-shifted or blue-shifted through varying the doping concentration and the external electric field. The obtained results can be used for designing optical fiber telecommunications operating at 1.55 μm. [ABSTRACT FROM AUTHOR]

A micro-opto-mechanical system (MOMS) technology for the fabrication of fiber-optic optoacoustic emitters is presented. The described devices are based on the thermoelastic generation of ultrasonic waves from patterned carbon films obtained by the controlled pyrolysis of photoresist layers and fabricated on miniaturized single-crystal silicon frames used to mount the emitters on the tip of an optical fiber. Thanks to the micromachining process adopted, high miniaturization levels are reached in the fabrication of the emitters, and self-standing devices on optical fiber with diameter around 350 µm are demonstrated, potentially suited to minimally invasive medical applications. The functional testing of fiber-optic emitter prototypes in water performed by using a 1064 nm Q-switched Nd-YAG excitation laser source is also presented, yielding broadband emission spectra extended from low frequencies up to more than 40 MHz, and focused emission fields with a maximum peak-to-peak pressure level of about 1.2 MPa at a distance of 1 mm from the devices. [ABSTRACT FROM AUTHOR]

An optical fiber doped with bismuth was fabricated using the modified chemical vapor deposition method. Both the linear and nonlinear optical properties of the glass fiber were investigated. The resonant nonlinear refractive index was found to be around 3 × 10−15 m2 W−1, which is five orders of magnitude larger than the non-resonant nonlinear refractive index of commercially available single-mode silica fiber. The high resonant nonlinearity of the Bi-doped fiber was attributed as arising from Si-related bismuth active centers. [ABSTRACT FROM AUTHOR]

Radio on free space optics—RoFSO—has gained momentum in research because of its cost effectiveness and efficiency in transferring data at a high rate that is comparable to that for optical fiber media. While the transmission data rate is limited in fiber due to dispersion and nonlinearity, such effects do not prevail in FSO communication links. The data rate depends mainly on the switching speed of the optoelectronic devices. With the characteristics of free space being random in nature, the performance of RoFSO is primarily governed by atmospheric conditions. In this paper, we evaluate the performance of the orthogonal frequency division multiplexing (OFDM) signal in free space and compare against its counterpart radio frequency (RF) wireless communication systems. Simulations have been done on the atmospheric conditions by means of modeling the scintillation effect using log-normal distribution. The performance of the proposed system under two different base-band modulations, namely OFDM–PSK (phase shift keying) and QAM (quadrature amplitude modulation) in weak turbulence conditions is studied. It is found that PSK performs better than QAM. Also the M-ary performance analysis shows that 3–5 dB improvement in the signal to noise ratio is obtained for OFDM based FSO transmission compared to RF based wireless transmission. [ABSTRACT FROM AUTHOR]

Anniversaries call for celebrations. Since it is now fifty years since the first semiconductor lasers were reported, it is highly appropriate to celebrate this anniversary with a Special Issue dedicated to the topic. The semiconductor laser now has a major effect on our daily lives since it has been a key enabler in the development of optical fibre communications (and hence the internet and e-mail), optical storage (CDs, DVDs, etc) and barcode scanners. In the early 1960s it was impossible for most people (with the exception of very few visionaries) to foresee any of these future developments, and the first applications identified were for military purposes (range-finders, target markers, etc). Of course, many of the subsequent laser applications were made possible by developments in semiconductor materials, in the associated growth and fabrication technology, and in the increased understanding of the underlying fundamental physics. These developments continue today, so that the subject of semiconductor lasers, although mature, is in good health and continues to grow. Hence, we can be confident that the pervasive influence of semiconductor lasers will continue to develop as optoelectronics technology makes further advances into other sectors such as healthcare, security and a whole host of applications based on the global imperatives to reduce energy consumption, minimise environmental impact and conserve resources. The papers in this Special Issue are intended to tell some of the story of the last fifty years of laser development as well as to provide evidence of the current state of semiconductor laser research. Hence, there are a number of papers where the early developments are recalled by authors who played prominent parts in the story, followed by a selection of papers from authors who are active in today's exciting research. The twenty-fifth anniversary of the semiconductor laser was celebrated by the publication of a number of papers dealing with the early achievements in the June 1987 Special Issue of IEEE Journal of Quantum Electronics. The Millennium Issue of IEEE Journal of Selected Topics in Quantum Electronics presented a further set of articles on historical aspects of the subject as well as a 'snapshot' of current research in June 2000. It is not the intention here to duplicate any of this historical material that is already available, but rather to complement it with personal recollections from researchers who were involved in laser development in the USA, France, Russia and the UK. Hence, in addition to fascinating accounts of the discovery of the theoretical condition for stimulated emission from semiconductors and of the pioneering work at IBM, there are two complementary views of the laser research at the Lebedev Institute, and personal insights into the developments at STL and at Bell Laboratories. These are followed by an account of the scientific and technological connections between the early pioneering breakthroughs and the commercialisation of semiconductor laser products. Turning to the papers from today's researchers, there is coverage of many of the current 'hot' topics including quantum cascade lasers, mid-infrared lasers, high-power lasers, the exciting developments in understanding and exploiting the nonlinear dynamics of lasers, and photonic integrated circuits with extremely high communication data capacity, as well as reports of recent progress on laser materials such as dilute nitrides and bismides, photonic crystals, quantum dots and organic semiconductors. Thanks are due to Jarlath McKenna for sterling support from IOP Publishing and to Peter Blood for instigating this Special Issue and inviting us to serve as Guest Editors. [ABSTRACT FROM AUTHOR]

This paper reports that the designed optical polarization mode dispersion compensator shows a good performance under the real-time variation of differential group delay, state of polarization and principal state of polarization in a (40x43)-Gb/s dense-wavelength-multiplexing, 1200-km enhanced return-to-zero differential-quadrature-phase-shift-keying (RZ-DQPSK) system. The polarization mode dispersion tolerance of the system is improved by 26 ps using the optical polarization mode dispersion compensator. The short and long time stabilities are tested with the bit error ratio recorded. [ABSTRACT FROM AUTHOR]

We report on the acousto-optic Q-switched operation of a Ho:YAP laser double-passed pumped by an all-fiber Tm-doped fiber laser. The output characteristics of the Ho:YAP laser are studied at pulse repetition frequencies of 1, 2, 5 and 10 kHz. The shortest pulse width, the maximum pulse energy and the highest peak power are measured to be 17 ns, 1.71 mJ and 71.25 kW, respectively. The output wavelength is centered at 2115.96 nm with a bandwidth of about 1.5 nm. The beam quality factor M2 = 2.41 +- 0.03 at the output power of 3.54 W was measured by using the traveling knife-edge method. [ABSTRACT FROM AUTHOR]

We propose an efficient phase-encoding quantum secret key generation scheme with heralded narrow-band single photons. The key information is carried by the phase modulation directly on the single-photon temporal waveform. We show that when the technique is applied to the conventional single photon phase-encoding BB84 and differential phase shift (DPS) quantum key distribution schemes, the key generation efficiencies can be improved by factors of 2 and 3, respectively. For N([?]3)-period DPS systems, the key generation efficiency can be improved by a factor of N. The technique is suitable for quantum-memory-based long-distance fiber communication systems. [ABSTRACT FROM AUTHOR]