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Optoelectronics, IEE Proceedings -

Issue 6 • Date Dec 1998

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Displaying Results 1 - 20 of 20
  • Characterisation of waveguide microcavities using high-resolution transmission spectroscopy and near-field scanning optical microscopy

    Publication Year: 1998 , Page(s): 379 - 383
    Cited by:  Papers (4)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (600 KB)  

    A wide variety of optoelectronic devices with novel gratings are being created for which knowledge of the internal optical fields and details of loss mechanisms are highly desirable. To this end, the authors have built apparatus and developed techniques for simultaneous measurement of the scattered and evanescent fields in the near-field of the active region of grating-type waveguide devices combined with high-resolution transmission measurements. Here they report characterisation and analysis of a waveguide microcavity. The transmission scans improve greatly on previous measurements, showing detailed the authors which the authors explain using transfer matrix simulations including the structure of the input and output waveguides of the device. They also present near-field scanning optical microscopy data of the active region of the waveguide. Internal mode patterns and height dependent data, when compared with simulations of the electric field magnitude, show that the device may suffer from significant scattering at the grating interfaces View full abstract»

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  • Light power behaviour when bending plastic optical fibres

    Publication Year: 1998 , Page(s): 313 - 318
    Cited by:  Papers (8)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (548 KB)  

    The redistribution of light power and the radiated power along a bent section of a plastic optical fibre (POF) is analysed. In this context, a theoretical approach for understanding, and hence optimising. The major factors that affect the behaviour of sensors utilising losses in fibre bends is reported. With a brief comment on the bend loss model used, an implementation of a three-dimensional analysis for bend losses is presented, and the theoretical analysis is compared with experimental results. The spatial redistribution of the optical power in the bend is also calculated View full abstract»

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  • Analysis of multiple-rib waveguide structures by the discrete-spectral-index method

    Publication Year: 1998 , Page(s): 365 - 371
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (584 KB)  

    The discrete-spectral-index (DSI) method is extended to analyse multiple-rib waveguide structures which contain 𝒬 (⩾2) ribs. By constructing general Fourier series for the E-field variation parallel to the semiconductor layers within the waveguide, a simple transcendental equation is derived for the propagation constants of the structure. The modal indices calculated via DSI for symmetric 4-rib and 5-rib waveguide arrays are within 0.001% of those computed by the semivectorial finite difference method; thus the validity of the formulation is established. Together with the ability of the DSI method to calculate overlap integrals directly, the accurate analysis and design of multiple-rib waveguide couplers have thereby been simplified and made more efficient View full abstract»

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  • In-plane microcavity resonators with two-dimensional photonic bandgap mirrors

    Publication Year: 1998 , Page(s): 373 - 378
    Cited by:  Papers (11)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (580 KB)  

    Two-dimensional photonic bandgap structures are used to form one-dimensional microcavities in a GaAs/AlGaAs laser-like heterostructure. Photoluminescence from InAs quantum dots embedded in the optical waveguide is used to probe these cavities. At resonance, peak transmission values greater than 30%, are observed for modes of 8 nm half-width, associated with estimated reflectivities in excess of 90%,. The mode volume is limited by the penetration depth of the field into the PBG mirrors, which amounts to approximately one lattice spacing or 0.25 μm. This value of penetration depth approaches the ultimate limit on compactness that can be achieved with the particular photonic lattice parameters used in the experiments. The dependence of the resonance wavelength on cavity length compares satisfactorily with a theoretical supercell model View full abstract»

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  • Three-dimensional photon confinement in photonic crystals of low-dimensional periodicity

    Publication Year: 1998 , Page(s): 384 - 390
    Cited by:  Papers (21)  |  Patents (3)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1024 KB)  

    Photonic crystals of one- or two-dimensional periodicity can be used to achieve three-dimensional photon confinement in dielectric waveguides with modal volumes of the order of a cubic half-wavelength. Since photonic crystals of low-dimensional periodicity do not have full three-dimensional bandgaps. The microcavities undergo increasing radiation losses with decreasing modal volumes. High-𝒬 resonant modes can be generated by reducing the strength of the photon confinement. Increasingly, larger modal volumes lead to lower radiation losses and more efficient coupling to waveguide modes outside the cavity View full abstract»

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  • Photoluminescence characterisation of Er3+/Yb3+ co-implanted alumina (Al2O3) thin films and sapphire crystals

    Publication Year: 1998 , Page(s): 325 - 330
    Cited by:  Papers (3)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (532 KB)  

    Successful incorporation both erbium and ytterbium in alumina by ion implantation is reported. Some evidence for indirect pumping of erbium through the transfer of energy from ytterbium has been observed. Both plasma-enhanced CVD deposited alumina thin films and sapphire crystals were employed as substrates, Yb3+ and Er3+ concentrations ranged from 2.4 at.%, to 8 at.% and from 0.4 at.% to 0.8 at.%, respectively. The samples show strong, broad, room-temperature photoluminescence at λ=1.53 μm corresponding to the intra-4f transitions between the 4I13/2 (first excited) and the 4I15/2 (ground) state of Er3+. The full width at half maximum of the emission spectrum is as high as 67 nm for the Al2O3 thin films, for the sapphire crystals it is 45 nm. The fluorescence lifetime of the samples has been measured to be as high as 4.2 ms at 50 mW pump power View full abstract»

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  • Design of externally tuned asymmetric fibre Fabry-Perot electroabsorption optical modulators

    Publication Year: 1998 , Page(s): 344 - 352
    Cited by:  Papers (2)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (692 KB)  

    The authors present design criteria for a fibre-matched, externally tunable, asymmetric Fabry-Perot, electroabsorption optical modulator. The theoretical analysis includes the effects of angular distribution of the incident power, optical bandwidth of the source, cavity finesse, and the power-coupling loss for both singlemode and multimode step-index fibre devices. The results indicate that, for a laser source, fibre with standard numerical aperture of 0.2 and a fibre front-end reflectivity of 0.6, a cavity length less than four times of the working wavelength is required to obtain 10 dB contrast for both singlemode- and multimode-fibre devices, while an optimum cavity length exists for minimum insertion loss. A theoretical analysis of degradation of device performance with mirror-surface roughness, mirror mismatch and mirror misalignment is also presented, showing that such devices have high tolerance to these deviations View full abstract»

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  • Quasi-three-dimensional simulation of carrier dynamics in quantum well DFB lasers

    Publication Year: 1998 , Page(s): 339 - 343
    Cited by:  Papers (2)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (404 KB)  

    A quasi-3D simulation for quantum well (QW) distributed feedback (DFB) laser diodes is presented. It takes into account both longitudinal effects such as the nonuniformity of the optical intensity, local gain, and spontaneous emission, and lateral effects such as current spreading, quantum carrier capture, and quantum carrier heating. The simulation is performed by solving several 2D cross-sections using Minilase II and a 1D longitudinal solver in a self-consistent fashion. As an example, an analysis of spatial hole burning in a λ/4 phase shifted DFB laser is given. It is shown that spatial hole burning (SHE) results in a local gain saturation, and is mainly restricted to electrons and holes within the QW. Investigations of electron heating at high power show that the temperature distribution of the bound electrons and the LO phonons in the QW is insignificant, and only slightly influenced by SHE View full abstract»

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  • Experimental investigation of layer-by-layer metallic photonic crystals

    Publication Year: 1998 , Page(s): 409 - 414
    Cited by:  Papers (2)  |  Patents (1)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (528 KB)  

    The authors have investigated the transmission properties and defect characteristics of layer-by-layer metallic photonic crystals. They have demonstrated experimentally that the metallicity gap of these crystals extends to an upper band-edge frequency, and no lower edge was detected down to 2 GHz. The defect structures built around these crystals exhibited high transmission peak amplitudes (100%) and high 𝒬 factors (2250). The crystals with low filling ratios (around 1-2%) were tested and were still found to possess metallic photonic crystal properties. These crystals exhibited high reflection rates within the metallicity gap and reasonable defect mode characteristics. A power enhancement factor of 190 was measured for the electromagnetic (EM) wave within planar cavity structures, by placing a monopole antenna inside the defect volume. These measurements show that detectors embedded inside a metallic photonic crystal can be used as frequently selective resonant cavity enhanced (RCE) detectors with increased sensitivity and efficiency when compared to conventional detectors View full abstract»

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  • Three-dimensional characterisation of a two-dimensional photonic bandgap reflector at midinfrared wavelengths

    Publication Year: 1998 , Page(s): 403 - 408
    Cited by:  Papers (1)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (820 KB)  

    A combined experimental and theoretical study is presented of the reflection properties of a two-dimensional photonic crystal in a three-dimensional optical environment. The crystal is a triangular lattice of cylindrical holes in bulk silicon. The reflection spectra are measured over a wide range of midinfrared wavelengths by using a Fourier-transform spectrometer with a convergent incident beam. Very high reflection coefficients are demonstrated for the first-order forbidden bands, reaching 98%, for the first band (λ≈6-8 μm) in TE polarisation following Γ-M direction. This result is of great promise for future applications of photonic bandgap reflectors in the midinfrared. From the comparison between the results of characterisation and those of numerical simulations, the contributions of the different effects that degrade the reflector performances are separated. The incident-beam divergence is shown to modify the shapes, widths and positions of the higher-order forbidden bands. Diffraction losses at the interface are found to be strongly dependent on the crystal termination and orientation, and can reach 60%, but only for the smallest wavelengths investigated. In turn, the fabrication inhomogeneities such as the small roughness of the interface or the hole-radius dispersion are shown to be the prime cause of degradation as long as diffraction effects remain weak View full abstract»

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  • Smallest possible electromagnetic mode volume in a dielectric cavity

    Publication Year: 1998 , Page(s): 391 - 397
    Cited by:  Papers (16)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (644 KB)  

    In photonic crystals, electromagnetic waves can be confined in all three dimensions leading to very small mode volumes. Computational search has been for smallest electromagnetic mode which occurs in two-dimensional dielectric photonic crystals slabs. The smallest mode volume found was V≈2(λ/2n)3, where n is the refractive index. This small mode volume can lead to significant enhancement of spontaneous emission rates in semiconductor nano-cavities due to the Purcell effect View full abstract»

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  • Broadband and low driving-voltage LiNbO3 optical modulators

    Publication Year: 1998 , Page(s): 360 - 364
    Cited by:  Papers (6)  |  Patents (4)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (500 KB)  

    Travelling-wave type LiNbO3 (LN) optical modulator structures employing thicker coplanar waveguide (CPW) electrodes are presented, for achieving a lower driving-voltage and broader bandwidth characteristic with an impedance and velocity match condition. To design a high-performance device-structure, a figure of merit p is introduced for travelling-wave type optical modulator, which can make an excellent modulator with a smaller value of p. Microwave characteristics of the modulators with a ridged type LN-substrate are theoretically compared to those of planar-type modulators. As a result, the characteristic-impedance and the product of driving-voltage and interaction-length of the electrodes in modulators are shown to be almost determined only by the buffer-layer thickness for any modulator-structure under a velocity-matching condition. By utilising a wider gap and accordingly thicker electrodes with a ridged LN-substrate, LN modulator structures having a 40 GHz-band and far less than a 3 V driving-voltage have been clarified under a 50 Ω characteristic-impedance system View full abstract»

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  • Optimal performance of coherent optical pulse CDMA systems based on code and phase synchronisation and interference cancellation

    Publication Year: 1998 , Page(s): 353 - 359
    Cited by:  Papers (4)  |  Patents (2)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (600 KB)  

    An optimal multi-user receiver is proposed for asynchronous coherent optical pulse code division multiple access (CDMA) systems. The optical code sequences of all transmitted users are locally generated and correlated to the received signal via a proper code and phase synchronisation process. The resultant correlation signals are integrated over the bit duration and then discriminated to recover the information data in the electrical domain. Since each data bit of any intended user is interfered with by two bits from all other users, multi-stage interference cancellation is employed. By estimation of the random ingredients of all user signals, the multiuser interference is rebuilt and removed from the desired bit. The key advantage within the context of a narrow pulse optical implementation is that the proposed interference cancellation can be performed in the electrical domain. The system implementation is described and the validity of the cancellation technique is theoretically proved. A numerical evaluation for the system performance is provided, and issues with respect to the system application are discussed. The predicted performance of the system offers a theoretical basis for the consideration of optical pulse CDMA systems for use in high-capacity optical fibre networks View full abstract»

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  • Interleaving two-dimensional lattices to create three-dimensional photonic bandgap structures

    Publication Year: 1998 , Page(s): 436 - 440
    Cited by:  Papers (2)  |  Patents (3)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (580 KB)  

    A two-dimensional lattice consists of an inhomogeneous dielectric medium which has periodic variations of permittivity along two linear directions, and is uniform in the third dimension. The paper considers a class of three-dimensional photonic band gap materials formed by the interleaving of a pair of two-dimensional lattices whose uniform directions are mutually orthogonal, the so-called `woodpile' geometry. It is shown by numerical calculations that the strong polarisation discrimination exhibited by the constituent two-dimensional sublattices in their respective stopbands leads to a completely three-dimensional photonic bandgap crystal in all polarisations when the two sublattices are interleaved. For a general class of such synthetic crystals having arbitrary two-dimensional distributions in the sublattices, many properties of the three-dimensional interleaved crystal can be predicted directly from a knowledge of the properties of the sublattices, thereby radically reducing the computational cost of their determination View full abstract»

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  • Clipping noise effects on the performance of digital baseband signal in lightwave hybrid digital and analogue transmission systems

    Publication Year: 1998 , Page(s): 335 - 338
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (312 KB)  

    Accurate modelling of the clipping effects in lightwave hybrid SCM carrier and digital baseband transmission systems is presented using newly derived formulae. Unlike the cases of bandpass transmission, the PDF of clipping noise in the baseband receiver is found to be asymmetrical. Numerical simulation is described for a system with a 140 Mb/s baseband channel and 33 microwave FM video channels. Results show that clipping noise can have a serious impact on the baseband transmission of digital signals, although its influence on analogue signals is moderate. It is also shown that the decision level may be adjusted to give an optimum BER in the presence of clipping noise View full abstract»

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  • Feedforward LF compensation for laser fibre optic transmitter

    Publication Year: 1998 , Page(s): 331 - 334
    Cited by:  Patents (2)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (272 KB)  

    Direct intensity modulation of laser optical power is probably the one most often used in practice. Classical circuits for semiconductor laser driving are not suitable for operation with unbalanced codes or data transmitted in the form of bursts. The reason for this is a low-frequency (LF) cut-off caused by the laser stabilising circuit. In the paper a concept is presented, along with practical realisation of the laser driving and stabilising circuit for unbalanced codes or burstmode transmission. A developed circuit allows modulation of the laser diode from DC up to the maximum modulation frequency of the laser. Experimental data including pulse tests, eye pattern test and thermal properties of proposed solutions are presented View full abstract»

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  • Experimental technique to determine the band structure of two-dimensional photonic lattices

    Publication Year: 1998 , Page(s): 398 - 402
    Cited by:  Papers (10)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (568 KB)  

    An experimental technique to probe the energy against wavevector dispersion curves of two-dimensional photonic bandgap structures is demonstrated from the study of 360 nm period air hole honeycomb lattices etched in AlGaAs waveguides. The technique relies on the coupling of external radiation incident on the lattices to the zone-folded bands of the photonic structures, This coupling is detected owing to observation of resonance features in the reflectivity spectra. Good agreement between experiment and theory is found for the second, third and fourth photonic bands, The technique allows external photons to be coupled into regions of the photonic dispersion curves close to the Brillouin zone boundaries, where the photon group velocity tends to zero, and thus has the potential to allow novel effects related to `heavy photons' to be investigated View full abstract»

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  • Experimental study and modelling of high-transmission defect modes in photonic crystals with graphite structure

    Publication Year: 1998 , Page(s): 415 - 419
    Cited by:  Patents (3)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (540 KB)  

    The authors present an experimental demonstration and a modelling of high-transmission defect modes in graphite type photonic crystals. Such two-dimensional crystals are of interest for applications in integrated optics due to the existence of a complete forbidden band of relatively low order. Experiments are currently performed at microwave frequencies using hexagonal lattices of alumina rods to provide an easier and more accurate analysis of point defect modes. The complete photonic bandgap extends from -57 to 61 GHz. By introducing a row of vacancies in the crystal, we show that a sharp resonance can be created ill the gap with a transmission level comparable to that obtained in the neighbouring transmission bands. Besides, a sharp resonance with a transmission of -6 dB (~25%,) is obtained in the lowest forbidden band when six defects are arranged to form a small hexagonal cavity. The transmission spectra are well reproduced by numerical simulations with a finite difference time domain model which also gives the field pattern of the defect modes View full abstract»

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  • Calculation of optical power emitted from a fibre grating laser

    Publication Year: 1998 , Page(s): 319 - 324
    Cited by:  Papers (4)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (540 KB)  

    The optical power that can be obtained from the facets of a fibre grating laser is calculated in the paper. This calculation is based on the effective transmission coefficient of the compound cavity of the device. The dependence of slope efficiency (of power per facet) on the reflectivities of the diode facets and grating, length of the laser diode and grating, and emission wavelength are considered. A configuration is suggested which allows the largest amount of optical power to be obtained from the back facet of the device when fibre gratings with strong reflectivities are used. It is also shown that a reduction in facet reflectivity results in significant reduction of the variation of output power slope efficiency due to changes in emission wavelength or length of the external cavity View full abstract»

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  • Parity, time-reversal and group delay for inhomogeneous dielectric slabs: application to pulse propagation in finite, one-dimensional, photonic bandgap structures

    Publication Year: 1998 , Page(s): 420 - 435
    Cited by:  Papers (2)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1428 KB)  

    The author considers the symmetries associated with the group-velocity delay of pulse transmission and reflection in a one-dimensionally inhomogeneous planar dielectric structure. From the principles of energy and parity conservation-as well as time-reversal symmetry-he derives the generalised Stokes reciprocity relations for such a slab. From these relations, he obtains very general equations relating the group delays and phases of the transmitted and reflected pulses. These results have important applications to pulse propagation through one-dimensional photonic bandgap structures View full abstract»

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