The RMS surface roughness of an optical polymer waveguide end facet cut by a milling router and measured by AFM is
investigated for a range of rotation speeds and translation speeds of the router. It was found that 1 flute (cutting edge)
routers gave significantly less rough surfaces than 2 or 3 flute routers. The best results were achieved for a 1 flute router
when the milling bit was inserted from the copper layer side of the board with a rotation speed of 15,000 rpm and a
translation speed of 0.25 m/min which minimized the waveguide core end facet RMS roughness to 183 ± 8 nm and gave
input optical coupling loss of 1.7 dB ± 0.5 dB and output optical coupling loss of 2.0 dB ± 0.7 dB. The relationship
between optical coupling loss at the input and output of the waveguides and waveguide end facet roughness is also
investigated in this paper. The ratio of RMS roughness to autocorrelation length of the roughness is shown to have a
quantified linear relationship with experimental measurements of optical insertion loss, input optical coupling loss and
output optical coupling loss. A new fabrication technique for cut waveguide end facet treatment has been proposed and
demonstrated which reduces the insertion loss by 2.60 dB ± 1.3 dB which is more than that achieved by the closest
available index matching fluid which gave 2.23 dB ± 1.2 dB and which is far more robust for use in commercial products.
A stable reproducible optical standard source for measuring multimode optical fiber attenuation is required as recent
round robin measurements of such fibers at several international companies and national standards organizations showed
significant variation when using a source having only the encircled flux in the near field emerging from it defined. The
paper presents and compares the far field modal power distributions for (i) 2 km and 3 km step-index multimode Hard
Plastic Cladding Fibers, HPCF, (SI-MMF) with 200 μm silica core diameter, 0.37 numerical aperture (NA) and polymer
cladding, (ii) a 10 m silica graded-index multimode fiber (GI-MMF) with 50 μm core diameter and 0.2 NA, and (ii) a
near field Encircled Flux Mode Convertor or “modcon”. A free space method for measuring the far field using a Lightemitting
diode (LED) centered at 850 nm wavelength with 40 nm 10 dB-bandwidth and a charge-coupled device (CCD)
camera is compared with a f-theta multi-element lens based far field pattern (FFP) system. Mandrels of different
diameter and different numbers of turns of the fiber around them were used to achieve an equilibrium mode distribution
(EMD) for the GI-MMF. The paper defines encircled angular flux (EAF) as the fraction of the total optical power
radiating from a multimode optical fiber core within a certain solid angle in the far field. The paper calculates the EAF
when the solid angle increases from the far field centroid.
Optical waveguides are considered and investigated for laser beam combining and colour homogenization for use in a three colour laser based display system. A recently developed laser is calibrated and the best method for designing an all-waveguide combining and homogenising system is investigated. Ray tracing simulations are performed and the results are presented. An optical combination of lenses is designed and used for laser beam focusing into a 200 μm fibre core diameter step index silica core multimode fibre. We also designed and simulated a slab waveguide for beam shaping and beam homogenizing.
Complex interconnection patterns in electrical PCBs have to use multiple layers of copper tracks. However, the same interconnections can be made in a single layer using optical waveguides as they cross on the same layer. Waveguide crossings where two waveguides intersect in the same optical layer are particularly important components as they offer OPCB layout designers additional flexibility to solve layout problems such as routing around cutout areas, electrical components and other obstacles on an OPCB. Use of waveguide crossings can also help to avoid sharp bends in the design as these bends are an important cause of optical loss. Despite all of the advantages of waveguide crossings, and although most of the light travels along the intended waveguide, a proportion of the optical power in one waveguide will couple into the crossing waveguide at each intersection point or couple out of the original waveguide and into the cladding. This coupling phenomenon causes optical loss and crosstalk in the system. In this paper, the results of an investigation of the optical loss due to the crossing of multimode polymer waveguide, fabricated on FR4 printed circuit boards, PCBs, as a function of crossing angles are presented theoretically and experimentally. The results from ray tracing simulation is compared with the experiment results and the contrast is discussed.
Electro-optical printed circuit board technology (EOCB) based on integrated planar polymer optical waveguides has been the subject of research and development for many years to provide a cost viable, fully integrated system embedded optical interconnect solution, however a number of constraints of this technology have yet to be overcome. Optical coupling loss at the input and output of the waveguides is one of the major issues and waveguide end facet roughness is one of the main sources of the coupling loss which is investigated in this paper. The results of a comprehensive investigation of the end facet roughness of multimode polymer waveguides, fabricated on FR4 printed circuit boards, PCBs, and its effect of optical loss are presented theoretically and experimentally. The waveguide end facet roughness was measured using an atomic force microscope, AFM, when the waveguides were cut using a milling router with various numbers of cutting edges called flutes. The optimized cutting parameters are derived and the optical coupling loss, between the laser source and the waveguide, due to the different roughness magnitudes is measured by experiment for the first time. To improve the surface quality and decrease the waveguide optical loss, a new fabrication technique for reducing the end facet roughness after cutting is proposed and demonstrated. The insertion loss was reduced by 2.60 dB ± 1.3 dB which is more than that achieved by other conventional methods such as index matching fluid.
Accurate, reliable and fast numerical modeling methods are required to design the optimum radial refractive index profile for single and multimode fibers to give specific dispersion characteristics prior to or even obviating costly experimental work. Such profiles include graded index and multiple concentric cladding layers. In this paper, a new numerical method is introduced which enables the derivatives of the propagation coefficient to be calculated analytically up to the third order of a single mode or multimode weakly guiding optical fiber with an arbitrary radial refractive index profile. These quantities are required to determine the group delay, τg, chromatic dispersion, D, and dispersion slope of the fiber. The expansion of the modal fields in terms of Laguerre-Gauss polynomials in the Galerkin method offers certain benefits. In particular, due to simplicity of the basis functions it is possible to carry out further analytical work on the results such as repeated differentiation of the matrix equation resulting from the Galerkin method to define up to the third-order derivatives of the propagation coefficients with respect to wavelength. This avoids approximation errors inherent in numerical differentiation, giving better accuracy and, at the same time, significantly reduces the computation time. A computer program was developed to demonstrate the proposed method for single and multimode fibers with radially arbitrary refractive index profiles. The paper provides simulation results to validate the approach.
In-plane bend loss represents the greatest commercial inhibitor to deploying multimode optical waveguides on densely
populated electro-optical printed circuit boards (OPCB) as the minimum bend radii currently possible are too large to
be practical in common designs. We present a concept and fabrication method for creating novel polymer optical
waveguide structures with reduced bend losses to enable higher density routing on an OPCB. These nested core
waveguide structures comprise a core surrounded by a thin shell of cladding, which allows for two-fold modal
containment by first a conventional low refractive index contrast (LIC) boundary followed by a secondary high
refractive index contrast (HIC) boundary. The purpose of this is to reduce the in-plane bend losses incurred on tightly
routed optical channels, while not incurring prohibitive dispersion, sidewall scattering and optical crosstalk penalties.
We have designed and fabricated nested core multimode polymer waveguides, evaluated their performance in
comparison to conventional step-index waveguides and simulated these structures using the beam propagation method.
Preliminary results are presented of our measurements and simulations.
The design, implementation and characterisation of an electro-optical backplane and an active pluggable optical
connector technology are presented. The connection architecture adopted allows line cards to mate and unmate from a
passive electro-optical backplane with embedded polymeric waveguides. The active connectors incorporate photonics
interfaces operating at 850 nm and a mechanism to passively align the interface to the embedded optical waveguides. A
demonstration platform has been constructed to assess the viability of embedded electro-optical backplane technology in
dense data storage systems. The electro-optical backplane is comprised of both copper layers and one polymeric optical
layer, whereon waveguides have been patterned by a direct laser writing scheme. The optical waveguide design includes
arrayed multimode waveguides with a pitch of 250 μm, multiple cascaded waveguide bends, non-orthogonal crossovers
and in-plane connector interfaces. In addition, a novel passive alignment method has been employed to simplify high
precision assembly of the optical receptacles on the backplane. The in-plane connector interface is based on a two lens
free space coupling solution, which reduces susceptibility to contamination. The loss profiles of the complex optical
waveguide layout has been characterised and successful transfer of 10.3 Gb/s data along multiple waveguides in the
electro-optical backplane demonstrated.
The paper describes the research in the £1.3 million IeMRC Integrated Optical and Electronic Interconnect PCB
Manufacturing (OPCB) Flagship Project in which 8 companies and 3 universities carry out collaborative research and
which was formed and is technically led by the author. The consortium's research is aimed at investigating a range of
fabrication techniques, some established and some novel, for fabricating polymer multimode waveguides from several
polymers, some formulations of which are being developed within the project. The challenge is to develop low cost
waveguide manufacturing techniques compatible with commercial PCB manufacturing and to reduce their alignment
cost. The project aims to take the first steps in making this hybrid optical waveguide and electrical copper track printed
circuit board disruptive technology widely available by establishing and incorporating waveguide design rules into
commercial PCB layout software and transferring the technology for fabricating such boards to a commercial PCB
manufacturer. To focus the research the project is designing an optical waveguide backplane to tight realistic constraints,
using commercial layout software with the new optical design rules, for a demonstrator into which 4 daughter cards are
plugged, each carrying an aggregate of 80 Gb/s data so that each waveguide carries 10 Gb/s.
Polymer tapered multimode waveguides were modeled using the finite difference wide-angle beam propagation method to investigate whether tapered input waveguide couplers decreasing in width away from the waveguide entrance give improved tolerance to lateral misalignments of an optical source compared to straight waveguides and whether there is any effect on angular misalignment tolerance for use in optical backplane interconnections. Input tapered couplers having a larger entrance and tapering down in width do indeed improve the lateral misalignment tolerance compared to straight waveguides but do so at the expense of an increased loss. Tapers have no effect on angular tolerance for strongly driven vertical cavity surface-emitting laser (VCSEL) sources although they cause a loss of angular tolerance for single-mode fiber sources and VCSELs at low drive currents.
Finite difference beam propagation modelling (FD-BPM) calculates the effect of thermal gradients in an optical backplane on the optical field propagation in a polymer multimode linearly tapered waveguide. Compared to straight waveguides, tapered entrances offer improved power coupling for a wide range of optical source lateral offset misalignments. However, surface temperature gradients of 0.5°C/μm across the taper were found to degrade this benefit due to the thermo-optic effect of the polymer. Higher surface temperature gradients improve power coupling in two discrete ranges of lateral source offset but the original tolerance to a wide range of source offsets is not recovered.
Optical detection systems usually rely on the intensity contrast (visible) or temperature difference (infrared) between target and background. Adding new dimensionality to the detection process is essential to enhance the sensitivity. This paper presents a novel theory for modeling the performance of an optical detection technique called Interferogram Phase Step Shift (IPSS), which relies on the coherence contrast between target and background to perform discrimination. The technique uses an interferometer to measure the self-coherence function of the input radiation, forming an interferogram, and an interference filter to produce an event marker (phase step) in it. The model predicts the displacement of the phase step in the interferogram, when a coherent target enters the system field of view, which is the kernel of the IPSS technique. The paper assesses the effects of the target to optical filter bandwidth ratio in the system responsivity, for optimization purposes, and models the experiments presented in a previous publication, predicting the experimental results theoretically to perform a comparison. It also includes the analytical derivation of the self-coherence functions of target and background as measured by the system's interferometer, and the computer modeling of the same self-coherence functions for an interference filter, with any arbitrary spectral response, considering the effects of the polarization of the light sources and optical components in the experiments. Finally, the theoretical curves for displacement vs. target-to-background power ratio, among others, are compared with the experimental results. Good agreement is demonstrated, and the causes of differences are discussed.
KEYWORDS: Signal detection, Target detection, Receivers, Data acquisition, Interferometers, Signal to noise ratio, Tungsten, Halogens, Photodetectors, Helium neon lasers
High detection probabilities with low false alarm rates, even in the presence of as much as - 28 dB signal to clutter can be obtained, if use is made of the coherence information contained in the input signal. Such a detection system is described and its Receiver Operating Curves (ROC) characterized from a reduced experimental set of data. The system has an input filter with a rectangular passband, an interferometer and a photodetector. The filter gives rise to a sinc function shaped interferogram envelope in optical path difference. The position of the interferogram's first null depends on the effective bandwidth of the scene in the instrument's field of view. The detection system is sensitive to the degree of coherence of the scene. The amount of the shift of the position of the first null is found to be a highly sensitive measure of the presence of a very narrow-band target within the instrument's input bandpass filter. An important characteristic of this system is that the shift is still detected even when an intense wideband incoherent source, which may be considered to be clutter, is introduced into the input. We measured the ROC curves for the optical detection system using a He-Ne laser narrowband coherent source target with a tungsten halogen bulb wideband incoherent background clutter. We show that the target source can be detected even when it is substantially weaker than the clutter at - 28 dB signal to clutter. We compare the predictions of our theoretical model with the experimental results and show good agreement.
Imaging spectrometers allowing spatially resolved targets to be spectrally discriminated are valuable for remote sensing and defense applications. The drawback of such instruments is the need to quickly process very large amounts of data. In this paper we demonstrate two imaging systems which detect a dim target in a bright background, using the coherence contrast between them, generating much less data but only operating over a limited optical bandwidth. Both systems use a passband filter, a Michelson interferometer, coupling optics and a CCD camera. The first uses the interferometer in a spatial mode, by tilting one of the mirrors to create a set of line fringes on the CCD array. The visibility of these fringes is proportional to the degree of coherence. The interferogram is displayed spatially on the CCD array, as a function of the path differences. The second system uses the interferometer in a temporal mode. A coherent point target and an extended background are imaged through the interferometer onto the CCD array, and one of the interferometer's mirrors is scanned longitudinally to vary the path difference in time. In both cases the coherent target is detected over a large dynamic range down to negative signal-to-background power ratios (in dB). The paper describes an averaging technique to improve the signal-to-noise ratio and correction techniques required to extract interferograms from the images. The spatial technique developed has the advantage of using no moving parts.
We present results on Resonant Cavity Light Emitting Diodes (RCLEDs) emitting at 650 nm, which have high efficiencies and low voltages. In particular, we report on the angular properties of these devices, and highlight the observation that overall spectral linewidth increases with collection angle. This unusual property of RCLEDs is largely a consequence of employing a microcavity in the design. An additional contributing factor is the relative distribution of gain amongst the cavity modes (i.e. the level of tuning or detuning of the underlying emission, defined with respect to the longitudinal cavity mode). We have used measurement techniques which spectrally resolve angular radiation profiles to determine the (de)tuning directly. Moreover, these profiles demonstrate how the overall spectral linewidth increases with collection angle. To this end, we have developed a semi- empirical method for determining the overall linewidth as a function of emission numerical aperture (NA). A 4 nm detuned device has been investigated and linewidths have been found to increase from 3.1 nm to 13.6 nm over a range of NA approximately equals 0 to NA equals 1, an increase by a factor of around 4. Obviously, a variable linewidth also implies a variable coherence length with NA. Consequently, the coherence length was found to decrease from 30 micrometer to 9 micrometer over the same range. Independent coherence length measurements were carried out by direct interferometric measurements, and confirmed the expected trends.
Using a new device which contains an array of microlenses whose focal lengths can be electrically varied, we have been able to control the input from one microlens to a single mode fiber using an applied voltage. For such a microlens array many closely-spaced focal spots can be generated in parallel, and electrically switched to address, potentially, an array of receiver fibers. We show how the particular switching characteristics of the device, whereby the lenses switch from diverging to converging, serves in turn to disperse light and to focus it into the fiber.
We present a simple theoretical model and compare it with experimental results for highly emissive sources having various coherence lengths in the presence of incoherent background illumination. Agreement between the experimental results and the theory is discussed.
We experimentally verified a modeled prediction of optimum polarizer and analyzer configuration for LCTV phase modulation while maintaining high transmission and restricted intensity modulation depth.
A method is presented for selecting the optimum polarizer configurations for a complex optical correlator. Two twisted nematic (TN) liquid crystal televisions (LCTVs) are used to modulate intensity and phase. The profiles of intensity and phase modulation vs. applied pixel voltage are determined using a Jones calculus model of the Philips P3.4 TN LCTV between polarizers. This allows selection of separate polarizer orientations which yield the highest ranges of intensity and phase modulation. The optimum configurations provide continuous intensity modulation with approximately 261 degrees of phase modulation.
A novel feature enhancement and similarity suppression algorithm is presented. This algorithm can be used for training matched filter based optical pattern recognition systems or for the correct recovery of noisy images from optical memories such as optical disks. The underlying theory and the results from computer simulations using human faces are presented. Some preliminary results have shown high probability of recognition in pattern recognition tests using the training data.
We have discovered for mutually pumped phase conjugation (MPPC) of two incoherent beams which are all remarkably insensitive to angular and lateral positional changes in either of the two input beams or in the photorefractive BaTiO3 crystal itself. By designed choosing the geometry such as the orientation between the pump beam and the crystal's c-axis the grating can be dominated and two lasers can be coupled efficiently. Three novel configurations can be formed in photorefractive crystals when the MPPC occurred. These configurations are part of a family of mutually pumped phase-conjugate mirrors (MPPCMs) all of which have similar internal beam paths within the crystal having three internal reflections. These configurations provide broad interaction, which offer with more efficient, and large angular and positional acceptance, and even high resolution and stability. We also experimentally demonstrate our discovery of a new double mutually pumped phase- conjugate mirror (DMPPCM) which phase conjugate three incident beams in pairs operating as two MPPCMs in the same crystals at the same time but in different orientations. These characteristics make these configurations particularly interesting for practical applications such as for phase locking lasers and for optical free space communications.
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