We have fabricated Long Period Grating filters in two Dual Concentric Core Fibers by an electric arc discharge technique. The gratings have induced coupling between the fundamental modes of the two cores. We have obtained filter with rejected band around 1220 nm and 1559 nm respectively, characterized by insertion loss lower than 0.5 dB. We also have investigated the inteest of using this fiber to implement a highly selective Mach-Zehnder interferometer with a 2.4 nm inter-fringe. Despite the high Ge-doping of the used fiber, thermal characterizations show a temperature sensitivity of the transmitted spectrum similar to that of the same grating written in a standard Single Mode Fiber.
Design and fabrication of superimposed fibre Bragg gratings are considered. Firstly, an accurate numerical model for computing the reflection and transmission spectra of the superimposed gratings is presented. The model employs the transfer matrix method and takes into account the photosensitivity profile of the fibre, which is understood as the dependence of the refractive index change versus UV light exposure dose. It is shown that the model accurately reproduces the impact of saturation of the photosensitivity profile on the reflection and transmission spectra of the superimposed grating.
Secondly, by varying the initial phase of each elementary grating comprising the superimposed one, the latter is optimised such that the total maximum refractive index change required is minimised. The optimisation is carried out by means of the genetic algorithm, a global optimisation technique. A method for an almost optimal use of the fibre photosensitivity is proposed. The method overcomes the problems related to the non-infinitesimal length of the interferometric pattern in the grating writing setup. A successful fabrication of optimised 8- and 16-channel filters for a 2.5 Gb/s DWDM system with 50 GHz channel separation is reported. It is shown that the optimised superimposed gratings have a smaller insertion loss than similar non-optimised ones.
Characterization of the complex relection spectrum and the spatial profile of fiber Bragg gratings using optical frequency domain reflectometry and the layer peeling algorithm is presented. The importance of correct scaling and polarization effects are discussed. The method gives accurate measurement of the spatial profile for grating with reflectivity < 98-99 %. Immunity to spurious reflections and high dynamic range in spectral measurements are achieved.
We have used a CO2 laser as a heating source to fabricate a single-mode fiber couplers. The process consists of fusing and stretching two standard single model fibers initially maintained in lateral contact. We have fabricated 3 dB couplers at 1500 nm with insertion loss < 0.5dB. We also have fabricated multiplexers such as a 100% coupler at 1310 nm and at 1550 nm. The channel isolation of these components are > 30dB with a bandwidth of 30 nm at -30dB. Finally, a coupler with a long constant waist have been fabricated.
The paper presents a new line of photonic fibers with reduce dcladding radius that permits to reduce the space occupancy of gain coils, fused fiber devices and fiber pigtails without affecting either the performance or the reliability. The proposed reduced cladding photonic fibers seem an effective answer to the demand for compact optical modules and small form factor amplifiers. Considering the advantages of the reduced cladding approach and that no-drawbacks are foreseeable, 80 microns cladding diameter is likely to become a new standard for all the photonic fibers used within optical modules.
We performed some experimental studies on a new type cost-effective power coupling method for large-core POFs, and proposed a novel 1×4 plastic optical fiber (POF) power coupler which is different from fused-taper type and mixing rod POF couplers. Loss factors of this coupler are also analyzed theoretically. This coupler is based on thermal feature of POF, the two end of coupler is composed of fused-drawn tapered fiber and thermal-contracted tapered fiber, respectively, which is characterized as simple configuration and easily fabrication. The performance parameters of this coupler including excess loss, insertion loss, and channel cross-talk were measured. The loss and cross-talk can meet the practical requirement, and the channel uniformity is not desirable. The main issue is technical process. In our experiments, heated-drawing and contracting is controlled manually, the uniformity is deteriorated greatly, moreover, the distortion of POF during drawing and contracting is inevitable, as a result it leads to large bending loss. In addition, the termination of the taper and the alignment of optical path influences seriously on performances. Compared with mixing rod coupler, the coupler has advantages of simple structure, easy fabrication due to the absence of the mixing rod middle component. The further research on this coupler is to draw and contract fiber mechanically rather than manually, to enhance the uniformity, and to reduce loss further.
A miniaturized optical spectrometer module has been developed and realized in polymer by injection moulding. The spectrometer is designed for the visible (VIS, 380 nm-750 nm) and near infrared spectral (NIR, 680 nm-1100 nm) range. The assembled module has a size of a match box with a spectral resolution (Rayleigh criterion) of <7 nm /10 nm for the visible and <7 nm/8 nm for the near infrared spectrum depending on the pixel width of the used detectors. The stray light has been reduced well below 0.5 % for the VIS-module (VIS: filter OG550, measured at 500 nm) and NIR-module (NIR: filter RG850, measured at 790 nm). To avoid a wavelength shift caused by a thermal expansion of the system, a passive temperature compensation unit is designed. As a result of this the temperature shift between -40 °C and +70 °C can be reduced to <0.03 nm/K. To guarantee a flexible application of the spectrometer the measurement signal is coupled into the spectrometer by a fibre to free-space coupling unit with a 90° beam deflection. In order to use injection moulded components for optical sensors, mould inserts with a high optical quality are required. A toroidal optical mirror with an average surface roughness of Ra<20 nm and a radial shape accuracy as high as 0.2 % (0.1 mm) and optical gratings for the visible and near infrared spectral range with a planarity of 4 μm/cm and an absolute diffraction efficiency as high as 80 % can be fabricated. LIGA-technology, ultra-precision machining and electro-forming processes are applied. All optical elements have been replicated in polycarbonate (PC) with comparable characteristics. The spectrometer set up is based on a modular concept. This enables a high position accuracy of the elements to each other (few tens of μm) and a variation of specification (wavelength and resolution).
Gain flatness of optical amplifiers over the communication bandwidth is a key requirement of high performance optical wavelength division multiplexing (WDM) communication systems. Most often, a gain flattening filter (GFF) with a spectral response matching the inverse gain profile is incorporated within the amplifier. The chirped fiber Bragg grating (CFBG) is an attractive technology to produce GFFs, especially in cases where very low error functions are required. Error functions smaller than or equal to ±0.1 dB for the full operating temperature range are now possible. Moreover, the systematic errors from cascaded filters are much smaller than for thin-film GFF, a factor of importance in a long chain of amplifiers. To achieve this performance level, the high-frequency ripples normally associated with CFBG-GFF have been reduced by combining state-of-the-art holographic phase masks and advanced UV-writing techniques. Lastly, to eliminate the residual low-frequency ripples and localized errors, we developed a laser annealing-trimming station. This fully automated station combines both the aging process and final trimming of the GFF refractive index profile to exactly match the required transmission spectra. The use of self-adjusting algorithms assures quick convergence of the error function within a very tight error band. The capital expenditure necessary to implement this new tool is small in relation to the gain in precision, reliability and manufacturing cycle time.
Fiber distributed feedback (F-DFB) lasers have proven to be attractive devices for interrogation of optical sensors with high frequency resolution, due to their very low frequency noise/narrow linewidth, low relative intensity noise (RIN), robust mode-hop free tunability, compact size, and flexible and accurate wavelength setting. It has also been demonstrated that F-DFB lasers can act as sensor elements for high resolution measurements of physical quantities causing strain, refractive index, or birefringence changes in the laser fiber. It has been demonstrated that F-DFB lasers can be used as fast tunable sources for high resolution and high accuracy spectral component characterization. They may also find applications in dense WDM transmission systems utilizing their potentials for accurate wavelength setting, easy wavelength tuning, semiconductor pump redundancy, or multiple wavelength operation. In this paper properties and applications of F-DFB lasers will be discussed, with emphasis on modeling, design and characterization of the devices. In particular, RIN and frequency noise properties, requirements on grating and gain medium quality, the design requirements for achieving singlemoded or (intentionally) multimoded laser operation, and the output characteristics of single- versus multimoded F-DFB laser devices will be treated.
We introduce the novel concept of using a fiber-optic coupler as a versatile displacement sensor. Comparatively long fiber-optic couplers, with a coupling region of approximately 10 mm, are manufactured using standard communication SM fiber and placed in a looped-back configuration. The result is a displacement sensor, which is robust and highly sensitive over a wide dynamic range. This displacement sensor resolves 1-2 μm over distances of 1-1.5 mm and is characterized by the essential absence of a 'spring constant' plaguing other strain gauge-type sensors. Consequently, it is possible to couple to extremely weak vibrations, such as the skin displacement affected by arterial heart beat pulsations. Used as a wrist-worn heartbeat monitor, the fidelity of the arterial pulse signal has been shown to be so high that it is possible to not only determine heartbeat and breathing rates, but to implement a new single-point blood pressure measurement scheme which does not squeeze the arm. In an application as a floor vibration sensor for the non-intrusive monitoring of independently living elderly, the sensor has been shown to resolve the distinct vibration spectra of different persons and different events.
A detailed study of distributed Raman amplification using a bi-directional pumping scheme is investigated in this paper. In the final part we present an unrepeatered transmission experiment with the use of distributed Raman amplification and key results are discussed.
In this paper a new method for reading out Bragg wavelength shifts experienced by fiber Bragg gratings is described. The system is based on a bi-polished silicon sample acting like a Fabry-Perot filter. The spectral response of the Silicon Fabry-Perot filter allows to convert the Bragg wavelength shift into a variation of the light intensity, which can be read by a photodiode. The method efficacy is proved monitoring dynamic strain characteristics of a simple structure. So, the vibration mode of an aluminium cantilever has been sensed by means a FBG sensor attached on the surface of the cantilever itself. In proposed demodulation method the sensitivity and accuracy depend on the spectral band width of the filter. The filter can be designed according to the range of the amplitude vibrations, assuring linear response of system, as function of the thickness of the silicon sample. Moreover, thank to great tuning capability of the Silicon Fabry-Perot filter, it is possible to place Bragg grating spectral response on the central portion of the linear region of the FP response.
We describe a global method for the generation of an amplitude modulated optical wave in the range of telecommunication frequencies. This method is based on the optical heterodyning of two DFB lasers. With the assumption of longitudinal multimode lasers, we study the influence of the laser ray width on the spectral purity of the signal generated by the photodiode. We describe the experimental set-up which permits to obtain a tunability of the source up to 275 GHz. We present the results obtained in reception by beat of two identical lasers in a large bandwidth photodiode. A study of the power and frequency stability of the generated signal in the quadratic receiver is carried out for several hours. An experimental fusion set-up by CO2 laser has been developed in our laboratory to ensure the realisation of fibre optical components. Studying these components behaviour under fast optical wave variation is a new approach which brings some useful information regarding component bandwidth. This property enables the validation of components for high rate transmissions. The lasers beating set-up is successfully used to test tapered fibres. The optical component is placed at the coupler output. Tapered fibre bandwidth is measured by an optical wave modulation frequency sweeping in the 0-40 GHz range.
A new class of photodetectors, whose active material is an organic semiconductor, has been developed. Thanks to the ease of deposition on any dielectric surface, the device may be built directly on the cleaved surface of an optical fiber, therefore realizing an on-fiber-detector (OFD). The photodetector is based on an organic semiconductor belonging to a new general class of neutral dithiolenes deposited onto a quartz substrate with microlithographically defined gold electrodes so to realize a metal-semiconductor-metal surface structure. First experimental results on a photodiode made of (monoreduced imidazolidine-2,4,5-trithione) having peak responsivity at 1014nm, have shown a time response down to 100microseconds, at present limited by the leakage current noise due to the poorly rectifying contacts. Differently from the vast majority of organic semiconductor materials, dithiolenes have shown extremely high chemical and thermal stability. The photoresponse of the dithiolenes in the liquid phase is shown to be wavelength selective with an absorption peak about 150nm wide that can be chemically tailored so to shift from almost 1000nm to 1700nm. Experimental measurements to prove that the absorption property is maintained in the solid state also at wavelengths around 1500nm, thus covering with a photodetector the spectrum of possible telecom applications, are under way.
Local mode coupling theory is used for the study of the tapered fibres with a variation of the core index along the propagation axis. An application to the diffusion of the fibre dopants during the realization by melting with a CO2 laser is done. The results show that the coupling coefficient between modes can be separated in a sum of two coefficients. The first one is the classic coefficient which takes into account the core radius variation along axis. The second one is reported in order to analyse the index core variation.
'The step from micro-technology to nano-technology requires more than a reduction of size by a factor of a thousand. If you want to move precisely in the nano-world, you don’t succeed by perfecting proven techniques.' Handelsblatt. This quotation shows a new approach to Nanotechnology that is somehow unusual. Exactly this new approach is realised in a new system of Nanorobotics. It allows the development of a production system that is by orders more precise than classical production lines. Moreover, it includes the adhesive bonding technology which oftentimes is the preferred joining technology on this scale. The described system is the result of an industrial demand to solve the actual problems of micro production technology.
Flame-fused type-II silica glass and synthetic type III silica glass were exposed to gamma radiation and then thermally poled. Second-harmonic generation (SHG) is reduced by several orders of magnitude in Suprasil glass for doses higher than 100 kGy and in Herasil glass for doses higher than a few MGy. In Herasil glass, however, SHG is unchanged for doses lower than about one MGy. In the latter case, Maker fringe measurements show that, like in the poled pristine glass, the second-order nonlinearity is located in a thin layer beneath the anodic surface. On the other hand, in the former case, Maker fringe measurements reveal the existence of weak bulk nonlinearity. This unexpected result suggests that dipole orientation takes place in the glass modified by gamma radiation.
We present experimental results of thermal polings performed on Suprasil I samples (Heraeus) under square alternative voltages at various frequencies. We report a large increase (×5 compared to a continuous voltage poling) of the second order non-linear coefficient within a sample poled at 1mHz.
Photonic crystal fibers (PCFs) have been receiving increasing attention over the past few years. They are single material fibers that use an array of air holes in the cladding to confine light to a core, instead of the more usual refractive index step within the solid material of a conventional fiber. As PCFs become more well-understood mainstream structures, the need arises to develop techniques to process them post-fabrication to form all-fiber devices. We have chosen to study heat-treatment processes analogous to the tapering of conventional fibers, except that in PCFs there is a second degree of freedom to exploit. Not only can the fiber be stretched to locally reduce its cross-sectional area, the air holes can be changed in size by heating alone under the effect of surface tension.
This persentation gives an overveiw of the field of microwave photonics with an emphasis on new fiber based devices which we belive have a real practical potential. Microwave photonics can be considered as the fruitful meeting point bewteen optics and microwave engineering, where optoelectronic devices and systems are used both for processing at microwave rates and for signal handling in microwave systems. The use of specialty fibers, glass poling and naturally fiber Bragg gratings opens new perspectives for the realization of low-cost devices with appropriate functionality. The application field for optical microwave transmission and processing spans from radar technology to cable TV and mobile communications systems. Over the last few years very much attention has been directed towards radio-over-fiber systems for the next-generation mobile communications infrastructure as well as hybrid fiber radio for picocell systems at 60 GHz or above. As a matter of fact, the higher the microwave frequencies, the greater are the similarities with the optical carrier and the more there is to be gained by processing the microwave signal in the optical domain. Other important application examples are beamforming networks for phased array antennas and subcarrier processing for routing in optical networks.
A consortium of UK, Australian and Japanese groups is designing a fibre-fed near IR (J & H band) multi-object spectrograph (FMOS) for the Subaru telescope. The prime focus of the telescope will support a 400-fibre multi-object positioning system, ECHIDNA. However, the IR spectrographs (of which there are two) are to be located close to the Nasmyth platform, so an interconnecting optical feed is required to deliver light from ECHIDNA. The Astronomical Instrumentation Group at the University of Durham is undertaking the design and construction of a suitable fibre-optic downlink. To allow the prime focus unit that houses ECHIDNA to be removed, the fibre cable is to include a connectorized break, located at the telescope top-end ring. The optical design also calls for a change in focal ratio from that delivered by ECHIDNA in order to couple light to the spectrograph with the greatest efficiency. This will be achieved in the connector coupling by means of an array of high-efficiency GRIN microlenses. The connector will additionally incorporate an integral back-illumination system for on-telescope testing and calibration. This paper describes the preliminary design of the fibre system that is to be constructed in Durham.
A non-destructive technique for characterising couplers by means of a local perturbation is described. A CO2 laser beam is scanned along the coupler length inducing a local perturbation to the coupler eigenmodes. Asymmetric and symmetric perturbations give respectively, accurate mapping of power-evolution and coupler-waist shape. Using this technique, both the information of the power distribution and coupling profile along the coupler waist are obtained. The method is studied theoretically and verified experimentally by characterising different types of fused fibre-couplers namely: half-cycle, full-cycle and non-uniform half-cycle couplers. Add-drop multiplexers based on the inscription of Bragg gratings in the waist of fibre couplers have attracted some attention lately. The correct mapping of the power evolution along the coupler length gives the exact positions in the coupler waist where Bragg gratings should be written in order to obtain optimum add-drop operation. Recently, it was shown that non-uniform coupler structures based on two highly coupled lateral regions and a weakly coupled central region, can provide an ideal add-drop multiplexing operation. These couplers are characterised using the described coupler characterisation technique and the weakly coupled region, where the grating should be written, is clearly identified. This non-destructive method for characterising fibre couplers can be used as a tool for accessing the uniformity of the fabricated couplers waist or the influence of the tapered transition regions in the coupler performance. Additionally it can be used to optimise the performance of add-drop multiplexers based on the inscription of Bragg gratings in the waist of fibre-couplers.
In this article we present a new micromachined platform for coupling the optical fibers to waveguides of Mach-Zehnder interferometers, supporting fibers in the same wafer as a waveguide wafer. Using standard CMOS technologies, like a UV photolithography, KOH wet etching of Si(100) wafer, sputtering of metal layers, RIE etching the fabrication of a very precise platform is demonstrated.