الالياف البلورية الفوتونية

[عراقية]

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[عراقية]

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اختي عراقيّة

[محمد ابوزيد]

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اخوكم / محمد ابوزيد

[NADER]

مرحبا للجميع وتحياتنا لاخت عراقية على هذا الموضوع والذي تتوفر عليه معلومات بكثرة جدا على الانترنيت وبمختلف الانواع
على سبيل المثال هذا العمل المشترك
بامكانك زيارة الرابط اسفل الموضوع :

Infrared Photonic Crystal Fibers and optical bundles
During the last decade a new frontier has emerged, which deals with the control of the optical properties of materials by periodic structures implemented in the materials. One can engineer a material that prohibits propagation of light, or allows it only in certain directions at certain frequencies, or localize light in specified cavities.
In the visible and near infrared ranges, photonic crystal fibers (PCFs) have shown great potential in overcoming the limits of conventional fibers and achieving remarkable abilities that regular fibers cannot accomplish, such as high power transmittance, high non-linear effects or no non-linear effects at high powers, controlled dispersion, low losses, low bending losses and large core area single-mode operation over a wide transmission ********
In the mid-IR range, PCFs have not yet been implemented due to the lack of suitable materials and techniques. Design and fabrication of PCFs for the mid-IR can be extremely useful, not only as large core area endlessly single-mode fibers but also for other applications like polarization maintaining with low losses and high power transmittance.
Our research involves the design, fabrication and optical testing of photonic crystal fibers for the mid-IR, based on silver halide polycrystalline materials. The work includes numerical simulations and calculations, design of equipment and optical setups, fabrication of fibers and optical measurements. Special attention will be given to the development of endlessly single-mode photonic crystal fibers of various structures and optical propertiesNovel infrared photonic crystal fibers (PCFs) have been fabricated by multiple extrusions of silver halide crystalline materials. In contrast to regular PCFs, which are made from holey silica glass fibers, our PCFs are composed of two solid crystalline materials.

Students / staff
Arnon Millo (PhD)
Phone: 03-6408405
Fax: 03-6415850
Room: 308
Email: [email protected]

Itay Naeh (M.Sc)
Phone: 03-6408405
Email: [email protected]

Lilya Lobachinski (M.Sc)
Phone: 03-6408405
Email: [email protected]

Research
Introduction
Optical unclad fibers for the middle infrared (mid-IR) range have many medical, military, industrial and research applications. However, for many applications there is a need for more advanced fibers, such as graded index fibers, polarization maintaining fibers and especially single-mode fibers.
Single-mode fibers (SMF) are usually used for communication in the visible and near- IR range. In the mid-IR, SMF are needed for the fabrication of thermal imaging fiber bundles, IR heterodyne detection systems, mid-IR sensors and as spatial filter elements for the terrestrial planet finder (TPF) projects of NASA-JPL or of the European Space Agency (EPS). The fabrication of SMFs in the mid-IR has been delayed for many years due to technical difficulties.
In the visible and near infrared ranges, photonic crystal fibers (PCFs) have shown great potential in overcoming the limits of conventional fibers and achieving remarkable abilities that regular fibers cannot accomplish, such as high power transmittance, high non-linear effects or no non-linear effects at high powers, controlled dispersion, low losses, low bending losses and large core area single-mode operation over a wide transmission ********
In the mid-IR range, PCFs have not yet been implemented due to the lack of suitable materials and techniques. Design and fabrication of PCFs for the mid-IR can be extremely useful, not only as large core area endlessly single-mode fibers but also for other applications like polarization maintaining with low losses and high power transmittance.
Our research involves the design, fabrication and optical testing of photonic crystal fibers for the mid-IR, based on silver halide polycrystalline materials. The work includes numerical simulations and calculations, design of equipment and optical setups, fabrication of fibers and optical measurements. Special attention will be given to the development of endlessly single-mode photonic crystal fibers of various structures and optical properties.
Photonic Crystal Fibers
During the last decade a new frontier has emerged, which deals with the control of the optical properties of materials by periodic structures implemented in the materials. One can engineer a material that prohibits propagation of light, or allows it only in certain directions at certain frequencies, or localize light in specified cavities.
Photonic crystal fibers are periodic dielectric structures composed of several materials with high and low refractive indices. In the visible and near-IR, PCFs are usually made from silica fibers which have periodic arrays of air holes running along their entire length. The two main groups of PCFs are Total Internal Reflection PCFs (TIR-PCF, also called solid core PCFs) and Photonic Band Gap PCFs (PBG-PCF, also called hollow core PCFs).
TIR-PCFs: These solid core PCFs transmit light by total internal reflection (TIR). They behave in many ways like standard core-clad fibers, but have several advantages such as low losses, high power transmittance and large core-area. TIR-PCFs transmit light in a solid core of a high refractive index surrounded by a geometrical arrangement of rods (or holes) of a lower refractive index (see Fig. 1a). One unique feature of the PCFs is that a SMF made from such a structure can operate over a very wide wavelength range (called endlessly single-mode), even with a large core area.
PBG-PCFs: PBG fibers transmit light in a hollow core surrounds by a geometric arrangement of holes in a solid matrix (see Fig 1b). PBG structures rely on the interference of light reflected from the periodic structures in the cladding. The highly periodic structure of air holes creates the photonic bandgap. This means that frequencies within the PBG are not allowed to propagate out through the cladding and may be trapped at the defect in the core. An inherent feature of PBG is that it only guides light in a limited spectral region. PBG’s can be used for devices such as resonators, filters, optical switches, optical amplifiers and waveguides.
Calculations and simulations reveal that a photonic band gap is easily generated in certain lattice arrangements when the ratio between the refractive indices of the rods and the cladding material is large. In fact it is currently believed that the minimum requirement for a complete bandgap is nrods/ncladding > 1.3. This requirement cannot be fulfilled if silver halide materials are used in both the rods and the cladding, since nAgBr / nAgCl 1.09. We were not able to form air holes in silver halide fibers, because the holes collapsed during the extrusion. Therefore, we conclude that fabricating IR PBG fibers from silver-halides polycrystalline cannot be done using the usual extrusion methods and thus we concentrate our efforts in fabricating TIR-PCFs

Fig. 1. (top)
(a) an index guiding TIR-PCF. The holes/rods in the clad lower the effective index of the cladding and so the fiber acts as an index guiding fiber.
(b) a Photonic Bandgap fiber PBG-PCF with hollow core. The periodic arrangement of the holes in the clad creates a photonic energy bandgap which prevents the light in the core from penetrating the clad.
(c) a Bragg fiber, the radial periodic of the alternating layers of the clad creates a 1D photonic bandgap that keeps the light locked in the core area.
(bottom) a Cyanophrys remus butterfly as example for natural phtonic crystal. The wings of the male Cyanophrys remus are bright ****llic blue on one side, thought to attract mates, and a dull green on the other to act as camouflage. Each side of the wing contained different photonic structures. The ****llic blue colour is produced by scales that are photonic single crystals whereas the dull green is the result of a random arrangement of photonic crystals (press on image to show video).


Silver halide photonic crystal fibers
Fabrication:
We have fabricated several PCFs of different geometries. The optical characteristics of the PCFs depend on these parameters: the geometry of the lattice, the ratio between the rod-rod distance and the rod diameter, the core diameter and the refractive indices of the matrix and the rods. We used the simulation tools to design and test the behavior of these structures in different wavelengths. Fig. 3 shows a collection of microscope photographs of some of the PCFs that we fabricated.

Fig 2. Some of the silver-halide PCFs we fabricated.
Left: a circular PCFs (the picture shows a 8mm preform).
Middle: a triangular PCF (8mm preform).
Right: a segmented cladding fiber (8mm preform)


Simulation:
Due to the complex nature of photonic crystals, It is not possible to preform analytical calculations of the propagation constants and the modes fields. We therefore used several approximations and simulations methods. We are using the following methods:
1. The effective index method: The effective index method was recently used for the analysis of core-clad fibers, segmented fibers and circular PCFs. The accuracy of this method was verified by comparison to theoretical values and to the finite element method. Here we discuss a radial EIM (REIM) to calculate the propagation constants of the bound and leaky modes in silver-halide PCFs. Knowing the imaginary part of the propagation constant is important for the calculation of the minimum length of the single-mode fiber. Both leaky and bound modes are activated at the fiber’s input interface. However, the leaky modes decay exponantly. From the imaginary part of the propagation constant, one can calculate the attenuation of the leaky modes and therefore the minimum length needed for a single-mode operation. For photonic crystal fibers, all the modes are leaky (since there is no clearly defined clad). Therefore, to achieve a proper single-mode operation, the imaginary part of the propagation constant of the second leaky mode must be larger than the first mode, by several orders.
2. The finite differences method and final elements method: With FMD, we considered the PCFs to be constructed of dielectric materials, with no free charges or currents. We chose to find a solution for the magnetic field H, since all its components are continuous at the boundaries, due to the uniformity of the magnetic permeability. The finite element software we are using is Comsol 3.3, Multiphysics.
3. The MIT Photonics Bands, for computing the band structures (dispersion relations) and electromagnetic modes of periodic dielectric structures. MPB computes definite-frequency eigenstates (harmonic modes) of Maxwell’s equations in periodic dielectric structures for arbitrary wavevectors, using fully-vectorial and three-dimensional methods. It is especially designed for the study of photonic crystals (http://ab-initio.mit.edu/photons/) (a.k.a. photonic band-gap materials), but is also applicable to many other problems in optics, such as waveguides and resonator systems.

Fig. 3. (left) a simulation using the radial effective index method on segmented cladding fibers. The calculation involves changing the duty cycle of the fiber and calculating the effective index of the first two modes and the effective index of the clad. The fiber act as a single-mode fiber when the effective index of the first mode in above the effective index of the clad and the second mode is below.
(right) A finial difference simulation of the electrical fields of a triangular PCFs. The image show the energy distribution of the fundamental mode.


Measurements:
To investigate the optical properties of our PCFs, we employ several experimental techniques.
(1) Near field scanning: to obtain the near field power distribution output of the PCF we used a scanning system based on probes made from tapered silver halide fibers. The probes were fabricated from unclad silver halide fibers that had been extruded from AgClBr crystals. One end of the probe was polished and the other end was etched, to form a sharp tip of with an diameter aperture of 5-10μm. This tapered fiber scanned the distal end of the PCF in the X-Y plane, at a distance Z<10µm from the PCF.
(2) Thermal camera imaging: we used an IR camera to investigate the far-field radiation distribution of the PCFs. The radiation was inserted into the core of the PCFs by the use a pinhole and lens.
(3) Far field and field of view: we measure the far-field radiation of the fibers using a IR CCD camera (Spricon). We can also measure the fiber field of view using a rotating MCT detector that scans the output of the fiber at different angles.
(4) Other methods: we also utilize FEWS (Fiber evanescent wave spectroscopy) , transmittance measurements using the cut-back method, bending losses measurements and more.

Fig. 4. Measurements
(a) an Infrared camera image of the output of a square PCFs with 8 modes in the core.
(b) a field of view scan of two PCFs with and without an outer absorbing layer. The absorbing layer helps to strip the cladding modes from the fundamental mode.
(c) a far field CCD camera image of the output of a single-mode segmented cladding fiber


IR silver-halide fiber bundles
Ordered bundles of silver-halide fibers, which are highly transparent in the middle infrared, were fabricated by multiple extrusions from single crystals. We fabricated and characterized the optical properties of thin and flexible bundles of diameters 0.7-2.0mm which incorporated 100 individual fibers. The measurements included: attenuation, resolution, crosstalk, near field scanning of single fibers in the bundle and bending losses. Bundles of lengths of several meters transmitted thermal images of bodies whose temperature was near room temperature. These bundles would be useful for medical, industrial and military applications, and in particular for endoscopic thermal imaging.
We used multiple extrusion techniques to form thin, flexible and ordered bundles of infrared transmitting silver-halide fibers, of lengths of several meters. Optical and mechanical measurements on these bundles showed highly improved properties. These include low transmission losses, relatively high spatial resolution, negligible bending losses and negligible crosstalk. In addition these bundles are made of silver halides which are nontoxic, non-hygroscopic and are transparent over a very wide spectral range (3-30µm), which is important for practical applications. The ordered bundles discussed in this work can serve as building blocks of fiberoptic thermal imaging systems, and in particular for thermal imaging endoscopes.

Fig. 6. (left) A microscope image of a 10x10 fiber bundle. The bundle outer diameter is 0.7mm.
(right) an Infrared camera image of a heated ring (1000C) and the same image as transmitted by the fiber bundle.


Recent Publications
• A. Millo, I. Naeh and A. Katzir, “Single-mode segmented cladding fibers for the middle-infrared”, J. Lightwav. Tech., accepted to be published, Aug 2006.
• A. Millo, I. Naeh, Y. Lavi and A. Katzir, "Silver-halide segmented cladding fibers for the middle infrared", Appl. Phys. Lett., vol. 88, no. 25, p. 251101, Jan. 2006.
• A. Millo, I. Naeh, Y. Lavi and A. Katzir, “Segmented cladding fibers for the middle infrared”, Proc. SPIE, vol 6128 (2006)
• S. Shalem, A. Tsun, E. Rave, A. Millo, L. Nagli and A. Katzir, "Silver halide single-mode fibers for the middle infrared", Appl. Phys. Lett., vol. 87, no. 9, p. 091103, Aug. 2005.
• E. Rave, S. Sade, A. Millo and A. Katzir, "Few modes in infrared photonic crystal fibers", J. Appl. Phys., vol. 97, no. 3, p. 033103, Feb. 2005.
• E. Rave, A. Millo, S. Sade and A. Katzir, "Towards the realization of a single-mode photonic crystal fiber in the middle infrared’, Proc. SPIE, vol. 5733, pp. 214-221, Jan. 2005.
• Lavi, Y., Millo, A., and Katzir, A., "Flexible ordered bundles of infrared transmitting silver-halide fibers: design, fabrication, and optical measurements," Appl. Opt., vol. 45, no. 23, pp. 5808-5814, 2006.
• Lavi, Y., Millo, A., and Katzir, A., "Thin ordered bundles of infrared-transmitting silver halide fibers," Appl. Phys. Lett., vol. 87, no. 24, pp. 241122-1-241122-3 (2005).
• Rave, E., Ephrat, P., Goldberg, M., Kedmi, E., and Katzir, A., "Silver halide photonic crystal fibers for the middle infrared," Appl. Opt., vol. 43, no. 11, pp. 2236-2241 (2004)


Links

• The Electronics lab in Tel Aviv University
• Electronics projects competition site
• MIT photonic crystals site
• Britney Spears guide to photonic crystals
• Comsol multiphysics
• Amnon Yariv group homepage at Caltech
• K. S. Chiang web site at Hong Kong city university

url]http://www.tau.ac.il/~applphys/research_irpcf.htm[/url]

وبامكانكم الاطلاع على الروابط ادناه :

http://web.mit.edu/sdey/www/mit/photonic.pdf

http://en.wikipedia.org/wiki/Photonic-crystal_fiber

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thank you soooooooooooooo mch