Measurement techniques of tailored optical fibres

Ryszard Romaniuka, Jan Doroszb

aWarsaw University of Technology, bBiałystok University of Technology

rrom@ise.pw.edu.pl

ABSTRACT Tailored optical fibres (TOF) may have non-standard geometrical shapes and dimensions as well as very complex internal refraction and physical structure. Practice shows, that in numerable cases, the standard measurement methods have to be specially adapted to particular case of a fibre. In some cases, a class of novel methods has to be worked out. In this work we present a number of modified measuring methods that suit the needs of characterization of a few basic kinds of tailored optical fibers such like: nonstandard dimension fibers, noncircular core fibers, complex shape fibers, complex refraction fibers, multicore optical fibers, nonlinear and active fibers. The modified measuring methods of tailored optical fibers include, among others: refractometry, polarimetry reflectometry, scatterometry, nefelometry, measurements of dimensions and shapes. Modifications of classical measuring equipment require usage of optimized light coupling techniques. The characterization and measuring methods of such fibers have to be standardized to enable the designers of the photonic functional systems (such as telemetric, sensory and optical signal processing) to apply them in a reliable and repeatable way.

Keywords: Optical fibre measurements, specialty optical fibres, lightmetry, fibre characterization, light coupling to fibre

1. INTRODUCTION Optical fiber technology can be divided into two major fields – opto-telecommunication and fiber optics of specialty non-communication fibers. Optical fiber communications (OFC) investigates new generations of ultra-low-loss, linear, isotropic and broadband dispersion compensated, transmission fibers for DWDM technology (spanning from S, through C to L and XL optical bands). The OFC uses optimization technologies for current, more efficient usage of older fibers installed several years ago. Optical fibers for applications outside the trunk transmission field are called specialty or tailored ones. The latter fibers are used in optical communication as short-length, functional photonic components. They are also used in photonic systems for information processing and hybrid optoelectronic ICs as well as in optical fiber sensors. Tailored optical fibers are manufactured in this country, using hybrid methods like rod-in-tube (RiT), double crucible (DC) and modified multicrucible (MMC) but mainly MCVD. The modifications of MCVD rely on adding a few additional stages of the technological process like asymmetrical gaseous etching or preform grinding. These processes are necessary for introducing new geometrical, refractive, mechanical and thermal features into the preform like: optical anisotropy, open channels, etc. The MCVD methods and their hybrid variations are used for manufacturing of tailored optical fibers at the Fiber Optics Laboratory of UMCS University in Lublin. Soft-glass tailored optical fibers are manufactured by the MMC, mosaic, extrusion, RiT and complex, multistage methods consisting of the mentioned ones at ITME Warsaw and Technical University of Białystok. One of major problems with presented here tailored MMC optical fibers is handling these fibers and measuring them. They can be engineered to standard outside dimensions like 125µm or so, but the cores will remain non-standard. The latter crates a lot of initial difficulties with optical power coupling, connecting, etc. We tried to overcome some of these initial difficulties using some classical optical measurement methods.

2. ASSUMPTIONS FOR MEASUREMENTS OF TAILORED OPTICAL FIBRES

Specific measurement techniques of TOF are associated with differences in their construction form classical optical fibres. A comparatively large and constantly increasing family of Tailored Optical Fibres (TOF) may be divided to several main sub-families. TOF may have non-standard geometrical shapes and dimensions as well as very complex internal refraction and physical structure. Practice shows, that in numerable cases, the standard measurement methods have to be specially adapted to particular case of a fibre. In some cases, a class of novel methods has to be worked out. We have recently developed a number of modified measuring methods that suit the needs of characterization of a few basic kinds of tailored optical fibers such like: nonstandard dimension fibers, noncircular core fibers, complex shape fibers, complex refraction fibers, multicore optical fibers, nonlinear and active fibers. The modified measuring methods of tailored optical fibers include, among others: refractometry, polarimetry reflectometry, scatterometry, nefelometry, measurements of dimensions and shapes. Modifications of classical measuring equipment require usage of optimized light coupling and decoupling techniques. The characterization and measuring methods of such fibers have to be standardized to enable the designers of the photonic functional systems (such as telemetric, sensory and optical signal processing) to apply them in a reliable and repeatable way. Lack of measurement standardization is the major cause of that the results can not be compared mutually with each other among the labs, as it was with singlemode telecom fibre spectral attenuation measurements more than twenty years ago. To make a small step toward the measurement standardization we designed a versatile computer based, TOF oriented, measurement system co-working with measurement processing tools and database, fig. 1.

Fig. 1. Schematic diagram of the computerized measuring system and Graphical User Interface (GUI) to networked, programming, fiber measurement, fiber oriented signal processing and fiber database, integrated system. The interface was built in LabWindows and uses standard tools like MathCad and StatGraph. The tools are used for standardized, networked fibre analuysis.

3. LABORATORY EXPERIMENTS

3.1. Isotropic optical fibres

Manufacturing of an ideal isotropic optical fiber (maintaining isotropy in all conditions) is virtually impossible. It is a model analyzed theoretically for the purpose to differentiate it from real fibers manufactured by various methods. There are a number of factors, including statistical ones, introducing residual anisotropy in very high quality, ultra-low-loss, nearly-ideal, transmission optical fibers. These are: optical losses, backscatter, dimension fluctuations, residual core ellipticity, local fluctuations of glass chemistry, stresses induced by fibre bends and torsions, thermal perturbations. The real fiber behaves as if it were weakly anisotropic of random distribution of this anisotropy along its length. This anisotropy manifests itself by fiber birefringence. The internal component of this residual birefringence depends on the fiber manufacturing. The basic parameter of a high quality isotropic optical fibre is its refractive index profile and its class of homogeneity in the fibre cross-section as well as along the fibre. Thus, apart from classical measurements, the highly isotropic fibre requires

measurements of residual and second order effects. These measurements require high accuracy. Residual birefringence is of the order 10-6, residual core ellipticity 1,005.

3.2. Refractive optical fibres

One can distinguish several fundamental classes of RIPs in a TOF. Some of these RIPs are analogous to the classes for optical communication fibres. These are: step-index, gradient-index, monotonic and non-monotonic, among them W-profiles, ring-index, multi-ring-index, isotropic and anisotropic, single argument n(r) and double argument n(r,Θ), depending on the azimuth. The fibres of distant RIPs differ considerably among themselves. The profound differences are in: diameter in modal field, ability to butt-joint couple with other fibres, sensitivity to microbending, modal structure, modal cut-offs, character of the fundamental mode. It may not be the HE11 in the general case. Introduction of a complex RIP in an optical fibre has the aim to optimize the dispersion in terms of flattening, shifting, etc. It also gives more immunity of the fibre to microbending. The basic aim of complex RIP design in a TOF is to shape the modal structure of the fibre. This structure is optimized for a particular application. There are several parameters connected with this optimization process like: functional dependence of modal field diameter on normalized frequency, precise choice of cut-off point, shaping of the far field radiation characteristics, coupling capabilities with other fibres, microbending losses, mechanical-thermal characteristics, etc. The basic measurements of refractive TOF require very precise assessment of the RIP.

3.3. Birefringent Optical Fibres

Anisotropic optical fibres are a broad and fundamental class of TOF. The broadest interest among these fibres is in birefringent ones. Measurements of birefringent optical fibres require localization of fast and slow optical axes. When the state of polarization of the input wave is in coincidence with any of the fibre polarization axes, then it theoretically stays stationary along the waveguide. When the axis of polarization of the input wave makes an arbitrary angle with fibre axis of polarization, then the state of polarization changes periodically with the beat length. BOF are used as polarizers, single polarization transmission lines for coherent systems, sensors, single polarization couplers, etc. The measurement techniques of BOF have to carefully take into account natural, inbuilt as well as induced polarization characteristics of such fibres. HB fibres are more difficult in handling than classical ones. Butt-joining and splicing requires precise angular justation against the HB axis. All laboratory and application set-ups require good quality joints between transmission and HB polarization maintaining fibre which acts as a single polarization transmission channel or a distributed sensor. The best way is to make a low loss splice. It is possible, however, to obtain such a good quality splice only in the case of joining optical fibres of similar thermal characteristics. When one tries to join MCVD fibre with MMC fibre, other equivalent methods have to be applied. All fibre polarizer is built of linearly birefringent fibres. One of the polarized wave components (unwanted one) is removed in such a fibre by continuous discrimination by an internal or external factor. This polarization component has to be more susceptible to such a factor than the second one, which is expected to remain in the fibre unaffected and 60dB of polarization discrimination can be obtained in a linearly birefringent MMC optical fibre. The characteristic measurements embrace mainly level of polarization separation, level of polarization cross-talk, residual power in unwanted polarizant.

3.4. Elliptical Optical Fibres

Singlemode elliptical core optical fibres are a fundamental kind of a large family of non-cylindrical core optical fibres. They exhibit natural, geometrical, linear birefringence. Additional inbuilt birefringence can be applied, as in classical, cylindrical, linear, HB fibres, which would add the geometrical one. Optical fibres of elliptical cores are quite well known theoretically, but there are some problems with their stable and repeatable manufacturing, for strictly required elliptical core parameters. This concerns especially the MCVD method. Manufacturing of elliptical core optical fibres (multimode and singlemode) is possible also by the MMC method. The natural, linear birefringence, in elliptical MMC fibres, is on the level of B≈10-5. For particular, extreme parameters of the elliptical core (small diameter) the corresponding value was of the order B≈10-4 and bigger. Other possibility to increase the birefringence in a singlemode elliptical-core optical fibre is to apply a heterogeneous RIP, depending on azimuth. The measurements embrace: core ellipticity, differentiation of various sources of birefringence.

Fig.2. Localization of axes of core ellipticity in a TOF.

3.5. Holey optical fibres (HOF)

Holey optical fibres can be divided to several basic classes: with macro-holes, and with micro-holes. The fibres with microholes can be further divided to refractive structures – periodic and non-periodic and only periodic photonic bandgap ones. A big research effort is now with photonic crystal optical fibres –PCF. The refractive structure is built from ordered, filled with air, micro-tubes placed in volume glass or stretching along the fibre. A more general name seems to be holey optical fibres or HOF. The fibre is made of a single material and the propagation conditions are set by the distribution of holes (in the cross-section) and dimensional proportions between holes and filling material (distances between holes axes). The optical wave can be propagated in a HOF using two different mechanisms – refractive and photonic. The characteristic measured properties are: structure of glass grid, localization of EM fields in the grid, range of opacity in terms of optical frequency for photonic propagation.

3.6. Nonlinear optical fibres

Optical fibres used now for communications carry much more optical channels and thus much bigger optical power than previously. This large optical power modifies transmission properties of optical fibre leading to nonlinearities. A nonlinear effect is defined when the signal leaving the fibre for certain wavelength does not increase linearly with input power for the same wavelength at the fibre input. The nonlinearity leads to power conversion between various lambdas. Nonlinear measurements got very important with the advent of broadband OF amplifiers. This allows for further increase in the number of multiplexed channels on a single fibre. High optical power together with increased transmission speed, dispersion optimization through compensation of fibre lengths, and a lot of narrowly spaced channels facilitate the nonlinear effects like phase self-modulation, cross-phase modulation, FWM, SBS and SRS. The nonlinear effects appear after the threshold of power density is overcome, what in an optical fibre is easy because of power confinement to the extremely narrow region of fibre core. However, the distribution of power in fundamental mode is different from distribution of refractive properties. Thus, assessment of the distribution of effective field is necessary, analogous to the field diameter. Precise measurements of nonlinear fibre parameters are more and more important for communications (to get rid of distortions) as well as for building of nonlinear optical fibre functional components, for soliton systems and photonic nonlinear systems. There are measured the following parameters: nonlinear refraction n2, effective fibre surface Aeff (combined with distribution of modal field) and levels of stimulated scattering. The investigation and measurement of nonlinear effects in complex broadband optical fibre transmission systems as well as in instrumentation and/or sensory systems got more complicated because of application of different kinds of optical fibres of partially compensating and partially enhancing dispersion and nonlinear effects. Thus, there is a need for investigation of nonlinear as well as dispersion effects of higher order.

3.7. Active and high power optical fibres

The active optical fibres carry considerable amount of optical power. They require special multi-layer construction to enable optical pumping. The effective pumping and transmission of high power is required simultaneously. The fibre may be multimode for pump signal and singlemode for amplified signal. The refractive index profile is designed in such

a way so as to cumulate the optical power in a singlemode core. The EDFAs are still optimized though they are the most advanced among the OFAs. Optimization parameters are: amplification uniformity inside the operating bandwidth, noise, amplification and dynamic range. There are investigated other active OFs made of different materials like polymers doped with rare earths, compound glasses, IR glasses, like ZBLAN, halide, chalcogenide, sulfide, etc. Also fiber Raman amplifiers basing on SRS are intensely developed. The measured parameters are mentioned above and optical power and energy levels, stability of these levels, spectral changes, beam temporal and space fluctualtions.

3.8. Bragg optical fibres

Most of the Bragg gratings are done on sensitized CVD fibres but also on IR glass fibres and plastic ones. The gratings can be straight, periodical, chirped, skew, short, long, repeated. The measurements include: geometrical data of gratings, spectral filtering characteristics, slope steepness, slope function (for slope shaped filters), cross-talk to other filter (channel), in-band uniformity, out-of-band lobes.

3.9. Conical optical fibres

Optical fibres with functionally changing of external dimensions, for example conical ones, are equivalent to the ones with NA aperture and RIP changing along fibre length. A typical fibre narrowing, acting as a coupler or evanescent field sensor, is for particular length, a multimode clad-less optical rod waveguide. The measured parameters are: mechanical narrowing profile, optical power transmitted during the narrowing process, insertion losses, power dissipation, cross-talk to other adjacent fibre cone.

3.10. Plastic optical fibres

Multimode gradient index, plastic, optical fibres (similar to multimode glass fibres in signal properties sense) differ from them by elasticity, way of joining and building of local transmission systems. These differences are quite attractive and thus the further intense research is going on the development of these fibres. Now the most important problems are in the multimode area to stabilize the available technologies of cheap, low-loss fibres of optimal gradient index. The measurement parameters are: spectral losses per fibre length, RIP and multimode dispersion optimization. In the area of image transmission the basic work are to increase the contrast and resolution and decrease price of disposable imageguides for medical purposes. The measurement parameters are: image quality, resolution, contrast, transfer function, image deterioration with bending.

3.11. Sensitized optical fibres

Technologically sensitized optical fibres SOF (or desensitized ones) are a large group of fibres designed for particular application. For example, fibres of sensitive cladding to electrical fields are designed for photonic electrical values measurements. SOF can be divided to: sensitive cladding fibres, differential characteristics between core and cladding fibres, sensitive material fibres, and ones of complex internal structure. Additional stages of technological process may lead to sensitizing of optical fibre to certain reactions. Just opposite to telecom fibres where one expects small sensitivities to external fields like temperature changes and microbending. For example functions n(T) of core-clad set of glasses cross each other, giving the condition of NA=0 and then the effect of reversed waveguide. Similarly the dispersion curves of core-cladding materials may cross each other. The measured parameters are: sensitivities of the fibre parameters to various kinds of external reactions like thermal, mechanical, electrical, magnetic, chemical, optical.

3.12. IR optical fibres

IROF technology operates in the approximate spectral range 2÷30µm. The subject of interest to IROF are materials, glasses, polycrystals, crystals, polymers transparent in this region and susceptible to making out of them fully structured optical fibres. The following conditions may be defined for IROF glasses: 1) Stability to devitrification processes in sufficiently brad range of temperatures; 2) Sufficiently low viscosity to pull a fibre; 3) Sufficiently high Tg, higher than room temperature; 4) As big difference between Tk and Tg; 5) Susceptibility to ultrapurification; 6) Mechanical and chemical stability, especially with respect to water; 7) Nontoxic; 8) Susceptibility to be covered with other glassy material to form structured core-cladding fibre; 9) Broad range of optical transparency and low-losses; 10) Susceptibility to precise doping effectively changing the refractive properties of material. As hypothetical advantages of

IROF are mentioned: low-losses of fibres, increase of singlemode fibre dimensions, lower sensitivities of fibre to external reactions. The measured parameters are:

3.13. Complex optical fibres

Complex optical fibres are filaments of internal structure, which is difficult, or impossible to be done with classical MCVD. Many of such fibres have not yet been described. They are manufactured using hybrid methods or multistage combination of a few simpler processes. The literature treats such fibres in the quest for new optical and signal transmission properties. The measured characteristics are quite broad embracing: geometrical properties, mechanical properties, optical values, dispersion, birefringence, polarization holding ability, fundamental mode peoperties, fibr effective area, nonlinear effects and many more.

3.14. Multicore optical fibres

Measurements techniques of multicore optical fibers require adaptation of special coupling methods for excitation of individual cores and for optical power detection from individual cores. A method was adapted with multifiber fused tapers. If the core distribution in the multicore fiber is simple and homogeneous (like: linear equidistant, triangular equilateral, square, hexagonal, etc) it is comparatively straightforward to assembly relevant fiber taper composed of single core fibers. The fibers should usually have nonstandard proportions between core and cladding. In the case of nonhomogeneous core distribution in a MOF, to manufacture a relevant taper from individual fibers is quite complicated. Sometimes it is easier to use the same MMC process with lowered speed of fiber pulling to manufacture much thicker fibers and then make tapers out of these “preform” like fibers. A photograph of a multiple fiber taper is presented if fig.3. The taper was cut (cleaved) at a proper length for diameter and core spacing match with investigated multicore optical fiber. A typical optical power detection scheme with homodyne reference channel was applied to measure the output from MOF. Fig.2 shows a block diagram of our computer based measuring system for assessing the parameters of optical fibers. The system consists of major two parts: computer management and programming environment,

Fig.3. Multifiber taper for coupling with MOF.

TOF measurement method with interference differential contrast and negative phase contrast (striated fields)

Biolar P1 microscope is equipped for measurements in interference homogeneous striated field. The interference is realized by three birefringent Wollastone prisms of different diffraction angles. Microscope tube prism gives striated field of large split r of image (r=32µm for a lens of 10x magnification). The remaining two prisms invoke homogeneous interference area of small split (r=1,84µm for a lens with 10x magnification) and big split (r=6,5µm for a lens with magnification 10x). The split originating from microscope tube birefringent prisms is constant and characteristic for these prisms. Addition to the optical system of PI microscope of a lens with rotary Wollastone birefringent prism allows changing fluently the split, by rotating this lens against the optical axis. The relative position of all involved prisms: tube and lens is neutral for rotation angles equal to .315,225,135,45 0ooo=γ The wave split, in these cases, originates only from tube prism. Rotating the lens prism of the following angles 00 or 3600 and 1800 one can obtain additive and subtractive location of prisms. The split is respectively maximum when 21max rrr += and minimum

when 12min rrr −= , where 1r and 2r denote split contribution values from birefringent tube and lens prisms. The

rotation angles equal to 00 270,90=γ are equivalent crossed setting left and right-rotated. The value of resulting split

r, for crossed position, is equal to sqrt( 22

21 rr + ). When the slit of condensor diaphragm is narrow enough and set

perpendicularly to the resulting direction of the wave split, there appears effective interference in the image plane of the Biolar PI microscope. The vibrations of the split waves (by the lens prism), when entering the tube prism, must be in agreement with the main cross section of the latter prism. The wave leaving the polarizer has to vibrate in the plane, which creates angle of 450 with main cross section of tube prism. At neutral, subtractive and additive settings, the direction of setting of condensor slit should be parallel to the direction of straight lined striae from the background of the field of vision. At crossed settings, the slit should be rotated of such an angle so as to fit it to a position perpendicular to the resulting split of the front of optical wave. The maximal split of optical fiber images, which one can obtain at crossed setting of birefringent prisms (tube and lens), equals 500/β µm, where β is lens magnification. The measurements performed in the interference striae field, at the mentioned split, have this advantage, that the chosen value may be measured two times. One time in ordinary image, while the second in extraordinary image, what gives better accuracy of measurement. This kind of interference is, however, not very useful in all cases. This depends, for a particular interferometer, on the diameter of measured optical fiber and its core, on number of cores and their distribution in the fiber. At high density of cores, the interference image gets non-readable, especially for regions close to fiber axis. In such cases the same interference microscope may be used but switched for differential interference.

Differential interference is a particular case of interference with double image. It is obtained as a result of slight transverse splitting of optical wave transmitted through the tested object. The difference in optical path-lengths Λ, in differential interference image, between ordinary and extraordinary waves does not express directly the difference of optical paths δ in the subject, but a gradient of this difference dδ/dy in the direction r of splitting of interference waves. The value of this split is of this order, as is lens resolution in comparison with the diameter of the subject. This property is described by the simple relation: Λ=rdδ/dy. The differential interference contrast in striated field can be obtained with subtractive setting of birefringent tube prism against the lens prism. Dense distribution of strips is a result of this and small resulting split of image. At subtractive position of both prisms, the split of interfering waves takes place in the direction of optical fiber axis, which is set perpendicularly to the direction of striae in the background. Interference image, for anisotropic fibers, appears in the form of straight-line striae shifted in the area of fiber. This shift is a sum resulting from ordinary and extraordinary waves. For isotropic fibers, the shift is not observable, or in other words, it is equal to zero. In order to obtain, differential image in the striated field, useful for further investigations and evaluation, it is necessary to use a compensator allowing for continuous fluent change of direction and value of the image split. The other way out is to place the fiber under a very small angle to the striated field visible in the background. The interference image, created in this way, is characterized by the symmetric shifts of striae. The shift has opposite sign on both sides of optical fiber axis. The shift is zero exactly on the fiber axis, measured against striae chosen as a reference one. A characteristic feature of this kind of interference is zeroing of differential optical pathways on the fiber axis. The feature facilitates considerably identification of the order of striae shifted in optical fiber. Measurements of striae shift in differential interference image of MOF

The exemplary differential interference measurements were done on several kinds of MMC optical fibers. Schematic cross-sections of several of MOF were presented in [2]. These cross-sections were in agreement with real fibers manufactured by MMC. The data on striae shifts were read from interference images obtained by interference differential contrast method. Very small image split, of the order of single µm, which gives differential interferometry, allows for observing quite readable interference images. These images are a base for geometrical shift measurements in chosen core of the fiber. The measurements were done for optical fibers placed in immersion environment of standard refractive index 525,1=mn and for wavelength λ=546nm. This particular wavelength was chosen with the aid of interference filter. The shift for the striae of zero-th order, chosen as a measurement one, for triple-core optical fiber was assessed in all cores by rotation of the fiber by 1200. This measuring situation and the resulting striated field photographed from the microscope, for this fiber presents [2]. The same way was applied in the case of different MOF. The differences in measuring method may stem for example from the fact that generally the MOF may hava a quite complicated, additional core-cladding, on-axis structure. Rotating the fiber of 1200, measurements may be done for off axis cores. Some of MOF require rotation of approximately 720 to measure the shift of striae in off-axis cores. The shift data acquisition started usually form the fiber axis, where the value of derivative dR/dy was equal to zero. This characteristic place is comparatively easy to be found on the interference image. It is a crossing point of straight-line extension to any line from the striated field from the background (for example for zero-th interference order line) with the same striae shifted in the observed optical fiber.

Somehow more difficult was assessing of the shift for the on-axis located cores in the MOF optical fibers. But, on the other hand, one has to remember of zeroing of the derivative and of the symmetry in distribution of the outside

(off-axis) cores. The part of the central core is located in the central area of the differential interference image. The image of the central core is in the neighborhood of superimposed images of the off axis cores. Determining of striae shift in MOF with the central (on-axis) core starts from the fiber axis to the direction of its circumference with omitting of areas occupied by the off-axis cores. The shift measurement method assumes a few simplifications of fiber geometry: the fiber has perfect axial geometry, the fiber is perfectly circular, cores are circular and the refractive index in the core is constant np=const. The character of changes of the cladding index np, in the investigated optical fibers, was measured by placing the fiber in an immersion fluid of the refractive index value nm exactly equal to that of the cladding nm=np. In the measurements, for triple core MMC optical fiber, it is seen that the straight-line striae from the background maintains its rectilinarity in the whole area of fiber cladding. This means that np=const. Further measurements of fiber RIPs exhibit also this feature. The geometrical shifts in striated fields in MMC multicore optical fiber images were measured using standard graphical analysis software. The analyzed images were fed to appropriate image analysis environment ant the shift was measured either manually, through a pointing device or automatically using additional image analysis procedures with contrast enhancement. Determining of RIP in MOF

The refractive index profiles in manufactured samples of MOF optical fibers were determined by the differential interference method from striae shift dR/dy determining as a function of y measured from the fiber center. The output data for computer based measurements were: nm-refractive index of immersion fluid surrounding the fiber during recording of striated field, Dmp- initial distance between striae in the image background. The refractive index profile was determined from striated field in accordance with the following equation:

( ) ( )( )

−= ∫

1

2/122

1expu uy

dyyun ϕπ

, 10 ≤≤ u ,

r=u/n(u), ( )dy

ydRD

arcymp

)(sin λϕ −= ,

where: φ - shift function, y - output coordinate. The results of measurements and calculation for MOF RIP were presented in [2]. Optical fibers from the one group of samples are characterized by lower value of refractive index on the core axis. This lowering is equal to ∆n=0,007. The maximum value of refractive index in this sample group differed by the value ∆n=0,005. Similar character posses RIPs of optical fibers from other sample. The external cores in the fibers from this group have refractive dip on the core axis equal to ∆n=0,008, slightly bigger than for previous. Bigger is also the maximum value of refractive index. Averaged maximum value was nrmax=1,595. The on-axis core of some MOF had two layers. The maximum difference between refractive indices in the core is ∆n=0,033.

A considerable axial depletion of refractive index exhibit on-axis cores of optical fibers from next two sample groups, which is equal to ∆n=0,0111, and 0,0131. The off-axis cores of fibers from these samples exhibit small local changes of refractive index, which are located not axially and equal to ∆n=0,008. The off-axis cores of fibers from next sample group have only a slight axial refractive index depletion equal to ∆n=0,004. The changes of refractive index in the cladding of all fiber sample groups are comparatively very small. They are not exceeding ∆n=0,01 on the average. Thus, we take an apologized assumption that the refractive index of the cladding is constant, np=const. Analytical, and also simple visual, comparison of refractive index profiles n(r) between the fibers in all samples reveal very small differences among them. This result says that the MMC process builds the individual cores (during pulling) of the multicore optical fiber quite homogeneously, and the diffusion processes (if any) are homogeneous. On the contrary, the sensory fiber designer may be interested in hetero-core fiber manufacturing of strictly engineered differential characteristics between cores. Measurements of MOF geometrical parameters

The researched MOF are mainly isotropic and, due to their shape, behave in transverse direction as cylindrical lenses. The diameters of core and cladding images observed in image plane of the microscope depend, to some extent, on the refractive index of the environment the fiber is submerged in (immersion). The lens like behavior of the fiber (even a fiber of complex construction, either multicore of multi-layer) can be neutralized. This neutralization is needed at fiber geometrical parameters measurements. When one measures the outside fiber diameter, the fiber can be immersed in a fluid of refractive index very close (the best, slightly smaller) than the fiber layer directly adjacent to the immersion. The accuracy of determination of the optical path difference is considerably influenced by the diameter of

the fiber core in micro-interferometric methods. This has an important meaning at the refractive index profile measurements. The shift of the interference striae, in refractively nonhomogeneous optical fiber, is determined for strictly stated distance y from the fiber axis. Determination of a particular measurement place in the fiber is connected with the knowledge of some geometrical parameters of the fiber, namely core and cladding diameters: dr and dp.

Microscopic pictures of investigated optical fibers were also obtained using the negative phase contrast method. Phase contrast microscope hardware was used of KFA type. In order to obtain a good contrast phase picture of an optical fiber, one has to set coaxially, and very precisely, a ring diaphragm, against the corresponding phase ring of chosen lens. This diaphragm is in the turret disk of the condensor. The optical fiber has to be placed in the central view field of the microscope. One has to know approximately the refractive index of the cladding, what is possible from the technological material data. The immersion environment is then prepared to match this approximate value of refractive index. The immersion environment has a slight refractive index gradient. The cladding of the optical fiber gets invisible exactly at the spot where nm=np, , or the index of cladding is equal to the index of immersion. In other words, the lightness of both objects gets the same: fiber core and background. Human eye estimates very precisely the differential lightness of adjacent objects or areas. Thus, the location of the place where nm=np is known quite precisely. The phase-contrast image of the fiber core is bright because always in fiber np<nr and simultaneously nm=np. The condition of reliable measurement of optical fiber diameter is fulfilled. This measurement can be done either directly via the microscope eyepiece or with the aid of professional PC, image analysis software. The Biolar PI microscope has a specialized OK15MK binocular eyepiece for this purpose. The accuracies of determining of geometric parameters (dimensions and shapes of core and cladding) of MOF with the aid of microscopic negative phase contrast method, using the Biolar PI microscope (manufactured by PZO), for different settings of the optical system is presented in [2]. The geometrical parameters of optical fibers were determined, most frequently, at the magnification equal to 20X. The MOF can be manufactured with dimensions ranging approximately between 10-500 µm (outside), with cores from single µm to occupying nearly whole fiber, i.e. with very thin outer cladding.

Measurements of material dispersion in MOF

Estimation of material dispersion in a multicomponent glass optical fiber is necessary for evaluation of fiber propagation properties. Multicomponent glasses have individual values of material dispersion, which may differ substantially among sample glasses. This is unlike high silica optical fibers, where material dispersion is standardized and very well defined. A lot of practical applications of soft-glass optical fibers concern the usage of white light (or polychromatic light). At these applications, in some cases, the spectral dispersion of refractive index, and in particular the dispersion of fiber numerical aperture may have a crucial meaning. Material dispersion is responsible, in digital signal transmission fibers, in pulse broadening, which is generally proportional to the second derivative of refractive index against the wavelength 22 / λ∂∂ n . In signal transmission fibers, this effect is smaller than in multicomponent glass fibers. Here, for the soft-glass optical fibers, the value of absolute difference between cladding and core indices is a strong function of wavelength: ( ) ( ) ( )λλλ pr nnn −=∆ and at some cases (of wrong glass pairs choice) the basic propagation condition of optical fiber may be endangered. The propagating conditions in the fiber disappear when

( )0=∆ cn λ , where cλ is a critical value of the wavelength for the chosen multicomponent pair of glasses (which are core and cladding of a fiber). This condition, only at first sight looks trivial, but in reality is not, as many sensor oriented multicomponent glass MMC fibers have very small value of numerical aperture. This aperture is subject to strong dispersion. Thus, the material spectral dispersion characteristics of soft-glass optical fibers have to be assessed, researched and well known, prior to fiber application.

The material spectral dispersion of refractive indices and numerical apertures was investigated in several families of MOF: triple elliptical core optical fiber of different numerical apertures in each core; double elliptical core optical fiber with different numerical apertures in each core; double circular core optical fiber of cores with different diameters. The first represents a hetero-core optical fiber. The dimensions of elliptical cores in this fiber are nearly identical, within the accuracy of dimension measurements. The numerical apertures in the elliptical cores differ by a step of δNA=0,05. From the optical wave propagation point of view, this difference is quite considerable. It means more in the case of singlemode core than in the case of multimode fiber. The cores have different brightness. The numerical aperture increases clockwise form the upper left core with the biggest value of NA. All three cores were made of different low-loss fiber optic grade, glass. Thus, all cores have different material spectral dispersion. Therefore, they have different dispersion of numerical apertures. The cores have different sensitivities for external reactions. This and similar fibers are prepared for optimized optical fiber sensory applications.

The second sample is a twin hetero elliptical core optical fiber. Again, the cores have the same dimensions but considerably different numerical aperture. This results in different brightnesses of particular cores. The numerical apertures of the cores in this fiber differ by the factor of two NA1 =2(NA2), which is quite large. Such a large value of multiplication factor requires the careful choice of core glasses of quite different values of refractive indices but fitted thermally and mechanically. Frequently, this kind of task is connected with own specialized glass synthesis for this particular purpose. Glass synthesis research for sensory optical fibers seems to emerge as a separate interesting branch of material engineering. A potentially very complex model of hetero-core optical fiber is third sample. The fiber has, for example two cores, which are different in dimensions. The two cores can have different or identical optical parameters. This particular fiber has cores of different dimensions and different numerical apertures. The fiber idea of this kind may include, for example, a model of single-mode core coupled strongly with multimode one. Several different propagation or optical conditions may be fulfilled in such a fiber leading to important photonic sensing solutions. The fiber can have two singlemode cores with slightly different propagation characteristics. The differential characteristics can then be utilized for stable polarization maintenance of propagating wave, when the cores are strongly coupled, for signal switching or for sensory purposes, including interferometric ones.

The investigations of dispersive properties of optical fibers were done with the aid of shearing micro-interferometer and interference filters. Carl Zeiss Jena filters were used and Biolar PI interference microscope. The filters had the following nominal transmission wavelengths: λ=661µm, λ=546µm, λ=480µm. The RIPs were determined using transverse interference method with differential split of the front of optical wave. The spectral dispersion of refractive index in individual cores and dispersion of the NA value was expressed by mean dispersion, assuming the following spectral standard lines: F(λ=486µm) and C(λ=656µm). The RIP dispersion in the cores and claddings of investigated MOF and spectral dispersion of the NA were presented in [2]. The value of parameter )( C

iF

iCFi nnn −=

− in the cladding of first fiber sample is constant and equals

0,0125. The value of n(F-C) in particular cores of fiber diminishes continuously from the first to the third core, from n1(F-C)=0,0135, n2(F-C)=0,0125, n3(F-C)=0,0115. The mean dispersion of NA value ])()[( C

iF

ii

CF NANANA −=−

behaves similarly. The dispersion of refractive index in the cladding of second fiber sample equals to np(F-C)=0,0160 ± 0,0005 and is constant. The dispersion in the first cladding is equal to n1(F-C)=0,0500. The dispersion in the second core is equal to n2(F-C)=0,0165. The dispersion of the numerical aperture NA is different in each cores and equals: NA1

F-C=0,02345- for the first core, and NA2F-C=0,0910 for the second core. It is to be assumed that the reasons

of all these substantial changes in the measured dispersive parameters are diffusion processes, which take place during the process of MMC optical fiber pulling. These processes are, to some extent, similar to those, which occur in single-core optical fibers. Here, however, different than in the case of a single-core optical fiber, there are several sources of migrating glass ions. This may complicate the diffusion model especially when the cores are separated only very closely from each other. Measurements of MOF polarization properties

The birefringence in optical fibers is introduced during the technological process. This concerns mainly communication fibers and the CVD process. Two basic ways to introduce birefringence in the optical fiber are: circular geometry disturbance of the fiber and the core, and freezing of internal anisotropic stress around the core. The third way is to propagate the wave in a fiber with two (or more) strongly coupled cores (single-mode). Generally speaking, the resulting fiber birefringence is a sum of shape and stress components. The birefringence affects the state of polarization of lightwave propagating in the fiber. The simplest example of a birefringent optical fiber is the one with an elliptical core. This type is called internal geometric birefringence. Stress birefringence is induced in a fiber by one or two stress agents of different shapes, which are embedded in the fiber close to the core. The MMC process with crucible separation allows for manufacturing of a few groups of basic solutions to (highly) birefringent optical fibers (referred frequently as to HB fibers). During the MMC process with crucible separation, unlike its previous usage, the innermost crucible is filled with core glass. The intermediate crucible is filled with glass, from which stress agents are pulled. The outermost crucible is of course cladding. The MMC process in this realization has a very rich collection of technological tools to design the HB fibers. In particular, the properties of the stress agents (dimensions, shapes, mechanical and thermal properties, closeness to the core, etc) can be extremely subtly tuned. Thus, the excess stress distribution in close vicinity of the fiber core can be precisely engineered. This in turn leads to introducing of strictly controlled birefringence to the fiber.

Using standard laboratory setup for observing optical Fourier transforms, the MOF were observed. Optical fibers of complex internal structure involving several cores, of different dimensions, different numerical apertures, etc have very complex optical Fourier transforms consisting of several tens of striae covering the whole transform image. In zero position, the transform totally covers the conoscopic cross. On the other hand, the Fourier transform obtained after rotating the complex multicore fiber of 900 barely covers only the parts of the conoscopic cross. The first Fourier transform is equivalent to complex, hetero-core fiber position, where optical core of the biggest value of refractive index is positioned over the central on-axis core. The second Fourier transform is, when this core (the biggest one of the biggest dimensions and the biggest value of refractive index) is totally uncovered, and the rest of cores overlap partially. All multicore optical fibers create Fourier transform, which are more or less symmetrical, what was to be anticipated. They represent different geometrical constructions and optical properties. The Fourier transforms possess total, or slightly spaced, conoscopic cross, and are characterized by plenty of slim striae. The striae appear on the whole surface of the transform with bigger or smaller density. Multicore optical fibers have many Fourier transforms, which may be observed rotating the fibers in the object plane round their long axis. Two-core optical fibers have two characteristic Fourier transforms, which are transformed each into another through several intermediate stages. Three-core optical fibers possess three most characteristic Fourier transforms. From these transforms several characteristic features of the fiber can be read. Optical Fourier transforms of fibers with five, six and more cores are a set of straight linear striated images. The width of these striae is increasing toward the center of the image. The width of striae in the central part of the image depends on the dimensions of the cores located around the fiber axis. The observed Fourier transforms reveal polarization properties of observed optical fibers. The fibers are birefringent in the direction transverse to its main axis and these polarization properties change with fiber rotation of 900. The Fourier transform images allow for subtle classification of manufactured optical fibers according to their polarization properties. The precise qualitative and quantitative evaluation of the Fourier transform interferograms is done using image analysis software.

4. CONCLUSIONS

Tailored optical fibres (TOF) are more and more frequently subject to investigations and applications in photonics systems. Assessment of their signal processing properties requires specialized measurement equipment and application o modified measurement methods.

5. ACKNOWLEDGEMENTS

The work is partially supported by the National Scientific Committee (KBN) grant no 4-T08D-22 on tailored optical fibers (ring-index) from multicomponent glasses for instrumental photonics.

6. REFERENCES 1. R.Romaniuk, J.Dorosz, Temperature sensor based on double core optical fibre, Systems of Optical Security,

Warsaw 24-25 October 2001, Proceedings of SPIE, in press; 2. J.Dorosz, R.Romaniuk, Multi-crucible technology of tailored optical fibers, Optica Applicata, Vol.28, No 4, 1998,

pp.293-322; 3. R.Romaniuk, Tailored optical fibres, Proc. SPIE, Vol. 5028, Optical Fibers and Their Applications. Białowieża

January 2002; Published in Proc. SPIE, vol. 5064 (www.spie.org)