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Evanescent Field Characterisation of Tapered Optical Fibre Sensors in Liquid Environments Using Near-Field Scanning Optical Microscopy

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Evanescent Field Characterisation of Tapered Optical Fibre Sensors in Liquid Environments Using Near-Field Scanning Optical Microscopy AU0019018 Evanescent Field Characterisation of Tapered Optical Fibre Sensors in Liquid Environments using Near-Field Scanning Optical Microscopy S. T. Huntington, J. Katsifolis*, P. N. Moar*, A. Roberts, L. W. Cahill* and K. A. Nugent. School of Physics, The University of Melbourne, Parkville 3052, Australia * School of Electronic Engineering, La Trobe University, Bundoora 3083, Australia Abstract Near-field scanning optical microscopy is used to measure the evanescent field surrounding a tapered optical fibre sensor in air, water and dimethyl sulfoxide solution. These measurements are found to be in good agreement with those theoretically predicted using the finite difference — beam propagation method, demonstrating the validity of our model for a number of environments. 3 1-04 ( 1. INTRODUCTION A tapered optical fibre is identical to a standard fibre except that the overall diameter of the fibre has been substantially and gradually reduced in one localised region. This reduction is generally accomplished by heating and drawing the fibre. Typically the fibre diameter in the taper region is reduced by a factor of 10-15. At visible wavelengths this tapered structure is unable to support a first order mode and as a result the mode spreads out as it travels through the tapered region and extends into the medium surrounding the taper. The extension of the field into the region immediately surrounding the taper allows the fibre taper to be used in a variety of applications including interferometric devices, biosensors, fiber dye lasers, soliton research and communications. The power loss across the tapered region can be measured and information about the external environment can be inferred.1"2'3 In order to optimise the fabrication of these devices it is necessary to characterise and model the evanescent field structure. Previously, the evanescent field distribution for a tapered optical fibre sensor has been measured in air using near-field scanning optical microscopy.4 Similar measurements have also been made for D-shaped optical fibre sensors.5 In the present work, a near-field microscope has been modified to allow for measurements of the field within a range of liquids.6'7 These measurements are of particular interest from a modeling and fabrication point of view as they are performed in the environment for which the sensor is designed. 2. FABRICATION Tapered optical fibres are fabricated using the setup shown in figure 1. In this case the flame elongation method is used8. Figure 1 shows the taper rig used for the drawing process. It consists primarily of two translation stages controlled by DC motors and a glass rotation bracket. The special glass rotation bracket and clamp assembly are attached to the 2 stages as shown in Figure 1. The rotation bracket allows the taper to be rotated at any time without introducing any stress in the taper region. In addition, the glass bracket can be easily removed from the drawing rig for easy taper transportation after fabrication. The flame head is fed by a methane/oxygen mixture and is also mounted on a 2 axis translation stage. The fiber to be tapered is first fed through the clamp/bracket assembly and then the outer clamps are used to secure the fiber to the stages. Once the fiber is secured, the region to be tapered is stripped of its single acrylate protective coating and gently wiped clean with acetone, followed by ethanol. A 2-3mm flame is then used to heat the taper region and a force is exerted by the DC motors. Typically the pulling speed used is approximately 40|am/s. For the 125um cladding, 5um core, single mode optical fibre used in these experiments this corresponds to a taper waist diameter of approximately lOum and a core diameter of about 500nm. 3. THEORY The single mode fibre taper was modeled using a finite difference beam propagation method (FD-BPM).9'10 The fibre tapers examined here are in general non-adiabatic. As a result, a propagating analysis must be used to account for the changing value of the propagation constant as the mode progresses through the taper. The FD-BPM is a process whereby an electric or magnetic field is propagated through the taper and is adaptively recalculated at each point. A special case of the wave equation must be solved in order to utilize the FD-BPM, generally known as the Helmholtz equation, V2T + k2n2x¥ = 0 (1) where ¥ is the electric field, k=2n/X is the free-space wave number, n is the refractive index of the medium, A. is the wavelength of the monochromatic source and, in cylindrical coordinates, 2 2 2 2 _ d Id Id d V — i r i i r™ ~r r~ CJ\ *~\ 2. o 2. "~\ l ' "~\ 2. ' \ ) or r or r o(p~ oz The taper is axisymmetric throughout its length which means that power cannot couple between odd and even modes, hence the term involving 0 can be dropped. Thus we assume an electric field *F of the form (3) where no is the refractive, index. Substituting this back into equation 1 and applying the paraxial slowly varying approximation yields 1 JL-+ k-(n-(r,z)- n;)w (4) zr dr r dror where n2(r, z) defines the complete taper index profile. For the special case at the origin where r=0, L'Hospital's rule is required, leading to ^?(> (5) zr dr v J The FD method is used to replace the partial derivatives in the equations 4 and 5 by the finite differences that are evaluated at nodal points on a rectangular mesh. The finite differences are then substituted into the wave equation and common terms are collected to form a tridiagonal system of linear equations that are solved for the electric field at all z positions. 4. NEAR-FIELD SCANNING OPTICAL MICROSCOPY MEASUREMENTS IN LIQUIDS The near field optical microscope employs a sub-wavelength probe that is fixed within a piezo-electric tube scanning system. By placing this probe within the "near field" of the taper, the evanescent field can be directly measured since it is converted into a propagating field within the probe. The near-field optical probes are fabricated from single mode optical fibre that has an operating wavelength of 632.8nm." A Synrad Model D48-2-11SW CO2 laser is used to heat the fibre while being drawn by a modified Sutter Instruments Model P-87 pipette pulling machine. After drawing, the sides of the probes are coating with aluminium. During this coating process the probes are orientated so that the tip of the probe is left uncoated. The result is a near field optical fibre probe that is opaque except for a sub-wavelength aperture (~100nm) at the tip. Once fabricated the near field probe is secured with an epoxy resin within the piezoelectric ceramic tube. The five quadrants of this tube are connected to high voltage amplifiers that receive inputs from a computer. By incrementing the voltages on the tube the probe is scanned in either the x, y or z direction. In the z direction the scan range is limited to about 4|am. The piezo-scanning system is therefore mounted on a stage connected to a precision stepper motor. This stage has a range of several centimeters and a minimum step size of 50nm. For the case of measurement in liquid environments, the probe must be safely placed within the liquid whilst keeping the piezo-electric scanning system clear. A mounting system was designed specifically for this purpose. Figure 2 shows a cross section of the mount, fibre taper, alignment objective (N.A. = 0.65) and the liquid. The liquid is inserted into the mount reservoir using a hypodermic needle. The liquid is delivered to the reservoir until the meniscus connects with the alignment object as shown in figure 2. Due to the proximity of the objective to the sample mount, the surface tension of the liquid is adequate to hold the liquid 'cell' in place. Typically about a 1.5cm length of the taper can be immersed in this manner. The near field probe is scanned towards the taper in the direction perpendicular to the plane of the page. The probe's small size allows it to penetrate the surface of the liquid cell without compromising the surface tension. In this manner the near field probe can be scanned within the near field of the taper safely. Light from a lOOmW Nd-YAG laser operating at 514nm is coupled into the input end of the fibre taper using a microscope objective with a numerical aperture of 0.65. The light collected by the near field probe is detected using a photomultiplier tube and is stored by the computer. A schematic of the complete system is shown in figure 3. The near field microscope was used to measure the evanescent field outside the taper for air, water and dimethyl sulfoxide solution having refractive indices of 1.00, 1.33 and 1.40 respectively. Figure 4 shows the results of these measurements. Each scan contains 128 data points, with a total integration time of approximately 34 seconds. Figure 5 shows the curves calculated using the FD — BPM and the results are summarized in table 1. There is clearly a change in the decay length as the external index is increased as predicted by the FD — BPM. The theoretical and measured decay lengths are in reasonable agreement given that the precise refractive index profile of the taper region is not known. 5. CONCLUSION A near-field scanning optical microscope has been successfully modified to measure the evanescent field surrounding a tapered optical fibre sensor in air, water and dimethyl sulfoxide solution.
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