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Fiber Strain
  1 r 2  4 6 8 10 12 14 16 18 20 22 24 §   2 -3-4-5-6 -7.-8--9- -K) 4  6 8 K 12 14 16 18 20 22 24 frequency  as V. of F c 0  2 Fig.  3  Simulation results for 7th-order lowpass filter CMOS technology, could contain four independent universaladaptors, offering  any  filter combination between  one 15th- order filter  and  four separate second-order filters, with  no time-division-multiplexing.  We  believe that this universaladaptor development  is a  significant advance  in the  practicalrealisation of precision digital filters. Acknowledgments:  The authors acknowledge  the UK  Science& Engineering Research Council  and the  contribution  of Dr. H.M. Reekie  in  developing the wave-filter theory. N.  PETRIE  14th  October  1983 J.  MAVORS.  G.  SMITH Integrated Systems GroupDepartment  of  Electrical EngineeringUniversity  of  EdinburghEdinburgh  EH9 3JL,  Scotland References 1  FETTWEIS, A.:  Digital filter structures related  to  classical filter  net- works ,  Arch. Elect. Ubertrag.,  1971,  25, pp.  79-892  LAWSON,  s. s.:  implementation  of  wave digital filter structures , Proc.  /££,  1981,  128 pp.  224-226 3  REEKIE,  H.  M.,  MAVOR, j., PETRIE,  N.,  and  DENYER,  p. B.: 'An automa- ted design procedure  for  frequency selective wave filters . ISCAS,IEEE,  May  1983,  pp.  258-2614  RUBINFIELD,  L. p.:  'A  proof  of  the  modified Booth s algorithm  for multiplication ,  IEEE Trans.,  1975,  C-24 pp.  1014-1015 TEMPERATURE DESENSITISATION  OF DELAY  IN  OPTICAL FIBRES  FOR SENSOR APPLICATIONS Indexing terms: Optical fibres, Optical sensors We demonstrate  a  significant reduction  in  delay sensitivityagainst temperature  in  optical fibres, using  a  novel tight jack-eted fibre structure. This technique  has the  cabability  of  pro- ducing fibre  in  which  the  optical delay  is  insensitive  to temperature. This  has  potential  in the  field  of  sensors  and fibre devices. Introduction:  Optical fibres are useful as interferometric sensorelements owing  to  their inherent sensitivity  to  changes  in  tem-perature, strain, pressure, electric current  and  magnetic field. 1 However, most  of  these measurements  in  sensing require  the ability  to  distinguish between  the  parameter being sensed andthe other influences which  may  have  a  similar effect  on the fibre. This  is  often achieved by using  a  reference fibre which  is subjected  to  the same influences  as  the sensing fibre, except  for the  one to be  measured. This requires careful layout  in the design  of  the sensor. Some control element  is  also included  in the reference arm  to  keep track  of  the drift induced  by  differ-ential effects, especially when temperature is  a  noise source.  In this letter we demonstrate  a  fibre coated with  a  novel materialwhich  has the  effect  of  desensitising  the  optical delay  in the fibre against temperature. Theory:  Sensing  in  optical fibres  is  possible  as a  result  of  thechange  in the  optical path length  / due to  some influencingcondition such  as a  change  in  temperature. This has the effectof altering the group index  N  as  well  as the  physical length  L of the fibre. The delay in  a  fibre can be represented by   = NL (1) where  c  is the speed of light and  / =  NL. The variation of optical delay with temperature is given by dt__}_ £[L  dN_ dT~  c{ dT   dT  2) Here we consider the temperature derivative  of  group index  N as synonymous with the phase index  n,  since  the  temperaturevariation  of  wavelength dispersion  is  small.  The  change  in group index  and  length  can be  either additive  or  subtractive.The length change  is  dependent  on the  thermal expansioncoefficient  of  the fibre which  is  small  for  silica fibres  (~  10~ 6 ).The net value is generally positive with respect  to  temperature.However, when  a  fibre  is  strained,  the  change  in  opticaldelay with respect  to  strain is given by the relation dtdodLdodN\ —  \ do J  3) where  o  is  the stress induced  in the  fibre.  In  this equation, thetwo terms within  the  brackets have opposite signs  and  hencethe overall effect  is  slightly reduced. However,  the  total effectis positive  for  an increase in stress.Consider  a  fibre coated with  a  material which  has a  coeffi-cient  of  linear expansion opposite  to  that  of  the fibre. Whenthe fibre  is  subjected  to a  temperature change, two effects willoccur. One will  be  the strain induced  by  length change  as the net result  of  the competing coefficients  of  thermal expansionsof the fibre and coating. The other effect will  be  the change  in delay due  to  the change  in  the group index  N  as a  result of thestrain effect and because  of  the temperature dispersion  of  N.  It can  be  seen that with  the  appropriate choice  of  coatingmaterial,  the  overall change  in  optical delay with respect  to temperature can  be  reduced  to  zero.  It  can  be  shown  by  anal-ysis of the composite structure  of  fibre and coating that dt 1  dN  4) where  E f  is  Young's modulus  of  the fibre,  z f  its  linear expan-sion coefficient and K  =  (A c  E c  a c  +  A f  E f  x f )/ A c  E c A f  E f )  5) Here,  the  subscripts  c  and  /  refer  to the  coating  and  fibre,respectively.  A  is the  cross-sectional area,  £ is  Young'smodulus and x the thermal expansion coefficient.Using eqns.  4 and 5, we  arrive  at the  thermal expansioncoefficient  of  the coating  for  zero temperature sensitivity when dt/dT  =  0  in the following equation:a c =(l  +K f /K E £ dN\_K ll N  do  I K, (6) ELECTRONICS  LETTERS  4th November  1983 Vol 19  No 24 1039  Table 1 DiameterPrimary coating Coeff.  of expn.Young's modulusFibre125/im250  urn +  5  x 10 7 72GNm- 2 Polymer900 ^m — -3-7 x 10~ 6 (room temp, data)-7-96 x 10~ 6 (around —25 deg C)21 GNrrr 2 where  K c  = A C E C  and  K f  = A f E f .  Using typical values forsingle-mode silica fibre and using parameters of the coating asoutlined in Table 1, we arrive at the required thermal expan-sion coefficient of the coating to be approximately-9 x 10~ 6 . Experiment:  A tight extrusion coated package was made usingan oriented polymer. The property of this polymer is its lowthermal expansion coefficient. The extrusion conditions allowthe alteration of the a from a small negative to a small positivevalue. The coefficients of linear expansion have been mea-sured, and for one sample it was noted that the temperaturesensitivity of group delay was extremely low, being reducedfrom approximately 38 ps deg 1  km 1  for the bare fibre tonear zero for the composite fibre structure, at a temperaturearound  —  20°C. In order to verify the large reduction in sensi-tivity to temperature and its application in sensors, a single-mode fibre Michelson interferometer was made 2  with eacharm approximately 35 m of the coated fibre. One arm wasplaced in a stable temperature environment near room tem-perature while the other was temperature ramped around —  25°C. A fringe count was made at the output of the interfer-ometer in order to compare it with the bare fibre subjected toa similar temperature ramping. Results:  The sensitivity of the bare silica fibre has beenmeasured 3  to be approximately  40n  rad C~   m~ l  at 0-633 ^m.This result is in good agreement with our measurements.Extrapolating this value to 1-52  fim,  the temperature sensi-tivity becomes 8-33 fringes C ^ 1  (16-6671 radC ^ 1 ). A 50040055 300E rn  ount  i 1000-- + 7 +  / + /  + /  / / -30-28 -26 tempera ture,°C -24-22 Fig.  1  Fringe count plotted against temperature  for  coated fibre Michelson interferometer had 35-4 m of fibre in the measurement arm  and 35-2 m in the reference arm. The fringe count representsdata for double the sense arm length, i.e. 70-8 m of  fibre.  Thetemperature sensitivity of the coated fibre is therefore0-774 fringes C^m 1  at 1-52 /an measurement of fringe count for the coated fibre showed anaverage of 0-774 fringes  C 1  m 1  (1-55TT  rad C 1  m 1 ). Thisrepresents a reduction in sensitivity to less than 10% of thebare fibre value. The measured fringe count data as a functionof temperature is plotted in Fig. 1, where the hysterisis is dueto the temperature difference between the thermocouple usedto measure the temperature and the actual temperature dis-tribution along the whole length of the coated fibre. As can beseen from the Figure, the slope in both directions is similarapproximately 2 deg C after the start of the measurement ineach direction of the temperature excursion.This sensitivity could be further reduced by changing themelt and extrusion conditions which in turn alters the expan-sion coefficient of the material. With the correct expansivity ofthe coating material, it should be possible to desensitise thefibre to temperature fluctuations over other, more useful tem-perature ranges. It should also be possible to insulate this typeof structure to reduce acoustic sensitivity by extruding lowcompliance materials. This would allow the manufacture ofsensors to be built for specific applications. Conclusion:  We have demonstrated a dramatic reduction inthe sensitivity to temperature of a fibre coated with a negativeexpansion coefficient material. Measurement has revealed thesensitivity to have been reduced to less than 10% of a baresilica fibre. It should be possible to further desensitise the fibreto temperature by altering the extrusion conditions. It wouldthen be feasible to produce fibre packages which are thermallyinsensitive at any desired temperature range. This type ofstructure would be most useful in sensor applications, and forthe first time allow the construction of highly stable devicessuch as fibre external cavity single-mode lasers. Acknowledgments:  The authors would like to acknowledge T.G. Ryan of ICI pic for providing the coating material and theDirector of Research, BTRL, for permission to publish theletter. R.  KASHYAP  24th October 1983 S. HORNUNG M.  H. REEVES. A. CASSIDY British Telecom Research LaboratoriesMartlesham HeathIpswich, Suffolk 1P5 7RE, England References 1  GIALLORENZI T. G. BUCARO J. A. DANDRIDGE A. SIEGEL G. H. JUN. COLE, j. H., RASHLEIGH,  s. c, and  PRIEST, R.  G.:  'Optical fiber sensor technology',  IEEE J. Quantum Electron.,  1982,  QE-18,  pp. 626-665 2  KASHYAP,  R., and  NAYAR,  B.  K.   'A single-mode fibre Michelsoninterferometer sensor'. 1st International Conference on OpticalFibre Sensors, IEE, London, 26-28 April 1983; Conference Publ. 221 3  MUSHA,  T.,  KAMIMURA,  j. and  NAKAZAWA, M.:  'Optical phase fluc- tuations thermally induced in a single-mode optical  fibre',  Appl.Opt.,  1982,  21 R DIOMETER INPUT CIRCUITREQUIREMENTS  FOR  MICROW VETHERMOGR PHY Indexing terms: Biomedical electronics, Microwave thermogra- phy Radiometer system performance requirements and problemsrelevant to the technique of medical microwave thermogra- phy  are briefly reviewed. The Dicke-type radiometer inputcircuit normally used for this application is analysed todetermine the factors limiting measurement performance. It is  shown that input circuit losses combined with aerial tobody impedance mismatch can cause significant measure-ment  errors. 1040 ELECTRONICS LETTERS 4th November  1983 Vol 19 No 24
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