GB2339016A - All optical sensors with direct digital response - Google Patents

Description

2339016 i 0 kLLOPr."'ICAL SENSORS WITH DIR_T:'C_T_ DIGIF-IAAL RESPONSE

-,'n4Ls invention relates _o all optical sensors with a direct digita]_ response for sensing parameters such as temperature, pressure etc.

Ther is currenlv a high interest 'n -he e L-_ L L_ sub.ect of optical sensing which Is based on the oromise of thle technology for compact, passive, high performance sensors with a hish imnunity to elect.-ical interferance.

-Lhe develoDTn.ent of --he optical fibre art provides a ready means for communicating wi.-Lh remote sensors in hazardous 4. U__ L_ -s based and/c, electrically noisy environments. Component on optical interferomell'-'ers appear to be nmost appropriat-e for --formance, and f achieving the highest levels of per technologies which are capable of realis-ing commensurate levels cf st-a-'--ility, by integrating It'-'he constituent optical elements, are showing encouracing progress.

-,he two principal performance parameters of a sensor are its sensitivity and dYnamIc range. An aztractil'on of the optical- approach is that high sensitivity is often achievable by the use o_f7 sensor elements which incorporate many optical wavelengths, a-'be; while st'll phys-cally compact, together with the L.1 L accurate measurement of extremely small phase perturbations. However, sensitivity is usually of little value without good dynamic range and the two characteristics are normally iner-ependent.

L. L L _L CA 2 A third element of consideration is the exclusiveness of the response to the particular influence being sensed, i.e. temperature, pressure, rotation, etc., although this aspect of necessary differentiation between different physical stimuli is not addressed here.

-Lhe points mentioned above can be related to sensors realised in the two classical forms of int-erferometer, the Michelson and the Mach-Zehnder as shown in -Pigs. 1A and 1B. The stimulus to be measured is appropriately appilied to one arm of the interferometer, thus nhase unbalancing the optica.-I 'bridge' which then shows a change in the output inteinslity of %the light 4ncident on -he phol-odeecor. Such simple systems suffer the disadvantage that the output signal, which varies as a raised cosine function, is only linear over a small range o-F values around an offset bias of 7/ 2' Th,is p-roblem can be overco7-,,e bv arrang--'ng that the reZerence arm, o-F the interfer-ortieter can be voltage tuned, via, say, the electro-optic effects and hence operating the sensor in a phase locked loop by applying a modulation tone and the control offset bias to this arm. -hus the inter--Ferometer is continuously balanced, the phase locked loop feedback voltage being linearly related to the input signal. Tn practice the interferometer can 23 be balanced at the -hase zero poin or t-he rad' dian point according to cons i de ra"--. ions of minimum olDtical source .r coherence requi-rement or mini-murn detector quantunt noise. Unfortunately, 'his:-hase locked loop j',node, by requiring intereroMeter, - electrical connection to the sacrifices the desirable all optical feature of the sensor.

,ccord-ng to the present invention there is nrovided an oo'ical sensor structure comprising a plurality of optical interferometer devices encased in a common sensor enclosure, each optical interferozeter con-prising a reference arm an'd, a sensing arm, the sensing arms of all the inter-ferometers being of different L J-L sensitivity, means for coupling the plurality o'Lc interferometers to a source of optical power, and means for coupling the plurality of interferometers to photodetection means whereby the outputs of the individual interferomet-ers form a digital representa 1 4 on of an analogue input stimulus common to the plurality of interferometers.

Embodiments of the invention will now be described with reference to Figs. 2-4 of the accompanying drawings, wherein:

io Figs. 2A & 2B illustrate configurations of sensor ar a%s utilising mul"ple,Mchelson and Mac'-I.ehnde.- interferomieters respectively, Fic. 3 illustrates schematically a towed array 7.n.ultiplexed fibre optic sensor system., and Fig. 4 illustrates schema t i cally a complete array element of the system of Fig. 31.

7t is possible to configure an array of interferometers of varying sizes, as shown in Figs. 2P-_ & 2B, so that the photodet-ector outputs, after passing through a comnarator, form a digital representation of L L U - the analogue input stimulus. The principle of operation depends on the sensed stimulus effecting successively increasing phase unbalances L4, 2_", 4-A6, etc. in the active regions x, 2x, 4x, etc. of- the inter- ferometer sets. Thus an analogue disturbance applied to the set provides a Gray scale digital (after thresholding) am--l-itude and z)hase response on the pa-rallel optical outputts. 2 levels are provided via n interferom.eters. There are many com'01inations of sensor activating influences, i.e. pressure, displacement, etc., and particular optical arrangements but for the purposes of general illustration the arrangement in Fig. 2(a) can be implemented with the sensing arms of the 4 set configured as a e struct L waveguid -_ - Lu."e in a thin cantilever membrane for monitoring displacement whillst 'the scheme Of Fig. 2(b) can be applied, again as a thin waveguide structure, with suitable sensitising coating to the measurement of i-,,agnetic field.

4 - in the arrangement of Fig. 2A each Michelson type interferometer is formed by an input waveguide 20a, 20b and 20c respectively and an output waveguide 21a, 21b, 21c. Each input is optically coupled to the corresponding output by means of a 3dB coupler arrangement 22a, 22b, 22c whereby half the input optical signal is coupled from the input waveguide to the output waveguide. All the input waveguides terminate in a reflector and all are of equal length. All of the output wavecuides termi-inat-e in a reflector and have lengths of x, 2x, 4x etc. The equal lengths of the input waveguide terminations fc-m the reference arms oF the respective interferome'Cers and the unequal lenaths of the output L - I _ J_ Waveguides -Eor-m 'he sensing arms of the respective interferometers. The arrangement of Fig. 2A is conveniently fabricated as an integrated optics structure in a substrate, e.g. lithium niobate, the couplers 22a-22c being fo-rmed in a rigid, relatively thick portion 200a and the termninations being formed in a relatively thin, flexible portion 200b extending from the rigid portion. The reflecting terminations are readily formed by holes (not shown) through the thin portion 200b, the holes having smooth reflecting wells which intersect the waveguides.

Half the reflect-ed o-.)t_ica1 signal from each reference arm -will be coupled into the output optical fibre, and will appear at the output together with hall t-he -eflect-ed; signal from the corresponding sensing arm (each of the couplers 22a, 22b and 22c is a bidirectional coup'er so 'he± half the reflected signal from the L - L. I sensing arm will in fact be coupled back into the input fibre). Thus as t-he relative path lengths of the reference and sense arms change so there will be amplitude and phase variaCions of the signals from each arm which are combined in the outputs. Assume, for example, 'that the portion 200b is subjected to flexing.

T,here will be alternative cancellat:cns and L - 5 reinforcements of the optical signals reflected from each -pair of associated terminations, the rate of alternation for each pair in response to a common -flexing being determined by the relative lengths of the sensing arms.

-5 When all the interferometers are subjected to a common St4MUJUS analogue L the unequal lengths x, 2x, 4x etc. of the sensing arms will provide the aforementioned successively increasing phase unbalances of L16, 2Lb, 4 L6 etc.

io in the arrancement of Fig. 2B, a set of otherwise equal Mach-ZehAnder interferometers 23a, 23b & 23c have sensing zones of length 'x, 2x, 4x etc. which, in response to a com.mon analogue st-imulus, will provide the L " 2L6, 4 Lo' etc. phase unbalances Again, a convenientLm-:_Iemientat-ion is in an integrated opt-ics waveguide structure.

7n a simple mode of operation "ne to -he - _.L L %_ basic sensitivity of the sensor me char. i sm, is determained by the length of the interaction region of the longest Lnterferomieter, with the quantization steps (noise) L - 4- e - corresponding to a phase change of 7/z. As indicated, the nu7[iber of levels of the system is e_-ual to 2 n and m i t s a r i s ing f r oT,, c o n s i d e r a t- i o n o -F 3 r a c c a 1 6 Implementat--ion factors succest 2 (or 064) would be I about the maximuni conveniently achievable. Clearly, a '-mit of about 35 dB of dynamic range would not be L L I L adequate for many applications but, fortunately, this range can be extended, t)ossibly up to the physical limits of linearity of the materia-I itself, by t1he mode of addressing the sensor.

An interesting property of the type of sensor proposed here is that as the excitation excursion is increased to take the sensor beyond its una7mbicuous range of levels, an unambiguous measure of the sensor condition can nevertheless be obtained if the sampling rate is sufficiently high for no more than a half an unambicuous excursion to occur between samples, i.e. operat_ion is effectively in a differential mode with the sensor keeping track of its condition on a bit by bit basis.

Thus by oversampling the dynamic range of a given conver'C.erl can be extended.

Clearly, the proposed sensor, and any associated optical fibre feed system, has very high speed capability and is certainly able to sau.ple at rates in the Gbit range. Hence, for example, if one takes a 4 bit, 16 level, device and samples at I Gbit., i.e. operational capability u,, to 500 MHz in the fundal-dent-al mode, oversampling by a factor of 63 with a consequential effective 1000 level operation (80 dB), is possible to 8 ME,. Z T,',,-----e of the principal characteristics of the sensors discussed here rendel- them well suited to their a-c-olication in array system.s. These characteristics are (a) -he all optical feature, ('- -) potential for high sensitivity and d-qnam-Lc range and (c) the particular convenience of short pulse sampling. Consider the linear array of sensors illustrated schematically in Fig. 3 such as might be used in an underwater 't-owed array. The t.-ansmission medium is single mode opt--Ica! 'Libre driven by a laser 30 under the control of a pulse generator 31. T_Jght is launched into the "go' -fibre 32 and is coupled into the sensor elein.ents S., S etc. Each L 21 S3 senso.- element is of the type shown in Fig. 2 and com-Drises a set of, say four, interferometers which are coup-led to the fibre 32 by a common coupler 41. ThU e U,ou-s of the elements are coupled back via a "return" ou L 1 fibre 33 which is terminated at a photodetector 34. The ao and return fibres can be enclosed in a common cable structure 37 which can be routed through a opening in the shins hull 38. With individual return fibres - for each interferomet-er a single short optical pulse will poll the sensor elements and return to -the receiver carrying sampled data on the state of the sensors at each location. For a 'Libre length of lkT-n t'A.-, ;e total transit tire is 10" L, s which would enable such a system to operate -at sampling rates of up'to 100kHz. For 500 uniformly spaced transducers a 20 nsec optical pulse will resolve the output of each sensor into contiguous time domain resr)onses at the receiver. Typically, the acoustic spectrum of interest extends up to only around 500 Hz and hence the allowable oversampling ratio (up to 100) could lead to over 60 dB dynamic range with 4 bit elements.

A practical inconvenience in the above system would arise from the necessity for four individual return lines from each sensor in the array (not shown in F;:.c,. 3), each carrying one bit of data. towever, each data bit couId be time multiDlexed within the 20 nS slot by the use of a 5 nsec light pulse, together with incremented 5 nS delays 41a, 41b, 41c, 41d inserted into successive bit outputs.;-. complete sensor in this form is illustrated schematicallY in Fig. 4. The 5 nsec oLticall r)ulse effectively samples each successive bit of successive int-erf'erometers to provide a serial time- contiguous output for the whole array. optical detection and subsequent electronic signal. processing is accomplished all the terminal station of the array, the iDhotodetector 34 z"eeding a dei-aultiplexer 35 from which the signals go to a processor 36.

---'.-.e sensor scheme disclosed, in its most arrays, comprehens-1ve i-mplennentat ion, in distributed - 1 represents a revolutionary approach to such systei-ns. The -main features of the invention are: (i) implementation can be in an all optical 30 configuration. (ii) The sensor output is, after optical detection and thresholding (via electronic ComjDarators) in L I digital form.

(iii) Extremelly fast sampling is possible because of the short resoonse time of the sensor and wide bandwidth of the transmission medium.

(iv) The dynamic range can be increased by oversampling, i.e. the inherent number of levels of the basic sensor multiplies up by the ratios of actual sampling rate to the!yquist limit.

(v) The sensor is compatible with the requirements for serial operation in an array of many units when opera-led in a time division multiplexed node with a short pulse polling the whole array within the sampling period.

(vi) According to the particular application various archiLectures are possible depending on (a) the nature of the environment being sensed, i.e.

maanetLic, pressure, temperature, etc., (b) tye It---ype of inter.'Eeromet-er which is most suitable and (c) the apportioning in the system of number of sensor bilts and oversampling ratke.

Claims (1)

1. 9 CLAIMS: - .in optical sensor structure comnris4nc a plurality of optical

   interferometer devices encased in a common sensor enclosure, each optical interferomete.- comprising a reference arm and a sensing arm, thesensing arms of all the interferometers being of different sensitivity, means for coupling the plurality of L interferometers to a source of opt-ical power, and means For coupling -he -lu-al'-y of interfLerome4-ers to T)hotodetection means whereby the outputs of the individual interferometers form a dicital rep-resentation of an analogue innut stimulus cortimon to the plurality of interferomete.,s.

   2. An onti-a7 sensor structure according to claiml wherein the plurality of interferometer devices are fibre onkic coupled to a common optical source and to a common photodetector means, the couplings from the ou".-puts of the interferometers each including a different incremental time delay whereby a single pulse of light 2C coupled into the interzeronneters r)roduces a series of L. k- - L time multiplexed outputs at 'the common photodetector means.

   3. An optical sensor s'ructure accordinq --o claim 1 or 2 wl-erein -he interferomeers are of the michelson type and each comprises an input optical waveguide terminating in a reflector and an output optical waveguide terminating in a reflector, the input and output optical waveguides being bidi.rectional-Ily coupled via a 3dB coupling means, all t-he interferometers hav-zng the same optical path length between the coupling means and one waveguide termination and different optical path lengths between t"he coupling means and the other waveguide term-ination, with all the te.-minat ions being encased in IChe common enclosure.

   4. An optical sensor structure according to claim I or 2 wherein the interferometers are of the 1.',ach-Zehnder type and comprise substantially ide.ntical optical path - 10 interferometers with each interferometer having its sensing arm coupled to a common analogue stintulus via a sensing zone, the sensing zones of all the interferometers being of different length.

   5. An optical sensor structure according to claim 3 or 4 wherein the different path or zone lengths are in a geometrical progression.

   6. An optical sensor structure substantially as described with -reference to Fig. 2a, 2b or 4 of the accompanying drawings.

   7. 7,n ontical sensor ar.ray, structure including a number of sensor elements each c6mprised of an optical sensor structure as claimed in any one of claims 1-6.

   6. An optical sensor array structure according to claim 7 wherein all the sensor elements are coupled to a common -input. optical fibre and to a cor,-Lmon output fibre, tI.-Ie elements including timme delay means whereby all the I 's in 'he array will produce time mul-Jp'exed element L L-1.L outputs in response to a single input pulse of light.

   9. An optical sensor'array structure substantially as described with -reference to Fig. 3 of the accompanying drawings.

   10. 7, method of operating an optical sensor array comr)rlsing a series of sensor elements each consisting of a set of' ontical. interferometers arranged to provide a digital representation of an analogue stimulus, wherein the inter-Ferometers of each sensor element and the series of ellements are connected to a conmon source of optical 'Cower and a comn.on photodetector means on a time multiplexed basis, characterised in that the array is operated in an oversampled mode whereby the inherent number of levels of a basis sensor element multiples up by the ratios of the sampling rate to the Nyquist limit t.o increase the dynamic range of the element.

   I I Amendments to the claims have been riled as follows interferometers with each interferometer having its sensing arm coupled to a common analogue stimulus via a sensing zone, the sensing zones of all the interferometers being of different length.

   5. An optical sensor structure according to claim 3 or 4 wherein the different path or zone lengths are in a geometrical progression.

   6. An optical sensor structure substantially as described with reference to Fig. 2a, 2b or 4 of the accompanying drawings.

   7. An optical sensor array structure including a number of sensor elements each comprised of an optical sensor structure as claimed in any one of claims 1-6.

   8. An optical sensor array structure according to claim 7 wherein all the sensor elements are coupled to a common input optical fibre and to a common output fibre, the elements including time delay means whereby all the elements in the array will produce time multiplexed outputs in response to a single input pulse of light.

   9. An optical sensor array structure substantially as described with reference to Fig. 3 of the accompanying drawings.

   10. A method of operating an optical sensor array comprising a series of sensor elements each consisting of a set of optical interferometers arranged to provide a digital representation of an analogue stimulus, wherein the interferometers of each sensor element and the series of elements are connected to a common source of optical power and a common photodetector means on a time multiplexed basis, characterised in that the array is operated in an oversampled mode whereby the inherent number of levels of a basic sensor element is multiplied by the ratio of the sampling rate to the Nyquist limit to increase the dynamic range of the element.