referring in more particularity to fig1 the present invention contemplates a passive optical tap for a single - mode optical fiber which comprises a semiconductor wafer 10 of n - type material , preferably silicon , having greater than 3 ohm - cm with ( 100 ) orientation and being 250 um in thickness . a passage well or cavity 12 is formed from one surface of the wafer 10 to the other surface by means of a uv driven dry photolytac etching process utilizing a focused laser beam produced by an ar + laser ( 350 nm line ) to induce photodissociation of gaseous cl 2 ( 3 ). the photodissociated cl 2 removes silicon reactively , forming volatile silicon chloride products . to produce a passage which can accommodate the fiber &# 39 ; s 80 um cladding diameter , the wafer is held at room temperature in 450 torr of cl 2 and irradiated for six minutes with a one watt laser beam focused to a 2 um beam waist . the diameter of the passage is approximately 120 um . once the passage is etched , the remaining processing is compatible with standard integrated circuit fabrication . ( 4 ) a p - n junction detector 14 is formed inside the passage by spinning boron - containing polymer onto the surface of the wafer formed by a high temperature drive - in . electrical contact is made to the junction at the top of the passage by maskless evaporation of aluminum to form electrical contact 16 . optical fiber 18 , which is a corning single mode fiber with 4 um glass core , 80 um glass cladding , a 250 um plastic buffer , was prepared by mechanically removing under a microscope an approximately 125 um cylindrical section of the plastic buffer . the fiber was then submersed in a buffered hf etchant solution . the plastic buffer acted as a mask so that only the section of fiber with exposed cladding was etched , along with areas of the fiber adjacent to the removed plastic buffer section . the depth of the notch was controlled by the length of the submersion time , with a 30 minute submersion time resulting in a notch depth of approximately 20 um . after etching , the plastic buffer was chemically removed with methylchloride . the characteristic taper of the notch is due to the diffusion of etchant from the exposed section of cladding to cladding underneath the plastic buffer , to thereby form a circularly symmetrical notch having an hour glass shape . the semiconductor wafer containing the detector cavity is then mounted on a standard integrated circuit package , which in turn is connected to a micro - manipulator at an adjustable tilt angle to facilitate the fiber insertion . the silicon chip containing the detector cavity is mounted beneath an 80 × microscope , on the x - y stage of the probe station . the fiber containing the fiber tap was held in a vacuum chuck mounted on a 3 - axis micromanipulator such that the vacuum chuck supported the fiber just above the fiber tap . the fiber was then threaded through the detectors cylindrical passage and the fiber tap was positioned near the top surface of the semiconductor wafer . the vacuum chuck was disengaged and replaced with a newport fm - 1 mode scrambler 20 , which provided adequate mechanical support for the fiber , at a point 4 cm above the wafer . mechanically chopped 830 nm light from a laser diode was injected into the other end of the fiber . microbends were induced in the fiber with the mode scrambler , and the notched section of the fiber was positioned within the passage of the semiconductor wafer for maximum coupling with the detector by monitoring photocurrent on an oscilloscope which received , through its 1 m ohm input , the anode and cathode of the detector , while the fiber was moved in relation to the semiconductor wafer . when the maximum coupling was achieved , the fiber was affixed to the wafer with epoxy . the size of the glass core in the fiber for a particular application is selected in dependence on the wavelength of light used in the application . accordingly , a fiber having a glass core of 4 um is used for a wavelength of 830 nm , whereas a glass core of 9 um would be used for a wavelength of 1300 nm , for example . the wafer may of course be of another semiconductor such as gaas ( gallium arsenide ), in which case the electroconductive metal could be auge ( gold germanium ), to compliment the substrate material . an optical power meter was positioned at the downstream end of the fiber ( optical bus ) to monitor the optical power remaining on the fiber after the tap . the fiber end face was covered with index matching gel to prevent back reflections which could influence the measurements . the mode - scrambler was adjusted so as to apply minimal microbending to the fiber , and the resulting fiber bus signal measured at the power meter was 660 uw with a variation of ± 1 %. the tapped signal was 0 . 4 na , or an optical power of 0 . 16 uw , which corresponds to a tapping ratio of 36 db ( fig5 a ). this low level of power was just observable above the 0 . 1 uw noise equivalent power . as microbending was increased , the tapped signal correspondingly increased without a measurable difference in bus output . fig5 b and 5c show the tapped signals ( 1 . 6 uw and 2 . 6 uw , respectively ), and the bus signals ( 660 uw for both cases ) for increasing amounts of microbending corresponding to tapping ratios of 26 db and 24 db , respectively . actual measurements of excess loss were hindered by the large uncertainty in the transmitted ( bus ) power , i . e ., ± 6 . 6 uw . a worst case estimate , however , can be made . since no excess loss was observed , the maximum value of lost power can be no greater than the difference between the power uncertainty ( 13 . 2 uw ) and the tapped power ( 2 . 6 uw ). hence , the lost power may be as large as 10 . 6 uw . this corresponds to a maximum loss ( defined as the fraction of power input which is lost ) of 1 . 62 % ( 0 . 07 db loss ), which is on par with the best commercially available taps . the magnitude of the microbending controls the amount of power transferred from the core to the cladding . in this way the tapping ratio may be continuously adjusted . when the microbending is small , a high impedence tap is obtained . its properties of low loss and high impedance make the integrated fiber - optic tap an appropriate choice for systems which distribute optical signals to a large number of nodes on a repeaterless single - mode fiber bus . the underlying principles which permit the integrated fiber - optic tap to transfer optical power from the core to the cladding , and to remove this power in a controlled , low - loss fashion , are described below . these principles give rise to certain functional features which are unique to this integrated fiber - optic tap . in particular , a proportionality exists between excess loss and tapping ratio . single - mode fiber may be modelled as two superimposed waveguides : a low - loss single - mode waveguide defined by the core radius and a high - loss multimode waveguide defined by the cladding radius . the presence of the core has negligible effect upon the guiding properties of the cladding provided core radius is much less than the cladding radius , and the normalized difference between the core and cladding index is much less than 1 . ( 5 ) the higher order cladding modes are substantially lossier than the guided core mode , due to the poor optical quality of the exterior cladding interface . high - loss modes are uncoupled from the low - loss modes according to coupled power theory ( 6 ), hence the cladding and core modes are uncoupled in a single - mode fiber . however , when the fiber is deformed , the losses of the core and cladding modes are comparable . as a consequence , power is coupled into the cladding . ( 5 , 7 ) the fraction of optical power in the core coupled into the cladding is a function of the magnitude of the deformation and length over which coupling occurs . the proposed device taps cladding modes , therefore the amount of optical power available for tapping is equal to the power coupled into the cladding . severe deformation of the fiber results in coupling of cladding modes to radiation modes , which contribute to the excess loss of this device . therefore , the fiber should be deformed in a way that maximizes the core to cladding coupling while minimizing the cladding to radiation coupling . experimental results ( 8 ) indicate that deformations applied via microbending fulfill this requirement . however , the poor optical quality of the cladding &# 39 ; s outer interface results in some loss to radiation modes . the fraction of optical power in the cladding , lr , which is radiated and lost in this manner is related to the intrinsic loss due the poor optical quality of the cladding - air interface expressed in db / cm , and the distance , in cm , between the point where cladding modes are induced by microbending and the point where they are tapped . radiation loss lr represents one source of loss in the integrated fiber - optic tap , and results in a decreased cladding power . radiation losses are reduced in the integrated tap structure by minimizing the distance between the point where cladding modes are induced and where they are tapped . ideally , this distance should be zero ; that is , the cladding modes should be induced at the tap itself . however , mechanical constraints dictated by the photodetector and microbender structures usually prevent this . light is removed from the cladding at the fiber tap , which is circularly symmetric notch etched into the cladding . the notch causes most of the remaining optical power traveling in the cladding to be radiated . the transfer of power from the cladding modes to radiation modes may be described in two ways : rays which impinge upon the tapered edge of the notch are removed as described by snell &# 39 ; s law , and the decreased cladding radius in the notch supports fewer modes . in any event , the result is the preferential removal of the higher order cladding modes . another form of loss intrinsic to the integrated fiber - optic tap results from cladding power not removed at the notch . because the notch radius , is greater than the core radius , some of the cladding power will continue to propagate beyond the notch , and will be eventually lost as radiation modes . the fraction of optical power in the cladding entering the notch which is not removed by the notch , l p , is a monotonically increasing function of radius provided core radius is less than notch radius which is less than cladding radius . if the radius at the notch were allowed to equal the radius of the core , all of the cladding power entering the notch would be tapped and the loss of optical power of the notch would equal zero . although this would improve device performance with respect to loss , it would likely introduce a significant increase in fabrication complexity . when the notch radius approaches the radius to which the core mode &# 39 ; s evanescent field extends into the cladding , tapping occurs by evanescent coupling to the core power as well as radiative coupling to the cladding power . because the evanescent field decreases exponentially with radius , tapping both core and cladding power results in higher sensitivity to variations in the notch radius than when only cladding power is tapped . therefore , notch radii which are less than the penetration radius of the evanescent field impose stricter tolerances upon the fabrication of the notch . to illustrate the significant effect that the two intrinsic loss mechanisms have upon the operation of the integrated fiber - optic tap , the loss parameters will be used to predict the macroscopic operating parameters of such a device . the total optical power lost , p 1 , due to the two intrinsic loss mechanisms is : where c = the fraction of optical power in the core coupled to the cladding , and p in = initial optical power guided by the core . clearly , the sum of the lost and tapped power must equal the power coupled into the cladding . the total power tapped , p t , is then : a more meaningful definition of overall excess loss is the fraction of initial optical power which is lost : likewise , the total tapping ratio , t , may be expressed as the fraction of initial optical power which is tapped : note that both l and t are directly proportional to c , therefore the loss and tapping ratio may be controlled by microbending . as c is varied , the modal power distribution within the cladding remains constant provided the coupling between the individual cladding modes has achieved a steady state distribution . ( 9 ) a constant modal distribution guarantees that l p , l r , and consequently a remain constant with varying c ; that is , loss is directly proportional to tapping ratio . to obtain a steady state modal power distribution , a mode - scrambler is used as the microbender . a mode - scrambler not only induces cladding modes , but causes the redistribution of these cladding modes into a steady state distribution . the advantages of the fiber optic tap according to the invention become apparent when the performance of a system using the tap is quantified . for these calculations , a repeaterless linear optical bus with n nodes is assumed ( fig2 a ). bus power entering node n from the previous node is p n - 1 . at each node a fraction t n of this bus power is tapped , and a fraction l n is lost ( fig2 b ). bus power entering the next node is therefore assuming t n is identical for all nodes ( t n = t ), the minimum tapped power will be at the last node , n . this will determine the number of receivers ( for a given sensitivity ) that can be supported by the bus : p tn , the equation for the tapped power at the last node (&# 34 ; tapped power equation &# 34 ; referred to later ), is : here , p 0 is the initial power injected into the bus , n is the total number of nodes , t is the tapping ratio , and l is the node fractional loss . fiber attenuation , connector losses , and splice losses are ignored . ( for applications such as vlsi optical clock distribution this is expected to be realistic ). bus optimization by equating the power tapped at each node , rather than by equating the tapping ratio at each node as we have done , will result in a bus which can support more nodes for a given injected power . however , our intention here in this analysis is to highlight the differences between conventional taps and the integrated fiber - optic tap , not to design the most efficient bus , and the assumption of equal tapping ratios results in expressions which are much more illustrative . p tn can be maximized with respect to the tapping ratio . consider an ideal bus , i . e ., a bus comprised of lossless taps . taking the derivative of this latter equation with l = 0 , and setting this expression equal to zero yields an optimal value for t : the first term on the right hand side of this expression is the injected power , the second represents the power tapped at the previous n - 1 nodes , and the third is the tapping ratio . for real taps , the value of t opt is sensitive to the loss mechanism , as demonstrated below . case 1 . assume that the fractional losses at each node are fixed at some value l , regardless of the tapping ratio t . this describes typical commercial devices , i . e ., l = 0 . 023 ( 0 . 1 db loss ) for a good fused coupler . taking the first derivative of the &# 34 ; tapped power equation &# 34 ; and setting to zero yields an optimal value for t : or in db , [ p tn ] opt ( dbm )= p 0 ( dbm )+( n - 1 ) 10log ( 1 - 1 / n )+ n10log ( 1 - l )+ 10log1 / n case ii . assume that the losses at each node are proportional to the tapping ratio ( l = at ), as is the case with the integrated fiber - optic tap . substituting at for l in the power at the last station is or in db , [ p tn ] opt ( dbm )= p 0 ( dbm )+( n - 1 ) 10log ( 1 - 1 / n )+ 10log ( 1 /( a + 1 ))+ 10log ( 1 / n ). the property of proportionality between loss fraction and tapping ratio constitutes a fundamental difference between the integrated fiber - optic tap and conventional taps such as fused couplers . comparison of the case of proportional loss , and the case of fixed loss , with the ideal case , highlights the advantage of the integrated fiber - optic tap : the case of fixed loss differs from the ideal case by a factor of 10nlog ( 1 - l ), which grows with order n , while the case of proportional loss differs from the ideal case by a factor of 10log ( 1 / a + 1 ), independent of n . the contrast is marked by system performance when n is large ( greater than 100 ), as illustrated in fig3 . when l does not equal zero and is fixed , the system performs closely to the ideal case for n small , but deviates drastically as the accumulation of losses becomes prohibitively large for n greater than 100 . when losses are proportional to tapping ratios , the system performance parallels the ideal case regardless of the number of stations and the impact of excess loss is muted ; for a tap efficiency of 50 %( a = 1 ), power is down 3 db from the ideal case ; for an efficiency of 33 %( a = 2 ), power is down only 5 db . as illustrated in fig3 this becomes very important for n greater than 100 . although loss considerations indicate that an arbitrarily large number of taps may be cascaded on the bus given an arbitrarily small t opt , there is a limit for n dictated by the minimum detectable power of the photodetectors . for an optical wavelength of 850 nm , a bit error rate of 10 - 9 , and a transmission rate of 500 mb / s , the minimum detectable power is found for three general classes of detectors : the maximum number of stations is determined by replacing [ p tn ] opt with the minimum detectable power in equations above and solving for n . fig4 illustrates the significant difference between a conventional fixed loss tap and an integrated fiber - optic tap according to the invention with respect to maximum allowable number of nodes . although the two horizontal axes of fig4 a and l , have no direct relation , they both span reasonable ranges for comparative purposes : fixed losses range from zero to 10 %, and loss proportionally , a , ranges from zero to 4 ( corresponding to a tap efficiency of 20 %). for the ideal case of zero loss both types of taps converge to the same limit . however , for non - zero losses the number of nodes supported by the integrated fiber - optic tap decreases slowly while the number of nodes supported by the fixed loss tap decreases rapidly . the photodetector responsivity is lower than expected for a silicon p - n junction by one or two orders of magnitude . one reason for this appears to be related to the surface spin - on technique used to fabricate the junction . this method of impurity deposition is probably not optimal for the particular cylindrical well or cavity geometry and probably results in a junction depth which narrows substantially with well depth . the shallow junction depth and the large absorption length of 830 nm light would certainly account for the poor responsivity . since the detector fabrication may be compatible with standard integrated circuit processing , the detector &# 39 ; s associated circuitry , such as a preamplifier and filters , could be located on the chip in close proximity to the well or cavity . this would likely alleviate the effects of the emi and electronic cross - talk associated with sending a small photocurrent across lengths of wire , and take full advantage of the tap &# 39 ; s integrated structure . in summary , the use of cladding modes in a single - mode fiber for optical tapping has been shown to exhibit tapping ratios as high as 36 db , and holds the potential of exhibiting tapping ratios limited only by the fiber &# 39 ; s intrinsic loss . excess loss on par with the best commercially available devices is obtainable ( 0 . 07 db loss ). the property of loss proportional to tapping ratio allows the present invention to be employed to support significantly more stations on a continuous , repeaterless bus than conventional , fixed loss devices . for a large number of stations , the amount of tapped power at the last in a series of n stations decreases with order n for the case of fixed losses , but decreased independent of n for the case of proportional losses . the ability to easily control and adjust the tapping ratio allows an optical bus to be expanded in situ without forfeiting optimal tapping ratio . an active tap , one which can be turned on or off by electro - mechanical control of the microbending mechanism ( by a piezoelectrical device , e . g . ), is also possible , as well as a tap which can be dynamically adjusted to account for changing network conditions . the implementation of this tapping technique via the novel integration of photodetector and fiber tap provides mechanical stability and compatability with current integrated circuit technology . the processes involved in fabricating this device do not hinder the use of standard circuit processing techniques . this compatability may provide for the assimilation of this device into large vlsi multiprocessor systems as the means for optical distribution of timing signals . the integrated fiber - optic tap may also be useful as a receiver for optical interconnection between chips , particularly in micro - area networks . ( 10 ) the large number of stations which can be supported by a bus incorporating this device makes its use in high bandwidth broadcast networks , such as digital video broadcasting , also particularly appealing . although a preferred embodiment of the invention has been disclosed and described herein , it should be understood that the present invention is in no sense limited thereby and its scope is to be determined by that of the appended claims . malcolm h . hodge and james a . moras , &# 34 ; a hard clad silica fiber multi - tap bus system ,&# 34 ; presented at foc lan &# 39 ; 86 , orlando , fla . 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