Abstract:
An optical tap is provided that includes an input waveguide having a first width for receiving an optical signal and a tap waveguide having a second width. The tap waveguide is coupled to the input waveguide in a junction region. An output waveguide, which has a third width, is coupled to the input waveguide in the junction region defined by the intersection of the input and tap waveguides. The input waveguide, tap waveguide and output waveguide respectively have input, tap and output longitudinal, centrally disposed optical axes. The input and tap axes define a first acute angle therebetween and the input and output axes define a second acute angle therebetween. A tapping ratio is defined by a ratio of optical output power from the tap waveguide to optical output power from the output waveguide. The tapping ratio is determined at least in part by the first, second and third widths and the first and second angles. The first, second and third widths and the first and second angles have values selected to produce a specified tapping ratio.

Description:
FIELD OF THE INVENTION 
   The present invention relates to the fabrication of an optical waveguide device for tapping out a small amount of power from a light signal guided in a planar waveguide. The invention discloses a compact and low-loss optical structure that taps light out with low excess loss. The response of the optical tap structure can also be substantially independent of the wavelength and the polarization of the light signal. 
   BACKGROUND OF THE INVENTION 
   The manipulation of input and output light signals to and from optical fiber transmission lines generally requires that the signals be processed in some fashion, examples of which might include amplification, power splitting or the addition and/or dropping of signals. With the persistent trend towards miniaturization and integration, the optical circuits which best serve these processing functions are more and more being integrated on optical chips as a single module. The resulting optical circuits, which carry channel waveguides as their fundamental light-guiding elements, are generally referred to as planar lightwave circuits or PLCs. Current planar waveguide technology typically prepares a PLC by depositing a sequence of three glass films (lower cladding, core and upper cladding) on a rigid planar substrate and utilizing photolithography to pattern the required waveguide and component designs into the core layer. The refractive index of the core composition is chosen to be larger than those of the cladding layers to ensure good optical confinement within the core waveguides. 
   In optical networks it is necessary to monitor the level of the propagating light signal at several points in the system. As more and more functions are integrated in photonic lightwave circuits, integrated tapping devices, tapping a small fraction of the light, are needed to monitor the signal power. Although Y-branching circuits with equal power division are fundamental building blocks for optical signal processing devices, any asymmetric adaptation of this form with a branching angle large enough to achieve compactness is unable to tap out a sufficient power fraction for many applications. An optical tap representing the current art typically comprises a pair of side-by-side channel waveguides, or directional couplers, in which structure the light signal in one waveguide is evanescently coupled to the other waveguide. The fraction of light tapped-out (tap efficiency) is controlled by the distance between the two waveguides and the by the length along which they couple. Unfortunately, the optical response of a directional coupler in general depends strongly both on the polarization and wavelength of the light signal to be tapped, a characteristic that is undesirable for a versatile optical network component. 
   Two types of integrated optical taps have been proposed that are both polarization independent and wavelength insensitive.  FIG. 1  illustrates the optical tap proposed by Henry et al. (U.S. Pat. No. 5,539,850). The invention comprises two directional couplers  101  and  102  in series in which the second coupler  102  compensates for the wavelength and polarization dependencies of the first coupler  101 . The light signal is input at port  103  and most of it exits at port  104 , while a small amount is tapped off to port  105 . This design, however, has several disadvantages. For example, the size of such a coupler cascade is large (typically a few mms), and it also possesses an inherent loss mechanism due to light dumped from port  106  of the device. A different design for a compact integrated tap has been disclosed by Adar et al. (U.S. Pat. No. 5,276,746) and is illustrated in FIG.  2 . It utilizes the guide-interaction properties of an X waveguide crossing to tap out a low level (−20 dB to −60 dB) signal. Light signal is input in port  201 , passes through the X-crossing  202  and most of the light exits at port  203  while a small amount of power is tapped off to port  204 . Due to symmetry, light can also be input at port  205 , in which case most of the light exits at port  204  and a small amount will be tapped off to port  203 . This design is also polarization independent, but the signal power fraction that can be tapped out using a crossing angle large enough to achieve device compactness is (as is the case for the Y-junction) insufficient for many applications. Moreover, the low index contrast between the cladding and the waveguide core materials, combined with the large crossing angle (&gt;10 degrees), results in a low tap efficiency. 
   The mechanism of light transfer between the arms of a pair of intersecting waveguides is, at least for small crossing angles, qualitatively similar to that of a directional (i.e. evanescent) coupler with variable inter-guide separation. At the X-branch geometric crossover between two guides A and B, the incoming optical field (say in branch A) can be pictured as the sum of equal-amplitude symmetric and antisymmetric component fields in the two incoming branches. Where they begin to interact on approach to the junction, these two component fields will in general develop different velocities (and possibly different rates of attenuation). In the output branches the two fields (minus their radiative and absorption losses) can be recombined taking their relative phase shifts into account. A phase shift of π/2, for example, would cause light to be wholly transferred from A to B. More generally the degree of transfer from A to B at any point of the crossover will depend on the phase difference accumulated to that point and, for small crossing angles (with a large interaction length) the light power may alternate back and forth several times before emerging from the crossing. The final degree of transfer therefore depends on the total phase difference accumulated over the entire crossover region. In this simple picture (see, for example, Bergmann et al.,  Applied Optics  23, 3000-3003 (1984)) the fractional power transferred between the waveguides is approximately periodic in the reciprocal of the crossing angle θ with a period that depends sensitively on the magnitude of the guide refractive index contrast Δn=n(core)−n(cladding) in the crossing regime. As a result of this sensitivity, most of the current applications of waveguide crossing structures are in the field of optical switches, and are based on the use of an external (electro-optic, magneto-optic, acousto-optic or thermo-optic) stimulus to modulate Δn in the region of the crossing. 
   At crossing angles larger than a degree or two the periodicity in 1/θ ceases and the power-fraction transferred from the signal waveguide to the tap waveguide decreases rapidly to extremely small values at larger crossing angles. Unfortunately, this is the angular region of relevance for the formation of compact waveguide-crossing taps. 
   SUMMARY OF THE INVENTION 
   The present invention demonstrates a manner in which the X-geometry of the simple waveguide crossing can be modified to greatly increase the fractional power tapped out in the angular regime appropriate for use with compact taps. Significantly, this same modification does not increase the loss (or fractional power transfer from channeled to radiative modes) associated with the tap. 
   The invention is directed to an integrated optical tap comprising an input waveguide, a tap waveguide, and an output waveguide, all meeting at a common junction. The input waveguide carries a light signal, from which the tap waveguide carries away a small amount of power, while another, an output waveguide, also originating from the junction, carries away most of the power. Another, a ‘blind’, waveguide may originate from the junction positioned on the opposite side of the input waveguide from the tap waveguide. The offset between the center axes of the tap waveguide and the blind waveguide can be adjusted to increase both the magnitude of the tapped power and a ‘figure of merit’ defined by the ratio of tapped-out power to scattering (radiative) loss. A taper may be added near the intersection of any two waveguides near the junction to increase further the fractional power tapped out and to decrease scattering losses. The response of an optical tap of this kind is substantially independent of the wavelength and the polarization of the light signal propagating in the waveguide. 
   In accordance with one aspect of the present invention, an optical tap is provided that includes an input waveguide having a first width for receiving an optical signal and a tap waveguide having a second width. The tap waveguide is coupled to the input waveguide in a junction region. An output waveguide, which has a third width, is coupled to the input waveguide in the junction region defined by the intersection of the input and tap waveguides. The input waveguide, tap waveguide and output waveguide respectively have input, tap and output longitudinal, centrally disposed optical axes. The input and tap axes define a first acute angle therebetween and the input and output axes define a second acute angle therebetween. A tapping ratio is defined by a ratio of optical output power from the tap waveguide to optical output power from the output waveguide. The tapping ratio is determined at least in part by the first, second and third widths and the first and second angles. The first, second and third widths and the first and second angles have values selected to produce a specified tapping ratio. 
   In accordance with another aspect of the invention, the second acute angle is nonzero and the input axis and the output axis intersect in the junction region at a point offset from an intersection between the tap axis and the input axis in the junction region. 
   In accordance with another aspect of the invention, at least one of the first, second and third widths differ from the other widths. 
   In accordance with another aspect of the invention, the first, second and third widths are substantially equal to one another. 
   In accordance with another aspect of the invention, the selected values of the first, second and third widths and the first and second angles are further selected to enhance a tapping figure of merit defined by a ratio of tap efficiency to scattering loss. 
   In accordance with another aspect of the invention, the junction region includes at least one tapered waveguide section. 
   In accordance with another aspect of the invention, the optical tap also includes at least one power transfer enhancing (PTE) waveguide having a fourth width and a PTE longitudinal, centrally disposed optical axis. The PTE waveguide is coupled to the input waveguide in the junction region. The PTE waveguide couples therethrough substantially none of the optical signal. The PTE axis and the input axis define a third acute angle therebetween. 
   In accordance with another aspect of the invention, the PTE axis and the output axis are nonparallel. 
   In accordance with another aspect of the invention, the PTE axis and the input axis intersect at a point offset from the intersection of the tap axis and the input axis. 
   In accordance with another aspect of the invention, an optical tap is provided that includes an input waveguide having a first width for receiving an optical signal and a tap waveguide having a second width. The tap waveguide is coupled to the input waveguide in a junction region. An output waveguide, which has a third width, is coupled to the input waveguide in the junction region defined by the intersection of the input and tap waveguides. The input waveguide, tap waveguide and output waveguide respectively have input, tap and output longitudinal, centrally disposed optical axes. The input and tap axes define a first acute angle therebetween. The input and output axes define a second acute angle therebetween. The junction region includes at least one tapered waveguide section. 
   In accordance with another aspect of the invention, a method is provided for tapping a desired portion of optical power from an optical signal. The method begins by providing an optical tap that includes an input waveguide having a first width for receiving an optical signal, a tap waveguide having a second width and being coupled to the input waveguide in a junction region, and an output waveguide having a third width and being coupled to the input waveguide in the junction region defined by the intersection of the input and tap waveguides. The input waveguide, tap waveguide and output waveguide respectively have input, tap and output longitudinal, centrally disposed optical axes. The input and tap axes define a first acute angle therebetween and the input and output axes define a second acute angle therebetween. The method continues by directing the optical signal though the input waveguide of the optical tap. Values for each of the first, second and third widths and the first and second angles are selected to produce a specified tapping ratio that gives rise to the desired portion of optical power at an output of the tap waveguide. The tapping ratio defines a ratio of optical output power directed through a tap waveguide to optical output power directed through an output waveguide. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  Schematic of a prior art optical tap comprising two cascaded directional couplers. 
       FIG. 2  Schematic of a prior art optical tap comprising a waveguide crossing. 
       FIG. 3  Schematic of a waveguide configuration that defines the geometry of the optical waveguide tap of the present invention. 
       FIG. 4  Schematic of a waveguide configuration of  FIG. 1  with the addition of a blind waveguide. 
       FIG. 5  Schematic of a waveguide configuration of  FIG. 2  where the input and output waveguides are aligned. 
       FIG. 6  A plot of the fractional power tapped T and the fractional power L lost by scattering out of the guide channels as a function of offset distance for one specific embodiment of the invention with angles α 1  and α 2  of  FIG. 5  both equal to 8 degrees. 
       FIG. 7  A plot of tap efficiency and loss as functions of reciprocal angle 1/α for
         an embodiment of the invention where angles α 1  and α 2  of  FIG. 5  are both equal to α.       
       FIG. 8  Schematic of an embellishment of the optical tap configuration of a)  FIG. 4 and b )  FIG. 3  showing the addition of triangular tapers positioned to enhance tap performance. 
       FIG. 9  Schematic of an embellishment of the optical tap configuration of  FIG. 4  showing the addition of a four triangular tapers with pairwise parallel edges, positioned to enhance tap performance. 
       FIG. 10  Schematic of an embellishment of the optical tap configuration of  FIG. 4  showing the addition of a single triangular taper positioned to enhance tap performance. 
       FIG. 11  A plot of tap efficiency and loss as a function of taper thickness H for an embodiment of the invention including one taper. 
       FIG. 12  A plot of tap efficiency and loss as a function of wavelength for incoming light signals with TE and TM polarization, using a specific embodiment of the invention including a taper. 
       FIG. 13.   a  Schematic cross-sectional top view of an integrated optical tap monitor. 
       FIG. 13.   b  Schematic cross-sectional side view of an integrated optical tap monitor. 
   

   DETAILED DESCRIPTION 
   It is worthy to note that any reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment. 
   Embodiment 1 of the invention is a waveguide structure as shown in FIG.  3 . The structure comprises an input waveguide  301  of width w s1 , a tap waveguide  302  of width w t , and an output waveguide  303  of width w s2 . The three waveguides meet at a junction  304 . We denote the acute angle enclosed by the input and the tap waveguides by α, and the acute angle enclosed by the input and the output waveguides by β. The operation of the tap is as follows. A light signal is input at the input port  305 , propagates through waveguides  301  and  303 , and most of the signal power is transmitted to the output port  307 . In the junction region  304 , some of the signal power is transferred into the tap waveguide and travels to the tap port  306 . 
   Embodiment 2 of the invention is a waveguide structure as shown in FIG.  4 . The structure consists of input, tap and output waveguides  401 ,  402  and  403 , with widths w s1 , w t1 , and w s2 , respectively, as in the previous embodiment. We add a blind waveguide  404  of width w t2  to the optical tap to improve its performance. The blind waveguide is a waveguide section which couples substantially zero portion of the light signal. The blind waveguide preferably ends in a non-reflecting waveguide termination  408  so that light is not reflected from the optical tap structure should there be a light signal propagating from tap port  406  or from the output port  407 . The acute angle enclosed by the input and the tap waveguides is denoted by α 1 , the acute angle enclosed by the input and the blind waveguides is denoted by α 2 , while the acute angle enclosed by the input and the output waveguides is denoted by β. The light signal enters the optical tap structure at input port  405 , propagates through waveguide  401 , and most of the signal power is transmitted to the output port  407 . In the junction region  409 , some of the signal is transferred into the tap waveguide and travels to the tap port  406 . The blind waveguide aids in optimal power transfer to the tap waveguide by turning the signal wavefront towards it. 
   Embodiment 3 of the invention is a waveguide structure as shown in FIG.  5 . This embodiment is a specific case of Embodiment 2, where the angle β is zero. In this case the blind and tap waveguides  502  and  504  are parallel to each other. The center axis of the blind waveguide  504  can be offset with respect to the center axis of the tap waveguide  502  to achieve optimal power transfer to the tap waveguide. The offset dimension is defined as the distance between the intersection  510  of the center axes of the input and tap waveguides and the intersection  511  of the center axes of the input and blind waveguides. The offset can take either positive or negative values, depending on whether the intersection  510  is closer or farther away than intersection  511  to the input port  505 . Therefore if the offset is positive, as is the case in  FIG. 5 , the device is effectively shorter than with zero offset. More generally, a deviation of the angular ratio α 1 /α 2  from unity can be added to the offset as a second optimization variable. 
   Embodiment 4 of the invention is a specific case of Embodiment 3, where the angles α 1  and α 2  are both equal to α. The optical tap is constructed from a set of channel waveguides made of a doped silica glass of refractive index of 1.45177 embedded in a silica cladding material with a refractive index of 1.444 at 1.55 μm. The width of all waveguides is the same: w s1 =w t1 =w s2 =w t2 =3 μm and the angle α=8°. Working throughout at a vacuum light wavelength of 1.55 μm, we calculate the tap efficiency T and the scattering loss L as a function of the waveguide offset using the two-dimensional beam propagation method (see for example, C.L. Xu et al.,  Journal of Lightwave Technology,  12, 1926-1931 (1994)). The quantities T and L are expressed in dimensionless form as a fraction of the power P i  into the input port in the form
 
 T=P   t   /P   l   ; L=[P   l   −P   t   −P   o   ]/P   i ;
 
   where P i  and P o  are respectively the powers exiting through the tap and output ports. These quantities are plotted in  FIG. 6  as a function of the waveguide offset. At zero offset, where the optical tap structure is similar to a common X waveguide crossing, the tap efficiency is below 0.01%. However, if we set the waveguide offset to +30 μm, the tap efficiency increases to 5.8%. Although the tap efficiency improves by a large factor, the scattering loss does not change significantly even after the introduction of a large offset. 
   The response of this optical tap structure cannot be described in terms of simple coupled-mode theory as has been done for simple waveguide crossings in the prior art. To demonstrate this, in  FIG. 7  we plot the tap efficiency and the scattering losses as functions of the inverse angle 1/α, for 3 μm wide waveguides with zero offset. While, for large values of 1/α (small angles), the functional form of the tapped power is sinusoidal as predicted by coupled-mode theory, for 1/α&lt;0.15, this periodicity clearly breaks down. In the region of larger angles, where compact optical taps are possible, the tapped power in  FIG. 7  is seen to be extremely small. However, in this same regime, the tapped power is a strong function of the offset between the tap and the blind waveguides, as exemplified in FIG.  6 . In this regime the physical behavior of the optical tap is more appropriately described by taking into account the full set of local guided and radiation modes. As the guided light in the input waveguide enters the junction region, the mode will couple to a large set of radiation modes in addition to the guided modes existing there. At the far end of the junction, both local guided and radiation modes combine to couple to the guided modes in the tap and the output waveguides. Finally, they also couple to radiation modes, causing the observed scattering losses. 
   Embodiment 5 of the invention is illustrated in  FIG. 8   a . The optical tap consists of the waveguide structure in Embodiment 2, with input, tap, output and blind waveguides  801 ,  802 ,  803  and  804 , respectively. To improve the performance of the optical tap, we add a set of triangular tapers  806 ,  807 ,  808 , and  809  near the waveguide junction  805 . The taper can be made of the same core material as the waveguides. Tapers  806  and  808  assist in redirecting a portion of the light traveling in the input waveguide  801  into the tap waveguide  802  by turning the wavefront of the light signal towards the tap waveguide. At the same time, by effectively increasing the overall width of the waveguides near the junction, the tap enables the accommodation of more guided modes in the primary coupling regime and thus reduces scattering losses. The dimensions of the taper can be appropriately designed such that scattering losses are minimized while maintaining relatively high tap efficiency. Nothing in this embodiment is intended to imply that the geometric shape of the taper be restricted to the linear or straight edge triangular form depicted in FIG.  8 . The taper can have any other functional shape without departing significantly from the spirit of the invention. 
   The optical tap of Embodiment 1 can also be modified in the same manner by adding tapers near the junction to improve its performance as illustrated in  FIG. 8   b . 
   Embodiment 6 of the invention is illustrated in FIG.  9 . This embodiment is a specific case of Embodiment 5, where each of the four tapers is a triangle and the two sides  901  and  902 , as well as the two sides  903  and  904  are parallel. 
   Embodiment 7 of the invention is illustrated in FIG.  10 . The optical tap is a specific case of Embodiment 5, where only the taper between the input and the tap waveguides has nonzero dimensions. The taper is a triangle  1001  bounded by sides  1002 ,  1003  and  1004  near the junction  1005 . We denote the angle enclosed by the input waveguide and the tap waveguide by 2φ. The dimensions of the taper can be defined with reference to  FIG. 10  by the angle φ+γ enclosed by the sides  1002  and  1003  (with −φ&lt;γ&lt;φ being a measure of the deviation of the taper from isosceles triangular form γ=0), and by the length H of the angular bisector of the obtuse angle opposite side  1002 . 
   Embodiment 8 is a specific case of Embodiment 7, where the waveguides and the taper are constructed using the material system of Embodiment 4 with the same waveguide widths, waveguide offset and angles. We plot the response of the optical tap against H with γ=−1° in FIG.  11 . As H is increased from 0 to H=1.4 μm, the tap efficiency doubles from 5.8% to 9.4%, while at the same time the scattering loss decreases from 4.1% to 2.5%. As a cumulative measure of optimizing the waveguide offset and the taper, a figure of merit for the optical tap, defined as the ratio of the tap efficiency to the scattering loss, increased from 0.01%/4.1%≈0.0024 to 9.4%/2.5%=3.76, or more than three orders of magnitude. 
   Embodiment 9 of the invention is a specific case of Embodiment 7, with the following parameters. The material system is the same as in Embodiment 4, while the waveguide widths are w s1 =w s2 =5, w t1 =w t2 =3 μm, the angles are α 1 =α 2 =10°, β=8°, and γ=0°, the waveguide offset is +12 μm, and the height of the taper is H=1 μm. We plot the response of the optical tap as a function of wavelength both for TE and TM polarization of the incoming light signal in FIG.  12 . The tap efficiency is substantially independent of wavelength in a large 200 nm wavelength range. Moreover, the response is also substantially independent of the polarization of the light signal. The difference between the tap efficiencies for TE and TM polarizations is about 0.1 dB across the entire wavelength range sampled, which is sufficiently small for most purposes. 
   Embodiment 10 of the invention is an integrated optical tap monitor illustrated in FIG.  13 . The monitor first comprises an optical tap  1301  of Embodiment 5.  FIG. 13.   a  is a schematic cross-sectional top view of the integrated optical tap monitor in the plane of the optical tap structure  1301 . With reference to  FIG. 13.   a , the optical tap comprises an input waveguide  1302 , an output waveguide  1304 , a blind waveguide  1305 , and a tap waveguide  1303  ending in a waveguide termination  1308 .  FIG. 13.   b  is a schematic cross-sectional side view of the integrated optical tap monitor in the plane defined by the axis of the tap waveguide  1303 . With reference to  FIG. 13.   b , the waveguides of the optical tap are enclosed by a lower cladding  1309  and an upper cladding  1310 . The monitor further comprises a turning mirror  1311  created by first etching a wedge-like opening  1312  through the upper and lower claddings  1309  and  1310  and through the tap waveguide  1303 . The opening  1312  has a first facet  1313  that vertically terminates the tap waveguide  1303  in the waveguide termination  1308  as well as a second facet  1314 , angled at about 45 degrees from the plane of the waveguides. On the second facet  1314  metal comprising the turning mirror  1311  is deposited to make it reflecting. The monitor further comprises a photodiode  1315  that is mounted above the turning mirror  1311 . The light signal enters the input port  1306 , and most of the signal travels to output port  1307 . Some of the light is tapped of by the optical tap into the tap waveguide  1303  and this tapped light signal travels to the waveguide termination  1308 . The tapped light signal encounters the metallized turning mirror  1311 , which reflects the signal out of the plane of the optical tap  1301  toward the photodiode  1315 , where the tapped light signal is collected and detected.