Abstract:
Photonic signals of high bandwidth are input into a planar spiral of waveguide. The spiral has a number of waveguide loops having a series optical tap areas wherein optical energy is leaked from the loops. The leaked optical energy is received by optical tap waveguides that carry the light for further processing.

Description:
BACKGROUND 
   An optical delay line can be used to separate the particular colors of a chromatic spectrum. An example of such a device is an array waveguide grating (AWG). If a conventional AWG is designed for extremely high color resolution, the AWG must have great physical size. This size can easily exceed that of a common silicon wafer. 
   There is thus a need for an optical delay line that provides high color resolution but that is compact. For many applications it is desirable that such a delay line be realizable in planar form. An optical delay line having these attributes can enable a very high resolution AWG system (a hyper-resolution AWG system). 
   SUMMARY 
   Photonic signals of high bandwidth are input into a spiral of waveguide. The spiral has a number of waveguide loops that provide a series optical taps at the surface of the spiral. Optical energy is released at the taps into tap waveguides that carry the light for further processing. 
   Other objects, advantages and new features will become apparent from the following detailed description when considered in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A and 1B  illustrate a partial view of a multiple-tapped optical delay line according to the description herein, shown without and with tap waveguides, respectively. 
       FIG. 2  shows a view of an elongated multiple-tapped optical delay line system. 
       FIG. 3  illustrates a cross section from the perspective of the plane “A” of  FIG. 2 , emphasizing optical waveguide loops and accompanying tap waveguides. 
       FIG. 4  depicts an optional tuning section as may be used with the multiple-tapped optical delay line according to the description herein. 
       FIG. 5  depicts another optional tuning section as may be used with the multiple-tapped optical delay line according to the description herein. 
   

   DESCRIPTION 
   Referring now to  FIG. 1A , a representative delay line  10  is shown. Delay line  10  includes a spiral  12  of optical waveguide  14 . Example delay line  10  has spiral waveguide loops  16   1 – 16   n , shown disposed within a single plane. 
   Broadband optical energy  18 , for example, is received at an input end  20  to waveguide spiral  12  and is released through optical tap areas  22   1 – 22   n  in waveguide loops  16   1 – 16   n , respectively. 
   End  24  of waveguide  14  can be a waveguide termination such as a matched load that decreases reflection. It should be noted that the number of waveguide loops shown in  FIG. 1A  is for illustration purposes. While waveguide loop numbers of least of order  10  are suitable for many purposes, waveguide loop numbers of order  100  will provide greater frequency resolution fidelity. The minimum curvature of the waveguide loops will be dictated by the bending limits suggested by the waveguide manufacturer. 
   In an understood manner, the length by which optical signals travel in a waveguide affects the phase of the traveling light and hence provides a mechanism by which the colors of the incoming light can be separated. In essence, different waveguide phase lengths permit different frequencies of light to be segregated from other frequencies of light. It is thereby possible to spread the colors of light out by creating a phase length difference that corresponds to a particular desired travel time of the light, wherein 1 divided by this desired travel time creates the approximate upper limit to the frequency resolution of a hyper-resolution AWG. 
   For example, to create a frequency resolution of approximately 100 MHz (0.1 GHz), a path length (in a vacuum) of approximately 10 feet of travel or 10 nanoseconds of light travel time is required. Because the index of refraction of glass (waveguide) differs from that of a vacuum, a shorter length of fiber is suitable to accomplish this delay. In this instance, approximately six feet of waveguide, between the first and last optical taps  16  (tap  16   1  and tap  16   n  of  FIG. 1A ) provides a 0.1 GHz resolution. 
   To create a 1 GHz frequency resolution, approximately 0.6 feet of optical waveguide between the first and last optical taps is needed. To create a 10 GHz frequency resolution, approximately 0.06 feet of optical fiber between the first and last optical taps is needed. Higher resolution can be achieved by lengthening the distance between these first and last optical taps. 
   As will be further explained, light radiated in the proximity of areas  22  of spiral  12  is received for further processing. Referring now to  FIG. 1B , there is shown delay line  10  used in conjunction with tap waveguides  26   1 – 26   n  placed in a close, predetermined, proximity to areas  22  of spiral  12 . The spatial relationship between tap waveguides  26  and optical tap areas  22  will be described further herein, but it should be noted here that the tap waveguides are placed close enough to tap areas  22  of loops  16  so as to receive light radiating from the loops. 
   In  FIG. 2 , there is illustrated a planar, multiple-tapped optical delay line system  28 . In the embodiment shown, delay line  10  is elongated to enhance light collection therefrom.  FIG. 2  illustrates a waveguide foundation  30 , greater details to be shown, that is positioned to orient loops  16  with respect to tap waveguides  26 . At locations where loops  16  and tap waveguides  26  cross, one may add additional waveguide cladding as desired to minimize undesired cross-talk. 
     FIG. 3  is a sectional view taken from the perspective of “A” of  FIG. 2 , and shows details of the spatial relationship between tap waveguides  26  and the location of optical tap areas  22  of spiral  12 . Also shown in  FIG. 3 , not to scale, is waveguide foundation  30  that can be used to maintain a desired proximity between tap waveguides  26  and optical tap areas  22  of loops  16 . Foundation  30  is shown comprising a substrate  32 , which can be, for example, silicon or silica; a base  34 , which can be, for example, silica approximately 15 μ-meters thick; a first cladding layer  36  which can be, for example, SiO 2  having for example Phosphorous and Boron doping wherein layer  36  can for example be of the order 5–30 μ-meters thick; and a second cladding layer  38  which can, for example, be SiO 2  with for example Phosphorous and Boron doping, wherein layer  38  can for example be approximately 10–30 μ-meters thick. Spiral  12  and tap waveguides  26  can be, for example, SiO 2  with Phosphorous doping, for example. Example processing and design considerations are, wherein T is for temperature and n is for refractive index, Tbase  34 &gt;Tcore  22 &gt;Tclad  36 &gt;Tcore  26 &gt;Tclad  38 , nclad  36 =nbase  34 =nclad  38  (all approximately equal), and ncore  22  and  26 &gt;nbase 36 , nclad 38 . 
   A similar “foundation” is described in the article titled: “Silica-based optical integrated circuits” by Y. P. Li and C. H. Henry, IEE Proc.-Optoelectron, Vol. 143, No. 5, October 1996. 
   Spiral  12  and tap waveguides  26  can be of the order of 3 μ-meters thick and 3 μ-meters tall, for example. Tap waveguides  26  can be placed directly above areas  22  of spiral  12 , be slightly laterally offset or significantly offset, the latter as illustrated in  FIG. 3 . A variety of combinations of geometries is possible. These combinations are designed so that the amount of light energy coupled from each optical tap area  22   i  to each tap waveguide  26   i  is roughly equal to 1 divided by the total number of tap waveguides. For example, a suitable approximate distance between taps  22  and tap waveguides  26  is 3 μ-meters to 10 μ-meters. 
   Referring now to  FIG. 4 , there is shown an optional waveguide tuning section  60 , an outline of which is shown in  FIG. 2 . One example of tuning section  60  comprises waveguide variable path lengths  62   1-i , which are on the order of 1 to 5 mm long, for example. When current is passed through metallized resistors  64   1-i , the corresponding underlying variable path length sections  62   1-i  are heated to elevated temperatures. The elevated temperatures change the optical path lengths and thereby the phase of light passing through variable path lengths  62   1-i . 
   Referring to  FIG. 5 , tuning section  60  may also comprise voltage controllable optical attenuators  66   1-i  which are on the order of 1 to 5 mm long. The optical attenuators may be used to set or adjust the amplitude weights of the optical power levels of the different tap waveguides. Similarly as with the embodiment of tuning section  60  of  FIG. 4 , when current is passed through metallized resistors  68  of the attenuators, the underlying waveguide sections  70  are heated to elevated temperatures. The elevated temperatures change the optical path lengths and thereby the amplitude of light passing through the optical attenuators  66 . 
   The tuning sections have utility in tapped delay systems which are used in very high resolution systems, i.e. systems capable of resolving frequencies more narrowly spaced than about 30 GHz. 
   Obviously, many modifications and variations are possible in light of the above description. It is therefore to be understood that within the scope of the claims the invention may be practiced otherwise than as has been specifically described.