Abstract:
An optical apparatus including a 360-degree star coupler with derivative structure(s) and applications to optical imaging, optical communications and optical spectroscopy.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention is directed to an optical imaging apparatus and in particular to a 360-degree star coupler and derivative structures/devices thereof with application to optical imaging, optical spectroscopy and/or optical communications. 
       BACKGROUND OF THE INVENTION 
       [0002]    A wide variety of optical apparatuses have been employed in the fields of imaging, telecommunications, and medicine—to name a few. Consequently, new optical apparatuses which may advantageously impact these fields are desired. 
       SUMMARY OF THE INVENTION 
       [0003]    An optical apparatus including a 360-degree star coupler is described and applied to optical imaging and spectroscopy. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0004]    A more complete understanding of the present invention may be realized by reference to the accompanying drawings in which: 
           [0005]      FIG. 1A  and  FIG. 1B  are schematic diagrams of an optical arrangement according to an aspect of the present invention with inset ( 1 A) showing a planar lightwave circuit of central structure ( 1 B); 
           [0006]      FIG. 2  is set of a plots of transmissivity vs. displacement d from the center for perfectly conducting cylinders of various radii a for (a) the flower and (b) a conventional imaging system having NA=0.9; 
           [0007]      FIG. 3(   a ) is a schematic image showing a probe tip in the x-z plane; 
           [0008]      FIG. 3(   b ) is a schematic image showing the y-z plane; 
           [0009]      FIG. 4  shows the normalized transmissivity (dB) vs. probe position; 
           [0010]      FIG. 5  is a plot showing the cross-section of the transmissivity (on a linear scale) from  FIG. 4  along the x axis; and 
           [0011]      FIG. 6  is a schematic diagram of an alternative optical arrangement according to an aspect of the present invention; 
           [0012]      FIG. 7  is a schematic diagram of another alternative optical arrangement according to an aspect of the present invention; 
           [0013]      FIG. 8  is a schematic diagram of another alternative optical arrangement according to an aspect of the present invention; and 
           [0014]      FIG. 9  is a schematic diagram of a spherical optical arrangement according to an aspect of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    The following merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. 
         [0016]    Furthermore, all examples and conditional language recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. 
         [0017]    Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure. 
         [0018]    Thus, for example, it will be appreciated by those skilled in the art that the diagrams herein represent conceptual views of illustrative structures embodying the principles of the invention. 
         [0019]    By way of some additional background, it may be appreciated that in an imaging system using conventional lenses, the resolution is limited to a feature size of approximately λ/NA, where λ is the wavelength in the material, and NA is the numerical aperture. As is known, the NA is related to the capture angle of the lens, and the maximum achievable NA from a conventional lens is ˜0.9. 
         [0020]    One reason that the resolution is limited is because objects that are significantly smaller than a wavelength scatter light in all directions. Only part of the scattered light can be collected using a conventional lens. The inability to collect all the scattered light from small features results in information loss that unfortunately prevents resolving such features. By analogy, this situation is like trying to capture a high-speed signal using a band-limited detector. 
         [0021]    According to an aspect of the present invention, an optical apparatus is provided which surrounds the object imaged thereby permitting the collection of all scattered light. Advantageously, image features smaller than a wavelength may be resolved while operating in the far field portion of the spectrum and over a wide wavelength range, limited only by the accuracy with which the inverse scattering problem can be solved. 
         [0022]    With initial simultaneous reference to  FIG. 1A  and  FIG. 1B , there is shown a planar lightwave circuit (PLC)  100 , according to an aspect of the present invention. The central portion of the PLC comprises a star coupler  110  (See, e.g., Dragone, U.S. Pat. No. 6,195,482 the entire contents of which are incorporated by reference as if set forth at length) having a 360° capture angle—i.e., a “360° star”. As shown, an input/output waveguide  120  optically feeds all the inlets  130 [ 1 ] . . .  130 [n] of the 360° star via waveguides having equal path lengths  140 [ 1 ] . . .  140 [i] through the effect of a series of cascaded of y-branch couplers  150 . 
         [0023]    Shown in  FIG. 1A and 1B , a small aperture  160  is positioned in the middle of the star coupler  110 . An object (not specifically shown) to be imaged (or measured or spectroscopically analyzed or simply counted) is placed in the aperture  160 . Operationally, light enters through the input/output waveguide  120 , enters the 360° star  110 , focuses at the center of the 360° star  110  where it is absorbed/scattered by the object positioned within the aperture  160  and after scattering exits the 360° star  110 , and is conveyed out of the PLC via input/output waveguide  120 . An optical circulator  170  may be employed to separate the outgoing light from the incoming light. Thermooptic phase shifters  175 [ 1 ] . . .  175 [ 4 ] are positioned on four of the long waveguides which may be used to correct/adjust fabrication-induced phase errors. Of course, those skilled in the art will readily understand that different types/number of phase shifters may be employed as different fabrication techniques are employed while constructing devices according to these aspects of the present invention. 
         [0024]    As the object is moved within the aperture  160 , transmissivity measurements may be performed and recorded and thereby used to interpret the image of the object. In the configuration shown, the connecting waveguides exhibit equal path length(s), and therefore the imaging is wavelength-insensitive. 
         [0025]    As noted previously, each of the individual waveguide light paths  140 [ 1 ] . . .  140 [n] which branch from the input/output waveguide  120  through the effect of the plurality y-branch couplers  150  have equal path lengths. In a preferred embodiment, each of the y-branch couplers  150  will exhibit the same 50/50 splitting/coupling ratios, although particular variations may be possible depending upon the application of the overall apparatus. When configured in this manner wherein each of the branch couplers  150  have the same coupling ratio(s) and the path lengths of each waveguide  140 [ 1 ] . . .  140 [n] is the same, imaging resulting from this “flower” structure is advantageously wavelength insensitive. 
         [0026]    At this point, consider only electric fields polarized perpendicular to the plane of the PLC  100 . If the vertical guidance is weak, then this corresponds to transverse magnetic (TM) polarization. As a result, the coupler inlets  130 [ 1 ] . . .  130 [n] create an approximately cylindrical wave that propagates into the center of the star coupler (not specifically shown—but substantially at aperture  160 ), passes through the center, and re-enters the inlets  130 [ 1 ] . . .  130 [n]. For a continuous-wave input with frequency ω, this creates a standing wave with a field distribution (using complex notation), 
         [0000]        Ē ( r,  φ)= {circumflex over (z)}J   0 ( kr )   [1] 
         [0027]    where k is the propagation constant in the slab waveguide and r and φ are the polar coordinates from the star coupler center. With a finite number of star coupler inlets P, Eq. [1] holds true for only the center diffraction zone, which has a diameter of P/k. As in an arrayed waveguide grating (AWG), the stronger the mutual coupling between the inlets, the less power appears in the higher diffraction orders. 
         [0028]    In a conventional lensing system, NA≈4/(kD), where D is the 1/e 2  width of the focal spot. For the 360° star such as that which is an aspect of the present invention, a first impression might be that the Bessel function in Eq. [1] is the “focal” spot, which results in NA=1.14. This is only slightly better than the best conventional lenses, which is disappointing. However, upon further investigation one finds that this definition of NA is not applicable to an omnidirectional system like the 360° star. 
         [0029]    Now, consider an example whereby a perfectly conducting cylinder having radius a is positioned with its center at a distance d from the center of the 360° star, which has radius R. The exact transmissivity of the device may then be analytically calculated using—for example—scattering theory. 
         [0030]    We know from the above that in the absence of the conducting cylinder, the field in the free-space region is J 0 (kr). Let us change coordinate system from a polar one centered at the 360° star center to a polar one centered at the cylinder center by expanding it in an orthonormal basis set. 
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         [0031]    The scattered field from the cylinder must be a circular, out-going wave centered on the cylinder and thus has the form: 
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         [0032]    The only reflection in the system is the cylinder, so the total field must be the field given by [2] plus the field given by [4]. To find the constants b m , we use the condition that the field must be zero on the cylinder boundary, r′=a. Thus the total field in the star coupler is 
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         [0033]    Thus the power transmissivity through the flower (from the input/output waveguide back to the input/output waveguide) is 
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         [0034]    where we subtracted off the wave coming from the inlets into the free-space region using the latter term in (6) so that we are left with just the wave re-entering the inlets. 
         [0035]      FIG. 2(   a ) shows a plot of T vs. d/λ for various a/λ. P=16 and R=30/λ, but any P&gt;˜4 and any R&gt;˜5/λ gives substantially the same plots. When a=0 (i.e., no cylinder) T=0 dB for all d. When the cylinder is at the star center (d=0), T=0 dB, which is expected because the incoming wave is reflected intact back to the inlets. As the cylinder is moved away from the origin, T shows a significant dip, even for radii as small as 0.05λ. Note that this is not direct imaging, and some signal processing would be needed in order to reconstruct the image, just as it is in other interferometric imaging systems. 
         [0036]    Compare this to a conventional lens system, where a long cylinder of radius a is moved through the focal spot of a two-lens system with linearly polarized beams.  FIG. 2(   b ) shows a plot of T vs. d (d is shortest distance from the focal center to the cylinder axis) for various a for such a system, assuming lenses with NA=0.9 and a Gaussian beam. 
         [0037]    As one can see, the conventional imaging system cannot reasonably detect cylinders with a &lt;˜0.15λ. If we consider a 3-dB drop in transmissivity as a positive detection, then the 360° star imaging system can detect perfectly conducting cylinders ˜4 times smaller in radius than a conventional imaging system. If we consider a 1-dB drop as positive detection, then the flower detects cylinders ˜9 times smaller in radius than a conventional imaging system. 
         [0038]    Unfortunately, as is well known in scattering theory we presently are unable to find a reasonable analytic solution for more complicated structures, such as two perfectly conducting cylinders. This is due—in part—because such structures exhibit scattering of scattering light. 
         [0039]    Note that for the TM-polarized light case analyzed above, the beam at the 360° star focus is like a focused beam with a purely longitudinal polarization. Those skilled in the art will appreciate that such a polarization gives the smallest focal spot in conventional imaging, obtained by starting with a radially polarized beam. 
         [0040]    A PLC flower—such as that shown schematically in FIG.  1 .—was constructed in 0.8%-index-step silica waveguides on a silicon substrate—substantially using the layout shown in that figure wherein the number of inputs, P is 32. Thermooptic phase shifters were positioned on four of the waveguides at the top and bottom in  FIG. 1  to correct any fabrication-induced phase errors. 
         [0041]    To access the center of the 360° star and create an aperture, a slot was cut through part of the PLC using a dicing saw with a width of ˜20 μm and a depth of ˜125 μm, starting from the right-hand side and stopping the cut just after passing through the star center. Unfortunately, such sawing damages one or more of the inlets, so in the future alternatives—i.e., etch a circular hole in the center of the 360° star—may be employed to minimize any damage. 
         [0042]    A titanium probe needle with a tip size of 6×8 μm 2  (outer dimensions) was used to evaluate the structure. Schematic images of the tip of the probe from orthogonal points of view are shown in  FIG. 3 . To fully test the imaging capability of the structure, a smaller probe may be used. The tip of the probe needle is slightly larger than the central diffraction zone diameter, which is 5.4 μm. 
         [0043]    A fiber was connected to the flower input/output, and using an optical circulator an amplified spontaneous emission was launched from an erbium-doped fiber amplifier (center wavelength of 1545 nm) into the flower and detected the returned light using an optical power meter. The trench was filled with index matching oil. Three of the four thermooptic phase shifters were driven to maximize the transmission. The total insertion loss, including the circulator, is 8 dB 
         [0044]    The probe tip was lowered into the trench. Using piezoelectric transducers, the tip was moved in the x- and y-directions and a computer to record the transmissivity vs. x and y. The results are shown in  FIG. 4 . The trench walls are parallel to the x-axis, and unfortunately the trench was slightly offset from the coupler center causing the probe to hit the wall before it moved out of the focus in the negative y direction. 
         [0045]    From this  FIG. 4  one can see that the probe is successfully detected by the flower, with an increased attenuation up to 14 dB when the probe is near the coupler center. Although the probe is much bigger than a wavelength, transitions in the image that are significantly smaller than a wavelength may be observed. For example,  FIG. 5  shows a cross section along the x axis. The transmissivity falls to half the background level in less than 0.3 wavelengths in the oil. Those skilled in the art will appreciate that such a sharp transition is not readily achievable in a conventional imaging arrangement. 
         [0046]    Turning now to  FIG. 6 , there is shown an alternative imaging arrangement according to another aspect of the present invention. More particularly, imaging system  600  includes a 360° star coupler  610  optically coupled via a plurality of waveguide arms  630 [ 1 ] . . .  630 [n] to the output of star coupler  620 . As shown in this  FIG. 6 , each of the elements may advantageously be integrated or otherwise fabricated on a common planar lightwave substrate  605 . 
         [0047]    Input/output to the star coupler  620  may be provided by optical fiber  617 . By employing a sufficient number of waveguide arms  630 [ 1 ] . . .  630 [n], a focal spot may be produced at center aperture  640 , which was shown previously to be positioned at substantially the center of the 360 star coupler  610 . Preferably, at least 4 waveguide arms are employed its upward bound is only limited by they number of physical couplings which may be made to both the 360 star coupler  610  and the star coupler  620 . 
         [0048]    Operationally, light (not specifically shown) enters the apparatus via optical fiber  617  and then routed to waveguide arms  630 [ 1 ] . . .  630 [n] through the effect of star coupler  620  where it is directed to 360 star coupler  610 . Upon entering 360 star coupler, the light is absorbed/scattered by substance positioned at aperture  640 , and subsequently coupled back into waveguide arms  630 [ 1 ] . . .  630 [n] and guided back to star coupler  620  and redirected out of optical input/output fiber  617 . In a preferred embodiment, the entire system  600  may advantageously be integrated onto a common substrate or chip,  605 . 
         [0049]    In this exemplary structure, by employing a sinusoidal chirp in the waveguide arms (grating arm lengths) the focus of the light in the 360° star coupler may be steered in a straight line with changes in wavelength. Alternatively, the focus may be moved in another direction either by adding another chirp or by employing a sinusoidal chirp of Rα and moving the input focus. As can be appreciated, when a sample is positioned within the aperture, it may be within an index matched material, or alternatively flowing through a tube having a desirable index. Light exiting the structure may be measured and sample plots derived from that exiting light. With such a flowing tube arrangement, a continuous run of sample(s) may be made through the tube while collecting optical spectra therefrom or a simple count of particles may be obtained. 
         [0050]    Turning now to  FIG. 7 , there is shown an alternative embodiment of an imaging apparatus  700  according to yet another aspect of the present invention. In this embodiment, 360 star coupler  710  having a central aperture  740  is optically connected to a series of 1×2 couplers  755 [ 1 ] . . .  755 [n] by a number of waveguide arms  730 [ 1 ] . . .  730 [n]. The 1×2 couplers  755 [ 1 ] . . .  755 [n] are in turn optically coupled to of 1×n switches  760 [ 1 ],  760 [ 2 ] by waveguide sets  765 [ 1 ] . . .  765 [n] and  767 [ 1 ] . . .  767 [n], respectively. 
         [0051]    Accordingly, when a sample is positioned within the aperture  740 , light entering the system  700  via input  790  is directed to the aperture  740  through the effect of input 1×n switch  760 [ 1 ], 1×2 couplers  765 [ 1 ] . . .  765 [n] and waveguide arms  730 [ 1 ] . . .  730 [n]. 
         [0052]    Light scattered or otherwise transmitted by sample in aperture  740  is then redirected back via waveguide arms  730 [ 1 ] . . .  730 [n], 1×2 couplers  755 [ 1 ] . . .  755 [n], waveguides  7671 ] . . .  767 [n] to a second 1×n switch  760 [ 2 ] and then output at waveguide/fiber  792 . Advantageously, a 1×2 coupler  770  in conjunction with waveguide/fiber  775 and 90-degree hybrid  780  facilitates differential measurements between input and output. 
         [0053]    Yet another embodiment is shown in  FIG. 8 . In this embodiment, the light source is placed above (or below) the 360° star coupler and is focused into the aperture. Some of the light that is scattered or generated by optically pumped fluorescence (if such fluorescent molecules are placed in the aperture) is captured by the waveguides of the 360° star coupler and is routed to either a coupler or a switch and then is routed off chip to a photodetector (optionally through an optical filter first). The coupler could be one large coupler or a tree arrangement of smaller couplers. By using the switch or by using different path lengths of all the waveguides and the coupler, one can determine the amplitude and possibly the phase in each arm, allowing one to use inverse scattering theory to reconstruct the image of the object in the 360 star° coupler 
         [0054]    Finally, the 360° star coupler can be extended to 3 dimensions, creating a 4π-steradian coupler. Turning to  FIG. 9 , there is shown such a spherical coupler  900  Such a coupler looks like a sphere with waveguides sticking out normal to the sphere surface  910 [ 1 ] . . .  910 [N]. For simplicity, the waveguides are shown as “dots” on the surface of the sphere. Advantageously, it can be made in a material such as glass using a method such as femtosecond laser waveguide writing, which allows 3-dimensional waveguide structures to be formed. The advantage of a 4π-steradian coupler is that all 3 dimensions of an object could be imaged. The waveguides could be collected in a coupler or switch (which could be 2 dimensional), as in the 2-dimensional case. As with those earlier described 360 degree couplers, an aperture  920  running through a central axis of the sphere permits the placement of a sample at substantially the center of the sphere for imaging/spectroscopic analysis. 
         [0055]    Summarizing, a 360 star coupler permits the construction of 2-D imaging arrangement using a PLC and a 4p-steradian coupler permits the construction of a 3-D imaging arrangement using 3-D integrated optics, thereby permitting the detection of subwavelength features with high sensitivity. It operates in the far field and works over a large wavelength range. 
         [0056]    At this point, while we have discussed and described the invention using some specific examples, those skilled in the art will recognize that our teachings are not so limited. For example, the structures shown may employ various number(s) of optical couplers, inputs/outputs etc. They may be integrated onto a single substrate, or fabricated from individual components. Accordingly, the invention should be limited only by the scope of the claims attached hereto.