Abstract:
A ring resonator structure includes a semiconductor substrate, a core, and a cladding. Either the core or the cladding comprises chalcogenide glass to improve electromagnetic confinement in the ring resonator structure.

Description:
PRIORITY INFORMATION 
     This application claims priority from provisional application Ser. No. 60/699,316 filed Jul. 14, 2005, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to the field of ring resonators, and in particular to ring resonators having chalcogenide glass to improve confinement. 
     In recent years, optical switches are finding increasing applications in DWDM optical communication systems. They are also indispensable components for programmable optical circuits. Current optical switches often employ an interferometer configuration (e.g., Mach-Zehnder or Sagnac interferometer), which turns light on and off by either constructive or destructive interference. However, since refractive index change of most materials either due to thermo-optic or electro-optic effect is typically small, interferometer switch schemes often require relatively long device length to achieve switching effect. The development of microring resonator structure provides a unique solution to optical switching. By its light confining nature and hence the high optical power stored in the ring, a small index change can lead to a relatively large resonant wavelength shift and high on-off ratio. In addition, it features a compact, in the order of a few 10 microns, and flexible for all-optical switching. The index change can be introduced by thermo-optic, eletro-optic effects or optical nonlinearity. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, there is provided a ring resonator structure. The ring resonator structure includes a semiconductor substrate, a core, and a cladding. Either the core or the cladding comprises chalcogenide glass to improve electromagnetic confinement in the ring resonator structure. 
     According to another aspect of the invention, there is provided a method of fabricating a ring resonator structure. The method includes providing a semiconductor substrate, forming a core, and forming a cladding. Either the core or the cladding comprises chalcogenide glass to improve electromagnetic confinement in the ring resonator structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing an arrangement of a ring resonator switch; 
         FIG. 2  is a schematic diagram illustrating the cross-section of the ring resonator structure of  FIG. 1  having chalcogenide as trimming coating material  20 ; 
         FIG. 3A  is a TEM demonstrating a fabricated Ge 23 Sb 7 S 70  channel waveguide which shows a rough surface resulting from the fabrication process;  FIG. 3B  is a TEM demonstrating a Ge 23 Sb 7 S 70  channel waveguide reflowed at 450° C. for 5 min showing that the reflow process effectively eliminated surface roughness; 
         FIG. 4  is a schematic diagram illustrating a cross-section of a ring resonator structure having chalcogenide glass as a cladding; 
         FIG. 5A  is a TEM graph of the TE mode ring resonator structure of  FIG. 4 ;  FIG. 5B  is a TEM graph of the TM mode for the ring resonator structure of  FIG. 3 ;  FIG. 5C  is a graph showing the relationship between the confinement factor (F-factor) and core height for the ring resonator structure of  FIG. 3 ; 
         FIG. 6  is a schematic diagram illustrating a cross-section of a ring resonator structure having chalcogenide glass as a core; 
         FIG. 7A  shows a TEM diagram of the TE and TM confinement in the core of the ring resonator structure of  FIG. 5 ;  FIG. 7B  is a graph demonstrating improvement in the F-factor associated with confinement of the ring resonator structure of  FIG. 6 ;  FIG. 7C  is a graph showing radiative loss of the ring resonator structure of  FIG. 6 ; 
         FIG. 8  is a schematic diagram illustrating a ring resonator structure having CMOS compatibility formed in accordance with the invention; and 
         FIG. 9A  shows a graph demonstrating the advantages of using a core having Si 3 N 4  for the ring resonator structure shown in  FIG. 8 ;  FIG. 9B  is a graph that demonstrates how an insulating layer thickness can effect the F-factor associated with the ring resonator structure of  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In a ring resonator, the resonant wavelength is readily determined by the material refractive indices and structure geometry. A refractive index change induced either by photoinduced refractive index change (PRC) effect or optical nonlinearity shifts the resonance on or off the working wavelength (e.g. 1.31 μm or 1.55 μm), which defines the optical ‘on’ and ‘off’ states and thus optical switching is achieved. Besides optical switching, tunability of the resonant wavelength also provides large optical system design flexibility and a number of other device applications, such as modulators, tunable add-drop filters and ring resonator trimming. 
     The photoinduced refractive index change (PRC) effect, which refers to the refractive index change of chalcogenide glasses under near bandgap light illumination of appropriate. The term chalcogenide glasses defines a large family of vitreous materials fabricated from metals and/or nonmetals, such as As, Ge, Sb, in conjunction with the heavier elements in the oxygen family, such as the chalcogens S, Se, Te. 
     Many chalcogenide glasses exhibit large optical nonlinearity and significant PRC effects that are very promising for optical switching, high-speed modulation and ring resonator trimming. Chalcogenide glasses are one of the materials known with largest third-order nonlinear optical effects, which is promising for ultra-high-speed optical switching. PRC effect is another efficient way of tuning chalcogenide material index. An index change in the order of 0.01 is reported in As 2 S 3  glass films, which suggests large device tunability potential. Depending on the exposure wavelength and power, PRC effect can be either irreversible or reversible by thermal annealing to near glass transition temperature, which offers the possibility for programmable optical circuit components and reversible ring resonator trimming. 
     The extinction ratio and/or modulation depth of the device is approximately proportional to the square of the ring&#39;s quality factor Q. However, at present, the Q of high-index-contrast (HIC) microrings is limited by the scattering loss resulting from sidewall roughness, and also significant sidewall roughness after etching is observed in chalcogenide glass waveguides. In resolving this issue, thermal reflow technique has previously demonstrated a reduction of sidewall roughness of organic polymer waveguides. There are also precedents of using thermal reflow techniques to fabricate chalcogenide microlens arrays. Therefore, one can utilize thermal reflow techniques to reduce sidewall roughness in chalcogenide core waveguides and microring resonators, which can lead to high-Q resonator structures. 
     In this invention, examples are provided for applications of PRC effect in tunable resonator structures. 
       FIG. 1  shows an arrangement of a ring resonator switch  2 . The ring resonator switch  2  includes an input waveguide  6  having an input port  10  and throughput port  12 , output waveguide  4 , and a ring resonator structure  8 . The input waveguide  6  uses its input port  10  to receive optical signals having various optical wavelengths. Both the input waveguide  6  and output waveguide  4  are coupled to the ring resonator structure  8 . By coupling the ring resonator structure  8  with the input waveguide  6 , optical signals having selective wavelengths are permitted to pass the ring resonator structure  8  to the output waveguide  4 . Those optical signals whose wavelengths are not permitted to pass are sent to the throughput port  12 . The output waveguide  4  and input waveguide  6  are comprised of two parallel ridge waveguides, however, other waveguides can be used. 
     Note the ring resonator in this embodiment includes chalcogenide as trimming coating material.  FIG. 2  shows a cross-section of the ring resonator structure  8  having chalcogenide as trimming coating material  20 . As shown in  FIG. 2 , the ring resonator structure includes a channel region  18 , cladding layers  22 , and a trimming coating layer  20 . 
     Given the refractive index of the glass is typically between  2  and  3 , the cladding material can be comprised of SiO x , SiN x  or polymers such as PMMA. Note the cladding layers  22  are divided into three regions having the same materials. 
     For ultra-fast switching purposes, glasses with high optical nonlinearity are suitable candidates for the cladding. For switching devices utilizing PRC effects, the trimming coating layer  20  can be made of chalcogenide glass whose bandgap is slightly larger than the signal wavelength. In both cases, by shining light of appropriate intensity and wavelength, combined with local annealing in PRC-based devices, the index of the trimming coating layer can be shifted, resulting in output and throughput intensity change and hence the switching effect. The trimming coating layer  20  can be formed by thermal evaporation or other film deposition techniques, such as sputtering or CVD. 
     In this embodiment, the cladding layers  22  can be formed by thermal oxidization of Si wafers to form a thermal oxide layer, such as SiO 2 . The channel layer  18  can be formed by deposition of poly-Si on one of the cladding layers  22  and using photolithography to define its shape. The internal layer is formed between the channel layer and trimming coating layer using deposition techniques. 
     The invention uses a thermal reflow process in which glass or polymer materials or devices are annealed above their glass transition temperature in order to induce morphological modifications due to the materials&#39; surface tension. One can successfully utilized the process to remove surface roughness on chalcogenide waveguides resulting from patterning process. A typical reflow process includes an anneal of the chalcogenide waveguides at a temperature ranging from 250° C.-500° C. for a time of a few minutes to a few hours.  FIGS. 3A-3B  shows two AFM images that compare the surface morphology of a Ge 23 Sb 7 S 70  waveguide before, shown in  FIG. 3A , and after thermal reflow, as shown in  FIG. 3B . Quantitative analysis of the image revealed that the top surface rms roughness had been reduced from 1.9 nm to below 0.5 nm and a significant roughness reduction was achieved. Ge 23 Sb 7 S 70  channel waveguides with propagation loss as low as 4.5 dB/cm at the wavelength of 1550 nm has been fabricated using this technique. 
       FIG. 4  shows a cross-section of a ring resonator structure  30  having chalcogenide glass as a cladding. The resonator structure includes a core  38  having Si, an insulating substrate  36 , and a cladding layer  34 . The core  38  is formed on the insulating substrate  36 , which can be comprised of SiO 2 . The cladding layer  34  totally encompasses the top and side regions of the core  38 . Note the cladding layer  34  includes chalcogenide glass, which is formed using thermal evaporation or other film deposition techniques, such as sputtering or CVD. The aspect ratio for this ring resonator structure  30  is fixed at  2 , while the core  38  can have varied heights. The cladding layer  34  is sized to be approximately 1 μm. Loss is determined by the sidewall roughness of the core  38 . Note the cladding layer  34  provides separation between the core  38  and air  32 . The insulating substrate  36 , in this embodiment, is comprised of SiO 2  but other insulating substrates can be used. 
       FIG. 5A  is a TEM graph of the TE mode and  FIG. 4B  is a TEM graph of the TM mode for the ring resonator structure  30  of  FIG. 3 . Both  FIGS. 5A and 5B  illustrate strong confinement of both the TE modes and TM modes.  FIG. 5C  shows the relationship between the confinement factor (F-factor) and core height for the ring resonator structure  30 . Note  FIG. 5C  shows that a larger core size leads to better confinement and smaller index effective index change for both the TE and TM modes. 
       FIG. 6  shows a cross-section of a ring resonator structure having chalcogenide glass as a core. The ring resonator structure includes a cladding layer  44  having SiO x , SiN x  or polymers such as PMMA. The core  48  is formed on a Si substrate  46  using thermal evaporation or other film deposition techniques, such as sputtering or CVD. After the deposition of the chalcogenide glass, etching is used to make the shape and dimension of the core  48 . Note the cladding layer  44  provides separation between the core  48  and air. The ring resonator structure  40  provides a large confinement factor (F-factor), which is suitable for waveguiding. Also, the ring resonator structure  40  provides low radiative loss, which is excellent for fabricating ultra-low loss waveguide structures. Moreover, the ring resonator structure  40  provides flexibility in the materials used to form the cladding layer  44 . Note the cladding layer  44  provides separation between the core  48  and air  42 . 
       FIG. 7A  shows a TEM diagram of the TE and TM confinement in the core  48  of the ring resonator structure  40 .  FIG. 7B  shows a graph demonstrating improvement in the F-factor associated with confinement. Moreover,  FIG. 7B  demonstrates that the larger the core  48  the better the confinement.  FIG. 7C  shows the how negligible radiative loss is when the core  48  is made larger. The reason this occurs is the use of the chalcogenide glass and low scattering loss associated with such materials. 
       FIG. 8  shows a ring resonator structure  50  having CMOS compatibility formed in accordance with the invention. The ring resonator structure  50  includes an insulating substrate  58  where a core  60  having Si or Si 3 N 4  core is formed. An insulating layer  56  is formed on those surfaces not facing the insulating substrate  58 . Also, the thickness of the insulating layer  56  should preferable be small so as to increase the F-factor. A chalcogenide glass layer  54  is formed on the insulating layer  56  using thermal evaporation or other film deposition techniques, such as sputtering or CVD. The chalcogenide glass layer  54  acts as a cladding layer. The insulating layer  56 , in this embodiment, is comprised of SiO 2  but other insulating materials can be used. The insulating substrate  58 , in this embodiment, is comprised of SiO 2  but other insulating substrates can be used. Note the chalcogenide glass layer  54  provides separation between the core  60  and air  52 . The chalcogenide glass material can be deposited and processed by similar techniques and tools as are currently used for CMOS processing. 
       FIG. 9A  shows a graph demonstrating the advantages of using a core having Si 3 N 4  for the ring resonator structure  50  shown in  FIG. 8 . It is clearly apparent that a core  60  having Si 3 N 4  used in ring resonator structure  50  has a higher F-factor then a core  60  having Si.  FIG. 9B  is a graph that demonstrates how the insulating layer  56  thickness can effect the F-factor associated with the ring resonator structure  50  of  FIG. 8 . 
     Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.