Abstract:
An apparatus and method for splitting a received optical signal into its orthogonal polarizations and sending the two polarizations on separate dual integrated waveguides to other systems on chip for further signal processing. The present invention provides an apparatus and method for facilitating the processing of optical signals in planar waveguides received from optical fibers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority from U.S. Provisional applications No. 60/389,845 filed Jun. 19, 2002 and No. 60/390,047 filed Jun. 19, 2002. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to an apparatus and method for facilitating the processing of optical signals from single mode optical fibers by integrated circuits.  
       BACKGROUND OF THE INVENTION  
       [0003]     Optical fibers have been widely used for the propagation of optical signals, especially to provide high speed communications links. Optical links using fiber optics have many advantages compared to electrical links: large bandwidth, high noise immunity, reduced power dissipation and minimal crosstalk. Optical signals carried by optical fibers are processed by a wide variety of optical and optoelectronic devices, including integrated circuits. Optical communications signals in optical fibers are typically in the 1.3 μm and 1.55 μm infrared wavelength bands. Optoelectronic integrated circuits made of silicon are highly desirable since they can be fabricated at low cost in the same foundries used to make VLSI integrated circuits. The optical properties of silicon are well suited for the transmission of optical signals, due to its transparency in the infrared wavelength bands of 1.3 μm and 1.55 μm and its high refractive index. As a result, low loss planar silicon optical waveguides have been successfully built in silicon integrated circuits.  
         [0004]     Optical signals traveling in optical fiber frequently need to be coupled to optoelectronic circuits and this can be done through a variety of known techniques and devices. There are many advantages to directly coupling optical signals on fiber with integrated optoelectronic silicon based circuits. The flat end of an optical fiber can be directly connected to the edge of a silicon integrated circuit, so the optical signal can be coupled to a flat end of a planar waveguide. An optical signal in a fiber can be coupled to a planar waveguide through the top surface of an integrated circuit using a grating coupler. The efficiency of such fiber to chip connections depends on many factors, including the number and types of optical modes in the fiber and in the integrated waveguide. Once an optical signal is on a chip, it can be processed either as an optical signal or converted to an electronic signal for further processing.  
         [0005]     An optical beam traveling in a single mode fiber (SMF) with circular cross section will typically have two optical modes, with one mode polarized in the x direction and a second mode polarized in the y direction. These two orthogonal polarizations have approximately the same propagation constant and approximately the same group velocity. Some refer to these two modes as a single mode with two polarization components. Within this discussion of the present invention, the two orthogonal polarizations are referred to as two modes.  
         [0006]     Similarly, two orthogonal polarization modes are preset in standard forms of polarization maintaining fibers. These two modes have sufficiently different phase and group velocities to prevent light from coupling back and forth between the two modes. However, the differences are slight enough that they can usually be treated in a similar manner to SMF, when used as an input to polarization splitting elements.  
         [0007]     In theory and under ideal conditions, there is no exchange of power between the orthogonal polarizations. If an optical signal is directed into only one polarization, then all the power should remain in that polarization. But in actual practice, imperfections or strains in the fiber cause random power transfer between the two polarizations. The total power is thus divided between the two polarizations, and this may not be a problem in some applications, but in many situations, this can be a major problem. In some cases, there can be a great deal of fluctuation and power transfer between the two polarizations. Such random fluctuations could cause the power delivered on one polarization, to be close to zero, which would result in some loss of signal, if only that polarization is being received.  
         [0008]     Single mode optical fiber with a circular cross section has two optical modes, although due to the rotational invariance of a single mode fiber, one mode is difficult to describe without referencing the presence of the other. Planar waveguides have a different type of modal configuration, where there are two primary types of modes: the transverse electric (TE) and the transverse magnetic (TM), which describe which field of the mode is oriented purely transversely to the direction of propagation. This is strictly true only for 2 dimensional ideal waveguides, however this naming convention is also used for real world three dimensional waveguides, which are only approximately TE or TM. Future references herein will make the common assumption that quasi-TE or quasi-TM modes are understood as TE or TM modes.  
         [0009]     It is difficult to connect an optical signal from an optical fiber to a planar waveguide due to differences in: cross sectional geometry, polarization characteristics and the number of optical modes. An SMF optical fiber has a circular cross section with a core diameter of less than ten microns. A nanophotonic planar waveguide can be substantially smaller, and as a result, contain modes that vary substantially in cross-sectional geometry. An SMF fiber will typically have two polarizations with essentially the same phase and group velocities. The polarizations in a planar waveguide can have very different phase and group velocities, or the planar waveguide could support only a single polarization mode. The number of optical modes in an SMF fiber is two when operated at the appropriate wavelength. A typical planar waveguide can have many optical modes within it, or it could have only one, depending on the design.  
         [0010]     Waveguides are designed for use over a particular wavelength range, so a single mode waveguide at one wavelength very often becomes multimodal at substantially shorter wavelengths. When one skilled in the art refers to waveguide operation, it is commonly understood that a particular wavelength range is being referenced with respect to single or multimode operation.  
         [0011]     The design process for optical paths comprises construction of maskworks. Maskworks include shapes, layout, data structures, netlists, and alignment marks and other elements of the design which are typically stored as digital data on a computer system. In addition, these electronic representations of the designs are transferred to a set of many physical masks which are used during the fabrication of the components. These many masks are included in the definition of maskworks.  
         [0012]     As a result of the many differences in characteristics between optical fibers and planar waveguides, it has been difficult to connect optical signals from one to the other.  
       SUMMARY OF THE INVENTION  
       [0013]     One embodiment of the present invention is an apparatus and method for splitting a received optical signal into its orthogonal polarizations and sending the two polarizations on separate dual integrated single mode waveguides to other systems on chip for further signal processing. The present invention provides an apparatus and method for facilitating the processing of optical signals in planar waveguides received from optical fibers. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a block diagram of an apparatus, according to one embodiment of the present invention.  
         [0015]      FIG. 2  is a block diagram of an apparatus, according to an alternate embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0016]      FIG. 1  is a block diagram of an apparatus, according to one embodiment of the present invention. Optical signal  101  is input to polarization splitting element  102 . Polarization splitting element  102  splits the received optical signal into two orthogonal polarizations or modes which are sent to output ports as output signals  103  and  113 .  
         [0017]     Output signal  103  is sent to the input of waveguide  104 . The optical signal received by the waveguide  104  propagates through it and appears as output signal  105 . Output signal  105  is input to optoelectronic signal processing system  106 , which processes the received signal and generates output  107 .  
         [0018]     Output signal  113  is sent to the input of waveguide  114 . The optical signal received by the waveguide  114  propagates through it and appears as output signal  115 . Output signal  115  is input to optoelectronic signal processing system  116 , which processes the received signal and generates output  117 .  
         [0019]     Waveguides  104  and  114  are fabricated as identical waveguides within the limitations of the particular semiconductor processing technology used to make them. The two waveguides are of the same length, width and height and made of the same materials, during the same semiconductor processing steps. To minimize differences in the waveguides due to local variations in an integrated circuit, the waveguides  104  and  114  can be fabricated in close proximity to each other on the same integrated circuit. In an exemplary embodiment, the waveguides  104  and  114  are no more than five microns apart. Just as matched transistors on an integrated circuit have to be built in close proximity to each other, the waveguides  104  and  114  have to be fabricated close together in order to be considered a matched pair of waveguides.  
         [0020]     In one embodiment, the polarization splitting element and the two waveguides are disposed on the same integrated circuit. In an alternate embodiment, the two waveguides are disposed on the same integrated circuit.  
         [0021]     If we consider waveguides  104  and  114  as identical waveguides, then their operating characteristics would be identical. Two optical modes propagated separately through the two waveguides will encounter the same optical environment, and any change in the two signals due to the waveguides will be the same. Optoelectronic signal processing systems  106  and  116  are also designed to be identical in operation, and have the same impact on the two separate optical modes. Systems  106  and  116  can be any of a general type of signal processing systems, which can process signals optically, electronically or optoelectronically. The two identical signal processing systems can be any one of the following types of devices, such as: photodetectors, filters, modulators, demodulators, amplifiers, pulse shapers, multiplexers, demultiplexers, etc., and other kinds of signal processors. The two identical signal processing systems can be output devices, such as chip to fiber couplers. In an alternate embodiment, the two systems  106  and  116  can be replaced by one signal processing system with two input ports and one output port.  
         [0022]     The outputs of the two signal processing systems can be combined together to generate a single output signal.  
         [0023]     Thus, the two modes can travel down paths, which are very similar before independently reaching the signal processing system, and the sum of the effects of the identical waveguides can be expected to produce a nearly polarization independent effect.  
         [0024]     A matched pair of optical waveguides can have two basic forms. In the first form, they are exactly identical copies of each other, placed along the same orientation, in proximity to each other. The second form is where the two copies are mirror images of each other, along some line in plane with the substrate, and the two copies are in proximity to each other.  
         [0025]     A particularly advantageous aspect of the present invention is that splitting the optical input signal into its two orthogonal polarizations and processing the two polarizations in an identical manner enables the two planar waveguides operating together to receive an optical signal without being adversely affected by the polarization characteristics of the optical waveguide.  
         [0026]     When waveguides are fabricated, there are two main sources of fabrication error: film thickness and etch process variation. Both of these two effects are dependent on the distance between two structures that are desired to be identical. The proper distance is best quantified with a number called the autocorrelation length of the variable. The autocorrelation length is a distance over which one thickness, for example, is known to be correlated to another, and beyond this distance, the relative thicknesses become increasingly non-deterministic. This autocorrelation function is well known to those skilled in the art, and can also be applied to the variations in process bias that create asymmetries between waveguides or optical circuits. A particularly relevant way to express the effects of a combination of effects, each with their own autocorrelation length is to combine them as a number representing the autocorrelation length of the phase of the optical signal.  
         [0027]      FIG. 2  is a block diagram of an apparatus, according to one embodiment of the present invention. Optical signal  201  is input to polarization splitting element  202 . Polarization splitting element  202  splits the received optical signal into two orthogonal polarizations or modes which are sent to output ports as output signals  203  and  213 .  
         [0028]     Output signal  203  is sent to the input of waveguide  204 . The optical signal received by the waveguide  204  propagates through it and appears as output signal  205 . Output signal  205  is input to photodetector  206 , which processes the received signal and generates electrical output signal  207 .  
         [0029]     Output signal  213  is sent to the input of waveguide  214 . The optical signal received by the waveguide  214  propagates through it and appears as output signal  215 . Output signal  215  is input to photodetector  216 , which processes the received signal and generates electrical output signal  217 .  
         [0030]     Electrical output signals  207  and  217  can be combined together by summing circuit  208  to generate electrical output signal  209 . In an alternate embodiment, the summing circuit  208  and the two photodetectors  206  and  216  can be replaced by one photodetector with two input ports and one output port.  
         [0031]     A particularly advantageous aspect of the present invention is the generation of an electrical output signal which corresponds to the total power of the two polarizations of the optical input signals, regardless of how the power is randomly transferred between the two polarizations.  
         [0032]     Although the present invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made without departing from the spirit and scope of the invention.