Abstract:
A design for a polarization independent ridge waveguide structure is shown. The inventive waveguide structure features discontinuous trenches formed on either side of the waveguide in which the waveguide has a first polarization characteristic absent the trenches and a second polarization characteristic when the trenches are present. The length of the trenches and the distance between the trenches are chosen to provide a desired amount of attenuation to each of the TE and TM modes. Additionally, this method is used to cause a predetermined polarization in order to compensate for the polarization of another optical component.

Description:
[0001]    This application claims the benefit of U.S. Provisional Application No. 60/472,775 filed May 23, 2003. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to integrated waveguide devices used in optical communications that are produced from planar waveguide substrates and more particularly to polarization compensation in weakly confined optical waveguides.  
         BACKGROUND OF THE INVENTION  
         [0003]    Optical components are extensively used in data communication networks. Typically these components are designed to propagate optical signals. In order to preserve signal integrity it is important to maintain certain optical characteristics. One of these characteristics is polarization dependent loss. Polarization dependant loss causes differing optical signals having substantially equal wavelengths to experience different attenuation characteristics in optical components. Since attenuation is critical for a variety of reasons it is necessary to ensure that the intensity of an optical signal is predictable in order to ensure the signal to noise ratio is within an acceptable range.  
           [0004]    This is a key concern for optical network designers because different lasers have different polarization characteristics. Additionally, it is known that many different types of integrated optical components suffer very high polarization dependant loss. As more of these components are introduced into an optical network problems arising from polarization dependant loss are complicated.  
           [0005]    Clearly, it would be beneficial to have integrated optical components which are polarization insensitive.  
         SUMMARY OF THE INVENTION  
         [0006]    The invention teaches an integrated waveguide structure comprising:  
           [0007]    a first waveguide for weakly confining a first optical signal propagating therein, said first waveguide having an optical propagation axis and a characteristic length as measured along said optical propagation axis, said first waveguide disposed on a substrate;  
           [0008]    a first trench formed within said substrate on a first side of said first waveguide, said trench having a characteristic length as measured parallel to said optical propagation axis of the first waveguide, said characteristic length of the trench being substantially shorter than said characteristic length of the first waveguide;  
           [0009]    a second trench formed on a second side of the first waveguide, said second trench disposed opposite said first trench, said second trench having a characteristic length approximately equal to that of the first trench;  
           [0010]    a third trench formed within said substrate on a first side of said first waveguide, said third trench having a characteristic length as measured parallel to said optical propagation axis of the first waveguide, said characteristic length of the third trench being substantially shorter than said characteristic length of the first waveguide; and,  
           [0011]    a fourth trench formed on a second side of the first waveguide, said fourth trench disposed opposite said third trench, said fourth trench having a characteristic length approximately equal to that of the third trench,  
           [0012]    said trenches having a low index of refraction material therein,  
           [0013]    said characteristic length of the first trench, said characteristic length of the third trench, and a distance between the first trench and the second trench being chosen to provide a predetermined polarization characteristic to the waveguide structure.  
           [0014]    Further, the invention provides a method of providing a polarization characteristic to an optical signal comprising:  
           [0015]    propagating the optical signal along a first section of a waveguide having a first set of trenches disposed sufficiently proximate the waveguide, the trenches for dispersing an optical signal coupled from the waveguide to any of the first set of trenches;  
           [0016]    propagating the optical signal along a second section of the waveguide absent trenches; and,  
           [0017]    propagating the optical signal along a third section of a waveguide having a second set of trenches disposed sufficiently proximate the waveguide, the second set of trenches for dispersing an optical signal coupled from the waveguide to any of the second set of trenches,  
           [0018]    such that the first section, the second section and the third section each has a predetermined polarization characteristic per unit length and the lengths of the first section, second section and third section are chose to provide a predetermined polarization characteristic to the optical signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    The invention is now described with respect to the figures in which:  
         [0020]    [0020]FIG. 1 is a section view of a waveguide structure that is polarization dependant;  
         [0021]    [0021]FIG. 2 is a section view of a prior art waveguide structure;  
         [0022]    [0022]FIG. 3 a  is a top view of a first embodiment of the invention;  
         [0023]    [0023]FIG. 3 b  is a section view of the first embodiment of the invention in which no trenches are present;  
         [0024]    [0024]FIG. 3 b  is a section view of the first embodiment of the invention in which trenches are present; and,  
         [0025]    [0025]FIG. 4 is a second embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]    Referring to FIG. 1 a cross section view of a prior art waveguide structure formed on a planar substrate  100  is shown. The waveguide structure comprises a waveguide core layer  101 , an upper cladding layer  102  with a ridge  102   a,  a lower cladding layer  107  and a planar substrate  106 . A horizontal axis  103  and a vertical axis  104  are also shown. The line  105  corresponds to region of equal optical intensity for light propagating within the waveguide of the waveguide structure. This waveguide structure has been designed to propagate only a lowest order single mode of light. Light having other higher order modes will dissipate within this structure far faster than light propagating along the waveguide and having the desired single mode. The waveguide core layer  101  has a higher index of refraction than the upper cladding layer  102 , the lower cladding layer  107 , and planar substrate  106 . The ridge  102   a  of the upper cladding layer serves to weakly confine light propagating within the waveguide. Light propagating within the waveguide is subjected polarization due to differences in propagation of light associated with a TE mode and a TM mode. Typically, it is very difficult to design a waveguide substrate in which the two polarization states are equal without incurring other undesired properties, such as very high insertion loss or very high cost.  
         [0027]    Referring to FIG. 2, a prior art waveguide structure is shown. This waveguide structure includes a complex set of layers that form a polarization compensated ridge waveguide. The waveguide structure comprises a waveguide substrate  10 , a lower cladding layer  12 , a waveguide core layer  14  having a set of quantum well layers  18 , and an upper cladding layer  16 . The manufacture of this waveguide involves a large number of steps using very expensive equipment. Additionally, the tolerances necessary to produce the desired optical performance are often difficult to maintain. For example, the quantum well layers incorporated in this structure are not easily produced rapidly and therefore the production of this waveguide device typically necessitates using a very expensive piece of equipment for an extended period of time. Clearly, this cost is reflected in the finished cost of the waveguide substrate.  
         [0028]    Referring to FIG. 3 a,  a top view of ridge waveguide structure according to a first embodiment of the invention is shown. This waveguide structure features a waveguide substrate  300 , a ridge  301  and a plurality of trenches  302 . The figure also shows a first section line  304  and a second section line  305 . A person of skill in the art of integrated waveguide design will be aware that this type of structure is useable for a semi-conductor waveguide, such as an Indium Phosphide waveguide or a Galium Arsenide Waveguide. Additionally, such a person will be aware that the trenches  302  are easily produced usilng a deep etching process.  
         [0029]    Referring to FIG. 3 b,  a cross section view corresponding to the first section line  304  is shown. The section view includes: a waveguide substrate  310 , a core layer  311 , a lower cladding layer  313  and an upper cladding layer  312  with ridges  301 . The intensity of light corresponding to the TE mode profile  320  and TM mode profile  321  are also shown. This section is highly analogous to the section view of a prior art waveguide provided by FIG. 1. Clearly, if this section corresponded to the entire length of the waveguide then the waveguide would have undesirable polarization consistent with that of the prior art in FIG. 1. It is readily apparent that the TE mode profile  320  and the TM mode profile  321  have slightly different shapes. As is well known to those of skill in the art, as an optical signal propagates within the waveguide structure of FIG. 3 a  it is partially attenuated and the TE mode will undergo less attenuation than the TM mode in the waveguide structure of FIG. 3 b.    
         [0030]    Referring to FIG. 3 c,  a cross section view corresponding to the second section line  305  is shown. The cross section view clearly shows the waveguide substrate  310 , a core layer  311  and an upper cladding layer  312  with ridges  301 . The trenches  302  are also present. Lines  320  and  321  correspond to the TE mode profile and the TM mode profile, respectively. The trenches  302  are disposed to cause a small portion of the TE mode to couple into the trenches  302 . A substantially smaller portion of the TM mode also couples into the trenches  302 . The result is an optical signal propagating within the waveguide structure shown in FIG. 3 b  will be partially attenuated and a portion of the optical signal corresponding to the TE mode experiences a higher degree of attenuation than a portion corresponding to the TM mode.  
         [0031]    As is well known to those of skill in the art, the attenuation of a waveguide is dependent upon an attenuation characteristic per unit length multiplied by the length through the waveguide that the optical signal propagates. Thus, once the characteristics of the waveguides of FIG. 3 b  and FIG. 3 c  have been determined, it is a simple matter to determine the ratio of lengths of the waveguide structures shown in FIG. 3 b  and FIG. 3 c  in order to produce a waveguide structure having a desired polarization dependent loss. Thus, a first design of a waveguide structure consistent with FIG. 3 a  includes an optical path having negligible polarization dependant loss. A second design of a waveguide structure consistent with FIG. 3 includes an optical path having a predetermined polarization dependant loss that has been chosen to compensate for the polarization characteristic of another optical component, for example, a semiconductor optical amplifier (SOA) or an erbium doped waveguide array (EDWA).  
         [0032]    As a person of skill in the art will be aware, the depth of the trench should be sufficiently deep that the attenuation characteristic of the corresponding waveguide is affected. The trench is optionally much deeper. In fact, the invention will support a trench that penetrates the entire depth of the substrate. Thus, for the purposes of this document, a trench should be considered as a feature disposed proximate a waveguide having sufficient depth to attenuate one of the TE and TM modes more than the other. Referring to FIG. 4 a second embodiment of the invention is shown having a feature intended to substantially duplicate some of the effects of the trenches as described with reference to FIG. 3 a,    3   b  and  3   c.  The waveguide structure of FIG. 4 features a waveguide core having a periodically varying width. In a region in which the waveguide core is narrow the waveguide is sufficiently narrow that the mode profiles of the lowest order TE and TM modes are affected. In a region where the width of the waveguide core is a maximum the waveguide core is sufficiently wide that the mode profiles of the lowerst order TE and TM mode experience a different attenuation characteristic than in the narrow region. Optionally, in the region where the waveguide core width is maximized, the lower order TE and TM mode are not substantially affected by the presence of the trench.  
         [0033]    A person of skill in the art will appreciate that embodiments of the invention are useful for both active and passive waveguide structures. Although a wide variety of optical components will produce polarization dependent loss, it is felt that the invention is particularly useful in compensating for polarization dependent loss in active optical components such as laser sources and optical amplifiers. Since a deep etching process is commonly used step in producing ridge waveguide structures, the costs of providing polarization compensated components is minimal using the teaching of the invention. A person of skill will be aware that indium phosphide (IP) and gallium arsenide (GaAs) are commonly used materials for building laser sources and optical amplifiers with ridge waveguides.  
         [0034]    Although various embodiments of the invention are shown featuring a ridge waveguide structure it will be apparent to one of skill in the art that a buried waveguide structure is easily modified to vary the polarization of a single mode optical signal optical signal propagating within such a buried waveguide. Numerous other embodiments of the invention may be envisioned by a person of skill in the art of waveguide design without departing from the spirit or scope of the invention.