Abstract:
A planar lightwave circuit having an optical filter, which comprises: a first waveguide; a second waveguide separated from the first waveguide, in which the width of a first cross-section opposite to the first waveguide is wider than that of a second cross-section of the other side; and a trench located between the first and second waveguides, in which the optical filter is positioned.

Description:
CLAIM OF PRIORITY 
   This application claims priority to an application entitled “Planar Lightwave Circuit Having Optical Filter,” filed in the Korean Intellectual Property Office on Nov. 16, 2004 and assigned Serial No. 2004-93493, the contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a planar light wave circuit, and more particularly to a planar lightwave circuit having an optical filter. 
   2. Description of the Related Art 
   Recently, communication networks of FTTH (Fiber-To-The-Home) types and FTTx (x: Premises, Home, Business, etc.) in which a central base station is linked to subscribers through optical fibers are used. The optical communication method for these FTTH or the FTTx networks require various types of optical transmitting and receiving devices for performing bi-directional communications. 
   The optical transmitting and receiving devices used for bi-directional communications typically include detecting data after converting optical signals of the corresponding waves to electrical signals and then transmitting the data to the receiving side after the conversion of electrical data into optical signals. An integrated planar lightwave circuit is mainly used as the optical transmitting and receiving device. More specifically, planar lightwave circuits having wave division multiplexing or demultiplexing filters mounted thereto are being used. Recently, planar lightwave circuits equipped with optical filters having various functions have been deployed for various applications in the optical communication network. 
     FIG. 1  shows a planar lightwave circuit having an optical filter. Referring to  FIG. 1 , the planar lightwave device  100  comprises first and second waveguides  111  and  112  separated from each other, a trench  120  which separates the first waveguide  111  from the second waveguide  112  by a predetermined distance, and an optical filter  130  positioned in the trench  120 . The planar lightwave circuit  100  forms the first and second waveguides  111  and  112  using a lower clad, a core, and an upper clad that are sequentially stacked on the semiconductor substrate. 
   The trench  120  is formed by dicing the lower clad, the core, and the upper clad, and as a result, it separates the first waveguide  111  from the second waveguide  112  by a predetermined distance. The optical filter  130  is positioned in the trench  120  and between the first and second waveguides  111  and  112 , and a wave division demultiplexing filter for demultiplexing the inputted optical signals is used. However, since the waveguides are separated from each other by the trench, the proceeding optical signals are diffused in the trench and as a result, some of the signals fail to converge toward the other side waveguide, thereby causing insertion losses of the optical signals. 
   To address the above problem, a planar lightwave circuit using a multi-mode waveguide has been suggested as a means for restraining the generation of insertion losses. However, the multimode waveguide does not provide a sufficient coupling efficiency suitable for a real product. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art and provides additional advantages, by providing a planar lightwave circuit for restraining the loss generated by mounting an optical filter. 
   In one embodiment, there is provided a planar lightwave circuit having an optical filter, which includes: a first waveguide; a second waveguide separated from the first waveguide, in which the width of a first cross-section opposite to the first waveguide is narrower than that of a second cross-section of the other side; and a trench located between the first and second waveguides, in which the optical filter is positioned. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  illustrates a planar lightwave circuit having a conventional optical filter; 
       FIG. 2  illustrates a planar lightwave circuit according to an embodiment of the present invention; 
       FIG. 3  is a view for showing the planar lightwave circuit shown in  FIG. 2 ; 
       FIG. 4  is a view for showing the planar lightwave circuit shown in  FIG. 2 ; and 
       FIGS. 5 and 6  are graphs comparing optical signal coupling losses of the planar lightwave circuit according to the present invention with those of a conventional planar lightwave circuit. 
   

   DETAILED DESCRIPTION 
   Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. For the purposes of clarity and simplicity, a detailed description of known functions and configurations incorporated herein will be omitted as it may make the subject matter of the present invention unclear. 
   Referring to  FIG. 2 , a planar lightwave circuit  200  according to the present invention includes first and second waveguides  211  and  212 , a trench  220  which separates the first waveguide from the second waveguide, and an optical filter  230  positioned in the trench  220 . 
   Referring to  FIG. 3 , the planar lightwave circuit  200  includes a substrate  201 , a lower clad  240 , a core( 210 ), and an upper clad( 250 ) sequentially stacked on the substrate  201 . The first wave guide  211  is a single mode waveguide and includes a linear transmission waveguide  221   a  and a reflection waveguide  211   b . The reflection waveguide  211   b  is identical to the second waveguide  212 . The transmission waveguide  211   a  is a single mode waveguide and outputs the inputted optical signals to the optical filter. The reflection waveguide  211   b  has a shape of a parabolic horn or polynomial curve and transfers the light reflected in the optical filter  230 . 
   The trench  220  is formed by dicing portions of the lower clad  240 , core( 210 ), and an upper clad( 250 ), and serves to separate the second waveguide  212  from the first waveguide  211 . 
   The trench is inclined by a predetermined angle a with respect to an optical axis and determined according to the relation between the optical filter  230  and the first and second waveguides  211  and  212 . 
   Referring to  FIG. 4 , the second waveguide  212  has a profile according to Equation 1 or Equation 2. The optical signals inputted through the first waveguide  211  are diffused while passing through the second waveguide  212 . Therefore, the second waveguide compensates for the insertion loss generated in the trench. 
   Equation 1 (Parabolic Horn Structure)
 
 W ( t )= W   1 +√(α tL     h   ),(0 ≦t ≦1)
 
   In Equation 1, W(t) is a function which represents the width change to the lengthwise direction parallel to the optical axis of the second waveguide  212 , and L h  represents the length of the second waveguide  212 , and W 1  represents the width of one surface of the second waveguide  212  opposite to the first waveguide  211 . Here, α is a shape-controlling parameter for the line width of the second waveguide  212 . 
   Equation 2 (Polynimial Structure)
 
 W ( t )= W   1   +a ( tL )+ b ( tL ) 2   +c ( tL ) 3   + . . . +z ( t L ) n  
 
   In Equation 2, W(t) is a function which represents width change to the lengthwise direction parallel to the optical axis of the second waveguide  212 , L h  represents the length of the second waveguide  212 , and W 1  represents the width of one surface of the second waveguide  212  opposite to the first waveguide  211 . W(t) is a polynomial function, and the second waveguide  212  has a structure in which the width thereof is gradually wider according to Equation 2. 
     FIGS. 5 and 6  are graphs of a comparison of optical signal coupling losses between the planar lightwave circuit according to the present invention and a conventional planar lightwave circuit. More specifically,  FIG. 5  compares the planar lightwave circuit according to the present invention with the conventional planar lightwave circuit in case a refractive index difference between the cores and the clads is 0.3%, and  FIG. 6  compares the present invention and the prior art in case the refractive index difference is 0.75%. The trench widths of the planar lightwave circuit shown in  FIGS. 5 and 6  can be variously applied with 20 to 40 μm. 
   Referring to  FIG. 5 , in the planar lightwave circuit according to the present invention, W 1  is 8 μm, α is 0.21, L h  is 274 μm, and the profile is determined in Equation 1 and Equation 2. 
   The dotted line in the graph represents the insertion loss of a planar lightwave circuit which is not according to the teachings of the present invention. It shows the insertion loss between within 0.03 to 0.06 dB. In contrast, the solid line represents the insertion loss change of the planar lightwave circuit which includes the horn waveguide at the second waveguide according to the present invention, and the insertion loss change is between 0.01 to 0.02 dB which is a value less than 50% of the insertion loss of the dotted line. 
   Referring to  FIG. 6 , the graph shows the test result when W 1  is 4 μm, α is 0.78, L is 277.5 μm, and the profile is determined in Equation 1. 
   The dotted line represents the insertion loss of the planar lightwave circuit which has no horn structure as in the present invention. In the dotted line, the insertion loss is 0.1 dB in case the width of the trench is 20 μm, and the insertion loss is 0.25 dB in case the width of the trench is 40 μm. Note that the insertion loss I is increased by 25 times compared with that of the trench having a width of 20 μm in case that the width of the trench is 40 μm. 
   Meanwhile, the solid line represents the insertion loss of the planar lightwave circuit having a horn structure according to the teachings of the present invention. In the solid line, the insertion loss is within 0.02 to 0.03 dB and is decreased by under about 1/10 compared with the case in which the planar lightwave circuit having no horn structure. 
   Namely, as shown in  FIGS. 5 and 6 , the insertion losses of the planar lightwave circuit using the waveguide of horn structure is decreased in comparison with those of the conventional planar lightwave circuit. On the other hand, the insertion loss of the planar lightwave circuit including the waveguide of horn structure of  FIG. 6  is 0.02 to 0.03 dB, and is similar to that of the planar lightwave circuit which has refractive index difference of 0.3% which is shown in  FIG. 5 . 
   Accordingly, the insertion loss of the planar lightwave circuit which comprises a waveguide of the profile according to the present invention is remarkably smaller than that of the conventional device. As the loss compensation of the planar lightwave circuit having a waveguide of high refractive index difference is large, the structure of the present invention can be applied to the planar lightwave circuit having refractive index difference larger than 0.3%. 
   Therefore, as the horn structure is applied to a waveguide having a large refractive index difference of 0.75%, the compensation of the insertion loss generated by a trench for mounting the optical filter is larger, thus the present invention can be applied to high density integration of the planar lightwave circuit and to an HIC (high index contrast) platform to which an optical filter is mounted. 
   Furthermore, while the insertion loss of the planar lightwave circuit which does not include a waveguide of the horn structure is largely changed according to the width of the trench, the insertion loss of the planar lightwave circuit which includes the waveguide of the horn structure is smoothly changed regardless of the width of the trench. 
   Namely, the planar lightwave circuit according to the present invention has a structure insensible to the width of the trench, and thus the width of the trench can be variously applied. Therefore, the planar lightwave circuit according to the present invention can use various types of optical filters. 
   Table 1 below is a table for comparing insertion losses of the planar lightwave circuit which can perform multiplexing/demultiplexing functions in which the light of some waves are reflected and the light of some waves are transmitted. The insertion losses of the planar light wave circuit which has refractive index difference of 0.75% and includes waveguides with the horn structure are compared with those of the planar light wave circuit which has no horn structure. 
   The planar lightwave circuit applied to Table 1 has an optical filter of multiplexing/demultiplexing type in which the light having a wave of 1310 nm can be transmitted and in which the light having a wave of 1550 nm can be reflected. The optical filter used in the experiment of Table 1 has a substrate whose thickness is 4 μm, and has a overall thickness of 22 μm including the thickness of a thin film. 
   
     
       
             
             
             
           
             
             
             
           
         
             
                 
                 
             
             
                 
               Conventional waveguide 
               Waveguide with horn 
             
             
                 
               (refractive index 
               structure (refractive index 
             
             
                 
               difference of 0.75%) 
               difference of 0.75%) 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
               Transmission 
               ~1.0 dB 
               &lt;0.5 dB 
             
             
               loss (T) 
             
             
               Reflection 
               ~1.2 dB 
               &lt;0.8_dB 
             
             
               loss (R) 
             
             
                 
             
           
        
       
     
   
   In case the refractive index difference is 0.75% the transmission and reflection losses of the planar lightwave circuit of the horn structure are decreased by more than 50%, compared with the planar lightwave circuit which does not have the waveguide of horn structure. In addition, even when a plurality of optical filters are connected in type of cascade, the overall insertion loss can be restrained. 
   While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.