Abstract:
A wavelength selector to be used in WDM networks is provided. The wavelength selector is composed of a circulator, an arrayed waveguide grating (AWG), which is used for wavelength demultiplexing and multiplexing part, and an electro-optical (EO) switching part. The input light after the circulator is demultiplexed through the AWG part and each channel of the demultiplexed lights is modulated and reflected through the EO switching part which is formed as Michelson type interferometer with mirror parts returning the light to the AWG. The modulated and reflected light is multiplexed through the AWG and the direction is changed to the output through the circulator.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a wavelength selector to be used in wavelength-division multiplexing (WDM) networks, and more particularly, to a wavelength selector to be used in WDM networks using an electro-optic (EO) switch.  
           [0003]    2. Description of the Related Art  
           [0004]    [0004]FIG. 1 illustrates an example of a conventional wavelength selector to be used in WDM networks. Referring to FIG. 1, the conventional wavelength selector  100  to be used in WDM networks includes a demultiplexing part  110 , an optical switching part  120 , and a multiplexing part  130 .  
           [0005]    The wavelength demultiplexing part  110  has a structure in which a demuplexer  114  comprised of an arrayed waveguide grating (AWG) is formed on a substrate  112 . The AWG is formed of silica, polymer, or a semiconductor material. The demultiplexer  114  demultiplexes light IN input from an input waveguide into light of each wavelength such as λ 1 , λ 2 , λ 3 , . . . , λ n−1 , and λ n , and outputs the light of the wavelengths to output waveguides.  
           [0006]    The optical switching part  120  has a structure in which a number N of semiconductor optical amplifiers (SOA)  124  are formed on a substrate  122 . Each of the optical amplifiers  124  is connected to each output waveguide of the demultiplexer  114 . Light of predetermined wavelength selected from different wavelengths such as λ 1 , λ 2 , λ 3 , . . . , λ n−1 , and λ n , output by the demultiplexer  114  passes through the SOA  124 , and light of the other wavelengths does not pass through the SOA  124 .  
           [0007]    The wavelength multiplexing part  130  has a structure in which a multiplexer  134  comprised of an AWG is formed on a substrate  132 . The AWG is formed of one of silica, polymer, or semiconductor materials. The multiplexer  134  outputs light to an output light OUT of a predetermined wavelength λ k  selected by the SOA  124 .  
           [0008]    The wavelength selector comprised of the wavelength demultiplexing part  110 , the optical switching part  120 , and the wavelength multiplexing part  130  includes a monolithic structure and a hybrid structure. The wavelength selector having a monolithic structure includes the wavelength demultiplexing part  110 , the optical switching part  120 , the wavelength multiplexing part  130  that are formed on a single substrate, i.e., on an InP (as a semiconductor material) substrate or polymer which can be electro-optically modulated. The wavelength selector having a hybrid structure includes the wavelength demultiplexing part  110 , the optical switching part  120 , and the wavelength multiplexing part  130  that are formed on a separate substrate and bonded to one another.  
           [0009]    It is advantageous that a wavelength selector of a monolithic structure is implemented by forming the SOA on the InP substrate, but the wavelength selector having a monolithic structure has a problem of complex fabrication processes and its high cost.  
           [0010]    Polymer material can be used also for the wavelength selector with a merit of simple fabrication process. Polymer materials for EO modulation, however, have a problem of very high propagation loss. So, it is desirable to compose a wavelength selector as a hybrid structure, in case of polymeric device. The wavelength demultiplexing part  110  and the wavelength multiplexing part  130  are formed on a substrate with a low optical loss in case of the wavelength selector of a hybrid structure. A problem for the hybrid structure is that an attachment process in which the wavelength demultiplexing part  110 , the optical switching part  120 , and the wavelength multiplexing part  130  should be aligned with one another and attached to one another, should be added to a fabrication process of the wavelength selector. This attachment process is a critical factor increasing the cost of product since it is usually performed using a high-priced aligning machine at quite a long process time. And the demultiplexer and the multiplexer should have the same characteristics in distributing the wavelength since there should be a critical loss and crosstalk if their characteristics are different. So, the couple of AWG should be chosen with a great care and should be tuned with a highly sensitive temperature controller so as for the couple of AWG operated with the same characteristics for the wavelengths.  
         SUMMARY OF THE INVENTION  
         [0011]    It is an objective of the present invention to provide a wavelength selector of a hybrid structure with a reduced attachment process and to overcome the complexity in tuning the couple of AWG with each other by using only one AWG.  
           [0012]    The wavelength selector includes an input, a wavelength demultiplexing part coupled to the input, which demultiplexes input light or distributes the lights as the wavelengths and outputs a plurality of output light of each wavelength, and an optical switching part including an electro-optic (EO) switch which transmits the plurality of output light from the wavelength demultiplexing part, and a mirror that reflects light transmitted from the EO switch to the opposite direction and selects the light of predetermined wavelengths by Michelson-type interferometry using the interference between the couple of light reflected from the couple of mirror.  
           [0013]    The input includes an input optical waveguide connected to WDM networks, from which the light is input, a transmission optical waveguide which transmits the light to the wavelength demultiplexing part and transmits the light after the reflection to the opposite direction, and a circulator including an output optical waveguide which outputs the light after the wavelength selection.  
           [0014]    It is also preferable that the wavelength demultiplexing part, connected to the input, outputs a plurality of light of different wavelengths through a plurality of optical waveguides and that a thermo-optic switch is to be connected to the optical waveguides to which the plurality of light is output.  
           [0015]    It is also preferable that the EO switch of the optical switching part is formed in an electro-optic (EO) polymer layer.  
           [0016]    It is also preferable that the EO switch includes a first optical waveguide, and a second optical waveguide whose refractive index is varied depending on bias voltage applied to it. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    The above objects and advantages of the present invention will become more apparent by describing the preferred embodiments with reference to the attached drawings in which:  
         [0018]    [0018]FIG. 1 illustrates an example of a conventional wavelength selector to be used in WDM networks;  
         [0019]    [0019]FIG. 2 illustrates a wavelength selector to be used in WDM networks, using an electro-optic (EO) switch;  
         [0020]    [0020]FIG. 3 is a cross-sectional view taken along line III-III of FIG. 2; and  
         [0021]    [0021]FIG. 4 illustrates a wavelength selector to be used in wavelength division multiplexed networks according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    The present invention will be described in detail with reference to the accompanying drawings in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be limited to the embodiments set forth herein.  
         [0023]    [0023]FIG. 2 illustrates a wavelength selector to be used in WDM networks, using an electro-optic (EO) switch, and FIG. 3 is a cross-sectional view taken along line III-III of FIG. 2.  
         [0024]    Referring to FIG. 2, the wavelength selector  200  to be used in WDM networks, using an electro-optic (EO) switch includes a wavelength demultiplexing part  210 , an optical switching part  220 , and a wavelength multiplexing part  230 .  
         [0025]    The wavelength demultiplexing part  210  has a structure in which a demuplexer  214  comprised of an arrayed waveguide grating (AWG) formed of polymer with little loss is formed on a substrate  212 . The demultiplexer  214  demultiplexes light IN incident from an input waveguide into light having different wavelengths such as λ 1 , λ 2 , λ 3 , . . . , λ n−1 , and λ n , and outputs the light having the wavelengths to output waveguides.  
         [0026]    The optical switching part  220  has a structure in which a number N of electro-optic (EO) switches  224  are formed on a substrate  222 . As shown in FIG. 3, each of the EO switches  224  is comprised of a first optical waveguide  224   a , a second optical waveguide  224   b , an upper electrode  224   c , a lower electrode  224   d , and a polymer cladding layer  224   e . The first optical waveguide  224   a  and the second optical waveguide  224   b  are formed on the polymer layer  224   e  on the substrate  222 . The lower electrode  224   d  is disposed between the polymer cladding layer  224   e  and the substrate  222 . The upper electrode  224   c  overlaps only with the second optical waveguide  224   b  on the polymer cladding layer  224   e . The second optical waveguide  224   b  which overlaps with the upper electrode  224   c  is formed of an electro-optic (EO) material whose refractive index can be varied depending on an applied bias voltage. The first optical waveguide  224   a  which does not overlap with the upper electrode  224   c  may be also formed of an electro-optic (EO) material.  
         [0027]    The EO switches  224  constitute a Mach-Zehnder type interferometry. That is, each light having different wavelengths such as λ 1 , λ 2 , λ 3 , . . . , λ n−1 , and λ n , output from the demultiplexer  214  is transmitted through the first optical waveguide  224   a  and the second optical waveguide  224   b  of each of the EO switches  224  by a predetermined mirror system. In this case, the existence of a phase difference between light passing through the first optical waveguide  224   a  and light passing through the second optical waveguide  224   b  is determined depending on whether a bias voltage is applied between the upper electrode  224   c  and the lower electrode  224   d  of the EO switches  224 . If there is a phase difference of π or odd multiple of π between the light passing through the first optical waveguide  224   a  and the light passing through the second optical waveguide  224   b , the light is radiated by destructive interference. On the contrary, if there is no phase difference between the light passing through the first optical waveguide  224   a  and the light passing through the second optical waveguide  224   b  or there is a phase difference of even multiple of π between the light passing through the first optical waveguide  224   a  and the light passing through the second optical waveguide  224   b , the light is transmitted to the next stage by the constructive interference.  
         [0028]    The wavelength multiplexing part  230  has a structure in which a multiplexer  234  comprised of an arrayed waveguide grating (AWG) is formed on a substrate  232 . The multiplexer  234  outputs light having a predetermined wavelength λ k  selected by the EO switches  222  to the output light OUT.  
         [0029]    Even though the EO switches  224  show very fast switching speed due to transmission speed in units of several ns, an attachment process among the wavelength demultiplexing part  210 , the optical switching part  220 , and the wavelength multiplexing part  230  is still required. In addition, the waveguide with the EO polymer material shows a optical loss as high as several (2 to 3) dB/cm while the length of the EO switches  224  should be more than several cm.  
         [0030]    [0030]FIG. 4 illustrates a wavelength selector to be used in WDM networks according to the present invention. Referring to FIG. 4, the wavelength selector  400  according to the present invention can be connected to WDM networks. The input light IN from the WDM networks is transferred to the circulator  402  through an input optical waveguide  402   a . The circulator  402  is connected to a wavelength demultiplexing part  410  and a transmission optical waveguide  402   b.    
         [0031]    The wavelength demultiplexing part  410  has a structure in which a demultiplexer  414  comprised of an arrayed waveguide grating (AWG) is formed on a substrate  412 . The demultiplexer  414  distributes the input light IN from the circulator  402  separately as the wavelengths such as λ 1 , λ 2 , λ 3 , . . . , λ n−1 , and λ n , and transfers the light of the each wavelength to output waveguides.  
         [0032]    Each light of different wavelengths such as λ 1 , λ 2 , λ 3 , . . . , λ n−1 , and λ n , and λ n , from the demultiplexer  414  is branched into a couple of optical waveguides  424   a  and  424   b  and is transmitted to the optical switching part  420 .  
         [0033]    The optical switching part  420  has a structure in which a number N of electro-optic (EO) switches  424  are formed on a substrate  422 . Each of the EO switches  424  is comprised of the first optical waveguide  424   a , the second optical waveguide  424   b , and the upper electrode  424   c . Each of the EO switches  424  further includes a lower electrode (not shown) and a polymer cladding layer (not shown). The upper electrode  424   c  overlaps only with the second optical waveguide  424   b . The second optical waveguide  424   b  which overlaps with the upper electrode  424   c  is formed of an electro-optic (EO) material whose refractive index can be varied depending on the bias voltage applied to it. The first optical waveguide  424   a  which does not overlap with the upper electrode  424   c  is formed with the same material. Both the first optical waveguide  424   a  and the second optical waveguide  424   b  are connected to a mirror  426  that is vertically disposed. The mirror  426  can be formed by coating a metal layer after providing a vertical facet, on which a mirror is to be formed, by etching.  
         [0034]    The EO switches  424  constitute a Michelson type interferometry so as for the length of the EO switches  424  to be relatively minimized. That is, each light having different wavelengths such as λ 1 , λ 2 , λ 3 , . . . , λ n−1 , and λ n ,output from the demultiplexer  414  is branched into the first optical waveguide  424   a  and the second optical waveguide  424   b  and is transmitted to the optical switching part  420 . Each transmitted light is reflected from the mirror  426  and is returned to the opposite direction through each optical waveguide  424   a  and  424   b . In this case, the magnitude of a phase difference between the light passing through the first optical waveguide  424   a  and the light passing through the second optical waveguide  424   b  is determined depending on whether a bias voltage is applied to the upper electrode  424   c . The light returned after reflection through the first optical waveguide  424   a  and that through the second optical waveguide  424   b  interferes at the joining part. As a result of the interference, only the light of selected wavelength is returned to the demultiplexer  414  of the wavelength demultiplexing part  410 . In this case, the demultiplexer  414  serves as a multiplexer and the light through it is transferred to the output through the output optical waveguide  402   c  of the circulator  402 .  
         [0035]    An electrode  416  for phase error correction is disposed on the second optical waveguide  424   b  in the wavelength demultiplexing part  410  adjacent to the optical switching part  420 . Phase errors may occur between the first optical waveguide  424   a  and the second optical waveguide  424   b  in the wavelength demultiplexing part  410  and between the first optical waveguide  424   a  and the second optical waveguide  424   b  in the optical switching part  420 . The phase errors may occur after the attachment process of the wavelength demultiplexing part  410  and the optical switching part  420 . When the phase errors occur, the phase errors can be corrected by the electrode  416  for phase error correction. That is, the phase errors are corrected by applying a bias voltage inducing a thermo-optic modulation in which the refractive index of the material is varied due to heat caused by the applied bias voltage.  
         [0036]    The optical waveguide of the wavelength demultiplexing part  410  is formed in a polymer layer with little loss, and the optical waveguide of the optical switching part  420  is formed in an electro-optic (EO) polymer layer. It is well known that an optical loss of the electro-optic (EO) polymer material is as high as  10  times of that of the passive polymer material forming passive devices or thermo-optic switches. Accordingly, in order to reduce the total optical loss of the wavelength selector  400 , the least portion of the optical waveguide for electro-optic (EO) switching operation is formed with the electro-optic (EO) polymer material, and the other part of optical waveguide is formed with the passive polymer material of a low loss.  
         [0037]    As described above, the wavelength selector to be used in WDM networks according to the present invention has the following advantages.  
         [0038]    First, the length of each of the electro-optic (EO) switches can be reduced by implementing the optical switching part as a Michelson type interferometry, and thus switching speed is improved by the decrease in the electric capacitance with the short length.  
         [0039]    Second, the electro-optic (EO) polymer layer with a relatively high optical loss is used by the least length in forming the electro-optic (EO) switches, and thus a total optical loss of the wavelength selector can be reduced.  
         [0040]    Third, only one relatively high-priced arrayed waveguide grating (AWG) is used, and thus the attachment point is decreased compared with that case using a couple of AWG. It can decrease the process time for the attachments, and thus manufacturing costs can be reduced.  
         [0041]    Fourth, It is not necessary to tune the wavelength property of AWG as in the case of previous techniques using a couple of AWG s in which the wavelength property should be tuned with each other. So, the selection of operation of AWG is much more simple.  
         [0042]    Fifth, the phase errors which can occur when the wavelength demultiplexing part is attached to the optical switching part, are corrected by the thermo-optic modulation, and thus the reliability of the wavelength selector can be improved.  
         [0043]    While this invention has been particularly shown and described with reference to 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.