Abstract:
A wavelength selective switch is provided. The wavelength selective switch according to the present invention comprises: an optical demultiplexer for separating an incident light with a plurality of wavelengths multiplexed into a plurality of wavelength lights and outputting the separated wavelength lights; an optical amplifier for selectively amplifying or absorbing the separated wavelength lights; an optical deflector for selectively deflecting outputs of the optical amplifier; and an optical multiplexer for multiplexing the selectively deflected lights and outputting the multiplexed lights.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
       [0001]    This application claims the benefit of Korean Patent Application No. 10-2007-0111014, filed on Nov. 1, 2007, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a wavelength selective switch (WSS), and more particularly, to a WSS which is capable of electrically selecting or switching a wavelength without any physical displacement. 
         [0004]    This work was partly supported by the IT R&amp;D program of MIC/IITA [2007-S-011-01, Development of Optical Switches for ROADM]. 
         [0005]    2. Description of the Related Art 
         [0006]    Initially, a wavelength division multiplexing (WDM) transmission system was used as a point-to-point transmission apparatus for connecting high-capacity nodes together. However, as the application areas of WDM transmission systems have been expanded and diversified, an optical add-drop multiplexing (OADM) technology capable of adding/dropping one or more wavelengths in each node is further required. An OADM has a structure of input/output ports and wavelength-add/drop ports where a plurality of wavelengths are multiplexed. Thus, an OADM is used in connecting intermediate nodes in a transmission path in units of wavelengths to expand the network connectivity and to increase efficiency. In an early OADM network, an F-OADM (fixed OADM) was adopted which is capable of adding/dropping only fixed wavelengths. In the network employing the F-OADM, network resources are wasted since a number of wavelengths are not used in an actual traffic transmission. Therefore, in the application of the F-OADM, the traffic needs to be predicted before the network is established and add/drop wavelengths need to be designed to be suitable for the prediction. Moreover, when a new wavelength is assigned, operating expenditures (opex) increase and a node connection status cannot be checked in real-time. 
         [0007]    Reconfigurable OADM (R-OADM) has been cited as an alternative technology for overcoming the limitations of the F-OADM since the early stages of WDM. R-OADM is reconfigurable by freely adding/dropping wavelengths of a node in a remote location and efficiently reconstructing the wavelength connection status of the entire network, thereby flexibly coping with a change in the traffic situation. Therefore, the maintenance costs (opex) can be reduced. In this regard, R-OADM has been regarded as an alternative technology for reducing capital expenditures (capex) for initial equipment purchase of the network, which reaches a limit at present. 
         [0008]    R-OADM can be largely classified as a broadcast and select system or a switch-based system. The broadcast and select system uses a dynamic channel equalizer (DCE) or a wavelength blocker (WB). Specifically, since the DCE-based system has less loss in a transmission path, compared to the switch-based system, a plurality of nodes can be accommodated in the DCE-based system. 
         [0009]    The switch-based R-OADM has defects of difficulty to be realized in a system using a full matrix and of a big volume. It is also expensive. Moreover, when the R-OADM uses a 2×2 switch, the number of pairs of transmitter/receiver needs to be as many as the number of used wavelengths, or a full matrix switch needs to be further added to an add/drop path. However, the R-OADM of the switch-based system has the advantage of mass-production and low expense of a multiplexer (MUX)/a demultiplexer (DeMUX), variable optical attenuator (VOA), and an integrated planar lightwave circuit (iPLC) structure formed by combining the 2×2 or 1×2 switches, as the PLC technology using a silicon substrate is developed. 
         [0010]    Another form of the switch-based system uses a WSS, whereby input/output between nodes can be freely selected. Thus, this form has the highest flexibility among the systems. A R-OADM switch using a WSS has been developed using an micro electromechanical system (MEMS) or a liquid crystal (LC) and can be in the form of a two-dimensional system or a three-dimensional system. 
         [0011]    A conventional WSS structure based on the 3D MEMS is disclosed in U.S. Pat. No. 6,625,346, U.S. Pat. No. 7,236,660, etc. Another waveguide structure is disclosed in U.S. Pat. No. 6,389,199. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention provides a non-displaceable wavelength selective switch (WSS) which is capable of switching without any physical displacement and makes a bulk-type optical component unit as a 2D hybrid or monolithic integration, to simplify operation and configuration thereof. 
         [0013]    According to an aspect of the present invention, there is provided a wavelength selective switch comprising: an optical demultiplexer for separating an incident light with a plurality of wavelengths multiplexed into a plurality of wavelength lights and outputting the separated wavelength lights; an optical amplifier for selectively amplifying or absorbing the separated wavelength lights; an optical deflector for selectively deflecting outputs of the optical amplifier; and an optical multiplexer for multiplexing the selectively deflected lights and outputting the multiplexed lights 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
           [0015]      FIG. 1  is a block diagram of a wavelength selective switch (WSS), according to an embodiment of the present invention; 
           [0016]      FIG. 2  illustrates an 1×N WSS structure, according to an embodiment of the present invention; and 
           [0017]      FIG. 3  illustrates a monolithically integrated 1×N WSS structure based on an InP, according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. 
         [0019]      FIG. 1  is a block diagram of a wavelength selective switch (WSS), according to an embodiment of the present invention. Referring to  FIG. 1 , the WSS comprises an optical demultiplexer  1 , an optical amplifier  2 , an optical deflector  3 , and an optical multiplexer  4 . 
         [0020]    The optical demultiplexer  1  separates an incident light into a plurality of wavelength signals. The optical amplifier  2  amplifies or absorbs the separated wavelength signals, respectively. The optical deflector  3  controls an angle of deflection of the light output by the optical amplifier  2  and changes an orientation of the light. The optical multiplexer  4  multiplexes the plurality of signals. 
         [0021]    According to an embodiment of the present invention, the optical demultiplexer  1  or the optical multiplexer  4  may use an arrayed waveguide grating (AWG) or concave grating (CG). 
         [0022]    An AWG uniformly changes the phases of waveguide modes with respect to a plurality of optical signal wavelengths so that the corresponding wavelength is selected to reach a specific position through a change of a radiation angle or angular dispersion. This is called linear dispersion. 
         [0023]    A CG diffracts an incident beam for each wavelength in a periodic grating structure to change the phase of the incident beam by a concave structure so that the phase of the incident beam is focused in an output direction, to obtain the angular dispersion and linear dispersion. 
         [0024]    In the current embodiment of the present invention, the optical amplifier  2  is an SOA (semiconductor optical amplifier) manufactured as a semiconductor device. The optical amplifier of the optical semiconductor device obtains an optical gain from a semiconductor medium by a current injection and amplifies or attenuates the incident beam. In the optical amplifier, a gain medium, that is, an active layer is grown on a substrate formed of generally GaAs, InP, or sapphire, and the material composition of the substrate and the active layer may vary according to an operating wavelength. 
         [0025]    According to the current embodiment of the present invention, the SOA operates in a C-band (1530 to 1570 nm) or in an L-band (1570 to 1612 nm). Accordingly, the active layer of an InGaAsP or InGaAs material is grown on the InP substrate. 
         [0026]    As a core element for selecting a wavelength, the optical deflector  3  performs a variable prism function with respect to the current injection in an optical waveguide structure. Since the optical deflector  3  is implemented in an integrated form within an free propagation region (FPR) of the AWG or a slab optical waveguide of the CG, its pattern needs to be designed to generate optical deflection of a radiated beam. However, in the AWG including the optical deflector in a triangular shape or any shape, the phase of the radiated beam may be adjusted by additionally modifying the phase of deflected signals by partially changing the waveguide structure through, for example, etching or re-growing and the like, on a part of the FPR, or by additionally inserting a structure which is capable of controlling the phase into the waveguide array positioned between two FPRs. Furthermore, in the CG including the optical deflector in any shape, deflective characteristics may be partially changed by partially etching a part of the slab waveguide or controlling a pitch of the grating. Therefore, the pattern of the optical deflector in the current embodiment of the present invention may be embodied in the structure to deflect the light by changing a refractive index of the waveguide by voltage application in addition to the current injection. 
         [0027]    As described above, to realize the WSS, the SOA is required as a tunable switch which employs the optical deflector  3  in the AWG or CG, and a variable optical attenuator (VOA) to amplify or attenuate a specific channel signal. 
         [0028]      FIG. 2  illustrates a 1×N WSS structure according to an embodiment of the present invention. In  FIG. 2 , the same numbers as those used in  FIG. 1  refer to the same elements. Reference numeral  11  indicates a silica AWG used as an optical demultiplexer  1  and reference numeral  20  indicates an InP CG chip. 
         [0029]    The AWG  11  comprises an input waveguide  110  of a single channel, a first FPR  111 , a waveguide array  112 , a second FPR  113 , and an output waveguide  114  of a plurality of channels and separates an input single channel into a plurality of channels. Since the technology of the AWG  11  having the above-described structure is well known to those skilled in the art, a detailed description thereof will not be provided herein. 
         [0030]    The CG chip  20  comprises an input waveguide  201 , an optical amplifier  2 , an optical deflector  3 , a CG  4  as an optical multiplexer, and an output waveguide  202 . The CG  4  further comprises a grating  41  and a slab waveguide  42 . 
         [0031]    In the structure as illustrated in  FIG. 2 , the AWG  11  and the CG  20  are realized by hybrid integration. 
         [0032]    In general, a silica-based AWG is a very stable device with respect to manufacturability and characteristics, but it cannot be used in the manufacture of an optical active device such as the SOA. In the current embodiment of the present invention, the AWG  11  demultiplexes an input optical signal. 
         [0033]    The signals demultiplexed into corresponding channels by the AWG  11  are transferred to the input waveguide  201  of the CG chip  20  from the output waveguide of the AWG  11 , and the SOA  2  performs the function of the VOA. 
         [0034]    The optical deflector  3  is embodied on a plane of optical incidence of the CG  4  to transfer the incident light from the SOA  2  to the grating  41  through the slab waveguide  42 . When a current is not supplied, the optical deflector  3  concentrates the light of each channel to any single point of the grating  41  and the grating  41  outputs the concentrated lights as a single beam to any one channel of the output waveguide  202 . 
         [0035]    When a current is supplied to the optical deflector  3 , the light of each channel is deflected to be incident on the grating  41  at a different incident angle and is radiated by the grating  41  so as to be output to each different channel of the output waveguide  202 . 
         [0036]    A dotted line portion in  FIG. 2  indicates a Rowland circle-based CG for convenience. Then, a trace in a straight line, oval, or any shape is possible according to the pitch of the grating  41 . 
         [0037]    The optical deflector  3  comprises an m deflector array having a plurality of deflectors. Each deflector is independently operated. When no current is injected, the grating  41  is designed to have the same angle of diffraction β 1  with respect to the wavelengths (λ 1  to λ m ) and the incident angles (α 1  to α m ) of the incident beam of each channel, and to multiplex a plurality of wavelengths being incident. When an electrical signal is applied to the optical deflector  3 , the incident angles of each of the corresponding channels are changed by refractive index changes within the deflection pattern, and more specifically, by refractive index reductions. Therefore, it is critical to design the grating  41  so that a diffraction angle moves within a range of β 1  to β N  with respect to an incident angle change. 
         [0038]    In order to increase the number of channels N of the output waveguide  202  in the structure of the CG  4 , the structure may be designed to generate a relatively large change in the refractive index with respect to the same current injection into the optical deflector  3  or to have a pattern for generating a relatively large beam deflection with respect to the same change in the refractive index. 
         [0039]    In order to increase variation in the refractive index, a core where the light traverses in the waveguide may be manufactured of a material with a band gap wavelength being close to an operation wavelength, that is, a channel wavelength. In this case, however, since an optical loss may increase, the material needs to be selected considering the optical loss. The structure of a pattern in which a beam deflects greatly is to be designed so that the right and left of a deflection pattern are as asymmetric as possible with respect to a beam traverse direction. Furthermore, in order to increase a difference of the variation in the refractive index within the deflection pattern, namely a value relevant to a difference of phase variation of radiation beams, a position of the deflection pattern needs to be designed to be spaced, as far as possible, from an end of the input waveguide of the deflector array. However, since the position of the deflection pattern needs to be far enough for the radiation beams to be received, if designed to be spaced excessively far, the deflection pattern becomes large and thus, the spaces between the input waveguides of the deflector array nearby the slab waveguide  42  become large, so that the entire size of the CG  4  may increase. 
         [0040]    In the above-designed or optimized waveguide structure and deflection pattern, a method of decreasing a grating radius of the grating  41  or reducing a grating order may be employed in order to additionally increase the number of channels of the output waveguides  202 . However, this method is not preferable because it broadens a spectral passband of the CG  4 . Thus, it is preferable to reduce the spaces between output waveguides on the Rowland circle in the designed structure of the CG  4 , resulting that more output waveguides can be included to receive the light with respect to a predetermined diffraction angle change. Furthermore, the spaces between the output waveguides of the CG  4  may be arranged so as to be smaller than the spaces between the input waveguides of the CG  4 , to enable broad switching at a narrow deflection angle. 
         [0041]    The optical deflector  3  may be inserted into the FPRs  111  and  113  of the AWG  11  instead of the CG 4 . Furthermore, the AWG  11  or CG  4  may also be embodied with a polymer or SOI (silicon-on-insulator) material in addition to the silica material. 
         [0042]      FIG. 3  illustrates a monolithically integrated 1×N WSS structure  5  based on InP, according to another embodiment of the present invention. 
         [0043]    Referring to  FIG. 3 , the WSS structure  5  is with an input waveguide  51  and an output waveguide array  57  positioned on the same plane  70  which is anti-reflection-coated. 
         [0044]    As illustrated in  FIG. 3 , an incident light of m different wavelengths multiplexed passes through the input waveguide  51  and is incident onto a first CG  52  with an incident angle α at the end of the input waveguide  51 . In the first CG  52 , m signals are transferred to a first output waveguide array  53  with diffraction angles β 1  to β m  which corresponds to each channel wavelength. The first CG  52  and second CG  54  illustrated in  FIG. 3  have the same structure as the CG  4  illustrated in  FIG. 2  and operate in the same manner. 
         [0045]    The SOA  2  amplifies or attenuates the light of each channel which is incident from the first output waveguide array  53 . The optical deflector  3  is implemented in the second CG  54  and deflects the incident light so as to have incident angles δ 1  to δ m . A grating of the second CG  54  is designed to have a diffraction angle γ 1  so as to output the incident light to a first output port  56  when the optical deflector  3  does not operate. As the optical deflector  3  operates, the incident light is deflected so as to have a diffraction angle γ 1  to γ N  and to be output for each channel. 
         [0046]    In this case, when an i-th channel wavelength is output, a light of the corresponding channel is obtained in the first output port  56  by turning on an i-th amplifier only. When the i-th channel wavelength is switched to another output port, a light of the corresponding channel is obtained at the other output port by injecting a current into an i-th deflector among the deflector array so that the i-th deflector deflects the incident light and changes the incident angle. The output port for each channel is connected to a corresponding channel of a second output waveguide array  57 . A method for selectively switching a desired channel can be applied to the WSS illustrated in  FIG. 2 . 
         [0047]    Accordingly, since the structure illustrated in  FIG. 3  can be manufactured as a single chip, compared to the structure illustrated in  FIG. 2 , the time and cost required for the 2D hybrid integration are reduced, the size of the device is small, and the reliability is relatively high. 
         [0048]    In the WSS according to the present invention, since the optical demultiplexer, optical amplifier, optical deflector and optical multiplexer can select the channel wavelength by the current injection to the optical amplifier array for an input optical signal of different channels and perform switching of the selected channel wavelength by applying an electric signal into the deflector, the higher reliability and smaller volume can be achieved, compared with the conventional bulk-type switching devices with physical displacement. Furthermore, in the WSS according to the present invention, since an optical component unit is two-dimensionally hybrid or monolithically integrated, an optical arrangement between the component units can be minimized or removed. In addition, the WSS according to the present invention is structurally stable and is capable of significantly increasing switching speed, compared with the conventional mechanical switching structure. 
         [0049]    While optimum exemplary embodiments of the present invention have been particularly shown and described with reference to accompanying drawings thereof, the specific terms used herein are for the purpose of describing the invention and are not intended to define the meanings thereof or be limiting of the scope of the invention set forth in the claims. Therefore, it will be understood by those of ordinary skill in the art that various changes and equivalent embodiments in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the claims. Furthermore, the scope of true technical protection of the present invention shall be defined by the technical idea of the following claims.