Abstract:
This invention relates to optical add/drop devices. These optical add/drop devices are all based on waveguide grating-based wavelength selective switches. Four types of optical switches (S-, L-. X-, and O-type) are disclosed and used to build optical add/drop devices. In addition to the universal advantage of requiring no multiplexers and demultiplexers, each type of switches has its own advantages to build add/drop devices. A simple add/drop device can be made by using only two switches. A large-scale add/drop device can also be built upon same switches. Since the switches are integrated and fabricated on a silicon-based substrate, the size and cost of the add/drop devices are also significantly reduced.

Description:
RELATED APPLICATIONS  
       [0001]    Priority is hereby claimed under 35 U.S.C. §120 to U.S. Provisional Patent Application Serial No. 60/338,927 filed Oct. 22, 2001, U.S. Provisional Patent Application Serial No. 60/346,066 filed Jan. 3, 2002, U.S. Provisional Patent Application Serial No. 60/373,803 filed Apr. 19, 2002, U.S. patent application Ser. No. 10/104,273 filed Mar. 22, 2002, and U.S. patent application Ser. No. 10/___,___ filed Jun. 19, 2002, each of which is hereby incorporated by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates generally to technologies for switching and routing optical signals, and more particularly, relates to add/drop devices comprising waveguide grating-based wavelength selective switches.  
           [0004]    2. Description of the Related Art  
           [0005]    Optical wavelength division multiplexing (WDM) is a very important method used in modern optical fiber communication systems to dramatically increase data transmission rate in an optical network. In WDM systems, the data travels on a number of different wavelength optical signals (wavelength channels). Each wavelength channel carries its own data information. Therefore, with WDM technology, a single optical fiber can transmit a number of distinguishable optical signals simultaneously. The result is a significant increase in the effective bandwidth of the optical fiber and data transmitting rate of the communication system.  
           [0006]    In the WDM networks of the past, adding, dropping or cross connecting of individual wavelength channels has involved conversion of the optical signal back to the electrical domain. Development of all-optical switches for applications ranging from add-drop functionality to large-scale cross-connects is key to adding intelligence to the optical layer of the optical networking systems. However, with current technical limitations, all fiber network systems implemented with optical switches are still quite expensive.  
           [0007]    To employ WDM technology in an optical communication system, optical demultiplexers, switches, multiplexers, and add/drop devices are important. Current state of the art in optical switching and signal transmission systems are limited to optical switching of an entire spectral range without wavelength differentiation or selection. Due to the lack of wavelength selection, an optical switch operation must frequently operate with a wavelength de-multiplexing and multiplexing device to transfer optical signals of different wavelengths to different ports. This requirement leads to more complicated system configurations, higher manufacture and maintenance costs, and lower system reliability. For this reason, even though optical switches provide an advantage that the optical signals are switched entirely in the optical domain without converting them into the electrical domain, the cost and size of application cannot be easily reduced.  
           [0008]    An add/drop device is used to inject (add) or extract (drop) one or more wavelength channels to or from a WDM network. Current optical add/drop devices usually consist of various types of optical switches and require optical multiplexers and demultiplexers, as shown in the prior art of FIGS. 1A and 1B. FIG. 1A shows a typical block diagram of an optical add/drop device. Through the optical add/drop device, wavelength channels can be added or dropped to or from the main optical transmission trunk.  
           [0009]    [0009]FIG. 1B illustrates the construction of a typical prior art optical add/drop device. This optical add/drop device requires a demultiplexer and a multiplexer to carry out wavelength selective switching operations in order to accomplish the add/drop functions. The requirement of a demultiplexer and a multiplexer makes the prior art optical add/drop devices complex and costly to build. For a simple add/drop matrix, this requirement of demultiplexer and a multiplexer is a significant burden. In addition, for a larger add/drop matrix, these prior art optical add/drop devices suffer from their rapidly increasing complexity as the matrix size grows. 
       
    
    
     BRIEF DESCRIPTIONS OF THE DRAWINGS  
       [0010]    The present invention can be better understood with reference to the following drawings. The components within the drawings are not necessarily to scale relative to each other, emphasis instead being placed upon clearly illustrating the principles of the present invention.  
         [0011]    [0011]FIG. 1A is a prior art optical add/drop device.  
         [0012]    [0012]FIG. 1B is a schematic diagram showing a prior art optical add/drop device using a demultiplexer and a multiplexer.  
         [0013]    [0013]FIGS. 2A and 2B are schematic diagrams showing the on/off switching functions of a Bragg grating wavelength selective bridge waveguide.  
         [0014]    [0014]FIGS. 3A to  3 B are cross sectional views showing the coupling configurations of a wavelength-selective bridge waveguide coupled between a bus waveguide and an outbound waveguide in S-type switches.  
         [0015]    [0015]FIG. 3C shows an add/drop device according to the present invention constructed using two S-type switches.  
         [0016]    [0016]FIGS. 4A to  4 B are functional diagrams for showing a wavelength selective bridge waveguide coupled between intersecting waveguides for switching and re-directing optical transmission of a selected wavelength using L-type switches.  
         [0017]    [0017]FIG. 4C shows an add/drop device according to the present invention constructed from two L-type switches.  
         [0018]    [0018]FIG. 4D shows a symbolic diagram of the structure shown in FIG. 4C.  
         [0019]    [0019]FIG. 5 shows an X-type switch used as an add/drop device.  
         [0020]    [0020]FIG. 6A shows an O-type switch disclosed in this invention.  
         [0021]    [0021]FIG. 6B is a schematic diagrams for showing an optical add/drop device implemented with two O-type switches.  
         [0022]    [0022]FIG. 6C is a schematic diagrams for showing an optical add/drop device implemented with multiple O-type switches.  
         [0023]    [0023]FIGS. 7A, 7B,  7 C, and  7 D are schematic diagrams for showing optical add/drop devices implemented with L-type switches.  
         [0024]    [0024]FIGS. 8A, 8B,  8 C, and  8 D are schematic diagrams for showing alternative optical add/drop devices implemented with L-type switches.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]    In the following description, numerous specific details are provided, such as the identification of various system components, to provide a thorough understanding of embodiments of the invention. One skilled in the art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In still other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of various embodiments of the invention. Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.  
         [0026]    The present invention utilizes MEMS-actuated waveguide grating-based wavelength intelligent switches as disclosed in our co-pending patent applications noted above. The switch is fabricated on a silicon substrate and the switching action is based on electrostatic bending of a part of waveguide with an integrated Bragg gratings built in its cladding layer. The waveguide with the integrated Bragg gratings, termed as a “bridge waveguide”, functions as a switching element. When the bridge waveguide is electro-statically bent close enough to an input waveguide, the wavelength which meets the Bragg phase-matching condition is coupled into the bridge waveguide. Through the bridge waveguide, the selected wavelength is then directed into a desired output waveguide.  
         [0027]    Electrostatic bending of the bridge waveguide can be implemented by applying a voltage between a silicon substrate and an electrode. This can greatly simplify the production of large-scale optical switches, compared with the prior art micro-mirror based MEMS approach. The integrated Bragg grating is formed by physically corrugating a waveguide. Thus, it does not rely upon a photorefractive index change, which enables building Bragg gratings in materials that are not photo-refractive. Further, the integrated Bragg grating can be made smaller, and packed closer together than fiber-optic devices.  
         [0028]    [0028]FIGS. 2A and 2B are schematic diagrams showing the on and off states respectively of a wavelength-selective bridge waveguide  120  relative to a multi-channel bus waveguide  110 . A multiplexed optical signal is transmitted in a bus waveguide  110  over N multiplexed wavelengths λ 1 , λ 2 , λ 3 , . . . , λ N , where N is a positive integer. In FIG. 2A, the wavelength selective bridge waveguide  120  is moved to an on-position and coupled to the waveguide  110 . An optical signal with a central wavelength λ i  particular to the Bragg gratings  125  disposed on the bridge waveguide  120  is guided into the wavelength selective bridge waveguide  120 . The remainder optical signal of the wavelengths λ 1 , λ 2 , . . . , λ i−1 , λ i+1 , . . . , λ N  is not affected and continues to transmit over the waveguide  110 . The Bragg gratings  125  have a specific pitch for reflecting the optical signal of the selected wavelength λ i  onto the wavelength selective bridge waveguide  120 . In FIG. 2B, the wavelength selective bridge waveguide is pulled off from the waveguide  110  to a “bridge-off” position. There is no “detoured signal” entering into the bridge waveguide. The entire multiplexed signal over wavelengths λ 1 , λ 2 , λ 3 , . . . , λ N  continue to transmit on the bus waveguide  110 .  
         [0029]    [0029]FIG. 3A shows structure of an “S” type switch. A wavelength selective bridge waveguide  220  is coupled between a bus waveguide  210  and a second waveguide  230 . A multiplexed optical signal is transmitted in a bus waveguide  210  over N multiplexed wavelengths λ 1 , λ 2 , λ 3 , . . . , λ N , where N is a positive integer. The wavelength selective bridge waveguide  220  has a first set of Bragg gratings disposed on a first “bridge on-ramp segment”  225 - 1  for coupling to the bus waveguide  210 . An optical signal with a central wavelength λ i  particular to the Bragg gratings  225  disposed on the bridge waveguide  220  is guided through the first bridge ramp segment  225 - 1  to be reflected into the wavelength selective bridge waveguide  220 . The remainder optical signal of the wavelengths λ 1 , λ 2 , . . . , λ i−1 , λ i+1 , . . . , λ N  is not affected and continues to transmit over the waveguide  210 . The Bragg gratings  225  have a specific pitch for reflecting the optical signal of the selected wavelength λ i  onto the wavelength selective bridge waveguide  220 . The wavelength selective bridge waveguide  220  further has a second set of Bragg gratings as a bridge off-ramp segment  225 - 2  coupled to an outbound waveguide  230 . The second set of Bragg gratings has a same pitch as the first set of Bragg gratings. The selected wavelength λ i  is guided through the bridge off-ramp segment  225 - 2  to be reflected and coupled into the outbound waveguide  230 . The bridge off-ramp segment  225 - 2  is disposed at a distance from the bridge on-ramp segment  225 - 1 . The bridge waveguide  220  can be an optical fiber, waveguide or other optical transmission medium connected between the bridge on-ramp segment  225 - 1  and the bridge off-ramp segment  225 - 2 .  
         [0030]    [0030]FIG. 3B shows another structure of “S” type switches. A wavelength selective bridge waveguide  220 ′ is coupled between a bus waveguide  210  and a second waveguide  230 ′. A multiplexed optical signal is transmitted in a bus waveguide  210  over N multiplexed wavelengths λ 1 , λ 2 , λ 3 , . . . , λ N , where N is a positive integer. The wavelength selective bridge waveguide  220 ′ has a first set of Bragg gratings disposed on a first “bridge on-ramp segment”  225 - 1  for coupling to the bus waveguide  210 . An optical signal with a central wavelength λ i  particular to the Bragg gratings  225 - 1  disposed on the bridge waveguide  220 ′ is guided through the first bridge ramp segment  225 - 1  to be reflected into the wavelength selective bridge waveguide  220 ′. The remainder optical signal of the wavelengths λ 1 , λ 2 , . . . , λ i−1 , λ i+1 , . . . , λ N  is not affected and continues to transmit over the waveguide  210 . The Bragg gratings  225 - 1  have a specific pitch for reflecting the optical signal of the selected wavelength λ i  into the wavelength selective bridge waveguide  220 ′. The wavelength selective bridge waveguide  220 ′ further has a bridge off-ramp segment  225 - 2 ′ coupled to an outbound waveguide  230 ′ near a section  235  of the outbound waveguide  230 . The section  235  on the outbound waveguide  230 ′ has a second set of Bragg gratings having a same pitch as the first set of Bragg gratings. The bridge off-ramp segment  225 - 2 ′ is disposed at a distance from the bridge on-ramp segment  225 - 1 . The bridge waveguide  220  can be an optical fiber, waveguide or other optical transmission medium connected between the bridge on-ramp segment  225 - 1  and the bridge off-ramp segment  225 - 2 ′.  
         [0031]    [0031]FIG. 3C shows a simple add/drop device that uses two “S” type switches. A wavelength selective bridge waveguide  220 - 1  is coupled between a bus waveguide  210  and a second waveguide  230 . A multiplexed optical signal is transmitted in a bus waveguide  210  over N+1 multiplexed wavelengths λ 1 , λ 2 , λ 3 , . . . , λ N , λ d , where N is a positive integer. The wavelength selective bridge waveguide  220 - 1  has a first set of Bragg gratings disposed on a first “bridge on-ramp segment”  225 - 1  for coupling to the bus waveguide  210 . An optical signal with a central wavelength λ d  particular to the Bragg gratings  225 - 1  disposed on the bridge waveguide  220 - 1  is guided through the first bridge ramp segment  225 - 1  to be reflected into the wavelength selective bridge waveguide  220 - 1 . The remaining optical signal of the wavelengths λ 1 , λ 2 , . . . , λ N  is not affected and continues to transmit over the waveguide  210 .  
         [0032]    The Bragg gratings  225 - 1  have a specific pitch for reflecting the optical signal of the selected wavelength λ d  into the wavelength selective bridge waveguide  220 - 1 . The wavelength selective bridge waveguide  220 - 1  further has a bridge off-ramp segment  225 - 2  coupled to second waveguide  230 . The bridge off-ramp segment  225 - 2  has a second set of Bragg gratings having a same pitch as the first set of Bragg gratings. The bridge off-ramp segment  225 - 2  is disposed at a distance from the bridge on-ramp segment  225 - 1 . The bridge waveguide  220  can be an optical fiber, waveguide or other optical transmission medium connected between the bridge on-ramp segment  225 - 1  and the bridge off-ramp segment  225 - 2 . Using the bridge off-ramp segment  225 - 2 , the optical signal λ d  can be dropped.  
         [0033]    Further, a wavelength selective bridge waveguide  220 - 2  is coupled between the bus waveguide  210  and second waveguide  230 . An optical signal to be added λ a  progagates along the second waveguide  230 . The wavelength selective bridge waveguide  220 - 2  has a first set of Bragg gratings disposed on a first “bridge on-ramp segment”  225 - 3  for coupling the optical signal λ a  to the bridge waveguide  220 - 2 . The optical signal λ a  is guided through the bridge ramp segment  225 - 3  to be reflected into the input waveguide  210  by a bridge off-ramp segment  225 - 4 .  
         [0034]    The Bragg gratings on the bridge off-ramp segmen  225 - 4  have a specific pitch for reflecting the optical signal of the selected wavelength λ a  into the input waveguide  210 . The bridge waveguide  220 - 2  can be an optical fiber, waveguide or other optical transmission medium connected between the bridge on-ramp segment  225 - 3  and the bridge off-ramp segment  225 - 4 . Using the bridge off-ramp segment  225 - 4 , the optical signal λ a  can be added.  
         [0035]    [0035]FIG. 4A shows structure of an “L” type switch. A wavelength selective bridge waveguide  320  is coupled between a bus waveguide  310  and an intersecting waveguide  330 . Note that the intersecting waveguide  330  may be “physically” intersecting, i.e., sharing the same physical waveguide at the intersection point. However, in other embodiments, the intersecting waveguide  330  may be intersecting in the sense that it crosses over or below the bus waveuide  310 . Thus, the term intersecting as used herein is meant to mean crossing over, crossing under, or physically intersecting.  
         [0036]    A multiplexed optical signal is transmitted in a bus waveguide  310  over N multiplexed wavelengths λ 1 , λ 2 , λ 3 , . . . , λ N , where N is a positive integer. The wavelength selective bridge waveguide  320  has a first set of Bragg gratings disposed on a first “bridge on-ramp segment”  325 - 1  for coupling to the bus waveguide  310 . An optical signal with a central wavelength λ i  particular to the Bragg gratings  325  disposed on the bridge waveguide  320  is guided through the first bridge ramp segment  325 - 1  to be reflected into the wavelength selective bridge waveguide  320 . The remainder optical signal of the wavelengths λ 1 , λ 2 , . . . , λ i−1 , λ i+1 , . . . , λ N  is not affected and continues to transmit over the waveguide  310 . The Bragg gratings  325  have a specific pitch for reflecting the optical signal of the selected wavelength λ 1  into the wavelength selective bridge waveguide  320 . The wavelength selective bridge waveguide  320  further has a second set of Bragg gratings  325  as a bridge off-ramp segment  325 - 2  coupled to an outbound waveguide  330 . The bridge off-ramp segment  325 - 2  is disposed at a distance from the bridge on-ramp segment  325 - 1 . The bridge waveguide  320  can be an optical fiber, waveguide or other optical transmission medium connected between the bridge on-ramp segment and the bridge off-ramp segment  325 - 2 .  
         [0037]    [0037]FIG. 4B shows another structure of “L” type switches. This structure is similar to that shown in FIG. 4A with the bus waveguide  310  disposed in a vertical direction and an intersecting outbound waveguide  330  disposed along a horizontal direction.  
         [0038]    [0038]FIG. 4C shows an add/drop device disclosed in this invention, which is constructed by two L-type switches. The add/drop device  300  consists of a bus waveguide  310 , an outbound waveguide  330 - 1 , an inbound waveguide  330 - 2 , and two bridge waveguides  320 - 1  and  320 - 2 .  
         [0039]    The bridge waveguide  320 - 1  has Bragg gratings formed on both ends of the waveguide that are adjacent the outbound waveguide  330 - 1  and the bus waveguide  310 . The Bragg gratings on the bridge waveguide  320 - 1  have a periodicity adapted to reflect a drop wavelength λ d . Similarly, the bridge waveguide  320 - 2  has Bragg gratings formed on both ends of the waveguide that are adjacent the inbound waveguide  330 - 2  and the bus waveguide  310 . The Bragg gratings on the bridge waveguide  320 - 2  have a periodicity adapted to reflect an add wavelength λ a .  
         [0040]    The add/drop device  300  operates as a compact optical add/drop device. Assume that the bus waveguide  310  carries a multiplexed optical signal having wavelength channels of λ 1 , λ 2 , . . . , λ N , λ d , then the optical signal with its central wavelength λ d  particular to the Bragg gratings of bridge waveguide  320 - 1  is coupled into the bridge waveguide  320 - 1  and further coupled into outbound waveguide  330 - 1 . As a result, the wavelength channel λ d  is extracted or “dropped” from the input terminal of bus waveguide  310  to the output terminal of outbound waveguide  330 - 1 . The remaining optical signal of the wavelength channels λ 1 , λ 2 , . . . , λ N  is not affected and continues propagating along the bus waveguide  310 .  
         [0041]    Similarly, a wavelength channel λ a  propagating along inbound waveguide  330 - 2  is coupled into bridge waveguide  320 - 2  and then coupled into bus waveguide  310  and is transmitted towards the output end of bus waveguide  310 . As a result, wavelength channel λ a  is “added” from inbound waveguide  330 - 2  to output terminal of bus waveguide  310 . This simple structure, constructed by two L-type switches, demonstrates its inherent simplicity of constructing an add/drop device that requires neither demultiplexers nor multiplexers.  
         [0042]    [0042]FIG. 4D shows a symbolic diagram of the structure shown in FIG. 4C and will be used later to illustrate other more complex structures. The building block of L-type wavelength selective switch is symbolized as an “L” around the intersection of two waveguides. The circles  350 - 1  and  350 - 2  denote the L-type wavelength selective switches are in the “on” position.  
         [0043]    [0043]FIG. 5 shows an X-type switch  500  that can serve as an add/drop device. The structure  500  consists of a bus waveguide  510 , a second waveguide  530 , and a bridge waveguides  520 , which forms a cross-type waveguide with four Bragg gratings segments  525 - 1 ,  525 - 2 ,  525 - 3 , and  525 - 4 . With Bragg gratings  525 - 1  and  525 - 2  set to drop wavelength λ d  and Bragg gratings  525 - 3  and  525 - 4  set to add wavelength λ a , this structure  501  performs as a compact add/drop device.  
         [0044]    Assuming that the bus waveguide  510  carries a multiplexed optical signal of wavelength channels λ 1 , λ 2 , . . . , λ N +λ d , then the optical signal with its central wavelength λ d  particular to Bragg gratings  525 - 1  is coupled into the wavelength selective bridge waveguide  520  by Bragg gratings  525 - 1  and then coupled again into the second waveguide  530  by Bragg gratings  525 - 2 . Therefore, the wavelength channel λ d  is extracted or “dropped” from bus waveguide  510  to the second waveguide  530 . The remaining wavelength channels λ 1 , λ 2 , . . . , λ N  are not affected and continues to propagate through the waveguide  510 .  
         [0045]    Similarly, a wavelength channel λ a  transmitting along second waveguide  530  is coupled into bridge waveguides  520  by Bragg gratings  525 - 3  and then coupled into bus waveguide  510  by Bragg gratings  525 - 4  and is transmitted towards the output end of bus waveguide  510 . As a result, wavelength channel λ d  is dropped and wavelength channel λ a  is added. This structure demonstrates its inherent simplicity of constructing an add/drop device—requiring neither demultiplexers nor multiplexers. Note that the Bragg grating  525 - 1  is “downstream” from Bragg grating  525 - 4 .  
         [0046]    [0046]FIG. 6A shows an O-type switch that uses a closed-loop wavelength selective bridge waveguide  620  coupled between a bus waveguide  610  and a second waveguide  630 . A multiplexed optical signal is transmitted in a bus waveguide  610  over N multiplexed wavelengths λ 1 , λ 2 , . . . , λ i−1 , λ i , λ i+1 , . . . , λ N  where N is a positive integer. The wavelength selective bridge waveguide  620  has a first set of Bragg gratings  625 - 1  for coupling to the bus waveguide  610 . An optical signal with a central wavelength λ i  particular to the Bragg gratings  625 - 1  propagating on the bus waveguide  610  is guided through the first Bragg gratings  625 - 1  segment and is reflected into the wavelength selective bridge waveguide  620 .  
         [0047]    The remainder optical signal of the wavelengths λ 1 , λ 2 , . . . , λ i−1 , λ i+1 , . . . , λ N  is not affected and continues to transmit over the waveguide  610 . The Bragg gratings  625 - 1  have a specific pitch for reflecting the optical signal of the selected wavelength λ i  onto the wavelength selective bridge waveguide  620 . The wavelength selective bridge waveguide  620  further has a second set of Bragg gratings  625 - 2  to couple λ i  into an outbound waveguide  630 . The second set of Bragg gratings  625 - 2  is disposed at a distance from the first Bragg gratings  625 - 1 . The bridge waveguide  620  can be an optical fiber, waveguide or other optical transmission medium connected between first Bragg gratings  625 - 1  and second Bragg gratings  625 - 2 .  
         [0048]    [0048]FIG. 6B shows an add/drop device constructed with two O-type switches described in FIG. 6A. Two closed-loop wavelength selective bridge waveguides  620 - 1  and  620 - 2  are coupled between a bus waveguide  610  and a second waveguide  630 . With Bragg gratings  625 - 1  and  625 - 2  set to drop wavelength λ d  and Bragg gratings  625 - 3  and  625 - 4  set to add wavelength λ a , this structure  601  can perform as an add/drop device.  
         [0049]    Similar to the operating functions described above for FIG. 6A, a multiplexed optical signal is transmitted in a bus waveguide  610  over N+1 multiplexed wavelengths λ 1 , λ 2 , . . . , λ N +λ d , where N is a positive integer. The optical signal with a central wavelength λ d  particular to the Bragg gratings  625 - 1  disposed on the bus waveguide  610  is guided through the first Bragg gratings  625 - 1  segment and is reflected into the wavelength selective bridge waveguide  620 . The remaining optical signals of wavelengths λ 1 , λ 2 , . . . , λ N  are not affected and continue to transmit over the waveguide  610 . Using the Bragg gratings  625 - 2 , the optical signal λ d  can be dropped.  
         [0050]    With the addition of bridge waveguide  620 - 2 , the wavelength λ a  transmitting along second waveguide  630  can be coupled into bridge waveguide  620 - 2  by Bragg gratings  625 - 4  and then coupled into bus waveguide  610  by Bragg gratings  625 - 3 . Thus the wavelength λ a  can be added. This structure is a simple add/drop device—requiring neither demultiplexers nor multiplexers.  
         [0051]    [0051]FIG. 6C further illustrates capability of structure expansion of add/drop devices based on O-type switches. This add/drop device  602  consists of a bus waveguide  610 , a second waveguide  630 , and four bridge waveguides  620 - 1 ,  620 - 2 ,  620 - 3 , and  620 - 4 , which have their Bragg gratings set to λ 1 , λ 2 , λ 3 , and λ 4 , respectively. With multiplexed wavelengths λ 1 , λ 2 , λ 5 , λ 6  provided as input into the input terminal of bus waveguide  610 , bridge waveguides  620 - 1  and  620 - 2  extract or “drop” wavelengths λ 1 , λ 2  to the second waveguide  630 .  
         [0052]    Similarly, bridge waveguides  620 - 3  and  620 - 4  inject or “add” wavelengths λ 3 , λ 4  traveling along the second waveguide  630  to bus waveguide  610 . The optical signals exiting from the output terminal of bus waveguide  610  are λ 3 , λ 4 , λ 5 , λ 6 , which are the combination of the remaining input signals λ 5 , λ 6  and the added signals λ 3 , λ 4 . Further expansion can be achieved by adding more bridge waveguides between bus waveguide  610  and the second waveguide  630 .  
         [0053]    For simplicity of illustrations FIGS. 7A to  8 D show only exemplary wavelengths λ 1 , λ 2 , λ 3 , λ 4 , λ 5 , λ 6 , instead of generalized N wavelengths λ 1 , λ 2 , λ 3 , . . . , λ N . Similarly, these illustrative drawings show only exemplary waveguides of the same type in a given structure, instead of generalized N waveguides.  
         [0054]    [0054]FIG. 7A shows in symbolic form an embodiment of the present invention. An add/drop device  710  can be constructed by combining four L-type switches, which are detailed in FIG. 4A. The add/drop device  710  comprises a bus waveguide  751  and another four waveguides  701 ,  702 ,  703 , and  704 . The add/drop device  710  further includes four L-type wavelength selective switches  791 ,  792 ,  793 , and  794  located at the intersections between bus waveguide  751  and waveguides  701 ,  702 ,  703 , and  704 , in which the Bragg gratings inside the L-type wavelength selective switches  791 ,  792 ,  793 , and  794  are preset to wavelength λ 1 , λ 2 , λ 3 , and λ 4 , respectively. Similar operation principles as described previously for the add/drop device shown in FIGS. 4C and 4D apply to the add/drop device  710 .  
         [0055]    With multiplexed input optical signal consisting of wavelength channels λ 1 , λ 2 , λ 5 , λ 6  provided into input terminal of bus waveguide  751 , L-type wavelength selective switches  791  and  792  extract or “drop” wavelength channels λ 1  and λ 2  to the waveguides  701  and  702 , respectively. Similarly, L-type wavelength selective switches  793  and  794  inject or “add” wavelengths channels λ 3  and λ 4  to the bus waveguide  751 , respectively.  
         [0056]    As a result, the optical signals exit from the output terminal of bus waveguide  751  are λ 3 , λ 4 , λ 5 , λ 6 , which are the combination of the remainder of the input signals λ 5 , λ 6  and the added signals λ 3 , λ 4 . Further expansion is achieved by adding more waveguides and associated L-type wavelength selective switches accordingly.  
         [0057]    Additional embodiments of add/drop devices employing L-type wavelength selective switches are shown in FIGS. 7B, 7C, and  7 D. In FIG. 7B, an add/drop device  720  is constructed by adding an add waveguide  755  and two associated L-type wavelength selective switches to the add/drop device  710  disclosed in FIG. 7A. Same basic operation principles as described previously for the add/drop device shown in FIGS. 4C and 4D apply to this add/drop device  720 . With the addition of add waveguide  755 , add wavelength channels λ 3  and λ 4  now come from the same input terminal of waveguide  755 .  
         [0058]    In FIG. 7C, an add/drop device  730  is constructed by adding a drop waveguide  756  and two associated L-type wavelength selective switches to the add/drop device  710  disclosed in FIG. 7A. With the addition of drop waveguide  756 , drop wavelength channels λ 1  and λ 2  are extracted to the output terminal of waveguide  756 .  
         [0059]    In FIG. 7D, an add/drop device  740  is constructed by adding both a drop waveguide  756  and an add waveguide  755  to the add/drop device disclosed in FIG. 7A. With the addition of both drop waveguide  756  and add waveguide  755 , drop wavelength channels λ 1  and λ 2  and add wavelength channels λ 3  and λ 4  will appear at drop and add terminals, respectively.  
         [0060]    [0060]FIGS. 8A, 8B,  8 C, and  8 D shows alternative embodiments of add/drop devices. In FIG. 8A, based on the structure  710  disclosed in FIG. 7A, an add/drop device is constructed by connecting two waveguides  851  and  852  to form a U-type waveguide. The function and the number of L-type wavelength selective switches required in this add/drop device  810  is identical to the add/drop device  710  in FIG. 7A. Basically, the addition of the structure  810  provides flexibility for manufacturing and integration. Similarly, the add/drop devices  820 ,  830 , and  840  disclosed in FIGS. 8B, 8C and  8 D respectively are alternative embodiments with U-type waveguide to the add/drop devices  720 ,  730 , and  740  disclosed in FIGS. 7B, 7C and  7 D respectively.  
         [0061]    Although the present invention has been described in terms of the presently preferred embodiment, it is to be understood that such disclosure is not to be interpreted as limiting. Various alternations and modifications will no doubt become apparent to those skilled in the art after reading the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alternations and modifications as fall within the true spirit and scope of the invention.