Patent Publication Number: US-2006002653-A1

Title: Apparatus for an optical circuit having a flat wavelength response

Description:
FIELD OF THE TECHNOLOGY  
      The application relates generally to Mach-Zehnder interferometer based planar lightwave circuits, and, more particularly, to planar integrated optical add/drop multiplexers with a flat wavelength response.  
     BACKGROUND  
      Many planar lightwave circuits are designed to have as flat a wavelength response as possible. That is, the optical circuit does not vary its response based on the wavelength of the optical signal. For example, a Mach-Zehnder Interferometer (MZI) based Optical Add-Drop Multiplexer (OADM) using Bragg gratings receives a multiplexed optical signal having multiple wavelengths or multiple wavelength channels. The OADM adds (multiplexes) and drops (demultiplexes) different wavelengths or channels from the optical signal. A 50/50 differential coupler couples the input light to two arms of the interferometer, each arm having a grating tuned to a resonant wavelength, otherwise known as the wavelength to be dropped. The gratings in each arm reflect and separate that wavelength from the optical signal. The reflected wavelength is coupled via constructive interference to a drop port which is coupled to the first coupler. The remaining wavelengths are transmitted to another 50/50 differential coupler and outputted to a cross state output via constructive interference. Additional light of the dropped wavelength is provided via an add port (corresponding to a bar state output) coupled to the second coupler and reflected by the gratings out the cross state output.  
      A balanced MZI was used for the MZI-based OADM, in which the optical path lengths of each arm were equalized to provide no effective optical path length difference. UV trimming was sometimes used in one of the arms before the grating to attenuate the dropped wavelength. In effect, the OADM was intended to drop only the resonant wavelength, without affecting the remaining wavelengths, thereby minimizing signal dB loss and signal degradation. However, although the 50/50 differential couplers could couple or split a particular wavelength according to a 50:50 ratio, the couplers were often wavelength dependent. For example, a coupler would couple a first wavelength according to the 50:50 ratio, but couple a second wavelength according to a 49:51 ratio. This resulted in band-narrowing of the OADM output which resulted in strong insertion loss variations across the optical bandwidth. Generally, band-narrowing is to be avoided in optical communications having a multiplexed optical signal, such as the conventional band (1530-1565 nm) and the long band (1570-1600 nm). Although the signal loss in a single OADM is minimal and generally acceptable, this signal loss was accentuated when multiple, cascaded OADMs were used to (de)multiplex multiple wavelengths. Due to the resultant wavelength response of each OADM, some wavelength channels experienced more signal loss than others.  
      While some signal loss may be unavoidable, the signal loss should be uniform across the output bandwidth. While attempts have been made to manufacture broadband couplers having little or no wavelength dependence, otherwise referred to as having a flat wavelength response, such couplers are often very long and difficult to realize in practice due to difficulties in manufacturing and use. However, in practice, even broadband couplers rarely had a perfect coupling ratio with no wavelength dependence. Generally, this was due to manufacturing defects, variations in substrate temperature, imperfect glass density and lack of uniformity, for example. Both couplers used in an MZI-based OADM were generally made in the same manufacturing process, which may include the same materials batch or the same processing batch, for example, and thereby had the same defects. In effect, the couplers were nominally identical and exhibited the same wavelength dependences. In addition to OADMs, thermo-optic switches and variable optical attenuators also utilized Mach-Zehnder interferometers with wavelength dependent couplers, and experienced the same wavelength response and band-narrowing issues mentioned with respect to the OADM. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic diagram illustrating an example of a broadband Mach-Zehnder based optical add/drop multiplexer;  
       FIG. 2  is schematic diagram illustrating a one-half wavelength effective path length difference in an arm of the optical add/drop multiplexer of  FIG. 1 ;  
       FIG. 3  is schematic diagram illustrating a multiple wavelength optical add/drop multiplexer using cascaded optical add/drop multiplexers of  FIG. 1 ;  
       FIG. 4  is a schematic diagram illustrating a second example of a broadband Mach-Zehnder based optical add/drop multiplexer;  
       FIG. 5  is schematic diagram illustrating a first optical coupler of the optical add/drop multiplexer of  FIG. 4 ;  
       FIG. 6  is schematic diagram illustrating a second optical coupler of the optical add/drop multiplexer of  FIG. 4 ;  
       FIG. 7  is schematic diagram illustrating a multiple wavelength optical add/drop multiplexer using cascaded optical add/drop multiplexers of  FIG. 4 ;  
       FIG. 8  is a chart illustrating the bar state output of the optical add/drop multiplexer of  FIG. 1 ;  
       FIG. 9  is a chart illustrating the cross state output of the optical add/drop multiplexer of  FIG. 1 ;  
       FIG. 10  is a chart illustrating the bar state output of the optical add/drop multiplexer of  FIG. 4 ;  
       FIG. 11  is a chart illustrating the cross state output of the optical add/drop multiplexer of  FIG. 4 ;  
       FIG. 12  is a schematic diagram illustrating an example of a broadband Mach-Zehnder based thermo optic switch; and  
       FIG. 13  is a schematic diagram illustrating an example of an optical system incorporating an optical add/drop multiplexer and a thermo-optic switch. 
    
    
     DETAILED DESCRIPTION OF THE EXAMPLES  
      Examples of an optical circuits having a flat wavelength response are shown generally in  FIGS. 1 and 4 . Although the optical circuits are particularly well suited for optical add/drop multiplexers, or the like, the teachings of the instant patent are not limited to any particular type of optical circuit. On the contrary, the teachings of the patent can be employed with virtually any optical circuit, including planar lightwave circuits. Thus, although the optical circuit will be described below primarily in relation to an optical add/drop multiplexer, the apparatus could likewise be used with optical switches such as broadband thermo-optic switches, variable optical attenuators and various combinations involving the above-mentioned optical circuits such as reconfigurable optical add/drop multiplexers.  
      Referring to  FIG. 1 , an optical add/drop multiplexer  10  may be provided as an optical circuit, such as a planar lightwave circuit. The optical add/drop multiplexer  10  is based on a Mach-Zehnder interferometer, which includes an input port  12  and a drop port  14  each optically coupled to a first optical coupler  16 . The input port  12 , the drop port  14  and the optical coupler  16  may be provided as waveguides disposed on a substrate. Input light from a light source, as indicated by the arrow labeled λ 1-4  superimposed on the input port  16 , is coupled via the input port  12  to the optical coupler  16 . The input light λ 1-4  may be a multiplexed signal from a fiber optic transmission or a laser. A multiplexed signal may include input light having multiple wavelengths, or multiple wavelength channels each of which may include a wavelength bandwidth of one or more wavelengths. Although described herein as multiple wavelengths (λ 1 , λ 2 , λ 3 , λ 4 ), it should be understood that the following description may refer to multiple wavelength channels as well.  
      The optical coupler  16  is a directional coupler which splits the input light into first and second portions according to a coupling phase or ratio. As such, although referred to as an optical coupler, the optical coupler  16  is a coupler/splitter. In one example, the optical coupler  16  is a 50/50 wavelength dependent coupler such that different wavelengths are split into first and second portions according to different phases or ratios. For instance, the optical coupler  16  may couple a first wavelength λ 1  at equally distributed at a 50:50 ratio and couple a second wavelength λ 2  at a 49:51 ratio. The optical coupler  16  may include two waveguides with one waveguide optically coupled to the input port  12  and the other waveguide optically coupled to the drop port  14 . The two waveguides are positioned proximate to each other to cause evanescent coupling between the waveguides. However, different optical couplers, including fused waveguide couplers, may be utilized.  
      The optical coupler  16  is optically coupled to an upper arm  18  and a lower arm  20  of the interferometer. The upper and lower arms  18 ,  20  may be provided as waveguides disposed on the substrate similar to the input port  12 , the drop port  14  and the optical coupler  16 . One portion of the input light is coupled to the upper arm  18  and a second portion of the input light is coupled to the lower arm  20  according to the coupling ratio of the optical coupler  16 . Each of the input light portions may include the multiple wavelengths λ 1-4  as provided in the input light, though, as mentioned, the coupling ratio may vary among the various wavelengths.  
      The upper and lower arms  18 ,  20  are provided as optical waveguides, each having an optical path length. The upper arm  18  receives one portion of the input light from the optical coupler  16 , and the lower arm  20  receives the other portion of the input light from the optical coupler  16 . Each arm  18 ,  20  also includes a grating  22 ,  24 , such as a Fiber Bragg grating (FG), each of which has a reflective resonance tuned to reflect a particular wavelength λ 1  and transmit the remaining wavelengths λ 2-4  with little or no effect on the remaining wavelengths λ 2,3,4 . Different gratings or methods of selecting particular wavelengths may also be used in place of the gratings  22 ,  24 . The gratings  22 ,  24  may also be disposed on the substrate, and may be provided by modifying the upper and lower arm waveguides  18 ,  20 .  
      As each portion of input light is incident on the gratings  22 ,  24 , the resonant wavelength λ 1  is reflected back to the optical coupler  16  and coupled in the optical coupler  16 . As light is cross-coupled through the coupler  16 , which includes light coupled from the input port  12  to the lower arm  20  or from the upper arm  18  to the drop port  14 , the light undergoes a 90° (π/2) phase shift. As such, there is a 180° (π) phase difference between the two portions of the reflected wavelength λ 1  at the input port  12 , resulting in destructive interference. Conversely, there is no phase difference between the two portions of the reflected wavelength λ 1  at the drop port  14 , resulting in constructive interference. The reflected wavelength λ 1  is thereby reflected out the drop port  14  and not the input port  12 , and separated from the transmitted wavelengths λ 2,3,4 .  
      The remaining wavelengths in each portion of the input light are propagated through the arms  18 ,  20  of the interferometer. The light through the upper arm  18  propagates through a first optical path length and the light through the lower arm  20  propagates through a second optical path length. Referring to  FIG. 2 , the upper and lower arms  18 ,  20  have an effective path length difference of one-half wavelength (λ/ 2 ) to cause a phase difference of 180° (π) between the two portions of the transmitted wavelengths λ 2,3,4 . As such, the transmitted wavelengths λ 2,3,4  in the lower arm  20  have a one-half wavelength differential relative to the input light and the transmitted light in the upper arm  18 . The actual path length difference may be greater than one-half wavelength, though the transmitted wavelengths λ 2,3,4  experience the same effective path length difference. The path length difference may be provided in various ways, including ultraviolet trimming. Alternatively, the effective path length difference may be accomplished by additional methods of varying the refractive index, including thermal or voltage applications, or by varying the width of the waveguide. The interferometer is thereby an unbalanced interferometer.  
      As shown in  FIG. 2 , the effective path length difference is applied to the lower arm  20  after the grating  24 . However, the effective path length difference may be applied in the upper arm  18  and may be applied either before or after the gratings  22 ,  24 . In one example, the upper arm  18  may be provided with UV trimming before the grating  22  and the lower arm  20  may be provided with UV trimming after the grating  24 , or vice versa. While the UV trimming in the lower arm  20  may be used to provide the one-half wavelength effective path length difference, the UV trimming in the upper arm  18  before the grating  22  may be provided to attenuate or switch the dropped wavelength λ 1 .  
      Referring again to  FIG. 1 , the upper arm  18  and the lower arm  20  are optically coupled to an optical coupler  28 . The optical coupler  28  is similar to the optical coupler  16 , in that the optical coupled  28  is a directional coupler/splitter having the same wavelength dependence and coupling ratio as the optical coupler  16 . The optical coupler  28  may also be a 50/50 evanescent coupler or a fused waveguide coupler, and the coupling ratio may vary depending on the wavelength. For example, the coupling ratio may be 50:50 at wavelength λ 1  and 49:51 at wavelength λ 2 . In one example, the optical couplers  16 ,  28  are nominally identical and may be formed from the same material and in the same manufacturing process. The transmitted wavelengths λ 2,3,4  in the upper arm  18  and the lower arm  20  are coupled to the optical coupler  28  with a phase difference of 180° (π).  
      The optical coupler  28  is optically coupled to an output port  30  and an add port  32 . The optical coupler  28 , the output port  30  and the add port  32  may each be provided as waveguides disposed on the substrate, similar to the other elements  12 ,  14 ,  15 ,  18 ,  20 ,  22 ,  24 . The transmitted wavelengths λ 2,3,4  that are cross-coupled through the optical coupler  28 , which includes light coupled from the lower arm  20  to the output port  30  or from the upper arm  18  to the add port  32 , undergo a 90° (π/2) phase shift. Due to the 90° (π/2) phase shift from cross-coupling in the optical coupler  16 , the 180° (π) phase differential in the lower arm  20  and the 90° (π/2) phase shift from cross-coupling in the optical coupler  28 , the transmitted wavelengths λ 2,3,4  from the lower arm  20  interfere constructively with the transmitted wavelengths λ 2,3,4  from the upper arm  18  (which undergo no phase shift) at the output port  30 . Conversely, there is a 180° (π) phase difference between the two portions of the transmitted wavelengths λ 2,3,4  at the add port  32 , resulting in destructive interference.  
      Light of the same wavelength as the dropped wavelength λ 1  is input to the add port  32  and coupled to the optical coupler  28 . The optical coupler  28  couples a portion of the added wavelength λ 1  to the upper arm  18  and another portion to the lower arm  20  according to the coupling ratio of the optical coupler  28 . Each portion of added wavelength λ 1  is incident on the gratings  22 ,  24  and, as the resonant wavelength, the added wavelength λ 1  is reflected back to the optical coupler  28 . As the added wavelength λ 1  is cross-coupled through the coupler  28 , which includes light coupled from the add port  32  to the upper arm  18  or from the lower arm  20  to the output port  30 , the added wavelength λ 1  undergoes a 90° (π/2) phase shift. As such, there is a 180° (π) phase difference between the two portions of the added wavelength λ 1  at the add port  32 , resulting in destructive interference. Conversely, there is no phase difference between the two portions of the added wavelength λ 1  at the output port  30 , resulting in constructive interference. Light of the same wavelengths λ 1-4  as the input light is thereby transmitted out the output port  30  which corresponds to a bar state port, and the add port  32  corresponds to a cross state port. By providing the output port as the bar state port, the various wavelengths λ 1-4  may be multiplexed and demultiplexed independent of the wavelength dependence of the optical couplers  16 ,  28 . The optical add/drop multiplexer  10  may thereby provide a flat wavelength response in the output.  
      Referring to  FIG. 3 , a multiple wavelength optical add/drop multiplexer  100  is shown which includes multiple optical add/drop multiplexers  102 ,  104 ,  106 ,  108  as described with respect to  FIG. 1 . The add/drop multiplexers  102 ,  104 ,  106 ,  108  may be provided on the same substrate, and the multiple wavelength optical add/drop multiplexer  100  may be provided as a planar lightwave circuit. Each add/drop multiplexer  102 ,  104 ,  106 ,  108  may include gratings having different reflective resonances tuned to reflect a different wavelength and transmit the remaining wavelengths with little or no effect on the remaining wavelengths. The output port of each optical add/drop multiplexer  102 ,  104 ,  106 ,  108  may be optically coupled to the input port of the next optical add/drop multiplexer, providing a cascaded arrangement of the optical add/drop multiplexers  102 ,  104 ,  106 ,  108 .  
      Each optical add/drop multiplexer  102 ,  104 ,  106 ,  108  may thereby drop a different wavelength, and the multiple wavelength optical add/drop multiplexer  100  may be used to multiplex and demultiplex various wavelengths within a multiplexed optical signal. For example the first optical add/drop multiplexer  102  may drop wavelength λ 1 , the second optical add/drop multiplexer  104  may drop wavelength λ 2 , the third optical add/drop multiplexer  106  may drop wavelength λ 3 , and the fourth optical add/drop multiplexer  102  may drop wavelength λ 4 . The number of optical add/drop multiplexers may depend on the number of wavelengths in the input light signal, or on the number of wavelengths to be dropped. Because each optical add/drop multiplexer  102 ,  104 ,  106 ,  108  operates independently of the wavelength dependence of the optical couplers, the multiple wavelength optical add/drop multiplexer  100  may provide an output signal without band-narrowing, and with an even wavelength response.  
       FIGS. 8 and 9  correspond to the output to the bar state (the output port  30 ) and the output to the cross state (the add port  32 ), respectively. The Y-axis in each of  FIGS. 8 and 9  corresponds to the wavelength dependence of the first optical coupler  16 , and the X-axis in each of  FIGS. 8 and 9  corresponds to the wavelength dependence of the second optical coupler  28 . The wavelength dependence of each optical coupler  16 ,  28  is shown as the coupling ratio, or coupling phase, of each optical coupler  16 ,  28 . The grayscale of  FIGS. 8 and 9  correspond to the intensity of the light being transmitted out the output port  30  and the add port  32 , respectively.  
      As shown in  FIG. 8 , if the optical couplers  16 ,  28  have substantially the same wavelength dependence, all of the light is output through the bar state port (the output port  30 ). Because the optical couplers  16 ,  28  are generally manufactured in the same process, which may include the same batch of materials or the same manufacturing batch, for example, the optical couplers  16 ,  28  will generally be nominally identical and have the same imperfections. As such, as the coupling ratios may vary depending on wavelength, the optical couplers  16 ,  28  experience the same wavelength dependence. As seen in  FIG. 8 , if the coupling ratios of the optical couplers  16 ,  28  are substantially the same, all light will be output through the output port  30  even if the coupling phase vary from zero degrees (100:0) to 90 degrees (0:100). Likewise, if the coupling ratios of the optical couplers  16 ,  28  are substantially the same, no light is transmitted through the add port  32 , as seen in  FIG. 9 .  
      Referring to  FIG. 4 , another example of an optical add/drop multiplexer  200  is shown. As with the optical add/drop multiplexer  10  above, the optical add/drop multiplexer  200  may be provided as an optical circuit, such as a planar lightwave circuit, based on a Mach-Zehnder interferometer. The optical add/drop multiplexer  200  includes an input port  202  and a drop port  204  each optically coupled to a first optical coupler  206 , and each of which may be disposed on a substrate. Input light, which may be a multiplexed optical signal having multiple wavelengths λ 1-4  is coupled via the input port  202  to the optical coupler  206 .  
      The optical coupler  206  is a directional coupler/splitter which splits the input light into first and second portions according to its coupling phase or ratio. The optical coupler  206  may have a wavelength dependence resulting in different coupling phases or ratios for different wavelengths. The optical coupler  206  may include two waveguides with one waveguide optically coupled to the input port  202  and the other waveguide optically coupled to the drop port  204 . The two waveguides are positioned proximate to each other to cause evanescent coupling between the waveguides, though fused waveguide couplers may be utilized.  
      The optical coupler  206  is optically coupled to an upper arm  208  and a lower arm  210  of the interferometer. The upper and lower arms  208 ,  210  may be provided as waveguides disposed on the same substrate as the input port  202 , the drop port  204  and the optical coupler  206 . One portion of the input light is coupled to the upper arm  208  and a second portion of the input light is coupled to the lower arm  210  according to the coupling ratio of the optical coupler  206 .  
      As with the optical add/drop multiplexer  10  above, the upper arm  208  receives one portion of the input light from the optical coupler  206 , and the lower arm  210  receives the other portion of the input light from the optical coupler  206 . Both arms  208 ,  210  have an equal effective optical path length, resulting in no effective optical path length difference, thereby resulting in a balanced interferometer. UV trimming, thermal control or voltage control may be applied to one of the arms  208 ,  210  before the grating to attenuate or switch the dropped wavelength λ 1 . Each arm  208 ,  210  also includes a grating  212 ,  214 , such as a Fiber Bragg grating (FG), each of which have a reflective resonance tuned to reflect a particular wavelength λ 1  and transmit the remaining wavelengths λ 2-4  with little or no effect on the remaining wavelengths λ 2,3,4 , though other gratings or methods of selecting particular wavelengths may be utilized. The gratings  212 ,  214  may also be disposed on the substrate.  
      As each portion of input light is incident on the gratings  212 ,  214 , the resonant wavelength λ 1  is reflected back to the optical coupler  206  and coupled through the drop port  204  due to the double 90° (π/2) phase shift. The remaining wavelengths in each portion of the input light are propagated through the arms  208 ,  210  of the interferometer. The light through the upper arm  208  propagates through a first optical path length and the light through the lower arm  210  propagates through a second optical path length.  
      The upper arm  208  and the lower arm  210  are optically coupled to an optical coupler  216 . The optical coupler  216  is a directional coupler/splitter and may be made from the same materials, process or batch, for example as the optical coupler  206 . The optical coupler  216  may also be a 50/50 evanescent coupler or a fused waveguide coupler, and the coupling ratio may vary depending on the wavelength. However, the optical coupler  216  has a wavelength dependence opposite the wavelength dependence of the optical coupler  206 . For example, both optical couplers  206 ,  216  may have a coupling ratio of 50:50 for wavelength λ 1 , though the optical coupler  206  may have a coupling ratio of 49:51 for wavelength λ 2  whereas the optical coupler  16  will have a coupling ratio of 51:49 for wavelength λ 2 . The transmitted wavelengths λ 2,3,4  in the upper arm  208  and the lower arm  210  are coupled to the optical coupler  216  with zero phase difference due to the balanced interferometer.  
      As respectively shown in  FIGS. 5 and 6 , the first and second optical couplers  206 ,  216  each have a different coupling length. In the example shown, the second optical coupler  216  has a coupling length, L 2 , that is three times as long as the coupling length, L 1 , of the first optical coupler  206 . As seen in  FIG. 5  and referring to the first region wherein light is coupled from one waveguide to another, input light from the input port  202  is partially cross-coupled by the optical coupler  206  to the lower arm  210  in the first coupling region.  
      By contrast,  FIG. 6  shows that the second optical coupler  206  fully couples light from the upper arm  208  to the opposing waveguide and then partially couples the light back again. This may be referred to as over-coupling whereby light is coupled according to the coupling phase or ratio of the second optical coupler  216  in the second coupling region. Light from the lower arm  210  may likewise be coupled by the second optical coupler  216 . The light is coupled in the first optical coupler  206  according to a first phase or ratio, and the light is coupled in the second optical coupler  216  according to a second phase or ratio that is opposite the first phase or ratio due to coupling in the second coupling region. For example, the coupling phase in the first optical coupler  206  may be increasing with the wavelength of the light while the coupling phase in the second optical coupler  216  may be decreasing with the wavelength of the light.  
      Although  FIGS. 5 and 6  demonstrate that the optical couplers  206 ,  216  may have opposite wavelength dependence by a coupling length, L 2 , three times longer than the coupling length, L 1 , the actual coupling length of the second optical coupler  216  may vary in relation to the optical coupler  206 , but still provide an effective coupling length that is three times the effective coupling length of the first optical coupler  206 . The effective coupling lengths of each optical coupler  206 ,  216  may thereby produce coupling according to opposite coupling phases or ratios, and hence opposite wavelength dependence. Although described as having modified the second optical coupler  216 , it should be understood that opposite wavelength dependences may be accomplished by providing the first optical coupler  206  with the longer effective coupling length. In addition, opposite wavelength dependence may be accomplished by varying the waveguide width or the waveguide spacing (which decreases exponentially with the coupling coefficient) of the optical couplers  206 ,  216 . For example, the waveguide spacing of the second optical coupler  216  may be one-third of the waveguide spacing of the first optical coupler  206 .  
      Referring again to  FIG. 4 , the optical coupler  216  is optically coupled to an output port  218  and an add port  220 . The optical coupler  216 , the output port  218  and the add port  220  may each be provided as waveguides disposed on the substrate. Due to the coupling in the second coupling region of the optical coupler  216 , the transmitted wavelengths λ 2,3,4  undergo a 90° (π/2) phase shift, as in the first optical coupler  206  although according to an opposite coupling phase/ratio. Due to the 90° (π/2) phase shift from cross-coupling in the optical coupler  206 , and the 90° (π/2) phase shift from cross-coupling in the optical coupler  216 , the transmitted wavelengths λ 2,3,4  from the lower arm  210  interfere destructively with the transmitted wavelengths λ 2,3,4  from the upper arm  208  (which undergo no phase shift) at the add port  220 . Conversely, there is a no phase difference between the two portions of the transmitted wavelengths λ 2,3,4  at the output port  218 , resulting in constructive interference.  
      Light of the same wavelength as the dropped wavelength λ 1  is input to the add port  220  and coupled to the optical coupler  216 . The optical coupler  216  couples a portion of the added wavelength λ 1  to the upper arm  208  and another portion to the lower arm  210 , which is then reflected by the gratings  212 ,  214  back to the optical coupler  216 . The added wavelength λ 1  is transmitted through the output port  218  and combined with the transmitted wavelengths λ 2,3,4 . Light of the same wavelengths λ 1-4  as the input light is thereby transmitted out the output port  218  which corresponds to a cross state port, and the add port  220  corresponds to a bar state port. However, by providing optical couplers  206 ,  216  with opposing wavelength dependences, any effect, such as band-narrowing, on the wavelengths λ 1-4  by the first optical coupler  206  is compensated by the second optical coupler  216 . The optical add/drop multiplexer  200  may thereby provide a flat wavelength response in the output.  
      Referring to  FIG. 7 , a multiple wavelength optical add/drop multiplexer  300  is shown which includes multiple optical add/drop multiplexers  302 ,  304 ,  306 ,  308  as described with respect to  FIG. 4 . The add/drop multiplexers  302 ,  304 ,  306 ,  308  may be provided on the same substrate, and the multiple wavelength optical add/drop multiplexer  300  may be provided as a planar lightwave circuit. Each add/drop multiplexer  302 ,  304 ,  306 ,  308  may include gratings having different reflective resonances tuned to reflect a different wavelength and transmit the remaining wavelengths with little or no effect on the remaining wavelengths. The output port of each optical add/drop multiplexer  302 ,  304 ,  306 ,  308  may be optically coupled to the input port of the next optical add/drop multiplexer, providing a cascading arrangement of the optical add/drop multiplexers  302 ,  304 ,  306 ,  308 .  
      As with the multiple wavelength add/drop multiplexer  100  described above, each optical add/drop multiplexer  302 ,  304 ,  306 ,  308  may drop a different wavelength, and the multiple wavelength optical add/drop multiplexer  300  may be used to multiplex and demultiplex various wavelengths within a multiplexed optical signal. The number of optical add/drop multiplexers may depend on the number of wavelengths in the input light signal, or on the number of wavelengths to be dropped. Because each optical add/drop multiplexer  302 ,  304 ,  306 ,  308  has optical couplers of opposing wavelength dependences, the multiple wavelength optical add/drop multiplexer  300  may provide an output signal without band-narrowing.  
       FIGS. 10 and 11  correspond to the output to the bar state (the add port  220 ) and the output to the cross state (the output port  218 ), respectively. The Y-axis in each of  FIGS. 10 and 11  corresponds to the wavelength dependence of the first optical coupler  206 , and the X-axis in each of  FIGS. 10 and 11  corresponds to the wavelength dependence of the second optical coupler  216 . The wavelength dependence of each optical coupler  206 ,  216  is shown as the coupling ratio, or coupling phase, of each optical coupler  206 ,  28 . The grayscale of  FIGS. 10 and 11  correspond to the intensity of the  206 ,  216  light being transmitted out the output port  218  and the add port  220 , respectively.  
      As shown in  FIG. 10 , if the optical couplers  206 ,  216  have opposite wavelength dependence, all of the light is output through the cross state port (the output port  218 ). Because the optical couplers  206 ,  216  are generally manufactured in the same process, they will generally be nominally identical and have the same imperfections. However, by designing the optical couplers  206 ,  216  as discussed above, the optical couplers  206 ,  216  may have substantially opposite wavelength dependence. As seen in  FIG. 10 , the output from the output port  218  remains constant as the coupling phase of the optical couplers  206 ,  216  remain inversely proportional. Likewise, no light transmitted through the add port  220 , as seen in  FIG. 11 .  
      Referring to  FIG. 12 , an example of a thermo-optic switch  400  is shown. The thermo-optic switch  400  may be built on a substrate as a planar lightwave circuit. The thermo-optic switch  400  is designed from a Mach-Zehnder interferometer and includes a first and second input port  402 ,  404  and an optical coupler  406  optically coupled to the input port  402 ,  404 . Input light, which may a single or multiple wavelengths λ 1-4 , is coupled via one or both of the input ports  402 ,  404  to the optical coupler  406 . The optical coupler  406  is a directional coupler/splitter which splits the input light into first and second portions according to its coupling phase or ratio.  
      An upper arm  408  and a lower arm  410  are optically coupled to the optical coupler  406 , and each arm  408 ,  410  receives a portion of the input light. One of the arms  408 ,  410  includes a heating element  412 , such as a thin film heater, disposed on the arm, which may include disposing the heating element  412  on the waveguide. Utilizing the thermo-optic response of sol-gel materials may also be utilized. The heating element  412  may be coupled to an electrical source via electrical leads  414 ,  416  which may be operatively coupled to a controller to control the heating element  412 . By applying heat to one of the arms  408 ,  410 , the refractive index may be tuned to achieve a particular phase shift in the portion of light being transmitted therein.  
      The light is transmitted through each arm  408 ,  410  to an optical coupler  418 , including any phase shift applied to one of the light portions based on the heating element  412 . The optical coupler  418  is optically coupled to a first output port  420  and a second output port  422 . In effect, the thermal-optic switch  400  may be provided as a switch to switch output between the first and second output ports  422 . Alternatively, the thermo-optic switch may be provided as a variable optical attenuator, which controllably and gradually varies the output of each output port  420 ,  422  based on the refractive index tuning provided by the heating element  412 .  
      As with the optical add/drop multiplexers  10 ,  200  shown in  FIGS. 1 and 4 , the optical couplers  406 ,  418  of the thermo-optic switch  400  may have a wavelength dependence that varies the coupling ratio according to the wavelength of the light. The optical couplers  406 ,  418  may therefore have opposing wavelength dependences, as discussed above, or the upper and lower arms  208 ,  210  may have an effective optical path length difference of one-half wavelength, as also discussed above.  
      The output from the output ports  420 ,  422  may vary depending on the method used to compensate for the wavelength dependence. For example, absent any effects from the heating element  412 , input light provided from the input port  402  will provide an output at the output port  422  (corresponding to a cross state port) if the optical couplers  406 ,  418  have opposing wavelength dependencies, and provide an output at the output port  420  (corresponding to a bar state port) if the lower arm  410  has an effective optical path length difference of one-half wavelength compared to the upper arm  408 . It should be understood the output from the output ports  218 ,  220  may also vary depending on the input port  402 ,  404  being utilized and on the arm being modified (if applying an effective path length difference of one-half wavelength) based on applicable phase shifts and interference.  
      Referring to  FIG. 13 , an example of an optical system  500  implementing an optical add/drop multiplexer, such as the optical add/drop multiplexer disclosed with respect to  FIGS. 1 and 4 . The optical add/drop multiplexers may further include cascaded optical add/drop multiplexers for multiplexing and de-multiplexing multiple wavelengths, such as those disclosed with respect to  FIGS. 3 and 7 . The optical system  500  may further implement a thermo-optic switch, such as the thermo-optic switch disclosed with respect to  FIG. 12 . The optical system  500  may be utilized in optical communication systems. In one example, the optical system  500  may be provided as a line card, such as a transponder line card, an amplifier line card or an aggregation line card. Alternatively, the optical system  500  may be provided as part of a router, such as an aggregation router or edge router, which may include a line card. The optical system  500  may be utilized in optical communications systems, optical networks or other systems involving optical transmissions. In addition, the optical add/drop multiplexers disclosed above and the thermo-optic switch as disclosed above, may be utilized in various optical systems where multiplexing, de-multiplexing or switching of optical signals is performed.  
      The optical system  500  includes an optical add/drop multiplexer  502  for receiving a multiplexed optical signal λ 1-4 . The receiving optical add/drop multiplexer  502  may de-multiplex one or more of the wavelengths from the remaining wavelengths. If de-multiplexing multiple wavelengths, the receiving optical add/drop multiplexer  502  may be a multiple wavelength add/drop multiplexer that includes multiple cascaded optical add/drop multiplexers, each one designed with a grating to separate and drop a particular wavelength from a multiplexed optical signal.  
      The optical system  500  further includes an optical add/drop multiplexer  504  for transmitting a multiplexed optical signal λ 1-4 . The transmitting optical add/drop multiplexer  504  may multiplex multiple, disparate wavelengths into a single signal. If multiplexing multiple wavelengths into a multiplexed optical signal λ 1-4 , the transmitting optical add/drop multiplexer  504  may be a multiple wavelength add/drop multiplexer that includes multiple cascaded optical add/drop multiplexers, each one designed to receive and add an optical signal wavelength and combine it with other optical signal wavelengths.  
      Although disclosed as separate optical add/drop multiplexers, in one example the optical add/drop multiplexers  502 ,  504  of the optical system  500  may be provided as the same optical add/drop multiplexer. In such a case, the optical add/drop multiplexer may multiplex optical signals being transmitted by providing the associated signal wavelength through an add port and transmitting the multiplexed signal through an output port. The optical add/drop multiplexer may de-multiplex optical signals being received by receiving the multiplexed signal through the input port and separating the particular signal wavelength via the drop port.  
      A switch  506  may be optically and operatively coupled to the optical add/drop multiplexers  502 ,  504 . In one example, the switch  506  may include one or more thermo-optic switches to provide an N×N switch. The number of thermo-optic switches may thereby depend on the number of wavelengths being multiplexed and de-multiplexed by the optical add/drop multiplexers  502 ,  504 . The switch  506  may receive an optical signal from the receiving optical add/drop multiplexer  502 , and provide a selected optical signal to a line card  508 . The optical signal may be selected via control signals arranged to vary the heating element and bias the output of the thermo-optic switch. Likewise, the switch  506  may receive an optical signal from the line card  508 , and provide the optical signal to the transmitting optical add/drop multiplexer  504 . As above, the optical signal may be selected via control signals which may bias the output of the thermo-optic switch.  
      The line card  508  may include an electrical-optical (E-O) interface which receives and transmits optical signals, and provides an interface with a physical medium for receiving and transmitting electrical data signals. The particulars of the line card  508  may depend on the application of the line card  508 . For example, the line card  508  may be a transponder line card, an aggregation line card or an optical amplifier line card. Although disclosed as separate elements, in one example the optical add/drop multiplexers  502 ,  504  and the switch  506  may be provided as part of the line card  508 . The line card  508  may thereby be used to provide multiplexing, de-multiplexing and switching functions for optical signals.  
      The line card  508  is coupled to a switch fabric  510  via one or more backplane interfaces  512 . The line card  508  may thereby receive optical signals via an optical input from the switch  506  and transmit electrical signals to the switch fabric  510  corresponding to the optical signals. In addition, the line card  508  may receive electrical signals from the switch fabric  510  via the backplane interfaces  512  and transmit optical signals corresponding to the electrical signals to the switch  506  via an optical output.  
      Although certain apparatus constructed in accordance with the teachings of the invention have been described herein, the scope of the invention is not limited thereto. On the contrary, the invention includes all embodiments of the teachings of the invention fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents.