Wavelength division multiplexing

Abstract

An arrayed wavelength grating device has a series of waveguiding channels ( 3 ) interconnecting two free space regions ( 2,4 ) across which light inputting and outputting the device passes. There are differences in optical path length between any two of said channels ( 3 ) lying adjacent to one other in said series, which differences are defined by respective optical path length increments. The optical path length progression across said channels ( 3 ) is non-linear, not being defined by any one value of the path length increments, nor by a plural number of values of the path length increments each defining a linear optical path length progression of a subset of at least three of said channels ( 3 ).

1 . An arrayed waveguide grating device having a series of waveguiding channels interconnecting two free space regions across which light inputting and outputting the device passes, there being differences in optical path length between any two of said channels lying adjacent to one other in said series which differences are defined by respective optical path length increments, wherein the optical path length progression across said channels is non-linear, not being defined by any one value of the path length increments, nor by a plural number of values of the path length increments each defining a linear optical path length progression of a subset of at least three of said channels.  
     
     
         2 . A device according to  claim 1 , wherein the optical path length increments successively increase from a minimum increment value, starting at one of said channels.  
     
     
         3 . An arrayed waveguide grating device having a series of waveguiding channels interconnecting two free space regions across which light inputting and outputting the device passes, there being differences in optical path length between any two of said channels lying adjacent to one other in said series which differences are defined by respective optical path length increments, there being means operable to change the optical path length increments so that the optical path length increments can be made to follow an optical path length progression across said channels which is non-linear, not being defined by any one value of the path length increments, nor by a plural number of values of the path length increments each defining a linear optical path length progression of a subset of at least three of said channels.  
     
     
         4 . A device according to  claim 3  and comprising means operable to change the optical path length increments so that the optical path length increments can be made to successively increase from a minimum increment value, starting at one of said channels.  
     
     
         5 . An arrayed waveguide grating device having a series of waveguiding channels interconnecting two free space regions across which light inputting and outputting the device passes, there being differences in optical path length between any two of said channels lying adjacent to one other in said series which differences are defined by respective optical path length increments, wherein the optical path length increments successively increase from a minimum increment value, starting at one of said channels.  
     
     
         6 . An arrayed waveguide grating device having a series of waveguiding channels interconnecting two free space regions across which light inputting and outputting the device passes, there being differences in optical path length between any two of said channels lying adjacent to one other in said series which differences are defined by respective optical path length increments, there being means operable to change the optical path length increments so that the optical path length increments can be made to successively increase from a minimum increment value, starting at one of said channels.  
     
     
         7 . A device according to  claim 4  or  6 , wherein said optical path length increment changing means comprises an input for receiving an electrical control signal for changing said optical path length increments in a manner defined by the level of the control signal.  
     
     
         8 . A device according to  claim 4 ,  6  or  7 , further comprising means operable to change the optical path lengths of the channels so as to vary each of the optical path length increments by a substantially equal amount.  
     
     
         9 . A device according to  claim 8 , wherein said optical path length changing means comprises an input for receiving a further electrical control signal the level of which defines said substantially equal amount.  
     
     
         10 . A device according to  claim 4 ,  6 ,  7 ,  8  or  9 , constructed so that the or each control signal is a current signal.  
     
     
         11 . A device according to  claim 4 ,  6 ,  7 ,  8  or  9 , constructed so that the or each control signal is a voltage signal.  
     
     
         12 . A device according to  claim 2  or  4  or any one of  claims 5  to  11 , wherein the starting channel is a channel at or close to the end of the series, whereby the optical path length increments successively increase across substantially all of the channels of the series.  
     
     
         13 . A device according to  claim 12 , wherein the starting channel is at an end of the series at which end the channels have lower optical path lengths.  
     
     
         14 . A device according to  claim 12 , wherein the starting channel is at an end of the series at which end the channels have higher optical path lengths.  
     
     
         15 . A device according to  claim 2  or  4  or any one of  claims 5  to  14 , wherein said starting channel is a channel at or close to the mid point of the series, whereby the optical path length increments successively increase towards both ends of the series.  
     
     
         16 . A device according to  claim 2  or  4  or any one of  claims 5  to  15 , wherein said successively increasing optical path length increments are defined by a function proportional to channel number in said series, counted from said starting channel, raised to the power of a factor P.  
     
     
         17 . A device according to  claim 16 , wherein the factor P=1.85 to 2.25.  
     
     
         18 . A device according to  claim 16 , wherein the factor P is approximately two, to provide a parabolic progression of optical path length increment.  
     
     
         19 . A device according to  claim 16 , wherein the factor P has a value approximately equal to one of 0.5, 0.8, 1.5, 2.1 and 3.4.  
     
     
         20 . A device according to  claim 16 , wherein the factor P is greater than one.  
     
     
         21 . A device according to  claim 16 , wherein the factor P is less than one and greater than zero.  
     
     
         22 . A device according to  claim 1  or  3 , wherein the path length increments vary irregularly across the series of channels.  
     
     
         23 . An arrayed waveguide grating device having a series of waveguiding channels interconnecting two free space regions across which light inputting and outputting the device passes, there being differences in optical path length between any two of said channels lying adjacent to one other in said series which differences are defined by respective optical path length increments, wherein the optical path length increments vary irregularly across the series of channels.  
     
     
         24 . An arrayed waveguide grating device having a series of waveguiding channels interconnecting two free space regions across which light inputting and outputting the device passes, there being differences in optical path length between any two of said channels lying adjacent to one other in said series which differences are defined by respective optical path length increments, there being means operable to change the optical path length increments so that the optical path length increments vary irregularly across the series of channels.  
     
     
         25 . A device according to  claim 22 ,  23  or  24 , wherein some of the optical path length increments are positive and some are negative in value.  
     
     
         26 . A device according to  claim 22 ,  23 ,  24  or  25 , wherein the values of the irregularly varying optical path length increments are such that the device has a passband which at least approximates to a function known, from one of signal theory and aperture theory, to provide filtering properties.  
     
     
         27 . A device according to  claim 26 , wherein the filtering properties are one of passband broadening, passband equalisation, passband segmentation into at least two passbands and wavelength-selective amplification/attenuation across the passband.  
     
     
         28 . A method of designing an arrayed waveguide device according to  claim 26  or  27 , the method comprising: 
 selecting a function defining a desired passband of the device;  
 finding a solution to a set of non-deterministic equations to find a phase deviation from a linear phase profile for each of a plurality of waveguiding channels of the device with which a passband at least approximating to the desired passband can be obtained; and  
 computing from the phase deviation the magnitude and sign of the path length increments for each of the channels of the series.  
 
     
     
         29 . A method according to  claim 28  using holographic techniques.  
     
     
         30 . A method according to  claim 28  using simulated annealing techniques.  
     
     
         31 . An arrayed waveguide grating device comprising two free space regions, a series of waveguiding channels interconnecting the two free space regions, and input and output channels each connected to one of the two free space regions, there being means for modulating the amplitude of light passing through the series of waveguiding channels so as to provide an amplitude modulation profile across the series of channels which is non-uniform.  
     
     
         32 . A device according to  claim 31 , wherein the amplitude modulation means comprises means for attenuating and/or amplifying the level of light travelling within the channels.  
     
     
         33 . A device according to  claim 31  or  32 , wherein the amplitude modulation means comprises means for modulating the efficiency with which light is coupled into and/or out of the channels, respectively from and/or to the free space regions.  
     
     
         34 . A device according to  claim 31 ,  32  or  33 , wherein the profile provides a device passband which at least approximates to a function known, from one of signal theory and aperture theory, to provide filtering properties.  
     
     
         35 . A device according to  claim 34 , wherein the filtering properties are one of passband broadening, passband equalisation, passband segmentation into at least two passbands and wavelength-selective amplification/attenuation across the passband.  
     
     
         36 . A device according to  claim 34 , wherein the filtering property is passband broadening, the broadened passband having a shape approximating to a rectangular or square wave function.  
     
     
         37 . A device according to any one of  claims 31  to  36 , wherein said profile is defined by one of a sinc function and a raised cosine function.  
     
     
         38 . A device according to any one of  claims 31  to  37 , wherein the level of amplitude modulation at one end of the series of channels approaches that at the other end of the series of channels.  
     
     
         39 . A device according to any one of  claims 31  to  38 , wherein the profile is at least approximately symmetrical about the mid point of the series of channels.  
     
     
         40 . A device according to any one of  claims 31  to  39 , wherein the series of waveguiding channels incorporates a gain medium.  
     
     
         41 . A device according to  claim 40 , wherein the device includes an input channel for receiving an optical pump beam and a further input channel for receiving a signal conveying beam.  
     
     
         42 . A device according to  claim 40  or  41 , wherein the gain medium is doped with a rare earth element.  
     
     
         43 . A device according to  claim 42 , wherein the rare earth element is erbium.  
     
     
         44 . A device according to any one of  claims 1  to  27  and  31  to  43 , wherein the optical path lengths of the channels are directly proportional to the geometric path lengths.  
     
     
         45 . A device according to any one of  claims 1  to  27  and  31  to  43 , wherein the refractive index in respect of light propagation along the channels varies, or is variable in use, amongst the channels.  
     
     
         46 . An arrayed wavelength grating device substantially as hereinbefore described with reference to FIGS.  1  to  8 , FIGS.  9  to  14 , FIGS.  15  to  18 , FIGS.  19  to  22 , FIGS.  23  to  25  or FIG. 26 of the accompanying drawings.  
     
     
         47 . A device according to any one of  claims 1  to  27  having the additional features of any one of  claims 31  to  43 .