Patent Publication Number: US-2012027370-A1

Title: Optical device

Description:
The invention relates to an optical device comprising a coupler and a connection network. 
     BACKGROUND OF THE INVENTION 
     Publication “Athermal InP-Based 90°-Hybrid Rx OEICs with pin-PDs&gt;60 Ghz for Coherent DP-QPSK Photoreceivers” (R. Kunkel, H.-G. Bach, D. Hoffmann, G. G. Mekonnen, R. Zhang, D. Schmidt and M. Schell, IPRM 2010, 22nd International Conference on Indium Phosphide and Related Materials, May 31-Jun. 4, 2010, Takamatsu Symbol Tower, Kagawa, Japan) discloses an optical device having the features of the preamble of claim  1 . The device comprises a coupler having coupler inputs and coupler outputs, and a connection network. The connection network comprises connecting waveguides which connect the coupler outputs with outputs of the connection network. One of the waveguides of the connection network crosses two other waveguides while those other waveguides have just one crossing. 
     Devices like those described in the cited publication require low-loss crossings in order to achieve sufficiently low imbalances within the output waveguides of the connection network. To this end, the mentioned publication proposes a precise technological fabrication process. 
     However, the precision required for achieving sufficiently low imbalances, is very hard to achieve in mass production. 
     OBJECTIVE OF THE PRESENT INVENTION 
     An objective of the present invention is to provide a device which requires less fabrication accuracy than prior art devices, but nonetheless reaches low imbalances. 
     A further objective of the present invention is to provide a device which can be fabricated at lower costs than prior art devices but show a comparable optical behaviour. 
     BRIEF SUMMARY OF THE INVENTION 
     An embodiment of the invention relates to an optical device comprising a coupler having coupler inputs and coupler outputs, and a connection network, wherein said connection network comprises connecting waveguides which connect said coupler outputs with outputs of the connection network, and wherein at least one connecting waveguide of the connection network crosses at least one other connecting waveguide of the connection network, characterized in that at least one connecting waveguide, which crosses other connecting waveguides less often than the connecting waveguide with the maximum number of crossings with other connecting waveguides, is attenuated by an optical attenuation element. 
     The attenuation of a waveguide, i.e. the attenuation of the signal transmitted by the waveguide, may be caused by any optical attenuation material (e. g. metal) which is arranged in direct or indirect contact with the electromagnetic waves guided by the waveguide. For instance, the optical attenuation element may be formed by an attenuating layer on top or under the waveguide in order to cause optical losses. 
     However, in a preferred embodiment, the optical attenuation element comprises or is formed by a dummy waveguide which crosses the at least one connecting waveguide. In this manner, the optical attenuation element may be fabricated together with the connecting waveguides without further effort. Thus, additional costs for the fabrication of the optical attenuation element may be completely avoided. 
     The dummy waveguide may have unconnected ends which are separate from the coupler outputs and the outputs of the connection network. As such, the optical influence of the dummy waveguides may be restricted to the attenuation of the assigned waveguide. 
     Preferably, the waveguide width of the dummy waveguide corresponds to the waveguide width of the corresponding connecting waveguide. 
     Further, all connecting waveguides, which cross other connecting waveguides less often than the connecting waveguide with the maximum number of crossings with other connecting waveguides, are preferably each connected to at least one optical attenuation element. 
     Furthermore, all connecting waveguides, which cross other connecting waveguides less often than the connecting waveguide with the maximum number of crossings, are preferably each connected to a specific number of optical attenuation elements, wherein said specific number corresponds to the difference between said maximum number and the number of waveguide crossings of the respective connecting waveguide. 
     According to another preferred embodiment, the coupler comprises four coupler outputs and four connecting waveguides, each connecting waveguide connecting one of the coupler outputs with a corresponding output of the connection network. The first connecting waveguide may cross the second and third waveguides each once and the fourth connecting waveguide stands preferably clear of any crossing with any other connecting waveguide. 
     Preferably a dummy waveguide crosses the second connecting waveguide under the same angle as the first connecting waveguide crosses the third waveguide. A further dummy waveguide may cross the third connecting waveguide under the same angle as the first connecting waveguide crosses the second waveguide. 
     Furthermore, the fourth waveguide may be connected to the first and second optical attenuation elements, wherein the first optical attenuation element may be a dummy waveguide which crosses the fourth connecting waveguide under the same angle as the first connecting waveguide crosses the second waveguide, and wherein the second optical attenuation element may be a dummy waveguide which crosses the fourth connecting waveguide under the same angle as the first connecting waveguide crosses the third connecting waveguide. 
     The coupler may have two inputs and four outputs wherein signals leaving the outputs have phase differences between each other of 90° or multiple thereof. For instance, the coupler may be a multimode interference coupler. 
     The device may further comprise four photodetectors and two differential amplifiers, each of the amplifiers being connected to two photodetectors, wherein each photodetector is connected to one of the outputs of the connection network. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the manner in which the above-recited and other advantages of the invention are obtained will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are therefore not to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail by the use of the accompanying drawings in which 
         FIG. 1  shows a first exemplary embodiment of an inventive device; and 
         FIG. 2  shows a second exemplary embodiment of an inventive device. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The preferred embodiment of the present invention will be best understood by reference to the drawings, wherein identical or comparable parts are designated by the same reference signs throughout. 
     It will be readily understood that the present invention, as generally described and illustrated in the figures herein, could vary in a wide range. Thus, the following more detailed description of the exemplary embodiments of the present invention, as represented in  FIGS. 1-2 , is not intended to limit the scope of the invention, as claimed, but is merely representative of presently preferred embodiments of the invention. 
       FIG. 1  shows a first embodiment of a device  10  according to the invention. The device  10  comprises a multimode waveguide coupler  20  which forms a six-port 90° optical hybrid device. Instead of a multimode waveguide coupler, any other type of coupler may be incorporated into device  10 , such as other types of 90°-hybrids or other types of couplers, for instance couplers based on internal 3 dB-splitters and internal phase shifters. 
     The waveguide coupler  20  has two optical inputs I 1  and I 2  and four optical outputs O 1 , O 2 , O 3  and O 4 . 
     Inputs I 1  and I 2  may be used to enter a QPSK-modulated signal Si and a local oscillator signal Slo into the coupler  20 . 
     The optical signals S 1 -S 4 , which leave the coupler outputs O 1 -O 4 , have phase differences between each other of 90° or multiple thereof. Supposing that signal S 1 , which leaves output O 1 , has a phase of 180°, signals S 2 , S 3  and S 4 , which leave outputs O 2 , O 3  and O 4 , will have phases of 270°, 90°, and 0°, respectively. 
     The multimode coupler  20  is connected to a connection network  30  which comprises four connecting waveguides  41 ,  42 ,  43  and  44 . These waveguides connect the coupler outputs O 1 , O 2 , O 3  and O 4  with outputs O 1 ′, O 2 ′, O 3 ′, and O 4 ′ of the connection network  30 . 
     The outputs O 1 ′, O 2 ′, O 3 ′, and O 4 ′ of the connection network  30  are connected to photodiodes  51 - 54  which absorb the electromagnetic signals S 1 -S 4  transmitted by waveguides  41 ,  42 ,  43  and  44 , respectively, and generate electrical signals S 1 ′-S 4 ′. Two differential amplifiers  61  and  62  that are each connected to two of the photodiodes  51 - 54 , generate demodulated electrical QPSK signals I and Q. 
     As can be seen in  FIG. 1 , the first connecting waveguide  41  crosses the second waveguide  42  at a first crossing  71 , and the third waveguide  43  at a second crossing  72 . As such, if the fabrication process is not perfect, signal S 1  that is transmitted via the first connecting waveguide  41 , will suffer additional attenuation compared to signals S 2 -S 4  in the other waveguides as the latter have to pass just one crossing (signals S 2  and S 3 ) or no crossing at all (signal S 4 ). Thus, the signal amplitudes at the outputs O 1 ′, O 2 ′, O 3 ′, and O 4 ′ may slightly differ. 
     In order to address this problem, waveguides  42 ,  43 , and  44  are in direct or indirect contact with optical attenuation elements. In the embodiment shown in  FIG. 1 , the optical attenuation elements are dummy waveguides  81 - 84  which cross the assigned connecting waveguide each under a predetermined angle γ 1  or γ 2  and thus cause additional attenuation. 
     The waveguide width of the dummy waveguides  81 - 84  preferably corresponds to the waveguide width of the connecting waveguides  41 - 44 . 
     The dummy waveguide  81  that attenuates the second connecting waveguide  42 , crosses the second waveguide  42  under the same angle γ 2  as the first connecting waveguide  41  crosses the third connecting waveguide  43  at the second crossing  72 . As such, the additional loss caused by dummy waveguide  81  corresponds to the additional loss caused by the second crossing  72 , and the signal strength of signal S 2  will better match with the signal strength of signal S 1 . 
     The dummy waveguide  82  that attenuates the third connecting waveguide  43 , crosses the third waveguide  43  under the same angle γ 1  as the first connecting waveguide  41  crosses the second connecting waveguide  42  at the first crossing  71 . As such, the additional loss caused by dummy waveguide  82  corresponds to the additional loss caused by the first crossing  71 , and the signal strength of signal S 3  will better match with the signal strength of signal S 1 . 
     The fourth waveguide  44  is attenuated by a first dummy waveguide  83  and a second dummy waveguide  84 . The first dummy waveguide  83  crosses the fourth connecting waveguide  44  under the same angle γ 1  as the first connecting waveguide  41  crosses the second waveguide  42  at the first crossing  71 , and the second dummy waveguide  84  crosses the fourth connecting waveguide  44  under the same angle γ 2  as the first connecting waveguide  41  crosses the third waveguide  43  at the second crossing  72 . As such, the additional losses caused by dummy waveguides  83  and  84  correspond to the additional losses caused by the first and second crossings  71  and  72 , and the signal strength of signal S 4  will better match with the signal strength of signal S 1 . 
     As apparent from the above, dummy waveguides  81 - 84  add additional optical losses to waveguides  42 - 44  which cross other waveguides less often than waveguide  41 . 
       FIG. 2  shows a second embodiment of a device  10  according to the invention. In this embodiment the optical attenuation elements are formed by metal layers  91 - 94  which are arranged on top or below waveguides  42 - 44 . The optical radiation of signals S 2 -S 4  overlaps at least partly with the metal layers and is therefore attenuated. 
     In the embodiment shown in  FIG. 2 , the length and position of metal layer  91  is chosen such that the estimated, measured or simulated optical loss caused by metal layer  91  corresponds to the estimated, measured or simulated optical loss caused by crossing  72 . 
     The length and position of metal layer  92  is chosen such that the estimated, measured or simulated optical loss caused by metal layer  92  corresponds to the estimated, measured or simulated optical loss caused by crossing  71 . 
     The length and position of metal layer  93  is chosen such that the estimated, measured or simulated optical loss caused by metal layer  93  corresponds to the estimated, measured or simulated optical loss caused by crossing  71 , and the length and position of metal layer  94  is chosen such that the estimated, measured or simulated optical loss caused by metal layer  94  corresponds to the estimated, measured or simulated optical loss caused by crossing  72 . 
     As such, metal layers  91 - 94  cause additional optical losses to waveguides  42 - 44  which cross other connecting waveguides less often than waveguide  41  that has the maximum number of two crossings with other connecting waveguides. 
     In the embodiment shown in  FIG. 2 , metal layers  93  and  94  are separate units. Alternatively, a single metal layer may be applied which causes optical losses comparable to those of crossings  71  and  72 . 
     REFERENCE SIGNS 
       10  device 
       20  optical coupler 
       30  connection network 
       41 - 44  connecting waveguide 
       51 - 54  photodiode 
       61 - 62  differential amplifier 
       71  first crossing 
       72  second crossing 
       81 - 84  dummy waveguide 
       91 - 94  metal layer 
     I 1 , I 2  optical input 
     I, Q demodulated electrical QPSK signal 
     O 1 -O 4  optical output 
     O 1 ′-O 4 ′ outputs of the connection network 
     S 1 -S 4  electromagnetic signal 
     S 1 ′-S 4 ′ electrical signal 
     Si QPSK-modulated signal 
     Slo local oscillator signal 
     γ 1 , γ 2  angle of waveguide crossing