Abstract:
A Mach-Zehnder Interferometer having a waveguide including an input portion for receiving an optical signal and having a first channel width, a first splitter for separating the optical signal into at least first and second paths, a first arm for the first path and having a second channel width that supports a single transverse optical mode, a second arm for the second path and having a third channel width that supports a single transverse optical mode, a second splitter portion for combining optical signals of the at least first and second paths, an output portion for transmitting a resultant combination of optical signals from the second splitter and having a fourth channel width, and wherein the second channel width is larger than the fourth channel width.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a Mach-Zehnder Interferometer optical modulator device and, more particularly, to a Mach-Zehnder Interferometer with an increased extinction ratio or an interaction region with an increased length.  
           [0003]    2. Background of the Related Art  
           [0004]    Mach-Zehnder Interferometers (MZI&#39;s) used as optical modulators are of great interest for high data rate fiber optic communication systems. A great deal of research has been carried out to develop MZI&#39;s since their introduction in the mid 1970&#39;s. The practicality of Ti-diffused LiNb0 3  waveguide systems has allowed the introduction of MZI&#39;s into communication systems.  
           [0005]    Parameters of interest for a MZI, which are indicative of the device&#39;s performance, include modulation efficiency, drive voltage, optical insertion losses due to fiber coupling efficiency, optical wavelength operating range and extinction ratio. Modulation efficiency can be achieved by microwave design and optical-microwave phase velocity matching techniques. Drive voltages can be lowered and/or made more effective by maximizing electro-optic overlap in the interaction region of the MZI. Optical insertion losses may be improved by advances in waveguide technology that enable mode matching of the optical field at the waveguide-fiber interface. Broad optical wavelength operating ranges can be achieved by controlling waveguide dispersions within the MZI. The extinction ratio is a measure of how well an optical device is turned off, usually given as a ratio in decibels (dB) between the output of a signal when the device is turned on and the output of a leakage signal when the device is turned off.  
           [0006]    The extinction ratio directly affects the “eye diagram,” which is a display on an oscilloscope of modulated known “1” and “0” signals to determine if the signals are being properly modulated by the device. An “open” eye shows a good signal with distinct differences between 1s and 0s. A “closed” or “fuzzy” eye means that some 0s could be seen as is and vice versa. Therefore, it is important that a high extinction ratio is achieved to lower bit-error-rates in optical communication systems.  
           [0007]    [0007]FIG. 1 illustrates a plan view of a MZI of the related art. As shown in FIG. 1, a waveguide  1  is formed in an electro-optic substrate  2 . The waveguide is comprised of an input portion  3 , a first splitter portion  4 , a first arm  5 , a second arm  6 , a second splitter portion  7  and an output portion  8 . The input portion  3  is connected to the first splitter portion  4 , which separates into a first branch  4   a  and a second branch  4   b.  The dashed line AA′ in FIG. 1 indicates where the input portion  3  ends and the first splitter portion  4  begins. The dashed line AA′ is where the sides of the input portion  3  are no longer in parallel or where the sides of the first splitter portion  3  are no longer curving. The first arm  5  is connected to the first branch  4   a  of the first splitter portion  3  and the second arm  6  is connected to the second branch  4   b  of the first splitter portion  4 . The dashed lines BB′ and CC′ in FIG. 1 indicate where the first splitter portion  3  ends, and where the first arm  5  and second arm  6  begin, respectively. The dashed lines BB′ and CC′ are where the sides of the first arm  5  and second arm  6  are no longer in parallel, or where the sides of the first splitter portion  3  are no longer curving. A first branch  7   a  of the second splitter portion  7  is connected to the first arm  5  and a second branch  7   b  of the second splitter portion  7  is connected to the second arm  6 . The dashed lines DD′ and EE′ in FIG. 1 indicate where the second splitter portion  7  ends, and where the first arm  5  and second arm  6  end, respectively. The dashed lines DD′ and EE′ are where the sides of the first arm  5  and second arm  6  are no longer in parallel, or where the sides of the second splitter portion  7  are no longer curving. The branches of the second splitter portion  7  come together and the output portion  8  is connected to the second splitter portion  7 . The dashed line FF′ in FIG. 1 indicates where the output portion  8  begins and the second splitter portion  7  ends. The dashed line FF′ is where the sides of the output portion  8  are no longer in parallel or where the sides of the second splitter portion  7  are no longer curving. Optical fiber lines (not shown) are respectively attached to the input portion  3  and the output portion  8  of the waveguide of the related art MZI.  
           [0008]    Typically, as shown in FIG. 1, the channel widths W 1  of the waveguide&#39;s arms  5  and  6 , and the channel widths W 2  of the input/output portions  3  and  8  are adjusted such that the channel widths of the arms support a single transverse optical mode over a predetermined optical wavelength operating range. All of the channel widths W 1  of the waveguide&#39;s arms  5 ,  6  and the channel widths W 2  of the input/output portions  3 ,  8  are the same (i.e., W 1 =W 2 ). Both the input portion  1  and the output portion  8  of the waveguide in the related art MZI have the same length L 1  measured from an end of the waveguide to a splitter for symmetry in fabrication of the device. For purposes of subsequent explanation of the invention, the MZI is bifurcated into an input side where the input signal is received and an output side where the output signal emanates.  
           [0009]    Operation of the MZI is shown FIGS. 2 a  and  2   b.  Specifically, FIG. 2 a  illustrates the operation of a MZI in an on-state (i.e., Δφ=0) and FIG. 2 b  illustrates that operation of a MZI in an off-state (i.e., Δφ=Π). Both of FIGS. 2 a  and  2   b  show how a single transverse optical mode is input to the input portion  3  of the waveguide on the input side of the device. Furthermore, FIGS. 2 a  and  2   b  show that the single transverse optical mode is divided into equal powers by the splitter portion  4 , which separates the optical mode into the arms  5  and  6  of the interferometer.  
           [0010]    As shown in FIG. 2 a,  when no differential phase shift Δφ is applied to the optical modes in the separated paths (i.e., Δφ=0), the second splitter  7  combines the separated optical modes and outputs a first order transverse optical mode. Consequently, an optical output fiber (not shown) connected to the output portion  8  picks up this first order transverse optical mode.  
           [0011]    As shown in FIG. 2 b,  when a differential phase shift Δφ of modulus ii (i.e., Δφ=Π) is applied to the optical modes in the separated paths by voltages applied through electrodes (not shown) in the vicinity of the separated arms  5  and  6 , the second splitter  7  combines the separated optical modes to form a second order transverse mode. Since the output portion  8  should only support a first order transverse optical mode, the second order transverse optical mode should be cut-off and radiate into the substrate. Therefore, the second order transverse optical mode should neither be outputted nor transmitted to an optical output fiber (not shown) connected to the output portion  8 . The efficiency with which the second order transverse optical mode is radiated into the substrate and not into an optical output fiber is an important factor affecting the extinction ratio of an MZI.  
         SUMMARY OF THE INVENTION  
         [0012]    Accordingly, the present invention is directed to a Mach-Zehnder Interferometer (MZI) that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.  
           [0013]    One aspect of the invention is a Mach-Zehnder Interferometer with an improved extinction ratio for the input and output portions having a predetermined length.  
           [0014]    Another aspect of the invention is a Mach-Zehnder Interferometer in which the overall device length with respect to desired extinction ratio is minimized.  
           [0015]    Also, another aspect of the invention is a Mach-Zehnder Interferometer in which a substrate size can be optimized with increased drive voltage efficiencies while maintaining a predetermined extinction ratio.  
           [0016]    Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.  
           [0017]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings:  
         [0019]    [0019]FIG. 1 is a plan view of a related art Mach-Zehnder Interferometer;  
         [0020]    [0020]FIG. 2 a  illustrates the operation of a Mach-Zehnder Interferometer in an on-state;  
         [0021]    [0021]FIG. 2 b  illustrates the operation of a Mach-Zehnder Interferometer in an off-state;  
         [0022]    [0022]FIG. 3 a  is a plan view of the output side of a Mach-Zehnder Interferometer;  
         [0023]    [0023]FIG. 3 b  is a cross-sectional view of the output side of a Mach-Zehnder Interferometer;  
         [0024]    [0024]FIG. 4 a  is a mode dispersion diagram for the first two transverse optical modes in the output side of a Mach-Zehnder Interferometer;  
         [0025]    [0025]FIG. 4 b  shows the cutoff angle a in an electro-optic substrate of the Mach-Zehnder Interferometer as function of the channel widths of the output portion illustrated in FIG. 4 a;    
         [0026]    [0026]FIG. 5 is a plan view of the output side of an exemplary Mach-Zehnder Interferometer according to the first embodiment of the present invention;  
         [0027]    [0027]FIG. 6 is a plan view of the output side of an exemplary Mach-Zehnder Interferometer according to the second embodiment of the present invention;  
         [0028]    [0028]FIG. 7 is a plan view of the output side of an exemplary Mach-Zehnder Interferometer according to the third embodiment of the present invention; and  
         [0029]    [0029]FIG. 8 is a graph showing exemplary improvements in the extinction ratio of a Mach-Zehnder Interferometer in accordance with the present invention;  
         [0030]    [0030]FIGS. 9 a,    9   b  and  9   c  respectively show the plan views of fourth, fifth and sixth exemplary embodiments in accordance with the present invention; and  
         [0031]    [0031]FIGS. 10 a  and  10   b  are illustrative of other exemplary types of Mach-Zehnder Interferometers in which the extinction ratio can be improved in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]    [0032]FIGS. 3 a  and  3   b  respectively show plan and cross-sectional views of the output side of a MZI. Particularly, FIG. 3 a  shows an electro-optic substrate  32  of a waveguide having, a first arm  35 , a second arm  36 , a second splitter portion  37  and an output portion  38 . FIG. 3 a  further shows an optical output fiber  39  connected to the output portion  38 . FIG. 3 b  illustrates that the cutoff angle α is measured between the cut-off second order transverse optical mode (2TOM) radiating into the substrate  32  and the substrate surface. Together, FIGS. 3 a  and  3   b  illustrate that both the cutoff angle α and the length L 1  of the output portion  38  must be sufficiently large to prevent any of the cut-off second order transverse optical mode that is radiating into the substrate from being transmitted into the optical output fiber  39 .  
         [0033]    [0033]FIG. 4 a  is a mode dispersion diagram for the first two transverse optical modes in the output portion of a MZI. This diagram shows the mode effective index, nff, for each mode as a function of the channel width W 2  of the output portion. The substrate has a bulk index of refraction nb and a surface index of refraction ns. The surface index of refraction ns is a result of the waveguide in the surface of the substrate. FIG. 4 b  is a cross-sectional view of an electro-optic substrate  42  of a MZI connected to an optical output fiber  49  showing the cutoff angle α as a function of the channel widths of the output portion illustrated in FIG. 4 a.  As shown in FIGS. 4 a  and  4   b,  when the second order mode is just at cutoff (W=W cutoff ) for the substrate, the cutoff angle is zero (α=0) degrees as represented by horizontal arrow. As W is decreased below W cutoff , the cutoff angle α increases and the second order transverse optical mode radiates into the substrate at a larger angle.  
         [0034]    Applicant has learned that when designing a MZI, the length L of the input and output portions can be shortened so as to increase the interaction length of the MZI (i.e., the length of the arms  35  and  36 ). For example, the length L of an output portion would be about  3 - 8 mm in a Ti:LiNbO 3  waveguide at 1.55 μm operating wavelength. A larger cutoff angle α allows for a shorter output portion while still preventing the optical output fiber from directly receiving the cut-off second order transverse optical mode. Otherwise, the extinction ratio would be degraded by the optical output fiber receiving the cut-off second order transverse optical mode.  
         [0035]    An output portion with a narrower channel width Wo than the channel width Wa of the arms can maintain an extinction ratio while using a shorter output portion length, improve the extinction ratio for a predetermined output portion length, or improve the extinction ratio and enable the use of a shorter output portion length. In the interaction region of a MZI, between the splitters, drive voltage may be improved with tight optical confinement by keeping the channel width W of the arms large and close to their second order transverse optical mode cutoff Wcutoff value for the waveguide. Both of these constraints can be satisfied by using two different channel widths in the waveguide of the interferometer, a wider one Wa in the interaction region (i.e. the arms  5  and  6 ), and a narrower one Wo in the output channel. Furthermore, it is also useful to use a narrower channel in the input portion of the MZI because it minimizes the possibility of input coupling into the second order transverse optical mode of the input side of the device, which could also degrade the extinction ratio. A designer may choose to apply the present invention to one of or both the input and output sides of a MZI. In addition, a designer of optical modulators may take the extra length gained from the input and output portions, and use the extra length in the interaction region. Increasing the length of the interaction region of a MZI increase the drive voltage efficiency of electrodes overlying the arms of a MZI. Therefore, the present invention can increase the extinction ratio, increase drive voltage efficiency or increase both the extinction ratio and the drive voltage efficiency.  
         [0036]    [0036]FIG. 5 shows a plan view of the output side of an exemplary MZI in accordance with a first embodiment of the present invention. FIG. 5 shows an electro-optic substrate  52  with a waveguide including, a first arm  55 , a second arm  56 , a second splitter portion  57 , an output portion  58  and an optical output fiber  59 . The channel widths Wa of the waveguide&#39;s arms  55  and  56  are adjusted such that the channels of the arms support or only support a single transverse optical mode over a predetermined optical wavelength operating range. As shown in FIG. 5, the channel width change between the width Wa of the channels in arms  55  and  56 , and the width Wo of the channel of the output portion  58  is obtained through reduction of the channel widths in the second splitter  57 .  
         [0037]    The arm ends  57   a  and  57   b  of the second splitter  57  have a channel width Wa and the output portion end  57   c  of the second splitter  57  has a channel width Wo, which is smaller than Wa. The output portion  58  has a constant channel width Wo and is connected to an optical output fiber  59 . The length L 5  shown in FIG. 5, for example, can be about 3 to 8 millimeters. Exemplary values of the channel width Wo and the channel width Wa for a 1.55 μm wavelength are 4 to 8 μm and 6 to 10 μm, respectively.  
         [0038]    [0038]FIG. 6 shows a plan view of the output side of an exemplary MZI in accordance with a second embodiment of the present invention. FIG. 6 shows an electro-optic substrate  62  with a waveguide including a first arm  65 , a second arm  66 , a second splitter portion  67 , a tapered output section  60 , an output portion  68  and an optical output fiber  69 . The channel widths Wa of the waveguide&#39;s arms  65  and  66  are adjusted such that the channels of the arms support or only support a single transverse optical mode over a predetermined optical wavelength operating range. As shown in FIG. 6, the channel width change between the width Wa of the channels in arms  65  and  66 , and channel width Wo of the output portion  68  is obtained through a width reduction by the tapered section  60  having linear sides.  
         [0039]    The arm ends  67   a  and  67   b  of the second splitter  67  have a channel width Wa and the tapered section end  67   c  of the second splitter  67  also has a channel width Wa. The tapered output section  60  has a first end  60   a  having a channel width Wa connected to the second splitter  67  and a second end  60   b  connected to the output portion  68  having a channel width Wo, which is smaller than the channel width Wa of the arms  65 ,  66 . Although a channel width Wa is indicated above for the first end  60   a  of the tapered output section, the first end  60  a may have an intermediate channel width between Wa and Wo can be used. In such a case, the tapered section end  67   c  of the second splitter  67  would also have the same intermediate channel width between Wa and Wo. The output portion  68  has a constant channel width of Wo. The other end of the output portion  68  is connected to an optical output fiber  69 . The length L 6  shown in FIG. 6, for example, can be about 3 to 8 millimeters. Exemplary values of the channel width Wo and the channel width Wa for a 1.55 μm wavelength are 4 to 8 μm and 6 to 10 μm, respectively.  
         [0040]    [0040]FIG. 7 shows a plan view of the output side of an exemplary MZI in accordance with a third embodiment of the present invention. FIG. 7 shows an electro-optic substrate  72  with a waveguide including, a first arm  75 , a second arm  76 , a second splitter portion  77 , a tapered output portion  70  and an optical output fiber  79 . The channel widths Wa of the waveguide&#39;s arms  75  and  76  are adjusted such that the channels of the arms support or only support a single transverse optical mode over a predetermined optical wavelength operating range. As shown in FIG. 7, the channel width change between the width Wa of the channels in arms  75  and  76 , and channel width Wo at the end  70   b  of the tapered output portion  70  is obtained through reduction in the tapered output portion  70  having linear sides.  
         [0041]    The arm ends  77   a  and  77   b  of the second splitter  77  have a channel width Wa and the output portion end  77   c  of the second splitter  77  has the same channel width Wa. The tapered output section  70  has first end  70   a  having a width Wa connected to the second splitter  77  and a second end  70   b  having a width Wo, which is smaller than Wa. Although a channel width Wa is indicated above for the first end  70   a  of the tapered output portion, an intermediate channel width between Wa and Wo can be used. In this case, the tapered output portion end  77   c  of the second splitter  77  would also have the same intermediate channel width between Wa and Wo. The second end  70   b  of the tapered output portion  70  is connected to an optical output fiber  79 . The length L 7  shown in FIG. 7, for example, can be about 3 to 8 mm. Exemplary values of the channel width Wo and the channel width Wa for a 1.55 μm wavelength are 4 to 8 μm and 6 to 10 μm, respectively.  
         [0042]    Exemplary improvements in the extinction ratio are shown in FIG. 8, where an extinction ratio for a variety of MZI&#39;s is plotted as a function of the change in channel width, Wa-Wo, as described above. For example, the extinction ratio is seen to be improved from a range of 20-25 dB to a range of 25-35 dB when the output channel width is reduced 2-3 microns from the usual single mode width close to the second mode cutoff. This data was taken with Ti:LiNbO 3  channels with (unreduced) widths Wa of 8 to 9 μm at 1.55 μm wavelength. All devices had the same output length of approximately 6 millimeters that is measured from the second splitter to an end of the output where an optical fiber can be connected. In the alternative, as discussed previously, the length of the output portion could be reduced below 6 mm without extinction ratio loss.  
         [0043]    [0043]FIG. 9 a  is a plan view of an exemplary MZI in accordance with a fourth embodiment of the present invention. FIG. 9 a  shows an electro-optic substrate  92  with a waveguide having an input portion  93   a,  a first splitter  94   a,  a first arm  95   a,  a second arm  96   a,  a second splitter portion  97   a  and an output portion  98   a.  FIG. 9 a  further shows an optical output fiber  99  connected to the output portion  98   a  and an optical input fiber  91  connected to the input portion  93   a.  The drive voltage efficiency is increased by the arms  95   a,    96   a  having longer lengths as a result of the output portion  98   a  and the input portion  93   a  being designed to have a shorter length La′ instead of a length La, as shown in FIG. 9 a.  In addition, the extinction ratio of a device having an output portion length La is maintained.  
         [0044]    [0044]FIG. 9 b  is a plan view of an exemplary MZI in accordance with a fifth embodiment of the present invention. FIG. 9 b  shows an electro-optic substrate  92  with a waveguide having an input portion  93   b,  a tapered input section  901 , a first splitter  94   b,  a first arm  95   b,  a second arm  96   b,  a second splitter portion  97   b,  a tapered output section  902  and an output portion  98   b.  FIG. 9 b  further shows an optical output fiber  99  connected to the output portion  98   b  and an optical input fiber  91  connected to the input portion  93   b.  The drive voltage efficiency is increased by the arms  95   a,    96   a  having longer lengths as a result of the output portion  98   a  and the input portion  93   a  being designed to have a shorter length Lb′ instead of a length Lb, as shown in FIG. 9 b.  In addition, the extinction ratio is improved.  
         [0045]    [0045]FIG. 9 c  is a plan view of an exemplary MZI in accordance with a sixth embodiment of the present invention. FIG. 9 c  shows an electro-optic substrate  92  with a waveguide having a tapered input portion  903 , a first splitter  94   c,  a first arm  95   c,  a second arm  96   c,  a second splitter portion  97   c  and a tapered output portion  904 . FIG. 9 c  further shows an optical output fiber  99  connected to the tapered output portion  904  and an optical input fiber  91  connected to the tapered input portion  903 . The extinction ratio is improved compared to other related art MZI&#39;s having input and output portions with a length Lc, as shown in FIG. 9 c.  In the alternative, the overall device length (i.e., length of the substrate) could be shorter at the expense of the improvement in the extinction ratio.  
         [0046]    Generally, the transition length, as indicated in FIGS. 5, 6 and  7 , over which a channel width is changed, either in a splitter, tapered section or tapered output portion, should be sufficiently long so that it is adiabatic in the sense that mode conversion to unwanted radiation modes is minimized. This generally would require transition lengths on the order of about 1 millimeter or more.  
         [0047]    The channel width Wa of the arms is large and close to the second order transverse optical mode cutoff value Wcutoff of the waveguide in the surface of the substrate. In addition, the present invention can be implemented in a variety of MZI&#39;s. For example, the present invention can be used in a MZI having more than two arms. As shown in FIG. 10 a,  appropriate splitters would have to be used to accommodate the additional arms. Furthermore, the present invention can be used in a compound MZI  101  including a plurality of inner MZI&#39;S  102  and  103 , as shown in FIG. 10 b.  For example, the invention can be applied to one of the two inside MZI&#39;s  102  and  103 , both of the two inside MZI&#39;s  102  and  103 , just the outside MZI  101 , all of the MZI&#39;s or portions of each of the MZI&#39;s.  
         [0048]    The waveguide structures in FIGS. 9 a,    9   b  and  9   c  are disclosed as being symmetrical end-to-end. However, it is to be understood that any combinations of the above-disclosed embodiments can be used in a MZI. For example, the first splitter could enlarge from a smaller channel width in the input portion to a larger channel width in the arms on the input side of the device and a tapered output portion can be used on the output side of the device.  
         [0049]    It will be apparent to those skilled in the art that various modifications and variations can be made in the Mach-Zehnder Interferometer of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.