Abstract:
A packaged integrated optical component comprises a substrate or chip housed in the package and having an optical waveguide within which an optical signal propagates. A photodetector disposed relative to the waveguide collects light that leaks from the optical waveguide and scattered into the substrate. In particular, the photodetector is disposed below the substrate waveguide or below a plane of a lower surface of the substrate so that its field of detection for scattered light in the substrate is sufficiently enhanced to provide for successful optical signal monitoring.

Description:
REFERENCE TO RELATED APPLICATION  
         [0001]    This application claims priority benefits of prior filed co-pending British patent application No. 00 02276.4, filed Feb. 1, 2000, entitled, INTEGRATED OPTICAL COMPONENTS, and is incorporated herein by reference thereto.  
         FIELD OF THE INVENTION  
         [0002]    This invention relates to a packaged integrated optical component and in particular to such a component having an optical waveguide formed in a substrate, mounted within a package.  
         BACKGROUND OF THE INVENTION  
         [0003]    The manufacture of integrated optical components intended for use in the telecommunications industry for the transmission of data with optical signals is known. A typical manufacturing technique for such a component employs a lithium niobate substrate cut from a wafer, which substrate is also referred to as a chip. An optical waveguide of a required configuration may be formed in the substrate such as by a selective titanium diffusion process. The substrate is then processed in a manner to provide it with the required operating characteristics. For example, an electrode structure may be formed on the surface of the substrate, spatially with the longitudinal extent of the formed optical waveguide so that electrical signals supplied to the electrode structure may influence the propagation and characteristics of an optical signal along the waveguide. The substrate is then mounted in a package appropriate with input and output optical and electrical connections providing for signal coupling and controlling of the operation of the component. The packaged component may, then, be deployed within a communication system.  
           [0004]    During operation of the communication system, it is usually desired to monitor the effect the optical component is having on the optical signal propagating through the component and to adjust the operating parameters for the optical component to bring the output signal of the component to within a required range of operation for that component. See, for example, U.S. Pat. No. 5,259,044 and the article of Y. Kubota et al., entitled “10 Gb/s Ti:LiNbO 3  Mach-Zehnder Modulator with Built-in Monitor Photo-diode Chip”,  ECOC  97, pp. 167-170, Sep. 22-27, 1997. For this purpose, the optical signal may be monitored externally of the component, or a portion of the optical signal may be led out of the component, for example, by means of an internal optical tap or optical coupler provided in a manner well known in the art. The extracted portion of the signal may then be monitored and appropriate corrective action taken in the event that the signal falls outside a desired range of operation.  
           [0005]    A disadvantage of the known techniques is that the use of a optical tap or coupler reduces the signal strength of the remaining optical signal, i.e., it is an addition of loss to the optical component. As a consequence, it has been proposed to gather some of the signal which is naturally lost from the waveguide by, for example, scattering within the LiNbO 3  chip. To achieve this goal, a photodiode has been arranged on the upper surface of the substrate or chip or on the output end face of the substrate. Unfortunately, these arrangements do not particularly work effectively, at least in part, because the power of the lost optical signal is very low and only a small part of the total lost light can be monitored because most of this scattered light goes into so-called substrate mode, where most of the light is scattered into and lost in the substrate. In any event, designers of such integrated optical component have attempted to reduce or minimize these scattering and other insertion losses from the waveguide which further reduces the total power of any scattered light in the substrate which might be available for photodetection component feedback control.  
           [0006]    It is an object of this invention to provide a means through which more light lost from an optical signal propagating in a waveguide formed in a substrate and scattered into the substrate may be effectively utilized for the purposes of enhanced signal detection so that appropriate corrective action taken when the optical signal deviates from a desired range of operation.  
         SUMMARY OF THE INVENTION  
         [0007]    According to this invention, a packaged integrated optical component comprises a substrate or chip housed in the package and having an optical waveguide within which an optical signal propagates. A photodetector disposed relative to the waveguide collects light that leaks from or is lost from the waveguide and is scattered into the substrate. In particular, the photodetector is disposed below the waveguide or a plane of a lower surface of the substrate so that its field of detection for scattered light in the substrate bulk is sufficiently enhanced to provide for successful optical signal monitoring. More particularly, the photodetector positioned within the package integrated optical component is supported below the plane of the surface of the optical waveguide and in proximity to the substrate bulk lower surface or in a plane below the lower surface of the substrate within a region in proximity to the substrate lower surface.  
           [0008]    Advantageously, embodiments of the present invention have benefited from investigations into alternative approaches to the collection of light lost from a waveguide in a substrate along which an optical signal is propagated to permit the monitoring of that light and overall control of the operation of the device. It had been found that the light scattered and lost generally downwardly from the optical waveguide, i.e. into the bulk of the substrate, can more easily be detected by a photodiode than the light scattered and lost upwardly and directed to the chip face from which the waveguide was formed or from the output or forward end face of the substrate. Thus, by detection of scattered light downwardly into the substrate improves the ability for detection of the propagating optical signal. Scattered light from the optical signal into the substrate and elsewhere is from optical losses due to the waveguide, but is especially true in the case when a device drives the light into the substrate with only a minimal optical output from the waveguide, such as, for example, extinction of the optical signal in the waveguide structure as occurs in the case of an optical component comprising a Mach-Zehnder modulator.  
           [0009]    The photodetector may be comprised of a photodiode or other light detecting component, but the use of a photodetector will be primarily be used throughout the description. Such a photodiode may be supported below a lower surface of the substrate, but in proximity to the substrate&#39;s lower surface. A preferred position for the photodiode may be determined empirically during the manufacture of a prototype component of any given design, but once established for a component of that design, may remain substantially constant for that design so that automated manufacture on a production basis can be achieved.  
           [0010]    Regardless of the preferred position for the photodiode, it is preferable that such a photodetector be located within the package of the optical component.  
           [0011]    To accommodate the photodiode below the lower surface of the substrate, the base of the package in which the substrate is secured is provided with a recess to provide a place for the location of the photodiode. In another embodiment, the substrate may be mounted relative tot he package base by means of a substrate support having a sufficient thickness to accommodate the presence of the photodiode between the substrate and the base wall of the package.  
           [0012]    In another embodiment, the photodiode may be conveniently mounted directly on the lower surface of the substrate or in a recess provided in the lower surface of the substrate, though it is also possible to mount the photodiode on the base of the package, adjacent to its lower surface of the substrate. Preferred mounting positions and arrangement may be determined empirically, at the time of prototyping of a design for a particular optical component.  
           [0013]    The optical component may be passive optical component such as, for example, an optical coupler, a combiner or a splitter, or may be an active component such, for example, a modulator or wavelength converter provided that, in each case, there is sufficient light scattering from the optical signal propagating in the optical component is realized. In the case of an active component, the component may be provided with electrodes on the upper surface of the substrate that are driven with suitable electrical signals when the component is deployed, in an optical system or in an optical telecommunication system.  
           [0014]    One such active optical component is a Mach-Zehnder optical modulator and tests have shown that the location of the photodiode below the output section of the waveguide formed in the substrate, such as a LiNbO 3  substrate or chip, between the second Y-junction of the interferometer and the output end plane of the chip, provides particularly good results. However, it has further been determined that satisfactory results may be obtained by locating the photodiode beyond the output end face of the substrate or chip adjacent the lower region of the lower portion of the substrate or substrate surface. This is believed to be the case because the light scattering is greater in a forward and downward direction, i.e., in a direction away from the upper surface of the substrate an its optical waveguide and towards the base of the package supporting the substrate, compared to other light scattering directions. 
       
    
    
       [0015]    The various features of the present invention and its preferred embodiments may be better understood by referring to the following discussion and the accompanying drawings in which like reference numerals refer to like elements in the several figures. The contents of the following discussion and the drawings are set forth as examples only and should not be understood to represent limitations upon the scope of the present invention.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a schematic vertical sectional view through a first embodiment of this invention.  
         [0017]    [0017]FIG. 2 is a schematic plan view of the embodiment shown in FIG. 1.  
         [0018]    [0018]FIG. 3 is a plan view of a second embodiment of a component configured as a Mach-Zehnder interferometer.  
         [0019]    [0019]FIG. 4 is a schematic side elevation of a third embodiment of this invention.  
         [0020]    [0020]FIG. 5 is a plan view of the third embodiment of this invention shown in FIG. 4.  
         [0021]    [0021]FIG. 6 is a schematic side elevation of a fourth embodiment of this invention.  
         [0022]    [0022]FIG. 7 is a plan view of the fourth embodiment of this invention shown in FIG. 6. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    Referring to FIG. 1, there is shown a vertical section through a part of a packaged integrated optical component  10  which may be, for example, a combiner, a splitter or an active component such as, for example, an electro-optical modulator. The packaged component  10  include a base  10 A with side and end walls (not shown), and is covered with a top wall (not shown) which is sealed to the side and end walls under, for example, hermetically controlled conditions. The package for component  10  is typically machined from a block of metal forming a rigid base  10 A. At the time of machining the metal block to form base  10 A to support an optical component substrate or chip  11 , a recess  12  is formed at a predetermined location along the length of the base.  
         [0024]    [0024]FIGS. 1 and 2 illustrate component substrate  11  in the form of a chip of lithium niobate cut from a wafer. Substrate  11  has formed in its surface an optical waveguide  13  by techniques well known and understood in the art. Briefly, the waveguide  13  may be formed by diffusion of a metal, such as titanium, into the lithium niobate so that the refractive index of the lithium niobate is changed within the area of diffusion.  
         [0025]    The substrate  11  is mounted on base  10  of the component package by means of a chip attachment  14 . Typically, this may comprise a resilient mount material which adheres to the undersurface of substrate  11  and to the upper face of base  10 A. Alternatively, substrate  11  may be directly secured to the upper surface of package base  10 A.  
         [0026]    A photodiode  15  is secured to a lower surface  16  of substrate  11 , i.e., the surface of substrate  11  opposite to upper surface  17  into which optical waveguide  13  has been diffused. The photodiode  15  is so secured prior to attachment of substrate  11  to package base  10 A. Although, the required position for photodiode  15  should be determined empirically during prototyping of optical component  10 , photodiode  15  is preferably disposed below waveguide  13  and partially along the length of the waveguide  13 , between the input and output end faces  18 ,  19  of substrate  11 . In an alternative arrangement (not shown), photodiode  15  may be secured directly to package base  10 A within the recess  12  to provide a clearance between the upper face of photodiode  15  and the lower surface  16  of substrate  11 .  
         [0027]    By providing a photodiode  15  below a lower surface  16  of substrate  11 , it has been discovered that photodiode  15  may collect a sufficient amount of scattered light in substrate  11  lost from waveguide  13  during operation of the component or device to permit adequate monitoring of the light propagating along waveguide  13 . Furthermore, the collection of such light does not interfere with the normal operation of component  10 .  
         [0028]    [0028]FIG. 3 shows a second embodiment illustrating an alternative possible waveguide configuration in lieu of linear optical waveguide  11  shown in FIG. 2. Referring to FIG. 3, component  20  comprises waveguide  20 A has an input section  23 A which splits into two branch waveguides  21 ,  22  that recombine into an output section  23 B. Such an arrangement is well known in the art as Mach-Zehnder configuration. A suitable electrode structure (not shown) is subsequently formed on the upper surface  17  of substrate  11  in the vicinity of or in proximity of branch waveguides  21 ,  22  to influence the propagation of the light within the branch waveguides so that interference takes place on recombining of the light from the two branch waveguides in output section  23 A, i.e., the light is modulated according to the driving signals applied to the electrodes as is well known in the art.  
         [0029]    In the arrangement of FIG. 3, photodiode  15  is positioned below output section  23 B to receive light scattered in the bulk of substrate  11  beneath that section, permitting monitoring of the modulated light leaving optical component  20 . In the case here, however, recess  12  in the package base  10  is formed nearer output end face  19  of substrate  11  when mounted in the package, rather than at the position shown in FIG. 1, i.e., it is positioned to receive scattered modulated light from beneath substrate surface  16  at output end  19  of substrate  11 . In all other respects, the arrangement employing the waveguide configuration of FIG. 3 is similar to that shown in FIGS. 1 and 2.  
         [0030]    [0030]FIGS. 4 and 5 disclose a third embodiment of a packaged component  20  in which photodiode  15  is accommodated in yet another alternative position. Here, the photodiode  15  is disposed to one side of the substrate  11 , that is, positioned outside the area of the substrate  11 , beyond an output end face  19  of the substrate. To permit the positioning of the photodiode below the lower surface  16  of the substrate, a recess  24  is machined at a suitable location in the package base  10  as shown in FIG. 4. The photodiode  15  will receive light scattered into the substrate bulk from the waveguide  13  and exiting substrate  11  through substrate output end face  19 .  
         [0031]    Referring to FIGS. 6 and 7 there is shown side sectional and plan views of a fourth embodiment of optical component  20  in which a lithium niobate substrate  11  has a recess  26  within its bottom surface to accommodate the positioning of photodiode  15 . It can be appreciated that photodiode  15  is disposed beneath the output waveguide  23 A and, accordingly, can collect and detect modulated light for the purposes of signal monitoring.  
         [0032]    Although the above embodiments make reference to a “base” of a package, it will be appreciated by those skilled in the art that a “base” is a suitable surface upon which the substrate can be mounted.  
         [0033]    Even though the above embodiments have been described with reference to the use of a single photodetector, the present invention is not limited to such arrangements. It will be appreciated that embodiments can be realized in which more than one photodetector may be employed in the several embodiments described. For example, two such photodiodes could be utilized in connection with any one recess  12 ,  24  or  26 , or one photodiode may be utilized at one recess location and another utilized at another recess location. In each of these cases, the two photodiodes can be coupled together to provide a stronger detected monitoring signal for use in a system feedback arrangement for signal monitoring and correction.  
         [0034]    Although the invention has been described in conjunction with one or more preferred embodiments, it will be apparent to those skilled in the art that other alternatives, variations and modifications will be apparent in light of the foregoing description as being within the spirit and scope of the invention. Thus, the invention described herein is intended to embrace all such alternatives, variations and modifications as that are within the spirit and scope of the following claims.