Abstract:
Methods and apparatus for isolating a modulation region of an external modulator that has a semiconductor waveguide structure that comprises at least one integral isolation region.

Description:
FIELD OF THE INVENTION 
     The invention relates to optical waveguides that include electro-absorption, and more particularly to such waveguides that modulate the intensity of optical beams with high speed electrical data signals. 
     BACKGROUND OF THE INVENTION 
     Optical signal transmission of data can have advantages over electrical transmission of data, particularly when data must be transmitted at very high rates. These advantages are usually associated with the wide bandwidth required in the transmission system, wherein transmission lines used for electrical data transmission are more subject to noise and signal attenuation than the optical fibers used for optical data transmission. 
     Light sources for such optical data systems can be directly or indirectly modulated by electrical signals that represent the transmitted data. Indirect modulation involves the use of an optical modulator that responds to the electrical signals that represent the data to be transmitted. Such optical modulators are typically of the electro-absorptive, electro-dispersive, or phase-shift type. Regardless of type, the optical modulator is usually configured to cause a variation or shift in the intensity of the optical beam from the light source in some relationship to the electrical signals. 
     Compact optical data transmitters of the indirectly modulated type preferably comprise a laser diode light source that is coupled to an electro-absorptive waveguide type modulator. Typically, the electro-absorptive modulator operates by utilizing the shift in transmissivity of the modulator&#39;s waveguide to a longer wavelength due to the application of a strong electric field. 
     This shift occurs with modulator waveguides of several different semiconductor structures, such as when the structure of the waveguide comprises a bulk semiconductor, a single isolated quantum well, multiple isolated quantum wells, or multiple coupled quantum wells (a superlattice). With this type of modulator, the wavelength at which the modulator&#39;s waveguide changes from relatively transmissive to relatively opaque changes as a function of the potential of the electrical input signals applied to it. 
     Thus, for any given instantaneous potential applied to the electrical input of the modulator, there is a range of wavelengths of light that may be passed through the modulator&#39;s waveguide with relatively low absorption, a range of wavelengths with relatively high absorption, and a narrow range of wavelengths at which the characteristics of the waveguide shift from relatively transmissive to relatively absorptive, an “electro-absorptive edge” region. 
     If an operating wavelength for the light source is selected so that the change in potential of the electrical input signals, plus any applied bias potential, shifts or varies the electro-absorptive edge about this operating wavelength, a modulated optical signal that has a relatively large depth of modulation is generated by the modulator. The modulator is biased to shift the wavelengths of the electro-absorptive edge with respect to the operating wavelength so that a relatively small change in input signal causes a relatively large change in absorption of the modulator. 
     Absorption modulators that have a semiconductor waveguide structure of the superlattice type exhibit very high contrast because of the high absorption to transmission ratio of light that is transmitted through this type of modulator using a small shift in modulator input signal potential. For instance, better than 30 db of contrast is achievable with as little as 3 volts shift in modulator input signal potential. 
     The optimal operating point in terms of contrast change as a function of modulation signal potential shift is in a region of relatively high absorption. To minimize insertion loss, a short modulation region is desirable, typically in the range of 100 to 200 microns in length. A short modulation region also improves the frequency response of the modulator. 
     One problem with such a modulator is that it has dimensions that make it difficult to handle and package. Another problem is that an external modulator of this type can induce optical feedback to a laser light source to which it is coupled. Variation in the feedback to a diode laser source during modulation can unstabilize the output of the diode laser and cause it to mode hop. 
     SUMMARY OF THE INVENTION 
     The invention comprises an absorption type modulator that has a semiconductor waveguide structure, at least an input isolation region coupled to the input of the modulator that has an index of absorption that changes with an applied bias potential and a short modulation region that is coupled to the output of the isolation region. The modulator may also have a second isolation region coupled to the output of the modulation region that has an index of absorption that changes with an applied bias potential. 
     The isolation region that is coupled to the input of the modulator serves to minimize optical feedback between the modulation region and a laser light source that is coupled to the input of the modulator. Since the isolation region can have a relatively long length, it also allows the overall dimensions of the modulator to be of reasonable size for easy handling and packaging while allowing the modulator region to be of small dimensions. An additional isolation region that couples the modulation region to the output of the modulator helps to minimize feedback of the modulated signal back into the modulator from downstream reflections. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a typical indirectly modulated optical data transmitter that is suitable for incorporating the invention. 
     FIG. 2 shows an external optical modulator of the electro-absorption type that is configured according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a typical indirectly modulated optical data transmitter  2  that is suitable for incorporating the invention. The transmitter  2  typically comprises a light source  4 , an external electro-optical modulator  6 , and a mounting substrate or sub-mount  8 . 
     The light source  4  preferably comprises a laser diode that is butt-coupled to the modulator  6 . Of course, the light source  4  may comprise a separate solid state laser or other source that is suitably coupled to the modulator  6 , such as by lens coupling through free space or by an optical fiber. 
     The modulator  6  is preferably of the waveguide type, and it has a modulator region with an electrical signal input that changes the absorption or refractive index of the modulator&#39;s waveguide in some relationship to the application of an electrical signal that represents data to the modulator&#39;s electrical signal input. The modulator  6  also has at least one, and preferably two, isolator regions that control the transmission of light between the modulator region and the modulator input, the modulator output, or both. 
     The light source  4  preferably has an output facet  10  that is butt-coupled to a corresponding input facet  12  of the modulator  6  to most effectively couple the light generated by the light source  4  into the modulator  6 . The modulator  6  has at least one input terminal  14  that is coupled to a first isolation region  16  of the modulator  6 . 
     The first isolation region  16  of the modulator  6  controls the transmission of light from the light source  4  to a modulation region  18  of the modulator  6  in proportion to a first electrical bias signal generated by a first bias source  20  that is coupled to the input terminal  14  via a signal line  22 . The circuit return between the first bias source  20  and the modulator  6  is typically made via a return wire  23  that is coupled between the first bias source  20  and a return terminal  24  on the substrate  8 . The return terminal  24  is coupled to the return circuit of the modulator  6  in this case. 
     The modulation region  18  has at least one input terminal  26  to allow the potential of an electrical data signal that is generated by a data source  28  and connected to the input terminal  24  via a signal wire  30  to modulate the intensity of the light that is coupled into it from the light source  4 . The circuit return between the data source  28  and the modulator  6  is typically made via a return wire  32  that is coupled between the data source  28  and the return terminal  24  on the substrate  8 . 
     The modulator  6  may optionally have a second isolation region  34  that has at least one input terminal  36  that is connected to a second bias source  38  via a signal line  40 . The second isolation region  34  controls the transmission of modulated light from the modulation region  18  to an output facet  42  of the modulator  6  in proportion to the potential of a second electrical bias signal that is generated by the second bias source  38  and applied to the input terminal  36  via the signal wire  40 . The circuit return between the second bias source  38  and the modulator  6  is typically made via a return wire  44  that is coupled between the second bias source  38  and the return terminal  24  on the substrate  8 . 
     The output facet  42  that is butt-coupled to an end facet  46  of an optical transmission fiber  48  to effectively couple modulated light into the optical fiber. The output of the modulator  6  may also be coupled to the optical fiber  48  by other means, such as by lens coupling through free space. 
     FIG. 2 shows the modulator  6  as described in connection with FIG. 1, wherein it is constructed according to a preferred embodiment of the invention. The modulator  6  preferably utilizes electro-absorption for modulation of the light received from the light source  4 , although the modulator  6  may use any other known modulation technique. 
     Although the modulator  6  is described, for convenience, as specifically a GaAs/Al x Gal 1−x As type of absorption device, it will be apparent to those skilled in the art that the structure may comprise alternate semiconductor compositions, depending on the operating wavelength of the light provided by the light source  4 , as recognized by those skilled in the art. 
     The modulator  6  is shown with the input facet  12  visible. The modulator  6  is fabricated on a modulator substrate  50 . The substrate  50  conveniently comprises conductive GaAs. A GaAs buffer layer  52  is grown to a thickness of approximately 0.25 micrometer on one planar surface of the substrate  50 , although a thickness anywhere in the range of approximately 0 to 2 micrometers is acceptable. The layer  52  is doped with donor ions, typically Si, to a concentration of approximately n=1*10 18  per cubic centimeter, although a concentration anywhere in the range of n=1*10 17  to 4*10 18  per cubic centimeter, or a graded layer, is also acceptable. 
     Next, a lower cladding layer  54  of Al x Ga 1−x As of composition X clad  and thickness h 1c , wherein X clad  and h 1c  are described below. The lower cladding layer  54  is also doped with donor ions, such as Si, to a concentration of approximately n=1*10 18  per cubic centimeter, although a concentration of approximately n=1*10 17  to 4*10 18  per cubic centimeter, or a graded layer, is also acceptable. Over the lower cladding layer  54  is then grown a waveguide core layer  56  that may have a single or multiple isolated quantum well Al x Ga 1−x As/GaAs structure, a multiple coupled quantum well Al x Ga 1−x As/GaAs structure, or a bulk Al x Ga 1−x As structure of composition x core  and thickness h core , wherein x core  and h core  are described below. The waveguide core layer  56  is left substantially undoped. A superlattice structure is generally preferred due to lower static insertion loss. 
     Over the waveguide core layer  56  is grown an inner upper cladding layer  58 , followed by an outer upper cladding layer  60 . The layers  58  and  60  are both Al x Ga 1−x As of composition x clad . The layer  58  has a thickness h uc1  and the layer  60  has a thickness of h uc2 , wherein h uc1  and h uc2  are described below. Typically, the layer  58  is not intentionally doped so that it forms an intrinsic region within the core of the modulator  6 . A thin cap layer  62  of approximately 0.1 micrometer thickness is grown over the layer  60 . 
     The layers  60  and  62  are doped with acceptor ions, such as Be, C, Mg or Zn. Typically it is Be-doped to a concentration of approximately p=1*10 18  per cubic centimeter for layer  60  and p=8*10 18  per cubic centimeter for layer  62 , although a concentration in the range of approximately p=1*10 17  to 2*10 19  per cubic centimeter is also acceptable. Layers  58 ,  60  and  62  are etched to form at least one rib  64 . To provide electrical isolation the rib  64  depth must be etched to below the dopant boundary between layers  58  and  60 . Of course, instead of etching, a suitable index step can be provided by selective oxidation or impurity induced layer disordering (IILD), as well known in the art. 
     The thicknesses (h 1c , h core , h uc1 , h uc2 ) and compositions (x clad , x core ) of the layers  52  through  60  described above are typically chosen so that light is guided in the core region of the modulator  6 , as constrained along the core layer  56 . Lateral confinement of the light within the core layer  56  is provided by the rib  64 . Additionally, an absorption layer (not shown) is selected so that its electro-absorption edge straddles the chosen operating wavelength to provide a useful absorption to input signal amplitude transfer curve. This absorptive layer may be located within the core layer or within the cladding layers. The core layer and/or cladding layer structures may comprise, for instance, a bulk semiconductor composition, a single isolated quantum well, multiple isolated quantum wells, or multiple coupled quantum wells (a superlattice). 
     The thicknesses of the layers  58  and  60 , h uc1  and h uc2  respectively, are chosen to strike a balance between a large electric field and a single lateral optical mode when the rib  64  is etched into the layers  58 ,  60  and  62 . For a given applied input signal potential to the modulator  6 , the electric field increases as h uc1  decreases, and thus it is advantageous to decrease h uc1  to decrease the operating potential. To provide electrical isolation of the rib  62 , etching must extend below the dopant boundary between layers  58  and  60 . If the etch is too deep, multiple lateral modes will be allowed, and this may adversely affect performance. 
     The input terminal  14  for the first isolation region  16  of the modulator  6  extends in length from the vicinity of approximately the plane of the input facet  12  to the vicinity of a leading edge  66  of the input terminal  26  for the modulation region  18 . The length of the input terminal  26  from the leading edge  66  to a trailing edge  68  of the input terminal  26  generally ranges from approximately 100 to 500 micrometers. If no second isolation region  34  is utilized, the trailing edge  68  of the input terminal  26  extends to the vicinity of approximately the plane of the output facet  42  of the modulator  6 . 
     If the second isolation region  34  is utilized, the trailing edge  68  of the input terminal  26  extends to the vicinity of a leading edge  70  of the input terminal  36  for the second isolation region  34 . A trailing edge of the input terminal  26  extends to the vicinity of approximately the plane of the output facet  42 . 
     Returning to FIG. 1, the first isolation region  16  and the second isolation region  34  may be used as a high data rate optical signal detector, because for a given bias potential as provided by the first bias source  20  to the first isolation region  16  and the second bias source to the second isolation region  34 , any change in intensity of light that passes through either of the isolation regions  16 ,  34  will cause a change in current that is supplied by the bias sources  20 ,  38 . Thus, an optical signal that is modulated in intensity can be detected by either of the isolation regions  16 ,  34  as a corresponding change in bias current amplitude. 
     When the transmitter  2  is so used for receiving purposes, at least the first isolation region  16 , and preferably the modulator region  18  as well, are biased to be highly absorptive when the second isolation region  34  is used for detection purposes. This allows the light source  4  to remain powered at all times. The level of the absorption by the first isolation region  16  and the modulator region  18  prevent the output of the light source  4  from reaching the second isolation region  34 , so that the second isolation region  34  only responds to the amplitude of an optical signal received from the fiber  48 . 
     For normal transmitting purposes, at least one of the isolation regions  16 ,  34  is biased to provide the desired degree of isolation for the modulator region  18 . The switching between receiving and transmitting states does not require that the light source  4  be disabled, since the absorptively biased first isolation region  16  and the modulator region  18  block substantially all the light generated by the light source  4  from reaching the second isolation region. 
     Thus, there has been described herein a waveguide type electro-absorptive optical modulator that has a semiconductor waveguide structure, at least an input isolation region coupled to the input of the modulator that has an index of absorption that changes with an applied bias potential and a short modulation region that is coupled to the output of the isolation region. The modulator may also have a second isolation region coupled to the output of the modulation region that has an index of absorption that changes with an applied bias potential. The embodiments described above should not be construed as limiting the scope of the invention because they are only made as specific examples of the implementation of the invention as claimed. It should be understood that various changes in the details, parts, materials, processing and fabrication of the invention as described above may be made while remaining within the scope of the claimed invention.