Abstract:
The present invention provides a bi-directional optical module with an optical isolator to prevent stray light from entering the laser diode (LD). The module includes a distributed feedback LD (DFB-LD), a photodiode (PD), a wavelength division multiplexed (WDM) filter, and a polarization independent isolator placed between the WDM filter and the optical fiber. The stray light emitted from the LD and scattered by optically discontinuous interface is prevented from returning to the LD by the isolator. Although the isolator shifts the optical axis of the receiving optical signal emitted from the optical fiber, the PD with a wide optical sensitive surface may receive almost whole portion of the receiving optical signal.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an optical module, in particular, the invention relates to a bi-directional optical sub-assembly that installs with a polarization independent optical isolator. 
         [0003]    2. Related Prior Art 
         [0004]    Some optical communication system adopt the PON (Passive Optical Network) system for the subscriber network to realize a high speed and a large capacity communication in relatively low cost. This PON system not only cuts a number of fibers to be used by applying a bi-direction module, but a plurality of subscribers commonly owns the single fiber, which enables the high speed and the low cost service comparable to the system using the conventional metal cables. The PON system also adopts the WDM (wavelength division multiplexing) communication where the optical signals with wavelengths of 1.31 μm and 1.55 μm (and/or 1.49 μm) are transmitted or received in the single fiber. 
         [0005]    The bi-direction optical module applied in such PON system has an arrangement that the transmitting light emitted from a laser diode (hereafter denoted as LD) may be optically coupled with the optical fiber, while, the receiving light emitted from the fiber to a photodiode (hereafter denoted as PD). Various arrangements of the LD, the PD, and other optical components for such bi-directional optical module have been presented. 
         [0006]    One example of such bi-directional module is, what is called, a two-package bi-directional module, where the module installs a transmitted optical sub-assembly (hereafter denoted as TOSA) and a receiver optical sub-assembly (hereafter denoted as ROSA) with packages independent to each other, and a wavelength division multiplexed filter (hereafter denoted as WDM filter) between the TOSA and the ROSA. The transmitting light emitted from the LD within the TOSA couples with the fiber after passing the WDM filter, while, the receiving light emitted from the fiber couples with the PD after the reflection by the WDM filter. Such bi-directional module with two packages has an inherent week point to raise the production cost. 
         [0007]    A bi-directional module with single package, which is common to the TOSA and the ROSA, installs all devices or components, such as an LD, a PD, a lens, a mirror, a WDM filter and others, for the optical communication within one package. The United States Patent Application, US 2006-269197A, has disclosed one type of such bi-directional module, in which the light emitted from the LD couples with the optical fiber reflected by the WDM filter and concentrated by the lens, while the light emitted from the fiber couples with the PD condensed by the lens and passing through the WDM filter. 
         [0008]    The LD typically applied in the bi-directional module is a type of a distributed feedback laser diode (hereafter denoted as DFB-LD) that integrates a diffraction grating to sharpen the optical spectrum of the light emitted therefrom within the body of the LD. While, in spite of its sharpened output optical spectrum, the DFB-LD has an inferior tolerance to the optical noise. That is, when the stray light, which is emitted from the LD and scattered or reflected at the end face of the optical fiber or at the other optically discontinuous interface, enters the cavity of the LD again, it makes the oscillation of the LD in stable to widen the width of the output spectrum. 
         [0009]    Accordingly, the DFB-LD is necessary to provide an optical means between the LD and the optical fiber to prevent the stray light from entering the body of the LD. A typical device to show such function is an optical isolator, which passes the forward light advancing from the LD to the optical fiber and prevents the backward light returning from the fiber to the LD. Japanese Patent Applications, published as JP-2004-170798A or JP2005-215219A, has disclosed an optical module built with an optical isolator therein. 
         [0010]    Two types of optical isolators are well known, one of which is the polarization dependent isolator, while, the other is the polarization independent isolator.  FIG. 6A  schematically illustrates a mechanism of the polarization independent isolator where the forward light mass pass the isolator, while,  FIG. 6B  schematically illustrates the mechanism where the backward light is prevented from passing through the isolator. The polarization independent isolator generally includes two birefringent plates,  62   a  and  62   b , that sandwiches the Faraday rotator  61  there between. The crystal axis of the birefringent plates,  62   a  or  62   b , is shown by the arrow “D”. 
         [0011]    The forward light has two polarizations, Er and Or, one of which is parallel to the z-direction, while the other of which is parallel to the y-direction, respectively. The former polarization is called as the extraordinary ray, while the latter is called as the ordinary ray. When such forward light enters the first birefringent plate  62   a , this birefringent plate  62   a  divided it into two rays and shifts only the extraordinary ray Er by a separation along to the z-direction at the output surface  72   a . The separation depends on the birefringent characteristic of the crystal  62   a  and the thickness of the plate. 
         [0012]    The Faraday rotator  61  rotates the polarization of both rays, Er and Or, by −45° at the exit surface  71  thereof. Further, the other birefringent plate  62   a  shifts only the extraordinary ray Er at the exit surface  72   b , while, it keeps the optical axis of the ordinary ray. Accordingly, the ordinary ray may optically couple with the optical fiber. The extraordinary ray does not optically couple with the fiber and it becomes the stray light, denoted as P, because its axis is sifted by the two birefringent plates,  62   a  and  62   b.    
         [0013]    For the backward light, which has also two polarizations, Er and Or, one of which Er is in parallel to the crystal axis D of the birefringent plate  62   b , while the other of which Or is in perpendicular to the crystal axis at the entrance surface  70 , as shown in  FIG. 6B . The birefringent plate  62   b  divides these into two rays, Er and Or, and shifts only the optical axis of the extraordinary ray Er at the exit surface  72   b  thereof. 
         [0014]    The Faraday rotator rotates the polarization of both rays, Er and Or, by −45° at the exit surface  71  thereof, which converts the ordinary ray Or at the entrance surface  70  of the first birefringent plate  62   b  into the extraordinary ray Er at the exit surface  71  of the Faraday rotator  61 . Then, the other birefringent plate  62   a  shifts the axis of the extraordinary ray Er, which is originally the ordinary ray Or as explained above, to the direction determined by the crystal axis of the birefringent plate  62   a . Thus, the polarization independent isolator shifts the optical axis of the backward light in both polarizations thereof, which is denoted by circle Q in  FIG. 6B . 
       SUMMARY OF THE INVENTION 
       [0015]    One aspect of the present invention relates to a bi-directional optical module. The module includes a semiconductor laser diode that emits transmitting light to an optical fiber, a photodiode that receives receiving light emitted from the optical fiber, which has a wavelength different to the transmitting light, a wavelength division multiplexed filter that reflects the transmitting light and transmits the receiving light, or transmits the transmitting light and reflects the receiving light, and an isolator placed between the wavelength division multiplexed filter and the optical fiber. The present optical module has features that the isolator is a type of the polarization independent isolator; the laser diode is positioned on the optical axis of the ordinary light of the isolator, while the photodiode is positioned on the optical axis of the extraordinary light of the isolator. 
         [0016]    The optical module of the present invention may have an optical device with a co-axial package that is constituted of a stem where the laser diode and the photodiode are mounted and a cap attached to the stem, wherein the stem and the cap forms a cavity into which the laser diode and the photodiode are installed. The optical module may have a sleeve assembly and a joint sleeve. The sleeve assembly receives an external optical fiber to which the laser diode and the photodiode optically couple. The joint sleeve optically aligns the sleeve assembly to the optical device to optically couple the laser diode and the photodiode with the optical fiber. The optical module may install the isolator in the inner side of the joint sleeve and the cap of the optical device may provide a condenser lens in a ceiling thereof. 
         [0017]    The polarization independent isolator of the present invention includes a pair of birefringent plates and a Faraday rotator put between the birefringent plates. In the optical module, two birefringent plates and the Faraday rotator have a slab shape with uniform thicknesses, respectively, and the optical axis of the isolator may be inclined to the optical axis of the optical fiber. In a modification, the optical isolator has a uniform thickness comprised of two birefringent plats with linearly varying thickness and the Faraday rotator with a uniform thickness. In this arrangement of the isolator, the normal of two surfaces of the isolator may be inclined to the optical axis of the optical fiber, or the normal of two surfaces of the isolator is in parallel to the optical axis of the fiber but the normal of two surfaces of the Faraday rotator may be inclined to the optical axis of the optical fiber. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0018]      FIG. 1  is a cross section of the package of the bi-directional optical module according to the present invention; 
           [0019]      FIG. 2A  schematically describes the process of the transmitting light emitted from the LD to optically couple to the optical fiber, and  FIG. 2B  schematically explains the mechanism of the polarization independent isolator for the transmitting light; 
           [0020]      FIG. 3A  schematically describes the process of the scattered backward light not to return to the LD, and  FIG. 3B  schematically explains the mechanism of the polarization independent isolator to shift the optical position of backward light; 
           [0021]      FIG. 4  shows behaviors of the beam shift S of the scattered backward light with respect to the thickness of the polarization independent isolator T; 
           [0022]      FIG. 5  schematically describes the process of the receiving light to optically couple with the PD; and 
           [0023]      FIG. 6A  explains the mechanism for the forward light to pass the polarization independent isolator, while  FIG. 6B  explains the mechanism for the backward light to prevent from passing through the isolator. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    Next, one packaged bi-direction module (hereafter denoted as BiD module) with a polarization independent isolator will be described in detail.  FIG. 1A  is a cross section of the BiD module  1 ,  FIG. 1B  is a cross section of the polarization independent isolator, and  FIG. 1C  magnifies the primary portion of the BiD module  1 . The BiD module  1  includes a disk shaped stem  11 , a DFB-LD  32  and a PD  36 . The DFB-LD  32  emits the transmitted light to the optical fiber that is not shown in  FIG. 1A , while, the PD  36  receives the received light with a wavelength different from that of the transmitted light. These two optically active devices, the DFB-LD  32  and the PD  36 , are mounted on the stem  11 . That is, the DFB-LD  32  is mounted on an optical axis for the ordinary ray of the polarization independent isolator  20 , while, the PD is mounted on the other axis for the extraordinary ray of the polarization independent isolator  20 . 
         [0025]    The stem  11  further provides a plurality of lead terminals  12  and a cap  13  accompanied with a lens  31 . The stem  11  and the cap  13  form a space within which the DFB-LD  32 , the PD  36  and other optical and electrical components are installed. The stem  11  and the cap  13  with the lens constitute an optical device  19  with a CAN-type package. The sleeve member  16 , which includes various members such as a bush  16   c , a stub  16   d  with a coupling fiber  16   e  in a center thereof, a sleeve  16   b , and a sleeve cover  16   a , and a joint sleeve  14  that mechanically couples the optical device  19  with the stem  11  and the cap  13  to the sleeve member  16  to optically couple the DFB-LD  32  and the PD  36  with the optical fiber in the stub  16   d  of the sleeve member  16 . The sleeve member  16  together with the optical device  19  constitutes the BiD module with a co-axial shape. 
         [0026]    One end of the joint sleeve  14  is fixed to the stem  11  such that the cylinder portion of the joint sleeve  14  wraps the side of the cap  13 . The other end of the joint sleeve  14  mounts the sleeve member  16  thereon. The isolator  20  is attached to the inner surface of the joint sleeve  14 , which is opposite to the end surface on which the sleeve member  16  is mounted, via the holder  15 . The isolator  20  has a function to prevent the backward light returning to the DFB-LD  32 . The backward light means the light advancing toward the LD  32 , while, the forward light means the light emitted from the LD  32  to advance toward the sleeve member  16 . The polarization independent isolator  20  provides a Faraday rotator  21  and a pair of birefringent plates,  22   a  and  22   b.    
         [0027]    These two birefringent plates,  22   a  and  22   b , in  FIG. 1A  has a slab shape with a uniform thickness and put the Faraday rotator  21  there between, but the optical axis of the isolator  20 , which is the normal of the slab shaped birefringent crystals,  22   a  and  22   b , is inclined with respect to the optical axis connecting the coupling fiber  16   e  with the optical device  19 . In an alteration, the birefringent plates,  22   a  and  22   b , may have a wedge shape with a linearly increasing thickness but the total thickness of the isolator  20  is uniform, where the normal of the Faraday rotator is in parallel to the optical axis connecting the coupling fiber  16   e  and the optical device  19 , while, the normal of the outer surface of the birefringent plates,  22   a  and  22   b , is inclined to the optical axis. 
         [0028]    Still further modification of the isolator  20  may be applicable, where the birefringent plates,  22   a  and  22   b , have the wedge shape but the normal of the outer surface thereof is in parallel to the optical axis, while the Faraday rotator has a constant thickness but the normal of the rotator is inclined to the optical axis. This modified arrangement of the isolator  20 , which is illustrated in  FIG. 2A , makes it possible to change the separation between the ordinary ray and the extraordinary ray of the isolator  20 . 
         [0029]    The condenser lens  31  may be a ball lens, which concentrates the transmitted light emitted from the DFB-LD  32  on the coupling fiber  16   e  and the received light emitted from the coupling fiber  16   e  on the PD  35 . The DFB-LD shows a good chirping characteristic. The WDM filter  33  reflects the transmitted light from the LD  32  to the coupling fiber  16   e  and passes the received light emitted from the coupling fiber  16   e  to the PD  36 . The sub-mount  34  mounts the DFB-LD  32  thereon, while, another sub-mount  41   a  mounts the PD  36 . The optical device  19  may provide another PD  35  for monitoring a portion of the back light emitter from the back facet of the DFB-LD  32 . The monitor PD  35  is mounted on the third sub-mount  41   b.    
         [0030]    The electronic circuit  37  controls the DFB-LD  32  and the PD  35 . The first block with a slant surface supports the WDM filter  33  on this slant surface, while, the second block  42   b  with a slant surface mounts the assembly of the monitor PD  35  and the sub-mount  41   b  on this slant surface. Accordingly, the light-receiving surface of the monitor PD  35  is inclined to the optical axis of the DFB-LD  32 . These first and second blocks,  42   a  and  42   b , are built in the stem  11  of the optical device  19 . 
         [0031]    Next, two mechanisms of the polarization independent isolator  20  will be explained, one of which is that the transmitted light from the DFB-LD  32  may couple with the fiber  16   e  in  FIG. 2 , while the other of which is that the reflected light of the transmitted light or the received light, which is the backward light, may uncouple from the DFB-LD  32 , which is shown in  FIG. 3 . As shown in  FIGS. 2 and 3 , the isolator has the arrangement that two birefringent plates have the wedge shape but the outer surface thereof is in parallel to the optical axis, while, the surface of the Faraday rotator which has the slab shape with an uniform thickness is inclined to the optical axis. This arrangement of the isolation  20  may change the separation between the ordinary ray and the extraordinary ray. 
         [0032]    Although  FIGS. 2B and 3B  simply illustrate the birefringent plates,  22   a  and  22   b , as the slab shape with the uniform thickness; they are practically the wedge shaped birefringent plate. 
         [0033]    As shown in  FIG. 2A , the transmitted light with a wavelength of 1310 nm is reflected at the WDM filter  33 , concentrated by the lens  31  and finally passes the polarization independent isolator  20 . The ordinary ray Or of the transmitted light passing through the isolator  20  rotates the polarization thereof by −45° and enters the fiber  16   e . Thus, the transmitted light may pass the isolator with substantially no loss to couple with the fiber  38 . 
         [0034]    On the other hand, the backward light, which is a portion of the transmitted light reflected at the end of the fiber  38 , passes the isolator  20 . However, one of the birefringence crystal  22   b  shifts the axis of the extraordinary ray Er, and the Faraday rotator  21  rotates the polarization of both the ordinary ray Or and the extraordinary ray Er by −45°. Finally, the other birefringent plates  22   a  shifts the axis of the extraordinary ray to a location marked Q in  FIG. 3B . Thus, the backward light may not enter the LD  32 . In  FIG. 3 , the symbol S means the separation of the backward light at the facet of the LD  32 , while the other symbol c denotes the center of the LD  32 , which corresponds to the optical axis of the LD  32 . 
         [0035]    Next, the relation between the separation S and the thickness T of the isolator will be described as referring to  FIG. 4 . The behaviors L 1  to L 3  denote the separation of the beam S (μm) from the original position with respect to the thickness T (mm) of the isolator under conditions of the magnification factor of the image, 1.5, 2.0 and 3.0, respectively. The embodiments aforementioned, which has the magnification factor about 2.0 and the thickness of the isolation about 1 mm. In this case, the beam shift becomes about 20 μm, which is enough separation to prevent the backward beam from re-entering the DFB-LD  32 , which prevents the DFB-LD  32  from being degraded in the oscillation performance thereof. 
         [0036]      FIG. 5  illustrates a condition where the received light, which is the backward light, may optically couple with the PD as passing through the isolator. The received light emitted from the fiber  38  and has a wavelength of 1490 nm passes through the polarization independent isolator  20  and enters the PD  36 . In this condition, the received light shifts its entering position to the PD  36  from the center R thereof by about 20 μm (S=20 μm), similar to the case for the scattered light of the transmitted light mentioned above. However, because the sensing area of the PD  36  has a diameter greater than 50 μm, typically 50 to 80 μm, the PD  36  may receive almost whole portion of the received light even if the light shifts the position thereof by the isolator  20 . 
         [0037]    Thus, according to the present arrangement of the bi-directional optical module, which installs the polarization independent isolator in addition to the DFB-LD and the PD, the scattered backward light does not re-enter in the DFB-LD and the received backward light with the wavelength different from the scattered backward light does enter the PD with substantially negligible optical loss. The scattered backward light may not enter the PD because the WDM filter installed in front of the PD reflects the substantially whole portion thereof. 
         [0038]    Additional modifications and improvements of the present invention may also be apparent to those of ordinary skill in the art. Thus, the particular combination of parts described and illustrated herein is intended to represent only certain embodiments of the present invention, and is not intended to serve as limitations of alternative devices within the spirit and scope of the invention.