Patent Publication Number: US-2012033976-A1

Title: Optical communication module

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. P2010-174287, filed on Aug. 3, 2010; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments of the present invention relate to an optical communication module. 
     BACKGROUND 
     An optical communication module is known which transmits and receives optical signals for data communications through a free space. 
     In a conventional optical communication module, an optical receiver receives an optical signal and external disturbing light such as sun light or illumination light. However, external disturbing light included in an optical signal adversely influences the performance of an optical transmitter/receiver. For this reason, to improve the performance of the optical transmitter/receiver, there have been developed various methods of increasing an S/N ratio of an optical signal received by the optical receiver by removing the external disturbing light from the optical signal. 
     Meanwhile, it is known that a light emitting diode (LED) or a surface emitting laser (VCSEL: Vertical Cavity Surface Emitting Laser) is used as a light emitting element for the optical communication module. Particularly, in comparison with an edge emitting-type laser diode, the VCSEL has such features that the drive current is low, that characteristic inspection at a wafer level is possible, and that the two-dimensional arrangement is easy. Accordingly, the VCSEL is widely used as the light source in optical information processing and optical communications. 
     The VCSEL is used while being sealed in a can package or a resin package to which an optical system such as a lens or an optical fiber is attached. Thus, a laser beam emitted from the VCSEL is transmitted to a receiver module through the optical system such as the lens and the optical fiber. The laser beam, however, has a problem that the intensity of the outer periphery of the laser beam is reduced unless the optical system is positioned accurately in a transmitter module. Particularly, a region which receives a low-intensity laser beam is so susceptible to external disturbing light that the laser beam received therein is unstable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing the configuration of an optical communication module according to embodiments. 
         FIG. 2  is a conceptual diagram for exemplifying a process of extracting a first optical signal S 1  from a first optical receiving signal and a second optical receiving signal. 
         FIG. 3  is a schematic top view of a transmitter module according to a first embodiment seen from the above. 
         FIG. 4  is a schematic cross-sectional view showing the transmitter module  20  according to the first embodiment. 
         FIG. 5  is a schematic top view of a receiver module  30  according to the first embodiment seen from the above. 
         FIG. 6  is a schematic cross-sectional view for illustrating the relationship between an emission angle and the properties of a first lens (second lens) according to the first embodiment. 
         FIG. 7  is a schematic cross-sectional view showing a first lens (second lens) according to a second embodiment. 
         FIG. 8  is a schematic cross-sectional view showing a first lens (second lens) according to a third embodiment. 
         FIG. 9  is a cross-sectional view showing a transmitter module  20  according to a fourth embodiment. 
         FIG. 10  is a top view of a receiver module  30  according to the fourth embodiment seen from the above. 
     
    
    
     DETAILED DESCRIPTION 
     An optical communication module according to embodiments of the present invention is characterized by including: a surface emitting element configured to convert an electrical signal into an optical signal and emit the optical signal; a lens provided at a predetermined distance from the surface emitting element in a direction of an optical axis of the surface emitting element so that an emission angle of the optical signal from the surface emitting element is 30° or lower, the lens configured to output a first optical signal and a second optical signal; a first polarizing plate provided on the center of an optical axis of the lens, and configured to change the first optical signal in a specific direction and pass the first optical signal therethrough; and further a second polarizing plate provided on the center of another optical axis of the lens and adjacently to the first polarizing plate, and configured to polarize the second optical signal at a polarization angle different from that of the first optical signal by 90°, and pass the second optical signal therethrough. 
     Hereinafter, embodiments of the invention will be described with reference to the drawing. 
       FIG. 1  is a block diagram showing the configuration of an optical communication module  100  according to the embodiments. The optical communication module  100  includes: an optical transmitter module  20  which creates an optical signal and outputs two signals  51  and S 2  different from each other in polarization angle; and an optical receiver module  30  which receives the two optical signals S 1  and S 2  outputted from the optical transmitter module  20  through a light transmission space. In the embodiments, the optical transmitter module and the optical receiver module are provided separately from each other by approximately 10 cm to 1 m. 
     The optical transmitter module  20  outputs the first optical signal  51  and the second optical signal S 2  which are obtained by polarizing an optical signal into specific directions with a first polarizing plate  22  and a second polarizing plate  23 , the optical signal being emitted as a laser beam by converting a predetermined current control signal (for example, bias current IBIAS, modulation current IMOD) with a light emitter  21 . The first polarizing plate  22  and the second polarizing plate  23  differ from each other in polarization angle by approximately 90°. Thus, the optical transmitter module  20  outputs the first optical signal S 1  and the second optical signal S 2  which have different polarization angles from each other by approximately 90° and have low interference with each other. 
     The optical signals inputted into the optical receiver module  30  include external disturbing light N 1  and N 2  such as indoor lighting and sun light in addition to the first optical signal Si and the second optical signal S 2  outputted from the optical transmitter module  20 . Hence, the first optical signal Si and the first external disturbing light Ni are inputted into a first receiver polarizing plate  31  of the optical receiver module  30 , while the second optical signal S 2  and the second external disturbing light N 2  are inputted into a second receiver polarizing plate  32 . 
     Since the first external disturbing light N 1  and the second external disturbing light N 2  are natural light, the oscillation orientation of each component is not fixed, and there is no polarization dependency as a whole. For this reason, the first external disturbing light N 1  and the second external disturbing light N 2  pass through the respective polarizing plates without any influence. 
     Moreover, since the first receiver polarizing plate  31  and the second receiver polarizing plate  32  are disposed adjacently to each other, the first optical signal S 1  and the first external disturbing light incident on the first receiver polarizing plate  31  can be considered to be equivalent to the second optical signal S 2  and the second external disturbing light N 2  incident on the second receiver polarizing plate  32 . Hereinafter, a signal in which the first external disturbing light N 1  is superposed on the first optical signal Si having passed through the first receiver polarizing plate  31  is called a first optical receiving signal OS 1 , while a signal in which the second external disturbing light N 2  is superposed on the second optical signal S 2  having passed through the second receiver polarizing plate  32  is called a second optical receiving signal OS 2 . 
     After the first optical receiving signal OS 1  and the second optical receiving signal OS 2  enter an optical receiver  33 , the first external disturbing light N 1  and the second external disturbing light N 2  are removed by a post-processing unit  34 . The post-processing unit  34  may further includes an amplifier circuit for amplifying the first optical signal S 1  from which the external disturbing light has been removed. In this case, the amplifier circuit may be a circuit using an operational amplifier, or may be a known circuit using known transformer, impedance, and amplifier to reduce a noise. 
       FIG. 2  is a conceptual diagram for exemplifying a process of extracting the first optical signal Si (or the second optical signal S 2 ) by removing the first external disturbing light and the second external disturbing light from the first optical receiving signal OS 1  and the second optical receiving signal OS 2 . In the diagram, an arrow in a circle denotes a polarization direction. The first polarizing plate  22  and the second polarizing plate  23  differ from each other in polarization angle by approximately 90°. Similarly, the first receiver polarizing plate  31  and the second receiver polarizing plate  32  are also polarizing plates that differ from each other in polarization angle by approximately 90°. The first optical signal S 1  outputted from the first polarizing plate  22  and the second optical signal S 2  outputted from the second polarizing plate  23  are respectively inputted into the first receiver polarizing plate  31  and the second receiver polarizing plate  32 , together with the first external disturbing light Ni and the second external disturbing light N 2 . The optical receiver  33  receives these signals in the form of the first optical receiving signal OS 1  and the second optical receiving signal OS 2 , respectively. Then, the post-processing unit  34  removes the external disturbing light N 1  and N 2 . In the embodiments, the phase of the second optical receiving signal OS 2  is inverted, and the first optical receiving signal OS 1  is multiplexed with the second optical receiving signal OS 2  whose logic has been inverted. Thereby, the first external disturbing light N 1  is extracted. After the phase of the first external disturbing light N 1  thus extracted is inverted, the first external disturbing light Ni is again multiplexed with the first optical receiving signal OS 1  to cancel the first external disturbing light N 1 . Thereby, the first optical signal S 1  is extracted. 
     FIRST EMBODIMENT 
       FIG. 3  is a top view of a transmitter module  20  according to a first embodiment seen from the above. As shown in the drawing, on an external light interference  10  within a ceramic board  1  stacked on an electrode terminal  3 , surface emitting elements  5   a ,  5   b  and a drive circuit  4  are fixed with a conductive adhesive and connected to each other with gold wires  11 . Moreover, a microchip capacitor  12  is disposed on the external light interference  10 . 
     The drive circuit  4  is a circuit for controlling the intensity of an optical signal through current control on the surface emitting elements  5   a ,  5   b  on the basis of a predetermined current control signal (for example, bias current IBIAS, modulation current IMOD). The configuration of the circuit may be known, as long as the circuit has a frequency characteristic of approximately 3 GHz. 
     The surface emitting elements  5   a  and  5   b  convert an electrical signal created by the drive circuit  4  into an optical signal. The surface emitting elements  5   a  and  5   b  may be a VCSEL, for example. The surface emitting element  5   a  (surface emitting element  5   b ) emits a laser beam having a frequency corresponding to the transmission distance of the optical signal. When the transmission distance is short, for example, an 850-nm laser beam is outputted. This embodiment includes two surface emitting elements  5   a  and  5   b , as well as polarizing plates  22  and  23  respectively thereabove. The polarizing plates  22  and  23  differ from each other in polarization angle by approximately 90°, as will be described later. 
       FIG. 4  is a cross-sectional view showing the transmitter module  20  according to the first embodiment, the cross-sectional view showing the section taken along the line A-A in the top view of the transmitter module in  FIG. 3 . In  FIG. 3 , a stacked electrical circuit  2  has vias formed in the stacked ceramic board  1 , so that each electrode terminal  3  can supply a signal to an unillustrated power supply, ground, and so forth. 
     As described above, the first surface emitting element  5   a  and the second surface emitting element  5   b  are mounted on the electrical circuit pattern of the ceramic board  1  with the conductive adhesive. The drive circuit  4  similarly mounted supplies an electrical signal which is converted into a modulation signal for the first surface emitting element  5   a  and the second surface emitting element  5   b , and then converted into an optical signal by driving the surface emitting elements  5   a  and  5   b . In this case, the modulation is performed with the same signal for the two surface emitting elements. 
     A first lens  7   a  and a second lens  7   b  are attached to the ceramic board  1  with a first cap  6   a  and a second cap  6   b  by a method such as electric resistance welding. The first lens  7   a  and the second lens  7   b  are optically disposed on the centers of the optical axes of the first surface emitting element  5   a  and the second surface emitting element  5   b , respectively. As will be described later, the properties of the first lens  7   a  (second lens  7   b ), such as lens structure, refractive index, curvature, and positional relation to the surface emitting element, are selected in relation to the emission angle and the spot diameter on a receiver module  30 . 
     The first polarizing plate  22  and the second polarizing plate  23  are fixed to a cap with lenses attached to polarizing plate-holding plates  8   a ,  8   b . As described above, the first polarizing plate  22  and the second polarizing plate  23  are set to have polarization angles with an approximately 90° difference to prevent interference of waveforms outputted from both of the polarizing plates. Moreover, each of the polarizing plates is disposed at a position where light spreading on the optical axis of the corresponding surface emitting element and on the optical axis of the lens sufficiently passes. In a case where the first cap  6   a  and the second cap  6   b  are not used, the polarizing plates may be fixed on the ceramic board  1  with a UV curable resin or the like. 
       FIG. 5  is a top view of the receiver module  30  according to the first embodiment seen from the above. An electrical circuit and a mount terminal  42  are formed on a glass epoxy board  41 . On the electrical circuit pattern, a first light receiving element  44   a  and a second light receiving element  44   b  are disposed respectively on insulating bases  45   a  and  45   b  for adjusting the height positions, which are made of a material such as a ceramic. The insulating bases  45   a  and  45   b  have upper and lower surfaces with a metal film formed thereon, allowing current to flow through wire bonding or the like. In addition, a light-receiving drive circuit  43  which performs amplification or the like on an electrical signal from the light receiving elements having received an optical signal is disposed on the circuit pattern, and connected to the light receiving elements and so forth with gold wires or the like. Note that, as noise measure, a microchip capacitor  46  having upper and lower electrodes is disposed near the light receiving element  44   a , the microchip capacitor  46  being formed between top surface and back surface sides. These circuits are further sealed with a trapezoidal structure made of a transparent resin. Moreover, polarizing plates  31  and  32  respectively adhere and are fixed to the first light receiving element  44   a  and the second light receiving element  44   b  with a transparent UV curable resin or the like, and specify the polarization directions of light that the light receiving elements receive. 
       FIG. 6  is a drawing for illustrating the relationship between the properties of the first lens  7   a  and the emission angle. Hereinafter, only the first lens  7   a  will be described for the convenience of the description, but the same applies to the second lens  7   b .  FIG. 6  shows an example in which the first lens  7   a  having a diameter of 2 mm and a refractive index of 1.55 is provided at a distance of 1.65 mm away from the light emitting surface of the first surface emitting element  5   a  (second surface emitting element  5   b ). In this case, the emission angle of light from the first surface emitting element  5   a  (second surface emitting element  5   b ) is 30°, and a spot having a diameter of approximately 270 mm is obtained on the light receiving element of the optical receiver module  30  located at a distance of 500 mm the first surface emitting element  5   a . Meanwhile, in a case where the first lens  7   a  having a diameter of 2 mm is provided at a distance of less than 1.65 mm from the light emitting surface of the first surface emitting element  5   a  (second surface emitting element  5   b ), the emission angle is 7°, and a spot having a diameter of approximately 60 mm is obtained on the light receiving element of the optical receiver module  30  located at a distance of 500 mm from the first surface emitting element  5   a.    
     As described above, in a case where the refractive index and curvature diameter of a lens used as the first lens  7   a  are fixed, the emission angle and the spot diameter are decreased as the first lens  7   a  and the first surface emitting element  5   a  becomes closer to each other. In addition, if the lens used as the first lens  7   a  is changed to a lens having a larger refractive index but maintaining the same curvature diameter and position relative to the surface emitting element, the light emission angle and the spot diameter are further decreased. Note that the closer the first polarizing plate  22  is to the first lens  7   a , the longer the interval between slits of the polarizing plate  22  becomes. Accordingly, it is more preferable to dispose the first polarizing plate  22  away from the lens  7   a . Note that, in this embodiment, the first polarizing plate  22  is disposed above the first lens  7   a  because of the space in the unit. 
     As in this embodiment, by appropriately setting the position, the lens diameter, and the refractive index of the first lens  7   a  (second lens  7   b ) as well as the positional relation to the first polarizing plate  22  (second polarizing plate  23 ), a laser beam outputted from the first surface emitting element  5   a  (second surface emitting element  5   b ) can be made close to parallel light as much as possible, and the light emission angle and the spot diameter can be decreased. As a result, entering of external disturbing light other than the optical signal outputted from the optical transmitter module  20  can be suppressed, while the laser beam intensity obtained in the optical receiver module  30  is made stable. 
     Second Embodiment 
       FIG. 7  is a drawing showing a first lens  7   a  according a second embodiment. 
     In this embodiment, each of the first lens  7   a  and a second lens  7   b  is in a cylindrical shape. The lens effect is obtained in a Z axis direction where the cylindrical axis is set as an X axis direction. A first polarizing plate  22  is provided on a Y axis direction that is a direction along the center of the optical axis of the first lens  7   a . The polarization plane of the first polarizing plate  22  is arranged perpendicularly to the Z axis direction where the lens effect is obtained. 
     By providing cylindrical lenses as in this embodiment, the first polarizing plate  22  serves as a slit, and the emission angle from the first light emitting element  5   a  is suppressed. Moreover, the percentage of laser beams passing through a slit of a receiver polarizing plate  34  is increased, and the S/N ratio is improved in comparison with a case where the first lens  7   a  is in a spherical shape with its planar surface parallel to the polarizing plate. 
     Third Embodiment 
       FIG. 8  is a drawing showing a first lens  7   a  according a third embodiment. 
     In this embodiment, as the first lens  7   a , a rod lens (registered trademark) is used. The rod lens is a rod-shaped gradient index lens obtained by cutting an optical fiber having a refractive index distribution in a radial direction into a certain length. 
     By using the rod lens as the first lens  7   a , a first optical signal behaves more like parallel light in an optical receiver module  30 . 
     Fourth Embodiment 
       FIG. 9  is a cross-sectional view of a transmitter module  20  according to a fourth embodiment. In this embodiment, a single light emitting element  51  is mounted on a glass epoxy board  52  and fixed thereto with a conductive epoxy  53 . Moreover, the surface emitting element  51  is connected to a drive IC, and has a structure capable of converting an electrical signal into an optical signal (unillustrated). The top surface of the surface emitting element  51  is sealed with a transparent epoxy resin  54  on the glass epoxy board  52 , and a portion of the transparent epoxy resin  54  near the surface emitting element  51  is thin. 
     Lenses  55  and  56  according to this embodiment are provided above the thin portion, that is, on the center of the optical axis of an optical signal outputted from the surface emitting element  51 . The lenses  55  and  56  collect the optical signal emitted from the surface emitting element  51 . Specifically, a microprism lens is disposed as the lens  55 , which branches the optical signal. Similarly, a prism lens as the lens  56  corrects the optical axis directions thereof, and guides the optical signals to polarizing plates  22  and  23 . 
     Each of the polarizing plates  22  and  23  is provided at a position where light spreading on the optical axis of the surface emitting element  51  and on the optical axes in the lenses sufficiently passes. The polarizing plates  22  and  23  differ from each other in polarization angle by 90°. 
       FIG. 10  is a cross-sectional view showing an optical receiver module according to the fourth embodiment. As shown in  FIG. 10 , two light receiving elements  61 ,  62  are integrally formed with a light receiving circuit  63 . The light receiving elements convert an optical signal received by the light receiving elements into a current. Furthermore, the light receiving elements perform amplification, demodulation, and so forth on the electrical signal. The light receiving circuit  63  is mounted on a glass epoxy board  64  and fixed thereto with a conductive epoxy  65 , and sealed with a transparent epoxy resin  66  on the glass epoxy board  64 . A portion of the transparent epoxy resin  66  near the light receiving circuit  63  is thin. 
     In this embodiment, a microprism lens as a lens  67  is disposed at the thin portion, and a first receiver polarizing plate  31  and a second receiver polarizing plate  32  are fixed thereon. Similarly to the optical transmitter module  20 , the polarization angle difference of 90° and the polarization direction are set so that two kinds of polarized optical signals emitted from the transmitter side can be received separately. The lens  67 , the first receiver polarizing plate  31 , and the second receiver polarizing plate  32  are fixed with a transparent UV curable adhesive or the like. In addition, these polarizing plates can be made more compact by being formed with a photonic crystal. The prism lenses and the like may also be formed using a photonic crystal. 
     With such a structure, when a first optical receiving signal OS 1  and a second optical receiving signal OS 2  emitted from the transmitter side enter simultaneously, only an optical signal that corresponds to one of the polarization angles reaches the prism lens  67  and is thus collected. 
     As in this embodiment, optical signals of two polarization angles from a single surface emitting element can be received by the optical receiver module  30 . In this manner, by providing a single surface emitting element, not only can the number of components be reduced, but also influences of individual differences among surface emitting elements such as laser beam intensity and emission angle are eliminated. Thus, the S/N ratio in the optical receiver module  30  is improved. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention.