Abstract:
A device capable of equalizing optical powers of optical signals in a passive optical network, the device comprising a first optical coupler for receiving optical signals having different optical power levels, an optical circulator capable of directing the optical signals from the first optical circulator, a laser diode capable of generating equalized optical signals having a predetermined range of optical power levels in response to the optical signals directed from the optical circulator, and a second optical coupler for receiving the equalized optical signals.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to optical transmission and, more particularly, to an optical power equalizer capable of equalizing the optical power of optical signals in a passive optical network. 
         [0002]    The increasing demand for faster and higher capacity information processing and transmission has accelerated the development and research in optical fiber networks and systems. Information may be transported through optical systems in audio, video, data, or other signal formats analogous to electrical systems. Furthermore, optical systems may be used in telephone, cable television, local area network (“LAN”) and wide area network (“WAN”) systems as well as other communication systems. Optical systems may also offer various communication services such as voice over internet protocol (“VoIP”) and internet protocol television (“IPTV”) services in a fiber-to-the-X (“FTTX”) architecture, including fiber-to-the-home (“FTTH”), fiber-to-the-premise (“FTTP”), fiber-to-the-curb (“FTTC”) or the like. 
         [0003]    A passive optical network (“PON”) is one of optical network systems used for the FTTX architecture for introducing optical communications.  FIG. 1  is a block diagram of a conventional PON  10 . Referring to  FIG. 1 , the PON  10  includes an optical line termination (“OLT”)  11 , an optical splitter  12  and a plurality of optical network units (“ONUs”)  13 - 1  to  13 -N. Each of the ONUs  13 - 1  to  13 -N transmits optical signals upstream to the OLT  11  through the optical splitter  12 . However, since the distance between the OLT  11  and the ONUs  13 - 1  to  13 -N may be different from each other, optical signals  14 - 1  to  14 -N may reach the OLT  11  with different optical powers due to, for example, signal path attenuation. The different power levels may disadvantageously result in an incorrect detection of the optical signals at the OLT  11 . Generally, a burst mode receiver (not shown) may be provided in the OLT  11  to equalize the optical powers of the optical signals. The burst mode receiver is required to detect a relatively wide range of powers, for example, 21 dB to 24 dB, and support dynamic adjustment of decision threshold values, which may complicate the OLT structure and reduce the bandwidth efficiency. 
         [0004]    It may be therefore desirable to have an optical power equalizer that is able to equalize the optical power of optical signals in a passive optical network. It may be also desirable to have an optical power equalizer that is able to cost-efficiently equalize optical signals at an OLT side. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    Examples of the invention may provide a device capable of equalizing optical powers of optical signals in a passive optical network, the device comprising a first optical coupler for receiving optical signals having different optical power levels, an optical circulator capable of directing the optical signals from the first optical circulator, a laser diode capable of generating equalized optical signals having a predetermined range of optical power levels in response to the optical signals directed from the optical circulator, and a second optical coupler for receiving the equalized optical signals. 
         [0006]    Examples of the invention may also provide a device capable of equalizing optical powers of optical signals in a passive optical network, the device comprising a laser diode capable of generating equalized optical signals having a predetermined range of optical power levels in response to optical signals having a first wavelength, and an optical circulator capable of directing the optical signals having a first wavelength to the laser diode and bypassing optical signals having a second wavelength different from the first wavelength. 
         [0007]    Some examples of the invention may also provide a device capable of equalizing optical powers of optical signals in a passive optical network, the device comprising a multiplexer/demultiplexer, a laser diode capable of generating equalized optical signals having a predetermined range of optical power levels in response to optical signals having a first wavelength from the multiplexer/demultiplexer, and an optical circulator capable of directing the optical signals having the first wavelength to the laser diode. 
         [0008]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0009]    The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings examples consistent with the invention. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
           [0010]    In the drawings: 
           [0011]      FIG. 1  is a block diagram of a conventional passive optical network (“PON”); 
           [0012]      FIG. 2  is a schematic diagram of a passive optical network consistent with an example of the present invention; 
           [0013]      FIG. 3A  is a schematic diagram of an optical power equalizer (“OPE”) consistent with an example of the present invention; 
           [0014]      FIG. 3B  is a schematic diagram of an OPE consistent with another example of the present invention; 
           [0015]      FIG. 4A  is a schematic diagram of an optical line termination (“OLT”) and the OPE illustrated in  FIG. 3A ; 
           [0016]      FIG. 4B  is a schematic diagram of an OLT and the OPE illustrated in  FIG. 3B ; 
           [0017]      FIG. 4C  is a schematic diagram of an OPE incorporated in an OLT consistent with an example of the present invention; 
           [0018]      FIGS. 5A and 5B  are characteristic diagrams of a Fabry-Perot laser diode; 
           [0019]      FIGS. 6A to 6D  are diagrams illustrating output spectrums at different input power levels; 
           [0020]      FIGS. 7A to 7D  are eye diagrams of uplink input powers before equalization; and 
           [0021]      FIGS. 8A to 8D  are eye diagrams of uplink input powers after equalization consistent with an example of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    Reference will now be made in detail to the present embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like portions. 
         [0023]      FIG. 2  is a schematic diagram of a passive optical network (“PON”)  20  consistent with an example of the present invention. Referring to  FIG. 2 , the PON  20  includes an optical line termination (“OLT”)  21 , an optical splitter  22 , a plurality of optical network units (“ONUs”)  23 - 1  to  23 -N and an optical power equalizer (“OPE”)  26 . In the present example, the OPE  26  is separate from the OLT  21 . In other examples, however, the OPE  26  is incorporated in the OLT  21 . The optical splitter  22 , located between the OLT  21  and the ONUs  23 - 1  to  23 -N, is connected to the OLT  21  through an optical path  27  and connected to the ONUs  23 - 1  to  23 -N through optical paths  27 - 1  to  27 -N. Each of the ONUs  23 - 1  to  23 -N transmits uplink optical signals to the OLT  11  through the optical splitter  22 . The optical signals  24 - 1  to  24 -N may exhibit different power levels due to different distances between the OLT  21  and the ONUs  23 - 1  to  23 -N. The OPE  26  equalizes the optical signals  24 - 1  to  24 -N having different power levels to provide optical signals  26 - 1  to  26 -N with substantially the same power level. 
         [0024]    The plurality or ONUs  23 - 1  to  23 -N include transceivers  25 - 1  to  25 -N, respectively, for transmitting uplink signals to the OLT  21  or receiving downlink signals from the OLT  21 . The OPE  26  includes a laser diode  28  capable of equalizing uplink optical signals. Each of the transceivers  25 - 1  to  25 -N and the laser diode  28  includes substantially the same laser diode. In one example consistent with the present invention, each of the transceivers  25 - 1  to  25 -N and the laser diode  28  includes a Fabry-Perot laser diode (“FP-LD”). Furthermore, distinct wavebands are used in the PON  20  for transmitting optical signals. In one example, uplink data are transmitted in a 1310 nanometer (nm) band, downlink data are transmitted in a 1490 nm band, and image data are transmitted in a 1550 nm band. 
         [0025]      FIG. 3A  is a schematic diagram of an optical power equalizer (“OPE”)  30  consistent with an example of the present invention. Referring to  FIG. 3A , the OPE  30  includes a laser diode  31 , an optical circulator  32 , a first optical coupler  33 - 1  and a second optical coupler  33 - 2 . In an uplink transmission, the first optical coupler  33 - 1  receives uplink signals having a first wavelength sent from transceivers of optical network units (“ONUs”), and directs the uplink signals to the optical circulator  32 , which in turn directs the uplink signals to the laser diode  31 . The laser diode  31 , which is substantially the same as those included in the transceivers, equalizes the uplink signals in power level and provides equalized uplink signals to the optical circulator  32 . The second optical coupler  33 - 2  receives the equalized uplink signals from the optical circulator  32  and sends the same to the OLT. In a downlink transmission, the second optical coupler  33 - 2  receives downlink signals having a second wavelength from the OLT and sends the downlink signals to the first optical coupler  33 - 1 , bypassing the optical circulator  32 . In one example consistent with the present invention, the laser diode  31  includes an FP-LD. Each of the first optical coupler  33 - 1  and the second optical coupler  33 - 2  includes a wavelength division multiplexed (“WDM”) coupler. Furthermore, the first wavelength and the second wavelength are 1310 nm and 1490 nm, respectively. 
         [0026]      FIG. 3B  is a schematic diagram of an OPE  30 - 1  consistent with another example of the present invention. Referring to  FIG. 3B , the OPE  30 - 1  includes a laser diode  31  and an optical circulator  32 - 1 . The optical circulator  32 - 1  is capable of band selection, thereby eliminating the first and second optical coupler  33 - 1  and  33 - 2  illustrated in  FIG. 3A . Specifically, in an uplink transmission, the optical circulator  32 - 1  directs uplink signals having a first wavelength at a first port labeled “1” to the laser diode  31 , receives equalized uplink signals from the laser diode  31  at a second port labeled “2”, and provides the equalized uplink signals to an OLT at a third port labeled “3”. In a downlink transmission, the optical circulator  32 - 1  directs downlink signals having a second wavelength at the third port “3” to the first port “1”, bypassing the laser diode  31 . 
         [0027]      FIG. 4A  is a schematic diagram of an optical line termination (“OLT”)  40  and the OPE illustrated  30  in  FIG. 3A . Referring to  FIG. 4A , the OLT  40  includes a light source (LS)  41 , a photodetector (PD)  42  and a multiplexer/demultiplexer (MUX/DEMUX)  43 . The light source  41 , for example, a laser diode, generates downlink optical signals to be transmitted to ONUs. The MUX/DEMUX  43  multiplexes the downlink optical signals for the downstream transmission, and demultiplexes equalized uplink optical signals sent from the second optical coupler  33 - 2 . The photodetector  42  detects the demultiplexed equalized uplink optical signals. In one example consistent with the present invention, the MUX/DEMUX  43  includes a WDM coupler. 
         [0028]      FIG. 4B  is a schematic diagram of an OLT  40  and the OPE  30 - 1  illustrated in  FIG. 3B . Referring to  FIG. 4B , the optical circulator  32 - 1  directs multiplexed downlink optical signals sent from the MUX/DEMUX  43  to the first port “1”, bypassing the laser diode  31  in a downlink transmission. In an uplink transmission, the optical circulator  32 - 1  directs uplink optical signals at the first port “1” to the laser diode  31 , receives equalized uplink optical signals from the laser diode  31  at the second port “2”, and provides the equalized uplink optical signals to the MUX/DEMUX  43  at the third port “3”. 
         [0029]      FIG. 4C  is a schematic diagram of an OPE  40 - 1  incorporated in an OLT  30 - 2  consistent with an example of the present invention. Referring to  FIG. 4C , the optical circulator  32  of the OPE  40 - 1  is connected to the MUX/DEMUX  43  at the first port “1”, and connected to the photodetector  42  at the third port “3”. In an uplink transmission, the optical circulator  32  directs demultiplexed uplink optical signals sent from MUX/DEMUX  43  at the first port “1” to the laser diode  31 , receives equalized uplink optical signals at the second port “2”, and sends the equalized uplink optical signals to the photodetector  42 . 
         [0030]      FIGS. 5A and 5B  are characteristic diagrams of a Fabry-Perot laser diode (“FP-LD”). Although FP-LDs are generally used as transceivers in ONUs for transmitting 1310-nm uplink signals, for the purpose of convenience, 1510-nm FP-LDs are used in the experimental design. Skilled persons in the art will understand that FP-LDs have similar optical characteristics, despite the applications in different wavelengths.  FIG. 5A  illustrates the output power of an FP-LD as a function of bias current. The FP-LD is controlled at a temperature of approximately 22 degrees Celsius (° C.), and has a mode spacing of approximately 1.3 nm. Referring to  FIG. 5A , it can be seen that the threshold current (I THRES ) of the FP-LD is approximately 9.5 milliampere (mA), at which the corresponding output power is approximately −10.1 dBm. When the bias current (I BIAS ) is 9 mA, the corresponding output power is approximately −13.6 dBm. The FP-LD enters an excited state as I BIAS  exceeds I THRES , and enters a saturation state as I BIAS  exceeds approximately 20 mA. 
         [0031]      FIG. 5B  illustrates the total power of an FP-LD at different uplink input powers. With a bias current I BIAS  of approximately 9 mA smaller than the I THRES , the FP-LD does not but is ready to enter the excited state. Considering an insertion loss of approximately 14 dB and transmission loss over optical paths, the power level of an uplink optical signal that reaches an OLT may range from approximately −17 dBm to −25 dB. Furthermore, according to the standards for PON, the minimum power level available for an OLT is approximately −25 dBm. As a result, power levels between −15 and −25 dBm are of interest. Referring to  FIG. 5B , for uplink input powers ranging from −15 to −25 dBm, the total output power of a mode-locked FP-LD ranges from approximately −14.9 to −15.1 dBm, resulting in an output power variation of approximately 0.2 dBm. Furthermore, for the same uplink input powers, the total output power of an FP-LD without mode-locked ranges from approximately −16.9 to −17.3 dBm, resulting in an output power variation of approximately 0.4 dBm. Accordingly, a mode-locked FP-LD has a better performance in equalization than an FP-LD without mode-locked. 
         [0032]      FIGS. 6A to 6D  are diagrams illustrating output spectrums at different input power levels. It can be seen from  FIGS. 6A to 6D  that the output powers corresponding to uplink input powers of −8.5, −11.5, −13.5 and −15.5 dBm are −14.1, −14.5, −14.7 and −14.9 dBm, respectively, given a mode-locked FP-LD. Accordingly, an OPE according to the present invention is able to equalize uplink optical signals having power levels ranging from −8.5 to −25 dBm with approximately 1 dBm power variation. 
         [0033]      FIGS. 7A to 7D  are eye diagrams of uplink input powers before equalization. In the experiment, phase modulation is conducted in an electro-optical modulator made of lithium niobate (LiNbO 3 ) in a 20 gigabits per second (Gbit/s) non-return to zero (“NRZ”) system. Referring to  FIGS. 7A to 7D , the measured extinction ratios (“ERs”) of the uplink input powers of −8.5, −11.5, −13.5 and −15.5 dBm are 9.96, 9.41, 9.01 and 8.35 dB, respectively. It can be found that the ER decreases as the uplink input power decreases. 
         [0034]      FIGS. 8A to 8D  are eye diagrams of uplink input powers after equalization consistent with an example of the present invention. Referring to  FIGS. 8A to 8D , the measured extinction ratios of the uplink input powers of −8.5, −11.5, −13.5 and −15.5 dBm are 7.01, 7.41, 7.22 and 7.03 dB, respectively, which are greater than 6 dB, a value required by the standards for PON. Furthermore, the eye openings are substantially the same in  FIGS. 7A to 7D . 
         [0035]    It will be appreciated by those skilled in the art that changes could be made to one or more of the examples described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular examples disclosed, but it is intended to cover modifications within the scope of the present invention as defined by the appended claims. 
         [0036]    Further, in describing certain illustrative examples of the present invention, the specification may have presented the method and/or process of the present invention as a particular sequence of steps. However, to the extent that the method or process does not rely on the particular order of steps set forth herein, the method or process should not be limited to the particular sequence of steps described. As one of ordinary skill in the art would appreciate, other sequences of steps may be possible. Therefore, the particular order of the steps set forth in the specification should not be construed as limitations on the claims. In addition, the claims directed to the method and/or process of the present invention should not be limited to the performance of their steps in the order written, and one skilled in the art can readily appreciate that the sequences may be varied and still remain within the spirit and scope of the present invention.