Abstract:
A receiver device and receiving method for a coherent optical communication system are disclosed. The disclosed device includes: an optical splitter configured to split a received optical signal into at least two paths; a first amplifier configured to amplify a second optical signal from among a first optical signal and the second optical signal split into two paths; a second amplifier configured to amplify the output signal of the first amplifier; and a coherent receiver module configured to mix and detect the output signal of the second amplifier and the first optical signal. The disclosed device provides the advantages of suppressing the occurrence of frequency offsets and minimizing phase noise, as well as allowing manufacture with a low cost without requiring either an additional optical source or additional optical fibers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of Korean Patent Application No. 10-2015-0008778, filed with the Korean Intellectual Property Office on Jan. 19, 2015, the disclosure of which is incorporated herein by reference in its entirety. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    Embodiments of the present invention relate to a receiver device in an optical communication system, more particularly to a receiver device employed in an optical communication system that utilizes a coherent method. 
         [0004]    2. Description of the Related Art 
         [0005]    The current optical communication system uses an IM-DD method with which the transmitter unit converts an electrical signal into an optical signal and then directly applies intensity modulation (IM) before transmitting the signal, and the receiver unit applies direct detection (DD) on the received optical signal. 
         [0006]    While the capacity of data transmissions for broadband total communication networks in future applications is expected to range from hundreds of gigabytes per second to tens of terabytes per second or even higher, the IM-DD method may not be easy to apply systems having a transmission capacity of 10 Gb/s or higher, because it does not allow phase modulation, so that the efficiency of bandwidth use is low, and the reception sensitivity is also low. 
         [0007]    Due to such requirements, coherent optical communication is again gaining interest, as it allows improved reception sensitivity by mixing a local oscillator signal with a received signal and allows improved bandwidth efficiency through phase modulation. 
         [0008]    In order to utilize coherent optical communication in practical applications for high-capacity, long-distance communication, the spectral linewidth of the semiconductor laser used for the optical sources in the transmitter and local oscillator must be very narrow, and the central frequency must be stable. 
         [0009]    Also, in order to perform coherent optical communication in a stable manner, the frequency of the received optical signal and the frequency of the local oscillator signal must be the same, with precise phase control required between the two signals. 
         [0010]    However, due the surrounding environment, physical properties, etc., it is virtually impossible to prepare two optical sources (e.g. laser diodes) that emit completely identical frequencies. Therefore, a frequency offset occurs between the local oscillator signal and the received optical signal. 
         [0011]    Also, the phase may randomly change due to the properties of the laser diode having a linewidth, resulting in the occurrence of phase noise which distorts signals. 
         [0012]    Therefore, the receiver device in a coherent optical communication system may require an additional technique for precisely controlling the frequencies and phases of two different optical sources or for compensating the distortions. 
         [0013]    To address the problems above, the self-homodyne coherent optical communication system was proposed. The self-homodyne coherent optical communication system has the transmitter device divide the optical carrier and provide the optical source that will be used for the local oscillator in the coherent receiver. 
         [0014]    However, the existing self-homodyne coherent optical communication system requires additional optical fibers for transmitting the optical source from the transmitter device for use as the local oscillator signal, so that the efficiency is lowered and the cost is increased due to the installing of the optical fibers. 
       SUMMARY OF THE INVENTION 
       [0015]    One aspect of the present invention is to provide a receiver device for a coherent optical communication system that can suppress the occurrence of frequency offsets and minimize phase noise. 
         [0016]    Another aspect of the present invention is to provide a receiver device for a coherent optical communication system that can be manufactured with a low cost without requiring either an additional optical source or additional optical fibers. 
         [0017]    An aspect of the present invention provides a receiver device for a coherent optical communication system that includes: an optical splitter configured to split a received optical signal into at least two paths; a first amplifier configured to amplify a second optical signal from among a first optical signal and the second optical signal split into two paths; a second amplifier configured to amplify the output signal of the first amplifier; and a coherent receiver module configured to mix the output signal of the second amplifier and the first optical signal and perform detection on the mixed signal. 
         [0018]    The first amplifier may amplify the second optical signal such that the minimum value of the output signal is within the saturation region of the second amplifier. 
         [0019]    The second amplifier may output a signal similar to a uniform signal. 
         [0020]    The first amplifier and the second amplifier may include a semiconductor optical amplifier (SOA). 
         [0021]    Another aspect of the present invention provides a receiver device for a coherent optical communication system that includes: an optical splitter configured to split a received optical signal into at least two paths; an amplifier configured to amplify a second optical signal from among a first optical signal and the second optical signal split into two paths; and a coherent receiver module configured to mix an output signal of the amplifier and the first optical signal and perform detection on the mixed signal. 
         [0022]    Still another aspect of the present invention provides a receiving method in a coherent optical communication system that includes: (a) splitting a received optical signal into at least two paths; (b) amplifying a second optical signal from among a first optical signal and the second optical signal split into two paths; (c) amplifying an output signal resulting from said step (b); and (d) mixing an output signal resulting from said step (c) and the first optical signal and performing detection on the mixed signal. 
         [0023]    An embodiment of the invention can provide the advantages of suppressing the occurrence of frequency offsets and minimizing phase noise, as well as allowing manufacture with a low cost without requiring either an additional optical source or additional optical fibers. 
         [0024]    Additional aspects and advantages of the present invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  is a block diagram illustrating the structure of a receiver device for a coherent optical communication system according to the related art. 
           [0026]      FIG. 2  is a block diagram illustrating the structure of a receiver device for a coherent optical communication system according to an embodiment of the invention. 
           [0027]      FIG. 3  illustrates the waveform of an optical signal inputted into a first amplifier in a coherent optical communication system according to an embodiment of the invention. 
           [0028]      FIG. 4  illustrates the output waveform of the first amplifier in a coherent optical communication system according to an embodiment of the invention. 
           [0029]      FIG. 5  illustrates the output waveform of the second amplifier in a coherent optical communication system according to an embodiment of the invention. 
           [0030]      FIG. 6  is a flow diagram illustrating the operation of a receiver device for a coherent optical communication system according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    As the invention allows for various changes and numerous embodiments, particular embodiments will be illustrated in the drawings and described in detail in the written description. However, this is not intended to limit the present invention to particular modes of practice, and it is to be appreciated that all changes, equivalents, and substitutes that do not depart from the spirit and technical scope of the present invention are encompassed in the present invention. In describing the drawings, like reference numerals are used for like elements. 
         [0032]    Certain embodiments of the invention will be described below in more detail with reference to the accompanying drawings. 
         [0033]      FIG. 1  is a block diagram illustrating the structure of a receiver device for a coherent optical communication system according to the related art. 
         [0034]    Referring to  FIG. 1 , a receiver device in a coherent optical communication system based on the related art may include an optical receiver unit  100 , a local oscillator  110 , an optical mixer  120 , and an optical detector  130 . 
         [0035]    The optical receiver unit  100  may receive optical signals transmitted from outside. An optical signal can be received through an optical transmission line such as an optical cable. 
         [0036]    The local oscillator  110  may serve to generate a separate optical signal that has the same frequency as that of the received signal. The local oscillator signal generated at the local oscillator  110  is a signal that does not include a data component and only has the same frequency as that of the received signal. 
         [0037]    The received optical signal and the local oscillator signal generated at the local oscillator  110  may be mixed through the optical mixer  120 . The optical mixer  120  may perform the mixing such that the received optical signal and the local oscillator signal of the local oscillator  110  are summed together. 
         [0038]    When the optical mixing is performed at the optical mixer, the optical detector  130  may perform detection on the mixed optical signal. A typical photodetector can be used for the optical detector. The optical detector  130  may convert the optical signal into an electrical signal, and the coherent optical system&#39;s receiver device can perform additional signal processing on the converted electrical signal. 
         [0039]    Although it is not shown in  FIG. 1 , a polarization splitter for splitting the received optical signal and the local oscillator signal according to polarization components can be used additionally. 
         [0040]    As illustrated in  FIG. 1 , the receiver device of a conventional coherent optical communication system requires a separate local oscillator  110 . Although the frequency of the local oscillator signal of the local oscillator  110  and the frequency of the received optical signal must be identical, it is difficult in actual practice to generate a local oscillator signal that has the same frequency as the received signal. As a result, a frequency offset may occur between the received optical signal and the local oscillator signal, causing distortions in the signal. 
         [0041]    Moreover, as the receiver device of the conventional coherent optical communication system requires a separate local oscillator, there is also the problem of increased manufacturing costs. 
         [0042]      FIG. 2  is a block diagram illustrating the structure of a receiver device for a coherent optical communication system according to an embodiment of the invention. 
         [0043]    Referring to  FIG. 2 , a receiver device for a coherent optical communication system according to an embodiment of the invention may include an optical splitter  200 , a first amplifier  210 , a second amplifier  220 , and a coherent receiver module  230 . 
         [0044]    The optical splitter  200  may be joined with an optical cable for transmitting a coherent optical signal that is transmitted from the transmission end, and may serve to split a received signal into two paths: a first path  202  and a second path  204 . The intensities of the first optical signal split to the first path  202  and the second optical signal split to the second path  204  can be controlled by the optical splitter  200 . 
         [0045]    The first optical signal split to the first path  202  may be inputted to the coherent optical receiver module  230  without separate processing. 
         [0046]    The second optical signal split to the second path  204  may be inputted to the first amplifier  210 . The first amplifier  210  may amplify the second optical signal such that the smallest signal of the second optical signal is greater than or equal to a preconfigured signal level. According to an embodiment of the invention, the first amplifier  210  can include a semiconductor optical amplifier (SOA). 
         [0047]      FIG. 3  illustrates the waveform of an optical signal inputted into the first amplifier in a coherent optical communication system according to an embodiment of the invention. 
         [0048]    Referring to  FIG. 3 , the second optical signal inputted to the first amplifier  210  may have multiple signal levels. The intensity of the second optical signal can be adjusted by the optical splitter  200 . The amplification factor of the first amplifier may be determined based on the saturation region of the second amplifier. To be more specific, the amplification factor of the first amplifier may be configured such that the output signal level of the first amplifier is within the saturation region of the second amplifier. 
         [0049]    An amplifier performs amplification linearly in a particular input signal level region, but when the input signal level exceeds a particular threshold, the amplification is no longer linear. Such signal level region in which the amplification is not performed linearly is referred to as the saturation region, and the first amplifier  210  may amplify the input signal such that the output signal level of the first amplifier  210  belongs to the saturation region of the second amplifier  220 . 
         [0050]      FIG. 4  illustrates the output waveform of the first amplifier in a coherent optical communication system according to an embodiment of the invention. 
         [0051]      FIG. 4  shows signal level plotted against time, and due to the limit on the amplifier&#39;s amplification factor, the output signal does not exceed a predetermined threshold. 
         [0052]    As described above, the first amplifier  210  may perform the amplification such that the minimum value of the output signal belongs to the saturation region of the second amplifier. To configure the amplification factor of the first amplifier  210 , it may be necessary to predict the minimum value of the optical signal. 
         [0053]      FIG. 5  illustrates the output waveform of the second amplifier in a coherent optical communication system according to an embodiment of the invention. 
         [0054]    Referring to  FIG. 5 , the output of the first amplifier  210  according to an embodiment of the invention, such as that shown in  FIG. 4 , may be inputted to the input end of the second amplifier  220 . In  FIG. 5 , it can be seen that the output signal of the first amplifier  210  is within the saturation region of the second amplifier. 
         [0055]    Since the amplification is not performed linearly if the levels of the input signal entering the second amplifier  220  all belong to the saturation region, the levels of the output signals of the second amplifier  220  may be given values similar to one another. Consequently, the output signal of the second amplifier  220  may be a signal similar to a uniform signal that does not carry data, and such output signal of the second amplifier  220  may have the same form as the local oscillator signal of a local oscillator. As the output signal of the second amplifier  220  is a signal converted from the received optical signal, it has an identical frequency as that of the received optical signal, making it possible to resolve the problem of frequency offsets occurring that occurs in the conventional coherent optical receiver device. 
         [0056]    Also, since there is no separate optical fiber needed for connecting to the transmission end, there is the advantage of lower implementation cost compared to the existing self-homodyne coherent system. 
         [0057]    The coherent receiver module  230  may serve to mix the first optical signal split at the optical splitter  200  and the signal outputted from the second amplifier  220  and perform detection on the mixed signal. The operation of the coherent receiver module  230  may be the same as the operation of a module used in a receiver device of a typical coherent optical communication system. The coherent receiver module  230  may perform a mixing operation on the output signal of the second amplifier  220  and the first optical signal, by adopting the output signal of the second amplifier  220  as a local oscillator signal, and may perform an optical detection operation of converting the mixed signal into an electrical signal. 
         [0058]    Although  FIG. 2  illustrates a structure in which two amplifiers  210 ,  220  are used for the amplification, the skilled person would readily understand that a signal similar to a uniform signal can be outputted with just one amplifier if the intensity of the second optical signal is sufficiently great. 
         [0059]    In cases where only one amplifier is used, the level of the inputted second optical signal should be greater than or equal to a preconfigured threshold, and preferably, the level of the second optical signal should be within the saturation region of the amplifier used. The output signal of this amplifier may have a form similar to a uniform signal, as is the case for the output signal of the second amplifier in  FIG. 2 . 
         [0060]      FIG. 6  is a flow diagram illustrating the operation of a receiver device for a coherent optical communication system according to an embodiment of the invention. 
         [0061]    Referring to  FIG. 6 , a received optical signal may be split into two paths (step  600 ). The splitting of the received optical signal can be performed by an optical splitter, and the first optical signal and second optical signal thus split may be processed independently. 
         [0062]    From among the split signals, a first amplification may be performed on the second optical signal by using a first amplifier (step  602 ). The first amplifier may perform the amplification such that the minimum value of the output power is located within the saturation region of the second amplifier. 
         [0063]    When the first amplification is performed, the first amplified signal may be inputted into the second amplifier to perform a second amplification (step  604 ). Since the minimum value of the first amplified signal obtained by the first amplification is located within the saturation region of the second amplifier, the amplification may be performed in the saturation region. 
         [0064]    Linear amplification does not occur in the saturation region, and therefore the second amplified signal may be given the form of a uniform signal in which all signal levels are similar. 
         [0065]    A coherent optical mixing may be performed for the optical signal split by the splitter and the second amplified signal that has undergone the second amplification (step  606 ). The second amplified signal may be used as the local oscillator signal of a conventional coherent optical communication system. Since the first optical signal and the second amplified signal are signals that have been split from the same signal, they have identical frequencies, making it possible to suppress the occurrence of frequency offsets. 
         [0066]    When the optical mixing is performed, detection may be performed on the mixed optical signal (step  608 ). The optical signal detection can be performed by using a photodetector, etc. 
         [0067]    While the present invention has been described above using particular examples, including specific elements, by way of limited embodiments and drawings, it is to be appreciated that these are provided merely to aid the overall understanding of the present invention, the present invention is not to be limited to the embodiments above, and various modifications and alterations can be made from the disclosures above by a person having ordinary skill in the technical field to which the present invention pertains. Therefore, the spirit of the present invention must not be limited to the embodiments described herein, and the scope of the present invention must be regarded as encompassing not only the claims set forth below, but also their equivalents and variations.