Abstract:
An optical transmitter includes: a light source configured to generate CW light; a drive signal generator configured to generate a drive signal; an optical modulator configured to modulate the CW light with the drive signal so as to generate a first optical signal; a combiner configured to combine the first optical signal and a second optical signal generated by using another light source; and a detector configured to detect a frequency difference between a frequency of the CW light and a center frequency of the second optical signal. The drive signal generator includes: a mapper configured to generate an electric field information signal based on input data; and a frequency controller configured to modify the electric field information signal based on the frequency difference such that the frequency of the CW light matches the center frequency of the second optical signal to generate the drive signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2016-145791, filed on Jul. 25, 2016, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments discussed herein are related to an optical transmitter, an optical receiver and an optical transmission method. 
       BACKGROUND 
       [0003]    In recent years, there has been a need for an optical network system that is able to accommodate various applications of different communication requirements. Thus, a communication technology has been discussed that multiplexes optical signals transmitted and received using different communication schemes. For example, with respect to an application that transmits large volumes of data, a higher-order modulation scheme is used. In this case, a generation and a demodulation of a modulated optical signal are performed using a digital signal processing technology. Note that the “higher-order” indicates that the number of bits per symbol is larger, for example, two or more. On the other hand, with respect to an application that requires a low-latency data transmission (or a data transmission of a small transmission delay), a transmission technology is used that minimizes the number of “optical-electric-optical” conversions in a communication route (such as intensity modulation/direct detection (IM/DD)). 
         [0004]    As a related technology, an optical communication system is proposed that is able to add a transmission system of a differently-modulated signal to a single optical fiber network (for example, Japanese Laid-open Patent Publication No. 2011-244436). Further, an optical multiplexer is proposed that multiplexes pieces of information densely and transmits densely-multiplexed information (for example, Japanese Laid-open Patent Publication No. 2013-51541). Furthermore,
   Documents 1 and 2 below respectively disclose an optical network that performs a communication in a data center and a communication between data centers.   Document 1: Payman Samadi et al., Virtual Machine Migration over Optical Circuit Switching Network in a Converged Inter/Intra Data Center Architecture, OSA Optical Fiber Communication Conference 2015, Th4G.6   Document 2: Payman Samadi et al., Experimental Demonstration of Converged Inter/Intra Data Center Network Architecture, In 17th International Conference on Transparent Optical Networks, IEEE, We.B.3.3   
 
         [0008]    As described above, when data volume is large, an optical signal is generated by a higher-order modulation scheme in order to transmit data efficiently. In general, a transmittable distance is short when a data transmission is performed by a higher-order modulation scheme, so a large number of relay devices are required between a transmitter and a receiver. Further, in many cases, the data transmission performed by a higher-order modulation scheme is realized using digital signal processing whose processing delay is large. Thus, it is difficult to lower a latency of a data transmission performed by a higher-order modulation scheme. Specifically, a latency becomes larger in the configuration in which an intensity modulated optical signal is converted into an electric signal and a modulated optical signal is generated from the electric signal in a higher-order modulation scheme using digital signal processing. 
         [0009]    For example, an intensity modulation/direct detection (IM/DD) is used in order to realize a low-latency data transmission. In this case, a latency is low because an optical signal generated by intensity modulation is transmitted up to a destination without any conversion. However, the number of bits transmitted in one symbol is small, which results in reducing the communication resource utilization efficiency. 
       SUMMARY 
       [0010]    According to an aspect of the present invention, an optical transmitter includes: a light source configured to generate continuous wave light; a drive signal generator configured to generate a drive signal based on input data; an optical modulator configured to modulate the continuous wave light with the drive signal so as to generate a first optical signal; a combiner configured to combine the first optical signal and a second optical signal generated by using another light source that is different from the light source; and a frequency difference detector configured to detect a frequency difference between a frequency of the continuous wave light and a center frequency of the second optical signal. The drive signal generator includes: a mapper configured to generate an electric field information signal based on the input data; and a frequency controller configured to modify the electric field information signal based on the frequency difference such that the frequency of the continuous wave light matches the center frequency of the second optical signal so as to generate the drive signal. 
         [0011]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0012]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]      FIG. 1  illustrates an example of an optical network system; 
           [0014]      FIG. 2  illustrates an example of a communication using an express path; 
           [0015]      FIGS. 3A and 3B  illustrate examples of an optical transmission circuit; 
           [0016]      FIGS. 4A and 4B  illustrate examples of an optical reception circuit; 
           [0017]      FIG. 5  illustrates an example of a communication using a general communication path; 
           [0018]      FIGS. 6A-6E  schematically illustrate optical spectra of a communication between data centers; 
           [0019]      FIG. 7  illustrates an example of a frequency controller; 
           [0020]      FIG. 8  illustrates an example of a mapper; 
           [0021]      FIG. 9  illustrates an example of a separator; 
           [0022]      FIG. 10  illustrates an example of the communication between data centers; and 
           [0023]      FIGS. 11 and 12  are a flowchart that illustrates an example of the communication between data centers. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0024]      FIG. 1  illustrates an example of an optical network system according to embodiments of the present invention. In the example illustrated in  FIG. 1 , data centers  100  ( 100 A,  100 B) arranged in different locations are connected through an optical fiber link  200 . 
         [0025]    Each of the data centers accommodates a plurality of servers  110 . In the example illustrated in  FIG. 1 , the data center  100 A accommodates servers  110   a  to  110   c.  The data center  100 B accommodates servers  110   d  to  110   f.  Each of the servers  110   a  to  110   f  is configured to include a plurality of server elements. 
         [0026]    A user can access a desired server  110 . In this case, data may be transmitted between servers in response to this access. For example, a data communication may be performed between servers that are accommodated in one data center. Alternatively, a data communication may be performed between data centers. For example, it is assumed that the server  110   d  stores backup data for the server  110   a.  In this case, when data stored in the server  110   a  is updated, a communication is performed between the servers  110   a  and  110   d,  and the data in the server  110   d  is also updated. Alternatively, the user can move data stored in a certain server to another server. 
         [0027]    As described above, an optical network system transmits data between servers in response to a request from a user. However, desired communication requirements differ by application. For example, a communication that is able to transmit large volumes of data efficiently may be requested. Further, a low-latency communication may also be requested. Thus, an optical network system is desired to satisfy various communication requirements. 
         [0028]    Each server  110  is accommodated in a server rack  140 . The server rack  140  accommodates a top-of-rack electric switch  120  and an optical transceiver  130  in addition to the server  110 . The top-of-rack electric switch  120  has a function that aggregates signals transmitted from a plurality of server elements and a function that distributes received signals to the plurality of server elements. The optical transceiver  130  generates an optical signal from an output signal of the top-of-rack electric switch  120  and guides the optical signal to a signal processor  150 . In addition, the optical transceiver  130  converts the optical signal received from the signal processor  150  into an electric signal and guides the electric signal to the top-of-rack electric switch  120 . 
         [0029]    The signal processor  150  includes a switch circuit  151  and a transceiver circuit  152 . The switch circuit  151  provides an optical path according to the communication requested by a user. For example, when a request to move data stored in the server  110   a  to the server  110   b  is provided, the switch circuit  151  guides an optical signal transmitted from a server rack that accommodates the server  110   a  to a server rack that accommodates the server  110   b.  When a request to move data stored in the server  110   a  to the server  110   d  is provided, the switch circuit  151  guides an optical signal transmitted from the server rack that accommodates the server  110   a  to the data center  100 B. 
         [0030]    The transceiver circuit  152  processes a modulated optical signal that is transmitted between the data centers  100 A and  100 B. The transceiver circuit  152  includes an optical transmitter that transmits a modulated optical signal and an optical receiver that receives a modulated optical signal. In this example, the modulated optical signal is generated by a higher-order modulation scheme (that is, a modulation scheme that transmits two or more bits of data for each symbol). 
         [0031]    As an example, when a request to move data stored in the server  110   a  to the server  110   d  is provided, the transceiver circuit  152  of the data center  100 A converts an optical signal transmitted from the server rack that accommodates the server  110   a  into an electric signal. The transceiver circuit  152  generates a modulated optical signal from this electric signal by digital signal processing and transmits the generated modulated optical signal to the data center  100 B. Then, in the data center  100 B, the transceiver circuit  152  demodulates the received modulated optical signal so as to recover data. Further, the transceiver circuit  152  generates an optical signal that transmits the recovered data and guides the optical signal to the server rack that accommodates the server  110   d.    
         [0032]    In the optical network system illustrated in  FIG. 1 , communication resources of an optical fiber link  200  are desired to be used efficiently. Thus, a communication between the data centers  100 A and  100 B is realized using the transceiver circuit  152 . In other words, an optical signal generated by a higher-order modulation scheme is transmitted between the data centers  100 A and  100 B. 
         [0033]    However, a communication that uses the transceiver circuit  152  includes converting an optical signal output from the optical transceiver  130  into an electric signal, generating a drive signal from the electric signal by digital signal processing, demodulating a modulated optical signal by coherent detection, and converting data obtained by the demodulation into an optical signal to transmit the optical signal to the optical transceiver  130 . Thus, it is difficult to perform a low-latency communication. Therefore, when a request to perform a low-latency communication is made between the data centers  100 A and  100 B, the optical network system transmits an optical signal between the optical transceivers  130  and  130  through the switch circuit  151  without using the transceiver circuit  152 . 
         [0034]    In the following descriptions, an optical path that transmits a modulated optical signal between the data centers  100 A and  100 B using the transceiver circuit  152  may be referred to as a “general communication path”. Further, an optical path that transmits an optical signal between the data centers  100 A and  100 B without using the transceiver circuit  152  may be referred to as an “express path”. 
         [0035]      FIG. 2  illustrates an example of a communication using an express path. In the example illustrated in  FIG. 2 , it is assumed that data is transmitted from the data center  100 A arranged at a location A to the data center  100 B arranged at a location B. The express path is realized by, for example, intensity modulation (IM)/direct detection (DD). 
         [0036]    Each optical transceiver  130  includes an optical transmission circuit  131  and an optical reception circuit  132 . As illustrated in  FIG. 1 , the optical transceiver  130  is provided in each server rack  140 . In  FIG. 2 , the optical reception circuit  132  implemented in the optical transceiver  130  of the data center  100 A is omitted, and the optical transmission circuit  131  implemented in the optical transceiver  130  of the data center  100 B is omitted. 
         [0037]    In the data center  100 A, data Y is provided to the optical transmission circuit  131 . The data Y is output from the server  110  accommodated in the server rack  140 . The optical transmission circuit  131  includes a light source (LD)  131   a.    
         [0038]    Then, the optical transmission circuit  131  generates an optical signal that transmits the data Y using the light source  131   a.  In this example, the optical transmission circuit  131  generates an intensity modulated optical signal (optical signal Y) from the data Y by intensity modulation (or on/off keying). In this case, for example, the light source  131   a  is driven by direct modulation. Here, the light source  131   a  may be a frequency tunable laser light source. In the following descriptions, the intensity modulated optical signal may be referred to as an “IM optical signal”. 
         [0039]    An IM optical signal Y generated by the optical transmission circuit  131  is transmitted to the data center  100 B by the switch circuit  151 . In the data center  100 B, the switch circuit  151  guides the received IM optical signal Y to the optical reception circuit  132 . The optical reception circuit  132  includes a photo detector (PD)  132   a.  The optical reception circuit  132  converts the IM optical signal Y into an electric signal using the photo detector  132   a.  Here, the power or the amplitude of an output signal of the photo detector  132   a  represents a value of each bit of the data Y. Thus, the data Y is recovered by converting the IM optical signal Y into an electric signal. 
         [0040]      FIGS. 3A and 3B  illustrate examples of an optical transmission circuit that generates an IM optical signal. As illustrated in  FIG. 3A , the optical transmission circuit  131  includes the light source  131   a.  The light source  131   a  is, for example, a frequency tunable laser light source. In this configuration, an IM optical signal is generated by direct modulation. In other words, the light source  131   a  is driven by input data. An IM optical signal output from the light source  131   a  may be guided to a specified optical port by performing spatial transmission using an optical antenna  131   c.  The optical antenna  131   c  is, for example, implemented by an optical system that includes a mirror and a lens. 
         [0041]    As illustrated in  FIG. 3B , the optical transmission circuit  131  may be configured to include the light source  131   a  and an optical modulator  131   b.  In this case, the light source  131   a  generates continuous wave light. The optical modulator  131   b  modulates the continuous wave light with input data so as to generate an IM optical signal. The optical modulator  131   b  generates an IM optical signal from input data by, for example, intensity modulation (on/off keying). 
         [0042]    As described above, the optical transmission circuit  131  generates an IM optical signal according to input data. However, the embodiments of the present invention are not limited to this configuration. For example, when the optical transmission circuit  131  is realized by the configuration of  FIG. 3B , the optical modulator  131   b  may generate a modulated optical signal by phase shift keying (mPSK) or quadrature amplitude modulation (mQAM). 
         [0043]      FIGS. 4A and 4B  illustrate examples of the optical reception circuit  132 . The optical reception circuit  132  receives an optical signal transmitted from the optical transmission circuit  131  of  FIG. 3A or 3B . As illustrated in  FIG. 4A , the optical reception circuit  132  includes the photo detector (PD)  132   a.  When the received optical signal is an intensity modulated signal, data is recovered by converting the received optical signal into an electric signal using a photo detector. An optical signal that arrives at an input optical port may be guided to the photo detector  132   a  using an optical antenna  132   b.    
         [0044]    As illustrated in  FIG. 4B , the optical reception circuit  132  may be configured to include a coherent receiver  132   c  and a digital signal processor (DSP)  132   d.  It is preferable that the optical reception circuit  132  be configured to recover data Y by performing coherent detection when a modulated optical signal is generated by phase shift keying or quadrature amplitude modulation. 
         [0045]      FIG. 5  illustrates an example of a communication using a general communication path. The general communication path is established by the transceiver circuit  152  illustrated in  FIG. 1 . The transceiver circuit  152  includes an edge transmitter  10  and an edge receiver  60 . In  FIG. 5 , the edge receiver  60  of the data center  100 A is omitted, and the edge transmitter  10  of the data center  100 B is omitted. 
         [0046]    The edge transmitter  10  includes an O/E circuit  11 , an optical switch (SW)  12 , a mapper (MAP)  13 , a distortion corrector (DIST)  14 , a frequency controller (FREQ CONT)  15 , a D/A converter (DAC)  16 , a light source (LD)  17 , and an IQ modulator (IQM)  18 . The mapper  13 , the distortion corrector  14 , and the frequency controller  15  are implemented by, for example, a digital signal processor. In this case, the mapper  13 , the distortion corrector  14 , and the frequency controller  15  may be implemented by one processor or by two or more processors. Alternately, the mapper  13 , the distortion corrector  14 , and the frequency controller  15  mat be implemented by a digital signal processing circuit. 
         [0047]    The O/E circuit  11  converts an optical signal transmitted from the server rack  140  in the data center  100 A into an electric signal. When an intensity modulated optical signal is transmitted from the server rack  140 , the O/E circuit  11  is implemented by a photo detector. The optical switch  12  guides the electric signal output from the O/E circuit  11  to the mapper  13 . The optical switch  12  may have an aggregation function. In other words, the optical switch  12  may aggregate a plurality of electric signals and guide the aggregated signal to the mapper  13 . In the following descriptions, data provided to the mapper  13  may be referred to as “data X”. 
         [0048]    The mapper  13  generates an electric field information signal from the data X according to a specified modulation scheme. The electric field information signal indicates a phase and an amplitude of a modulated optical signal output from the IQ modulator  18 . Thus, this electric field information signal is configured by an I-component signal and a Q-component signal. The distortion corrector  14  corrects the electric field information signal generated by the mapper  13  such that a chromatic dispersion of the optical fiber link  200  is compensated for. In other words, the distortion corrector  14  performs pre-equalization or pre-compensation in order to compensate for a chromatic dispersion of the optical fiber link  200 . 
         [0049]    The frequency controller  15  is started when a general communication path and an express path are multiplexed in a wavelength channel. The frequency controller  15  will be described later. 
         [0050]    The D/A converter  16  converts the electric field information signal into an analog signal so as to generate a drive signal. In other words, a drive signal is generated by the mapper  13 , the distortion corrector  14 , the frequency controller  15 , and the converter  16 . That is, the mapper  13 , the distortion corrector  14 , the frequency controller  15 , and the converter  16  operate as a drive signal generator that generates a drive signal. Alight source  17  is implemented by a frequency tunable laser light source in this example. In other words, the light source  17  outputs continuous wave light of a specified frequency. The IQ modulator  18  modulates the continuous wave light output from the light source  17  with a drive signal, so as to generate a modulated optical signal. The modulated optical signal generated by the IQ modulator  18  is transmitted to the data center  100 B through the optical fiber link  200 . 
         [0051]    Note that the mapper  13  generates an electric field information signal from data X according to a modulation scheme that transmits two or more bits of data for each symbol. In other words, a modulated optical signal output from the IQ modulator  18  transmits two or more bits of data for each symbol. Therefore, in the following descriptions, the modulated optical signal output from the IQ modulator  18  may be referred to as a “multi-level optical signal”. 
         [0052]      FIG. 6A  illustrates a spectrum of a multi-level optical signal X generated by the edge transmitter  10 . An optical frequency fx corresponds to a frequency of continuous wave light output from the light source  17 . In other words, the optical frequency fx represents a center frequency of the multi-level optical signal X. The width of the spectrum (that is, the bandwidth of a modulated optical signal) depends on a baud rate and a modulation scheme of the multi-level optical signal X. 
         [0053]    It is assumed that a request to perform a low-latency data transmission is made when the general communication path described above is established. In other words, it is assumed that an IM optical signal Y is transmitted from the optical transmission circuit  131  of  FIG. 2  when the multi-level optical signal X generated by the edge transmitter  10  is being transmitted. 
         [0054]    In this case, in order to save the communication resources of the optical fiber link  200 , a general communication path that transmits a multi-level optical signal X and an express path that transmits an IM optical signal Y are multiplexed in one wavelength channel. In the configuration illustrated in  FIG. 5 , the multi-level optical signal X and the IM optical signal Y are combined by an optical coupler  19  and guided to the optical fiber link  200 . 
         [0055]      FIG. 6B  illustrates a spectrum of an IM optical signal Y generated by the optical transmission circuit  131 . An optical frequency fy corresponds to a center frequency of an output optical signal of the light source  131   a  illustrated in  FIG. 2, 3A , or  3 B. In other words, the optical frequency fy represents a center frequency of the IM optical signal Y. The width of the spectrum (that is, the bandwidth of a modulated optical signal) depends on, for example, a baud rate of the IM optical signal Y. 
         [0056]    Here, in order to multiplex a general communication path and an express path in one wavelength channel, the center optical frequency fx of the multi-level optical signal X and the center optical frequency fy of the IM optical signal Y need to match each other. Thus, for example, the optical frequency of the output light of the light source  17  is controlled such that fx and fy match each other. However, an oscillating frequency of a laser has an error. A maximum frequency error of a general laser light source is about 1.5 GHz. Thus, fx and fy rarely completely match each other. For example, in the examples illustrated in  FIGS. 6A and 6B , the center optical frequency fy of the IM optical signal Y is slightly higher than the center optical frequency fx of the multi-level optical signal X. 
         [0057]    Thus, the edge transmitter  10  has a function to match the optical frequency fx and the optical frequency fy. In other words, the edge transmitter  10  includes an optical splitter (SPL)  21 , an optical coupler (CPL)  22 , and a frequency difference detector (ΔF)  23  in addition to the circuit components described above. 
         [0058]    The optical splitter  21  splits an IM optical signal Y transmitted from the optical transmission circuit  131  of the server rack  140  and guides the IM optical signal Y to the optical coupler  22 . The optical coupler  22  combines continuous wave light output from the light source  17  and the IM optical signal Y guided from the optical splitter  21 . The frequency difference detector  23  includes a photo detector that converts output light of the optical coupler  22  into an electric signal. Then, according to the electric signal output from this photo detector, the frequency difference detector  23  detects a frequency of a beat component of an output light of the optical coupler  22 . The frequency of the beat component of the output light of the optical coupler  22  indicates a difference between the center optical frequency fx of the multi-level optical signal X and the center optical frequency fy of the IM optical signal Y. Note that the frequency difference detector  23  may detect the frequency of a beat component by digital signal processing. In this case, the frequency difference detector  23  may include an FFT circuit. 
         [0059]    The frequency controller  15  modifies an electric field information signal according to a frequency difference detected by the frequency difference detector  23 .  FIG. 7  illustrates an example of the frequency controller  15 . In the example of  FIG. 7 , the frequency controller  15  is implemented by a digital signal processor. 
         [0060]    The frequency controller  15  includes an integrator circuit  31  and a rotation operation circuit  32 . A function f(t) and an electric field information signal are provided to the frequency controller  15 . The function f(t) is an optical frequency difference Δf detected by the frequency difference detector  23 . The electric field information signal is generated by the mapper  13  and configured by an I-component signal and a Q-component signal. 
         [0061]    The integrator circuit  31  integrates an optical frequency difference Δf over time. Then, the integrator circuit  31  outputs the following phase information θ(t) as an integration result. 
         [0000]      θ( t )=∫2 πf ( t ) dt  
 
         [0062]    The integrator circuit  31  may include a mod2π circuit. In this case, an output value of the integrator circuit  31  is converted into a value included between 0 and 2π. 
         [0063]    The rotation operation circuit  32  modifies the I-component signal and the Q-component signal using the following calculations by use of the phase information θ(t). In other words, the rotation operation circuit  32  controls a phase indicated by the I-component signal and the Q-component signal according to the optical frequency difference Δf detected by the frequency difference detector  23 . I and Q respectively represent input signals of the rotation operation circuit  32 . I′ and Q′ respectively represent output signals of the rotation operation circuit  32 . 
         [0000]        I′=I· cos θ( t )− Q· sin θ( t )
 
         [0000]        Q′=I· sin θ( t )+ Q ·cos θ( t )
 
         [0064]    An output signal of the frequency controller  15  is converted into an analog signal by the D/A converter  16  and provided to the IQ modulator  18 . Then, the IQ modulator  18  modulates the continuous wave light output from the light source  17  with the output signal of the operation circuit  32 , so as to generate the multi-level optical signal X. 
         [0065]    As described above, the electric field information signal is modified according to the optical frequency difference Δf. Thus, the center frequency of the multi-level optical signal X generated by the IQ modulator  18  is fx+Δf. fx represents a frequency of the continuous wave light output from the light source  17 . Here, the optical frequency difference  4   f  corresponds to a difference between the frequency fx of the continuous wave light output from the light source  17  and the center optical frequency fy of the IM optical signal Y. Thus, when the electric field information signal is modified by the frequency controller  15 , the center frequency of the multi-level optical signal X generated by the IQ modulator  18  matches the center optical frequency fy of the IM optical signal Y, as illustrated in  FIG. 6C . 
         [0066]    However, the signal band of the IM optical signal Y illustrated in  FIG. 6B  and the signal band of the multi-level optical signal X illustrated in  FIG. 6C  overlap in the frequency domain. Thus, if the IM optical signal Y and the multi-level optical signal X are multiplexed in frequency domain, it will be difficult for an optical receiver to separate these optical signals. Thus, when an express path is established in addition to a general communication path, the edge transmitter  10  generates an electric field information signal of a modulated optical signal transmitted through the general communication path such that the signal band of the express path and the signal band of the general communication path do not overlap. 
         [0067]      FIG. 8  illustrates an example of the mapper  13 . In this example, the mapper  13  includes a switch  41 , a mapper  42 , a distributor  43 , mappers  44 - 1  and  44 - 2 , frequency controllers  45 - 1  and  45 - 2 , an adder  46 , and a selector  47 . An express path instruction that indicates whether to establish an express path is provided to the mapper  13 . 
         [0068]    When an instruction to establish an express path is not provided, the switch  41  guides input data to the mapper  42 . On the other hand, when the instruction to establish an express path is provided, the switch  41  guides the input data to the distributor  43 . The distributor  43  distributes the input data to the mappers  44 - 1  and  44 - 2 . Here, for example, the distributor  43  alternately distributes the input data to the mappers  44 - 1  and  44 - 2  for each N bits. N corresponds to a modulation scheme. 
         [0069]    The mapper  42  generates an electric field information signal from the input data according to a specified modulation scheme. Each of the mappers  44 - 1  and  44 - 2  generates an electric filed information signal from the input data by a modulation scheme that is to be determined according to a modulation scheme of the mapper  42 . When the mapper  42  performs mapping according to a modulation scheme of 2 k bits/symbol, each of the mappers  44 - 1  and  44 - 2  performs mapping according to a modulation scheme of k bits/symbol. For example, when k is 2 and the mapper  42  performs mapping by 16 QAM, each of the mappers  44 - 1  and  44 - 2  performs mapping by QPSK. In this case, the distributor  43  alternately distributes the input data to the mappers  44 - 1  and  44 - 2  for each 2 bits. When k is 4 and the mapper  42  performs mapping by 256 QAM, each of the mappers  44 - 1  and  44 - 2  performs mapping by 16 QAM. In this case, the distributor  43  alternately distributes the input data to the mappers  44 - 1  and  44 - 2  for each 4 bits. 
         [0070]    The frequency controller  45 - 1  modifies an output signal of the mapper  44 - 1  such that a center frequency of a modulated optical signal generated according to the output signal of the mapper  44 - 1  is higher by Δf 1 . On the other hand, the frequency controller  45 - 2  modifies an output signal of the mapper  44 - 2  such that a center frequency of a modulated optical signal generated according to the output signal of the mapper  44 - 2  is lower by Δf 1 . The configurations and the operations of the frequency controllers  45 - 1  and  45 - 2  are substantially the same as the configuration and the operation of the frequency controller  15  illustrated in  FIG. 7 . However, +Δf 1  and −Δf 1  are respectively provided to the frequency controllers  45 - 1  and  45 - 2  as a function f(t). Δf 1  is determined such that a spectrum of the multi-level optical signal X and a spectrum of the IM optical signal Y do not overlap when the center optical frequency of the multi-level optical signal X and the center optical frequency of the IM optical signal Y match each other. Δf 1  is, for example, about 5 GHz. 
         [0071]    The adder  46  combines an output signal of the frequency controller  45 - 1  and an output signal of the frequency controller  45 - 2 . The selector  47  selects an output signal of the mapper  42  when an instruction to establish an express path is not provided. On the other hand, the selector  47  selects an output signal of the adder  46  when the instruction to establish an express path is provided. Then, the mapper  13  outputs the signal selected by the selector  47  as an electric field information signal. 
         [0072]      FIG. 6D  illustrates a spectrum of a multi-level optical signal X generated by the IQ modulator  18  when an express path is established. X 1  represents an optical signal generated according to the electric field information signal output from the mapper  44 - 1 , and X 2  represents an optical signal generated according to the electric field information signal output from the mapper  44 - 2 . The center frequency of X 1  is fy+Δf 1 , and the center frequency of X 2  is fy-Δf 1 . 
         [0073]      FIG. 6E  illustrates a spectrum of an output optical signal of the edge transmitter  10  when a general communication path and an express path are multiplexed. As illustrated in  FIG. 6E , even if the general communication path and the express path are multiplexed, the signal band of the express path and the signal band of the general communication path do not overlap. In other words, the amount of frequency shift Δf 1  is determined such that the signal band of the express path and the signal band of the general communication path do not overlap. Here, the signal bandwidth of a modulated optical signal is estimated according to a baud rate and a modulation scheme of the modulated optical signal. Thus, the mapper  13  can generate an electric field information signal from input data such that the signal band of the express path and the signal band of the general communication path do not overlap. 
         [0074]    As described above, when an express path is not established, the edge transmitter  10  of the data center  100 A transmits the multi-level optical signal X illustrated in  FIG. 6A  to the data center  100 B. On the other hand, when the express path is established, the edge transmitter  10  transmits a multi-level optical signal X (X 1 , X 2 ) and a IM optical signal Y to the data center  100 B, as illustrated in  FIG. 6E . 
         [0075]    An optical signal transmitted from the data center  100 A through the optical fiber link  200  is split by an optical splitter  51  in the data center  100 B, and the optical signal is guided to an optical switch  52  and an edge receiver  60 . The optical switch  52  guides the received optical signal to an optical reception circuit  132  of a server rack that accommodates a destination server. 
         [0076]    For example, the optical reception circuit  132  converts the received optical signal into an electric signal using the photo detector  132   a,  as illustrated in  FIG. 2 . Here, it is assumed that an express path is established in addition to a general communication path. That is to say, the optical reception circuit  132  receives the multi-level optical signal X (X 1 , X 2 ) generated by the IQ modulator  18  and the IM optical signal Y transmitted from the optical transmission circuit  131 , as illustrated in  FIG. 6E . In this case, the IM optical signal Y is substantially a baseband signal in the electric domain. Thus, when the IM optical signal Y is converted into an electric signal by the photo detector  132   a,  data Y is recovered. On the other hand, the center frequency of the multi-level optical signal (X 1 , X 2 ) is shifted by Δf 1  with respect to the baseband in the electric domain. Thus, if the bandwidth of the photo detector  132   a  is lower than Δf 1 , a signal component of the multi-level optical signal X will not be detected by the photo detector  132   a.  In other words, if the bandwidth of the photo detector  132   a  included in the optical reception circuit  132  is configured to be lower than Δf 1 , the optical reception circuit  132  can recover the data Y transmitted by the IM optical signal Y from the optical signal illustrated in  FIG. 6E . In order to remove the multi-level optical signal X with a high degree of accuracy, a low-pass filter may be provided on the output side of the photo detector  132   a.    
         [0077]    As illustrated in  FIG. 5 , the edge receiver  60  includes a coherent receiver (Rx)  61 , an A/D converter (ADC)  62 , an FFT processor  63 , a separator (SEPA)  64 , a demodulator (DEMOD)  65 , an electric switch (SW)  66 , and an E/O circuit  67 . The FFT processor  63 , the separator  64 , and the demodulator  65  are implemented by, for example, a digital signal processor. In this case, the FFT processor  63 , the separator  64 , and the demodulator  65  may be implemented by one processor or by two or more processors. Alternately, the FFT processor  63 , the separator  64 , and the demodulator  65  may be implemented by a digital signal processing circuit. 
         [0078]    The coherent receiver  61  generates an electric field information signal that represents a received optical signal using a local light source. When the coherent receiver  61  receives the multi-level optical signal X illustrated in  FIG. 6A , the frequency of the local light source is controlled at fx. Further, when the coherent receiver  61  receives the multi-level optical signal X and the IM optical signal Y illustrated in  FIG. 6E , the frequency of the local light source may be controlled at fy. The A/D converter  62  converts the electric field information signal generated by the coherent receiver  61  into a digital signal. The FFT processor  63  converts an output signal of the A/D converter  62  into a frequency-domain signal. 
         [0079]    As illustrated in  FIG. 9 , the separator  64  includes a frequency separator  64   a,  a decision unit  64   b,  an interference calculator  64   c,  an interference remover  64   d.  The separator  64  receives an input signal that includes a signal component transmitted by the multi-level optical signal X (data-X signal component) and a signal component transmitted by the IM optical signal Y (data-Y signal component) and extracts the data-X signal component from the input signal. 
         [0080]    The frequency separator  64   a  extracts the data-X signal component and the data-Y signal component from the frequency-domain signal output from the FFT processor  63 . The data-X signal component is guided to the interference remover  64   d,  and the data-Y signal component is guided to the decision unit  64   b.  The decision unit  64   b  decides a value for each symbol according to the data-Y signal component. Here, the data Y is transmitted by the optical transmission circuit  131  illustrated in  FIG. 3A or 3B  by intensity modulation (or on/off keying). Thus, the decision unit  64   b  detects “1” or “0” for each symbol. 
         [0081]    The interference calculator  64   c  calculates an interference component between the multi-level optical signal x and the IM optical signal Y. Specifically, the interference calculator  64   c  calculates a phase variation of the multi-level optical signal X due to the IM optical signal Y. Here, when the IM optical signal is in an OFF state, the phase variation of the multi-level optical signal X due to the IM optical signal Y is substantially zero. On the other hand, when the IM optical signal Y is in an ON state, the phase of the multi-level optical signal X varies due to the IM optical signal Y. The amount of the phase variation can be calculated according to the features and the length of the optical fiber link  200 . Thus, the interference calculator  64   c  calculates a phase variation of the multi-level optical signal X due to the IM optical signal Y according to the ON/OFF state of the IM optical signal Y. 
         [0082]    The interference remover  64   d  removes an interference component from the data-X signal component. The removal of an interference component is realized by, for example, subtraction. In other words, the separator  64  extracts an electric field information signal that represents a multi-level optical signal X and removes an interference component from the electric field information signal. 
         [0083]    The separator  64  does not always have to remove an interference component. In this case, the separator  64  does not have to include the decision unit  64   b,  the interference calculator  64   c,  and the interference remover  64   d.  The separator  64  extracts and outputs an electric field information signal that represents the multi-level optical signal X. In this case, the separator  64  operates as a frequency filter. 
         [0084]    Return to  FIG. 5 . The demodulator  65  demodulates each symbol according to the electric field information signal output from the separator  64 . Here, the demodulator  65  performs a demapping that corresponds to a mapping performed by the mapper  13  of the data center  100 A. As a result, data X is recovered. The switch  66  guides the data X to an E/O circuit  67  that corresponds to a destination server. In other words, the electric switch  66  operates as a router. The E/O circuit  67  converts the data X into an optical signal. The E/O circuit  67  is realized by, for example, the configuration illustrated in  FIG. 3A or 3B . Then, the optical signal that carries the data X is transmitted to a server rack that accommodates the destination server of the data X. 
         [0085]    As described above, a data communication between data centers is usually realized through a general communication path in order to save the communication resources of the optical fiber link  200 . The general communication path transmits a modulated optical signal generated by a higher-order modulation scheme. In addition, when a low-latency data communication is performed between data centers, a general communication path and an express path are multiplexed in one wavelength channel. The express path is realized by, for example, IM/DD. Here, electric field information on the general communication path is modified by digital signal processing such that the center wavelength of the general communication path and the center wavelength of the express path match each other, and the signal band of the general communication path and the signal band of the express path do not overlap. Thus, it is possible to realize a low-latency data communication as needed while saving the communication resources between data centers. 
         [0086]    Further, the edge receiver  60  can suppress a phase variation due to an adjacent optical signal when the edge receiver  60  extracts an electric field information signal of a target optical signal from a multiplexed optical signal. Thus, even if a general communication path and an express path are multiplexed in one wavelength channel, data transmitted through a general communication path can be recovered with a high degree of accuracy. 
       EXAMPLE 
       [0087]      FIG. 10  illustrates an example of a communication between data centers. The data center  100  illustrated in  FIG. 10  corresponds to the data center  100 A or  100 B illustrated in  FIG. 1 . Thus, the data center  100  includes a plurality of server racks  140 . The server  110 , the top-of-rack electric switch  120 , and the optical transceiver  130  are accommodated in each of the server racks  140 . In this example, a WDM optical signal is transmitted in a communication performed between data centers. 
         [0088]    The data center  100  includes an optical frequency difference detector (ΔF)  71 , an edge transceiver  72 , an optical switch  81 , an optical transceiver  82 , and an electric switch  83 . The optical frequency difference detector  71  and the edge transceiver  72  correspond to the transceiver circuit  152  illustrated in  FIG. 1 . The optical switch  81 , the optical transceiver  82 , and the electric switch  83  correspond to the transceiver circuit  151  illustrated in  FIG. 1 . 
         [0089]    The optical frequency difference detector  71  corresponds to the frequency difference detector  23  in the example of  FIG. 5 . In other words, the optical frequency difference detector  71  detects a difference between a center frequency of an optical signal transmitted through a general communication path and a center frequency of an optical signal transmitted through an express path. The edge transceiver  72  includes the edge transmitter  10  and the edge receiver  60  illustrated in  FIG. 5 . The optical frequency difference detector  71  may be implemented within the edge transceiver  72 . 
         [0090]    A channel allocation controller  91  selects an unused wavelength channel when a request to perform a communication between data centers is made by a user. Then, the channel allocation controller  91  controls the edge transceiver  72 , the optical switch  81 , the optical transceiver  82 , the electric switch  83 , and the optical transceiver  130  using a channel allocation instruction, such that the selected wavelength channel is established. 
         [0091]    Each optical transceiver  130  can transmit and receive an optical signal by IM/DD. A center frequency of an optical signal transmitted from the optical transceiver  130  is controlled by a channel allocation instruction. The communication within a data center is realized by the optical switch  81 . For example, a data transmission from the server  110   a  to the server  110   c  is realized by an optical transceiver  130   a,  the optical switch  81 , and an optical transceiver  130   c.    
         [0092]    Each optical transceiver  82  can also transmit and receive an optical signal by IM/DD. In other words, the optical transceiver  130  and the optical transceiver  82  can transmit and receive an optical signal through the optical switch  81  by IM/DD. 
         [0093]    The communication between data centers is realized by a general communication path or an express path depending on an application. For example, when there is no need for low latency, a general communication path is established in order to save the communication resources between data centers. In this case, a multi-level optical signal is transmitted and received using the edge transceiver  72 . For example, when data X accommodated in the server  110   a  is transmitted to another data center, the data X is guided to the edge transceiver  72  through the optical transceiver  130   a,  the optical switch  81 , the optical transceiver  82 , and the electric switch  83 . Then, the edge transceiver  72  generates a multi-level optical signal X using the edge transmitter  10  (such as the mapper  13 , the distortion corrector  14 , and the IQ modulator  18  in  FIG. 5 ), and transmits the generated multi-level optical signal X to a specified data center through the optical fiber link  200 . 
         [0094]    An optical signal received through a general communication path is guided to the edge transceiver  72  by the optical switch  81 . Then, the edge transceiver  72  recovers data using the edge receiver  60  (the coherent receiver  61 , the FFT processor  63 , the separator  64 , and the demodulator  65  in  FIG. 5 ). The recovered data is forwarded to a target server through the electric switch  83 , the optical transceiver  82 , and the optical switch  81 . 
         [0095]    When there is a need for low latency, an express path is established. In this case, an IM optical signal transmitted from the optical transceiver  130  is guided to the optical fiber link  200  by the optical switch  81 . The IM optical signal received through the express path is guided to a target server by the optical switch  81 . In other words, the IM optical signal is transmitted between the optical transceivers  130  and  130  without passing through the edge transceiver  72 . 
         [0096]    When a general communication path and an express path are multiplexed in one wavelength channel, the optical frequency difference detector  71  detects a difference between a center frequency of a multi-level optical signal transmitted through a general communication path and a center frequency of an IM optical signal transmitted through an express path. The edge transmitter  10  modifies an electric field information signal according to the detected frequency difference. Here, the mapping correction illustrated in  FIG. 6D  is performed in the mapper  13 , and the shifting of a center frequency illustrated in  FIG. 6C  is performed in the frequency controller  15 . Then, the edge transmitter  10  generates a multi-level optical signal according to the modified electric field information signal. 
         [0097]      FIGS. 11 and 12  are a flowchart that illustrates an example of a communication between data centers. For example, the processing of this flowchart is performed when a request to perform a communication between data centers is made by a user. In this example, it is assumed that a request to perform a data transmission from a server (a transmission source server) accommodated in a data center to another server (a destination server) accommodated in another data center is made. In the following descriptions, the data center that accommodates a transmission source server may be referred to as a “source data center”. Further, the data center that accommodates a destination server may be referred to as a “destination data center”. 
         [0098]    In S 1 , the channel allocation controller  91  selects an available wavelength channel in the optical fiber link  200 . In the following descriptions, a center frequency of the selected wavelength channel may be referred to as “fz”. In S 2 , the channel allocation controller  91  controls an optical frequency of the edge transceiver  72  according to the selected wavelength channel. In other words, in the source data center, an optical frequency of the light source  17  of the edge transmitter  10  is controlled at fz. In the destination data center, an optical frequency of the local light source used by the coherent receiver  61  of the edge receiver  60  is controlled at fz. 
         [0099]    In S 3 , the channel allocation controller  91  controls the optical switch  81 , the optical transceiver  82 , and the electric switch  83 , and sets up a general communication path. In other words, in the source data center, the optical switch  81 , the optical transceiver  82 , and the electric switch  83  are controlled such that data read from the transmission source server is guided to the edge transceiver  72 . In the destination data center, the optical switch  81 , the optical transceiver  82 , and the electric switch  83  are controlled such that data recovered from a received optical signal is guided to the destination server. Then, in S 4 , a communication between data centers using a general communication path is started. 
         [0100]    In S 5 , the channel allocation controller  91  waits for a request to perform a low-latency communication between the data centers to be made. When this request is received, the channel allocation controller  91  controls the optical switch  81  and sets up an express path in S 6 . 
         [0101]    In S 7 , the channel allocation controller  91  controls an optical frequency of a corresponding optical transceiver  130  according to the wavelength channel selected in S 1 . In other words, in an optical transceiver  130  corresponding to a transmission source server that performs a low-latency communication, an optical frequency of the light source  131   a  is controlled at fz. Accordingly, a center frequency of the general communication path established in advance and a center frequency of the newly established express path are both controlled at fz. However, there exists an oscillating frequency error of a laser light source, so the center frequency of the general communication path and the center frequency of the express path do not completely match. 
         [0102]    In S 8 , the optical frequency difference detector  71  detects a difference Δf between the center frequency of the general communication path and the center frequency of the express path. The frequency controller  15  of the edge transmitter  10  modifies an electric field information signal according to a result of the detection performed by the optical frequency difference detector  71  such that the center frequency of the general communication path matches the center frequency of the express path. Here, the frequency controller  15  modifies the electric field information signal such that a center frequency of an output optical signal of the IQ modulator  18  is shifted by Δf. 
         [0103]    In S 9 , the mapper  13  of the edge transmitter  10  modifies an electric field information signal of a multi-level optical signal transmitted through the general communication path such that optical spectra of the general communication path and the express path do not overlap. Here, the mapper  13  modifies the electric field information signal such that a spectrum of the multi-level optical signal transmitted through the general communication path is separated into two spectra, for example, as illustrated in  FIG. 6D . In other words, the mapper  13  generates the electric field information signal from input data using the mappers  44 - 1  and  44 - 2  illustrated in  FIGS. 8 . S 8  and S 9  may be performed in parallel. Further, S 9  is performed before S 8 . Then, in S 10 , a communication in which the general communication path and the express path are multiplexed are performed between data centers. 
         [0104]    In S 11 , the channel allocation controller  91  waits for a request to stop the low-latency communication to be made. When this request is received, the data center  100  stops a data transmission using the express path in S 12 . 
         [0105]    In S 13 , the frequency controller  15  stops modifying an electric field information signal. This results in returning the center frequency of the general communication path to the state before the express path is established. In S 14 , the mapper  13  returns a mapping pattern for generating an electric field information signal to the state before the express path is established. In other words, the mapper  13  generates an electric field information signal from input data using the mapper  42  illustrated in  FIG. 8 . 
         [0106]    As described above, according to the optical network system of the embodiments of the present invention, a general communication path that transmits a multi-level optical signal and an express path that realizes a low latency are multiplexed. This results in improving the communication resource utilization efficiency in an optical communication that satisfies a plurality of different communication requirements. 
         [0107]    All examples and conditional language provided herein are intended for the pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although one or more embodiments of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.