Abstract:
An object of the present invention is to provide a temperature-independent optical frequency shifter for generating sub-carriers with a miniaturizable configuration, as well as to provide an all-optical OFDM modulator using the same that is compact, has low temperature dependence, and is even compatible with different frequency grids. Provided is an optical frequency shifter and an optical modulator using the same, the optical frequency shifter comprises one input optical port, a 1-input, 2-output optical coupler optically connected thereto, two Mach-Zehnder modulation units individually optically connected to the two outputs thereof, a 2-input, 2-output optical coupler optically connected to the individual outputs thereof, and two output optical ports optically connected to the outputs thereof, wherein the two Mach-Zehnder modulation units are driven by periodic waveforms at the same frequency whose phases differ from each other by (2p+1)π/2 (p: integer).

Description:
TECHNICAL FIELD 
     The present invention relates to an optical frequency shifter and an optical modulator using the same, and more particularly, to an optical frequency shifter that shifts input continuous light into two optical frequencies, and an all-optical frequency-division multiplexing optical modulator using the same. 
     BACKGROUND ART 
     Due to vigorous communication demands, investigations towards increasing the capacity of backbone networks are being actively conducted. With increases in transmission capacity, if wavelength-division multiplexing (WDM) is used together with raising the per-wavelength symbol rate (the modulation symbol delivery speed), the effects of wavelength dispersion and polarization mode dispersion increase sharply. Furthermore, the optical intensity for obtaining a required reception sensitivity for transmission increases, and signal quality degradation due to four-wave-mixing, cross-phase modulation, self-phase modulation, and the like produced inside the optical fiber also become problematic. 
     In order to solve such problems, technology that uses orthogonal frequency-division multiplexing (OFDM) on each wavelength channel and multiplexes the above with WDM is being investigated as a multiplexing technology with excellent dispersion resistance and high bandwidth utilization efficiency. With OFDM, by encoding N carriers (where N is an integer equal to or greater than 2) orthogonal to each other, the symbol rate can be lowered to 1/N compared to the case of a single carrier, and the dispersion resistance can be improved. OFDM is a general-purpose technology in the field of radio. 
     As a technology that OFDM modulates an optical signal, there is a method that electrically generates an OFDM signal similarly to radio and drives an optical modulator (see PTL 1). The optical system is simple if this technique is used, but since the modulator and the modulator driving unit demand bands of approximately N times the symbol rate, there is a problem in that these bands become a limiting factor. 
     Meanwhile, all-optical OFDM that multiplexes sub-carrier light pre-modulated by an optical modulator has been proposed (see PTL 2 and 3). As illustrated in  FIG. 1 , first, multiple sub-carrier light beams are generated with a multi-carrier generation circuit (optical sub-carrier generator)  101 . Next, these sub-carrier light beams are discriminated into individual sub-carrier light beams with an optical separation unit  102 , and after being respectively data-modulated by optical orthogonal modulators  103   a  and  103   b , are multiplexed by an optical multiplexer  104  to obtain a modulated output. As disclosed in PTL 3, the optical separation unit  102  may comprise delayed interferometers  105 ,  106   a , and  106   b . In so doing, a high extinction ratio can be obtained, even in the case where the optical frequency grid of the WDM signals (the optical frequency interval between WDM optical signals) and the sub-carrier interval differ to some degree. Although  FIG. 1  illustrates the case of two sub-carriers, the optical circuit on the transmitting side is also comparatively simple in this case, and thus is promising as a next-generation high-speed transmission technology. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Patent Laid-Open No. 2005-311722 
     PTL 2: Japanese Patent Laid-Open No. 2009-017320 
     PTL 3: Japanese Patent Laid-Open No. 2009-198914 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, with the above configuration of the all-optical OFDM modulator, it is necessary to use delayed interferometers  105  and  106  in the optical separation unit  102  for the purpose of sub-carrier discrimination, and there is a problem in that the circuit size becomes larger for this reason. In order to set the frequency grid of the WDM to 100 GHz, it is necessary to set the free spectrum range (FSR) of the delayed interferometers to approximately 50 GHz (see PTL 3). If the delayed interferometers are manufactured with a silica optical waveguide (approximately N=1.49), the optical path differential of the delayed interferometers becomes approximately 4 mm. In order to set the frequency interval of the wavelength channels to the 50 GHz interval that is recently being adopted, the optical path differential becomes double at approximately 8 mm, which requires an optical separation unit with a large circuit size. 
     Also, since the lithium niobate waveguides or silica optical waveguides constituting the delayed interferometers typically have an index of refraction that is temperature-dependent, there is a problem in that the center wavelength of the delayed interferometers changes according to the environmental temperature. In order to resolve the above, it is necessary to perform temperature adjustment or make the delayed interferometers temperature-independent. Temperature adjustment complicates the implementation of the modulator module, and also has the problem increasing power consumption (typically several Watts). Temperature independence has the problem of inducing increased loss (typically around 1 dB). 
     Furthermore, since it is necessary to set the FSR of the delayed interferometers to match the optical frequency grid and the sub-carrier interval, it is necessary to modify the design of the delayed interferometers for different frequency grids, and there is a problem in that different optical separation units become necessary. 
     The present invention, being devised in light of related technology like the above, takes as an object to provide a temperature-independent optical frequency shifter that generates sub-carriers with a miniaturizable configuration, as well as to provide an all-optical OFDM modulator using the same that is compact, has low temperature dependence, and is even compatible with different frequency grids. 
     Solution to Problem 
     An optical frequency shifter of the first mode of the present invention for solving the above problem comprises one input optical port; a 1-input, 2-output optical coupler optically connected to the one input port; two Mach-Zehnder modulation units individually optically connected to the two outputs of the 1-input, 2-output optical coupler; a 2-input, 2-output optical coupler optically connected to the individual outputs of the two Mach-Zehnder modulation units; and two output optical ports optically connected to the two outputs of the 2-input, 2-output optical coupler; wherein the two Mach-Zehnder modulation units are driven by periodic waveforms at the same frequency whose phases differ from each other by (2p+1)π/2 (p: integer). 
     Also, an optical frequency shifter of the second mode is the optical frequency shifter of the first mode that the biases of the two Mach-Zehnder modulation units are adjusted such that the individual outputs become 0 when not driven. 
     Also, an optical frequency shifter of the third mode is the optical frequency shifter of the first mode that, provided that the half-wave voltage of the Mach-Zehnder modulation units is Vπ, the full voltage amplitude values of the periodic waveforms that drive the Mach-Zehnder modulation units are within 60% to 120% inclusive of 2Vπ. 
     Also, an optical frequency shifter of the fourth mode is the optical frequency shifter of the first mode that the 1-input, 2-output optical coupler is a Y-optical branch coupler, and the 2-input, 2-output optical coupler is a 2-input, 2-output multimode interference optical coupler. 
     Also, an optical frequency shifter of the fifth mode is the optical frequency shifter of the first mode that a modulation electrode provided in one Mach-Zehnder modulation unit from between the two Mach-Zehnder modulation units and a modulation electrode provided in the other Mach-Zehnder modulation unit are cascade-connected, and a delay of π/2 in the periodic waveform is provided on an electrical line that connects the modulation electrode provided in the one Mach-Zehnder modulation unit and the modulation electrode provided in the other Mach-Zehnder modulation unit. 
     Also, an optical frequency shifter of the sixth mode is the optical frequency shifter of the fifth mode that the length of the modulation electrode provided in one Mach-Zehnder modulation unit from between the two Mach-Zehnder modulation units and of the modulation electrode provided in the other Mach-Zehnder modulation unit is shorter for the modulation electrode provided in the Mach-Zehnder modulation unit closer to the electrical input, and longer for the modulation unit provided in the Mach-Zehnder modulation unit farther from the electrical input. 
     Also, an optical modulator of the seventh mode of the present invention for solving the above problem is an optical modulator that generates an optical signal, the optical modulator comprising a 1-input, 2-output optical frequency shifter unit; two optical modulation units individually optically connected to the two outputs of the 1-input, 2-output optical frequency shifter unit; and an optical multiplexing unit optically connected to the individual outputs of the two optical modulation units; wherein the 1-input, 2-output optical frequency shifter unit is provided with one input optical port, a 1-input, 2-output optical coupler optically connected to the one input port, two Mach-Zehnder modulation units individually optically connected to the two outputs of the 1-input, 2-output optical coupler, a 2-input, 2-output optical coupler optically connected to the individual outputs of the two Mach-Zehnder modulation units, and two output optical ports optically connected to the two outputs of the 2-input, 2-output optical coupler, wherein the two Mach-Zehnder modulation units are driven by periodic waveforms at the same frequency whose phases differ from each other by (2p+1) π/2 (p: integer). 
     Also, an optical modulator of the eighth mode is the optical modulator of the seventh mode that the two modulation units are individual optical orthogonal modulation units, and the optical multiplexing unit is a 2-input, 1-output optical coupler. 
     Also, an optical modulator of the ninth mode is the optical modulator of in the seventh mode that the two modulation units are individual polarization multiplexing optical orthogonal modulation units, and the optical multiplexing unit is a 2-input, 1-output optical coupler. 
     Also, an optical modulator of the tenth mode is the optical modulator of the seventh mode that the two modulation units are double optical orthogonal modulation units individually optically connected to the two outputs of an individual 1-input, 2-output optical coupler and a 1-input, 2-output optical coupler, the optical multiplexer comprises a first 2-input, 1-output optical coupler that multiplexes one output from each of the two double optical orthogonal modulation units, a second 2-input, 1-output optical coupler that multiplexes the other output from each of the two double optical orthogonal modulation units, and a polarization multiplexer that polarization multiplexes the output of the first optical coupler and the output of the second optical coupler, and a polarization converter is provided between one of either the output of the first optical coupler and the output of the second optical coupler, and the polarization multiplexer. 
     Also, an optical modulator of the eleventh mode is the optical modulator of the seventh mode that the biases of the two Mach-Zehnder modulation units provided in the optical frequency shifter are adjusted such that the individual outputs become 0 when not driven. 
     Also, an optical modulator of the twelfth mode is the optical modulator of the seventh mode that, provided that the half-wave voltage of the Mach-Zehnder modulation units provided in the optical frequency shifter is Vπ, the full voltage amplitude values of the periodic waveforms that drive the Mach-Zehnder modulation units provided in the optical frequency shifter are within 60% to 120% inclusive of 2Vπ. 
     Also, an optical modulator of the thirteenth mode the optical modulator of the seventh mode that the 1-input, 2-output optical coupler provided in the optical frequency shifter is a Y-optical branch coupler, and the 2-input, 2-output optical coupler provided in the optical frequency shifter is a 2-input, 2-output multimode interference optical coupler. 
     Also, an optical modulator of the fourteenth mode is the optical modulator of the seventh mode that a modulation electrode provided in one Mach-Zehnder modulation unit from between the two Mach-Zehnder modulation units provided in the optical frequency shifter and a modulation electrode provided in the other Mach-Zehnder modulation unit provided in the optical frequency shifter are cascade-connected, and a delay of π/2 in the periodic waveform that drives the optical frequency shifter is provided on an electrical line that connects the modulation electrode included in the one Mach-Zehnder modulation unit and the modulation electrode provided in the other Mach-Zehnder modulation unit provided in the optical frequency shifter. 
     Also, an optical modulator of the fifteenth mode is the optical modulator of the fourteenth mode that the length of a modulation electrode provided in one Mach-Zehnder modulation unit from between the two Mach-Zehnder modulation units provided in the optical frequency shifter and of the modulation electrode provided in the other Mach-Zehnder modulation unit is shorter for the modulation electrode provided in the Mach-Zehnder modulation unit closer to the electrical input, and longer for the modulation unit provided in the Mach-Zehnder modulation unit farther from the electrical input. 
     Advantageous Effects of Invention 
     By providing one input optical port; a 1-input, 2-output optical coupler optically connected to the one input port; two Mach-Zehnder modulation units individually optically connected to the two outputs of the 1-input, 2-output optical coupler; a 2-input, 2-output optical coupler optically connected to the individual outputs of the two Mach-Zehnder modulation units; and two output optical ports optically connected to the two outputs of the 2-input, 2-output optical coupler; and by driving the two Mach-Zehnder modulation units by periodic waveforms at the same frequency whose phases differ from each other by π/2, it is possible to provide an optical frequency shifter and an optical modulator that do not require delayed interferometers, are compact, do not have temperature dependency, and do not depend on a wavelength grid. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating a configuration of an OFDM modulator of the related art; 
         FIG. 2  is a diagram illustrating a configuration of an optical frequency shifter according to the first embodiment of the present invention; 
         FIG. 3  is a diagram illustrating modulation amplitude dependence of frequency shift components in an optical frequency shifter according to the first embodiment of the present invention; 
         FIG. 4  is a diagram illustrating frequency shift components in an optical frequency shifter according to the first embodiment of the present invention; 
         FIG. 5  is a diagram illustrating an output signal obtained with an optical frequency shifter according to the first embodiment of the present invention; 
         FIG. 6  is a diagram illustrating a configuration of an optical frequency shifter according to a modification of the first embodiment of the present invention; 
         FIG. 7  is a diagram illustrating a configuration of an optical frequency shifter according to the second embodiment of the present invention; 
         FIG. 8  is a diagram illustrating a simulation of an output signal obtained with an optical frequency shifter according to the second embodiment of the present invention; 
         FIG. 9  is a diagram illustrating a configuration of an optical modulator according to the third embodiment of the present invention; 
         FIG. 10  is a diagram illustrating a configuration of an optical orthogonal modulation unit; 
         FIG. 11  is a diagram illustrating a configuration of an optical modulator according to the fourth embodiment of the present invention; 
         FIG. 12  is a diagram illustrating a configuration of a polarization multiplexing optical orthogonal modulation unit; 
         FIG. 13  is a diagram illustrating a configuration of an optical modulator according to the fifth embodiment of the present invention; and 
         FIG. 14  is a diagram illustrating a configuration of an optical modulator according to a modification of the fifth embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described with reference to the drawings and mathematical formulas. 
     First Embodiment 
       FIG. 2  illustrates a configuration of an optical frequency shifter  210  according to the first embodiment of the present invention. The optical frequency shifter  210  of the first embodiment of the present invention comprises an input port  211 , a 1-input, 2-output optical coupler  212  optically connected to the input port  211 , two Mach-Zehnder modulation units (hereinafter, MZ modulation units)  213   a  and  213   b  respectively and optically connected to the two outputs of the optical coupler  212 , a 2-input, 2-output optical coupler  214  individually and optically connected to the two MZ modulation units  213   a  and  213   b , and output optical ports  215   a  and  215   b  individually and optically connected to the two outputs of the 2-input, 2-output optical coupler  214 . 
     The two MZ modulation units  213   a  and  213   b  are driven via electrical amps  218   a  and  218   b  by electrical signals produced by a signal generator  216 , but as illustrated in  FIG. 2 , the driving unit of the MZ modulation unit  213   a  is provided with an electrical delay line  217  having a phase shift of π/2. As a result, the two MZ modulation units  213   a  and  213   b  become driven by identical electrical waveforms whose phase differs by π/2. 
     Herein, in the optical frequency shifter  210  according to the first embodiment of the present invention illustrated in  FIG. 2 , a Y-optical branch coupler is used as the 1-input, 2-output optical coupler  212 . This is because taking such a configuration makes it possible to provide a 1-input, 2-output optical coupler with a wide range of operating wavelengths and small splitting ratio instability. However, the present invention is not limited to this example, and for the 1-input, 2-output optical coupler, a 1-input, 2-output multimode interference optical coupler may also be used, and additionally a directional coupler, a 2-input, 2-output multimode interference optical coupler, or one of the input ports of a 2-input, 2-output optical coupler such as an asymmetric X-coupler may also be used. 
     Also, in the optical frequency shifter  210  according to the first embodiment of the present invention illustrated in  FIG. 2 , a multimode interference optical coupler is used as the 2-input, 2-output optical coupler  214 . This is because taking such a configuration makes it possible to provide a 2-input, 2-output optical coupler with a wide range of operating wavelengths. However, the present invention is not limited to this example, and obviously the use of another coupler, such as a directional coupler, an asymmetric X-coupler, or a wideband optical coupler using a lattice configuration, is also acceptable. 
     Also, in the optical frequency shifter  210  according to the first embodiment of the present invention illustrated in  FIG. 2 , a Y-optical branch coupler with two in-phase optical outputs is used as the 1-input, 2-output optical coupler  212 , and a multimode interference optical coupler that produces a 90 degree phase difference between two optical outputs is used as the 2-input, 2-output optical coupler  214 . This is not only because a Y-optical branch coupler and a multimode interference optical coupler are suitable as the respective couplers for the first embodiment of the present invention, but also because taking such a combination also has the merit of making it unnecessary to insert an optical delay in the optical arm coupling the Y-optical branch coupler  212  and the multimode interference optical coupler  214 . However, as illustrated in detail in a modification of the first embodiment, the present invention is not limited to this example. 
     Next, operation of the optical frequency shifter  210  according to the first embodiment of the present invention will be described. Herein, an input waveform into the optical frequency shifter  210  is expressed as E=E 0 ( t ). At this point, since input light is guided by the Y-optical branch coupler  212  to the MZ modulation units  213   a  and  213   b  while keeping the same phase, the input optical fields Eain(t) and Ebin(t) of the MZ modulation units  213   a  and  213   b  are respectively expressed as in Eq. 1 and Eq. 2 below. 
     
       
         
           
             
               
                 
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     At this point, the driving waveform Db(t) of the MZ modulation unit  213   b  is expressed as in Eq. 3 below, assuming a sine wave for simplicity.
 
Math. 3
 
 D   b ( t )= m  sin(2π f )  Eq. 3
 
     Herein, m is a proportionality coefficient, and f is the frequency of the driving waveform. The driving waveform Da(t) of the MZ modulation unit  213   a  receives a delay of π/2 from the electrical delay line  217 , and thus becomes like Eq. 4 below. 
     
       
         
           
             
               
                 
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     Now assume that the MZ modulation units  213   a  and  213   b  are bias-adjusted so as to indicate a sinusoidal response to the respective driving waveforms. At this point, the optical field outputs Eaout(t) and Ebout(t) of the MZ modulation units  213   a  and  213   b  are respectively expressed as in Eq. 5 and Eq. 6 below. 
     
       
         
           
             
               
                 
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     Herein, Jn is an nth order Bessel function of the first kind. These two optical fields are multiplexed by the multimode interference optical coupler  214 . At this point, in the multimode interference optical coupler  214 , the combined light is given a phase shift of π/2, and thus the optical fields E 1 ( t ) and E 2 ( t ) obtained from the output ports  215   a  and  215   b  are respectively given as in Eq. 7 and Eq. 8 below. 
     
       
         
           
             
               
                 
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     Ignoring the higher-order terms and focusing on the n=0 term, E 1 ( t ) and E 2 ( t ) respectively become like Eq. 11 and Eq. 12 below.
 
Math. 11
 
 E   1 ( t )≈− E   0   J   1 ( m )exp(− j 2π ft )  Eq. 11
 
Math. 12
 
 E   2 ( t )≈− jE   0   J   1 ( m )exp(+ j 2π ft )  Eq. 12
 
     Eq. 11 and Eq. 12 above demonstrate that E 1 ( t ) is given a frequency shift of −f from the original frequency, while E 2 ( t ) is given a frequency shift of +f from the original frequency. 
     Herein, in the optical frequency shifter  210  according to the first embodiment of the present invention illustrated in  FIG. 2 , the driving waveform Da(t) of the MZ modulation unit  213   a  is taken to receive a delay of π/2 from the electrical delay line  217 , but obviously it is also acceptable to provide the electrical delay line  217  on the side of the MZ modulation unit  213   b  and apply the π/2 delay to the driving waveform Db(t). In this case, f indicated in the formulas is replaced with −f. Furthermore, generally the advantageous effects of the present invention can be exhibited if a phase difference of (2p+1) π/2 is applied between Da(t) and Db(t), where p is an integer. 
     Also, in the optical frequency shifter  210  according to the first embodiment of the present invention illustrated in  FIG. 2 , the driving waveforms Da(t) and Db(t) of the MZ modulation units  213   a  and  213   b  are assumed to be sine waves, but this is because the generation of such waveforms is easy, and furthermore because the load on the driving electrical system can be reduced since the waveform is narrow. However, the present invention is not limited to this example, and obviously a waveform other than a sine wave is also acceptable. In this case, the coefficients applied to the Bessel functions in Eq. 9 and Eq. 10 will change. 
       FIG. 3  is a diagram illustrating the relationship  300  between the driving amplitude m of an optical frequency shifter according to the first embodiment of the present invention, and the obtained optical frequency components. In the drawing, an f component, a 3f component, and a 5f component are depicted. Of these, the f component becomes important for the operation of the optical frequency shifter, and the drawing demonstrates that the f component is maximized when m=1.17π. Meanwhile, since there is an aspect of the load on the driving electrical system increasing as the driving amplitude increases, a driving amplitude m from 60% to 120% of π is desirable. This is equivalent to setting the full amplitude from 60% to 120% of 2Vπ, provided the half-wave voltage of an MZ modulation unit is Vπ. 
       FIG. 4  is a diagram illustrating the results  400  of calculating the optical frequency spectrum obtained at the output optical port  215   b  when driving an optical frequency shifter according to the first embodiment of the present invention. The horizontal axis represents the optical frequency normalized to f, while the vertical axis is the optical power. Also, the driving amplitude m is taken to be 1.17π. Eq. 10 demonstrates that the optical frequency after passing through the optical frequency shifter becomes +f, −3f, +5f, −7f, and so on. 
       FIG. 5  is a diagram illustrating a waveform  500  obtained when configuring an optical frequency shifter according to the first embodiment of the present invention and actually driving.  FIG. 5  demonstrates that by using an optical frequency shifter of the present invention, output whose optical frequency is respectively shifted by −f and +f is obtained at the output optical ports  215   a  and  215 , respectively. 
     With this configuration, optical delayed interferometers for discriminating the ±f optical frequency components become unnecessary, thus making it possible to provide an optical frequency shifter of small size, in which it is unnecessary to take into account changes in the characteristics of the delayed interferometers due to temperature. In addition, since there are no optical delayed interferometers, it is possible to provide an optical frequency shifter that is not limited to operation on a specific wavelength grid, but is capable of operating on any frequency grid. 
     Modification of First Embodiment 
       FIG. 6  illustrates a configuration of an optical frequency shifter  610  according to a modification of the first embodiment of the present invention. The optical frequency shifter  610  according to a modification of the first embodiment of the present invention comprises an input port  611 , a 1-input, 2-output optical coupler  612  optically connected to the input port  611 , two Mach-Zehnder modulation units (hereinafter, MZ modulation units)  613   a  and  613   b  individually and optically connected to the two outputs of the optical coupler  612 , a 2-input, 2-output optical coupler  614  individually and optically connected to the two MZ modulation units  613   a  and  613   b , and output optical ports  615   a  and  615   b  individually and optically connected to the two outputs of the 2-input, 2-output optical coupler  614 . 
     The two MZ modulation units  613   a  and  613   b  are driven via electrical amps  618   a  and  618   b  by electrical signals produced by a signal generator  616 , but as illustrated in  FIG. 6 , the driving unit of the MZ modulation unit  613   b  is provided with an electrical delay line  617  having a phase shift of π/2. As a result, the two MZ modulation units  613   a  and  613   b  become driven by identical electrical waveforms whose phase differs by π/2. In the optical frequency shifter according to the first embodiment of the present invention illustrated in  FIG. 2 , the electrical delay line  217  is provided on the driving system of the MZ modulation unit  213   a , but obviously the advantageous effects of the present invention can be exhibited even if the electrical delay line  617  is provided for the driving unit of the MZ modulation unit  613   b , as with the optical frequency shifter  610  according to a modification of the first embodiment of the present invention illustrated in  FIG. 6 . 
     Also, in the optical frequency shifter  610  according to a modification of the first embodiment of the present invention illustrated in  FIG. 6 , a 1-input, 2-output multimode interference optical coupler is used as the 1-input, 2-output optical coupler  612 . In the optical frequency shifter  210  according to the first embodiment of the present invention illustrated in  FIG. 2 , a Y-optical branch coupler is used as the 1-input, 2-output optical coupler  212 , but obviously the advantageous effects of the present invention can be exhibited even if a 1-input, 2-output multimode interference optical coupler is used as the 1-input, 2-output optical coupler  612 , as with the optical frequency shifter  610  according to a modification of the first embodiment of the present invention illustrated in  FIG. 6 . 
     Furthermore, in the optical frequency shifter  610  according to a modification of the first embodiment of the present invention illustrated in  FIG. 6 , a 2-input, 2-output X-coupler having outputs with different waveguide widths as illustrated in  FIG. 6  is used as the 2-input, 2-output optical coupler  614 . This is because an X-coupler that uses adiabatic mode evolution has low-loss characteristics over a wide band, making it possible to provide a wideband, low-loss optical frequency shifter. In addition, since with an X-coupler the phases between the combined light become 0 and π, a π/2 optical delay line  619  is inserted between the MZ modulation unit  613   b  and the 2-input, 2-output optical coupler  614  in order to compensate. In the optical frequency shifter  210  according to the first embodiment of the present invention illustrated in  FIG. 2 , a 2-input, 2-output multimode interference optical coupler is used as the 2-input, 2-output optical coupler  214 , but obviously the advantageous effects of the present invention can be exhibited even if an X-coupler is used as the 2-input, 2-output optical coupler  614 , and an optical delay line  619  is used to compensate for the phase, as with the optical frequency shifter  610  according to a modification of the first embodiment of the present invention illustrated in  FIG. 6 . 
     Second Embodiment 
       FIG. 7  illustrates a configuration of an optical frequency shifter  710  according to the second embodiment of the present invention. The optical frequency shifter  710  of the second embodiment of the present invention comprises an input port  711 , a 1-input, 2-output optical coupler  712  optically connected to the input port  711 , two MZ modulation units  713   a  and  713   b  individually and optically connected to the two outputs of the optical coupler  712 , a 2-input, 2-output optical coupler  714  individually and optically connected to the two MZ modulation units  713   a  and  713   b , and output optical ports  715   a  and  715   b  individually and optically connected to the two outputs of the 2-input, 2-output optical coupler  714 . 
     Herein, in the optical frequency shifter  710  according to the second embodiment of the present invention illustrated in  FIG. 7 , two MZ modulation units  713   a  and  713   b  are provided. The two MZ modulation units  713   a  and  713   b  are individually equipped with modulation electrodes  720   a  and  720   b . These modulation electrodes  720   a  and  720   b  are connected by an electrical line  721 , with an electrical delay line  722  provided between the modulation electrodes  720   a  and  720   b  such that an electrical delay of π/2 is applied between the driving waveforms of the modulation electrodes  720   a  and  720   b . In addition, the electrical line  721  connecting the modulation electrodes  720   a  and  720   b  is ultimately terminated by a terminating resistor  723 . The optical frequency shifter  710  is driven via an electrical amp  718  by a driving waveform generated by a signal generator  716 . With this configuration, one electrical amp is sufficient to drive the MZ modulation units, making it possible to provide an optical frequency shifter  710  with low power consumption. 
     Herein, it is noted that although modulation electrodes are obviously also provided in the MZ modulation units  213   a ,  613   a  and  213   b ,  613   b  included in the optical frequency shifter  210  according to the first embodiment of the present invention and the optical frequency shifter  610  according to a modification of the first embodiment of the present invention illustrated in  FIG. 2  and  FIG. 6 , modulation electrodes are omitted from  FIG. 2  and  FIG. 6  which illustrate configurations. 
     In addition, in the optical frequency shifter  710  according to the second embodiment of the present invention illustrated in  FIG. 7 , the MZ modulation units  713   a  and  713   b  comprise x-cut lithium niobate, and the modulation electrodes  720   a  and  720   b  are single-electrode driving electrodes. However, the present invention is not limited to this example, and the MZ modulation units  713   a  and  713   b  may comprise z-cut lithium niobate and polarization inversion, and the modulation electrodes  720   a  and  720   b  may be single-electrode driving electrodes. Alternatively, the MZ modulation units  713   a  and  713   b  may comprise z-cut lithium niobate, and the modulation electrodes  720   a  and  720   b  may be dual-electrode driving electrodes. Obviously, MZ modulation units  713   a  and  713   b  comprised of other types of materials are also acceptable. 
     In addition, in the optical frequency shifter  710  according to the second embodiment of the present invention illustrated in  FIG. 7 , the length of the modulation electrode  720   b  closer to the electrical input may be made shorter than the length of the modulation electrode  720   a  farther from the electrical input in order to account for loss on the electrical line from the modulation electrode  720   b  to the modulation electrode  720   a  and obtain the same degree of modulation with the MZ modulation units  713   a  and  713   b . However, the present invention is not limited to this example. 
     Furthermore, in the optical frequency shifter  710  according to the second embodiment of the present invention illustrated in  FIG. 7 , a Y-optical branch coupler is used as the 1-input, 2-output optical coupler  712 . This is because taking such a configuration makes it possible to provide a 1-input, 2-output optical coupler with a wide range of operating wavelengths and a small splitting ratio instability. However, the present invention is not limited to this example, and for the 1-input, 2-output optical coupler, a 1-input, 2-output multimode interference optical coupler may also be used, and additionally a directional coupler, a 2-input, 2-output multimode interference optical coupler, or one of the input ports of a 2-input, 2-output optical coupler such as an asymmetric X-coupler may also be used. 
     Also, in the optical frequency shifter  710  according to the second embodiment of the present invention illustrated in  FIG. 7 , a multimode interference optical coupler is used as the 2-input, 2-output optical coupler  714 . This is because taking such a configuration makes it possible to provide a 2-input, 2-output optical coupler with a wide range of operating wavelengths. However, the present invention is not limited to this example, and obviously the use of another coupler, such as a directional coupler, an asymmetric X-coupler, or a wideband optical coupler using a lattice configuration, is also acceptable. 
       FIG. 8  is a diagram illustrating simulation values for an optical spectrum obtained by an optical frequency shifter according to the second embodiment of the present invention. As illustrated in  FIG. 8 , two frequency-shifted optical outputs can still be obtained with such a configuration. 
     Third Embodiment 
       FIG. 9  illustrates a configuration of an optical modulator  900  according to the third embodiment of the present invention. The optical modulator  900  according to the third embodiment of the present invention illustrated in  FIG. 9  comprises an optical frequency shifter  910 , optical orthogonal modulation units  924   a  and  924   b  individually and optically connected to the two outputs of the optical frequency shifter  910 , and a 2-input, 1-output optical coupler  925 , optically connected to the outputs of the optical orthogonal modulation units  924   a  and  924   b , that multiplexes the two outputs. Herein, an optical frequency shifter according to the first embodiment of the present invention is used as the optical frequency shifter  910 . 
     Herein, in the optical modulator  900  according to the third embodiment of the present invention illustrated in  FIG. 9 , a Y-optical branch coupler is used as the 2-input, 1-output optical coupler  925 . This is because taking such a configuration makes it possible to provide a 2-input, 1-output optical coupler with a wide range of operating wavelengths and a small splitting ratio instability. However, the present invention is not limited to this example, and for the 2-input, 1-output optical coupler, a 2-input, 1-output multimode interference optical coupler may also be used, and additionally a directional coupler, a 2-input, 2-output multimode interference optical coupler, or one of the output ports of a 2-input, 2-output optical coupler such as an asymmetric X-coupler may also be used. 
     In addition, in the optical modulator  900  according to the third embodiment of the present invention illustrated in  FIG. 9 , although the optical frequency shifter  210  according to the first embodiment of the present invention illustrated in  FIG. 2  is used, obviously it is also acceptable to use the optical frequency shifter  610  according to a modification of the first embodiment of the present invention illustrated in  FIG. 6 , or to use the optical frequency shifter  710  according to the second embodiment of the present invention illustrated in  FIG. 7 . 
     The optical orthogonal modulation units  924  in  FIG. 9  may be realized with the configuration illustrated in  FIG. 10 . The optical orthogonal modulation unit  924  illustrated in  FIG. 10  comprises an input optical port  1026 , a 1-input, 2-output optical coupler  1027  optically connected to the input optical port  1026 , two MZ modulation units  1028   a  and  1028   b  individually and optically connected to the two outputs of the 1-input, 2-output optical coupler, a 2-input, 2-output optical coupler  1030  optically connected to the outputs of the two MZ modulation units  1028   a  and  1028   b , an output optical port  1031  optically connected to one of the outputs of the 2-input, 2-output optical coupler  1030 , a monitor optical port  1032  optically connected to the other output, and an optical monitor  1033  optically connected to the monitor optical port  1032 . Additionally, an optical delay line  1029  that applies a π/2 delay to light is provided between one of the MZ modulation units (in the case of  FIG. 10 ,  1028   b ) and the 2-input, 2-output optical coupler  1030 . 
     By taking such a configuration, light whose optical frequency is shifted by −f is guided to the optical orthogonal modulation unit  924   a , while light whose optical frequency is shifted by +f is guided to the optical orthogonal modulation unit  924   b , as described using the formulas in the first embodiment. Consequently, by setting the optical frequency shift magnitude f equal to half the symbol rate, an all-optical OFDM signal is obtained as the output of the 2-input, 1-output optical coupler  1025 . 
     With this configuration, optical delayed interferometers for discriminating the ±f optical frequency components become unnecessary, thus making it possible to provide an optical modulator of small size, in which it is unnecessary to take into account changes in the characteristics of the delayed interferometers due to temperature. In addition, since there are no optical delayed interferometers, it is possible to provide an optical modulator that is not limited to operation on a specific wavelength grid, but is capable of operating on any frequency grid. 
     Fourth Embodiment 
       FIG. 11  illustrates a configuration of an optical modulator  1100  according to the fourth embodiment of the present invention. The optical modulator  1100  according to the fourth embodiment of the present invention illustrated in  FIG. 11  comprises an optical frequency shifter  1110 , polarization multiplexing optical orthogonal modulation units  1134   a  and  1134   b  individually and optically connected to the two outputs of the optical frequency shifter  1110 , and a 2-input, 1-output optical coupler  1123 , optically connected to the outputs of the optical orthogonal modulation units  1134   a  and  1134   b , that multiplexes the two outputs. Herein, the optical frequency shifter  210  according to the first embodiment of the present invention is used as the optical frequency shifter  1110 . 
     Herein, in the optical frequency shifter  1110  according to the fourth embodiment of the present invention illustrated in  FIG. 11 , a Y-optical branch coupler is used as the 2-input, 1-output optical coupler  1123 . This is because taking such a configuration makes it possible to provide a 2-input, 1-output optical coupler with a wide range of operating wavelengths and a small splitting ratio instability. However, the present invention is not limited to this example, and for the 2-input, 1-output optical coupler, a 2-input, 1-output multimode interference optical coupler may also be used, and additionally a directional coupler, a 2-input, 2-output multimode interference optical coupler, or one of the output ports of a 2-input, 2-output optical coupler such as an asymmetric X-coupler may also be used. 
     In addition, in the optical modulator  1100  according to the fourth embodiment of the present invention illustrated in  FIG. 11 , although the optical frequency shifter  210  according to the first embodiment of the present invention illustrated in  FIG. 2  is used, obviously it is also acceptable to use the optical frequency shifter  610  according to a modification of the first embodiment of the present invention illustrated in  FIG. 6 , or to use the optical frequency shifter  710  according to the second embodiment of the present invention illustrated in  FIG. 7 . 
     Furthermore, the polarization multiplexing optical orthogonal modulation units  1134  in  FIG. 11  may be realized with the configuration illustrated in  FIG. 12 . The polarization multiplexing optical orthogonal modulation unit  1134  illustrated in  FIG. 12  comprises an input optical port  1235 , a 1-input, 2-output optical coupler  1236  optically connected to the input optical port  1235 , optical orthogonal modulation units  1224   a  and  1224   b  individually and optically connected to the two outputs of the 1-input, 2-output optical coupler  1236 , a 2-input, 1-output polarization multiplexer  1238  optically connected to the outputs of the two optical orthogonal modulation units  1224   a  and  1224   b , and an output optical port  1239  optically connected to the output of the 2-input, 1-output polarization multiplexer  1238 . Additionally a polarization converter  1237  that converts the optical polarization to an orthogonal polarization is provided between one of the optical orthogonal modulation units (in the case of  FIG. 12 ,  1124   a ) and the polarization multiplexer  1238 . Herein, the optical orthogonal modulation units  1224   a  and  1224   b  may take the configuration illustrated in  FIG. 10 . 
     By taking such a configuration, light whose optical frequency is shifted by −f is guided to the polarization multiplexing optical orthogonal modulation unit  1134   a , while light whose optical frequency is shifted by +f is guided to the polarization multiplexing optical orthogonal modulation unit  1134   b , as described using the formulas in the first embodiment. Consequently, by setting the optical frequency shift magnitude f equal to half the symbol rate, a polarization-multiplexed all-optical OFDM signal is obtained as the output of the 2-input, 1-output optical coupler  1123 . 
     Fifth Embodiment 
       FIG. 13  illustrates a configuration of an optical modulator  1300  according to the fifth embodiment of the present invention. The optical modulator  1300  according to the fifth embodiment of the present invention illustrated in  FIG. 13  comprises an optical frequency shifter  1310 , double optical orthogonal modulation units  1340   a  and  1340   b  individually and optically connected to the two outputs of the optical frequency shifter  1310 , and a multiplexer  1342 , optically connected to the outputs of the double optical orthogonal modulation units  1340   a  and  1340   b , that multiplexes the two outputs. Herein, the optical frequency shifter  210  according to the first embodiment of the present invention is used as the optical frequency shifter  1310 . 
     In addition, the double optical orthogonal modulation unit  1340   a  comprises a Y-optical branch coupler  1341   a  that splits input light in two, and optical orthogonal modulation units  1324   a  and  1324   b  optically connected to the two outputs of the Y-optical branch coupler  1341   a . The double optical orthogonal modulation unit  1340   b  comprises a Y-optical branch coupler  1341   b  that splits input light in two, and optical orthogonal modulation units  1324   c  and  1324   d  optically connected to the two outputs of the Y-optical branch coupler  1341   b.    
     Furthermore, the multiplexer  1342  comprises a Y-optical branch coupler  1343   a  that multiplexes one of respective outputs of the double optical orthogonal modulation units  1340   a  and  1340   b , Y-optical branch coupler  1343   b  that multiplexes the other respective output of the double optical orthogonal modulation units  1340   a  and  1340   b , and a polarization multiplexer  1345  that polarization multiplexes the outputs of the Y-optical branch couplers  1343   a  and  1343   b . Also, a polarization converter  1344  that converts the optical polarization to an orthogonal polarization is provided between one of the Y-optical branch couplers  1343   a  and  1343   b  (in this case,  1343   a ) and the polarization multiplexer  1345 . 
     Herein, in the optical modulator  1100  according to the fifth embodiment of the present invention illustrated in  FIG. 11 , although the optical frequency shifter  210  according to the first embodiment of the present invention illustrated in  FIG. 2  is used, obviously it is also acceptable to use the optical frequency shifter  610  according to a modification of the first embodiment of the present invention illustrated in  FIG. 6 , or to use the optical frequency shifter  710  according to the second embodiment of the present invention illustrated in  FIG. 7 . 
     By taking such a configuration, light whose optical frequency is shifted by −f is guided to the double polarization multiplexing optical orthogonal modulation unit  1340   a , while light whose optical frequency is shifted by +f is guided to the double polarization multiplexing optical orthogonal modulation unit  1340   b , as described using the formulas in the first embodiment. Consequently, by setting the optical frequency shift magnitude f equal to half the symbol rate, a polarization-multiplexed all-optical OFDM signal is obtained as the output of the multiplexer  1342 . 
     Note that besides the configuration illustrated in  FIG. 13 , the configuration of the fifth embodiment of the present invention illustrated in  FIG. 13  obviously can still exhibit the advantageous effects of the present invention even if the spatial layout of the double optical orthogonal modulation units is disposed so as to be nested, as in  FIG. 14 . 
     REFERENCE SIGNS LIST 
     
         
           210 ,  610 ,  710 ,  910 ,  1110 ,  1310 ,  1410  Optical frequency shifter 
           211 ,  611 ,  711 ,  911 ,  1026 ,  1111 ,  1235 ,  1311 ,  1411  Input optical port  212 ,  612 ,  712 ,  912 ,  1112 ,  1027 ,  1236 ,  1312 ,  1341   a ,  1341   b ,  1412 ,  1441   a ,  1441   b  1-input, 2-output optical coupler 
           213   a ,  213   b ,  613   a ,  613   b ,  713   a ,  713   b ,  913   a ,  913   b ,  1028   a ,  1028   b ,  1113   a ,  1113   b ,  1313   a ,  1313   b ,  1413   a ,  1413   b  Mach-Zehnder modulation unit 
           214 ,  614 ,  714 ,  914 ,  1030 ,  1114 ,  1314 ,  1414  2-input, 2-output optical coupler 
           215   a ,  215   b ,  615   a ,  615   b ,  715   a ,  715   b ,  915   a ,  915   b ,  1031 ,  1115   a ,  1115   b ,  1239 ,  1315   a ,  1315   b ,  1415   a ,  1415   b  Output optical port 
           216 ,  616 ,  716 ,  916 ,  1116 ,  1316 ,  1416  Signal generator 
           217 ,  617 ,  722 ,  917 ,  1117 ,  1317 ,  1417  Electrical delay line 
           218   a ,  218   b ,  618   a ,  618   b ,  718  Electrical amp 
           619 ,  1029  Optical delay line 
           720   a ,  720   b  Modulation electrode 
           721  Electrical line 
           723  Terminating resistor 
           924   a ,  924   b ,  1224   a ,  1224   b ,  1324   a ,  1324   b ,  1324   c ,  1324   d ,  1424   a ,  1424   b ,  1424   c ,  1424   d  Optical orthogonal modulation unit 
           925 ,  1123 ,  1343   a ,  1343   b ,  1443   a ,  1443   b  2-input, 1-output optical coupler 
           1032  Monitor optical port 
           1033  Optical monitor 
           1134   a ,  1134   b  Polarization multiplexing optical orthogonal modulation unit 
           1237 ,  1344 ,  1444  Polarization converter 
           1238 ,  1345 ,  1445  Polarization multiplexer 
           1340   a ,  1340   b ,  1440   a ,  1440   b  Double optical orthogonal modulation unit 
           1342 ,  1442  Multiplexer