Abstract:
The present invention discloses an art for eliminating the dependency of the optical modulator on the polarization degree by using an orthogonal mode light which is a mixture of two lights that are perpendicular to each other in polarization direction as a light source. An optical communication system of the present invention comprises: a first optical generator and a second optical generator for generating a first single mode light and a second single mode light, respectively, wherein the first single mode light and the second single mode light are orthogonal and non-interfering with each other, and have the same wavelength; a mixing means for mixing the first single mode light and the second single mode light to output an orthogonal mode light; and an optical modulator for amplifying and modulating the orthogonal mode light after receiving the orthogonal mode light through an optical fiber so as to produce a constantly amplified optical signal regardless of a deflection degree of the orthogonal mode light.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention generally relates to an optical communication system using an optical modulator having polarization dependency, and more specifically, to a technology of removing non-uniform amplification factors of modulating signals of the optical modulator having polarization dependency by mixing two lights whose polarization directions are vertical each other and using them as a light source.  
         [0003]     2. Description of the Drawings  
         [0004]      FIG. 1  is a mimetic diagram illustrating a conventional optical communication system using an optical modulator having polarization dependency. Since a reflective semiconductor optical amplifier is usefully applied in a Wavelength Division Multiplexer-Passive Optical Network (hereinafter, referred to as “WDM-PON”) as disclosed in “Semiconductor Laser Amplifier-Reflector for the Future FTTH Applications” (Vol. 2, p. 196) written by N. Buldawwo, S. Mottet, F. le Gall, D. Siggogne, D. Meichenin, S. Chelle in European Conference on Optical Communication, an example where the reflective semiconductor optical amplifier is used as an optical modulator having polarization dependency.  
         [0005]     In general, the optical communication system comprises a light source  100 , an optical circulator  200 , a reflective semiconductor optical amplifier  300  and a photodiode  400 .  
         [0006]     An optical generator  110  in the light source  100  outputs a continuous wave (hereinafter, abbreviated as “CW”) laser light λ s  having a predetermined strength, and the representative example of the optical generator  110  is a Distributed Feedback Laser Diode (hereinafter, referred to as “DFB-LD”). The laser light λ s  from the optical generator  110  is projected into the reflective semiconductor optical amplifier  300  through the optical circulator. Then, the reflective semiconductor optical amplifier  300  receives the laser light λ s  to generate a modulated optical signal λ m  that has the same wavelength as that of the projected laser light λ s . The optical signal λ m  modulated by the semiconductor optical amplifier  300  is inputted in the photodiode  400  through the optical circulator  200 , and then transformed into an electric signal.  
         [0007]      FIG. 2  is a diagram illustrating the operation principle of the reflective semiconductor optical amplifier  300  of  FIG. 1 . The projected light λ s  inputted through the optical waveguide  310  is transmitted into the active layer waveguide  320 , reflected in the high reflection coating film  330 , and outputted to the optical waveguide  310  again. Here, while the projected light λ s  is processed along the active waveguide  330 , the projected light λ s  is amplified depending on inputted signal current. Since a reflected output light copies the signal current, the reflective semiconductor optical amplifier  300  converts the light λ s  into the upstream optical signal λ m  having the same wavelength as that of the light λ s .  
         [0008]     Referring to  FIG. 3 , the active layer waveguide  320  has a quantum well structure which is formed of alternately deposited materials having a large energy band gap and a small energy band gap at tens of Ω. In this case, the state available in a Momentum space is two-dimensionally distributed, and the state depending on energy levels is intensively distributed in a specific energy level to increase photoelectric conversion efficiency. However, since a quantum well layer has its horizontal structure different from its vertical structure, the distribution in the momentum direction of excited electrons is directional. A light projected into the semiconductor optical amplifier having a quantum well structure has different photoelectric conversion efficiency depending on its polarization direction. That is, in general, optical amplification gain by the semiconductor optical amplifier having a quantum well depends largely on the polarization direction of the projected light.  
         [0009]     For example, as shown in  FIG. 4 , the optical amplification gain is 20 dB when the projected light is vertically polarized while the optical amplification gain is 10 dB when the projected light is horizontally polarized.  
         [0010]     Meanwhile, the laser light λ s  generated from the optical generator  110  by duplicating photons has polarization because a polarization characteristic is also duplicated during the above generation process. As a result, when a single mode laser light having the polarization is used in an optical communication network, its polarization direction can be polarized due to distortion of an optical fiber. However, when the polarized laser light is inputted and then modulated in the reflective optical amplifier  300  having large polarization dependency of the optical amplification gain, the modulated optical signal has a difference of the optical amplification gain as described above, which causes instability of the optical transmission system.  
       SUMMARY OF THE INVENTION  
       [0011]     Accordingly, it is an object of the present invention to provide an optical communication system which removes instability of optical transmission due to polarization dependency even when a reflective optical amplifier having large polarization dependency.  
         [0012]     According to one aspect of the present invention, there is provided an optical communication system comprising: a first optical generator and a second optical generator for generating a first single mode light and a second single mode light, respectively, wherein the first single mode light and the second single mode light are orthogonal and non-interfering with each other, and have the same wavelength; a mixing means for mixing the first single mode light and the second single mode light to output an orthogonal mode light; and an optical modulator for amplifying and modulating the orthogonal mode light after receiving the orthogonal mode light through an optical fiber so as to produce a constantly amplified optical signal regardless of a deflection degree of the orthogonal mode light.  
         [0013]     According to another aspect of the present invention, there is provided an optical communication method comprising: a first step of generating a first single mode light and a second single mode light, wherein the first single mode light and the second single mode light are orthogonal and non-interfering with each other, and have the same wavelength; a second step of mixing the first single mode light and the second single mode light to generate an orthogonal mode light; and a third step of receiving the orthogonal mode light through an optical fiber, and amplifying and modulating the received orthogonal mode light regardless of a deflection degree of the orthogonal mode light to produce a constantly amplified optical signal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     Other aspects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the drawings in which:  
         [0015]      FIG. 1  is a mimetic diagram illustrating a conventional optical transmission system using a reflective semiconductor optical amplifier;  
         [0016]      FIG. 2  is a diagram illustrating the operation principle of the reflective semiconductor optical amplifier of  FIG. 1 ;  
         [0017]      FIG. 3  is a diagram illustrating an active layer waveguide having a quantum well structure in the reflective semiconductor optical amplifier of  FIG. 1 ;  
         [0018]      FIG. 4  is a diagram illustrating optical power amplification gain when two single mode lights each having different polarization directions are projected into the reflective semiconductor optical amplifier of  FIG. 1 ;  
         [0019]      FIG. 5  is a diagram illustrating an optical communication system according to a first embodiment of the present invention;  
         [0020]      FIG. 6  is a graph illustrating an orthogonal mode light deflected by θ rad. in horizontal and vertical directions;  
         [0021]      FIG. 7  is a diagram illustrating an example where the first embodiment is applied in WDM-PON; and  
         [0022]      FIG. 8  is a diagram illustrating an optical communication system according to a second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]     The present invention will be described in detail with reference to the accompanying drawings. Herein, the same reference numbers and arrows will be used throughout the drawings and the following description to refer to the same parts shown in FIGS.  1  to  4 .  
         [0024]      FIG. 5  is a diagram illustrating an optical communication system according to a first embodiment of the present invention. The optical communication system of  FIG. 5  is an example obtained by improving the structure of the light source  100  in the common optical communication system of  FIG. 1 . Hereinafter, the detailed explanation on the elements such as the optical circulator  200 , the reflective semiconductor optical amplifier  300  and the photodiode  400  in the common optical communication system is omitted.  
         [0025]     In the first embodiment, a light source  100  comprises optical generators  111  and  112 , and a 3 dB coupler  120 . The first optical generator  111  generates a first single mode laser light λ v,s  polarized in a vertical direction through the above-described photon duplication process. In the same way, the second optical generator  112  generates a second single laser light λ h,s  polarized in a horizontal direction. Here, the two laser lights λ v,s  and λ h,s  are orthogonal and non-interfering with each other, and have the same wavelength. The orthogonal laser lights λ v,s  and λ h,s  are transmitted into two input terminals  121  and  122 , and then branched into two output terminals  123  and  124  in the 3 dB coupler  120 , so that the laser lights λ v,s  and λ h,s  has a half power, respectively. The first single mode laser light λ v,s  and the second single laser light λ h,s  that are outputted from the output terminal  123  of the coupler  120  are mixed while they do not interfere each other, so that an orthogonal mode laser light λ v+h,s  is generated.  
         [0026]     The light for the orthogonal mode upstream signal can secure stability of the signal even when the light is modulated and amplified by a reflective semiconductor optical amplifier having polarization dependency. More specifically, the orthogonal mode laser light λ v+h,s  is transmitted into a first port  210  of an optical circulator  200 . Then, the orthogonal mode laser light λ v+h,s  is outputted to a second port  202 , and transmitted through an optical fiber into a reflective semiconductor optical amplifier. Here, the orthogonal mode laser light λ v+h,s  which is transmitted through the optical fiber is deflected by θ rad. shown in  FIG. 6  depending on the state of the optical fiber as described above. The deflected the orthogonal mode laser light λ v+h,s  is transmitted into the reflective semiconductor optical amplifier  300  to be modulated. However, the non-uniformity of the optical amplification gain shown in the prior art is not generated even when the optical signal modulated through the reflective semiconductor optical amplifier having polarization dependency is generated.  
         [0027]     The specific reasons will be explained with reference to  FIGS. 6 and 7 . First, suppose that the semiconductor optical amplifier  300  of  FIG. 5  has the characteristic of  FIG. 4 , the powers of the single mode laser light λ v,s  and λ h,s  are P, an optical fiber loss is L dB, and the optical fiber has no polarization dependency. If the polarization direction of the orthogonal mode laser light λ v+h,s  for an upstream signal is maintained, the power where the light polarized in the vertical and horizontal directions is projected into the optical amplifier  300  is P/2·10 −L/10 , and its electromagnetic strength is (P/2·10 −L/10 ) 1/2 . Here, an optical power gain of the vertical polarization is 20 dB, and a strength gain of an electromagnetic wave is 10 dB. Therefore, the strength of the optical wave where a vertical element of the orthogonal mode laser light λ v+h,s  for an upstream signal is modulated in the optical amplifier  300  is (P/2·10 −L/10 ) 1/2 ·10 10/10  that is reduced by an optical power to obtain P/2·10 −L/10 ·10 20/10 . An optical power gain of the horizontal polarization is 10 dB, and a strength gain of an electromagnetic wave is 5 dB. The optical power where a horizontal element of the orthogonal mode laser light λ v+h,s  for an upstream signal is modulated in the optical amplifier  300  is P/2·10 −L/10 ·10 10/10 . As a result, the addition of the optical powers of the vertical element and the horizontal element of the modulated optical signal λ v+h,m  is P/2·10 −L/10 ·(10 10/10 +10 20/10 )=P/2·10 −L/10 ·110.  
         [0028]     If the polarization direction of the orthogonal mode laser light λ v+h,s  for an upstream signal is deflected by θ rad. as shown in  FIG. 6  until it reaches the reflective semiconductor optical amplifier  300 , the optical power of the modulated optical signal can be obtained as follows. Since the electromagnetic strength of the vertical element of the projected light λ v+h,s  that is not deflected is (P/2·10 −L/10 ) 1/2 , the electromagnetic strength of the vertical element of λ v,s  of the projected lights in  FIG. 6  is (P/2·10 −L/10 ) 1/2 ·cos θ and the electromagnetic strength of the horizontal element is (P/2·10 −L/10 ) 1/2 ·sin θ. In the same way, the electromagnetic strength of the horizontal element of λ h,s  of the projected lights in  FIG. 6  is (P/2·10 −L/10 ) 1/2 ·cos θ and the electromagnetic strength of the horizontal element is (P/2·10 −L/10 ) 1/2 ·sin θ. Meanwhile, since the optical power gain of the vertical element is 20 dB and the optical power gain of the horizontal element is 10 dB, the amplified optical power of the whole vertical elements of the projected light λ v+h,s  of  FIG. 6  is P/2·10 −L/10 ·(sin 2 θ+cos 2 θ)· 10   20/10 =P/2·10 −L/10 ·10 20/10 , and the amplified optical power of the whole horizontal elements of the projected light λ v+h,s  of  FIG. 6  is P/2·10 −L/10 ·(sin 2 θ+cos 2 θ)·10 20/10 =P/2·10 −L/10 ·10 10/10 . As a result, the modulated optical signal has a predetermined optical power even when the projected light is deflected by a random angle and projected into the optical amplifier. In other words, the optical transmission system has a predetermined optical power gain regardless of polarization directions even when a reflective semiconductor optical amplifier having large polarization dependency is used.  
         [0029]      FIG. 7  is a diagram illustrating an example where the first embodiment is applied in the WDM-PON. The two single mode laser lights that are orthogonal with each other having the same wavelength are inputted respectively into two input terminals of a 3 dB coupler, mixed as an orthogonal mode laser light having a half power, and then outputted into two output terminals of the 3 dB coupler. The orthogonal mode laser light, which is used as a light for an upstream signal, is wavelength-division-multiplexed through two WDM  510  and  520 , and then transmitted into a subscriber&#39;s station. That is, the orthogonal mode laser light λ i,v+h,s  generated from the light source  100  is wavelength-division-multiplexed in the WDM  500  with a light having a different wavelength which is generated for other subscribers. Thereafter, the wavelength-division-multiplexed light Σλ i,v+h,s  is split by a 1×n optical splitter. The split light Σλ i,v+h,s  for an upstream signal is transmitted into the j th  one of a plurality of Optical Line Terminals (hereinafter, referred to as “OLT”) in a base station. Then, the split light Σλ i,v+h,s  for an upstream signal is transmitted into the WDM  530  at the subscriber&#39;s side through the first port  201  and the second port  202  in the optical circulator  200  so that lights may be separated in each subscriber, and transmitted into an Optical Network Unit (hereinafter, referred to as “ONU”)  700  of the i th  subscriber. The light Σ i,v+h,s  for an upstream signal is projected into the reflective semiconductor optical amplifier  300  in the ONU  700 . An upstream optical signal λ i,v+h,m  generated from the reflective semiconductor optical amplifier  300  is transmitted into the WDM  530  through the optical fiber where the light λ i,v+h,s  for an upstream signal is transmitted, wavelength-division-multiplexed with upstream optical signals of other subscribers by the WDM  530 , and then transmitted into an OLT  800  of the base station. The wavelength-division-multiplexed upstream optical signal λ i,s+h,m  is projected into the second port  202  of the optical circulator  200  in the OLT  800 , and then outputted into a third port  203 . After the outputted upstream optical signal Σλ i,v+h,m  is split in each subscriber in the WDM  540 , the optical signal Σλ i,v+h,m  is transmitted into the photodiode  400  and converted into an electric signal.  
         [0030]      FIG. 8  is a diagram illustrating an optical communication system according to a second embodiment of the present invention. The light source  100  also comprises optical generators  111  and  112  like in the first embodiment, and single mode laser lights that are orthogonal with each other are generated. The single mode laser lights λ v,s  and λ h,s  are inputted in output terminals  141  and  142  of a Polarization Beam Splitter (hereinafter, referred to as “PBS”)  130 , and outputted to an input terminal of the PBS  130 . Unlike the first embodiment, a light λ v+h,s  is obtained by mixing lights λ v,s  and λ h,s  which have initial optical powers. Thereafter, the same process where the light λ v+h,s  is outputted to the optical circulator  200 , modulated in the optical amplifier and then converted into an electric signal in the photodiode of the first embodiment is repeated in the second embodiment. The second embodiment can be used in the same optical communication network such as the WDM-PON like the first embodiment.  
         [0031]     As described above, in an optical transmission system and a method thereof according to an embodiment of the present invention, even when an optical modulator having polarization dependency like a reflective semiconductor optical amplifier in an optical transmission network is used, a constantly amplified optical signal can be obtained regardless of deflection of a light inputted in the optical modulator, which results in stable optical transmission.  
         [0032]     While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and described in detail herein. However, it should be understood that the invention is not limited to the particular forms disclosed. Rather, the invention covers all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined in the appended claims.