Abstract:
Provided are a semiconductor optical amplifier and an optical signal processing method using the same. The reflective semiconductor optical amplifier includes: an optical signal amplification region operating to allow a downward optical signal incident from the external to obtain a gain; and an optical signal modulation region connected to the optical signal amplification region and generating a modulated optical signal. The downward optical signal is amplified through a cross gain modulation using the modulated optical signal and is outputted as an upward optical signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This U.S. non-provisional patent application claims priority under 35 U.S.C. §119 of Korean Patent Application No. 10-2008-0099613, filed on Oct. 10, 2008, the entire contents of which are hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention disclosed herein relates to a semiconductor optical amplifier and an optical signal processing method using the same, and more particularly, to a semiconductor optical amplifier using a cross gain modulation and an optical signal processing method using the same. 
         [0003]    As high speed internet and diverse multimedia services are introduced lately, Fiber To The Home (FTTH) technology for connecting a telephone station to a home through an optical fiber has been actively developed in order to provide a large amount of information to a user. To commercially use this technology, it is necessary to transmit a large amount of information and achieve a low cost for realizing it. 
         [0004]    In general, because a Passive Optical Network (PON) is based on a passive device, it is advantageous in an aspect of network maintenance and management. Furthermore, since a large number of subscribers share an optical fiber in the PON, it is economical. Especially, according to a Wavelength Division Multiplexed-Passive Optical Network (WDM-PON), since respectively different wavelengths are allocated to each subscriber, its security and expandability are excellent. 
         [0005]    However, according to the WDM-PON, a very expensive optical source such as Distributed Feedback Laser Diode (DFB-LD) is required for each subscriber to use respectively different wavelengths. Furthermore, to achieve a rapid troubleshooting and effective wavelength management, a provider of the WDM-PON preliminarily prepares optical sources of a specific wavelength allocated to each subscriber. That is, if a wavelength dependable optical source is used, a price competitive WDM-PON may be difficult to realize. 
         [0006]    Accordingly, researches for using a low-cost wavelength independent optical source such as a reflective semiconductor optical amplifier as an optical source of an Optical Network Terminal (ONT) in the WDM-PON have been actively progressed. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention provides an inexpensive and high operating speed reflective semiconductor amplifier. 
         [0008]    Embodiments of the present invention provide reflective semiconductor optical amplifiers including: an optical signal amplification region operating to allow a downward optical signal incident from the external to obtain a gain; and an optical signal modulation region connected to the optical signal amplification region and generating a modulated optical signal, wherein the downward optical signal is amplified through a cross gain modulation using the modulated optical signal and is outputted as an upward optical signal. 
         [0009]    In some embodiments, the optical signal amplification region includes a semiconductor amplifier and the optical signal modulation region includes a laser diode, and the optical signal amplification region and the optical signal modulation region include a first electrode and second electrode, respectively, the first electrode and second electrode being configured to allow independent current injection. 
         [0010]    In other embodiments, a direct current is applied to the first electrode of the optical signal amplification region and a Radio Frequency (RF) current is applied to the second electrode of the optical signal modulation region. 
         [0011]    In still other embodiments, the optical signal modulation region generates an optical signal modulated by the RF current, and the optical signal amplification region performs the cross-gain modulation on the downward optical signal through the modulated optical signal and then outputs the cross-gain modulated downward optical signal as an upward optical signal. 
         [0012]    In even other embodiments, a wavelength band of the upward optical signal is included in a gain band of the semiconductor amplifier and is different from that of the downward optical signal. 
         [0013]    In yet other embodiments, the optical signal modulation region includes a diffraction grating. 
         [0014]    In further embodiments, the optical signal modulation region oscillates a light of a single wavelength. 
         [0015]    In still further embodiments, the reflective semiconductor optical amplifiers further include: an anti-reflective layer disposed adjacent to the optical signal amplification region; and a reflective layer disposed adjacent to the optical signal modulation region. The optical signal amplification region and the optical signal modulation region are interposed between the anti-reflective layer and the reflective layer. 
         [0016]    In even further embodiments, the anti-reflective layer receives the incident downward optical signal and outputs the upward optical signal. 
         [0017]    In yet further embodiments, the reflective semiconductor optical amplifiers further include a spot-size converter interposed between the optical signal amplification region and the anti-reflective layer, wherein the anti-reflective layer is optically connected to one optical fiber. 
         [0018]    In other embodiments of the present invention, reflective semiconductor optical amplifiers include: an anti-reflective layer serving as an input/output path of light; a reflective layer spaced apart from the anti-reflective layer, the reflective layer reflecting the light; and an optical amplification modulation region having a resonant structure between the anti-reflective layer and the reflective layer. 
         [0019]    In some embodiments, the resonant structure of the optical amplification modulation region generates a resonant frequency of a single wavelength. 
         [0020]    In other embodiments, the optical amplification region includes a semiconductor amplifier operating to allow the light to obtain a gain and a laser diode generating a modulated signal, the semiconductor amplifier and the laser diode being connected in series. 
         [0021]    In still other embodiments, a band of an oscillation wavelength of the laser diode is included in a gain band of the semiconductor amplifier and is different from a wavelength band of the light. 
         [0022]    In still other embodiments of the present invention, methods for processing an optical signal of a wavelength division multiplexed-passive optical network (WDM-PON) including a central office and an optical network terminal include: receiving a downward optical signal from the central office; generating a modulated optical signal; and amplifying the downward optical signal through a cross gain modulation using the modulated optical signal and outputting the amplified downward optical signal as an upward optical signal. 
         [0023]    In some embodiments, a wavelength band of the modulated optical signal is included in a gain band of the amplification but is different from that of the downward optical signal. 
         [0024]    In other embodiments, a wavelength band of the modulated optical signal is another wavelength band that is not used in the WDM-PON. 
         [0025]    In still other embodiments, the upward optical signal is transmitted to the central office through one optical fiber. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0026]    The accompanying figures are included to provide a further understanding of the present invention, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present invention and, together with the description, serve to explain principles of the present invention. In the figures: 
           [0027]      FIG. 1  is a conceptual diagram of a RSOA according to an embodiment of the present invention; 
           [0028]      FIG. 2  is a block diagram illustrating a WDM-PON based on a RSOA according to an embodiment of the present invention; 
           [0029]      FIG. 3  is a cross-sectional view illustrating a RSOA according to an embodiment of the present invention; 
           [0030]      FIG. 4  is a cross-sectional view illustrating a RSOA according to an embodiment of the present invention; 
           [0031]      FIGS. 5A and 5B  are views illustrating a cross gain modulation in a SOA according to an embodiment of the present invention; and 
           [0032]      FIGS. 6 and 7  are cross-sectional views illustrating a RSOA according to other embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0033]    Preferred embodiments of the present invention will be described below in more detail with reference to the accompanying drawings. The present invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. 
         [0034]    In the specification, it will be understood that when a layer (or film) is referred to as being ‘on’ another layer or substrate, it can be directly on the other layer or substrate, or intervening layers may also be present. Also, in the figures, the dimensions of layers and regions are exaggerated for clarity of illustration. Also, though terms like a first, a second, and a third are used to describe various regions and layers in various embodiments of the present invention, the regions and the layers are not limited to these terms. These terms are used only to discriminate one region or layer from another region or layer. Therefore, a layer referred to as a first layer in one embodiment can be referred to as a second layer in another embodiment. An embodiment described and exemplified herein includes a complementary embodiment thereof. 
         [0035]      FIG. 1  is a conceptual diagram of a Reflective Semiconductor Optical Amplifier (RSOA) according to an embodiment of the present invention. 
         [0036]    Referring to  FIG. 1 , the RSOA includes a Spot-Size Converter (SSC)  102  and an amplification modulation region  103 . The amplification modulation region  103  may include a Semiconductor Optical Amplifier (SOA) and a Laser Diode (LD). The SSC, the SOA, and the LD may be optically connected to each other through a butt joint. 
         [0037]    An anti-reflective layer  105 , optically connected to an optical fiber  101 , may be disposed on one end of the SSC  102  in order to reduce light reflection. The RSOA may be connected to a central office through one optical fiber  101  in order to reduce a manufacturing cost. For this, a high reflective layer  104  for reflecting an incident light is disposed on one end of the amplification modulation region  103 , facing the anti-reflective layer  105 . 
         [0038]    A downward optical signal λ in  incident from the optical fiber  101  to the amplification modulation region  103  is amplified through an optical gain according to a modulation of a current inputted into the amplification modulation region  103 , and then is reflected by the high reflective layer  104  to be transmitted as an upward optical signal λ out  of the optical fiber  101 . 
         [0039]      FIG. 2  is a block diagram illustrating a Wavelength Division Multiplexed-Passive Optical Network (WDM-PON) based on a RSOA according to an embodiment of the present invention. Referring to  FIG. 2 , the WDM-PON includes a central office (CO)  10 , an optical fiber  40 , a remote node  20 , and an Optical Network Terminal (ONT)  30 . The ONT  30  may include a plurality of ONTs ONT 1  . . . ONT N . 
         [0040]    The CO  10  includes an optical source unit Tx for transmitting the downward optical signal, an optical receiving unit Rx for receiving the upward optical signal, an optical coupler/distributor  50 , and an optical multiplexer/de-multiplexer (Mux/DeMux)  12 . In general, a single mode optical source (e.g., Distributed FeedBack (DFB)-LD)) is used as the Tx of the CO  10 . 
         [0041]    The downward optical signal is inputted to the remote node  20  through the optical fiber  40 , and is divided by each wavelength through an optical Mux/DeMux  21  of the remote node  20 . Then, the downward optical signal divided by each wavelength is transmitted into the plurality of independent ONTs. Each of the ONTs may include an optical coupler/distributor  50 , a RSOA for transmitting the upward optical signal as an optical source, and an Rx for receiving the downward optical signal. The upward optical signal modulated in the RSOA is inputted to the CO  10  through the remote node  20  and the optical fiber  40 . According to embodiments of the present invention, since the RSOA is configured to have a resonant structure and utilize a cross gain modulation, a high speed operating characteristic can be provided. 
         [0042]      FIG. 3  is a cross-sectional view illustrating a RSOA  100  according to one embodiment of the present invention. 
         [0043]    An amplification modulation region  103  may include a SOA  122  and an LD  121 . The SSC  102  may include a passive waveguide  112  for improving an optical coupling efficiency with an optical fiber  101  and the amplification modulation region  103  may include a gain waveguide  115  for converting an optical signal. 
         [0044]    The passive waveguide  112  and the gain wave guide  115  may be provided on a substrate  110 . A clad layer  113  is provided on the substrate  110 , the passive waveguide  112 , and the gain waveguide  115 . The passive waveguide  112  and the gain waveguide  115  may be surrounded by the clad layer  113  and the substrate  110 . The gain waveguide  115  may include a first gain waveguide  115   a  and a second gain waveguide  115   b.  The first gain waveguide  115   a  may constitute the LD  121  and the second gain waveguide  115   b  constitute the SOA  122 . 
         [0045]    A first upper electrode  116  is provided on the clad layer  113  above the LD  121 , and a second upper electrode  117  is provided on the clad layer  113  above the SOA  122 . Each ohmic layer  114  is provided between the clad layer  113  and the first and second upper electrodes  116  and  117 . A lower electrode  118  is provided below the substrate  110 . The first and second upper electrodes  116  and  117  are separated from each other and then used as upper electrodes of the LD  121  and the SOA  122 . Accordingly, the LD  121  and the SOA  122  may be configured to have possible independent current injection. 
         [0046]    The LD  121  may be a DFB-LD. A diffraction grating constituting the LD  121  may be provided above or below the first gain waveguide  115   a.  Likewise, according to embodiments of the present invention, the LD  121  may include a resonant structure allowing cross gain modulation through a Radio Frequency (RF) modulation of an injection current. Accordingly, the RSOA according to embodiments of the present invention has a resonant frequency and also operates at a high speed of more than about 1.25 Gbps or about 2.5 Gbps, which is far faster than about 1.25 Gbps. 
         [0047]    The substrate  110  may be formed of n-InP. The gain waveguide  115  may be formed of InGaAsP where a band gap of a bulk or quantum well structure is about 1.55 μm. The passive waveguide  112  may be formed of InGaAsP where a band gap is about 1.1 μm to 1.3 μm. The clad layer  113  may be formed of p-InP, and the ohmic layer  114  may be formed of p + -InGaAs. A current blocking structure limiting a path of a current injected from the upper electrodes  116  and  117  may be formed around the gain waveguide  115  in the clad layer  113 . The current blocking structure may be a buried heterostructure formed of at least one of p-InP and n-InP. A high reflective layer  104  and the anti-reflective layer  105  may have a stacked layer of a titanium oxide layer and a silicon oxide layer and may have an appropriate thickness with respect to a wavelength of light. 
         [0048]    Referring to  FIG. 4 , the passive waveguide of a SSC  102  may be diagonally formed at a an angle θ of about 5° to about 30° with respect to the anti-reflective layer  105 , in order to provide more reduced facet reflectivity. Furthermore, the passive waveguide  112  may be tapered to have a width similar to an optical mode of the optical fiber  101 , such that optical coupling efficiency is improved. The SSC  102  and the amplification modulation region  103  may be optically connected to each other through a butt joint  109 . 
         [0049]      FIGS. 5A and 5B  are views illustrating a cross gain modulation in a SOA according to an embodiment of the present invention.  FIG. 5A  illustrates a downward optical signal λ in , an upward optical signal λ out , and a modulated laser signal λ laser  in the SOA. P_λ in , P_λ out , and P_λ laser  represent an intensity of the downward optical signal, an intensity of an upward optical signal, and an intensity of a modulated layer signal, respectively, and a horizontal-axis represents time. In  FIG. 5B , an x-axis represents an intensity of an input beam (e.g., the incident optical signal) and a y-axis represents an intensity amplification rate (i.e., a gain) of an output light (e.g., an upward optical signal) with respect to the input light. 
         [0050]    Referring to  FIGS. 5A and 5B , when a current of a predetermined amplitude is injected in the SOA, a gain is uniform at a low light intensity but is gradually decreased at above a saturation input light intensity P SAT . This is called a gain saturation phenomenon. A continuous wave (CW) having a uniform intensity, i.e., the downward optical signal λ in , and a laser signal λ laser  having the modulated light intensity may be simultaneously injected in the SOA. A gain of the CW may be reduced due to the modulated light intensity. Especially, the gain reduction of the CW downward optical signal λ in  is more clearly shown if the intensity sum of the downward signal λ in  and the modulated light is more than the saturation input light intensity P SAT  in the SOA. Therefore, the CW downward optical signal λ in  may be modulated to have information of a laser signal λ laser  having a modulated light intensity while passing through the SOA. This process is called a cross Gain Modulation (XGM). 
         [0051]    The XGM is used for including information of the modulated light in another wavelength light. The XGM is effective at a high speed of more than about 40 Gbps. Accordingly, the RSOA using the XGM according to another embodiment of the present invention can provide a high speed operating characteristic. 
         [0052]    Referring to  FIG. 1  to  FIGS. 5A and 5B , according to embodiments of the present invention, a current of a predetermined amplitude can be injected into the SOA  122  and in this case, the downward optical signal may obtain a gain. The LD  121  is configured to modulate the downward optical signal and for this, a modulated current is inputted. Information of the modulated laser signal generated in the LD  121  can be transmitted in the downward optical signal from the CO  10 , through the XGM. The upward optical signal generated through this process is reflected by the high reflective layer  104 , and then is transmitted into the CO  10  through the optical fiber  101 . As well-known, the XGM is effective at an operating speed of about 40 Gbps. Consequently, each ONT can transmit the upward optical signal of more than about 10 Gbps to the CO  10  through the RSOA according to one embodiment of the present invention. 
         [0053]    For effective laser oscillation, an oscillation wavelength λ of the LD  121  may be within a gain band width of the SOA  122 . Furthermore, for effective filtering of a modulated wavelength (performed in an optical MuX/DeMux in a remote node and a central office), the oscillation wavelength λ may be a wavelength that is not used in a corresponding optical network (i.e., different from the wavelength band of the downward and upward optical signals). That is, the oscillation wavelength λ may be selected not to overlap a wavelength band used in another ONT. As well-known, the oscillation wavelength λ of the LD  121  can be adjusted by changing the grating interval L of the diffraction grating  123  as shown in Equation below. Accordingly, the above-mentioned required wavelength band may be effectively selected by changing the interval L of the diffraction grating  123 . An effective refractive index n eff  can be calculated including refractive indexes of a waveguide and an adjacent region through a light expanding and progressing to the waveguide and the adjacent region. 
         [0000]      λ= L· 2 n   eff  (λ: oscillation wavelength,  L:  pitch of diffraction grating,  n   eff : effective refractive index) 
         [0054]      FIGS. 6 and 7  are cross-sectional views illustrating a RSOA according to other embodiments of the present invention. An LD may be a Distributed Bragg Reflector (DBR)-LD. Except for a structure and disposition based on a difference between the DBR-LD and the DFB-LD of the above-mentioned embodiment, technical features related to the DFB laser are similarly or identically applicable to the DBR-LD. Accordingly, for concise description, overlapping technical features may be omitted because their functions and features are similar or identical even when different numerical references are used. 
         [0055]    Referring to  FIG. 6 , a RSOA  200  according to another embodiment of the present invention may include a SSC  202  and an amplification modulation region  203 . The amplification modulation region  203  may include a SOA  222  and a DBR-LD  221 . 
         [0056]    Between a high reflective layer  104  and an anti-reflective layer  105 , a first passive waveguide  212   a,  a first gain waveguide  215   a,  a second passive waveguide  212   b,  a second gain waveguide  215   b,  and a third passive waveguide  212   c  may be disposed alternately. The first gain waveguide  215   a  and the first and second passive waveguides  212   a  and  212   b  at the both sides of the first gain waveguide  215   a  constitutes the DBR-LD and the second gain waveguide  215   b  constitutes the SOA  222 . In order to realize the DBR-LD, a resonant structure such as a diffraction grating  223  may be disposed above or below the first and second passive waveguides  212   a  and  212   b.    
         [0057]    As mentioned above, an oscillation wavelength of the DBR-LD  221  may be a wavelength band which is within a gain band width of the SOA  222  and is not used in a corresponding optical network (i.e., different from a wavelength band of the downward optical signal and the upward optical signal. Technical requirements related to the above oscillation wavelength may be accomplished through an adjustment of an interval of the diffraction grating  223 . 
         [0058]    Referring to  FIG. 7 , the RSOA  300  according to further another embodiment of the present invention may include a SSC  302  and an amplification modulation region  303 . The amplification modulation region  303  may include a SOA  322  and a DBR-LD  321 . 
         [0059]    Between a high reflective layer  104  and an anti-reflective layer  105 , a first gain waveguide  315   a,  a first passive waveguide  312   a,  a second gain waveguide  315   b,  and a second passive waveguide  312   b  are disposed alternately. The first gain waveguide  315   a  and the first passive waveguide  312   a  constitute DBR-LD and the second gain waveguide  315   b  constitutes the SOA  322 . In some embodiment of the inventive concept, the third passive waveguide  312   b  may constitute the SOA  322 . In order to realize the DBR-LD  321 , a resonant structure such as the diffraction grating  323  may be disposed above or below the first passive waveguide  312   a.    
         [0060]    As mentioned above, an oscillation wavelength of the DBR-LD  321  may be a wavelength band which is within a gain band width of the SOA  322  and is not used in a corresponding optical network (i.e., different from a wavelength band of the downward optical signal and the upward optical signal. Technical requirements related to the above oscillation wavelength may be accomplished through an adjustment of an interval of the diffraction grating  323 . 
         [0061]    According to above-mentioned embodiments of the present invention, a single mode operating optical source and a RSOA are integrated together. The single mode operating optical source may be directly modulated up to an operating speed of about 10 Gbps and a cross gain modulation is effective at an operating speed of about 40 Gbps. The RSOA according to the embodiments of the present invention may be realized at an operating speed of more than about 10 Gbps. Furthermore, since the RSOA according to the embodiments of the present invention is a reflective type using one optical fiber, a system using the same may be realized with a low cost. 
         [0062]    The above-disclosed subject matter is to be considered illustrative, and not restrictive, and the appended claims are intended to cover all such modifications, enhancements, and other embodiments, which fall within the true spirit and scope of the present invention. Thus, to the maximum extent allowed by law, the scope of the present invention is to be determined by the broadest permissible interpretation of the following claims and their equivalents, and shall not be restricted or limited by the foregoing detailed description.