Abstract:
A semiconductor optical amplifier for amplifying input optical signals is disclosed. The optical amplifier includes a substrate; a first active layer laminated on the substrate for generating pumping lights; a second active layer laminated on the substrate being gain-clamped by the pumping light and amplifying the input optical signals; and a grating formed on an upper portion of the substrate, adjacent to a boundary between the first active layer and the second active layer, for partially allowing the transmission of the pumping lights to the second active layer and partially reflecting the pumping lights.

Description:
CLAIM OF PRIORITY 
     This application claims priority to an application entitled “SEMICONDUCTOR OPTICAL AMPLIFIER AND OPTICAL AMPLIFYING APPARATUS USING THE SAME,” filed in the Korean Intellectual Property Office on Feb. 25, 2004 and assigned Serial No. 2004-12563, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical-transmission apparatus and, more particularly, to an optical-amplifying apparatus used to amplify optical signals. 
     2. Description of the Related Art 
     Generally, optical-amplifying apparatuses are used to amplify the intensity of input optical signals and to transmit the amplified optical signals over a long distance with minimum errors. Such optical-amplifying apparatuses have important operation characteristics, such as gain, noise figure, and saturation output power. As erbium doped fiber amplifiers are known to have high gain, low noise figure, and large saturation output power, they have been widely utilized in many backbone networks or metro networks. However, the erbium doped fiber amplifiers have disadvantages in that they are expensive, and have relatively large sizes and limited amplification bands. 
     In contrast, semiconductor optical amplifiers are capable of solving the short comings of the erbium doped fiber amplifiers in that their costs are low, their sizes are large, and their amplification bands can be relatively and easily adjusted. However, as the semiconductor optical amplifiers have high noise figures, their actual application is inevitably limited. 
     The conventional semiconductor optical amplifiers tend to be small sizes and their amplification bands can be changed by adjusting the composition of gain materials. Moreover, they can be manufactured at a low cost. 
     In the case of general gain-clamped semiconductor optical amplifiers, they have good gain precedence and saturation output power. However, as they have a very high noise figure of more than 8 dB, they have many limitations when they are used in transmission systems. In the case of Raman amplifiers, they have very low noise figures. However, since they have a very low-light amplification efficiency, they require high power laser diodes in order to obtain enough gain. 
     In order to solve these disadvantages, a new optical-amplifying apparatus technology has been recently proposed, which has a structure that combines a Raman amplifier and a semiconductor optical amplifier. In such a combined structure, an optical signal obtains Raman gain before being inputted to the semiconductor optical amplifier, so that effective gain is increased. In addition, a signal-to-noise ratio (SNR) of the semiconductor optical amplifier can be improved by the Raman gain, so that the effective noise figure is reduced. 
       FIG. 1  shows a conventional optical-amplifying apparatus. As shown, the optical-amplifying apparatus includes a single mode fiber (SMF)  110 , a Raman optical amplifier constructed with a wavelength selective coupler (WSC)  130  and a laser diode (LD)  120 , and a semiconductor optical amplifier (SOA)  140 . 
     In operation, the laser diode  120  outputs a pumping light S 2  having a wavelength of 1400 nm to 1500 nm. The wavelength selective coupler  130  has a first port connected to the single mode fiber  110 , a second port connected to the semiconductor optical amplifier  140 , and a third port connected to the laser diode  120 . Thus, the wavelength selective coupler  130  provides the single mode fiber  110  with the pumping light S 2  inputted to the third port, and outputs a Raman-amplified optical signal S 1  inputted to the first port to the second port. 
     The single mode fiber  110  is pumped by the pumping light S 2 , and then Raman-amplifies and outputs the optical signal S 1  inputted via a stimulated Raman-scattering effect. The semiconductor optical amplifier  140  amplifies and outputs the Raman-amplified optical signal S 1 . 
     However, the conventional optical-amplifying apparatus as described above has a hybrid structure, defined by the semiconductor optical amplifier and the Raman optical amplifier, and integrates an expensive laser diode with the semiconductor optical amplifier, thereby increasing the material cost and the manufacturing cost. In addition, a separate wavelength selective coupler is required in order to integrate two devices with each other, and optical loss occurring in the connection portion between the devices deteriorates the optical amplification characteristic. Furthermore, a larger space is required in order to install the combined devices. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art and provides additional advantages, by providing an optical-amplifying apparatus and a semiconductor optical amplifier that are economical and capable of improving the output characteristic. 
     According to one aspect of the present, there is provided a semiconductor optical amplifier comprising: a substrate; a first active layer laminated on the substrate for generating pumping lights; a second active layer laminated on the substrate and coupled to the first active layer, which is being gain-clamped by the pumping light, and for amplifying and outputting an inputted optical signal; and a grating formed on an upper portion of the substrate, which is adjacent to a boundary between the first active layer and the second active layer, the grating partially allowing the transmission of the pumping lights to the second active layer and partially reflecting the pumping lights. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows a conventional optical-amplifying apparatus; 
         FIG. 2  is a view illustrating a basic operation principle of the present invention; 
         FIG. 3  is a graph illustrating an output characteristic of the optical-amplifying apparatus shown in  FIG. 2 ; 
         FIG. 4  shows the construction of an optical-amplifying apparatus according to a preferred embodiment of the present invention; 
         FIG. 5  is a detailed view of the semiconductor optical amplifier shown in  FIG. 4 ; and, 
         FIG. 6  is a graph illustrating the output characteristic of the optical-amplifying apparatus shown in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, an embodiment according to the present invention will be described with reference to the accompanying drawings. For the purposes of clarity and simplicity, a detailed description of known functions and configurations incorporated herein will be omitted as it may make the subject matter of the present invention unclear. 
       FIG. 2  shows a simplified schematic diagram illustrating the basic operation principle according to the teachings of the present invention. As shown, the optical-amplifying apparatus includes a dispersion-compensated fiber (DCF)  210 , a first and a second wavelength selective coupler  230  and  250 , a first and a second pumping light source  220  and  240 , and a semiconductor optical amplifier  260 . 
     The first pumping light source  220  outputs a first pumping light S 4  having a wavelength of 1400 nm to 1500 nm. The second pumping light source  240  outputs a second pumping light S 5  having a wavelength of 1400 nm to 1500 nm. 
     The first and the second pumping light source  220  and  24  each may include laser diodes. 
     The first wavelength selective coupler  230  has a first port connected to the dispersion compensated fiber  210 , a second port connected to the second wavelength selective coupler  250 , and a third port connected to the first pumping light source  220 . The first wavelength selective coupler  230  provides the dispersion compensated fiber  210  with the first pumping light S 4  inputted to the third port, and outputs a Raman-amplified optical signal S 3  inputted to the first port to the second port. 
     The dispersion compensated fiber  210  is pumped by the first pumping light S 4 , and Raman-amplifies and outputs the optical signal S 3  inputted by the stimulated Raman-scattering effect. Further, the dispersion compensated fiber  210  compensates for the dispersion of the optical signal S 3  and may employ a single-mode fiber. In that case, the single-mode fiber Raman-amplifies and outputs an inputted optical signal, but does not compensate for the dispersion of the optical signal. 
     The second wavelength selective coupler  250  has a first port connected to the second port of the first wavelength selective coupler  230 , a second port connected to the semiconductor optical amplifier  260 , and a third port connected to the second pumping light source  240 . Further, the second wavelength selective coupler  250  provides the semiconductor optical amplifier  260  with the second pumping light S 5  inputted to the third port, and outputs the Raman-amplified optical signal S 3  inputted to the first port to the second port. 
     The gain of the semiconductor optical amplifier  260  is clamped by the second pumping light S 5 , and the amplifier  260  has an active layer for amplifying and outputting the inputted Raman-amplified optical signal S 3 . Note that as the second pumping light S 5  has a wavelength that is substantially shorter than the gain wavelength of the semiconductor optical amplifier  260 , the second pumping light S 5  plays a role of clamping the gain of the semiconductor optical amplifier  260 . 
       FIG. 3  is a graph illustrating the output characteristic of the optical-amplifying apparatus shown in  FIG. 2 . 
     A first gain curve  310  represents the gain of the optical-amplifying apparatus when the second pumping light source  240  is in an off-state, and a second gain curve  320  represents the gain of the optical-amplifying apparatus when the second pumping light source  240  is in an on-state. A first noise figure curve  340  represents the noise figure of the optical-amplifying apparatus when the second pumping light source  240  is in an off-state, and a second noise figure curve  330  represents the noise figure of the optical-amplifying apparatus when the second pumping light source  240  is in an on-state. Accordingly, the optical amplifying apparatus according to the teachings of the present invention can obtain a gain-clamping effect of the semiconductor optical amplifier  260  using the second pumping light S 5  as shown in  FIG. 3 . 
       FIG. 4  illustrates the construction of an optical-amplifying apparatus according to an embodiment of the present invention, and  FIG. 5  illustrates the semiconductor optical amplifier shown in  FIG. 4  in more detail. As shown, the optical-amplifying apparatus includes a dispersion compensated fiber (or a single mode fiber)  410  and a semiconductor optical amplifier  420 . 
     The semiconductor optical amplifier  420  includes a substrate  422 , a first and a second active layer  424  and  426 , a clad  428 , a low-reflection coating layer  460 , and an anti-reflection coating layer  470 . Further, the semiconductor optical amplifier  420  includes an oscillation area  430  and an amplifying area  450  in relation to the first active layer  424  and the second active layer  426 . The oscillation area  430  generates a pumping light and the amplifying area  450  amplifies an inputted Raman-amplified optical signal S 6 . 
     The substrate  422  has a grating  440  at an upper portion thereof. Herein, the grating  440  belongs to the oscillation area  430  and is formed so as to be adjacent to the boundary between the first active layer  424  and the second active layer  426 . 
     The first active layer  424  belongs to the oscillation area  430  and is laminated on the substrate  422  to generate pumping lights having a wavelength of 1400 nm to 1500 nm according to the input of electric current. Further, a lasing direction of the first active layer  424  is set to be opposite to the input direction of the Raman-amplified optical signal S 6 . 
     The second active layer  426  belongs to the amplifying area  450  and is laminated on the substrate  422 , and it amplifies and outputs the inputted Raman-amplified optical signal S 6  according to the input of electric current. The first active layer  424  and the second active layer  426  are coupled to each other using a butt-joint method. 
     Since the first active layer  424  has an oscillation wavelength (i.e., wavelength of the pumping light) shorter than the wavelength of the Raman-amplified optical signal S 6 , the Raman-amplified optical signal S 6  is not absorbed into the first active layer  424 , but is inputted to the second active layer  426 . 
     The grating  440  stabilizes the oscillation wavelength of the first active layer  424 , thereby stabilizing a Raman-gain characteristic obtained by the pumping light. A portion of the pumping lights generated by the first active layer  424  are partially transmitted by the grating  440 , and the other portion of the pumping lights are reflected by the grating  440 . The transmitted pumping light S 8  is inputted to the second active layer  426 . Herein, since the transmitted pumping light S 8  has a wavelength that is much shorter than a gain wavelength of the second active layer  426 , the transmitted pumping light S 8  plays a role of clamping the gain in the second active layer  426 . The reflected pumping light S 7  is provided to the dispersion compensated fiber  410 . 
     The clad  428  is laminated on the first and the second active layer  424  and  426 , and confines a light inside the first and second active layers  424  and  426  together with the substrate  422 . 
     The low-reflection coating layer  460  is coated at one end surface of the semiconductor optical amplifier  420 , which is adjacent to the oscillation area  430 , and has a low-reflection factor. 
     The antireflection coating layer  470  is coated on the other end surface of the semiconductor optical amplifier  420 , which is adjacent to the amplifying area  450 , and has a reflection factor with a value near to 0. 
     In operation, the dispersion compensated fiber  410  is pumped by the reflected pumping light S 7 , and Raman-amplifies and outputs the optical signal S 6  inputted by the stimulated Raman-scattering effect. Further, the dispersion compensated fiber  410  compensates for the dispersion of the optical signal S 6  and may employ a single-mode fiber. In that case, the single-mode fiber Raman-amplifies and outputs an inputted optical signal but does not compensate for the dispersion of the optical signal. 
       FIG. 6  is a graph illustrating the output characteristic of the optical-amplifying apparatus shown in  FIG. 4 . For the purpose of comparison with  FIG. 3 ,  FIG. 6  shows a third gain curve  510  and a third noise figure curve  520  of the optical-amplifying apparatus together with the second gain curve  320  and the second noise figure curve  330 . 
     The third gain curve  510  represents the gain of the optical-amplifying apparatus, and the third noise figure curve  520  represents the noise figure of the optical-amplifying apparatus. As shown in  FIG. 6 , the optical-amplifying apparatus can obtain a gain-clamping effect of the second active layer  426  and a Raman-amplification effect by the transmitted pumping light S 8 . Further, the optical-amplifying apparatus has a noise figure smaller than 3 dB due to the Raman-amplification effect. 
     As described above, an optical amplifying apparatus according to the present invention uses a gain-clamped semiconductor optical amplifier combining the functions of a Raman amplifier and a pumping-light source, thereby reducing the noise figure and increasing the effective gain. 
     Further, according to the present invention, since the semiconductor optical amplifier simultaneously performs the functions of a pumping light source and an optical amplifier while having the same module structure as that of the existing semiconductor optical amplifier, an optical-amplifying apparatus using the semiconductor optical amplifier has a small size and low cost in comparison with the conventional hybrid-type optical-amplifying apparatus. 
     While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.