Abstract:
Disclosed is an optical semiconductor device that provides an optical gain or optical loss depending on application of electric current. The optical semiconductor device comprises: a lower clad layer; an active layer disposed on the lower clad layer, the active layer generating optical gain or optical loss depending on injection of carriers; an upper clad layer disposed on the active layer, the upper clad layer serving to trap light in the active layer in cooperation with the lower clad layer; and a temperature control part for controlling the temperature distribution of the active layer along the light propagation axis in such a manner that temperature of the active layer varies depending on positions in the active layer.

Description:
CLAIM OF PRIORITY 
     This application claims priority to an application entitled “BROAD-BAND OPTICAL SEMICONDUCTOR DEVICE,” filed in the Korean Intellectual Property Office on Feb. 3, 2005 and assigned Serial No. 2005-10094, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a broad-band optical semiconductor device containing an active layer. In particular, the present invention relates to a broad-band optical semiconductor device which has an extended gain band. 
     2. Description of the Related Art 
     Generally, a wavelength division multiplexing passive optical network (WDM-PON) requires a light source that emits light over a broad range of wavelengths. To meet this requirement, a WDM-PON is provided with broad-band optical semiconductor devices, such as semiconductor optical amplifiers (SOA), semiconductor lasers, or other devices that can operate over an extended gain band. 
       FIG. 1  is a sectional view showing a conventional broad-band optical semiconductor device. The broad-band optical semiconductor device  100  shown in  FIG. 1  includes a lower electrode  110 , a lower clad layer  120 , an active layer  130 , an upper clad layer  140 , and an upper electrode  150 . 
     The lower clad layer  120  is an n-type compound semiconductor. The lower electrode  110  placed at the bottom of the lower clad layer  120  is connected to the ground and formed from a conductive metal. The active layer  130  placed on the top of the lower clad layer  120  is comprised of the 1st through the nth quantum wells  130 - 1 ˜ 130 -N, having a thickness increasing gradually from the 1st quantum well to the nth quantum well. The upper clad layer  140  placed on the top of the active layer  130  is a p-type compound semiconductor. The upper electrode  150 , where an electric current is applied, is placed on the top of the upper clad layer  140  and formed from a conductive metal. 
       FIG. 2  is a graph showing the gain curve as a function of wavelengths of the conventional broad-band optical semiconductor device  100  described above. As shown in  FIG. 2 , the gain v. wavelength curves  210 - 1 ˜ 210 -N of the 1st through the nth quantum wells  130 - 1 ˜ 130 -N overlap with one another resulting in a broad-band gain curve  220 . 
     As noted above, the conventional broad-band optical semiconductor device  100  is provided with an active layer  130  that includes multiple quantum wells having thickness that increases gradually to provide an extended gain band. However, one of the problems faced by the broad-band optical semiconductor device  100  described above is that it is impossible to obtain uniform gain. In other words, the broad-band optical semiconductor device  100  shows decrease in efficiency of the gain because carriers (electrons and holes) are injected non-uniformly, depending on the position of a quantum well. 
     Another problem attributable to the conventional broad-band optical semiconductor device  100  is that use of the active layers containing the quantum wells results in a great change in the gain with respect to the direction of polarization of the incident light. 
       FIG. 3  is a sectional view of another conventional broad-band optical semiconductor device. The broad-band optical semiconductor device  300  includes a lower electrode  310 , a lower clad layer  320 , an active layer  330 , an upper clad layer  340 , a first upper electrode  350 , and a second upper electrode  355 . 
     The lower clad layer  320  is an n-type compound semiconductor. The lower electrode  310  comprising a conductive metal is placed at the bottom of the lower clad layer  320  and connected to the ground. The active layer  330  is placed on the top of the lower clad layer  320  and generates optical gain depending on the injection of carriers. The upper clad layer  340 , a p-type compound semiconductor, is placed on the top of the active layer  330 . The first and the second upper electrodes  350  and  355 , which are conductive metals and to which electric current is applied, are placed on the top of the upper clad layer  340 . The active layer  330  is divided into a first region  370 , to which carriers  360  are injected by way of the first upper electrode  350 , and a second region  375 , to which carriers  365  are injected by way of the second upper electrode  355 . 
     The first level of current I 1  and second level of current I 2 , different from each other, are applied to the first upper electrode  350  and the second upper electrode  355 , respectively. Therefore, the first region  370  and the second region  375  have a different number of carriers, and the first region  370  and the second region  375  have different gain bands. 
       FIG. 4  is a graph showing the gain curve depending on wavelengths in the above-described broad-band optical semiconductor device  300 . As shown in  FIG. 4 , the gain curve  410  of the first region  370  and the gain curve  420  of the second region  375  overlap with each other, resulting in a broad-band gain curve  430 . 
     As described above, the broad-band optical semiconductor device  300  provides an extended gain band compared to other conventional optical semiconductor devices by varying the carrier number distribution along the light propagation axis. However, the broad-band optical semiconductor  300  is incapable of ensuring sufficient gain band, as change in the carrier number results in a change in the gain band. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art and provide additional advantage by providing a broad-band optical semiconductor device which provides an extended gain band simultaneously with a high and uniform gain. 
     In accordance to one aspect of the present invention an optical semiconductor device that shows optical gain or optical loss depending on application of electric current is provided. The optical semiconductor device comprises: a lower clad layer; an active layer disposed on the lower clad layer, the active layer generating optical gain or optical loss depending on injection of carriers; an upper clad layer disposed on the active layer, the upper clad layer serving to trap light in the active layer in cooperation with the lower clad layer; and a temperature control part for controlling temperature distribution of the active layer along the light propagation axis in such a manner that temperature of the active layer varies depending on positions in the active layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above features and advantages of the present invention will be more apparent from the detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a sectional view showing a conventional broad-band optical semiconductor device; 
         FIG. 2  is a graph showing the gain curve depending on wavelengths in the broad-band optical semiconductor device as shown in  FIG. 1 ; 
         FIG. 3  is a sectional view showing another conventional broad-band optical semiconductor device; 
         FIG. 4  is a graph showing the gain curve depending on wavelengths in the broad-band optical semiconductor device as shown in  FIG. 3 ; 
         FIG. 5  is a sectional view showing a broad-band optical semiconductor device according to one aspect of the present invention; 
         FIG. 6   a  is a graph showing the temperature distribution of the active layer, depending on the positions in the active layer along the light propagation axis; 
         FIG. 6   b  is a graph showing the gain wavelength depending on the positions in the active layer along the light propagation axis; 
         FIG. 7  is a sectional view showing a broad-band optical semiconductor device according to another aspect of the present invention; and 
         FIG. 8  is a sectional view showing a broad-band optical semiconductor device according to another aspect of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, several aspects of the present invention will be described with reference to the accompanying drawings. For purposes of clarity and simplicity, a detailed description of known functions and configurations incorporated herein are omitted, as they may make the subject matter of the present invention unclear. 
     The present invention provides optical semiconductor devices containing an active layer that provide a broad-band gain. The present invention utilizes the principle that gain of an optical semiconductor device varies significantly as the band gap of the active layer contained in the optical semiconductor device changes. Moreover, the present invention utilizes the principle that the band gap is affected greatly by change in the temperature, and the fact that the band gap may also be affected by the composition of the active layer and the thickness of a quantum well. 
       FIG. 5  is a sectional view showing a broad-band optical semiconductor device according to one aspect of the present invention. The broad-band optical semiconductor device  500  includes a lower clad  530 , a lower electrode  520 , an active layer  540 , an upper clad  550 , an upper electrode  560 , and temperature control parts  510 ,  570  and  575 . 
     The temperature control parts  510 ,  570  and  575  control the temperature distribution of the active layer  540  along the light propagation axis (the light propagation direction or the longitudinal direction of the active layer  540 ). In addition, the temperature control parts  510 ,  570  and  575  comprise a heat sink  510 , a first electric resistance layer  570 , and a second electric resistance layer  575 . 
     The heat sink  510  discharges the heat transferred from the lower electrode  520  to the exterior. Herein, the heat conductivity at the interface of the heat sink  510  and the lower electrode  520  is maintained at a constant value. In other words, the heat quantity distribution  580  transferred from the lower electrode  520  to the heat sink  510  has a uniform value along the light propagation axis. 
     The lower electrode  520 , a metal, is disposed on the heat sink  510  and connected to ground. 
     The lower clad layer  530 , an n-type semiconductor, is disposed on the lower electrode  520 . 
     The active layer  540  is disposed on the lower clad layer  530  and generates optical gain depending on the injection of carriers. The active layer  540  has a non-uniform temperature distribution along the light propagation axis due to the temperature control parts  510 ,  570  and  575 . 
       FIG. 6A  shows the temperature distribution along the light propagation axis within the active layer  540 .  FIG. 6B  shows the gain wavelength along the light propagation axis within the active layer  540 . As shown in  FIGS. 6A and 6B , the temperature distribution of the active layer is such that the temperature increases gradually from the first end to the second end (i.e. along the light propagation axis) of the active layer  540 . Accordingly, the gain wavelength of the active layer  540  shifts from a shorter wavelength to a longer wavelength from the first end to the second end (i.e. along the light propagation axis). In  FIG. 6A , the graph showing the temperature distribution depending on positions of the active layer  540  is obtained by linear approximation. 
     The upper clad layer  550 , a p-type semiconductor, is disposed on the active layer  540  and traps light within the active layer  540  in cooperation with the lower clad layer  530 . 
     The upper electrode  560 , a conductive metal and to which electric current is applied via the first and the second electric resistance layers  570  and  575 , is disposed on the upper clad layer  550 . 
     The first and the second electric resistance layers  570  and  575 , disposed on the upper electrode  560  and apart from one another, serve as resistance heaters. In addition, the first level of electric current I 1  and a second level of electric current I 2 , the levels that differ from one another, are applied to the first and the second electric resistance layers  570  and  575 , respectively. 
     Herein, the first and the second electric resistance layers  570  and  575  have the same electric resistance value, and the second electric current I 2  is higher than the first electric current I 1 . In principle, Joule heat is proportional to the square of an applied electric current under a constant electric resistance value. Therefore, the heat quantity of the second electric resistance layer  575  is greater than that of the first electric resistance layer  570 . 
     Further, heat emitted from the first and the second electric resistance layers  570  and  575  contributes to non-uniform temperature distribution of the active layer  540 , as shown in  FIG. 6A . 
       FIG. 7  is a sectional view showing a broad-band optical semiconductor device according to another aspect of the present invention. The broad-band optical semiconductor device  600  includes a lower clad  630 , a lower electrode  620 , an active layer  640 , an upper clad  650 , an upper electrode  670 , and temperature control parts  610  and  660 . The temperature control parts  610  and  660 , comprised of a heat sink  610  and an electric resistance layer  660 , control the temperature distribution of the active layer  640  along the light propagation axis. 
     The heat sink  610  discharges the heat transferred from the lower electrode  620  to the exterior. Herein, the heat conductivity at the interface of the heat sink  610  and the lower electrode  620  remains constant. In particular, the heat quantity distribution  680  transferred from the lower electrode  620  to the heat sink  610  has a uniform value along the light propagation axis. 
     The lower electrode  620 , a metal, is disposed on the heat sink  610  and connected to the ground. 
     The lower clad layer  630 , an n-type semiconductor, is disposed on the lower electrode  620 . 
     The active layer  640  is disposed on the lower clad layer  630 , and generates optical gain depending on the injection of carriers. The active layer  640  has a non-uniform temperature distribution along the light propagation axis due to the temperature control parts  610  and  660 . In particular, the temperature distribution of the active layer  640  is such that the temperature increases gradually from the first end to the second end (i.e. along the light propagation axis) of the active layer  640 . Accordingly, the gain wavelength of the active layer  640  shifts from a shorter wavelength to a longer wavelength from the first end to the second end (i.e. along the light propagation axis). 
     The upper clad layer  650 , a p-type semiconductor, is disposed on the active layer  640  and traps light within the active layer  640  in cooperation with the lower clad layer  630 . 
     The electric resistance layer  660  is disposed on the upper electrode  650 , and the electric resistance distribution is such that the electric resistance increases gradually from the first end to the second end (i.e. along the light propagation axis) of the electric resistance layer  660 . In principle, Joule heat is in inverse proportion to the electric resistance value under a constant voltage. Therefore, the heat quantity of the first end is greater than that of the second end. Further, the heat emitted from the electric resistance layer  660  contributes to the non-uniform temperature distribution of the active layer  640  along the light propagation axis. 
     The upper electrode  670 , a conductive metal and to which a voltage V is applied, is disposed on the electric resistance layer  660 . 
       FIG. 8  is a sectional view showing a broad-band optical semiconductor device according to another aspect of the present invention. The broad-band optical semiconductor device  700  includes a lower clad  740 , a lower electrode  730 , an active layer  750 , an upper clad  760 , an upper electrode  770 , and temperature control parts  710  and  720 . 
     The temperature control parts  710  and  720 , comprised of a heat sink  710  and a thermal resistance layer  720 , controls the temperature distribution of the active layer  750  along the light propagation axis. 
     The heat sink  710  functions to discharge the heat, transferred from the lower electrode  730  via the thermal resistance layer  720 , to the exterior. Herein, the heat quantity distribution transferred to the heat sink  710  along the light propagation axis via the thermal resistance layer  720  is not uniform. In particular, the heat quantity decreases gradually from the first end to the second end (i.e. along the light propagation axis) of the heat sink. Accordingly, the non-uniform distribution of the heat quantity contributes to a non-uniform temperature distribution of the active layer  750  along the light propagation axis. 
     The thermal resistance layer  720  is disposed on the heat sink  710  and has a thermal resistance distribution where the resistance increases gradually from the first end to the second end (i.e. along the light propagation axis). In particular, the heat passing through the first end is greater than the heat passing through the second end. Such non-uniform thermal resistance distribution of the thermal resistance layer  720  causes the non-uniform distribution of the heat transferred to the heat sink  710 , resulting in the non-uniform temperature distribution of the active layer  750  along the light propagation axis. 
     The lower electrode  730 , a conductive metal, is disposed on the thermal resistance layer  720  and connected to the ground. 
     The lower clad layer  740 , an n-type semiconductor, is disposed on the lower electrode  730 . 
     The active layer  750  disposed on the lower clad layer  740  generates optical gain depending on the injection of carriers. The active layer  750  has a non-uniform temperature distribution along the light propagation axis due to the temperature control parts  710  and  720 . In particular, the active layer  750  has a temperature distribution where the temperature increases gradually from the first end to the second end (i.e. along the light propagation axis) of the active layer. Accordingly, the gain wavelength of the active layer  750  shifts from a shorter wavelength to a longer wavelength from the first end to the second end (i.e. along the light propagation axis). 
     The upper clad layer  760 , a p-type compound semiconductor, is disposed on the active layer  750  and traps light within the active layer  750  in cooperation with the lower clad layer  740 . 
     The upper electrode  770 , a conductive metal and to which electric current I is applied, is disposed on the upper clad layer  760 . 
     As noted above, the present invention utilizes the principle that change in temperature of the active layer results in change in the band gap, particularly the principle that increase in the temperature of the active layer causes decrease in the band gap followed by a shift in the gain wavelength toward a longer wavelength. 
     The present invention may also be applied to an optical semiconductor device that includes an active layer and that provides optical loss. In this case, the principle regarding change in band gaps due to variations in the temperature of an active layer may be applied in the same manner. In particular, an increase in the temperature of the active layer results in a decrease in the band gap, followed by a shift in the loss wavelength toward a longer wavelength. 
     Therefore, the present invention may be applied not only to an optical semiconductor device requiring a broad-band gain but also to an optical semiconductor device requiring a broad-band loss. 
     As noted above, the broad-band optical semiconductor device according to the present invention, which includes a temperature control part and controls the temperature distribution of an active layer along the light propagation axis within the active layer, can provide an extended gain band simultaneously with a high and uniform gain. 
     In addition, the broad-band optical semiconductor device according to the present invention may be applied not only to an optical semiconductor device requiring a broad-band gain but also to an optical semiconductor device requiring a broad-band loss. 
     While the invention has been shown and described with reference to certain aspects 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.