Abstract:
A reflective semiconductor optical amplifier light source is disclosed. The reflective semiconductor optical amplifier light source includes a transmissive type semiconductor optical amplifier for creating and amplifying spontaneous emission light, a reflector for reflecting amplified spontaneous emission light outputted from the semiconductor optical amplifier such that amplified spontaneous emission light is reflected back into the semiconductor optical amplifier, and a bandpass filter having a predetermined wavelength band width for limiting wavelength bands of the amplified spontaneous emission light capable of passing through the bandpass filter. In one aspect of the invention, the bandpass filter is interposed between the semiconductor optical amplifier and the reflector. In another aspect, a polarization filter is imposed to limit the reflected emission light to a predetermined polarization.

Description:
CLAIM OF PRIORITY  
       [0001]     This application claims priority, pursuant to 35 U.S.C. §119, to that patent application entitled “Reflective Semiconductor Optical Amplifier Light Source,” filed in the Korean Intellectual Property Office on Nov. 18, 2003 and assigned Serial No. 2003-81474, the contents of which are hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a light source, and more particularly to a broadband light source for outputting Amplified Spontaneous Emission (ASE) light.  
         [0004]     2. Description of the Related Art  
         [0005]     Erbium-doped fiber amplifier light sources, super luminescent diodes, and reflective semiconductor optical amplifier light sources have been suggested as broadband light sources for outputting incoherent light through wide wavelength bands. Although the erbium-doped fiber amplifier light sources produce stable high-power polarization-insensitive light, the produced wavelength band is limited so that the erbium-doped fiber amplifier light sources are unsuitable for the broadband light sources. In addition, the size of the erbium-doped fiber amplifier light sources is larger than the size of semiconductor devices and it is further difficult to reduce the manufacturing cost even if the erbium-doped fiber amplifier light sources are mass-produced. Super Luminescent Diodes (SLDs), on the other hand, have a large optical bandwidth and can be manufactured at a low cost. However, SLDs have a problem in that the output power is limited. Reflective Semiconductor Optical Amplifier (RSOA) light sources output spontaneous emission light that has been amplified by a highly reflective coating layer deposited on a first terminal or end of a Semiconductor Optical Amplifier (SOA). Thus, it is possible to achieve high-power broadband light sources at a low cost with RSOAs. However, if reflectivity of a second terminal of the semiconductor optical amplifier is not extremely low, a Fabry-Perot resonator is formed between the first and second terminals so that the spectrum of output light varies based on the wavelength of the output light. This variation is referred to as a “gain ripple”. Thus, it is difficult to obtain incoherent light having a uniform spectrum.  
         [0006]     In order to solve the gain ripple problem of the reflective semiconductor optical amplifier light sources, another broadband light source has been suggested, in which an external broadband reflector is connected to the SOA and a high reflective coating layer is not deposited on the first terminal of the semiconductor optical amplifier.  
         [0007]      FIG. 1  is a schematic view showing a structure of a conventional reflective semiconductor optical amplifier light source  100 . The conventional reflective semiconductor optical amplifier light source  100  includes a semiconductor optical amplifier (SOA)  110  and an external reflector (R)  120 , which are connected to each other through an optical waveguide  130 .  
         [0008]     The semiconductor optical amplifier  110  includes a gain medium  112  and first and second anti-reflective layers  114  and  116  coated on both side ends of the gain medium  112 . Incoherent amplified spontaneous emission light  140  is outputted through the first and second anti-reflective layers  114  and  116 .  
         [0009]     The external reflector  120  is optically connected to the first anti-reflective layer  114  through optical waveguide  130  and is used to reflect back the incoherent amplified spontaneous emission light  140  outputted through the first anti-reflective layer  114 , such that the incoherent amplified spontaneous emission light  140  emitted is returned into the semiconductor optical amplifier  110 . The distance between the semiconductor optical amplifier  110  and the external reflector  120  is preset or predetermined such that the incoherent amplified spontaneous emission light  140  returns to the semiconductor optical amplifier  110  by traveling over a length referred to as the coherence length. Coherence length is well-known in the art and need not be explained in detail herein.  
         [0010]     The structure shown in  FIG. 1  has an advantage in that a reflectivity condition needed of the first and second anti-reflective layers  114  and  116  to achieve a small gain ripple is attenuated. Thus, as a small gain ripple is generated even if reflectivity of the first and second anti-reflective layers  114  and  116  is not extremely low, an anti-reflection coating for the first and second anti-reflective layers  114  and  116  is easily achieved.  
         [0011]     However, the above-mentioned reflective semiconductor optical amplifier light source  100  has an output power level lower than that of the erbium-doped fiber amplifier light source, so that there is a need for increasing the output power of the reflective semiconductor optical amplifier light sources and to provide a reflective semiconductor optical amplifier light source having a higher output power.  
       SUMMARY OF THE INVENTION  
       [0012]     A high-output RSOA light source is disclosed. The RSOA comprises a transmissive type semiconductor optical amplifier for creating and amplifying spontaneous emission light, a reflector for reflecting amplified spontaneous emission light outputted from the semiconductor optical amplifier such that amplified spontaneous emission light is reflected back into the semiconductor optical amplifier and a bandpass filter having a predetermined wavelength band width for limiting wavelength bands of the amplified spontaneous emission light capable of passing through the bandpass filter. In one aspect of the invention, the bandpass filter is interposed between the semiconductor optical amplifier and the reflector. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     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:  
         [0014]      FIG. 1  is a schematic view showing a structure of a conventional reflective semiconductor optical amplifier light source;  
         [0015]      FIG. 2  is a schematic view for explaining the concept of a reflective semiconductor optical amplifier light source according to the principles of the present invention;  
         [0016]      FIG. 3  is a graph showing various output power characteristics of a reflective semiconductor optical amplifier light source as a function of a position of a bandpass filter as referred to in  FIG. 2 ;  
         [0017]      FIG. 4  is a schematic view showing a structure of a reflective semiconductor optical amplifier light source according to a first embodiment of the present invention;  
         [0018]      FIG. 5  is a schematic view showing a structure of a reflective semiconductor optical amplifier light source according to a second embodiment of the present invention; and  
         [0019]      FIG. 6  is a schematic view showing a structure of a reflective semiconductor optical amplifier light source according to a third embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the following description of the present invention, the same reference numerals are used to designate the same or similar components. Furthermore, for the purpose of clarity and simplicity, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention unclear.  
         [0021]      FIG. 2  is a schematic view for explaining a concept of a reflective semiconductor optical amplifier light source  200  according to the principles of the present invention, and  FIG. 3  represents a graph illustrating the output power characteristics of the reflective semiconductor optical amplifier light source  200  as a function of a position of a bandpass filter (BPF) included therein.  
         [0022]     The reflective semiconductor optical amplifier light source  200 , shown in  FIG. 2 , includes a transmissive type semiconductor optical amplifier (SOA)  210 , a reflector (R)  220 , and an optical isolator (ISO)  230 . The optical isolator  230  is located at an output terminal of the reflective semiconductor optical amplifier light source  200  in order to reduce a gain ripple phenomenon caused by light incident into the reflective semiconductor optical amplifier light source  200  from an exterior source.  
         [0023]      FIG. 3  illustrates a first output power spectrum  310  achieved when a bandpass filter is not added to the reflective semiconductor optical amplifier light source  200  (first case), a second output power spectrum  320  achieved when the bandpass filter is imposed at a position shown and referred to as “A” in  FIG. 2 , (second case), a third output power spectrum  330  achieved when the bandpass filter is imposed at a position shown and referred to as “B” in  FIG. 2  (third case), and a fourth output power spectrum  340  achieved when the bandpass filter is imposed at a position shown and referred to as “C” in  FIG. 2  (fourth case).  
         [0024]     In the first case, the first output power spectrum  310  is determined based on a gain curve of the semiconductor optical amplifier  210  and has an uneven distribution through a wide wavelength bands In the second case, second output power spectrum  320  is distributed within a pass band of the bandpass filter and has a output power magnitude larger than a output power magnitude of the first output power spectrum  310 . In the third case, the third output power spectrum  330  has a reduced output as compared with the output power magnitude of the first output power spectrum  310  due to an insertion loss of the bandpass filter. And, in the fourth case, the fourth output power spectrum  340  has a output power magnitude similar to the magnitude of the first output power spectrum  310  within the pass bandwidth of the bandpass filter. This is because a reflection is created by the bandpass filter so that output power magnitude is increased within the pass band width of the bandpass filter.  
         [0025]     The increased output power magnitude is saturated corresponding to the magnitude of the first output power spectrum  310  due to output saturation of the semiconductor optical amplifier  210 . As illustrated the position of the bandpass filter, shown and referred to as position A (second case) provides the greatest increase in the output. This is because a spectrum of amplified spontaneous emission light  250 , which is reflected from the reflector  220  back to the semiconductor optical amplifier  210  is limited within the pass bandwidth of the bandpass filter, so the output power within the pass bandwidth of the bandpass filter is selectively increased. That is, since the output power out of the pass band width of the bandpass filter is reduced, the output power in the pass band width of the bandpass filter is relatively increased. In addition, a distribution of the spectrum in the pass band width of the bandpass filter depends on a spectrum characteristic of the bandpass filter as well as the gain curve of the semiconductor optical amplifier  210 , so it is possible to uniformly distribute the second output power spectrum  320  by adjusting the spectrum characteristic of the bandpass filter. Embodiments of the present invention are now described with regard to the placement of an appropriate bandpass filter in the optical path between the SOA  210  and reflector  220 , at the position referred to as position “A”.  
         [0026]      FIG. 4  is a schematic view showing a structure of a reflective semiconductor optical amplifier light source  400  according to a first embodiment of the present invention. The reflective semiconductor optical amplifier light source  400  includes a semiconductor optical amplifier  410 , a bandpass filter  420 , a reflector  430 , and an optical isolator  440 .  
         [0027]     The semiconductor optical amplifier  410  has a gain medium  412  and first and second anti-reflective layers  414  and  416  coated on both side ends of the gain medium  412 , as previously described. Incoherent amplified spontaneous emission light  460  is outputted through the first and second anti-reflective layers  414  and  416 . The reflector  430  is optically connected to the first anti-reflective layer  414 , via optical medium  150 , which reflects the incoherent amplified spontaneous emission light  460 , back into the semiconductor optical amplifier  410 . As discussed previously, the distance between the semiconductor optical amplifier  410  and the reflector  430  is preset to be a coherence length.  
         [0028]     The bandpass filter  420  is imposed in the optical path between the semiconductor optical amplifier  410  and the reflector  430  and has a predetermined pass bandwidth for limiting wavelength bands of the incoherent amplified spontaneous emission light  460 . A distribution of the output power spectrum of the reflective semiconductor optical amplifier light source  400  may be adjusted by controlling a spectrum characteristic of the bandpass filter  420 .  
         [0029]     The optical isolator  440  is located at an output terminal of the reflective semiconductor optical amplifier light source  400  in order to prevent the gain ripple phenomenon caused by light incident into the reflective semiconductor optical amplifier light source  400  from an exterior source.  
         [0030]      FIG. 5  is a schematic view showing a structure of a reflective semiconductor optical amplifier light source  500  according to a second embodiment of the present invention. The reflective semiconductor optical amplifier light source  500  includes a semiconductor optical amplifier  510  having a gain medium  512  and first and second anti-reflective layers  514  and  516  coated at both side ends of the gain medium  512 , a reflector  540 , a bandpass filter  520 , a 45° polarization rotator (λ/8)  530 , and an optical isolator  550 . The reflective semiconductor optical amplifier light source  500  is similar to the reflective semiconductor optical amplifier light source  400  shown in  FIG. 4 , with the inclusion of the 45° polarization rotator  530  located between the reflector  540  and the bandpass filter  520 . Hence, only the 45° polarization rotator  530  needed be explained in detail to understand the embodiment shown in  FIG. 5 .  
         [0031]     In this exemplary embodiment, 45° polarization rotator  530  is provided to improve the polarization characteristic of the reflective semiconductor optical amplifier light source  500 , such that the semiconductor optical amplifier  510  gain varies based on the polarization state of the reflected light. In addition, the reflective semiconductor optical amplifier light source  500  outputs amplified spontaneous emission light  570  having a random polarization state due to the 45° polarization rotator  530 . The amplified spontaneous emission light  570  outputted from the bandpass filter  520  passes through the 45° polarization rotator  530  twice by means of the reflector  540  so that a polarization direction of the amplified spontaneous emission light  570  is rotated at a right angle (90 degrees) from an initial polarization direction. In this 90 degree rotated state, the amplified spontaneous emission light  570  is inputted into the semiconductor optical amplifier  510 .  
         [0032]      FIG. 6  is a schematic view showing a structure of a reflective semiconductor optical amplifier light source  600  according to a third embodiment of the present invention. The reflective semiconductor optical amplifier light source  600  includes a semiconductor optical amplifier  610  having a gain medium  612  and first and second anti-reflective layers  614  and  616  coated at both side ends of the gain medium  612 , a reflector  640 , a bandpass filter  630 , a 45° polarization rotator (λ/8)  620 , and an optical isolator  650 . The reflective semiconductor optical amplifier light source  600  is similar to the reflective semiconductor optical amplifier light source  500  shown in  FIG. 5 , except that the 45° polarization rotator  620  is located between the semiconductor optical amplifier  610  and the bandpass filter  630 . Again, only the operation of the 45° polarization rotator  620  in this position need be discussed to understand the operation of the embodiment shown in  FIG. 6 .  
         [0033]     In this exemplary embodiment, amplified spontaneous emission light  670  outputted from the semiconductor optical amplifier  610  passes through the 45° polarization rotator  620  twice by means of the reflector  640  so that a polarization direction of the amplified spontaneous emission light  670  is rotated at a right angle from an initial polarization direction thereof. In this rotated state, the amplified spontaneous emission light  670  is reflected back into the semiconductor optical amplifier  610 .  
         [0034]     While the present 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.