Patent Publication Number: US-2002003655-A1

Title: L-band optical fiber amplifier

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to an optical fiber amplifier, specifically, to an L-band optical fiber amplifier which increases amplification gain of L-band optical signals by using an amplified spontaneous emission ASE backwardly output from an erbium doped fiber EDF as a pumping light in a manner that the backward ASE is re-injected into EDF through a reflecting means.  
       [0003] 2. Description of the Related Art  
       [0004] In general, optical communication systems apply optical signals having wavelength of 1530 nm to 1560 nm, namely, C-band. However, there have been studied to use optical signals having wavelengths of 1570 nm to 1610 nm, namely, L-band, according to recent demands for transmitting information of huge volume and for accommodating advanced communication techniques.  
       [0005] Meanwhile, it is necessary to provide an optical fiber amplifier compensating for the gain drop of optical signals, caused while transmitting through an optical cable in the optical communication system. The optical fiber amplifier generally applies an erbium doped fiber EDF which doped with an erbium ion, the rare earth ion. The optical fiber amplifier applying EDF has a structure that optical signals and a pumping light are supplied to EDF. This fundamental structure is adapted both to the optical fiber amplifiers for C-band and for L-band.  
       [0006] The methods for constructing the L-band optical fiber amplifier are well known such that a method for using a pumping light of 1550 nm, a method for setting length of EDF to be long, and the like. Here, the method for using a pumping light of 1550 nm needs a laser diode for generating the pumping light. The laser diode has some drawbacks that the manufacturing process is very complicated, and therefore its cost is increased too much. Accordingly, the methods for extending the length of EDF have been widely considered.  
       [0007]FIG. 1 shows a structure of a conventional L-band optical fiber amplifier. A pumping laser diode  2  generates a pumping light (P) of 980 nm for example, the same way with that of C-band optical fiber amplifier. The pumping light (P) output from the laser diode  2  is supplied to a wavelength division multiplexer WDM  3 . An input light (S) is applied to WDM  3  through an isolator  1  as an optical signal. Then, WDM  3  forwards the input signal (S) and the pumping light (P) to EDF  4 , which has a length of 70 m in general adapted to the L-band optical fiber amplifier. The optical signal (S) amplified by EDF  4  is output through an isolator  5 . When the pumping light of 980 nm is input to EDF  4 , the rare earth ion, i.e., erbium ion, doped on EDF is excited to produce amplified spontaneous emission ASE having a bandwidth of 1530 nm to 1560 nm. Here, since the length of EDF  4  is set to be long, the ASE forwarding through EDF  4  can be used as the pumping light repeatedly, thus amplifying the optical signals of 1570 nm to 1610 nm substantially.  
       [0008] As shown in FIG. 2, the amplification gain according to the above structure becomes flat relatively in the bandwidth of 1570 nm to 1610 nm.  
       [0009] However, since the pumping light having a bandwidth of 1530 to 1560 nm is generated through EDF in the above structure, the length of EDF gets longer, thus increasing the manufacturing cost of the L-band optical fiber amplifier.  
       BRIEF SUMMARY OF THE INVENTION  
       [0010] Accordingly, the present invention is directed to an L-band optical fiber amplifier that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.  
       [0011] An object of the present invention is to provide an L-band optical fiber amplifier which can amplify an optical signal having a long-wavelength bandwidth, not providing an additional expensive pumping laser diode.  
       [0012] Another object of the present invention is to provide an L-band optical fiber amplifier which can reduce length of EDF and pumping power applied therein as much as compared with that of conventional L-band optical fiber amplifier.  
       [0013] To accomplish an object in accordance with a first aspect of the present invention, there is provided an L-band optical fiber amplifier comprising: an erbium doped fiber EDF, provided on an optical path, for amplifying optical signals transmitted through the optical path; a pumping light generating means generating a predetermined pumping light; a coupling means coupling the pumping light to the optical path; and a reflecting means, established in front of EDF, for re-injecting a backward amplified spontaneous emission (ASE), output backwardly from EDF, to EDF.  
       [0014] To accomplish another object in accordance with the first aspect of the invention, the reflecting means includes: a coupling means for guiding the backward ASE from the optical path to a reflecting mirror and for coupling an output light reflected from the reflecting mirror to the optical path; and a reflecting mirror reflecting the ASE input from the coupling means.  
       [0015] To accomplish another object in accordance with a second aspect of the present invention, there is provided an L-band optical fiber amplifier comprising: an erbium doped fiber, provided on an optical path, for amplifying optical signals transmitted through the optical path; a pumping light generating means generating a predetermined pumping light; a coupling means coupling the pumping light to the optical path; and a reflecting means, established in front of the erbium doped fiber, for reflecting and re-injecting a predetermined wavelength light among the backward ASE, output backwardly from the erbium doped fiber, to the erbium doped fiber.  
       [0016] Furthermore, to accomplish another object in accordance with the second aspect of the invention, the predetermined wavelength light is set selectively within a range of 1530 nm to 1560 nm, preferably, to a wavelength around 1550 nm.  
       [0017] Moreover, to accomplish another object in accordance with the second aspect of the invention, the reflecting means includes: a first coupling means for guiding the backward ASE from the optical path to a second coupling means, and for coupling an output light from the second coupling means to the optical path; a second coupling means for transmitting the backward ASE from the first coupling means to a tunable filter, and for coupling an output light from the tunable filter to the first coupling means; and a tunable filter filtering a predetermined wavelength light among the backward ASE input from the second coupling means.  
       [0018] Besides, to accomplish another object in accordance with the second aspect of the invention, the reflecting means includes: a coupling means for providing the backward ASE from the optical path to a fiber bragg grating, and for coupling an output light from the fiber bragg grating to the optical path; a fiber bragg grating reflecting a predetermined wavelength light among the backward ASE input from the coupling means.  
       [0019] In addition, to accomplish another object in accordance with the second aspect of the invention, the reflecting means further includes a fiber bragg grating, provided on the optical path, for reflecting a predetermined light.  
       [0020] According to the present invention, the backward ASE having a bandwidth of 1530 nm to 1560 nm output backwardly from EDF, preferably, only the backward ASE of a wavelength around 1550 nm, is reflected by the reflecting means and re-injected to EDF, thus acting as the pumping light. Therefore, it is possible to increasing the amplification gain for L-band optical signal remarkably, without any additional expensive laser diode for supplying the pumping light.  
       [0021] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0022] The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention:  
     [0023] In the drawings:  
     [0024]FIG. 1 shows a structure of a conventional L-band optical fiber amplifier;  
     [0025]FIG. 2 is a graph showing an amplification characteristic of the L-band optical fiber amplifier in FIG. 1;  
     [0026]FIG. 3 illustrates a structure of an L-band optical fiber amplifier in accordance with a first embodiment of the present invention;  
     [0027]FIG. 4 depicts another structure of an L-band optical fiber amplifier in accordance with a second embodiment of the present invention;  
     [0028]FIG. 5 shows another structure of the L-band optical fiber amplifier in FIG. 4;  
     [0029]FIG. 6 denotes another structure of an L-band optical fiber amplifier in accordance with a third embodiment of the present invention;  
     [0030]FIG. 7 shows a filtering characteristic of a fiber bragg grating depicted in FIG. 6;  
     [0031]FIG. 8 illustrates another structure of an L-band optical fiber amplifier in accordance with a fourth embodiment of the present invention; and  
     [0032] FIGS.  9 (A) and  9 (B) are graphs demonstrating test results of amplification gain according to total powers and wavelengths of input optical signals with the preferred structure in FIG. 8.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0033] Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.  
     [0034] Now referring to FIG. 3, illustrating a structure of an L-band optical fiber amplifier in accordance with a first embodiment of the invention, an optical signal (S) is input to a second WDM  33  by way of a first isolator  31  and a first WDM  32 . A pumping light having a wavelength of 980 nm, for example, output from a pumping laser diode  34  is supplied to the second WDM  33 . The second WDM  33  forwards the input light (S) and the pumping light (P) to an L-band EDF  35 . An output light of EDF  35  travels through a second isolator  36 . Here, the present structure amplifies the input light (S) in the same manner with that of FIG. 1 that the pumping light (P) and the input signal (S) are transmitted to EDF  35  through the second WDM  33 . However, according to the first embodiment of the invention, the length of EDF  35  is set to approximately 50 m to 60 m, shorter than that of FIG. 1, approximately 70 m.  
     [0035] Meanwhile, a reflecting mirror  37  is provided in front of the first WDM  32 . The reflecting mirror  37  reflects and re-injects the light, output backwardly from EDF  35  and input through the second and first WDMs  33  and  32  in sequence, to the first WDM  32 .  
     [0036] As is generally known, when the pumping light output from the laser diode  34  is supplied to EDF through the second WDM  33 , the pumping light stimulates the erbium ion of EDF to produce the amplified spontaneous emission ASE having a bandwidth of 1530 nm to 1560 nm. Here, the ASE includes a forward ASE travels in the same direction with the optical signal and a backward ASE transmitted backwardly in the opposite direction to the optical signal. The forward ASE is transmitted through EDF  35  and acts as the pumping light, thus amplifying the L-band optical signal, the same way with the conventional L-band optical fiber amplifier.  
     [0037] The backward ASE output from EDF  35  is supplied to the first WDM  32  through the second WDM  33 . The first WDM  32  guides the backward ASE to the reflecting mirror  37 . Next, the reflecting mirror  37  reflects the backward ASE to EDF  35  through the second WDM  33 . Consequently, the reflected ASE re-injected to EDF  35  acts as the pumping light to amplify the L-band optical signal. That is, the backward ASE output from EDF  35  is re-injected to EDF  35  through a reflecting means including the first WDM  32  and the reflecting mirror  37 , thus acting as the pumping light.  
     [0038] According to the above structure, since the amplification gain of EDF  35  can be increased by means of the backward ASE, it is possible to reduce the length of EDF  35  as much as compared with the conventional EDF, maintaining the same amplification efficiency.  
     [0039] Meanwhile, the reflecting mirror of the above structure reflects all of the backward ASE output from EDF  35 , which ensures a sufficient supply of pumping light to EDF  35 , thus increasing the amplification gain most effectively. However, the ASE output from EDF  35  may include a light of 1530 nm or less, or a light of 1560 nm or more, other than the light of 1530 nm to 1560 nm. Here, the light of 1560 nm or more corresponds to the region of the optical signal for the L-band optical fiber amplifier. If the ASE of 1560 nm or more is input to EDF  35 , corresponding ASE doesn&#39;t act as the pumping light, but contribute to exhausting the pumping light on the contrary, thus deteriorating the amplification efficiency. Besides, the ASE of 1560 nm or more functions as a noise against a normal optical signal.  
     [0040]FIG. 4 illustrates another structure in accordance with a second embodiment of the invention, the structure being made in consideration of the drawbacks discussed in the above structure. In FIGS. 3 and 4, identical components are indicated by identical reference numerals, and the detailed description will be omitted. With reference to FIG. 4, the reflecting means has a ring-shaped structure comprising a coupler  41 , a third isolator  42 , an attenuator  43  and a tunable filter  44 . That is, the backward ASE transmitted through the first WDM  32  is supplied to the third isolator  42  by means of the coupler  41 . Then, the backward ASE is output from the third isolator  42  is transmitted to the tunable filter  44  through the attenuator  43 . Here, the attenuator  43  attenuates the backward ASE power to obtain a preferable value of the amplification gain. The tunable filter  44  selects a specified wavelength bandwidth of passing light, such as a wavelength bandwidth in the range of 1530 nm to 1560 nm, more desirably, a light having a wavelength around 1550 nm. As the inventor&#39;s test results, the best amplification efficiency was observed when the light around 1550 nm among the backward ASE is supplied to EDF  35  through the tunable filter. Next, the output light of the tunable filter  44  is sent to the first WDM  32  by way of the coupler  41 . Then, the light is re-injected to EDF  35  by the second WDM  33 , the same way with the previous embodiment.  
     [0041] In the above structure, the ASE backwardly output from EDF  35  is transmitted to the coupler  41  by way of the second and first WDM  33  and  32 . Then, the backward ASE input to the coupler  41  is forwarded to the tunable filter  44  by way of the third isolator  42  and the attenuator  43 . The tunable filter  44  filters the backward ASE input from the attenuator  43  to filter the light having a wavelength around 1550 nm. Next, the output light of the tunable filter  44  is re-injected to EDF  35  by way of the coupler  41 , the first WDM  32  and the second WDM  33  in sequence. According to the above preferred embodiment of the invention, it is possible to provide a more stabilized optical fiber amplifier, in which the most desired light is passed by the tunable filter  44  and used as the pumping light.  
     [0042] Now referring to FIG. 5, a circulator  51  is applied instead of the coupler  41  of FIG. 4. The backward ASE transmitted from the first WDM  32  is supplied to the tunable filter through the attenuator  42 . Then, the light having a wavelength around 1550 nm output from the tunable filter  43  is returned to the first WDM  32 . The preferred embodiment depicted in FIG. 5 is operated in the same manner with that of FIG. 4, and the detailed description will be omitted.  
     [0043] With reference to FIG. 6 showing another structure of an L-band optical fiber amplifier in accordance with a third embodiment of the invention, a fiber bragg grating FBG  61  is applied as the reflecting means. The backward ASE transmitted from the first WDM  32  is supplied to FBG  61 . Then, the reflected light from FBG  61  is forwarded to EDF  35  through the first and second WDMs  32  and  33  in sequence, thus acting as pumping light.  
     [0044] Referring to FIG. 7, FBG  61  has a characteristic that the filtering factor deteriorates remarkably in a specified bandwidth, λa to λa′. That is, FBG  61  doesn&#39;t pass a light of specified bandwidth, λa to λa′, but reflects. Here, the reflected wavelength of FBG  61  can be set when manufacturing easily. In the present embodiment, the reflected wavelength is set within the range of 1530 nm to 1560 nm, preferably, to a wavelength around 1550 nm. Moreover, FBG  61  is configured in a manner that its edge is non-reflectively coated so that the passed light of FEG  61  can not be returned. According to the present embodiment, the backward ASE output from EDF  35  is supplied to FBG  61  through the second and first WDMs  33  and  32  in sequence. Then, the light having a wavelength around 1550 nm reflected from FBG  61  is re-injected to EDF  35  by way of the first and second WDMs  32  and  33  in sequence. The reflected light from FBG  61  to EDF  35  acts as the pumping light, thus increasing the amplification efficiency of EDF  35  for the L-band signal.  
     [0045] With reference to FIG. 8, showing another structure of an L-band optical fiber amplifier in accordance with a fourth embodiment of the invention, FBG  81  is provided between the first isolator and WDM  33 . Here, FBG  81  is set to intercept a wavelength bandwidth in the range of 1530 nm to 1560 nm, more desirably, a wavelength around 1550 nm. When an input light having a long-wavelength bandwidth is input through the first isolator  31 , the light is forwarded to EDF  35  by way of FBG  81  and WDM  33 . Here, a pumping light of 980 nm output from a pumping laser diode  34  is supplied to EDF  35  through WDM  33 . Meanwhile, when the pumping light of 980 nm is input to EDF  35  in the same manner with the previous embodiment, the backward ASE having a bandwidth of 1530 nm to 1560 nm is output from EDF  35  and input to FBG  81  through WDM  33 . Then, the intercepted light having a wavelength around 1550 nm of FBG  81  is reflected and re-injected to EDF  35  through WDM  33 . However, the other backward light passed through FBG  81  is applied to the first isolator  31  and removed therein. Whereas, the reflected ASE is re-injected to EDF  35  and acts as the pumping light, thus increasing the amplification efficiency of EDF  35  for the L-band signals.  
     [0046] According to the present structure, it is possible to simplify the structure of the L-band optical fiber amplifier by simply establishing FBG  81  on the optical path, without any additional WDM for extracting the backward ASE of EDF  35  from the optical path.  
     [0047] Now referring to FIGS.  9 (A) and  9 (B), demonstrating test results of amplification gain according to total powers and wavelengths of input optical signals with the preferred structure discussed with reference to FIG. 8, FIG. 9( a ) denotes a test result where FBG  81  is not applied, that is, the backward ASE output from EDF  35  is not supplied to EDF  35 , whereas, FIG. 9(B) illustrates a test result where FBG  81  is provided, that is, the backward ASE output from EDF  35  is re-injected to EDF  35 . Besides, the test results shown in FIGS.  9 (A) and  9 (B) are obtained under the conditions that the length of EDF  35  is 50 m, the light wavelength reflected by FEG  81  is 1545 nm, and the pumping light applied is of 170 mW 980 nm. Here, the power (Pin) of the input light (S) is variously given as −5 dBm, 15 dBm, −10 dBm, and −15 dBm. Furthermore, the curves shown at the lower part of the graphs denote the noise figures measured according to wavelengths and input powers.  
     [0048] As shown in FIGS.  9 (A) and  9 (B), in case that FBG  81  is not applied, the amplification gains measured according to total powers and wavelengths of input optical signals are approximately 6 dB to 16 dB, whereas, in case that FBG  81  is used, the amplification gains are approximately 14 dB to 24 dB, by which it is proved that the structure in accordance with the invention can increase the amplification gain for L-band light prominently. Meanwhile, according to the preferred embodiments discussed above, the reflecting means for reflecting the backward ASE output from EDF  35  are provided just in front of WDM  33  for supplying the pumping light output from the pumping laser diode  34  to the optical path. However, it is not restricted to position the reflecting means to a specified place. That is, the reflecting means may be positioned between WDM  33  and EDF  35 . According to the present invention, it is possible to install the reflecting means in any place where the backward ASE output from EDF  35  can be reflected and re-injected to EDF  35 .  
     [0049] Therefore, the L-band optical fiber amplifier according to the present invention can increase the amplification gain efficiently without applying an expensive laser diode for generating a pumping light of 1550 nm for example.  
     [0050] Furthermore, the L-band optical fiber amplifier, according to the invention constructed in a manner that the backward ASE output from EDF is re-injected to EDF, can increase the amplification gain noticeably, thus reducing the length of EDF and pumping power applied therein as much as compared with the that of conventional EDF for the same amplification gain.  
     [0051] It will be apparent to those skilled in the art that various modifications and variations can be made in the L-band optical fiber amplifier of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.