Title of Invention

"A MODIFIED OPTICAL FIBER AMPLIFIER FOR ACHIEVING HIGH GAIN OF SMALL SIGNAL"

Abstract

1. A modified optical fiber amplifier for achieving high gain of a small signal, which amplifies a weak incident signal light ,comprising: a pump laser diode (LD)(210) for applying a pumping light to amplify the incident signal light; a wavelength division multiplexer (WDM)(220) for coupling the incident signal light and the pumping light output from the pump LD into an optical fiber; a fibre amplification medium for amplifying the incident light signal output from the WDM (220)using the pumping light of the pump LD(210), and also a feedback signal light reentering the fiber amplification medium from a coupler(240); a coupler(240) attached in the form of a loop mirror , for reflecting the incident signal light, which has already been amplified by the fiber amplification medium, in a loop shape and sending the reflected signal light back to the fiber amplification medium; and a circulator placed in front of the WDM(220), for receiving the weak incident signal light and transmitting the weak incident signal light to the WDM(220), and outputting the feedback signal light, amplified by the fiber amplification medium and passed through the WDM(220).

Full Text

1. Field of the Invention The present invention relates to a modified optical fiber amplifier, and more particularly, to an optical fiber amplifier for improving the gain of a small signal having a low luminous intensity by connecting an optical coupler in the form of a loop mirror. 2. Description of the Related Art In a general repeater for long distance communications, a conventional optical communications repeat is performed in such a manner that a weakened light signal is transformed into an electrical signal, the electrical signal is amplified, and then this amplified electrical signal is transformed back into a light signal. Such a repeating method excessively increases the size of a repeating amplifier system and strongly depends on transmission speed. An optical fiber amplifier for amplifying an optical signal is required as a repeater to overcome the above problem and effectively amplify an optical signal. An erbium doped fiber amplifier (EDFA) being such a fiber amplification repeater is attracting attention as a next-generation optical repeater for optical communications. The EDFA periodically amplifies an optical signal to prevent attenuation of the optical signal due to long distance transmission, when much data is transmitted over a long distance through an optical fiber. FIG. 1 is a block diagram showing the configuration of a normal single forward EDFA comprising an erbium doped fiber (EDF) 130 being an optical signal amplifying medium, a pump laser diode (LD) 120, a wavelength division multiplexer (WDM) 110 and 140. Here, the pump LD 120 functions as a light source for exciting erbium ion of a ground state in the EDF 130. The WDM 110 couples a signal light and a pump light into a fibre and transmits it to a single line. The isolators 100 and 140 prevent an optical signal from proceeding backward. The WDM 110 couples a signal light and a pump light into a fiber and transmits the fiber to the EDF 130. The optical isolators 100 before the WDM 110 prevents the optical signal from being amplified inefficiently by amplified spontaneous emission (ASE), generated by the EDF 130, which is reflected by an optical device such as a signal input connector and reenters the EDF 130. Similarly, the isolator 140 after the EDF 130 prevents the amplification efficiency of the EDFA from being degraded due to ASE which is reflected by an optical device such as a signal output connector and again incident upon the EDF 130. The pump LD 120 has a wavelength of 980nm, pumps laser forward (i.e., in the direction the signal light proceeds), and generates fixed power of 120mW. The EDF 130 is an EDF doped with high aluminum, and the erbium concentration is 290ppm. Meanwhile, the optical fiber amplifier transforms pump light energy into signal light energy using the principles of a laser. Accordingly, when the intensity of the signal light is sufficiently low, the normal optical fiber amplifier cannot effectively change the erbium sufficiently excited by a pump light of high intensity into a ground state, since the signal light passes through the EDF once. Thus, the amplification efficiency of the general optical fiber amplifier is not high. That is , the energy of the pump light cannot be effectively transformed into the energy of the signal light. Therefore, there is a limit in increasing the gain of a small signal, and the difference in gain between the wavelengths of the signal light is comparatively wide. SUMMARY OF THE INVENTION To solve the above problems, it is an objective of the present invention to provide an erbium doped fiber amplifier (EDFA) for achieving high again of small signal, including an optical coupler behind an EDF in the form of a loop mirror, for again amplifying a signal light output via the loop mirror using the EDF and finally outputting the amplified signal light using an optical circulator, in order to increase the gain of a small signal in the entire wavelength range of the EDFA and reduce the difference in gain between the wavelength range of the EDFA and reduce the difference in gain between the wavelengths of the signal light. Accordingly, to achieve the above objective, there is provided an optical fiber amplifier for a achieving high gain of a small of a small signal, which amplifies a weak incident signal light, comprising: a pump laser diode (LD) for applying a pumping light to amplify the incident signal light; a wavelength division multiplexer (WDM) for coupling the incident signal light and the pumping light output from the pump LD into an optical fiber; a fiber amplification medium for amplifying the incident light signal output from the WDM using the pumping light of the pump LD, and also a feedback signal light reentering the fiber amplification medium from a coupler; a coupler attached in the from of a loop mirror, for reflecting the incident signal light, which has already been amplified by the fiber amplification medium, in a loop shape and sending the reflected signal light back to the fiber amplification medium; and a circulator placed in front of the WDM, for receiving the weak incident signal light and transmitting the weak incident signal light to the WDM, and outputting the feedback signal light, amplified by the fiber amplification medium and passed through the WDM. It is preferable that the optical fiber amplifier further comprises a filter installed in the coupler functioning as a loop mirror or between the fiber amplification medium and the coupler, for preventing amplified spontaneous emission (ASE) to obtain higher gain of a small signal. Also, preferably, the optical fiber amplifier further comprises a gain equalizer installed in the coupler functioning as a loop mirror or between the fiber amplification medium and the coupler, for equalizing the gains of the optical fiber amplifier. BRIEF DESCRIPTION OF THE DRAWINGS The above objective and advantage of the present invention will become more apparent by describing in detail a preferred embodiment thereof with reference to the attached drawings in which: FIG. 1 is a block diagram illustrating the configuration of a conventional single forward erbium doped fiber amplifier (EDFA); FIG. 2 is a block diagram illustrating the configuration of an optical fiber amplifier for achieving high gain of a small signal, according to an embodiment of the present invention; and FIG. 3 is a graph illustrating gains according to wavelengths for the comparison of small signal gains of two kinds of EDFAs. FIG. 4 and FIG 5. illustrating 'filter', 'gain equalizer' and 'inside layout of the coupler'. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 2, an optical fiber amplifier for achieving high gain of a small signal according to an embodiment of the present invention is comprised of a pump laser diode (LD) 210, a wavelength division multiplexer (WDM) 220, an erbium doped fiber (EDF) 230, a coupler 240 and a circulator 200. The pump LD 210 applies a pumping light being a light source for exciting erbium ion of a base state in the EDF being an amplifying medium, in order to amplify an input signal light of low intensity. The WDM 220 couples the input signal light and the pumping light output from the pump LD 210 into one optical fiber, and applies the coupled input signal light to the EDF 230. The EDF 230 amplifies the input signal light from the WDM 220 using the pumping light of the pump LD 210, and also amplifies a feed back signal light reentering the EDF 230 from the coupler 240. The coupler 240, as a 1550n m 50:50 coupler, is attached in the form of a loop mirror and reflects the incident signal light, which has already been amplified by passing through the EDF 230, to the EDF 230 in a loop shape. The circulator 200 is placed in front of the WDM 220 and receives the incident signal light of low intensity and transmits it to the WDM 220, and finally outputs the feedback signal light amplified by the EDF 230 and passed through the WDM 220. The ports 1 and 2 of the circulator 200 transmit the input signal light, and the ports 2 and 3 transmit the output signal light. The insertion losses of the former and the latter are 1.2dB and 1.7dB, respectively. The ports 2 → 1 and ports 3 → 2 each have an isolation of 40dB or higher, and serve as an isolator used as input and output ports in a convention EDFA. Accordingly, there is no need to use an additional isolator. In order to compare the result of the present invention with a conventional single forward EDFA shown in FIG. 1, the pump LD 210 has a wavelength of 980nm, sets its power to be 120mW as in the conventional EDFA, and pumps laser forward, i.e., in the direction the signal light travels. The WDM 220 is a fused type and the used EDF is a 5m-long high Al-doped EDF having an erbium concentration of 29099m. The operation of the present invention will now be described based on the above-described configuration. First, an incident signal light passes through the ports 1 and 2 of the circulator 200, is coupled with a pump light by the WDM 220, and is primarily amplified by the EDF 230. The amplified signal light is separated in a ratio of 50 to 50, and the separated lights make a round and are coupled to each other in the coupler 240. The signal light fed back via a loop mirror is again amplified by the EDF 230, and the amplified signal light is finally output via the ports 2 and 3 of the circulator 200. In the present invention, the intensity of the input signal light used to measure the properties of the EDFA is set to be -30dBm, and the wavelength and intensity of the pump light are respectively set to be 980n m and 120mW. An optical device such as the WDM, and the EDF in the present invention are the same as those used in the conventional EDFA, and the two EDFs are the same in length. Gain was measured at intervals of 5nm in the wavelengths of the input signal light between 1530nm and 1560nm, and the result of measurement is shown in FIG. 3 together with the measured gains of the conventional EDFA. As shown in FIG. 3, the EDFA using the loop mirror method small signal gain which is higher in wavelengths between 1530nm and 1560nm than the conventional single forward EDFA, and the difference in gain according to the wavelengths is small, i.e. , between 6dB and 13dB. The EDFA according to the present invention achieves gain of a small signal which is higher by 7dB or more, particularly in the wavelength of 1560nm. Meanwhile, a filter ( as shown in FIG 4 or 5) is placed in the coupler 240 serving as the loop mirror or between the EDF 230 and the coupler 240, and can achieve higher gain of a small signal by preventing ASE. Also, a gain equalizer (as shown in FIG 4 or 5) can be installed in the coupler 240 functioning as the loop mirror or between the fiber amplification medium and the coupler to equalize the gains of the optical fiber amplifier. According to the present invention, high gain of a small signal can be achieved in the entire wavelength range between 1530nm and 1560nm by using the 1550nm-use 50:50 coupler as the loop mirror. The difference in gain between wavelengths can be significantly narrower in the EDFA according to the present invention than the conventional EDFA using an EDF having the same length as that of the EDF in the present invention. The loop mirror method being the structure of the EDFA used in the present invention is a double pass method, thus allowing reduction in the length of the EDF compared with existing single pass methods since a signal light passes through the EDF twice. Furthermore, since the interval between amplifiers becomes wider with an increase in the gain of the EDFA in long-distance transmission, fewer amplifiers can be used for a given distance. Thus, the loop mirror method according to the present invention is economical and can reduce the probability of errors being generated in the amplifiers. We Claim: 1. A modified optical fiber amplifier for achieving high gain of a small signal, which amplifies a weak incident signal light ,comprising: a pump laser diode (LD)(210) for applying a pumping light to amplify the incident signal light; a wavelength division multiplexer (WDM)(220) for coupling the incident signal light and the pumping light output from the pump LD into an optical fiber; a fibre amplification medium for amplifying the incident light signal output from the WDM (220)using the pumping light of the pump LD(210), and also a feedback signal light reentering the fiber amplification medium from a coupler(240); a coupler(240) attached in the form of a loop mirror , for reflecting the incident signal light, which has already been amplified by the fiber amplification medium, in a loop shape and sending the reflected signal light back to the fiber amplification medium; and a circulator placed in front of the WDM(220), for receiving the weak incident signal light and transmitting the weak incident signal light to the WDM(220), and outputting the feedback signal light, amplified by the fiber amplification medium and passed through the WDM(220). 2. The optical fiber amplifier for achieving high gain of a small signal as claimed in claim 1, comprising: a filter(240) preferably installed in the coupler functioning as a loop mirror or between the fiber amplification medium and the coupler, for preventing amplified spontaneous emission (ASE) to obtain higher gain of a small signal. 3. The optical fiber amplifier for achieving high gain of a small signal as claimed in claim 1, comprising: a gain equalizer(240) preferably installed in the coupler functioning as a loop mirror or between the fiber amplification medium and the coupler, for equalizing the gains of the optical fiber amplifier. 4. The optical fiber amplifier for achieving high gain of a small signal as claimed in claim 1, wherein the fiber amplification medium is an erbium doped fiber. 5. The optical fiber amplifier for achieving high gain of a small signal as claimed in claim 1, wherein the coupler is a 1550nm-use 50:50 coupler. 6. An optical fiber amplifier for achieving high gain of a small signal substantially as herein described with reference to the foregoing description and the accompanying drawings.

Documents:

1774-del-1998-abstract.pdf

1774-del-1998-assignment.pdf

1774-del-1998-claims.pdf

1774-del-1998-correspondence-others.pdf

1774-del-1998-correspondence-po.pdf

1774-del-1998-description (complete).pdf

1774-del-1998-drawings.pdf

1774-del-1998-form-1.pdf

1774-del-1998-form-19.pdf

1774-del-1998-form-2.pdf

1774-del-1998-form-3.pdf

1774-del-1998-form-4.pdf

1774-del-1998-form-6.pdf

1774-del-1998-gpa.pdf

1774-del-1998-petition-137.pdf

1774-del-1998-petition-138.pdf