OPTICAL FIBER AMPLIFIER FOR CONTROLLING GAIN FLATNESS

Abstract: An optical fiber amplifier for controlling gain flatness including an optical fiber amplifying unit (120) for amplifying input signal light according to a predetermined amplification control value, a measuring unit (130) for measuring the power level of amplified spontaneous emission (ASE) at a predetermined wavelength band output from the optical fiber amplifying unit (120), and a gain controller (140) for controlling the amplification control value of the optical fiber amplifying unit (120) according to the measured power level of the ASE to flatten the gain of the optical fiber amplifying unit (120). The gain flatness of amplified signal light is monitored and controlled, thereby obtaining gain-flattened signal light even when power level of signal light varies from wavelength to wavelength.

Description: OPTICAL FIBER AMPLIFIER FOR CONTROLLING GAIN FLATNESS Technical Field The present invention relates to an optical fiber amplifier for controlling gain flatness, and more particularly, to an optical fiber amplifier for controlling gain by measuring the gain flatness thereof. Background Art Conventional optical fiber amplifiers are optimized to give the highest level of gain flatness before being installed on a transmission path. In an optical fiber amplifier, automatic gain control or automatic level control is achieved by monitoring the power levels of all input signals and the power levels of all output signals. Due to a change in the signal power level for each channel that is added or dropped during add/drop multiplexing (ADM) as well as a change in loss at each different wavelengths which depends on the properties of an optical fiber used as a transmission path, there exhibits a great difference in the quality of signals of the respective channels received by a reception terminal. In general, an optical fiber used as a transmission path exhibits a great difference in loss per channel. If a light signal in the wavelength band of 1545.32 to 1557.36 nm, which is generally used in a transmission system having 16 channels at intervals of 100 GHz, is transmitted over e.g. , 100 Kim, the loss experienced by a light signal in the 1545.32 nm channel is 0.3 dB greater than that experienced by a light signal in the 1557.36 nm channel. Also, in a network system in which ADM is used, since the output power levels of a typical laser diode are not uniform, it is quite difficult to attain coincidence of power levels between a channel signal transmitted through an ADM node and a channel signal add-multiplexed after being drop-multiplexed. Thus, since the power level and the signal-to-noise ratio vary greatly between channels of the light signal received by the reception terminal, the signal quality of a channel compared to that as designed may be considerably degraded. If this phenomenon repeatedly occurs, the problem becomes severe. Disclosure of the Invention To solve the above problems, it is an objective of the present invention to provide an optical fiber amplifier for use in a long-distance transmission system or an add/drop multiplexing (ADM) system, for controlling gain flatness by measuring the gain flatness of an output signal of the optical fiber amplifier and controlling the amplification gain thereof. Accordingly , to achieve the above objective , there is provided an optical fiber amplifier for controlling gain flatness including an optical fiber amplifying unit for amplifying input signal light according to a predetermined amplification control value, a measuring unit for measuring the power level of amplified spontaneous emission (ASE) at a predetermined wavelength band output from the optical fiber amplifying unit, and a gain controller for controlling the amplification control value of the optical fiber amplifying unit according to the measured power level of the ASE to flatten the gain of the optical fiber amplifying unit. Brief Description of the Drawings The above objective and advantages 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 schematic diagram of an optical fiber amplifier for controlling gain flatness according to the present invention; and FIG. 2 shows the relationship between the gain characteristics and the wavelength of the first and second filters shown in FIG. 1. Best mode for carrying out the Invention Referring to FIG. 1, an optical fiber amplifier for controlling gain flatness according to the present invention includes an input port 100, a first measuring unit 110, an optical fiber amplifying unit 120, a second measuring unit 130, a gain controller 240 and an output port 150. The first measuring unit 110 includes a first light divider 111 for dividing light signals incident from the input port 100 in a predetermined ratio, and a first light receiving element 112 for converting light signals output from the first light divider 111 into electrical signals. The optical fiber amplifying unit 120 includes first and second pump light sources 121 and 122 for generating pump light, first and second wavelength-division multiplexers 123 and 124 for multiplexing the pump light and signal light, an amplification optical fiber 125 for amplifying the signal light according to the pump light, and first and second isolators 126 and 127 for preventing amplified spontaneous emission (ASE) generated from the amplification optical fiber 125 from being reflected at an input port 100 and an output port 150 and re-entering the amplification optical fiber 125. The second measuring unit 130 includes a second divider 134 for dividing the signal light amplified by the optical fiber amplifying unit 120, a third light divider 131 for sub-dividing the signal light divided by the second light divider 134 in a ratio of 1 : 1 , a first filter 132a connected to one output port of the third light divider 131, for allowing signal light of a predetermined wavelength band to pass therethrough, a first light receiving element 133a for converting signal light output from the first filter 132a into an electrical signal and outputting the converted signal to the gain controller 140, a second filter 132b connected to the other output port of the third light divider 131, for allowing signal light of a predetermined wavelength band to pass therethrough, and a second light receiving element 133b for converting signal light output from the second filter 132b into an electrical signal and outputting the converted signal to the gain controller 140. Here, the first and second filters 132a and 132b are narrow bandpass filters having different central frequencies so as to be capable of measuring short- wavelength ASE and long- wavelength ASE from the signal light output from the optical fiber amplifying unit 120. The optical fiber amplifier having the aforementioned configuration operates as follows. The first light divider 111 divides light signals incident from the input port 100 in a predetermined ratio. The first light receiving element 112 converts some of the divided light signal from the input port 100 into an electrical signal and outputs the converted electrical signal to the gain controller 240. The gain controller 240 detects the power level of the input electrical signal. The remaining light signal divided by the first light divider 111 is incident into the amplification optical fiber 125 via the first isolator 126 and the first wavelength-division multiplexer 123. The first and second pump light sources 121 and 122 generate pump light whose power level is controlled according to amplification control values output from the gain controller 140. The amplification optical fiber 125 amplifies the signal light incident from the first wavelength-division multiplexer 123 according to the pump light incident through the first and second wavelength-division couplers 123 and 124. The second light divider 134 divides the amplified signal light in a predetermined ratio and outputs the divided signal light to the output port 150 and the third light divider 131, respectively. The first and second filters 132a and 132b connected to two output ports of the third light divider 131, filter the signal light in a predetermined wavelength band. Here, as shown in FIG. 2, the central wavelengths of the first and second filters 132a and 132b exist properly outside the range of a wavelength region 202 in which a signal channel exists and exist properly inside a wavelength band in which ASE exists. Also, the first and second filters 132a and 132b have central wavelengths which exist outside a wavelength region in which signal channels exist and which exist properly inside a short-wavelength region 200 or a long-wavelength region 204. The power level of ASE reflects the gain characteristics which are dependent on the wavelengths of the optical fiber amplifying unit 12. Thus, the gain of the optical fiber amplifying unit 120 can be flattened by measuring the power levels of the short- and long-wavelength ASEs and controlling the output power level of the pump light according to the measured power levels. The second and third light receiving elements 133a and 133b convert signal light filtered by the first and second filters 132a and 132b into electrical signals to then output the electrical signals to the gain controller 140. The gain controller 140 checks the power level of the ASE from the output signals of the second and third light receiving elements 133a and 133b and supplies amplification control values to the first and second pump light sources 121 and 122 so as to adjust the output power level of the pump light. For example, if the power level of the short- wavelength ASE is greater than any other wavelength ASE, the amplification control value for reducing the power level of the pump light is supplied. If the power level of the long-wavelength ASE is greater than any other wavelength ASE, the amplification control value for increasing the power level of the pump light is supplied. Also, the gain controller 140 determines the power level of an input light signal by measuring the output signal of the first light receiving element 112, thereby obtaining gain characteristics of the output light signal to the power level of the input light signal. According to the present invention, the gain flatness of amplified signal light is monitored and controlled, thereby obtaining gain-flattened signal light even when power level of signal light varies from wavelength to wavelength. The optical fiber amplifier according to the present invention can reduce a difference in the transmission characteristic of each channel in a long-distance optical transmission system or an ADM network including ADM, in which optical fibers having a great loss characteristic for each channel are used as transmission paths. Industrial Applicability The optical fiber amplifier according to the present invention can be used in a long-distance optical transmission system or an ADM network including ADM, in which optical fibers having a great loss characteristic for each channel are used as transmission paths.

Claims: What is claimed is: 1. An optical fiber amplifier for controlling gain flatness comprising: an optical fiber amplifying unit for amplifying input signal light according to a predetermined amplification control value; a measuring unit for measuring the power level of amplified spontaneous emission (ASE) at a predetermined wavelength band output from the optical fiber amplifying unit; and a gain controller for controlling the amplification control value of the optical fiber amplifying unit according to the measured power level of the ASE to flatten the gain of the optical fiber amplifying unit. 2. The optical fiber amplifier according to claim 1, wherein the measuring unit comprises: a first light divider for dividing the output signal light of the optical fiber amplifying unit; a second light divider for sub-dividing the light signal divided by the first light divider in a predetermined ratio; a first filter for filtering the ASE of a first predetermined wavelength band from one output of the second light divider; a first light receiving element for converting the output signal of the first filter into an electrical signal and outputting the electrical signal to the gain controller; a second filter for filtering the ASE of a second predetermined wavelength band from the other output of the second light divider; and a second light receiving element for converting the output signal of the second filter into an electrical signal and outputting the electrical signal to the gain controller. 3. The optical fiber amplifier according to claim 2, wherein the central wavelength of the first or second filter exists outside a wavelength region in which a signal channel exists and covers a wavelength region in which ASE exists. 4. The optical fiber amplifier according to claim 3, wherein the central wavelength of the first or second filter exists inside a short- wavelength band outside a wavelength region in which a signal channel exists. 5. The optical fiber amplifier according to claim 3, wherein the central wavelength of the first or second filter exists inside a long- wavelength band outside a wavelength region in which a signal channel exists.

Inventors: HWANG SEONG-TAEK, JUNG RAE-SUNG, KIM SUNG-JUN, YOON SOO-YOUNG, HWANG, SEONG-TAEK, JUNG, RAE-SUNG, KIM, SUNG-JUN, YOON, SOO-YOUNG
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO., LTD.
Application year: 1999
Main classification: H01S3
IPC codes: G02F1, H01S3, H04B10