# US 5,706,126 A · Optical amplifier > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 5,706,126 A - Document ID: US5706126A - Status: Granted - Kind: A - Application number: 08/707,391 - Filed: 1996-09-04 - Granted: 1998-01-06 - Earliest claimed: 1995-09-14 - Assignees: NEC Corporation - Inventors: Masayuki Fujita - Web page: https://patentgrep.com/patent/US5706126A ## CPC - H01S 5/0687: Stabilising the frequency of the laser - H01S 3/13013: {by controlling the optical pumping} - H01S 3/1305: {Feedback control systems} - H01S 3/06754: {Fibre amplifiers (H01S3/06708 takes precedence)} - H01S 5/50: Amplifier structures not provided for in groups H01S5/02 - H01S5/30 - H01S 2301/02: ASE (amplified spontaneous emission), noise; Reduction thereof - H01S: DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL ## Abstract A second optical branching coupler and a second light sensor are additionally provided at the output side of the optical amplifier, in addition to a first optical branching filter for branching part of the light outputted from the optical amplifier and a first light sensor for measurement of the level of the branching light. Provided between the first light sensor and the second light sensor is a band-pass filter which allows passage of only light with specific wavelengths, for measurement of the level of the light with specific wavelengths. The difference between the levels of the outputted light measured by the first light sensor and the light with specific wavelengths measured by the second light sensor is calculated by a subtracting process to control the gain of the optical amplifier on the basis of the result of the subtracting process. The band-pass filter is provided for removal of amplified spontaneous emission from the optical output. The band-pass filter allows only the amplified spontaneous emission to pass. Since the feedback is controlled free from the influence of the amplified spontaneous emission, a constant optical output is maintained with high precision. ## Claims ### Claim 1 An optical amplifier comprising: light-amplifying means for directly amplifying an inputted optical signal which has been inputted using a stimulated emission phenomena, to generate an amplified optical signal; first optical branching means for branching pan of said amplified light to output first branching light; first light-sensing means for converting said first branching light into a first electrical signal to output said signal; second optical branching means for branching part of said amplified light to output second branching light; light-pass filter means which allows light, of said second branching light, with wavelengths in a predetermined specific band to pass, to output light with specific wavelengths; second light-sensing means for converting said light with specific wavelengths into a second electrical signal to output said signal; an operational amplifying medium for subjecting said first electrical signal and said second electrical signal to a subtracting process to output a differential signal; and gain control means for controlling a gain of said light-amplifying means on the basis of said differential signal so as to maintain a level of said optic;it signal at a constant value. ### Claim 2 (depends on 1) An optical amplifier as claimed in claim 1, wherein said specific wavelength band includes wavelengths other than the center wavelength of said inputted signal light. ### Claim 3 (depends on 1) An optical amplifier as claimed in claim 1, wherein said operational amplifying medium comprises subtracting means for subtracting a value obtained by multiplying an optical output of said light with specific wavelengths by a predetermined constant, from the optical output of said first branching light so as to remove the amplified spontaneous emission from said first branching light. ### Claim 4 (depends on 3) An optical amplifier as claimed in claim 3, wherein said light-amplifying means includes an optical fiber amplifier comprising an amplifying optical fiber and a pumping light source for inputting pumping light to said amplifying optical fiber. ### Claim 5 (depends on 4) An optical amplifier as claimed in claim 4, wherein said gain control means comprises control means for controlling current to be injected into said pumping light source. ### Claim 6 (depends on 3) An optical amplifier as claimed in claim 3, wherein said light-amplifying means comprises a semiconductor optical amplifier. ### Claim 7 (depends on 6) An optical amplifier as claimed in claim 6, wherein said gain control means comprises control means for controlling current to be injected into said semiconductor optical amplifier. ### Claim 8 An optical amplifier comprising: light-amplifying means for directly amplifying an inputted optical signal which has been inputted using a stimulated emission phenomena, to generate an amplified optical signal; first optical branching means for branching part of said amplified light to output first branching light; first light-pass filter means which allows light, of said first branching light, with wavelengths in a predetermined specific band to pass, to output first light with specific wavelengths; second optical branching means for branching part of said amplified light to output second branching light; second light-pass filter means which allows light, of said second branching light, with wavelengths in a predetermined specific band to pass, to output second light with specific wavelengths; second light-sensing means for converting said light with specific wavelengths into a second electrical signal to output said signal; an operational amplifying medium for subjecting said first electrical signal and said second electrical signal to a subtracting process to output a differential signal; and gain control means for controlling a gain of said light-amplifying means on the basis of said differential signal so as to maintain a level of said optical signal at a constant value. ### Claim 9 (depends on 8) An optical amplifier as claimed in claim 8, wherein said first specific wavelength band is set as a band which includes wavelengths other than or equal to the center wavelength of said inputted signal light, and said second specific wavelength band is set as a band which includes wavelengths other than the center wavelength of said inputted signal light. ### Claim 10 (depends on 8) An optical amplifier as claimed in claim 8, wherein said operational amplifying medium comprises subtracting means for subtracting a value obtained by multiplying an optical output of said light with specific wavelengths by a predetermined constant, from an optical output of said first branching light so as to remove the amplified spontaneous emission from said first branching light. ### Claim 11 (depends on 10) An optical amplifier as claimed in claim 10, wherein said light-amplifying means includes an optical fiber amplifier comprising an amplifying optical fiber and a pumping light source for inputting pumping light to said amplifying optical fiber. ### Claim 12 (depends on 11) An optical amplifier as claimed in claim 11, wherein said gain control means comprises control means for controlling current to be injected into said pumping light source. ### Claim 13 (depends on 12) An optical amplifier as claimed in claim 12, wherein said light-amplifying means comprises a semiconductor optical amplifier. ### Claim 14 (depends on 13) An optical amplifier as claimed in claim 13, wherein said gain control means comprises control means for controlling current to be injected into said semiconductor optical amplifier. ### Claim 15 An optical amplifier comprising: light-amplifying means for directly amplifying inputted signal light which has been inputted using a stimulated emission phenomena, to generate amplified light; amplified signal light-extracting means for removing amplified spontaneous emmission from said amplified light to extract the amplified signal light with the same wavelengths as said inputted signal light; level-measuring means for measuring a level of said amplified signal light to optput the level of the amplified signal light; and gain control means for controlloing a gain of said light-amplifying means so as to maintain an optical output of said amplified light at a constant level on the basis of said level of the amplified signal light, wherein said amplified signal light-extracting means comprises a first coupler for receiving said amplified light and for producing first and second outputs based on said amplified light, and a second coupler for recieving said second output of said first coupler and for producing first and second outputs, and wherein said level-measuring means comprises a first light sensor for receiving said first output of said first coupler, wherein said amplified signal light-extracting means further comprises a first bandpass filter coupled between said first light sensor and said first coupler, and a second bandpass filter for receiving said first output of said second coupler, wherein said level-measuring means further comprises a second light sensor coupled to an output of said second bandpass filter, and wherein said gain control means comprises an operational amplifier for receiving outputs from said first and second light sensors.