WO1996029627A1 - Optical fiber amplifier - Google Patents

Abstract

An optical fiber amplifier used for optical communication, and provided with a pumping light source (1), a first wave combining means (2) for combining signal light with pumping, a first light amplifying fiber (3) through which the combined light is passed, a wave separator (4) provided on the output side of the first optical amplifying fiber so as to separate the wave-combined light into signal light and excitation light, an optical element (5) disposed in a path through which the separated signal light passes, a second wave combining means (7) for combining the signal light with the pumping light again, and a second light amplifying fiber (8) through which this combined light is passed. The optical element (5) includes at least either a dispersion compensation fiber or an optical filter.

Description

 Description Optical fiber amplifier Technical field

 The present invention relates to an optical amplifier used for optical communication and an optical communication system using the optical amplifier. Background art

 In an optical communication system, an optical fiber is used as a transmission path connecting a transmitting device and a receiving device. Since the optical fiber has an inherent dispersion characteristic, a dispersion compensating element is arranged on the transmission line to compensate for the dispersion. If the transmission line is long, an optical amplifier for compensating transmission loss is inserted in the middle, but a gain flattening optical filter is arranged to flatten the gain of this optical amplifier. Conventionally, these dispersion compensating elements and gain flattening optical filters have been independently arranged outside the optical amplifier. For example, a dispersion compensating fiber as a dispersion compensating element is arranged immediately before a receiving device or immediately after a transmitting device, and the total amount of dispersion of a transmission line is compensated collectively.

 Also, among optical communication systems, in a multi-repeater optical transmission system having a plurality of repeater amplifiers, since the gain to be provided for each repeater amplifier differs, the gain is set by the pump light power in each repeater amplifier. Was. Disclosure of the invention

 However, introducing a passive optical element such as a dispersion compensation fiber / gain flattening optical filter into an optical transmission system causes a loss of signal light power. In addition, when these optical elements are arranged on the input side of the optical amplifier, the noise figure is degraded. On the other hand, when these optical elements are arranged on the output side of the optical amplifier, the output optical power is reduced.

Therefore, optical elements such as dispersion compensation fiber and gain flattening filter are optically amplified. It is conceivable that the signal light is demultiplexed from the pump light by a demultiplexer inside the optical amplifier and passed through an optical element inside the optical amplifier. However, in this demultiplexer, a phenomenon occurs in which the signal light leaks slightly to the pumping light side. If the leakage of the signal light is not sufficiently suppressed, there is a problem in that a plurality of paths of the signal light exist, causing interference and deteriorating the error rate. In particular, in an optical amplifier having a configuration in which there is no path through which only the pumping light passes after the pumping light and the signal light are demultiplexed, it is difficult to take measures to suppress the signal light leakage light. We have to rely on the demultiplexing performance of this.

 In addition, when an optical amplifier is used as a relay amplifier in a multi-repeater optical transmission system, the intervals between the repeaters are generally not constant. The light power will be different. In this case, there is a problem that the wavelength dependence of gain and noise differs in each repeater.

 Accordingly, an object of the present invention is to introduce a passive optical element such as a dispersion compensation fiber and a gain flattening filter into an optical communication system together with an optical amplifier in order to prevent the noise figure and the output optical power of the optical amplifier from deteriorating. The purpose is to suppress interference between light and leaked light.

 Another object of the present invention is to equalize the wavelength dependence of gain and noise in each repeater when using the optical amplifier as a repeater amplifier in a multi-repeater optical transmission system.

The above object is to provide a first multiplexer for multiplexing a signal light and a pump light, a first optical amplifier fiber through which the multiplexed light passes, and a multiplexed light disposed at an output side of the first optical amplifier fiber. A demultiplexer for re-demultiplexing the signal light and the pump light, and at least one of a dispersion compensation fiber and an optical filter arranged in a path through which the demultiplexed signal light passes. This is achieved by an optical fiber amplifier including a second multiplexer for re-multiplexing and a second optical amplifying fiber for transmitting the combined light. Here, the first and second optical amplification fibers are, for example, erbium-doped optical fibers. The first multiplexer, demultiplexer, and second multiplexer are each replaced by a multiplexer / demultiplexer with the same configuration. Yes <Brief description of drawings

 FIG. 1 is a configuration diagram showing one embodiment of the present invention.

 FIG. 2 is a configuration diagram showing an example of the multiplexer / demultiplexer.

 FIG. 3 is a configuration diagram showing an example of a variable optical attenuator.

 FIG. 4 is a block diagram showing another embodiment of the present invention.

 FIG. 5 is a diagram showing a configuration and a forward operation of the optical isolator.

 FIG. 6 is a diagram showing the configuration and the reverse operation of the optical isolator.

 FIG. 7 is a configuration diagram showing an example of the dispersion compensation fiber.

 FIG. 8 is a configuration diagram showing another embodiment of the present invention.

 FIG. 9 is a configuration diagram showing an example of an optical filter for flattening an amplifier gain.

 FIG. 10 is a block diagram showing another embodiment of the present invention.

 FIG. 11 is a block diagram showing another embodiment of the present invention.

 FIG. 12 is a block diagram showing another embodiment of the present invention.

 FIG. 13 is a block diagram showing another embodiment of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a configuration diagram showing one embodiment of the present invention. As shown in the figure, the pumping light (for example, wavelength 980ηπι) from the pumping light source 1 and the signal light (for example, wavelength 1530 nm-1560 nm) input from the outside are multiplexed by the multiplexer / demultiplexer 2. The optical amplification fiber 3 is, for example, an erbium-doped fiber, and is excited by the excitation light to amplify the signal light. The multiplexer / demultiplexer 4 demultiplexes the signal light and the pump light. The demultiplexed signal light passes through the optical element 5. The optical element 5 includes at least one of a dispersion compensation fiber and an optical filter. By passing the optical signal through the optical element 5, the dispersion is compensated for, or the wavelength dependence of the amplifier gain is compensated. The signal light that has passed through the optical element 5 is multiplexed by the multiplexer / demultiplexer 7 with the pumping light that has passed through the optical fiber 6. On the other hand, the pumping light passing through the optical fiber 6 Signal light leakage light. However, by passing through the multiplexer / demultiplexer 7, the mixing of signal light leakage light is suppressed. The multiplexed signal light is amplified by the optical amplification fiber 8 pumped by the pump light, and is output after compensating for the loss in the optical element 5.

 Here, the multiplexers / demultiplexers 2, 4, and 7 are realized, for example, as shown in FIG. Fig. 2 shows an example in which the multiplexed light of wavelengths 1 and 2 is separated, but the light can also be multiplexed and the two lights of wavelengths 1 and 2 can be multiplexed. This utilizes the fact that the light intensity periodically changes from one waveguide to the other and vice versa between the adjacent waveguides 9 and 10. Because the cycle of the light intensity change differs depending on the wavelength, it is necessary to output two wavelengths of light input from the same input port to different ports, and output two wavelengths of light input to different ports to the same boat. It is possible. Therefore, this type of multiplexer / demultiplexer has the function of a multiplexer / demultiplexer depending on how it is used.

When the optical fiber amplifier of the present invention is used as a repeater amplifier in a multi-repeater optical transmission system, for example, as shown in FIG. 12, the signal light is supplied to an optical attenuator or a variable attenuator arranged on the output side of the optical amplifier. The light is attenuated by the optical attenuator 36 and output. This attenuation is set so that the sum of the loss and the loss in the optical fiber for transmission to the next relay amplifier is constant in each span. As a result, the signal light input power to the amplifier can be kept constant in each repeater while the pump light power of the repeater amplifier is kept constant. Here, the variable optical attenuator 36 can be realized, for example, as shown in FIG. The light emitted from the optical fiber 11 is collimated by the lens 12 and guided to the optical fiber 15 by the lens 14 via the prism 13. The attenuation rate can be changed by moving the prism 13 as indicated by the arrow in the figure. FIG. 4 is a block diagram showing another embodiment of the present invention. The optical fiber amplifier according to the present embodiment particularly performs dispersion compensation. The pump light source 1 generates pump light for the optical amplification fiber. As the excitation light source 1, for example, a laser diode emitting light having a wavelength of 980 mn is used. The pumping light generated from the pumping light source 1 is multiplexed with the signal light by the multiplexer / demultiplexer 2 to excite the optical amplification fiber 3. As an optical amplification fiber For example, an erbium-doped fiber is used. After that, the pump light is demultiplexed with the signal light by the multiplexer / demultiplexer 4, is again multiplexed with the signal light by the multiplexer / demultiplexer 7 via the optical fiber 6, and excites the optical amplification fiber 8. On the other hand, the signal light input to the optical amplifier is multiplexed with the pump light by the multiplexer / demultiplexer 2 via the optical isolator 16 and amplified by the optical amplifier fiber 3. Then, the light is demultiplexed with the pump light by the multiplexer / demultiplexer 4, and the fiber dispersion in the transmission line is compensated by the dispersion compensating fiber 18. Thereafter, the signal light is multiplexed with the pump light again by the multiplexer / demultiplexer 7, amplified by the optical amplification fiber 8, and output via the optical isolator 17. By arranging the optical lasers 16 and 17, the reflected return light at each section and the spontaneous emission light of the optical amplification fiber do not return to the input stage.

 Here, the optical isolators 16 and 17 are realized, for example, as shown in FIG. 5 and FIG. In FIG. 5, of the light incident from the left side of the figure, the light transmitted through the polarizer 19 rotates the polarization plane by 45 degrees by the 45-degree Faraday rotator 20 and can pass through the polarizer 21. On the other hand, in Fig. 6, of the light incident from the right side of the figure, the light that has passed through the polarizer 21 rotates the polarization plane by 45 degrees by the 45-degree Faraday rotator 20, and the polarizer 19 It cannot pass through.

 The dispersion compensating fiber 18 is a fiber having a negative dispersion value (ie, normal dispersion). This is realized, for example, as shown in FIG. The dispersion compensating fiber shown on the left side of the figure has a structure in which a core 22 is formed inside a clad 23, similarly to a fiber used for a transmission line. However, in comparison with the relationship between the core 24 and the clad 25 in the fiber used for the transmission line shown on the right side of the figure, the dispersion of the dispersion compensating fiber is controlled by reducing the diameter of the core 22 to control the structural dispersion. Is realized.

FIG. 8 is a configuration diagram showing another embodiment of the present invention. The optical fiber amplifier according to the present embodiment particularly performs amplifier gain flattening. The pumping light generated from the pumping light source 1 is multiplexed with the signal light by the multiplexer / demultiplexer 2 to excite the optical amplification fiber 3. After that, the signal light is demultiplexed with the signal light by the multiplexer / demultiplexer 4, is again multiplexed with the signal light by the multiplexer / demultiplexer 7 via the optical fiber 6, and the optical amplification fiber 8 is excited. on the other hand, The signal light input to the optical amplifier is multiplexed with the pumping light by the multiplexer / demultiplexer 2 via the optical isolator 16 and amplified by the optical amplifier fiber 3. Then, the light is demultiplexed with the pump light by the multiplexer / demultiplexer 4, and the wavelength dependence of the amplifier gain is compensated by the optical filter 26. After that, it is multiplexed with the pump light again by the multiplexer / demultiplexer 7, amplified by the optical amplification fiber 8, and output via the optical isolator 17.

 Here, the optical filter 26 used for the amplifier gain flattening is realized, for example, as shown in FIG. The light from the optical fiber 27 is collimated by the lens 28. As for the parallel light, only light in a specific wavelength range is transmitted by the optical filter element 29 and guided to the optical fiber 31 by the lens 30. The optical filter element 29 is realized, for example, by forming a thin film on the surface. By controlling the angle of the optical filter element 29, the wavelength range to be transmitted can be changed.

 FIG. 10 is a block diagram showing another embodiment of the present invention. The optical fiber amplifier according to this embodiment performs dispersion compensation and amplifier gain flattening. The pumping light generated by the pumping light source 1 is multiplexed with the signal light by the multiplexer / demultiplexer 2 to excite the optical amplifier fiber 3. After that, the signal light is demultiplexed with the signal light by the multiplexer / demultiplexer 4, is again multiplexed with the signal light by the multiplexer / demultiplexer 7 via the optical fiber 6, and excites the optical amplification fiber 8. On the other hand, the signal light input to the optical amplifier passes through the optical isolator 16, is multiplexed with the pump light by the multiplexer / demultiplexer 2, and is amplified by the optical amplifier fiber 3. Then, the light is demultiplexed with the pump light by the multiplexer / demultiplexer 4, the optical dispersion is compensated by the dispersion compensating fiber 18 via the optical isolator 32, and the wavelength dependence of the amplifier gain is compensated by the optical filter 26. Sex is compensated. Thereafter, the light is again multiplexed with the pump light by the multiplexer / demultiplexer 7, amplified by the optical amplifier fiber 8, and output via the optical isolator 17. Due to the optical isolators 16, 17, and 32, the reflected return light at each section and the spontaneous emission light of the optical amplifier fiber do not return to the input stage.

FIG. 11 is a block diagram showing another embodiment of the present invention. The optical fiber amplifier according to this embodiment performs dispersion compensation in particular. The pumping light generated by the pumping light source 1 is multiplexed with the signal light by the multiplexer / demultiplexer 2 to excite the optical amplification fiber 3. That After that, the signal light is demultiplexed with the signal light by the multiplexer / demultiplexer 4, the signal light component leaked to the pumping light side by the multiplexer / demultiplexer 4 is cut off by the optical filter 3 3, and the signal light is again split by the multiplexer / demultiplexer 7. The light is multiplexed to excite the optical amplification fiber 8. On the other hand, the signal light input to the optical amplifier passes through the optical isolator 16, is multiplexed with the pump light by the multiplexer / demultiplexer 2, and is amplified by the optical amplifier fiber 3. Then, the light is demultiplexed with the pump light by the multiplexer / demultiplexer 4, and the fiber dispersion in the transmission line is compensated by the dispersion compensating fiber 18. After that, it is multiplexed with the pumping light again by the multiplexer / demultiplexer 7, amplified by the optical amplification fiber 8, and output via the optical isolator 17.

 FIG. 12 is a block diagram showing another embodiment of the present invention. The present embodiment relates to an optical repeater amplifier using bidirectional excitation, and particularly performs dispersion compensation. The pumping light generated by the pumping light source 1 is multiplexed with the signal light by the multiplexer / demultiplexer 2 to excite the optical amplification fiber 3. Thereafter, the optical signal is demultiplexed with the signal light by the multiplexer / demultiplexer 4 and is again multiplexed with the signal light by the multiplexer / demultiplexer 7 to excite the optical amplification fiber 8. The pumping light generated from the pumping light source 34 is multiplexed with the signal light by the multiplexer / demultiplexer 35 to excite the optical amplification fiber 8. As the excitation light source 34, for example, a laser diode that emits light having a wavelength of 1480 nm is used. On the other hand, the signal light input to the optical amplifier passes through the optical isolator 16, is multiplexed with the pump light by the multiplexer / demultiplexer 2, and is amplified by the optical amplifier fiber 3. Then, the light is demultiplexed with the pump light by the multiplexer / demultiplexer 4, passes through the optical isolator 32, and is compensated for the fiber dispersion in the transmission line by the dispersion compensating fiber 18. After that, it is multiplexed with the pump light again by the multiplexer / demultiplexer 7, amplified by the optical amplification fiber 8, demultiplexed with the pump light by the multiplexer / demultiplexer 35, passed through the optical isolator 17, and passed through the variable optical attenuator. According to 36, the output is attenuated to the output optical power reference value unique to each repeater and output. It goes without saying that bidirectional excitation can be applied to embodiments other than this embodiment.

FIG. 13 is a block diagram showing another embodiment of the present invention. This embodiment relates to a multi-repeater optical transmission system using an optical fiber amplifier according to the present invention. The transmitter 37 1 forms a transmitter 37 together with the booster 37 2. An optical fiber amplifier using the configuration of the present invention is used as the booster 372. Booster 3 The optical signal amplified by 72 is transmitted as an output from the transmitting device 37. The signal from the transmitting device 37 reaches a relay amplifier 39 via an optical fiber 38 which is a transmission path. As the relay amplifier 39, an optical fiber amplifier using the configuration of the present invention is used. The relay amplifier 39 compensates for the transmission loss of the signal light, compensates for the dispersion or the wavelength dependence of the amplifier gain, and sends it out again to the transmission line. As described above, the signal light repeats passing through the optical fiber and the relay amplifier, and reaches the receiving device 42 after passing through the last optical amplifier 40 and the last transmission line optical fiber 41. The receiving device 42 includes a preamplifier 4 21 and a receiver 4 22. An optical fiber amplifier using the configuration of the present invention is used as the preamplifier 421. The signal light amplified by the preamplifier 4 21 is received by the receiver 4 22. In the optical transmission system of this embodiment, since the optical fiber amplifier of the present invention is used as the booster 372, the relay amplifiers 39, 40, and the preamplifier 421, wavelength multiplexed optical transmission is performed. In this case, a wide band can be used, the dispersion of the transmission line can be compensated, and the noise figure and output optical power in the optical amplifier are preserved. The gain, noise, and wavelength dependence of each optical amplifier can be made equal.

 In the optical fiber amplifier of the present invention, the dispersion compensating fiber is arranged between the two optical amplification fibers inside the optical fiber amplifier, and the signal light is passed through the optical fiber amplifier so that the noise figure and the output optical power of the optical amplifier are maintained. Dispersion compensation can be performed. In addition, an optical filter is placed between the two optical amplification fibers inside the optical fiber amplifier, and signal light is passed to compensate for the wavelength dependence of the optical amplifier gain while maintaining the noise figure and output optical power of the optical amplifier. can do.

 In addition, by providing a path in the optical amplifier through which pumping light passes and signal light does not pass, the multiplexer / demultiplexer reduces the amount of signal light leaking to the pumping light side using an optical filter, thereby reducing interference. Error rate can be prevented from deteriorating.

In a multi-repeater optical transmission system, an optical attenuator is installed immediately after the optical fiber amplifier, so that the pump light power of the optical fiber amplifier is equalized in each relay amplifier, and the output optical power is set by the optical attenuator. And the gain of each optical fiber amplifier The wavelength dependence of noise can be made equal. This characteristic makes it possible to use a wide wavelength range as a signal band, and is particularly effective when performing wavelength division multiplexing optical transmission.

Claims

The scope of the claims 1. A pump light source for generating pump light, a first multiplexer for multiplexing the signal light and the pump light, and a first optical amplifier for inputting an output of the first multiplexer and amplifying the signal light A fiber; a demultiplexer that demultiplexes the output of the first optical amplification fiber into pump light and signal light; an optical element that receives the signal light demultiplexed by the demultiplexer as input; A second multiplexer for re-multiplexing the output of the optical element and the pump light demultiplexed by the demultiplexer, and a second optical amplifier fiber for inputting the output of the second multiplexer and amplifying the signal light An optical fiber amplifier comprising:  2. The optical fiber amplifier according to claim 1, wherein a dispersion compensating fiber is used as the optical element.  3. The optical fiber amplifier according to claim 1, wherein an optical filter is used as the optical element.  4. The optical fiber amplifier according to claim 1, wherein a combination of two or three of a dispersion compensating fiber, an optical filter, and an optical isolator is used as the optical element.  5. An optical filter that transmits the pump light but does not transmit the signal light is provided on a path between the demultiplexer and the second multiplexer and through which the pump light demultiplexed from the signal light passes. The optical fiber amplifier according to any one of claims 1 to 4, 6. The optical fiber amplifier according to any one of claims 1 to 5, further comprising an optical attenuator or a variable optical attenuator on an output side of the second optical amplifier fiber.  7. An optical communication system comprising a transmitting device, a receiving device, and an optical fiber connecting the transmitting device and the receiving device, wherein any of claims 1 to 6 is provided in the middle of the optical fiber. An optical communication system comprising the above optical fiber amplifier as a relay amplifier. 8. The transmission device according to any one of claims 1 to 6, 8. The optical communication system according to claim 7, comprising an amplifier amplifier as a booster.  9. The optical communication device according to claim 7, wherein the receiving device includes the optical fiber amplifier according to any one of claims 1 to 6 as a preamplifier. system.  10. The optical communication system according to claim 8, wherein the receiving device includes the optical fiber amplifier according to any one of claims 1 to 6 as a preamplifier.  11. An optical communication system comprising a transmitting device, a receiving device, and an optical fiber connecting the transmitting device and the receiving device, wherein the transmitting device is any one of claims 1 to 6. An optical communication system comprising the optical fiber amplifier according to claim 1 as a booster.  12. The optical communication device according to claim 11, wherein the receiving device includes the optical fiber amplifier according to any one of claims 1 to 6 as a preamplifier. system.  13. An optical communication system comprising a transmitting device, a receiving device, and an optical fiber connecting the transmitting device and the receiving device, wherein the receiving device is any one of claims 1 to 6. An optical communication system, comprising: the optical fiber amplifier according to (1) as a preamplifier. 14. A transmission device comprising the optical fiber amplifier according to any one of claims 1 to 6 as a booster.  15. A receiving device comprising the optical fiber amplifier according to any one of claims 1 to 6 as a preamplifier.