JP2005011989A - Optical amplifier - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high gain optical amplifier suppressing a microscopic ocsillation. <P>SOLUTION: An optical circulator 14 is connected between an optical fiber 12 for transmission and an erbium doped optical amplifier fiber (FDF) 16. A fiber Bragg grating (FBG) 28 for selectively reflecting an excitation light is connected to a port C of the circulator 14. The circulator 14 lets a signal light in through from the fiber 12 to the EDF 16. The excitation light outputted from laser diodes 22, 24 enter into the EDF 16 through an optical coupler 26 and an optical circulator 18. The excitation light not absorbed by the EDF 16 and a part of an ASE light generated from the EDF 16 enter into the FBG 28 by the circulator 14. The excitation light reflected by the FBG 28 enters into the fiber 12 through the circulator 14 to generate the Raman amplification at the fiber 12. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical amplifier having a serially connected Raman amplification medium and a rare earth-doped optical amplification fiber.
[0002]
[Prior art]
In long-distance optical fiber transmission, an optical amplification repeater transmission system in which an optical repeater amplifier is arranged between transmission optical fibers is used. The optical repeater amplifier comprises an erbium-doped fiber amplifier (EDFA) having an optical amplification fiber (EDF) doped with erbium and an excitation light source for exciting erbium ions of the optical amplification fiber. In general, an optical fiber doped with a rare earth element other than erbium can also be used as an optical amplification fiber.
[0003]
An optical amplification repeater transmission system that amplifies signal light by causing Raman amplification in a transmission optical fiber is also known.
[0004]
Broadband optical amplification can be realized by serially connecting optical amplifiers having different amplification bands. Also, a high gain as a whole can be realized by serially connecting a plurality of optical amplifiers having the same amplification band.
[0005]
A configuration in which optical amplifiers are connected in multiple stages is described in US Pat. No. 6,396,623, US Pat. No. 6,424,457, and US Pat. No. 6,456,426. In particular, US Pat. No. 6,396,623 discloses a configuration in which a Raman amplifier and an EDFA are serially connected.
[0006]
In the high gain optical fiber amplifier, when the input power level to the optical fiber amplifier is low, the gain increases due to the gain compression effect. The ASE light generated in the EDF is Rayleigh scattered by the transmission optical fiber at the front stage or the rear stage and re-enters the EDF.
[0007]
In particular, an optical amplification configuration in which an optical fiber for transmission is operated as a Raman amplifier and an EDFA is disposed at the subsequent stage has the following problems. That is, part of the ASE light generated in the EDF is incident on the transmission optical fiber that causes Raman amplification, where it is amplified and backward Rayleigh scattered, and reenters the EDF. The ASE light re-entering the EDF re-enters the transmission optical fiber (Raman amplification medium) due to backward Rayleigh scattering in the EDF. In such a process, if the gain of the EDF is larger than the reflection attenuation amount (power ratio of incident light and reflected light) due to the backward Rayleigh scattering effect in the transmission optical fiber and the EDF, a resonator is formed by the transmission optical fiber and the EDF. And an oscillation phenomenon due to the resonator may occur.
[0008]
FIG. 2 shows an output spectrum example when the Raman amplifier and the EDFA are serially arranged. The horizontal axis indicates the wavelength, and the vertical axis indicates the light intensity. It can be seen that there is a fine oscillation.
[0009]
It is known that this oscillation phenomenon can be suppressed by disposing an optical isolator in front of the EDFA. It is also known that the noise figure can be improved. For example, P.I. B. Hansen et al, “High sensitive 1.3 μm optically received receiving Raman amplification”, Electronics Letters, vol. 32, pp. 2164-2165 (1996).
[0010]
[Problems to be solved by the invention]
When the Raman amplifier and the EDFA are serially connected, a Raman amplification excitation light source and an EDF excitation light source are separately prepared in the configuration described in US Pat. No. 6,396,623. However, if the transmission optical fiber is excited with a single excitation light source to cause Raman amplification and the EDF is excited, the cost can be reduced.
[0011]
In order to suppress the oscillation phenomenon, it is necessary to provide an optical isolator between the Raman amplifier and the EDF, which suppresses return light propagating in the opposite direction to the signal light, such as reflected light and scattered light generated by the EDF. Such an optical isolator also blocks the excitation light, making it impossible to share one excitation light source for EDF and Raman amplification.
[0012]
U.S. Pat. No. 6,424,457 describes a configuration in which a bypass path for bypassing an optical isolator that blocks ASE light from the rear optical fiber amplifier is provided and excitation light for the rear EDFA is bypassed by the bypass path. .
[0013]
If a WDM (wavelength division multiplexing) optical coupler is placed on the input side and output side of the optical isolator placed between the Raman amplifier and the EDF so that the pump light can bypass the optical isolator, the pump light source can be shared. Become. However, in this configuration, the signal light passes through the WDM optical coupler and the optical isolator on the input side and output side of the optical isolator, and the excitation light passes through the WDM optical coupler on the input side and output side of the optical isolator. Since the signal light passes through the three optical devices and the excitation light passes through the two optical devices, the insertion loss increases for both.
[0014]
Another solution is to use an optical isolator that allows reverse transmission for a particular wavelength or wavelength band. However, in this configuration, the wavelength of the excitation light cannot be changed later, and the restriction is too strong.
[0015]
An object of the present invention is to propose an optical amplifier that is less likely to oscillate, including a serially connected Raman amplification medium and a rare earth-doped optical amplification fiber.
[0016]
[Means for Solving the Problems]
An optical amplifier according to the present invention includes a Raman amplification medium, a rare earth-doped optical amplification fiber, a pumping light source that generates pumping light having an excitation wavelength for exciting the Raman amplification medium and the rare earth-doped optical amplification fiber, and the pumping light source. An optical coupler that supplies the pumping light to be output to the rare earth-doped optical amplifying fiber from behind, a reflector that selectively reflects the light having the pumping wavelength, the Raman amplifying medium, and the rare earth-doped optical amplifying fiber. An optical circulator disposed between the optical amplifiers, the signal light output from the Raman amplification medium is transferred to the rare earth doped optical fiber, and the excitation light and ASE light from the rare earth doped optical fiber are transferred to the reflector. And an optical circulator for transferring the excitation light reflected by the reflector to the Raman amplification medium.
[0017]
Preferably, the reflector comprises a fiber Bragg grating.
[0018]
【Example】
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0019]
FIG. 1 is a schematic block diagram of an embodiment of the present invention. Signal light S (wavelength λs) is input to the input terminal 10 from an optical transmitter (not shown). The wavelength λs is, for example, the 1550 nm band. The signal light S is input from the input terminal 10 to the transmission optical fiber 12. The transmission optical fiber 12 functions as a Raman amplification medium as will be described later, and amplifies the signal light S.
[0020]
The signal light propagated through the transmission optical fiber 12 is input to the port A of the optical circulator 14 and input from the port B to the erbium-doped optical fiber (EDF) 16. The optical circulator 14 is an optical device that outputs the input light of the port A from the port B, outputs the input light of the port B from the port C, and outputs the input light of the port C from the port A.
[0021]
The EDF 16 amplifies the signal light S. The signal light S amplified by the EDF 16 is input to the port B of the optical circulator 18 and output from the port C. The optical circulator 18 is an optical device that outputs the input light of the port A from the port B and outputs the input light of the port B from the port C. The amplified signal light output from the port C of the optical circulator 18 is supplied from the output terminal 20 to a transmission optical fiber or an optical receiver (not shown).
[0022]
In principle, Raman amplification has a gain at a wavelength that is about 100 nm longer than the wavelength of the pumping light, so a wavelength shorter than the signal wavelength by about 100 nm is used in the wavelength band that can be used as the pumping light for an optical fiber amplifier Just do it. For signal light in the 1550 nm band, for example, 1460 nm is adopted as the wavelength of the excitation light.
[0023]
The laser diodes 22 and 24 oscillate in the 1460 nm band. The optical coupler 26 combines the output lights of the laser diodes 22 and 24 and applies the combined light to the port A of the optical circulator 18 as excitation light. When the optical coupler 26 is simply an optical element that combines the output lights of the laser diodes 22 and 24, the laser oscillation wavelengths of the laser diodes 22 and 24 are slightly different in order to avoid interference. When the optical coupler 26 is a polarization beam splitter that combines the output lights of the laser diodes 22 and 24 with polarizations orthogonal to each other, the laser oscillation wavelengths of the laser diodes 22 and 24 may be the same. The laser diodes 22 and 24 and the optical coupler 26 constitute an excitation light source. Let λp be the wavelength of the excitation light output from this excitation light source.
[0024]
The optical circulator 18 outputs the excitation light input to the port A from the port B toward the EDF 16. The EDF 16 is excited by the excitation light from the port B of the optical circulator 18 and amplifies the signal light S as described above. At that time, the EDF 16 generates ASE light. Half of the ASE light generated in the EDF 16 is statistically propagated in the opposite direction to the signal light S.
[0025]
The excitation light that could not be absorbed by the EDF 16 and the ASE light generated in the EDF 16 and propagating in the opposite direction to the signal light S are input to the port B of the optical circulator 14 and output from the port C. Connected to the port C of the optical circulator 14 is a fiber Bragg grating (FBG) 28 that selectively reflects the wavelength λp of the excitation light and absorbs other light, particularly ASE light generated by the EDF 16. The FBG 28 reflects only the excitation light out of the light from the port C of the optical circulator 14 (excitation light and ASE light generated by the EDF 16) and absorbs the rest.
[0026]
The excitation light reflected by the FBG 28 reenters the port C of the optical circulator 14 and is output from the port A. The pumping light output from the port C of the optical circulator 14 is input to the transmission optical fiber 12 and causes Raman amplification in the transmission optical fiber 12. By this Raman amplification, the signal light S is optically amplified as described above.
[0027]
In this embodiment, the signal light only needs to pass through the optical circulator 14 between the transmission optical fiber 12 serving as the Raman amplification medium and the EDF 16, so that the signal light passes through two WDM optical couplers and an optical isolator. , Less loss. The loss of the pumping light is only a loss caused by passing through the optical circulator 14 twice, and is smaller than the case of passing through the two WDM optical couplers and the optical isolator. The fiber Bragg grating 28 can significantly reduce the ASE light to a level substantially close to zero.
[0028]
The fiber Bragg grating 28 can be connected to the optical circulator 14 by fusion. Therefore, the exchange is easy. When changing the wavelength λp of the excitation light, the fiber Bragg grating 28 may be changed to a fiber Bragg grating that reflects the new excitation wavelength λp.
[0029]
Obviously, instead of the fiber Bragg grating 28, it is also possible to use a reflector having a selective reflection function as well.
[0030]
FIG. 3 is a spectrum example of the optical amplification output according to this embodiment. It can be seen that fine oscillation is suppressed. The horizontal axis indicates the wavelength, and the vertical axis indicates the optical power (dBm). The excitation power measured at the connection point between the port B of the optical circulator 18 and the EDF 16 is 175 mW.
[0031]
【The invention's effect】
As can be easily understood from the above description, according to the present invention, it is possible to realize an optical amplifier that does not oscillate with high gain. By adjusting the amplification band of Raman amplification, a wide amplification band can be easily secured as a whole. Since there is little loss of signal light and pump light, pump light power can be used effectively. That is, if the total gain is the same, the pump light power can be reduced, and if the pump power is the same, the total gain can be increased. Since the propagation of ASE light from the rare earth-doped optical amplification fiber to the Raman amplification medium is blocked, a low-noise optical amplifier can be realized.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of an embodiment of the present invention.
FIG. 2 is a spectrum example of an optical amplification output causing oscillation.
FIG. 3 is a spectrum example of an optical amplification output according to the present embodiment.
[Explanation of symbols]
10: input terminal 12: transmission optical fiber 14: optical circulator 16: erbium-doped optical amplification fiber (EDF)
18: Optical circulator 20: Output terminals 22, 24: Laser diode 26: Optical coupler 28: Fiber Bragg grating

Claims (2)

A Raman amplification medium (12);
A rare earth-doped optical amplification fiber (16);
An excitation light source (22, 24, 26) for generating excitation light having an excitation wavelength for exciting the Raman amplification medium (12) and the rare earth-doped optical amplification fiber (16);
An optical coupler (18) for supplying the pumping light output from the pumping light source (22, 24, 26) to the rare earth-doped optical amplification fiber (16) from the rear;
A reflector (28) that selectively reflects light of the excitation wavelength;
An optical circulator (14) disposed between the Raman amplifying medium (12) and the rare earth doped optical amplifying fiber (16), wherein the signal light output from the Raman amplifying medium (12) is used as the rare earth doped The pump light and the ASE light from the rare earth doped optical fiber (16) are transferred to the reflector (28), and the pump light reflected by the reflector (28) is transferred to the optical amplification fiber (16). An optical amplifier comprising an optical circulator for transfer to the Raman amplification medium (12). The optical amplifier according to claim 1, wherein the reflector comprises a fiber Bragg grating.

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