JP2005070522A - Compound optical amplifier - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To easily recover an original gain profile even when a Raman gain is reduced due to the individual difference or secular deterioration of a transmission optical fiber, or cracks on the optical fiber. <P>SOLUTION: A transmission optical fiber 12 optically amplifies signal light with Raman amplification. An erbium doped optical fiber (EDF) 22 further amplifies the signal light optically. A pumping light source 30 generates pumping light to pump the transmission optical fiber 12 and the EDF 22. A pumping light supplying device 26 supplies the pumping light to the EDF 22 from the signal light output side of the EDF 22. A standby pumping light source 40 stands ready to generate secondary pumping light to pump the transmission optical fiber 12. Signal light power detectors 20, 22 monitor input signal light power of the EDF 22. When the Raman gain is reduced, a driving circuit 44 drives the standby pumping light source 40 in accordance with a result of detection of the signal light power detectors 20, 22 so as to make the input signal light power of the EDF 22 be a desired value. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a composite optical amplifier including a serially connected Raman amplifier and a rare earth-doped optical amplifier.

  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 includes 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.

  An optical amplification repeater transmission system that amplifies signal light by causing Raman amplification in a transmission optical fiber is also known.

  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.

  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.

  Two power controls are applied to the excitation light source of the optical amplifier. That is, the output power of the laser diode that is the excitation light source is monitored, and the automatic power control (APC) that controls the drive current so that the power is constant, and the power of the amplified signal light is constant. Automatic level control (ALC) for controlling the excitation power.

The following configuration is known as a configuration for adjusting the gain profile of an optical amplifier to a desired shape. That is, a variable gain equalizer is disposed at the output of the optical amplifier, and the optical powers of the short wavelength light and the long wavelength light of the output light of the variable gain equalizer are measured so that the optical powers of both are equal to each other. The gain profile of the variable gain equalizer is controlled.
US Pat. No. 6,396,623 US Pat. No. 6,424,457 US Pat. No. 6,456,426

  The composite optical amplifier using both the EDFA and the Raman amplifier has the following problems. That is, since the Raman gain depends on the effective cross-sectional area and transmission loss of the transmission optical fiber, the gain profile of the composite optical amplifier varies depending on the individual difference (effective cross-sectional area and / or individual difference of transmission loss) of the transmission fiber. In addition, the gain of the Raman amplifier decreases and the gain profile of the composite optical amplifier greatly collapses due to the insertion of the optical fiber accompanying the repair of the optical cable and the increase of the fiber loss due to the aging of the transmission optical fiber. In particular, when a cable failure occurs immediately before the EDFA, and an optical fiber having an effective area larger than that of the transmission optical fiber is inserted between the EDFA and the transmission optical fiber functioning as a Raman amplifier, the Raman gain is remarkably increased. Decrease.

  FIG. 2 shows an example of the gain profile before and after the optical fiber is inserted just before the EDFA. The vertical axis represents the net gain, and the horizontal axis represents the wavelength. A solid line indicates a gain profile before the optical fiber is inserted, that is, a gain profile during normal operation, and a broken line indicates a gain profile after the optical fiber is inserted. Before the optical fiber is inserted, the gain profile is almost flat, but after the optical fiber is inserted, the gain profile is greatly broken.

  By arranging the variable gain equalizer in the output stage, the gain profile can be adjusted. For this purpose, a large number of optical elements such as a 3 dB optical coupler, two optical filters, and two photodiodes are used. Is required.

  The present invention can easily cope with individual differences in transmission optical fibers, and maintains a gain profile in a desired shape even when there is an increase in the loss of the transmission optical fiber that has been inserted as an optical fiber or a Raman amplifier. An object of the present invention is to provide a composite optical amplifier that can be used.

  It is another object of the present invention to provide a composite optical amplifier that can easily cope with individual differences in transmission optical fibers and maintain a gain profile in a desired shape.

  The composite optical amplifier according to the present invention includes a Raman amplification medium that optically amplifies signal light, a rare earth-added optical amplification medium that optically amplifies signal light amplified by the Raman amplification medium, the Raman amplification medium, and the rare earth addition An excitation light source that generates excitation light for exciting the optical amplification medium, and an excitation light that supplies excitation light output from the excitation light source to the rare earth-doped optical amplification medium from the signal light output side of the rare earth-doped optical amplification medium And a signal light power detector for monitoring the input signal light power of the rare earth-doped optical amplifying medium, and sub-excitation light for exciting the Raman amplifying medium. In accordance with the detection result of the standby excitation light source and the signal light power detector, the standby excitation light source is driven so that the input signal light power of the rare earth-doped optical amplification medium becomes a desired value. Characterized by comprising a driving circuit.

  Thus, by preparing the standby excitation light source in advance, even if the gain of Raman amplification decreases, the output signal power of Raman amplification can be easily recovered to a desired value by the standby excitation light source. Since the signal input power of the subsequent rare earth doped optical amplifier can be easily maintained at a desired value, it becomes easy to support the entire gain profile. Therefore, the gain profile can be easily maintained, and a stable amplification operation can be expected.

  The composite optical amplifier according to the present invention is further a pumping light / signal light transfer device disposed between the Raman amplifying medium and the rare earth-doped optical amplifying medium, wherein the signal light optically amplified by the Raman amplifying medium is transmitted. A signal light transfer path for transferring toward the rare-earth-doped optical amplifying medium and hindering optical transmission in the reverse direction, and a bypass for transferring excitation light not absorbed by the rare-earth-doped optical amplifying medium toward the Raman amplifying medium And a pumping light / signal light transfer device.

  Such an excitation light / signal light transfer device can prevent the ASE light generated in the rare earth light amplifying medium from entering the Raman amplifying medium and reentering the rare earth light amplifying medium. Thereby, the deterioration of the oscillation phenomenon and the noise index can be suppressed. In addition, the Raman amplification medium can be excited with excitation light that has not been absorbed by the rare earth-doped optical amplification medium. Excitation light can be used effectively.

  The composite optical amplifier according to the present invention further includes an optical coupler for coupling the output light of the standby pumping light source to the bypass path. Thereby, the output light of the standby excitation light source can be easily supplied to the Raman amplification medium when necessary.

  Preferably, the wavelength of the preliminary excitation light is substantially equal to the wavelength of the excitation light output from the excitation light source. This makes it easy to achieve a gain profile similar to that when only the excitation light source is used.

  According to the present invention, even if the output signal power of the Raman amplifier decreases due to individual differences in the transmission optical fiber, an increase in the loss of the transmission optical fiber, or a new optical fiber, the output of the Raman amplifier is reduced by the standby pumping light source. The signal power can be recovered to a desired value. Since the signal input power of the subsequent rare earth doped optical amplifier can be easily maintained at a desired value, it becomes easy to support the entire gain profile. Therefore, the gain profile can be easily maintained, and a stable amplification operation can be expected. It is only necessary to monitor the power of the signal light input to the rare earth-doped optical amplifying medium, but it is only necessary to add one photodetector, which can be realized at low cost.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows a schematic block diagram of an embodiment of the present invention. Signal light is input to the input terminal 10 from an optical transmission terminal station or an optical transmission line (not shown). The signal light is input to the transmission optical fiber 12 from the input terminal 10 and is optically amplified by Raman amplification at the signal light output side portion of the transmission optical fiber 12, and then the WDM optical coupler 14, the optical isolator 16, and the WDM optical coupler 18. Is input to the duplexer 20. The duplexer 20 applies most of the signal light input from the WDM optical coupler 18 to the EDF 22 and the rest to the light receiver 24. The signal light supplied from the duplexer 20 to the EDF 22 is optically amplified while propagating through the EDF 22 and is input to the port B of the optical circulator 26. The optical circulator 26 supplies the signal light input to the port B from the port C to the output terminal 28.

  The excitation light source 30 includes laser diodes 32 and 34 having a wavelength of about 1460 nm and a multiplexer 36 that combines the output lights of the laser diodes 32 and 34. The laser beam combined by the multiplexer 36 is applied to the port A of the optical circulator 26 as the excitation light of the EDF 22 and the excitation light of Raman amplification of the transmission optical fiber 12. The optical circulator 26 supplies the pumping light input from the pumping light source 30 to the port A to the EDF 22 from the port B. The excitation light propagates in the EDF 22 in the opposite direction to the signal light, and excites erbium added to the EDF 22 during that time. The excited erbium optically amplifies the signal light.

  The excitation light remaining without being absorbed by the EDF 22 is output from the EDF 22, passes through the multiplexer 20, and is input to the WDM optical coupler 18. Between the WDM optical coupler 14 and the WDM coupler 18, a bypass path 38 that bypasses the optical isolator 18 with light having an excitation wavelength (here, 1460 nm) is connected. The pumping light remaining without being absorbed by the EDF 22 passes through the WDM optical coupler 18, the bypass path 38 and the WDM optical coupler 14 and enters the transmission optical fiber 12. The pumping light input to the transmission optical fiber 12 causes Raman amplification in the transmission optical fiber 12, whereby the signal light is optically amplified as described above.

  The WDM optical coupler 14, the optical isolator 18, the WDM coupler 18, and the bypass path 38 constitute a pumping light / signal light transfer apparatus. With this pumping light / signal light transfer device, it is possible to prevent the ASE light generated in the EDF 22 from entering the transmission optical fiber 12 and reentering the EDF 22. Thereby, the deterioration of the oscillation phenomenon and the noise index can be suppressed. In addition, since the pump light that has not been absorbed by the EDF 22 is used as pump light for Raman amplification in the transmission optical fiber 12, the pump light can be used effectively.

  In this embodiment, in preparation for an increase in the loss of the transmission optical fiber 12 or an interruption of the optical fiber between the transmission optical fiber 12 and the WDM optical coupler 14, a standby standby pumping light source 44 for Raman amplification is provided. A multiplexer 46 for optically coupling the output light (sub-excitation light) of the standby excitation light source 44 to the bypass 38 and a drive circuit 44 for driving the standby excitation light source 40 are prepared. The wavelength of the standby excitation light source 40 is substantially equal to the wavelength of the excitation light source 30 (here, 1460 nm). Thereby, it becomes easy to realize the same gain profile as when only the excitation light source 30 is used.

  In other words, the light receiver 24 measures the power of the signal light output from the transmission optical fiber 12. The comparison circuit 46 compares the output of the light receiver 24 with a predetermined reference value Vref. The drive circuit 44 drives the standby excitation light source 40 with a drive current such that the comparison result of the comparison circuit 46 becomes zero. Thereby, the standby pumping light source 40 outputs pumping light having a power such that the signal light power after Raman amplification becomes a predetermined value.

  Of course, at the beginning of operation, the pumping light (main pumping light) output from the pumping light source 30 can cause Raman amplification with sufficient gain in the transmission optical fiber 12, so that the standby pumping light source 40 and the drive circuit 44 are Paused. The comparison circuit 46 may also be paused. When the threshold value of the signal light power after Raman amplification that does not require the standby pumping light source 40 is input to the comparison circuit 46 and the output voltage of the light receiver 24 is equal to or higher than the threshold value, the drive circuit 44 and thus the standby pumping light source 40 is You may make it pause.

  If the loss of the transmission optical fiber 12 increases after the start of use, or if another optical fiber is inserted between the transmission optical fiber 12 and the WDM optical coupler 14 due to a cable failure or the like, The input power of the EDF 22 is greatly deviated from the design value, and the gain profile of the EDF 22 is deformed. If the output power of the excitation light source 30 is simply adjusted, it is very difficult to make the overall gain profile flat.

  In this embodiment, the Raman gain can be adjusted, that is, increased without changing the output power of the excitation light source 30 by utilizing the standby excitation light source 40. The standby pumping light source 40, the drive circuit 44, and the comparison circuit 46 may be set to a substantially operating state in which auxiliary pumping light having a certain level of optical power is output from the standby pumping light source 40. Then, the Raman amplification gain is adjusted by the comparison circuit 46 and the drive circuit 44 so that the signal light power after Raman amplification, that is, the input signal light power of the EDF 22 becomes equal to a desired value, for example, a design value. Since the output power of the pumping light source 30 is not changed and the input signal light power of the EDF 22 is equal to the design value, the optical amplification characteristics at the EDF 22 are maintained as they are. As a result, the entire gain profile can be easily readjusted to an appropriate shape.

  Although the invention has been described with reference to specific illustrative embodiments, various modifications and alterations may be made to the above-described embodiments without departing from the scope of the invention as defined in the claims. This is obvious to an engineer in the field to which the present invention belongs, and such changes and modifications are also included in the technical scope of the present invention.

It is a schematic block diagram of one Example of this invention. It is an example of the change of the gain profile by a cable failure.

Explanation of symbols

10: input terminal 12: transmission optical fiber 14: WDM optical coupler 16: optical isolator 18: WDM optical coupler 20: duplexer 22: erbium-doped optical fiber 24: light receiver 26: optical circulator 28: output terminal 30: excitation light source 32, 34: laser diode 36: multiplexer 38: bypass path 40: standby excitation light source 42: multiplexer 44: drive circuit 46: comparison circuit

Claims (4)

A Raman amplification medium (12) for optically amplifying signal light;
A rare earth-doped optical amplification medium (22) for optically amplifying the signal light optically amplified by the Raman amplification medium (12);
An excitation light source (30) for generating excitation light for exciting the Raman amplification medium (12) and the rare earth-doped optical amplification medium (22);
Excitation light supply element (26) for supplying excitation light output from the excitation light source (30) to the rare earth addition optical amplification medium (22) from the signal light output side of the rare earth addition optical amplification medium (22)
A composite optical amplifier comprising:
A signal light power detector (20, 22) for monitoring the input signal light power of the rare earth-doped optical amplification medium (22);
A standby excitation light source (40) waiting to generate sub-excitation light for exciting the Raman amplification medium (12);
In accordance with the detection result of the signal light power detector (20, 22), a drive circuit for driving the standby pumping light source (40) so that the input signal light power of the rare earth-doped optical amplification medium (22) becomes a desired value. 44)
A composite optical amplifier comprising:   Furthermore, a pumping light / signal light transfer device disposed between the Raman amplifying medium (12) and the rare earth-doped optical amplifying medium (22), the signal light optically amplified by the Raman amplifying medium (12). Is transferred toward the rare earth-doped optical amplifying medium (22), and a signal light transmission path that inhibits optical transmission in the reverse direction, and excitation light that is not absorbed by the rare earth-doped optical amplifying medium (22) 2. The composite optical amplifier according to claim 1, further comprising a pumping light / signal light transfer device (14, 16, 18, 38) having a bypass path (38) for transferring toward the medium (12).   The composite optical amplifier according to claim 2, further comprising an optical coupler (42) for coupling output light of the standby pumping light source (40) to the bypass path (38).   The composite optical amplifier according to any one of claims 1 to 3, wherein the wavelength of the preliminary pumping light is substantially equal to the wavelength of the pumping light output from the pumping light source (30).

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