Apparatus and method for amplification medium performance simulation, and optical amplifier Number:7,154,664 from the United States Patent and Trademark Office (PTO) owispatent An amplification medium simulation apparatus comprises a basic data retaining unit 21, an input signal beam information retaining unit 22, and a simulation executing unit 31 approximating and calculating an output signal beam power at each signal beam wavelength outputted from the amplification medium involving a fluctuation in ion population at the metastable energy level in the amplification medium due to input of the input signal beam, by using contents retained in the basic data retaining unit 21 and the input signal beam information retaining unit 22, and outputting a result of calculation as a result of simulation of performance of the amplification medium.<H amplification, performance, Apparatus, and, m, 7154664.html, Patent, pto, 7154664

Title: Apparatus and method for amplification medium performance simulation, and optical amplifier

Abstract: An amplification medium simulation apparatus comprises a basic data retaining unit 21, an input signal beam information retaining unit 22, and a simulation executing unit 31 approximating and calculating an output signal beam power at each signal beam wavelength outputted from the amplification medium involving a fluctuation in ion population at the metastable energy level in the amplification medium due to input of the input signal beam, by using contents retained in the basic data retaining unit 21 and the input signal beam information retaining unit 22, and outputting a result of calculation as a result of simulation of performance of the amplification medium.

Patent Number: 7,154,664 Issued on 12/26/2006 to Nishihara, et al.

Inventors: Nishihara; Masato (Kawasaki, JP), Sugaya; Yasushi (Kawasaki, JP), Hayashi; Etsuko (Kawasaki, JP)
Assignee: Fujitsu Limited (Kawasaki, JP)

Appl. No.: 11/187,938

Filed: July 25, 2005

Current U.S. Class: 359/341.42 ; 359/337.1; 398/37

Current International Class: H01S 3/00 (20060101); H04B 10/08 (20060101)

Field of Search: 359/341.42,337.1 398/37

References Cited [Referenced By]

U.S. Patent Documents


6144486	November 2000	Bennett et al.
6631027	October 2003	Gerrish et al.
6690508	February 2004	Tian et al.
6798567	September 2004	Feldman et al.
6836355	December 2004	Lelic et al.
6891662	May 2005	Sugaya et al.
2002/0041433	April 2002	Terahara
2002/0191276	December 2002	Onaka et al.
2004/0156094	August 2004	Kawahara et al.
2005/0254119	November 2005	Nishihara et al.
2006/0087723	April 2006	Takeyama et al.

Foreign Patent Documents


1033834	Sep., 2000	EP
1089477	Apr., 2001	EP
2000-261078	Sep., 2000	JP
2000-261079	Sep., 2000	JP
WO 98/36294	Aug., 1998	WO

Other References

Jou et al. Application of SPICE Simulation to Study WDM and SCM Systems Using EDFAs with Chirping. IEEE Transactions On Education, vol. 45, No. 3, Aug. 2002. cited by examiner .
R. C. Giles, et al., "Modeling Erbium-Doped Fiber Amplifiers", Journal of Lightwave Technology, vol. 9, No. 2, Feb. 1991. cited by other .
M. Nishihara, et al., "Characterization and New Numerical Model of Spectral Hole Burning In Broadband Erbium-Doped Fiber Amplifier", 2003 Optical Society of America. cited by other .
T.Aizawa, et al., "Effect of Spectral-Hole Burning on Multi-Channel EDFA Gain Profile", In: Proceedings of Conference on Optical Communication 1999, (OFC'99), WGI, 1999, p. 102-104. cited by other .
E. Desurvire, et al., "Erbium-Doped Fiber Amplifiers: Device and System Developments", John Wiley & Sons, 2002, p. 265-277. cited by other .
P. C. Becker, et al., "Erbium-Doped Fiber Amplifiers: Fundamentals and Technology", Academic Press, 1999, p. 156-161, 429-449. cited by other.

Primary Examiner: Hughes; Deandra M.
Attorney, Agent or Firm: Staas & Halsey LLP

Parent Case Text

This application is a continuation application, filed under 35 USC 111(a), of International Application PCT/JP2003/008219, filed Jun. 27, 2003.

Claims

The invention claimed is:

1. An amplification medium performance simulation apparatus for simulating performance of an amplification medium excited by a pump light from a pumping source to amplify a signal light comprising: a basic data retaining unit for retaining basic data of said amplification medium; an input signal light information retaining unit for retaining a total power and a power at each wavelength of an input signal light as information on the input signal light to be inputted to an amplification medium to be simulated; and a simulation executing unit for reckoning a fluctuation in ion population at metastable energy levels corresponding to said input signal wavelength and a wavelength which is specific to said amplification medium, in said amplification medium caused by input of said input signal light, and approximating and calculating an output signal light power at each signal light wavelength outputted from said amplification medium on a basis of a result of the reckoning, by using contents retained in said basic data retaining unit and said input signal light information retaining unit, and outputting a result of the calculation as a result of simulation of the performance of said amplification medium.

2. The amplification medium performance simulation apparatus according to claim 1, wherein said simulation executing unit comprises: a population inversion rate calculating unit for calculating a population inversion rate on the basis of a signal light power according to a position with a coordinate in the longitudinal direction of said amplification medium; a population inversion rate change quantity calculating unit for calculating a quantity of a change in population inversion rate which may occur due to a fluctuation in ion population at the metastable energy level of said amplification medium caused by input of said input signal light, as a function of a wavelength of the input signal light and a position in the longitudinal direction of said amplification medium, by using the population inversion rate calculated by said population inversion rate calculating unit and contents retained in said basic data retaining unit and said input signal light information retaining unit; a signal light power change calculating unit for performing calculation of a change in optical power of the signal light propagating through said amplification medium from a signal light input end in said amplification medium in each of minute propagation ranges started from the signal light input end and terminated at a signal light output end, by using a differential equation defined by the quantity of a change in population inversion rate calculated by said population inversion rate change quantity calculating unit and the contents retained in said basic data retaining unit and said input signal light information retaining unit; a signal light power calculating unit for adding, in order, changes in optical power in the minute propagation ranges from a change in optical power in the minute range at the signal input end as a starting point to a change in optical power in the minute range at the signal light output end as a terminating point calculated by said signal light power change calculating unit to the power value of the input signal light retained in said input signal light information retaining unit between the signal light input end and the signal light output end, to calculate a signal light power according to a position with a coordinate in the longitudinal direction of the signal light propagating in said amplification medium including the fluctuation in ion population at the metastable energy level in said amplification medium caused by input of said input signal light; and an outputting process unit for outputting a result of calculation of the power of the signal light outputted from the signal light output end calculated by said signal light power calculating unit as a result of simulation of the performance of said amplification medium.

3. The amplification medium performance simulation apparatus according to claim 2, wherein said population inversion rate change quantity calculating unit uses at least one or more Gaussian functions as functions for calculating the quantity of a change in the population inversion rate.

4. The amplification medium performance simulation apparatus according to claim 2, wherein said population inversion rate change quantity calculating unit for calculating a quantity of a change in the population inversion rate comprises: a first function operating unit for operating a first function having a first wavelength band in a gain saturation state as a center; a second function operating unit for operating a second function comprised of a function having a second wavelength band characteristic of said amplification medium as a center; and an adding unit for adding results of the operations from said first function operating unit and said second function operating unit.

5. The amplification medium performance simulation apparatus according to claim 4, wherein said first function operated by said first function operating unit is composed of a total of Gaussian functions given according to respective wavelengths of the input signal light, and said second function operated by said second function operating unit is composed of a total of a plurality of Gaussian functions.

6. The amplification medium performance simulation apparatus according to claim 5, wherein the Gaussian function given according to each wavelength of the input signal light in the first function is determined as a value expressed in terms of a center wavelength which is a wavelength of the input signal light and a full width half maximum according to said amplification medium, each of the Gaussian functions in the second function is determined as a value expressed in terms of a center wavelength which is in a second wavelength band characteristic of said amplification medium and a full width half maximum according to said amplification medium, and the full width half maximum of each of the Gaussian functions in the first function and the second function is retained in said basic data retaining unit.

7. The amplification medium performance simulation apparatus according to claim 5, wherein a depth of each of the Gaussian functions in the first function or the second function is defined by a depth function which increases as a total power of the input signal light increases, and saturates above a predetermined value.

8. The amplification medium performance simulation apparatus according to claim 7, wherein the depth function of each Gaussian function given according to each wavelength of the input signal light in the first function is defined by a function having a wavelength .lamda..sub.i of the input signal light, an optical power P.sub.i(z) at a position with a coordinate z in the longitudinal direction of said amplification medium at the wavelength .lamda..sub.i of the input signal light and a total power P.sub.total(z) of the input signal light at a position with a coordinate z in the longitudinal direction of said amplification medium as variables; the depth function of each Gaussian function in the second function is defined by a function having a wavelength .lamda..sub.i in the second wavelength band, a total power P.sub.total(z) of the input signal light at a position with the coordinate z in the longitudinal direction of said amplification medium and a population inversion rate n(z) of said amplification medium as variables; and coefficients defining the depth functions of the Gaussian functions in the first function and the second function are retained in said basic data retaining unit.

9. The amplification medium performance simulation apparatus according to claims 2, wherein the basic data retaining unit retains, as the basic data of said amplification medium, at least an overall length of said amplification medium, a gain coefficient g(.lamda.), an absorption coefficient a(.lamda.) and a loss l(.lamda.) expressed as functional equations with respect to each input signal light wavelength, and a population inversion rate n(z) not added thereto the fluctuation in ion population at the metastable energy level in said amplification medium; said signal light power change calculating unit calculates the population inversion rate n(z) from a signal light power according to a position with a coordinate in the longitudinal direction of the signal light propagating in said amplification medium calculated by said signal light power calculating unit, and calculates a minute change in optical power of the signal light propagating at a position with the coordinate z in the longitudinal direction of said amplification medium, by using a change in optical power in each minute unit of the length in the longitudinal direction of said amplification medium dP(z)/dz={(g(.lamda.)+.alpha.(.lamda.))(n(z).DELTA.n.sub.SHB(.lamda.,z))-- (.alpha.(.lamda.)+1 (.lamda.))}P(z) using the population inversion rate n(z), the change quantity .DELTA.n.sub.SHB(.lamda.,Z) of the population inversion rate calculated by said population inversion rate change quantity calculating unit and the basic data retained in said basic data retaining unit.

10. The amplification medium performance simulation apparatus according to claims 1, wherein said simulation executing unit approximates and calculates gain deviation among signal light wavelengths caused by spectral hole burning.

11. An optical amplifier comprising: a pumping source for outputting a pump light; a signal light amplification medium excited by the pump light from said pumping source to amplify an input signal light; and a gain equalizer for equalizing a gain of an output signal light outputted from said signal light amplification medium; wherein said gain equalizer has a gain equalization characteristic so that it compensates gain deviation due to a fluctuation in ion population at a metastable energy level in said amplification medium caused by input of the input signal light, on the basis of a result of simulation outputted from an amplification medium performance simulation apparatus for simulating the performance of said amplification medium excited by the pump light from said pumping source to amplify the signal light, said amplification medium performance simulation apparatus comprising: a basic data retaining unit for retaining basic data of said amplification medium; an input signal light information retaining unit for retaining a total power and a power at each wavelength of the input signal light as information on the input signal light to be inputted to said amplification medium to be simulated; and a simulation executing unit for reckoning a fluctuation in ion population at a metastable energy levels corresponding to said input signal wavelength and a wavelength which is specific to said amplification medium, in said amplification medium caused by input of the input signal light, and approximating and calculating an output signal light power at each signal light wavelength outputted from said amplification medium on a basis of a result of the reckoning, by using contents retained in said basic data retaining unit and said input signal light information retaining unit, and outputting a result of calculation as a result of simulation of the performance of said amplification medium.

12. An optical amplifier comprising: a pumping source for outputting a pump light; a signal light amplification medium excited by the pump light from said pumping source to amplify an input signal light; and a pumping source controlling unit for controlling said pumping source; said pumping source controlling unit controlling said pumping source so that it compensates gain deviation due to a fluctuation in ion population at a metastable energy level in said amplification medium caused by input of the input signal light, on the basis of a result of simulation outputted from an amplification medium performance simulation apparatus for simulating the performance of said amplification medium excited by the pump light from said pumping source to amplify the signal light, said amplification medium performance simulation apparatus comprising: a basic data retaining unit for retaining basic data of said amplification medium; an input signal light information retaining unit for retaining a total power and a power at each wavelength of the input signal light as information on the input signal light to be inputted to said amplification medium to be simulated; and a simulation executing unit for reckoning a fluctuation in ion population at a metastable energy levels corresponding to said input signal wavelength and a wavelength which is specific to said amplification medium, in said amplification medium caused by input of the input signal light, and approximating and calculating an output signal light power at each signal light wavelength outputted from said amplification medium on the basis of a result of the reckoning, by using contents retained in said basic data retaining unit and said input signal light information retaining unit, and outputting a result of calculation as a result of simulation of the performance of said amplification medium.

13. The optical amplifier according to claim 12, wherein said pumping source controlling unit comprises: a first power monitor for monitoring powers of the input signal light and the output signal light; a wavelength allocation information obtaining unit for obtaining wavelength allocation information on a signal light propagating in said amplification medium; an automatic gain control unit for outputting a signal for controlling said pumping source obtained on the basis of the powers of the input signal light and the output signal light monitored by said first power monitor so that a gain of said optical signal amplification medium is constant; and a correcting unit for correcting a control quantity for said pumping source in said automatic gain control unit on the basis of the wavelength allocation information obtained by said wavelength allocation information obtaining unit so that gain deviation in a wavelength band due to spectral hole burning decreases.

14. The optical amplifier according to claim 13, wherein said wavelength allocation information obtaining unit is comprised of a spectrum analyzer monitoring wavelength allocation of a signal light inputted to or outputted from said amplification medium.

15. The optical amplifier according to claim 13, wherein said wavelength allocation information obtaining unit obtains the wavelength allocation information from a control signal light transmitted together with the signal light.

16. The optical amplifier according to claim 12, said pumping source controlling unit comprises: a second power monitor for obtaining powers of the input signal light and the output signal light in each of a plurality of bands divided on the basis of a result of the simulation obtained by said amplification medium performance simulation apparatus; and an automatic average gain control unit for outputting a signal for controlling said pumping source on the basis of the powers of the input signal light and the output signal light in each of the bands obtained by said second power monitor so that average gains in said bands are equalized.

17. The optical amplifier according to claim 12, wherein said simulation executing unit comprises: a population inversion rate calculating unit for calculating a population inversion rate on the basis of a signal light power according to a position with a coordinate in the longitudinal direction of said amplification medium; a population inversion rate change quantity calculating unit for calculating a quantity of a change in population inversion rate which may occur due to a fluctuation in ion population at the metastable energy level of said amplification medium caused by input of said input signal light, as a function of a wavelength of the input signal light and a position in the longitudinal direction of said amplification medium, by using the population inversion rate calculated by said population inversion rate calculating unit and contents retained in said basic data retaining unit and said input signal light information retaining unit; a signal light power change calculating unit for performing calculation of a change in optical power of the signal light propagating through said amplification medium from a signal light input end in said amplification medium in each of minute propagation ranges started from the signal light input end and terminated at a signal light output end, by using a differential equation defined by the quantity of a change in population inversion rate calculated by said population inversion rate change quantity calculating unit and the contents retained in said basic data retaining unit and said input signal light information retaining unit; a signal light power calculating unit for adding, in order, changes in optical power in the minute propagation ranges from a change in optical power in the minute range at the signal input end as a starting point to a change in optical power in the minute range at the signal light output end as a terminating point calculated by said signal light power change calculating unit to the power value of the input signal light retained in said input signal light information retaining unit between the signal light input end and the signal light output end, to calculate a signal light power according to a position with a coordinate in the longitudinal direction of the signal light propagating in said amplification medium including the fluctuation in ion population at the metastable energy level in said amplification medium caused by input of said input signal light; and an outputting process unit for outputting a result of calculation of the power of the signal light outputted from the signal light output end calculated by said signal light power calculating unit as a result of simulation of the performance of said amplification medium.

18. The optical amplifier according to claim 12, wherein said population inversion rate change quantity calculating unit uses at least one or more Gaussian functions as functions for calculating the quantity of a change in the population inversion rate.

19. The optical amplifier according to claim 17, wherein said population inversion rate change quantity calculating unit for calculating a quantity of a change in the population inversion rate comprises: a first function operating unit for operating a first function having a first wavelength band in a gain saturation state as a center; a second function operating unit for operating a second function comprised of a function having a second wavelength band characteristic of said amplification medium as a center; and an adding unit for adding results of the operations from said first function operating unit and said second function operating unit.

20. The optical amplifier according to claim 19, wherein said first function operated by said first function operating unit is composed of a total of Gaussian functions given according to respective wavelengths of the input signal light, and said second function operated by said second function operating unit is composed of a total of a plurality of Gaussian functions.

21. The optical amplifier according to claim 20, wherein the Gaussian function given according to each wavelength of the input signal light in the first function is determined as a value expressed in terms of a center wavelength which is a wavelength of the input signal light and a full width half maximum according to said amplification medium, each of the Gaussian functions in the second function is determined as a value expressed in terms of a center wavelength which is in a second wavelength band characteristic of said amplification medium and a full width half maximum according to said amplification medium, and the full width half maximum of each of the Gaussian functions in the first function and the second function is retained in said basic data retaining unit.

22. The optical amplifier according to claim 20, wherein a depth of each of the Gaussian functions in the first function or the second function is defined by a depth function which increases as a total power of the input signal light increases, and saturates above a predetermined value.

23. The optical amplifier according to claim 22, wherein the depth function of each Gaussian function given according to each wavelength of the input signal light in the first function is defined by a function having a wavelength .lamda..sub.i of the input signal light, an optical power P.sub.i(z) at a position with a coordinate z in the longitudinal direction of said amplification medium at the wavelength .lamda..sub.i of the input signal light and a total power P.sub.total(z) of the input signal light at a position with a coordinate z in the longitudinal direction of said amplification medium as variables; the depth function of each Gaussian function in the second function is defined by a function having a wavelength .lamda..sub.i in the second wavelength band, a total power P.sub.total (z) of the input signal light at a position with the coordinate z in the longitudinal direction of said amplification medium and a population inversion rate n(z) of said amplification medium as variables; and coefficients defining the depth functions of the Gaussian functions in the first function and the second function are retained in said basic data retaining unit.

24. The optical amplifier according to claims 17, wherein the basic data retaining unit retains, as the basic data of said amplification medium, at least an overall length of said amplification medium, a gain coefficient g(.lamda.), an absorption coefficient .alpha.(.lamda.) and a loss l(.lamda.) expressed as functional equations with respect to each input signal light wavelength, and a population inversion rate n(z) not added thereto the fluctuation in ion population at the metastable energy level in said amplification medium; said signal light power change calculating unit calculates the population inversion rate n(z) from a signal light power according to a position with a coordinate in the longitudinal direction of the signal light propagating in said amplification medium calculated by said signal light power calculating unit, and calculates a minute change in optical power of the signal light propagating at a position with the coordinate z in the longitudinal direction of said amplification medium, by using a change in optical power in each minute unit of the length in the longitudinal direction of said amplification medium dP(z)/dz={(g(.lamda.)+.alpha.(.lamda.))(n(z)+.DELTA.n.sub.SHB(.lamda.,z))- -(.alpha.(.lamda.)+1 (.lamda.))}P(z) using the population inversion rate n(z), the change quantity .DELTA.n.sub.SHB(.lamda.,z) of the population inversion rate calculated by said population inversion rate change quantity calculating unit and the basic data retained in said basic data retaining unit.

25. The optical amplifier according to claims 12, wherein said simulation executing unit approximates and calculates gain deviation among signal light wavelengths caused by spectral hole burning.

Description

TECHNICAL FIELD

The present invention relates to an apparatus and a method for amplification medium performance simulation, and an optical amplifier.

On the basis of a rapid increase in data communication traffic with a recent rapid spread of the Internet, focused is a technique relating to a wavelength division multiplexing transmission technique which is a technique for increasing the speed and capacity of the network, and a photonic network which is a network in which each wavelength transmitted by means of the wavelength division multiplexing transmission technique is supposed to be one communication path.

The present invention relates to an apparatus and a method for amplification medium performance simulation suitable for use in simulation of performance of an amplification medium applied when a photonic network is configured, and an optical amplifier made on the basis of a result of simulation obtained by this apparatus.

BACKGROUND ART

Because of expectation for realization of an optical network (photonic network) having high flexibility, it is required for a node configuring the network to cope with a large change in the number of wavelengths, which are supposed to be a communication path. Particularly, an optical amplifier which is a constitutional element of a node is required to cope with a large change in allocation and the number of wavelengths to stabilize the amplification characteristic.

The wavelength characteristic of an amplification medium such as an EDFA (Erbium Doped Fiber Amplifier) used in a known optical network system, in which allocation and the number of the wavelengths are assumed not to be largely changed, can be presupposed to depend upon only population inversion by a single band approximation (refer to non-patent document 1). Namely, the wavelength characteristic can be approximated and grasped according to the value of the population inversion rate, with the whole amplification band of the EDFA being one unit.

In concrete, as shown in FIG. 24, a pattern of relative gain coefficients as being the wavelength characteristic over the whole range of the amplification bandwidth (wavelengths from 1500 to 1580 nm of an input signal light in the drawing) of the EDFA can be grasped for each population inversion rate. Accordingly, wavelength flatness of the EDF in C band (Conventional Band) is realized by combining the automatic gain control by which the population inversion is controlled to be constant, and the gain equalizer according to the relative gain coefficient distribution corresponding to the population inversion that is controlled to be constant.

FIG. 25 shows an example of the structure of an optical repeater 100 used in a known optical network system in which wavelength allocation and the number of wavelengths are not largely changed. The optical repeater 100 shown in FIG. 25 is configured by inserting an optical attenuator (VOA: Variable Optical Attenuator) 102 between two EDFA amplifying units 101-1 and 101-2 serially connected.

Each of the EDFA amplifying units 101-1 and 101-2 comprises branching couplers 101a and 101b, an EDFA 101c, photodiodes (PD: Photo Diode) 101d and 101e, and a control circuit 101f. In each of the EDFA amplifying units 101-1 and 101-2, the input/output powers are monitored by the respective photodiodes 101d and 101e, and an optical signal amplified by the EDFA 101c under the automatic gain control by the control circuit 101f is outputted.

As shown in FIG. 26, for example, when the input power of the optical repeater 100 is changed from the first level to the second level, the output power of the optical repeater 100 is made constant by adjusting the quantity of loss in the variable optical attenuator 102 while keeping the gain in each of the EDFA amplifying units 101-1 and 101-2 constant.

If the amplification characteristic of the EDFA 101c in each of the EDFA amplifying units 101-1 and 101-2 is assumed to be an optical network that can be approximated to a single band, the gain wavelength characteristic can be always kept constant by keeping the gain of each of the EDFA 101c constant. Accordingly, it becomes possible to make the gain wavelength characteristic of the optical repeater 100 flat irrespective of the input power, by disposing a gain equalizer whose loss characteristic is appropriately designed in the following stage of the EDFA amplifying units 101-1 and 101-2.

Namely, since it is supposed that the known optical repeater is applied to an optical network in which wavelength allocation and the number of wavelengths are not largely changed, the gain equalizer arranged in the following stage of the EDFA amplifying units 101-1 and 101-2 is designed on the assumption that the wavelength characteristic of an amplifying medium as above is approximated to a single band.

However, in an optical network recently demanded in which wavelength allocation and the number of wavelengths are largely changed, the gain deviation due to an effect of spectral hole burning (SHB: Spectral-Hole Burning) which is a local gain saturation effect in the wavelength region cannot be ignored when the selected wavelengths are arranged to be particularly gathered in a narrow band. Since the effect of this SHB differs according to wavelength allocation supposed in the optical network, it is necessary to analyze the gain deviation caused by SHB according to the wavelength allocation beforehand supposed when the apparatus is designed.

FIG. 27 shows gain deviation characteristic due to SHB of an EDFA. When a gain wavelength characteristic A in the saturated state where a saturation signal at 1540 nm (signal saturating the gain of the EDFA) is compared with a gain wavelength characteristic B in the non-saturated state where no saturation signal is inputted, it can be confirmed that, in the saturated state, the gain in the vicinity of the saturation signal wavelength and 1530 nm is decreased (refer to a gain difference C between the characteristics A and B) to make holes.

This phenomenon occurs due to a local gain saturation phenomenon of a gain medium having inhomogeneous broadening. In the known single band approximation, this local change in gain wavelength characteristic is ignored.

As a model of EDFA in which SHB is considered, there have been reported a model (refer to non-patent document 2) which separately deals with the absorption/emission process and the saturation process between energy levels formed by the inhomogeneous broadening, and a model which adds the quantity of gain fluctuation due to SHB derived from a result obtained by separately measuring the gain wavelength characteristic obtained by means of single band approximation (refer to non-patent document 3).

As techniques relating to the present invention, there are also techniques described in Patent Document 1 and Patent Document 2 shown below.

However, the technique described in Non-Patent Document 2 provides a very complex calculation formula for analyzing the gain deviation, thus has a disadvantage that the process requires a long time. The technique described in Non-Patent Document 3 considers only the neighborhood of the signal wavelength, thus has a disadvantage that the gain fluctuation in the vicinity of 1530 nm cannot be modeled.

As a method of measuring the amplification characteristic of an amplification medium, there is a method (hardware simulation) for measuring the amplification characteristic from an actually formed optical amplifier other than the method of calculating through numerical value calculation described above. However, some sorts of hardware simulation take a long time or require much labor to measure entirely the wide operation conditions of an optical repeater.

In the light of the above problems, an object of the present invention is to provide an apparatus and a method for amplification medium performance simulation, and an optical amplifier, which introduce a simple approximate expression, thereby modeling gain fluctuation in a range other than the neighborhood of the signal wave within a short time.

Non-Patent Document 1: C. R. Giles, et al., "Modeling Erbium Doped Fiber Amplifiers," IEEE J. of Lightwave Tchnol., pp. 271 283, vol. 9, no. 2, Feb., 1991;

Non-Patent Document 2: E. Desurvire, "ERBIUM-DOPED FIBER AMPLIFIERS Principles and Applications," John Wiley & Sons, Inc., Chapter 4, 1994;

Non-Patent Document 3: T. Aizawa, et al., "Effect of Spectral-Hole Burning on Multi Channel EDFA Gain Profile," OFC'99, WG1, 1999;

Patent Document 1: Japanese Patent Application Laid-Open No. 2000-261078; and

Patent Document 2: Japanese Patent Application Laid-Open No. 2000-261079.

DISCLOSURE OF INVENTION

To attain the above object, the present invention provides an amplification medium performance simulation apparatus for simulating performance of an amplification medium excited by a pump beam from a pumping source to amplify a signal beam comprising a basic data retaining unit for retaining basic data of the amplification medium, an input signal beam information retaining unit for retaining a total power and a power at each wavelength of an input signal beam as information on the input signal beam to be inputted to an amplification medium to be simulated, and a simulation executing unit for reckoning a fluctuation in ion population at a metastable energy level in the amplification medium caused by input of the input signal beam, and approximating and calculating an output signal beam power at each signal beam wavelength outputted from the amplification medium, by using contents retained in the basic data retaining unit and the input signal beam retaining unit, and outputting a result of the calculation as a result of simulation of the performance of the amplification medium.

The simulation executing unit may comprise a population inversion rate calculating unit for calculating a population inversion rate on the basis of a signal beam power according to a position with a coordinate in the longitudinal direction of the amplification medium, a population inversion rate change quantity calculating unit for calculating a quantity of a change in population inversion rate which may occur due to a fluctuation in ion population at the metastable energy level of the amplification medium caused by input of the input signal beam, as a function of a wavelength of the input signal beam and a position in the longitudinal direction of the amplification medium, by using the population inversion rate calculated by the population inversion rate calculating unit and contents retained in the basic data retaining unit and the input signal beam information retaining unit, a signal beam power change calculating unit for performing calculation of a change in optical power of the signal beam propagating through the amplification medium from a signal beam input end in the amplification medium in each of minute propagation ranges started from the signal beam input end and terminated at a signal beam output end, by using a differential equation defined by the quantity of a change in population inversion rate calculated by the population inversion rate change quantity calculating unit and the contents retained in the basic data retaining unit and the input signal beam information retaining unit, a signal beam power calculating unit for adding, in order, changes in optical power in the minute propagation ranges from a change in optical power in the minute propagation range at the signal input end as a starting point to a change in optical power in the minute range at the signal output end as a terminating point calculated by the signal beam power change calculating unit to the power value of the input signal beam retained in the input signal beam information retaining unit, to calculate a signal beam power according to a position with a coordinate in the longitudinal direction of the signal beam propagating in the amplification medium including the fluctuation in ion population at the metastable energy level in the amplification medium caused by input of the input signal beam, and an outputting process unit for outputting a result of calculation of the power of the signal beam outputted from the signal beam output end calculated by the signal beam power calculating unit as a result of simulation of the performance of the amplification medium.

Preferably, the population inversion rate change quantity calculating unit uses at least one or more Gaussian functions as functions for calculating the quantity of a change in the population inversion rate.

The population inversion rate change quantity calculating unit for calculating a quantity of a change in the population inversion rate may comprise a first function operating unit for operating a first function having a first wavelength band in a gain saturation state as a center, a second function operating unit for operating a second function comprised of a function having a second wavelength band characteristic of the amplification medium as a center, and an adding unit for adding results of the operations from the first function operating unit and the second function operating unit.

Preferably, the first function operated by the first function operating unit is composed of a total of Gaussian functions given according to respective wavelengths of the input signal beam, and the second function operated by the second function operating unit is composed of a total of a plurality of Gaussian functions.

In this case, the Gaussian function given according to each wavelength of the input signal beam in the first function is determined as a value expressed in terms of a center wavelength which is a wavelength of the input signal beam and a full width half maximum according to the amplification medium, each of the Gaussian functions in the second function is determined as a value expressed in terms of a center wavelength which is in a second wavelength band characteristic of the amplification medium and a full width half maximum according to the amplification medium, and the full width half maximum of each of the Gaussian functions in the first function and the second function is retained in the basic data retaining unit.

A depth of each of the Gaussian functions in the first function or the second function may be defined by a depth function which increases as a total power of the input signal beam increases, and saturates above a predetermined value.

In this case, the depth function of each Gaussian function given according to each wavelength of the input signal beam in the first function may be defined by a function having a wavelength .lamda..sub.i of the input signal beam, an optical power P.sub.i(z) at a position with a coordinate z in the longitudinal direction of the amplification medium at the wavelength .lamda..sub.i of the input signal beam and a total power P.sub.total(z) of the input signal beam at a position with a coordinate z in the longitudinal direction of the amplification medium as variables, the depth function of each Gaussian function in the second function may be defined by a function having a wavelength .lamda..sub.i in the second wavelength band, a total power P.sub.total(z) of the input signal beam at a position with the coordinate z in the longitudinal direction of the amplification medium and a population inversion rate n(z) of the amplification medium as variables, and coefficients defining the depth functions of the Gaussian functions in the first function and the second function may be retained in the basic data retaining unit.

The basic data retaining unit may retain, as the basic data of the amplification medium, at least an overall length of the amplification medium, a gain coefficient g(.lamda.), an absorption coefficient .alpha.(.lamda.) and a loss l(.lamda.) expressed as functional equations with respect to each input signal beam wavelength, and a population inversion rate n(z) not added thereto the fluctuation in ion population at the metastable energy level in the amplification medium, the signal beam power change calculating unit may calculate the population inversion rate n(z) from a signal beam power according to a position with a coordinate in the longitudinal direction of the signal beam propagating in the amplification medium calculated by the signal beam power calculating unit, and calculate a minute change in optical power of the signal beam propagating at a position with the coordinate z in the longitudinal direction of the amplification medium, by using a change in optical power in each minute unit of the length in the longitudinal direction of the amplification medium dP(z)/dz={(g(.lamda.)+.alpha.(.lamda.))(n(z)+.DELTA.n.sub.SHB(.lamda., z))-(.alpha.(.lamda.)+1(.lamda.))}P(z) using the population inversion rate n(z), the change quantity .DELTA.n.sub.SHB(.lamda.,z) of the population inversion rate calculated by the population inversion rate change quantity calculating unit and the basic data retained in the basic data retaining unit.

Preferably, the simulation executing unit approximates and calculates gain deviation among signal beam wavelengths caused by spectral hole burning.

The present invention further provides an amplification medium performance simulation method for simulating performance of an amplification medium excited by a pump beam from a pumping source to amplify a signal beam, comprising a population inversion rate change quantity calculating step of calculating a quantity of a change in population inversion rate which may occur due to a fluctuation in ion population at a metastable energy level of the amplification medium caused by input of the signal beam, an optical power change calculating step of performing calculation of a change in optical power of the signal beam propagating in the amplification medium from a signal beam input end of the amplification medium in each of minute propagation ranges started from the signal beam input end and terminated at a signal beam output end, by using a propagation equation of the amplification medium on the basis of a corrected population inversion rate corrected with the quantity of a change in population inversion rate calculated at the population inversion rate change quantity calculating unit, an output signal beam power calculating step of performing successive addition of each change in optical power in the minute propagation range calculated at the optical power change calculating step to the power value of the input signal beam between the signal beam input end and the signal beam output end, to calculate an output signal beam power outputted from the amplification medium including the fluctuation in ion population at the metastable energy level in the amplification medium due to input of the input signal beam, and an outputting process step of outputting a result of calculation calculated at the output signal beam power calculating step as a result of simulation of the performance of the amplification medium.

The present invention still further provides an optical amplifier comprising a pumping source for outputting a pump beam, a signal beam amplification medium excited by the pump beam from the pumping source to amplify an input signal beam, and a gain equalizer for equalizing a gain of an output signal beam outputted from the signal beam amplification medium, wherein the gain equalizer has a gain equalization characteristic so that it compensates gain deviation due to a fluctuation in ion population at a metastable energy level in the amplification medium caused by input of the input signal beam, on the basis of a result of simulation outputted from an amplification medium performance simulation apparatus for simulating the performance of the amplification medium excited by the pump beam from the pumping source to amplify the signal beam, the amplification medium performance simulation apparatus comprising a basic data retaining unit for retaining basic data of the amplification medium, an input signal beam information retaining unit for retaining a total power and a power at each wavelength of the input signal beam as information on the input signal beam to be inputted to the amplification medium to be simulated, and a simulation executing unit for reckoning a fluctuation in ion population at a metastable energy level in the amplification medium caused by input of the input signal beam, and approximating and calculating an output signal beam power at each signal beam wavelength outputted from the amplification medium, by using contents retained in the basic data retaining unit and the input signal beam information retaining unit, and outputting a result of calculation as a result of simulation of the performance of the amplification medium.

The present invention still further provides an optical amplifier comprising a pumping source for outputting a pump beam, a signal beam amplification medium excited by the pump beam from the pumping source to amplify an input signal beam, and a pumping source controlling unit for controlling the pumping source, the pumping source controlling unit controlling the pumping source so that it compensates gain deviation due to a fluctuation in ion population at a metastable energy level in the amplification medium caused by input of the input signal beam, on the basis of a result of simulation outputted from an amplification medium performance simulation apparatus for simulating the performance of the amplification medium excited by the pump beam from the pumping source to amplify the signal beam, the amplification medium performance simulation apparatus comprising a basic data retaining unit for retaining basic data of the amplification medium, an input signal beam information retaining unit for retaining a total power and a power at each wavelength of the input signal beam as information on the input signal beam to be inputted to the amplification medium to be simulated, and a simulation executing unit for reckoning a fluctuation in ion population at a metastable energy level in the amplification medium caused by input of the input signal beam, and approximating and calculating an output signal beam power at each signal beam wavelength outputted from the amplification medium, by using contents retained in the basic data retaining unit and the input signal beam information retaining unit, and outputting a result of calculation as a result of simulation of the performance of the amplification medium.

In this case, the pumping source controlling unit may comprise a first power monitor for monitoring powers of the input signal beam and the output signal beam, a wavelength allocation information obtaining unit for obtaining wavelength allocation information on a signal beam propagating in the amplification medium, an automatic gain control unit for outputting a signal for controlling the pumping source obtained on the basis of the powers of the input signal beam and the output signal beam monitored by the first power monitor so that a gain of the optical signal amplification medium is constant, and a correcting unit for correcting a control quantity for the pumping source in the automatic gain control unit on the basis of the wavelength allocation information obtained by the wavelength allocation information obtaining unit so that gain deviation in a wavelength band due to spectral hole burning decreases.

The wavelength allocation information obtaining unit may be comprised of a spectrum analyzer monitoring wavelength allocation of a signal beam inputted to or outputted from the amplification medium. The wavelength allocation information obtaining unit may obtain the wavelength allocation information from a control signal beam transmitted together with the signal beam.

The pumping source controlling unit may comprise a second power monitor for obtaining powers of the input signal beam and the output signal beam in each of a plurality of bands divided on the basis of a result of the simulation obtained by the amplification medium performance simulation apparatus, and an automatic average gain control unit for outputting a signal for controlling the pumping source on the basis of the powers of the input signal beam and the output signal beam in each of the bands obtained by the second power monitor so that average gains in the bands are equalized.

As above, according to the apparatus and method for amplification medium performance simulation of this invention, the simulation executing unit introduces a simple approximate expression to be able to output gain deviation occurring due to a local fluctuation in ion population in a wavelength region at a metastable energy level in each minute unit of the length in the longitudinal direction of the amplification medium, involving gain deviation in a region other than the neighborhood of the signal wave, through a process within a short period of time, as a result of simulation.

In the optical amplifier according to this invention, it is possible to control the pumping source by the pumping source controlling unit or designed the gain equalizer on the basis of a result of highly accurate simulation obtained through a process within a short period of time from the simulation executing unit of the amplification medium performance simulation apparatus of this invention, whereby gain deviation due to a local fluctuation in ion population in a wavelength region at a metastable energy level in the amplification medium is compensated. It is thus possible to largely improve the stability of the automatic gain control.

Particularly, in an optical amplifier which is a constitutional element of a node in a photonic network in which the wavelength allocation of a signal beam can be dynamically changed, it is possible to improve the stability of the amplification characteristic according to a large change in wavelength allocation and the number of wavelengths.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing an amplification medium performance simulation apparatus according to a first embodiment of this invention;

FIGS. 2 and 3 are diagrams for illustrating an operating process in the apparatus according to this invention.

FIG. 4 is a flowchart for illustrating an operation of the amplification medium performance simulation apparatus according to the first embodiment of this invention;

FIGS. 5 through 7 are diagrams in each of which a results of simulation by the amplification medium performance simulation apparatus according to the first embodiment is compared with measured values obtained by experiment;

FIG. 8 is a block diagram showing an optical amplifier according to a second embodiment of this invention;

FIG. 9 is a diagram for illustrating a gain equalization characteristic of a gain equalizer disposed in the following stage of an EDFA shown in FIG. 8;

FIG. 10 is a block diagram showing an optical amplifier according to a third embodiment of this invention;

FIG. 11 is a block diagram showing a modification of the third embodiment;

FIG. 12 is a flowchart for illustrating an operation of the optical amplifier according to the third embodiment;

FIGS. 13 through 18 are diagrams for illustrating a working effect given by superimposing correction of a fluctuation in gain due to SHB on an automatic gain control in the optical amplifier according to the third embodiment;

FIG. 19 is a block diagram showing an optical amplifier according to a fourth embodiment of this invention;

FIG. 20 is a block diagram showing an optical amplifier according to a fifth embodiment of this invention;

FIG. 21 is a flowchart for illustrating an operation of the optical amplifier according to the fifth embodiment of this invention;

FIGS. 22 and 23 are diagrams for illustrating a working effect given by the optical amplifier according to the fifth embodiment of this invention;

FIG. 24 is a diagram for illustrating an example where a wavelength characteristic (gain spectrum) is approximated and grasped according to a value of a population inversion rate with the whole amplification band of an EDFA being one unit;

FIG. 25 is a block diagram showing an example of the structure of an optical repeater used in a known optical network system in which wavelength allocation and the number of wavelengths are assumed not to be largely changed;

FIG. 26 is a diagram for illustrating an example where the output power of the optical repeater is made constant by adjusting a loss quantity in a variable optical attenuator shown in FIG. 25; and

FIG. 27 is a diagram showing a gain deviation characteristic due to SHB of an EDFA.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, description will be made of embodiments of the present invention with reference to the drawings.

(a) Description of First Embodiment

FIG. 1 is a block diagram showing an amplification medium performance simulation apparatus 1 according to a first embodiment of this invention. The amplification medium performance simulation apparatus 1 shown in FIG. 1 simulates the performance of an amplification medium. Particularly, the amplification medium performance simulation apparatus 1 can carry out simulation of the output power characteristic and gain characteristic of an amplification medium in an optical amplifier applied to an apparatus configuring a photonic network.

When an optical amplifier assumed to be applied to an optical network in which wavelength allocation and the number of wavelengths are largely changed is designed, the gain wavelength characteristic of an amplification medium to be evaluated is accurately grasped, whereby the input/output power characteristic and the characteristics of a gain equalizer are so designed as to secure the gain flatness.

In the amplification medium performance simulation apparatus 1, an EDFA, for example, can be used as the amplification medium to be simulated. Hereinafter, a case where an EDFA is used as the amplification medium, but another amplification medium other than the EDFA can be used.

The amplification medium performance simulation apparatus 1 according to the first embodiment can simulate the gain deviation characteristic of an EDFA occurring due to SHB as above by obtaining basic data of the EDFA and information on the input signal beam. Now, description will be made of a principle of calculation of the gain deviation characteristic of the EDFA due to SHB in the amplification medium performance simulation apparatus 1.

As described above with reference to FIG. 27, it is found that the effect of inhomogeneous broadening at the wavelength level in the gain saturated state in an EDFA is significant particularly at the signal beam wavelength and in a 1530 nm band. Giving attention to the effect of the bands in which the inhomogeneous broadening is particularly large, the amplification medium performance simulation apparatus 1 according to this embodiment calculates the quantity of a change in the population inversion rate. Hereinafter, the gain fluctuation occurring at the signal beam wavelength will be referred to as "main hole," whereas the gain fluctuation occurring in the vicinity of 1530 nm will be referred to as "second hole."

FIGS. 2 and 3 are diagrams for illustrating an operating process in the apparatus 1 according to this embodiment. The gain fluctuation due to SHB at a certain wavelength is caused by that the number of ions of ER.sup.3+ at the metastable energy level of transition corresponding to the length of the optical wavelength changes, that is, the population inversion rate changes from its average value.

Assuming that the quantity of a change in the population inversion rate causing gain fluctuation due to SHB is .DELTA.n.sub.SHB when the signal beam propagates from a position with a coordinate z by a minute portion .DELTA.z (refer to FIG. 2) in the longitudinal direction of an EDF 50, the optical power P (z+.DELTA.z) at the minute portion .DELTA.z is expressed by an equation (1). In the equation (1), n represents the population inversion rate at a position with the coordinate z in the longitudinal direction of the EDF 50, P(z) represents the signal beam power at a position with the coordinate z in the longitudinal direction of the EDF 50, and G(