JP5144561B2 - Gain-flattened cascaded optical fiber amplifier, apparatus for designing the gain equalizer of the amplifier, and method for designing the gain equalizer of the amplifier - Google Patents

Description

  The present invention provides a gain-flattened cascaded optical fiber amplifier, a design apparatus for designing a gain equalizer incorporated in the gain-flattened optical fiber amplifier, and a method for designing the gain equalizer About.

  In recent years, the transmission capacity of optical fiber communication has increased rapidly, and the transmission band of wavelength division multiplexing (WDM) signals has increased. Accordingly, expansion of the amplification band of optical fiber amplifiers used as repeaters has been studied. In the current optical transmission system, an erbium-doped optical fiber amplifier (EDFA) that can amplify the C band (1530 to 1565 nm) and the L band (1565-1625 nm) and uses a silica-based glass as a host glass of an amplification fiber is mainstream. It is. As optical fiber amplifiers other than the C and L bands, a praseodymium-doped fluoride optical fiber amplifier (PDFA) in the O band (1260 to 1360 nm), a thulium doped fluoride optical fiber amplifier (TDFA) in the S band (1460 to 1530 nm) And is expected as an optical fiber amplifier of the next generation signal band.

  When performing broadband and multi-relay transmission, the transmission characteristics deteriorate due to the gain deviation of the optical fiber amplifier. Therefore, it is necessary to flatten the gain spectrum of each stage of the optical fiber amplifier using a gain equalizer. is there. When the gain equalizer is installed at the front stage of the optical fiber amplifier, the noise becomes high, and when it is installed at the rear stage, the efficiency is low. For this reason, the amplification medium of the optical fiber amplifier is divided into two to form a cascade type (series multi-stage type) configuration, and a gain equalizer is installed in the middle of the multi-stage amplifier to simultaneously realize low noise and high efficiency. Often take. For example, Non-Patent Document 1 describes an example of an optical amplifier in which a gain equalizer is inserted between multistage amplifiers.

H Masuda, A Mori, K. Shikano, K. Oikawa, K. Kato, M. Shimizu, "Ultra-wide-band hybrid tellurite / silica fiber Raman amplifier", Optical Fiber Communication Conference and Exhibit 2002 (OFC 2002: Optical fiber International Conference and Exhibition on Communications) Proceedings, pp. 388-390, published in March 2002 "Erbium-doped optical fiber amplifier" p222-223, Editor: Shoichi Sudo, Optronics "Erbium-doped fiber amplifier" p33, editor: Shoichi Sudo, Optronics B. R. Judd, "Optical absorption intensities of rare-earth ions," Phys. Rev., vol. 127, no.3, pp.750-761 (1962) G. S. Oflet, "Intensities of crystal spectra of rare-earth ions," J. Chem. Phys., Vol. 37, no.3, pp. 511-520 (1962)

  When a gain equalizer is installed in a quartz-type EDFA cascade amplifier, a design method for the gain equalizer has been established. However, if the type of ions to be added or the type of host glass is changed, this established method cannot be applied as it is. For cascaded amplifiers other than quartz-based EDFAs (for example, cascaded optical fiber amplifiers using amplification media containing rare earth ions with three or more levels that should be considered for ion density), the above design method is used as is. Even if this is the case, flattening of the gain spectrum cannot be realized.

  Therefore, an object of the present invention is to provide a design method for a gain amplifier in the case where a gain equalizer is installed in a cascade type amplifier other than a silica-based EDFA.

  Another object of the present invention is to provide a gain equalizer designed by using the above design method and an optical amplifier incorporating the gain equalizer.

  Furthermore, this invention aims at providing the design apparatus used for the said design method.

  The above object is solved by the present invention described below.

  A first aspect of the present invention is a gain-flattened cascade type (series multi-stage type) rare earth addition comprising a plurality of amplification media or amplifiers and a gain equalizer provided at at least one of the intermediate positions. The present invention relates to an optical fiber amplifier. This cascade-type rare earth doped optical fiber amplifier is characterized in that the gain equalizer has a loss spectrum shape and a gain equalization band set in the following process.

i) WDM signal light is input from the signal light input section of a cascade type rare earth doped optical fiber amplifier in which a variable optical attenuator is inserted in place of the gain equalizer at the installation position of the gain equalizer, and the gain spectrum is measured. To do,
ii ) Having the spectrum shape of the gain spectrum measured in i ), and the loss with respect to the total power of the WDM signal light when passing through the variable optical attenuator becomes equal to the loss when passing through the gain equalizer Deriving the loss spectrum of the gain equalizer that satisfies the two conditions
iii) (a) The gain spectrum after gain equalization in calculation is the sum of the gain spectrum measured in i) and the loss spectrum of the variable optical attenuator (converted in dB) to the gain equalizer derived in ii) Defining a difference from the loss spectrum (dB conversion) and deriving a calculated gain equalization band from the gain spectrum after gain equalization derived by this definition, or
(B) deriving a calculated gain equalization band from the loss spectrum derived in ii);
To calculate a calculated gain equalization band, and compare the calculated gain equalization band with a desired gain equalization band preset for the cascaded amplifier. Determining whether or not a gain equalization band of a predetermined gain equalization band set in advance for a cascaded amplifier matches.
iv) As a result of the comparison of iii), when the calculated gain equalization band is narrower than a desired gain equalization band preset for the cascaded amplifier, the variable optical attenuator Increasing the loss and reducing the loss of the variable optical attenuator if wider than the desired gain equalization band;
v) Repeat the above steps i) to iv) until the desired gain equalization band is obtained, and determine the shape of the loss spectrum of the gain equalizer and the gain equalization band.

  In the cascade optical fiber amplifier of the present invention, a correction function can be incorporated when deriving the gain spectrum after the gain equalization of iii).

  Further, in the cascade type optical fiber amplifier of the present invention, the rare earth ions involved in amplification added to the amplification medium when in the excited state or the amplified state have three or more energy levels that cannot approximate the ion density to zero. It is characterized by having.

In the cascade optical fiber amplifier of the present invention, the rare earth ions involved in amplification added to the amplification medium when in the excited state or the amplified state have a relaxation rate (the sum of the emission relaxation rate and the non-emission relaxation rate) of l. It is characterized by having three or more energy levels that are × 10 ⁴ s ⁻¹ or less.

  The second aspect of the present invention relates to a design method for a gain equalizer that is inserted into at least one of a plurality of amplification media constituting a cascade-type rare earth-doped optical fiber amplifier or an intermediate position of the amplifier. The design method of the present invention includes the following steps.

i) WDM signal light is input from the signal light input section of a cascade type rare earth doped optical fiber amplifier in which a variable optical attenuator is inserted in place of the gain equalizer at the installation position of the gain equalizer, and the gain spectrum is measured. To do,
ii ) Having the spectrum shape of the gain spectrum measured in i ), and the loss with respect to the total power of the WDM signal light when passing through the variable optical attenuator becomes equal to the loss when passing through the gain equalizer Deriving the loss spectrum of the gain equalizer that satisfies the two conditions
iii) (a) The gain spectrum after gain equalization in calculation is the sum of the gain spectrum measured in i) and the loss spectrum of the variable optical attenuator (converted in dB) to the gain equalizer derived in ii) Defining a difference from the loss spectrum (dB conversion) and deriving a calculated gain equalization band from the gain spectrum after gain equalization derived by this definition, or
(B) deriving a calculated gain equalization band from the loss spectrum derived in ii);
To calculate a calculated gain equalization band, and compare the calculated gain equalization band with a desired gain equalization band preset for the cascaded amplifier. Determining whether or not a gain equalization band of a predetermined gain equalization band set in advance for a cascaded amplifier matches.
iv) As a result of the comparison of iii), when the calculated gain equalization band is narrower than a desired gain equalization band preset for the cascaded amplifier, the variable optical attenuator Increasing the loss and reducing the loss of the variable optical attenuator if wider than the desired gain equalization band;
v) Repeat the above steps i) to iv) until the desired gain equalization band is obtained, and determine the shape of the loss spectrum of the gain equalizer and the gain equalization band.

  In the design method of the gain equalizer of the present invention, a correction function can be incorporated when deriving the gain spectrum after gain equalization of iii).

  Furthermore, in the design method of the gain equalizer of the present invention, the rare earth ions involved in amplification added to the amplification medium when in the excited state or the amplified state have three energy levels that cannot approximate the ion density to zero. It is characterized by having it above.

In the design method of the gain equalizer of the present invention, the rare earth ions involved in amplification added to the amplification medium when in the excited state or the amplified state have a relaxation rate (the sum of the emission relaxation rate and the non-emission relaxation rate). It has three or more energy levels which are ¹ × 10 ⁴ s ⁻¹ or less.

  A third aspect of the present invention relates to an apparatus for designing a gain equalizer that is inserted into at least one position among a plurality of amplifying media or amplifiers constituting a cascade-type rare earth doped optical fiber amplifier. The design apparatus of the present invention includes a variable optical attenuator, a photodetector for measuring the gain of WDM signal light, a calculation circuit, and a variable optical attenuator control circuit.

  In the designing apparatus of the present invention, the shape of the loss spectrum and the gain equalization band of the target gain equalizer are calculated by the following process.

i) WDM signal light is input from the signal light input section of a cascade type rare earth doped optical fiber amplifier in which a variable optical attenuator is inserted in place of the gain equalizer at the installation position of the gain equalizer, and the gain spectrum is measured. To do,
ii ) Having the spectrum shape of the gain spectrum measured in i ), and the loss with respect to the total power of the WDM signal light when passing through the variable optical attenuator becomes equal to the loss when passing through the gain equalizer Deriving the loss spectrum of the gain equalizer that satisfies the two conditions
iii) (a) The gain spectrum after gain equalization in calculation is the sum of the gain spectrum measured in i) and the loss spectrum of the variable optical attenuator (converted in dB) to the gain equalizer derived in ii) Defining a difference from the loss spectrum (dB conversion) and deriving a calculated gain equalization band from the gain spectrum after gain equalization derived by this definition, or
(B) deriving a calculated gain equalization band from the loss spectrum derived in ii);
To calculate a calculated gain equalization band, and compare the calculated gain equalization band with a desired gain equalization band preset for the cascaded amplifier. Determining whether or not a gain equalization band of a predetermined gain equalization band set in advance for a cascaded amplifier matches.
iv) As a result of the comparison of iii), when the calculated gain equalization band is narrower than a desired gain equalization band preset for the cascaded amplifier, the variable optical attenuator Increasing the loss and reducing the loss of the variable optical attenuator if wider than the desired gain equalization band;
v) Repeat the above steps i) to iv) until the desired gain equalization band is obtained, and determine the shape of the loss spectrum of the gain equalizer and the gain equalization band.

  The gain equalizer designing apparatus of the present invention may include a circuit incorporating a correction function when deriving the gain spectrum after the gain equalization of iii).

  The apparatus for designing a gain equalizer of the present invention has three or more energy levels in which rare earth ions involved in amplification added to an amplification medium when in an excited state or an amplified state cannot approximate the ion density to zero. It is characterized by that.

In the gain equalizer designing apparatus of the present invention, the rare earth ions involved in amplification added to the amplification medium in the excited state or in the amplified state have a relaxation rate (the sum of the emission relaxation rate and the non-emission relaxation rate). It has three or more energy levels which are ¹ × 10 ⁴ s ⁻¹ or less.

  By using the gain equalizer manufactured by the method of the present invention, an amplification medium other than the quartz-based EDF (for example, an amplification medium containing rare earth ions having three or more levels to which ion density should be considered). It is possible to easily design a gain equalizer even in a cascade-type rare earth-doped optical fiber amplifier using the above. In addition, the design method of the present invention can be easily implemented as a device, and can be automated.

It is a figure for demonstrating the design method of the gain equalizer of the cascade type rare earth addition optical fiber amplifier of quartz type EDFA. It is a figure for demonstrating the behavior at the time of applying the design method of the gain equalizer of the cascade type rare earth addition optical fiber amplifier of quartz type EDFA to the cascade type rare earth addition optical fiber amplifier of TDFA. It is a figure for demonstrating the reason why the design method of the gain equalizer of the cascade type rare earth doped optical fiber amplifier of silica-based EDFA cannot be applied to the cascade type rare earth doped optical fiber amplifier of TDFA. It is a graph which shows the gain spectrum at the time of using a variable optical attenuator as a virtual gain equalizer (GEQ). It is a figure for demonstrating the difference in the ion density distribution of quartz type EDFA and fluoride EDFA. It is a flowchart for demonstrating the design method of the gain equalizer of this invention. It is a figure for demonstrating the design method of the gain equalizer of this invention. It is a graph which shows the correlation of the maximum value and correction value of the equalization gain of the gain equalizer of this invention. It is a graph which shows the relaxation rate dependence of a gain deviation. It is a figure for demonstrating the relaxation rate dependence of conversion efficiency. It is the schematic which shows an example of the cascade type amplifier of this invention. 1 is a schematic diagram of a gain equalizer designing apparatus according to the present invention. FIG. FIG. 6 is a schematic view of another design apparatus for a gain equalizer according to the present invention. It is the graph which showed the loss spectrum of the gain equalizer. It is a graph which shows the gain and noise figure spectrum after gain equalization at the time of using the gain equalizer designed by the design method of this invention.

  In the quartz-based EDFA, a design method for a gain equalizer of a cascade type (also referred to as a serial multistage type) rare earth-doped optical fiber amplifier has been established. However, in many cases, the method cannot be applied if the type of ion or host glass to be added is changed. The present invention provides a design method for a gain equalizer of a cascade-type rare-earth doped optical fiber amplifier to which a conventional design method cannot be applied, and also includes a gain equalizer designed based on this design method. A rare earth-doped optical fiber optical amplifier and a design apparatus for the gain equalizer are provided.

  In this specification, first, a design method of a gain equalizer of a quartz-based EDFA cascade-type rare earth-doped optical fiber amplifier (also referred to as a cascade-type silica-based EDFA in this specification) will be described. Next, as examples in which the design method cannot be applied, an S-band thulium-doped optical fiber amplifier (TDFA) in which the doped rare earth ion is thulium, and a cascaded rare earth-doped optical fiber amplifier of fluoride EDFA (in the present specification, conventionally, For simplicity, a cascade-type rare earth-doped optical fiber amplifier to which the design method (1) is not applied is also referred to as “cascade-type amplifier”. After that, the design method of the gain equalizer of the present invention will be described, and the cascade amplifier of the present invention including the gain equalizer designed by this design method and the design apparatus of the gain equalizer will be described.

1. Design Method for Cascade-type Silica-based EDFA Gain Equalizer The design of the gain equalizer currently used in cascade-type quartz-based EDFA is to connect multiple quartz-based EDFAs without interposing a gain equalizer. This is realized by measuring the gain spectrum of the multistage amplifier and setting the excess gain portion of the desired gain equalization band as the loss spectrum of the gain equalizer.

  Taking a case of a two-stage cascaded optical fiber amplifier as an example, a method for designing a gain equalizer of a two-stage cascaded quartz-based EDFA will be specifically described with reference to FIG. The design procedure is as follows.

  First, as shown in FIG. 1A, an amplifier is prepared in which EDFAs are connected in two stages and a gain equalizer is not inserted in the middle.

(1) The gain spectrum (I) of this amplifier is measured.
(2) The loss spectrum (III) of the gain equalizer represented by the difference ((I)-(II)) between the target gain equalization spectrum (II) and the gain spectrum (I) measured in (1). calculate. Here, the calculated loss spectrum (III) of the gain equalizer is the same as that of the gain equalizer (GEQ) virtually illustrated in the latter stage of the EDFA in FIG. Has a wavelength band.

  (3) A gain equalizer (GEQ) having the calculated gain equalizer loss spectrum (III) is produced, and the gain equalizer is inserted into the intermediate stage of the cascade optical fiber amplifier to obtain the gain spectrum ( IV) is measured.

  The measured gain spectrum (IV) after gain equalization coincides with the target spectrum (II) after gain equalization calculated in (2) above.

  Thus, the gain equalizer obtained by the above-described design method of the cascade-type quartz-based EDFA gain equalizer gives the same gain equalization spectrum at the time of design and when actually incorporated in an amplifier (however, loss The gain equalizer, which is a medium, differs in position between the virtually described insertion position and the actually inserted intermediate stage. Therefore, the gain equalizer is designed (FIG. 1 (a)) and actually incorporated (FIG. 1). The pumping light power of 1 (b) is different.)

2. In designing a gain equalizer of a cascade amplifier, the above 1. 2. When the method is applied 2-1. Cascade type TDFA
As an example of an amplifier to which the design method of a cascade-type quartz-based EDFA gain equalizer cannot be applied, an example of a cascade-type TDFA is given. FIG. 2 shows a conceptual diagram when a quartz-based EDFA method is applied to TDFA. Above 1. The gain equalizer for the two-stage TDFA is manufactured according to the method (FIG. 2 (a)), and when the obtained gain equalizer is inserted in the intermediate stage of the cascade amplifier, the target gain flattened gain is obtained. A spectrum cannot be obtained (FIG. 2 (b)).

  The reason why a gain spectrum flattened by TDFA cannot be obtained as described above will be described with reference to FIG.

In order to design a gain equalizer, it is necessary to set the target gain spectrum and keep it constant. For this purpose, the ions of the amplification start level and end level of the rare earth ions that generate the amplification action are required. It is necessary to keep the density constant. That is, when the ion density of the amplification start level is N ₁ and the ion density of the amplification end level is N ₂ , N ₁ = constant and N ₂ = constant must be satisfied. When the gain equalizer designed by the method of designing a cascade type quartz EDFA gain equalizer is inserted in a position between the amplification medium of the cascade amplifier or between the amplifiers, the input signal light to the subsequent amplification medium or amplifier The power changes, and the ion density at each level changes. In the case of a quartz-based EDFA, as shown in FIG. 3A, the level at which the ion density should be considered in determining the gain spectrum (the level at which the ion density cannot be approximated to zero) is the amplification start level ( ³ I _{13 / 2)} and (because it is only two ³ I _15/2), the input signal power variation (FIGS. 3 (a) amplification final energy level exhibited when increased in) to ion density of each semi-position (FIGS. 3 (a) (1) to (2)), if the ion density of one level is set to a desired value by adjusting the pumping light power of the subsequent amplification medium or amplifier, The ion density of one level is also a desired value (FIGS. 3A, 2, and 3). For example, if the ion density N _{2 at the} final amplification level is set to a desired value by adjusting the pumping light power, the ion density N _{1 at the first} amplification level is set to a desired value (FIG. 3A ), Formula (1)).

On the other hand, in the case of TDFA, as shown in FIG. 3 (b), as the level that should consider the ion density, in addition to the amplification start level ( ³ H ₄ ) and the amplification final level ( ³ F ₄ ), there is another one. There are two levels ( ³ H ₆ ). For this reason, the ion density of each level cannot be kept constant by adjusting the excitation light power as used in a quartz-based EDFA. More specifically, when the input signal light power is changed (shown in FIG. 3 (b) is increased) (FIG. 3 (b) (1) to (2)), the desired N ₂ is obtained. When the pumping light power is adjusted, the pumping speed of the first stage ( ³ H ₆ → ³ F ₄ ) of the up-conversion pumping also changes at the same time, and the ion density (N ₀ ) of the ground level changes (FIG. 3B ( 2) to (3)). As a result, the total ion density of the amplification start level ⁽³ _{H 4)} and the amplification final energy level _{^{(3 F 4) (N 1}} + N 2) to change occurs, the ion density _{N 1} of the desired values of the amplification final energy level Cannot be obtained (FIG. 3B, equation (2)).

  FIG. 4 shows a case where a wavelength-independent variable optical attenuator (ATT) is inserted in the intermediate stage of the TDFA in a two-stage cascaded TDFA, and the loss value of the variable optical attenuator is changed to input signal light to the subsequent amplifier. When the power is changed, the gain spectrum is a sum of the gains of the thulium-doped optical fibers of the former stage and the latter stage. The pumping light power to the subsequent optical fiber was adjusted so that the gains of 1482 nm coincided. If there are only two levels for which the ion density should be considered, such as a quartz-based EDFA, the loss value of the variable optical attenuator can be changed in the measured wavelength band (1480-1510 nm) by adjusting the excitation light power. The same gain spectrum is obtained. However, in the cascade type TDFA, as can be seen from FIG. 4, as the loss value of the variable optical attenuator increases, the gain on the long wavelength side decreases, and the same gain spectrum cannot be obtained. This is because in cascaded TDFA, the gain equalizer obtained by the design method of cascaded quartz-based EDFA cannot be used to set the ion density of the amplification start level and amplification end level to a desired value. Represents. That is, the design method of the gain equalizer of the cascade type quartz EDFA cannot be applied to the cascade type TDFA.

2-2. Cascade type fluoride EDFA
Next, an example of a cascade type fluoride EDFA will be given as an example of an amplifier to which the cascade equalizer type EDFA gain equalizer design method cannot be applied. In this example, the case of applying the cascade type quartz EDFA method will be described with reference to FIG. Since the behavior of the gain spectrum when designing the gain equalizer shown in FIG. 2 for the cascade type fluoride EDFA is the same as that of the cascade type TDFA, only the ion density in the amplified state is compared here. FIG. 5 shows a result of excitation from the ground level of ⁴ I _15/2 to ⁴ I _11/2 when _{0.98 μm} excitation light is used. In the quartz-based EDFA, the relaxation rate from ⁴ I _11/2 to ⁴ I _13/2 is so high that N ₃ is almost zero. For this reason, there are two levels ( ⁴ I _12/3 and ⁴ I _15/2 ) for which the ion density should be considered. On the other hand, in fluoride EDFA, since the relaxation rate from ⁴ I _11/2 to ⁴ I _13/2 is small, the ion density of N ₃ cannot be ignored. That is, in the fluoride EDFA, there are three levels ( ⁴ I _11/2 , ⁴ I _12/3 and ⁴ I _15/2 ) for which the ion density should be considered. Therefore, the cascade type fluoride EDFA cannot apply the gain equalizer design method of the cascade type quartz EDFA, similarly to the cascade type TDFA.

3. 3. Method for Designing Cascade Amplifier Gain Equalizer of the Present Invention As described above, there are three levels (levels where the ion density cannot be approximated to zero) in which the ion density is considered in the excited state or amplified state of the cascade amplifier. In one or more cascaded rare earth-doped optical fiber amplifiers, when the input signal light power to the subsequent amplification medium or amplifier changes, the total amount of the ion density of the amplification start level and the final level changes. For this reason, it is difficult to design a gain equalizer from a gain spectrum in a state where no gain equalizer is inserted. Therefore, the present invention provides a new gain that solves the above problem by preventing the input signal light power to the subsequent amplification medium or amplifier from changing when the gain equalizer is designed and when the gain equalizer is inserted. An equalizer design method is proposed.

  In the present invention, when designing the gain equalizer, a cascade type rare earth doped optical fiber amplifier in which a variable optical attenuator is inserted in place of the gain equalizer at the installation position of the gain equalizer of the cascade type amplifier is used. The following processes i) to v) are carried out with a cascade type rare earth doped optical fiber amplifier in which such a variable optical attenuator is inserted, and the shape and wavelength band of the loss spectrum of the gain equalizer are obtained.

i) WDM signal light is input from the signal light input section of a cascade type rare earth doped optical fiber amplifier in which a variable optical attenuator is inserted in place of the gain equalizer at the installation position of the gain equalizer, and the gain spectrum is measured. To do,
ii) Having a spectral shape of the gain spectrum measured in i) in a desired wavelength band, and loss with respect to the total power of the WDM signal light when passing through the variable optical attenuator passes through the gain equalizer. Deriving the loss spectrum of the gain equalizer that satisfies the two conditions of being equal to the loss of time,
iii) (a) The gain spectrum after gain equalization in calculation is the sum of the gain spectrum measured in i) and the loss spectrum of the variable optical attenuator (converted in dB) to the gain equalizer derived in ii) Defining a difference from the loss spectrum (dB conversion) and deriving a calculated gain equalization band from the gain spectrum after gain equalization derived by this definition, or
(B) deriving a calculated gain equalization band from the loss spectrum derived in ii);
To calculate a calculated gain equalization band, and compare the calculated gain equalization band with a desired gain equalization band preset for the cascaded amplifier. Determining whether or not a gain equalization band of a predetermined gain equalization band set in advance for a cascaded amplifier matches.
iv) As a result of the comparison of iii), when the calculated gain equalization band is narrower than a desired gain equalization band preset for the cascaded amplifier, the variable optical attenuator Increasing the loss and reducing the loss of the variable optical attenuator if wider than the desired gain equalization band;
v) Repeat the above steps i) to iv) until the desired gain equalization band is obtained, and determine the shape of the loss spectrum of the gain equalizer and the gain equalization band.

  Here, the sum of the gain spectrum measured in i) and the loss spectrum of the variable optical attenuator when calculating the target gain equalization band in the above step iii) is based on dB conversion, and is converted to transmittance. If it is, it is a product. Further, the difference between the above sum and the loss spectrum of the gain equalizer derived in ii) is based on the dB conversion, and is excluded when the transmittance conversion is performed. In the description of the present specification, the case of conversion by dB is described, but the present invention includes the case of conversion by transmittance.

  Each of the above processes will be described in detail with reference to FIGS. FIG. 6 is a flowchart showing the concept of the design method of the present invention. FIG. 7 is a schematic diagram for explaining the design procedure (i) to v)). FIG. 7 shows the case of a two-stage type.

  First, as shown in FIG. 7 (a), a variable optical attenuator (ATT or ATT in the art) in which a gain equalizer is assumed between the amplifying medium or amplifier of a cascade amplifier (denoted as amplifier in FIG. 7). An amplifier in which a VOA is also inserted is prepared (in this specification, a variable optical attenuator (ATT) inserted between amplification media of the cascade amplifier is also referred to as an ATT insertion amplifier).

  Next, as shown in FIG. 6, the design is started (S602), and the ATT value and pumping light power of the ATT insertion amplifier are prepared (S604). A desired loss spectrum (I) is set as the first loss spectrum of ATT. The loss spectrum (I) of the ATT is arbitrarily set to a desired value from the gain equalization band of the desired cascade amplifier.

(A) Process i)
The WDM signal light is input from the signal light input unit to the ATT insertion amplifier after adjusting the loss spectrum (I) of the ATT, and the gain spectrum (II) is measured (FIG. 6, S606).

(B) Process ii)
Process ii) identifies the shape of the loss spectrum of the gain equalizer.

  Specifically, in the desired wavelength band, having the spectrum shape of the gain spectrum measured in i), the loss with respect to the total power of the WDM signal light when passing through the variable optical attenuator is designed for gain equalization The loss spectrum (III) of the gain equalizer that satisfies the two conditions of being equal to the loss when passing through the filter is derived (FIG. 6, S608). The loss spectrum of the gain equalizer is calculated by the following procedure.

When the variable optical attenuator is used, the total output signal optical power of the n-wave WDM signal output from the variable optical attenuator is P _total-ATT (the total output signal optical power at point A in FIG. 7A. Or obtained from the loss of ATT.) When the total output signal optical power of the n-wave WDM signal output from the gain equalizer when using the gain equalizer is P _total-GEQ , the following equation (3 ) To (5). Thereby, the loss with respect to the total signal power generated when the WDM signal passes through the variable optical attenuator matches the loss when passing through the gain equalizer, and the desired gain spectrum (II) measured in i) above is obtained. The loss spectrum (III) of the gain equalizer can be derived so as to be equal to the spectrum shape of the wavelength band.

Here, P _s (λ _i ) is the power (unit: dBm) of the i-th WDM signal light input to the variable optical attenuator or gain equalizer, and L is the loss (unit: wavelength-independent variable optical attenuator). : DB), M (λ _i ) is the loss of the gain equalizer at the i-th WDM signal wavelength position (unit: dB), G _flat is the gain of each signal wavelength after gain equalization, and G (λ _i ) is This is the gain (unit: dB) of the i-th WDM signal in the entire cascade amplifier when the variable optical attenuator is inserted at the design stage.

Calculation of M (lambda _i) after setting the design procedure i) with L (variable optical attenuator loss values. FIGS. 7 (a) of (I)), obtained from _{G flat} set arbitrarily M ( λ _i ) is calculated by adjusting G _flat until P _total−ATT = P _total−GEQ is satisfied. Here, G (λ _i ) is the gain of the i-th WDM signal in the entire cascade amplifier when the variable optical attenuator measured in step i) is inserted (i-th in (II) of FIG. 7A). WDM signal gain). From each M (λ _i ) of the above calculation result, the loss spectrum (III) of the gain equalizer is calculated, and the shape of the loss spectrum of the gain equalizer can be specified.

  In the present invention, the loss with respect to the total signal power is targeted for the loss spectrum shape between the loss spectrum of the gain equalizer reflecting the shape of the gain spectrum and the wavelength-independent loss of the wavelength-independent variable optical attenuator. Therefore, even if the integrated values of the losses are simply made equal, the total signal light powers output to the subsequent amplifiers are not equal.

Instead of the power of the WDM signal light output from the variable optical attenuator, the WDM signal light power (P _s (λ _i )) input to the variable optical attenuator is measured, and the equations (3) to (5) are used. It is possible to design the gain equalizer of the present invention. In the above description of the present invention, the loss characteristic of the variable optical attenuator is a wavelength-independent one. However, if the loss characteristic at the time of variable can be grasped, the L value of Expression (5) is used. It is also possible to design the gain equalizer of the present invention by setting for each wavelength.

(C) Process iii)
In step iii), the wavelength band of the gain equalizer is derived (FIG. 6, S610). In the present invention, two methods can be used for deriving the wavelength band.

  In the first procedure, (a) the gain spectrum derived by ii) is calculated as the sum (dB conversion) of the gain spectrum measured in i) and the loss spectrum of the variable optical attenuator after the gain equalization in calculation (a). A difference (dB conversion) from the loss spectrum of the equalizer is defined as a value, and a calculated gain equalization band is derived from the gain spectrum after gain equalization derived by this definition.

A specific derivation procedure will be described with reference to FIG. The loss spectrum (I) of the variable optical attenuator is added (dB conversion) to the gain spectrum (II) measured in the above step i), and the gain spectrum ((I) + (II) not including the loss of the variable optical attenuator) ) Is calculated. The gain spectrum obtained by (I) + (II) corresponds to a gain spectrum (dB conversion) obtained by adding the gain spectrum of the preceding stage and the subsequent stage amplification medium or amplifier excluding the variable optical attenuator. Next, an expected gain is calculated from the gain spectrum not including the loss of the variable optical attenuator represented by (I) + (II) and the loss spectrum (III) of the gain equalizer derived in the above process ii). The gain spectrum after equalization is defined as the following equation (6).
Gain spectrum after gain equalization (dB) = (I) + (II) − (III) (6)

  This equation (6) corresponds to deriving the gain spectrum (IV) after gain equalization in FIG. That is, as shown in FIG. 7, after gain equalization, the loss spectrum (III) of the gain equalizer is subtracted from the gain spectrum ((I) + (II)) not including the loss of the variable optical attenuator. The gain spectrum (IV) is obtained. A gain equalization band is obtained from the lower limit and the upper limit of the wavelength of the flattened portion of the gain spectrum (IV).

  In the present specification, the “gain equalization band” refers to a concept including a wavelength range from a lower limit value to an upper limit value of a wavelength at which gain equalization is performed. Specifically, for example, in the case of a gain equalization band of 1480 to 1510 nm, “gain equalization band” refers to a concept including a wavelength range between a lower limit value 1480 nm and an upper limit value 1510 nm in addition to a bandwidth of 30 nm. .

  In the second procedure, (b) a calculated gain equalization band is derived from the loss spectrum derived in ii) above. Specifically, the loss spectrum of the gain equalizer derived in the above ii) (for example, the loss spectrum of (III) in FIG. 7A) can be seen from the above formulas (3) to (5). The shape of the loss spectrum and the wavelength range for gain equalization, that is, the gain equalization band. Therefore, the gain equalization band of the gain equalizer can be obtained from the loss spectrum obtained in ii).

  Comparing the calculated gain equalization band obtained by either the first or second procedure described above with a desired gain equalization band preset for the cascade amplifier, and calculating the gain equalization It is determined whether or not a desired gain equalization band preset for the cascade amplifier matches the band (FIG. 6, S612). The desired gain equalization band set in advance for the cascade amplifier is a gain equalization band required for the gain equalizer to be designed, and is preset as a characteristic of the gain equalizer set for the cascade amplifier. Target value.

(D) Process iv)
If, as a result of the comparison in iii), the compared calculated gain equalization band matches the desired gain equalization band preset for the cascade amplifier, the gain equalization obtained in ii) above The shape of the loss spectrum of the equalizer and the gain equalization band obtained in iii) above are defined as the shape of the loss spectrum of the gain equalizer and the gain equalization band (FIG. 6, S614).

  On the other hand, if the calculated gain equalization band is not equal to the desired gain equalization band, the variable optical attenuator if the calculated gain equalization band is narrower than the desired gain equalization band preset for the cascade amplifier. The loss of the variable optical attenuator is reduced if the loss is wider than the desired gain equalization band.

(E) Process v)
The above i) to iv) are repeated by the ATT insertion amplifier in which the loss of the variable optical attenuator is adjusted until the calculated gain equalization band matches the desired gain equalization band preset for the cascade amplifier. (FIG. 6, NO in S612).

  In the present invention, a gain equalizer having the shape of the loss spectrum and the gain equalization band obtained by the processes i) to v) is manufactured.

  A conventional method may be applied to manufacture a gain equalizer having a target gain equalization band. For example, the methods disclosed in (Non-Patent Document 2) can be used, and these methods may be set so as to have the shape of the loss spectrum and the gain equalization band obtained by the above process.

  The gain flattening of the caskate-type rare earth doped optical fiber amplifier is realized by using the manufactured gain equalizer. The variable optical attenuator that can be used in the method of the present invention preferably has a flat wavelength characteristic for ease of calculation. For example, the following variable optical attenuators can be used. Examples of mechanical variable optical attenuators include a method in which an attenuation plate with a transmission loss distribution is inserted during optical beam coupling between fibers and the attenuation plate is moved, and a method in which the gap between fibers is variable Etc. Non-mechanical examples include those using an electro-optic effect, those using a magneto-optic effect, and those using a thermo-optic effect.

  In addition, it is preferable to further improve the design accuracy by incorporating a correction function when deriving the gain spectrum after gain equalization in the step iii). The correction function can be raised as shown in FIG. 8, for example, and can be derived from experimental values. Specifically, when the loss spectrum of the gain equalizer is deviated, the value at which the blur has occurred is measured and plotted as a correction function. Note that the correction function shown in FIG. 8 is an example and varies depending on conditions and the like.

  The design method of the present invention described above can be automated. For example, using a computer-controllable optical attenuator (ATT) as a variable optical attenuator, using an optical spectrum analyzer as a gain spectrum detector, and using a computer to derive a gain spectrum after gain equalization It is possible to automate the design. The correction function can also be automated by incorporating it into a computer or electronic circuit.

  The above description is an example in which the cascade amplifier has two stages, and the gain equalizer is installed between the two stages. However, the design method of the present invention is a multistage cascade amplifier having three or more stages, In other words, the present invention can be applied even when three or more amplification media or amplifiers are included and a plurality of gain equalizers are included. For example, a gain equalizer having a desired gain equalization band may be designed by applying the design method in order from the gain equalizer on the WDM signal input side.

  In the present invention, as described above, the amplification medium or amplifier has three or more levels (levels in which the ion density cannot be approximated to 0) in consideration of the ion density in the excitation state or amplification state of the cascade amplifier. Any method can be applied to the method of the present invention. Examples of cascade-type rare earth-doped optical fiber amplifiers include those described in Non-Patent Document 3, and specifically, for example, in a combination of the following host glass and rare earth ions, an excited state or an amplified state There are three or more levels in which the ion density should be taken into consideration (levels where the ion density cannot be approximated to 0).

  Types of host glass: tellurite glass, germanate glass, fluoride glass, chalcogenide glass, fluorophosphate glass.

Kinds of rare earth ions: Tb ³⁺ , Dy ³⁺ , Ho ³⁺ , Er ³⁺ , Tm ³⁺ , Yb ³⁺ , Pr ³⁺ , Nd ³⁺ , Pm ³⁺ , Sm ³⁺ , Eu ^{3+ and the} like.

  Furthermore, the thing containing the amplification medium shown in Table 1 mentioned later can be mentioned.

  The cascade amplifier according to the present invention includes a pumping means for pumping the amplification medium, an optical attenuator (having a role different from that used when designing the gain equalizer), an isolator, a wavelength multiplexing coupler, a pumping light source, etc. Conventional means may be included.

Next, the levels for which the ion density should be considered will be described in more detail. The ion density in the excited state or amplified state is greatly affected by the relaxation rate of each level. At a level with a very high relaxation rate, ions immediately relax to the lower level, so that the ion density at that level becomes almost zero, and the amplification characteristic of the amplifier is ignored by ignoring the ion density at that level. Can be discussed. On the other hand, in a level with a low relaxation rate, the time during which ions remain in the level becomes longer and the ion density becomes higher. Therefore, discussion must be made in consideration of the ion density of the level. Therefore, in the present specification, the latter case is referred to as a level where ion density should be considered (in this specification, a level where the ion density cannot be approximated to 0 or a level involved in amplification). Defined. The relaxation rate varies greatly depending on the type of rare earth ions, the energy interval between transition levels, the type of host glass, and the like. For this reason, as described above, even if the same EDFA (at 0.98 μm excitation), when the host glass is quartz glass, there are two levels ( ⁴ I _13/2 , ⁴ I _15/2 ), but in the case of fluoride glass, there are three ( ⁴ I _11/2 , ⁴ I _13/2 , ⁴ I _15/2 ). In FIG. 9, a 0.98 μm-pumped EDFA is obtained by applying a gain equalizer manufactured by applying a quartz EDFA gain equalizer design method to a relaxation rate of ⁴ I _11/2 to a cascade type optical fiber amplifier. The figure which plotted the gain deviation (The gain flatness of a desired zone | band is shown. Since it is trying for the gain deviation to be 0 in the design is equivalent to the deviation | shift from a design value) is shown. The relaxation rate is defined as the sum of the emission relaxation rate (also referred to as spontaneous emission rate) and the non-emission relaxation rate, and when it can be measured, it is equal to the reciprocal of the fluorescence lifetime of the actual measurement value. The relaxation rate formula (7) is shown below.

τ _m represents the measured fluorescence lifetime, and τ _R is the fluorescence lifetime obtained by JO analysis (Non-Patent Document 4 or Non-Patent Document 5) (the reciprocal of this value indicates the emission relaxation rate). , _WNR is the non-light emission relaxation rate. As can be seen from FIG. 9, when the relaxation rate becomes smaller than 1.0 × 10 ⁴ s ⁻¹ , the gain deviation increases rapidly (eg, about 2 dB for fluoride glass). This is because the ion density of ⁴ I _11/2 cannot be ignored. Therefore, a design method for a gain equalizer of a quartz-based EDFA cannot be applied with a relaxation rate smaller than 1.0 × 10 ⁴ s ⁻¹ . This is because rare earth ions other than erbium (Er) have a high relaxation rate (depending on the energy interval, etc.) involved in amplification, or a glass with a low non-luminescence relaxation rate (a glass with a low phonon energy, eg, a chalcogenide glass The same tendency is observed in optical fiber amplifiers using fluoride glass or the like as host glass, that is, optical fiber amplifiers having three or more levels that should be considered for ion density in the excited state and the amplified state. . For example, in the case of the TDFA described in the previous example (when fluoride glass is used as the host glass), the amplification initial level ( ³ H ₄ ) is 700 s ⁻¹ and the amplification final level is about 100 s ^−1. The ion density is distributed in three levels of an amplification start level, an amplification end level, and a ground level. The design method of the present invention is expected as a design method for a gain equalizer of a cascade amplifier having the above-described relaxation rate.

Further, FIG. 10 shows the relaxation rate dependence (in the case of TDFA) of the conversion efficiency from signal light to pumping light. The conversion efficiency increases until the relaxation rate is around 1 × 10 ³ s ⁻¹ . Therefore, the method of the present invention is expected as a method of designing a gain equalizer corresponding to the cascade amplifier of the present invention such as an amplification medium of 1 × 10 ³ s ⁻¹ or less, for example, thulium-doped fluoride fiber.

4). Cascade type amplifier of the present invention A cascade type amplifier of the present invention comprises a plurality of amplification media or amplifiers and a gain equalizer provided at at least one of the intermediate positions. Specifically, as shown in FIG. 11A, the cascade amplifier (1100A) of the present invention includes a first amplifying medium or amplifier (1102), a first gain equalizer from the WDM signal input side. (1104) at least a second amplification medium or amplifier (1106). The cascade amplifier of the present invention can further include a plurality of gain equalizers and amplification media or amplifiers in this order on the output side, and can be a multistage cascade amplifier (FIG. 11 (1100B)). In the above example, the case where all gain equalizers are introduced at positions between the amplification media or amplifiers has been described. However, it is not always necessary to introduce the gain equalizer at any location between all the amplification media or amplifiers. . For example, as illustrated in FIG. 11 (1100C), when a three-stage amplification medium or amplifier is provided, the gain equalizer is installed only between the first amplification medium or amplifier and the second amplification medium or amplifier. You can also

  In the present invention, a plurality of amplifying media or amplifiers include conventional means such as pumping means, optical attenuators (whose roles are different from those used when designing gain equalizers), isolators, wavelength multiplexing couplers, and pumping light sources. Can do.

  The gain equalizer of the cascade amplifier according to the present invention is designed by the above-described method for designing the gain equalizer of the cascade amplifier. That is, the gain equalizer of the cascade amplifier according to the present invention has the loss spectrum shape and gain equalization band of the gain equalizer set in the following process.

i) WDM signal light is input from the signal light input section of a cascade type rare earth doped optical fiber amplifier in which a variable optical attenuator is inserted in place of the gain equalizer at the installation position of the gain equalizer, and the gain spectrum is measured. To do,
ii) Having a spectral shape of the gain spectrum measured in i) in a desired wavelength band, and loss with respect to the total power of the WDM signal light when passing through the variable optical attenuator passes through the gain equalizer. Deriving the loss spectrum of the gain equalizer that satisfies the two conditions of being equal to the loss of time,
iii) (a) The gain spectrum after gain equalization in calculation is the sum of the gain spectrum measured in i) and the loss spectrum of the variable optical attenuator (converted in dB) to the gain equalizer derived in ii) Defining a difference from the loss spectrum (dB conversion) and deriving a calculated gain equalization band from the gain spectrum after gain equalization derived by this definition, or
(B) deriving a calculated gain equalization band from the loss spectrum derived in ii);
To calculate a calculated gain equalization band, and compare the calculated gain equalization band with a desired gain equalization band preset for the cascaded amplifier. Determining whether or not a gain equalization band of a predetermined gain equalization band set in advance for a cascaded amplifier matches.
iv) As a result of the comparison of iii), when the calculated gain equalization band is narrower than a desired gain equalization band preset for the cascaded amplifier, the variable optical attenuator Increasing the loss and reducing the loss of the variable optical attenuator if wider than the desired gain equalization band;
v) Repeat the above steps i) to iv) until the desired gain equalization band is obtained, and determine the shape of the loss spectrum of the gain equalizer and the gain equalization band.

  When the cascade amplifier of the present invention includes a plurality of gain equalizers, a gain equalizer having a desired gain equalization band is designed by applying the above design method in order from the gain equalizer on the WDM signal input side. do it.

  The gain equalizer of the present invention may be manufactured by applying a conventional method. For example, it can be manufactured using the methods disclosed in (Non-Patent Document 2) and the like, and these methods may be set to have the shape of the loss spectrum and the gain equalization band obtained by the above process. .

  Moreover, it is preferable that the gain equalizer of the cascade amplifier of the present invention further enhances the design accuracy by incorporating a correction function when deriving the gain spectrum after the gain equalization in the process iii). The correction function is as described above.

  As described above, the cascade amplifier of the present invention has three or more levels (levels in which the ion density cannot be approximated to 0) in consideration of the ion density in the excited state or the amplified state of the cascade amplifier as described above. Any thing is acceptable. Examples of cascade-type rare earth-doped optical fiber amplifiers include those described in Non-Patent Document 3, and specifically, for example, in a combination of the following host glass and rare earth ions, an excited state or an amplified state There are three or more levels in which the ion density should be taken into consideration (levels where the ion density cannot be approximated to 0).

  Types of host glass: tellurite glass, germanate glass, fluoride glass, chalcogenide glass, fluorophosphate glass.

Kinds of rare earth ions: Tb ³⁺ , Dy ³⁺ , Ho ³⁺ , Er ³⁺ , Tm ³⁺ , Yb ³⁺ , Pr ³⁺ , Nd ³⁺ , Pm ³⁺ , Sm ³⁺ , Eu ^{3+ and the} like.

  Furthermore, the thing containing the amplification medium shown in Table 1 mentioned later can be mentioned.

Furthermore, in the cascade amplifier of the present invention, the rare earth ions involved in amplification added to the amplification medium when in the excited state or the amplified state have a relaxation rate (the sum of the emission relaxation rate and the non-emission relaxation rate) of l ×. It has three or more energy levels that are 10 ⁴ s ⁻¹ or less.

5). Apparatus for designing gain equalizer of cascade amplifier of the present invention An apparatus for designing the gain equalizer of the cascade amplifier of the present invention will be described with reference to FIG. 12 and FIG. As shown in FIG. 12, the design apparatus (1200) of the present invention includes an amplification medium or amplifier (referred to as an amplifier in FIGS. 12 and 13) (1202, 1204), a variable optical attenuator (1206), and a detector (1208). ), And a calculation circuit and a variable optical attenuator control circuit (1210). When driving the design apparatus, the WDM light source (1212) is connected to the input side. As a detector, as shown in FIG. 13, a photodetector (1302) corresponding to the WDM signal wavelength can be used.

  When the design of the gain equalizer of the cascade amplifier of the present invention is automated, the design apparatus of the present invention can be configured using an optical attenuator, an optical spectrum analyzer, and a computer that can be controlled by a computer.

  Also, a computer can be made into an electronic circuit and incorporated into an optical spectrum analyzer. Furthermore, the correction function shown in FIG. 8 can be incorporated into a computer or an electronic circuit to constitute a design apparatus using the correction function.

  The variable optical attenuator usable in the gain equalizer designing apparatus of the present invention preferably has a flat wavelength characteristic for ease of calculation. For example, the following variable optical attenuators can be used. Examples of mechanical variable optical attenuators include a method in which an attenuation plate with a transmission loss distribution is inserted during optical beam coupling between fibers and the attenuation plate is moved, and a method in which the gap between fibers is variable Etc. Non-mechanical examples include those using an electro-optic effect, those using a magneto-optic effect, and those using a thermo-optic effect.

  In the gain equalizer designing apparatus of the present invention, as described above, the level that should consider the ion density in the excited state or the amplified state of the cascade amplifier (a level where the ion density cannot be approximated to 0) is used as the amplification medium. Anything can be used as long as there are three or more. Examples of cascade-type rare earth-doped optical fiber amplifiers include those described in Non-Patent Document 3, and specifically, for example, in a combination of the following host glass and rare earth ions, an excited state or an amplified state There are three or more levels in which the ion density should be taken into consideration (levels where the ion density cannot be approximated to 0).

  Types of host glass: tellurite glass, germanate glass, fluoride glass, chalcogenide glass, fluorophosphate glass.

Kinds of rare earth ions: Tb ³⁺ , Dy ³⁺ , Ho ³⁺ , Er ³⁺ , Tm ³⁺ , Yb ³⁺ , Pr ³⁺ , Nd ³⁺ , Pm ³⁺ , Sm ³⁺ , Eu ^{3+ and the} like.

  Furthermore, the thing containing the amplification medium shown in Table 1 mentioned later can be mentioned.

  The gain equalizer designing apparatus of the present invention obtains the loss spectrum shape and gain equalization band of the target gain equalizer through the following process. The following process is as described in the method of designing a gain equalizer of the present invention.

i) WDM signal light is input from the signal light input section of a cascade type rare earth doped optical fiber amplifier in which a variable optical attenuator is inserted in place of the gain equalizer at the installation position of the gain equalizer, and the gain spectrum is measured. To do,
ii) Having a spectral shape of the gain spectrum measured in i) in a desired wavelength band, and loss with respect to the total power of the WDM signal light when passing through the variable optical attenuator passes through the gain equalizer. Deriving the loss spectrum of the gain equalizer that satisfies the two conditions of being equal to the loss of time,
iii) (a) The gain spectrum after gain equalization in calculation is the sum of the gain spectrum measured in i) and the loss spectrum of the variable optical attenuator (converted in dB) to the gain equalizer derived in ii) Defining a difference from the loss spectrum (dB conversion) and deriving a calculated gain equalization band from the gain spectrum after gain equalization derived by this definition, or
(B) deriving a calculated gain equalization band from the loss spectrum derived in ii);
To calculate a calculated gain equalization band, and compare the calculated gain equalization band with a desired gain equalization band preset for the cascaded amplifier. Determining whether or not a gain equalization band of a predetermined gain equalization band set in advance for a cascaded amplifier matches.
iv) As a result of the comparison of iii), when the calculated gain equalization band is narrower than a desired gain equalization band preset for the cascaded amplifier, the variable optical attenuator Increasing the loss and reducing the loss of the variable optical attenuator if wider than the desired gain equalization band;
v) Repeat the above steps i) to iv) until the desired gain equalization band is obtained, and determine the shape of the loss spectrum of the gain equalizer and the gain equalization band.

  In the present invention, it is preferable to further improve the design accuracy by incorporating a correction function when deriving the gain spectrum after gain equalization in step iii). The correction function is as described above.

Furthermore, the apparatus for designing a gain equalizer of a cascade amplifier according to the present invention has a relaxation rate (a light emission relaxation rate and a non-light emission relaxation) when a rare earth ion involved in amplification added to an amplification medium is in an excited state or an amplified state. The total of the energy levels is 1 × 10 ⁴ s ⁻¹ or less.

  A cascade amplifier of the present invention as shown in FIG. 11 (with two or three amplification media or amplifiers) was produced. This cascade type amplifier has the configuration of additive ions, host glass, and cascade type amplifier shown in Table 1. A design apparatus as shown in FIG. 12 or 13 is constructed using a general variable optical attenuator, and the shape of the loss spectrum of the gain equalizer and the gain equalization band are obtained, and this value is used as the gain equalizer. Set. The gain equalizer was obtained by the procedure described in Non-Patent Document 2 based on the obtained set value. Table 1 also shows the gain deviation when a gain equalizer designed by a conventional method of designing a gain equalizer applicable to a cascade type quartz EDFA is installed.

  When a gain equalizer manufactured using the method of the present invention is used, the characteristics of a cascade amplifier when a gain equalizer is manufactured using a conventional quartz-based EDFA gain equalizer design method are shown. It is shown in comparison with 1.

  As an example, FIG. 14 and FIG. 15 show the loss spectrum of the gain equalizer manufactured in Example 1 in Table 1 as a cascade amplifier and the gain spectrum measured by incorporating the gain equalizer in the cascade optical fiber amplifier. Show. Further, Example 9 in Table 1 shows the result of improving the accuracy of gain equalization of the optical fiber amplifier of Example 1 by incorporating a correction function. The correction function is shown in FIG. 8, and correction can be performed by adding the correction value on the vertical axis to the loss spectrum of the gain equalizer calculated in the first embodiment. The correction function shown in FIG. 8 was derived from experimental values. However, the correction function shown in FIG. 8 is an example, and changes variously depending on conditions and the like. Any of the cascade amplifiers shown in Examples 1 to 9 achieves a gain deviation of 0.5 dB or less by using the gain equalizer manufactured by the method of the present invention. Proved.

  As described above, by using the gain equalizer manufactured by using the method of the present invention, a cascade type using an amplification medium containing rare earth ions having three or more levels to which ion density should be considered. Even in an optical fiber amplifier, a gain equalizer can be easily designed. In addition, the design method of the present invention can be easily implemented as a device, and can be automated. When designing without using the method of the present invention, a gain equalizer is actually manufactured, mounted on an optical fiber amplifier, the amplification characteristic is evaluated, and gain equalization is repeated many times until the desired characteristic is obtained. It is anticipated that it will be necessary to recreate the device, and the present invention does not require repetitive fabrication of such a gain equalizer.

DESCRIPTION OF SYMBOLS 1100 Cascade type | mold amplifier 1102 Amplification medium or amplifier 1104 Gain equalizer 1106 Amplification medium or amplifier 1200 Gain equalizer design apparatus 1202 Amplification medium or amplifier 1204 Amplification medium or amplifier 1206 Variable optical attenuator 1208 Detector 1210 Calculation circuit and variable optical attenuator control circuit 1212 WDM light source 1302 Detector

Claims (10)

A gain-flattened cascade type rare earth-doped optical fiber amplifier comprising a plurality of amplification media or amplifiers and a gain equalizer provided at at least one of the intermediate positions, the gain equalizer But,
i) WDM signal light is input from the signal light input section of a cascade type rare earth doped optical fiber amplifier in which a variable optical attenuator is inserted in place of the gain equalizer at the installation position of the gain equalizer, and the gain spectrum is measured. To do,
ii ) Having the spectrum shape of the gain spectrum measured in i ), and the loss with respect to the total power of the WDM signal light when passing through the variable optical attenuator becomes equal to the loss when passing through the gain equalizer Deriving the loss spectrum of the gain equalizer that satisfies the two conditions
iii) (a) The gain spectrum after gain equalization in calculation is the sum of the gain spectrum measured in i) and the loss spectrum of the variable optical attenuator (converted in dB) to the gain equalizer derived in ii) Defining a difference from the loss spectrum (dB conversion) and deriving a calculated gain equalization band from the gain spectrum after gain equalization derived by this definition, or
(B) deriving a calculated gain equalization band from the loss spectrum derived in ii);
To calculate a calculated gain equalization band, and compare the calculated gain equalization band with a desired gain equalization band preset for the cascaded amplifier. Determining whether or not a gain equalization band of a predetermined gain equalization band set in advance for a cascaded amplifier matches.
iv) As a result of the comparison of iii), when the calculated gain equalization band is narrower than a desired gain equalization band preset for the cascaded amplifier, the variable optical attenuator Increasing the loss and reducing the loss of the variable optical attenuator if wider than the desired gain equalization band;
v) The above steps i) to iv) are repeated until the target gain equalization band is obtained, and the gain equalization set in the process of determining the shape of the loss spectrum of the gain equalizer and the gain equalization band. A gain-flattened cascade-type rare earth doped optical fiber amplifier characterized by having a loss spectrum shape and gain equalization band.   2. The cascade type optical fiber amplifier according to claim 1, wherein a correction function is incorporated in deriving the gain spectrum after gain equalization in iii).   3. The rare earth ions involved in amplification added to the amplification medium have three or more energy levels in which the ion density cannot be approximated to zero when in an excited state or an amplified state. Cascade type optical fiber amplifier. The energy level of the rare earth ions involved in amplification added to the amplification medium when in the excited state or the amplified state has a relaxation rate (the sum of the emission relaxation rate and the non-emission relaxation rate) of 1 × 10 ⁴ s ⁻¹ or less. 4. The cascade type rare earth doped optical fiber amplifier according to claim 3, wherein the cascade type rare earth doped optical fiber amplifier has three or more units. A method of designing a gain equalizer that is inserted into at least one of a plurality of amplifying media or amplifiers constituting a cascade-type rare earth-doped optical fiber amplifier,
i) WDM signal light is input from the signal light input section of a cascade type rare earth doped optical fiber amplifier in which a variable optical attenuator is inserted in place of the gain equalizer at the installation position of the gain equalizer, and the gain spectrum is measured. To do,
ii ) Having the spectrum shape of the gain spectrum measured in i ), and the loss with respect to the total power of the WDM signal light when passing through the variable optical attenuator becomes equal to the loss when passing through the gain equalizer Deriving the loss spectrum of the gain equalizer that satisfies the two conditions
iii) (a) The gain spectrum after gain equalization in calculation is the sum of the gain spectrum measured in i) and the loss spectrum of the variable optical attenuator (converted in dB) to the gain equalizer derived in ii) Defining a difference from the loss spectrum (dB conversion) and deriving a calculated gain equalization band from the gain spectrum after gain equalization derived by this definition, or
(B) deriving a calculated gain equalization band from the loss spectrum derived in ii);
To calculate a calculated gain equalization band, and compare the calculated gain equalization band with a desired gain equalization band preset for the cascaded amplifier. Determining whether or not a gain equalization band of a predetermined gain equalization band set in advance for a cascaded amplifier matches.
iv) As a result of the comparison of iii), when the calculated gain equalization band is narrower than a desired gain equalization band preset for the cascaded amplifier, the variable optical attenuator Increasing the loss and reducing the loss of the variable optical attenuator if wider than the desired gain equalization band;
v) The steps i) to iv) are repeated until a target gain equalization band is obtained, and includes a process of determining the shape of the loss spectrum of the gain equalizer and the gain equalization band. Design method.   6. The design method according to claim 5, wherein a correction function is incorporated in deriving a gain spectrum after gain equalization in iii). A design apparatus for designing a gain equalizer to be inserted into at least one of a plurality of amplifying media constituting a cascade type rare earth-doped optical fiber amplifier or an amplifier, a variable optical attenuator, and a WDM signal light Including a photodetector, a calculation circuit, and a variable optical attenuator control circuit for measuring the gain of the circuit; i) a variable optical attenuator is inserted in place of the gain equalizer at the installation position of the gain equalizer; WDM signal light is input from the signal light input section of the cascaded rare earth-doped optical fiber amplifier, and the gain spectrum is measured.
ii ) Having the spectrum shape of the gain spectrum measured in i) , and the loss with respect to the total power of the WDM signal light when passing through the variable optical attenuator becomes equal to the loss when passing through the gain equalizer Deriving the loss spectrum of the gain equalizer that satisfies the two conditions
iii) (a) The gain spectrum after gain equalization in calculation is the sum of the gain spectrum measured in i) and the loss spectrum of the variable optical attenuator (converted in dB) to the gain equalizer derived in ii) Defining a difference from the loss spectrum (dB conversion) and deriving a calculated gain equalization band from the gain spectrum after gain equalization derived by this definition, or
(B) deriving a calculated gain equalization band from the loss spectrum derived in ii);
To calculate a calculated gain equalization band, and compare the calculated gain equalization band with a desired gain equalization band preset for the cascaded amplifier. Determining whether or not a gain equalization band of a predetermined gain equalization band set in advance for a cascaded amplifier matches.
iv) As a result of the comparison of iii), when the calculated gain equalization band is narrower than a desired gain equalization band preset for the cascaded amplifier, the variable optical attenuator Increasing the loss and reducing the loss of the variable optical attenuator if wider than the desired gain equalization band;
v) The steps i) to iv) are repeated until the target gain equalization band is obtained, and the shape of the loss spectrum of the gain equalizer and the gain equalization band are determined to determine the loss spectrum of the gain equalizer. A design apparatus characterized by obtaining a shape and a gain equalization band.   8. The design apparatus according to claim 7, wherein a correction function is incorporated in deriving a gain spectrum after the gain equalization in iii).   9. The rare earth ion involved in amplification added to the amplification medium has three or more energy levels that cannot approximate the ion density to zero when in an excited state or an amplified state. Design equipment. The energy level of the rare earth ions involved in amplification added to the amplification medium when in the excited state or the amplified state has a relaxation rate (the sum of the emission relaxation rate and the non-emission relaxation rate) of 1 × 10 ⁴ s ⁻¹ or less. The design apparatus according to claim 9, wherein the design apparatus has three or more positions.

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