JP2006245334A - Optical fiber amplifier - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical fiber amplifier capable of satisfactorily controlling the gain spectrum of a thorium-doped optical fiber amplifier (TDFA), using a simple configuration or a simple algorithm. <P>SOLUTION: The optical fiber amplifier is provided with an amplifying optical fiber (201) doped with thorium; an excitation light source (204) for emitting an excitation light to be incident into the amplifying optical fiber; a detecting means (230) for detecting the loss amount of the excitation light in the amplifying optical fiber; an error signal extracting means (240) for outputting an error signal between the loss amount detected in the detecting means and the loss amount of the excitation light equivalent to a previously stored gain value; and a control means (250) for controlling the excitation light source so that the power of the excitation light varies and the error signal becomes zero. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical fiber amplifier, and more particularly to an optical fiber amplifier capable of controlling a gain to be constant.

  2. Description of the Related Art Conventionally, in an optical fiber communication system, long distance transmission is possible by inserting optical fiber amplifiers at appropriate transmission distances and amplifying optical signals that are attenuated by transmission through optical fibers. An optical fiber amplifier using stimulated emission can simultaneously amplify a plurality of signal lights having different wavelengths within an amplification band, and is used in a wavelength division multiplexing optical transmission system (WDM transmission system). In addition, photonic networks based on the WDM transmission system have been actively studied.

  In general, when an optical amplifier typified by an erbium doped fiber amplifier (EDFA) is applied to WDM transmission, a plurality of signals having different frequencies are amplified and output with substantially uniform power. The gain spectrum of the amplifier is preferably flat. Generally, a flat gain spectrum is generally realized by using a gain equalizer having an inverse characteristic of the gain spectrum of an optical amplifier.

  However, in an optical amplifier, generally, when the input changes, the gain spectrum changes. That is, even if gain flattening is achieved under certain input signal power conditions, if the input power to the amplifier changes due to changes in span loss of the optical fiber or changes in the number of WDM channels, the output between channels varies between channels. A WDM signal having a certain value is output. As a result, the channel with high output power is degraded by the nonlinear optical effect generated in the subsequent optical fiber transmission line. Also, the low output channel causes the optical signal-to-noise ratio to deteriorate in the optical amplifier at the next stage. That is, the optical amplifier needs to keep the gain spectrum constant against the input power fluctuation.

As a method for suppressing such an adverse effect, a method for controlling the gain to be constant is widely used in the EDFA. The gain spectrum G (λ) of the EDFA is such that the erbium ion density in the EDFA is N _tot , of which the erbium ion density in the amplified starting state is N ₂ and the erbium ion density in the amplified terminal state is N _1. (N _tot = N ₂ + N ₁ ), represented by the following formula (1).

Here, L is the length of the erbium-doped fiber (EDF), g ^* (λ) is the gain in the completely inverted distribution state per unit length, and α (λ) is the absorption per unit length. g, α, and L are constant regardless of the pumping light power or the input signal power. Only N ₂ is a value that depends on the pumping light power and the signal light input power, and the gain spectrum G (λ) of the EDFA is uniquely determined by the value of N ₂ . N ₂ can be controlled to be constant by changing the pumping light power and the like, whereby the gain spectrum can be controlled to be constant.

On the other hand, the situation is complicated in the case of a thulium doped fiber amplifier (TDFA). The energy level of thulium ions is shown in FIG. The 1400 nm amplification of TDFA uses ³ H ₄ → ³ F ₄ stimulated emission in the four-position system. The total density of thulium ions in the thulium-doped fiber (TDF) is N _tot , and the thulium densities in the excited starting position, the amplified final position, and the ground supporting position in the excited state are N ₂ , N ₁ , N ₀ , respectively. Then (N _tot = N ₂ + N ₁ + N ₀ ), the gain of TDFA is

It is represented by Where L is the TDF length, g ^* (λ) is the gain per unit length when thulium is all excited to ³ H ₄ , and α (λ) is the case where all thulium is excited to ³ F ₄ . It represents absorption per unit length. N ₂ and N ₁ are values that depend on the pumping light power and the signal light input power, respectively, and vary independently to some extent. Therefore, in order to control the gain spectrum to be constant, it is necessary to control two variables N ₂ and N ₁ , respectively.

As a method for controlling N ₂ and N ₁ independently, a method has been proposed in which TDFA is excited with two wavelengths and the two wavelengths are controlled independently (see, for example, Non-Patent Document 1).

As another method of independently controlling N ₂ and N ₁ , a method of providing another signal light for gain control and controlling the input power of the signal light has been proposed (for example, see Non-Patent Document 2). ). Regarding this method, a method of monitoring the gain of the gain control signal light has been proposed as a method for extracting a control error signal.

W. J. Lee et al., “Gain excursion & tilt compensation algorithm for TDFA using 1.4 um / 1.5 um Dual Wavelength Pump Control.” Proc. OFC 2002, Paper ThZ3, pp. 571-572, 2002. S. Aozasa and M. Shimizu, “Novel gain spectrum control method for input power and temperature variations in S-band thulium doped fiber amplifier,” Proceedings of 29th European Conference on Optical Communication (ECOC2003), paper We4.p.10, 2003

  However, in the above-described method in which TDFA is excited with two wavelengths and the two wavelengths are controlled independently, it is not clear what control error signal is used to control these two wavelengths.

  In addition, the above method of providing another signal light for gain control and controlling the input power of the signal light requires complicated control settings because the control target value must be constant and empirical fitting parameters must be entered. It was necessary to use complicated initial setting and amplifier configuration such that the wavelength fluctuation of the signal light becomes a fatal control error.

  The present invention has been made in view of such problems, and an object of the present invention is to provide an optical fiber amplifier capable of controlling the gain spectrum of TDFA satisfactorily with a relatively simple configuration or algorithm. There is.

  In order to achieve the above object, the present invention provides an optical fiber amplifier comprising an amplification optical fiber to which thulium is added, and excitation light incident on the amplification optical fiber. A pumping light source that emits light, a detecting unit that detects a loss amount of the pumping light in the optical fiber for amplification, a loss amount detected by the detecting unit, and a pumping light loss amount corresponding to a gain value stored in advance. Error signal extracting means for outputting an error signal and control means for controlling the excitation light source so that the power of the excitation light is varied and the error signal becomes zero are provided.

  A second aspect of the present invention is the optical fiber amplifier according to the first aspect, wherein the pumping light has a wavelength of 760 nm to 840 nm, 980 nm to 1120 nm, or 1320 nm to 1520 nm.

  A third aspect of the present invention is the optical fiber amplifier according to the first aspect, wherein a second pumping light that has a wavelength different from a wavelength of the pumping light and is incident on the amplification optical fiber is emitted. The excitation light source is further provided.

  The invention according to claim 4 is the optical fiber amplifier according to claim 3, characterized in that the power of the second pumping light incident on the amplification optical fiber is constant.

  According to a fifth aspect of the present invention, in the optical fiber amplifier according to the third aspect, the second pumping light is incident from the signal light output end of the amplification optical fiber toward the signal light input end. It is characterized by that.

  A sixth aspect of the present invention is the optical fiber amplifier according to the third aspect, wherein the second detection means detects the amount of loss of the second excitation light in the amplification optical fiber, the second Second error signal extraction means for outputting a second error signal between the loss amount detected by the detection means and the loss amount of the second excitation light corresponding to the gain value stored in advance, and the first It further comprises second control means for controlling the second pumping light source so that the power of the second pumping light is varied and the second error signal becomes zero.

  A seventh aspect of the present invention is the optical fiber amplifier according to the third to sixth aspects, wherein the pumping light has a wavelength of 760 nm to 840 nm, 980 nm to 1120 nm, or 1320 nm to 1520 nm, The excitation light has a wavelength of 1100 nm to 1300 nm, or 1500 nm to 18000 nm.

  The invention according to an eighth aspect is the optical fiber amplifier according to the first to seventh aspects, wherein the amplification optical fiber is added with 3000 ppm wt% or more of thulium.

  As described above, according to the present invention, it is possible to provide an optical fiber amplifier that can satisfactorily control the gain spectrum of TDFA with a relatively simple configuration or algorithm.

As shown in FIG. 1, the thulium ion density is determined based on the absorption of the ground position by excitation light or signal light, absorption of stimulated position by stimulated light and stimulated emission, non-luminous interaction between thulium ions (cross relaxation), and 800 nm. It is determined by the balance of amplified spontaneous emission light (ASE light). In particular, when used in a region where the 800 nm band ASE is growing strongly (the peak wavelength of TDFA is relatively short and the gain is high), the influence of the ground level absorption of the excitation light and signal light is affected. The ratio becomes relatively small, and the ratio of N ₂ and N ₀ is almost determined by stimulated absorption / stimulated emission of 800 nm ASE light. For this reason, since N ₁ and N ₂ can be controlled to be substantially constant by simply controlling one parameter (a parameter including information on N ₁ ), a certain degree of accuracy (within a few dB) from Equation (2). Gain control is possible.

One of the parameters including N ₁ information is the loss of excitation light in the TDF. The loss of excitation light in TDF consists of ground level absorption and excitation level absorption, but the ratio of excitation level absorption is much larger than that of ground level absorption, and the ground level absorption can be almost ignored. That is, by making the loss in the TDF of the pumping light constant, N ₁ can be made constant, and as a result, the gain spectrum can be controlled to a certain extent.

Furthermore, the control accuracy is improved by adding the second excitation light. As described above, the ratio of N ₂ to N ₀ is almost determined by stimulated absorption / stimulated emission of 800 nm ASE light, but it is not completely free from the influence of ground level absorption of excitation light and signal light. In particular, when the power of the input signal light is strong, the power of the pump light becomes strong because the gain is controlled at a constant level, and the ground level absorption of the pump light and signal light is larger than when the power of the input signal light is small. As a result, the gain control error becomes larger. By adding the second excitation light, which has a high base-spot absorption efficiency and relatively high power, the base-potential absorption of the first excitation light and signal light can be ignored, and the spectral accuracy increases. . In particular, when comparing the ground level absorption of the pump light and the signal light, the ground level absorption of the signal light causes a larger error, so the output of the optical fiber for amplification becomes stronger as the signal light is amplified. Inputting the second excitation light from the end toward the input end is more effective for improving the control accuracy. In this case, even if the second pumping light is input at a constant power, a certain degree of gain control accuracy can be secured, but if the second pumping light absorption can be kept constant, the control accuracy will be further improved. .

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

(First embodiment)
FIG. 2 shows the configuration of the optical fiber amplifier according to the first embodiment of the present invention. In the optical fiber amplifier of this embodiment, a pumping light source 204 that emits pumping light, a branching coupler 205 that splits the pumping light into two, and one of the pumping lights branched by the branching coupler 205 are input via an optical isolator 202a. The WDM coupler 203a that combines the signal light combined with the signal light, the TDF 201 that receives the signal light combined with the pump light, the WDM coupler 203b that separates the pump light and the signal light that has passed through the TDF 201, and the WDM coupler 203b. An optical amplifying unit 200 provided with an optical isolator 202b through which the waved signal light passes and is output; a PD (Photo Diode) 206 that receives the other of the pumping light branched by the branch coupler 205; and a WDM coupler 203b. PD 207 that receives the excitation light demultiplexed in step 1 and the power level of the excitation light output from PD 206 and PD 207 (input Pumping light loss 231 that outputs the amount of pumping light loss (pumping light loss amount monitoring value) in the TDF 201 based on the monitoring value and the output monitoring value), the pumping light corresponding to a certain gain value A storage unit (not shown) that stores the loss amount as a target value in advance, and an error signal extraction that outputs an error signal by comparing the target value stored in advance with the pump light loss monitor value output from the pump light loss 231 And a control unit 250 that controls the power of the pumping light emitted from the pumping light source 204 based on the error signal output from the error signal extracting unit 240.

In the optical amplifying unit 200, the input signal light having a wavelength between 1400 nm and 1530 nm and the excitation light emitted from the excitation light source 204 are combined by the WDM coupler 203 a and input to the TDF 201. The excitation light activates Tm ³⁺ ions in the TDF 201, and the signal light passes through the Tm ³⁺ ions to cause stimulated emission, whereby the signal light is amplified and output.

  The input power of the excitation light to the TDF 201 is monitored by inputting the excitation light branched by the branch coupler 205 arranged between the excitation light source 204 and the branch coupler 203a to the PD 206 of the detection unit. The output power of the pumping light from the TDF 201 is monitored by inputting the pumping light separated from the signal light by the WDM coupler 203b disposed between the TDF 201 and the optical isolator 202b to the PD 207 of the detection unit. .

  In the detection unit 203, based on the input monitor value of the excitation light measured by the PD 206 to the TDF 201 and the output monitor value from the TDF 201 measured by the PD 207, the loss amount of the excitation light in the TDF 201 (excitation light loss monitor value). ) And output to the error signal extraction unit 240.

  The error signal extraction unit 240 sets a pumping light loss amount corresponding to a certain gain value as a target value in advance, compares the target value with the pumping light loss amount monitor value output from the detection unit, and generates an error signal. Is extracted and output to the control unit 250.

  The control unit 250 inputs the error signal from the error signal extraction unit 240 and controls the excitation light source 204 so that the error signal becomes zero. When error signal = excitation light loss monitoring value−target value is defined, when the error signal is positive, the power of the excitation light is increased. When the error signal is negative, the power of the excitation light is decreased. In the direction, the excitation light source 204 is controlled.

  In the configuration of this embodiment, a forward pumping configuration in which pumping light is input in the same direction as the signal light is used, but a backward pumping configuration in which pumping light is input so as to propagate in the opposite direction to the signal light can be employed. Further, the present invention can also be applied to a bidirectional excitation configuration in which excitation light is input from both ends of the TDF 201. Regarding control accuracy, the backward excitation configuration or the bidirectional excitation configuration is better than the forward excitation configuration.

  For example, more specifically, as the excitation light source 204, it is possible to use excitation light having a wavelength of 800 nm ± 40 nm (760 nm to 840 nm, the same applies hereinafter), 1050 nm ± 70 nm, or 1420 nm ± 100 nm. At these wavelengths, a high-power semiconductor LD can be used. As the branch coupler 205 that monitors the power of the pumping light input to the TDF 201, a coupler such as a 10: 1 coupler or a 20: 1 coupler can be used. Also, here, an example is shown in which the power of the pumping light to be input is measured using the branch coupler 205, but a monitor for monitoring the output at the end opposite to the output end of the LD in a semiconductor LD or the like. In many cases, a PD is provided, and this monitor PD can be used in place of the branch coupler 205 and the PD 206.

In addition, the configuration of the present embodiment can be used regardless of the core addition amount of Tm ³⁺ in TDF201. However, in order to obtain high efficiency by one-wavelength excitation, the addition amount of Tm ³⁺ is 3000 ppm. It is desirable to set it above.

FIG. 3 shows the gain of the WDM signal when the signal input fluctuates in the optical fiber amplifier of this embodiment. In TDF201, 6000 ppm of Tm3 ⁺ was added to the core, the cutoff wavelength was 1000 nm, the relative refractive index difference Δn was 1.6%, and the fiber length was 3 m. A high output LD having a wavelength of 1390 nm was used as the excitation light source 204. In the signal light, the WDM signal of 9 waves (wavelength interval 10 nm) from 1460 to 1540 nm is changed between −11 to −31 dBm / ch so that the error signal becomes zero, that is, in the TDF 201 of the excitation light. The power of the excitation light source 204 was controlled so that the loss was constant. When the optical fiber amplifier of the present embodiment is implemented as exemplified above, the gain spectrum can be controlled with an error of about 2.5 dB as shown in FIG.

(Second embodiment)
FIG. 4 shows an optical fiber amplifier according to the second embodiment of the present invention. The configuration of the optical fiber amplifier according to the present embodiment is a bidirectional pump configuration in which pump lights having different wavelengths are input from both ends of the TDF. Compared with the configuration of the optical fiber amplifier of the first embodiment shown in FIG. 2, the configuration of the optical fiber amplifier of the second embodiment further includes a pumping light source that emits pumping light input in the opposite direction to the signal light. Is provided.

  The optical fiber amplifier of the present embodiment includes a pumping light source 404 that emits pumping light A, a branching coupler 405 that splits the pumping light A into two, and one of the pumping light A branched by the branching coupler 405 via an optical isolator 402a. A WDM coupler 403a for combining the signal light input in this manner, a TDF 401 for inputting the signal light combined with the pumping light A, a WDM coupler 409 for splitting the pumping light A and the signal light passing through the TDF 401, A pump light source 408 having a wavelength different from that of the pump light A and emitting the pump light B input to the TDF 401 in the opposite direction to the signal light through the WDM couplers 410 and 409, and the signal light demultiplexed by the WDM coupler 409 are An optical amplifying unit 400 including an optical isolator 402b that passes through and outputs, and a PD 406 that receives the other of the pumping light A branched by the branching coupler 405, and The pump light A demultiplexed by the WDM coupler 409 is input based on the PD 407 input via the WDM coupler 410 and the power levels (input monitor value and output monitor value) of the pump light A output from the PD 406 and PD 407, respectively. The detection unit 430 includes a pumping light loss 431 that outputs the pumping light A loss amount (pumping light loss amount monitor value) in the TDF 401, and stores the pumping light loss amount corresponding to a certain gain value as a target value in advance. (Not shown) a storage unit, an error signal extraction unit 440 that compares a target value stored in advance with an excitation light loss monitor value output from the excitation light loss 431 and outputs an error signal, and an error signal extraction unit And a control unit 450 that controls the power of the pumping light A emitted from the pumping light source 404 based on the error signal output from the 440.

In the optical amplifying unit 400, the input signal light having a wavelength between 1400 and 1530 nm and the pumping light A emitted from the pumping light source 404 are combined by the WDM coupler 403 and input to the TDF 401. The excitation light A activates Tm ³⁺ ions in the TDF 401, and the signal light passes through the Tm ³⁺ ions to cause stimulated emission, whereby the signal light is amplified and output.

  In addition, excitation light B from the excitation light source 408 having a wavelength different from that of the excitation light source 404 is input from the output side end of the TDF 401 via the WDM coupler 409.

  The input power of the excitation light A to the TDF 401 is monitored by inputting the excitation light A branched by the branch coupler 405 disposed between the excitation light source 404 and the WDM coupler 403 to the PD 406 of the detection unit 430. . The output power of the pump light A from the TDF 401 is WDM that separates the pump light A separated from the signal light by the WDM coupler 409 disposed between the TDF 401 and the optical isolator 402b, and further separates the pump light A and the pump light B. It is monitored by taking it out via the coupler 410 and inputting it to the PD 407 of the detection unit 430.

  In the detection unit 430, based on the input monitor value of the excitation light A measured by the PD 406 to the TDF 401 and the output monitor value from the TDF 401 measured by the PD 407, the loss amount of the excitation light A in the TDF 401 (excitation light A loss) (Quantity monitor value) is calculated and output to the error signal extraction unit 440.

  In the error signal extraction unit 440, the loss amount of the pumping light A corresponding to a certain gain value is set in advance as a target value, and this target value is compared with the pumping light A loss amount monitor value output from the detection unit. The error signal is extracted and output to the control unit 450.

  The control unit 450 receives the error signal from the error signal extraction unit 440 and controls the excitation light source 404 so that the error signal becomes zero. When error signal = excitation light A loss monitoring value−target value is defined, the power of pumping light A is increased when the error signal is positive, and the power of pumping light A when the error signal is negative. The excitation light source 404 is controlled in the direction of lowering. During this period, the pumping light B is input to the TDF 401 with a constant power.

  In this embodiment, the pumping light A is input in the same direction as the signal light, but the present control method has a backward pumping structure in which the pumping light A is input so as to propagate in the opposite direction to the signal light, A bidirectional excitation configuration in which excitation light A is input from both ends of the TDF 401 is also applicable. Regarding control accuracy, the backward excitation configuration or the bidirectional excitation configuration is better than the forward excitation configuration. For the excitation light B, either forward excitation or backward excitation is effective, but the control accuracy is higher when the excitation light B is input in the direction opposite to the signal traveling direction.

  As the excitation light source 404, an excitation light source that emits excitation light A having a wavelength of 800 nm ± 40 nm (760 nm to 840 nm, the same applies hereinafter), 1050 nm ± 70 nm, or 1420 nm ± 100 nm can be used. The wavelength of the excitation light source 408 is preferably 1200 nm ± 100 nm or 1650 nm ± 150 nm. At these wavelengths, a high-power semiconductor LD can be used. As the branch coupler 405 for monitoring the power of the input pumping light A to the TDF, a coupler such as a 10: 1 coupler or a 20: 1 coupler can be used. Further, in this embodiment, an example is shown in which the input pumping light power is measured using the branch coupler 405. However, in a semiconductor LD or the like, a monitor that monitors the output at the end opposite to the output end of the LD. In many cases, a PD is provided, and this monitor PD can be used instead of the branch coupler 405 and the PD 406.

In addition, the configuration of the present embodiment can be applied regardless of the value of the core addition amount of Tm ³⁺ in TDF401. However, in order to obtain high efficiency by one-wavelength excitation, the addition amount of Tm ³⁺ is set to It is desirable to set it to 3000 ppm or more.

FIG. 5 shows the gain of the WDM signal when the signal input fluctuates in the optical fiber amplifier of this embodiment. In TDF401, 6000 ppm of Tm ³⁺ was added to the core, the cutoff wavelength was 1000 nm, the relative refractive index difference Δn was 1.6%, and the fiber length was 3 m. A high-power LD having a wavelength of 1390 nm was used as the excitation light source 404, and an LD having a wavelength of 1200 nm was used as the excitation light source 408. The power of the excitation light source B was about 45 mW. The WDM signal of 9 waves (wavelength interval 10 nm) from 1460 to 1540 nm is changed to -11 to -31 dBm / ch to the signal light so that the error signal becomes zero, that is, the loss of the pump light A in the TDF 401 Is controlled so that the power of the excitation light source 404 is constant. When the optical fiber amplifier of this embodiment is implemented as illustrated above, the gain spectrum can be controlled with an error of about 0.6 dB as shown in FIG. Compared to FIG. 3, it can be seen that the control accuracy is improved by adding the second excitation light (excitation light B) of 1200 nm.

(Third embodiment)
FIG. 6 shows an optical fiber amplifier according to a third embodiment of the present invention. The configuration of the optical fiber amplifier according to the present embodiment is a bidirectional pump configuration in which pumping lights having different wavelengths are input from both ends of the TDF, similarly to the optical fiber amplifier illustrated in the second embodiment. Compared with the configuration of the optical fiber amplifier of the second embodiment shown in FIG. 4, the configuration of the optical fiber amplifier of the third embodiment has a loss amount in the TDF of pumping light input in the opposite direction to the signal light. A component for detecting and controlling the power of the excitation light is further provided.

  The optical fiber amplifier of this embodiment includes a pumping light source 604 that emits pumping light A, a branching coupler 606 that splits the pumping light A into two, and one of the pumping light A branched by the branching coupler 405 via a WDM coupler 605. The signal light input through the optical isolator 602a, the WDM coupler 603 to be combined, the TDF 601 to which the signal light combined with the excitation light A is input, and the excitation light A and the signal light that have passed through the TDF 601 are separated. Waveform WDM coupler 608 and pumping light having a wavelength different from that of pumping light A, bifurcated by branching coupler 610, and emitting pumping light B input to TDF 601 in the opposite direction to the signal light via WDM couplers 609 and 608 An optical amplifying unit 600 including a light source 607 and an optical isolator 602b through which the signal light demultiplexed by the WDM coupler 609 passes and is output, a branching coupler The PD 611 that receives the other of the pump light A branched at 06, the PD 612 that receives the pump light A demultiplexed by the WDM coupler 608, and the pumps that are output from the PD 611 and PD 612, respectively. A pumping light A loss 631 that outputs a pumping light A loss amount (pumping light A loss amount monitoring value) in the TDF 601 based on the power level of the light A (input monitor value A, output monitor value A), and a branch coupler PD 614 that receives the other of the excitation light B branched at 610 as an input, PD 613 that receives the excitation light B that has been demultiplexed by the WDM coupler 603, via the WDM coupler 605, and the excitation that is output from PD 613 and PD 614, respectively. Based on the power level of the light B (input monitor value B, output monitor value B), the excitation light B in the TDF 601 A detection unit 630 including a pumping light B loss 632 that outputs a loss (pumping light B loss monitoring value), and each of the pumping light A loss amount and the pumping light B loss amount corresponding to a certain gain value is set as a target in advance. A storage unit (not shown) that stores the value A and the target value B, and compares the target value A stored in advance with the pumping light A loss monitoring value output from the pumping light A loss 631 and outputs an error signal A From the error signal extraction unit 640 that compares the target value B stored in advance with the pump light B loss amount monitor value output from the pump light B loss 632 and outputs the error signal B, and the error signal extraction unit 640 The power of the excitation light A emitted from the excitation light source 604 is controlled based on the output error signal A, and the excitation light B emitted from the excitation light source 607 based on the error signal B output from the error signal extraction unit 640 is controlled. A control unit 650 for controlling power.

In the optical amplifying unit 600, the input signal light having a wavelength between 1400 and 1530 nm and the excitation light A emitted from the excitation light source 604 are combined by the WDM coupler 603 and input to the TDF 601. The excitation light activates Tm ³⁺ ions in the TDF 601 and the signal light passes through the Tm ³⁺ ions to cause stimulated emission, whereby the signal light is amplified and output.

  In addition, excitation light B from the excitation light source 607 having a wavelength different from that of the excitation light source 604 is input from the output side end of the TDF 601 via the WDM coupler 608.

  The input power of the pump light A to the TDF 601 is monitored by inputting the pump light A branched by the branch coupler 606 disposed between the pump light source 604 and the WDM coupler 605 to the PD 611 of the detector 630. . The output power of the pumping light A from the TDF 601 is the WDM that separates the pumping light A separated from the signal light by the WDM coupler 608 disposed between the TDF 601 and the optical isolator 602b, and further separates the pumping light A and the pumping light B. It is monitored by taking it out via the coupler 609 and inputting it to the PD 612 of the detection unit 630.

  The input power of the pumping light B to the TDF 601 is monitored by inputting the pumping light B branched by the branching coupler 610 disposed between the pumping light source 607 and the WDM coupler 609 to the PD 614 of the detection unit 630. . The output power of the pumping light B from the TDF 601 is the WDM that separates the pumping light B separated from the signal light by the WDM coupler 603 disposed between the TDF 301 and the optical isolator 602a, and further separates the pumping light A and the pumping light B. It is monitored by taking it out through the coupler 605 and inputting it to the PD 613 of the detection unit 630.

  In the detection unit 630, the loss amount of the excitation light A in the TDF 601 (excitation light) based on the input monitor value A of the excitation light A measured by the PD 611 to the TDF 601 and the output monitor value A from the TDF 601 measured by the PD 612. A loss amount monitor value) is calculated and output to the error signal extraction unit 640. Further, based on the input monitor value B of the excitation light B measured by the PD 614 to the TDF 601 and the output monitor value B from the TDF 601 measured by the PD 613, the loss amount of the excitation light B in the TDF 601 (excitation light B loss amount). Monitor value) is calculated and output to the error signal extraction unit 640.

  In the error signal extraction unit 640, the loss amounts of the pumping light A and the pumping light B corresponding to a certain gain value are set as target values in advance, and the target value and the pumping light A loss amount output from the detection unit are set. The monitor value and the pumping light B loss amount monitor value are respectively compared, and the error signal A and the error signal B are extracted and output to the control unit 650.

  The control unit 650 controls the excitation light source 604 and the excitation light source 607 so that the error signal A and the error signal B input from the error signal extraction unit 640 become zero. When error signal = monitor value−target value is defined, when the error signal is positive, the power of pumping light A or pumping light B is increased, and when the error signal is negative, pumping light A or pumping light. The excitation light source 604 and the excitation light source 607 are controlled in the direction in which the B power is lowered. Since the control loop of the excitation light A and the excitation light B is nested, the control speed of the control light B is sufficiently slower than the control speed of the control light A.

  In this embodiment, the pumping light A is input in the same direction as the signal light, but the present control method has a backward pumping structure in which the pumping light A is input so as to propagate in the opposite direction to the signal light, A bidirectional pumping configuration in which pumping light A is input from both ends of the TDF 601 is also applicable. Regarding control accuracy, the backward excitation configuration or the bidirectional excitation configuration is better than the forward excitation configuration. For the excitation light B, either forward excitation or backward excitation is effective, but the control accuracy is higher when the excitation light B is input in the direction opposite to the signal traveling direction.

  As the excitation light source 604, an excitation light source that emits excitation light A having a wavelength of 800 nm ± 40 nm (760 nm to 840 nm, the same applies hereinafter), 1050 nm ± 70 nm, or 1420 nm ± 100 nm can be used. The wavelength of the excitation light source 607 is preferably 1200 nm ± 100 nm or 1650 nm ± 150 nm. At these wavelengths, a high-power semiconductor LD can be used. As the branch couplers 606 and 610 for monitoring the power of the input pumping light A to the TDF, a coupler such as a 10: 1 coupler or a 20: 1 coupler can be used. Further, in many cases, a semiconductor LD or the like is provided with a monitor PD for monitoring the output at the end opposite to the output end of the LD. The combination of the branch coupler 606 and PD 611 or the branch coupler 610 and PD 614 Instead of this combination, it is also possible to use the power monitor PD with a built-in LD.

Further, the configuration of the present embodiment can be applied regardless of the core addition amount of Tm ³⁺ of TDF6-1. However, in order to obtain high efficiency by one-wavelength excitation, addition of Tm ³⁺ The amount is desirably 3000 ppm or more.

It is a figure which shows the energy level of thulium ion. It is a block diagram of the optical fiber amplifier of embodiment which concerns on this invention. It is a figure which shows the gain of the WDM signal in the optical fiber amplifier of embodiment which concerns on this invention. It is a block diagram of the optical fiber amplifier of embodiment which concerns on this invention. It is a figure which shows the gain of the WDM signal in the optical fiber amplifier of embodiment which concerns on this invention. It is a block diagram of the optical fiber amplifier of embodiment which concerns on this invention.

Explanation of symbols

201, 401, 601 Thulium-doped fiber (TDF)
202a, 202b, 402a, 402b, 602a, 602b Optical isolators 203a, 203b, 403, 409, 603, 605, 608, 609 WDM couplers 204, 404, 408, 604, 607 Excitation light sources 205, 405, 410, 606, 610 Branch coupler 206, 207, 406, 407, 611, 612, 613, 614 PD

Claims (8)

An optical fiber for amplification with thulium added,
An excitation light source that emits excitation light incident on the amplification optical fiber;
Detecting means for detecting a loss amount of the excitation light in the amplification optical fiber;
Error signal extraction means for outputting an error signal between the loss amount detected by the detection means and the pump light loss amount corresponding to the gain value stored in advance; and the error signal is generated by varying the power of the pump light. An optical fiber amplifier comprising control means for controlling the pumping light source so as to be zero.   2. The optical fiber amplifier according to claim 1, wherein a wavelength of the pumping light is 760 nm to 840 nm, 980 nm to 1120 nm, or 1320 nm to 1520 nm.   2. The optical fiber amplifier according to claim 1, further comprising a second pumping light source that emits second pumping light that has a wavelength different from the wavelength of the pumping light and is incident on the amplification optical fiber.   4. The optical fiber amplifier according to claim 3, wherein the power of the second pumping light incident on the amplification optical fiber is constant.   4. The optical fiber amplifier according to claim 3, wherein the second pumping light is incident from a signal light output end of the amplification optical fiber toward a signal light input end. 5. A second detection means for detecting a loss amount of the second excitation light in the amplification optical fiber;
Second error signal extraction means for outputting a second error signal between the loss amount detected by the second detection means and the loss amount of the second excitation light corresponding to the gain value stored in advance; And a second control means for controlling the second pumping light source so that the power of the second pumping light is varied and the second error signal becomes zero. 4. An optical fiber amplifier according to item 3.   The wavelength of the excitation light is 760 nm to 840 nm, 980 nm to 1120 nm, or 1320 nm to 1520 nm, and the wavelength of the second excitation light is 1100 nm to 1300 nm, or 1500 nm to 18000 nm. The optical fiber amplifier according to 3 to 6.   The optical fiber amplifier according to any one of claims 1 to 7, wherein the amplification optical fiber is doped with thulium in an amount of 3000 ppmwt% or more.

Images