JP2004103929A - Optical fiber amplifier - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical fiber amplifier for keeping the gain spectrum of TDFA constant. <P>SOLUTION: There are provided a thorium-added fiber TDF, a first stimulation light source 11 that emits stimulation light-1 of wavelength λp1, and a second stimulation light source 12 that emits stimulation light-2 of wavelength λp2. The λp1 and the λp2 are assumed to be 1380nm and 1450nm, respectively, to satisfy λp2≥λp1+25nm, 1495nm≥λp1≥1360nm, and 1520nm≥λp2≥1385nm, so that the thorium-added fiber TDF is excited with two different stimulation light of wavelengths near 1.4μm. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical fiber amplifier, and is particularly useful when applied to an optical fiber communication system that performs communication using an optical fiber.
[0002]
[Prior art]
FIG. 10 shows a configuration of a thulium (Tm) -doped fiber amplifier (TDFA) according to the related art. FIG. 10A shows the case of the 1.4 μm band excitation (Non-Patent Document 1), and FIG. 10B shows the case of the 1.4 / 1.5 μm band excitation (Non-Patent Document 2). This TDFA has a Tm-doped fiber (TDF) as a gain medium and one pump light source 1 (FIG. 10A) or two different wavelength pump light sources 2 and 3 (FIG. 10B). However, in FIG. 10, reference numerals 4, 5, and 6 denote multiplexers, and reference numerals 7 and 8 denote drive circuits, and obvious optical components such as optical isolators are omitted for simplicity.
[0003]
The wavelength of the excitation light in the case of FIG. 10A is about 1420 nm. Also, TDF with high concentration is used as TDF. This TDFA has a feature that its structure is simpler than that of the TDFA of FIG.
[0004]
On the other hand, in the case of FIG. 10B, the wavelengths of the pump light sources 2 and 3 are 1420 nm and 1560 nm, respectively, and those pump lights are multiplexed with the signal light using the multiplexers 5 and 6, respectively. The signal light wavelength is approximately 1480-1510 nm. That is, the multiplexer 5 multiplexes the signal light and the pump light having a shorter wavelength than the signal light. The multiplexer 6 multiplexes the signal light and the pump light having a longer wavelength than the signal light.
[0005]
[Non-patent document 1]
S. Aozasa et al. , OFC, PD1, 2001
[Non-patent document 2]
Kasamatsu et al., IEICE Electronics Society Conference, C-3-91, p. 217,2000
[0006]
[Problems to be solved by the invention]
These TDFAs operate under the condition that the input signal light power (Psin) is constant. However, in a wavelength division multiplexing (WDM) system, the input signal light power (Psin) generally changes due to the number of signal light wavelengths, the change in transmission fiber loss with time, and the like. Then, the gain spectrum of the TDFA changes according to the input signal light power (Psin). For this reason, the conventional technique has a drawback that the gain spectrum of the TDFA cannot be kept constant, thereby deteriorating the system performance.
[0007]
An object of the present invention is to provide an optical fiber amplifier that can keep a gain spectrum of a TDFA constant in view of the above-mentioned conventional technology.
[0008]
[Means for Solving the Problems]
The configuration of the present invention that achieves the above object is as follows.
[0009]
The first optical fiber amplifier has a thulium-doped fiber, a first pumping light source that emits pumping light of wavelength λp1, and a second pumping light source that emits pumping light of wavelength λp2, where λp2 and λp1 are: An optical fiber amplifier that satisfies the following relationships: λp2 ≧ λp1 + 25 nm, 1495 nm ≧ λp1 ≧ 1360 nm, 1520 nm ≧ λp2 ≧ 1385 nm. That is, the present optical fiber amplifier is characterized mainly in that the thulium-doped fiber is pumped by pumping light having two different wavelengths near 1.4 μm.
[0010]
The second optical fiber amplifier includes a thulium-doped fiber, at least one pump light source, two splitters for splitting input and output signal light powers at two different signal light wavelengths (λ1, λ2), and the splitter. The two photodetectors for receiving the received signal light power, and the gain at λ1 and λ2 are calculated from the received signal light power, and the pumping light power of the at least one pumping light source is maintained so as to keep the gains constant. An optical fiber amplifier characterized by having a control circuit for changing the value. That is, the main characteristic of the present optical fiber amplifier is to perform gain detection and gain constant control by monitoring the input / output signal light power.
[0011]
3. The optical fiber amplifier according to claim 2, wherein the third optical fiber amplifier emits, as the at least one excitation light source, a first excitation light source that emits excitation light of wavelength λp1 and an excitation light of wavelength λp2. Λp2 and λp1 satisfy the following relationships: λp2 ≧ λp1 + 25 nm, 1495 nm ≧ λp1 ≧ 1360 nm, 1520 nm ≧ λp2 ≧ 1385 nm.
[0012]
The fourth optical fiber amplifier includes a thulium-doped fiber, a first pumping light source that emits pumping light of wavelength λp1, a second pumping light source that emits pumping light of wavelength λp2, and the first pumping light source. A first photodetector for detecting the power of the pumping light to be emitted, a second photodetector for detecting the power of the pumping light emitted from the first pumping light source and passing through the thulium-doped fiber, and the second photodetector. A third photodetector that detects excitation light power emitted from the excitation light source, a fourth photodetector that detects excitation light power that has passed through the thulium-doped fiber after being emitted from the second excitation light source, The transmissivity (T (λp1)) of the pumping light of wavelength λp1 in the thulium-doped fiber is calculated from the first and second light receiving levels, and the pumping light of wavelength λp2 is calculated from the light receiving levels of the third and fourth light receivers. The tree of the above The transmissivity (T (λp2)) of the W-doped fiber is calculated, and the power of the pump light emitted from the first and second pump light sources is controlled so that T (λp1) and T (λp2) become constant. An optical fiber amplifier comprising a control circuit. That is, the present optical fiber amplifier is mainly characterized in that the power of the pumping light having two different wavelengths is monitored, thereby detecting the transmittance of the pumping light and controlling the gain to be constant.
[0013]
The fifth optical fiber amplifier is the optical fiber amplifier according to claim 4, wherein λp2 and λp1 satisfy the following relationships: λp2 ≧ λp1 + 25 nm, 1495 nm ≧ λp1 ≧ 1360 nm, 1520 nm ≧ λp2 ≧ 1385 nm. An optical fiber amplifier.
[0014]
The sixth optical fiber amplifier is the optical fiber amplifier according to claim 5, wherein the signal light and the pumping light having the wavelength λp1 propagate in the thulium-doped fiber in the same direction. A multiplexer installed at the previous stage for multiplexing the pumping light of the wavelength λp1 and the signal light, and the thulium so that the signal light and the pumping light of the wavelength λp2 propagate in the thulium-doped fiber in opposite directions. An optical fiber amplifier, comprising: a multiplexer that is provided at a stage subsequent to the doped fiber and multiplexes the pump light having the wavelength λp2 and the signal light. The main characteristic of the optical fiber amplifier is that it has a low noise figure.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0016]
[First Embodiment]
FIG. 1 is a block diagram showing an optical fiber amplifier according to the first embodiment of the present invention. As shown in the figure, the optical fiber amplifier is similar to the optical fiber amplifier according to the prior art shown in FIG. 10 (a), but the following points are mainly different, and the other points are the same. . Therefore, the same portions as those in FIG. 10A are denoted by the same reference numerals, and duplicate description is omitted.
[0017]
In the prior art, the wavelength of the excitation light is basically one, but in the present embodiment, two wavelengths are used. The two wavelengths are set in the TDFA absorption wavelength range (about 1360-1440 nm) and the amplification wavelength range (about 1450-1520 nm). In FIG. 1, the wavelengths of the excitation light from the excitation light sources 11 and 12 (referred to as excitation light -1 and -2, respectively) are 1380 nm and 1450 nm. After the pumping light-1 and the pumping light-2 are multiplexed by the multiplexer 13, they are further multiplexed with the signal light by the multiplexer 4.
[0018]
FIG. 6 shows the dependence of the gain coefficient spectrum of the TDFA on the upper level ratio. Here, the upper level ratio is a ratio of the upper level of amplification to the sum of the upper level and lower level of amplification. The gain in dB is proportional to this gain factor. The upper level ratio depends on the input pump light power and the input signal light power, the pump light wavelength, and the signal light wavelength. For example, the spectrum when strongly excited by the excitation light of 1380 nm is close to the spectrum when the upper level ratio is 63%. The spectrum when strongly excited by the excitation light of 1450 nm is close to the spectrum when the upper level ratio is 45%. Therefore, when the input signal light power changes during the operation of the system, the gain spectrum can be kept constant by appropriately changing the pump light powers at 1380 nm and 1450 nm. For example, when the input signal light power is increased and the pumping light power is constant, the gain spectrum changes toward the spectrum when the upper level ratio is low. At this time, the pumping light power of 1380 nm and 1450 nm This spectral change can be eliminated by increasing the ratio of the pumping light powers of. That is, the disadvantage that the gain spectrum changes according to the change in the input power, which is a problem in the related art, can be eliminated.
[0019]
If the two excitation light wavelengths (λp1 and λp2, where λp1 <λp2) are too close, the difference between the spectra when strongly excited at each wavelength is small, and the spectral controllability is small. Therefore, it is desirable that λp2−λp1 ≧ 25 nm.
[0020]
When λp2 is close to the signal light wavelength and is in the above-mentioned amplification wavelength range (about 1450 to 1520 nm), the pumping light-2 may be amplified in the TDF by the pumping light-1. Also in this case, the above-mentioned gain spectrum control can be performed similarly.
[0021]
[Second embodiment]
FIG. 2 is a block diagram illustrating an optical fiber amplifier according to a second embodiment of the present invention. As shown in this figure, the amplifier using the optical fiber is similar to the prior art shown in FIG. 10B, except for the following points. Therefore, the same portions as those in FIG. 10B are denoted by the same reference numerals, and redundant description is omitted.
[0022]
In this embodiment, splitters 21 and 22 for monitoring the signal light power are provided before and after the conventional optical fiber amplifier. The signal light split by the splitters 21 and 22 is further split by the splitters 23 and 24. The signal lights split into two by the splitters 23 and 24 are detected by the photodetectors 29, 30, 31, and 32 through optical filters 25, 26, 27, and 28, respectively. Here, the optical filters (25, 26) and (27, 28) allow signal lights of different wavelengths (λ1 and λ2) to pass.
[0023]
The signal light gain G (λ1) at this signal light wavelength is calculated from the signal light power of λ1 received by the light receivers 29 and 30 installed before and after the present optical fiber amplifier. Similarly, the signal light gain G (λ2) at this signal light wavelength is calculated from the signal light power of λ2 received by the light receivers 31 and 32 installed before and after the present optical fiber amplifier. The branching devices 21, 22, 23, and 24 are specifically, for example, fiber couplers. The optical filters (25, 27) and (26, 28) are specifically, for example, dielectric multilayer filters. The signal light wavelengths λ1 and λ2 are, for example, 1480 nm and 1510 nm, respectively.
[0024]
When the operating condition of the system changes and the input signal light power to the present optical fiber amplifier changes, the pump light powers (P1 and P1 respectively) of the pump light sources 2 and 3 where the gains G (λ1) and G (λ2) become constant. And P2). For example, first, using a control circuit, P1 is changed at high speed so that G (λ1) is constant, and then G (λ1) is changed at high speed so that G (λ1) is constant. P2 is changed more slowly than P1 so that λ2) is constant. By this control, G (λ1) and G (λ2) become constant, and thus the gain spectrum becomes constant. The above control can eliminate the disadvantage that the gain spectrum changes according to the change in input power, which is a problem in the related art.
[0025]
[Third embodiment]
FIG. 3 is a block diagram showing an optical fiber amplifier according to a third embodiment of the present invention. As shown in the figure, the optical fiber amplifier is similar to that of the second embodiment shown in FIG. 2, but differs mainly in the following points. Therefore, the same portions as those in FIG. 2 are denoted by the same reference numerals, and duplicate description will be omitted.
[0026]
In the second embodiment, the excitation light sources 2 and 3 are 1420 nm and 1560 nm, but in this embodiment, the excitation light sources 35 and 36 are 1380 nm (λp1) and 1450 nm (λp2). . After the pumping light (pumping light-1 and -2) from the pumping light sources 35 and 36 is multiplexed by the multiplexer 33, it is multiplexed with the signal light by another multiplexer 34 adjacent to the TDF. The gain spectrum when strongly pumped with 1380 nm pump light has a gain peak on the short wavelength side. On the other hand, the gain spectrum when strongly pumped with 1450 nm pump light has a gain peak on the long wavelength side. Therefore, by exciting the TDF with these two wavelengths, a change in gain spectrum caused by a change in input signal light power can be avoided as follows.
[0027]
As in the case of the second embodiment, when the operating conditions of the system change and the input signal light power to the optical fiber amplifier changes, the gains G (λ1) and G (λ2) become constant. The excitation light powers (P1 and P2, respectively) of the excitation light sources 35 and 36 are changed. For example, first, using a control circuit, P1 is changed at high speed so that G (λ1) is constant, and then G (λ1) is changed at high speed so that G (λ1) is constant. P2 is changed more slowly than P1 so that λ2) becomes constant. By this control, G (λ1) and G (λ2) become constant, and thus the gain spectrum becomes constant.
[0028]
By such control, it is possible to eliminate the disadvantage that the gain spectrum changes according to the change in the input power, which is a problem in the related art.
[0029]
When λp2 is close to the signal light wavelength and is in the above-mentioned amplification wavelength range (about 1450 to 1520 nm), the pumping light-2 may be amplified in the TDF by the pumping light-1. Also in this case, the above-mentioned gain spectrum control can be performed similarly.
[0030]
[Fourth embodiment]
FIG. 4 is a block diagram showing an optical fiber amplifier according to a fourth embodiment of the present invention. As shown in the figure, the optical fiber amplifier is similar to that of the third embodiment shown in FIG. 3, but differs mainly in the following points. Therefore, the same portions as those in FIG. 3 are denoted by the same reference numerals, and duplicate description will be omitted.
[0031]
In the third embodiment, signal light gains of two different wavelengths are monitored and control is performed by feedback. However, in the present embodiment, pump light losses of two different wavelengths (λp1 and λp2) are monitored. Control is performed by feedback. That is, in the present embodiment, the control is performed such that the pumping light losses of two different wavelengths (L (λp1) and L (λp2), respectively) become constant.
[0032]
The excitation light power of 1380 nm emitted from the excitation light source 35 is monitored by the light receiver 41. The power of the excitation light of 1450 nm emitted from the excitation light source 36 is monitored by the light receiver 42. Examples of the light receivers 41 and 42 are photodiodes installed on the rear end face of the laser diode of the laser diode module. The excitation light emitted from the excitation light sources 35 and 36 enters the TDF and is absorbed according to the operating conditions of the TDF. Thereafter, the signal light is separated from the signal light by the demultiplexer 43 behind the TDF, and further guided to the photodetectors 45 and 45 via the demultiplexer 44. The light receiving level difference between the light receiving devices 41 and 45 and the light receiving level difference between the light receiving devices 42 and 46, and the multiplexing of the excitation light from the excitation light sources 35 and 36 to the light receiving devices (41, 45) and (42, 46). The TDF loss can be calculated from the optical component (other than TDF) loss such as the devices 33 and 34 and the duplexers 43 and 44.
[0033]
FIG. 7 shows the dependence of the excitation light absorption coefficient at 1380 nm and 1450 nm on the upper level ratio obtained from the relationship shown in FIG. The amount of excitation light absorption in dB units is proportional to their absorption coefficients. Both absorption coefficients for the two wavelengths decrease as the upper level ratio increases. FIG. 8 shows the dependence of the upper level ratio on the absorption coefficient ratio obtained from the relationship of FIG. 7, where the absorption coefficient ratio is the ratio of the absorption coefficient at 1450 nm to the absorption coefficient at 1380 nm. It is.
[0034]
From FIG. 8, the absorption coefficient ratio and the upper level ratio correspond to one to one. If the absorption coefficient ratio is constant, the upper level ratio becomes constant, and the gain spectrum is kept constant. According to the control of the present embodiment, since L (λ1) and L (λ2) are constant, the absorption coefficient ratio is constant, and thus the gain spectrum is kept constant. The relationship between FIG. 7 and FIG. 8 generally holds for pumping light wavelengths other than 1380 nm and 1450 nm.
[0035]
FIG. 9 shows a gain / loss spectrum at a typical upper level ratio (37%). The case where the signal light gain at 1490 nm is 30 dB is shown. At this time, a gain of 20 dB or more is obtained in a wavelength range of about 1475 to 1520 nm. The pumping light losses at the pumping light wavelengths of 1380 nm and 1450 nm are about 43 dB and 30 dB, respectively. Therefore, in the configuration of FIG. 4, the above-described gain spectrum control system can be configured using ordinary optical components and electrical components. That is, when the power of the pumping light input to the TDF is set to about 20 dBm, and the above-described pumping light loss and the optical component loss to the pumping light (about 7 dB) are considered, the pumping light power detected by the photodetectors 45 and 46 is: They are about -30 dBm and about -13 dBm, respectively. This is a sufficiently high optical power when considering the dark current level of a typical receiver.
[0036]
When λp2 is close to the signal light wavelength and is in the above-mentioned amplification wavelength range (about 1450 to 1520 nm), the pumping light-2 may be amplified in the TDF by the pumping light-1. Also in this case, the above-described gain spectrum control can be performed in the same manner by reading the pumping light loss L (λp2) as a gain. Therefore, the above-described pumping light loss is referred to as transmittance (L (λp1) and L (λp2)), including the case where the gain of the pumping light occurs.
[0037]
[Fifth Embodiment]
FIG. 5 is a block diagram showing an optical fiber amplifier according to a fifth embodiment of the present invention. The optical fiber amplifier according to this embodiment is similar to the fourth embodiment shown in FIG. 4, but differs mainly in the following points. Therefore, the same portions as those in FIG. 4 are denoted by the same reference numerals, and duplicate description will be omitted.
[0038]
In the fourth embodiment, the 1380 nm pump light and the 1450 nm pump light are propagated in the same direction, that is, in the forward direction of TDF (the same direction as the propagation direction of the signal light) (forward pumping). The 1380 nm excitation light is used for forward excitation, and the 1450 nm excitation light is used for backward excitation. The 1380 nm pump light strongly pumps near the signal light input end of the TDF, and most of the TDF is absorbed during propagation, and the power is small near the TDF output end. On the other hand, the pumping light of 1450 nm strongly pumps near the signal light output end of the TDF, and most of the TDF is absorbed during propagation, and the power is small near the TDF input end. Since a lower noise figure can be obtained when the wavelength of the pump light in which the pump light power is dominant near the signal light input end of the TDF is lower, the present embodiment has an advantage that the noise figure is lower than that of the fourth embodiment. Having. In the figure, reference numeral 51 denotes a demultiplexer, and 52 and 53 denote multiplexers / demultiplexers.
[0039]
【The invention's effect】
As described above, according to the embodiment of the present invention, there is a problem in the related art. The disadvantage that the gain spectrum of TDFA cannot be kept constant and system performance deteriorates can be solved.
[Brief description of the drawings]
FIG. 1 is a block diagram showing an optical fiber amplifier according to a first embodiment of the present invention.
FIG. 2 is a block diagram illustrating an optical fiber amplifier according to a second embodiment of the present invention.
FIG. 3 is a block diagram showing an optical fiber amplifier according to a third embodiment of the present invention.
FIG. 4 is a block diagram showing an optical fiber amplifier according to a fourth embodiment of the present invention.
FIG. 5 is a block diagram showing an optical fiber amplifier according to a fifth embodiment of the present invention.
FIG. 6 is a characteristic diagram showing the upper level ratio dependency of the gain coefficient spectrum of TDFA.
FIG. 7 is a characteristic diagram showing the upper level ratio dependency of the excitation light absorption coefficient at 1380 nm and 1450 nm, obtained from the relationship of FIG.
8 is a characteristic diagram showing the absorption coefficient ratio dependency of the upper level ratio obtained from the relationship of FIG. 7;
FIG. 9 is a characteristic diagram showing a gain / loss spectrum at a typical upper level ratio (37%).
FIG. 10 is a block diagram showing an optical fiber amplifier according to the related art.
[Explanation of symbols]
2, 3, 11, 12, 35, 36 Excitation light sources 4, 5, 6, 13, 33, 34 Multiplexers 21, 22, 23, 24 Splitters 25, 26, 27, 28 Optical filters 29, 30, 31 , 32, 41, 45 Photodetectors 43, 44, 51 Demultiplexers 52, 53 Multiplexers

Claims (6)

It has a thulium-doped fiber, a first pumping light source that emits pumping light of wavelength λp1, and a second pumping light source that emits pumping light of wavelength λp2. An optical fiber amplifier that satisfies the relationship of 1360 nm, 1520 nm ≧ λp2 ≧ 1385 nm. A thulium-doped fiber, at least one pump light source, two splitters for splitting input and output signal light powers at two different signal light wavelengths (λ1, λ2), and receiving the split signal light power And a control circuit for calculating the gain at λ1 and λ2 from the received signal light power and changing the excitation light power of the at least one excitation light source so as to keep the gains constant. An optical fiber amplifier characterized by the above-mentioned. The optical fiber amplifier according to claim 2, wherein
As the at least one excitation light source,
It has a first pumping light source that emits pumping light of wavelength λp1 and a second pumping light source that emits pumping light of wavelength λp2. An optical fiber amplifier satisfying a relationship of ≧ 1385 nm. A thulium-doped fiber, a first excitation light source that emits excitation light of wavelength λp1, a second excitation light source that emits excitation light of wavelength λp2,
A first light receiver for detecting excitation light power emitted from the first excitation light source;
A second photodetector that detects the power of the excitation light that has passed through the thulium-doped fiber after being emitted from the first excitation light source;
A third light receiver for detecting an excitation light power emitted from the second excitation light source;
A fourth photodetector that detects the power of the excitation light that has passed through the thulium-doped fiber after being emitted from the second excitation light source,
The transmissivity (T (λp1)) of the excitation light having the wavelength λp1 in the thulium-doped fiber is calculated from the light reception levels of the first and second light receivers, and the wavelength is calculated from the light reception levels of the third and fourth light receivers. calculating the transmittance (T (λp2)) of the pumping light of λp2 in the thulium-doped fiber;
An optical fiber amplifier comprising a control circuit for controlling the power of pumping light emitted from the first and second pumping light sources so that T (λp1) and T (λp2) become constant. The optical fiber amplifier according to claim 4, wherein
An optical fiber amplifier, wherein λp2 and λp1 satisfy the following relationships: λp2 ≧ λp1 + 25 nm, 1495 nm ≧ λp1 ≧ 1360 nm, 1520 nm ≧ λp2 ≧ 1385 nm. The optical fiber amplifier according to claim 5, wherein
A multiplexing device that combines the pump light of the wavelength λp1 and the signal light, which is provided in a stage preceding the thulium-doped fiber, so that the signal light and the pump light of the wavelength λp1 propagate in the same direction in the thulium-doped fiber. Vessels,
A multiplexing device that combines the pumping light of the wavelength λp2 and the signal light, which is provided after the thulium-doped fiber, so that the signal light and the pumping light of the wavelength λp2 propagate in the thulium-doped fiber in opposite directions. An optical fiber amplifier comprising:

Images