JP3551155B2 - Optical fiber amplifier and optical amplifier including the same - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical fiber amplifier and an optical amplifier including the same, and more particularly, to an optical fiber amplifier capable of simultaneously achieving a sufficient gain shift effect, low noise characteristics, and high operating efficiency, and an optical amplifier including the same. It is.
[0002]
[Prior art]
Conventionally, research and development of rare earth element-doped optical fiber amplifiers have been actively performed as key devices in large-capacity wavelength division multiplexing communication.
In recent years, in order to effectively utilize the entire low-loss band (1450 to 1650 nm) of a quartz optical fiber transmission line as a wavelength resource, research and development of an optical fiber amplifier doped with Tm (thulium), a kind of rare earth element, has been conducted. Has been done.
In one example of this Tm-doped optical fiber amplifier, it has been reported that a gain band can be formed in a 1.47 μm band by using a laser beam in a 1.05 μm band as an excitation light source (Japanese Patent No. 2688303). No.).
[0003]
In another example of the Tm-doped optical fiber amplifier, light having two wavelengths of 1.05 μm and 1.56 μm is used for pumping, so that the gain band (1.47 μm band) originally possessed by Tm is changed to a 1.49 μm band. (Optics Letters, vol. 24, p. 1684, 1999 (Optics Letters vol. 24 p. 1684, 1999). )reference).
FIG. 5 shows transition of the first pumping light having a wavelength of 1.05 μm and the second pumping light having a wavelength of 1.56 μm at the energy level of Tm.
[0004]
FIG. 6 is a block diagram showing an example of a conventional two-wavelength pumped Tm-doped optical fiber amplifier. Tm-doped optical fiber modules 31a to 31c are connected in series, and pump light is introduced in front of the Tm-doped optical fiber module 31a. The first pumping light source 32a having a wavelength of 1.05 μm and the second pumping light source 33a having a wavelength of 1.56 μm are connected via an appropriate wavelength multiplexing optical coupler (not shown), and a Tm-doped optical fiber module 31b The first pumping light source 32b and the second pumping light source 33b are connected to each other via an appropriate wavelength multiplexing optical coupler (not shown).
[0005]
In the two-wavelength pumped Tm-doped optical fiber amplifier, an input signal 34 input to the Tm-doped optical fiber module 31a is amplified while sequentially passing through the Tm-doped optical fiber modules 31a to 31c, and output from the Tm-doped optical fiber module 31c. It is output as a signal 35.
In this two-wavelength pumped Tm-doped optical fiber amplifier, a gain of 25 dB and a noise figure of 5 dB are achieved in a band of 1475 to 1510 nm.
[0006]
Generally, the pumping configuration in an optical fiber amplifier is a forward pumping in which pumping light is introduced from the front of the optical fiber in the same direction as the signal light, a backward pumping in which pumping light is introduced in the opposite direction to the signal light, or a bidirectional combination of these. There is excitation. The feature of forward pumping is low noise, the feature of backward pumping is high output and high efficiency, and the feature of bidirectional pumping is a combination of features of forward pumping and backward pumping. In particular, in backward pumping, the pumping intensity distribution in the longitudinal direction in the Tm-doped optical fiber matches the signal intensity distribution that is amplified while propagating, and the operating efficiency is high because the pumping intensity is high where the signal intensity is high.
[0007]
FIG. 7 is a configuration diagram showing an example of a conventional multi-stage rare-earth-doped optical fiber amplifier such as Er (erbium). A front-stage section 41 includes a rare-earth-doped optical fiber module 51a and an excitation light source for introducing excitation light. The rear part 42 is composed of a rare earth-doped optical fiber module 51b, pumping light sources 52b and 52c for bidirectional pumping and an isolator 53b, and the former part 41 and the latter part 42 are made of an isolator. 53c are connected in series.
[0008]
In this rare earth-doped optical fiber amplifier, an input signal 43 input to the rare earth-doped optical fiber module 51a via the isolator 53a is amplified while sequentially passing through the rare earth-doped optical fiber modules 51a and 51b, and an output signal 44 is output from the isolator 53b. As a result, the front stage 41 is pumped forward by the pumping light source 52a to realize low noise, and the rear stage 42 is bidirectionally pumped by the pumping light sources 52b and 52c to realize high gain and high efficiency. .
The Er-doped optical fiber amplifier is described in Electronics Letters, Vol. 34, p. 567, 1998 (Electronics Letters, vol. 34, p. 567, 1998), Electronics Letters, Vol. 34, p. 1747, 1998 (Elec
tronics Letters, vol. 34, p. 1747, 1998).
[0009]
[Problems to be solved by the invention]
By the way, in the above-mentioned conventional two-wavelength pumped Tm-doped optical fiber amplifier, the efficiency (ratio of the amplifier output power to the pumping power) is as low as about several percent or less. Therefore, even when the pumping power is about 500 mW, the signal is low. There was a problem that the amplifier output at a wavelength of 1500 nm was only about +6 dBm, and the amplifier output was insufficient.
For example, assuming a typical wavelength-division multiplexed optical communication system of about 16 waves, when an amplifier output required for a transmitter or a repeater is roughly estimated, an input power per signal channel is −25 dBm / ch, and a fiber amplifier is used. When the gain is 25 dB, an amplifier output of +12 dBm is required, and the amplifier output is currently insufficient.
[0010]
Here, the reason why the efficiency of the two-wavelength pumped Tm-doped optical fiber amplifier is lower than that of the Er-doped optical fiber amplifier is considered to be a forward pumping configuration. In the optical fiber amplifier, it is considered to apply backward pumping or bidirectional pumping in which pumping light is introduced from behind the Tm-doped optical fiber modules 31a to 31c.
However, in the conventional two-wavelength pumped Tm-doped fiber amplifier, when backward pumping or bidirectional pumping is performed at two wavelengths, the noise figure is significantly deteriorated (increased), and only extremely poor characteristics are obtained as an optical fiber amplifier.
[0011]
SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and is an optical fiber capable of simultaneously achieving a sufficient gain shift effect, low noise characteristics, and high operation efficiency even in a two-wavelength pumped Tm-doped optical fiber amplifier. It is an object of the present invention to provide a fiber amplifier and an optical amplifier including the same.
[0012]
[Means for Solving the Problems]
In order to solve the above problems, the present invention employs the following optical fiber amplifier and an optical amplifier including the same.
That is, in the optical fiber amplifier according to claim 1 of the present invention, a plurality of glass fibers doped with a rare earth element are connected in series to form a multistage gain medium, and the energy level of the rare earth ions is higher than the ground level. A multi-stage optical fiber amplifier using stimulated emission transition between two energy levels having high energy, wherein the two energy levels are provided in front of a glass fiber of each of the plurality of stages except at least the last stage. A first excitation light source that forms an inversion distribution between the first excitation light source and an oscillation wavelength different from the oscillation wavelength of the first excitation light source, and from a ground level to a laser lower level of the two energy levels. A second excitation light source for exciting ions is provided to excite from the front, and the first excitation light source and the second excitation light source are provided at least in front of the last-stage glass fiber. Provided with a light source, provided with the first excitation light source behind the glass fiber is characterized in that a configuration for exciting the glass fiber of the final stage from the bidirectional.
[0014]
The optical fiber amplifier according to claim 2 is the optical fiber amplifier according to claim 1 , wherein the rare earth element is one selected from thulium (Tm), holmium (Ho), erbium (Er), and praseodymium (Pr). It is characterized by being.
[0015]
An optical fiber amplifier according to a third aspect is the optical fiber amplifier according to the first or second aspect, wherein the rare earth ions added to the glass fiber are thulium ions (Tm ³⁺ ).
[0016]
An optical fiber amplifier according to a fourth aspect is the optical fiber amplifier according to the first, second or third aspect , wherein the glass fiber is a rare earth element-doped fluoride glass fiber.
[0017]
According to a fifth aspect of the present invention, in the optical fiber amplifier according to any one of the first to fourth aspects, an optical isolator for a signal wavelength band is provided between the stages.
[0018]
An optical fiber amplifier according to claim 6 is the optical fiber amplifier according to any one of claims 1 to 5 , wherein a wavelength of light output from the first pumping light source is 1.04 to 1.07 μm, The wavelength of the light output from the second excitation light source is set to 1.53 to 1.90 μm.
[0019]
Billing optical amplifier of claim 7, wherein, in the optical amplifying device formed by connecting a plurality of optical fiber amplifiers in series and / or parallel, any one of claims 1 to at least one of said optical fiber amplifier 6 1 The optical fiber amplifier described in the item is characterized.
[0020]
In the optical fiber amplifier of the present invention, the light output from the first pumping light source forms a population inversion between desired energy levels, and achieves a laser amplification operation in the stimulated emission transition. Further, by increasing the lower level number density of the laser by the light output from the second pumping light source, the effective gain peak wavelength is shifted to the longer wavelength side.
For example, a Tm-doped optical fiber is forward-pumped at two wavelengths to realize a gain shift and low noise, and is also pumped by light output from a first pumping light source from the rear to efficiently amplify signal light, Achieve high efficiency and high gain.
[0021]
It is not advisable to perform two-wavelength pumping from the rear because the noise figure is significantly deteriorated. The reason for the deterioration of the noise figure is that the light output from the second pumping light source from the rear degrades the population inversion of the first half of the optical fiber more than necessary, and at the same time, does not contribute to the gain in the second half. This is because components are generated.
Also, in the first half of the Tm-doped optical fiber, high gain and low noise can be realized in the gain shift band, so that the overall noise of the optical fiber amplifier can be reduced. The reason is that the noise figure of the optical fiber amplifier is almost determined by the population inversion at the entrance of the optical fiber to be amplified.
[0022]
Further, in order to realize higher gain and efficiency together with the gain shift effect, a Tm-doped optical fiber having an appropriate length is divided into a plurality of fibers, and a first-stage portion has a forward pumping configuration using two wavelengths and a second-stage optical fiber. After the section, it is effective to adopt a bidirectional pump configuration in which forward pumping by two wavelengths and backward pumping by the first pumping light source are combined. Also in this case, since the noise figure is almost determined in the first half (previous stage) of the Tm-doped optical fiber, forward pumping that can realize low noise in the first half is adopted.
[0023]
Further, by connecting the Tm-doped optical fiber that is appropriately pumped as described above, the fiber length can be lengthened, and high gain and high efficiency can be achieved.
In the case of a multi-stage configuration, it is important to insert an optical isolator in the signal wavelength band between the stages for high stability and high efficiency of the optical fiber amplifier. The reason is that ASE (Amplified Spontaneous Emission: Amplified Spontaneous Emission) generated in the second stage and subsequent stages propagates in the opposite direction to the signal, and the inversion distribution in the preceding fiber is reduced more than necessary. This is because the characteristics of the entire optical fiber amplifier are degraded.
[0024]
BEST MODE FOR CARRYING OUT THE INVENTION
Embodiments of the optical fiber amplifier of the present invention will be described with reference to the drawings.
[0025]
[First Embodiment]
FIG. 1 is a configuration diagram showing a two-wavelength pumped Tm-doped optical fiber amplifier according to a first embodiment of the present invention. In front of a Tm-doped optical fiber module (glass fiber) 1, a pump for introducing pumping light is provided. A first pumping light source 2a having a wavelength of 1.05 μm and a second pumping light source 3 having a wavelength of 1.56 μm are connected via an appropriate wavelength-division multiplexing optical coupler (not shown). A first pumping light source 2b having a wavelength of 1.05 μm for introducing light is connected via an appropriate wavelength multiplexing optical coupler (not shown). Further, isolators 4a and 4b are connected to the signal input port and the output port of the Tm-doped optical fiber module 1 in order to suppress undesirable laser oscillation due to return light.
[0026]
Tm-doped optical fiber module 1 is a fluoride optical fiber doped with Tm in the base material consisting of fluoro zirconate glass, as amplifying fiber the Tm-doped fluoride fiber, ³ F ₄ → ³ H ₄ transition ( (1.47 μm band).
This Tm-doped optical fiber module 1 has a module form in which the concentration of Tm is 2000 ppm, the core diameter is 2.0 μm, and the fiber length is 20 m.
[0027]
The first pumping light sources 2a and 2b are used to form a population inversion in the Tm-doped optical fiber module 1. For example, a Yb (ytterbium) optical fiber laser having a wavelength of 1.05 μm (maximum output 500 mW) is suitable. Used for
The second excitation light source 3 is set so as to be included in the ground level absorption transition from the ground level ³ H _{6 of} Tm to ³ H ₄ , and this absorption transition has a peak at a wavelength of about 1.65 μm. And extends from 1.53 μm to 1.9 μm. For example, a 1.56 μm band Er-doped optical fiber laser (500 mW at maximum) corresponding to the bottom of the absorption transition is preferably used.
[0028]
In this optical fiber amplifier, a semiconductor laser having a wavelength variable from 1.45 μm to 1.55 μm and a signal input power of −30 dBm was used as a signal light source for gain measurement. A saturated signal having a wavelength of 1500 nm and a signal power of -10 dBm was input simultaneously with the above signal, and the amplified output of the saturated signal was defined as the output of the optical fiber amplifier.
In this optical fiber amplifier, after passing through the isolator 4a, the input signal 5 is amplified while passing through the Tm-doped optical fiber module 1, and is output as an output signal 6 from the isolator 4b.
[0029]
FIG. 2 is a diagram showing the gain and the noise figure of this optical fiber amplifier. In the figure, the spectrum a indicates that the output power of the first pumping light source 2a is 200 mW, the output power of the first pumping light source 2b is 100 mW, The gain and the noise figure when the output power of the light source 3 is 40 mW, and the spectrum b is the output power of the first pumping light source 2a at 200 mW, the output power of the first pumping light source 2b at 100 mW, and a Tm-doped optical fiber. These are the output power of 40 mW by the second pumping light source 3 to the module 1 and the gain and noise figure when 40 mW pumping light of the second pumping light source (not shown) is introduced from the rear.
[0030]
According to FIG. 2, it can be seen that the gain does not change much in both the spectra a and b, but the noise figure in the spectrum b is significantly deteriorated in the wavelength range of 1460 to 1520 nm. Here, degradation of the noise figure of approximately 5 dB to 10 dB was observed.
In the spectrum a, the gain at a wavelength of 1500 nm of the saturation signal is as low as about 13 dB, but this depends on the number of Tm ions in the optical fiber.
The output (= saturation signal output) of this optical fiber amplifier is about +3 dBm when the total pumping power is 340 mW, which is about twice (3 dB) as compared with the amplifier output in +2 dB forward pumping which is +0 dBm. Was able to achieve.
[0031]
[Second embodiment]
FIG. 3 is a block diagram showing a multistage two-wavelength pumped Tm-doped optical fiber amplifier according to a second embodiment of the present invention. In front of the Tm-doped optical fiber module 1a, two wavelengths (a first pump light and a second In order to excite using the second excitation light, the first excitation light source 2a having a wavelength of 1.05 μm and the second excitation light source 3a having a wavelength of 1.56 μm for introducing the excitation light are appropriately wavelength-multiplexed optical couplers ( (Not shown), and an isolator 4 a is connected to a signal input port of the Tm-doped optical fiber module 1 in order to suppress undesired laser oscillation due to return light. ing.
[0032]
Also, in order to introduce pumping light in front of the Tm-doped optical fiber modules 1b and 1c connected in series in order to perform pumping using two wavelengths (first pumping light and second pumping light) as described above. The first pumping light source 2b of the wavelength 1.05 μm band and the second pumping light source 3b of the wavelength 1.56 μm are connected via an appropriate wavelength multiplexing optical coupler (not shown), and the Tm-doped optical fiber module 1b, Behind 1c, a first pumping light source 2c having a wavelength of 1.05 μm for introducing pumping light is connected via an appropriate wavelength multiplexing optical coupler (not shown).
[0033]
An isolator 4 b is connected to a signal output port of the Tm-doped optical fiber module 1 c to suppress undesired laser oscillation due to return light, and serves as a second step portion 12.
Here, the Tm-doped optical fiber modules 1b and 1c connected in series may be replaced by a module having a length of 20 m in which the concentration of the Tm-doped optical fiber module 1b is doubled (4000 ppm).
Between the first step 11 and the second step 12, an ASE (Amplified Spontaneous Emission) generated in the second step 12 and traveling in the opposite direction to the signal is amplified. An isolator 4c for preventing entry into the first step 11 is inserted.
[0034]
In this optical fiber amplifier, the length of the Tm-doped optical fiber is increased in order to increase the gain as the amplifier. The reason is that the gain cannot be expected even if the Tm optical fiber length is simply increased, so that the pumping intensity distribution in the optical fiber is almost eliminated when the optical fiber becomes longer to some extent (about twice the absorption length). This is because it is necessary to connect a plurality of excited optical fibers of an appropriate length to increase the length.
[0035]
In this optical fiber amplifier, in the first stage 11, the Tm-doped optical fiber module 1a is excited by two-wavelength excitation from the front using the first excitation light source 2a and the second excitation light source 3a.
In the second step section 12, the Tm-doped optical fiber modules 1b and 1c are excited by two wavelengths from the front using the first excitation light source 2b and the second excitation light source 3b, and the first excitation light source 2c from the rear. To excite.
Accordingly, the input signal 5 is amplified while passing through the Tm-doped optical fiber modules 1a to 1c, and is output as the output signal 13 from the isolator 4b.
[0036]
FIG. 4 is a diagram showing the gain and noise figure of this optical fiber amplifier. The output power of each of the first pumping light sources 2a and 2b is 200 mW, the output power of the first pumping light source 2c is 100 mW, and the Tm-doped optical fiber The gain and the noise figure when the pump light having an output power of 40 mW is introduced into the module 1a by the second pump light source 3a and the pump light having the output power of 40 mW is introduced into the Tm-doped optical fiber module 1b by the second pump light source 3b. is there.
[0037]
According to FIG. 4, a gain of 25 dB or more and a noise figure of 5 dB or less were obtained. In addition, the output of the amplifier was at least +15 dBm. Also, when the backward pumping by the first pumping light source 2c of the second stage section 12 was removed, it was confirmed that the amplifier output was +6 dBm even with the same pumping power.
As described above, the output of the amplifier was significantly improved.
[0038]
Further, since the isolator 4c for preventing the ASE from entering the first step 11 is inserted between the first step 11 and the second step 12, the operation of the optical fiber amplifier is performed. And the efficiency of gain shift can be improved.
[0039]
The embodiments of the optical fiber amplifier according to the present invention have been described above with reference to the drawings. However, the specific configuration is not limited to the present embodiment, and a design change or the like may be made without departing from the gist of the present invention. Is possible.
For example, in the above-described embodiment, an example in which one or three Tm-doped optical fiber modules are used has been described. However, the number, length, shape, Tm concentration, and the like of the Tm-doped optical fiber modules are desired. It can be changed appropriately so as to obtain a gain and an amplifier output, and is not limited to the above embodiment.
[0040]
In the above embodiment, the optical fiber module doped with Tm is used. However, the element to be added may be a rare earth element having an emission transition using stimulated emission transition between energy levels higher than the ground level. However, the present invention is not limited to only Tm.
Further, the wavelength of the first pumping light sources 2a to 2c may be appropriately selected for the light emission transition, and may be any as long as it can constitute an optical fiber amplifier. The wavelengths of the second pumping light sources 3, 3a, 3b may be any as long as they have a wavelength suitable for pumping from the ground level to the laser lower level.
[0041]
In the above embodiment, the first pumping light sources 2a to 2c are Yb optical fiber lasers, and the second pumping light sources 3, 3a, 3b are Er-doped optical fiber lasers. A laser, a combination of a semiconductor laser and an optical fiber amplifier (for example, a 1.55 μm band DFB laser and an Er-doped optical fiber amplifier), or various solid-state lasers (titanium sapphire, semiconductor laser-pumped Nd: YAG laser, etc.) may be used. Good.
Further, by connecting the optical fiber amplifier of the present invention and a conventional optical fiber amplifier in series and / or in parallel, a wide band gain can be realized.
[0042]
【The invention's effect】
As described above, according to the optical fiber amplifier of the present invention, it is possible to realize an optical fiber amplifier with high efficiency, low noise, and a long gain peak wavelength shifted. Therefore, by using the optical fiber amplifier of the present invention together with the optical fiber amplifier that does not shift the gain peak, it is possible to cope with an increase in capacity that requires a wide amplification wavelength band. Wavelength multiplex communication with little transmission loss can be performed.
[Brief description of the drawings]
FIG. 1 is a configuration diagram illustrating an optical fiber amplifier according to a first embodiment of the present invention.
FIG. 2 is a diagram illustrating a gain and a noise figure of the optical fiber amplifier according to the first embodiment of the present invention.
FIG. 3 is a configuration diagram illustrating an optical fiber amplifier according to a second embodiment of the present invention.
FIG. 4 is a diagram illustrating a gain and a noise figure of an optical fiber amplifier according to a second embodiment of the present invention.
FIG. 5 is a schematic diagram showing energy levels of Tm and transitions of first and second excitation light sources.
FIG. 6 is a configuration diagram showing a conventional two-wavelength pumped Tm-doped fiber amplifier.
FIG. 7 is a configuration diagram showing a conventional multi-stage optical fiber amplifier.
[Explanation of symbols]
1,1a-1c Tm doped optical fiber module (glass fiber)
2a to 2c First excitation light source 3, 3a, 3b Second excitation light source 4a to 4c Isolator 5 Input signal 6, 13 Output signal 11 First step 12 Second step

Claims (7)

A plurality of glass fibers doped with a rare earth element are connected in series to form a multi-stage gain medium, and the stimulated emission transition between two energy levels of the rare earth ion energy levels higher than the ground level is used. A multi-stage optical fiber amplifier,
A first pumping light source that forms a population inversion between the two energy levels in front of the glass fibers of at least the last stage of the plurality of stages, and an oscillation wavelength of the first pumping light source. It has a different oscillation wavelength, and comprises a second excitation light source that excites ions from the ground level to the laser lower level of the two energy levels, and the glass fiber is configured to be excited from the front,
A first excitation light source and a second excitation light source provided at least in front of the last stage glass fiber, and the first excitation light source provided behind the glass fiber; An optical fiber amplifier characterized in that it is configured to excite light from both directions. The optical fiber amplifier according to claim 1, wherein the rare earth element is one selected from thulium (Tm), holmium (Ho), erbium (Er), and praseodymium (Pr). The rare earth ions added to the glass fiber are thulium ions (Tm ³⁺ 3. The optical fiber amplifier according to claim 1, wherein 4. The optical fiber amplifier according to claim 1, wherein the glass fiber is a rare earth element-doped fluoride glass fiber. The optical fiber amplifier according to any one of claims 1 to 4, wherein an optical isolator for a signal wavelength band is provided between each of the stages. The wavelength of light output from the first excitation light source is 1.04 to 1.07 μm, and the wavelength of light output from the second excitation light source is 1.53 to 1.90 μm. The optical fiber amplifier according to any one of claims 1 to 5, wherein Multiple optical fiber amplifiers connected in series and / or parallel Optical amplifier
7. An optical amplifying device, wherein at least one of the optical fiber amplifiers is the optical fiber amplifier according to claim 1.

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