WO2007034563A1

WO2007034563A1 - Optical amplifier

Info

Publication number: WO2007034563A1
Application number: PCT/JP2005/017620
Authority: WO
Inventors: Tomoaki Takeyama; Keiko Sasaki; Tatsuya Tsuzuki
Original assignee: Fujitsu Limited
Priority date: 2005-09-26
Filing date: 2005-09-26
Publication date: 2007-03-29
Also published as: JPWO2007034563A1

Abstract

An optical amplifier comprising an optical amplification medium added with a rare earth element and having an incident end and an exit end with L-band signal light impinging on the incident end, a first light source generating first light for exciting the amplification medium, a second light source generating second light for accelerating radiation in the amplification medium, a multiplexer for multiplexing the first light and the second light, and a section for supplying multiplexed light from the multiplexer to the incident end or the exit end.

Description

Specification

Optical amplifier

Technical field

The present invention relates to an optical amplifier, and more particularly to an optical amplifier that amplifies an L-band WDM (Wavelength Division Multiplex) optical signal.

Background art

In recent years, with the development of Internet technology, the demand for information services has increased dramatically. In trunk-line optical transmission systems, there is a demand for further large capacity and flexible network formation.

[0003] WDM is the most effective transmission technology to meet such system demands. WDM is a transmission system that multiplexes light of different wavelengths and transmits multiple signals simultaneously using a single optical fiber. Currently, commercialization of WDM is already in progress mainly in North America.

[0004] On the other hand, there is an EDFA (Erbium-Doped Fiber Amplifier) as a key component for realizing a WDM system. EDFA is erbium (Er ³⁺⁾添Ka卩fiber (EDF: Erbium- Do ped Fiber) and the amplifying medium, which can collectively amplifying the optical signal wavelength-multiplexed by using a wide gain band light It is an amplifier.

[0005] The C band (Conventional band: about 1530 to about 1560 nm) with the highest amplification efficiency was first commercialized, and now the L band (Longer wavelength b and: about 1570 to the next highest amplification efficiency). 1610nm) EDFA has also been commercialized.

[0006] The most basic L-band EDFA configuration has a forward excitation configuration as shown in FIG. In Fig. 1, an L-band EDFA is a multiplexer that combines, for example, an excitation light source (laser diode: LD) 1 that emits excitation light (pump light) of 0.98 m, and L-band signal light and excitation light. 2 and EDF3 into which the combined light is incident.

[0007] The characteristics of L-band optical amplification in EDFA will be briefly described. Figure 2 shows the wavelength characteristics of the EDF gain per unit length (gain coefficient) with respect to the inversion distribution rate t (t = 0.0 to 1.0). Figure 3 shows the C band and L It is a figure which shows the gain band of a band. As shown in FIG. 2 and FIG. 3, in the C band, the gain characteristic becomes flat when the population inversion ratio is about 0.7 (t = 0.65 in FIG. 3), and the gain of the C band dominates. (Almost constant). On the other hand, when the population inversion ratio is about 0.7, the wavelength of the L band is tilted. For this reason, in the L-band amplification, about 0.4 (t = 0.39 in Fig. 3) is used as the inversion distribution rate at which the gain characteristics are almost flat (constant). In other words, in the L-band optical amplification, the 0.98-111 band and 1.48-m band excitation light used in the C-band amplification is supplied to the EDF under the setting of the inversion ratio lower than that of the C band Is done. In this case, since the inversion distribution rate is low, the amplification rate per unit length is low. Therefore, the EDF for L-band amplification is lengthened to supply almost the same gain to the L-band signal light and to have the same gain as the C-band signal light gain.

In the EDFA configuration as shown in FIG. 1, when L-band signal light and excitation light are incident on the EDF3, the amount of absorption per unit length at the excitation light wavelength is less than that at the L-band signal light wavelength. Due to the very small amount of radiation per unit length, as shown in the graph of 0. ge ^ mlSOmW in Fig. 6, the inversion distribution rate rises in the vicinity of 2 to 6 m (5 m) EDF length. . Incidentally, in the case of C-band signal light, the amount of radiation per unit length is not as small as that of L-band signal light, so this rise is not as pronounced as in the L-band. A large amount of C-band ASE (Amplified Spontaneous Emission) occurs due to the rise of the reversal distribution rate near the EDF length of 5 m. Of the generated ASE, the ASE traveling toward the entrance is further amplified by the excitation light, and the entrance edge force is also emitted as a back ASE. With the above mechanism, in the L-band EDFA as shown in Fig. 1, the conversion efficiency deteriorates due to being used for amplification of the excitation light ¾ack ASE, and the conversion efficiency as high as the C-band EDFA cannot be obtained. .

[0010] Note that the inversion distribution ratio in the backward excitation is in a state opposite to that in the forward excitation.

[0011] As a method for suppressing the efficiency degradation peculiar to the L band as described above, for example, there are techniques disclosed in Patent Documents 1 and 2. In Patent Document 1, in the L-band EDFA, the optical signal in the C-band EDF amplified by 0.98 m excitation light and the specific wavelength included in the ASE (the highest efficiency in the C-band is 1.55 m) A configuration in which this amplified light is used as a tap coupler or pumping light for an L-band optical signal is disclosed.

However, the technique disclosed in Patent Document 1 has the following problems. First, C van EDF hardly amplifies L-band signal light. For this reason, the overall EDFA NF (Noise Figure) is almost equivalent to the loss caused by optical components such as tap couplers and filters placed between C-band EDF and L-band EDF, and isolators to prevent oscillation. : Noise figure) will deteriorate. Second, two EDFs are required: a first EDF that generates C-band ASE and a second EDF with L-band amplification. This leads to an increase in cost.

[0013] Patent Document 2 discloses a method of using 0.98 m residual pumping light output from C-band EDF and 1.55 m band ASE as excitation light for L-band EDF using a circulator. Yes.

However, the technique disclosed in Patent Document 2 has the following problems. First, the L-band EDF that amplifies the signal light is backward pumped. For this reason, the inversion distribution rate at the entrance to determine NF is greatly reduced, and it is thought that NF degradation will become severe. Second, a special part called a circuit is required. Furthermore, as in the technique of Patent Document 1, two EDFs are required, a first EDF that generates C-band ASE and a second EDF for L-band amplification.

Patent Document 1: Japanese Patent Laid-Open No. 2003-243755

Patent Document 2: Japanese Patent Laid-Open No. 2002-94158

Disclosure of the invention

Problems to be solved by the invention

[0015] An object of the present invention is to provide an L-band optical amplifier capable of improving efficiency while suppressing an increase in cost.

Means for solving the problem

The present invention employs the following configuration in order to achieve the above object.

That is, the present invention includes a rare earth element-doped optical amplification medium having an incident end and an output end, and an L-band signal light is incident on the incident end.

A first light source that generates first light for exciting the rare earth element-doped optical amplification medium; a second light source that generates second light that promotes radiation in the rare earth element-doped optical amplification medium;

A multiplexer that multiplexes the first and second lights from the first and second light sources; And a supply unit that supplies the light combined by the multiplexer to the incident end or the output end.

Preferably, in the optical amplifier according to the present invention, the supply unit supplies the light combined by the multiplexer to the incident end,

A third light source for generating the first light;

A fourth light source for generating the second light;

A second multiplexer that combines the first and second lights from the third and fourth light sources; and a second multiplexer that supplies the light combined by the second multiplexer to the emission end. And a supply unit.

Preferably, in the optical amplifier according to the present invention, the first light has a wavelength shorter than a C-band wavelength,

The second light has a wavelength longer than the first light and shorter than the L band.

[0020] Preferably, in the optical amplifier according to the present invention, the second light has a wavelength near a radiation peak in the rare earth element-doped amplification medium.

[0021] Preferably, in the optical amplifier according to the present invention, the wavelength of the second light is about 1.

49-: L 55 m. More preferably, it is about 1.53 m.

[0022] Preferably, in the optical amplifier according to the present invention, the power of the second light is about

— 30-20dBm.

[0023] Preferably, in the optical amplifier according to the present invention, a power ratio between the second light and the first light is about -50 to OdB.

[0024] The rare earth element-doped optical amplifying medium of the optical amplifier according to the present invention is an erbium-doped fiber or an erbium-doped optical waveguide.

The invention's effect

[0025] According to the present invention, it is possible to provide an L-band optical amplifier capable of improving efficiency while suppressing an increase in cost.

Brief Description of Drawings

FIG. 1 is a diagram showing a configuration example of a conventional EDFA.

[Figure 2] Figure 2 shows the wavelength distribution of the gain coefficient per unit length of EDF and the inversion distribution ratio (t = 0 to 1.0).

FIG. 3 is a diagram showing gain bands of C band and L band.

FIG. 4 is a diagram showing a configuration example of an embodiment of an optical amplifier according to the present invention.

[FIG. 5] FIG. 5 is a graph showing the relationship between wavelength and absorbed Z radiation of EDF.

FIG. 6 is a graph showing the relationship between EDF length and population inversion rate.

FIG. 7 is a graph showing the relationship between the wavelength at the incident end of the EDF and the back ASE.

FIG. 8 is a graph showing the relationship between wavelength and gain.

FIG. 8 is a table summarizing the characteristics of Examples and Comparative Examples 1 and 2.

FIG. 9 is a graph showing wavelengths that can be taken as second excitation light.

FIG. 11 is a graph showing a range that can be taken as the second pumping light power.

FIG. 12 is a graph showing a range that can be taken as a power ratio between the first excitation light and the second excitation light.

FIG. 13 is a diagram showing a modification of the embodiment, and shows a configuration example when the present invention is applied to a backward excitation EDFA.

FIG. 14 is a diagram showing a modification of the embodiment, and shows a configuration example when the present invention is applied to a bidirectional excitation EDFA.

Explanation of symbols

[0027] 10 · · · Erbium-doped fiber amplifier (EDFA)

11 · · 'Erbium-doped fiber (EDF)

12,12Α · · · First light source

12Β · · · 3rd light source

13,13Α… Second light source

13Β · · · 4th light source

14,14Α, 15,15Α

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described with reference to the drawings. The configuration in the embodiment is an exemplification, and the present invention is not limited to the configuration of the embodiment.

<Configuration of optical amplifier> FIG. 4 is a diagram showing an embodiment (configuration example) of an optical amplifier according to the present invention. Fig. 4 shows an EDFA that amplifies L-band signal light by forward pumping as an optical amplifier.

In FIG. 4, the EDFA 10 includes an EDF 11 as a rare earth element-doped optical amplification medium having an incident end and an output end, a first light source 12, a second light source 13, a first light source 12, and A multiplexer 13 that multiplexes two wavelengths from the second light source 13, a L-band signal light, and a multiplexed light from the multiplexer 13, and combines them as a supply unit that is sent to the incident end of the EDF 15. It consists of waver 15.

[0031] The EDF 11 is an EDF for L-band amplification, and has an EDF length that is sufficiently longer than the C-band EDF so that the gain of the L-band signal light can be obtained at a predetermined value (for example, 14 dB) or more.

The first light source 12 is configured using, for example, a semiconductor laser (LD). The first light source 12 emits first light that excites EDF. That is, the first light source generates pumping light having a wavelength shorter than the C band as pumping light (pump light) for the L band signal light. As the first light source 12, for example, a general-purpose 0.88, 0.88 or 1.48 m band excitation light (first light) used as an L-band excitation light source is applied. be able to.

[0033] The second light source 13 is configured using, for example, an LD. The second light source 13 emits light having a wavelength that promotes radiation in the EDF 11 (second light). The second light source 13 generates, as the second light, light having a wavelength longer than the wavelength of the excitation light output from the first light source 12 and shorter than the L band. For example, the wavelength of the second light is set to be near the radiation peak of EDF11.

FIG. 5 is a graph showing the relationship between the wavelength and the absorbed Z radiation of EDF. In Fig. 5, the lower graph shows the absorption characteristics of EDF when EDF excitation light (for example, 0.98 / zm) is not incident, and the upper graph shows the excitation light incident on EDF. Shows the radiation characteristics of EDF.

Here, focusing on the upper graph, the wavelength near the peak (radiation peak) in this graph is set as the wavelength of the second light. In the example shown in Fig. 5, the wavelength around 1530 nm (about 1.53 ^ m) is set as the wavelength of the second light. As the second light source 13, an lmW class low output LD that generates the second light having a desired wavelength can be used.

[0036] As the multiplexers 14 and 15, for example, a WDM optical fiber force bra is applied. The multiplexer 14 multiplexes the first light (excitation light) from the first light source 12 and the second light (referred to as “radiation promoting light”) from the second light source 13, Send to 15. The multiplexer 15 has one wave or wavelength multiplexed L-bar. Signal light is incident. The multiplexer 15 combines the L-band signal light and the light combined by the multiplexer 14 and enters the incident end of the EDF 11.

According to the optical amplifier (EDFA10) shown in FIG. 4, the first light from the first light source 12 (light that generates excitation light of EDF11 (absorption (increased inversion distribution rise): for example, 0.98 / ζ πι)), The second light from the second light source 13 (light having a wavelength in the vicinity of the emission peak of EDF 11 (about 1.53 m) (radiation-promoting light (radiation-induced light)) is multiplexed by the multiplexer 14. The light combined by the multiplexer 14 is combined with the L-band signal light by the multiplexer 15 and is incident on the incident end of the EDF 11.

[0038] As shown in FIG. 5, the wavelength having the highest radiation characteristic is selected as the wavelength of the radiation promoting light (second light). This wavelength is the most effective factor that causes (or induces) radiation in EDF11, and the inversion distribution near the EDF11 entrance (2-6 m, more specifically around 5 m), as shown in Figure 6. Rise is suppressed. Suppressing the rise of the population inversion means that the generation of ASE near 5m is suppressed. This suppresses the generation of back ASE that travels in the direction of the entrance and is amplified by the excitation light and radiated at the entrance edge. As a result, the excitation light (first light) can be efficiently used to amplify the L-band signal light without being used to amplify the back ASE. Therefore, the conversion efficiency (amplification efficiency) of EDFA is improved.

[0039] The EDFA 10 can be configured by adding a low-power LD (second light source 13) and a multiplexer 14 to the configuration of the conventional EDFA. Therefore, efficiency can be improved at low cost.

[0040] It should be noted that the first and second lights described above both mean light that contributes to amplification of signal light.

, “First excitation light” and “second excitation light”.

Next, an embodiment of the EDFA 10 having the configuration shown in FIG. 4 will be described. In the examples, EDF having a length of 28.5 m was applied as EDF11.

[0042] In addition, as the L-band signal light amplified by the EDFA 10, 32 waves (channels) at intervals of 100 GHz from 1577.03 nm to 1603.17 nm, and light of -20 dBmZch was applied. As the first light source 12, a pumping LD that outputs 0.98 m and 130 mW light was used. [0043] The EDF emission peak wavelength depends on the material doped with erbium (Er) (such as A1). The emission peak wavelength of EDF11 applied in this example was 1.53 / zm. For this reason, an LD that outputs 1.53 / zm light (radiation promoting light) was used as the second light source 13. The power of radiation promotion light was lmW.

[0044] FIG. 6 is a graph showing the relationship between the EDF length measured for the example and the population inversion ratio. FIG. 7 is a graph showing the relationship between the wavelength and backASE measured for the example. FIG. 8 is a graph showing the relationship between the wavelength and gain measured for the example.

FIG. 6 and FIG. 7 show the measurement results for Comparative Example 1 as an example to be compared with the Example, and FIG. 8 shows the measurement results for Comparative Example 2 as an example to be compared with the Example. It is shown. FIG. 9 is a table summarizing the measurement results of (2) Examples, (1) Comparative Example 1 and (3) Comparative Example 2. Details of Comparative Example 1 and Comparative Example 2 are as follows.

<< Comparative Example 1 >> As Comparative Example 1, an EDFA having the configuration shown in FIG.

Only the excitation light of ^ m, 130mW is incident on EDF11 (1.53m is not incident). Conditions other than the conditions relating to the incident light are the same as in the example.

<< Comparative Example 2 >> As Comparative Example 2, in addition to the configuration of Comparative Example 1, an excitation light force consisting of 5 C-band 5 nm-spacing 5 waves was made incident on the incident end of DF11. This excitation light is assumed to use ASE that spreads over the entire C band for excitation of EDF. Conditions other than those relating to incident light are the same as in the example.

FIG. 6 shows the measurement results when the light according to the example and the comparative example 1 is incident. As shown in Fig. 6, in Comparative Example 1 (conventional EDFA), the inversion distribution rate near the EDF11 entrance (about 2 to 6 m) rises, and then the inversion distribution rate gradually increases toward the output end. Decreases. As shown in Fig. 7, this rising part appears at the incident end of EDF11 as a backASE in the C-band (peak: about 1530 nm) that causes deterioration of the excitation efficiency.

On the other hand, in the example, as shown in FIG. 6, the portion of the rising force S near the entrance that existed in Comparative Example 1 disappears, and is flat near the inversion distribution ratios 0 and 5. . As shown in FIG. 7, the backASE centered at about 1530 nm, which is present in Comparative Example 1, is reduced due to the disappearance of the rising portion. Therefore, in the example, the inversion distribution ratio near the EDF entrance is improved. It can be seen that the occurrence of backASE that adversely affects efficiency is suppressed.

[0050] Also, referring to (2)-(1), the difference in NF between (2) Example and (1) Comparative Example 1 shown in FIG. Thus, it can be seen that the degradation of NF is sufficiently small.As described above, according to the present invention, it is possible to improve the efficiency while suppressing the degradation of NF.

[0051] Also, according to FIG. 9, the addition of radiation promoting light (1.53 μm) in the example improves the conversion efficiency by about 4 dB (4.2 dB), indicating that The

FIG. 8 shows a comparison between Example and Comparative Example 2. In this comparison, the total power of the ASE (C band 7 waves) used in Comparative Example 2 and the radiation promoting light (1.53 m) in the example was set to lmW. A comparison of the conversion efficiency between Example and Comparative Example 2 is shown in FIG.

As shown in FIG. 9, the conversion efficiency in Comparative Example 2 was 9.5 dB, showing the same result as in Comparative Example 1. In other words, ASE does not improve efficiency. This is because, as shown in Fig. 8, the pump light power of 0.98 m is used to amplify the pump light in the C-band long wavelength band.

FIG. 10 is a graph showing the relationship between the excitation wavelength (second light wavelength), the conversion efficiency, and the EDF radiation coefficient. In Fig. 10, we focus on the graph showing the relationship between conversion efficiency and wavelength. As a result, it can be seen that conversion efficiency is higher than those of Comparative Examples 1 and 2 in the wavelength band of 1490 nm to 1550 nm (about 1.49 to 1.55 m).

In FIG. 10, the conversion efficiency has a peak at a wavelength of 1532 nm (about 1.53 μm).

This is referred to as the peak of the EDF radiation coefficient graph shown in Figure 10. In other words, the conversion efficiency is maximized at the EDF radiation peak. Therefore, in the present invention, as the second light (radiation promoting light), the wavelength near the radiation peak (for example, around 1.53 m) is most preferably selected as the wavelength of the radiation peak. Is next preferred. In addition, selecting a wavelength near 1.53 m is preferable from the viewpoint of efficiently using excitation light of 0.98 m in view of the comparison result with Comparative Example 2.

However, from the viewpoint of improving efficiency, the wavelength of the second light can be selected from a predetermined range centered on the radiation peak. In the example shown in Fig. 10, the peak wavelength (1532nm: -5.3dB) force is also reduced when the conversion efficiency decreases by ldB, 2dB, and 3dB. m), 1518-1545 (approximately 1.51-: L 54 μm), It was 1512-1548 nm (about 1.51-: L55 / zm). One wavelength in these ranges can also be selected as the second light.

FIG. 11 is a graph showing the relationship between the power of 1.53 / z m pumping light (second light) and the conversion efficiency. In FIG. 11, from the viewpoint of obtaining the best conversion efficiency, 7 dBm, which is the power when the conversion efficiency reaches the peak, can be selected as the optical power.

[0058] Further, from the viewpoint of improving the conversion efficiency, the power of the second light can be selected as about -30 to 20 dBm as shown in FIG. Furthermore, in FIG. 11, the optical power ranges when the conversion efficiency is reduced by 1 dB, 2 dB, and 3 dB are −10 to 17 dBm, 19 to 20 dBm, and −25 to 22 dBm. The power of the second light can be selected from any of these ranges.

FIG. 12 is a graph showing the power ratio between the second light (1.53 μm) and the first light (0.98 μm). In Fig. 12, from the viewpoint of obtaining the best conversion efficiency, 1 ldB, which is the power ratio when the conversion efficiency reaches its peak, can be selected.

[0060] From the viewpoint of improving the conversion efficiency, as the power ratio, as shown in FIG. 12, an approximately -50 to OdB force can be selected. Further, in FIG. 12, the optical power ranges when −1 dB, 2 dB, and 3 dB respectively decrease from the peak conversion efficiency were −32 to −6 dB, −40 to 1 ldB, and −47 to 0 dB. The power ratio can be selected from any force in these ranges.

[0061] Modified Examples>

Figure 4 shows the EDFA10 with forward excitation. However, the present invention can also be applied to an EDFA that performs backward excitation or bidirectional excitation. FIG. 13 is a diagram illustrating a configuration example of the EDFA 10A that performs backward excitation, and FIG. 14 is a diagram illustrating a configuration example of the EDFA 10B that performs bidirectional excitation.

[0062] In FIG. 13, an EDFA 10A combines a first light source 12A that emits first light, a second light source 13A that emits second light, and a first light and a second light. A multiplexer 14A and a multiplexer 15A as a supply unit for sending the combined light to the output end of the EDF 11 are provided. The same components as those shown in Fig. 4 can be applied.

[0063] Also, in FIG. 14, the EDFA 10B includes a configuration related to forward excitation shown in FIG. And the configuration related to the backward pumping shown. In other words, in addition to the configuration shown in FIG. 4, the EDFA 10B transmits the third light source 12B that generates the first light, the fourth light source 13B that generates the second light, and the first and second lights. A multiplexer 14B as a second multiplexer for multiplexing and a multiplexer 15B as a second supply unit for supplying the light combined by the multiplexer 14B to the emission end of the EDF 11 are provided.

The components shown in FIGS. 13 and 14 can be the same as the components shown in FIG. 4 (first light source 12, second light source 13, multiplexers 14 and 15). Therefore, description of each component is omitted.

Further, in the present embodiment, an example in which EDF is applied as a rare earth element-doped optical amplification medium has been described. Instead of EDF, it is also possible to apply an optical waveguide doped with erbium.

[0066] In the present embodiment, the optical amplifier to which erbium is applied as the rare earth element has been described. However, it is conceivable that the present invention is applied to an optical amplifier using an amplification medium doped with a rare earth element different from erbium (for example, prasedium or thulium).

Claims

The scope of the claims

[1] A rare earth element-doped optical amplifying medium having an incident end and an output end, and an L-band signal light is incident on the incident end;

A multiplexer that combines the first and second lights from the first and second light sources, and a supply unit that supplies the light combined by the multiplexer to the incident end or the exit end; And an optical amplifier.

[2] The supply unit supplies the light combined by the multiplexer to the incident end,

A third light source for generating the first light;

A fourth light source for generating the second light;

A second multiplexer that combines the first and second lights from the third and fourth light sources; and a second multiplexer that supplies the light combined by the second multiplexer to the emission end. The optical amplifier according to claim 1, further comprising: a supply unit.

[3] The first light has a wavelength shorter than the C-band wavelength,

The optical amplifier according to claim 1 or 2, wherein the second light has a wavelength longer than the first light and shorter than the L band.

[4] The second light has a wavelength near a radiation peak in the rare earth element-doped amplification medium.

The optical amplifier according to claim 1.

[5] The wavelength of the second light is about 1.49-: L 55 m

The optical amplifier according to claim 1.

[6] The power of the second light is about 30 to 20 dBm.

The optical amplifier according to claim 1.

[7] The power ratio between the second light and the first light is about −50 to OdB.

The optical amplifier according to claim 1.

[8] The rare earth element-doped optical amplification medium is an erbium-doped fiber. The optical amplifier according to claim 1.

[9] The optical amplifier according to any one of [1] to [7], wherein the rare earth element-doped optical amplification medium is an erbium-doped optical waveguide.