JP2002246673A - Optical amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical fiber amplifier which suppresses the amplification of spontaneous emission light of a wavelength 0.8 μm band, which is a cause for deteriorating the amplification efficiency of the light of the wavelength 1.47 μm band, and whose amplification efficiency of the light of the wavelength 1.47 μm band is satisfactory. SOLUTION: An optical fiber where thulium and neodymium are added to a core is made to be a gain medium. The spontaneous emission light of the wavelength 0.8 μm band, which occurs at the time of optical amplification by thulium, is absorbed by neogim, and the light of the wavelength 1.47 μm band is efficiency amplified.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a thulium (Tm)
Amplifier using an optical fiber having a core doped with Nb as a gain medium, in particular, co-doping neodymium (Nd) with a wavelength of 1.47
The present invention relates to an optical amplifier capable of improving amplification efficiency in the μm band.

[0002]

2. Description of the Related Art For the purpose of amplifying light in a wavelength band of 1.47 μm, an optical amplifier using an optical fiber (hereinafter abbreviated as “TDF”) doped with thulium as a gain medium is used. This optical amplifier amplifies signal light in the 1.47 μm band using pump light in the 1.05 μm band. As is apparent from the energy level of the thulium ion (Tm ³⁺⁾ shown in FIG. 2, in the wavelength 1.47μm band, ³
H ₄ is the upper level, ³ F ₄ becomes lower level, after being pumped by the pumping light of wavelength 1.05μm band, the transition from ³ H ₄ to ³ F _4, light of wavelength 1.47μm band is amplified You.

[0003]

[SUMMARY OF THE INVENTION However, with the optical amplifier, occur transition from ³ H ₄ to ³ H _6, spontaneous emission light in the wavelength 0.8μm band is generated by this transition. This wavelength 0.8μ
The ratio of the emission of the spontaneous emission light in the m band to the emission of the amplified light in the 1.47 μm band (hereinafter abbreviated as “branching ratio”) is found to be about 9: 1 in the case of the fluoride optical fiber. (Michel JFDigonnet (ed.) “Rare earth doped fi
ber lasers and amplifiers ”Marcel Dekker Inc (199
3) Page 111, Table 22). As described above, the spontaneous emission light in the wavelength of 0.8 μm is large and the spontaneous emission in the wavelength of 0.8 μm is more easily generated and amplified in the gain medium than the light in the wavelength of 1.47 μm. As a result, there has been a problem that the density of electrons at the energy level of ³ H ₄ decreases, and the amplification efficiency of light in the 1.47 μm wavelength band deteriorates. The present invention has been made in view of such circumstances, and has a wavelength of 1.
Suppresses the amplification of spontaneous emission light in the 0.8 μm band, which is a cause of deteriorating the amplification efficiency of 47 μm light,
An object of the present invention is to provide an optical fiber amplifier having good amplification efficiency of light in the 1.47 μm band.

[0004]

In order to solve the above-mentioned problems, according to the first aspect of the present invention, a thulium-doped glass is used as a gain medium, and pump light and signal light are multiplexed with the gain medium. An optical amplifier for amplifying an incident signal light, wherein neodymium is co-doped in a gain medium. According to a second aspect of the present invention, in the first aspect, the gain medium is an optical waveguide. According to a third aspect of the present invention, in the first or second aspect, neodymium is co-added at a concentration of 200 ppm or more.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.
FIG. 1 shows an example of an optical fiber amplifier according to the present invention. FIG. 1 is a configuration diagram of an optical fiber amplifier according to this example. The optical transmission line 1 is, for example, an optical transmission line for transmitting signal light in the 1.47 μm band. The optical transmission line 1 is connected to an input port of the WDM coupler 2. An excitation light source 3 is connected to another input port of the WDM coupler 2,
An output port of the M coupler 2 is an optical fiber (hereinafter, referred to as “TN”) in which thulium and neodymium, which are gain media, are added to a core.
DF "). This T
The other end of the NDF 4 is connected to the optical transmission line 1. In this example, the connection between the optical components is performed by fusion splicing.

Here, as a gain medium, silica-based TNDF in which thulium and neodymium are added to a core of silica-based fiber in which Ge and Al are added to a core is used based on silica glass. Silica-based TNDF
The relative refractive index difference between the core and the cladding is 0.5-2%,
The concentration of Al is 0 ppm to 15000 ppm, and the core diameter is 2 μm to 5 μm.
μm, the cut-off wavelength is 1.3 μm or less, the concentration of thulium added to the core is 500 ppm to 5000 ppm, and the concentration of neodymium added to the core is 200 ppm or more, preferably 200 p.
pm to 1000 ppm and a fiber length of 5 m to 20 m are used. Note that the TNDF used here is not limited to silica-based TNDF, but TNDF based on glass other than quartz, such as fluoride-based TNDF.
It may be.

The excitation light source 3 has a wavelength of 1.05 μm
A pump that emits band excitation light and has an output of 0.5 W to 2.0 W is used. Specifically, an ytterbium-doped fiber laser or the like is used. As the WDM coupler 2, an optical fiber fusion type coupler or the like is used.

Next, the operation of the optical fiber amplifier according to this embodiment will be described. For example, a wavelength of 1.47 sent from the optical transmission line 1
The signal light in the μm band is supplied to the excitation light source 3 in the WDM coupler 2.
Multiplexed with the pumping light having a wavelength of 1.05 μm. The signal light and the pump light are input to one end of the TNDF 4, where they are optically amplified and output from the other end of the TNDF 4 to the optical transmission line 1. Neodymium is added to the core of the TNDF 4 in order to improve the amplification efficiency of signal light in the 1.47 μm band. Due to the addition of neodymium, the wavelength 1.47
The principle by which the amplification efficiency of the signal light in the μm band can be improved will be described with reference to FIG.

FIG. 2 shows the energy levels of thulium ions and neodymium ions (Nd ³⁺ ). First, the energy level is changed by the excitation light in the wavelength of 1.05 μm.
After a transition from the ³ H ₆ level to the ³ H ₅ level, a non-radiative transition causes a transition to the ³ F ₄ level. ^Since the excitation life of the ³ F ₄ level is relatively long, the excitation light in the wavelength band of 1.05 μm is again absorbed and ³ F ₂
After being excited to a level, non-radiative transition causes a transition to the ³ H ₄ level. This pumping from the ³ F ₄ level to the ³ F ₂ level shortens the effective excitation lifetime of the ³ F ₄ level, thereby reducing the energy between the ³ H ₄ level and the ³ F ₄ level. A population inversion is formed,
Equivalent to the energy difference between these two energy levels,
Amplification of light in the wavelength band of 1.47 μm becomes possible.

At this time, since a transition from the ³ H ₄ level to the ³ H ₆ level also occurs, natural light having a wavelength of 0.8 μm corresponding to the energy difference between the two energy levels is emitted. This natural light is absorbed by neodymium added to the core. Since the energy difference between the ⁴ I _9/2 level and the ⁴ F _5/2 level corresponds to the energy of light in the wavelength of 0.8 μm band, neodymium ions emit light in the wavelength of 0.8 μm band generated by thulium ions. Neodymium ions absorb ⁴ I _9/2
The transition from the level to ⁴ F _5/2 level.

The absorption spectrum of neodymium has a remarkable absorption peak A near a wavelength of 0.8 μm as shown in FIG.
Therefore, by adding neodymium to the core, in the amplification of light in the 1.47 μm band, the wavelength 0.8 μm
Amplification of natural light in the band can be effectively suppressed. Thus, by suppressing the amplification of natural light in the wavelength band of 0.8 μm, it is possible to prevent the density of electrons existing at the ³ H ₄ level of thulium ions from decreasing, and to reduce the wavelength to 1.47 μm.
Band light can be efficiently amplified.

In the optical amplifier of this example, since an optical fiber in which thulium and neodymium is added to the core is used as a gain medium, amplification of spontaneous emission light of 0.8 μm band generated at the time of light amplification of 1.47 μm band light is performed. Suppress, wavelength 1.47μ
An optical amplifier having good amplification efficiency for light in the m band can be realized. Hereinafter, specific examples will be described.

EXAMPLE An optical fiber amplifier having the structure shown in FIG. 1 was manufactured. As the excitation light source 3, an ytterbium-doped fiber laser was used, and an excitation light having a wavelength of 1.05 μm and an output of 2 W was emitted. Further, TNDF4 has a relative refractive index difference between the core and the cladding of 1.2%, and Al of 11700 pp.
m, doping 1000 ppm of thulium, core diameter 3.3 μm,
A cut-off wavelength of 0.93 μm and a fiber length of 5 m were used. In an optical amplifier using a fluoride TDF as a gain medium, the power of the pump light is usually about several hundred mW to about 1 W, but in the embodiment of the present invention, the power of the pump light is set to 2 W.
By doing so, a high-output amplified light can be obtained.

Next, with respect to the neodymium addition concentration, the wavelength
A study was made on how the gain of TNDF4 for 1.47 μm band light was improved, and the results are shown in FIG. The gain was improved with an increase in the added amount of neodymium, and the gain became almost constant when the added amount exceeded 200 ppm. If the added amount of neodymium is 200 ppm or more,
It was found that the gain was increased by 2.5 dB as compared with the case where neodymium was not added. For this reason, the minimum amount of neodymium added was set to 200 ppm.

In addition, G added to the core described here
e, Al concentration, TNDF core diameter, cutoff wavelength,
The values of the concentrations of thulium and neodymium and the fiber length added to the core are merely examples, and other values may be used as long as the object of the present invention is achieved. Also, the wavelength
For the light source used as the excitation light source in the 1.05 μm band,
What is described here is merely an example, and other light sources may be used as long as the object of the present invention is achieved. Here, the expression of the wavelengths of the pump light and the signal light as in the 1.05 μm band is not limited to only light having a wavelength of 1.05 μm, but also includes wavelengths near 1.05 μm.

[0016]

As described above, according to the present invention,
Since the gain medium is an optical fiber with thulium and neodymium added to the core, it suppresses the amplification of spontaneous emission light in the 0.8 μm band, which occurs when the light in the 1.47 μm band is amplified,
An optical amplifier having good amplification efficiency for light in the 1.47 μm wavelength band can be provided.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an optical fiber amplifier according to this example of the present invention.

FIG. 2 is a diagram illustrating the energy levels of thulium and neodymium used in the optical fiber amplifier according to this embodiment of the present invention, and a mechanism for absorbing natural light in the 0.8 μm band with neodymium.

FIG. 3 is a diagram showing an absorption spectrum of neodymium.

FIG. 4 is a graph showing the effect of improving the gain with respect to the concentration of added neodymium.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Optical transmission line, 2 ... WDM coupler, 3 ... Excitation light source, 4 ...
TNDF

Claims (3)

[Claims] 1. An optical amplifier in which thulium-doped glass is used as a gain medium and pump light and signal light are multiplexed and incident on the gain medium to amplify the signal light, wherein neodymium is co-doped into the gain medium. An optical amplifier characterized in that: 2. The optical amplifier according to claim 1, wherein said gain medium is an optical waveguide. 3. The optical amplifier according to claim 1, wherein said neodymium is co-doped at a concentration of 200 ppm or more.